Abstract:
This study investigates the effects of Z-pinning on the delamination performance in
opening and shear loading modes in woven fabric reinforced / epoxy composite
materials, as well as the effects of friction between specimen crack faces and the Z-pin
failure mechanisms involved in mode II delamination.
Mode I and mode II delamination tests are carried out on Z-pinned unidirectional (UD)
and woven laminates. Both UD and woven laminates exhibit enhanced delamination
resistance and crack propagation stability through Z-pinning. The effects of various
structural and Z-pin parameters on the mode I and mode II delamination behaviour are
separately assessed.
The 4ENF testing configuration is deemed as the appropriate mode II configuration for
the testing of Z-pinned laminates. A new basic friction rig is used to measure the
friction coefficient between crack faces in woven laminates. An additional friction
effect attributed to fibre architecture is identified. A specially designed delamination
specimen is used to overcome the difficulty of accurately measuring crack propagation
in Z-pinned woven fabric materials and aid data reduction using the available analytical
methods.
The failure mechanisms involved in the mode II delamination of Z-pinned laminates
have been investigated with the implementation of a new test. Z-pins fail under shear
loading through a combination of resin crushing, laminate fibre breakage, pin shear, pin
bending and pin pullout. The balance of the failure mechanisms is shown to be a
function of the crack opening constraint, material type, stacking sequence, Z-pin angle
and insertion depth to Z-pin diameter ratio.
Z-pin and material parameters influencing Z-pinning quality are identified, categorised
and quantified. The importance of controlling Z-pin insertion depth is underlined and
updated manufacturing procedures are proposed. Partial pinning appears as an
advantageous alternative.
A reduction in in-plane stiffness and in-plane strength in UD and woven fabric
composites is measured, whilst no significant change of in-plane shear stiffness of UD
materials is observed. A reduction in the fibre volume fraction is the single most
important parameter affecting the in-plane stiffness.
The performance of a Z-pinned sub-structural component is investigated. Enhanced
loading carrying capacity and damage tolerance is achieved through Z-pinning.
