The more tests the more you know and understand (these things have oodles of information collected)...With one succesful test you only know that it's good enough. With multiple tests you know by how much.
lomapaseo,
In the aircraft industry, structural qualification tests are not really performed to collect data or to investigate how a particular component will respond to loads. Instead, the primary purpose of structural qualification testing is to validate the analysis used to design the component. As you noted, there is lots of data acquired during a structural test, and this data is used to improve the analytical models. A complex structure like an aircraft wing might be analyzed for 50 or more different load cases, but it is not tested for each of those load cases. I don't mean to discount the value of testing, but when an aircraft design is certified it is the analysis results that really count. The testing is really just a check to ensure that the analysis work was accurate.
As for structural cracks, there should not be any if the aircraft is maintained and operated within design parameters. However, aircraft structural designers acknowledge that cracks may occur for any number of reasons, and they perform detailed analyses just for these conditions. All critical aircraft structures have a Fracture Control Plan, which outlines how the structure will respond to cracks, how cracks will be prevented, and how cracks can be detected when they occur. Aircraft companies employ analysts specializing in fracture mechanics, as well as numerous QA and manufacturing process engineers to develop fracture control plans.
DTDHandbook | Guidelines for Damage Tolerance Design and Fracture Control Planning | Guidelines for Damage Tolerance Design and Fracture Control Planning