Showing papers on "Crack closure published in 2009"

A thermoplastic/thermoset blend exhibiting thermal mending and reversible adhesion.

Abstract: In this paper, we report on the development of a new and broadly applicable strategy to produce thermally mendable polymeric materials, demonstrated with an epoxy/poly(-caprolactone) (PCL) phase-separated blend. The initially miscible blend composed of 15.5 wt % PCL undergoes polymerization-induced phase separation during cross-linking of the epoxy, yielding a "bricks and mortar" morphology wherein the epoxy phase exists as interconnected spheres (bricks) interpenetrated with a percolating PCL matrix (mortar). The fully cured material is stiff, strong, and durable. A heating-induced "bleeding" behavior was witnessed in the form of spontaneous wetting of all free surfaces by the molten PCL phase, and this bleeding is capable of repairing damage by crack-wicking and subsequent recrystallization with only minor concomitant softening during that process. The observed bleeding is attributed to volumetric thermal expansion of PCL above its melting point in excess of epoxy brick expansion, which we term differential expansive bleeding (DEB). In controlled thermal-mending experiments, heating of a cracked specimen led to PCL extrusion from the bulk to yield a liquid layer bridging the crack gap. Upon cooling, a "scar" composed of PCL crystals formed at the site of the crack, restoring a significant portion of the mechanical strength. When a moderate force was applied to assist crack closure, thermal-mending efficiencies exceeded 100%. We further observed that the DEB phenomenon enables strong and facile adhesion of the same material to itself and to a variety of materials, without any requirement for macroscopic softening or flow.

Effect of strain ratio and strain rate on low cycle fatigue behavior of AZ31 wrought magnesium alloy

Abstract: Magnesium alloys are increasingly used in automotive and aerospace industries for weight reduction and fuel economy improvement. Low cycle fatigue (LCF) behavior of these alloys is an important consideration for the structural applications. The objective of the present investigation was to identify influences of strain ratio and strain rate on cyclic deformation characteristics and fatigue life of an AZ31 extruded alloy. As the strain ratio decreased, stronger cyclic hardening rate, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed due to increasing compressive stresses. This was considered to be associated with the twinning during cyclic deformation in the compressive phase, and detwinning in the tensile phase. The residual twins acting as barriers to dislocation slip and pile-up were considered to be the main cause for the occurrence of cyclic hardening. Fatigue life increased with decreasing strain ratio and increasing strain rate. Fatigue crack initiation occurred at the specimen surface due to the presence of larger grains near the surface, and fatigue crack propagation was characterized by a mixture of striations and dimple-like ductile fracture features.

Environmentally Enhanced Fracture of Glass: A Historical Perspective

Abstract: In this paper, we review the phenomenon of delayed failure, a life-limiting process for glasses that are subjected to tensile stresses. With the development of crack-weakening theories (Ingles and Griffith) and the observation that surface damage enhances delayed failure, the scientific community recognized that delayed failure in glass is caused by the growth of cracks that are subjected to tensile stresses. Fracture mechanics techniques were used to quantify crack growth rates in terms of applied stress, temperature, and the chemical environments that cause subcritical crack growth. We review the theories that have been developed to rationalize subcritical crack growth data, including theories based on plastic deformation at the crack tip, chemical adsorption of the reacting species, and direct chemical reaction of the environment with the strained bonds at the crack tip. The latter theory seems to be most consistent with the finding that water reacts directly with the strained Si–O bond because of the ability of water to donate both electrons and protons to the strained bond. Other chemicals having this characteristic also cause subcritical crack growth. Finally, we review the quantum mechanical calculations that have been used to quantify the chemical reactions involved in subcritical crack growth.

When and how do cracks propagate

Abstract: Crack propagation in an isotropic 2d brittle material is widely viewed as the interplay between two separate criteria. Griffith's cap on the energy release rate along the crack path decides when the crack propagates, while the Principle of Local Symmetry PLS decides how, that is, in which direction, that crack propagates. The PLS , which essentially predicts mode I propagation, cannot possibly hold in an anisotropic setting. Further it disagrees with its competitor, the principle of maximal energy release, according to which the direction of propagation should coincide with that of maximal energy release. Also, continuity of the time propagation is always implicitly assumed. In the spirit of the rapidly growing variational theory of fracture, we revisit crack path in the light of an often used tool in physics, i.e. energetic meta-stability of the current state among suitable competing crack states. In so doing, we do not need to appeal to either isotropy, or continuity in time. Here, we illustrate the impact of meta-stability in a 2d setting. In a 2d isotropic setting, it recovers the PLS for smooth crack paths. In the anisotropic case, it gives rise to a new criterion. But, of more immediate concern to the community, it also demonstrates that 2d crack kinking in an isotropic setting is incompatible with continuity in time of the propagation. Consequently, if viewing time continuity as non-negotiable, our work implies that the classical view of crack kinking along a single crack branch is not correct and that a change in crack direction necessarily involves more subtle geometries or evolutions.

Fatigue crack initiation detection by an infrared thermography method

Abstract: In this paper, the study of the temperature variation during fatigue tests was carried out on different materials (steels and aluminium alloys). Tests were performed at ambient temperature using a piezoelectric fatigue system (20 kHz). The temperature field was measured on the surface of the specimen, by means of an infrared camera. Just at the beginning of the test, it was observed that the temperature increased, followed by a stabilization which corresponds to the balance between dissipated energy associated with microplasticity and the energy lost by convection and radiation at the specimen surface and by conduction inside the specimen. At the crack initiation, the surface temperature suddenly increases (whatever the localization of the initiation), which allows the determination of the number of cycles at the crack initiation and the number of cycles devoted to the fatigue crack propagation. In the gigacycle fatigue domain, more than 92% of the total life is devoted to the initiation of the crack. So, the study of the thermal dissipation during the test appears a promising method to improve the understanding of the damage and failure mechanism in fatigue and to determine the number of cycles at initiation.

Mechanisms for Solidification Crack Initiation and Growth in Aluminum Welding

Abstract: In the present work, mechanisms are proposed for solidification crack initiation and growth in aluminum alloy 6060 arc welds. Calculations for an interdendritic liquid pressure drop, made using the Rappaz–Drezet–Gremaud (RDG) model, demonstrate that cavitation as a liquid fracture mechanism is not likely to occur except at elevated levels of hydrogen content. Instead, a porosity-based crack initiation model has been developed based upon pore stability criteria, assuming that gas pores expand from pre-existing nuclei. Crack initiation is taken to occur when stable pores form within the coherent dendrite region, depending upon hydrogen content. Following initiation, crack growth is modeled using a mass balance approach, controlled by local strain rate conditions. The critical grain boundary liquid deformation rate needed for solidification crack growth has been determined for a weld made with a 16 pct 4043 filler addition, based upon the local strain rate measurement and a simplified strain rate partitioning model. Combined models show that hydrogen and strain rate control crack initiation and growth, respectively. A hypothetical hydrogen strain rate map is presented, defining conceptually the combined conditions needed for cracking and porosity.