Through-thickness 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.

Effect of Post-Cured through Thickness Reinforcement on Disbonding Behavior in Skin-Stringer Configuration.

An experimental investigation of interlaminar toughness for post-cured through-thickness reinforcement (PTTR) skin-stringer sub-element is presented. The improvement in the crack resistance capability of skin-stringer samples was shown through experimental testing and finite element analysis (FEA) modeling. The performance of PTTR was evaluated on a pristine and initial-disbond of the skin-stringer specimen. A macro-scale pin-spring modeling approach was employed in FEA using a non-linear spring to capture the pin failure under the mixed-mode load. The experimental results showed a 15.5% and 20.9% increase in strength for the pristine-PTTR and initial-disbond PTTR specimens, respectively. The modeling approach accurately represents the overall structural response of PTTR laminate, including stiffness, adhesive strength, crack extension scenarios and progressive pin failure modes. This modeling approach can be beneficial for designing damage-tolerant structures by exploring various PTTR arrangements for achieving improved structural responses.

Effects of carbon-fibre Z-pins on the through-thickness tensile strength of curved composite laminates under four-point bending

This study investigates the influence of discrete through-thickness reinforcement, i.e. Z-pins, on the through-thickness tensile strength (TTS) of curved laminates through four-point bending experiments. Three types of samples are considered: unpinned, and Z-pinned with 0.27 % and 0.54 % areal densities. HexPly® IM7/8552 carbon/epoxy unidirectional prepreg with 0/±45 layup and 0.28 mm diameter T300/BMI pins were employed to manufacture the specimens. The Z-pinned laminates have comparable TTS with the unpinned samples in terms of the first observable load-drop. However, the TTS corresponding to the ultimate load-drop for low-density and high-density Z-pinned samples are 29 % and 38 % lower than for the unpinned ones, respectively. Z-pinned samples show less scatter than unpinned ones in terms of the ultimate TTS values. All specimens failed suddenly, i.e. with no evident damage progression. This implies that Z-pins were not able to form a progressive bridging zone to dissipate mechanical energy. Through a detailed meso-scale finite element analysis (FEA), it was found that the high through-thickness tensile residual stress in the Z-pin neighbourhood generated from the cool-down stage of cure is an important factor in causing the reduction in TTS of Z-pinned laminates. CT scan images of tested Z-pinned specimens show that the carbon-fibre pins experience fracture inside the laminates.

SOUND AND THERMAL INSULATION PROPERTIES OF SANDWICH COMPOSITES MADE OF WASTE KEVLAR® MATERIALS

This paper examines the thermal and acoustic insulation characteristics of sandwich composites with waste Kevlar® fiber-reinforced face materials and polyurethane/paper cardboard cores. Waste Kevlar® short fibers (carding waste) were reinforced into the sandwich composites’ core part in varying ratios (2%, 5%, and 10%). Kevlar® fabric edge waste (waste of weaving process) was used to produce the face materials of sandwich composites. Sandwich composites were also stitched using Kevlar® yarns to observe the effect of the through-thickness reinforcement on sound and thermal insulation properties. The sound insulation test results showed that reinforcement of short Kevlar® fibers into the core parts of sandwich composites somewhat raised their sound absorption coefficients. Because the stitching holes created air spaces for sound vibrations, the sound absorption coefficient values improved. The sound transmission losses of sandwich composites were also increased up to 30 dB after short Kevlar® fiber addition. The thermal conductivity coefficient of sandwich composites decreased, indicating that the addition of Kevlar® fibers increased their insulation properties.

An evaluation of large diameter through-thickness metallic pins in composites

There is increasing demand for functional through-thickness reinforcement (TTR) in composites using elements whose geometry exceeds limitations of existing TTR methods like tufting, stitching, and z-pinning. Recently, static insertion of large diameter TTR pins into heated prepreg stacks has proven a feasible and robust reinforcement process capable of providing accurate TTR element placement with low insertion forces and lower tow damage compared with existing methods for similar element sizes (>1mm diameter) like post-cure drilling. Local mechanical performance and failure mechanics of these pinned laminates are reported here. Laminates with a single statically inserted pins (1.2, 1.5, and 2.0 mm) can mostly retain their in-plane integrity alongside a local improvement in mode I delamination toughness in carbon fibre-benzoxazine laminates. Tensile strength is mostly unaffected by the pins resulting from delamination suppression, whereas there is up to a doubling of Young’s modulus. Compressive strength is significantly diminished (up to 42 %) in pinned laminates. Interlaminar toughness is improved, and peak toughness is pushed ahead of the crack as pin diameter increases. The lack of significant deterioration in in-plane tensile properties in pinned laminates produced using static insertion can expand the range and forms of materials that can be inserted compared to existing TTR.

A study on the formation and failure mechanisms of CF/PPS-metal induction welding joints strengthened by micro-pins

This study proposed a novel hybrid joining technique that combines through-thickness reinforcement (TTR) and induction welding methods to address the challenges of composite-metal joining. The effects of geometrical parameters of micro-pins on the formation and bearing performance of hybrid joints were investigated by combing the experimental and numerical simulation approaches. Two simulation models which included the induction heating transfer and joint tensile failure process were established by COMSOL Multiphysics and Abaqus/Explicit. Subsequently, digital image correlation (DIC) was used to monitor the deformation process of different types of joints under tensile load, and a scanning electron microscope (SEM) was used to observe the welding interface of failed joints. By comparing the experimental and simulation results, it is found that adding pins can significantly improve the mechanical performance of welded joints, with maximum increases of 159% and 1758% in ultimate strength and energy absorption respectively compared to welded joints without interlock structures. This technique presents a potential solution for achieving high-quality metal-composite welded structures.

Acoustic Emission Analysis of Mode II Interlaminar Fracture Toughness of 3D Reinforced CFRP

The use of composites in industry is increasing due to their ability to replace traditional materials. Carbon fiber-reinforced polymers offer a favorable strength-to-weight ratio, making them advantageous in numerous applications. Delamination is a common failure mode for composite materials, making it a crucial factor in ensuring material safety during service life. While fiber orientation in composites is designed for specific directional reinforcement, out-of-plane loads are often neglected, posing a critical challenge. Implementing through-thickness reinforcement, such as tufting, can enhance out-of-plane resistance, enabling more accurate structural designs. Non-destructive testing methods, particularly acoustic emission, play a significant role in ensuring component safety by detecting early damage and flaws. This study focused on monitoring mode II interlaminar fracture toughness and end-notched flexure (ENF), using acoustic emissions to compare the performance of samples with different through-thickness reinforcements against that of nonreinforced samples. The research analyzed acoustic emission patterns during testing, revealing a strong correlation with failure stages and the resistance induced by reinforcements. This approach provided valuable insights into damage characterization, supported by fractography analysis, especially concerning the final stages of failure due to damage, and the effects of different thread reinforcements. Acoustic emission proved crucial for real-time monitoring, enabling informed decisions to be made regarding component repair and lifespan extension in composite materials.

Flexural Behaviour of Foam Cored Sandwich Structures with Through-Thickness Reinforcements

Composite sandwich structures are well-suited for applications requiring high bending strength, flexural rigidity, crashworthiness, and light weight. However, skin–core debonding and core failure remain a barrier to optimal structural performance when polymeric foams are used as core materials. Suppressing or compartmentalising these failure modes can enhance the structural integrity of sandwich structures. In this paper, the flexural response of a sandwich structure was improved by adding carbon fibre-reinforced plastic in the form of through-thickness ribs during the manufacturing process. The effect of the position of the ribs was investigated using a quasi-static three-point bend test. A camera was used to capture failure events, while the digital image correlation technique provided the full-strain field at different stages of loading. Improved flexural performance was obtained when a reinforcement was placed on either side of the loading roller. With this configuration, skin–core debonding was restricted to a confined portion of the panel, resulting in a more localised and stable fracture process, which involved enhanced foam crushing and hardening. A simple FEA approach has been adopted in this paper and has proven to be an effective approach for capturing the details of the failure process, including the debonding in the composite foam structures, without the need for complex and computationally expensive interface modelling.

Improving the delamination bridging performance of Z-pins through the use of a ductile matrix

This paper presents a characterisation of the effect of varying the polymer matrix in Z-pin through-thickness reinforcement in pre-preg based laminates. Four matrix systems of increasing elongation at break are considered, namely: 1) a low glass-transition temperature (LTG) epoxy; 2) a high glass-transition temperature (HTG) epoxy; 3) a bismaleimide triazine (BT); 4) a bismaleimide (BMI). The last matrix is that used in commercially available Z-pins. The manufacturing of T300 carbon-fibre Z-pins employing the first three matrices via micro-pultrusion is discussed. The BT resin is considered as a benchmark for the manufacturing process. A preliminary screening of the mode II bridging performance of through-thickness reinforcement manufactured using the three matrix systems is carried out. A novel experimental set-up based on an acrylic glass carrier, which allows the failure mode of the through-thickness reinforcement to be visualised, is introduced. The preliminary tests reveal a 7-fold increase of work-to-failure for the candidates with the highest elongation at break, LTG Z-pins, compared to their baseline BMI-based counterparts. LTG Z-pins were then inserted in quasi-isotropic E-glass epoxy laminates and their bridging performance characterised across the full mode-mixity range. The experimental results indicate that LTG Z-pins provide a peak mode I interlaminar fracture toughness of the order of 40 kJ/m2, compared to the 28 kJ/m2 yielded by BMI Z-pins. Moreover, the transition from full pull-out to rupture for the LTG pins occurs at a mode-mixity of 0.55, whereas BMI Z-pins start failing at a mode-mixity of 0.2. The superior bridging performance of the LTG Z-pins is correlated with the enhanced ductility and toughness of the constituent matrix via detailed fractographic observations.

Novel method for interlaminar reinforcement using polymer/fibre pins

A common theme in the application of through-thickness reinforcement is the desire to minimize/eliminate any reduction in in-plane properties. In this work a novel method of reinforcement is introduced using a polymer/fibre pin. Similar in concept to the traditional metalworking process of riveting these pins are placed through the thickness with deliberate excess length. The application of heat and pressure deforms the exposed ends of the pin against the part surface. The resultant preform maintains its shape and may be re-formed with further heat and pressure with the reinforcement conforming to the desired shape. Samples manufactured using this method are tested under quasi-static tensile loading and show no significant change in properties due to the pin addition. Samples tested under mode 1 show further refinement of the manufacture method is required, with shallow pin angles resulting in sub-optimal performance.

Design considerations in the strengthening of composite lap joints using metal z-pins

Through-thickness reinforcements are an effective technique to improve the structural properties of bonded fibre-polymer composite lap joints. However, there is a lack of understanding of the effectiveness of this technique for internal step-lap joints. This study examines the strengthening effects of z-pins on different types of composite step-lap joints. Co-cured single, quadruple and multi-step lap joints made of carbon-epoxy laminate were reinforced with steel z-pins. The results reveal that z-pins were highly effective at increasing the ultimate strength (+20%) and elongation limit (+250%) of the single lap joint under tensile loading by suppressing catastrophic failure through plastic shear deformation following fracture of the co-bonded interface. However, the z-pins were less effective at improving the strength (+7.5%) and maximum strain (+18%) for the quadruple lap joint and did not provide any strengthening for the multi-step lap joint. The mutually exclusive relationship between the number of bonded interfaces and the z-pin strengthening efficacy is discussed.

FEM analysis of unit cell using single I-Fiber stitching process

The development and use of composite materials for several manufacturing industries require reliable adhesives and joints. However, the use of composite joints decreases the strength in the z-direction where composite laminates have lower strength. In this way, several methods have been proposed to reinforce CFRP laminates in the z-direction. Based on previous studies, a new technology in the through-thickness reinforcement for composite laminates called I-Fiber stitching process has been introduced, and the development continues under investigation. In the present research, the experimental results obtained from a unit-cell specimen under pull-out load with three different thicknesses and stitched with a single yarn using the I-Fiber stitching process were analyzed using the finite element method. Microscope image analysis was performed to obtain the best approach of the numerical model. Stress distribution of the I-Fiber and interface analysis using cohesive elements were achieved to understand the performance of the single stitch for each unit-cell case. Finally, a failure load comparison was performed between the experiments and the numerical analysis. The results showed the complexity of the I-Fiber shape under different laminate thicknesses and how the fiber volume of the reinforcement can vary along the thickness using the same stitching yarn for all cases.

Influence of glass fibre reinforced polymer composite pin reinforcement on shear and dynamic behaviour of composite joint manufactured through co-cure technique

The co-cure technique efficiently joins composite components in wings, ailerons, skins, ribs and spars in aeronautical applications. The present study investigates the effect of through-thickness reinforcement of glass fibre reinforced polymer composite pin on shear and dynamic properties of co-cured composite joints. The experimental results exhibited that through-thickness reinforcement with 2% composite pin volume significantly improved the shear strength and elongation limit by 38.4% and 102.4% compared to unpinned joints. Furthermore, composite pin reinforcement changed the catastrophic failure of co-cured joints to progressive failure due to the tougher bridging traction of composite pins. The comparison of fractured surfaces exposed that debonding, composite pin pull-out and shear fracture of pins are the major failure mechanisms involved in enhanced shear strength and progressive failure of co-cured composite joints. The experimental modal analysis avowed that through-thickness reinforcement with 2% composite pin volume improved the fundamental natural frequency of the co-cure joints, while co-cure joint reinforced with higher volume (4%) of composite pins had higher damping value due to excessive fibre damage, which increased interaction between composite adherent and adhesive.

Insertion of large diameter through-thickness metallic pins in composites

Existing Through-Thickness Reinforcement (TTR) methods for laminated composites using semi or fully rigid reinforcing elements, like tufting, stitching, and z-pinning, present limitations on reinforcing element geometry, strength, and stiffness. Where these application envelopes are exceeded, TTR element insertion results in unacceptable levels of damage to both the composite and/or TTR elements. Here, we demonstrate that low-speed insertion of rigid reinforcements into heated prepreg preforms is a feasible and robust reinforcement process capable of providing accurate TTR element placement with minimal tow disturbance compared with existing methods for similar pin sizes. The insertion process is characterised with respect to insertion forces, and mesoscale laminate deformation/damage for carbon-benzoxazine prepreg preforms. The research investigates the influence of pin leading edge on insertion for a range of pin diameters (1.2, 1.5, and 2.0 mm) and preform consolidation states, describing low insertion forces and good quality laminate preforms. Insertion forces increase with pin diameter, typically resulting from increased pin-tow contact area and friction. Large diameter sizes and low insertion forces expand the range and forms of materials that can be inserted compared to existing TTR methods and show that this method can potentially be transferred to benefit work on composite hole creation, joining, and repair.

Enhancing the Out-of-Plane Compressive Performance of Lightweight Polymer Foam Core Sandwiches

This study investigates the flatwise compressive performance of the developed sandwiches with core stitching modification. Polyvinyl chloride (PVC) foam cores with a density of 0.048 g/cm3 were stitched with 600-tex glass fiber yarns. Then the core materials were sandwiched with carbon/epoxy and glass/epoxy face sheets, respectively. The composite sandwich structures were produced by applying the vacuum bag method, and the composite layers were co-cured. The non-stitched foam core sandwiches were also manufactured as the benchmark. The developed sandwich panels were subjected to flatwise compression tests per the ASTM C365 Standard to obtain compressive properties. The mechanical testing showed that the stitched foam core sandwiches carried dramatically higher compressive loads and presented additional load peaks. The increments in the compressive strength of the developed sandwich panels were found above 80% without a significant weight penalty. Core stitching, providing the sandwich panels to have better out-of-plane strength, is a simple and less time-consuming through-thickness reinforcement process among the other core modification methods.

Z-Pinned composites with combined delamination toughness and delamination Self-Repair properties

Damage to z-pinned laminates and other types of composite materials with through-the-thickness fibre reinforcement (e.g. 3D woven yarns, stitches, tufts) cannot be repaired using conventional methods. To overcome this problem, we have developed a new type of z-pinned composite that uniquely incorporates thin thermoplastic mendable filaments which, upon activation using heat, can repair delamination damage. We demonstrate this technology using a z-pinned composite material containing carbon fibre based ply layers containing a low fraction of thermoplastic filaments composed of poly[ethylene-co-(methacrylic acid)] (EMAA). The inclusion of mendable EMAA filaments in the carbon plies increased the mode I interlaminar fracture toughness of the z-pinned composite material due to their capacity to pin, deflect and branch delamination cracks. The mode I interlaminar toughness values for crack initiation and crack propagation were increased greatly (by over 230% and 750% respectively) by the inclusion of EMAA in the carbon fabric to the z-pinned composite relative to the control specimen. Upon moderate heating, the EMAA activated a mendable healing process that repaired the delamination damage and partially restored the mode I toughness properties. This novel repair process has potential application for any type of composite containing through-thickness reinforcement, including 3D woven, stitched, tufted and z-anchored materials.

Analysis of crack-arrest design features for adhesively bonded composite joints: An experimental and numerical study

In this paper a systematic experimental and numerical study on mechanical design features to arrest fatigue crack growth in adhesive joints is performed. A material model for fatigue crack growth analysis based on a cohesive zone element formulation is presented together with a methodology to compare design features and to identify and quantify fundamental crack-arrest mechanisms. Results show that the influence of crack-arrest design features on fatigue crack growth can be successfully modelled using the developed material models and a mesoscopic representation of design features. A combination of reduced crack front loading via local reinforcement and Mode I reduction from through-thickness reinforcement are necessary to efficiently arrest crack growth. A simple local reduction of peel stresses does not lead to a sustainable crack-arrest and should therefore be avoided. Additionally, the two-dimensional growth of a crack in an adhesive joint can be exploited by increasing the effective crack front length. Finally, it is shown that a compression preload applied to the mechanical design features introduces through-thickness stresses to the adhesive that significantly reduce local fatigue crack growth rates.

A novel model of Z-pin insertion in prepreg based on fracture mechanics

Through-thickness reinforcement is a promising solution to the problem of delamination susceptibility in laminated composites. Modeling Z-pin–prepreg interaction is essential for accurate robotics-assisted Z-pin insertion. In this paper, a novel Z-pin insertion force model combining the classical cohesive finite element (FE) method with a dynamic analytical fracture mechanics model is proposed. The velocity-dependent cohesive elements, in which the fracture toughness is provided by the analytical model, are implemented in Z-pin insertion FE model to predict the crack initiation and propagation. Then Z-pin insertion experiments are performed on prepreg sample with metallic Z-pins at different velocities to identify the analytical model parameters and validate the simulation predictions offered by the model. Dynamics of Z-pin interaction with inhomogeneous prepreg is described and the effects of insertion velocity on prepreg contact force are studied. Results show that the force model agrees well with experiments and the fracture toughness rises with the increasing Z-pin insertion velocity.

Maximizing the Performance of 3D Printed Fiber-Reinforced Composites

Fiber-reinforced 3D printing technology offers significant improvement in the mechanical properties of the resulting composites relative to 3D printed (3DP) polymer-based composites. However, 3DP fiber-reinforced composite structures suffer from low fiber content compared to the traditional composite, such as 3D orthogonal woven preforms solidified with vacuum assisted resin transfer molding (VARTM) that impedes their high-performance applications such as in aerospace, automobile, marine and building industries. The present research included fabrication of 3DP fiberglass-reinforced nylon composites, with maximum possible fiber content dictated by the current 3D printing technology at varying fiber orientations (such as 0/0, 0/90, ±45 and 0/45/90/−45) and characterizing their microstructural and performance properties, such as tensile and impact resistance (Drop-weight, Izod and Charpy). Results indicated that fiber orientation with maximum fiber content have tremendous effect on the improvement of the performance of the 3DP composites, even though they inherently contain structural defects in terms of voids resulting in premature failure of the composites. Benchmarking the results with VARTM 3D orthogonal woven (3DOW) composites revealed that 3DP composites had slightly lower tensile strength due to poor matrix infusion and voids between adjacent fiber layers/raster, and delamination due to lack of through-thickness reinforcement, but excellent impact strength (224% more strong) due to favorable effect of structural voids and having a laminated structure developed in layer-by-layer fashion.

Influence of stitching on the out-of-plane behavior of composite materials – A mechanistic review

Fiber-reinforced polymer composites are widely used in the aerospace industry due to their high stiffness and strength-to-weight ratios. However, their applicability can be limited by their relatively low interlaminar properties when compared to metallic alternatives. Through-thickness reinforcement approaches, such as stitching, z-pinning, needling, tufting, and three-dimensional weaving, have been developed in recent decades to enhance the interlaminar properties of composites. Stitching is considered to be an efficient and cost-effective method to reinforce composites in the through-thickness direction. Additionally, stitch parameters (stitch density, linear thread density, thread material, pretension, etc.) highly influence the in-plane and out-of-plane properties. This paper summarizes results from over one hundred papers on the influence of stitch parameters on fracture energy, interlaminar strength, and impact characteristics of stitched composite laminates, sandwich composites, and high-temperature composites. Much of the research on the influence of stitch parameters has focused on thermoset polymer matrix composites (PMCs), while fewer studies have investigated the impact of stitch parameters on high temperature or sandwich composites. Modification of existing and new test methods have been developed to adequately measure the effectiveness of stitching on the out-of-plane behavior of PMC panels. Results demonstrate that out-of-plane properties of PMCs are highly dependent on stitch parameters and can be enhanced by through-thickness stitching.