[en] The volume effect and stress heterogeneity effect (i.e., the effect of loading type) on the ultimate strength are analyzed for fiber-reinforced composites. The main failure mechanisms are assumed to be fiber breakage and fiber pull-out. Depending on the load redistribution around a broken fiber, two different regimes can be obtained. The results are applied to the prediction of ultimate strengths of SiC fiber-reinforced composites subjected to tension, pure flexure and three-point flexure

[en] In this paper, it is proposed to identify a damage law based upon kinematic fields. The equilibrium gap method is used to determine the parameters of an anisotropic damage model. The approach is applied to a practical biaxial test performed on a C/C composite.

[en] Displacement field measurements based on digital image correlation are discussed for 2D pictures in terms of principles and possible exploitations. The presentation will focus on the study of cracked media.

[en] Thermal shocks are applied to a 304L austenitic stainless steel plate with a pulsed laser. A stroboscopic reconstruction is used for infrared (IR) and visible light camera measurements. The displacement fields are measured with a digital image correlation (DIC) technique. Different IR devices are used to measure the temperature variations (i.e. medium wave camera and short wave pyrometry). Several ways of determining the emissivity or absorptivity are discussed. The complete 3D thermal loading is numerically determined by minimising the difference between experimental measurements and finite element analyses of thermal fields. An elastoplastic model is then used to compute mechanical fields that are compared with DIC measurements. (authors)

[en] The present study aims at proposing a new generation of experimental protocol for analysing crack propagation in quasi brittle materials. The boundary conditions are controlled in real-time to conform to a predefined crack path. Servo-control is achieved through a full-field measurement technique to determine the pre-set fracture path and a simple predictor model based on linear elastic fracture mechanics to prescribe the boundary conditions on the fly so that the actual crack path follows at best the predefined trajectory. The final goal is to identify, for instance, non-local damage models involving internal lengths. The validation of this novel procedure is performed via a virtual test-case based on an enriched damage model with an internal length scale, a prior chosen sinusoidal crack path and a concrete sample. Notwithstanding the fact that the predictor model selected for monitoring the test is a highly simplified picture of the targeted constitutive law, the proposed protocol exhibits a much improved sensitivity to the sought parameters such as internal lengths as assessed from the comparison with other available experimental tests. (authors)

[en] Thermal fatigue tests on AISI 316L(N) austenitic stainless steel samples are performed through pulsed laser on specimen that can be subjected to an additional static mechanical load. These tests are carried out in Helium environment with a dedicated and heavily instrumented set-up. The fatigued surface is monitored by a hybrid multiview system composed of two visible light and one infrared cameras that, through 3D-registration, provides in-situ access to the 3D surface displacement fields and 2D temperature fields. At a fatigue frequency of 1 Hz, the surface temperature range covered per cycle can be varied from 150 degrees C to 250 degrees C, conditions that allow surface damage to be reproduced. The multiview system reveals the time-resolved mechanisms of surface damage, from significant cyclic plasticity with persistent slip bands to microcrack initiation and growth, leading to their quantitative characterization (microcrack density, length of major crack, orientation) all along the test. These observations are confirmed at a few check points where the test is interrupted for optical microscopy inspection of the surface. Finally, the thermal fatigue data are compared to purely mechanical isothermal uniaxial fatigue data through the use of an equivalent strain, and an excellent (and conservative) agreement is obtained. (authors)

[en] Thermal fatigue cracking is observed in some components of nuclear power plants. The residual lifetime prediction of cracked components is necessary to determine maintenance programs. An automatic procedure is developed for 2D modeling of multiple crack propagation within the finite element software Code_Aster[reg] to evaluate the crack growth rate and shielding effects in a multicracked structure in thermomechanical fatigue. It consists in a global remeshing method to model crack growth and includes a propagation strategy based on the crack length increment. A set of parametric studies is analyzed for a cracked pipe to evaluate the influence of geometrical and loading parameters on the residual lifetime of the crack network

[en] Three-dimensional (3-D) tomographic images of a nodular graphite cast iron obtained using a laboratory X-ray source were used to analyze the opening of a fatigue crack during in situ mechanical loading. Direct image analysis and digital image correlation are utilized to obtain the 3-D morphology and front location of the crack, as well as the displacement fields in the bulk of the specimen. From digital image correlation results it is possible to extract the crack opening displacement (COD) map in the whole sample cross-section and to compute stress intensity factors (SIFs) all along the crack front, even for COD values that are less than the image resolution. The comparison of COD maps with local values of the SIF enabled for an estimation of the opening SIF (K_op) equal to 6 MPa m^1/2.

[en] Synchrotron X-ray tomography was performed during in situ fatigue crack propagation in two small-size specimens made of nodular graphite cast iron. While direct image analysis allows us to retrieve the successive positions of the crack front, and to detect local crack retardation, volume correlation allows for the measurement of displacement fields in the bulk of the specimen. The stress intensity factors (SIFs), which are extracted from the measured displacement fields and the corresponding local crack growth rate all along the front, are in good agreement with published results. In particular, it is possible to link the non-propagation of a crack with crack closure in the crack opening displacement maps or with a local value of the measured SIF range. It is shown that a non-uniform closure process along the crack front induces an asymmetric arrest/growth of the crack.