Structural fatigue and fracture of shape memory alloy actuators: Current status and perspectives

Shape Memory Alloy (SMA)-actuators are efficient, simple, and robust alternatives to conventional actuators when a small volume and/or large force and stroke are required. The analysis of their failure response is critical for their design in order to achieve optimum functionality and performance. Here, (i) the existing knowledge base on the fatigue and overload fracture response of SMAs under actuation loading is reviewed regarding the failure micromechanisms, empirical relations for actuation fatigue life prediction, experimental measurements of fracture toughness and fatigue crack growth rates, and numerical investigations of toughness properties and (ii) future developments required to expand the acquired knowledge, enhance the current understanding, and ultimately enable commercial applications of SMA-actuators are discussed.

A review of crack growth models for near-neutral pH stress corrosion cracking on oil and gas pipelines

This paper presents a review of four existing growth models for near-neutral pH stress corrosion cracking (NNpHSCC) defects on buried oil and gas pipelines: Chen et al.’s model, two models developed at the Southwest Research Institute (SwRI) and Xing et al.’s model. All four models consider corrosion fatigue enhanced by hydrogen embrittlement as the main growth mechanism for NNpHSCC. The predictive accuracy of these growth models is investigated based on 39 crack growth rates obtained from full-scale tests conducted at the CanmetMATERIALS of Natural Resources Canada of pipe specimens that are in contact with NNpH soils and subjected to cyclic internal pressures. The comparison of the observed and predicted crack growth rates indicates that the hydrogen-enhanced decohesion (HEDE) component of Xing et al.’s model leads to on average reasonably accurate predictions with the corresponding mean and coefficient of variation (COV) of the observed-to-predicted ratios being 1.06 and 61.2%, respectively. The predictive accuracy of the other three models are markedly poorer. The analysis results suggest that further research is needed to improve existing growth models or develop new growth models to facilitate the pipeline integrity management practice with respect to NNpHSCC.

Relationship Between Intrinsic Threshold Stress Intensity And Near-Tip Residual Stress: A Reference Material Property

The relationship between intrinsic, closure-free threshold stress intensity and near-tip residual stress, characterizes the effect of load magnitude as well as load history on near-threshold fatigue crack growth rates. It serves as a reference against which precise closure data can be extracted from growth rates to calibrate analytical estimates. These possibilities were subjected to rigorous experimental verification involving threshold and near-threshold fatigue response under overloads, underloads with load-shedding on a steel prone to oxide debris formation. The study reveals why conventional load shedding practice to characterize threshold stress intensity is prone to yield unconservative and misleading results.

The Effect of Loading Rate on the Environment-Assisted Cracking Behavior of AA7075-T651 in Aqueous NaCl Solution

The influence of loading rate on the environment-assisted cracking (EAC) behavior of AA7075-T651 immersed in 0.6 and 1.0 M NaCl solution was assessed at applied potentials ranging from −800 to −1200 mVSCE via a slow-rising stress intensity (K) testing methodology. Measured crack growth rates under rising K loading are compared to those obtained using a fixed K protocol, which revealed that rising K-based testing consistently yields increased crack growth rates relative to static K approaches across all tested conditions. However, relative to other alloy systems, EAC in AA7075-T651 is only modestly loading rate-dependent, as demonstrated by testing conducted at fixed dK/dt ranging from 0.25 to 2.0 MPaÖm/h. The implications of the observed results are considered in the context of current EAC testing specifications, with specific focus on the conservatism and efficiency of rising K-based approaches.

ANALYSIS OF REDUCING THE GROWTH RATE OF FATIGUE CRACKS FROM OVERLOADS OF VARIOUS VALUE AND CHARACTER IN ALUMINUM ALLOY 2024-T3

This work highlights the results of tests with overloads and underloads on aluminum samples in the region of near-threshold crack growth rates. Crack growth curves and fatigue life values were obtained that the classical fatigue life prediction models of Elber, Barsom, Wheeler and Willenborg could not explain. Fractographic analysis verified the correctness of the fatigue fracture curves. The chemical analysis of the oxygen content on the fractured surfaces of the sample by the method of energy dispersive X-ray spectroscopy showed the possibility of a relationship between the weight fraction of active particles and residual stresses in the material and confirmed some of the provisions of the theory of brittle microfracture on the effect of oxygen and hydrogen on fatigue crack propagation.

Advantageous Description of Short Fatigue Crack Growth Rates in Austenitic Stainless Steels with Distinct Properties

In this work two approaches to the description of short fatigue crack growth rate under large-scale yielding condition were comprehensively tested: (i) plastic component of the J-integral and (ii) Polák model of crack propagation. The ability to predict residual fatigue life of bodies with short initial cracks was studied for stainless steels Sanicro 25 and 304L. Despite their coarse microstructure and very different cyclic stress–strain response, the employed continuum mechanics models were found to give satisfactory results. Finite element modeling was used to determine the J-integrals and to simulate the evolution of crack front shapes, which corresponded to the real cracks observed on the fracture surfaces of the specimens. Residual fatigue lives estimated by these models were in good agreement with the number of cycles to failure of individual test specimens strained at various total strain amplitudes. Moreover, the crack growth rates of both investigated materials fell onto the same curve that was previously obtained for other steels with different properties. Such a “master curve” was achieved using the plastic part of J-integral and it has the potential of being an advantageous tool to model the fatigue crack propagation under large-scale yielding regime without a need of any additional experimental data.