Failure of Sandwich Structures with Sub-Interface Damage
This thesis addresses some aspects concerned with fatigue behaviour and damage tolerance
of foam core sandwich structures with composite faces. Two different foam cores
typically used in aeronautical and marine applications are considered. The focus is on
face/core interface cracks (debonds) and sub-interface impact damage in the core; the
face sheet is regarded undamaged.
The first part of the thesis (Papers A, B and C) is concerned with fatigue behaviour of
cellular foams under constant and shedding load-amplitude. Mode I and mode II fatigue
crack growth is experimentally investigated. Stress intensity factors at the crack tip are
computed using the finite element method and assuming validity of linear elastic fracture
mechanics. The fatigue crack propagation is characterised in terms of the Paris’ law
equations and the fatigue threshold ?Kth is experimentally evaluated. The obtained crack
growth rates are shown to be very high, especially under mode I loading, suggesting that a
conventional damage tolerance approach may be impractical for design purposes. Instead,
a design based on keeping the applied stress intensity range below the fatigue threshold
?Kth may be recommended.
In the next part (Papers D and E) the focus is shifted to analysis of residual static
strength and failure mechanisms of sandwich beams with impact damage. An idealised
2D damage configuration is investigated. Two typical failure modes are considered; shear
failure of the core and local buckling of the face sheet. Failure mechanisms of impactdamaged
beams and the effect of bridging in the peripheral regions of impact damage are
thoroughly examined. The properties of the crushed core, underlying the impact damage,
are experimentally determined. A parametrised finite element model of impact damage
with implemented crushed core and bridging conditions in the peripheral regions of impact
damage is developed. A point-stress criterion is used for post-impact fracture analysis.
The model shows good agreement with post-impact experimental results.
The last paper links the two aforementioned parts of the thesis addressing the fatigue
life of impact-damaged sandwich beams under constant load-amplitude. A tentative approach
is suggested for estimation of the threshold load levels for sandwich beams with
known impact damage size. It is based on a modified point-stress criterion coupled with
experimentally measured fatigue threshold stress intensity range ?Kth. The proposed approach
shows good correlation with experiments and may be applied for evaluation of
maximum allowable load levels during the preliminary design of sandwich structures.
School:Kungliga Tekniska högskolan
Source Type:Doctoral Dissertation
Keywords:sandwich structure; foam core; fatigue; crack growth; stress intensity factor; fatigue threshold; fracture toughness; damage tolerance; impact damage; crushed core; bridging; fracture analysis; point-stress criterion
Date of Publication:01/01/2001