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The Forming limit diagram (FLD) is a powerful tool for describing the formability of sheet materials in the automobile industry, which provides fundamental data for die design and Finite Element (FE) simulation. However, traditional FLD testing is typically conducted at quasi-static strain rates from 0.001/s to 0.01/s, which are much lower than the industrial stamping process with strain rates about 1-10/s. In this research, FLDs at various punch speeds (from 1mm/s to 100mm/s or 120mm/s) were obtained for three kinds of AHSS, Quenched and Partitioned steel, Dual Phase 980 and Dual Phase 590 and three kinds of conventional steels, Low Alloy High Strength steel, Bake Hardening steel and IF steel. The results show that FLDs at a typical industrial stamping speed (100mm/s or 120mm/s) are considerably lower than the quasi-static test speed for the Advanced High Strength Steels (AHSS).

Hot forming process of ultrahigh strength boron steel 22MnB5 is widely applied in vehicle industry. It is one of the most effective approaches for vehicle light weighting. Dynamic recovery is the major softening mechanism of the boron steel under austenite state at elevated temperatures. Deformation mechanism of the boron steel can be revealed by investigation on the behavior of dynamic recovery, which could also improve the accuracy of forming simulations for hot stamping. Uniaxial tensile experiments of the boron steel are carried out on the thermo-mechanical simulator Gleeble3800 at elevated temperatures. The true stress-strain curves and the relations between the work hardening rate and flow stress are obtained in different deformation conditions. The work hardening rate decreases linearly with increasing the flow stress.

Adhesive bonding has many applications in the automotive industry. The single-lapped bonded joint is the most typically used among various bonding types. This paper presents experimental research for determining the strain field of the single-lapped joint under tensile loading. The materials for the joint are epoxy-based structural adhesive and low-carbon electrolytic zinc steel plate. In the study, a DIC (digital image correlation) system was adopted to measure the strain distribution of the bonded joint during a tensile test. The bonded steel coupons in the tensile test were prepared according to the ASTM standard. During the measurement, images of the coupon joint were taken before and after the deformation process. Then the DIC system measured the strain of bonded joint by comparing two consecutive images. The measured data from the DIC was compared to data taken simultaneously from a traditional extensometer.

The maximum equivalent plastic strain (EPSmax), which can be achieved in the gauge region of a cruciform specimen during in-plane biaxial tensile tests, is limited due to early fracture on the cruciform specimen arm. In this paper, a theoretical model was proposed to determine the factors related to the EPSmax of a cruciform specimen following ISO 16842: 2014. Biaxial tensile tests were carried out to verify the theoretical analyses. Results show that the material strength coefficient (k) has no effect on the EPSmax, and EPSmax increases with the increase of the material hardening exponent (n) and the cross-sectional-area ratio (c) of the arm region to the gauge region. It is found that the applied load ratio (α) has an effect on EPSmax, which decreases as the load ratio increases from 0:1 (i.e. uniaxial tension) to 1:2 (i.e. plane strain state) and then increases as the load ratio increases to 1:1 (i.e. balanced biaxial tension).

In this paper, a new method for determining the forming limit diagram (FLD) of thin sheet metals, called DIC-Grid method, is proposed based on digital image correlation (DIC) technique. It's assumed that there exists one virtual grid with an initial diameter of 2.5mm, which is usually the same dimension as the grid in traditional circular grid analysis, close to the crack of specimen, and the limit strain point on FLD is determined by the deformation of this virtual grid. The DIC-Grid method has been compared with traditional circular grid analysis and the standard ISO/FDIS 12004-2 in Nakajima tests. The results show that the forming limit strains obtained by the newly proposed method are more stable and precise. Furthermore, DIC-Grid method can avoid the measurement error which exists in the circular grid analysis. Meanwhile, it overcomes the shortcomings of time-consuming data processing and non-applicable for unrealistic strain distribution in the method of ISO standard.

Similar laser welded blanks with same material and same gauge have been extensive applied in automobile body for improving the material utilization and extending maximum coil size. It is known that, for TWBs with dissimilar material and thicknesses, the difference of material properties and/or thickness of the welded blanks, change of the material properties in the weld seam and heat-affected zones (HAZ) as well as location and orientation of the weld seam are reasons for reduced formability. However, the plastic deformation capacity of TWBs is reduced even when the material and thickness are the same. The aim of this paper is to evaluate the deformation behaviors of similar laser welded joints. Uniaxial tensile of five laser welded joints, with 90°,60°,45°,30°and 0°weld orientations, were tested by using optical measurement-DIC (Digital Image Correlation). Strain /strain ratio distribution and evolution of each joint was analyzed and compared with base material.