Composites- Failure Mechanisms Flashcards
Types of failure
Matrix failure
Fibre failure
Interfacial adhesion failure
Failure of adhesion between plies (delamination)
Often a mixture of more than 1 of these
Loading modes failure can happen under
Tension, compression, shear, flexure
How does failure occur in short and long fibre composites when fibres are aligned or random?
Failure processes in short fibre generally similar to long fibre composites.
Aligned fibres: under longitudinal loading if fibres are sufficiently long they can fail, under transverse loading failure occurs at the interface with the matrix.
Randomly aligned fibres: loads are resolved along or across the fibres and generally the composite fails as a result of a combination of multiple failure events
The two cases for unidirectional tensile failure under longitudinal loading
Fibres are more brittle than the matrix
Matrix is more brittle than the fibres
How does tensile failure proceed under longitudinal loading
Get an initial fracture at a point. Crack propagation speeds up and leads to an increasingly rough surface. Radial lines emanate from the fracture origin. Sequence of mirror->mist->hackle from close to origin to further away as surface gets rougher. This sequence can occur in individual fibres and overall for the whole composite
Broom-like failure
Unidirectional tensile failure under longitudinal loading. Weak fibre matrix interface leads to broom-like failure. Lots of fibres being pulled out of matrix so lots of exposed fibre ends
Non-brittle failure under longitudinal loading
Some materials fail in a non-brittle fashion. E.g aramid fibres have a fibrillar failure mechanism
Two cases for unidirectional tensile failure under transverse loading
The interfacial bonding is weaker
The matrix is weaker
As with uniaxial loading the weaker of the two determines how the composite will fail. Occurs as much lower load than longitudinal load
What determines which failure type occurs first for off-axis loading?
Loads are resolved along or across the fibres and the critical conditions for each direction determines which occurs first. Often transverse failure first
Unidirectional compressive failure
In compression individual fibres suffer from micro-buckling which eventually lead to the formation of kink bands (bands of broken fibre segments) and eventual composite failure. Kink-band formation can be initiated by the presence of a void
Kink-band formation models
Rosen: kink-bands form when the load applied is higher than the support provided by neighbouring fibres. Buckling of multiple adjacent fibres before kink-band.
Argon: some ply splitting occurs before the kink-bands form. Splitting, buckling, kink-band
Unidirectional flexure failure
Fibres on one side are compressed and on the other side in tension. Has a neutral axis in between. Both tensile and compressive failure events can be observed with opposing directions about a neutral direction
Sequence of events to failure for cross-ply laminate under tension
1- Ply splitting (transverse matrix cracking) occurs in transverse plies, the axial plies often remain undamaged as there is a low stress on the transverse plies.
2- as applied stress increases matrix cracking in axial plies occurs, caused by Poisson’s contractions, where cracks on axial and transverse intersect delamination can begin.
3- final failure occurs as the applied stress reaches a critical level
Why doesn’t delamination have to be across the same interface?
A delamination crack along one interface can be steered by the presence of a transverse matrix crack through a transverse ply to its other interface
Cross-ply laminate failure under compression sequence
1- Delamination at matrix cracks, unequal stresses in the 0 and 90 plies.
2- micro-buckling and kink-band formation, delamination removes support for the plies which causes the fibres to buckle into the space.
Transverse matrix cracking may be caused by delamination as in the tensile case or the buckling of fibres
Angle-play laminate failure under tension example of 0/+45/-45/90
Fails through a combination of previous mechanisms.
a) matrix cracking in 90s
b) delamination of 90s
c) matrix cracking of 45s
d) delamination of 45s
e) final failure of 0s
Angle-ply laminates under compression and flexure
Under compression and flexure they are prone to delamination which splits the laminate into a series of sub-laminates.
Under compression: each sub-laminate can form a kink-band or undergo further delamination.
Under flexure: each sub-laminate has one surface under tension and one under compression
Example of failure sequence for 0/+45/-45 angle ply laminates
When angle-ply loaded, generate shear forces
a) an off-axis ply split occurs in a 45 layer adjacent to a 0 layer.
b) longitudinal ply splits develop in the 0 layer in response.
c) support is lost and the fibres can micro-buckle into the split creating a transverse ply split.
d) the process is repeated creating a stepped failure process.
Woven composite failure
Complicated failure mechanisms due to their structure. In tension matrix cracking occurs in weft tows first which initiates failure in the load-bearing warp tows. In compression the existing crimp in the tows influences the failure process by promoting micro-buckling of the fibres, delamination and ultimately failure. Additionally, interstitial sites in woven composites act as weak points and promote failure.
