Delamination properties of z-pinned composites in hot–wet environment

The effect of hot–wet environment (75 °C and 85% relative humidity) on the delamination fracture properties and interlaminar toughening mechanisms of z-pinned carbon fibre–epoxy composite is investigated. The absorption rate of water from the hot–wet environment into the composite is accelerated slightly by z-pins, although the pins did not change the saturation limit of the material. Absorbed water weakens the pin/composite interface and this lowers the ultimate elastic traction load generated by z-pins under mode I interlaminar loading. However, once the pin/composite interface has failed, the traction load and energy required to pull-out the z-pins is not affected by absorbed water. The mode I interlaminar fracture toughness and low-energy impact damage resistance of z-pinned composites is not degraded significantly by exposure to hot–wet environment, and this is because absorbed water does not affect the pull-out traction properties of z-pins.     

Influence of z-pin length on the delamination fracture toughness and fatigue resistance of pinned composites

The effect of z-pin length on the mode I and mode II delamination toughness and fatigue resistance of z-pinned carbon-epoxy composites is investigated. Experimental testing and mechanical modelling reveals that both the mode I fracture toughness and fatigue resistance increase with the z-pin length due to increased bridging traction loads generated by elastic stretching and pull-out of the pins. The opposite trend occurs for mode II toughness, which decreases with increasing z-pin length due to lower traction loads arising from restrictions on the shear-induced rotation and pull-out of the pins. The mode II fatigue resistance is increased by z-pinning, although it is not dependent on the z-pin length. Increasing the z-pin length beyond a critical size also changes the mode I and mode II delamination fracture and fatigue processes from single to multiple cracking. The effect of z-pin length on the delamination toughening and fatigue strengthening mechanisms is determined.     

Environmental durability of z-pinned carbon fibre–epoxy laminate exposed to water

The influence of z-pins on the water absorption properties of a quasi-isotropic carbon fibre–epoxy laminate is assessed. Fibrous composite pins accelerate the moisture absorption rate and increase the total absorbed moisture concentration when the laminate is immersed in water. However, the moisture absorption properties of the laminate are not affected significantly by pins when exposed to hot and humid air. Water diffusion into the z-pinned laminate is aided by interfacial cracks between the pins and laminate. Also, the axial alignment of fibres within the composite pins in the through-thickness direction increases the water absorption rate. Pin pull-out tests reveal that water absorption reduces the mode I crack bridging traction load generated by pins by reducing the shear strength of the pin-laminate interface. This indicates that the mode I delamination toughness induced by pinning is weakened by moisture absorption.     

Experimental determination of the mode I delamination fracture and fatigue properties of thin 3D woven composites

The mode I delamination fracture toughness and fatigue strength of thin-section three-dimensional (3D) woven composite materials is experimentally determined. The non-crimp 3D orthogonally woven carbon–epoxy composites were thin (2 mm) and consequently their through-thickness z-binder yarns were inclined at a very steep angle (about 70°) from the orthogonal direction. The steep z-binder angle has a marked effect on the delamination toughening and fatigue strengthening mechanisms. Experimental testing revealed that the fracture toughness and fatigue resistance increased progressively with the volume content of z-binders. However, the steep angle caused the z-binder yarns bridging the delamination crack to deform and fail in shear and through-thickness tension, rather than in-plane tension which usually occurs in thick 3D woven composites. Mode I pull-off tests on a single woven z-binder yarn embedded within the composite revealed that the crack bridging traction load, strain energy absorption and failure mechanism were strongly affected by the steep angle.     

Experimental analysis of the through-thickness compression properties of z-pinned sandwich composites

This paper presents an experimental investigation into the flat-wise compression properties, strengthening mechanisms and failure modes of sandwich composite materials reinforced with orthogonal z-pins. The compression modulus of the sandwich composite increases rapidly with the volume content of z-pins due to their high longitudinal stiffness, however acoustic emission monitoring and X-ray computed tomography reveal that some z-pins are damaged during elastic loading. The compression stress to induce core crushing is increased greatly by z-pinning (up to nearly 700%), although a large percentage of the z-pins fail close to the elastic stress limit by longitudinal splitting and/or kinking. The total absorbed compressive strain energy of the sandwich composite is also improved greatly by z-pinning (more than 600%) due to the z-pins resisting core crushing, even though they are severely damaged. The results and observations presented in this paper have implications on the mechanical modelling of sandwich materials reinforced with brittle z-pins.     

Structurally stitched NCF CFRP laminates. Part 1: Experimental characterization of in-plane and out-of-plane properties

The insertion of local through-thickness reinforcements into dry fiber preforms by stitching provides a possibility to improve the mechanical performance of polymer-matrix composites perpendicular to the laminate plane (out-of-plane). Three-dimensional stress states can be sustained by stitching yarns, leading to increased out-of-plane properties, such as impact resistance and damage tolerance. On the other hand, 3D reinforcements induce dislocations of the in-plane fibers causing fiber waviness and the formation of resin pockets in the stitch vicinity after resin infusion which may reduce the in-plane stiffness and strength properties of the laminate.In the present paper an experimental study on the influence of varying stitching parameters on in-plane and out-of-plane properties of non-crimp fabric (NCF) carbon fiber/epoxy laminates is presented, namely, shear modulus and strength as well as compression after impact (CAI) strength and mode I energy release rate. The direction of stitching, thread diameter, spacing and pitch length as well as the direction of loading (which is to be interpreted as the direction of the three rail shear loading or the direction of crack propagation in case of mode 1 energy release rate testing) were varied, and their effect on the mechanical properties was evaluated statistically.The stitching parameters were found to have ambivalent effect on the mechanical properties. Larger thread diameters and increased stitch densities result in enhanced CAI strengths and energy release rates but deteriorate the in-plane properties of the laminate. On the other hand, a good compromise between both effects can be found with a proper selection of the stitching configurations.     

The effect of stitching on FRP cylindrical shells under axial compression

FRP composites have usually poor through-thickness mechanical properties and, therefore, are able to sustain more loads in tension than in compression and fail as a consequence of buckling. The through-thickness reinforcement is carried out by stitching to improve the delamination strength and to reduce the in-plane crack growth rate.Experiments were performed on both stitched and unstitched laminated plates which were prepared by using woven roving glass fibre mat and chopped strand glass fibre mat with polyester resin, and the effect of stitching was studied. It is observed that stitching increases delamination strength to a great extent. There are losses in the in-plane mechanical properties due to in-plane fibre damage and creation of resin-rich pockets.Various energy-absorbing modes observed during progressive crushing of both stitched and unstitched FRP cylindrical shells under axial compression were studied both experimentally and theoretically. Analytical expressions for the calculation of energy absorption in various modes and average crush stress were derived, and the results thus obtained were compared with the experiments.     

Compression fatigue properties of z-pinned quasi-isotropic carbon/epoxy laminate with barely visible impact damage

This paper presents an experimental study into the use of z-pins to improve the compression fatigue properties of quasi-isotropic carbon fibre–epoxy composite containing barely visible impact damage (BVID). The study investigates the effect of increasing volume content of z-pins (up to 4%) on the barely visible impact damage resistance, post-impact compression fatigue properties, and fatigue damage mechanisms of a quasi-isotropic carbon–epoxy material. The study reveals new insights into the impact damage resistance of z-pinned composites. Z-pins induce different responses in the compression fatigue properties of the quasi-isotropic composite following low or high-energy impact loading. Z-pins proved ineffective at increasing the fatigue properties when the quasi-isotropic composite contained low-energy BVID. However, z-pins were effective at improving the fatigue performance of the composite with high-energy BVID, with the post-impact fatigue life and fatigue endurance limit increasing with the pin content. The improvement in fatigue performance is due solely to the increased resistance against high-energy impact damage imposed by the z-pins. It is also found that z-pins do not affect the fatigue mechanism or fatigue damage growth rate of the composite containing BVID.     

Multifunctional composites using reinforced laminae with carbon-nanotube forests

Traditional fibre-reinforced composite materials with excellent in-plane properties fare poorly when out-of-plane through-thickness properties are important. Composite architectures with fibres designed orthogonal to the two-dimensional (2D) layout in traditional composites could alleviate this weakness in the transverse direction, but all of the efforts so far have only produced limited success. Here, we unveil an approach to the 3D composite challenge, without altering the 2D stack design, on the basis of the concept of interlaminar carbon-nanotube forests that would provide enhanced multifunctional properties along the thickness direction. The carbon-nanotube forests allow the fastening of adjacent plies in the 3D composite. We grow multiwalled carbon nanotubes on the surface of micro-fibre fabric cloth layouts, normal to the fibre lengths, resulting in a 3D effect between plies under loading. These nanotube-coated fabric cloths serve as building blocks for the multilayered 3D composites, with the nanotube forests providing much-needed interlaminar strength and toughness under various loading conditions. For the fabricated 3D composites with nanotube forests, we demonstrate remarkable improvements in the interlaminar fracture toughness, hardness, delamination resistance, in-plane mechanical properties, damping, thermoelastic behaviour, and thermal and electrical conductivities making these structures truly multifunctional.     

Macroscopic analysis of interfacial properties of flax/PLLA biocomposites

This study presents results from a study of the mechanical behaviour of flax reinforced Poly(l-Lactic Acid) (PLLA) under in-plane shear and mode I interlaminar fracture testing. Slow cooling of the unreinforced polymer has been shown to develop crystalline structure, causing improvement in matrix strength and modulus but a drop in toughness. The in-plane shear properties of the composite also drop for the slowest cooling rate, the best combination of in-plane shear performance and delamination resistance is noted for an intermediate cooling rate, (15.5 °C/min). The values of G_Ic obtained at this cooling rate are higher than those for equivalent glass/polyester composites. These macro-scale results have been correlated with microdroplet interface debonding and matrix characterization measurements from a previous study. The composite performance is dominated by the matrix rather than the interface.     

Improving bearing performance of composite bolted joints using z-pins

The effect of z-pins on the bearing properties and damage tolerance of composite bolted joints is experimentally studied in this paper. The region around bolt-holes in carbon/epoxy laminates was reinforced in the through-thickness direction with different volume contents and sizes of fibrous z-pins. Bearing test results show that the z-pins improved the bearing stiffness (by 7.5–9.6%), ultimate load (7.7–12.8%), failure strength (7.4–9.8%), and elastic strain energy absorption to bearing failure (8.5–16.3%) of the composite joints. The bearing properties increased at a quasi-linear rate with the z-pin content, but were not dependent on pin diameter. Stiffness is improved by z-pins increasing the through-thickness tensile modulus around the bolt-hole of the joint. Post-mortem microstructural examination of the failed joint specimens revealed that z-pins improve the bearing strength by reducing cracking near the bolt-hole via an interlaminar bridging toughening mechanism that involves debonding and frictional sliding of pins within the damaged region. The elastic strain energy to failure is increased by the through-thickness stiffening and toughening provided by the z-pins. This study proves that the reinforcement of bolt-holes with z-pins increases the bearing properties without the weight penalty incurred with the traditional strengthening method of thickening the laminate around holes.     

Composite toughness enhancement with interlaminar reinforcement

An experimental investigation of a newly proposed through-thickness reinforcement approach aimed to increase interlaminar toughness of laminated composites is presented. The approach alters conventional methods of creating three-dimensional fiber-reinforced polymer composites in that the reinforcing element is embedded into the host laminate after it has been cured. The resulting composite is shown to possess the benefits of a uniform surface quality and consolidation of the original unreinforced laminate. This technique was found to be highly effective in suppressing the damage propagation in delamination double-cantilever beam (DCB) test samples under mode I loading conditions. Pullout testing of a single reinforcing element was carried out to understand the bridging mechanics responsible for the improved interlaminar strength of reinforced laminate and stabilization and/or arrest of delamination crack propagation. The mode I interlaminar fracture of reinforced DCB samples was modeled using two-dimensional cohesive finite-element scheme to support interpretation of the experiments.     

Assessment of transverse impact damage in GF/EP laminates of conductive nanoparticles using electrical resistivity tomography

GF/EP composite laminates with an epoxy matrix modified by carbon black (CB) of 2.0 wt.% and copper chloride (CC) were manufactured by the vacuum assisted resin infusion (VARI) technique. The effects of CB nanoparticles and CC on improvement in Modes I and II interlaminar fracture toughness and impact damage resistance and on the electrical conductivity of GF/EP laminate composites were investigated. Delamination growth was calibrated by in situ electrical resistance changes during interlaminar fracture tests. The relationship between growth of delamination and change in electrical resistance was characterised. A damage index based on the change in electrical resistance was introduced, and a new method of electrical resistivity tomography was developed to access transverse impact damage in GF/EP laminates based on a matrix of conductive points in both in-plane and through-thickness directions. The damage images from in-plane and through-thickness electrical resistivity tomography were finally estimated with the corresponding C-scan.     

Finite element model for compression after impact behaviour of stitched composites

A finite element (FE) model using coupling continuum shell elements and cohesive elements is proposed to simulate the compression after impact (CAI) behaviour and predict the CAI strength of stitched composites. Continuum shell elements with Hashin failure criterion exhibit the composite laminate damage behaviour; whilst cohesive elements using traction-separation law characterise the laminate interfaces. Impact-induced delamination is explicitly modelled by reducing material properties of damaged cohesive elements. Computational results have demonstrated the trend of increasing CAI strength with decreasing impact-induced delamination area. Spring elements are introduced into the model to represent through-thickness stitch thread in the composite laminates. Results in this study validate experimental finding that CAI strength is improved when stitching is incorporated into the composite structure. The proposed FE model reveals good CAI strength predictions and indicates good agreement with experimental results, making it a valuable tool for CAI strength prediction of stitched composites.     

Prediction of stiffness and stresses in z-fibre reinforced composite laminates

The mechanical properties of z-pinned composite laminates were examined numerically. Finite element calculations have been performed to understand how the through-thickness reinforcement modifies the engineering elastic constants and local stress distributions. Solutions were found for four basic laminate stacking sequences, all having two percent volume fraction of z-fibres. For the stiffness analysis, a micro-mechanical finite element model was employed that was based on the actual geometric configuration of a z-pinned composite unit cell. The numerical results agreed very well with some published solutions. It showed that by adding 2% volume fraction of z-fibres, the through-thickness Young's modulus was increased by 22–35%. The reductions in the in-plane moduli were contained within 7–10%. The stress analysis showed that interlaminar stress distributions near a laminate free edge were significantly affected when z-fibres were placed within a characteristic distance of one z-fibre diameter from the free edge. Local z-fibres carried significant amount of interlaminar normal and shear stresses.     

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.     

Experimental determination of the structural properties and strengthening mechanisms of z-pinned composite T-joints

This paper presents an experimental investigation into the efficacy of z-pins to improve the structural properties of stiffened joints made of carbon/epoxy composite. Pull-off tests were performed on T-joints without z-pins or reinforced along the skin–stiffener bond-line with z-pins to volume contents of 0.5%, 2% or 4%. Testing was performed at different pull-off load angles between 0° and 45° to the stiffener to induce different proportions of normal (through-thickness) tensile and in-plane secondary bending stresses along the skin–stiffener bond-line. It was found that z-pins do not improve the stiffness or failure initiation load of T-joints, but they are effective at raising the ultimate failure strength, failure displacement, and absorbed energy capacity. These properties increase rapidly with the z-pin content, and maximum improvements of about 75% to the ultimate strength and over 600% to the total absorbed energy capacity were achieved at the highest pin content (4% by volume). The percent improvements to the structural properties are approximately the same for the different load angles, revealing that z-pins are equally effective at resisting bond-line cracking under normal tensile or secondary bending stresses. Fractographic analysis revealed that z-pins increase the joint properties by creating bridging tractions across the bond-line crack between the stiffener and skin. The z-pins ultimately fail by a combination of debonding/pull-out from the adherends.     

Improving the through-thickness thermal and electrical conductivity of carbon fibre/epoxy laminates by exploiting synergy between graphene and silver nano-inclusions

Fibre-reinforced polymer composites typically feature low functional (e.g., electric and thermal conductivity) and structural (e.g. mechanical strength and fracture toughness) properties in the laminate’s thickness direction. In the event of lightning strikes, overheating, and impact by foreign objects, composite laminates may suffer wide spread structural damage. This research explores the synergistic physical interaction between two-dimensional nanostructured (graphene nano-platelets) and, zero- or one-dimensional conductive fillers (silver nanoparticles or silver nanowires, respectively) when both are dispersed in fibre–polymer laminates. The results reveal a synergistic improvement in the through-thickness thermal conductivity that is more than the additive improvements by each constituent. Specifically, the simultaneous inclusion of graphene nano-platelets and silver nanoparticles/nanowires at a combined loading of 1 vol% resulted in approximately 40% enhancement in the through-thickness thermal conductivity while the inclusion of graphene nano-platelets alone at the same loading resulted only in 9% improvement. Similarly, the through-thickness electrical conductivity of carbon fibre/epoxy laminates incorporating graphene nano-platelets together with silver nanoparticles/nanowires was notably higher (⩾70%) than can be achieved by graphene nano-platelets alone (∼55%). These results demonstrate that the presence of nano-reinforcements exhibiting varied phonon transport and electron transfer pathways, and geometric aspect ratios promote synergistic physical interactions. Small improvements were found in the mechanical properties, including tensile, flexural or compressive properties of the carbon fibre-reinforced laminates, due to the relatively low concentrations of the nano-fillers.     

Ballistic impact and explosive blast resistance of stitched composites

The effectiveness of stitching in increasing the damage resistance of polymer composites against ballistic projectiles and explosive blasts is determined. Glass-reinforced vinyl ester composites stitched in the through-thickness direction with thin Kevlar®-49 yarn were impacted with a bullet travelling at 0.9 km s⁻¹ or an underwater explosive shock wave moving at 1.5 km s⁻¹. The amount of delamination damage to the composite caused by a ballistic projectile was reduced slightly with stitching. Stitching was highly effective in increasing the damage resistance against explosive blast loading. The increased damage resistance was due to the stitching raising the Mode I interlaminar fracture toughness of the composite. While the stitched composites experienced slightly less damage, their flexural modulus and strength was similar to the properties of the unstitched composite after ballistic impact testing. The post-blast flexural properties of the stitched composites, on the other hand, were degraded less than the properties of the unstitched material.