Chapter 8

Chapter 8

FATIGUE Fatigue = failure under cyclic stress. compression on top specimen bearing bearing counter motor flex coupling tension on bottom Stress varies with time. --key parameters are S and m Key points: Fatigue... Adapted from Fig. 8.16, Callister 6e. (Fig. 8.16 is from Materials Science in Engineering, 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.) max m S time min

--can cause part failure, even though max < c. --causes ~ 90% of mechanical engineering failures. Chapter 8-17 FATIGUE DESIGN PARAMETERS Fatigue limit, Sfat: S = stress amplitude unsafe --no fatigue if S < Sfat S fat 103 Sometimes, the fatigue limit is zero! safe 105 107 109 N = Cycles to failure S = stress amplitude unsafe safe 103 case for steel (typ.) Adapted from Fig. 8.17(a), Callister 6e. case for Al (typ.) Adapted from Fig. 8.17(b), Callister 6e.

105 107 109 N = Cycles to failure Chapter 8-18 FATIGUE MECHANISM Crack grows incrementally typ. 1 to 6 da m K dN ~ a increase in crack length per loading cycle crack origin Failed rotating shaft --crack grew even though Kmax < Kc --crack grows faster if Adapted from increases Fig. 8.19, Callister crack gets longer 6e. (Fig. 8.19 is D.J. Wulpi, loading freq. increases.from Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.)

Chapter 8-19 IMPROVING FATIGUE LIFE 1. Impose a compressive surface stress S = stress amplitude Adapted from Fig. 8.22, Callister 6e. (to suppress surface cracks from growing) near zero or compressive m moderate tensilem larger tensile m N = Cycles to failure --Method 1: shot peening --Method 2: carburizing shot put surface into compression 2. Remove stress concentrators. bad bad C-rich gas better better Adapted from

Fig. 8.23, Callister 6e. Chapter 8-20 CREEP Occurs at elevated temperature, T > 0.4 Tmelt Deformation changes with time. strain, INCREASING T 0 tertiary t primary secondary elastic 0 T < 0.4 Tm time Adapted from Figs. 8.26 and 8.27, Callister 6e. Chapter 8-21 SECONDARY CREEP Most of component life spent here. Strain rate is constant at a given T, --strain hardening is balanced by recovery stress exponent (material parameter) Q c

. n s K 2 exp activation energy for creep RT (material parameter) strain rate material const. Strain rate increases for larger T, applied stress 200 100 Adapted from Fig. 8.29, Callister 6e. 427C (Fig. 8.29 is from Metals Handbook: 538C Properties and Selection: Stainless Steels, Tool Materials, and 649C Special Purpose Metals, Vol. 3, 9th ed., D. Benjamin (Senior Ed.), 1 American Society for rate (%/1000hr) Metals, 1980, p. Chapter 8-22 131.) Stress (MPa) 40

20 10 10-2 10-1 Steady state creep s CREEP FAILURE Failure: Estimate rupture time S 590 Iron, T = 800C, = 20 ksi 100 applied stress From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.32, p. 87, John Wiley and Sons, Inc., 1987. (Orig. source: Pergamon Press, Inc.) Time to rupture, tr T(20 logtr ) L temperature function of applied stress time to failure (rupture) Stress, ksi along grain boundaries. g.b. cavities 20 10

data for S-590 Iron Adapted from Fig. 8.45, Callister 6e. (Fig. 8.45 is from F.R. Larson and J. Miller, Trans. ASME, 74, 765 (1952).) 1 12 16 20 24 28 L(103K-log hr) 24x103 K-log hr T(20 logtr ) L 1073K Ans: tr = 233hr Chapter 8-23 SUMMARY Engineering materials don't reach theoretical strength. Flaws produce stress concentrations that cause premature failure. Sharp corners produce large stress concentrations and premature failure. Failure type depends on T and stress: -for noncyclic and T < 0.4Tm, failure stress decreases with: increased maximum flaw size, decreased T, increased rate of loading. -for cyclic : cycles to fail decreases as increases. -for higher T (T > 0.4Tm): time to fail decreases as or T increases. Chapter 8-24

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