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.     

A new approach for user-independent determination of formability of a steel sheet sheared edge

It is common to use a forming limit curve (FLC) for a feasibility study of a deep-drawn steel part based on a finite element analysis (FEA). However, in such an approach a neglected fact is that a blank edge in industrial production is often produced by shear cutting. Especially, for many high strength steel grades, this cutting process notably reduces edge formability. An overestimation of formability of the blank edge, with an FLC, is the consequence that may lead to cracks at the sheared edge of a part. The following paper describes a new approach to determine formability of a sheet-steel sheared edge by hole expansion test that uses an FLC tool set. This approach delivers a hole expansion ratio with considerably lower scattering compared to the hole expansion according to ISO 16630. Additionally, information on the planar isotropy, flow and necking behavior of the material, is supplied. Finally, a pragmatical way of transferring test results into an FEA of the forming process for a sheet blank with a sheared edge is presented.     

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.     

Punching of ultra-high strength steel sheets using local resistance heating of shearing zone

A punching process using local resistance heating of a shearing zone was developed to shear ultra-high strength steel sheets. The shearing zone was heated by passing electric current between the sheet holder and the knockout in order to decrease the flow stress in the shearing, and the heating of the die and punch was prevented by no contact with the sheet during the heating. Electrode pins having an individual spring were employed to attain uniform heating of the shearing zone. The welding resistance of the heads of the electrode pins to the sheet by the heating was examined for Ag-W, Cu-W, Ag + WC and W. The Cu-W pins having the highest welding resistance were employed in a punching experiment of 980 MPa level ultra-high strength steel sheets. The punching load was considerably reduced by the heating, e.g., about 1/5 of the cold punching load at 800 °C. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased.     

Factors governing hole expansion ratio of steel sheets with smooth sheared edge

Stretch-flangeability measured using hole expansion test (HET) represents the ability of a material to form into a complex shaped component. Despite its importance in automotive applications of advanced high strength steels, stretch-flangeability is a less known sheet metal forming property. In this paper, we investigate the factors governing hole expansion ratio (HER) by means of tensile test and HET. We correlate a wide range of tensile properties with HERs of steel sheet specimens because the stress state in the hole edge region during the HET is almost the same as that of the uniaxial tensile test. In order to evaluate an intrinsic HER of steel sheet specimens, the initial hole of the HET specimen is produced using a milling process after punching, which can remove accumulated shearing damage and micro-void in the hole edge region that is present when using the standard HER evaluation method. It was found that the intrinsic HER of steel sheet specimens was proportional to the strain rate sensitivity exponent and post uniform elongation.     

Improvement of stretch flangeability of ultra high strength steel sheet by smoothing of sheared edge

Stretch flangeability of an ultra high strength steel sheet having small ductility was increased by improving quality of a sheared edge. The effect of the quality of the sheared edge on the stretch flangeability of the high strength steel sheets was examined from expansion of a sheared hole with a conical punch. The quality of the sheared edge varied with the clearance between the punch and die, and the quality influenced the limiting expansion ratio. It was shown from measurement of surface of the sheared edge that the limiting expansion ratio of the sheared ultra high strength steel sheet is dependent on the macroscopic unevenness and hardness of the sheared edge and not on the microscopic roughness. The macroscopic unevenness of the sheared edge was eliminated by smoothing the edge with a conical punch. It was found that the smoothing of the sheared edge is effective in improving the stretch flangeability of the ultra high strength steel sheet.     

Analysis of sheared edge formability of aluminum

Edge quality produced by shearing processes often leads to reduced material formability which was observed in multiple studies and summarized in the reference literature. The intention to make the sheared edge performance more predictable has motivated development of several experimental techniques such as the hole expansion test and the half dogbone tensile test. The paper presents a detailed review of published results for both of these techniques and illustrates very limited research dedicated to sheared edge performance of aluminum alloys. The experimental study, performed on a broadly used aluminum alloy, 6111-T4, illustrated the effects of cutting clearance on longitudinal, transverse and diagonal orientations of the trim line relative to the rolling direction. For all sheet orientations, increasing the cutting clearance resulted in a substantial reduction in material stretchability along the sheared surface. However, for all investigated conditions a cutting clearance of 5% of material thickness resulted in stretching performance similar to the standard tensile test. In this case the sheared edge does not affect the stretching behavior of tested material. The analysis of material prestrain on sheared surface stretchability for a variety of combinations of minor and major strains indicated that for the widely accepted industry standard gap of 10% of the material thickness, the prestrain has significant effects on stretchability which only gets stronger with increased thinning of the sheet in the prestraining process. For an extended clearance of 40%, the effect of prestrain was less visible indicating that the sheared edge has a stronger effect on these cutting conditions than prestrain.Analysis of the effect of the cutting angle on stretchability indicated that higher elongations were observed with cutting angles of 10° and 20° for broadly used 10% clearance compared to orthogonal cutting with an identical clearance.The results of half dogbone tensile tests were compared with the results of hole expansion tests performed on the same sheet material. This comparison indicated that a substantial amount of localization occurs in the hole expansion test and leads to a much higher hole expansion ratio for small cutting clearances compared to the total elongations observed in tensile tests. However, the local strains measured in the area adjacent to fracture in the tensile test were above the hole expansion ratio.     

聚能射流穿甲后超高强度钢靶板的损伤特征及其机理

超高强度钢靶板在聚能射流穿甲后损伤特征的研究表明,在穿甲过程中,弹孔周围白层的温度超过了相变点,形成了晶粒尺度在20nm左右的马氏体 残余奥氏体混合组织.绝热剪切带（ASB）中的剪切仅发生在相当窄的层面上,其宽度约为2μm,平均切应变量为110,应变速率≥2.24×106s-1;光学显微镜下显示的剪切带宽度是集中剪切变形区（LSDZ）及其热影响区（HAZ）的总宽度.聚能射流穿甲过程中,靶板材料破坏包含了如下几个相互重叠的过程:与射流接触的靶板周围局部区域材料沿射流方向上的整体协调变形;射流与靶板之间的剪切断裂和弹孔表面部分靶材的熔化;集中在极薄层面卜高度局域化的剪切变形;应力波在靶表面反射造成的正向开裂.     

An overview of stabilizing deformation mechanisms in incremental sheet forming

In incremental sheet forming (ISF) strains can be obtained well above the forming limit curve (FLC) that is applicable to common sheet forming operations like deep drawing and stretching. This paper presents an overview of mechanisms that have been suggested to explain the enhanced formability. The difference between fracture limit and necking limit in sheet metal forming is discussed. The necking limit represents a localized geometrical instability. Localized deformation is an essential characteristic of ISF and proposed mechanisms should stabilize the localization before it leads to fracture. In literature six mechanisms are mentioned in relation to ISF: contact stress; bending-under-tension; shear; cyclic straining; geometrical inability to grow and hydrostatic stress. The first three are able to localize deformation and all but the last, are found to be able to postpone unstable growth of a neck. Hydrostatic pressure may influence the final failure, but cannot explain stability above the FLC.     

Design and analysis of new test method for evaluation of sheet metal formability

A new test method including the tool shape and test procedure was developed to evaluate sheet metal formability using the finite element method (FEM). This method is intended to generate the various modes of deformation and to control the onset of failure independently under each mode so that the forming limit diagram (FLD) achieves a good representation of a wide range of strains.A blank holder force-punch stroke diagram with three failure loci is introduced to define the optimum process condition and the formability index by which each material is quantitatively evaluated. The test procedure of this method consists of three steps: drawing a blank holder force (BHF)-punch stroke diagram, measuring strains from the part stamped at the optimum process condition, and grading the test materials using the formability index. In numerical simulations under optimum process conditions, sheet metals can fail due to multi-mode rupture; this failure leads to a widely balanced strain distribution in the FLD such that strains are developed near the forming limit over a wide range of forming modes.Experiments were conducted on three grades of steel sheets to validate the proposed method. Stamping results yield well-defined strain signatures having a wide range of strain distribution in the FLD in all materials tested. The outcomes of the shape and strain behaviors agree well with the numerical results.     

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 500 μm or 31% of the thickness. The fracture processes and involved mechanisms were discussed.     

Predicting breakage on a die radius with a straight bend axis during sheet forming

As the use of high strength sheet steels for automotive applications increases, there have been increasing occurrences of breakage on die radii at strains that are less than predicted by a conventional forming limit curve. To develop a better understanding of the problem, data from an angular stretch bend test were analyzed to determine the applied stress on a bend that caused breakage for a range of die radius, sheet thickness, and steel grade. The forming limit stress was calculated using a modified version of Stoughton's stress-based forming limit curve. The ratio of the applied failure stress to the forming limit stress is defined as the failure stress ratio. Regression analysis was used to determine the effect of the sheet thickness to die radius ratio (T/R) on the failure stress ratio for the different steel grades ranging from DDQ+ to DP980. The results were grouped due to commonality among various steel grades. Three predictive equations for (1) DP800, (2) DP980, and (3) all the other steel grades describe the behavior for all the steels in the study. It is found that breakage on a die radius for high strength steels such as DP600, DP800, and DP980 is typically a result of low values of the forming limit stress.     

Warm and hot punching of ultra high strength steel sheet

Warm and hot punching using resistance heating was developed to improve the quality of sheared edges of an ultra high strength steel sheet. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased. The rollover depth and burr height of the sheared edge became large above 800 °C. Although the roughness of the burnished surface was almost constant, the roughness of the fracture surface increased from 650 °C. The punching load was extremely reduced by the heating, i.e. 40% of the cold punching load at 650 °C and 15% at 1070 °C. The local resistance heating of the shearing region was efficient for the warm and hot shearing. It was found that the warm and hot shearing of ultra high strength steel sheets is effective for improving the quality of the sheared edge and in reducing the shearing load.     

切角板坯对纯铜薄板矩形盒拉深影响的试验及数值分析

在大量纯铜薄板矩形盒拉深试验的基础上,结合有限元计算,分析了矩形盒的拉深特性。指出,矩形切角板坯使法兰曲边变形分布得以改善,非拉深变形抵抗弱化,提高了拉深成形性。有限元模拟结果显示矩形板坯拉深断裂点产生在凸模肩部转角附近,且始终承受两向不等拉伸,应变计算值与实际拉深测定值相符;切角板坯的拉深断裂点则转移至接近凹模口的侧壁处,且始终处于压剪组合变形状态,断裂时板厚应变相当小,认为倾向于剪切断裂。切角板坯断裂点的拉、压应力组合效应使该点板厚几乎不变,是提高拉深极限的重要因素之一。     

The mechanics of incremental sheet forming

The deformation mechanism of incremental sheet forming (ISF) is examined experimentally through forming specially prepared copper sheets. Strain distributions through the thickness of the sheets are measured for two configurations of ISF: two-point incremental forming (TPIF) and single-point incremental forming (SPIF), and a comparison is made to pressing. The measurements show that the deformation mechanisms of both SPIF and TPIF are stretching and shear in the plane perpendicular to the tool direction, with shear in the plane parallel to the tool direction. Strain components increase on successive laps, and the most significant component of strain is shear parallel to the tool direction. Increasing stretching and shear perpendicular to the tool direction account for differences between the sine law prediction and measured wall thickness for both SPIF and TPIF. The observed mechanisms of SPIF and TPIF differ from a mechanism of pure shear that has previously been assumed.     

差厚高强钢激光拼焊板拉伸失稳行为研究

对0.7mm/1.0mm厚度组合的B170P1钢激光拼焊板试件进行半球凸模胀形试验,分析不同应变状态下拼焊板的变形、失稳特点及应变分布情况,研究其拉伸失稳规律。研究表明,拼焊板试件的变形失稳主要发生于薄侧母材,且随应变状态由单拉向平面等双拉的转变,应变分布趋于均匀,失稳位置向焊缝靠近;在变形过程中,靠近焊缝的薄侧母材在平行于焊缝方向的变形受到厚侧母材及焊接区的影响,其应变路径快速向平面应变漂移,达到成形极限状态,降低了拼焊板的成形性能。焊缝的存在导致差厚激光拼焊板各部分变形不均匀,在差厚激光拼焊板的实际应用中,应采取适当措施抑制薄侧母材的局部变形,增加厚侧母材塑性变形的比例,提高差厚拼焊板的冲压成形性。     

An investigation of tearing failure in fine-blanking process using coupled thermo-mechanical method

In fine-blanking processes, localized deformation is prone to cause tearing failure at the surface of shearing edges, which can influence the quality and functionality of the blanked part. In this paper, a numerical simulation of a fine-blanking process has been conducted by using a coupled thermo-mechanical finite element method. The thermal effect on the material properties has been taken into account in the simulation. On the basis of the results of the simulation, the distributions of equivalent plastic strain and stresses in a fine-blanking process have been analyzed. Moreover, examinations of metallurgical microstructure by means of optical and scanning electron microscopy have been performed. It has been observed that the grains near the shearing edge were highly distorted within a narrow shearing zone. The formation of microvoids under large shear deformation supports the argument that material damage initiates and develops around this local area during the fine-blanking process. It is suggested that tearing failure could initiate from the damaged area of the workpiece adjacent to the tool tips where cracking occurs due to excessive local tensile stress produced by fine blanking attributes and interfacial friction.     

行李箱内板零件冲压成形分析

在材料力学性能测试和成形极限分析的基础上,采用网格应变分析技术研究了行李箱内板零件冲压过程危险区域的金属流动规律,评价了两种钢板的成形效果。结果表明：行李箱内板危险部位的变形方式为胀形一深拉延变形,r值和n值较高的冷轧薄板具有较高的成形极限,相应地其冲压成形安全裕度也较大。     

Mechanism investigation of friction-related effects in single point incremental forming using a developed oblique roller-ball tool

Single point incremental forming (SPIF) is a highly versatile and flexible process for rapid manufacturing of complex sheet metal parts. In the SPIF process, a ball nose tool moves along a predefined tool path to form the sheet to desired shapes. Due to its unique ability in local deformation of sheet metal, the friction condition between the tool and sheet plays a significant role in material deformation. The effects of friction on surface finish, forming load, material deformation and formability are studied using a newly developed oblique roller ball (ORB) tool. Four grades of aluminum sheet including AA1100, AA2024, AA5052 and AA6111 are employed in the experiments. The material deformation under both the ORB tool and conventional rigid tool are studied by drilling a small hole in the sheet. The experimental results suggest that by reducing the friction resistance using the ORB tool, better surface quality, reduced forming load, smaller through-the-thickness-shear and higher formability can be achieved. To obtain a better understanding of the frictional effect, an analytical model is developed based on the analysis of the stress state in the SPIF deformation zone. Using the developed model, an explicit relationship between the stress state and forming parameters is established. The experimental observations are in good agreement with the developed model. The model can also be used to explain two contrary effects of friction and corresponding through-the-thickness-shear: increase of friction would potentially enhance the forming stability and suppress the necking; however, increase of friction would also increase the stress triaxiality and decrease the formability. The final role of the friction effect depends on the significance of each effect in SPIF process.     

压料面形式对矩形盒拉深成形影响的数值模拟

针对3种压料面形式下矩形盒的拉深成形过程进行数值模拟,得出相应板坯曲边的剪切应力及角对称线上等效应力、等效应变的分布规律,并对其进行了对比分析.模拟结果表明矩形盒法兰曲边采用锥面压料面形式时的成形性能最好,板坯整体变形最均匀;采用柱面压料时成形极限比平面压料时更低.该研究结果对非回转对称拉深的试验研究及实际生产具有一定的参考价值.