Composites- Failure Mechanisms

Question 1

Types of failure

Answer 1

Matrix failure
Fibre failure
Interfacial adhesion failure
Failure of adhesion between plies (delamination)
Often a mixture of more than 1 of these

Question 2

Loading modes failure can happen under

Answer 2

Tension, compression, shear, flexure

Question 3

How does failure occur in short and long fibre composites when fibres are aligned or random?

Answer 3

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

Question 4

The two cases for unidirectional tensile failure under longitudinal loading

Answer 4

Fibres are more brittle than the matrix
Matrix is more brittle than the fibres

Question 5

How does tensile failure proceed under longitudinal loading

Answer 5

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

Question 6

Broom-like failure

Answer 6

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

Question 7

Non-brittle failure under longitudinal loading

Answer 7

Some materials fail in a non-brittle fashion. E.g aramid fibres have a fibrillar failure mechanism

Question 8

Two cases for unidirectional tensile failure under transverse loading

Answer 8

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

Question 9

What determines which failure type occurs first for off-axis loading?

Answer 9

Loads are resolved along or across the fibres and the critical conditions for each direction determines which occurs first. Often transverse failure first

Question 10

Unidirectional compressive failure

Answer 10

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

Question 11

Kink-band formation models

Answer 11

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

Question 12

Unidirectional flexure failure

Answer 12

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

Question 13

Sequence of events to failure for cross-ply laminate under tension

Answer 13

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

Question 14

Why doesn’t delamination have to be across the same interface?

Answer 14

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

Question 15

Cross-ply laminate failure under compression sequence

Answer 15

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

Question 16

Angle-play laminate failure under tension example of 0/+45/-45/90

Answer 16

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

Question 17

Angle-ply laminates under compression and flexure

Answer 17

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

Question 18

Example of failure sequence for 0/+45/-45 angle ply laminates

Answer 18

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.

Question 19

Woven composite failure

Answer 19

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.