Shengde Zhang

Creep-Fatigue Life Assessment Under Multiaxial Strain State

This paper studies the multiaxial creep-fatigue life for type 304 stainless steel at elevated temperature. Strain controlled biaxial tension-compression creep-fatigue tests were carried out using cruciform specimens under four strain waves at three principal strain ratios. The strain wave and the principal strain ratio had a significant effect on creep-fatigue life of the cruciform specimen. The creep-fatigue life ratio decreased as the principal strain ratio increased which indicates that larger creep damage occurred at larger principal strain ratio. The effects of the strain wave and principal strain ratio were discussed in relation to the observations of surface crack and void area density in the gage part of the specimen. Creep-fatigue lives were discussed in relation to the principal stress amplitude calculated by finite element analysis and creep-fatigue damage was evaluated by linear damage rule.Copyright

Multiaxial Low Cycle Fatigue for Ni-Base Single Crystal Super Alloy at High Temperature

This paper presents multiaxial low cycle fatigue lives of YH61 Ni-base single crystal superalloy at elevated temperature. Tension-torsion low cycle fatigue tests were performed using hollow cylinder specimens with aligning the specimen axis to [100] direction at 1173 K. Biaxial tension-compression low cycle fatigue tests were also performed using cruciform specimens whose x and y axes were aligned to \(\langle 110\rangle \) directions. The effect of strain multiaxiality on the multiaxial low cycle fatigue lives was discussed. Couples of multiaxial stress and strain parameters were applied to the experimental data for correlating the multiaxial low cycle fatigue lives of both types of the specimens. The strain parameters yielded a large scatter in the correlation and Mises equivalent stress was only the parameter to give a satisfactory correlation of multiaxial low cycle fatigue lives of the superalloy.

Multiaxial low cycle fatigue life of Mod.9Cr-1Mo steel circumferential notched specimen under nonproportional loading

This paper presents the notch effect in low cycle fatigue of Mod.9Cr-1Mo steel under multiaxial loading. Tension- torsion multiaxial low cycle fatigue tests were performed using three types of circumferential notched specimens of the steel and crack initiation, failure and propagation lives were experimentally obtained. Two proportional and nine nonproportional strain waveforms were used in low cycle fatigue tests. The superposition of torsion loading on tension loading reduced the three lives but they little decreased with elastic stress concentration factor. The three lives in nonproportional loading were smaller than those in proportional loading but showed smaller reduction with elastic stress concentration factor. Local strains at the notch root were estimated by finite element analyses and Neubers rule. Mises strain range and maximum principal strain range conservatively estimated the crack initiation lives of the notched specimens.

Creep-fatigue life assessment for three kinds of solders

Abstract This paper presents a creep-fatigue life evaluation for three kinds of solders of Sn–37Pb, Sn– 3.5Ag and Sn – 3.0Ag – 0.5Cu. Strain controlled push – pull creep-fatigue tests were performed for solid bar specimens under fast–fast, fast–slow, slow–fast and strain–hold strain waveforms and the effect of strain waveforms on creep-fatigue lives is discussed. Sn– 3.5Ag and Sn– 3.0Ag – 0.5Cu solders showed stronger creep-fatigue resistance compared to Sn –37Pb solder. The applicability of the linear damage rule, the strain range partitioning method and the grain boundary sliding model is discussed for predicting creep-fatigue lives of the three solders. The strain range partitioning method and the grain boundary sliding model satisfactorily predicted creep-fatigue lives within a small scatter.

Effect of Non-Proportional Loading on Fatigue Crack Initiation and Growth Behavior of Maraging Steel

This paper deals with multiaxial low cycle fatigue (LCF) crack behavior of maraging steel under non-proportional loading. Strain controlled LCF tests under proportional and non-proportional loading with 90° out-of-phase shift between the axial and shear strains were carried out on tube specimens at room temperature. In the correlation with Mises strain, LCF lives under non-proportional loading were underestimated by a factor of 10 from those under proportional loading. Observation of the specimen surface during fatigue tests showed that a large number of cracks were initiated in shear loading and non-proportional loading compared with uniaxial loading. Finite element analyses for estimating the stress concentration around inclusions revealed that the inclusion under shear loading and non-proportional loading had more locations to initiate cracks than the inclusion under uniaxial loading. The crack initiation life under proportional loading did not significantly differ from that under non-proportional loading. Fatigue cracks under non-proportional loading, however, propagated faster than those under proportional loading. The faster fatigue crack propagation under nonproportional loading presumably results from higher strain intensity at crack tips than that under proportional loading. The fatigue lives were predicted from the crack propagation analysis. Most calculated lives were within a factor of 2 scatter band so that the correlation of the test results appears satisfactory. Therefore, the main cause of shorter fatigue life under non-proportional loading resulted from the faster crack growth due to the higher strain intensity.Copyright