Introduction to composite materials and structures

Introduction to Composite Materials and Structures

Nachiketa Tiwari

Indian Institute of Technology Kanpur

Lecture 28

Failure of Unidirectional Composites

Introduction

•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 composite s appearance are

in composite’s behavior and observed.

•The internal failure of a composite sample could manifest as:–Breaking of fibers

B ki f fib

–Development of micro‐cracks in matrix

–Debonding between fibers and matrix

–Delamination, i.e.

Delamination i e separation of different layers of a laminate

•Here, we explore different failure mechanisms of composites, first at ply level, and subsequently at level.

level laminate level

of Unidirectional Layers Failure

y p p,p,•In many composite samples, before failure occurs at macroscopic scale, the material response changes significantly. This change in material

response, which precedes actual material failure, is a manifestation of a large of micro failures as as non linear behavior.

number“micro‐failures”well‐material behavior

•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 Al i l h f il l d f idi i l l ld b h l load at which the composite fractures.

of Unidirectional Layers Failure

•If failure in unidirectional lamina is considered to be composite material’s proportional limit, then such a failure criteria ensures a significant margin of safety.

•However, in a very large number of composites the stress‐strain responses curve is fairly straight to the point of failure. This is so, because of a

combination two reasons:

of

–Most of the fibers used in modern composite laminates exhibit brittle failure.

–The fiber volume fraction in a very large number of composites is maximized, and hence, the fiber failure and composite failure occur at same load level.

•However, in materials which have either very low fiber volume fraction or where matrix material is very stiff (vis‐à‐vis fiber), the overall stress‐strain response of composite may be considered as its proportional limit.

•In following slides, different failure modes of unidirectional layers have been discussed.