Life prediction of stainless steels by cyclic and stable hysteresis curves

Abstract

This study develops an analytical expression to describe the cyclic stress‐strain curve obtained from a series of fully‐reversed fatigue tests. A set of stress‐strain relationships is proposed to simulate the tensile branch of the stable hysteresis loop. The complete shape of the stable hysteresis loop is then constructed and the associated theoretical plastic work calculated by integrating the area within the enclosed curve. The theoretical plastic work is employed to predict the fatigue lives of the investigated materials on the basis of their respective stable plastic work per cyclelife curves. In this paper, the current mathematical derivations are based upon the endochronic theory of plasticity. The accuracy of the proposed set of stress‐strain relationships is verified by conducting fully‐reversed constant strain amplitude fatigue tests on AISI 316 and AISI 304 stainless steels. The experimental and simulation results are found to be in good agreement, hence confirming the accuracy of the proposed analytical stress‐strain relationships. Again, comparing the obverted and predicted fatigue lives, a good agreement is found between the two sets of results.     

Remote nondestructive evaluation technique using infrared thermography for fatigue cracks in steel bridges

Long‐standing infrastructure is subject to structural deterioration. In this respect, steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair. However, the inaccessibility of such structures makes inspection time consuming and labour intensive. Therefore, there is an urgent need for developing high‐performance nondestructive evaluation (NDE) methods to assist in effective maintenance of such structures. Recently, use of infrared cameras in nondestructive testing has been attracting increasing interest, as they provide highly efficient remote and wide area measurements. This paper first reviews the current situation of nondestructive inspection techniques used for fatigue crack detection in steel bridges, and then presents remote NDE techniques using infrared thermography developed by the author for fatigue crack detection and structural integrity assessments. Furthermore, results of applying fatigue crack evaluation to a steel bridge using the newly developed NDE techniques are presented.     

The use of stress sweep tests for asphalt mixtures nonlinear viscoelastic and fatigue damage responses identification

Asphaltic materials are known to present a behavior that can be approximated by the theory of viscoelasticity. For these materials it is essential to characterize fatigue damage. An important aspect therein is the separation between nonlinear viscoelastic and fatigue damage responses. This is a complex issue, since both nonlinearity and damage have a similar effect on the overall material mechanical behavior, i.e. decrease in the stiffness and increase in the phase angle. This paper presents an experimental and a mathematical procedure to separate the nonlinear viscoelastic from the fatigue damage response for asphaltic materials. Stress sweep tests were used to characterize a hot mixture asphalt at nine conditions (three temperatures and three frequencies). Once all strain values were obtained in a stress controlled sweep test, a statistical analysis was used to find the maximum stress that can be applied to the material without invoking the damage response. The results showed that the transition stress value is directly associated with material properties, the stiffness being an important factor in this result. Consequently, stress, temperature and frequency determine together the mechanical response of the material (linear or nonlinear viscoelastic, fatigue damage and/or plastic deformation). Results from this study can be associated with other fatigue damage approaches in order to better select the stress or strain amplitude that should be used in fatigue tests, and to eliminate the amount of energy that is dissipated in the nonlinear viscoelastic region.     

Design methods for reliable fatigue assessment of PM components

The use of PM materials is rapidly expanding with an increasing concentration on highly loaded structural parts such as synchroniser hubs, gears, sprockets or shifting forks. The successful implementation of PM materials for such parts depends on a reliable fatigue design concept. Such a design concept has to consider the local durability, especially in fatigue critical sharply notched areas, depending on the local density of the material and stress gradients. This paper summarises different design methods in order to transfer the fatigue behaviour of specimens to components by considering sharply notched areas. Four different local approaches have been investigated: the highly stressed volume approach, the stress gradient approach, the critical distance method and the stress averaging method according to Neuber. The design methods have been analysed on the basis of fatigue testing results of unnotched and notched fatigue specimens and of synchroniser hubs made from a 4% Ni diffusion‐alloyed steel material (Distaloy AE+0.6%C). The transferability of characteristic fatigue properties from specimens to a sharply notched component, a synchroniser hub, is presented and the practicability of the design methods demonstrated and discussed. These investigations showed that the most reliable concept was the highly stressed volume approach. The accuracy of the approach can be comprehended separating statistical and so called material support effect.     

Hip stem fatigue test prediction

The accuracy of fatigue test prediction methods for the standard fatigue testing of hip stems was evaluated against the experimental results of static and fatigue tests. Axial unnotched strain-controlled material fatigue tests provided the required cyclic material properties. Finite element analysis of the hip stems predicted a maximum tensile stress to within 3–7% of strain gauge measurements. The four methods investigated accurately predicted hip stem fatigue strength at 5 million cycles (?1% to ?9% errors). The strain–life methods successfully predicted fatigue life (factors 1/7.0–9.2 of the test) at high and low stress amplitudes of 352 and 315 MPa, respectively. The classical stress–life method was only accurate (factor 1/1.9) for the low stress level. The current study has demonstrated that fatigue test prediction methods can be applied with confidence to support standard fatigue testing of hip stems. Further studies can expand the understanding of these methods and their clinical relevance by investigating effects due to variable amplitude loading and environment.     

On the failure condition in strain-controlled low cycle fatigue

Axial-strain controlled low cycle fatigue tests were performed on several materials in different metallurgical conditions using various test-piece geometries, strain ranges, temperatures and frequencies in order to arrive at a proper choice for the definition of failure. Several alternative failure criteria proposed in the literature were examined in terms of load response measurements and the shape of the hysteresis loop in the compressive portion of the cycle. A 20% fall in the saturation stress in tension and cusp formation in the compressive portion of the cycle have been identified as the two best criteria for defining the failure life of laboratory specimens in strain-controlled low cycle fatigue testing.     

Low‐cycle fatigue life prediction of various metallic materials under multiaxial loading

This paper proposed a simple life prediction model for assessing fatigue lives of metallic materials subjected to multiaxial low‐cycle fatigue (LCF) loading. This proposed model consists of the maximum shear strain range, the normal strain range and the maximum normal stress on the maximum shear strain range plane. Additional cyclic hardening developed during non‐proportional loading is included in the normal stress and strain terms. A computer‐based procedure for multiaxial fatigue life prediction incorporating critical plane damage parameters is presented as well. The accuracy and reliability of the proposed model are systematically checked by using about 300 test data through testing nine kinds of material under both zero and non‐zero mean stress multiaxial loading paths.     

Simulation of cyclic stress/strain evolutions for multiaxial fatigue life prediction

This study deals with simulation for cyclic stress/strain evolutions and redistributions, and evaluation of fatigue parameters suitable for estimating fatigue lives under multiaxial loadings. The local cyclic elastic–plastic stress–strain responses were analyzed using the incremental plasticity procedures of ABAQUS finite element code for both smooth and notched specimens made of three materials: a medium carbon steel in the normalized condition, an alloy steel quenched and tempered and a stainless steel, respectively. Emphasis is on the studying of ‘intelligent’ material behaviors to resist fracture, such as stress redistribution and relaxation through plastic deformations, etc. For experimental verifications, a series of tests of biaxial low cycle fatigue composed of tension/compression with static and cyclic torsion were carried out on a biaxial servo-hydraulic testing machine (Instron 8800). Different multiaxial loading paths were used to verify their effects on the additional cyclic hardening. The comparisons between numerical simulations and experimental observations show that the FEM simulations allow better understanding on the evolutions of the local cyclic stress–strain and it is shown that strong interactions exist between the most stressed material element and its neighboring material elements in the plastic deformations and stress redistributions. Based on the local cyclic elastic–plastic stress–strain responses, the energy-based multiaxial fatigue damage parameters are applied to correlating the experimentally obtained lives. Improved correlations between the predicted and the experimental results are shown. It is concluded that the improvement of fatigue life prediction depends not only on the fatigue damage models, but also on the accurate evaluations of the cyclic elasto-plastic stress/strain responses.     

Fatigue crack growth through a residual stress field introduced by plastic beam bending

We present predictions and measurements of fatigue crack growth rates in plastically bent aluminium 2024‐T351 beams. Beam bending and fatigue were carefully controlled to minimize factors other than residual stress that could affect the fatigue crack growth rate, such as large plastic strains or residual stress relaxation. The residual stress introduced by bending was characterized by a bending method and by the slitting method, with excellent agreement between the two methods. Crack growth rates were predicted by three linear elastic fracture mechanics (LEFM) superposition based methods and compared to experimental measurements. The prediction that included the effects of partial crack closure correlated with experimental data to within the variability normally observed in fatigue crack growth rate testing of nominally residual stress free material. Therefore, we conclude that crack growth through residual stress fields may be predicted using the concept of superposition as accurately as crack growth through residual stress free material, provided that the residual stress is accurately known, the residual stress remains stable during fatigue, the material properties are not changed by the introduction of residual stress, and that the effect, if any, of partial crack closure is taken into account.     

Thermoelastic Phase Analysis (TPA): a new method for fatigue behaviour analysis of steels

In literature, there are already well‐established thermal methods which allow for the estimation of fatigue limit, in particular for metallic materials such as austenitic steels. These methods are based on heat source generation analysis or on surface temperature evaluation of material subjected to different types of cyclic loading. General application of methodology found limitation in those cases in which temperature changes on material related to fatigue damage were very low and, furthermore, thermal methods require high‐performance equipment and a difficult setup. This is the case, for instance, with brittle materials (such as martensitic steels), welded joints and aluminium alloys. In this work, a new thermal method named Thermoelastic Phase Analysis is used to evaluate the fatigue limit of martensitic steels. This thermal method is based on an empirical approach. The main idea is that phase of thermoelastic response of the material subjected to fatigue loading is influenced by the presence of a heat source due to dissipative phenomena related to damage. Monitoring of the phase parameter provides a more stable setup and an independent means of identifying the fatigue limit of material. The method has also proven to be potentially one order of magnitude faster than traditional thermal methods.     

Residual stress/strain evolution due to low‐cycle fatigue by removing local material volume and optical interferometric data

Three experimental methods, based on optical interferometric measurements of deformation response to local material removing, have been implemented for residual stresses determination. Two first techniques are employed to characterize initial residual stress values and their evolution near welded joints of aluminium plates under low‐cycle fatigue. The hole‐drilling method gives high‐accurate dependencies between residual stress components and number of cycles. The second approach comprises cracks modelling by narrow notches to describe residual stress distributions in more wide spatial range near the weld. The results demonstrate residual stress evolution is of complex character and cannot be uniquely qualified as a gradual relaxation. Besides, the secondary hole drilling method is developed and used as a fast and reliable tool to quantify the redistribution of residual strains near cold‐expanded holes due to low‐cycle fatigue. Dependencies of circumferential residual strains along the secondary hole edge versus number of cycles are constructed.     

Stress–strain curves of metallic materials and post‐necking strain hardening characterization: A review

For metallic materials, standard uniaxial tensile tests with round bar specimens or flat specimens only provide accurate equivalent stress–strain curve before diffuse necking. However, for numerical modelling of problems where very large strains occur, such as plastic forming and ductile damage and fracture, understanding the post‐necking strain hardening behaviour is necessary. Also, welding is a highly complex metallurgical process, and therefore, weldments are susceptible to material discontinuities, flaws, and residual stresses. It becomes even more important to characterize the equivalent stress–strain curve in large strains of each material zone in weldments properly for structural integrity assessment. The aim of this paper is to provide a state‐of‐the‐art review on quasi‐static standard tensile test for stress–strain curves measurement of metallic materials. Meanwhile, methods available in literature for characterization of the equivalent stress–strain curve in the post‐necking regime are introduced. Novel methods with axisymmetric notched round bar specimens for accurately capturing the equivalent stress–strain curve of each material zone in weldment are presented as well. Advantages and limitations of these methods are briefly discussed.     

The influence of cyclic material properties on fatigue life prediction by amplitude transformation

The procedure of amplitude transformation for fatigue life prediction combines elements of the nominal stress and the local stress/strain methods. An essential part of this technique, the conversion from nominal stress to local stress and strain conditions, requires an input of the cyclic material properties. A practical problem may arise if the exact material properties are unknown at the time a fatigue life prediction has to be made (eg in the design stage). This paper outlines the procedure of amplitude transformation and discusses the dependence of fatigue life predictions on the cyclic material properties in terms of the parameters that define the cyclic stress/strain curve. Fatigue life predictions based on the nominal stress approach, on the local stress/strain technique and on amplitude transformation are compared with experimental results for typical stress histories.     

A fatigue damage indicator parameter for P91 chromium‐molybdenum alloy steel and fatigue pre‐damaged P54T carbon steel

Two grades of structural steel were subjected to fully reversible, constant stress amplitude cyclic loading. The local strain response of the material was measured and recorded during the test, with the applied testing technique enabling the monitoring of hysteresis loop variation for the narrowest cross‐section of the hourglass specimen. Changes in hysteresis loop width, representing the local inelastic response of the material, were recorded in order to monitor the density of structural imperfections. Material ratcheting behaviour was observed as changes in the mean strain for selected load cycles. Ratcheting was attributed to local deformation of the material in the vicinity of imperfections such as voids or inclusions, as well as deformation induced by the propagation of microcracks. Definitions of a damage indicator parameter and damage parameter were proposed. The fatigue behaviour of the two investigated grades of steel was finally illustrated in the form of damage curves for different stress amplitudes and for undamaged and fatigue pre‐damaged material.     

Identification of local cyclic stress‐strain curves at material inhomogeneities on the basis of digital image correlation

Low‐cycle fatigue (LCF) tests on butt‐welded joints revealed that material inhomogeneities become decisive for the fatigue behavior at elastic‐plastic strain amplitudes and the geometrical notch is no longer the failure location – as observed in high‐cycle fatigue (HCF) regime. Digital image correlation techniques enable the measurement of local strains at the surface of the welded region. A new method is proposed that is able to calculate related local stresses and identifies local cyclic material properties in structures of inhomogeneous material and geometry. This method combines strain measurements and Finite Element simulations. It is a fast iteration procedure that is able to perform the identification within less than ten iterative steps at several hundred local points. Transient material behavior is easy to capture and an application at high temperatures is anticipated due to the method is based on materials mechanics.     

Concurrent ratcheting–fatigue damage analysis of uniaxially loaded A‐516 Gr.70 and 42CrMo Steels

This study intends to investigate the concurrent interaction of fatigue damage and ratcheting strain in two commonly used steel alloys of (American Society for Testing and Materials) ASTM A‐516 Gr.70 and 42CrMo, respectively for pressure vessels and high grade machinery parts over uniaxial stress cycles. Ratcheting extension and fatigue damage progress were both characterized cycle‐by‐cycle over life cycles of tested materials. The interaction of ratcheting and fatigue damage was defined based on mechanistic parameters involving the effects of mean stress, stress amplitude and cyclic softening/hardening response of materials. The extent of ratcheting effect was defined by product of average ratcheting strain per cycle, and maximum stress value during a cycle, while fatigue damage was analysed based on earlier developed energy‐based models of Xia–Ellyin, and Smith–Watson–Topper. Overall damage due to ratcheting and fatigue was calibrated through a weighting factor at various mean/ cyclic amplitude stresses. An algorithm was developed to evaluate overall damage due to ratcheting and fatigue stress cycles of materials subjected to various mean and amplitude stresses. The estimated lives at different mean stresses and stress amplitudes for ASTM A‐516 Gr.70 and 42CrMo samples showed good agreements as compared with those of reported experimental data.     

Fatigue of rare‐earth containing magnesium alloys: a review

Lightweighting in ground vehicles is today considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment‐damaging and climate‐changing emissions. Magnesium (Mg) alloy, as a strategic ultra‐lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength‐to‐weight ratio, dimensional stability, good machinability and recyclability. However, the hexagonal close‐packed crystal structure of Mg alloys gives only limited slip systems and develops sharp deformation textures associated with strong mechanical anisotropy and tension–compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavourable influence on the material performance. This problem could be conquered through weakening the texture via addition of rare‐earth (RE) elements. Thus, a number of RE‐containing Mg alloys have recently been developed. To guarantee the structural integrity, durability and safety of highly loaded structural components, understanding the characteristics and mechanisms of cyclic deformation and fatigue fracture of such RE‐Mg alloys is of vital importance. In this review, the available fatigue properties including stress‐controlled fatigue strength, strain‐controlled cyclic deformation characteristics and fatigue crack propagation behaviour are summarized, along with the microstructural change and crystallographic texture weakening in the RE‐containing Mg alloys in different forms (cast, extruded and heat‐treated states), in comparison with those of RE‐free Mg alloys.     

Monitoring mechanical damage in structural materials using complimentary NDE techniques based on thermography and acoustic emission

This work deals with nondestructive evaluation (NDE) of the fracture behavior of metallic materials by combining thermographic and acoustic emission (AE) characterization. A new procedure, based on lock-in infrared (IR) thermography, was developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue. The thermography results on crack growth rate were found to be in agreement with measurements obtained by the conventional compliance method. Furthermore, acoustic emission was used to record different cracking events. The rate of incoming signals, as well as qualitative features based on the waveform shape, was correlated with macroscopically measured mechanical parameters, such as load and crack propagation rate. Additionally, since the failure modes have distinct AE signatures, the dominant active fracture mode was identified in real time. The application of combined NDE techniques is discussed for characterizing the damage process which leads to catastrophic failure of the material, thereby enabling life prediction in both monolithic aluminum alloys and aluminum alloy/SiC particle (SiC_p) reinforced composites.