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


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.


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



Thermomechanical efficiency


Latent heat of fusion


Crack tip opening displacement (COD)


COD at end of skin retraction


COD under remote loading


COD under uniform cohesive stress


Crack tip opening rate


Initial COD rate during skin retraction

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

Failure strain


Thermal diffusivity


Fibril draw ratio


Mass density


Relative craze density


Cohesive stress or craze stress


Remote stress


Uniform normal cohesive traction


Crack length


Thickness of a TPB specimen


Load point compliance


Craze length


Non-dimensional compliance


Longitudinal wave speed


Specific heat


Displacement of the striker on a TPB specimen


Young’s modulus


Secant modulus


Load measured during a TPB test


Surface heat transfer coefficient


Finite volume cell number


Thermal conductivity


Stress intensity factor under remote loading


Stress intensity factor under uniform cohesive stress


Length of a TPB specimen


Cell size of finite volume model


Rate of heat generated per unit area


Geometry function of stress intensity factor under remote loading


Span of a TPB specimen


Skin thickness of a bilayer specimen


Critical thickness of the melt layer in the adiabatic decohesion model






Initial test temperature


Transition temperature of a bilayer structures


Brittle/tough transition temperature


Failure time predicted by the adiabatic decohesion model


Temperature of failure of the skin of a bilayer structure


Melting temperature


Geometry function of stress intensity factor under uniform cohesive stress


Width of a TPB specimen


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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