Shear-affected Zone Research Articles

Damage Evolution in Complex-Phase and Dual-Phase Steels during Edge Stretching.

The role of microstructural damage in controlling the edge stretchability of Complex-Phase (CP) and Dual-Phase (DP) steels was evaluated using hole tension experiments. The experiments considered a tensile specimen with a hole at the center of specimen that is either sheared (sheared edge condition) or drilled and then reamed (reamed edge condition). The damage mechanism and accumulation in the CP and DP steels were systematically characterized by interrupting the hole tension tests at different strain levels using scanning electron microscope (SEM) analysis and optical microscopy. Martensite cracking and decohesion of ferrite-martensite interfaces are the dominant nucleation mechanisms in the DP780. The primary source of void nucleation in the CP800 is nucleation at TiN particles, with secondary void formation at martensite/bainite interfaces near the failure strain. The rate of damage evolution is considerably higher for the sheared edge in contrast with the reamed edge since the shearing process alters the microstructure in the shear affected zone (SAZ) by introducing work-hardening and initial damage behind the sheared edge. The CP microstructures were shown to be less prone to shear-induced damage than the DP materials resulting in much higher sheared edge formability. Microstructural damage in the CP and DP steels was characterized to understand the interaction between microstructure, damage evolution and edge formability during edge stretching. An analytical model for void evolution and coalescence was developed and applied to predict the damage rate in these rather diverse microstructures.

Predicting Failure during Sheared Edge Stretching Using a Damage-Based Model for the Shear-Affected Zone

Predicting Failure during Sheared Edge Stretching Using a Damage-Based Model for the Shear-Affected Zone

Characterization of mechanically sheared edges of dual phase steels

In recent years advanced high strength steels (AHSS) received increased interest for light structures with improved performance, but they are often sensitive to edge cracking during sheet metal forming. In this study mechanically sheared edges were characterized for three dual phase steels (DP600, DP780 and DP980), sheared with three die clearances (5%t, 10%t, 15%t) and along rolling and transverse directions. Microstructures of the materials were provided first, and then the sheared edges were examined by optical microscopy and scanning electron microscopy that reveal the morphology and random feature of the sheared edges. A factorial analysis was performed to reveal the general trends of the processing parameters on four edge zones. A new strain measurement method was used for characterizing strain distribution in the sheared region, which shows the peak strain to be higher than 3. The strain quickly decreases from sheared edge to interior, leaving a shear-affected zone of about 500μm or 31% of the thickness. The fracture processes and involved mechanisms were discussed.

Effect of a Strain-Hardening Rate at Uniform Elongation on Sheared Edge Stretching

Edge failure during stretching of sheared edges limits the use of sheet steels in a number of product applications. The shearing process causes a highly strained region adjacent to the shear face, called the shear-affected zone. In the present study, the strain-hardening rate at uniform elongation, Z, is used as an empirical measure of cohesive strength at the interface of the various phases in steel microstructures. The higher the value of Z, the lower the macro strain when voids begin to form that lead to decohesion of the interface and subsequent failure. The data from four different studies are used to show that the true circumferential strain at failure in a hole expansion is a direct function of Z for most microstructural conditions. Sheet steels that exhibit better performance than that which would be expected for their Z values have one or more of the following characteristics—an increase in ferrite strength, lower carbon martensite in DP steels, or TRIP steels. A hot-rolled ferrite/pearlite microstructure is the only case of decreased true circumferential strain at failure for a given value of Z.

Review of the Shearing Process for Sheet Steels and Its Effect on Sheared-Edge Stretching

Failure in sheared-edge stretching often limits the use of advanced high-strength steel sheets in automotive applications. The present study analyzes data in the literature from laboratory experiments on both the shearing process and the characteristics of sheared edges. Shearing produces a surface with regions of rollover, burnish, fracture, and burr. The effect of clearance and tensile strength on the shear face characteristics is quantified. Higher strength, lower ductility steels exhibit an increase in percent fracture region. The shearing process also creates a zone of deformation adjacent to the shear face called the shear-affected zone (SAZ). From an analysis of data in the literature, it is concluded that deformation in the SAZ is the dominant factor in controlling failure during sheared-edge stretching. The characteristics of the shear face are generally important for failures during sheared-edge stretching only as there is a correlation between the characteristics of the shear face and the characteristics of the SAZ. The effect of the shear burr on shear-edge stretching is also related to a correlation with the characteristics of the SAZ. In reviewing the literature, many shearing variables that could affect sheared-edge stretching limits are not identified or if identified, not quantified. It is likely that some of these variables could affect subsequent sheared-edge stretching limits.

Failure During Sheared Edge Stretching

Failure during sheared edge stretching of sheet steels is a serious concern, especially in advanced high-strength steel (AHSS) grades. The shearing process produces a shear face and a zone of deformation behind the shear face, which is the shear-affected zone (SAZ). A failure during sheared edge stretching depends on prior deformation in the sheet, the shearing process, and the subsequent strain path in the SAZ during stretching. Data from laboratory hole expansion tests and hole extrusion tests for multiple lots of fourteen grades of steel were analyzed. The forming limit curve (FLC), regression equations, measurement uncertainty calculations, and difference calculations were used in the analyses. From these analyses, an assessment of the primary factors that contribute to the fracture during sheared edge stretching was made. It was found that the forming limit strain with consideration of strain path in the SAZ is a major factor that contributes to the failure of a sheared edge during stretching. Although metallurgical factors are important, they appear to play a somewhat lesser role.