M. Nabil Bassim

Study of the formation of adiabatic shear bands in steels

Abstract The formation and evolution of adiabatic shear bands at high strain rate deformation of AISI 4340 steel was investigated using a torsional split Hopkinson bar system. The process was also modeled by finite element methods. Factors affecting the occurrence of the adiabatic shear bands such as specimen geometry were also examined. Depending on these factors, it was found that the adiabatic shear bands occur more likely when there is differences in properties and microstructure than in the rest of the sample and that a specific specimen geometry and dimensions favor the occurrence of high strain rates and hence the formation of the adiabatic shear bands.

Characterization of the compression properties of rail steels at high temperatures and strain rates

Abstract This paper describes a study of the mechanical behaviour of several rail steels at elevated temperatures and strain rates exceeding 10 3 s −1 . Dynamic compression testing was performed at high strain rates on several plain carbon and low alloy eutectoid rail steels in the temperature range 25–680 °C using a modified version of the direct impact Hopkinson bar to generate adiabatic stress-strain curves. It was found that the flow stress dependences on the initial testing temperature and interlamellar spacing of pearlite at high strain rates and 25 °C follow the same established trends reported for the low strain rate regime. Impacts at low deformation temperatures (25–180 °C) were found to induce localized catastrophic shearing at the strain rate tested in steels with a fine pearlitic structure.

Variation in dislocation cell size with local strain in a low alloy steel

Abstract The role of dislocation cell structure is well established in the work hardening of metallic materials. The cell diameter in transmission electron microscopy specimens taken at various places along the gauge length of broken specimens of a low alloy steel containing niobium was correlated with the local strain. Also, microhardness measurements were made and the variation in hardness with strain was obtained. It is shown that the cell size decreases with increasing strain until a critical size is reached. After this critical size, the cell size remains constant, even while work hardening continues but apparently with decreasing rate as in stage III. These results are consistent with the principle of “similitude” in stage II and support the hypothesis of the mesh length theory of work hardening that stage III arises when dislocation cells cease to shrink and similitude breaks down.

Evaluation of fracture toughness of HSLA80 Steel at high loading rates using stretch zone measurements

Abstract There is increased interest in using the stretch zone width (SZW) as a parameter for evaluation of the fracture toughness of ductile metallic materials. This approach is particularly useful in the case of fracture at high loading rates where evaluation of fracture toughness criteria such as JIc from compliance measurements is not feasible. In the current study, specimens of HSLA80 were fractured at very high loading rates, corresponding to K larger than 106MPa m 1 2 s−1 using a modified Split Hopkinson Bar system. These tests were performed on compact tension specimens of 12.6 mm thickness at −30°C. Other experiments were performed using a drop weight tester on specimens of 25 mm thickness at −40, −20, 0 and 20°C. The stretch zone width was determined using scanning electron microscopy. It was found that the steel retains a high fracture toughness at −30°C, albeit lower than at 20°C. In measurement of the SZW, for the 12.6mm specimens there was no significant difference between using a nine-point approach (ASTM) and a three-point approach, while for the 25 mm specimens, the nine-point approach gave a much lower value.

Study of dislocation cell structures from uniaxial deformation of AISI 4340 steel

Abstract A considerable interest has focused on the role of substructure during the uniaxial deformation of pure metals and alloys. To elucidate this role further, the evolution of dislocation cell structures during the tensile straining of AISI 4340 steel was investigated by transmission electron microscopy. It was found that substructural evolution reflects the inhomogeneous distribution of stress and strain in a composite material consisting of ferrite and pearlite. Cell formation occurs at different stages in the phases, with a more advanced state of cell development in ferrite grains indicating higher dislocation mobility. The strength of 4340 steel appears to be largely due to the pearlite. Work hardening by cell mechanisms occurs in the ferrite only after stresses exceed the pearlite strength. The dependence of dislocation cell size on flow stress σ was also studied and σ is found to be linearly related to the inverse cell size, indicating the operation of similitude in the “meshlength” theory of work hardening over most of the strain range. The breakdown of this relationship at higher strains is related to the transition to stage IV hardening behaviour, where increased dislocation mobility through profuse cross slip results in a very low hardening rate and nearly constant cell size.

The observation of dislocation structures during the fracture of prestrained AISI 4340 steel

Abstract The role that dislocation structures in a low alloy steel which had been subjected to various amounts of prestraining play in the fracture toughness of this steel has been investigated previously. In the study reported in this paper the presence and development of dislocation structures in tensile specimens of the same steel which were loaded to fracture are examined. The material used is an AISI 4340 steel which was predeformed, before tensile pulling, to values ranging from 0% to 15% prestrain by cold rolling. It was found that small amounts of prestraining introduce mobile dislocations into the material. These mobile dislocations contribute to the initial process of cell formation. They also have an effect on the extent of cell size reduction which occurs until final fracture takes place.

USE OF THE STRETCH ZONE FOR THE CHARACTERIZATION OF DUCTILE FRACTURE

Abstract There is an increasing effort to use the stretch zone ahead of a fatigue crack as a measure of the fracture toughness of ductile materials, particularly in cases where alternative means of measuring the toughness are not possible, e.g. at high strain rates. The stretch zone (SZ) is hence related to parameters such as the J -integral. It was, however, found that the relationship between (SZ) and J IC is affected by other factors including fatigue pre-straining of the material. In this study, the effect of initial pre-straining on the evolution of the stretch zone in a number of ductile steels was investigated. Actual measurements of the deformation of the stretch zone as it is loaded were undertaken using electron microscopy. It is shown that both prior pre-straining and fatigue pre-cracking have a significant work-hardening effect, which controls the width of the stretch zone and hence the measured fracture toughness.

Dislocation cell structures in copper in torsion and tension

Abstract A comparative study of the deformation of commercial copper in torsion and in tension was undertaken. Specimens for both tests had the same grain size and underwent the same head treatment. In tensile specimens, dislocation cell structures were observed in specimens cut in the direction of the applied load while specimens were obtained parallel to the longitudinal axis in torsion. This was followed by transmission electron microscopy examination. It was observed that in tension, the work hardening rate is much higher than in torsion. The dislocation structures in tension are characterized by the formation of sharp subgrain boundaries while in torsion, laminar dislocation structures consisting of paired sheets evolved together with other paired sheets forming checkerboard patterns. These patterns were exhibited up to strains of about 440%. The resistance to bowing out of free dislocation segments in the two cases explains the difference in the work hardening for both tension and torsion.

The role of dislocations in the fracture of prestrained AISI 4340 steel

Abstract It has earlier been established that AISI 4340 steel shows an increase in fracture toughness when it is predeformed to strains of less than 5%. The reason for such an increase in toughness was related to dislocation behaviour at the crack tip which causes significant blunting to take place. In the present study the role of dislocations in the fracture process is analysed quantitatively. It is proposed that, at prestrains lower than 5%, mobile dislocations interact with the crack to cause blunting while part of the plastic work is expended in dislocation cell formation. At values of prestrains larger than 5%, dislocation cell structures are already formed and the crack has to interact with these cell structures as the crack advances. In both cases the crack advances in heavily deformed material with a dislocation density in excess of 10 15 m −2 .

Investigation of dislocation structures in predeformed AISI 4340 steel

Abstract It has been previously established that the presence of mobile dislocations due to small amounts of prestraining in AISI 4340 steels contribute to a significant increase in the toughness of this material. This process is in the present paper further analysed by examination of the development of dislocation structures from various cross sections along the gauge length of fractured specimens which were subjected to prestraining ranging from 0% to 15%. It is shown that the dislocation cell size decreases with the increase in local reduction in area until the local reduction in area reaches a critical value. The cell size then remains constant. In contrast, the critical value of the reduction in area decreases with the increase in initial prestraining. It is shown that the principle of similitude, which is part of the mesh length theory of strain hardening, is valid for this steel. Mobile dislocations, on prestraining less than 5%, are shown to play a major role in contributing to the process of similitude and also to the reported increase in fracture toughness.