CAE-Companion-2018-2019

Modeling of Materials & Connections WISSEN CAE

and gives the limits of safe loading. These criteria have points determined from coupon tests and assumptions are used to complete the envelope. TheMaximum Stress-Strain criteria assumes no interaction of stress components on failure, whereas the quadratic criteria more correctly assumes stress interactions but can be unrealistic for some load combina- tions. In recent years the physically based Puck criteria for UD composites has gained popularity and does overcome many of these limitations.

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Max. Stress

Puck

Tsai ‐ Wu

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For impact and crash classical criteria are not appropriate and progressive ply damage laws should be used since these allow the different ply failure modes to be represented and damaged independently. For a UD composite failure modes may be fibre tension or compression failure, transverse ma- trix tension or compression failure, or matrix shear failure. In addition delamination of the weak resin layer between plies may occur. The popular approach to model ply damage is via damage variables that modify initial linear stiffness; delamina- tion is usually approximated with damaging spring elements that tie plies together and absorb resin fracture energy during progressive failure.

A further important crash mode is axial crushing. Here ply and delamination models are not applicable and specialized techniques to model composites fragmentation are needed. Fragmentation is initiated via a trigger device (usually a chamfer) that creates local micro-cracking; this then steadily propagates through the part as it is crushed. An important feature is that material at the crash front is fundamentally different to the intact undamaged materials and both must be properly represented in the numerical model.

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CAEWissen by courtesy of Dr. Anthony Pickett, IFB Univer - sity of Stuttgart.

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