by

Dr. Md Tanwir Alam(Assistant Professor)

Department of Mechanical Engineering

Glocal University, Saharanpur(India)

4/23/2020 4:20 AM 1

Failure of Composites UNIT-4

FAILURE MECHANISMS

Failure in fibers (ductile-matrix composites, e.g., polymer-matrix and metal-matrix composites), so a high interfacial strength is desired.

Failure in matrix (brittle-matrix composites, e.g., ceramic-matrix and carbon-matrix composites), so a low interfacial strength is desired (to allow cracks to deflect along fiber-matrix interface, thereby allowing fibers to pull out for the purpose of increasing the toughness)

Introduction to Failure

Failure of composites is, unlike metals, a complex multi‐stage process. Failure of a composite sample may get triggered in a certain “mode”, but its propagation and final failure modes may be significantly different.

In a large number of cases, composite failure gets initiated internally, and it is only once failure has propagated beyond a certain extent, that changes in composite’s behavior and appearance are observed.

4/23/2020 4:20 AM 3

4/23/2020 4:20 AM 4

The internal failure of a composite sample could be:

Breaking of fibers

Development of micro‐cracks in matrix

Debonding between fibers and matrix

Delamination, i.e. separation of different layers of a laminate

4/23/2020 4:20 AM 5

It is observed that many defects which may arise from composite materials in fibers, matrix and lamina. These defects, if they exist include misalignment of fibers, cracks in matrix, non uniform distribution of the fibers in the matrix, voids in fibers and matrix, delaminated regions, and initial stress in the lamina as a result of its manufacture and further treatment. These defects tend to propagate as the lamina is loaded creating an accelerated rate of failure.

4/23/2020 4:20 AM 6

Causes of Failure in Composites1. Fibre-matrix debonding 2. Fiber and/or Matrix breaking3. Development of microcracks in matrix and/or fiber4. Fibre misalignment 5. Density variation (due to resin distribution) 6. Cut or broken fibres7. Improper curing of resin 8. Delamination 9. Impact damage (tool drop) 10. Voids and Inclusions 11. Abrasion and scratches 12. Machining problems

4/23/2020 4:20 AM 7

Failure mechanisms in fiber composites are a function of many parameters such as constituent properties, lamination geometry, state of stress, etc.

Failure can be viewed at the microscopic level of fiber matrix interaction, at the single lamina level and at the macroscopic laminate level.

The basic failure mechanisms at the microscopic level include tensile, compressive or shear fracture of the matrix, bond failure of the fiber matrix interface and tensile or compressive (buckling) failure of the fibers.

4/23/2020 4:20 AM 8

In angle ply laminates the first failure in the form of cracks parallel to the fibers occurs when the strain limit of the weakest ply (usually at 90 degree to the load) is exceeded. The subsequent behavior of the laminate depends on its notch sensitivity and interlaminar shear strength.

Failure patterns are highly influenced by free edges where a three dimensional state of stress exists with high localized interlaminar shear and normal stresses.

A variety of failure modes, not always predictable by theory, can be observed around notches in boron/epoxy, glass/epoxy and graphite/epoxy composites using experimental strain analysis techniques.

4/23/2020 4:20 AM 9

In uniaxial loaded quasi-isotropic laminates with holes, regions of high strain concentration with nonlinear response develop.

Failure starts at off-axis points in the form of local delamination and fiber breakage and propagates across the width of the plate.

Failure at the top of the crack usually takes the form of a damage zone consisting of ply subcracking along fiber directions, local delamination and fiber breakage.

Final failure occurs when this damage zone reaches some critical size.

FiberMatrix Fiber pull-out

Failure of Unidirectional Laminates

For unidirectional laminates, the stress‐strain behavior is typically linear. However, as externally applied load exceeds a certain threshold, the stress‐strain response curve may become increasingly nonlinear. This “threshold” could be one definition of the failure load of unidirectional ply.

Alternatively, the failure load of a unidirectional ply could be the actual load at which the composite fractures.

4/23/2020 4:20 AM 11

Failure of Unidirectional Layer due to Longitudinal Tension

Fig. Different Failure Modes of a Unidirectional Lamina in Longitudinal Tension (Left: Brittle Failure, Center: Brittle Failure with Fiber Pullout, Right: Brittle Failure with Debonding and/or Matrix Cracking)

4/23/2020 4:20 AM 12

Stress-strain diagram for brittle matrix-ductile fiber composite

4/23/2020 4:20 AM 13

Specific strength = strength/density

4/23/2020 4:20 AM 15