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.
Blistering: Blistering can occur anywhere in the lamina and is caused by the expansion of trapped gases within the lamina. Surface blisters can occur due to chemical attacks or localized heating of the matrix
Contamination: The inclusion of foreign materials in the laminate such as peel ply or backing paper, usually between plies, during fabrication of the component. Depending on the size and extent of the contamination, there will be a varying effect on the component
Corner crack: Matrix crack, either perpendicular or translaminar to the ply. Cracks are caused by the same reasons as corner splitting.
Corner/edge splitting: An edge delamination is typically due to edge impact. These delaminations are cracks between plies that run parallel to the ply interface.
Corner radius delaminations: Matrix cracks running parallel to the fiber axis in the corner radius of a component, usually a stiffener
Cracks: Matrix cracks are characterized by localized partial through-the-thickness cracking. Many cracklike failures in continuous fiber composites and their laminates are contained within the interface planes, where the matrix material properties dominate the fracture response.
Creep: The plastic deformations caused by sustained loading, usually at high temperatures. Creep is a matrix dominated failure, and therefore it tends to affect compression and shear performance to a greater extent than tension
Crushing: Local indentations or surface dents caused by impact damage. It may be a sign that there is further internal damage, such as delaminations, fiber fracture, or matrix cracking.
Cuts and scratches: Cuts and scratches can be treated as surface damage. The severity of surface scratches and notches depends on their width, depth, and orientation to the fibers or loading direction.
Damaged filaments: Broken filaments knots, splices, split tow, fiber separation, hollow fibers, or interrupted fibers all come under the heading of damaged filaments.
Delaminations: Delaminations are one of the most frequently encountered types of damage found in advanced composite materials. Delaminations are a matrix defect, where in-plane matrix cracks propagate between plies of a laminate or within a laminate, where cracks run parallel to the fiber direction.
Dents: Indentations at the point of contact of a moving body. They are distinct from crushing because no fibers are broken.
Edge damage: Caused by the mishandling of components. Common features of edge damage are splitting and delaminations. Edge delaminations can arise from high out-of-plane normal or shearing stresses produced in the vicinity of free edges
Excessive ply overlaps: Occurs when the ply is not correctly trimmed during assembly. This can result in laminate dimensional tolerance errors
Fastener holes: There is a wide range of fastener hole defect types e.g. Fastener removal and reinstallation, Hole elongation, Improper fastener installation and seating, Missing fasteners, Overtorqued fasteners, tilted countersink holes, etc.
Fiber distribution variance: Unevenness of fiber distribution or improper yarn spacing could change the laminate properties to the extent that the laminate load response will be different from design requirements.
Fiber kinks: Sharp edge buckling of fibers within the matrix. Previous studies have concluded that kinking is a direct consequence of micro buckling.
Fiber/matrix debonds Separation at the fiber/matrix interface, this will result in loss of shear transfer and degradation of the overall strength of the laminate.
Fiber misalignment: Occurs when there is either misorientation of the ply, deviation from predetermined winding patterns, or washout of fiber from excessive resin flow.
Fracture: Any type of cracking in the fibers, such as fiber kinks. Severe matrix cracking is often associated with fracture.
Impact damage: The principal cause of penetration and the amount of damage depends on the energy level of the projectile involved.
Marcelled fibers: Marcelling is the waviness of the fibers (Figure A2.29). Waves in the fibers will degrade the lamina compression strength due to a decrease in micro buckling resilience.
Matrix crazing: Multiple cracks in all directions within the resin. Nonstructural matrix materials are more prone to this type of defect.
Missing plies: An incorrect stacking sequence is the consequence of missing plies
Moisture pickup: Moisture is absorbed into the laminate through the matrix. Moisture contamination is usually contained in the outer plies, where degradation of the resin properties such as softening will reduce the stiffness and the fiber/matrix interface bond strength.
Over-aged prepreg: A situation where the B-staged prepreg resin has aged or partially set to a point where the final cure will not provide adequate fiber/matrix adhesion and volatile evacuation. Reduction in the strength or stiffness of the laminate will result.
Over/under cure: Occurs when the curing process is too long or too short in time and/or when too high or low a temperature is used
Pills or fuzz balls: These are prepreg deviations, also known as “furring” of the fibers
Ply underlap or gap: Occurs when the ply size is too short. This will cause localized inadequate strength and stiffness load response of the component as opposed to the design requirements
Porosity: Evidenced by the presence of numerous bubbles (voids) within the laminate,
Surface damage: Notches or any other surface irregularity resulting from mishandling or poor release procedures are termed as surface damage
Surface oxidation: Can result from lightning strikes or battle damage.
Surface swelling: Blisters caused by the use of undesirable solvents on the outer ply are examples of surface swelling
Thermal stresses: Although thermal stresses are not a true defect as such, the resulting residual stresses in the laminate are extraneous to the component’s design and thus may affect the component’s structural performance
Translaminar cracks: Through-the-thickness cracks where fibers are broken are translaminar cracks
Debond: Debonds are separations in a secondary adhesive bond or sandwich facing.
Variation in density: Variation in the density of the laminate is associated with resin inconsistencies, voids, or porosity.
Variation in the fiber-volume ratio: Resin-rich or resin-starved areas produce variations in the fiber-matrix ratio (fiber-volume ratio). The fiber-volume ratio is an important parameter for determining the strength and stiffness of a laminate by micromechanics.
Voids: Voids are trapped air or other volatiles in the resin. Voids are a single bubble, whereas porosity is a cluster of several microscopic voids
Warping: Warping is a result of a detailed or assembly part mismatch. Also, residual thermal stresses remaining in the laminate after fabrication can produce warping
Wrong materials: Wrong materials used in the fabrication of the component are a blueprint error
Composite Structure Repair Procedure
Advanced composite materials provide the necessary damage tolerance through relatively low, applied design strains. However, defects and damage still occur in composite materials, and it is the assessment of defect and damage criticality and the subsequent repair requirements that are currently challenging for operators of composite materials. When composite materials components are damaged or defective in some way, the engineer/technician needs to determine the size, shape, depth, type, and extent of the anomaly and restitution approach. A typical repair procedure is shown below
Locate the damaged area
Assess the extent of damage
Evaluate the stress state of the damaged area stress state
Design the repair scheme
Remove damage and repair structure
Fabricate and prepare the repair scheme
Apply the repair scheme
Conduct post-repair quality checks
Document repair procedures
Monitor the repair region
Of immediate importance is the ability to identify the damage and determine its extent by some suitable nondestructive inspection (NDI) technique. Most, if not all of the standard NDI techniques currently used require high levels of operator experience to successfully apply the NDI technique and interpret the results. This book is written to provide an in-depth study of defects and damage in composite materials. It is significantly focused on the defect and associated structural response to the presence of defects.
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