Specifying Fracture Toughness and Reference Temperature for Successful Stamping Operations

The goals of this project were: (a) to measure the tensile and fracture toughness properties of two high-strength thin steel sheets and (b) to determine the reference temperature (T₀) of each steel sheet using in-situ Automated Ball Indentation (ABI®) tests conducted at room temperature. For comparison, miniature tensile specimens were machined from each sheet in the longitudinal and transverse orientations. All mechanical tests were conducted at room temperature (22°C).

The room-temperature ABI® tests produced true-stress versus true-plastic-strain curves that are in good agreement with those from the miniature tensile specimens. The yield and ultimate strength values from the ABI® tests were within ±2% of those from the tensile specimens.

The ABI-determined T₀ values for the two steels (A and B) were -16°C and -14°C, respectively. Per ASTM Standard E1921, and since the T₀ values were within 20°C for the two steels, they are considered of the same material and the average T₀ for both steels is -15°C. This T₀ value of -15°C shows that stamping at room temperature falls in the transition region (ductile-to-brittle transition) with large variations in fracture toughness values (approximately from 100 MPa√m on the 5% probability limit curve to 240 MPa√m on the 95% limit curve).

This explains why cracking occurred at different places during the stamping of these sheets; although the two steels meet the specified tensile strength values and ductility properties. Hence, this proves that tensile properties requirements alone are not sufficient and a Reference Temperature (T₀) range should also be specified for steel acquisition.

However, because the steel sheets are thin, destructive fracture toughness specimens cannot be machined or tested. In contrast, the ABI® tests produce valid fracture toughness results and valid Reference Temperature even for 1-mm thick sheets using a 0.76-mm diameter ball indenter (because the final indentation depth of 0.09-mm is less than 10% of the thickness of the thin steel sheet).

Using the Median Fracture Toughness Master Curve Equation per ASTM Standard E1921 (KJc = 30 + 70 × e^[0.019(T-T₀)]), the median fracture toughness for a T₀ of -15°C would be 171.4 MPa√m while a T₀ of -40°C would produce a median fracture toughness value of 257.4 MPa√m (i.e., a 50% increase). A lower Reference Temperature increases the median and the 5% probability values of fracture toughness at the stamping operation temperature and reduces the formation of cracks at high stress intensity value sites during the stamping operation. This will obviously improve the stamping production efficiency and reduces the number of reject-steel coils.

To prevent crack formation during the stamping of thin sheets, the highest stress intensity factor (at the severest bend of the stamping die) must be lower than the initiation fracture toughness of the steel sheet. For high strength steel sheets, although the probability of crack formation is small, the two concerns of potential low fracture toughness and/or relatively high reference temperature must be addressed.