Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . Introduction 3

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Current Fatigue-Design Procedures 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Analysis of Predicted Service 4

3.1 Stress-Life Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Strain-Life Procedure 5

3.2.1 Step 1: Relationship Between Loads, Stresses and Strains . . . . . . 5 3.2.2 Step 2: Development of Stress-Strain Histogram . . . . . . . . . . . . . . . 6 3.2.3 Step 3: Determination of Fatigue Damage . . . . . . . . . . . . . . . . . . . . 6 3.2.4 Stress-Strain Hysteresis Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2.5 Strain Life Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 . Determination of Allowable Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Material Behavior 7

4.1.1 Data Procurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Trends with Strength 9

4.1.3 Variability of Fatigue Properties Within a Steel Grade . . . . . . . . . . 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Effects of Fabricating Processes 11

4.2.1 Forming or Cold Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2.2 Effect of Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2.3 Other Fabricating Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.3 Effects of Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3.1 Corrosion Effects on Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3.2 Elevated Temperature Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 . Fatigue-Design Trends 13 6 . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

The material presented in this publication has been prepared in accordance with rec- ognized engineering principles and is for general information only . This information should not be used without first securing competent advice with respect to its suitability for any given application . The publication of the material contained herein is not intended as a representation or warranty on the part of American Iron and Steel lnstitute-or of any other person named herein-that this information is suitable for any general or par- ticular use or of freedom from infringement of any patent or patents . Anyone making use of this information assumes all liability arising from such use .

I . Introduction often not easily separable. However, in most ground transportation vehicles, failure is considered to have occurred as soon as a fatigue crack has initiated or becomes visible. Thus, the crack- propagation life is ignored and provides a safety factor against catastrophic failure.

This brochure discusses the fatigue (or crack-initiation) life of sheet steel used in manufacturing ground vehicle compo- nents, the major factors affecting their fatigue life, and the methods used to ana- lyze their past or future performance. The intent is to review sheet steel properties as related to the current design proce- dures used by transportation industry engineers in their efforts to prevent fa- tigue failure, with special emphasis on the strain-life fatigue-design approach.

2. Current Fatigue-Design Engineers in the ground transportation

industry are very much concerned with the reliability and safety of their products: cars, trucks, agricultural equipment, etc. These products are vttal to the national economy and are used by operators in all walks of life. The concerns of the design engineer focus especially on the overall structure as well as its components when exposed to service conditions that result in numerous fluctuating loads, stresses or strains which may result in fatigue failure.

Fatigue is failure under repeated loads. There are three stages in a fatigue failure: crack initiation, crack propaga- tion, and final fracture. These stages are

Procedures The current fatigue-design procedures

used to design sheet steel components have evolved primarily from experience based on proven performance of a struc- ture or component. When new designs were developed, newly acquired engi- neering principles were applied.

A summary of steps or sequence of events in developing a design for a com- ponent is shown in Figure 1. Design of a component is affected by past service or performance analysis of comparable components (solid line branching around

-4 OUTLINE DESIGN ,h---------- --

Predicted Service Material Properties H Fabrication Processes Analysis Environment

Cold-Forming Welding

Other Processes

Load or Vibration Spectrum Fatigue Testing

- Cyclic Stress-Strain Curve

-Crack Initiation - Strain-Life and Stress-

Life Curves - Notch Sensitivity

Corrosion Temperature I High

Temperature Fatigue Fatigue

Analysis or Experiment

Service Stress

Allowable Stress

Fatigue Life Prediction -+Inadequate--------+

I Adequate

Full-Scale-Component Fatigue-Life Test Inadequate- ----- -- 1 Adequate

, I Products I

Figure 1. Fatigue-Design Flow Chart

left side of flow diagram) andlor a previ- ously performed inadequate fatigue-life prediction or test (dashed line branching around right side of flow diagram). The design requires the development of two types of information: ( I ) the anticipated service stresses as determined from ac- tual field or test track measurements or by theoretical analyses (left downward branch in Figure I ) , and (2) allowable stresses determined from considerations of the material properties, manufacturing processes, and the environment (right downward branch of Figure 1). As shown in the flow diagram, the fatigue properties of a given sheet steel material may be af- fected by the fabrication processes and the environment. If all the required steps are properly taken, a newly designed component should perform adequately under the anticipated fatigue conditions.

Design of fatigue resistant ground vehicle components has benefitted in recent years from two parallel develop- ments: improved material-character- ization procedures and analytical techniques of fatigue life prediction. Cur- rent procedure for the design of fatigue critical components emphasizes finite-life behavior and views fatigue as a problem in cyclic deformation. The primary inputs to component life prediction through the use of analytical techniques are: (1) a set of material properties related to cyclic de- formation, and (2) a loading history of the component. The major emphasis in this booklet is the discussion of the relevant material cyclic properties of sheet steels used in the ground transportation indus- try. The following is a more detailed discussion of the design steps shown in . the flow chart (Figure 1).

3. Analysis of Predicted Service

The service stresses of a component are derived from a load, displacement, or strain (or stress) vs. time record obtained during test-track or service tests. Stress or strain spectra at critical locations are then determined by use of computer programs and finite element analysis. The two principal fatigue life prediction methods are stress life and strain life.

3.1 Stress-Life Procedure Stress-life fatigue design procedures

have been widely used in the past and are still used today. The procedures are based on nominal stress fluctuations in the component. For specific steel grades, fabricating processes and environmental exposures, constant-amplitude stress-

-

- Figure 2.

Fatigue-Life Cycles (2a) Semi-Log S-N Curve

Numerically Minimum Stress, in Cycle, ksi (2b) Modified Goodman Diagrams

Cycles to Failure, N (Log Scale)

(2c) Stress-Range Concept

Stress-Life Concepts

life (S-N) fatigue data are summarized as either semi-log or log-log curves devel- oped for given stress ratios, R (minimum stress divided by maximum stress), as shown in Figure 2a. The stress-life fa- tigue data used by designers also are expressed in the form of Goodman or constant-life diagrams, which show life- cycle design curves for a combination of different minimum and maximum stresses for the constant-amplitude stress cycles, Figure 2b. The stress- range concept shown in Figure 2c has been used to a lesser extent1'; however, most of data available in the literature show only the total life of the specimen. Consequently, S-N type data do not pro- vide the crack initiation life information desired by transportation engineers, Figure 3.

Figure 3 is a schematic S-N curve in which the total fatigue life is subdivided into the number of cycles necessary to initiate a fatigue crack and the number of cycles necessary to propagate the crack through the test specimen2. S-N type data are usually obtained from smooth specimens tested either axially or in bending. For low stress ranges, the fa- tigue life of a specimen consists primarily of crack initiation life. As the magnitude

.. of the applied alternating stress in-

) creases, the total fatigue life decreases and the percent of the fatigue life to initi- ate a crack decreases. At a high value of applied alternating stress, the number of cycles expended in crack initiation is negligible.

Stress raisers or notches in structural components result in stress levels that are greater in the vicinity of the notch than the nominal stress level. Thus, the material at the tip of the notch is sub- jected to the maximum stress magnitude and fluctuation. Consequently, this mate- rial is most susceptible to fatigue-crack initiation. The higher the stress concen- tration for a given stress cycle, the shorter is the fatigue-crack-initiation life. Under identical test conditions, the total fatigue life of a notched specimen is shorter than that of a smooth specimen or a specimen containing a less severe stress raiser.

3.2 Strain-Life Procedure With the recent emphasis on increased

fuel economy through reduced car weight, coupled with stringent product liability requirements, more sophisticated fatigue design procedures utilizing strain-life fatigue data have received more attention. Because today many transportation-equipment designers are

- I I I I I I I I / I I 1 1 1 1 1 1 1 1 I 1 1 1 1 1 ' - - - - - - - - -- Initiation Life - - Propagation Life

\ - Total Life

-------- /-

Fatigue Limit

Number of Cycles to Failure, N, Log Scale

Figure 3. lnitiation and Propagation Life for a Schematic S-N Curve

using this fatigue design procedure3, the remainder of this booklet deals with fa- tigue data needed for the strain-life approach.

In analyzing the fatigue performance of a component, three major steps are required: (1) determine the relationship between applied loads and resulting stresses and strains, (2) develop a stress-strain histogram and (3) deter- mine the fatigue damage or the fatigue life of the design.

3.2.1 Step 1 : Relationship Between Loads, Stresses and Strains

The relationship between applied loads and the resulting stresses or strains can be determined by any one of three different methods.

One method is to attach a strain gauge to the detail (notch) and then subject the part to the given loads. The fatigue be- havior of the localized plastic zone is then simulated by testing smooth speci- mens under strain-controlled conditions, Figure 4, such that the minimum cross section for the specimen is some fraction of the plastic-zone size2. However, suit- able correction factors must be used to account for differences in stress state, size, and strain gradient between the smooth specimen and the plastic zone for the structural detail of interest4.

A second method to obtain the desired load-stress-strain relationships is the use of a finite-element computer program with elastic elements. The calculated

Free Body AU of Stfucture

Structure Under Stress-Controlled Conditions

Figure 4. Strain-Controlled Test Specimen Simulation for Stress Concentrations in Structures

'See References

elastic stresses at the notch are used to determine the theoretical stress concen- tration factor, K,. One of the most popular means of determining Kt is the Neuber5 approach in which K, is determined by the geometric mean value of the elastic stress and strain concentration factors Ka and KE, respectively. Thus,

Because in real materials the stresses and strains at the root of notches are plastic and not elastic and because real materials have varying responses to cyc- lic loads in the presence of a high stress concentration, a fatigue notch factor6, K,, is derived from the Neuber relationship as:

where Au and AE are thelocal stress and strain ranges at the notch root and AS and Ae arenominal or global stress and strain ranges remote from the notch. It is convenient to incorporate the modulus of elasticity, E, and Hooke's relationship be- tween AS and he and rewrite Equation ( 2 ) in the form:

Kt = (Aa AE E)WAS (3)

Thus, the fatigue notch factor is affected by the product of the local stress and strain range, the modulus of elasticity, and the global stress range. Values of Kf are experimentally determined for various materials by testing small notched speci- mens in the laboratory for all stress ranges, AS, in the stress-range histogram.

A third method of determining the rela- tionship between loads, stresses and strains is to use an elastic-plastic finite- element analysis. This allows direct determination of the true strains and stresses at the notch; however, many costly computer iterations are required, and the results may still contain un- certainties about the material effects on the fatigue concentration factor.

In spite of disadvantages of the second and third methods, it is expected that the finite-element-analysis approaches will play an important role in the fatigue analysis in the future.

3.2.2 Step 2: Development of Stress-Strain Histogram

The second step in solving the fatigue

design problem is to determine from the true-stress-strain relationship a histogram of strain cycles by using an appropriate counting method. Several methods are available'; however, the two most popular are the rainflow method and the range- mean count. Both appear to lead to compatible fatigue-life predictions.

3.2.3 Step 3: Determination of Fatigue Damage

In the last step, Miner's rule and the previously developed constant-amplitude strain-life data are then used to deter- mine the cumulative fatigue damages. If the mean-stress effects are not to be neglected, the fatigue damage must be calculated for each individual stress- strain loop defined by the strain range and the mean strain. The cumulative damage is then converted to predicted fatigue life cycles in terms of hours or years of service. The calculations are performed by a computer through a vari- ety of in-house or commercially available computer programsg.

3.2.4 Stress-Strain Hysteresis Loops

The strain-life approach is based on the cyclic properties of the material under constant-strain fluctuations. As described in Section 4.1 .l, Data Procurement, these properties are established by subjecting a smooth specmen to fully reversed cyclic strain amplitudes of constant mag- nitude. Such a fully reversed cycle results in a stress-strain hysteresis loop10 such as that shown in Figure 5.

The total strain range, AE, for a hyste- resis loop is equal to twice the strain amplitude, c, (i.e., Ac = 2ca), and the total stress range, Au, is equal to twice the stress amplitude u, (i.e., Au = 2 ~ ~ ) . More- over, the total strain amplitude can be represented as the sum of its elastic and plastic components, Figure 5 , such that

because Ace = A x , where E is Young's modulus. E

3.2.5 Strain Life Analysisl1

A log-log plot of the stable plastic- strain amplitude, A r J 2 , versus the number of reversals to failure, 2N,, generally results in a straight-line rela- tionship, Figure 6, given by the equation

Figure 5. Schematic of a Stress-Strain Hysteresis Loop

where elf = fatigue-ductility coefficient

c = fatigue-ductility exponent

N~ = number of cycles to failure; therefore 2Nt is equal to the number of reversals to failure.

Similarly, a log-log plot of the stable stress amplitude, Aul2, versus the num- ber of reversals to failure, 2N,, results in a straight-line relationship given by the e~uat ion

where u, = true stress amplitude of t = fatigue-strength

coefficient b = fatigue-strength

exponent

Dividing Equation 6 by the modulus of elasticity E, gives the elastic-strain amplitude in terms of the fatigue-strength coefficient, the fatigue-strength exponent and the fatigue life:

Reversals to Failure, 2 Nf , Log Scale

Figure 6. Strain- Life Plot

Combining Equations 4, 5. and 7 results in the strain-life relationship

) which is represented as a heavy curve in Flgure 6

The transition-fatigue I~fe, 2N,, obtamed when the elast~c and plastic components of the total strain are equal (~.e., A E J A E , = 1) IS given by the relat~on

Equations 8 and 9, and Figure 6 show that total fatigue lives less than 2N, are governed primarily by the plastic-strain amplitude (ductility), whereas the total fa- tigue lives greater than 2N, are governed primarily by the elastic-strain amplitude (static strength). Moreover, it is claimed that the fatigue-ductility coefficient, E ' , ,

and the fatigue-strength coefficient, a',, determined from regression analysis of fatigue data, can be approximated by the true fracture ductility, E , , and the true frac- ture strength, a,, respectively, obtained from a monotonic tension test1'.

True stresses (a) and true strains (r) are used in the foregoing relationships; however, engineering stresses (S) and engineering strains (e) are obtained from the hysteresis loops and may be converted as follows:

provided that uniform elongation is occurring. Since the engineering strains of the hysteresis loops are small ( E < lo-'), the error in assuming that cr = S and E = e is small and is therefore usually neglected.

4. Determination of Allowable Stresses

Allowable stresses for components exposed to loads that vary with time, i.e. are exposed to fatigue conditions, can be affected by the type of steel used in the structure (material properties) and the interrelationships with the fabricating pro- cesses and the environment, Figure 1. The emphasis of this discussion will be

on those factors that significantly affect the fatigue performance rather than on those that do not appreciably affect fatigue behavior (such as fracture toughness).

4.1 Material Behavior

4.1.1 Data Procurement

The test specimen for the strain-life approach generally has the same thick- ness as the sheet steel being tested, Figure 7. The surfaces may be left in the as produced condition or may be a machined finish. These differences in surface finish may cause differences in long-life behavior (a rougher surface cor- responding to a lower fatigue limit than a smooth machined finish) but are thought to be inconsequential for short life behav- ior ( l o3 to 105 reversals).

The constant-strain-amplitude tests are conducted under carefully controlled alignment of the specimen axis and the applied loads. Woods-metal gripping methods help to insure this critical align- ment. The strain measured with an extensometer across the gage length of the specimen is used as a feedback sig- nal in a closed-loop hydraulic testing machine. During the test the stress and strain are recorded as a function of the number of cycles (hysteresis loops, Sec- tion 3.2.4).

Many steels subjected to constant cyclic-strain amplitude exhibit an initial transient behavior, but reach an essentially cyclically stable stress-strain behavior that corresponds to a constant hysteresis loop. This stable behavior is usually reached in less than 50 percent of the total fatigue life of the specimen. The stabilized stress values are plotted at the corresponding strain values to con-

\ Thickness

I as Received

Figure 7. Strain-Controlled Fatigue Test Specimen

Stable Hysteresis Loops for Three Strain Amplitudes

Strain Amplitude

Cyclic Stress-Strain Curve

Monotonic u Cyclic

Cyclically Softening Cyclically Hardening Cyclically Stable Complex Behavior

Figure 8. Cyclic Stress-Strain Curves for Various Behaviors

struct a cyclic stress-strain curve similar to a monotonic stress-strain curve, Fig- ure 8(a).

Another means of producing the cyclic stress-strain curve consists of subjecting a specimen to blocks of gradually de- creasing and then increasing strain amplitudeslO. A maximum strain ampli- tude of t 1.5 to 2.0 percent is usually sufficient to cyclically stabilize the metal quickly without the danger of causing the specimen to neck, fail, or buckle before a stable state is achieved. The cyclic stress-strain curve is then determined by the locus of superimposed hysteresis loop tips.

When the cyclic stress-strain and the monotoniccurves of the metal are com- pared, they fall into one of the following categories:

1. cyclically stable behavior 2. cyclically hardening behavior 3, cyclically softening behavior, or 4, complex cyclic behavior. These behaviors can be illustrated by

observing the variation in stress as a smooth specimen is subjected to com-

pletely reversed, constant-amplitude strain fluctuations with a zero mean value, as shown in Figure 8(b). If the stress magnitude required to apply the constant strain cycles remains constant, the metal is cyclically stable, which implies that the metal's cyclic stress- strain properties are identical to its monotonic stress-strain properties (engi- neerina stress and strain ropert ties

increases, the metal cyclically hardens; if it decreases, the metal cyclically softens. A metal exhibits complex cyclic behavior when cyclic softening, hardening, or stable behavior occur for different strain ranges.

As previously discussed, the response of steels to cyclic loading in the presence of a plastically-deforming notch is differ- ent from that predicted from elastic anal-

~tained from a tension test): If the stress yses; thus the fatigue notch factor, K,, is

Drill .I00 in dia Hole - 1 1 p.5 ,IO in 1 250 2 ,001 in

As Received 3.0 2 .10 in

Figure 9. Notch-Sensitivity Specimen (K, = 2.5)

Dual Phase 80 Steel ( K , = 1.56) For Kt = 2.5 Y \ I

950 Grade St?el (K, = 1.42)

;-L ( K t DQSK = 1.70) Steel 1

10 I I I I I 1 02 1 o3 1 o4 1 o5 1 o6 107

Reversals, 2 N,

Figure 10. Neuber Plots of Notched Fatigue-Test Data for DQSK, 950 Grade, and Dual Phase 80 Steels (to be published, J.M. HOI~)

determined for various steel grades. Specimens are prepared by drilling a 0.100-inch diameter hole in a 0.500-inch wide specimen, Figure 9. This specimen configuration results in a theoretical stress concentration factor, K, of 2.5. The notched specimen IS subjected to a stress amplitude, AS, and the results plotted against the reversals to failure, ZN,, Figure 10.

Most of the strain-life procedures and data analyses have been developed un- der the guidance of the Fatigue Design and Evaluation Committee of the Society of Automotive Engineers13 (SAE) and the American Society of Testing and Materials Committee E-9 on Fatiguet2. Because of the complexity of procedures and analyses, investigators are encour- aged to use and report experimental data utilizing data sheets similar to those de- veloped by the SAE, Figure l l . Most designers of vehicles have some type of fatigue-property data bank for various materials. Use of a standard method of reporting data makes the data more use- ful to these designers.

4.1.2 Trends with Strength

More and more compilations of strain- life data for various steels and for individual steel grades are becoming available in the open technical literature. The SAE has published an information report14 that summarizes the monotonic and cyclic properties for steels with ten- sile strengths ranging from 52 to 375 ksi. In one studyT5 several microalloyed steels exhibiting different cyclic harden- ing and softening behaviors showed improved long-life (> lo6 cycles) fatigue properties with increasing tensile strength for smooth specimens. The short-life properties, l o3 to lo4 reversals to failure, a ~ ~ e a r e d to be similar and in- de~endent of tensile strenath. However. the fatiaue notch factor. of these steels

MONOTONIC PROPEATIES:

/ Mod 01 EUst E GPa( x 103 ksi)

/ Yiea Strength. o 2 i i S, MPa( ks~)

Ultimate Strength, S, MPa( kst)

Strength Coefl , K MPa( ks i l

I Stratn Hard Exp , n

Red in Area. % RA

True Frac Strength, nl MPa( ksl)

True Frac Ducthty. C,

CYCLIC PROPERTIES: Correlation Coetf.. r

Y~eld Strength. 0 2% Sy MPa( ks~)

Strength Coetf , K MPa( kslQ

Stram Hard Exp , n'

Fatigue Strength Coefl , rr,' MPa( ksi)

Fat~gue Strength Exp . b

I Fatigue Duct~l~ty Coetf . e l '

( Fattgue Duwldy Exp , c P

I MICROSTRUCTURE:

I Gram S~ze

Hardness H

Material Source

Specimen Orientat~on

3ther Specs.

Reference

Dale

d - - was different in thatbot'h the shirt- and Figure 11. Material Characterization Data Sheet of SAE

Sheet n l -

long-life behaviors of notched specimens generally increased w~th tensile strength. Other studiesJ6 also have shown that various high-strength low-alloy steels (HSLA) have better long-life fatigue prop- erties than hot-rolled low-carbon steels. In general, as has been shown for long- life stress-controlled fatigue data for lives of 105 to lo6 cycles, in stram-controlled tests steels tend to show an increase in fatigue strength with increasing tensile strength1'. However, most investigators are reluctant to correlate or predict this relationship. In summary, smooth speci- mens of higher strength steels appear to have improved long-life fatigue proper- ties in comparison to lower strength steels. Generalizations concerning short- life behavior are not clear cut.

4.1.3 Variability of Fatigue Properties Within a Steel Grade

An early study comparing results from many heats of SAE-950 steel showed data scatter in life of at least an order of magnitudez4. However. several compre- hensive investigations have been conducted to determine the variability of strain-life properties in a single grade of ~ t e e l l ~ - ~ ~ . Both monotonic and cyclic properties have been investigated. For example, a studyZ1 of three heats including five coils of five different sheet thicknesses showed no statistically sig- nificant differences between the sets of data, Figure 12. Other studiesz0 that sampled material at the head, tail, and middle of a single coil have also shown the insensitivity of strain-life properties within a grade of steel. Studies have also shown?2 that no significant differences exist between longitudinal and trans- verse cyclic stress-strain behaviors in a sulfide-shape-controlled SAE-980X grade of steel. More recent studiesz3 of a dual phase 80 steel have led to this same conclusion, Figure 13. All of these inde- pendent studies lead to the same significant conclusion: the strain-life be- havior for a steel grade is insensitive to steel-processing variables and, further- more, sampling and testing a single location in a single coil of the grade is sufficient to characterize the steel grade. The significance and impact of this con- clusion to both the steel supplier and designer is obvious.

Studiesz5 have also been conducted to determine the effect of sulfide morphol- ogy on the strain life properties of steels. Strain-life properties of a Zr-treated (for sulfide shape control) steel were found to be similar to those corresponding to the lower 95-percent confidence limit of SAE

L~ne for Combined Data

/

Code Symbol 05C + 05T x 11 0 F 02 0 03 07 +

L

Runouts

10-4 L I I I I lo* 1 o3 1 o4 1 o5 10" 1 (

Reversals, 2Nf

Figure 12. Total-Strain Amplitude Versus Reversals to Failure for a 950 Grade Steel (Ref. 21)

Sheet Amount Code Axial Prestrain OIL 0 0 12L 4% 13L-1 A 0 13L-2 A 0

Line for Combined Data

J 16L-1 0 8% 16L-2 8% 16T * 8% 18L x 0 L = Long~tudinal T = Transverse

L

Runouts

Reversals, 2Nf

Figure 13. Total-Strain Amplitude Versus Reversals to Failure of a Dual-Phase 80 Steel (Ref. 23)

950X grade steel. In addition, data ob- tained26 for two different carbon levels of an SAE 980 grade of steel gave similar results. However, some caution should be exercised regarding such find~ngs. In general, the short life fatigue properties of steels are associated with the plastic behavior, and the long life properties with the elastic behavior of a steel. Thus, improvements In the ductility (plastic be- havior) through sulfide shape control or the reduction in sulfur level may have an effect upon the short life fatigue results of smooth specimens.

4.2 Effects of Fabricating Processes

The effects of fabricating processes on the fatigue life or fatigue performance of a component are recognized as being very important fatigue parameters1 be- cause these processes may cause geo- metric stress risers, residual stresses, and initiate discontinuities andior micro- structure changes that can mask all of the material effects discussed above. These fabricating processes include: ( I ) cold-forming such as bending, stretching, and drawing, (2) welding and mechanical fastening using drilled, punched, andior tapped holes, and (3) shearing, slitting and burning. Only some of the most im- portant processing parameters can be discussed here.

4.2.1 Forming or Cold Work

Sheet steel users are concerned with the effects of cold working (during forming) on the strain-life properties of the steel provided. One investigationz0 has shown insignificant changes in the cyclic stress-strain properties of an HSLA steel for 5 percent uniaxial tension pre- straining, 10 percent uniaxial tension prestraining, and 20 percent rolling, Fig- ure 14. Similarly, a second studyz7 showed minor increases in long-life strain-life behavior due to forming (prestraining). Some studiesz3 of modest (8 percent) levels of uniaxlal prestrain have shown insignificant effects on the strain life results of a dual phase 80 steel, Figure 13. Results1 from tests of fabri- cated members show no significant effect on fatigue results of the cold work introduced during bending and forming of the members, Figure 15. However, other ~ t u d i e s * ~ have shown significant in- creases in long4fe strain-controlled fatigue behavior for 40 percent balanced biaxial prestrain of SAE 1008 hot-rolled low-carbon strip, Figure 16. As shown by other inve~tigators2~, cold working can improve the long-life fatigue properties

- / Undeformed

0 0.004 0.008 0.012 0.016

Strain, E

Figure 14. Monotonic and Cyclic Stress-Strain Curves for an HSLA Steel, Both Underformed and Prestrained 10 Percent in Tension (Ref. 20)

Rolled Cold Formed Surfaces Corners \ \

Catgegory A 30 ksi Beams

\ Sheared Edges

101 I 1 I 1 1 1 1 1 1 I 1 I 1 1 1 1 1 1 I I 1 1 1 1 1 1 1 I I l l

103 I o4 I o5 I o6 Cycles to Failure, N L

Figure 15. Fatigue Test on 4-Inch-Deep Beams (Category A)(Ref. 1)

with a possible accompanying degrada- tion of short-life behavior because any decrease in ductility will almost certainly lower the strain-life curve at short lives. These investigators suggested that very large prestraining values can lead to ma- terial damage which, in turn, reduces the fatigue life. Thus additional studies are needed to clearly define the effects of cold work of forming on the strain-life be- havior of steels. Not only may the level or amount of cold work be of significance. but also the mode of deformation; i.e., uniaxial tension, biaxial tension, plane strain (rollmg) or drawing modes of deformation.

4.2.2 Effect of Welding

Even if the properties of formed sheets could be properly characterized by care- ful laboratory studies, the fatigue performance of a structural component will be significantly altered if welded dur- ing fabrication. Not only can the heating and cooling. resulting from welding, sig- nificantly alter the fatigue properties of the sheet steel in the heat-affected zone, but crack-like imperfections or disconti- nuities may occur in the weld. In addition, very severe stress concentrations may

result at the junction of the weld and the steel sheet. Predictions of the behavior of w e l d r n e n t ~ ~ ~ - ~ ~ by using strain-life pro- cedures have had limited success because of the complexities of modeling the geometric stress raisers and because of the lack of strain-life data for the weld metal and heat-affected-zone regions.

More recent studies33 of arc welding of four steels having a wide range of yield strengths have shown the fatigue behav- ior of the welded joints to be the same for all four steels. Extensive studies34 of fabricated details used for bridges have shown that the fatigue behavior of these structures is the same for all structural steels with yield strengths ranging from 36 to 100 ksi and that the fatigue life of the welded detail is predicted from the applied cyclic stress range and severity of the geometric detail (category). These findings are consistent with recent frac- ture mechanics studies35 that show if the initiation life of a structure is eliminated by introducing a crack or a crack-like imperfection (such as those found in welds), the life of a structure is governed solely by the crack propagation behavior which is the same forall steels. A recent study' has shown that the fatigue life of fabricated sheet steel structures can, in

fact, be predicted using these same procedures and structural geometry categories, Figure 15.

4.2.3 Other Fabricating Processes

Some of the effects of other fabricating processes such as shearing, slitting, burning, and the drilling and tapping of holes have been explored1 but need more attention especially concerning the use of the strain-life concept.

4.3 Effects of Environment

4.3.1 Corrosion Effects on Fatigue

Many of the steels used in automotive applications are subject to a corrosive environment that causes pitting.

Few data are available on the effects of corrosive attack and strain-life behav- ior. Studies have shown36, 37 that pits formed from corrosive attack produce a degradation of the fatigue behavior.

4.3.2 Elevated Temperature Effects

Limited data38 are also available on the effects of elevated temperature on strain- life behavior of sheet steels.

A- - No Prestrain

0- Balanced Biaxial Prestrain

10 1 02 1 o3 1 o4 I o5 1 o6 lo7

2 N,: Reversals to Failure

Figure 16. Elastic Strain-Life for 0 and 40% Balanced Biaxial Prestrain of a 1008 Hot-Rolled Low Carbon Strip (Ref. 28)

5. Fatigue- Design Trends

As seen from the previous discussion, trends in the design against fatigue fail- ure of components in transportation vehicles have led to more sophisticated procedures, both in design and in test procedues. Therefore, it is not surprising to observe different emphases ex- pressed and pursued by various groups of engineers performing the design- analyses and the testing of components or full-scale structures.

Confirmation of a proper fatigue- design concept lies in full-scale component testing in addition to small specimen testing described earlier. As mentioned previously, the testing of a de- sign is accomplished in laboratory tests, track tests, or in-service tests. These tests require that the design endures cer- tain bogey-test requirements defined in a variety of ways: (a) number of specified constant-amplitude stresses, (b) number of specified variable-amplitude stress spectra, (c) hours of test-track service, and/or (d) hours, days, months or years of in-service performance. Specific bogey-test requirements differ between products and fabricators; however, as shown by the flow chart in Figure 1,

adequate test performance will allow pro- gression of a new design towards production, while inadequate perfor- mance will initiate yet another cycle for a newly outlined design.

Some of the current trends to improve the reliability and safety of sheet steel products. especially as related to fatigue performance, are concentrated in one or several of the followinq areas:

Improvements 6 fatigue properties of sheet steels and of the methods used to join them Modification of pads and fabricating processes to reduce stress concentrations Reduction or relief of residual stresses due to fabricating processes Improvement of the fatigue-design methodology by extended use of computers, including variable- amplitude spectra representative of actual in-service conditions of a component, or full-scale structure. Development of new design procedures.

combining efforts in all of these areas is representative of the current design procedure of transportation industry en- gineers and is surely expected to lead to safer and more reliable sheet steel structures.

6. References 1. K. H. Klippstein, Fatigue Behavior of

Steel Sheet Fabrication Details, So- ciety of Automotive Engineers, Paper No. 81 0436,1981.

2. S. T. Rolfe and J. M. Barsom, Fracture and Fatigue Control in Structures-Application of Fracture Mechanics, Prentice-Hall Inc., Englewood Cliffs, New Jersey, 1977.

3. J. M. Barsom, K. H. Klippstein, and A. K. Shoemaker, "State-of-the-Art Report on Fatigue Behavior of Sheet Steels for Automotive Applications," American Iron and Steel Institute, Washington, D.C. Research Report SG-80-2, February, 1980.

4. L. F. Coffin. Jr., "Fatigue at High Temperature," Fatigue at Elevated Temperatures, ASTM STP 520, American Society for Testing and Materials, 1973, pp. 5-34.

5. H. Neuber, "Theory of Stress Concentration for Shear-Strained Prismatical Bodies with Arbitrary Nonlinear Stress-Strain Law," Trans. of the American Society of Mechani- cal Engineers, J. of Applied Mechanics, December, 1961, pp. 544-550.

6. T. H. Topper, R. M. Wetzel, and J. Morrow, "Neuber's Rule Applied to Fatigue of Notched Specimens," Journal of Materials, JMLSA, Vol. 4, No.1, March 1969, pp. 200-209.

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8. M. A. Miner, "Cumulative Damage in

Fatigue," Journal of Applied Mechnics, Vol. 12, No. 1, September 1 945.

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12. ASTM Standards, Part 10, Metals. - Physical, Mechanical, Corrosion Testing. (1 980)

13. Fatigue Under Complex Loading: Analyses and Experiments, SAE Advances in Engineering, Volume 6, R.M. Wetzel, ed., Society of Automo- tive Engineers, Inc., 1977.

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15. R. W. Landgraf and A. M. Sherman, "Fatigue Behavior of Microalloyed Steels," Microalloy 75 Proceedings, Union Carbide, 1977, pp. 498-502.

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17. R. B. Wilson, "Compilation of Mono-

tonic Tensile Properties and Strain Controlled Cyclic Properties of AlSl 1010 and Various High Strength- Low Alloy Steels Produced by Inland Steel (Compilation as of 12/31/72)," Inland Steel Research Laboratories

-, , ) Report. East Chicago. Indiana. 18. "Fatigue Properties of Van-80

Steel," Jones and Laughlin Report, Pittsburgh, Pennsylvania, March 23, 1973.

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21. J. M. Holt and B. K. Stewart, "Strain- Controlled Fatigue Properties of USS EX-TEN F 50 Steel," Society of Automotive Engineers, Paper No. 790460, 1 979.

22. T. R. Risbeck, "Variability in Fatigue Behavior as a Function of Tensile Strength in Maxi-Form 80 Steel," Republic Steel Corporation, Inde- pendence, Ohio, Research Report PR-12036-76-1, March 1976.

23. J. M. Holt and B. K. Stewart, "Varia- bility of Strain-Controlled Fatigue Properties of USS Dual Phase 80," Society of Automotive Engineers, ) Paper N o 801 400.1980,

24. R. T. Ault, "Variability of Tensile and

Fatigue Properties of Republic X-50-W and Maxi-Form 50 Steels," Republic Steel Corporation, lnde- pendence, Ohio, Research Report TR-12026-7, April 1973.

25. R. Pietrowski, "Variability in the Fa- tigue Properties of Dofascoloy 45F and Dofascoloy 50W," Dominion Foundries and Steel, Limited, Hamilton, Ontario, July 28, 1977.

26. R. Pietrowski, "Fatigue Properties of Dofascoloy 80F," Dominion Found- ries and Steel, Limited, Hamilton, Ontario, November 28, 1977.

27. R. Pietrowski, "The Effect of Forming on the Fatigue Properties of Dofascoloys 50F, 50W, and 502," Dominion Foundries and Steel, Lim- ited, Hamilton, Ontario, November 3, 1977.

28. T. E. Parker and G. L. Montgomery, "Effect of Balanced Biaxial Stretching on the Low Cycle Fatigue Behavior of SAE 1008 Hot Rolled Low Carbon Steel," Society of Automotive Engineers, Paper No. 750048,1975.

29. G. Z. Libertini. T. H. Topper, and B. N. Leis, "The Effect of Large Prestrains on Fatigue," Experimen- tal Mechanics, February 1977.

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Symposium on Fatigue Testing of Weldments, Toronto, Canada, May 2, 1977.

32. F.V. Lawrence, "Estimation of Fatigue-Crack Propagation Life in Butt Welds," Welding Research Supplement, May 1973, pp. 21 2-5 to 220-5.

33. K. W. Ewing, M. L. Wilson, R. A. Heimbuch, D. K. Watney, and A. F.Houchens, "Fatigue of Welded High Strength Low Alloy Steels," Society of Automotive Engineers, Paper No. 800374,1980.

34. J. W. Fisher, "Bridge Fatigue Guide -Design and Details," American In- stitute of Steel Construction, New York, New York, 1977.

35. J. M. Barsom, "Fatigue Behavior of Pressure-Vessel Steels," Welding Research Council Bulletin No. 194, 1 979.

36. J. R. Hiam and R. Pietrowski, "The Influence of Forming and Corrosion on the Fatigue Behavior of Automo- tive Steels," Society of Automotive Engineers, Paper No. 780040, 1978.

37. E. A. Lorai and G. W. Bush, "Fatigue Properties of Galvanized Steel and Hot Rolled Steel Before and After Exposure to Salt Spray ," Society of Automotive Engineers, Paper No. 740034, 1974.

38. C. E. Jaske, "Low-Cycle Fatigue of AISI 101 0 Steel at Temperatures up to 1200" F," Journal of Pressure Vessel Technology, Trans. ASME, August 1977.