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Cyclic mean stress relaxation behaviour of P91 steel: Experiments and constitutive modeling

Influence of strain amplitude on the development of dislocation structure during cyclic plastic deformation of 304 LN austenitic stainless steel

Low cycle fatigue tests were conducted on cast nickel-base superalloy IN738LC in vacuum and air and on specimens coated with a layer of NaCl and Na 2SO 4 at 900°C and two different strain rates. The fatigue life, determined in terms of the cyclic plastic strain, decreased markedly as the severity of the environment increased, ie in the order vacuum, air, NaCl + Na 2SO 4. The marked strain-rate effect found in air and saline environments disappeared when the experiments were conducted in a vacuum of 1.3 × 10 −2 Pa. Cyclic plastic deformation had the effect of producing coarsening which was associated with cyclic softening. Crack nucleation occurred in favourably oriented grains in vacuum while for air and for NaCl + Na 2SO 4 coated specimens the cracks always initiated at hot-corroded grain boundaries near the surface or generally at oxide spikes in surface-connected grain boundaries. The crack propagation from these nucleation sites was essentially transgranular for all environments studied.

Correlation between endurance limit and cyclic yield stress determined from low cycle fatigue test

Influences of strain rate on the low cycle fatigue behavior of gravity casting Al alloys

Cyclic deformation characteristics and fatigue crack growth behaviour of a newly developed aerospace superalloy Haynes 282

Most engineering components are subjected to cyclically varying loads with mean stress (or mean strain). Often the loads may result in a multiaxial stress state. It is well known that the fatigue process is sensitive to superposed tensile mean stress in both the high-cycle and low-cycle fatigue regimes. In an elastoplastic regime, a material which experiences a stress-controlled cyclic loading with a non-zero mean stress, will exhibit an accumulated ratcheting strain (cyclic creep strain). This ratcheting strain can cause an additional damage in the material. Thus, a complete fatigue theory should be capable to evaluate the effects of mean stress and ratcheting strain. A large number of multiaxial fatigue theories have been proposed in the past and are reviewed in the literature, for example, see Refs [1–2]. These theories may be divided into four categories, viz: equivalent-stress and -strain, energy-based criteria and the critical plane approaches. The first two approaches usually have difficulty to include the mean stress effect and to account for path dependency of the cyclic plastic deformation. This essential interaction between stress path and strain path is inherently included in the energy approach. In the present paper an energy-based fatigue theory proposed by Ellyin and his collaborators [3] is extended to include the mean stress and ratcheting strain effects on the fatigue life of materials.

High temperature low cycle fatigue behavior of a directionally solidified Ni-base superalloy DZ951

Failure assessment of a hard brittle coating on a ductile substrate subjected to cyclic contact loading

Magnesium alloys are greatly appreciated due to their high strength to weight ratio, stiffness, and low density; however, they can exhibit complex types of cyclic plasticity like twinning, de‐twinning, or Bauschinger effect. Recent studies indicate that these types of cyclic plastic deformations cannot be fully characterized using the typical tools used in cyclic characterization of steels and aluminium alloys; thus, it is required new approaches to fully capture their cyclic deformation and plasticity. This study aims to propose and evaluate a phenomenological cyclic elastic‐plastic approach designed to capture the cyclic deformation of magnesium alloys under multiaxial loading conditions. Series of experimental tests were performed to characterize the cyclic mechanical behaviour of the magnesium alloy AZ31BF considering proportional loadings with different strain amplitude ratios and a nonproportional loading with a 45° phase shift. The experimental results were modulated using polynomial functions in order to implement a cyclic plasticity model for the AZ311BF based on the phenomenological approach proposed. Results show good correlations between experiments and estimates.

Influence of phase transformation due to temperature on cyclic plastic deformation in 304LN stainless steel

In automotive electronic applications, solder joints are subjected to various loading conditions such as thermomechanical and vibrational loads. Local ther-momechanical passive loads acting on the solder often take place within the time scale of minutes to hours leading to low strain rate creep deformation. In contrast, under vibrational loads, high strain rate elastic and plastic deformations within a much shorter time scale are expected to govern the cyclic aging of the solder joint. The present work is focused on the investigation of the deformation and degradation behavior of the SnAgCu solder alloy in the high strain rate regime. This is achieved by performing tensile tests under varying strain rates and temperatures. Additionally, High Cycle Fatigue (HCF) experiments are performed using a frequency of 40 Hz at different stress amplitudes, mean stress and temperature levels. The experimental data reveals the high cycle fatigue behavior and, in particular, the impact of temperature and mean stress effects on the specimens lifetime. In order to describe the high cycle fatigue of SnAgCu in Finite Elements (FE) simulations we employ a viscoplasticity material model. We discuss the model parameter and propose a gradual calibration procedure using the HCF test results for the description of the complex viscoplastic deformation behavior in heigh cycle fatigue regime. Finally, the proposed finite element approach is applied for numerical modelling of the observed Woehler lifetime curves. The simulation results show the capability of the current modelling to map the mean stress and temperature effects on the high cycle fatigue failure of SnAgCu.

The fatigue life prediction model based on crack propagation from micro‐structural features is derived and presented for planar and randomly oriented Discontinuous Reinforced Metal Matrix Composites (DRMMCs). The model contains the influence of micro‐structural properties such as aspect ratio, volume fraction of particle/fibre and constraint between particle and the matrix. The effect of residual thermal stresses generated within the matrix during development of composite is considered. The particle/fibre plays a dominant role in the development of the cyclic plastic zone size ahead of the crack tip; moreover, it enhances the cyclic plastic deformation characteristics of DRMMC. The theoretical model‐based evaluations for low cycle fatigue in DRMMCs are within the proximity of the experimental results.

Perhaps the most thoroughly studied feature of cyclic plasticity has been persistent slip bands (PSBs) formed in some metals during cyclic loading. The aim of this paper is to delimit the conditions for the occurrence of the PSBs and to discuss their role in the fatigue process in all its stages, that is in cyclic hardening resulting in a saturated stress–strain response, in initiation of fatigue microcracks and in propagation of fatigue cracks. The PSBs are zones of high cyclic slip activity. Therefore the cyclic stress–strain response depends on their volume fraction and on the ratio of cyclic slip activities within the PSBs to those outside the PSB. The cyclic plastic deformation within the PSBs leads to the formation of surface extrusions and intrusions along the traces of the active slip planes; fatigue microcracks start from the surface intrusions. Thus the PSBs play a crucial role in the process of crack initiation. Stage I fatigue cracks usually run along the PSBs. At very early stages of crack propagation these PSBs are formed before the cracks are initiated; later the PSBs are formed in the plastic zones ahead of the cracks. Stage II fatigue cracks often follow the PSBs on the microscopic level in the threshold region.

Micromechanical modeling of cyclic plastic deformation of ZK60 Mg alloy using 3D full-field crystal plasticity coupled with computational homogenization