GRP Specimens Research Articles

Improving the mechanical properties of Glass Reinforced Plastics by slight mechanical compression

Glass reinforced plastics (GRPs) are composite materials that have been used widely in engineering. Mechanical properties of fabricated GRP products depend on the type and arrangement of reinforcement, the type of the plastic as well as the reinforcement to plastic ratio. Some of these factors are interdepended in determining the quality of the final product. In the present study, the influence of applying mechanically a slight pressure during fabrication on the properties of GRP specimens, was investigated. Specimens were fabricated using the hand lay-up method and were further processed either by a vacuum bag compression or a mechanically applied pressure. The properties of the produced composite specimens were then determined experimentally. The application of mechanical compression was found to improve their properties. The effect of the mechanically applied compression or vacuum was evident as a reduction of the specimens’ internal void volume compared to the non-compressed ones, resulting in an increase in the density by 9–12% and a reduction in the thickness of the specimens by 15–20%. Additionally, the tensile and flexural strength of the specimens were increased by more than 15% and 5% respectively when pressure was applied mechanically, reflecting an improvement in GRPs’ mechanical properties.

Spall strength of glass fiber reinforced polymer composites

In the present paper results of a series of plate impact experiments designed to study spall strength in glass–fiber reinforced polymer composites (GRP) are presented. Two GRP architectures are investigated—S2 glass woven roving in Cycom 4102 polyester resin matrix and a balanced 5-harness satin weave E-glass in a Ciba epoxy (LY564) matrix. The GRP specimens were shock loaded using an 82.5 mm bore single-stage gas-gun. A velocity interferometer was used to measure the particle velocity profile at the rear (free) surface of the target plate. The spall strength of the GRP was obtained as a function of the normal component of the impact stress and the applied shear-strain by subjecting the GRP specimens to normal shock compression and combined shock compression and shear loading, respectively. The spall strengths of the two GRP composites were observed to decrease with increasing levels of normal shock compression. Moreover, superposition of shear-strain on the normal shock compression was found to be highly detrimental to the spall strength. The E-glass reinforced GRP composite was found to have a much higher level of spall strength under both normal shock compression and combined compression and shear loading when compared to the S2-glass GRP composite. The maximum spall strength of the E-glass GRP composite was found to be 119.5 MPa, while the maximum spall strength for the S2 glass GRP composite was only 53.7 MPa. These relatively low spall strength levels of the S2-glass and the E-glass fiber reinforced composites have important implications to the design and development of GRP-based light-weight integral armor.

The influence of mechanical conditioning on the viscoelastic behaviour of short-fibre glass reinforced epoxy resin (GRP)

In this paper an attempt has been made to understand the influence of conditioning on the viscoelastic behaviour of glass reinforced epoxy resin (GRP) by comparing the creep characteristics of unconditioned and conditioned specimens. This has been done by experimentally measuring the creep strains and by calculating the stress/strain isochronals and the isochronous compliance/stress trajectories for both types of specimen. It is shown that the creep strains obtained from GRP specimens conditioned at 26 and 40% ultimate tensile strength fall within the creep strain scatter band obtained from the six unconditioned specimens. The isochronals clearly show, first, that the conditioning shifts the isochronals such that a greater strain is obtained for a given creep stress, and second, that the isochronals remain straight up to a creep stress equal to the conditioning stress. The isochronous compliance/stress trajectories of the conditioned specimens exhibit compliance saturation. This saturation is found only at elevated stresses in the unconditioned specimens. Compliance saturation is discussed in terms of the degradation of the in-situ matrix and subsequent fibre dominated composite deformation. An operational composite model that estimates a limiting initial compliance necessary for subsequent viscoelastic behaviour is tentatively proposed. By contrasting the isochronous compliance/stress trajectories of GRP with those of glass-fibre-reinforced thermoplastic, GM-PET, a qualitative analysis of the engineering applicability of three non-linear viscoelastic models is given.

Use of embedded optical fibre for early fatigue damage detection in composite materials

The use of embedded optical fibre to detect fatigue damage in GRP three-point bend specimens is described. A simple signal attenuation measurement system was used to indicate damage development. The stresses in the optical fibre were calculated using an existing ‘fibre in matrix’ model. The optical fibre fatigue strength was predicted using constants calculated from optical fibre embedded into the composite material. Early fatigue damage was successfully detected in GRP specimens.

繊維強度低下を防いで疲労強度を向上させる平織ＧＲＰの疲労挙動について

In the present report a particular attention is given to the fact that the fatigue damage of cloth GRP is caused by the lowering of fiber strength. From such a viewpoint, measurements were made on the residual strength of the GRP specimens and the strength of the constituting fiber strands in order to clarify the change in fiber strength during fatigue. It was found that the crack length obtained from Griffith strength agreed well with the predicted value calculated from the damage. Fatigue tests were made on the altered specimens whose weft strands were covered with polymer sheets. The warp strands were found to be protected from suffering damage by wefts at woven intersections. The fatigue strength increased by 2.26 times of the original value (at the 107-th cycles.), and thus it was possible to prepare the cloth GRP of high fatigue strength.

Deformation and fracture of a glass-reinforced plastic with �45� spiral reinforcement under simple and complex loadings

1. When tubular GRP specimens with a reinforcement angle of ±45° were placed in peripheral tension, it was found that the ML corresponds to the appearance of residual strains associated with the formation of microcracks. 2. The previous loading history affects the moment of appearance of the ML in the case of both simple and complex loadings.

A study of failure modes in grp specimens undergoing destructive short term tensile testing

In the short term tensile testing of 600 grp specimens to destruction, three types of abnormal failure were distinguished. Consistent differences were found between average values for groups of specimens differentiated by failure mode. The differences have been interpreted in terms of the forces experienced by specimens failing in each mode.

The Effect of Environmental-Loading History on the Transverse Strength of GRP Laminate

Loaded transverse GRP specimens and epoxy references were exposed to different environmental conditions. Tensile strength measurements conducted under exposure and after a drying period yielded the following main conclusions: Current and residual transverse strength was not affected much by cold-water and hot dry air during a short exposure period. Under hot-water exposure, transverse strength showed a significant drop with time, especially beyond 2 days of exposure and above 25% of the preload level. Based on earlier studies, the transverse-strength degradation of GRP specimens is attributable to attack by water molecules on the fiber-glass surfaces and the fiber-matrix interfacial bonds. This process is triggered and accelerated by heat and external loading. 1 Based on strength of reference specimens of Series B-0.

The stressed state and certain fracture characteristics of glass-reinforced plastics loaded in tension at an angle to the direction of reinforcement

The stressed state of glass-reinforced plastic specimens with various structures is investigated experimentally in the case of tensile loads acting at 45° to the direction of reinforcement. A relation is established between the stress distribution over the cross section and the “scale factor” associated with the relative width of the understressed zones along the edges of the specimen. The fracture characteristics of GRP specimens with different structures are analyzed. Recommendations are given for testing GRP with different structures in tension at an angle of 45° to the direction of reinforcement.