R. Brooks

Compression moulding of glass and polypropylene composites for optimised macro- and micro- mechanical properties II. glass-mat-reinforced thermoplastics

Processing of glass-mat thermoplastics (GMTs) by hot-flow compression moulding represents a complex physical process with non-isothermal and non-Newtonian conditions. To increase the understanding of process and property interactions, a series of statistically designed experiments have been conducted on flat plaques. Processing variables were ranked in order of importance for increased laminate properties. Processing models were generated using regression techniques describing the effect of processing variables on void content measured from the microstructure and laminate mechanical properties. Time at pressure had the greatest effect on strength and moduli.

Compression moulding of glass and polypropylene composites for optimised macro- and micro-mechanical properties. 4: Technology demonstrator — a door cassette structure

Abstract A systematic study of the effects of charge composition and process parameters is presented for a prototype automotive part (a door cassette) produced by compression moulding from glass fibre and polypropylene composites. This follows previous studies (Parts I–III) of the process-property relations for commingled glass and polypropylene fabrics, glass-mat thermoplastics (GMTs) and sandwiched structures of commingled fabric shins with a GMT core. The scope and function of the component is described together with details of the moulding experiments with GMT and fabric combined as a pre-laminated sandwich and a GMT charge with local areas of fabric to provide selective stiffening. Process parameters were varied to study the effects on proportional mould fill, pressure distributions, microstructure and flexural modulus. Statistical process models were generated from the results and these were used to define process windows. Of the parameters studied, the consolidation time dominated the flexural modulus and void content whilst increased press force and material temperature increased proportional mould fill.

Compression moulding of glass and polypropylene composites for optimised macro- and micro- mechanical properties 3. Sandwich structures of GMTS and commingled fabrics

The co-moulding of random-reinforced GMTs with commingled E-glass and polypropylene fabrics was investigated in a series of experiments which were designed statistically. Materials were heated in an infra-red oven prior to non-isothermal compression moulding. Maximum preheat times and temperatures were determined by gel permeation chromatography to monitor the onset of degradation of the polymer matrix. Laminate properties were mapped according to the processing parameters and optimum conditions for consolidation were identified. The process parameters were ranked in order of importance for maximum flexural modulus and time at pressure proved the most significant factor. Microscopy was used to investigate the impregnation mechanisms and the subsequent removal of voidage.

Impact properties of compression moulded commingled E-glass-polypropylene composites

Abstract The impact properties of commingled E-glass-polypropylene composites with varying fibre architectures have been investigated. The fabric structures include balanced and unbalanced twill weaves and a three-dimensional woven fabric. Comparative data for glass mat thermoplastics are included and additional comparisons with crossply continuous filament tape laminates are also made. All the materials were processed into flat panels via non-isothermal compression moulding. First, voidage was correlated with processing parameters to produce well consolidated laminates. Mechanical tests included tensile, flexural, and interlaminar shear. Impact tests include Charpy and falling weight; the latter ranged from 15 J to full penetration and were followed by IR thermography and microscopy to determine the extent of damage areas. The results demonstrate the evolution of damage with increasing impact energies and the mode of impact damage propagation for the different fibre architectures.

Characterization of transverse failure in composites using acoustic emission

Composites materials provide a strong, lightweight material suitable for engineering applications. Degradation of these materials is by propagation of a range of defects which ultimately control failure. Acoustic emission (AE), an ultrasonic method for materials characterization, provides one of the most sensitive techniques for monitoring this material degradation and has advantages over both conventional ultrasonic and radiographic methods. Since composite materials used for engineering applications are manufactured by cross-ply methods, transverse failure is an important damage mechanism in controlling the initiation of damage in a complete composite. These defects cause stiffness degradation which may be lifetime limiting in, for instance, stiffness dependent aerospace applications, ultimately leading to initiation of delamination. Transverse samples of glass-polyester resin have been monitored using acoustic emission as they are taken to mechanical failure. Results presented consider the AE produced by transverse composite material in cured and post-cured conditions. The pattern of AE indicates the fact that the resin toughness has increased due to post cure. Weibull statistics have been applied to acoustic emission event parameters in the case of a transverse composite, although this particular application of the statistics does not bear a fundamental relationship to the microscopic damage mechanisms. Weibull statistical parameters associated with AE signals have proved useful in signalling the changing condition of these materials and monitoring the onset of damage. Transverse cracking in fibre bundles is considered, with acoustic emission providing a sensitive method for real time detection of cracking. AE provides a unique view into the micromechanics of defect initiation and growth.

Influence of microstructural voids on the mechanical and impact properties in commingled E-glass/polypropylene thermoplastic composites

Abstract In recent years, the compression moulding of E-glass/polypropylene commingled composites has been thoroughly investigated. In particular, in the University of Nottingham, a number of studies have been carried out, trying to correlate moulding parameters with mechanical properties and microstructural void content. However, some aspects of commingled composites have received less coverage so far and are therefore dealt with in this paper. These concern the effect of the processing conditions of these materials on interlaminar shear strength and impact properties and the influence of the synergy between processing, microstructure and properties on the impact performance of commingled composite structures. With this aim, flat plaques of E-glass/polypropylene commingled composites with a different fibre architecture (two-and three-dimensional) were non-isothermally compression moulded under various moulding conditions and then tested. The test programme included falling weight impact tests with a staircase procedure, Charpy impact tests and interlaminar shear strength (ILSS) tests. To evaluate the consolidation of the laminates, void content measurement using optical microscopy was related to ILSS and impact test results. In particular, the specific issues arising in moulding laminates with added three-dimensional fibres were studied. These include correct placement of the tow, sufficient preheating of thick laminates and nesting of the layers during moulding. The results of these tests are discussed in the light of the moulding conditions and quality, and conclusions are drawn regarding optimum moulding conditions for impact performance. Finally, indications on the reliability and possible improvement of the moulding procedure to yield a sufficient moulding quality, even with large thickness, are also provided. The knowledge acquired on material consolidation properties is applied in the manufacture of an automotive side intrusion beam: problems due to the scale effect are also discussed.

Transverse cracking of fibre bundle composites studied by acoustic emission and Weibull statistics : effects of postcuring and surface treatment

Transverse failure in composite materials is a mechanism in the ultimate failure of the engineering composite. It is controlled by the strength of the fibre/matrix interface and improvement in the strength of this interface will improve the overall transverse strength. Transverse fibre bundle composites (TFBC) have been tested to failure, where the condition of the composite and the fibre/matrix interface have been modified. Progression to failure has been monitored using acoustic emission with the AE data analysed in a novel way using Weibull statistics. Although Weibull statistics have previously been used to characterise fibre bundle failure, where the concept of weakest link applies, this work extends this approach in an empirical way using an acoustic emission form of Weibull equations. The AE profile, when compared to stress/strain data, showed a “quiet-then-noisy” profile for room cured resin, which changed to “noisy-then-quiet” when the resin was post cured. Kevlar reinforced TFBC showed regular AE from low strains. The pattern of AE changed when specimens had been post cured and when the Kevlar fibres had been subjected to ultrasound treatment. Although individual AE events were highly variable, Weibull analysis of the AE parameters derived from a glass reinforced composite proved highly robust, with the AE ringdown count distributions moving to higher values for the more brittle, stronger post-cured resin. Measuring interfacial failure stress via the onset of AE, suggested the interface was weakened, but in a selective way, which did not necessarily show in the final failure stress of the composite.

Quantification of mesoscale variability and geometrical reconstruction of a textile

Automated image analysis of textile surfaces allowed determination and quantification of intrinsic yarn path variabilities in a 2/2 twill weave during the lay-up process. The yarn paths were described in terms of waves and it was found that the frequencies are similar in warp and weft directions and hardly affected by introduced yarn path deformations. The most significant source of fabric variability was introduced during handling before cutting. These resulting systematic deformations will need to be considered when designing or analysing a composite component. An automated method for three dimensional reconstruction of the analysed lay-up was implemented in TexGen which will allow virtual testing of components in the future.

Predictive Modeling of the Impact Response of Thermoplastic Composite Sandwich Structures

This article reports on work toward developing a practical methodology for the predictive modeling of the performance of thermoplastic composite (TPC) sandwich structures under impact loading. Explicit finite element analysis methods, using LS-DYNA® software, are described. Details of extensive materials characterization tests and material model parameter calibration for both the composite skin and polymer foam core are included. The simulations of deformation response, damage and failure of the sandwich structures is validated against experimental tests of the indentation and three-point bending of TPC sandwich beams. Good agreement between simulations and experiments has been achieved for indentation loading up to high degrees of core crush. The same is true for a significant part of the bending response including failure prediction. However, it has been necessary to introduce a principal strain fracture criterion to account for core shear and skin—core debonding failures at higher strains. For full predictive capability in this region, further experimental work is needed to obtain the necessary strain rate-dependent fracture data for the core and interface.

Thermal Degradation of Flax Fibres as Potential Reinforcement in Thermoplastic Composites

This work reports on a study of thermal degradation of flax fibres to gain an improved understanding of the use and limitations of flax fibres as reinforcement for thermoplastic composites manufactured by the vacuum forming process. The effect of heating on chemical decomposition and thermal stability was performed, using fourier transform infrared spectrometry (FTIR) and thermogravimetry (TG) techniques. In addition, the characterisation of micro structures of failure surface following tensile testing of the composites was conducted. The results show that the hemicelluloses decomposition of flax fibres during thermal degradation is a factor to have the detrimental effect on the thermal stability of fibres, particularly with low heating rate. The present investigation, A decrease of hemicellulose and pectin content of the fibres, a decrease of consolidation temperature and an increase of heating rate during the manufacturing of flax fibre thermoplastic composites should improve their mechanical performance.