Effects of processing conditions on the mechanical and water absorption properties of resin transfer moulded kenaf fibre reinforced polyester composite laminates.

This paper focuses on the mechanical and water absorption properties of kenaf fibre reinforced polyester laminates manufactured by resin transfer moulding. Varying processing conditions were considered as alternatives to fibre treatments, thereby potentially avoiding additional cost and complexity in the manufacturing process. Laminates were produced by altering fibre moisture content, mould temperature and mould pressure following injection. Tensile, flexural, impact and water absorption tests were conducted. Processing conditions were found to have little effect on properties except for pressurisation which increased tensile and flexural strength and decreased water absorption at low fibre volume fractions. Examinations using a scanning electron microscope showed that all the laminates failed by fibre pull-out.MvdH2016http://www.journals.elsevier.com/composites-part-a-applied-science-and-manufacturin

Analysis of stress concentration around a spheroidal cavity under asymmetric dynamic loading

AbstractThe fracture and fatigue properties of porous materials are strongly influenced by stress concentrations around the pores. In addition, failure of structural components initiates at locations of high stress concentration which is often caused by holes, inclusions or other discontinuities. In view of this, the stress concentration around a spheroidal cavity embedded in an elastic medium is studied under dynamic loading conditions. While solutions abound for static loads, only limited solutions exist for dynamic loads. The stress field around a spheroidal cavity is determined by using a hybrid methodology that combines the finite element technique with a spherical wave function expansion method. The stress concentrations within the matrix are found to be dependent on the frequency of excitation, aspect ratio of the cavity and the Poisson’s ratio of the matrix. The study reveals that dynamic stress concentrations can reach much higher values than those encountered under static loading

Effect of resin system on the mechanical properties and water absorption of kenaf fibre reinforced laminates.

The objective of this study is to compare the mechanical and water absorption properties of kenaf (Hibiscus cannabinus L.) fibre reinforced laminates made of three different resin systems. The use of different resin systems is considered so that potentially complex and expensive fibre treatments are avoided. The resin systems used include a polyester, a vinyl ester and an epoxy. Laminates of 15%, 22.5% and 30% fibre volume fraction were manufactured by resin transfer moulding. The laminates were tested for strength and modulus under tensile and flexural loading. Additionally, tests were carried out on laminates to determine the impact energy, impact strength and water absorption. The results revealed that properties were affected in markedly different ways by the resin system and the fibre volume fraction. Polyester laminates showed good modulus and impact properties, epoxy laminates displayed good strength values and vinyl ester laminates exhibited good water absorption characteristics. Scanning electron microscope studies show that epoxy laminates fail by fibre fracture, polyester laminates by fibre pull-out and vinyl ester laminates by a combination of the two. A comparison between kenaf and glass laminates revealed that the specific tensile and flexural moduli of both laminates are comparable at the volume fraction of 15%. However, glass laminates have much better specific properties than the kenaf laminates at high fibre volume fractions for all three resins used.Department of Science and Technology, South Africa under the Advanced Manufacturing Technology Strategy (AMTS). National Research Foundation (NRF). Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg.MvdH2016http://www.journals.elsevier.com/materials-and-desig

Guided Wave Testing

Guided waves can propagate long distances in thin-walled structures, such as pipelines or plates. This allows for the efficient monitoring and testing of large structures and for the detection of hidden or inaccessible defects. Guided wave propagation is dispersive and multi-modal, requiring a thorough understanding of the wave propagation and scattering phenomena from simulations. Guided wave dispersion diagrams, mode shapes, and typical signals are illustrated for the example of isotropic plates. Both low and high frequency guided waves have been used for the testing of plate structures, with different wave modes and applications including tomography and arrays for the detection and localization of defects. For multilayered and anisotropic structures, guided wave propagation becomes more complex, and often the fundamental guided wave modes are employed for defect detection. For pipelines different commercially available testing systems have been developed and long propagation distances up to 100 m have been achieved. Careful selection of guided wave mode and excitation frequency allows the minimization of attenuation due to viscoelastic coatings and in buried pipelines. Synthetic focusing using non-axisymmetric modes improves defect imaging and localization. Experimental methods differ from standard ultrasonic testing, as good control of the excited guided wave mode shape and signal are required to achieve improved sensitivity for small defects. In addition to contact piezoelectric transducers, electromagnetic and laser techniques allow for noncontact measurements. Finite Element Analysis is one of the numerical simulation techniques used to obtain a better understanding of guided wave testing and to improve defect characterization