•In-plane biaxial constraint effect on strain evolution is investigated.•Equibiaxial stress state imposes the largest constraint.•Large amplitude stress cycle suppresses subsequent strain accumulation.•Cyclic effect on membrane fatigue behavior is related to membrane stress state.The durability of a proton exchange membrane is affected by both mechanical degradation and chemical degradation. While fatigue and relative humidity cycling tests have been conducted to address mechanical degradation, the cyclic behavior that bridges the gap between the stress-strain response and fatigue behavior is not well established. The objective of this study is to understand the strain evolution during biaxial cyclic loading that resemble the actual stress state of the membrane. In particular, the effect of loading paths on strain evolution is examined to account for the stress state on strain accumulation. It is found that the constraint effect of stress in one direction on strain evolution in another direction strongly depends on the stress state of the membrane, and the equibiaxial stress state imposes the most significant constraint on strain evolution. Furthermore, the constraint effect induced by biaxial loading is more significant at higher relative humidity values. Moreover, high-stress amplitude cycle acts to retard strain accumulation in the subsequent low-stress amplitude cycle. The findings reported here will provide new evidence for an understanding of the fatigue behavior of a proton exchange membrane as well as durability modeling of proton exchange membrane fuel cells.Download high-res image (206KB)Download full-size image

Reflow voids created by solder oxidation reduce the reliability of lap joints. In situ visualization of reflow voids in Sn-3Ag-0.5Cu (SAC305) lap-shear solder joints under cyclic stressing was realized by X-ray computed tomography (CT), while the ratcheting deformation of the solder joints was monitored by a non-contact displacement detecting system (NDDS). The results revealed that the shape evolution of reflow voids in solder joints, as characterized by the sphericity of the voids, can be divided into three stages: i.e., the initial stage with a sharp drop, a stable stage, and a rapidly declining stage. A new evolution law for describing the progress of sphericity was proposed, and was further introduced into a viscoplastic constitutive model based on the OW-AF nonlinear kinematic hardening rule. The damage-coupled OW-AF model yielded an accurate estimation of the whole-life ratcheting behavior of Sn-3Ag-0.5Cu (SAC305) lap-shear solder joints.

Zirconium alloy is used as nuclear fuel cladding in nuclear reactors because it offers the merits of low neutron absorption cross-section, high corrosion resistance, excellent mechanical properties, and satisfactory creep resistance under operating conditions. However, the cladding experiences uniaxial and multiaxial stresses, which can lead to material failure due to the ratcheting effect. In this context, in this study, a series of uniaxial and multiaxial ratcheting tests were conducted on zirconium alloy tubes at 350 °C. The experimental results show that the temperature significantly affects the properties of zirconium alloy, and its ductility at 350 °C is poorer than that at room temperature. The axial ratcheting strain rate greatly increases with increase in the mean stress, stress amplitude, and peak stress holding time. In addition, the loading history also greatly affects the uniaxial and multiaxial ratcheting of zirconium tubes. A lower stress level after a loading history with higher stress level leads to shakedown of the ratcheting, while a higher mean stress after a loading history with lower mean stress brings down the ratcheting strain rate. The existence of a constant internal pressure significantly retards the failure of the zirconium tubes.

Total strain-controlled low cycle fatigue (LCF) tests of a nickel based superalloy were performed at 650 °C. Various hold times were introduced at the peak tensile strain to investigate the high-temperature creep-fatigue interaction (CFI) effects under the same temperature. A substantial decrease in fatigue life occurred as the total strain amplitude increased. Moreover, tensile strain holding further reduced fatigue life. The saturation phenomenon of holding effect was found when the holding period reached 120 s. Cyclic softening occurred during the LCF and CFI process and it was related to the total strain amplitude and the holding period. The relationship between life-time and total strain amplitude was obtained by combining Basquin equation and Coffin-Manson equation. The surface and fracture section of the fatigued specimens were observed via scanning electronic microscope (SEM) to determine the failure mechanism.

The evolutions of uniaxial tension properties and the ratcheting fatigue behavior of AZ31 magnesium alloy sheet pre-corroded in the phosphate buffered solution (PBS) stimulated physiological environment with increasing immersion time were investigated in this study. The corrosion behavior of AZ31 sheet was also studied. It was found that the corrosion rate decreased exponentially with immersion time until stabilized at about 0.05 g/m2 h after 28 days immersion. The corrosion behavior was characterized by pitting corrosion and progressive pitting corrosion in the direction of pits depth was gradually inhibited. The Young's modulus and the elongation of AZ31 sheet decreased exponentially with increasing immersion time and became low levels with reductions of around 10% and 20% respectively at 28 days immersion. The ultimate strength, however, decreased no more than 5%. With increasing immersion time, both the ratcheting strain and ratcheting strain rate in the stable stage of ratcheting strain evolution tended to increase. 28 days degradation decreased the fatigue lives by a factor of about 2.5–5. The pre-corroded damage defined as life reduction after degradation on the basis of the Miner's linear damage rule (LDR) was found to increase exponentially with immersion time regardless of the loading conditions. A ratcheting fatigue life prediction model based on the Miner LDR, the Smith–Watson–Topper (SWT) parameter and the Basquin's equation was proposed and yielded good prediction for the pre-corroded AZ31 sheet.

Low temperature sintered nano-silver paste can be used for connected chips that require high temperature operation and high heat dissipate ability because of its higher melting temperature, and better thermal/electrical conductivity than conventional solders and adhesive films. In this study, high temperature ratcheting behavior of the nano-silver paste sintered lap shear joint was examined by cyclic shear force at room temperature and elevated temperatures at 225 °C, 275 °C and 325 °C. The effects of shear stress amplitude, mean shear stress, peak stress dwell time and temperature on the ratcheting response of sintered lap shear joint were analyzed. Results show that (1) the ratcheting strain and ratcheting strain rate of the nano-silver sintered lap shear joint increase with increasing shear stress amplitude or mean shear stress; (2) the ratcheting strain increases with the increasing temperature or the peak stress dwell time; (3) the ratcheting strain shakes down after it accumulates to a certain extent at room temperature; (4) even though 225 °C is a destructive temperature for solder and adhesive films, the ratcheting evolution of nano-silver sintered joint is very mild no matter whether the sample dwells at peak stress or not; (5) the influence of shear stress amplitude, mean shear stress or peak stress dwell time is more and more obvious at high temperatures.