I. Roman

Failure modes and fracture mechanisms in flexure of Kevlar-epoxy composites

The results of testing in three-point bending of aramid fibre-reinforced epoxy composites are described. This loading mode has been chosen in order to increase the variety of failure modes and of fracture mechanisms. The main failure modes observed are tensile and delamination, with a transition at a fibre volume fraction of about 46%. This mode transition is detectable by monitoring various mechanical properties and acoustic emission data against the fibre volume fraction. The tensile mode comprises a fracture mechanism of fibre splitting and pull-out and the delamination comprises fibre bending, tearing off of fibre skin and shearing of individual filaments. Other effects such as the shifting of the neutral axis and the compressive failure at the compression side are also reported.

Tensile properties of real unidirectional Kevlar/epoxy composites

Abstract Mechanical behaviour, tensile strength and failure modes in real unidirectional Kevlar/epoxy composites, loaded parallel to the fibres, at volume fraction (Vf) range 0.26–0.73, were investigated. It was found that the measured tensile strengths deviated from the expected values calculated from the Rule of Mixture. The deviation, which was minimal at Vf of about 0.5, was mainly due to geometrical deficiencies typical of real composites. At Vf 0.5 the deviation was explained by the increasing lack of matrix between some adjacent fibres and by squeezing of fibres. The initial part of loading was typified by straightening out of non-axial fibres, accompanied by fibre/matrix debonding. The straightening process was completed at a stress level of about 0.6–0.7 of the composite strength. Matrix damage began at this stress level and continued to develop up to final failure. Failure of Kevlar fibres was noted to occur only at an extremely short loading interval coinciding with the catastrophic final failure. This was due to the small scatter of Kevlar fibre strength.

Simultaneous Determination of Shear and Young's Moduli in Composites

An experimental method is described to calculate both shear and Youngs moduli of composite materials by three-point bending. The proposed procedure includes appropriate corrections for certain extraneous effects. The importance of the two basic ratios, E/G, which is an indicator of the degree of anisotropy of the material, and L/d, the span-to-depth ratio, is underlined.

The effect of fibre content on the simultaneous determination of Young's and shear moduli of unidirectional composites

Abstract The method for simultaneous determination of Youngs and shear moduli proposed recently is applied to unidirectional composites with a wide range of fibre volume fractions. Also, interlaminar and translaminar modes of loading are compared. The results obtained fit well the Halpin-Tsai theoretical predictions of both Youngs and shear moduli. A limitation on the validity of the method is discussed in terms of suitable loading spans which produce significant flexure and shear contributions together.

Hybrid effects in the bending stiffness of graphite/glass-reinforced composites

The flexural modulus of graphite/glass-reinforced hybrids exhibits deviations from the rule-of-mixtures base-line. The deviation increases as the segregation of the glass and graphite layers increases, and it reaches a maximum when the two fibre types are arranged in 3 layers. When the stiffer fibres (graphite) are in the outer layers the deviation is positive but it is negative with the less stiff fibres (glass) on the outside. The extent of deviation is shown to be predictable by the analysis.

Analysis of several loading methods for simultaneous determination of Young's and shear moduli in composites

Abstract Several mechanical test methods suitable for the simultaneous determination of Youngs and shear moduli are analysed. These test methods are characterised by a loading configuration which simultaneously induces flexure and shear deflections, and by a two-step procedure with two different loading span/specimen depth ratios.

Stress Intensity Factor Measurements in Composite Sandwich Structures

In composite materials, the stress intensity factor is significantly affected by the reinforcement geometry. This geometry affects the degree of anisotropy of the material. It is maintained that in order to obtain valid stress intensity factor values, the compliance calibration procedure presented should be carried out for every new reinforcement geometry.

Determination of grain size in uranium-chromium alloys by ultrasonic attenuation

Ultrasonic attenuation was measured in cylindrical samples of uranium-chromium alloys used for nuclear fuel elements. The grain structure of the samples was previously refined by proper heat-treatment and the grain size in the range of 50 to 200 μm was determined by conventional metallographic methods. The attenuation coefficient,γ, was correlated with the mean grain diameter,¯D, for three ultrasonic frequencies: 4, 6 and 12 MHz. The experimental results were compared with existing theories and good agreement was found with the curves based on Merkulovs analysis for cubic metals. A more practical result of this work was the development of a non-destructive testing procedure for grain-size determination in nuclear fuel elements cast from these alloys. The highest frequency (12 MHz) is normally used as it is more sensitive in the lower grain-size limit (50 to 100 μm) and only in border-line cases would one switch to the other frequencies (6 and 4 MHz) which are more applicable to the higher grain-size limit (100 to 200 μm).

FRACTURE MECHANISMS IN BENDING OF UNIDIRECTIONAL KEVLAR-REINFORCED EPOXY COMPOSITES

Failure of unidirectional Kevlar-reinforced epoxy composites in bending occurs by tensile mode in low fibre contents and by delamination in higher fibre contents. With a loading span to depth ratio of 8 the mode transition takes place around a fibre content of 50%, and it is detectable by monitoring mechanical properties or acoustic emission data vs. fibre content. The present study describes the fracture mechanisms occuring under each failure mode and some of the extraneous mechanisms typical of the three-point bending technique. The tensile mode comprises a mechanism of fibre splitting and pull-out, and the delamination comprises fibre bending, skin tearing off and shearing of individual fibres. Other effects such as shifting of the neutral axis and compressive failure are also reported.

Analysis of elastic properties of symmetrically laminated beams in bending

Abstract Formulae for the bending stiffness, the static moment and the bending and shear stresses in symmetrically laminated beams are proposed. These beams are made of m different materials arranged in a total number of n layers. Such structures are designated ( m , n ) laminates. The particular case of bi-material laminates is presented as an example, and it is shown that synergistic effects may occur for the beam stiffness. The occurrence of these effects depends on the number of layers, the volume fractions of the materials and their modulus ratio. Intimate mixture of the materials yields a simple rule-of-mixtures behaviour.