Evaluation of Machining Defects in a Composite Laminate by Combining Non-destructive and Tensile Testing

Abstract

Although composite structures can be molded to nearly final shape, they generally require different finishing operations, such as drilling and trimming, to meet dimensional tolerances and enable assembly. Conventional drilling and trimming methods tend to induce internal damage to the composite, mainly in the form of edge delamination, matrix thermal degradation, fiber fracture or fiber pullout. As the damage mechanisms and progression are not yet fully understood, it is crucial that the appropriate tools be used to verify the structural integrity of composite parts. NDT techniques allow observing the extent of damage in composites; however they have limited resolution for shallow edge defects. On the other hand, well-designed tensile tests, made from coupons taken over extra materials intentionally left around a part, are sensitive to the reduction of mechanical properties of trimmed composite samples due to, among others, thermal degradation and smearing of the matrix. In that respect, the main objective of this work is to evaluate the possibility to detect and quantify small edge machining damage using the ultrasonic testing (UT) method and combine this approach with mechanical testing to determine the influence of machining on the quality of trimmed parts. Indeed, a combination of mechanical testing and NDT techniques could provide an interesting avenue to appropriately certify the quality of a machined composite part. A non-destructive testing (NDT) method and tensile mechanical testing will thus be used to characterize the extent and the influence of damage induced by the trimming operation of quasi-isotropic carbon/epoxy composites. First, the resolution and the precision of ultrasonic testing (UT) are evaluated with composite samples comprising internal (artificial) defects. Next, this method is used to evaluate machined samples prepared by using high performance machining tools for composites. It is shown that the UT technique is able to detect the very small edge defects induced by trimming. Then, mechanical testing of narrow tensile coupons is performed to compare impact of milling, abrasive cutting and sanding on the quality of trimmed surfaces. Results suggest that some type of damage could efficiently be quantified when coupons of reduced width is used to magnify the influence of defects on the measured strength.