Showing papers on "Paris' law published in 2022"

The dual role of martensitic transformation in fatigue crack growth

Abstract: Significance About 90% of all mechanical service failures are caused by fatigue. Avoiding fatigue failure requires addressing the wide knowledge gap regarding the micromechanical processes governing damage under cyclic loading, which may be fundamentally different from that under static loading. This is particularly true for deformation-induced martensitic transformation (DIMT), one of the most common strengthening mechanisms for alloys. Here, we identify two antagonistic mechanisms mediated by martensitic transformation during the fatigue process through in situ observations and demonstrate the dual role of DIMT in fatigue crack growth and its strong crack-size dependence. Our findings open up avenues for designing fatigue-resistant alloys through optimal use of DIMT. They also enable the development of physically based lifetime prediction models with higher fidelity.

Modelling fatigue crack growth in shape memory alloys

Abstract: We present a phase field-based framework for modelling fatigue damage in Shape Memory Alloys (SMAs). The model combines, for the first time: (i) a generalized phase field description of fracture, incorporating multiple phase field formulations, (ii) a constitutive model for SMAs, based on a Drucker–Prager form of the transformation surface, and (iii) a fatigue degradation function, with damage driven by both elastic and transformation strains. The theoretical framework is numerically implemented, and the resulting linearized system is solved using a robust monolithic scheme, based on quasi-Newton methods. Several paradigmatic boundary value problems are addressed to gain insight into the role of transformation stresses, stress-strain hysteresis, and temperature. Namely, we compute Δε − N curves, quantify Paris law parameters, and predict fatigue crack growth rates in several geometries. In addition, the potential of the model for solving large-scale problems is demonstrated by simulating the fatigue failure of a 3D lattice structure.

A Review on the Fatigue behaviour of AlSi10Mg alloy fabricated using Laser Powder Bed Fusion technique

Abstract: Laser powder bed fusion (LPBF) of AlSi10Mg alloy is widely studied for the aerospace and automotive applications. Considering safety over cost, fatigue life of the material is very critical for these applications. This article reviews the interrelationship between the LPBF process parameters-microstructure-crack initiation and crack growth mechanisms under fatigue loading conditions. It addresses current problems and potential opportunities in the fabrication of fatigue-resistant AlSi10Mg alloy for light weight structural applications. The methodology for mechanical testing techniques, specimen design guidelines, post-manufacturing treatments, and other aspects of AM parts ought to be standardised. It is possible to standardise the LPBF process thorough understanding of the interrelationships among process parameters, structural aspects such as microstructure of solidified material, and mechanical properties of the fabricated part. The deformation and fracture mechanism during the cyclic loading of influences the fatigue resistance of AlSi10Mg alloy. Influence of these microstructural features, grain morphology, texture, pore size, shape distribution, and surface roughness on the fatigue properties are vital for any applications that prioritize safety over cost. The hierarchical microstructure in the LPBF processed material showed an interesting crack growth mechanism, this mechanism of crack growth is an important novelty of this work. The influence of process of sample removal and post processing on the fatigue properties are significantly control the fatigue properties. Heating the substrate of the built sample and certain post processing conditions were observed to relieve the stress in the as-built material. Post-heat treatment observed to improve the fatigue property of the selective laser melted AlSi10Mg alloy owing to the homogeneous redistribution of Si particle from the cellular boundaries and stress relief. Hence, in this review, the inter-relationship between the LPBF process parameters-microstructure-crack initiation and crack growth mechanisms under cyclic loads were studied in detail. The major aspects reviewed in this article include influence of process parameters on fatigue life and their interaction with the formation of defects. Further, specific factors dictating the fatigue characteristics in as-built and post processed AlSi10Mg alloy are elaborately discussed, concluded by fatigue models detailing the fatigue failure mechanisms.

An efficient implementation of a phase field model for fatigue crack growth

Abstract: Abstract Recently, phase field modeling of fatigue fracture has gained a lot of attention from many researches and studies, since the fatigue damage of structures is a crucial issue in mechanical design. Differing from traditional phase field fracture models, our approach considers not only the elastic strain energy and crack surface energy, additionally, we introduce a fatigue energy contribution into the regularized energy density function caused by cyclic load. Comparing to other type of fracture phenomenon, fatigue damage occurs only after a large number of load cycles. It requires a large computing effort in a computer simulation. Furthermore, the choice of the cycle number increment is usually determined by a compromise between simulation time and accuracy. In this work, we propose an efficient phase field method for cyclic fatigue propagation that only requires moderate computational cost without sacrificing accuracy. We divide the entire fatigue fracture simulation into three stages and apply different cycle number increments in each damage stage. The basic concept of the algorithm is to associate the cycle number increment with the damage increment of each simulation iteration. Numerical examples show that our method can effectively predict the phenomenon of fatigue crack growth and reproduce fracture patterns.

Role of metastable austenite in the fatigue resistance of 304L stainless steel produced by laser-based powder bed fusion

Abstract: The fatigue crack growth (FCG) behavior and fatigue strength of 304L stainless steel (SS) manufactured by the laser powder bed fusion (LB-PBF) process were investigated. Effect of build orientation, microstructure, and temperature--considering that the alloy undergoes temperature-dependent stress-induced martensitic transformation (SIMT)--were determined. FCG rates were found to be broadly independent of the build orientation and microstructure, although it is reduced in shorter builds due to the presence of higher compressive residual stress in them. Microstructural investigations of the fatigue crack reveal that SIMT occurs at the crack tip of alloys tested at room temperature, whereas the same was negligible at 150 °C. SIMT induces dilatation and shear, which enhances crack closure and retards FCG rates at RT compared to that at 150 °C. Using the microstructural observations of the transformed zone, an estimate of the crack closure due to SIMT is provided. Finally, failure envelopes or the Kitagawa-Takahashi diagram was prepared for different temperatures to facilitate a damage tolerant design approach.

Uncertainty quantification in fatigue crack-growth predictions

Abstract: Abstract The reliability of the damage tolerance approach to engineering design is affected by numerous sources of uncertainty that can lead to unsafe predictions, in turn jeopardizing the safety of structures. This work presents a robust stochastic framework for fatigue crack-growth predictions applied to a round bar under tension–compression loading conditions. Multi-source uncertainties were taken into account to derive the lifespan distribution for the bar in such a way to cover the uncertainties typically appearing in a structural integrity assessment. The sensitivity of each input variable was obtained and the influences of variables on the life predictions were derived and ranked accordingly.

Coupling of phase field and viscoplasticity for modelling cyclic softening and crack growth under fatigue

Abstract: A coupled phase field-viscoplasticity approach was developed to model the deformation and crack growth in a nickel-based superalloy under fatigue. The coupled model has an advantage in predicting the cyclic softening behavior of the alloy caused by fatigue damage, overcoming a major limitation of the original cyclic viscoplasticity model. The coupled approach is also highly effective in predicting fatigue crack propagation under varied dwell times at peak load, an important behavior for crack growth under dwell fatigue. By incorporating the stress state factor, the coupled model is further utilized to investigate the growth behavior of 3D cracks under fatigue. Both the geometrical feature of the 3D crack front and the overall crack growth rate are well captured, confirming the predicative capability of the coupled model.

Coupling of phase field and viscoplasticity for modelling cyclic softening and crack growth under fatigue

Abstract: A coupled phase field-viscoplasticity approach was developed to model the deformation and crack growth in a nickel-based superalloy under fatigue. The coupled model has an advantage in predicting the cyclic softening behavior of the alloy caused by fatigue damage, overcoming a major limitation of the original cyclic viscoplasticity model. The coupled approach is also highly effective in predicting fatigue crack propagation under varied dwell times at peak load, an important behavior for crack growth under dwell fatigue. By incorporating the stress state factor, the coupled model is further utilized to investigate the growth behavior of 3D cracks under fatigue. Both the geometrical feature of the 3D crack front and the overall crack growth rate are well captured, confirming the predicative capability of the coupled model.