The application of advanced materials in components and structures has evolved due to the need to reduce structural weight and improve performance. Other attributes of composite materials, such as corrosion resistance, excellent surface profiles, enhanced fatigue resilience, and tailored performance, have also been significant contributors to the rapid rise in composite materials application. As a result, these new materials are required to perform at higher stress levels than previous applications while also providing adequate levels of damage tolerance.
Type of damages
Manufacturing and defects/damages
Manufacturing damage includes anomalies, such as porosity, microcracking, and delaminations resulting from processing discrepancies. It also includes such items as inadvertent edge cuts, surface gouges, and scratches, damaged fastener holes, and impact damage. Manufacturing defects include:
Resin starved areas
Resin rich areas
Blisters, air bubbles
Examples of flaws occurring in manufacturing include a contaminated bond-line surface or inclusions, such as prepreg backing paper or separation film, that is inadvertently left between plies during layup. Inadvertent (non-process) damage can occur in detail parts or components during assembly or transport or during operation.
A part is resin rich if too much resin is used, for nonstructural applications this is not necessarily bad, but it adds weight. A part is called resin starved if too much resin is bled off during the curing process or if not enough resin is applied during the wet layup process. Resin-starved areas are indicated by fibers that show to the surface. The ratio of 60:40 fiber to resin ratio is considered optimum. Sources of manufacturing
Improper cure or processing
Mislocation of holes or details
Damage can occur at several scales within the composite material and structural configuration. This ranges from damage in the matrix and fiber to broken elements and failure of bonded or bolted attachments. The extent of damage controls repeated load life and residual strength and is critical to damage tolerance.
Many honeycomb structures, such as wing spoilers, fairings, flight controls, and landing gear doors, have thin face sheets which have experienced durability problems that could be grouped into three categories: low resistance to impact, liquid ingression, and erosion. These structures have adequate stiffness and strength but low resistance to a service environment in which parts are crawled over, tools dropped, and service personnel is often unaware of the fragility of thin-skinned sandwich parts. In-service defects include:
Cracks from local overload
Damages to these components, such as core crush, impact damages, and disbonds, are quite often easy to detect with a visual inspection due to their thin face sheets. However, they are sometimes overlooked or damaged by service personnel who do not want to delay aircraft departure or bring attention to their accidents, which might reflect poorly on their performance record. Therefore, damages are sometimes allowed to go unchecked, often resulting in the growth of the damage due to liquid ingression into the core. Nondurable design details (e.g., improper core edge close-outs) also lead to liquid ingression.
Damage and Defect Description
Defects and damage in structural components are common occurrences, whether they arise during material processing, component fabrication, or in-service action. The effect of the defect or damage in the composite component’s structural integrity is essential in understanding the criticality of the defect. The defects can be listed in terms of developing a common stress state. These common stress states are delaminations, transverse matrix cracks, holes or fiber fracture, and design variance
Bearing surface damage: Occurs at the contact point between a pin (fastener) and the hole edge. The damage is likely to contain fiber fracture, delaminations, and matrix cracking, and is a result of improper fastener installation, joint overload, or loose fasteners.