Francis Collombet. Surface and machining induced damage characterization of abrasive water jet milled carbon/epoxy composite specimens and their impact on tensile behavior. Wear, Elsevier, 2017, 376-377 (Part B), p. a b s t r a c t Controlled depth milling of composites structures by abrasive water jet (AWJ) is a new area of machining being explored and knowledge on this is bare minimum. Hence it is essential to investigate surface quality and damage induced to ascertain their mechanical reliability. Here, the mechanism of material removal is manifested by erosive wear. In this study, carbon fiber reinforced plastic (CFRP) laminates are milled using AWJ process and surfaces generated by varying process parameters are characterized using roughness systems, X-ray tomography and scanning electron microscopy (SEM). SEM images reveal presence of damages in form of craters, ridges, broken fibers and embedded abrasive particles. Crater formation due to erosion phenomenon is affected by jet pressure. It is seen that the crater volume increases by around 500% when pressure varies from 80 MPa to 140 MPa. In the literature reviewed correlation between roughness of the machined surface and the mechanical behavior is ambiguous and remains an open problem. Hence, novel attempt has been made to analyze the influence of damage (crater volume) on tensile strength. Mechanical tests on specimens with varying surface texture and crater sizes reveals that tensile strength of machined specimens is more influenced by crater volume rather than surface roughness.
In this paper, experimental and numerical study of drilling of carbon fibre reinforced plastic (CFRP) laminate with aluminium alloy stacks has been carried out. Drilling of these multi-materials is a challenging task to manufacturing engineers because of different mechanical properties of materials. In this work, the impact of the machining parameters on the effect of twist drill and the geometry of double cone drill on cutting forces, holes quality and on CFRP/Al interface have been investigated. From the experimental study, it was found that the double cone drills induce less thrust force compared to the standard twist drill. From the numerical analysis based on the linear fracture mechanics of the CFRP and the plastic behaviour of the aluminium with isotropic hardening, on the one hand, the critical thrust force responsible for the delamination of the last ply as a function of the aluminium thickness has been identified and on the other hand, the maximum thrust force responsible for the interface separation of CFRP/Al has been predicted as a function of the aluminium thickness.
Thermal fatigue of a hot work tool steel (X38CrMoV5) is investigated under various test conditions. A microscopic interconnected heat-checking pattern forms on the oxidized surface of the specimen. The evolution of the crack density and morphology is characterized by Scanning Electron Microscopy (SEM) and image analysis. The effects of the initial hardness, the maximum temperature and the heating period of the thermal cycle are reported. It is shown that the saturated heat-checking density is independent of the maximum temperature of the thermal cycle, but is dependent on the heating rate. No significant effect of the initial hardness is observed. In the saturated regime, a linear relationship is established between the heat-checking density and the maximum heat-flux density applied to the specimen. The saturated crack density is explained by the difference between the thermo-mechanical strains of the oxide layer and the steel. A damage criterion, independent of the initial hardness of the steel, is proposed to describe the microscopic heat-checking life.
a b s t r a c tIn this study, the high temperature wear behavior of hot forming tool steel grades is investigated by successive sliding of a pre-alloyed Usibor1500P s strip heated at high temperature. Experimental tests are performed at high temperature on an instrumented Deep-Drawing Process Simulator (DDPS). This laboratory pilot is employed to rank different steel grades used as tool materials in the hot-stamping process. The wear damage of the tool (die radius) is characterized by profilometry and SEM observations, and three quantitative criteria are determined from 2D profile measurements to assess adhesive and abrasive wear. Under examined conditions at high temperature, a predominant transfer mechanism is observed, while abrasive wear appears as minor damage. When the surface hardness of the tool material is not great enough, the sub-surface of the die radius can exhibit a plastic shear deformation of about 10 mm in depth. This leads to emission of wear debris coming from the cumulated cyclic plastic deformation of the sub-surface. In contrast, for high surface hardness, the adhesive wear rapidly reaches an asymptotic state.
Surface cracking or heat-checking is investigated at a microscopic scale on a hot work tool steel (X38CrMoV5) tested under thermal fatigue. Thermal fatigue tests are periodically interrupted to observe the surface of the specimens by scanning electron microscopy (SEM). A non destructive and semi-automatic method is developed to assess and evaluate the two-dimensional crack pattern initiated on the oxide scale layer formed on the specimen surface. The crack pattern is characterized by image analysis in terms of density, morphological and topological features. This technique allows to determine the number of cycles to initiate the microscopic heat-checking and to follow its evolution.
In the Selective Laser Melting (SLM) process, residual stresses are a major problem because they impact the dimensional accuracy and mechanical properties of the manufactured parts. A new methodology, based on distortion measurements using the bridge curvature method (BCM), is developed for the quantitative assessment of residual stresses. The bending of the surface of the released specimen is approximated by a quadratic polynomial and quantitative criteria are determined on both profiles and surface topographies measured by noncontact 3D optical microscopy. The accuracy of the method is evaluated by a statistical analysis using repeatability tests. Focus variation microscopy (FVM) measurements show better repeatability than extended field confocal microscopy. Compared to the 2D measurements generally reported in the literature, 3D characterization provides relevant information as the orientation of the main distortion, which may help to highlight the effect of SLM process parameters. In fact, the flatness parameters and curvature attributes measured on surface topographies are much more robust and repeatable than the distortion magnitude measured on isolated profiles. In particular, 3D analysis helps to show that the distortions are maximum perpendicular to the path of the laser.
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