Modelling Dynamic Fracture in Polymers Using a Local Modulus Concept
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The present study deals with the experimental characterization of short crack propagation in SLM (selective-laser-melting) manufactured stainless steel. More specifically, the determination of cyclic R-curves is discussed. This describes the dependency of the crack propagation threshold on crack growth during the short crack propagation stage. For metals, the threshold, starting at a material-intrinsic value, increases until it reaches a value independent of the crack length due to crack closure phenomena which build up at that stage. The cyclic R-curve, when used in the frame of a cyclic R curve analysis, characterizes the resistance of a material to fatigue crack growth and the ability to arrest a physically short crack. Thus, it is the link between classical fatigue and fracture mechanics. In the high-cycle-fatigue range, the short crack propagation stage dominates the overall lifetime, i.e., the number of cycles until failure. Below the fatigue limit crack arrest of hitherto propagable micro-cracks will occur. The effort for the experimental characterization of the short fatigue crack propagation behavior and the cyclic R-curve is very high compared to experiments on long crack propagation. A very exact measurement of crack extension is required, since small increments need to be depicted. Pre-cracking must leave a closure free initial crack, since closure must be build up only by the cyclic R-curve. The closure-free status is achieved by compression pre-cracking. The aim of the present study is an insight into the influence of an AM process on the short crack propagation threshold. Cyclic R-curves are experimentally determined at different load-ratios for 316L austenitic steel specimens produced by SLM and conventional manufacturing. Residual stresses are measured in the crack plane and their influence on the cyclic R-curve is discussed.
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This chapter contains sections titled: Special Features of the Propagation of Microstructurally Short Fatigue Cracks Definition of Short and Long Cracks Transgranular Crack Propagation Crystallographic Crack Propagation: Interactions with Grain Boundaries Mode I Crack Propagation Governed by Cyclic Crack-Tip Blunting Influence of Grain Size, Second Phases and Precipitates on the Propagation Behavior of Microstructurally Short Fatigue Cracks Significance of Crack-Closure Effects and Overloads General Idea of Crack Closure During Fatigue-Crack Propagation Plasticity-Induced Crack Closure Influence of Overloads in Plasticity-Induced Crack Closure Roughness-Induced Crack Closure Oxide- and Transformation-Induced Crack Closure ΔK*/K*max Thresholds: An Alternative to the Crack-Closure Concept Development of Crack Closure in the Short Crack Regime Short and Long Fatigue Cracks: The Transition from Mode II to Mode I Crack Propagation Development of the Crack Aspect Ratio a/c Coalescence of Short Cracks Intercrystalline Crack Propagation at Elevated Temperatures: The Mechanism of Dynamic Embrittlement Environmentally Assisted Intercrystalline Crack Propagation in Nickel-Based Superalloys: Possible Mechanisms Mechanism of Dynamic Embrittlement as a Generic Phenomenon: Examples Oxygen-Induced Intercrystalline Crack Propagation: Dynamic Embrittlement of Alloy 718 Increasing the Resistance to Intercrystalline Crack Propagation by Dynamic Embrittlement: Grain-Boundary Engineering
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In this study, the crack initiation and propagation behavior of interfacial crack in bimaterial are discussed. Normal crack opening displacements(NCOD) and stresses are analyzed by finite element method using ANSYS and used for extracting fracture parameters. The energy release rates can not explained the initiation and crack propagation velocity of interfacial crack. Initial velocity of crack propagation is dependent upon the normal and shear stress behind of crack tip. The crack propagation velocity of interfacial crack is very dependent upon the normal and shear stress behind of crack tip. In case of negative shear displacements increase in interfacial crack, initiation delay of crack propagation is dependent upon the negative shear stress ahead of crack tip due to the suppressing of crack opening. In case of positive shear displacements increase in interfacial crack, initiation delay of crack propagation is dependent upon the stress behind of crack tip due to the stress decrease. The fracture toughness increase is due to the initiation delay of crack propagation.
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