Experimental study on z-pin bridging law by pullout test

Abstract

This paper presents an experimental study on the evaluation of bridging law for a z-pin. The relationship between the z-pin bridging force and its displacement was measured by z-pin pullout tests. The tests were carried out using three types of samples with: single small pin; 3×3 small-pins (three columns? three rows) and 3×3 big-pins. For 3×3 small-pins samples, a typical pullout curve with initial bonding, debonding and frictional sliding was obtained. A high peak value of the debonding force was reached before z-pin debonding started. After debonding was initiated, the pull-out force dropped rapidly to a lower value, the pins were then pulled out steadily against friction. However, for samples with 3x3 big-pins, it was difficult to discern the peak debonding force. The major results of this study are expected to provide a better physical understanding of the mechanics and mechanisms of z-pin bridging, aside from an efficient and accurate methodology to measure the crack-bridging law.

A bridging law and its application to the analysis of toughness of carbon nanotube-reinforced composites and pull-out of fibres grafted with nanotubes

“…These features are seen in the direct pull-out experimental force-displacement curves [31,35,37]. As indicated by Cooper et al [35], and also demonstrated by the experiments of Barber et al [31,37], the pull-out force-displacement curves of CNTs and their ropes from polymer matrices resemble typical pull-out force-displacement curves of microfibres in conventional cementitious and polymer composites, such as polymer fibres from a cement matrix [19,46], glass fibres from polymers [47], carbon fibre z-pins from carbon fibre-epoxy composites [29,48], metallic rods from carbon-epoxy composites [28], and Kevlar threads from a resin [49]. These force-displacement curves are often represented by an idealized simple tri-linear diagram shown in Fig.…”

Mode II delamination toughness of z-pinned laminates

“…Our current work on z-13 pin pullout test under mode I tension [23] shows that the values of δ a and T a for mode I bridging law are affected by many factors, such as, material and geometry properties of the pin and laminates, interfacial bonding and friction stresses. So far, there are no experimental data published for z-pin pullout under mode II shearing.…”

Influence of Z-pin embedded length on the interlaminar traction response of multi-directional composite laminates

“…The tangential friction force for the Z-pin material and geometry presented here was modelled with the power law relationship given in equation (12). The scaling constant λ of 1069 was fitted to the average of the experimental data, however the large scatter in the experimental data are highlighted by the approximate bounds power law curve with λ of 700 and 1300.…”

“…Where µ is the coefficient of coulomb friction, and pnl is the non-linear residual frictional force per unit length defined in equation (12).…”

“…The traction response of a Z-pin in a delaminated composite material when loaded in mode I is influenced by four parameters; the pin and the bond strength with the matrix, the frictional contact between the pin and the host material, the elastic modulus and the tensile strength of the pin if pull-out does not occur and pin fracture [6]. The bond strength and the subsequent frictional pull-out is a function of the pin embedded length in the material and its surface condition such as roughness [4,7,11,12]. The residual stresses in the host and the pin material due to manufacturing conditions also play a significant role in its response, which is particularly evident between composite laminates with unidirectional (UD) or multi-directional laminates [4,13].…”