Plastic zone evolution during fatigue crack growth: Digital image correlation coupled with finite elements method

International audienceNonlinearities effects at the crack tip, due to the elastic-plastic material behavior , impact the crack growth rate and path. This paper is devoted to the study of the plastic zone evolution in the crack tip region. The methodology relies on coupling an elastic-plastic Finite Elements Method (FEM) model and experimental displacements measured by Digital Image Correlation (DIC). These latter are introduced as Dirichlet boundary conditions in the finite elements analysis. The considered FEM domain is constant, i.e. the same mesh with a centered crack is moved to each new crack tip position deduced from DIC. The new boundary conditions are updated and the residual stresses and plastic strains of the former computation are interpolated and actualized on the mesh shifted to the new crack tip position in order to incorporate them in the numerical model. The coupling method allowed applying experimental boundary conditions in order to be as close as possible to real experimental conditions and to observe the plasticity evolution from small to large scale yielding conditions. A fatigue test was conducted to validate the proposed approach. The identification residues are proved to be lower than those obtained with an experimental displacements projection onto Williams' series basis, which is a method commonly used with DIC. The coupling results present an attractive similarity with Irwin's model regardless of the crack length. Thus, the definition of the mask needed for the displacements fields projection on Williams' model can be deduced with a reliable estimate of Irwin's plastic radius

Thermo-mechanical fatigue resistance characterization and materials ranking from heat-flux-controlled tests. Application to cast-irons for automotive exhaust part

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Thermal fatigue analysis of automotive Diesel piston: Experimental procedure and numerical protocol

International audienceA new thermal fatigue rig using High Frequency induction heating is developed to test automotive Diesel engine pistons. An adapted test piston is internally cooled by permanent water flow while its bowl sensitive to thermo-mechanical fatigue is subjected to cyclic induction heating. The temperature is measured in depth by thermocouples and in surface by a thermal infrared camera or a pyrometer. The crack initiation and propagation and the local deformations are provided by optical means. Thermo-mechanical loadings are calibrated by thermal measurements on the piston during engine operation and the entire test is modelled by finite elements. A constitutive model and a fatigue criterion for aluminium alloys are proposed to estimate the piston lifetime under severe cyclic loading. The proposed fatigue bench allows loading the piston in thermal fatigue scheme very similar to that encountered in engine operation conditions. Crack detection is facilitated by numerical modelling that helps to detect the most critical areas and also to reliably estimate the number of cycles for initiate cracks

Cyclic behaviour of short glass fiber reinforced polyamide: experimental study and constitutive equations

WOSInternational audiencePolymer matrix composites are widely used in the automotive industry and undergo fatigue loadings. The investigation of the nonlinear cyclic behaviour of such materials is a required preliminary work for a confident fatigue design, but has not involved many publications in the literature. This paper presents an extensive experimental study conducted on a polyamide 66 reinforced with 35 wt% of short glass fibres (PA66 GF35), at room temperature. The material was tested in two conditions: dry-as-moulded (DAM) and at the equilibrium with air containing 50% of relative humidity (RH50). An exhaustive experimental campaign in tensile mode has been carried out, including various strain or stress rates, complex mechanical histories and local thermo-mechanical recordings. Such an extended database allowed us to highlight several complex physical phenomena: viscoelastic effects at different time scales, irrecoverable mechanisms, non-linear kinematic hardening, non-linear viscous flow rule, cyclic softening. Taking into account this advanced analysis, a constitutive model describing the cyclic behaviour is proposed. As the experimental database only includes uniaxial tensile tests, the general 3D anisotropic frame is reduced to an uniaxial model valid for a specific orientation distribution. The robust identification process is based on tests which enable the uncoupling between the underlined mechanical features. This strategy leads to a model which accurately predicts the cyclic behaviour of conditioned as well as dry materials under complex tensile loadings

Fatigue crack growth in compacted and spheroidal graphite cast irons

International audienceThe present paper focuses on the fatigue life of a Compacted Graphite cast Iron (CGI) as compared to a Spheroidal Graphite cast Iron (SGI). Fatigue crack growth laws have been determined with digital image correlation. Main difference between the materials is that cracks propagate faster in the CGI than in the SGI. X-ray tomography was also used in order to assess graphite morphologies and crack shapes. A complex morphology was observed for the vermicular graphite with rounded edges that limit notch effects. The crack spreads easily in CGI via a quasi cleavage mechanism and by propagating through graphite mainly by breaking the vermicules

Probabilistic thermal-mechanical fatigue criterion for lost foam casting aluminium alloys based on 2D/3D porosities distribution

A thermal-mechanical fatigue criterion is proposed to assess the lifetime of aluminium alloys produced by a lost foam casting process. It is based on the observed size distribution of pores sizes which are considered as critical zones for cracks initiation and gives reliable results when both pores sizes are identified from X-ray tomography and mean stresses are taken into account. Lifetime probabilities are finally successfully compared with experimental results

Fatigue crack growth law identification by Digital Image Correlation and electrical potential method for ductile cast iron

International audienceIn this paper, a comparison between two methods used to identify fatigue crack propagation law is conducted: Digital Image Correlation (DIC) and Direct Current Potential Drop (DCPD). For this purpose, fatigue tests were conducted at R-ratio of 0.1 on a ductile cast iron commonly used for exhaust manifolds manufacturing. Results show a good agreement between the methods illustrating the accuracy of each technique for the analysis of fatigue crack growth. Moreover, an interest of DIC is also to allow studying the plasticity that occurs at the crack tip during the fatigue test

Multiaxial fatigue under complex non-relaxing loads

International audienceAutomotive anti-vibration parts undergo complex and multi-directional fatigue loadings, which must be considered and finally validated in their design phase. Some parts have to endure Road Load Data (RLD) test-loads (i.e. stochastic signals, representative for the actual service conditions of a part), meanwhile being already pre-charged by a constant load e.g. by the engine mass or by swaging. These conditions might result in positive minimal load values. For natural rubber, it is well known that this leads to material reinforcement which is usually related to strain induced crystallization. The reinforcement effect is well studied and illustrated in the so-called Haigh diagram by various previous studies of uni-axial tension tests Cadwell et al. 1940, Champy et al. 2015). However, to be able to determine a mean-load correction factor for a part or a specimen under complex stochastic loads, the concept of the Haigh diagram must be extended to various complex and multi-axial test conditions. It is crucial for its fundamental understanding to perform a dedicated experimental test campaign on specimens under different complex periodic loading cycles. Such fatigue tests are conducted on hourglass-shaped natural rubber specimens, loaded by coupled and aligned tension and torsion actuators, which permits to induce numerous complex load states. Finite Element Analysis (FEA) is used to determine the respective local mechanical state in the rubber specimens. Different strain, biaxiality ratio and critical plane orientation histories over a loading cycle are achieved in different test series. Finally, the number of cycles to crack-initiation and the final fracture surfaces are carefully analyzed using an optical microscope and a Scanning Electron Microscope (SEM). The results are then compared with the outcomes of non-relaxing fatigue tests in simple tension and torsion with the same specimens

Probabilistic thermal-mechanical fatigue criterion for lost foam casting aluminium alloys based on 2D/3D porosities distribution

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Experimental investigations about complex non-relaxing fatigue loads for carbon-black filled natural rubber

International audienceThis paper presents an extensive test campaign and analysis to better understand the fatigue behavior and the mechanisms at stake for natural rubber under complex non-relaxing loads. This database includes relaxing uniaxial tests under tension and torsion, relaxing and non-relaxing tension tests for 3 temperatures, relaxing multiaxial tension-torsion proportional tests, and non-relaxing tension-torsion multiaxial sequenced tests for different temperatures. The tests are conducted on hourglass-shaped natural rubber specimens, loaded by coupled and aligned tension and torsion actuators. Various strain ratios, uniaxial and biaxial loading sequences and critical plane orientation histories over a loading cycle are achieved. A total of 240 specimens were tested. Different temperatures are imposed to affect reinforcement and provide a comparison to previous results on crystallization. Furthermore, finite element analyses (FEA) are performed to determine first the respective local mechanical state in the specimens and then to infer the crack orientation predicted from a critical plane approach based on the maximum principal strain. Finally, the results are carefully analyzed based on three indicators: the relative improvement on fatigue lifetime, the cracks features (roughness and branching, from optical and SEM observations) and their comparison to the crack orientation predicted using the critical plane approach