Characterization of interlaminar fracture in composite materials : a case study approach

Composite materials are replacing standard engineering metals and alloys for many applications. Their inherent ability to be custom tailored for any application has made fiber reinforced composites a very viable material option. Their superior specific strength and stiffness characteristics have made them very competitive in the aerospace industry. The primary limitation of fiber reinforced composites is fracture toughness, specifically delamination. Delamination failures are common due to the nature of composite construction. A variety of manufacturing techniques are available to make composites. Generally, all these methods employ a layered stacking of fibers in a primary plane. The interface between these layers is typically not reinforced with fibers and is the source of delamination or interlaminar fracture. Porosity and other manufacturing related defects also introduce nucleation sites for delamination. Methods exist to evaluate and quantify inter-laminar fracture toughness, both experimentally and analytically. The material property that best represents resistance to delamination is the strain energy release rate (Gc). This can be experimentally obtained and analytically predicted with some success. The primary focus of this study was the development of a process that would characterize and address interlaminar fracture in composites. This common mode of failure is not easily accounted for or mitigated. The design process developed considered two distinct approaches. Both methods required a database of material properties to compiled. The primary design approach was a “screening” methodology that employed comparative testing to down select composite architectures based on design drivers and applications. Another approach that was also investigated was a “predictive” or analytical approach. This process consisted of using closed form solutions or specifically finite element modeling methods to determine the strain energy release rate for given modes of failure. It was determined that analytically predicting crack growth or damage in complex structures will require research and study beyond this thesis. However, the screening approach provided meaningful results repeatedly. This screening approach was applied to several case studies. Each case study was a separate project that investigated a unique topic relating to interlaminar fracture of composites. The process was used to satisfy sponsor needs and each project in turn provided a means to validate or improve the process. Each case study was also used to advance and validate the analytical techniques as well. Four case studies will be presented and the technical contributions of each will be discussed. 1. Evaluating composite Aerofan blade material for Pratt&Whitney 2. Investigating composite honeycomb fuel tanks for the X-33 3. Characterizing Aerospace resin systems for ACG 4. Understanding composite to metal bond behavior The four case studies were unique investigations that required interlaminar fracture characterization and analysis. In almost all cases delamination was the source of primary structure failure.