3D X-ray Microtomography Volume Correlation to Study Fatigue Crack Growth

International audienceGlobal digital volume correlation is used to analyze a series of computed tomography images of a nodular graphite cast iron specimen subjected in situ to a fatigue test. From the obtained displacement field, a specific procedure is implemented to extract stress intensity factors all along the crack front. The proposed methodology allows one to measure key parameters in fatigue crack propagation directly from 3D images

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

Effect of Fe and Mn Content on the Microstructures and Tensile Behaviour of AlSi7Cu3 Alloy: Thermal Analysis and Tensile Tests

The present study was performed on two AlSi7Cu3 alloys with different Fe and Mn contents (standard alloy and high-Fe/Mn alloy). The evolution of microstructures during solidification of the standard AlSi7Cu3 alloy was investigated by thermal analysis and interrupted quenching test. The effect of Fe and Mn content on the solidification reaction and sequence were studied. The results show that increasing the Fe and Mn content changes the precipitation sequence of the iron-intermetallic α-Al15 (Fe,Mn)3Si2 and β-Al5FeSi, leading to the precipitation of α + β phases at a higher temperature. Microstructural characterizations were also performed on the fully solidified alloys to study the effect of Fe and Mn content on the microstructure of AlSi7Cu3 alloy. Fe and Mn were found to promote the formation of Fe-intermetallics. With the increase of Fe/Mn content, Fe-intermetallics increased in both size and amount, while more small pores (Feret diameter < 200 µm) were also introduced. 3D networks of α-Al15(Fe,Mn)3Si2 and β-Al5FeSi phases were revealed by Lab X-ray Computed Tomography, however, it is difficult to perform a quantitative analysis of the respective volume fraction of α-Al15(Fe,Mn)3Si2 and β-Al5FeSi phase from their 3D morphology. Monotonic tensile tests on both alloys show the mechanical properties of the studied alloys were not sensitive to the Fe/Mn content, while the fractography analysis reveals that cracks growth and final fracture under monotonic load are more prone to occur through the eutectic Si, Al2Cu phases and iron-intermetallics than through aluminium matrix.The authors wish to thank the ANR (Agence Nationale de la Recherche) MatetPro project INDiANA (ANR-12RMNP-0011) for funding the study on Al-Si alloys, Pierre Osmond from PSA Peugeot Citroën for providing the material of this study and the China Scholarship Council (CSC) for funding the PhD thesis of Zaidao Li. The ISIS4D X-Ray CT platform which has been funded by International Campus on Safety and Intermodality in Transportation (CISIT), the Nord-Pas-de-Calais Region, the European Community and the National Center for Scientific Research is also acknowledged for microtomographic acquisition, and thank the Common Center of Microscopy (CCM) of Lille University for the SEM and EDS. The authors also thank Dr. Dan Luo for her assistance with casting and SEM analysis. UK Research & Innovation (UKRI) [Engineering & Physical Sciences Research Council (EPSRC) EP/N007638/1]

Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

International audienceFatigue failure of a gas-nitrided 4140 steel under axial cyclic loading results from a competition between surface crack initiation in the nitrided case and internal "fish-eye" cracking inside the core material. When nitriding is deep enough, the internal mechanism prevails in smooth specimens and fatigue strength improvement as compared to base metal is about 20%. In the present study, three V-notched specimens (blunt-medium-severe) are designed to be representative of the stress gradient (i) in a small rotary bending specimen, (ii) at the root of a gear tooth, and (iii) at the root of a very sharp notch. The cracking mechanism depends on the notch severity. The nitrided blunt notch fails from a fish-eye nucleated at the case/core boundary whereas the medium and sharp notches fail from surface cracks. The high-cycle fatigue strength improvement varies from 80% for the blunt notch and to more than 100% for the sharp notch. The notch fatigue behaviour of nitrided steel is discussed by comparing the evolutions of internal and surface fatigue strengths with relative stress gradient