M.N. Bassim

Cyclic stress-strain response and dislocation structures in polycrystalline aluminum

Abstract Fully reversed strain-controlled fatigue tests were performed on polycrystalline specimens of commercially 99.0% purity aluminum. The objective was to reveal the influence of plastic strain amplitude and fatigue cycles on dislocation arrangements and investigate the role of dislocation structures on cyclic deformation behavior of aluminum. The test specimens were cylindrical in shape having an effective gauge length section 6.5 mm in diameter and 25 mm long. The fatigue tests were run using symmetrical tension–compression loading under constant strain amplitude in laboratory air environment and room temperature. The longitudinal strain amplitudes used for the testing were in the range of 1.0×10 −3 –1.1×10 −2 with a constant strain rate of 0.0001 s −1 . Cyclic deformation behavior was characterized by analyzing the cyclic hardening response, and microstructural observations by means of transmission electron microscopy. The cyclic stress–strain curve of polycrystalline aluminum is characterized with the occurrence of cyclic strain hardening at which the saturation stress increases with plastic strain at all plastic strain amplitudes tested. In addition, the cyclic stress–strain behavior obtained in this study showed grain size dependence, which is in agreement with an equivalent Hall–Petch effect of grain size on cyclic deformation behavior. An investigation on the effect of changing strain amplitude on cyclic hardening reinforces the analysis that dislocation cell structures control fatigue properties. In all strain ranges investigated, microstructures are mainly formed by dislocation cells due to high stacking fault energy, which favors an activation of multiple glide systems and formation of three-dimensional dislocation structures. Persistent slip bands and labyrinth structures were not observed. The observed dislocation cellular structures are low energy structures, which govern plastic hardening (saturated stress) behavior of commercial purity polycrystalline aluminum.

Finite element modeling of the formation of adiabatic shear bands in AISI 4340 steel

Adiabatic shear bands (ASB) occur during the plastic deformation of metals at high strain rates in torsional split-Hopkinson bar tests. The formation of ASB in AISI 4340 steel was modeled using the finite element (FE) method. Both strain hardening and thermal softening were considered and attention was given to the initialization and growth of the ASB. The calculated results indicate that the ASB could initialize at material defects. The evolution of the ASB occurs in three stages and is significantly influenced by strain hardening and thermal softening. Also, thermal conduction plays an important role during the formation of the ASB.

Dynamic mechanical properties in relation to adiabatic shear band formation in titanium alloy-Ti17

A Split-Hopkinson Pressure Bar system was used to determine the dynamic mechanical properties in relation to the formation of adiabatic shear bands in titanium alloy-Ti17. Cylindrical and conical frustum specimens were impacted between the incident and transmitted bars. The experimental results showed that the dynamic yield stress (alpha(yd)) and impact strength (alpha(bd)) were both higher than the corresponding static values, and the failure was sensitive to the strain rate, but insensitive to the applied stress level. The critical strain rate for failure was epsilon(c) = 2000 s (-1). Microscopic examinations revealed that the break of specimen frequently occurred along the shear band. A hemicyclic shear band appeared on the transverse section of the conical frustum, while a straight shear band developed at the location of maximum shear stress and propagated along the trace of the maximum shear stress, which was determined by the analysis of the stress distribution

A study of the effect of initial grain size and strain rate on dislocation structures in copper

Abstract The effect of initial grain size and strain rate on the formation and subsequent shrinkage of dislocation cell structure was studied in commercial copper. As with earlier studies, transmission electron microscopy was performed on sections of the gauge length of broken specimens tested in tension and the dislocation cell size was plotted as a function of reduction in area. It was found that at low strains, material with the coarser grain size had larger cells than that with the finer grains for a given reduction in area. As the strain increased, this difference decreased until a critical cell size was reached. Further deformation resulted in the formation of subgrain boundaries. The cross-over between dislocation cell mechanism to a subgrain mechanism is explained in terms of a generalized Hall-Petch equation. The effect of strain rate is also examined using the same approach.

A note on dislocation-cell structures in commercial copper

Abstract The variation of dislocation-cell size with strain, expressed as a reduction in the area of broken specimens, was studied in copper specimens of commercial purity. Furthermore, the relationship between cell size and stress was also investigated at high strains and was compared with earlier work which had been performed at much lower stresses and strains. It was found that above a certain critical strain the dislocation cells did not decrease further in size and eventually transformed into subgrains before fracture occurred. Also, there is an indication that the saturation of dislocation density occurs simultaneously with the formation of cells of minimum size.

Evaluation of fatigue-crack initiation at inclusions in fully pearlitic steels

Abstract The effect of inclusions in rail steels on fatigue-crack initiation was investigated. A comparative study of fatigue-crack initiation in four-point bending specimens obtained from the rail head perpendicular to the rail direction was carried out. Four rail steels with different properties and inclusion type and distribution were considered. Acoustic emission was used to detect the onset of crack initiation during fatigue testing. Following testing, detailed microstructural observation and composition analysis were carried out by scanning electron microscopy to characterize the crack-initiation sites. The results reveal that crack initiation occurs at elongated inclusions. It was observed that there are three different mechanisms for initiation of fatigue cracks. They are: (1) microcrack initiation in a local deformation band ahead of the inclusion, (2) interfacial surface decoherence, and (3) the inclusion breaks and acts as a crack. The difference in the fatigue properties among the four rail steels was explained by these three fatigue-crack initiation mechanisms.

Ductile fracture mechanisms in AISI 4340 steel

Abstract The ductile fracture process in metallic materials is initiated by inhomogeneous plastic deformation, which in some metals is manifested as a dislocation cell substructure. The role of well-developed dislocation cells in the fracture process of AISI 4340 steel was investigated using transmission (TEM) and scanning electron microscopy (SEM). It was found that the ferrite and pearlite constitutes of the steel exhibit different dislocation substructures, with cells forming in ferrite grains and shear bands occurring in the ferrite lamellae of pearlite. The combined TEM and SEM results suggest that microcracks are initiated at cementite platelets in pearlite under the combined action of tensile loading and localized shear in the adjacent ferrite. Microcracks then propagate by the nucleation and coalescene of voids along the well-developed dislocation cell walls in the ferrite grains.

Mathematical prediction of dislocation cell sizes with strain using the mesh-length theory of work hardening☆

Abstract An example of the evolution of cell structures which is directly predicted by the principle of similitude in the mesh-length theory of work hardening is the reduction of dislocation cell size with total strain. In tensile samples, total strain can be easily represented by the reduction of area of the gauge length. The relationship between dislocation cell size D and reduction of area RA has been measured for several different metals using transmission electron microscopy and this information is of great significance in the interpretation of microstructural development during straining. This paper develops a mathematical representation for the relationship between D and RA using the dislocation density and the approximate shear stress. Although the agreement between the mathematical calculations and the experimental data in the copper and steel samples tested is not precise, the general trends are relatively well represented. Further refinements in the necessary models of materials properties could well improve this relationship considerably and lead to a mathematical model which would be very useful in the prediction of microstructural development and materials properties.

Finite element modeling of the influence of anisotropic material properties on dislocations in metals

Theories of work hardening have been based on the occurrence of dislocation cell structures whose formation has significant influence on material properties in metals. In the present work, finite element (FE) modelling was used to investigate the effect of anisotropy in the grains on the dislocation density during tensile deformation. The deformation within several grains was considered in this FE-model. In each grain, different anisotropic elastic moduli, yield strengths and the active slip systems were taken into account. The dislocation density was indirectly determined by simulating the distribution of the elastic strains after unloading. Such distribution was found to be similar to the occurrence of mosaic blocks and is influenced by the grain size, the changes of local anisotropic yield strengths and of the local active slip systems within each grain.

HIGH STRAIN RATE STUDY OF CERAMICS USING HOPKINSON BAR SYSTEM

There are at present several applications where high strength ceramics have replaced metals that are subjected to high speed impact from projectiles. This requires an evaluation of behavior of ceramics under impact at high strain rates. This current study provides information on high strain-rate behavior of alumina tested in shear using torsional Hopkinson bar. Dynamic stress-strain curves were generated to investigate deformation behavior prior to fracture while fractography of the broken specimens was carried out to establish the mode of failure. The results of this investigation are similar to what is obtainable in metallic materials in which mechanism of damage is controlled by strain localization and formation of adiabatic shear bands.