Shear-affected Zone Research Articles

A comparison of high strain rate response and adiabatic shear band formation in additively and traditionally manufactured Inconel 718

3D printing of Inconel 718 has been increasingly commercialized, as such research comparing the mechanical properties of additively manufactured (AM) and traditionally manufactured (TM) components in both quasi-static and high rate regimes should be conducted. Herein, the high strain rate material response, as well as the formation of adiabatic shear bands (ASBs) in AM and TM Inconel 718 are studied using a split Hopkinson pressure bar (SHPB) system. A top-hat cylindrical specimen geometry was used to convert the compressive load to shear and conduct high rate shear tests in the SHPB. The quasi-static and high rate stress-strain data reported herein indicates that TM components have at least a 70 MPa higher compressive yield strength. Interestingly, the AM specimens were more susceptible to ASB formation than their TM counterparts. The critical shear strain at which an ASB forms was higher for the TM specimens compared to the AM specimens. This was attributed to the TM material having a higher critical stress for dynamic recrystallization initiation, which is required to ASB formation. The microstructures of the shear zone in the top-hat specimens were examined using an optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). Dynamic recrystallization was observed at the center of the shear affected zone (SAZ) using TEM from material lifted out from the ASB. Due to the plastic deformation in the shear region, the material strain hardens and therefore has a higher hardness than the bulk material. At the center of the shear region, a peak in nano-hardness of 585 HV was recorded on the ASB. The outcome of this research will lead to the prevention, containment, and control of shear instability in AM structures.

Improving the stretch flangeability of ultra-high strength TRIP-assisted steels by introducing banded structure

A novel ultra-high strength transformation induced plasticity (TRIP)-assisted steel with a better combination of ductility and stretch flangeability was fabricated by introducing a banded structure into the resulting microstructure. The corresponding relationships between microstructures and mechanical properties were studied in detail by the tensile test and the hole expansion test. These tests reveal that some pancake-like ferrite grains obtained in cold rolling are retained during intercritical annealing and finally form a banded structure together with martensite/austenite (M-A) islands along the rolling direction. The steel can be further strengthened in the subsequent punching process, while uniformly-distributed bainite matrix with fine M-A islands dominates the overall microstructure after annealing at higher annealing temperatures. The banded structure of the soft ferrite phase will promote crack propagation along the rolling direction rather than in the thickness direction, thereby favoring improvement of the hole expansion ratio (HER). Instead, the mixed bainite-M-A islands microstructure led to increased micro-voids density at the edge of the shear affected zone (SAZ) after the hole punching process and a great number of cracks rapidly propagated through the thickness direction in the matrix, which is responsible for a decrease in HER. Excellent mechanical properties were obtained in the low-temperature annealed sample with an HER of 29–35%, elongation of 21–26%, tensile strength of 941–1016 MPa, and yield strength of 426–566 MPa.

Study of deformation and fracture of high strength steel sheet during conventional and robust trimming by conducting partial trimming tests

An instrumented trimming press equipped with D2 grade steel die inserts with sharp edges was used to conduct trimming tests on DP980 sheets with clearances of 10%, 20%, and 30% of the sheet thickness. Trimming tests were conducted either with or without a support pad placed under the scrap piece and the trimming force was measured as a function of penetration distance of the upper die. The progression of plastic deformation and fracture processes that occurred in the shear affected zone (SAZ) of DP980 were observed on the polished cross-sections of the sheet by performing partial trimming tests. During conventional trimming with 10% clearance, cracks propagated simultaneously from the upper and lower surfaces of the sheet toward each other. However, when the clearance was increased to 30%, the main crack was initiated from the upper surface and propagated toward the lower surface resulting in the formation of a tensile-type burr. At 30% clearance, the trimming test conducted with a support pad prevented bending of the scrap piece, resulting in a uniform damage distribution within the SAZ. Thus, an improved sheared edge quality is achieved at larger clearances.

Early crack initiation mode during tensile fracture process in a punch-processed precipitation-hardened steel plates and evaluation method to treat the crack as an equivalent pre-crack

For partially worked metals such as punching-processed steel plates, a process-affected zone designated “shear-affected zone” (SAZ) is formed. In this study, tensile tests of punched specimens were conducted to determine the strength of partially worked metals. In the fracture surface of the specimen, a semi-elliptical crack originating in the SAZ, or “shear crack,” was observed, inclined at 45° to the remote tensile stress direction. This shear crack changed the behavior of the metal during tensile loading, e.g., ductile-brittle transition. In specimens exhibiting macroscopic brittle fracture, microscopic brittle fractures cause shear cracks before the specimen reaches its macroscopic yield point. The shear cracks in ductile fractured specimens include a similar brittle-fractured region that can be considered a pre-crack; the punched material can be considered a pre-cracked material. This study highlights the requirement for a novel mechanical model based on linear fracture mechanics to evaluate and predict the fracture mode of punched materials.

Sheared Edge Formability Characterization of High Strength Aluminum Alloys at Room and Elevated Temperatures using Hole Expansion Tests

High strength 6000- and 7000-series aluminum alloys have significant potential for automotive light weighting due to their high strength-to-weight ratios. Alternate thermal processing routes and warm forming operations with rapid heating to 150 - 300°C are being explored to expand their forming process windows. While the constitutive and formability behavior of these alloys at elevated temperatures is an active area of research, limited data is available on their sheared edge stretchability, particularly under elevated temperature forming conditions. To this end, the influence of cutting clearance on the edge fracture limits of two precipitation hardening alloys, AA6013-T4 and AA7075-T6, and a non-heat treatable AA5182-O were first investigated at room temperature using the conical hole expansion test. A 12% clearance was found to be suitable for the three materials and used to process the hole expansion samples prior to warm forming. Microhardness tests were used to characterize the depth and severity of the shear affected zone (SAZ) for the AA7075-T6 before and after the warm forming cycle. The forming temperature was found to increase the hole expansion ratio by approximately 400%, 150%, and 520% for the AA5182-O, AA6013-T4, and AA7075-T6 respectively relative to the room temperature tests. The edge stretchability during W-temper forming of the AA6013 and AA7075 at room temperature was also assessed. It was observed that W-temper forming negatively influenced the hole expansion of AA6013 by approximately 30% relative to the T4 condition. Conversely, the hole expansion ratio for AA7075 when formed in W-temper increased by over 35% with respect to the T6 temper.

Erratum to “On the shear-affected zone of shear bands in bulk metallic glasses” [J. Alloys Compd. 837 (2020) 155494

Erratum to “On the shear-affected zone of shear bands in bulk metallic glasses” [J. Alloys Compd. 837 (2020) 155494

On the shear-affected zone of shear bands in bulk metallic glasses

Abstract Notched bars of bulk metallic glasses, Pd40Ni40P20 and Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105), were deformed under 3-point bending conditions, resulting in the formation of shear bands before failure. The immediate environment of shear bands was investigated in detail using fluctuation electron microscopy to extract information on the strain-induced modification of the medium-range order (MRO) and its lateral extension. Characteristic material-independent gradients were observed for the tensile and compressive sides of the samples indicating the impact of the local stress state on the MRO. Our results based on structural changes of the MRO suggest an upper limit of a few microns for the lateral extension of the shear-affected environments of shear bands.

Shear and laser cutting effects on hydrogen embrittlement of a high-Mn TWIP steel

The influence of shear and laser cutting on hydrogen embrittlement of a hot-rolled Fe-18Mn-0.4C-1.2Al-1.8Cr TWIP (twinning induced plasticity) steel sheet in 3 mm thickness was investigated by hydrogen pre-charging and subsequent slow strain rate tests. Due to the severe plastic deformation during shearing, the ductility of the shear-cut specimen was significantly reduced in tensile test compared to the laser-cut specimen. After hydrogen pre-charging, the laser-cut specimen exhibited less susceptibility to hydrogen embrittlement than the shear-cut specimen. The fractographic analysis revealed that the shear-affected zone acted as crack initiation site during tensile test, which exhibited a flat fracture zone with high stress triaxiality. In hydrogen-charged condition, the flat fracture zone transited from large primary dimples to the mixed features with dimples and quasi-cleavage features.

The effect of microalloying on the sheared edge ductility of ferritic steels

Sheared edge ductility plays a critical role in many automotive and structural applications. The effect of microalloying elements on the microstructure, mechanical properties, sheared edge quality and hole expansion properties of six thermomechanically laboratory hot rolled and coiled, approximately 4 mm thick, precipitation hardened ferritic steels have been investigated. The studied V-microalloyed steels had low carbon contents, 0.05 and 0.09 wt.%, and, in some cases, microalloying additions of either Nb or Nb+Ti. Yield strengths were in the range 500 MPa to 750 MPa and hardness values between 176 and 260 HV10 depending on the carbon content and level of precipitation strengthening. Detailed microstructural analysis using laser scanning confocal microscopy (LCSM), FESEM and FESEM-EBSD revealed that all the steels consisted of mainly single-phase ferrite. FESEM analysis revealed the presence of relatively coarse precipitates at grain boundaries and inside some of the ferrite grains, but the number density of visible precipitates varied greatly between grains. The addition of Nb or Nb+Ti reduced the ferrite grain size, especially the size of the largest grains as defined by the 90th percentile in the cumulative grain area distribution (d90%). Mean hole expansion (HE) ratios varied from 39 % to 76 % with the highest values associated with the lowest carbon contents. Despite raising the strength, microalloying with Nb or Nb+Ti had little if any detrimental effect on HE ratios compared to the V-microalloyed variants without Nb or Ti, presumably as a result of the grain refinement. Titanium nitrides acted as void nucleation sites close to the sheared edge. Microalloying with Nb and Nb+Ti led to a decrease in the depth of the shear affected zone (SAZ) and a smaller difference in hardness between the base material and the SAZ.

Study of the hydrogen uptake in deformed steel using the microcapillary cell technique

The microcapillary cell electrochemical method is capable of evaluating hydrogen (H) uptake in steel with respect to deformation, which is induced by various mechanical methods (cold rolling, bending and punching). A clear relation between the deformation degree and the local H-content is established for dual phase (DP600) steel. The magnitude of the deformation nearby a punched edge is quantitatively determined using electron backscatter diffraction technique. A shear-affected zone is identified at the edge of the punched hole. The dedicated local electrochemical measurements confirm the presence of high concentrations of local-H in this shear affected zone, which is likely detrimental for H-embrittlement.

Characterization of Sheared Edges in Warm Blanking of Magnesium Alloy AZ31B.

This research aims to characterize damage at the sheared edge caused by the blanking operation of magnesium alloy AZ31B sheets. Shearing tests were carried out on an in-house blanking die-set and mechanical press (universal testing machine) by varying punch–die clearance and temperature. Edge damage was distinguished by the geometrical features of the sheared edge and by the distribution of the edge strain hardening (ESH) index. In this account, optical microscopy and scanning electron microscopy were applied to examine the characteristic dimensions of the sheared edge, fracture profile, and sheared edge quality, while the Vickers hardness test was applied to observe the surface micro-hardness in the shear zone (SZ) and the shear affected zone (SAZ). It was concluded that the blanking of magnesium alloy sheets at room temperature results in sheared edge defects, due to premature fracture, referred to here as micro-cracks, loose particles, and a jagged-plus-curved fracture profile. However, such deformities were completely suppressed with the rise in temperature. In addition, based on optical morphology, micro-hardness tests, and microstructure evolution, the recommendation regarding blanking temperature for the magnesium alloy AZ31B has was proposed.

Effect of Prior Austenite Grain Size on Hole Expansion Ratio of Quenching and Partitioning Processed Medium-Mn Steel

The effect of the prior austenite grain size (PAGS) on the tensile properties and hole expansion ratio (HER) has been investigated. Starting from different PAGS values (4.7 μm and 15.2 μm) obtained by controlling the austenitizing temperature, microstructure consisting of martensite and austenite (Vγ of 0.09–0.18) was produced by the quenching and partitioning process. Increasing Vγ had a beneficial influence on the tensile elongation regardless of the PAGS, but deteriorated the HER. However, larger PGAS alleviated the degradation of the HER. The major influence of a larger PAGS on the HER results from the decreased population of interface between neighboring martensite in the shear-affected zone, because that interface is revealed to be a major site for void formation during hole expansion testing.

Role of shear-affected zone on shear-crack formation during tensile loading in punched hole metal

Role of shear-affected zone on shear-crack formation during tensile loading in punched hole metal

Experimental Techniques for Finite Shear Strain Measurement within Two Advanced High Strength Steels

There is a growing need to experimentally characterize local shear deformation in advanced high strength steels (AHSS) for the calibration of stress-state dependent fracture criteria and to better understand sheared edge cracking during secondary forming operations. Planar shear test specimens with digital image correlation (DIC) strain measurement are now commonly performed tests but may not be able to resolve the local strains during the final stage of fracture when the macroscopic shear band collapses to a micro-shear band with intense local strains. Studies of sheared edge stretching of AHSS have shown that the microstructure at the sheared edge experiences extreme local shear deformation with the shear-affected zone (SAZ) that can be much larger than the macroscopic strains reported using DIC on planar shear tests. In this work, two independent experimental techniques are proposed to characterize the residual strain distribution within the shear-affected zone for two AHSS grades with a similar strength level: a complex-phase (CP) steel, CP800, and a dual-phase (DP) steel, DP780. The first method uses finite strain theory to calculate the work-conjugate equivalent strain from grain rotations within the shear bands of interrupted in-plane shear tests. A comparison between the local DIC strain measurements and the grain rotation measurements were found to be in excellent agreement until just prior to failure. The second technique used micro-hardness measurements taken from the interrupted shear tests to develop correlations with the measured equivalent strain from the DIC system. The hardness and grain rotation techniques were then used to characterize the local strain distribution within the SAZ of hole expansion test specimens for punch clearances of 12% and 28%. Both methods provided similar strain distributions with the grain rotation method having the highest strain resolution. The residual strain field within the SAZ of both AHSS was found to be strongly dependent upon the punch clearance. Finally, a homogenization procedure was applied to the SAZ strain distributions to facilitate the length scale transition from the grain-level to length scales appropriate for finite-element modelling of sheet metal forming operations with sheared edges.

Maximizing the Expansion Ratio through Multi-stage Shear-cutting Process during Collar Forming

Previous investigations showed that a two-stage punching process reduced the edge crack sensitivity on high-strength multi-phase steels significantly compared to a single-stage process. This is caused by an alteration of the state of stress in the shear-affected zone during the second stage, which results in not only higher expansion ratios, but also higher collars during the collar forming test. Another preceding research project at the Chair of Metal Forming and Casting, Technical University of Munich, investigated the optimized values for cutting offset, and die clearance for both pre-cutting and re-cutting steps for a single-stage punching process.The aim of this research project was to combine those previous findings by increasing the number of stages in a punching process with each using the optimized parameters. The behavior of milled edges served as a reference for optimum performance. The number of stages influenced material properties, such as surface characteristics, material deformation in the shear-affected zone, and roughness of the cutting surface. The quality of the cutting surface increases drastically due to various factors, such as a changed surface hardness, which affects fatigue resistance and, therefore, the component’s lifetime. The validation of this punching strategy not only achieves an even higher expansion ratio in common sheet metals, but also makes highly edge-crack sensitive materials available for industrial applications.

Evaluation of wear-induced plastic deformation at the trimmed edge of DP980 steel sheets

In order to increase the use of advanced high strength steel (AHSS) for automotive light-weighting, the significant trimming-induced wear damage that AHSS sheets cause to the trim dies should be reduced so as to decrease die maintenance costs and deterioration of the quality of sheared edge. In this research, the wear characteristics in the upper AISI D2 die inserts used for trimming DP980-type AHSS sheets and the wear-induced plastic deformation at the sheared edges of DP980 were studied. Observation of wear profiles at the edge of the upper trim die revealed that material abrasion was the main damage feature. The trimmed edge quality of DP980 deteriorated with the number of strokes, as indicated by an increase in burr width. Plastic strains near the surface of the sheared edge estimated using the displacements of martensite plates, increased from 2.9 at 2.5 μm below the trimmed edge after 40,000th cycles, to 55 after 80,000th cycles. Micro-hardness tests performed to estimate the local flow stresses in the shear affected zone (SAZ) indicated that the flow stress at a distance of 4.5 μm below the trimmed edge increased with the number of trimming cycles to 1.73 GPa in the 80,000th trimmed part.

Ductile-to-brittle transition in tensile failure due to shear-affected zone with a stress-concentration source: a comparative study on punched-plate tensile-failure characteristics of precipitation-hardened and dual-phase steels

The effect of shear-affected zone (SAZ), with a stress-concentration source induced by the punching process, on tensile properties was investigated. Tests using honed specimens (which have the same shapes and stress-concentration without any SAZ) and smooth specimens were conducted to compare the effect with that of the punched specimens. Dual-phase steel, which has a high work-hardening ability and low yield strength, and precipitation-hardened steel, which has a low work-hardening ability and high yield strength, were used in the tests. Materials with two tensile strength grades were prepared from both types of steel. Only the precipitation-hardened steel with higher strength grade punched specimen showed a brittle fracture with extremely short fracture-elongation, whereas the other specimens showed a ductile fracture. The fracture surface analysis revealed that cracks initiated in the maximum shear stress plane of the SAZ under tensile loading at first. We call the crack “shear crack.” The steel which showed brittle fracture used in this study easily exhibited plastic-strain localization compared with the other steels. If the shear crack is sharp, then the transition from ductile to brittle failure tends to occur. Furthermore, the strength characteristics of the punched specimen depend on the crack length dependency of the strength resistance and the failure phenomenon of the original material.

Effect of shear-affected zone on fatigue crack propagation mode

It is well known that the fatigue limits of some precipitation-hardened steels with a punched hole do not increase, even if the tensile strengths of base-metal increases. This study suggested that the dominant reason for this strange phenomenon is a transition from the Mode I type fatigue crack propagation due to the cyclic plastic deformation to Mode II type propagation due to the damage accumulation by a quantitative analysis of influential factors based on fracture mechanics. This result can open a probability that the fatigue limit of the punched hole with shear-affected zone can be actively quantified.

Effect of quenching temperature on stretch flangeability of a medium Mn steel processed by quenching and partitioning

The stretch flangeability of a medium Mn steel prepared by quenching and partitioning was examined using a hole-expansion test. Samples were heat-treated at quenching temperatures 90 °C ≤ TQ ≤ 170 °C, at which no secondary martensite is formed upon final quenching. The hole-expansion test used a 60° conical punch. The hole at the center of the specimens was prepared by punching or wire cutting. In both sample preparation conditions, hole expansion ratio (HER) increased as TQ decreased. The dependence of HER on TQ was more pronounced in the samples prepared by punching than in samples prepared by wire cutting. While there is no clear correlation between HER and tensile properties, the HER decreased as retained austenite increased. By punching, the retained austenite transformed to strain-induced martensite in the shear-affected zone near the hole edge. This martensite has a negative impact on the HER.

Influence of shear-affected-zone due to punching on tensile characteristics of steel plate

In punched steel plate, “punched specimen” from here onwards, the region near the edge of the punched hole is called the shear-affected-zone (SAZ), where tensile characteristics are compromised due to the punching process. However, the specific characteristics and influence on the tensile characteristics of the SAZ are unknown. Therefore, it is difficult to predict the effects of punching on the specimen. Here, we evaluated two kinds of specimens: punched and “honed.” The hole of the honed specimen is formed by drilling and polishing, thus there is no SAZ on the honed specimens. In these specimens, we focused on the initiation and propagation of cracks under tensile loading and set the following objectives: (1) Determine whether any special events occur in the fracture mechanism of the punched vs honed specimens under tensile loading. (2) Find the reasons for any special events that occur. In our investigation, a punched specimen showed brittle fracture even within the static range of the strain rate. By failure surface observation of the punched specimens, we found that under tensile loading, cracks caused by the shear stress aligned with the tensile direction initiate in the SAZ. These cracks are the origin of the final fracture of the punched specimen. We assumed that whether the break is a ductile failure or brittle fracture depends on whether the crack tip becomes blunt. The differences in the fracture behaviors are investigated via detailed fracture surface observation.