Automated hole expansion test with pneumatic crack detection

Abstract

The hole expansion test according to ISO 16630:2009 is a widely used method for determining the edge crack sensitivity, e. g. for advanced high strength steels (AHSS). However, the optical crack detection causes a subjective stop of the test, which is also difficult to automate. These existing disadvantages are eliminated by a newly developed method to stop the hole expansion test using pressure drop at an edge crack through the entire sheet thickness. The pneumatic crack detection ensures a high degree of objectivity and automation. The functionality of the new stopping method was experimentally proven by the comparison with the optical method, the investigation of the influence of different parameters and by testing of good part and reject batches from series production. The pneumatic method stops the test when through-thickness cracks are less pronounced up to a hole expansion ratio λ of approx. 35%, i. e. for materials sensitive to edge cracks. Thus, this variant provides more objective results than the optical method. The test results between the two methods are comparable in the range of λ ≤ 100%.