Interfacial strength in thermoplastic composites - at last an industry friendly measurement method?

Many elegant techniques have been developed for the quantification of composite micro-mechanical parameters in recent years. Unfortunately, most of these techniques have found little enthusiastic support in the industrial product development environment, where they are viewed as time consuming, complex, inefficient, labour intensive, and in many cases unproven or inapplicable in 'real' systems. Despite this reaction, there is a real need for a 'user-friendly' micro-mechanics to aid the composites industry to move to the next level of development. A method for deriving values for τ (the interfacial shear strength) and ηo (a fibre orientation factor) from a simple combination of the composite tensile stress-strain curve and the fibre length distribution has been available for some time. Despite the recent wealth of activity in the development of micro-mechanical test techniques, there has been little follow-up on this older technique. In this paper we explore this analysis by its application to injection moulded glass-fibre-reinforced thermoplastic composites produced using three matrices (polypropylene, polyamide 6,6 and polybutyleneterephthalate) and containing different levels of glass-fibre. We furthermore show how the analysis can be extended to obtain another important micro-mechanics parameter, σuf, the fibre stress at composite failure. Values of τ and ηo obtained using this improved version of the original model are presented and discussed

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene : 6. the properties of injection moulded long fibre PP at high fibre content

The results of an investigation of the mechanical performance of injection moulded long glass fibre reinforced polypropylene with a glass fibre content in the range 0-73 weight % are presented. The composite modulus exhibited a linear dependence on fibre content over the full range of the study. Composite strength and impact resistance exhibited a maximum in performance in the 40-50 weight % reinforcement content range. The residual fibre length and fibre orientation in the samples has also been characterised. These parameters were also found to be fibre concentration dependent. Modeling of the composite strength using the measured fibre length and orientation data did enable a maximum in strength to be predicted. However, the position and absolute level of the predicted maximum did not correlate well with the experimental data. Further analysis indicated that deeper investigation of the dependence of the interfacial shear strength and fibre stress at composite failure on the fibre content are required to fully elucidate these results

The influence of fibre length, diameter and concentration on the impact performance of long glass-fibre reinforced polyamide 6,6

Results of an investigation of the mechanical performance of injection moulded long glass-fibre reinforced polyamide 6,6 composites are presented. The glass-fibre content in these composites was varied over the range of 10-50% by weight using fibres with average diameters of 10, 14 and 17 μm. Impact testing was carried out at −40, 23 and 80 °C on dry-as-moulded and boiling water conditioned samples. The results from these long fibre composites are compared with standard extrusion compounded short glass-fibre materials. Data on the influence of fibre diameter, fibre concentration, residual fibre length, hydrothermal conditioning and testing temperature on the composite performance in notched and unnotched pendulum impact tests and multiaxial instrumented impact tests are presented and discussed. All of the above parameters are shown to have significant influence on impact performance. However, the level of these effects is shown to depend on which type of impact test is being considered

Interfaces and interfacial effects in glass reinforced thermoplastics - Keynote Presentation

Optimization of the fibre-matrix interphase region is critical to achieving the required performance level in thermoplastic matrix composites. Due to its initial location on the fibre surface, the sizing layer is an important component in the formation and properties of the composite interphase. Consequently, any attempt to understand the science of the composite interphase must encompass an understanding of the science of sizing. In this paper the role of sizings from fibre manufacture through to performance of composite parts is reviewed. In particular the role of organosilane coupling agents and how the formation of a polysiloxane interphase is influenced by the surface properties of the fibre is examined. The influence of the sizing film former in terms of its level of interaction with the silane coupling agent is also examined. The importance of residual stresses in thermoplastic composites in the values obtained for the apparent adhesion levels in these systems is highlighted. These residual stresses are shown to play a significant role in determining the level of interfacial strength in thermoplastic composites and in particular in polyolefin matrices. By applying some of the available models for this phenomenon this analysis is extended to explore the effect of the anisotropic fibre microstructure of carbon, aramid and natural fibres on the apparent interfacial strength in thermoplastic composites

Why are natural fibres failing to deliver on composite performance?

The poor performance of natural fibres as composite reinforcements where the focus on chemical aspects has not yet delivered the "holy grail" of glass fibre replacement in volume applications is discussed. An explanation is proposed based on the anisotropic structure of these fibres and its influence the composite interphase

Understanding the impact performance of injection moulded long fibre reinforced polyamide

Short fibre reinforced thermoplastics have been used in the automotive industry for many years and there has recently been a strong growth in the use of polyamide based materials in under-the-hood applications. More recently there has been an increasing growth in the use of long fibre thermoplastic composite systems in semi-structural and engineering applications. Glass fibre reinforced polyamides are excellent composite materials in terms of their high levels of mechanical performance and temperature resistance. However the mechanical properties of polyamide based composites decrease markedly upon the absorption of water and other polar fluids. There also exist a number of well documented differences in the structure performance relationships of short fibre reinforced polyamide and polypropylene composites and it can be expected that there will also be differences when we compare these resins reinforced with long fibres. In this paper we present data on the mechanical performance of long fibre reinforced polyamide 6,6 which may be relevant to the above discussion. We have prepared injection moulded long fibre reinforced polyamide 6,6 samples with a range of glass contents (0-50 % wt) using glass fibres having average fibre diameters of 10, 14, and 17 μm. Mechanical performance has been determined for both "dry as moulded" state and after hydrolytic conditioning and compared with reference short fibre composites based on 10 μm diameter fibre in the same resin system. We will focus our discussion on the effects of fibre length, fibre diameter and fibre concentration on the impact performance of these composites. We will show how it is important to discriminate between notched (Figure 1) and unnotched (Figure 2) testing when discussing impact performance as these two properties show very different structure-performance relationships

Structure-property relationships in glass-reinforced polyamide, Part 3: Effects of hydrolysis ageing on the dimensional stability and performance of short glass-fiber-reinforced polyamide 66

We present results on an in-depth study of the effects of hydrolysis testing on the mechanical performance, weight change, and dimensional stability of injection moulded glass-fibre reinforced polyamide 66 based on two chopped fibre products with different sizing formulations. Composite and resin samples have been characterised both dry as moulded and after conditioning at either 120°C or 150°C for a range of times up to 1000 hours. The results reveal that hydrothermal ageing in water-glycol mixtures results in significant changes in the mechanical performance, weight, and dimensions of these materials. The negative effects of conditioning could be mitigated to some degree by the appropriate choice of the glass fibre sizing; however the sizing effect diminished with increasing conditioning time. All materials showed a weight increase due to conditioning at 120°C which was typical of a single Fickian diffusion process and there was clear evidence of multiple processes involved when conditioning at 150°C. It was not apparent that the glass fibre sizing affected the dimensional stability of the composites. We show that there is a strong correlation between the swelling of these samples and the level of fluid adsorption. Although the PA66 resin showed reasonably homogeneous swelling, the composites exhibited different levels of swelling depending on direction. These effects were well in line with the known effects of fibres on restriction of the matrix deformation (mechanical, thermal or moisture swelling) in the fibre direction. These differences correlate well with the average fibre orientation with respect to the various direction axes. Composite tensile strength and unnotched impact resistance appeared to scale inversely with the level of swelling of the material

The influence of fibre length, diameter and concentration on the modulus of glass fibre reinforced Polyamide 6,6

Results of an extensive investigation of the mechanical performance of injection moulded long glass fibre reinforced polyamide 6,6 composites are presented. The glass fibre content in these composites was varied over the range of 10-50% by weight using fibres with average diameters of 10, 14, and 17 μm. Mechanical testing was carried out at 23 and 150 °C on dry-as-moulded and boiling water conditioned samples. The results from these composites were compared with standard extrusion compounded short glass fibre materials. The influence of fibre diameter and concentration on the residual fibre length, fibre orientation distribution and composite modulus is presented and discussed in comparison to the predictions of some of the available micromechanical models

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene : 7. interface strength and fibre strain in injection moulded long fibre PP at high fibre content

The mechanical performance of injection moulded long glass fibre reinforced polypropylene with a glass fibre content in the range 0-73% by weight has been investigated. The composite modulus exhibited a linear dependence on fibre content over the full range of the study. Composite strength and impact resistance exhibited a maximum in performance in the 40-50% by weight reinforcement content range. The residual fibre length, average fibre orientation, interfacial shear strength, and fibre strain at composite failure in the samples have been characterised. These parameters were also found to be fibre concentration dependent. The interfacial shear strength was found to be influenced by both physical and chemical contributions. Theoretical calculations of the composite strength using the measured micromechanical parameters enabled the observed maximum in tensile strength to be well modelled

Interfacial strength in fibre reinforced thermoplastics

There has been a rapid growth in the development and application of fibre-reinforced thermoplastic polymer composites in recent years. Parallel to this growth has been the increasing recognition of the need to better understand and measure the micro-mechanical parameters which control the structure-property relationships in such composites. The properties of thermoplastic composites result from a combination of the fibre and matrix properties and the ability to transfer stresses across the fibre-matrix interphase. Optimization of the stress transfer capability of the fibre-matrix interphase region is critical to achieving the required performance level in thermoplastic matrix composites