Shear Bands In Steel Research Articles

Effect of strain rate on self-organisation of adiabatic shear bands in steel

ABSTRACTThe experiment was conducted by the radical collapse of thick-walled cylinders. Optical microscopic examination showed that sample ( = 7.3 × 104 s−1, εef = 0.45) failed to produce adiabatic shear bands (ASBs), while ASBs were obtained in samples ( = 1.48 × 105 s−1, εef = 0.86; = 1.18 × 105 s−1, εef = 0.81), which shows that the energy barrier in shear band formation was decreased with the increase of shear strain rate. With the increase of strain rate, the number of ASBs decreased and the spacing and the average length increased, and the propagation rate of ASBs increased from 213 m s−1 ( = 1.18 × 105 s−1) to 417 m s−1 ( = 1.48 × 105 s−1).

Effects of pre-strain on sensitization and interganular corrosion for 304 stainless steel

The degree of sensitization (DOS) and the susceptibility to interganular corrosion (IGC) were investigated for pre-strained 304 stainless steel (SS) following thermally aged at 750 °C for one hour. The maximum pre-strain applied to the steel is 20%.The microstructures of 304 SS subjected to pre-strain and thermal aging were observed. The volume fraction of strain-induced α'-martensite and the vickers hardness were measured, respectively. The double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were used to characterize the DOS as a function of various pre-strain levels. The tests in copper‑copper sulfate‑sulfuric acid (16%) solution were performed to evaluate the susceptibility to IGC. The morphologies of the cracks caused by IGC were observed. The results show, although pre-strained 304SS had been thermally aged at 750 °C for one hour, strain-induced α'-martensite and shear bands in steel are most retained and increase with pre-strain, causing the strengthening effect and leading to the increase in hardness. The DOS and the susceptibility to IGC also increase with pre-strain. The higher the value of the DOS is, the more serious IGC becomes. Obvious intergranular cracks as well as the dissolution of grain boundaries and the grains began to appear in the steel with 5% pre-strain, and the attack had been deteriorated with the increasing pre-strain. When the pre-strain is reached 20%, the longest crack is about 250 μm and its width is about 38 μm, indicating that the severe IGC had occurred. No obvious reversion of martensite to austenite and desensitization occurred for 304 SS withstood the maximum pre-strain up to 20% and thermally aged at 750 °C for one hour.

A new upper estimation for the blow-up time of solutions of a Volterra integral equation and its application to the modeling of the formation of shear bands

The estimation of the blow-up time of a class of Volterra integral equations of convolution type is studied. A new upper estimate of the blow-up time, expressed in terms of the convergence of a certain integral, is found. Moreover, it turns out that the aforementioned upper estimate can be applied to an even more general class of Volterra integral equations, like the one describing the formation of shear bands in steel.

Study of the formation of adiabatic shear bands in steel during impact

Both metals and other materials may exhibit the formation of narrow bands of extreme strains when impacted at high strain rates and large strains. These are known as Adiabatic Shear Bands (ASBs). They are observed during material processing such as forging and machining as well as wear and in armor plates during impact by projectiles. The prevailing theory for their formation is that they form in narrow bands because of two competing mechanisms occurring sequentially: strain hardening followed by thermal softening from the retained heat due to the impact. However, recent studies suggest that the formation of ASBs may be a simultaneous occurrence of different mechanisms which starts with the emergence of dislocations depending on the imposed local strain and strain rate. This study uses different methodologies to explore the microstructure of ASBs in a hardened low alloy steel. The study includes the effect of the initial microstructure on the formation of ASBs. The Focused Ion Beam technique was used to prepare transmission electron microscopy samples from regions within the shear bands to eliminate the induced further deformation which could be produced by conventional approaches of electropolishing. The present study reveals that each of these methodologies complements each other. Also more complex series of mechanisms including dislocation cell formation, texture development, dynamic recrystallization and carbide dissolution accommodate the excessive strains that occur during the evolution of the shear bands.

Study of the formation of adiabatic shear bands in steels

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.

Temperature measurements and photographic observations of evolving adiabatic shear bands

Experiments are described in which the local temperature and local strain distribution are measured during the formation of adiabatic shear bands in steels. The specimen employed is a short thin-walled tube loaded dynamically in a torsional Kolsky bar (split-Hopkinson bar). Local temperature is determined by measuring the infrared radiation emanating at 12 neighboring points on the specimen's surface, including the shear band area. Indium-antimode elements are employed for this purpose to give the temperature history during deformation. In addition, high-speed photographs are made of a grid pattern deposited on the specimen's surface, thus providing a measure of the strain distribution at various stages during shear band formation. The results provide a picture of the developing strain localization process, of the temperature history within the forming shear band, and of the consequence loss in the load capacity. It appears that plastic deformation follows a three-stage process that begins with a homogeneous strain state followed by a generally inhomogeneous strain distribution, and finally by a narrowing of the localization into a fine shear band. Experimental results are compared with predictions of various models.

Effect of Nominal Strain-Rates on the Initiation and Growth of Adiabatic Shear Bands in Steels

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Adiabatic shear bands in steel

Abstract : Bands of intense shear have been reported in steels which have been subjected to a high strain rate. It was proposed that these bands were caused by unstable adiabatic shear; that the rate of thermal softening (as heat is generated locally by plastic work) is greater than the rate of work hardening of the metal. Thus shear continues preferentially in these soft bands and, if the deformation leads to fracture, then fracture usually occurs within these bands. On metallographic examination the bands appear white when etched in picral or nital, and have always been assumed to consist of martensite. Work was undertaken to check this assumption. (Author)