Limiting Dome Height Test Research Articles

Effects of electrodeposited zinc coating on the formability of drawing-quality aluminum-killed sheet steel

Although sheet formability depends significantly on the properties of the base metal, surface coatings used for corrosion protection can affect the friction at the tool/sheet interface and, therefore, influence the amount of deformation. In an effort to better understand the effects of surface coatings on formability, limiting dome height testing was done on samples of drawing- quality aluminum- killed sheet steel with laboratory- produced electrodeposited zinc coatings of different characteristics. The results showed that the formability of zinc- coated products was reduced compared to the uncoated steel in all strain states. It was also shown that zinc coating crystallographic texture strongly influences sheet forming behavior and may be as important a parameter as surface roughness in friction characterization.

A tensile model analysis of punch stretch tests: The effects of friction and strain hardening on measurement sensitivity

A theoretical model is presented to predict punch height as a function of sample strain and friction at the sheet-punch interface for punch-stretch forming of narrow strips in a Limiting Dome Height (LDH) test. Examination of the strain distribution in narrow punch strips reveals the following trends in LDH test sensitivity when punch height at failure is the measure of formability. First, the sensitivity of out-of-plane punch tests to material deformation behavior decreases with punch travel. Second, the LDH test is more sensitive to variations in friction in lubricated tests than in dry tests, and thus, dry tests may appear as more reproducible than lubricated tests. Third, strain hardening differences between sheet materials are directly manifest through variations in the distribution of strain in the strip. This effect of strain hardening, coupled with the geometry of the LDH test, leads to an almost linear relation between sample strain at instability (or failure) and punch height as experimentally observed. Finally, the sensitivity of punch height measurements to variations in n decreases as n increases.

Surface preparation for the LDH test

The present work is aimed at developing a surface preparation technique that reduces or eliminates random variations often found in the limiting dome height (LDH) test without diminishing the ability of the test to discriminate between different batches of material. Degreasing with a nonpolar solvent (mineral spirits) followed by the application of a low viscosity mineral oil (mineral seal oil) gave the best results and was adopted as a standard. With this preparation, the LDH was as low as with sheets degreased with chlorinated solvents, while scatter was reduced. Surfaces prepared by wiping with a clean towel gave higher LDH values, corresponding to a surface coated with SAE 30 oil. None of the preparation methods could completely eliminate or mask residual films left by oils compounded with boundary or EP additives. Day-to-day variations were evident with all preparation techniques employed over a period of three months. However, when all LDH means were plotted as a function of sheet location in the coil, the means defined a curve which—within the limited coil length represented by the total sample quantity available—had a period of about 15 m (45 ft). Thus, random variations observed in the laboratory and press shop can be attributed to small variations in mechanical properties, revealed by the great sensitivity of the LDH test and not necessarily detectable by tensile tests. Surface preparation may contribute to random variations but such contribution can be eliminated by the standard preparation technique.

Variables affecting dome test results

The limiting dome height (LDH) test has been proposed as a simulative test to evaluate the stretch formability of sheet metals which shows good correlation with field forming performance. The present study investigates how deviations from the LDH test procedure and test geometry given in the literature affects the dome test results for aluminum body sheet alloys. The results should be helpful in explaining between-lab variability and helpful to laboratories which cannot perform the LDH test according to specification, because of equipment limitations, in interpreting their data.

Evaluating the GMR-Limiting Dome Height Test as a new measure of press formability near plane strain

A number of tests have been proposed as predictors of press shop formability. The most common of these are the Olsen cup, tensile, and hardness tests. However, none of these tests are very successful. Recently, A.K. Ghosh proposed the Limiting Dome Height test (LDH) to predict formability since this test modifies specimens to produce the same strain state at failure as found in a particular stamping. This report describes a correlation study in which specimens were collected from lots of steel and aluminum which exhibited either good or bad press performance in automotive stampings. The failure mode, in all cases, was splitting which initiated within ± 2 pct of plane strain. These specimens were evaluated by the LDH, Olsen, and tensile tests and compared to the actual press performance. Only the LDH test adequately separated the acceptable sheets from the bad.