International Journal of Fracture

, Volume 148, Issue 4, pp 315–329 | Cite as

Effect of skin fracture on failure of a bilayer polymer structure

Original Paper

Abstract

A thin skin of low tensile failure strain, if bonded to the tensile surface of an un-notched impact bend specimen of much tougher material, can change the global failure mode from ductile to brittle. A novel model of this well-known effect is developed and applied to results from impact tests on a tough core of polyamide-polyethylene blend, with a single skin of brittle EVOH. At a fixed crosshead speed, notched specimens of the blend become brittle at a relatively low temperature Tbt. Un-notched bilayer specimens continue to show skin fracture up to a considerably higher temperature Tfs; above this temperature they do not fail at all but below Tbt they too fail in a brittle manner. Within the temperature range from Tfs down to Tbt there is a transition from crack arrest, either at the skin/core interface or further into the core where a crack would not normally propagate, to brittle fracture. This brittle fracture temperature is predicted by modelling the process as a three-phase impact event. In the first phase, the striker bends the bilayer quasi-statically. The second phase begins with instantaneous fracture of the skin at its failure strain. The skin ends retract at finite speed, and a craze grows in the adjacent core material to accommodate the local strain singularity. The last phase is a striker-driven impact event similar to that in a notched bend specimen of the core material, except that the crack-tip craze already bears the adiabatic temperature distribution generated while it was driven open by skin retraction. The criterion for craze decohesion, and hence for a crack jump, is the same adiabatic decohesion criterion which accounts for the speed-dependence of impact fracture in notched monolayer specimens. Applied computationally, this model predicts whether a bilayer structure fails in a brittle way or whether cracks initiated in the skin are arrested, either temporarily or permanently, at the skin/core interface.

Keywords

Adiabatic decohesion Crack arrest Fracture mechanisms Impact TPB tests Polymeric bilayer material Rapid crack propagation Surface embrittlement 

Nomenclature

β

Thermomechanical efficiency

ΔHf

Latent heat of fusion

δ

Crack tip opening displacement (COD)

δF

COD at end of skin retraction

δr

COD under remote loading

δu

COD under uniform cohesive stress

\({\dot{\delta}}\)

Crack tip opening rate

\({\dot{\delta}_0}\)

Initial COD rate during skin retraction

\({\epsilon_{\rm fs}}\)

Failure strain

κ

Thermal diffusivity

λF

Fibril draw ratio

ρ

Mass density

ρ*

Relative craze density

σc

Cohesive stress or craze stress

σr

Remote stress

σu

Uniform normal cohesive traction

a

Crack length

B

Thickness of a TPB specimen

C

Load point compliance

c

Craze length

C*

Non-dimensional compliance

c0

Longitudinal wave speed

Cp

Specific heat

d

Displacement of the striker on a TPB specimen

E

Young’s modulus

Esec

Secant modulus

F

Load measured during a TPB test

h

Surface heat transfer coefficient

j

Finite volume cell number

k

Thermal conductivity

KIr

Stress intensity factor under remote loading

KIu

Stress intensity factor under uniform cohesive stress

L

Length of a TPB specimen

L0

Cell size of finite volume model

\({\dot{q^{\prime\prime}}}\)

Rate of heat generated per unit area

r(a)

Geometry function of stress intensity factor under remote loading

S

Span of a TPB specimen

s

Skin thickness of a bilayer specimen

sc

Critical thickness of the melt layer in the adiabatic decohesion model

T

Temperature

t

Time

T0

Initial test temperature

Tbts

Transition temperature of a bilayer structures

Tbt

Brittle/tough transition temperature

tdc

Failure time predicted by the adiabatic decohesion model

Tfs

Temperature of failure of the skin of a bilayer structure

Tm

Melting temperature

u(c,a)

Geometry function of stress intensity factor under uniform cohesive stress

W

Width of a TPB specimen

Fo

Fourier number

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Copyright information

© Springer Science+Business Media B.V. 2008

Authors and Affiliations

  1. 1.Department of Mechanical EngineeringImperial College LondonLondonUK

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