Failure of composites John Summerscales

Outline of lecture Strength Failure mechanisms Fractography Failure criteria Fracture mechanics

Strength strength = stress at failure failure may be yielding in metals non-recoverable loss of elastic response first-ply failure ultimate failure one material can have several different strengths

Strength Kelly-Tyson equation for UD composites: σc = σfVf + σm*(1-Vf) at high Vf, or σc < σm#(1-Vf) at low Vf where σm* is the tensile stress in the matrix at the failure strain of the fibre, and σm# is the maximum tensile strength of the matrix For small mis-alignments: σc = σfVf / cos2θ = σfVfsec2θ

Failure mechanisms matrix cracking fibre fracture debonding = interface failure delamination = interlayer failure fibre pullout micro-buckling kink bands cone of fracture

Failure strain of composites The key criterion for composite failure is the local strain to failure: ε’ a.k.a. elongation at break and not stress Note that ε’ for the fibre/matrix interface i.e. transverse fibres = ~0.25 %

Matrix cracking polyester resin ε’ = 0.9-4.0 % max min polyester resin ε’ = 0.9-4.0 % vinyl ester ε’ = 1.0-4.0 % epoxy resin ε’ = 1.0-3.5 % phenolic resin ε’ = 0.5-1.0 % data from NL Hancox, Fibre Composite Hybrid Materials, Elsevier, 1981.

Fibre fracture S/R-glass ε’ = 4.6-5.2 % …. E-glass ε’ = 3.37 % ……….… Kevlar 49 ε’ = 2.5 % ………………. HS-carbon ε’ = 1.12 % ………………… UHM-carbon ε’ = 0.38 % ………………….. data from NL Hancox, Fibre Composite Hybrid Materials, Elsevier, 1981.

Fibre-matrix debonding c a b Crack can run through (not shown), or around the fibre NB: ~12000 carbon or 1600 glass UD fibres / 1mm2

Fibre-matrix debonding:

Delamination of layers one layer is a lamina (plural = laminae) several layers in a composite is a laminate separation of the layers is delamination to avoid delamination 3-D reinforcement (often woven or stitched) Z-pinning

Stress whitening of GFRP both debonding (fibre/matrix separation) and delamination (layer separation) create internal defects which scatter light the consequence is that the transparency of the laminate becomes more opaque, referred to as “stress whitening” similar effects may be seen in other composites (e.g. at stitches in NCF CFRP)

Fibre pullout as parts of a fractured composite separate, the fibres which have debonded can fracture remote from principal fracture plane. energy is absorbed by frictional forces as the fibre is pulled from the opposite face debonding and pullout absorbs high energies and results in a tough material

Micro-buckling In bending tests, failure occurs due to: poor fibre/matrix adhesion in combination with the stress concentration at the loading roller

Kink bands (HM fibre composites) Compressive load causes buckling followed by co-operative failure of a group of fibres to produce short lengths of parallel mis-oriented fibre Image from http://coeweb.eng.ua.edu/ aem/people/samit/ nanoclay.htm

Cone of fracture (CFRP) the impacted face shows no sign of damage delamination occurs in a cone fibre spalling from the back face known as BVID barely visible impact damage difficult to detect unless reported

Fractography use of optical or electron microscopes to image the fracture surface:

Fracture mechanics stress intensity factor (Pa.m1/2 ) fracture toughness (critical stress intensity factor, Pa.m1/2 ) separate parameters in each plane mode I (x) II (y) III (z) JG Williams, Fracture mechanics of composite failure, Proc IMechE Part C: Journal of Mechanical Engineering Science, 1990, 204(4), 209-218. crack

Design to avoid failure Beware first ply failure dependent on laminate stacking sequence failure index (FI) of >1 = failure dependent on the failure criteria selected reserve factor (RF) <1 = failure for Tsai-Hill failure criteria, RF =1/√(FI)

Failure criteria failure occurs when local stress reaches a critical value: σi ≥ σi' or τij ≥ τij' (' indicates failure condition) von Mises yield criterion: critical distortional strain energy Tresca yield criterion: maximum shear stress Tsai-Hill criterion: an envelope in stress space …. and many others

Failure criteria those above plus many other criteria no agreement ! (see MJ Hinton, AS Kaddour and PD Soden, Failure criteria in fibre reinforced polymer composites: the world-wide failure exercise, Elsevier, Amsterdam, 2004.  ISBN 0-08-044475-x).