Investigation of CFRP laser milling using a 30 W Q-switched Yb:YAG fiber laser: Effect of process parameters on removal mechanisms and HAZ formation

A study on laser machining of Carbon Fibre Reinforced Polymer (CFRP) is presented. Experimental tests were carried out on a 4 mm thick CFRP sheet, using a Q-switched 30 W Yb:YAG fiber laser. The aim of the paper is to detect which process parameters and how they affect the laser beam–material interaction, and to explain the effect of the process parameters on the removal mechanisms and HAZ formation. The process parameters examined were: the laser beam scan speed, the pulse frequency, the number of repetitions of the geometric pattern, the distance between two consecutive scan lines and the scanning strategy. The ANalysis Of VAriance (ANOVA) was applied to a two-level factorial design, specifically developed for this aim. Experimental results showed the presence of different interaction mechanisms such as: ablation, matrix burning and mechanical effect. The damage mechanisms and the influence of the process parameters on the HAZ extent are discussed too.     

A composite index to quantify dispersion of carbon nanotubes in polymer-based composite materials

Several methods with various levels of sophistication exist to quantify dispersion of carbon nanotubes (CNTs) in a polymer matrix, such as the ASTM D2663 standard that is often used in engineering practice. However, most methods are limited by their accuracy, complexity of implementation, and scalability. In this paper, we present a new technique and index to quantify dispersion of CNTs in a composite material. The technique is partially based on a quadrat method, and takes into account the dispersion and agglomerate size distribution of the CNTs. This index is benchmarked against the ASTM index using computer generated images and images experimentally obtained from thirty CNT composite material specimens with different CNT loading rates up to twenty volume percent. The new index is shown to be more versatile and reliable than the ASTM index. It is easily implementable in engineering practice as opposed to other more sophisticated techniques available in the literature.     

Voids and their effect on the strain rate dependent material properties and fatigue behaviour of non-crimp fabric composites materials

The manufacture of composite structures is inevitably linked to the formation of voids. Several non-destructive techniques are potentially able of detecting defects, but just the exact knowledge of the effects of defects on the mechanical properties allows the definition of thresholds for the purpose of quality management. In this paper an experimental program for characterizing the effect of voids on the composite materials behaviour is presented. Therefore glass fibre non-crimp fabric reinforced epoxy composites were produced using vacuum assistant resin transfer moulding. For obtaining various void contents specially modified process parameters were used. Nominally defect free specimens are compared with flawed specimens. Tensile testing at different loading speeds and fatigue tests in tension-compression loading are performed.     

Triglyceride-based thermosetting resins with different reactive diluents and fiber reinforced composite applications

Recently, there has been a growing research interest on renewable composite due to sustainability concerns. This work demonstrates the possibilities of bio-based reactive diluents and thermoset resin systems in composite applications from an engineering point of view. Formulating bio-based resins and monomers with different reactive diluents can tailor the physical and mechanical properties of the polymer system, allowing them to be suitable for different applications. In addition to the traditional reactive diluents, bio-based methacrylated fatty acid (MFA) was used in the formulations. Increasing the MFA content increases the toughness and the bio-based content of thermosetting polymers while reducing the hazardous air pollutant emissions. Composites panels with fiber glass and natural fiber reinforcement using selected bio-based resin formulations showed good mechanical properties and exhibited similar physical performance to parts made with commercial petroleum derived resins.     

Influence of process parameters on the thermostamping of a [0/90]12 carbon/polyether ether ketone laminate

In this study, an experimental evaluation of the thermostamping process was made for a 3D part molded with a [0/90]₁₂ laminate composed of unidirectional carbon fiber/polyether ether ketone (CF/PEEK) plies. Using the Taguchi method, the effect of four operational parameters on the part thickness, interlaminar shear strength and the degree of crystallinity were investigated. These parameters are the preheating temperature in the oven, the mold temperature, the oven to mold transfer time and the stamping pressure. The results show that the mold temperature and stamping pressure have a significant effect on part consolidation. In addition, the interlaminar shear strength, measured at the base of the molded part, was higher for thinner parts compared to those having a greater thickness. These results were also confirmed by Differential Scanning Calorimetry analysis, which show that the degree of crystallinity is higher for thinner parts.     

The influence of a thermoplastic toughening interlayer and hydrothermal conditioning on the Mode-II interlaminar fracture toughness of Carbon/Benzoxazine composites

Carbon fibre/Benzoxazine laminates with and without non-woven polyamide (PA) fibre veils at the interlaminar regions were manufactured using vacuum assisted resin transfer moulding (VARTM). The effect of the interlaminar thermoplastic veils on the Mode-II critical strain energy release rate (G_IIC), under both wet and dry conditions, was determined using two commercially available Benzoxazine resins: a toughened system and an untoughened system. In all samples the toughened system outperformed the untoughened system. The overall resistance to Mode-II crack growth was significantly improved by the inclusion of the interlaminar veils due to an increase in the thickness of the matrix-rich interlaminar region, plastic deformation of the PA fibres and a crack-pinning mechanism. Moisture caused an increase in matrix ductility, which improved the resistance to crack initiation; however, this was counteracted by a reduction in fibre/matrix interfacial strength causing a reduction in resistance to crack growth.     

The influence of hydrothermal conditioning on the Mode-I,thermal and flexural properties of Carbon/Benzoxazine composites with a thermoplastic toughening interlayer

Carbon/Benzoxazine laminates with and without non-woven thermoplastic fibrous polyamide (PA) veils at the interlaminar regions were manufactured using Vacuum Assisted Resin Transfer Moulding (VARTM). The effect of the interlaminar thermoplastic veils on the Mode-I strain energy release rate (G_IC), flexural stiffness, glass transition temperature (T_g) and water absorption behaviour was determined using two commercially available Benzoxazine resins. Despite an increase in the maximum moisture content, the veils greatly enhanced G_IC by an increase in fibre bridging of PA fibres, with concurrent reductions in flexural stiffness. Water ingress resulted in large reductions in the T_g, although no significant change was observed due to the PA interlayers. Fibre bridging and fibre pull-out were the main mechanisms by which the veils assisted in resisting delamination. The presence of the water was observed to degrade mechanical properties due to a reduction in fibre/matrix interfacial strength, molecular degradation and plasticisation of the matrix.     

Uncertainty in the manufacturing of fibrous thermosetting composites: A review

Composites manufacturing involves many sources of uncertainty associated with material properties variation and boundary conditions variability. In this study, experimental and numerical results concerning the statistical characterisation and the influence of inputs variability on the main steps of composites manufacturing including process-induced defects are presented and analysed. Each of the steps of composite manufacturing introduces variability to the subsequent processes, creating strong interdependencies between the process parameters and properties of the final part. The development and implementation of stochastic simulation tools is imperative to quantify process output variabilities and develop optimal process designs in composites manufacturing.     

Development of spring-in angle during cure of a thermosetting composite

The development of spring-in angle during cure of AS4/8552 thermosetting composite is investigated by using a cure quench technique. C-shaped preforms are cured on the inner wall of an aluminium tube. The cure is interrupted at various points during the Manufacturer's Recommended Cure Cycle (MRCC) by quenching the tool tube into water. The diameters of the specimens cured in this way are measured and the spring-in angles for a 90° arc of the specimens are calculated. The test data show that the samples quenched at earlier stages of cure exhibit a larger spring-in and the spring-in angle reduces as the specimen is further cured.

A cure kinetics simulation is performed to understand the development of cure throughout the MRCC. It has been found that the vitrification of the specimen occurs approximately 45 min after the start of the 180 °C dwell period, and the specimens quenched before vitrification are observed to have larger spring-in.

An explanation of this observation is the fact that, before vitrification the specimen is in the rubbery state during the temperature range between the cure temperature and the instantaneous glass transition temperature, and in this state it has a larger thermal expansion coefficient compared to that in the glassy state, causing more contraction in the through-the-thickness direction, hence more spring-in.

The cure quench experiment provides an insight into the relative importance of the thermal contraction above and below the glass transition temperature and the cure shrinkage.     

Experimental measurement of wrinkle formation during draping of non-crimp fabric

A rig and image analysis methodology is described to characterise wrinkle formation during draping of non-crimp fabrics. The circular fabric blank is draped over a male hemispherical mould, partly constrained by a circular clamping ring around the periphery of the blank. The three-dimensional shape of the fabric is derived from a shape-from-focus analysis of a stack of photographs of the deformed blank. Wrinkles are identified from the deviation of the shape from a smoothed shape. Wrinkle formation is strongly dependent on the fabric architecture and increases progressively with increased punch displacement. Triaxial fabrics have the highest wrinkle amplitude, unidirectional and 0/90° biaxial fabrics the lowest amplitude. The clamping force reduces the wrinkling for some fabrics but, for the maximum force applied, is not effective at eliminating wrinkling.     

Development of a carbonization-in-nitrogen method for measuring the fiber content of carbon fiber reinforced thermoset composites

Carbon fiber reinforced thermoset composites such as carbon fiber epoxy composites are widely used in aircraft and aerospace, and are being increasingly used in automotive applications because of their lightweight characteristics, high specific strength, and stiffness. The carbon fiber content in the composite plays a critical role in enhancing structural performance. The carbon fibers contribute to the strength and stiffness; therefore, the mechanical properties of the composite are greatly influenced by the carbon fiber content. Measurement of carbon fiber content is essential for product quality control and process optimization. In this work, a novel carbonization-in-nitrogen (CIN) method is developed to characterize the fiber content in carbon fiber thermoset composites. A carbon fiber composite sample is carbonized in a nitrogen environment at elevated temperatures, alongside a neat resin sample. The carbon fibers are protected from oxidization while the resin (the neat resin and the resin matrix in the composite sample) is carbonized under nitrogen environment. The neat resin sample is used to calibrate the resin carbonization rate and calculate the amount of the resin matrix in the composite sample. The new method has been validated on several thermoset resin systems, and found to yield accurate estimation of fiber content in carbon fiber thermoset composites.     

Quality control and monitoring of NSM CFRP systems: E-modulus evolution of epoxy adhesive and its relation to the pull-out force

BackgroundIn Fibre-Reinforced Polymer (FRP) applications, the mechanical behaviour of the strengthening system strongly depends on the epoxy adhesive, particularly at early ages.MethodsThe present paper describes the application of an innovative technique (termed EMM-ARM: Elasticity Modulus Monitoring through Ambient Response Method) for continuous monitoring of the stiffening process of an epoxy adhesive used in structural reinforcements applications. A simultaneous study of direct pull-out tests with concrete specimens strengthened with near-surface mounted (NSM) carbon FRP laminate strips was carried out to compare the evolution of bond performance with the E-modulus of epoxy since early ages.ResultsThe peak pull-out force and the epoxy E-modulus obtained by EMM-ARM exhibit very similar evolution kinetics.ConclusionsA relationship between the evolution of epoxy E-modulus and the maximum pull-out force is assessed, highlighting the potential of applying EMM-ARM for quality control and decision-making assistance of NSM systems.     

Morphology, dynamic mechanical and thermal studies on poly(styrene-co-acrylonitrile) modified epoxy resin/glass fibre composites

Poly(styrene-co-acrylonitrile) (SAN) was used to modify diglycidyl ether of bisphenol-A (DGEBA) type epoxy resin cured with diamino diphenyl sulfone (DDS) and the modified epoxy resin was used as the matrix for fibre reinforced composites (FRPs) in order to get improved mechanical and thermal properties. E-glass fibre was used as the fibre reinforcement. The morphology, dynamic mechanical and thermal characteristics of the systems were analyzed. Morphological analysis revealed heterogeneous dispersed morphology. There was good adhesion between the matrix polymer and the glass fibre. The dynamic moduli, mechanical loss and damping behaviour as a function of temperature of the systems were studied using dynamic mechanical analysis (DMA). DMA studies showed that DDS cured epoxy resin/SAN/glass fibre composite systems have two T_gs corresponding to epoxy rich and SAN rich phases. The effect of thermoplastic modification and fibre loading on the dynamic mechanical properties of the composites were also analyzed. Thermogravimetric analysis (TGA) revealed the superior thermal stability of composite system.     

Durability study of pultruded CFRP plates immersed in water and seawater under sustained bending: Water uptake and effects on the mechanical properties

This paper presents the durability behavior of pultruded unidirectional carbon fiber reinforced polymer (CFRP) plates immersed in water and seawater at room temperature, under sustained bending strain of 30% and 50% ultimate strain. In this study, water absorption kinetics of CFRP composite and effects of moisture ingress on the mechanical properties, such as tensile properties and short beam shear strength, constitute integral parts of the investigation. The study reveals that seawater immersion leads to higher equilibrium moisture content than water immersion, due to the blister induced damages on the CFRP plate surfaces in seawater. However, diffusion coefficient in seawater immersion is shown to be lower compared to the water immersion, and is attributed to the high concentration of dissolved salts in seawater that retard water diffusion by osmosis. Increasing the bending strain reduces the free volume fraction of the resin matrix, which is responsible for the decreased water uptake and diffusion coefficient for both immersions. Immersion in both media leads to the pronounced degradation in the resin controlled property (i.e., short beam shear strength) of CFRP, but shows less or negligible effects on the fiber controlled properties (i.e., tensile strength and modulus). Both immersion media and 50% bending strain level show remarkable effects on the variation of the mechanical properties of CFRP.     

Mechanical characterisation of carbon fibre–PEEK manufactured by laser-assisted automated-tape-placement and autoclave

Obtaining autoclave-level mechanical properties using in-situ consolidation of thermoplastic composites by Automated Tape Placement (ATP) is challenging. However, relatively recent availability of high quality ATP grade pre-preg material and tape heads equipped with more efficient heat sources (e.g. lasers) offers an opportunity to achieve improved mechanical properties and deposition rates. In the present study, carbon fibre–PEEK laminates, manufactured by laser-assisted ATP (LATP) and autoclave, are compared. Analysis of the through-thickness temperature distribution during LATP processing using thermocouples indicates that LATP cooling rates are extremely rapid and suggests full through-thickness melting of the pre-preg tape may not occur. Inadequate crystallinity, in conjunction with voids, compromised mechanical properties compared to autoclaved laminates but was beneficial in terms of the toughness of LATP laminates. Optimisation of pre-preg properties and processing parameters is required to realise the full potential of the LATP process in terms of mechanical properties, energy requirements, cost and deposition rates.     

The impact of the nature of nanofillers on the performance of wood polymer nanocomposites

Wood polymer nanocomposites have been prepared from aspen wood using melamine-urea-formaldehyde (MUF) and montmorillonite nanoclay. The nanoparticles were ground with a ball-mill and mixed with the prepolymer to form suspensions that were subsequently impregnated into the wood and in situ polymerized. The influence of the nature of nanofillers and interphase interactions between nanoparticles and MUF on the physical/mechanical properties of the resulting wood polymer nanocomposites was investigated, using SEM, TEM and Electron Probe Micro-Analysis (EPMA) methods. Significant improvements in wood properties, including surface hardness, modulus of elasticity, dimensional stability and water repellence, were obtained with the addition of hydrophobic nanoparticles into the wood. The improved properties could be ascribed to inherent properties as well as better interphase between MUF and nanoparticles, and their co-reinforcement on the wood. Ball-mill treatment favored the dispersion of the nanoparticles into the wood, but broke down functional groups on the hydrophobic nanoclay surface, which was detrimental for the bonding between the nanoparticles and the MUF matrix.     

On avoiding thermal degradation during welding of high-performance thermoplastic composites to thermoset composites

One of the major constraints in welding thermoplastic and thermoset composites is thermal degradation of the thermoset resin under the high temperatures required to achieve fusion bonding of the thermoplastic resin. This paper presents a procedure to successfully prevent thermal degradation of the thermoset resin during high-temperature welding of thermoplastic to thermoset composites. The procedure is based on reducing the heating time to fractions of a second during the welding process. In order to achieve such short heating times, which are much too short for commercial welding techniques such as resistance or induction welding, ultrasonic welding is used in this work. A particularly challenging scenario is analysed by considering welding of carbon-fibre reinforced poly-ether-ether-ketone, with a melting temperature of 340 °C, to carbon-fibre reinforced epoxy with a glass transition temperature of 157 °C.     

Characterization of composite materials made from discontinuous carbon fibres within the framework of composite recycling

Recycling carbon fibres from waste composite materials would only be efficient if it were possible to separate the fibres and the matrix and to re-use the recycled fibres as new reinforcements. The challenge is to use non-continuous fibres to produce high-strength materials. The formation of defects in “semi-long” fibre composites has not yet been taken into account. In this paper the influence of fibre length and fibre alignment on the strength and the modulus of composite materials is illustrated. It is shown that the presence of defects may be modelled in order to understand what the quality of a second generation composite material would be.     

Forming induced wrinkling of composite laminates with mixed ply material properties; an experimental study

One disadvantage of multi-layer forming of unidirectional (UD) prepreg tape is the risk of out-of-plane wrinkling. This study aims to show how mixed ply material properties affect global wrinkling behaviour.An experimental study was performed using pre-stacked UD prepreg on a forming tool with varying cross sections. Parameters studied include local interply friction, effects of co-stacking and fibre stresses in critical fibre directions. Experimental evaluation was performed on out-of-plane defect height, type and location. The study shows that fibre stresses in some fibre directions were crucial for the shearing required to avoid wrinkling. The same fibre stresses may cause wrinkling if the lamina is stacked in a non-beneficial order. Changing the friction locally, or reducing the number of difficult combinations of fibre angles, improves the forming outcome slightly. However, in order to make a significant improvement, co-stacking or different fibre stacking is required.     

Deconsolidation in glass mat thermoplastic composites: Analysis of the mechanisms

During reheating and post-processing of thermoplastic-based composites, deconsolidation is often observed: the volume fraction fibre decreases and the void content increases. In this article, the phenomena leading to deconsolidation are investigated, with particular emphasis on the elastic release of stress in the preform, also called springback effect. A model is proposed to simulate the evolution of the specimen thickness with time. A comparison with model experiments consisting in relaxation of glass mats in polyethylene-glycol is provided. This result, together with reheating experiments of Glass Mat reinforced Thermoplastics GMT parts, showed that deconsolidation is mainly governed by the elastic behaviour of the fibre preform. It is also observed that the air initially dissolved in the matrix tends to coalesce during reheating due to diffusion, but also to tensile forces induced by the springback effect.