An investigation of the mechanical performance of Z-pin reinforced composites
File(s)
Author(s)
Fert, Marcin Maciej
Type
Thesis
Abstract
Fibrous composites, having excellent mechanical properties in the direction of the fibres, have lower mechanical properties in the through thickness direction, controlled by resin. Z-pinning improves the delamination toughness (up to 500%) with a relatively modest reduction to the in-plane mechanical properties (typically 5–15%).
This experimental study investigates the mechanical performance of Z-Pins bridging an existing delamination in fibre reinforced resin composites under pull-out (Mode I), shear-out (Mode II) and mixed mode loading conditions using a specially designed testing rig. In Mode II the opening displacement was restricted and measured by springs of three different stiffnesses.
A new technique of needle assisted Z-Pin insertion was developed, in which prepreg panels were perforated with a steel needle in order to insert Z-Pins. This technique ensured the desired orientation of Z-Pins, improved pinning quality and removed the necessity of costly preforms used in the traditional UAZ method.Test specimens were blocks (15 mm x 15 mm x 6mm thick) of carbon-epoxy IM7/8552 composite in unidirectional (UD) and quasi-isotropic (QI) stacking sequences, with PTFE delamination film in the mid-plane recreating an existing crack, bridged with a single T300/9310 Z-Pin or a group of four pins of either 0.28 mm or 0.51 mm diameter.
Three phases of pull-out were identified: Linear Phase (linear force-displacement curve), Crack Formation (unstable crack propagation phase) and Frictional Sliding (friction-controlled pull-out). Two phases of shear-out were identified: Linear Phase (with no energy loss) and Breaking Phase (where the fibrous structure of the Z-Pins is fractured, ending with Z-Pin breakage). In mixed mode specimens behaved similarly to pull-out for the pin angles up to 45°. For higher angles the behaviour was more similar to pure shear-out. The influence of the Z-Pin diameter, z-pinning depth, distance between adjacent Z-Pins, composite stacking sequence and pull-out speed on the Z-Pins behaviour were investigated. The results will be useful in the formulation of improved Z-Pin bridging laws for use in finite element models.
This experimental study investigates the mechanical performance of Z-Pins bridging an existing delamination in fibre reinforced resin composites under pull-out (Mode I), shear-out (Mode II) and mixed mode loading conditions using a specially designed testing rig. In Mode II the opening displacement was restricted and measured by springs of three different stiffnesses.
A new technique of needle assisted Z-Pin insertion was developed, in which prepreg panels were perforated with a steel needle in order to insert Z-Pins. This technique ensured the desired orientation of Z-Pins, improved pinning quality and removed the necessity of costly preforms used in the traditional UAZ method.Test specimens were blocks (15 mm x 15 mm x 6mm thick) of carbon-epoxy IM7/8552 composite in unidirectional (UD) and quasi-isotropic (QI) stacking sequences, with PTFE delamination film in the mid-plane recreating an existing crack, bridged with a single T300/9310 Z-Pin or a group of four pins of either 0.28 mm or 0.51 mm diameter.
Three phases of pull-out were identified: Linear Phase (linear force-displacement curve), Crack Formation (unstable crack propagation phase) and Frictional Sliding (friction-controlled pull-out). Two phases of shear-out were identified: Linear Phase (with no energy loss) and Breaking Phase (where the fibrous structure of the Z-Pins is fractured, ending with Z-Pin breakage). In mixed mode specimens behaved similarly to pull-out for the pin angles up to 45°. For higher angles the behaviour was more similar to pure shear-out. The influence of the Z-Pin diameter, z-pinning depth, distance between adjacent Z-Pins, composite stacking sequence and pull-out speed on the Z-Pins behaviour were investigated. The results will be useful in the formulation of improved Z-Pin bridging laws for use in finite element models.
Version
Open Access
Date Issued
2015-08
Date Awarded
2016-06
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Hitchings, Dennis
Robinson, Paul
Sponsor
Polish Air Force Association
Publisher Department
Aeronautics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)