Z-pin Reinforcement Research Articles

A novel method for through-thickness reinforcement of laminated composites using discrete micro-polarization-induced fiber injection (DMFI) approach

Conventional through-thickness reinforcement methods for laminated composites, such as Z-pin, encounter issues with in-plane property degradation and complex fabrication processes. To achieve rapid and low-damage reinforcement, a novel approach using short-chopped carbon fibers (SCFs) to form a micron-diameter interlaminate structure has been proposed. This method employs a discrete micro-polarization-induced fiber injection (DMFI) technique, where polarized SCFs are electrostatically oriented and injected at high speeds into pre-formed holes in the laminates. The insertion process of SCFs was thoroughly investigated, with optimal interlaminate conditions determined using high-speed cameras and other equipment. The toughening mechanism of SCFs was explored through various characterization methods, including metallurgical microscopy. This innovative method offers several advantages over the traditional Z-pin reinforced method. Notably, present method eliminates the need for prefabrication of Z-pins and fully leverages the excellent mechanical properties of individual carbon fiber in short length. It provides superior interlaminar mechanical properties, achieving a 392% improvement compared to the control group and a 15% improvement compared to 0.1 mm Z-pin reinforcement at the same insertion volume fraction. Additionally, it has minimal impact on the in-plane properties of the laminates, with only a 3.6% reduction in tensile strength and a 4.1% reduction in compression strength. Furthermore, it is environmentally friendly, allowing for the recycling and reuse of waste SCFs.

Combined loading of unreinforced and Z-pin reinforced composite pi joints: An experimental and numerical study

Blind predictions of experimental responses of non-reinforced and z-pin reinforced composite pi joints were established using a progressive damage and failure model. The finite element model used a novel mixed-mode cohesive formulation to model intra-laminar and inter-laminar damage. A deliberate meshing strategy was used for cohesive interlayers to capture the interaction between intra- and inter-laminar damage modes. A smeared cohesive zone modeling approach was implemented to efficiently include the effects of z-pinning for the z-pin reinforced specimens. Using material properties obtained through experimental correlation of simpler joint configurations, blind predictions for large element specimens subjected to combined loading (axial compression and push-off loading) were established. To assess the predictive capabilities of the model, experimental and numerical comparisons were made in terms of load-displacement response, critical loads, and failure progression. Comparisons between the experimental results and predictions for the unreinforced joints were found to be in good agreement across the range of compressive loads tested. For the z-pin reinforced joints, initial responses were accurately predicted; however, ultimate loads and the toughening effect of the z-pin reinforcement was overpredicted.

Experimental and numerical study on the influence of cure process on the bridging traction mechanism of z-pins

This paper focuses on the bridging mechanism of individual z-pins embedded in carbon/epoxy laminates with curing effects into account. Based upon experiments, partial debonding exists in the vicinity of z-pins before pullout loading. Despite scatter, bi-linear relationship from load–displacement data is collected to characterize the bridging responses of z-pins. Complete pull-out is the dominant failure mode of z-pins after loading, due to inferior interfacial adhesion. A representative volume element (RVE) model of a single z-pin reinforced composite laminates is established, with resin-rich pockets and interfacial behavior into consideration. A qualitative analysis is presented to evaluate the influence of curing defects on the bridging ability of z-pins. The variations of cure-dependent properties are explicitly modeled. Cohesive elements are applied to the z-pin and interface regions in terms of bi-linear constitutive law. The predictions show reasonably well agreement with the experiments. Remarkably, the mismatched curing residual stresses are responsible for the generation of interfacial cracking around z-pins. The pre-existing cracking provides a reduction of physical interfacial contact, thus adversely controlling the effectiveness of z-pin reinforcement. Furthermore, the influence of cure process on the bridging behavior of z-pins is explored using a parametric method.

Experimental investigation into influences of Z-pin and deltoid on structural properties and damage tolerance of CFRP T-joints

Composite T-joints bear low-velocity impact in service. The characteristics of their impact response and residual tensile performance are crucial to structural safety but have not been researched in sufficient detail. This study applies the Z-pin pre-hole insertion process in concert with two fillet radii to manufacture four types of carbon fiber reinforced plastics (CFRP) T-joints. The joints were then characterized by using robust low-velocity impact tests at different energies as well as post-impact quasi-static tensile tests . The results show that increasing the size of the deltoid improved the impact resistance of the joints but the deltoid itself was more vulnerable to damage, which reduced its influence on the residual tensile performance of the joints with increasing impact energy. The Z-pin reinforcement in the non-impact region had a limited influence on the low-velocity impact resistance, while its influence on the residual tensile performance of the T-joints was positively correlated with the degree of damage to them after impact. Moreover, the difference in influence between the deltoid and the Z-pin on the structural properties and damage tolerance of the T-joints was closely related to their respective reinforcement mechanisms, and can contribute to the lightweight design of the T-joint. We also used infrared thermography to observe the evolution of damage to the T-joints during the loading process. The significant fracture of the joint was predicted successfully by comparing different infrared characteristics.

Effect of Ø 0.11 mm Z-pinning on the properties of composite laminates via an ultrasound guided insertion process

This paper develops an ultrasound guided Z-pinning process, which can insert finer Z-pins of Ø 0.11 mm (in comparison with traditional Ø 0.28 mm - 0.50 mm) with minimal in-plane damage and substantially improve the interlaminar performance of Z-pinned composite laminates. The experimental results show that for unidirectional fibre/epoxy laminates reinforced by 0.1 vol% of Ø 0.11 mm carbon Z-pins, the in-plane strength reduction is only 1.31% in tension and 3.46% in compression, respectively, while the Mode-I fracture toughness can be improved by 11 times in comparison with the unpinned specimens. The results can be attributed to the small damage to the microstructures of the pinned area, where long and narrow resin-rich zone with small in-plane fibre waviness and neglectable out of plane fibre crimp are represented. The interlaminar toughening mechanism of fine Z-pinned laminates is elucidated by the bridging law with the establishment the DCB testing model based on the traditional Z-pin reinforcement mechanism.

Compression properties of z-pinned carbon-fibre/epoxy laminates reinforced with circumferentially notched z-pins

This paper represents results of the investigations on in-plane compression properties of unidirectional carbon-fibre/epoxy laminates with a three-dimensional z-pin reinforcement with circumferentially notched z-pins. Different notch designs - rectangular, circular, triangular and sinusoidal - were examined to identify the impact on the resulting compression strength and modulus. From the determinations it can be concluded that notching of z-pins has a significant impact on the compression strength, no matter of the notch design. Due to the changes in microstructure caused by the implementation of notches at the z-pin surface, eg partially lower in-plane fibre waviness as well as reduced swelling behaviour, the compression strength values are significantly higher compared to those of laminates reinforced with unnotched pins. With increasing notch depth and decreasing notch distances the compression strength values are rising. In contrast, the Young's modulus of the z-pinned laminate is not affected by notching of the z-pins. An influence of the notch design, depth and distance of circumferential notches on the Young's modulus was not found with the investigation carried out.

A numerical and experimental study on the multiple fracture progression of CFRP T-joints under pull-off load

This paper reports a combined numerical and experimental study on the multiple fracture evolution in carbon fiber reinforced polymer (CFRP) T-joints under pull-off load. The experimental study successfully captured the sequential fracture initiation and evolution involving a complex interplay among noodle cracking, noodle-skin and skin-stiffener delamination, and their correlation with the load-displacement curves. Numerically, two independent numerical methods, (a) the extend finite element method (X-FEM) available in ABAQUS, and (b) the augmented finite element (A-FEM) method, are used to cross-check their predictive capability in modeling progressive fracture evolution in the T-joints. It is found that, although the nonlinear load-displacement curves predicted by both methods are consistent with experimental data, their predictions on fracture progression responsible for final failure are different. The A-FEM prediction is in good agreement with experimental record while the X-FEM's prediction is inaccurate. Finally, the validated A-FEM model is coupled with the Binary Model to quantify the improvement in fracture resistance using the Z-pin reinforcement technique. The simulation results, which are validated by previously reported experimental data, show that properly arranged Z-pin reinforcement can improve the fracture tolerance and significantly delay the final failure of T-joints.

Impact resistance of Z-pin-reinforced sandwich composites

This study investigates the effect of face materials, Z-pin types and distribution densities on drop-weight impact properties of foam core sandwich composites. The novelty of this study is to eliminate damage of face part by only reinforcing the core part of sandwich structures. Impact test was performed at different energy levels (20–50 J). The addition of Z-pins into the sandwich composites decreased the elasticity and ductility while it increased the stiffness of sandwich composites. The Z-pin reinforcement increased the peak forces, but decreased the peak deformations of the sandwich composites. However, higher energy absorption was only observed at the higher Z-pin distribution density. The results showed that Z-pin distribution density, bonding between the face sheets/pins, and the face sheet material have a great influence on the impact behaviour of the Z-pin-reinforced sandwich composites besides the Z-pin types.

X-ray photo-electron spectroscopic studies of cryogenic and plasma surface-treated z-pins

The mechanical properties of composite materials are primarily dependent on the load transmission between their components. Especially, the fracture performance of a fibre-reinforced polymer made of a preimpregnated fibres (prepreg) with a three-dimensional z-pin reinforcement can be influenced by the interface characteristics between pins and laminate. X-ray photo-electron spectroscopic (XPS) analysis and scanning electron microscopic (SEM) studies of untreated, cryogenic and plasma surface-treated z-pins were carried out to determine the effect of surface treatments on chemical composition, formation of functional groups and topography of pin surface. Pullout tests were conducted to investigate the impact of a surface treatment on the bridging forces, which have significant influence on delamination toughness of a z-pinned laminate. It was pointed out that a cryogenic treatment cannot lead to an increase of oxygen or nitrogen functional groups at the z-pin surface. Nevertheless, the pullout forces can be increased which are caused by an increased surface roughness. However, with a plasma treatment, an oxygen and nitrogen functionalization can be reached. An increase of the O/C ratio with all plasma treatments can be determined. Summarizing the results, it can be shown that oxygen functional groups can be generated by a plasma treatment. These groups are able to establish covalent bonds between z-pin surface and bulk material, which can lead to a better load transmission between the pins and laminate and thus to higher fracture properties. The increased pullout forces result from a combination of improved adhesion between pins and laminate and increased interlocking effects, due to the higher surface roughness after plasma treatment.

Study on Tensile Mechanical Behavior of Composite T-Joints

Based on three kinds of composite T-joints with different connection way for tension test outside the plane, it was obtained contrastively that how the ordinary adhesive, Z-pin reinforcement and stitching reinforcement three different fitting influence tensile strength, damage failure process and failure mode of composite T-joints. The test results showed that compared with ordinary adhesive connection mode, tensile strength of the Z-pin reinforcement and stitching reinforcement T-joints increased by 13.6% and 11.4%, respectively; and the largest deformation increased by 19.2% and 15.1%, respectively. After reaching maximum load condition, the ordinary adhesive T-joints had brittle failures, but Z-pin reinforcement and stitching reinforcement T-joints all showed that the ductile damage behavior, corresponding to the load-displacement curve appeared saw-tooth wave platform. Obviously, the Z-pin reinforcement T-joints had the most significant reinforcement effect on tensile properties of composite laminates T-joints.

Interaction of Z-pins with Multiple Mode II Delaminations in Composite Laminates

The application of Z-pinning is a subject of great interest in the field of through-thickness reinforcement (TTR) of composite laminates. To date, the majority of Z-pin characterisation work has been conducted on fracture coupons containing a single embedded delamination, which is often not representative of real failure of reinforced composite structures in service. In this investigation a test procedure to produce two independent Mode II delaminations was developed to analyse their interaction with a region of Z-pin reinforcement. Initially numerical models were used to optimise the chosen configuration. Experimental results show in detail the response of Z-pins to two independent delaminations. These results highlight the ability of the Z-pins to effectively arrest mode II delaminations at multiple levels through the sample thickness. Additionally they provide a much needed data set for validation and verification of Z-pin numerical modelling tools.

Fatigue Properties of Aerospace Z-Pinned Composites

This paper presents an experimental study into the effect of through-thickness z-pin reinforcement on the in-plane and out-of-plane (delamination) fatigue properties of carbon-epoxy composites used in aerospace structures. The in-plane fatigue strength and fatigue life (load cycles-to-failure) of aerospace composite materials are reduced by z-pins. The in-plane compressive fatigue properties decrease when the volume content of z-pins is increased. Reductions to the in-plane fatigue properties are due to microstructural damage caused by the z-pins. However, the out-of-plane (delamination) fatigue properties of composites are increased greatly by z-pins. The mode I, mode II and mixed mode I/II delamination fatigue properties increase rapidly with increasing volume content of z-pins. The improvement is due to the z-pins forming a large-scale bridging zone along the delamination which resists fatigue crack growth. The work clearly reveals that a trade-off exists between the in-plane and out-of-plane fatigue properties of z-pinned composites. Improvements to the delamination fatigue properties come at the expense of lower in-plane fatigue performance, and this is a key consideration for the design of z-pinned aerospace composite structures.

Comparative study of the mode I and mode II delamination fatigue properties of z-pinned aircraft composites

This paper compares improvements to the mode I and mode II delamination fatigue resistance of an aerospace composite material achieved by z-pin reinforcement. Mode I (cyclic crack opening) and mode II (cyclic crack sliding) interlaminar fatigue tests were performed on a carbon fibre reinforced epoxy composite reinforced in the through-thickness direction with different volume contents and diameters of z-pins. Paris curves obtained from displacement-controlled fatigue tests reveal that z-pins are more effective at resisting the initiation and growth of delamination cracks under mode I than mode II conditions. Both the mode I and II fatigue resistance increase with the z-pin content due to the formation of a large-scale extrinsic crack bridging toughening zone, although fatigue strengthening is greater for mode I. Improvements to the mode I and mode II delamination fatigue resistance are also dependent on the z-pin diameter. Similarities and differences in the mode I and mode II fatigue properties are related to the fatigue strengthening mechanisms induced by z-pins for the two load conditions.

Flexural properties of z-pinned composite laminates in seawater environment

Unidirectional carbon-fibre reinforced composite laminates with and without z-pins were immersed in artificial seawater and exposed to two different temperature levels (−1.75 and 50 °C), as well as thaw–freeze cycles (+20/−20 °C). The investigation described is focused on the question to which degree seawater absorption, as well as bending properties are influenced by z-pin reinforcement. The results indicate an increasing influence of the z-pin reinforcement on the water sorption rate, while the sorption rate of unpinned laminates is lower. This is a result of the additional diffusion pathways of the moisture ingress into the laminate caused by the inserted z-pins which in turn change the micro-structure. Furthermore, the sorption rate depends on the immersion temperature. Laminates immersed into seawater with higher temperatures (50 °C) display a significantly higher diffusion rate than those immersed in colder seawater (−1.75 °C) or those immersed under thaw–freeze conditions (+20/−20 °C). Z-pin reinforced laminates with a unidirectional fibre orientation show a reduced bending strength by about 31 %, as well as a reduced flexural modulus by about 11 % in comparison to unpinned samples. Unpinned and z-pinned samples that were exposed to a seawater environment for 1344 h show a reduced flexural modulus depending on the immersion temperatures. As opposed to flexural modulus, flexural strength is not affected by immersion time or temperature. The overall bending strain energy that is necessary for a complete fracture of the unpinned samples under 4-point bending loads can be described with the value of the elastic bending strain energy. In contrast to this the overall bending strain energy of the z-pinned laminates is composed of two different components –the elastic bending strain energy and the post-fracture strain energy. The post-fracture strain energy occurs after exceeding the flexural strength. The overall bending strain energy of z-pinned and unpinned samples without immersion into seawater is around 7.2 J, while the percentage of the post-fracture energy of the pinned samples is 40 % with respect to the overall bending strain energy. The duration of the immersion into water and higher water temperatures increases the overall bending strain energy for both unpinned and pinned samples. The increase is higher for z-pinned samples and is mainly caused by the increase of the post-fracture energy.

Prediction of elastic properties for woven z-pinned composites

Delamination occurs easily due to impact loads caused by the weakness of out-of-plane properties in composite materials. In order to overcome this problem of suppressing delamination, which is a critical weakness for composite materials, z-pinned composite material with reinforced out-of-plane elastic properties was developed by inserting z-pins into out-of-plane directions. In order to overcome the stability within the mutually orthogonal yarn direction of z-pinned composite material, a woven z-pinned composite was developed. This paper examines the effects of z-pin reinforcement on the elastic properties of composite materials as well as the changes in elastic properties used for the diameter of the z-pin, the length of the resin rich zone, and the length of yarn thickness by incorporating the graded fiber waviness that is created by the z-pin and graded warp and fill. As a result, we observed an increase in the out-of-plane properties of the woven z-pinned composite. Positive correlations were observed between the numerical model and experimental results.

Mixed mode cohesive law for Z-pinned composite analyses

A coupled cohesive zone model is here developed for modeling the Z-pin reinforcement in composite laminates; both the interlaminar failure and the failure of Z-pins themselves were incorporated in the work. The mode I and mode II bridging response component of the Z-pins during mixed mode delamination are represented by two unrelated traction–separation laws, and a new method for modeling the Z-pin bridging response with cohesive elements is introduced. The standard mode I, II and mixed mode delamination toughness tests are analyzed for Z-pinned composite laminates. Comparison between the numerical simulations and experimental results demonstrates the applicability and validity of the present model. The modeling methodology is easy to be carried out and flexible enough to account for different Z-pin density and distribution, which can also be extended to simulation of other through thickness reinforcements or rivet joint. The present model showed path dependence in mixed mode delamination, which may be further utilized to account for the failure when normal and shear load is not increased proportionally with each other.

Z-pin Reinforcement on the Core Shear Properties of Polymer Foam Sandwich Composites

The influence of orientation of carbon fiber rods on the core shear properties of Z-pins reinforced foam core sandwich was investigated by a series of out-of-plane shear tests. The reinforcing pins were aligned along or opposite to the shear loading direction, or they were arrayed in an X-shaped configuration. Experimental results indicate that the compression pin and the tension pin have an equivalent contribution to core shear stiffness when they are arrayed opposite to each other. Pull-out of Z-pins from facesheets and foam core can absorb strain energy after debonding and hinder crack extension. Two shear failure modes were observed in the tests, and analytical formulae predicting the shear strength have been presented.

Investigation of the Effect of Z-Pin Reinforcement on the Collapse of Foam-Cored Sandwich Panels

Z-pin reinforced foam core sandwich is fabricated by reinforcing lightweight foam with a truss network of pultruded carbon fiber rods, and then laminated between composite facesheets to produce sandwich structure. Experimental and analytical studies on this novel sandwich composite under three-point bending were presented in this paper, and the failure models were investigated and compared with that of the unreinforced one. It is found that the bending properties of foam core sandwich are enhanced significantly by pins. The predictor formulas were developed to obtain the collapse loads of Z-pinned sandwich, and then the influence of pins on the flexural properties were presented. The collapse load is clearly affected by the inclination angle, volume fraction and Young's modulus of pin. The Z-pinned core was subsequently optimized from a minimized weight standpoint subjected to various failure constraints.

Experimental study and computer simulation on degradation of z-pin reinforcement under cyclic fatigue

This paper presents an experimental study on the fatigue degradation of the bridging forces of z-pins embedded in polymer laminates. Carbon/epoxy laminate specimens were reinforced in the through-thickness direction with two volume contents and two sizes of z-pins. The specimens were fatigue tested in the through-thickness (mode I) direction to measure the deterioration in the bridging forces of the pins. Degradation of the debonding and friction pull-out forces of the pins with the number of load cycles was determined over a range of fatigue displacements. The degradation of the bridging forces is dependent on the diameter of the pins. Reducing the diameter of the pins reduced the degradation rate of the debonding and friction forces. However, the friction force always degraded more rapidly under fatigue loading than the debonding force. Using the fatigue data and experimental observations, a micromechanics model for calculating the degradation of bridging tractions was developed and the lifetime of z-pinned laminates subjected to mode I fatigue was simulated. In the modeling, z-pins are treated as a series of external forces applied to the DCB laminates which provides bridging forces to the opening crack during delamination. Considering the degradation of bridging force under cyclic loading, the maximum bridging force in the bridging law is assumed to be a power law function decreasing with loading cycles N and maximum z-pin displacement δ max. The classic beam theory is utilized to calculate the energy release rate consumed by the newly created crack surface. The fracture energy method is used as the delamination criterion, in which the unpinned DCB fracture toughness versus loading cycles is assumed as an exponential function determined by previous test results. Computer simulation results show that z-pins can effectively reduce the delamination growth rate under both load-control and displacement-control conditions.

Cohesive modeling of delamination in Z-pin reinforced composite laminates

The mode I interlaminar fracture in Z-pin reinforced composite laminates is modeled using a cohesive volumetric finite element (CVFE) scheme. The test configuration used in this study is a Z-pin reinforced double cantilever beam specimen. A bilinear rate-independent but damage-dependent cohesive traction–separation law is adopted to model the fracture of the unreinforced composite and discrete nonlinear spring elements to represent the effect of the Z-pins. The delamination toughness and failure strength of the Z-pin reinforced composites are determined by a detailed comparison study of the numerical modeling results with experimental data. To further reduce the computational effort, we introduce an equivalent distributed cohesive model as a substitute for the discrete nonlinear spring representation of the Z-pins. The cohesive model is implemented on various test problems with varying failure parameters and for varying spatial Z-pin reinforcement configurations showing good agreement with the experimental results.