Low Cycle Fatigue Behavior of High-Strength Structural Steel under Biased Strain Control
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The low cycle fatigue (LCF) behavior of high-strength structural steel under biased strain control has been investigated. Seven strain ratios with constant maximum strain amplitude were adopted. Significant cyclic softening and mean stress relaxation were observed in all cases. The fatigue life under biased strain control could be well predicted by Coffin-Manson relationship. Fractography by scanning electron microscopy was used to determine the LCF failure mechanisms and fatigue crack propagation modes.Keywords:
Fractography
Low-cycle fatigue
Strain (injury)
Cyclic stress
Fractography
Low-cycle fatigue
Strain (injury)
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Abstract Two‐level cumulative low‐cycle fatigue lives of AISI 316 stainless steel notched specimens with different biaxial loading mode sequences were experimentally analyzed in this paper. Forty‐eight cases were conducted in the experimental program by considering the loading level sequence effect, the biaxiality of two levels and the cycle ratios. Results show that interlock effect caused by the characteristic fracture surfaces of the different biaxial states is beneficial to the cumulative fatigue lives. On the other hand, the tensile loading of the second level will accelerate the opening of cracks and decrease fatigue strength. Miner's rule predicts most fatigue lives within 30% error bands, and the loading level sequence effect was not found in this research involving complex fracture modes. Fractography of specimens in the cumulative fatigue tests is reported and discussed in this paper.
Fractography
Low-cycle fatigue
Interlock
Sequence (biology)
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Low-cycle fatigue
Cyclic stress
Lüders band
Strain (injury)
Fatigue limit
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Cyclic fatigue under rotary bending tests were conducted on partially stabilized zirconia (PSZ) from NGK and Nilsen, and silicon nitride from NGK and Norton. Fractography was performed on the failed specimens to determine the fracture structure and morphology. The results showed that the cyclic fatigue fracture was the same as the fracture structure previously observed in bending tests. The cyclic fatigue data indicated that structural ceramic could function in fatigue stress levels at a higher percentage of their average fast fracture strength than the fifty percent of ultimate strength used for wrought steels.
Fractography
Cyclic stress
Fatigue limit
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The low cycle fatigue (LCF) behavior of Q235 steel under mean strain control has been investigated. A serious of the strain controlled cyclic loading experiments with several combinations of strain amplitudes and mean strains have been performed. Significant cyclic hardening and mean stress relaxation were observed in all cases. Fractography by scanning electron microscopy (SEM) was used to determine the LCF failure mechanisms and fatigue crack propagation modes of the Q235 steel.
Fractography
Cyclic stress
Low-cycle fatigue
Strain (injury)
Hardening (computing)
Strain hardening exponent
Stress–strain curve
Stress relaxation
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A crack was founded in an engine box of an aeroplane during fatigue test by fluorescence penetrate testing.According to the results of macroscopical testing,fractography analysis,chemical analysis,hardness testing and metallographic analysis,it is concluded that the fracture model is low cycle fatigue,Nd segregation of the surface results in the fatigue fracture of the engine box and low material toughness promots the extending of the crack.Based on the analysis results,some measures are brought forward to avoiding the failure of the engine box.
Fractography
Low-cycle fatigue
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Abstract A six-cylinder crankshaft in a 12L diesel engine was locked after testing for about 840 hours in the bench test. The fractography investigation indicates that fatigue is the dominant failure mechanism of the crankshaft. It is found that the fatigue crack mainly initiated at the fillet of the crankweb between the 6 th main journal and 6 th crankpin. The fatigue crack initiation area lies outside the induction surface hardened zone. From detailed metallographic inspection, abnormal microstructure containing Widmannstatten was found in the fatigue crack initiation area and the 5 th main journal, while that was not found at the severely deformed crankweb. Since the region containing Widmannstatten has lower hardness, the root cause of the failure may be that the abnormal microstructure lowered the fatigue strength at the stress concentrated fillet. The crankshaft prematurely fractured under the complex stress condition in the bench test.
Fractography
Fillet (mechanics)
Fatigue limit
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Attempts are made on revealing the very-high cycle fatigue behavior and fatigue mechanism of high strength steels. GCr15 steel was used as testing material and rotating bending fatigue test was adopted. The fractography of fatigue failure was observed by optical microscopy and scanning electron microscopy. The observations shows that, for the number of cycles to fatigue failure between 10~6 and 4×10~8 cycles, fatigue cracks mostly initiated in the interior of specimen and originated at non-metallic inclusions. The results indicate that the fatigue life of specimens with crack origin at the interior of specimen is longer than that with crack origin at specimen surface. The experimental results and the fatigue mechanism were further analyzed in terms of fracture mechanics and fracture physics.
Fractography
Fatigue limit
Failure mechanism
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Fractography
Statistical Inference
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