Forming processes are among the most frequently used in the fabrication of perforated metals. Formability is the term used to evaluate the capacity of a material to withstand the stretch/draw stresses of forming before splitting occurs. We have had a long-standing interest in testing formability because the standard mechanical tests to evaluating strength in materials, such as the tensile tests, had not correlated well with what actually occurs in forming operations.
Standard formability tests, such as the Limiting Dome Height test (LDH) showed good correlation to press performance, but required high capital investment (upward of $200,000) and lacked the ability to provide consistent from one location or machine to another.
Ohio State University and Interlaken Technology Corporation, with sponsorshipp by the Edison Materials Technology Center (EMTEC) designed a reproducible, accurate formability test and an inexpensive formability testing machine. Following a study conducted by General Motors Corp. that found that 80% of splitting failures occur near the plane strain condition, the OSU Formability Test reliably reproduces this strain state.
After several years work, the team succeeded in designing and refining both the machine and tooling for measuring formability to yield testing results that had one tenth the variation found in the old LDH tests, with time to peform reduced by a factor of 5 to 10, and a machine capital cost of approximately $50,000. The formability test machine has now been commercialized and more than 10 of them are in use in industry during the first years of availability.
These improvements make formability testing a practical reality for all sizes of forming companies.
Measure the formability of perforated sheets of stainless steel and aluminum and to develop the following relationships:
Each sample was cleaned with acetone followed by methanol and drying. The sample was lubricated with oil in the region of punch contact. The sample was placed in the OSU formability tooling and clampled. The punch was then advanced at a rate of 1 mm/sec until failure occured.
For each test the measure of formability was the punch height at which the material sustained a maximum load was obtained from the recorded load versus displacement data. Three tests were performed on each of the 90 test conditions requested and the punch heights recorded for each were averaged.
Table 1 summarizes the materials and perforation spacing tested. For each hole diameter, different hole spacing giving rise to different percent open areas which have been summarized.
Table 2 summarizes formability measurements for the 90 tests performed. The results are grouped by the pattern and size of holes, then ordered by material and gauge. The mode of failure (either single failure,.S, or multiple failure, M) and the places where failure was observed (near die, D, near punch, P, or center, C). A blank in these columns means that the failure was not uniquely located. The effect of each of the variables on how they affect formability has been considered and explained with the help of plots and charts in the following sections.
The % open area was plotted against the punch height at maximum load. Charts 1-6 show this variation for each of the materials. Each plot contains a series of curves for various hole sizes and thickness, as noted in the legend of the plot. The hole size is given in mm and the thickness is given as the gauge number.
The results show that there is a drop in formability as the % open area increases. However for aluminum 3003, the trends were not very clear. The reason for this could be that the punch height at which these were failing was too small and the specimen failed in a very short time. Hence the scatter in the values is expected to be high and the test may not be sufficiently accurate to deduce a trend.
