5-4 Size effect of crack propagation of freestanding metallic nano-films
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We conducted crack propagation experiments on freestanding copper films with thicknesses ranging from about 800 to 100nm deposited by electron beam evaporation to clarify the size effect on fracture toughness in the nano-scale. It was found that initially, the crack propagated stably under loading, and then the crack propagation rate rapidly increased, resulting in unstable fracture. The fracture toughness K_C was estimated on the basis of the R-curve concept, showing a clear size effect where thinner films have smaller fracture toughness. The fracture surface suggested that the crack underwent large plastic deformation in the thicker 800-nm and 500-nm films, whereas it propagated with highly localized plastic deformation in the thinner 100-nm film. This size effect in fracture toughness will be related to a transition in deformation and fracture morphology near the crack tip.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.
Paris' law
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Titanium alloy
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Paris' law
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Abstract To study crack dynamic propagation behaviour and rock dynamic fracture toughness, a single cleavage triangle (SCT) specimen was proposed in this paper. By using these specimens and a drop‐weight test system, impact experiments were conducted, and the crack propagation velocity and the fracture time were measured by using crack propagation gauges. To examine the effectiveness of the SCT specimen and to predict the test results, finite difference numerical models were established by using AUTODYN code, and the simulation results showed that the crack propagation path agrees with the test results, and crack arrest phenomena could happen. Meanwhile, by using these numerical models, the crack dynamic propagation mechanism was investigated. Finite element code ABAQUS was applied in the calculation of crack dynamic stress intensity factors (SIFs) based on specimen dimension and the loading curves measured, and the curves of crack dynamic SIFs versus time were obtained. The fracture toughness (including initiation toughness and propagation toughness) was determined according to the fracture time and crack speeds measured by crack propagation gauges. The results show that the SCT specimen is applicable to the study of crack dynamic propagation behaviour and fracture toughness, and in the process of crack propagation, the propagation toughness decreases with crack propagation velocity, and the crack arrest phenomena could happen. The critical SIF of an arrest crack (or arrest toughness) was higher than the crack propagation toughness but was lower than the initiation toughness.
Compact tension specimen
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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
Embrittlement
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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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This paper reports the basic problems in aplication of fracture mechanics to concrete. The fracture toughness values obtained by usual toughness test are very higher than that calucurated by surface energy. Glucklich explained this fact by aggreate crack arrest effect. Authors have taken the same view-point and thought that the fracture toughness value of heterogeneous material like concrete is related the energy that is needed to extend macroscopic crack including crack arrest effect. Instead of considering the physical meaning of fracture toughness value furthermore, we have proposed several problems as follows. 1) Dependance of fracture toughness value on crack length 2) Dependance of fracture toughness value on stress field 3) Dependance of fracture toughness value on specimen size effect 4) Dependance of fracture toughness value on artificial flaw geometry 5) Relationship of fracture toughness value to usual non-crack introduced specimen strength As the first step, we have experimented fracture toughness test in tensile splitting and bending. In short crack length, fracture toughness value became very small. So we hypothesized branching crack that is stable crack extension before ultimate failure. Usual non-crack introduced specimen strength is related to fracture toughness and critical crack length. The critical crack length is thought the branching crack of non-crack introduced specimen and depends on aggregates and stress field. As a result of our experiments, we can aply fracture mechanics to concrete. However the fracture toughness value and critical crack length of concrete are very large. This means that concrete is insensitive to crack and it is no merit to aply fracture mechanics method to "macroscopic" failure of concrete.
Compact tension specimen
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