Okenwa I. Okoli

The effects of strain rate and failure modes on the failure energy of fibre reinforced composites

Abstract The utilisation of polymer composite materials in safety critical structures necessitates their full characterisation. This will bring about the much needed boost in confidence for their application to industrial situations, especially where high speed impact is a concern. Impact performance can in some way be measured by the energy absorbed or expended to failure of a material. Hence, establishing the rate effect on energy absorption is of paramount importance when designing for impact. Tensile, shear and 3-point bend tests were conducted on a woven glass/epoxy laminate at increasing rates of strain. The results suggest a linear relationship between expended energy and the log of strain rate in the laminate tested. Furthermore, it was found that a relationship exists between the flexural energy obtained at low strain rate and the high speed (2–4 m s −1 ) test values. Magnified views of the failed specimen surfaces, viewed under a scanning electron microscope, indicate a change in failure modes as strain rate is increased, which brought about the increase in energy observed.

The effect of strain rate and fibre content on the Poisson’s ratio of glass/epoxy composites

Abstract The utilisation of composite materials in structural applications has prompted the need for a full characterisation of their behaviour under dynamic loading conditions. The strain rate effects of most unfilled polymers can be described by the Eyring theory of viscosity which assumes that the deformation of a polymer involves the motion of a chain molecule over potential energy barriers. The Eyring model suggests that yield stress varies linearly with the logarithm of strain rate. In the present study, tensile tests were performed on a glass epoxy laminate at different rates of strain to determine the effects of strain rate on the Poisson’s ratio of the material. In addition, further tests were conducted at varying fibre contents to verify the relationship between fibre content and Poisson’s ratio. The findings from the experimental results suggest that Poisson’s ratio is not sensitive to strain rate. In addition, it was suggested that the absence of rate sensitivity in the Poisson’s ratio of the laminates tested is due to the presence of fibres in the composites.

Failure modes of fibre reinforced composites: The effects of strain rate and fibre content

The many aspects of high speed response of fibre reinforced composite materials have received the attention of a large number of investigators. Nevertheless, the understanding of the mechanisms governing failure under high speed loadings remain largely unknown. The effect of rate and fibre content on failure mechanisms was investigated by viewing fractured surfaces of tensile specimens using a scanning electron microscope (SEM). Tensile tests were conducted on a woven glass/epoxy laminate at increasing rates of strain. A second laminate (with random continuous glass reinforcement) was tested in tension at varying fibre volume fractions in order to ascertain the relationship between fibre content and failure mechanisms. The results suggest a brittle tensile failure in fibres of the woven laminate. In addition, the matrix was observed to play a greater role in the failure process as speed was increased, resulting in increased matrix damage and bunch fibre pull-out. The results also indicated that increasing the fibre volume fraction increased the likelihood of a matrix dominated failure mode.

Development of a triboluminescence-based sensor system for concrete structures

The triboluminescence phenomenon has been proposed as a sensor system for detecting and monitoring damage in aerospace and civil infrastructure systems (CIS). While significant work is being done in developing such systems for aerospace structures, little or no work is being done in developing triboluminescence-based sensor systems for the critical and aging CIS. This article reports some findings in the work being done by the authors to develop such a sensor system for civil infrastructure applications. A ZnS:Mn-based cementitious patch that emits light when stressed or fractured was developed and its triboluminescence performance under loading characterized. The results show that a ZnS:Mn concentration level of 10% gives the best triboluminescence response without adversely affecting the compressive strength of the patch, while also minimizing the use of the expensive ZnS:Mn crystals. The triboluminescence response increased as the concentration of ZnS:Mn in the system increased. The highest triboluminescence response was obtained at a concentration level of 25% but resulted in significant reduction in the system’s compressive strength. Nonetheless, the presence of ZnS:Mn affects the hydration process by slowing down the conversion of the needle-shaped crystals of calcium sulfoaluminate hydrate (ettringites) into the monosulfate hydrate that makes concrete vulnerable to sulfate attack.

A review of multiscale composite manufacturing and challenges

As the utilization of advanced composites in structural applications grows, the need for improving their through-thickness properties becomes imperative. Although the behavior of composite laminates under structural and thermal loads has received much attention with their growth in safety critical structures, more effort needs to go into selectively improving their electrical and thermal conductivities. Additionally, the ability to manufacture composite structures that can inherently monitor their own health will be exceedingly beneficial. This paper provides an overview of advances made towards multiscale composite manufacturing. Multiscale composites, especially with the use of carbon nanotubes, have been sought to provide enhanced structural (through-thickness) properties and increased electrical and thermal conductivities. This report will review the state of art in the manufacturing of multiscale composites, their scalability, and their inherent potential for multifunctionality. Current techniques mostly result in the application of carbon nanotubes throughout the entire laminates, rather than in selected areas. Subsequently, one of the main barriers, to the widespread use of carbon nanotube-applied composites, is an efficient mass-producible manufacturing process. This paper attempts to highlight the current knowledge gaps in this critical area of composites manufacturing.

Resin infusion between double flexible tooling: prototype development

The Resin Infusion between Double Flexible Tooling (RIDFT) technique is a novel two-stage process, which incorporates resin infusion and wetting with vacuum forming. The flow front of the infused resin is two-dimensional and avoids flow complexities prevalent in the three-dimensional flow seen in other liquid composite molding techniques. It employs a one-sided mold, which provides obvious cost benefits when compared with resin transfer molding. On-going prototype development of the RIDFT process has yielded positive results. Composite laminates with good surface quality, micro structural characteristics, and mechanical properties have been repeatedly produced with cost savings of 24% when compared with SCRIMP. This paper describes the RIDFT process, outlining its merits and presenting its challenges, whilst identifying potential benefits to industry. Current work being undertaken include the refining of production parameters, the construction of a larger prototype to examine the full extent of its suitability for the manufacture of large composite components and the incorporation of the UV curing technique to reduce the cycle time in the manufacture of large structures.

Enabling damage detection: manufacturing composite laminates doped with dispersed triboluminescent materials

Triboluminescent materials are being harnessed to address the gaps in current structural health monitoring systems. Their innate ability to emit light when stressed or broken makes them ideal candidates for the ubiquitous and in situ monitoring of structures. The increasing use of advanced composites in critical structures, where subsurface damage initiation may go unnoticed, further highlights the urgency in developing efficient online monitoring technologies. This work looked at the manufacturing of composite laminates that have been doped with various concentrations (0 to 10 %wt.) of a triboluminescent material (ZnS:Mn). Laminates were manufactured using a vacuum infusion process. Dispersing the ZnS:Mn particulates was cumbersome because their density was higher than the resin that caused settling during resin infusion. The dispersion of ZnS:Mn is critical to their use in the health monitoring of the host structure. As such, a method for mechanical agitation using a rotational vacuum infusion apparatus was developed employing centrifugal motion. The degree of dispersion in the resulting laminates was determined using scanning electron microscopy and the energy dispersive scanning feature of the electron microscope for elemental mapping. A quantitative metric was established by computations of the Euclidean distance of EDS mapping. Studies of the effect of ZnS:Mn concentration on the tensile strength of laminates showed that increasing the ZnS:Mn concentration reduced the tensile strength. Key processing parameters were studied, and determined that curing kinetics were not altered by ZnS:Mn inclusion.

High Strain Rate Characterization of a Glass/Epoxy Composite

High strain rate characterization of polymer composite materials has been limited due to difficulties in the measuring equipment. In particular, jigs and fixtures can be subject to inertial disturbances present at speeds up to and above 2 m/s. These disturbances are due to the phenomena of mechanical resonance and control problems that the test equipment acquires at high speeds. The amplitudes of the resulting oscillations increase with test speed and change the test sample response, making results difficult to analyze. These inertial problems may be avoided if the extrapolation of low to high strain data is applicable. Tests were performed measuring material properties at increasing rates of strain. These properties included the Young’s modulus, tensile and shear strengths, and the shear modulus of a glass-epoxy composite. The results show that the effect of the logarithm of the rate of strain on the material properties can be regarded as linear and extrapolated to provide the data at high strain rates.

Getting light through cementitious composites with in situ triboluminescent damage sensor

Triboluminescent damage sensors comprising highly efficient triboluminescent materials could allow simple, real-time monitoring of both the magnitude and location of damage. The inability to effectively capture and transmit the triboluminescent optical signals generated within opaque composites like concrete has, however, limited their damage monitoring applications. The in situ triboluminescent optical fiber sensor has been developed to enable the detection and transmission of damage-provoked triboluminescent emissions without having to position triboluminescent crystals in the host material. Flexural tests were performed on mortar and reinforced concrete beams having the in situ triboluminescent optical fiber sensor integrated into them. The intrinsic triboluminescent signals generated in the beams under loading were successfully transmitted through the optical fibers to the photomultiplier tube by side coupling. Successful side coupling will make a truly distributed in situ triboluminescent optical fiber sensor possible when the entire length of the sensor is mostly covered with the triboluminescent composite coating. The results show the viability of the in situ triboluminescent optical fiber sensor for the structural health monitoring of cementitious composites. Real-time failure detection was demonstrated in unreinforced mortar beams, while real-time damage (crack) detection was demonstrated in reinforced concrete beams. Preliminary work on reinforced concrete beams showed that the integrated in situ triboluminescent optical fiber sensor was able to detect multiple cracks caused by loading, thereby providing early warning of structural degradation before failure.

Failure in composite laminates: overview of an attempt at prediction

Abstract Success with the high strain rate testing of polymer composites has been limited by the ability to isolate the inherent inertial disturbances attributed to the test system. This necessitated the development of a technique for the prediction of high strain rate material property data. The resulting data were used in a finite element analysis (FEA) to simulate impact behaviour of glass fibre reinforced composites. High strain rate properties obtained by extrapolating results of experiments conducted at low to intermediate strain rates were used in the FEA of a simple three-point bend beam impact. Three point bend impact tests were performed on the laminates, and comparisons were made of the results predicted from this analysis and actual impact test data. The results show that the finite element model created may be used to predict the behaviour of woven glass laminates. However, the inclusion of flexible post-failure degradation rules to allow for progressive damage, will improve the accuracy of the analysis.