Failure is not one thing
A metal part usually fails in a limited number of ways, such as yielding, cracking or fatigue. A composite is a structure on a small scale, with fibers, matrix, interfaces and layers, and each of those can fail in its own way. A composite part may show several mechanisms at once, and damage often builds up gradually before the part finally gives way.
Knowing the main failure modes helps explain why composites are designed, tested and inspected the way they are. The descriptions below are qualitative. How and when each mode occurs depends on the specific materials, the layup and the loading.
Fiber breakage
The fibers carry most of the load, so fiber breakage is the failure mode that finally ends the useful life of a part under tension. Individual fibers have a spread of strengths, because each one contains small flaws. As load increases, the weakest fibers break first and their load passes to neighbors through the matrix. In a well-made laminate, isolated breaks are tolerated, and the part fails only when enough breaks gather to cause a fracture across the section.
Fiber breakage can also happen from sharp bending, from impact and from cutting or drilling damage. Carbon fibers are comparatively brittle and break with little warning, whereas aramid fibers absorb more energy, as the comparison of reinforcing fibers describes.
Matrix cracking
The matrix is generally weaker and more brittle than the fibers, and it is the first thing to crack in many loading cases. Matrix cracks often appear as fine cracks running parallel to the fibers in plies loaded across their fiber direction. They can form under repeated loading, under thermal cycling or under moderate overload.
A matrix crack in a single ply does not usually cause the part to fail immediately, since the fibers in other plies keep carrying load. But such cracks matter for several reasons. They reduce stiffness, they allow moisture or fluids to enter, and they can become starting points for delamination. The properties of the polymer matrix, including its toughness and its temperature limits, have a strong influence on how easily this happens.
Delamination
Delamination is the separation of layers within a laminate. It is often the signature failure of laminated composites. The bond between plies is held mostly by the resin, and the resin is relatively weak in the direction through the thickness of the laminate. Loads that act through the thickness, or that produce peel stresses near edges, holes and joints, can pull plies apart.
Common causes include:
- Impact. An object striking the surface can create a cone-shaped region of internal separation.
- Free edges. Stresses concentrate at cut edges and can start a split.
- Holes and fasteners. Drilling that is rough or too aggressive can start delamination at the hole. The issue is central to machining carbon fiber without delamination.
- Geometry. Curved sections under bending can generate through-thickness tension.
- Manufacturing defects. Voids, contamination and poor consolidation weaken the bond between layers. Process quality in layup and curing therefore has a direct effect.
Delamination may be internal and invisible, and it reduces compressive strength sharply because the separated plies can buckle more easily.
Fiber-matrix debonding and pullout
At a finer scale, fibers can separate from the matrix around them, which is called debonding, and can slide out of the matrix, which is called pullout. The strength of the interface controls this behavior. When the bond is very strong, cracks tend to run straight across fibers and the failure is brittle and flat. When the bond is moderate, cracks deflect along the fibers, and debonding and pullout absorb energy, giving a tougher failure with a rough, fibrous fracture surface. The balance is the subject of the discussion of the fiber-matrix interface.
Compression failures
Composites usually perform differently in compression than in tension. Under compressive load, fibers may micro-buckle, forming a narrow band where they kink. This is resisted by the matrix, so a stiff, well-bonded matrix helps. Damage such as delamination or a poorly aligned fiber lowers compressive strength much more than it lowers tensile strength. For this reason, compression after impact is a common design and test measure for structures such as aircraft skins.
Impact damage
Low-energy impact, such as a dropped tool or a stone strike, may leave very little visible mark on the surface while causing internal matrix cracking and delamination below it. This is sometimes called barely visible impact damage. It is a major reason designers set conservative allowable strains in primary structures and why inspection plans exist.
Different fibers respond differently. Tougher fibers and tougher resins can reduce the extent of damage, and layers of a more ductile material are sometimes added for protection. Sandwich panels, with thin skins over a light core, can be especially sensitive to local impact, which is part of the discussion of sandwich structures.
Fatigue and environment
Under repeated loading, composites accumulate damage in ways that differ from metals. Instead of a single dominant crack growing steadily, many small matrix cracks and delaminations may spread through the material, gradually reducing stiffness. Many carbon composites show good fatigue resistance, though results depend on the system and the load conditions.
Environment matters too. Moisture can reduce matrix properties, and heat can soften the resin. Ultraviolet exposure and chemical attack can degrade surfaces. Thermal cycling creates stress from differences in thermal expansion between plies and between materials. Residual stress left from curing adds to the load.
Other composite families
The same ideas apply, with differences, to non-polymer composites. In metal matrices, the matrix is often ductile and the failure modes include void growth and particle cracking. In ceramic matrix composites, the goal is for fibers to bridge cracks and prevent the sudden brittle failure that monolithic ceramics show. Concepts such as fracture toughness apply across all of these.
Finding and preventing damage
Several practices reduce the chance of unexpected failure:
- Design for damage tolerance, assuming that flaws or impact damage may exist and checking the structure can still carry its load.
- Control the process to limit voids, wrinkles and misalignment.
- Test coupons and components under realistic loads and environments, since materials testing is the basis of the design allowables.
- Inspect in service using ultrasonics, thermography, x-ray or tap testing. Nondestructive evaluation is the standard approach for finding internal damage.
- Analyze failures when they occur, using fracture surfaces and sections to identify the sequence of events, as in failure analysis.
The main point is that composites rarely fail in a single clean break. Understanding the stages of damage, from first matrix cracks to final fiber fracture, lets engineers design with appropriate margins and inspect where it counts.