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

A mechanistic interpretation of the comparative in-plane mechanical properties of 3D woven, stitched and pinned composites

A comparison of substantial published data for 3D woven, stitched and pinned composites quantifies the advantages and disadvantages of these different types of through-thickness reinforcement for in-plane mechanical properties. Stitching or 3D weaving can either improve or degrade the tension, compression, flexure and interlaminar shear properties, usually by less than 20%. Furthermore, the property changes are not strongly influenced by the volume content or diameter of the through-thickness reinforcement for these two processes. One implication of this result is that high levels of through-thickness reinforcement can be incorporated where needed to achieve high impact damage resistance. In contrast, pinning always degrades in-plane properties and fatigue performance, to a degree that increases monotonically with the volume content and diameter of the pins. Property trends are interpreted where possible in terms of known failure mechanisms and expectations from modelling. Some major gaps in data and mechanistic understanding are identified, with specific suggestions for new standards for recording data and new types of experiments

Tensile properties of carbon fibres and carbon fibre-polymer composites in fire

The effect of fire on the tensile properties of carbon fibres is experimentally determined to provide new insights into the tensile performance of carbon fibre-polymer composite materials during fire. Structural tests on carbon-epoxy laminate reveal that thermally-activated weakening of the fibre reinforcement is the dominant softening process which leads to failure in the event of a fire. This process is experimentally investigated by determining the reduction to the tensile properties and identifying the softening mechanism of T700 carbon fibre following exposure to simulated fires of different temperatures (up to 700 degrees C) and atmospheres (air and inert). The fibre modulus decreases with increasing temperature (above similar to 500 degrees C) in air, which is attributed to oxidation of the higher stiffness layer in the near-surface fibre region. The fibre modulus is not affected when heated in an inert (nitrogen) atmosphere due to the absence of surface oxidation, revealing that the stiffness loss of carbon fibre composites in fire is sensitive to the oxygen content. The tensile strength of carbon fibre is reduced by nearly 50% following exposure to temperatures over the range 400-700 degrees C in an air or inert atmosphere. Unlike the fibre modulus, the reduction in fibre strength is insensitive to the oxygen content of the atmosphere during fire. The reduction in strength is possibly attributable to very small (under similar to 100 nm) flaws and removal of the sizing caused by high temperature exposure

Experimental and numerical investigation of low velocity impacts of fibre-metal laminates

In this work, an experimental and numerical investigation into the impact behaviour of fibremetal laminates (FMLs) subject to low velocity impact is conducted. The study investigates the damage initiation and progression of FMLs in order to characterise and represent the complex damage responses and deformation that lead to strength and stiffness loss. Low velocity impact tests were experimentally conducted using a drop-test impact rig. Numerical analysis was conducted using Abaqus/Explicit, with damage models incorporated to represent in-plane composite damage, delamination, elastic-plastic behaviour of the metal layers at high strain rates, and failure in the adhesive layers. In addition to plastic deformation of the aluminium layers, matrix cracking, fibre failure and delamination of the composite glassepoxy prepreg were observed as critical damage modes. The numerical simulations exhibited good correlation with experimental impact tests in terms of kinematic response and damage development

Developments in structural proof testing for aircraft composite components

This paper evaluates structural proof testing methods for the detection of manufacturing defects in aircraft composite components. A static proof test method based on compliance and surface strain mapping and a dynamic (vibration) method based on mode shape curvature (MSC) analysis are evaluated for the detection of manufacturing defects. The evaluation of these structural proof methods was performed using finite element analysis to identify the test conditions best suited for damage/defect detection. The finite element modelling was validated by structural proof tests performed on T-joint composite specimens containing the manufacturing defects of large voids, porosity and delaminations. The coupled compliance and surface strain mapping technique was able to detect a delamination crack along the stiffener-skin bond-line or a void within the fillet region when the joint was elastically loaded, but failed to detect porosity at the concentrations which typically occur in joints with defects. The MSC technique successfully detected voids, porosity and delaminations in the T-joint excited by elastic stress waves induced by random frequency vibrations

Tensile strength modelling of glass fiber-polymer composites in fire

A thermal-mechanical model is presented to calculate the tensile strength and time-to-failure of glass fiber reinforced polymer composites in fire. The model considers the main thermal processes and softening (mechanical) processes of fiberglass composites in fire that ensure an accurate calculation of tensile strength and failure time. The thermal component of the model considers the effects of heat conduction, matrix decomposition and volatile out-gassing on the temperature-time response of composites. The mechanical component of the model considers the tensile softening of the polymer matrix and glass fibers in fire, with softening of the fibers analyzed as a function of temperature and heating time. The model can calculate the tensile strength of a hot, decomposing composite exposed to fire up to the onset of flaming combustion. The thermal-mechanical model is confined to hot, smoldering fiberglass composites prior to ignition. Experimental fire tests are performed on dry fiberglass fabric and fiberglass/vinyl ester composite specimens to validate the model. It is shown that the model gives an approximate estimate of the tensile strength and time-to-failure of the materials when exposed to one-sided heating at a constant heat flux. It is envisaged the model can be used to calculate the tensile softening and time-to-failure of glass-polymer composite structures exposed to fire

The efficiency of ultra-high molecular weight polyethylene composite against fragment impact

This paper presents an experimental investigation into the ballistic resistance of ultra-high molecular weight polyethylene (UHMW-PE) composite, and compares its performance against a range of common metallic and composite armour materials. An extensive experimental program was conducted to determine the ballistic limit velocity (V<inf>50</inf>) of UHMW-PE composite against 12.7 and 20 mm fragment simulating projectiles (FSPs) for a wide range of thicknesses. For protection against these projectiles, UHMW-PE composite was found to be consistently more mass efficient than rolled homogeneous armour steel (RHA), high hardness armour steel (HHA), aluminium alloy 5059-H131, and polymer composites reinforced with aramid, glass or carbon fibres. In terms of armour space claim, UHMW-PE composite was found to be less efficient than both steel types and glass fibre-reinforced plastic, though it was comparable to aramid fibre-reinforced plastic, and was more efficient than aluminium 5059-H131 and carbon fibre-reinforced plastic. Scaling effects were observed that showed metals were more effective against smaller projectiles in terms of armour mass required to stop a given projectile kinetic energy. These effects were not observed to the same extent for UHMW-PE composite, giving rise to a higher UHMW-PE mass efficiency against larger projectiles

Structural properties of self-healing microvascular fibre composites

This paper presents a finite element analysis and experimental study into the effect of microvascular fibres on the mechanical properties of self-healing carbon-epoxy composite materials. The major aim of this study is to quantify the beneficial and detrimental effects of microvascular fibres on the elastic modulus, strength and interlaminar fracture toughness properties of self-healing composites. Finite element modelling and experimentation showed that microvascular fibre networks reduce the tensile and compressive strengths but increase the mode I interlaminar fracture toughness by blunting and/or deflecting the crack tip. The localised ply waviness induces inter-ply splitting cracking under tensile loading and ply kinking under compression loading which lowers the strength properties; the larger the fibre diameter the greater the reductions in strength and stiffness

The effect of target thickness on the ballistic performance of ultra high molecular weight polyethylene composite

The ballistic performance of thick ultra-high molecular weight polyethylene (UHMW-PE) composite was experimentally determined for panel thicknesses ranging from 9 mm to 100 mm against 12.7 mm and 20 mm calibre fragment simulating projectiles (FSPs). Thin panels (similar to <10 mm thick) were observed to undergo large deflection and bulging, failing predominantly in fibre tension. With increased thickness the panels demonstrated a two-stage penetration process: shear plugging during the initial penetration followed by the formation of a transition plane and bulging of a separated rear panel. The transition plane between the two penetration stages was found to vary with impact velocity and target thickness. These variables are inter-related in ballistic limit testing as thicker targets are tested at higher velocities. An analytical model was developed to describe the two-stages of perforation, based on energy and momentum conservation. The shear plugging stage is characterised in terms of work required to produce a shear plug in the target material, while the bulging and membrane tension phase is based on momentum and classical yarn theory. The model was found to provide very good agreement with the experimental results for thick targets that displayed the two-stage penetration process. For thin targets, which did not show the initial shear plugging phase, analytical models for membranes were demonstrated as suitable