When performing structural analysis, the results are always considering the best possible conditions. However, in real life, everything is not perfect. That is why the components have discontinuities when performing due to defects that appear in manufacturing. These discontinuities are known as defects. These defects have a great impact in mechanical properties in composites materials. The design and analysis engineer must keep in mind this factor when designing or analyzing it. Moreover, when the piece is manufactured, several quality controls have to be meet to accept or reject it.
What is a defect in composite materials?
A defect is any unintentional local variation in the physical state of mechanical properties of the structure that may affect the structural behavior of the component.
When defects appear?
Defects occur during three different phases:
- component manufacture
- materials processing
- during service use.
Most common defects in composite materials
The following defects are the most common in composite materials:
- Bearing surface damage: a fastener or pin contacts with the laminate causing delamination, fiber fracture and matrix cracking.
- Blistering: when air is trapped within the laminate and cannot escape. It causes delamination.
- Contamination: foreign materials are introduced accidentally in the laminate during the manufacture of the component.
- Corners: it is important to make a good transition in corners because of several defects occur in composites. Some examples are: corner crack that is when the matrix have cracked. Corner splitting: delamination between the plies generally caused by out-of-plane stresses.
- Cracks: occur only in the matrix when it has been overstressed in various loading conditions. Produce stress concentration which can lead to delamination.
- Debond: excessive local shear-transfer stresses causes separation at the interface which results in loss of shear transfer and degradation of the overall strength of the laminate.
- Delamination: The separation of the layers of material in a laminate. It may occur at any time in the cure or subsequent life of the laminate and may arise from a wide variety of causes. It can be caused by impact damage, free edges (where the interlaminar stresses are high due to mismatched Poisson’s ratio) and arresting of through-the-thickness matrix cracks at a ply interface, after which the crack runs parallel to the interface,
- Fracture: when the fibers cracks. Impacts, cuts and penetration of laminate can affect to fracture fibers which reduces the tensile strength.
- Moisture: absorbed by the matrix which degrade its properties making it softer.
- Over-aged prepreg: prepreg resin has aged and when it is finally cured, it will not provide adequate interface adhesion.
- Porosity: caused by voids. This voids results from poor process control over-aged material, moisture in prepreg. They produce small stress concentrations which reduces the tension, compression, interlaminar and bearing properties, specifically compression at elevated temperatures.
- Surface swelling: Blisters caused by the use of undesirable solvents on the outer ply.
- Thermal stresses: they occur in the curing process. Residual stresses results when the components are removed from the autoclave during cooldown.
- Voids: are trapped air or other volatiles in the resin. They are caused by poor process control and can be localized or uniformly distributed (porosity). They are generated from: dissolved air within the resin, air stirred into the resin, trapped air in the filament bundle, residual solvent carrier, reaction products form the curing process and volatilization of low-molecular-weight components of the resin.
