Experimental Fatigue

Experimental FatigueOverview: Here is a little of what you will find on this page.

Background information on general causes and recognition of material failuresThe influence of different factors on material fatigueA sample experiment that may be performed at homeInsight into experiments dealing with fatigueFatigue considerations in real life

Causes and Recognition of Fatigue Failures

General Causes of Material Failures:

Design deficienciesManufacturing deficienciesImproper and insufficient maintenanceOperational overstressingEnvironmental factors (i.e. heat, corrosion, etc.)Secondary stresses not considered in the normal operating conditionsFatigue failures

Improper and insufficient maintenance seems to be one of the most contributing factors influencedby some improper designs such as areas that are hard to inspect and maintain and the need forbetter maintenance procedures. In many circumstances the true load is difficult to predict resultingin a structure being stressed beyond its normal capabilities and structural limitations. When astructure is subject to cyclic loads, areas subject to fatigue failure must be accurately identified.This is often very hard to analyze, especially in a highly composite structure for which analysishas a high degree of uncertainty. Thus, in general, experimental structural fatigue testing isfrequently resorted to.

Recognition of Fatigue Failure

Two fatigue zones are evident when investigating a fracture surface due to fatigue, the fatiguezone and the rupture zone. The fatigue zone is the area of the crack propagation. The area of finalfailure is called the rupture or instantaneous zone. In investigation of a failed specimen, therupture zone yields the ductility of the material, the type of loading, and the direction of loading.The relative size of the rupture zone compared with the fatigue zone relates the degree ofoverstress applied to the structure. The amount of overstressing can be determined from thefatigue zone as follows: highly overstressed if the area of the fatigue zone is very small comparedwith the area of the rupture zone; medium overstress if the size or area of both zones are nearlyequal; low overstress if the area of rupture zone is very small. Figure 1 describes these relationsbetween the fatigue and rupture zones.

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Figure 1 Fracture appearances of fatigue failures in Bending by Dr. Charles Lipson,"WhyMachine Parts Fail," Machine Design, Penton, Cleveland 13, Ohio. From Metal Fatigue: Theoryand Design, ed. A.F. Madayag, pg. 5.

The fatigue zone can be described as follows: a smooth rubbed, and velvety appearance, thepresence of waves known as "clam-shells" or "oyster-shells", "stop marks" and "beach marks," andthe herringbone pattern or granular trace which shows the origin of the crack. In general, stopmarks indicate the variations in the rate of crack propagation due to variations in stress amplitudein a cyclic application varying with time. Figure 2 is a schematic representation of the fatiguezone.

Figure 2 Typical fatigue zone with identifying marks. From Metal Fatigue: Theory and Design,ed. A.F. Madayag, pg. 3.

Design Considerations

Even if careful attention to good design practices is constantly the goal of design engineers,fatigue problems are sometimes introduced into the structure. Fatigue failures are often the resultof geometrical or strain discontinuities, poor workmanship or improper manufacture techniques,material defects, and the introduction of residual stresses that may add to existing service stresses.

Typical factors affecting fatigue include the following: Stress raisers, usually in the form of a


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notch or inclusion; most fatigue fractures may be attributed to notch effects, inclusion fatiguespecimens are rare. High strength materials are much more notch-sensitive than softer alloys.Corrosion is another factor that affects fatigue. Corroded parts form pits that act like notches.Corrosion also reduces the amount of material which effectively reduces the strength and increasesthe actual stress. Decarburization, the loss of carbon from the surface of the material, is the nextfactor. Due to bending and torsion, stresses are highest at the surface; decarburization weakens thesurface by making it softer. Finally, residual stresses which add to the design stress; the combinedeffect may easily exceed the limit stress as imposed in the initial design.

Influence of Processing and Metallurgical Factors on Fatigue

A myriad of factors affect the behavior of a material under fatigue loading. Obvious factorsinclude the sign, magnitude, and frequency of loading, the geometry and material strength level ofthe structure and the ambient service temperature. However, processing and metallurgical factorsare not often considered, but these factors determine the homogeneity of materials, the sign anddistribution of residual stresses, and the surface finish. Thus, processing and metallurgical factorshave an overriding influence on the performance of a structure.

Processing Factors

Stresses are normally highest at the surface of a structure, so it follows that fatigue usually initiatesat the surface. Stress raisers are more likely to be present as a result of surface irregularitiesintroduced by the design of the structure or produced in service or resulting from processing.Processing factors can introduce a detrimental or beneficial effect into a structure, usually in theform of effect on strength level or residual stress condition of the surface material. Therefore, theeffect of processing on the mechanical properties of a material, especially the surface of thematerial, directly affects fatigue properties. Processing factors that influence the fatigue life of astructure include the following: the process by which a part is formed, such as die casting; the heattreatment of a material, such as quenching, which builds up residual stresses and annealing, whichrelieves internal stress (see Figure 3); case hardening, such as carburization or nitriding, whichincreases surface hardness and strength (see Figure 4); surface finish, such as polished smooth byelectropolishing; cold working, which increases strength; also, cladding, plating, chemicalconversion coatings, and anodizing.


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Figure 3 Effect of hardness on the fatigue life of threads rolled before and after heat treatment.From Metal Fatigue: Theory and Design, ed. A.F. Madayag, pg. 82.

Figure 4 Bending fatigue test results on sections from crankshafts: endurance limit versus surfacetreatment. From Metal Fatigue: Theory and Design, ed. A.F. Madayag, pg. 70.

Metallurgical Factors

Metallurgical factors refers to areas within the material, wither on the surface or in the core, whichadversely affect fatigue properties. These areas may arise from melting practices or primary orsecondary working of the material or may be characteristic of a particular alloy system. Invirtually all instances the detriment to fatigue properties results from a local stress-raising effect.Therefore, metallurgical factors affecting fatigue include the following: surface defects,sub-surface and core defects, inhomogeneity, anisotropy, improper heat treatment, localizedoverheating, corrosion fatigue, and fretting corrosion.


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Experimental Analysis of Fatigue

Fatigue Life Curves

Failure due to repeated loading is known as fatigue. A small crack, a scratch, or some other suchminor defect causes localized deformation. This deformation leads to a small crack if one was notinitially present. After cyclic loading, that is, loading in the same way multiple times, the crackgrows, and eventually the material fails. A fatigue life curve is a graphical representation of thecyclic loading. Simply, a fatigue life curve, also known as an S-N curve is a plot of the stressamplitude versus the number of cycles the material goes through before it fails. That is, for acertain stress, the material will fail within a certain number of cycles. Figure 5 is an example of atypical fatigue life curve.

Figure 5 Typical Fatigue Life Curve. From Mechanical Behavior of Materials Laboratory,Prepared by Staff of Engineering Science and Mechanics, Virginia Polytechnic Institute, pg. 8-3.

To help understand the concept of fatigue life curves, an experiment that may be performed easilyby anyone is presented here.

Fatigue Life Curves Experiment

What you need:

4 Jumbo Paper ClipsA Copy of Figure 6

What to do:

Bend four paper clips at each of the four bending angles indicated in Figure 6 until eachfails, counting the number of times the paper clip is bent. For instance, bend a paper clip 45,then bend it to -45, then back to zero. This is one cycle.

1.

Record the data in Table 1.2.Using a spreadsheet program, plot the number of cycles versus the angle. The anglerepresents the stress amplitude.

3.


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Figure 6 Bending angle guide. Place paper clip in a vertical plate and bend in the middle usingthis guide to judge the angles. From Mechanical Behavior of Materials Laboratory, Prepared byStaff of Engineering Science and Mechanics, Virginia Polytechnic Institute, pg. 8-6.

Table 1: Table For Experimental DataCycles to Failure Angle,

? 90? 45? 20? 10

An example of this experiment:

Table 2: Experimental DataCycles to Failure Angle,

1.5 907 4529 2079 10


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Fatigue Crack Growth

If an engineering component contains a crack, and if a cyclic or repeated load is applied, the crackis likely to grow slowly with increasing number of load cycles. This process is known as fatiguecrack growth. In a fatigue crack growth experiment, the progress of a crack growing under a cyclicload is measured, and the results are plotted as a fatigue crack growth rate curve, da/dN versus K(that is, change in crack length divided by change in number of cycles to failure versus change infracture toughness). A typical fatigue crack growth curve is shown in Figure 7.

Figure 7 Crack growth rates obtained from adjacent pairs of a vs. N data points. From [Dowling


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93] p. 465; ©1993 by Prentice Hall, Upper Saddle River, NJ. From Mechanical Behavior ofMaterials Laboratory, Prepared by Staff of Engineering Science and Mechanics, VirginiaPolytechnic Institute, pg. 9-3.

In the simplest form of a fatigue crack growth rate test, a cyclic load is applied that has fixedmaximum and minimum loading levels. The test specimen is usually a plate of material in which acrack has already been started at the end of a V-bottom machined slot. In a typical fatigue crackgrowth experiment, the sample is loaded in a closed-loop servohydraulic testing machine and datafor crack length, number of cycles to failure, and fracture toughness is recorded. From this data themechanical behavior for a certain material can be described under fatigue crack growth loading bythe fatigue crack growth rate curve. This sort of experiment is useful for materials that wouldundergo high cyclic loading stresses such as an airplane wing or a helicopter rotor.

Low Cycle Fatigue

Low cycle fatigue is the repeated cyclic loadings that cause significant plastic deformation in amaterial and may cause fatigue cracking after a relatively small number of cycles-hundreds orthousands. Low cycle fatigue typically occurs as a result of repeated localized yielding near stressraisers, such as holes, fillets, and notches, despite the elastic deformation occurring over the bulkof the component. Uniaxial testing is performed on several smooth (unnotched) specimens underdifferent cyclic deformation levels in a typical low cycle fatigue test. Each specimen follows agiven constant stress amplitude, completely reversed, cyclic strain. That is, the mode of testing isstrain control instead of stress control. Stress response is monitored during cyclic loading, and thenumber of cycles to failure is recorded for these tests. The results from several tests are necessaryto determine the cyclic stress-strain curve and the strain life curve for the material. A schematicrepresentation of a completely reversed controlled strain test is shown in Figure 8.

Figure 8 Completely reversed controlled strain test and two possible stress responses, namelycycle-dependent hardening and softening. From [Landgraf 70]; ©ASTM. From Mechanical


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Behavior of Materials Laboratory, Prepared by Staff of Engineering Science and Mechanics,Virginia Polytechnic Institute, pg. 10-2.

If a sufficiently high strain level is reached, yielding may occur before the maximum strain isreached on each cycle of loading. Stress amplitude usually varies; if it increases, the material issaid to cyclically harden, if it decreases, the material is said to cyclically soften. However, thisbehavior tends to stabilize such that the variation in the stress amplitude is small after an initialperiod of transient hardening or softening. Once the behavior is stabilized, a closed stress-strainhysteresis loop is formed during each strain cycle. This hysteresis loop typically looks like Figure9. The area inside the hysteresis loop is the energy absorbed per unit volume of the material. Thisenergy mostly dissipates as heat. Fatigue failure results under repeated cycling and the life ismeasured by the number of strain cycles to failure. A strain life curve may be plotted from the datacontained in the hysteresis loop as is shown in Figure 10. This sort of experiment is useful inanalysis of parts where the material is strong enough to withstand the cyclic loading it may gothrough, but fails due to fracture at a bolt hole or other such stress raiser. An example of this is astructural member bolted to another structural member which fails due to a crack originating fromthe bolt hole.

Figure 9 Stable stress-strain hysteresis loop. From Mechanical Behavior of Materials Laboratory,Prepared by Staff of Engineering Science and Mechanics, Virginia Polytechnic Institute, pg. 10-2.


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Figure 10 Elastic, plastic, and total strain vs. Life curves. Adapted from [Landgraf 70]; ©ASTM.From Mechanical Behavior of Materials Laboratory, Prepared by Staff of Engineering Scienceand Mechanics, Virginia Polytechnic Institute, pg. 10-5.

Real Life-Design and Manufacturing Considerations

The following describes a relationship between factors that shape the S-N curves as they areinfluenced by design and manufacturing conditions and the effects of such conditions on thefatigue properties of materials, components, and structures.

Recommendations for Designs to Avoid Fatigue Failures

A designer can help to minimize the possibility of fatigue failure by proper design of structuralcomponents. Many fatigue failures may be attributed to lack of sufficient consideration of designdetails or a lack of appreciation of engineering principles. These principles, which are an integralpart of good design of structures subject to fatigue are well reported in literature, but thisinformation has been scattered throughout sources and may be inaccessible to a designer whoneeds to understand and utilize the principles. It is good design practice to seek out sources of thisinformation and to utilize the principles before, during and after the design process.

Fatigue Considerations in Helicopter Design and Service

The helicopter is subject to many fatigue considerations that fixed wing aircraft are not, such asthe significant oscillatory loads due to the harmonic content of aerodynamic loads from thecombined rotational and translation blade motion through the air, also the cyclic loading of therotor start-stop cycles, transient maneuver loads, gust loads, coriolis loads, and torque loads. Theseare just a small example of the many unique fatigue loading considerations in helicopter design.Thus one may realize the complexity of helicopter design and service. The helicopter is one ofmany vehicles and structures which undergo unique fatigue loading, therefore, fatigueconsiderations are an important part of the design process.


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Figure 11 A tension fatigue failure of a helicopter rotor blade flapping link. Fatigue crackoriginated at arrow B, propagated to arrows. From "Metal Fatigue and Its Recognition," CivilAeronautics Board, Bureau of Safety, Bulletin No. 63-I, April 1963, by Frank R. Stone, Jr. FromMetal Fatigue: Theory and Design, ed. A.F. Madayag, pg. 10.

Summary

In summary, fatigue plays an important role in all areas of the engineering process, from design tomanufacture to service during the life of a product or structure. All engineers must carefully studythe effects and the many types of fatigue, no matter what field of engineering and what part of theengineering process.

References

Metal Fatigue: Theory and Design. ed. A.F. Madayag, John Wiley & Sons, Inc., 1969.

Materials Science and Engineering, An Introduction. 3rd Edition, William D. Callister, Jr., JohnWiley & Sons, Inc., 1994.

Mechanical Behavior of Materials Laboratory. N.E. Dowling and R.A. Simonds, UniversityPrinting Service, 1995.

Table of Contents

Submitted by Chris Meyer

Virginia Tech Materials Science and Engineering

http://www.eng.vt.edu/eng/materials/classes/MSE2094_NoteBook/97ClassProj/exper/meyer/www/meyer.html

Last updated: 5/6/97


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Experimental Fatigue

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