Z-pinned Composite Laminates Research Articles

Concurrent multiscale modeling of in-plane micro-damage evaluation in Z-pinned composite laminates

Most of the unit cell models for Z-pinned composite laminates (ZCL) are mesoscopic in scale, where the fiber reinforced zone is modeled as homogeneous but without detailed characterization and failure analysis of the fibers and matrix. This paper proposes a new concurrent multiscale unit cell model to investigate the in-plane micro-damage evolution of ZCL. The novelty worth mentioning is the proposed element-based method to generate random microbuckling spatial fibers in the fiber reinforced zone, where the microbuckling is caused by fiber waviness and fiber crimp, and then the mesh of fiber, Z-pin, matrix, and the interface between Z-pin and matrix is regenerated. In-plane tensile tests (ASTM D3039) are conducted for verification. There is good agreement between experiment and simulation, only with deviations of −0.97% and 2.69% for tensile modulus and fracture initiation stress, respectively. The research indicates that in-plane micro-damage initiates from the interface between Z-pin and matrix. The stress concentration and higher local fiber volume fraction, caused by the microbuckling spatial fibers clustered near the Z-pin, play a major role in the evolution of interface damage, crack initiation and growth.

A mechanistic predictive model for ultrasound guided insertion process of Ø 0.11 mm Z-pins

The insertion of finer Z-pins is a key issue for utilization of their reinforcement potential in Z-pinned composite laminates. This paper aims to reveal the insertion mechanisms of the ultrasound guided insertion process of Ø 0.11 mm Z-pins. Taking into account the fibre deformation, fibre–matrix interface, needle tip cutting and needle-prepreg contact, a new mechanical model is proposed to predict the change of insertion force during the insertion process. It is found that the application of ultrasonic vibration can minimize frictional effect and reduce fibre fracture, thus significantly decreasing the insertion force. The traditional two-phase insertion mechanism is transformed into single-phase insertion mechanism due to ultrasonic vibration. Based on this model, the effects of process parameters (including vibration amplitude, vibration frequency, feed rate and needle geometry) on the insertion force can be determined. Then Z-pin insertion experiments are performed on the prepreg sample to verify the proposed model. Results show that the model can capture the insertion mechanisms during the ultrasound guided insertion and predictions are in excellent agreement with the experimental measurements.

Damage mechanisms in the CAI failure of thin z-pinned composite laminates

The results of an experimental investigation into the compression after impact (CAI) performance of thin z-pinned carbon-epoxy laminates are reported in this paper. Unpinned and z-pinned [02/902]S and [0/±45/90]S samples were impacted at energies varying between 2 J and 35 J and then subjected to compression loading until failure. The amount and nature of the damage induced by impact and the damage mechanisms leading to CAI failure were characterized by X-radiography and through visual observation of the sample surfaces. The study shows that the effect of z-pins may be beneficial or detrimental to the post-impact strength of the laminates depending on the energy of the impact. The sequence of the main damage mechanisms observed on the laminates during the compression tests is illustrated and discussed to clarify the role of z-pins in controlling the residual strength of the impacted laminates over the full range of examined impact energies.

Experimental investigation of high strain-rate, large-scale crack bridging behaviour of z-pin reinforced tapered laminates

Significant research exists on small-scale, quasi-static failure behaviour of Z-pinned composite laminates. However, little work has been conducted on large-scale, high strain-rate behaviour of Z-pinned composites at structural level. Small-scale testing is often at an insufficient scale to invoke the full crack bridging effects of the Z-pins. Full-scale testing on real components involves large length scales, complex geometries and resulting failure mechanisms that make it difficult to identify the specific effect of Z-pins on the component failure behaviour. A novel cantilever soft body impact test has been developed which is of sufficient scale to invoke large-scale delamination, such that behaviour in Z-pin arrays at high strain-rates can be studied. Laminates containing Z-pin arrays were subjected to soft-body gelatine impact in high-speed light gas-gun tests. Detailed fractographic investigation was carried out to investigate the dynamic failure behaviour of Z-pins at the microscopic scale.

Delamination and compression-after-impact properties of z-pinned composite laminates reinforced with circumferentially notched z-pins

The investigations performed here show a positive influence on the properties of z-pin reinforced laminates using circumferentially notches as a defined z-pin surface micro-structuring. In addition to significant improvements of the fracture mechanical properties under Mode-I crack opening loading, effective reductions of the resulting delamination areas that arise during an impact can be achieved. Furthermore, the residual strength properties of pinned laminates are more favourable than those of unpinned ones and can be further improved by the use of circumferentially notched z-pins. The determined values of the Mode-I critical energy release rate GIC can be increased by approx. 13% if micro-structured z-pins are used. Laminates reinforced with these z-pins also show reductions in the dimensions of the delamination area depending on the impact energy level. Also the compression-after-impact strength can be increased by 6% for low impact energy of 17 J in z-pinned laminates reinforced with circumferentially notched z-pins, compared to those reinforced with unnotched z-pins. For increasing impact energies the values of the pinned laminates with unnotched as well circumferentially notched z-pins converge. Therefore, it can be concluded that for higher impact energies the effect of a defined micro-structuring of the z-pins surfaces on the delamination area after impact and compression-after-impact properties is negligible.

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.

Comparison of laminate thickness on the low velocity impact behaviors for Z-pinned composite laminates

The Z-pinning technique can enhance the delamination resistance for composite laminates effectively. In this paper, the effect of laminate thickness on the low velocity impact responses of Z-pinned laminates with [02/902]2S, [02/902]4S, and [02/902]6S layup patterns was explored. These laminates had nominal thickness of 2.1 mm, 4.5 mm and 6.9 mm, respectively. The impact energy was set to 5 J, 10 J, 20 J and 30 J, and the mechanical curves during the experiment were recorded. Afterwards, ultrasonic C-scanner, scanning electron microscope (SEM) and X-ray micro-computed tomography (μCT) were used to detect the impact-induced damage. The theoretical models for predicting the delamination initiation threshold and related threshold impact energy were consistent with the experimental data. The results showed that Z-pins had almost no effect on the delamination initiation, but Z-pinning became significant for delamination suppression as the impact energy exceeded the threshold impact energy. In addition, due to the improved penetration resistance, the maximum central displacements were suppressed with Z-pin implantation as the noticeable fiber breakage occurred. Due to the fact that the dominant failure mode changed from fiber breakage to delamination with the increase of the laminate thickness, the delamination damage and internal defect suppression for thin laminate was inferior to that for the intermediate-thickness laminate. However, the suppression effectiveness became weak for the thick laminate at the same impact energy due to the smaller bridging force with slight Z-pin pull-out.

Tensile properties of Z-pinned composite laminates with low damage insertion technique

In this paper, the effects of Z-pin diameter and Z-pin length on the tensile properties of carbon fiber reinforced laminates were studied. A new Z-pin insertion technique which causes less microdamage damage than the original method was developed. Experimental testing reveals that the in-plane tensile strength of Z-pin specimens is 96.98%-105.32% of the control group. The best performance of in-plane tensile properties is TF18 group which pinned by full-depth Φ0.18mm diameter. The tensile strength increases by 5.32%, and the Young’s modulus increases by 4.71% relative to the control group. Compared with the group of large diameter Z-pin, the in-plane performance of the ultra-fine diameter group is better due to the less in-plane damage. In the half-depth group, delamination occurred between the middle Z-pin sublayer and the upper and lower layers, thus reducing the in-plane tensile property. The Poisson’s ratio increases in both full depth and half depth after Z-pinned and it is only related to insertion depth.

Experimental investigation on the double-position impact responses and damage mechanism for Z-pinned composite laminates

Z-pins can enhance the resistance to delamination for composites effectively under impact loads. In this paper, the effect of Z-pinning technique on double-position low velocity impact behaviors for composite laminates were explored. The two impact positions had the same distance from the center of the laminates with the impact distances of 0 mm, 10 mm, 20 mm and 40 mm in between. The two positions were sequentially impacted with 5 J, 10 J and 20 J impact energy, respectively. The mechanical curves and the delamination damage projected area (DDPA) were recorded during the impact tests. Afterwards, scanning electron microscope (SEM) and X-ray micro-computed tomography (μCT) techniques were performed for characterizing the damage statuses and internal defects. Experimental results demonstrated that Z-pinning was unable to delay the delamination initiation at low impact energy, which led the similar damage statuses of double impact positions by comparing to the unpinned laminates. But at high impact energy, Z-pinning could suppress the delamination damage and internal defects, which weakened the damage interference. In addition, at high impact energy, dominated fiber breakage damage mode occurred for the second impact at the impact distance of 0 mm, accompanied by larger overlapped delamination area with the first impact and lower proportion of internal defects. This resulted in similar values of maximum central displacement and peak energy moment between the unpinned and Z-pinned laminates.

Minimizing the in-plane damage of Z-pinned composite laminates via a pre-hole pin insertion process

The present work focuses on an experimental investigation on the efficiency of the Z-pin pre-hole insertion technique in minimizing the in-plane damage while improving the delamination resistance of quasi-isotropic carbon fibre/epoxy laminates. The results indicate that the Z-pin pre-hole insertion causes much less crimped and/or broken fibres leading to a minimal strength reduction of 3.50% in tension and 4.80% in compression respectively for composite laminates containing 1 vol.% of carbon Z-pins. The Mode-I fracture toughnesses of the composites reinforced with ~1 vol.% carbon and Kevlar Z-pins are about 12 times higher than that of the control samples, suggesting an efficient crack arrest mechanism by the implanted pins. The substantially enhanced through-thickness properties together with the minimal reduction in in-plane performance warrant the pre-hole insertion technique as a practical method in mitigating the delamination in fibre reinforced laminates.

Mode I delamination fatigue resistance of unidirectional and quasi-isotropic composite laminates reinforced with rectangular z-pins

This paper presents an experimental study on the mode I delamination properties of circular and rectangular z-pins in hot-cured carbon fiber/epoxy laminates. Specimens with two different ply stacking sequences (unidirectional and quasi-isotropic) are used to investigate the effect of different pin geometries on the interlaminar fracture toughness and fatigue resistance. Due to a change in the debonding behavior during the cooling stage after the curing process in an autoclave, the use of rectangular z-pins results in an increased mode I fracture toughness compared to circular pins in both unidirectional and quasi-isotropic hot-cured laminates. Furthermore, Paris curves obtained from delamination fatigue resistance tests reveal that rectangular z-pins are more effective at increasing delamination resistance than circular z-pins in quasi-isotropic laminates. In unidirectional laminates, a change in the cross-section geometry of the pins only slightly affects the mode I fatigue resistance of z-pinned composite laminates.

Progressive failure analysis of Z-pinned composite laminates under tension loading

Based on micromechanical characters of Z-pinned composite laminate, a finite element model is established to predict progressive failure properties of Z-pinned composite laminate under tension loading. For Z-pinned composite laminates, four failure modes: matrix cracking, matrix compressive failure, fibre breakage and fibre kinking are included in the present model. The calculated failure strength is compared with those obtained from experiments and other model. Failure mode pictures and stress-strain curves under tension load are obtained. The results indicate fibre breakage due to the matrix cracking is the main failure mode in Z-pinned carbon/epoxy composites under tension.

Progressive failure analysis of Z-pinned composite laminates under tension loading

Progressive failure analysis of Z-pinned composite laminates under tension loading

An interface approach based on moving mesh and cohesive modeling in Z-pinned composite laminates

Abstract An FE approach based Arbitrary Lagrangian-Eulerian (ALE) and cohesive fracture mechanics is implemented to investigate the effects of debonding mechanisms on the behavior of z-pinned composite laminates. The model is based on the combination of moving and discrete cohesive interface elements, which allow the simulation of interfacial damage or strengthening mechanisms produced by debonding phenomena or z-pinned techniques, respectively. Moreover, complex phenomena such as crack initiation, coalescence mechanisms are easily implemented in both static and dynamic frameworks. Despite existing approaches, available from the literature, the computational procedure is able to overcome difficulties concerning mesh dependence of the solution, numerical complexities and costs involved in the solving procedure. The numerical implementation of the model and its capability to predict debonding mechanisms are discussed with respect different laminate configurations and onset conditions. Moreover, comparisons with existing experimental results available from the literature are developed to investigate the relationship between strengthened and unstrengthened composite laminates.

An experimental investigation into multi-functional Z-pinned composite laminates

This paper investigates the feasibility of monitoring progressive delamination growth in Z-pinned composite laminates via the measurement of through-thickness electrical resistance. This novel health monitoring technique is based on connecting Z-pins both in series and in parallel by means of arrays of electrodes arranged on the laminate surfaces. This creates a multi-functional (through-thickness reinforcing and sensing) laminated structure. Experimental results on double-cantilever beam coupons demonstrate that the entire Mode I bridging response of Z-pins can be monitored, from the arrival of the delamination front to the complete pull-out of the through-thickness reinforcement. Hence the extent of delamination can be inferred from the through-thickness resistance. This is proved for both conductive (carbon fibre-reinforced) and non-conductive (glass fibre-reinforced) laminates. The premature Z-pin failure during progressive pull-out corresponds to an abrupt increase of through-thickness electrical resistance. The delamination sensing/suppression method presented in this paper can be readily applied to Z-pinned composites at structural level.

On the delamination self-sensing function of Z-pinned composite laminates

This paper investigates for the first time the usage of through-thickness reinforcement for delamination detection in self-sensing composite laminates. Electrically conductive T300/BMI Z-pins are considered in this study. The through-thickness electrical resistance is measured as the delamination self-sensing variable, both for conductive and non-conductive laminates. The Z-pin ends are connected to a resistance measurement circuit via electrodes arranged on the surface of the laminate. The delamination self-sensing function enabled by conductive Z-pins is characterised for Mode I/II delamination bridging, using single Z-pin coupons. Experiment results show that, if the through-thickness reinforced laminate is electrically conductive, the whole Z-pin pull-out process associated with delamination bridging can be monitored. However, for a non-conductive laminate, delamination bridging may not be sensed after the Z-pin is pulled out from one of the surface electrodes. Regardless of the electrical properties of the reinforced laminate, the through-thickness electrical resistance is capable of detecting Mode II bridging, albeit there exists an initial “blind spot” at relatively small lateral deformation. However, the Z-pin rupture can be clearly detected as an abrupt resistance increase. This study paves the way for exploring multi-functional applications of through-thickness reinforcement.

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.

Effect of z-pin surface treatment on delamination and debonding properties of z-pinned composite laminates

The effect of z-pin surface treatment on the delamination fracture properties of z-pinned unidirectional carbon fibre/epoxy prepreg laminate is presented in this paper. Cryogenic and plasma treatments were used to increase the pin/composite interface properties. Z-pin pullout tests were carried out to study the relations between the bridging force and the displacement. Mode-I double-cantilever beam tests were used to characterize the improvements in delamination toughness. It was pointed out that appropriate treatments could effectively increase the delamination fracture properties. Oxygen-containing functional groups could be induced on the pin surface through cold plasma treatment. An increasing surface energy is improving the wettability so that more chemical reactions can be generated between the epoxy group and z-pin surface. Furthermore, the surface roughness of z-pins can be extended with a plasma or cryogenic treatment. The pins obtained a larger surface area, which could wet by the epoxy matrix during the z-pin-insertion and curing process.

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.

Effect of Z-Pin on the through-the-Thickness Strength of Woven Composite Laminates

Delamination failure occurs due to the out-of-plane loading because of the weakness in the thickness directional properties of composite materials. The z-pinned composite has been developed to overcome such a problem. In this study, the mechanical properties of z-pinned composite laminates were examined using the analytical model. The effects of z-pins on the strength in the thickness direction were investigated. Fibers in the laminate are gradually waved by the formation of resin rich zone caused by inserting the z-pins. A constitutive model was developed for the predictions of strengths of woven z-pinned composites. The changes in the strengths in the in-plane and out-of-plane directions of the woven z-pinned composites and the effects of controlling parameters such as the diameter of z-pin and densities of z-pin on the strengths were identified. It was found that the out-of-plane tensile strength increases 14% relative to that of composite without z-pins when the diameter of z-pin is 0.25 mm. However, the in-plane tensile strength and in-plane and out-of-plane shear strengths were reduced to 9%, 7% and 8%, respectively, over the strengths of composite without z-pins. Qualitatively good correlations are obtained between the results of the suggested model and the experiments.