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Until now the hole expansion ratio has been generally regarded as a relative “local formability” parameter with limited application to edge-cracking analysis and prediction. In this study a constrained statistical test data analysis methodology is introduced, where the lower-bound hole expansion ratio is the basis for three practical edge-cracking failure criteria. The Maximum Edge Stretch Criterion is directly compatible with CAE simulation. The Edge Thinning Limit Criterion and the Critical Thickness Criterion are more useful in field work and post mortem laboratory failure analysis. Two case studies are described, where hole expansion test data are used to analyze edge cracking of Advanced High Strength Steel (AHSS) in real-world automotive seating applications.

Edge stretching performance was assessed with the conical-punch hole expansion test for a variety of automotive sheet steels. Included were: an ultra-low carbon IF steel, a dual-phase advanced high strength steel (DP 980), an austenitic stainless steel (204), an annealed martensitic stainless steel (410 AN), and a ferritic stainless steel (429 MOD). Various hole fabrication methods were considered: conventional piercing (shearing), water-jet cutting and laser cutting. With pierced holes, no effect of shearing clearance on the hole expansion ratio (HER) was observed. The dual-phase steel and the austenitic stainless steel exhibited relatively low hole expansion performance in the pierced-hole condition (HER ≤ 50%). However, these materials demonstrated tremendous potential for improvement with alternative edge preparation methods, and both benefitted more from laser cutting than from water-jet cutting.

The influence of strain rate on work hardening behavior has been determined for a variety of high strength steels including high-strength low-alloy (HSLA), dual phase (DP), and transformation-induced plasticity (TRIP) steels. Tensile testing was performed at true strain rates of 10-3 s-1 and 1.0 s-1 to represent laboratory testing conditions and dynamic press-forming operations, respectively. Work hardening behavior is described by the conventional strain hardening exponent (n-value), the work hardening rate (dσ/dε), and the Shape-Tilt-Strength (STS) equation as an alternative approach. The effects of deformation temperature and temperature rise during deformation (adiabatic heating) on work hardening are also evaluated. Increasing the strain rate generally increases the work hardening rate at smaller strains, which may contribute to a broader initial strain distribution in press forming.

Springback in sheet metal forming is becoming a very troublesome issue with the increased use of high strength steels in automobiles. The current trend for many applications is to reduce vehicle weight by down-gaging, that is, substituting higher strength, thinner steels for lower strength, thicker steels. The primary springback concern in sheet metal forming is variation in springback, rather than the magnitude of the springback. Even large springback can be accommodated if it can be consistently predicted. Variations in springback are caused by variations in mechanical properties and gage, and by fluctuations in the conditions of the forming process. This paper addresses the expected springback issues associated with the application of high strength sheet steels in light of strength and thickness uniformity. A simple expression is used to show how variations in yield strength and gage may be expected to influence springback in sheet metal forming.

Ultra-low carbon sheet steel with excess stabilizing elements such as titanium, niobium and vanadium can be strengthened via internal nitriding in an ammonia/nitrogen atmosphere. Strength can be imparted to the sheet either in coil form in an open coil annealing (OCA) furnace or after forming in a batch-type component nitriding process. The latter strengthening method presents several advantages over conventional high strength steels including lower forming loads and increased part complexity (enhanced formability). To illustrate the effects of the nitriding thermal cycle on the dimensional stability and residual stress levels of formed parts, a post-forming nitrided steel was compared to a conventional commercially produced high-strength low-alloy (HSLA) steel in laboratory simulations. A slit-ring technique was used to measure residual stresses and dimensional stability. The residual stress relief as a function of nitriding time was measured for various nitriding temperatures.

The effects of strain path on formability and microstructural evolution with strain in two low-carbon steels were examined. The steels include a 0.008 wt. pct. C batch annealed 0.81 mm thick sheet and a 0.031 wt. pct. C continuously annealed0.74 mm sheet with essentially equivalent mechanical properties (YS: 230 MPa; UTS: 350 MPa; n: 0.18). The steels were subjected to various increments of prestrain in either uniaxial or biaxial tension, and forming limits were assessed in the samples after a strain path change to biaxial or uniaxial tension, respectively. Biaxial stretching prestrain lowers the uniaxial tension forming limit, while uniaxial tensile prestrain raises the biaxial stretching forming limit. The differences in forming response were also correlated with distinct dislocation cell structures. The effects of strain path on formability were shown to correlate with predictions based on a redundant strain model and a critical thickness strain model.