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The specific behaviour of shape memory alloys (SMA) is due to a martensitic transformation [Shape Memory Materials. Cambridge University press, Cambridge] . This transformation consists mainly in a shear without volume change and is activated either by stress or temperature. The superelastic behaviour and the one-way shape memory effect are both due to the partition between austenite and martensite. The superelastic effect is obtained for fully austenitic SMA: loaded up to 5% strain, a sample recovers its initial shape after unloading with a hysteretic loop. The one-way shape memory effect is obtained when a martensitic SMA, plastically deformed, recovers its initial shape by simple heating. Superelasticity and one-way shape memory effect are useful for several three-dimensional applications. Despite all these phenomena are well known and modelled in 1D, the 3D behaviour, and especially the one-way shape memory effect, remains quite unexplored [Mater. Sci. Res. Int., 1 (1995) 260]. Actually, the development of complex 3D applications requires time-consuming iterations and expensive prototypes. Predictive phenomenological models are consequently crucial objectives for the design and dimensioning of SMA structures. Therefore, a series of 2D proportional and non-proportional, isothermal and non-isothermal tests have been performed. This database will be used to build a phenomenological model within the framework of irreversible processes. [ABSTRACT FROM AUTHOR]

The article discusses how to control testing machines with digital image correlation. It details the development and implementation of full-field measurement techniques. The article attempts to show the feasibility of controlling strains in a mechanical test using digital image correlation. A computer software, called Correli-Control, has been developed.

Abstract: This study concerns the pseudoelasticity of shape memory alloys. A series of tests under multiaxial loadings and a micro–macro model are used to show the validity of a conjecture concerning the relation between the volume fraction of martensite and the equivalent transformation strain. [Copyright &y& Elsevier]

It is proposed to develop and identify a probabilistic two-scale model for HCF that accounts for the failure of samples, but also for the thermal effects during cyclic loadings in a unified framework. The probabilistic model is based on a Poisson point process. Within the weakest link theory, the model corresponds to a Weibull law for the fatigue limits. The thermal effects can be described if one considers the same hypotheses apart from the weakest link assumption. A method of identification is proposed and uses temperature measurements to identify the scatter in anS/ Ncurve. The validation of the model is obtained by predictingS/ Ncurves for different effective volumes of a dual-phase steel. [ABSTRACT FROM AUTHOR]

Most of the theoretical studies devoted to multiaxial ratchetting are focused on the location of the macroscopic yield domain in order to predict the correct direction of plastic strain rate, since normality of the plastic strain rate to the yield surface is usually assumed. To the authors'' knowledge, the shape of subsequent yield surfaces was always kept constant in these models for simplicity reasons but unlike experimental observations. In a previous paper [J. Eng. Mater. Technol. 124(4) (2002) 402], the authors have explained the need to take into account yield surface distortion in macroscopic modeling and have therefore proposed such a constitutive model but only for biaxial loadings. In the present paper, the generalization of this distortional model is proposed for any loading paths. The model is then identified and validated on a large data base obtained firstly with an efficient polycrystalline model that can predict multiaxial ratchetting as well as yield surface distortion, and secondly with the experimental results of complex tests realized on a type 316L stainless steel. [Copyright &y& Elsevier]

This article deals with the multiphasic, anisothermal behavior of 16MND5 steel (used in French nuclear reactor vessels) under complex loading. We are focusing more specifically on transformation plasticity induced by proportional or nonproportional biaxial loading. The first part briefly reviews the experimental setup we have been using. The second part concerns the study of transformation-induced plasticity under constant stress in the cases of bainitic, martensitic and austenitic transformations. These tests enabled us to identify Leblond''s transformation plasticity model. The final part presents tests on the transformation plasticity induced by nonproportional biaxial loading and their comparison with the previously identified model. [Copyright &y& Elsevier]

This paper is concerned with the experimental behaviour of a 16MND5 steel (french vessel steel) under complex loading. A particular attention is paid to plasticity induced by phase transformation. We present an experimental set-up to apply thermo-mechanical loads under tension-torsion. This apparatus enables us to reach temperature of 1200 °C at a maximum heating rate of 60 °C s−1 and a high cooling rate of −30 °C s−1. A series of tests is performed in order to show the rule of loading on transformation plasticity. [Copyright &y& Elsevier]

The present paper deals with self‐heating phenomenon under cyclic loading at high loading frequency 20 kHz and low stress amplitude (i.e., far below the yield stress). The main goal is to propose two different methods to determine the heat source field at the origin of this temperature change. To do that the local expression of the 1D heat diffusion equation and experimental thermal data fields obtained by an infrared camera are used. The proposed methodology is applied to experimental results from tests performed on a ML340 maraging steel. [ABSTRACT FROM AUTHOR]

The purpose of this work is to extend the use of unconventional tests and full field measurements (kinematical and thermal) to the identification of the effect of a wide plastic pre-strain range on the high cycle fatigue properties of a dual-phase steel. An unconventional specimen is designed. The geometry of this specimen permits a constant gradient of pre-strain to be obtained after a monotonic tensile test. Then, a self-heating test under cyclic loading is carried out on the pre-strained specimen. During this cyclic test, the thermal field is measured using an infrared camera. Finally, a suitable numerical strategy is proposed to identify a given thermal source model taking into account the influence of plastic pre-strain. The results show that, with the unconventional test and the procedure developed in this work, the influence of a plastic pre-strain range on fatigue properties can be identified by using only one specimen when, for a classical fatigue campaign, a great number of specimens is required. [ABSTRACT FROM AUTHOR]

Abstract: The temperature of rubber-like materials increases under cyclic loadings, due to their dissipative behaviour and low thermal conductivity. This well-known phenomenon, called heat build-up, has attracted the attention of researchers for a long time. But, to our knowledge, no published studies have tried to link this temperature rise to fatigue behaviour, as already done for many metallic materials. Two main points are discussed in this paper. The first one is dedicated to heat build-up measurements: a specific experimental protocol was developed to capture the instantaneous heat build-up and, based on this protocol, a “heat build-up test” was defined in order to link the temperature rise to the principal maximum strain, which is a commonly used variable for fatigue criterion. A discussion on the correlation between these results and the fatigue behaviour is opened. This relation is illustrated for several industrial materials by a comparison between heat build-up measurements and fatigue life duration. The second point investigates the ability to couple X-ray tomography measurements presented elsewhere to the former heat build-up results in order to predict the initiation lifetime. An approach based on a critical energy criterion was proposed and the comparison to a classic Wöhler curve approach gave very good results. [Copyright &y& Elsevier]

Heterogeneous dissipation in steel sheets due to cyclic loading is difficult to measure, especially in the transverse direction because of the high conductivity and low thickness of the sheets. The goal of this article is thus to develop an experimental protocol allowing for the dissipation field determination from infrared thermography. The protocol is based on a specific differential measurement and an asynchronous acquisition. It reduces measurement artefacts due to coating, rigid body motion, convection, and optical deleterious effects. It is eventually applied to different specimens. [ABSTRACT FROM AUTHOR]

Abstract: The present paper deals with the superelastic behavior of both isotropic and anisotropic shape memory alloys (SMA). Recently, a macroscopic model, which permits to simulate the superelasticity of SMA under complex multi-axial loading, has been proposed by Bouvet et al. [Bouvet, C., Calloch, S., Lexcellent, C., 2004. A phenomenological model for pseudoelasticity of shape memory alloys under multi-axial proportional and non-proportional loadings. Eur. J. Mech. A Solids 23, 37–61]. In this model, a conjecture concerning the proportionality of the equivalent transformation strain with the martensite volume fraction has been adopted. The main goal of this study is to show the validity of this conjecture when the stress state is multi-axial. In a first part, the case of isotropic SMA is considered. An equivalent stress and an equivalent transformation strain are introduced. In the second section, the case of anisotropic SMA is considered. The previous equivalent stress and equivalent transformation strain are generalized to take into account the anisotropy of the material. The relation between the equivalent transformation strain and the martensite volume fraction is discussed by using, on one hand, experimental results under proportional tension-torsion loadings and, on the other hand, a polycrystalline model. [Copyright &y& Elsevier]

Abstract: A probabilistic two-scale model was recently developed to describe high-cycle fatigue (HCF) properties of steels associated with an identification method of the scatter based on thermal measurements. Up to now, this theoretical framework was used for uniaxial load histories. It is proposed to modify the probabilistic two-scale model to account for the effect of a multiaxial and proportional load histories on HCF properties. This modification consists in the introduction of an equivalent activation stress in the process controlling the plasticity kinetics. Thermal measurements are performed under biaxial tension–compression load histories to identify and validate an HCF criterion. [Copyright &y& Elsevier]

A probabilistic framework developed to model multiaxial high cycle fatigue tests is proposed. Up to now, with a probabilistic point of view (i.e., Weibull law), models account for the stress heterogeneity effect by introducing the concept of effective volume. It is proposed to extend this concept to multiaxial load histories. It consists in introducing a factor representing the distribution of activated slip directions. The non-proportionality effect is taken into account with no additional parameter with respect to traditional (i.e., Weibull) probabilistic approaches. The proposed model is validated on a large experimental data base and compared with other models. [ABSTRACT FROM AUTHOR]

Abstract: Two theoretical investigations, i.e. a phenomenological macroscopic model and a “micro-macro” model are developed for modelling the experimental surfaces of initiation of phase transformation in shape memory alloys. A possible initial anisotropy of the materials is taken into account. [Copyright &y& Elsevier]

• Cruciform welded joints are studied under cyclic loading with an infrared camera. • Thermoelastic stress analysis allows quantification of the structural stress. • Influence of weld toe radius over local stress is evaluated through TSA. • New approach based on image segmentation is proposed for fatigue crack monitoring. • This technique is successfully applied to study two mini-structure configurations. Fatigue assessment of welded assemblies is a key challenge both for industrial and academic research. This paper assesses possibilities of thermoelastic coupling quantitative analysis for the comprehension of fatigue phenomena occurring with welded structures. A test protocol has been set up to measure temperature field on cruciform as-welded specimens under cyclic loading thanks to an infrared camera. Identification of temperature variations associated to thermoelastic coupling (order of magnitude of 0.1 K) allows to map temperature which are directly correlated to stress tensor first invariant. From this result, analysis of thermoelastic coupling gradient allows to determine the part of bending stress due to specimen distortion (up to 20% higher than nominal stress). Besides, thermal conduction influence when evaluating a local stress at weld toe is studied thanks to a numerical thermal model. In addition, infrared images post-treatment based on image segmentation is proposed for fatigue crack monitoring at weld toes (minimal crack size detection of 1 mm). Experimental validation is performed through analysis of marked fracture surface. Finally, fatigue crack growth rates are evaluated with this approach for different load amplitudes. [ABSTRACT FROM AUTHOR]

• Uncertainties quantification of the fatigue properties determination. • Study of the ideal case and the classic staircase procedure as reference. • Proposition of two methods to analyse the staircases with reloaded specimens. • Definition of a validity domain for the censure-based method. • Sensitivity of the cumulative damage for the allowable stress calculation method. An evaluation of fatigue property determination using numerical simulation is presented for a variant of the staircase procedure with the reuse of unbroken specimens. The simulated staircases are analysed with the Maximum Likelihood Estimation method to obtain the fatigue parameters: the median fatigue limit and the coefficient of variation. For each condition, 1000 staircases are simulated and evaluated to obtain a statistical study. The gain on the fatigue scatter uncertainties with the variant procedure is highlighted. However, an assumption on the damage accumulation must be stated when reloading specimens up to their failure. Thus, staircases are simulated with damage consideration. [ABSTRACT FROM AUTHOR]

The combination of various residual stress measurement methods is a common practice to complete knowledge that a single measurement method cannot provide. In this study, incremental X-Ray diffraction is combined with the contour method to measure a bent notched specimen to study the methods robustness. A finite element analysis model is built and validated with strain measurement of the bending process thus providing prior knowledge of the residual stress field. Three-dimensional neutron diffraction residual stress measurements are also performed to obtain a reference measurement with a non-destructive method and to validate the simulated stress field. In-depth stress gradient measured by X-ray diffraction is corrected with four different methods that all show good correlation with neutron diffraction measurements. Correction methods, assumptions and uncertainties are discussed and differences are observed on the robustness of the methods. Contour method measurements are performed and results are also in agreement with neutron measurements. The results provided by the contour method are complementary to those of the X-Ray diffraction since, despite a lower accuracy on the edges where X-ray diffraction is performed, the contour method offers the complete cartography of longitudinal stress in a symmetry plane of the bent specimen. Uncertainty of the contour method due to the post-processing procedure is discussed. [ABSTRACT FROM AUTHOR]

Welded assemblies are commonly used in the shipbuilding industry. Because of the combination of stress concentration and cyclic loading, welded joints could be a critical area for fatigue damage. Thus, knowing stress and strain histories at the critical points of the structure is necessary, particularly when a confined plasticity occurs, to determine the fatigue life of welded assemblies. To avoid time-consuming nonlinear finite element analyses (FEA), simplified estimation methods of the elastic-plastic strain/stress can be used. In a previous work, an approach to estimate stress state at critical points was developed and employed in the case of double-notched specimens. The present paper focuses on welded joints in order to validate this strategy with the aim to estimate the fatigue crack initiation life of T-joints. To go further, a parametric approach has been adopted to take into account the local geometries of welded joints and to determine the constraint operator without any FEA. The results predicted by this approach are compared with experimental fatigue results. [ABSTRACT FROM AUTHOR]

The purpose of this work is to extend the use of full field measurements (kinematical and thermal) to the identification of the high cycle fatigue properties of seam weld joints. A self-heating test under cyclic loading is carried out on a seam weld joint. During the test, displacement and temperature fields of the specimen are measured by Stereo Digital Image Correlation (SDIC) and Infra-red (IR), respectively. Then a suitable numerical strategy is proposed to identify a given thermal source model. Thus, a correlation between the change of the seam weld dissipative source and the expected mean fatigue strength of the assembly is successfully recovered. [ABSTRACT FROM AUTHOR]

The main goal of this paper is to provide an experimental way to deduce the field of dissipated energy from the measurement of the temperature field. The case studied is a structural sample ( i.e. with a circular notch) in order to test the ability of the protocol to describe heterogeneous energy fields. The material chosen is a filled synthetic elastomer in order to investigate the case of large displacements. Moreover, very thin samples are used to reduce the solving of the thermo-mechanical problem to a 2D investigation. To reach high enough spatial and thermal resolutions the protocol suggested takes advantage of the low thermal conductivity of the material and of a very precise calibration of the infrared camera used to record the temperature fields. The paper suggests a protocol based on an adiabatic assumption and investigates its capabilities and range of validity. A very accurate description of the severe gradients of the fields of dissipated energy can be obtained. In order to evaluate the reliability of the experimental fields, a numerical approach is applied using a simple yet robust dissipation modelling. Finally, the experimental fields of displacement and of dissipated energy are compared with the ones obtained from the numerical simulation. [ABSTRACT FROM AUTHOR]

Design of SFRP parts against fatigue is clearly not an easy task due to the numerous dissipation mechanisms involved and to the strong anisotropy induced by the injection process. Another difficulty is to evaluate a relevant scale to compute the thermomechanical data, as the complex geometry of the parts can induce a variation within a distance comparable to or lower than the representative scale of the microstructure. In this study, injected samples with a geometry leading to a theoretical stress concentration factor Kt=2.5 are under consideration. The samples are submitted to several cyclic blocks and the temperature fields are measured using an infrared camera. Taking advantage of the good thermal and spatial resolutions, a first analysis is performed on the temperature fields, allowing to highlight the hot spot location and to follow the crack propagation. A specific adiabatic analysis is then suggested in order to measure directly the fields of dissipated energy. To predict this field, a numerical chain is then applied involving the simulation of the injection process and finite elements analysis using an anisotropic elasto-viscoplastic model. The predicted heat sources are compared to the experimental ones for several loading amplitudes and the ability of the approach to catch the experimental fields is finally discussed. [ABSTRACT FROM AUTHOR]

Accelerated ageing in natural sea water at different temperatures from 20 °C to 80 °C was performed on a silica filled polychloroprene rubber. Degradation when monitored with mechanical properties at both macroscopic and microscopic scale led to a large increase of the modulus coupled with a strong decrease of strain and stress at break. Data from tensile tests were used for lifetime prediction with an Arrhenius extrapolation for modulus, strain and stress at break. The validity of this lifetime prediction was evaluated using a 23 year-old sample aged in natural conditions. Strain at break could be predicted using an Arrhenius extrapolation with an activation energy of 50 kJ/mol. However, an extrapolation based on linear Arrhenius behaviour did not apply for modulus and stress at break due to the presence of a degradation profile across the sample. These observations confirmed that strain at break is not governed by bulk properties of samples but by the degradation rate at the external surface of the sample (i.e. in contact with water). [ABSTRACT FROM AUTHOR]

Abstract: The fatigue design of short fiber reinforced plastic (SFRP) components for structural applications in the automotive industry requires an accurate knowledge of the numerous factors affecting the fatigue lifetime (process, loading type, environment, etc.). It is therefore crucial to reduce the characterization duration, which is even higher than for metallic materials as the test frequency has to be limited in order to reduce the influence of the temperature rise. The goal of this paper is to apply a heat build-up protocol on a SFRP used for structural automotive applications (PA66GF50, a polyamide filled with 50% of glass fiber in mass) and to predict the fatigue properties from the temperature measurements. A first approach based on the temperature measurements only is proposed, providing a good correlation to the fatigue data obtained classically. In order to provide a relation between the full heat build-up curve and the Wöhler curve, the dissipated energy is then evaluated from the thermal measurements and the parameters of an energetic fatigue criterion are identified from the results obtained from one single sample. The fatigue curve predicted from the heat build-up curve is compared to the one obtained from a classical fatigue campaign and the agreement is very good. The energy based criterion as well as the identification of the dissipation sources from the temperature measurements are finally challenged by an experimental campaign on smaller samples, conditioned at the same humidity ratio of 35% and tested for the same load ratio. The agreement obtained is there again very good and clearly calls for further investigation on the ability of the approach to predict the influence of other factors (humidity, load ratio, fibers orientation, etc.). [Copyright &y& Elsevier]

Abstract: Micro-hardness testing is widely used to measure the materials local response and is very valuable to describe gradients of physical properties. For polymers, the most common use is to measure a scalar value (hardness or modulus), which gives access to useful qualitative information but can clearly not be used to identify local constitutive models, strongly needed to achieve the numerical simulation of heterogeneous massive parts. In this study, load/displacement curves obtained at a micro-scale are used to identify the parameters of an Edwards–Vilgis hyperelastic model. The protocol proposed is coupling FE simulations achieved with Abaqus and optimization procedures using the dedicated software Boss Quattro. In order to limit the microstructure and viscous effects, the material studied is an unfilled natural rubber which exhibits a behavior very close to perfect hyperelasticity. Several numerical parameters (indent geometry, friction, thickness, …) as well as experimental protocols were tested in order to check the protocol reliability. The identified parameters are used to simulate macroscopic tests (tensile, compression and pure shear tests). The agreement with experimental data is very good, which is rarely found in the literature and which validates several numerical assumptions. [Copyright &y& Elsevier]

Abstract: Based on the experimental identification of the phase transformation surface, a modelling of shape memory alloys anisothermal behaviour is proposed. Within the framework of the thermodynamics of irreversible processes, two internal variables are chosen: the stress-induced martensite volume fraction and the self-accommodating martensite volume fraction. A special attention is paid to take into account the asymmetry between tension and compression behaviours. [Copyright &y& Elsevier]

This paper is focused on variable amplitude loading spectra applied in the automotive industry, more specifically for the chassis system parts with respect to high cycle fatigue design. A first analytical model referred to as Heuler's model is considered. It is of the simplest use, with one parameter to be identified. A whole identification process, based on road measurement realized on carmaker's proving ground with instrumented vehicles, is developed and discussed. Once the model is identified, the cycles influence on damage production is also investigated. In the light of the obtained results with this model, a second one, noted modified Heuler's model, is introduced and investigated. This model, similar to the first, requires the identification of two parameters. Once again, the whole model identification process is applied, and the lifetime accuracy is assessed. A new tool is presented, enabling the visualisation on the loading spectrum representation of the most and less damaging cycles. Hence, a methodology is set to legitimize a gate implementation when studying a variable amplitude loading spectrum. [ABSTRACT FROM AUTHOR]