数字图像相关法的板料成形极限研究与应用

为了克服现有板料成形极限测定方法的不足,采用XJTUDIC三维数字散斑应变测量系统,实现了一种基于数字图像相关法的板料成形极限测定新方法.首先,将测量系统架构在杯突试验机上,记录从试验过程初始到破裂的一系列图像,实现应变在线、全场测定.然后,通过对破裂前的最后一张图像进行截线,拟合数据,对颈缩区域插值得到极限应变,从而绘制成形极限曲线图.最后,通过对SPCC36型号碳钢材料板料件成形极限的测量,验证了本文方法的可行性和有效性.     

数字图像相关法用于金属薄板成形性能研究

针对板料成形性能的两个关键指标:板料的塑性应变比和成形极限图FLD,基于数字图像相关方法,分别设计了相应的检测手段和实验流程,并介绍了涉及的关键技术。SPCC薄钢板、镁铝合金这两种材料的测试结果表明,所采用方法能够高效地得到被测试件表面的三维全场应变,为金属薄板成形性能的测试分析提供了一种有效的应变检测手段;所测定SPCC钢板的值不是定值,而是随着应变的增加而逐渐降低,表现出了各向异性;所测定的FLD有助于充分了解和利用材料的性能,为实际生产中零部件的合理选材提供重要参考依据。     

数字图像相关法的板料成形极限研究与应用

为了克服现有板料成形极限测定方法的不足,采用XJTUDIC三维数字散斑应变测量系统,实现了一种基于数字图像相关法的板料成形极限测定新方法。首先,将测量系统架构在杯突试验机上,记录从试验过程初始到破裂的一系列图像,实现应变在线、全场测定。然后,通过对破裂前的最后一张图像进行截线,拟合数据,对颈缩区域插值得到极限应变,从而绘制成形极限曲线图。最后,通过对SPCC36型号碳钢材料板料件成形极限的测量,验证了本文方法的可行性和有效性。     

数字图像分析法在薄板成形应变场测量中的应用

对薄板成形应变场的传统的测量方法进行了研究，指出其不中产误差的来源，并提出了数字图像分析法测量薄板成形中的应变场，对测量原理，新的测量方法对老方法的改进，以及如何降低误差进行了介绍，指出了这种方法的前景，同时提出了改进意见。     

测定钢板成形极限的三种方法与应用比对

近邻破裂点离线测量法是成形极限试验用来观察成形极限应变的最常用方法。随着光学视频技术在成形试验中的应用,通过测量网格临界破裂时应变的临界破裂点原位测量法,以及ISO12004标准中新推荐的破裂区截面线拟合法,均已进入到成形极限的测量领域。这两种测定成形极限的新方法弥补了紧邻破裂点测量法带来的近似效应和误差,极大提高了成形极限曲线的可信度。文章运用3种不同的方法,对一种高强度冷成形用钢进行成形试验,测定其在近似平面应变状态下的成形极限,并对结果进行比较。     

板料成形极限应变与极限应力的转换关系

因极限应变构成的FLD受应变路径的影响很大 ,在研究复杂应变路径的成形极限问题时带来极大不便。而以极限应力构成的成形极限应力图FLSD由于不依赖加载应变路径 ,已成为目前研究的热点。本文在分析板料极限应变与极限应力的基础上 ,得出了相互转换的公式 ,针对分散性失稳、凹槽失稳和平面应变漂移失稳等准则 ,进行了不同加载应变路径FLD转换FLSD。结果表明 ,对于同一失稳准则 ,在不同加载应变路径下FLSD几乎为同一条曲线 ,FLSD作为复杂加载路径板料成形极限判据更加方便。     

成形极限曲线测定GB/T24171-2009国家标准简介及应用建议

新的成形极限曲线(FLC)检测国家标准GB/T 24171-2009分为两个部分,第1部分GB/T 24171.1-2009和第2部分GB/T 24171.2-2009。第1部分的内容与传统的测试方法基本相同,适合大多数实验室及现场条件下的成形极限曲线检测,第2部分提出了一种更准确的实验室成形极限曲线测定方法,现阶段只有部分实验室能够满足试验条件要求。针对标准第2部分,对其中关于试样、试验装置、试验条件和应变分析等一些关键内容做了试验研究,就新标准的应用提出了一些建议。     

板料真实成形极限的测定及其应用

采用带圆弧侧边板料的单向拉伸试验方法及工具显微镜厚度测量法测定了St14铝镇静钢板向应变（ε2）较大的真实成形极限。并折算成ε2为0．4时的真成形极限的FLD－0．4，分析了FLD－0．4与常规单向拉伸试验性能的关系。     

高应变率成形极限图测试与仿真研究进展

高应变率成形是一种在极短时间内释放高能量而使金属变形的成形方法,研究表明高应变率能显著提高多种金属材料的成形极限能力.通过高速杯突实验绘制成形极限曲线,可反映金属在高应变率下的成形能力,从而指导冲压设计.文中介绍了目前常用的几种高速杯突实验设备,包含气压、液压和电磁等驱动方式,并对高应变率下的成形极限图进行了分析和总结...     

工业纯铝板料渐进成形极限图研究

板料渐进成形极限图和传统成形极限图有显著不同,到目前为止尚未有统一的测试方法.提出了一种测试板料渐进成形极限图的方法,成形工具分别沿往复圆弧和交替往复圆弧轨迹运动,对0.9mm工业纯铝板进行渐进成形.板料破裂后,获得了主应变和次应变,建立了该种板料渐进成形极限图.通过板料渐进成形分别成形了一些零件,测量了零件的主应变和次应变并绘制了它的渐进成形极限图.结果表明,所获得的工业纯铝渐进成形极限图能较为准确地预测渐进成形中板料的破裂,测试方法适用于渐进成形极限图的建立.     

基于三维全场应变摄影测量的板料成形极限动态在线测定

针对目前材料成形极限无法实现在线动态测定的不足,本文将数字图像相关法和双目立体视觉技术相结合,提出一种基于三维全场应变摄影测量的动态在线测定材料成形极限的方法,并研制了快速在线动态测定板料成形极限的实验装置。通过平面金属试件的成形极限测定实验,验证了该方法用于动态在线快速建立材料成形极限图FLD的有效性。     

一种建立板料成形极限应力图的新方法

基于塑性理论建立了比例加载条件下双向拉伸应力应变关系,结合Swift分散性失稳准则,提出了一种建立板料成形极限应力图的方法。分别应用Hill 48和Hosford屈服准则以及单向拉伸性能参数,建立了铝合金板(r<1)和薄钢板(r>1)两种材料的成形极限应力图(FLSD),分析表明,不同的屈服准则的选取对于成形极限应力曲线有不同的影响,对于不同类型的材料屈服准则的影响程度也不同。与由通常的成形极限图(FLD)转换所得到的成形极限应力图(FLSD)进行了对比分析,结果表明,所提出的方法计算过程更为简便,并能较为准确地建立成形极限应力图,可以作为复杂加载路径下的成形极限破裂判据。     

板料成形极限理论与实验研究进展

成形极限是板材成形领域中重要的性能指标和工艺参数。文章在阐述成形极限在板料成形中的意义的基础上,综述并分析了成形极限在理论和实验方面的研究进展。成形极限图受应变路径的影响,给工业生产应用带来极大不便。以极限应力构成的成形极限应力图不受应变路径的影响,作为复杂加载路径的成形极限判据更加方便和实用。FLSD研究与FLD相结合,成为精确地确定破裂判别准则的主要途径之一,是近来研究的热点。十字形双向拉伸是实现复杂加载路径有效实用的试验方法。最后对成形极限应力图和十字形双向拉伸试验需要解决的关键问题作了阐述。     

Logistic regression analysis for experimental determination of forming limit diagrams

The forming limit diagram (FLD) is probably the most common representation of sheet metal formability and can be defined as the locus of the principal planar strains where failure is most likely to occur. Experimental determination of the FLD consists in performing a set of formability tests on a sheet metal blank, where a regular grid has been previously etched. After each test, the deformation of the grid is measured and the relative strains computed. Strains observed closely at the fracture location are related to as ‘failed’ points, while strains observed on the sound areas of the specimens are labelled as ‘safe’ points. Starting from a set of experimental tests, the FLD should be empirically determined through a statistical analysis of collected data. In fact, statistical approaches (such as linear regression) are required to properly account for the internal randomness of failure occurrence. Linear regression, as well as most of the other empirical approaches in the scientific literature, takes into account only information related to the safe points.This paper proposes a different approach, the logistic regression, for the empirical determination of FLDs. Logistic regression allows to directly derive the probability of an event (e.g. the failure) as a function of different predictor variables (both the principal planar strains). Therefore, by using logistic regression, the process designer can directly associate the failure probability to the scrapping costs, in order to economically evaluate a new sheet metal forming operation.Logistic regression allows the determination of the FLD by including information concerning both safe and failed points.     

Measuring forming limit strains with digital image correlation analysis

Digital image correlation (DIC) analysis methods can provide very accurate strain measurement that is useful for identifying the onset of localized necking and thus determining the forming limit strains of sheet metals. Several different DIC procedures to establish a forming limit diagram (FLD) are compared including both position dependent and time dependent measurement methods where the latter is found to be more accurate for an Al–Mg–Si alloy. Since FLD standards are still being developed to include DIC methods, a new DIC-based time dependent measurement method for the forming limit strains is proposed in this research: (1) Surface topography is extracted from the DIC data and changes in surface height difference are obtained, of the test specimen; (2) The onset of localized necking is identified by the limit of stable flow of the surface height difference or the first derivative of the surface height difference near the development of a local neck when using the Marciniak test procedure. Compared to existing methods, this new procedure more accurately defines the onset of necking and the boundary of safe deformation of an FLD.     

板料成形极限的理论预测与数值模拟研究

本文针对目前板料成形极限的实验、理论计算和数值模拟方法以及成形极限应力图的研究进展 ,进行了综述与分析 ,提出了通过数值模拟方法预测板料成形极限所存在的一些问题。认为找到一种能够尽量减少对外部条件的依赖 ,从而更本质地反映材料性能的方法。     

变压边力对铝合金板冲压成形的影响研究

通过数值模拟和实验,研究了变压边力对铝合金板成形过程中应变路径的变化规律,基于MK法研究应变路径的变化对铝合金板成形极限(FLC)的影响,通过比较不同随时间变化的压边力对铝合金成形极限的影响,揭示变压边力改善铝合金板冲压成形性能的机理。实验和数值模拟结果表明,变压边力改善铝合金板成形性能的主要原因是变压边力对应变路径的影响,对杯形拉深来说,渐增型随时间变化的变压边力可以改善铝合金板的成形性能。     

Forming limit diagram of auto-body steel sheets for high-speed sheet metal forming

This paper is concerned with the uniaxial tensile properties and formability of steel sheets in relation to the strain rate effect. The elongation at fracture for CQ increases at a high strain rate while the elongation at fracture for DP590 decreases slightly in relation to the corresponding value for a quasi-static strain rate. The uniform elongation and the strain hardening coefficient decrease gradually when the strain rate increases. The r-value of CQ and DP590 was measured with a high-speed camera in relation to the strain rate. The r-value is slightly sensitive to the strain rate. Static forming limit curves (FLCs) and high-speed FLCs were constructed with the aid of punch-stretch tests with arc-shaped and square-shaped specimens. In addition, a high-speed crash testing machine with a specially designed high-speed forming jig was used for the high-speed punch-stretch tests. Compared with the static FLC, the high-speed FLC of CQ is higher in a simple tension region and lower in a biaxial stretch forming region. The high-speed FLC for DP590 decreases in relation to the static FLC throughout the entire region. The elongation at fracture appears to be closely related to the simple tension region of the FLC. The shear fracture is observed from SEM images of specimens tested in the biaxial stretch forming region under the high-speed forming condition. The dimples indicating the shear fracture have elongated horseshoe shape. The high-speed FLC is lower than the static FLC in the biaxial stretch forming region because the shear fracture induces the decrease of ductility. The results confirm that the strain rate has a noticeably influence on the formability of steel sheets. Thus, the forming limit diagram of high-speed tests should be considered in the design of high-speed sheet metal forming processes.