Effect of loading rate on fracture morphology in a high strength ductile steel

Fracture experiments in a high-strength ductile steel (2.3Ni-1.3Cr-0.17C) were conducted under static and dynamic loading conditions in a three-point bend and a one-point bend configurations. A qualitative description of the influence of loading rate on the microscopic features of the fracture surfaces and their role in the fracture initiation process was considered. The fracture surfaces consist of tunneled region and shear lips. The size of the shear lips increases wit increasing loading rate and is characterized by microvoids and cell structures. The tunneled region consists of large voids and micro-voids that coalesce by impingement. At high loading rates, localized molten zones are observed at the tunnel-shear lip interface

A novel reformulation of the Theory of Critical Distances to design notched metals against dynamic loading

In the present study the linear-elastic Theory of Critical Distances (TCD) is reformulated to make it suitable for predicting the strength of notched metallic materials subjected to dynamic loading. The accuracy and reliability of the proposed reformulation of the TCD was checked against a number of experimental results generated by testing, under different loading/strain rates, notched cylindrical samples of aluminium alloy 6063-T5, titanium alloy Ti–6Al–4V, aluminium alloy AlMg6, and an AlMn alloy. To further validate the proposed design method also different data sets taken from the literature were considered. Such an extensive validation exercise allowed us to prove that the proposed reformulation of the TCD is successful in predicting the dynamic strength of notched metallic materials, this approach proving to be capable of estimates falling within an error interval of ±20%. Such a high level of accuracy is certainly remarkable, especially in light of the fact that it was reached without the need for explicitly modelling the stress vs. strain dynamic behaviour of the investigated ductile metals

An investigation of dynamic failure in 2.3Ni-1.3Cr-0.17C steel

The dominant micromechanisms of dynamic failure initiation in high-strength ductile steels were investigated using 2.3Ni-1.3Cr-0.17C steel. Fracture experiments were conducted in three-point bend and one-point bend configurations. The influence of loading rate on the extent of each micromechanism in the fracture-initiation process was considered. The fracture surfaces consisted of a tunneled region and shear lips. The shear lips are characterized by microvoids. The tunneled region consists of large voids and microvoids that coalesced by impingement. At high loading rates, localized molten zones are observed at the tunnel-shear lip interface. The material-rate sensitivity causes a decrease in the size of the tunneled area at higher loading rates

On the dynamically stored energy of cold work in pure single crystal and polycrystalline copper

The thermo-mechanical response of single crystal and polycrystalline high purity copper is systematically compared at low and high strain rates. The mechanical response of each type of material is very different in terms of strain hardening, although both are distinctly strain rate sensitive. A simplified interpretation of the Taylor–Quinney coefficient, in which the strain dependence is not considered, shows a clear (almost linear) increase of this factor with the strain rate, while the two types show distinct trends. This factor increases with the strain rate but remains markedly lower than the classical value of 0.9. The stored energy of cold work is found to be relatively independent of the strain rate, with the polycrystal storing more energy than the single crystal. A microstructural study (transmission electron microscopy) of representative specimens of each type at low and high strain rates reveals a basically similar microstructure, despite dissimilar values of energy storage. It is proposed that a higher level of storage of the energy of cold work by polycrystalline copper is due to the presence of grain boundaries in this group

THE DYNAMIC PROPERTIES OF TWO-PHASE ALUMINA/GLASS CERAMICS

Les propriétés dynamiques des céramiques monolytiques se dégradent habituellement en présence d'une seconde phase. La différence d'impédance des constituants est le principal facteur responsable des dommages microstructuraux aux interfaces de phase. Des essais de restauration sous chocs ont montré que pour les alumine/verre, les microfissures étaient initiées à un niveau de contrainte inférieur à celui de la contrainte à rupture dynamique. Les premières microfissures ne sont pas liées entre elles et l'échantillon conserve sa rigidité élastique en dépit de sa perte de résistance à l'écaillage, qui, elle, est associée au dommage microstructural. La microfissuration a pour conséquence des concentrations locales de contraintes résiduelles et des échauffements locaux qui accompagnent la relaxation de l'enérgie élastique. La multiplication des dislocations et des glissements, une déformation plastique et une redistribution des différentes phases vitreuses, ainsi qu'une recristallisation partielle de la phase vitreuse sont parmi les conséquences de cet échauffement local.The dynamic properties of monolithic ceramics are in general degraded by the presence of a second phase. Impedance mismatch is the primary factor responsible for microstructural damage in the region of the phase boundaries. Planar shock recovery experiments have shown that microcracks in alumina/glass are generally initiated at a stress level below the observed dynamic failure stress. The initial microcracks are unconnected and the sample retains elastic rigidity, in spite of the loss of spall strength associated with the microstructural damage. Microcracking results in local residual stress concentrations and local heating which accompany the elastic energy release. Dislocation multiplication and glide, the flow and redistribution of any glassy phase, and partial crystallisation of the glass are among the possible consequences of this local heating

Materials Science and Engineering A xxx (2006) xxx--xxx

The mechanical behavior and microstructure of pure iron subjected to dominant shear loading has been characterized over a wide range of strain rates. Pure iron is found to be highly strain-rate sensitive. Iron exhibits marked strain softening at # 850 MPa that is unexpected for the annealed material, as characterized by TEM, but is identical to that of iron preshocked at 40 GPa [G.M. Weston, J., Mater. Sc. Lett. 11 (1992) 1361]. The microstructure is found to undergo significant refinement with increasing strain rate, from large initial grains (50 #m), through dislocation cells and large twinning, and finally micro-twins and dynamically recrystallized 200 nm grains at the higher strain rates. In situ temperature measurements indicate the release of an external heat source, other that the thermomechanical conversion of plastic work, which is identified as dynamic recrystallization. The present results suggest the operation of the # (BCC) # (HCP) phase transition that is known to occur during hydrostatic or shock loading at 13 GPa. The combination of the high strain-rate sensitivity and dominant shear loading conditions seem to trigger this phase transition, thus supporting recent work [K.J. Caspersen, A. Lew, M. Ortiz, M., E.A. Carter, Phys. Rev. Lett. 10 (2004) 115501] emphasizing the role of shear