Cyclic plastic deformation Cyclic plastic deformation and damage in 304LN and damage in 304LN stainless steelstainless steel

--Surajit Kumar Paul et al.

Reporter: Yong WangSupervisor: Professor Xu Chen

HighlightsHighlightsLCF and ratcheting response, LCF and ratcheting response,

ratcheting–LCF interaction, and ratcheting–LCF interaction, and LCF and ratcheting damage LCF and ratcheting damage evolution in 304LN stainless evolution in 304LN stainless steel are experimentally steel are experimentally investigated.investigated.

True stress controlled ratcheting True stress controlled ratcheting test procedure is adopted in this test procedure is adopted in this investigation to assess the investigation to assess the materials ratcheting materials ratcheting performance.performance.

ExperimentExperimentAISI 304LN austenitic stainless

steel was available in the form of pipe with 320mm outer diameter and 25mm thickness.

Cylindrical specimens of 7mm gauge diameter and 13mm gauge length were machined from the pipe in such a way that the loading axes of the specimens were parallel to the pipe axis.

Low cycle fatigueLow cycle fatigue LCF tests were performed at the total strain ranges of ±0.2%,

±0.5%, ±0.7%, ±0.85%, ±1.0%, ±1.2%, ±1.4%, ±1.6%, ±1.8% and ±2.0% with strain rate of 1×10−3 /s.

below the strain amplitude of 0.5%, the material displayed negligible initial cyclic hardening followed by progressive cyclic softening throughout its life

at strain amplitude of 0.7–1.2%,the initial pronounced cyclic hardening and followed by mild progressive cyclic softening is exhibited

at strain amplitude larger than 1.4%, the material exhibited very rapid cyclic hardening in initial few cycles and followed by mild gradual cyclic hardening almost without reaching its saturated value till final fracture

RatchetingRatchetingTrue stress controlled ratcheting responses

of 304LN stainless steel are examined in this investigation.

RatchetingRatcheting

RatchetingRatcheting

plastic strain amplitude and plastic strain energy are damaging parameters in cyclic plastic deformation, therefore damage growth is delayed in the presence of mean stress and hence the ratcheting life is prolonged.

Effect of pre-ratcheting on Effect of pre-ratcheting on subsequent LCF behaviorsubsequent LCF behavior Pre-ratcheting tests are performed at 0.1%,

0.25% and 0.5% of its ratcheting life, i.e. interrupted at 198, 450 and 990 cycles. After pre-ratcheting, LCF tests are conducted with 0.7% strain amplitude.

Reason behind choosing 0.7% strain amplitude is that average stress amplitude in LCF is close to the ratcheting stress amplitude, so that other effect in cyclic plastic deformation (i.e. strain range effect, loading dependent cyclic hardening/softening, etc.) can be avoided.

Effect of pre-ratcheting on Effect of pre-ratcheting on subsequent LCF behaviorsubsequent LCF behavior

total cyclic plastic damage evolution path in LCF with pre-ratcheting

remaining LCF life fraction versus ratcheting strain accumulated during pre-ratcheting

Effect of pre-LCF on Effect of pre-LCF on subsequent tensile behaviorsubsequent tensile behavior Pre-LCF experiments with 1.0% strain

amplitude are interrupted at 15, 200, 510 and 800 cycles, i.e. 0.015, 0.2, 0.5 and 0.78 of LCF life fractions. LCF life fractions can be determined by the ratio of current number of cycles and number of cycles to failure (for 1.0% strain amplitude is 1017).

Those pre-LCF damaged specimens are then used for tensile test.

Effect of pre-LCF on Effect of pre-LCF on subsequent tensile behaviorsubsequent tensile behavior

yield stress, ultimate tensile stress and uniform elongation variation with LCF life fractions

Effect of pre-LCF on Effect of pre-LCF on subsequent tensile behaviorsubsequent tensile behavior

Factographs and image processed factographs in pre-LCF (strain amplitude 1.0%) followed by tensile test: a1, b1, c1, d1 and e1 are factographs; a2, b2, c2, d2 and e2are image processed factographs at 0, 15, 200, 510 and 800 cycles pre-LCF.

Effect of pre-LCF on Effect of pre-LCF on subsequent tensile behaviorsubsequent tensile behavior

average dimple size versus number of cycles

dimple number in unit area versus number of cycles

Effect of pre-ratcheting on Effect of pre-ratcheting on subsequent tensile behaviorsubsequent tensile behavior

Pre-ratcheting experiments with mean stress of 120MPa and stress amplitude of 420MPa are interrupted at 50, 450 and 1500 cycles, i.e. 0.025, 0.23 and 0.76 of ratcheting life fractions

Effect of pre-ratcheting on Effect of pre-ratcheting on subsequent tensile behaviorsubsequent tensile behavior

yield stress, ultimate tensile stress and uniform elongation variation with ratcheting strain fractions

yield stress in tensile test of pre-ratcheting specimen, plastic strain amplitude and hysteresis loop area variation with number of cycles

Effect of pre-ratcheting on Effect of pre-ratcheting on subsequent tensile behaviorsubsequent tensile behavior

Factographs and image processed factographs in pre-ratcheting (mean stress 120MPa and stress amplitude 420MPa) followed by tensile test: a1, b1, c1 and d1 are factographs; a2, b2, c2 and d2 are image processed factographs at 0, 50, 450 and 1500 cycles pre-ratcheting.

Effect of pre-ratcheting on Effect of pre-ratcheting on subsequent tensile behaviorsubsequent tensile behavior

average dimple size versus number of cycles in pre-ratcheting

dimple number in unit area versus number of cycles

ConclusionsConclusions Ratcheting life improves with increasing mean stress.

Mean stress dependent hardening, i.e. decreasing hysteresis loop area and plastic strain amplitude with increasing mean stress, thought to be responsible for the improvement in ratcheting life.

Pre-ratcheting significantly reduce subsequent LCF life. Permanent ratcheting strain accumulation can be the root cause of LCF life reduction.

Pre-ratcheting alter the hardening/softening behavior in succeeding LCF tests

Tensile properties, i.e. yield stress, ultimate tensile stress and uniform elongation alter in a systematic manner with LCF and ratcheting damage.

Average dimple size increases and dimple number in unit area decreases with LCF damage whereas, average dimple size decreases and dimple number in unit area increases with ratcheting damage.

RevieReviewwEffect of pre-ratcheting on subsequent

LCF behaviorEffect of pre-LCF on subsequent

tensile behaviorEffect of pre-ratcheting on subsequent

tensile behavior

Effect of pre-tension(strain control and stress control) on subsequent LCF behavior

Effect of pre-tension(strain control and stress control) on subsequent ratcheting

Effect of pre-LCF on subsequent ratcheting

PlaPlann

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