Delamination of Z-pinned carbon fibre reinforced laminates

Conference: 11th International Fatigue Congress Advanced Materials Research, 2014

This paper presents an experimental investigation into the mode I interlaminar fatigue resistance of carbon fibre-epoxy laminate reinforced in the through-thickness direction With z-pins. The effects of the volume content, diameter and length of z-pins on the delamination fatigue resistance and crack bridging toughening mechanisms are determined. The delamination growth rate slowed when the volume content or length of the z-pins was increased or the z-pin diameter was reduced. Z-pinning is a highly effective technology for increasing the delamination fatigue resistance of composites.

Conference: 20th International Conference on Composite Materials (ICCM20), 2015

This paper presents an experimental study into improvements to the mode I (crack opening) and mode II (crack sliding) delamination fatigue properties of carbon fibre-epoxy laminate reinforced through-the-thickness with metal or carbon fibre rods. The z-pins were made of unidirectional carbon fibre composite, copper, stainless steel or titanium. The delamination fatigue properties were measured in cyclic displacement control using the double cantilever beam (DCB) test for mode I and end notch flexure (ENF) test for mode II. Both the metal and carbon z-pins are effective at resisting the initiation and growth of delamination cracks under cyclic mode I and mode II loads, although the fatigue strengthening effect was greater for mode I, irrespective of the z-pin material. The fatigue resistance increases due to the z-pins forming of a large-scale extrinsic bridging zone along the fatigue crack. The influence of the material properties of the z-pins on their fatigue strengthening capacity and fatigue failure mode is determined.

Conference: International Conference on Structural Integrity and Failure (SIF), 2013

This paper shows that z-pins are remarkably effective at increasing the mode I interlaminar fatigue resistance of fibre reinforced polymer composites. Z-pins increase the threshold cyclic stress intensity factor to initiate delamination crack growth by up to 15 times and slow the delamination fatigue crack growth rate by more than 1000 times. The efficacy of z-pins in improving the delamination fatigue properties is dependent on both their volume content and diameter.

Composites Science and Technology, 2011

This paper presents a cohesive zone model based finite element analysis of delamination resistance of z-pin reinforced double cantilever beam (DCB). The main difference between this and existing cohesive zone models is that each z-pin bridging force is governed by a traction-separation law derived from a meso-mechanical model of the pin pullout process, which is independent of the fracture toughness of unreinforced laminate. Therefore, two different traction-separation laws are used: one representing the toughness of unreinforced laminate and the other the enhanced delamination toughness owing to the pin bridging action. This approach can account for the large scale bridging effect and avoid using concentrated pin forces, thus removing the mesh dependency and permitting more accurate analysis solution. Computations were performed using a simplified unit strip model. Predicted delamination growth and load vs. displacement relation are in excellent agreement with the prediction by a complete model, and both models are in good agreement with test measured load vs. displacement relation. For a pinned DCB specimen, the unit strip model can reduce the computing time by 85%.

A new micro-mechanical model is proposed for describing the bridging actions exerted by through-thickness reinforcement on delaminations in prepreg based composite materials, subjected to a mixed-mode (I-II) loading regime. The model applies to micro-fasteners in the form of brittle fibrous rods (Z-pins) inserted in the through-thickness direction of composite laminates. These are described as Euler-Bernoulli beams inserted in an elastic foundation that represents the embedding composite laminate. Equilibrium equations that relate the delamination opening/sliding displacements to the bridging forces exerted by the Z-pins on the interlaminar crack edges are derived. The Z-pin failure meso-mechanics is explained in terms of the laminate architecture and the delamination mode. The apparent fracture toughness of Z-pinned laminates is obtained from as energy dissipated by the pull out of the through-thickness reinforcement, normalised with respect to a reference area. The model is validated by means of experimental data obtained for single carbon/BMI Z-pins inserted in a quasi-isotropic laminate.

Composites Science and Technology, 2012

A finite element model for predicting delamination resistance of z-pin reinforced laminates under the mode-II load condition is presented. End notched flexure specimen is simulated using a cohesive zone model. The main difference of this approach to previously published cohesive zone models is that the individual bridging force exerted by z-pin is governed by a specific traction-separation law derived from a unit-cell model of single pin failure process, which is independent of the fracture toughness of the unreinforced laminate. Therefore, two separate traction-separation laws are employed; one represents unreinforced laminate properties and the other for the enhanced delamination toughness owing to the pin bridging action. This approach can account for the so-called large scale bridging effect and avoid using concentrated pin forces in numerical models, thus removing the mesh-size dependency and permitting more accurate and reliable computational solutions.

Journal of Composites Science

This work reviews the effects of z-Pins used in composite laminates as through-the-thickness reinforcement to increase the composite’s properties in the out-of-plane direction. The paper presents the manufacture and microstructure of this reinforcement type while also incorporating the impact of z-Pins on the mechanical properties of the composite. Mechanical properties include tensile, compression, flexure properties in static, dynamic and fatigue loads. Additionally, mode I and mode II properties in both static and fatigue loading are presented, as well as hygrothermal, impact and compression after impact properties.

Next Generation Fiber-Reinforced Composites - New Insights [Working Title]

Delamination propagation in laminated composite materials is a common issue that always concerns us when we consider composites for structural purpose. Many possible solutions have been studied; the most famous is the three-dimensional (3D) woven composites materials, which have promising interlaminar fracture resistance but at the cost of increasing density, which for aerospace industry is very important. In this chapter, mode 1 double cantilever beam (DCB) interlaminar fracture toughness tests according to the American Society for Testing and Materials (ASTM) D5528 standard were performed on composite specimens made of E-Glass Saertex 830 g/m2 Biaxial (+/−45°) with Sypol 8086 CCP polyester resin with orthogonal z-axis oriented yarn woven of 0.22 mm diameter nylon monofilament. Four specimens were made with a longitudinal distance between the warp binders of 0.5, 1, 1.5, and 2 cm, respectively. A tensile test according to the ASTM D3039 standard was performed to study how z-binder ...

Composites Part B: Engineering, 2012

We present a non-dimensional analytical model for crack propagation in a z-pinned double cantilever beam specimen (DCB) under mode I loading. Effect of various design parameters on the crack bridging length and apparent fracture toughness are investigated using this model. The efficacy of the analytical model is evaluated by comparing the results with 3D finite element (FE) simulations of the DCB. In the FE model the z-pins are modeled as discrete nonlinear elements. Bi-linear cohesive elements are used ahead of the crack tip to account for the interlaminar fracture toughness of the composite material. The results for load-deflection and crack length obtained from the analytical model and the FE model are compared and found to be in good agreement. The proposed non-dimensional analytical model will be useful in the design and analysis of translaminar reinforcements for composite structures.

Materials, 2021

Dependence of the initiation values of the Strain Energy Release Rate, GCi, on the orientation of the reinforcement direction α relative to the delamination front was investigated for two laminates of different interfacial ply arrangements. In the case of the first laminate, the delamination was located at the interface of the layers reinforced with symmetric fabric and unidirectional fabric. In the case of the second laminate, the delamination was located at the interface of layers reinforced with symmetric fabric. In both laminates, the orientation of fibers in the layers separated by the delamination differed by 45° regarding the warp directions. The investigations were carried out for Mode I, Mode II, and Mixed-Mode I/II (GII/GI = 1 and GII/GI = 1.7) loadings using hybrid beam specimens. The major problem appearing in the intended tests was the inevitable lack of symmetry in the xz and xy planes of the specimens and the resulting deformation and stress–strain couplings, causing ...