May/June_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 0 2 6 test results between labs, the VDA has used its strength and agility to de- mand automotive suppliers test to their requirements, which, to use the previous example, would require the operator to test using the strain rate method (Method A1 or A2) of ISO6892-1 instead of EN10002, or even ISO 6892-1 (Method B). The VDA has also intro- duced strict requirements regarding many other aspects of the test setup, none of which conflict with the require- ments of ISO6892-1, but which add to them by defining the best setup param- eters rather than leaving these deci- sions in the hands of the test operator. The VDA’s tensile test requires test- ing to the following parameters: • Pre-stress • Test rate • Change of test rate • Data sampling frequency • Modulus of elasticity • Tensile strength • Plastic strain ratio (r-value) • Strain hardening exponent (n-value) The outcome of all these require- ments is that if operators implement them correctly, results between labs testing the same material may be con- siderably more consistent. During Instron’s round robin with the contrib- utors of the VDA/GDA standards, it was easy to see the benefits of adopting such an approach as the test data we examined were extremely comparable despite materials being tested using different test systems and software. The VDA method that covers testing to ISO 6892 and other related ISO methods is now available to any Instron customer with Bluehill Universal software, and we can help to advise on how best to meet these stringent requirements. In addition to tensile testing, the VDA—in consultation with the GDA— has also excelled in creating methods for determining formability properties of aluminum. The most common stan- dard is VDA 238-100, which is an adap- tation of the ISO 7438 bend test. This type of test is often referred to as a “hemming bend test.” The main differ- ence between these standards is the shape of the punches, or upper anvils. In VDA 238-100, the angle of the punch is much more acute than it is in a typi- cal ISO 7438 bend test. This is done to more closely simu- late all of the require- ments in the auto- motive industry dur- ing the manufactur- ing of new vehicles. However, a prob- lem came up when the VDA present- ed round robin data showing that the acute angle in punch size had been caus- ing significant vari- ability in results, and that results from labs using these fixtures were incredibly in- consistent. After fur- ther investigation, it was discovered that many labs were us- ing fixtures adapted from their existing ISO 7438 test setups, which turned out not to be sufficient. Due to the acute angles of the punches in the VDA standards, if a compressive load of 3kN is applied, the lateral force into the anvils can be sig- nificantly greater (two-to-three times) which will cause the fixture to deform during the test. In order to help reduce this error in results, VDA 238-100 now dictates that the device must meet a certain stiffness to prevent the fixture from deforming. Instron has worked with many customers to supply these fixtures, ensuring compliance, and re- ducing variability in the results. Metals testing standards are con- stantly evolving, and the VDA has taken an aggressive approach to implement- ing change for the better. Despite the challenges of meeting these new and more stringent requirements, automo- tive manufacturers and OEM suppliers will ultimately benefit by producing test results that are more repeatable across the entire automotive supply chain. ~AM&P For more information: Matt Spiret, High Force Applications, Instron, 825 Uni- versity Ave., Norwood, MA 02062, 800- 877-6674, Matthew_Spiret@instron.com, www.instron.com. Reference 1. https://www.vda.de/en/association/ about/future-comes-as-standard.html A bend fixture for VDA 238-100, also known as a hemming bend fixture. Hemming bend fixture with punch fully inserted.