The testing of properties for Composites laminates and structures can be tricky. Much of this is due to the complex base of ingredients and makeup of the laminate. When we are putting together a combination of resins, reinforcements, coring, fasteners, curing agents, fasteners, etc. we can radically affect the properties of the FRP structure. This can be beneficial and disadvantageous.
ASTM test specifications can be used to determine properties such as stiffness, chemical resistance, impact strength, etc. There are several designed specifically for FRP laminates. These can be very informative, especially when developing and comparing potential laminate designs.
House tests can also be developed to test for more specific properties on test panels and the overall structure.
A flat and square test speciment may test out great for certain properties. When it is put into actual application, it may have differenet features which make it stronger or weaker than the flat test panel. Things such as holes, notches, square corners, curves, and other geometric shapes have a wide range of effects. They may increase localized thickness from overlap joints, may cause stress concentrators, or may have thin areas because of an area being difficult to reach.
One of the projects I worked on recently involved the conversion of the processing from hand layup to resin infusion processing. For product liability we wanted to prove that we tested the laminates for strength, and doing an overall structure test was out of the question. But testing flat and square infusion panels would just give us a number. So we setup a baseline with the same size flat and square test panels made with the old laminate schedule using hand layup. This old laminate schedule had been very suitable in its application. We just needed to ensure that the new infused laminate schedule exceeded the strength of the original hand laid laminate schedule.
We setup this direct comparison between laminates by removing the variables of the part shape itself. We also added some safety factor to ensure for any other variables that may be involved. Our supplier tested the panels for us, giving us official ASTM results. We worked through a few different scenarios with the new laminate schedule, received test results, and made our decision. Future inquiries into our engineering work can be backed up with technical testing data.