Fatigue Testing - ASM International

1 Fatigue Testing Introduction Fatigue is the progressive, localized, perma-nent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fracture after a sufficient number of fluctuations. Fatigue fractures are caused by the simultaneous action of cyclic stress, tensile stress and plastic strain. If any one of these three is not present, Fatigue cracking will not initiate and propagate. The cyclic stress starts the crack; the tensile stress produces crack growth (propagation). Although compressive stress will not cause Fatigue , compression load may do so. The process of Fatigue consists of three stages: Initial Fatigue damage leading to crack nu-cleation and crack initiation Progressive cyclic growth of a crack (crack propagation) until the remaining uncracked cross section of a part becomes too weak to sustain the loads imposed Final, sudden fracture of the remaining cross section Fatigue cracking normally results from cyclic stresses that are well below the static yield strength of the material.

2 (In low-cycle Fatigue , however, or if the material has an appreciable work-hardening rate, the stresses also may be above the static yield strength.) Fatigue cracks initiate and propagate in re-gions where the strain is most severe. Because most engineering materials contain defects and thus regions of stress concentration that intensify strain, most Fatigue cracks initiate and grow from structural defects. Under the action of cy-clic loading, a plastic zone (or region of deforma-tion) develops at the defect tip. This zone of high deformation becomes an initiation site for a fa-tigue crack. The crack propagates under the ap-plied stress through the material until complete fracture results. On the microscopic scale, the most important feature of the Fatigue process is nucleation of one or more cracks under the influ-ence of reversed stresses that exceed the flow stress, followed by development of cracks at per-sistent slip bands or at grain boundaries.

3 Prediction of Fatigue life The Fatigue life of any specimen or structure is the number of stress (strain) cycles required to cause failure. This number is a function of many variables, including stress level, stress state, cy-clic wave form, Fatigue environment, and the metallurgical condition of the material. Small changes in the specimen or test conditions can significantly affect Fatigue behavior, making ana-lytical prediction of Fatigue life difficult. There-fore, the designer may rely on experience with similar components in service rather than on laboratory evaluation of mechanical test speci-mens. Laboratory tests, however, are essential in understanding Fatigue behavior, and current studies with fracture mechanics test specimens are beginning to provide satisfactory design criteria. Laboratory Fatigue tests can be classified as crack initiation or crack propagation. In crack initiation Testing , specimens or parts are sub-jected to the number of stress cycles required for a Fatigue crack to initiate and to subsequently grow large enough to produce failure.

4 In crack propagation Testing , fracture mechan-ics methods are used to determine the crack growth rates of preexisting cracks under cyclic loading. Fatigue crack propagation may be caused by cyclic stresses in a benign environ-ment, or by the combined effects of cyclic stresses and an aggressive environment (corrosion fa-tigue). Fatigue Crack Initiation Most laboratory Fatigue Testing is done either with axial loading, or in bending, thus producing only tensile and compressive stresses. The stress usually is cycled either between a maximum and a minimum tensile stress, or between a maximum tensile stress and a maximum compressive stress. 1 Atlas of Fatigue Curves Boyer, AuthorCopyright 1986 ASM International All rights Fatigue Testing The latter is considered a negative tensile stress, is given an algebraic minus sign, and therefore is known as the minimum stress. The stress ratio is the algebraic ratio of two specified stress values in a stress cycle.

5 Two commonly used stress ratios are the ratio, A, of the alternating stress amplitude to the mean stress (A= S"/ S ml and the ratio, R, of the min-imum stress to the maximum stress (R = Smin/ Smax). If the stresses are fully reversed, the stress ratio R becomes -I; if the stresses are partially re-versed, Rbecomes a negative number less than 1. If the stress is cycled between a maximum stress and no load, the stress ratio R becomes zero. If the stress is cycled between two tensile stresses, the stress ratio R becomes a positive number less than 1. A stress ratio R of 1 indicates no variation in stress, making the test a sustained-load creep test rather than a Fatigue test. Applied stresses are described by three pa-rameters. The mean stress, S m is the algebraic average of the maximum and minimum stresses in one cycle, Sm= (Smax+ Smin)/2. In the com-pletely reversed cycle test, the mean stress is zero. The range of stress, S, is the algebraic difference between the maximum and minimum stresses in one cycle, Sr = S max-S min The stress amplitude, S" is one half the range of stress, S" = S ,/ 2 = (Sm.))

6 ,-Sm;.)/2. During a Fatigue test, the stress cycle usually is maintained constant so that the applied stress conditions can be written Sm S a where Sm is the static or mean stress, and Sa is the alternating stress, which is equal to half the stress range. Nomenclature to describe test parameters in-volved in cyclic stress Testing are shown in Fig. 1. Fig. 1 Nomenclature to describe test parameters involved in cyclic stress Testing S-N Curves. The results of Fatigue crack initia-tion tests usually are plotted as maximum stress, minimum stress, or stress amplitude to number of cycles, N, to failure using a logarithmic scale for the number of cycles. Stress is plotted on either a linear or a logarithmic scale. The result-"' "-:;: VJ" ID "C ~ c. E "' 00 00 ~ iii 1100 1000 12340 ~teel o"i:, 48 HAC 150 900 ' (unnoto;hed)-125 BOO 700 600 500 400 300 200 100 "F ---:--T--: s attgue tmtt , 234~lsteel ~ '\.. 48 HAC _ "' '),. (notched) ~.

7 ,__ .. t- Fattgue It mit, s, Aluminum1 alloy --,; ~7075-TS -~ 100 75 50 25 0 Stress ratio (R) = -1 0 106 107 10' Number of cycles to fracture, N Fig. 2 Typical S-N curves for constant amplitude and sinusoidal loading ~ "" --' VJ" ID "C -~ c. E "' 00 00 1?. iii ing plot of the data is an S-N curve. Three typical S-N curves are shown in Fig. 2. The number of cycles of stress that a metal can endure before failure increases with decreasing stress. For some engineering materials such as steel (see Fig. 2) and titanium, the S-N curve be-comes horizontal at a certain limiting stress. Below this limiting stress, known as the Fatigue limit or endurance limit, the material can endure an infinite number of cycles without failure. Fatigue Limit and Fatigue Strength. The hor-izontal portion of an S-N curve represents the maximum stress that the metal can withstand for an infinitely large number of cycles with 50% probability of failure and is known as the Fatigue (endurance) limit, Sp Most nonferrous metals do not exhibit a Fatigue limit.

8 Instead, their S-N curves continue to drop at a slow rate at high numbers of cycles, as shown by the curve for aluminum alloy 7075-T6 in Fig. 2. For these types of metals, Fatigue strength rather than Fatigue limit is reported, which is the stress to which the metal can be subjected for a specified number of cycles. Because there is no standard number of cycles, each table offatigue strengths must specify the number of cycles for which the strengths are reported. The Fatigue strength of nonferrous metals at I 00 million ( 108) or 500 million (5 X 108) cycles is erroneously called the Fatigue limit. Low-Cycle Fatigue . For the low-cycle Fatigue region (N < 104 cycles) tests are conducted with controlled cycles of elastic plus plastic strain, Introduction rather than with controlled load or stress cycles. Under controlled strain Testing , Fatigue life be-havior is represented by a log-log plot of the total strain range, . 1 versus the number of cycles to fiihue (Fig.)

9 3). The total strain range is separated into elastic and plastic components. For many metals and alloys, the elastic strain range, A<, is equal to the stress range divided by the modulus of elasticity. The plastic strain range, A< P' is the difference be-tween the total strain range and the elastic strain range. Stress-Concentration Factor. Stress is concen-trated in a metal by structural discontinuities, such as notches, holes, or scratches, which act as stress raisers. The stress-concentration factor, K1, is the ratio of the area test stress in the region oft he notch (or other stress concentrators) to the corresponding nominal stress. For determina-tion of K1, the greatest stress in the region of the notch is calculated from the theory of elasticity, or equivalent values are derived experimentally. The Fatigue notch factor, Kp is the ratio of the Fatigue strength of a smooth (unnotched) speci-men to the Fatigue strength of a notched speci-men at the same number of cycles.

10 Fatigue notch sensitivity, q, for a material is determined by comparing the Fatigue notch fac-tor, K1, and the stress-concentration factor, K1, for a specimen of a given size containing a stress concentrator of a given shape and size. A com-mon definition of Fatigue notch sensitivity is q = (K1-1)/(K,-1), in which q may vary between zero (where K1= 1) and I (where K1= K,). This value may be stated as percentage. Fatigue Crack Propagation In large structural components, the existence of a crack does not necessarily imply imminent failure of the part. Significant structural life may remain in the cyclic growth of the crack to a size at which a critical failure occurs. The objective of Fatigue crack propagation Testing is to determine the rates at which subcritical cracks grow under cyclic loadings prior to reaching a size critical for fracture. The growth or extension of a Fatigue crack under cyclic loading is principally controlled by maximum load and stress ratio.