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transmitted to the crankshaft by connecting rods, and inertia load due to the mass of thecomponent and its attachments. Due to the dynamic nature of the system, both the gasand the inertia forces apply bending and torque to the component. The bendingmoments are reacted by journal bearings. Figure 3 shows the variation of bending andtorque with respect to crank angle, separately measured at the fillet of an eight cylinder

engine crankshaft (2).Identifying sources of failure of crankshaft is critical to its design and

optimization. On the crankshaft surface, fillets behave as stress raisers and cracks cannucleate at their surface due to combined cyclic bending and torsion and grow inward.Silva (3) separates failure sources into three categories; operating sources, mechanicalsources, and repairing sources. The operating sources include oil absence, defectivelubrication on journals, high operating oil temperature, and improper use of the engine.Mechanical sources include misalignments of the crankshaft on assembly, improper

journal bearings (wrong size), lack of control on the clearance between journals andbearings, and crankshaft vibration. Repairing sources include misalignments of the

journals due to improper grinding, misalignments of the crankshaft, high stress

concentrations due to improper grinding at the radius on either side of the journals, highsurface roughness due to improper grinding or wear, improper welding or nitriding, andstraightening operations.

In this review paper, first the conceptual design process for crankshafts and thetasks involved are discussed. Then, manufacturing process steps for cast and forgedcrankshafts are presented and the influence of residual stresses on fatigue behaviorfrom fillet rolling, shot peening, nitriding, and induction hardening are discussed. This isfollowed by a comparison of crankshaft material and manufacturing processtechnologies currently in use in terms of their durability performance. Several durabilityassessment procedures used for crankshafts are then presented, including discussionsof fatigue crack growth, geometry effects, and surface hardening. Bench testing andexperimental techniques to evaluate fatigue performance of crankshafts are alsopresented. Next, geometric optimization of crankshafts is briefly discussed, and finally,cost analysis and potential cost saving potentials from several studies in the literatureare presented.

DESIGN AND MANUFACTURING CONSIDERATIONS

A thorough conceptual design process for a crankshaft requires input design datafrom the engine specifications and operating conditions, design task including design for

rigidity, static strength and durability, and manufacturing processes and considerations.Figure 4 shows a chart of crankshaft design tasks prepared by Dubensky (4). Accordingto this flowchart, preliminary dimensions are specified based on the engine design dataand previously designed comparable components. This preliminary design should beverified for rigidity, deformations, static strength, and fatigue strength under differentload-case scenarios (i.e. bending, torsion, and combined bending and torsion loadingconditions) considering appropriate factors of safety. Other design factors such as


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lubrication requirements, frequencies of vibration in torsion and bending and enginesound level are to be included afterwards. When the basic requirements of thepreliminary design are fulfilled, alternative manufacturing processes should beevaluated to obtain the most feasible and cost effective manufacturing plan. Followingpreparation of manufacturing plan and simulating the processes, a prototype component

should be manufactured and tested to verify the design requirements. A detailedflowchart including a step-by-step calculation procedure for crankshaft conceptualdesign is provided by Dubensky (4).

Crankshafts are typically manufactured by casting and forging processes. Figure5 shows the crankshaft manufacturing process flow for sand casting and forging.Manufacturing by forging has the advantage of obtaining a homogeneous part thatexhibits less number of microstructural voids and defects compared to casting. Inaddition, directional properties resulting from the forging process help the part acquirehigher toughness and strength in the grain-flow direction. While designing the forgingprocess for crankshaft, the grain-flow direction can be aligned with the direction ofmaximum stress that is applied to the component (along the axis of the shaft and

related to bending).According to Shamasunder (1), there are three typical stages in crankshaft

forging; reducer rolling (to prepare a preform), blocker forging (to give a basic profile tothe preform), and finisher forging (to give the desired contours to the crankshaft). Ineach stage, a well-planned deformation is induced to ensure metal flow into the diecavity in both top and bottom dies. As the forging continues, top and bottom diessqueeze the billet. Each point in the workpiece moves in a particular direction with aspecific velocity as determined by the die cavity profiles. Metal flow pattern will result ina complete filling of the die cavity to produce a sound forging quality. Grain flow patternis an indicator of forging quality as well as directional strength.

Manufacturing of the blank forged crankshaft could be followed by a number ofpost-processing steps including machining, heat treatment, induction hardening, andsurface rolling. The heat treatment step is not applied if microalloyed steels are selectedas materials of construction for the crankshaft, lending to considerable cost saving. Atypical procedure of the post-processing stage of a 42CrMo4 alloy steel crankshaftincluding machining, heat treatment and surface hardening is shown in Figure 6.

According to this procedure (5), the forging process is followed by annealing to removethe unwanted residual stresses generated by the refinement procedure applied toforgings. The annealed parts are subject to straightening after quenching andtempering, to correct for deformations due to the heat treatment. Additional annealing isthen performed to remove the residual stresses generated during straightening. Next, in

the machining stage, the part is turned and ground to obtain the required dimensionsand tolerances. Finish grinding should be carefully selected to avoid the occurrence ofunfavorable tensile residual stress distributions that would remain in the material evenafter induction surface hardening and grinding and would reduce fatigue strength of thematerial. Induction surface hardening is followed by stress annealing if the depth of thesurface hardening is smaller than the depth of the damaged layer, since in this way it ispossible to change the unfavorable stress state in the surface layer induced by


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machining. After induction surface hardening, the magnitude and distribution of residualstresses should be such that they contribute to the fatigue strength of the material.Induction surface hardening is followed by finish grinding and non-destructive magneticinspection of the surface to reveal the possible existence of cracks.

Fillet rolling has traditionally been used to induce compressive residual stresses

at the crankshaft fillets. The compressive residual stress generated at this critical areaincreases fatigue life of the component. Optical measurements at the deep-rolledcrankshaft fillets show that the amount of compressive residual stress increasessignificantly in the axial direction, which coincides with the direction of bending stressesthe component is subject to (6). In addition, prior to rolling crankshaft fillets, the stressgradients are very high near the fillet surface such that the stress magnitude drops veryquickly with increasing distance away from the fillet surface (7). This makes the processof fillet rolling quite beneficial.

The influence of the residual stresses induced by the fillet rolling process on thefatigue process of a ductile cast iron crankshaft section under bending was studied byChien et al. (7) using the fracture mechanics approach. They investigated fillet rollingprocess based on the shadowgraphs of the fillet surface profiles before and after therolling process in an elasticplastic finite element analysis with consideration of thekinematic hardening rule. A linear elastic fracture mechanics approach was employed tounderstand the fatigue crack propagation process by investigating the stress intensityfactors of cracks initiating from the fillet surface. An effective stress intensity factor,which combines the stress intensity factors due to the bending moment and due to theresidual stress, was defined. The effective stress intensity factor range was thenapproximated and compared to an assumed threshold stress intensity factor range todetermine if the crack can continue to propagate for a given crack length. To validatethe computational results, resonant bending fatigue tests were performed and four-bubble failure criterion was employed. According to this criterion, oil is applied to thefillet surface during the resonant bending fatigue test. For a given bending moment, thecrankshaft fatigue life is determined when four pinhead-sized bubbles within a 6.35 mmarea appear on the fillet surface. At this point, the test is suspended. It was found thatthis failure criterion that is based on cracks nucleated in the surface of the fillet is tooconservative when the residual stress near the fillet due to rolling and the high stressconcentration near the fillet are accounted for.

Park et al. (8) studied the effect of nitriding and fillet rolling on fatigue life ofcrankshafts made of microalloyed steel and quenched and tempered alloy steel. Figure7 shows component test results of this study for various surface treatments, i.e.nitriding, fillet rolling (500 kgf and 900 kgf) and bare samples. These experiments

indicate that surface modification can increase endurance limit of crankshaftsignificantly. Both fillet rolled (at 900 kgf of rolling) and nitrided samples show more thana factor of 1.8 increase in fatigue limit. In addition, it was found that higher rolling forceinduces higher compressive residual stress on the crank surface, leading to betterendurance limit characteristics. Nevertheless, with residual stress reaching a certainoptimum level, additional load only results in plastic deformation that is detrimental tofatigue limit. This optimum residual stress could be found experimentally or numerically


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using FEA. It should be noted that Gligorijevic et al. (9) reported a factor of 1.3improvement in fatigue strength of nodular cast iron diesel engine crankshafts afternitriding.

Gundlach et al. (10) investigated the influence of fillet rolling and shot-peening inaustempered ductile iron that is commonly used as an alternative for forged steel in

manufacturing crankshafts and camshafts. They performed rotating bending fatiguetests on smooth and notched hour-glass shaped specimens shown in Figure 8, madefrom the cylindrical end of the crankshaft. Fillet specimens had tangential fillets of 1.02mm, 1.52 mm, or 3.05 mm radius. They found that fillet rolling improves the fatiguestrength of all three fillet geometries considered. They suggest that the fillet rolling forcebe increased as the fillet radius increases to attain the same degree of compressivestress and subsequently obtain comparable improvements in fatigue strength. Usingoptimum conditions for fillet rolling will improve the fatigue strength of the filletedspecimens equal to or even above that of the smooth specimens. They also found thatshot peening also improves the fatigue strength of the filleted specimens, but to a lesserdegree than fillet rolling. Although the compressive residual surface stress by shot

peening was higher than that of the fillet rolled specimens, the depth of penetration ofthe compressive stress developed by shot peening was appreciably shallower than byfillet rolling. Figure 9 shows the improvement of fatigue strength of the ADI specimensby fillet rolling or shot peening. However, they emphasized that both shot peening andfillet rolling decrease the notch sensitivity of the ADI and move the fracture from thefillets to the gage section of the test specimens.

Hoffmann and Turonek (11) examined forged steel crankshafts in terms of theirmechanical properties and fillet rolling. The general material grades they testedincluding heat treatments and hardness are listed in Table 1. Sections of thecrankshafts were tested to generate complete reverse bending fatigue data. Table 2lists the fatigue data for the steel grades developed separately. The percentageincrease in fatigue strength attributed to fillet rolling for different materials are 23% forcarbon steel (CS) and high strength steel (HS), 44% for microalloyed steel MA1 with 8%V, and 25% for microalloyed steel MA2 with 6% V. The mechanical property data aresummarized in Table 3.

Induction surface hardening is also used to induce compressive residual stressesat the critical locations of the crankshaft. A study by Grum (5) shows that significantfavorable residual stress ranging from 1020 to 1060 MPa at a depth of around 250 m,then slowly dropping to a depth of 3.5 mm to around 800 MPa, is generated at bearinglocations by induction hardening of a forged CrMo steel crankshaft. Therefore, by gentlyvarying the hardness and through compressive residual stresses in the transition area, it

is possible to reduce the detrimental effect of the notch induced by stress concentrationin parts under cyclic loading. However, according to the same study, a major difficulty ininduction surface hardening is to ensure a very slight variation in hardness and theexistence of compressive residual stresses in transition areas to the hardness of thebase material.


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PERFORMANCE COMPARISON OF COMPETING MANUFACTURING TECHNIQUES

The major crankshaft material and manufacturing processes currently in use areforged steel, nodular cast iron and austempered ductile iron (ADI). Chatterley andMurrell (12) compared fatigue strength of crankshafts made of forged steel, ductile ironand austempered ductile iron by conducting bending fatigue tests. Tests wereperformed on nitrided 1% chromium/molybdenum forged steel, fillet-rolled ductile ironwith 700 MPa tensile strength and 2% elongation, and fillet-rolled ADI (the austemperedversion of the ductile iron used). All crankshafts were designed to operate within a 4-cylinder turbo-charged diesel engine. The fatigue tests were carried out on a constantamplitude mechanical stroking machine. The crankshafts were firmly clamped with splitclamping blocks across two main bearing journals, with the bending moment beingapplied by means of a moment arm bolted onto an adapter press-fitted onto either thefront (nose) or flywheel end of the shafts. Results of the fatigue tests are summarized in4. Fatigue strength at ten million cycles of the rolled ADI crankshaft was found to beinferior to the forged steel crankshafts. However, the ADI showed better fatigue strengththan ductile iron.

Pichard et al. (13) investigated the possibility of replacing the traditional alloyedsteels and cast iron with control-cooled microalloyed steel in order to save cost andincrease productivity without compromising the mechanical properties including fatigue.Based on the results of their research, 35MV7 control-cooled microalloyed steel showssimilar tensile and rotating bending fatigue strength as AISI 4142 quenched andtempered steel, while an improved machinable version of 35MV7 shows 40% higherturning index* and 160% higher drilling index compared to AISI 4142 quenched andtempered steel. In addition, the behavior of 35MV7 microalloyed steel after inductionhardening is equivalent to that of the referenced quenched and tempered steel. For the

response to ion nitriding compared to the nitrided heat treated steels such as AISI 4142and AISI 1042 grades, the surface values are identical, the conventional nitrided depthis greater and the treated layers are more homogeneous for the 35MV7 steel. For shortnitriding treatments, the results obtained on the 35MV7 steel are much better than thoseobtained for AISI 1042 quenched and tempered steel, the fatigue limit increased byabout 135%. In comparison with a quenched and tempered highly alloyed steel used incrankshafts (32CDV13) and for short nitriding treatments, the fatigue limit of 35MV7steel is only 10% lower, while significant cost saving could be made by using 35MV7steel. When forged steel are compared to cast iron and alloyed ductile iron used incrankshafts, the fatigue properties of forged steels are better that that of cast iron. Table5 shows fatigue experiment results on ductile iron, alloyed ductile iron, quenched and

tempered steel and microalloyed steel specimens made of crankshafts. A 13% costreduction was obtained for the final component by replacing the traditional AISI 4142steel with 35MV7 control-cooled microalloyed steel. This includes 10% savings on the

*Cutting speed that causes flank wear of 0.2 mm on the tip of the tool in 20 min continuous machining.Cutting speed at which it is just possible to bore 50 holes in a plate 40 mm thick before the tool becomes unusable.


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unfinished piece, 15% saving on mechanical operations and 15% saving on ion nitridingtreatment.

Gligorijevic et al. (9) named a number of advantages of nodular cast iron incrankshaft applications. According to them, nodular cast irons combine the favorablecharacteristics of other ductile materials, such as steel, with other advantages, such as

easy machinability, design flexibility (i.e. the free selection of shape of the componentand thus the ability to integrate several functions in a single part), high dimensionalaccuracy of the raw castings and the resultant cost reduction in final machining. Theystate that nodular cast iron has higher damping capacity and lower notch sensitivity.However, these statements may have overlooked the generally inferior fatigueproperties of nodular cast iron compared to steels, and the significant role of castingvoids and defects in reducing the crankshaft fatigue strength.

In the study of Gundlach et al. (10) discussed earlier, notch sensitivity of an ADI,namely a grade between ASTM A897 Grade 1 and 2, was investigated. As the severityof the fillet radius increased from 3.0 to 1.5 to 1.0 mm, the fatigue strength at ten millioncycles decreased from 335 MPa to 282 MPa to 253 MPa. Figure 9 shows thecomparison of fatigue strengths of smooth and filleted specimens. The results of thisstudy do not agree with those of Chatterley and Murrell (12) discussed earlier withrespect to the effect of fillet rolling on durability performance of ADI.

DURABILITY ASSESSMENT OF CRANKSHAFTS

Durability assessment of crankshafts includes material and component testing,stress and strain analysis, and fatigue or fracture analysis. Material testing includeshardness, monotonic, cyclic, impact, fatigue and fracture (crack growth) tests on

specimens made from the component or from the base material used in manufacturingthe component. Component testing includes fatigue tests under bending, torsion, orcombined bending-torsion loading conditions. Dynamic stress and strain analysis mustbe conducted due to the nature of the loading applied to the component. Complexity ofthe geometry of most crankshafts necessitates employing finite element analysis tools.

Figure 10 illustrated the stress distribution obtained by static analysis due to gasload in the fillet of a crankshaft. Although static analysis is commonly used in designingcrankshafts, the nature of loading especially the torsional vibrations makes transientstress analysis inevitable. The fillet has shown to be the most critical location underprimary static loading in crankshafts. Nevertheless, performing transient analysis on athree dimensional solid model of a crankshaft is costly and time consuming.

Payer et al. (14) developed a two-step technique to perform nonlinear transientanalysis of crankshafts combining a beam-mass model and a solid element model.Using FEA, two major steps are used to calculate the transient stress behavior of thecrankshaft; the first step is the calculation of time dependent deformations by a step-by-step integration. Using a rotating beam-mass-model of the crankshaft, a time dependentnonlinear oil film model and a model of the main bearing wall structure, the mass,


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damping, and stiffness matrices are built at each time step. The system of resultingequations is then solved by an iterative technique. In the second step, those transientdeformations are enforced to a solid element model of the crankshaft to determine itstime dependent stress behavior. The major advantage of using the two steps isreduction of CPU time for calculations. This is because the number of degrees of

freedom for performing of step one is low and therefore enables an efficient solution.Furthermore, the stiffness matrix of the solid element model for step two needs only tobe built up once.

In order to estimate fatigue life of crankshafts, Prakash et al. (15) performedstress and fatigue analysis on three example parts, belonging to three different classesof engines. The classical method of crankshaft stress analysis (by representingcrankshaft as a series of rigid disks separated by stiff weightless shafts) and an FEMbased approach using ANSYS code were employed to obtain natural frequencies,critical modes and critical speeds, and amplitudes and stresses in the critical modes. Afatigue analysis was also performed and the effect of variation of fatigue properties ofthe material on failure of the parts was investigated.

Henry et al. (16) presented a procedure to assess crankshaft durability. Thisprocedure as shown in Figure 11 consists of four main steps. The first step is modelingand load preparation that includes mesh generation, calculation of internal static loads(mass), external loads (gas and inertia) and torsional dynamic response due to rotation.The second step is the finite element method calculation including generating input filesfor separate loading conditions. Third step is the boundary condition file generation. Thefinal step involves the fatigue safety factor determination. This procedure wasimplemented for a nodular cast iron diesel engine crankshaft. The initial (beforeoptimization) eight counterweight crankshaft design is illustrated in Figure 12. The worstload condition with respect to fatigue safety factor was at 3500 rpm engine speed. Atthis speed one of the fillets has the lowest fatigue safety factor. The variation ofcalculated torsional moment and bearing load with respect to engine speed is shown inFigure 13. Using the operating conditions of Figure 13, the stress variation for the mostcritical location (crank pin fillet) is obtained as shown in Figure 14.

Fatigue crack growth analysis of a diesel engine forged steel crankshaft wasinvestigated by Guagliano and Vergani (17) and Guagliano et al. (18). Theyexperimentally showed that with geometry like the crankshaft, the crack grows faster onthe free surface while the central part of the crack front becomes straighter. Based onthis observation, two methods were compared; the first considers a three dimensionalmodel with a crack modeled over its profile from the internal depth to the externalsurface. In order to determine the stress intensity factors concerning modes I and II a

very fine mesh near the crack tip is required which involves a large number of nodesand elements, and a large computational time. The second approach uses twodimensional models with a straight crack front and with the depth of the real crack,offering simpler models and less computational time. Comparison of the two differentapproaches based on KI and KII values show good agreement. In addition, the modelwas validated using strain-gage measurements during tests. Figure 15 shows the axialstrain pattern as a result of the two-dimensional plain analysis and its comparison with


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experimental results obtained by strain-gage measurements. In addition, they observeda linear empirical relation between the crack depth and its external dimension.

Regulskii et al. (19) investigated the effect of geometry and surface hardening onfatigue strength of crankshafts of a two-cylinder motorcycle engine made of VCh 50-2high-strength cast iron. Non-hardened crankshafts with a fillet, and hardened and non-

hardened crankshafts without a fillet were analyzed. Figure 16 shows the results offatigue testing of the crankshafts. The life of the crankshafts subjected to vibrationsurface hardening appears to be, on the average, 8% higher than that of the non-hardened crankshafts with a fillet. This gain is insignificant when it comes to cyclic lifeand attests to the low efficiency of the vibration surface hardening in this case, mostlybecause the scatter in the lives of these crankshafts is much higher than that of thecrankshafts with a fillet. The decrease in the stress concentration due to the 2 mmradius fillet caused an increase in life up to the level comparable to that provided byhardening the shafts without a fillet. The crack developed along the circumference of thecrankpin and, upon reaching the length of the order of 0.25 - 0.30 of its perimeter, itmoved to the central cheek and propagated in the direction normal to the crank plane.

In a number of cases, the occurrence of the first crack was shortly followed by theoccurrence of the second one initiated in a similar manner and then a competitivedevelopment of the two cracks took place. The decrease in the endurance of thecrankshafts tested compared to smooth specimens, as calculated from the ratio ofsmooth specimen fatigue limit to crankshaft fatigue limit was found to be in the range of3.0 to 4.3, due to the effect of geometry, treatment procedure and other technologicalfactors.

EXPERIMENTAL TECHNIQUES AND BENCH TESTING

Component testing is considered as a required step in durability assessment and

its results incorporate the effects of geometry, surface finish, residual stresses, anddirectionality of properties on fatigue behavior. Because of the high cost of thecomponent and test systems, and complex geometry of multiple-bearing one-piececrankshafts, fractions of the crankshafts consisting of one crank with two coaxiallylocated main journals fixed in the grips of the testing machine are commonly used forfatigue testing. Although this fractionizing somewhat deviates from the real componentservice conditions, it provides a considerable reduction in the number of specimens andthe cost of tests.

Yu et al. (20) and Chien et al. (7) performed resonant bending fatigue tests onSAE J434C D5506 cast iron crankshaft sections. In the first study, resonant frequencies

of a resonant bending system with notched crankshaft sections were obtainedexperimentally and numerically in order to investigate the effect of notch depth on thechange of resonant frequency of the system. Resonant frequencies of the resonantbending system with crankshaft sections were obtained before and after introduction ofthe notches and the frequency drops were compared. The crack propagation in thecomponent could then be related to the frequency drop due to existence of the notches.Figure 17 shows the setup for resonant bending fatigue tests where a crankshaft


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section is attached to two heavy steel tines and the system acts as a large tuning fork.The right tine is then excited by a shaker. In the second study by Chien et al. (7), asdescribed previously, the effect of residual stress on fatigue analysis of crankshafts wasinvestigated experimentally and numerically. Similar test arrangement and componentas in the study of Yu et al. (20) was used, but here the fatigue life of the component was

compared for rolled and un-rolled crankshafts. The four-bubble criterion wasimplemented where cyclic bending loads cause cracks to open and close, andconsequently, create oil bubbles as observed in the experiment. Based on the four-bubble failure criterion, crankshafts with the existence of the residual stresses due tothe fillet rolling process failed on the order of one million cycles under a bendingmoment of 508.4 N m (4500 lb-in.).

As described earlier, Regulskii et al. (19) performed fatigue tests on a ductilecast iron crankshaft of a motorcycle engine shown in Figure 18 using machines with acrank mechanism for the excitation of alternating stresses as shown in Figure 19. Thecantilever mounted crankshaft fragments were tested in bending on a machine withinertial load excitation by rotating unbalanced masses through the connecting rod and

crosshead in Figure 19. According to them, the shortcoming of the cantilever mountingof a crankshaft fragment in bending tests is that the loading pattern in such a mountingdiffers significantly from that in service, although it is in principle applicable forcomparative tests. By fragmenting the crankshaft, a number of masses connected to thesystem are removed affecting the dynamic response of the system. In addition, asapplied to the tests of motorcycle engine crankshafts, this arrangement provides noreduction in the number of the specimens tested, since only one specimen can be cutout from a crankshaft. Also, because of its small length, the whole crankpin would be inthe grips, in which case, the fracture zone can not be monitored.

They developed a special device for mounting the crankshaft in the working areaof the machine orthogonally toward the axial force produced by an elastic dynamic-displacement converter for this force to be applied to the central cheek of the crankshafthalfway between its supports. The main elements of the device itself are two springplates and a demountable clamp, which encircles the central crank cheek and isconnected to the active grip. The elastic converter enables converting the angulardisplacements of the crosshead with weights into the axial displacements of the activegrip of the machine. The elastic converter of the dynamometer connected to the passivegrip performs a reverse conversion, i.e., it converts the axial displacements of thepassive grip back into the angular displacements of the indicator of the load range. Theelastic spring plates, to which the main journals of the crankshaft are fastened, possessa reasonably high stiffness in tensioncompression but rather low bending stiffness,which is by more than an order of magnitude lower than that of the crankshaft. The

scheme chosen provides identical loading conditions for both crankpins when theposition of the axis along which the load acts remains unchanged. The loading conditionenabled obtaining the patterns of in-service fracture of crankshafts, i.e., across thecentral cheek in laboratory tests. However, it does not provide for bringing the testedcrankshaft fragments to separation because redistribution of the bending momentstaken up by the crankpins and main journals takes place as the crack grows.


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The tests were conducted with a fully reversed load cycle with a frequency of 20Hz at constant load amplitude. The appearance of the first fatigue crack of length from1.0 to 1.5 mm was taken as the criterion for the number of cycles to failure. The loadamplitude of Pa= 24 kN corresponding to the amplitude of the nominal bending stress ofa= 88 MPa was chosen so that the mean life of the crankshaft did not move into the

low cycle range of lives and that the results enabled plotting a portion of the fatiguecurve.

Another arrangement for fatigue testing of crankshafts was used by Matsumuraet al. (21). In this arrangement, samples were cut from the crankshafts and fixed at bothends, and load was applied at a center journal, as shown in Figure 20.

To determine fatigue strength of a V8 engine crankshaft, Jensen (2) outlined anddescribed a crankshaft strength evaluation program as shown in Figure 21. According tothis program, the load determination work begins with the sections of the crankshaft tobe analyzed. The critical sections of production V8 crankshafts are shown in Figure 22.Small holes are drilled along the axial centerlines of the main journals and crankpins toprovide a path for the strain gage lead wires to the front of the crankshaft. Strain gagesfor bending and torsion are placed at the fillet and crankpin areas, respectively, asshown in Figure 23. This arrangement is necessary to ensure accuracy of measurementdue to the specific loading mode. The crankshaft is carefully assembled in the engineand the engine is installed on a dynamometer stand. The gages are calibrated while theengine runs and stops consequently. Separate and combined bending and torsion loadsare determined using this arrangement.

Following load determination, the crankshaft is cut to allow mounting on theindividual test sections in the tuning forks. Due to lower relative importance of torsioncompared to bending loads as discussed previously, only bending tests were conductedon crankshafts. Two test pieces containing the critical sections are cut from each

crankshaft to be tested. Since experiments show the highest stresses at the criticalsections to occur in the crankpin fillets, the main journal fillets in the overlap area werepeened to prevent failure. Without peening, failure would occur in the main journal filletsdue to the very large stress concentrations introduced by the proximity of the rigidtuning forks. The tuning fork is driven by an electromagnetic vibrator mountedhorizontally in a support frame. The vibrator motion is transmitted to the tuning forkthrough a small drive rod attached to one of the tuning fork arms. The fatigue datarelated to the study are plotted in the form of a S-N curve in Figure 24. The slope of theline is obtained by running the fatigue samples at several load levels. The measuredand experimental data are then compared to obtain the crankshaft factor of safety.

GEOMETRY OPTIMIZATION

Mikulec et al. (22) discuss the calculation steps to optimize geometry of a vehiclecrankshaft using a powertrain computer model developed at Ford Powertrain andVehicle Research Laboratory. According to this program, in addition to material


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properties, a number of design parameters should be specified as input to theoptimization problem, as well as to analyze crankshaft stresses, torsional frequency andfatigue safety factor. These include crankshaft reciprocating weight, rotating weight,main cheek width, main cheek thickness, bore spacing, connecting rod length/crankradius, number of cylinders, arrangement of cylinders, number of main journals, number

of crank-pins, main fillet radius, rolled fillet fatigue strength improvement (due to rolling),and cheek relief thickness. Diameter of main journals, length of main journals, diameterof crank-pins and length of crank pins are considered as design variables. Theconstraints of the optimization problem are named as crank-pin pressure due to peakcylinder pressure or high speed inertia load, fatigue safety factor of the main cheek, firstmode torsional natural frequency, and the distance from oil hole to the nearest fillet.

Henry et al. (16) modified the web design of a nodular cast iron diesel enginecrankshaft based on the crankshaft durability assessment procedure they implemented,as described previously (i.e. Figure 11), to reduce maximum stress within the bounds ofreasonable geometries. This constraint allowed changes in the web width and pin sideweb profile only. The initial and modified webs are shown in Figure 25. This modified

design gave improvements in fatigue safety factor by 19% for the crank pin fillet studied.They attributed the stress reduction to two mechanisms, the first to be the geometricreinforcement of the webs, and the second to be the decrease in the maximum mainbearing reaction on the fifth journal due to the induced change in crank balancing.

COST ANALYSIS

A systematic cost estimation of crankshafts is provided in the work of Nallicheri etal. (23). Dividing the cost of crankshafts into variable and fixed cost, they evaluate and

compare the production cost of crankshafts made of nodular cast iron, austemperedductile iron, forged steel, and microalloyed forged steel. The common variable costelements are named as the costs of material, direct labor, and energy. The commonelements of fixed cost are named as the costs of main machine, auxiliary equipment,tooling, building, overhead labor, and maintenance. Based on a cost model andassuming estimation parameters for a particular crankshaft made of the four materialsand manufacturing processes, the production cost was obtained as shown in Figure 26and Table 6 for a production rate of 792,000 parts per year. While the material costs ofall the four processes are essentially similar, the labor cost contribution in the steelforging case is higher than that of nodular castings due to the more complex machiningprocess for steel. The breakdown shows that the ADI and forged steel crankshaft costs

are similar, with steel forging being 7% lower in cost than ADI. Sensitivity of the costwith respect to volume is evaluated and shown in Figures 27 and 28. Since the use ofmicroalloyed steel eliminates the need for heat treatment, it yields savings comparedwith other processes. Moreover, using microalloyed steel reduces machining costscompared to conventional forging due to eliminating part or all of heat treatment steps.

Nallicheri et al. (23) conclude that when designers are looking for high strengthcrankshafts, microalloyed forging steels are cost effective, high performance


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replacements for nodular castings. ADI crankshafts were considered cost effective inlow production runs (below 180,000 parts per year). According to them, the choice of acost effective production route for crankshafts is dependent upon two factors; theproduction volume and the requirements of the engine. If the engine design canaccommodate the properties offered by a nodular cast crankshaft, then the cast

crankshaft offers the most cost effective fabrication route. If the design calls for morestringent strength requirements, then the other alternatives must be considered. Atproduction volumes above 200,000 parts per year, microalloyed steel forgings offer themost cost effective, high performance crankshafts. It should be noted that moreemphasis on fuel efficiency and the rapidly growing need for optimization in recentyears, places stringent performance requirements on engine components, particularlythe rotating parts such as crankshafts. Therefore, microalloyed steel forgings offer thebest potential for achieving fuel efficient and high performance engines.

Hoffmann and Turonek (11) examined the cost reduction opportunities for forgedsteel crankshafts, with raw material cost, fatigue strength and machinability being theprimary factors evaluated. In their study, a cost model, supported by fatigue and

machinability data, was utilized to select the lowest cost among the material grades theyexamined including carbon steel (CS), alloy steel (AS), high strength steel (HS), andmicroalloyed steels (MA1 and MA2), as previously discussed and listed in Table 1.Table 7 shows a typical cost breakdown for the carbon steel crankshaft computed bythe previously developed cost model. The principal assumptions to determine the costwere steel cost (CS=$0.517/kg, CS-HS=$0.546/kg, AS=$0.572/kg, AS-HS=$0.600/kg,and MA1/MA2=$0.539/kg), machining waste (35% for four cylinder and 15% for sixcylinder), and the finished weight (16.8 kg for four cylinder and 21.4 kg for six cylinder).

Tables 8 and 9 illustrate costs at three production levels for both the four cylinderand six cylinder crankshafts. Table 8 shows that replacing carbon steel with highstrength carbon steel has no cost benefit. The savings realized by using bettermachining steel are offset by the higher cost of the raw material. Table 9, however,shows that for the higher strength application requiring alloy steel there are savings tobe gained by using high strength alloy steel. Both tables show that maximum savingsare realized from the microalloy grades. These savings are summarized in Table 10.The microalloy grade could reduce the finished cost by 11 to 19 percent compared tothe quenched and tempered alloy steel (SAE 4140), and by 7 to 11 percent comparedto the quenched and tempered carbon steel (SAE 1050) of their study. These microalloygrades meet or exceed the fatigue strength of the original materials for the applicationstudied, and have better machinability characteristics. Table 11 shows the potential costreduction that could be obtained both in machining and raw material (4.9 kg saved) byredesigning the four-cylinder crankshaft with cheeking (machining the sides of the

counterweights) and topping (machining the ends of the counterweights) eliminated.These savings are 7 percent for a carbon steel design and 13 percent if the material ischanged to a microalloy grade. In their study, high sulfur levels (0.1%) for quenched andtempered medium carbon and alloy steels, that enhance machining, show noappreciable reduction in fatigue strength. For alloy steels this has a potential cost savingof 2-6 percent, while for carbon steel the study indicated no savings.


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CONCLUSIONS

1. Crankshaft is one of the most critically loaded components of internal combustionengine and experiences cyclic bending and torsion loads during its service life. Themain sources of loading are gas load due to the combustion process transmitted tothe crankshaft by connecting rods, and inertia load due to the mass of thecomponent and its attachments.

2. Fillets in the crankshaft act as stress raisers and endure the highest level of stressunder service loading. They are the critical locations, where cracks can nucleate at afillet surface due to combined cyclic bending and torsion.

3. Failure sources of crankshafts include oil absence, defective lubrication on journals,high operating oil temperature, misalignments, improper journal bearings orimproper clearance between journals and bearings, vibration, high stressconcentrations, improper grinding, high surface roughness, and straighteningoperations.

4. Crankshafts are typically manufactured from forged steel, nodular cast iron andaustempered ductile iron (ADI). When forged steels are compared to cast iron andalloyed ductile iron used in crankshafts, the fatigue properties of forged steels aregenerally found to be better that that of cast iron.

5. Manufacturing by forging has the advantage of obtaining a homogeneous part thatexhibits less number of microstructural voids and defects compared to casting. Inaddition, directional properties resulting from the forging process helps in higherstrength in the grain-flow direction. Grain flow pattern is an indicator of forgingquality as well as directional strength.

6. Fillet rolling has traditionally been used to induce compressive residual stresses atthe crankshaft fillets, making the process of fillet rolling quite beneficial to fatiguestrength. Using optimum conditions for fillet rolling will improve the fatigue strengthof the filleted specimens equal to or even above that of the smooth specimens.

7. Shot peening also improves the fatigue strength of the fillet, but to a lesser degreethan fillet rolling. Although the compressive residual surface stress by shot peeningcan be higher than that of fillet rolling, the depth of penetration of the compressivestress developed by shot peening may be shallower than by fillet rolling.

8. Induction surface hardening is also used to induce compressive residual stresses atthe critical locations of the crankshaft. A challenge in induction surface hardening isto ensure a very slight variation in hardness and the existence of compressiveresidual stresses with proper magnitude and distribution in transition areas to thehardness of the base material.

9. Because of the high cost of the component and test systems and complex geometryof multiple-bearing one-piece crankshafts, fractions of the crankshafts consisting ofone crank with two coaxially located main journals fixed in the grips of a testingmachine are commonly used for fatigue testing. Due to lower relative importance of


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torsion compared to bending loads, only bending tests are often conducted oncrankshafts.

10. Diameter of main journals, length of main journals, diameter of crank-pins and lengthof crank pins have been considered as design variables in optimizing the geometryof crankshafts. Crank-pin pressure, fatigue safety factor of the main cheek, first

mode torsional natural frequency, and the distance from oil hole to the nearest fillethave been used as the constraints in such optimization.

11. The use of microalloyed steel eliminates the need for heat treatment, resulting incost savings compared to conventional forging of quenched and tempered alloy orcarbon steels, while meeting or exceeding the fatigue strength of these steels. Inaddition, elimination of the heat treatment reduces machining costs.

12. The choice of a cost effective production route for crankshafts is dependent upon theproduction volume and the requirements of the engine. For high performancecrankshafts with stringent strength requirements and at high volumes, microalloyedforging steels are most cost effective. ADI crankshafts may be cost effective in low

production runs. Cast crankshafts can be cost effective in high volume productions,only if the engine design can accommodate the lower strength.

REFERENCES

1. Shamasunder, S., 2004, Prediction of Defects and Analysis of Grain Flow in CrankShaft Forging by Process Modeling, SAE Technical Paper No. 2004-01-1499,Society of Automotive Engineers.

2. Jensen, E. J., 1970, Crankshaft Strength Through Laboratory Testing, SAETechnical Paper No. 700526, Society of Automotive Engineers.

3. Silva, F. S., 2003, An Investigation into the Mechanism of a Crankshaft Failure,Key Engineering Materials, Vols. 245-246, pp. 351-358.

4. Dubensky, R. G., 2002, Crankshaft Concept Design Flowchart for ProductOptimization, SAE Technical Paper No. 2002-01-0770, Society of AutomotiveEngineers.

5. Grum, J., 2003, Analysis of Residual Stresses in Main Crankshaft Bearings afterInduction Surface Hardening and Finish Grinding, Journal of AutomobileEngineering, Vol. 217, pp. 173-182.

6. Ren, W., Keyu. L., and Lee, Y. L., 2004, Optical Measurement of Residual Stress atthe Deep-Rolled Crankshaft Fillet, SAE Technical Paper No. 2004-01-1500, Societyof Automotive Engineers.

7. Chien, W. Y., Pan, J., Close, D., and Ho, S., 2005, Fatigue Analysis of CrankshaftSections Under Bending with Consideration of Residual Stresses, InternationalJournal of Fatigue, Vol. 27, pp. 1-19.


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8. Park, H., Ko, Y. S., and Jung, S. C., 2001, Fatigue Life Analysis of Crankshaft atVarious Surface Treatments, SAE Technical Paper No. 2001-01-3374, Society of

Automotive Engineers.

9. Gligorijevic, R., Jevtic, J., Vidanovic, G., and Radojevic, N., 2001, Fatigue Strengthof Nodular Iron Crankshafts, SAE Technical Paper No. 2001-01-3412, Society of


10. Gundlach, R. B., Semchyshen, M., and Whelan, E. P., 1998, Notch Sensitivity andFatigue in Austempered Ductile Iron, SAE Technical Paper No. 980685, Society of


11. Hoffmann, J. H. and Turonek, R. J., 1992, High Performance Forged SteelCrankshafts - Cost Reduction Opportunities, SAE Technical Paper No. 920784,Society of Automotive Engineers.

12. Chatterley, T. C. and Murell, P., 1998, ADI Crankshafts An Appraisal of TheirProduction Potentials, SAE Technical Paper No. 980686, Society of AutomotiveEngineers.

13. Pichard, C., Tomme, C., and Rezel, D., 1993, Alternative Materials for theManufacture of Automobile Components: Example of Industrial Development of aMicroalloyed Engineering Steel for the Production of Forged Crankshafts, InProceedings of the 26thISATA International Symposium on Automotive Technologyand Automation, Aachen, Germany.

14. Payer, E., Kainz, A., and Fiedler, G. A., 1995, Fatigue Analysis of CrankshaftsUsing Nonlinear Transient Simulation Techniques, SAE Technical Paper No.950709, Society of Automotive Engineers.

15. Prakash, V., Aprameyan, K., and Shrinivasa, U., 1998, An FEM Based Approach to

Crankshaft Dynamics and Life Estimation, SAE Technical Paper No. 980565,Society of Automotive Engineers.

16. Henry, J., Topolsky, J., and Abramczuk, M., 1992, Crankshaft Durability Prediction A New 3-D Approach, SAE Technical Paper No. 920087, Society of AutomotiveEngineers.

17. Guagliano, M. and Vergani, L., 1994, A Simplified Approach to Crack GrowthPrediction in a Crankshaft, Fatigue and Fracture of Engineering Materials andStructures, Vol. 17, No. 11, pp. 1295-1994.

18. Guagliano, M., Terranova, A., and Vergani, L., 1993, Theoretical and ExperimentalStudy of the Stress Concentration Factor in Diesel Engine Crankshafts, Journal of

Mechanical Design, Vol. 115, pp. 47-52.19. Regulskii, M. N., Pogrebnyak, A. D., and Balakovskii, O. B., 2002, Procedure and

Results of Investigation into Fatigue Strength Characteristics of Motorcycle EngineCrankshafts, Strength of Materials, Vol. 34, No. 6, pp. 629-635.

20. Yu, V., Chien, W. Y., Choi, K.S., Pan, J., and Close, D., 2004, Testing and Modelingof Frequency Drops in Resonant Bending Fatigue Tests of Notched Crankshaft


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Sections, SAE Technical Paper No. 2004-01-1501, Society of AutomotiveEngineers.

21. Matsumura, Y., Kurebayashi, Y., Konagaya, D., and Mizuno, K., 1999,Development of Nitrocarburizing Steels for Crankshafts, SAE Technical Paper No.1999-01-0601, Society of Automotive Engineers.

22. Mikulec, A., Reams, L., Chottiner, J., Page, R. W., and Lee, S., 1998, CrankshaftComponent Conceptual Design and Weight Optimization, SAE Technical Paper No.980566, Society of Automotive Engineers.

23. Nallicheri, N. V., Clark, J. P., and Field, F. R., 1991, Material Alternatives for theAutomotive Crankshaft; A Competitive Assessment Based on ManufacturingEconomics, SAE Technical Paper No. 910139, Society of Automotive Engineers.

24. Halderman J. D. and Mitchell, C. D., 2001, Automotive Engines, 1stedition, PrenticeHall, Inc.

25. Payer, 2000, Advanced Numerical Simulation Techniques for the Fatigue and NVH

Optimization of Engines, 2nd

MSC Worldwide Automotive Conference.


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Table 1 General steel grades tested, heat treatments and hardness in the study byHoffmann and Turonek (11).

Table 2 Hardness and bending fatigue strengths for the steels studied by Hoffmannand Turonek (11).

Table 3 Mechanical properties from crankshaft journals studied by Hoffmann andTuronek (11).


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Table 4 Summary of fatigue test results on forged and ADI crankshafts studied byChatterley and Murrell (12).

Table 5 Fatigue experiment results on specimens from competing crankshaftmaterials studied by Pichard et al. (13).


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Table 6 Breakdown of cost by manufacturing factor for fully machinedcrankshafts of alternative processes in 1991 currency from thestudy by Nallicheri et al. (23).

Table 7 Cost breakdown for four cylinder carbon steel crankshaft based onannual volume of 100,000 from the study by Hoffmann and Turonek(11).


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Table 8 Cost data for the four cylinder crankshaft of the study by Hoffmann andTuronek (11).

Table 9 Cost data for six cylinder crankshaft of the study by Hoffmann and Turonek(11).


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Table 10 Summary of potential savings for four and six cylinder engines of the study

by Hoffmann and Turonek (11).

Table 11 Cost reduction by eliminating cheeking and topping for the four cylindercrankshaft of the study by Hoffmann and Turonek (11).


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Figure 1 Typical crankshaft with main journals that support the crankshaft in the block.Rod journals are offset from the crankshaft centerline (24).

Figure 2 An automotive ductile cast iron crankshaft and a close-up of the fillet (7).


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Figure 3 Load data for the critical section (one of the fillets) of a V8 engine nodulariron crankshaft versus crank angle (2).

Figure 4 Crankshaft design tasks presented by Dubensky (4).


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(a)

(b)

Figure 5 (a) Casting and (b) forging process flows for manufacturing crankshafts (23).

Figure 6 Mechanical machining and heat treatment procedure from blank tocrankshaft (5).


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Figure 7 S-N curve rig test results with various surface treatments for CrMo alloy steel(8).


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Figure 8 Ductile iron crankshaft casting used for smooth and notched test specimensin the study by Gundlach et al. (10).

Figure 9 Comparison of fatigue strength of the original, fillet-rolled, shot-peened, andsmooth wall ground ADI specimens in the study by Gundlach et al. (10).


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Figure 10 Stress distribution due to maximum gas load at the fillet of a crankshaft (25).

Figure 11 Crankshaft durability assessment procedure used by Henry et al. (16).


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Figure 12 Solid model of the diesel engine crankshaft in the study of Henry et al. (16).

Figure 13 The calculated operating conditions in the study of Henry et al. (16).

Figure 14 Stress cycle for the crank pin fillet in the study of Henry et al. (16).


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Figure 15 Numerical versus experimental axial strains and pattern along the crank fillet(axis unit in mm) obtained by a plane model (18).

Figure 16 Results of constant amplitude fatigue testing of crankshafts: (1) non-hardened crankshafts without a fillet; (2) crankshafts without a fillet subjectedto vibration surface hardening; (3) non-hardened crankshafts with a fillet (19).


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Figure 17 Experimental setup for resonant bending fatigue tests of crankshaft (20,7).


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Figure 18 Schematic diagram of a crankshaft of a motorcycle engine in the study byRegulskii et al. (19).

Figure 19 Scheme of mounting a full scale motorcycle crankshaft on elastic elementssimulating two crankshaft bearings (a) and block diagram of a test bench for

testing crankshafts in bending in the crank plane (b) in the Regulskii et al.study: (1) elastic converter of torsional vibrations into axial ones; (2) activegrip; (3) fragment of the crankshaft; (4) passive grip; (5) elasticdynamometer; (6) optic monitor; (7), (8), and (10) diaphragms; (9) springplate; (11) crosshead; (12) connecting rod of the dynamic displacementexciter (19).


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Figure 20 Schematic view of bending fatigue test of a crankshaft (21).

Figure 21 Crankshaft strength evaluation program by Jensen (2).

Figure 22 V8 crankshaft critical sections in the study by Jensen (2).


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Figure 23 Strain gage bridge placement in the study by Jensen (2).

Figure 24 S-N diagram of crankshaft section strengths in the study by Jensen (2).


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Figure 25 Initial (left) and modified web designs to reduce maximum stress within thebounds of reasonable geometries in the study by Henry et al. (16) thisconstraint allowed changes in web width and pin side web profile only.

Figure 26 Cost breakdown for crankshafts of alternative processes based on 792,000units per year in 1991 currency in the study by Nallicheri et al. (23).


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Figure 27 As-formed piece costs as a function of annual production volume in the studyby Nallicheri et al. (23).

Figure 28 Finished piece costs alternative processes as a function of annualproduction volume in the study by Nallicheri et al. (23).

2005ZoroufiFatemi26FITC

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