Papers by Keyword: Crack Tip Shielding

Abstract: In order to investigate roles of grain boundaries on the improved fracture tough-ness in ultrafine-grained metals, interactions between crack tips, dislocations, and disclinationdipoles at grain boundaries are performed to aluminium bicrystal models containing a crackand h112i tilt grain boundaries using molecular dynamics simulations. A proposed mechanismto express the improved fracture toughness in ultrafine-grained metals is the disclination shield-ing effect on the crack tip mechanical field. The disclination shielding can be activated whena transition of dislocation sources from crack tips to grain boundaries and a transition of thegrain boundary structure into a neighbouring energetically stable boundary by emitting dis-locations from the grain boundary occur. The disclination shielding effect becomes large asdislocations are continuously emitted from the grain boundary without dislocation emissionsfrom crack tips. This mechanism can further shield the mechanical field around the crack tipand obtain the plastic deformation by dislocation emissions from grain boundaries, hence itcan be expected that the disclination shielding effect can improve the fracture toughness inultrafine-grained metals

Abstract: A sharply notched specimen of porous silicon carbide with porosity of 37% was fatigued under four-point bending. The opening displacement of a fatigue crack was measured at several positions along cracks by using scanning electron microscopy. The crack propagation curve was divided into stages I, II, and III. The crack propagation rate first decreased with crack extension in stage I and became constant in stage II. In stage III, the crack propagation rate increased again. The range of crack opening displacement measured in SEM was lower than that calculated from the applied load range by FEM, suggesting that the anomalous variation of the crack propagation rate with crack extension was caused by crack-tip shielding due to crack face contact. The crack-tip stress intensity factor was estimated as a true crack driving force from the relation between the crack opening displacement and the applied load. The amount of crack-tip shielding increased very quickly with crack extension, reducing the crack-tip stress intensity factor in stage I. In stage II, the increasing applied stress intensity factor is balanced by the increase in the crack-tip shielding. The crack-tip stress intensity factor increases with crack extension in stage III.

Abstract: The geometrical shielding produced by intergranular crack-tip branching in the fracture toughness tests of the Fe–V–P alloy is quantitatively assessed particularly with respect to the contribution of crack splitting. This process was evaluated by an identification of secondary intergranular cracks visualized on metalographical samples perpendicular to the fracture surface. The analysis of mixed trans/intergranular fracture revealed no special influence of triple-point branching (splitting) on the total crack tip shielding in cases of such highly spatially tortuous crack fronts. Thus, the previously reported results taking only the effect of crack tip kinking and meandering into account were proved to be correct.

Abstract: The topic of plasticity-induced closure and its role in shielding a crack tip from the full range of applied stress intensity factor has provoked considerable controversy over several decades. We are now in an era when full field measurement techniques, e.g. thermoelasticity and photoelasticity, offer a means of directly obtaining the stress field around a crack tip and hence the effective stress intensity factor. Nonetheless, without a clear understanding of the manner in which the development of plasticity around a growing crack affects the applied stress field, it will remain difficult to make crack growth rate predictions except through the use of an often highly conservative upper bound growth rate curve where closure is absent, or through semi-empirical approaches. This paper presents new evidence for an interpretation of plasticity-induced crack tip shielding as arising from two separate effects; a compatibility-induced interfacial shear stress at the elastic-plastic interface along the plastic wake of the crack, and a crack surface contact stress which will vary considerably as a function of stress state, load and material properties.

Abstract: The paper is focused on crack-tip shielding effects caused by crack deflection (tilting) processes in particle reinforced composites. A numerical analysis by means of the ANSYS code was employed in order to assess these effects caused by rigid particles and holes in the matrix. As the first step, the effective stress intensity factor Keff was calculated for many crack tip positions in front, and in between, two particles of the same kind. In spite of a significant crack tip tilting, the spherical holes induce a slight increase in the Keff factor (anti-shielding). On the other hand, the rigid particles cause a significant decrease in the Keff-value (shielding), while leaving the crack practically unaffected. In the second step, the effect of particles in the wake of the crack tip will be analyzed in order to obtain a complex picture of shielding.