Fibre Science and Technology最新文献

Moulding compounds were produced from nylon 6 granules and eight different grades of E-glass fibre roving. The fibres were of 10–16 μm diameter and had been finished with sizes of different formulation which in three cases omitted the conventional silane coupling agent. Tensile test bars were produced from the compounds by injection moulding and these were tested in a standard ‘dry’ condition and also after boiling in water for 48 h to saturate the material. Impact tests were also carried out. The results show that under ‘dry’ conditions the absence of the silane may result in significant reductions in stiffness, strength, and also in ductility, but in one case the omission of the silane had little effect. In the ‘wet’ condition all materials showed significant degradation of properties and those without the coupling agent were by far the worst.

The reductions in stiffness and strength were not accompanied by corresponding increases in ductility, except in the extreme cases, on the contrary the poorly coupled materials were generally rather brittle. This trend was also reflected in the impact test results.

The influence of coupled diffusion of heat and moisture on the transient stresses in a composite is investigated analytically where the moisture diffusion coefficient is taken to be temperature dependent while the thermal diffusion coefficient is kept constant. There is no a priori reason why moisture and temperature should be uncoupled such that each will obey the simple diffusion theory, particularly without reference made to the initial and boundary conditions of a particular situation. A study of the coupled diffusion equations was made by allowing for time-dependent changes in the humidity and temperature of the environment. The appropriate transient boundary conditions are specified on the surfaces of an infinite plate. Numerical calculations were carrie dout for the T300/5208 epoxy matrix of the graphite fibre-reinforced composite in which the non-uniformity of moisture and temperature is evaluated for sudden changes in the surface moisture and/or temperature. The coupling effect between temperature and moisture is found to be most significant when the plate undergoes a sudden change in surface temperature while the surface moisture concentration is held constant. This suggests the need to perform additional experiments for evaluating the coupled diffusion phenomenon and its influence on the mechanical behaviour of epoxy-resin composites.

Additional results are obtained for a plane body containing a circular cavity whose surface is subjected to the sudden change of temperature and/or moisture. The resulting stresses are found to fluctuate with time and used in conjunction with the strain energy density criterion for locating possible failure sites.

In composite material structures, one of the primary limitations in minimising structural weight is the joining of various structural components. In composite structures joining is largely limited to employing adhesive bonding¹ or mechanical fasteners,² such as bolts or rivets, or possibly a combination of the two. Since mechanical fastening requires intermittent load paths, resulting in large stress concentrations, as well as the cutting of fibres to make the necessary holes, adhesive bonding is considered far more desirable for joining composite structural components to either other composite or metallic structural components.

Concerning various adhesive systems, in order to design, analyse and optimise bonded structures^3–13 it is necessary to know the mechanical properties of the adhesives in both shear and tension. Furthermore, it is necessary to know these properties for each temperature and moisture level to which the structure will be subjected, because the adhesives today are polymeric, and therefore subjected to ‘hygrothermal’ effects, i.e. the deleterious effects of combined high moisture and high temperature.^8,10,11

Unfortunately, not many of the moderate temperature adhesives, which are mostly epoxies, have been characterised. However, a few dozen have been characterised to some extent.^{11–13,15,16} One of the reasons for this lack of material property data is the lack of standardization of a suitable shear and tension test-piece configuration. However, more and more, the thick adherend shear test specimen is being accepted as the means to obtain shear properties because of the constancy of the shear across most of the length of the joint in this test configuration.^{11–13,15,16}

The fabrication of a composite structure with a fibre/resin reinforced system simultaneously requires:

• 1.

  (a) a thermal cycle to cure the matrix,

• 2.

  (b) a pressure to compact the system.

The optimisation of composites with high mechanical properties from a curing cycle is a compromise between the effect of the thermal cycle on the resin and that of pressure on the prepreg compacting capabilities.

With dynamic thermomechanical methods of tests, it is possible to follow a given cycle for the progress of resin curing and to determine the starting point in time of gellification. This point is the point appropriate to pressure application on the lay-up so as to obtain the best possible composite material.

A study has been made of the effect of a local fibre distribution in the vicinity of a crack on the fracture toughness G_c of glass fibre reinforced epoxy resins, which have ten kinds of fibre volume fraction including very low fraction, by means of fracture mechanics and fractography.

The results obtained are summarized as follows:

• 1.

  (1) There exists a critical fibre volume fraction V_fc; below V_fc, the strengthening effect due to fibre does not affect the fracture toughness of the material. The fracture toughness of the FRP having V_f larger than V_fc increases linearly with V_f on the log-log diagram.

• 2.

  (2) A phenomenon of local pop-in at a pre-crack front precedes a global advancement of a crack front. The local pop-in occurs preferentially at some particular locations along the pre-crack front where the fibre spacing is locally wide.

• 3.

  (3) The fracture toughness of FRP depends not only on the macroscopic fibre volume fraction V_f in the material but also on the local volume fraction V_fL and on the local fibre spacing D_ave in the vicinity of the pre-crack front. Both increase of the V_fL and decrease of the D_ave have the same effect of increasing the fracture toughness of FRP.

• 4.

  (4) The scatter of fracture toughness values in a conventional fracture toughness test can be well explained by considering the effect of V_fL and D_ave upon the fracture toughness.

It has been found possible to use adhesives in place of other techniques for joining composite materials with a polyester matrix, reinforced with fibre-glass, which are used for making body parts for the transport industry. Research has therefore been centred round bonding advanced composite materials (with fibres arranged in specifically chosen directions) which, in the face of increasing demand for light-weight vehicles, may possibly be the answer to the problem of manufacturing highly stressed parts.

The fibres (glass or carbon) in these materials (polyester or epoxy matrix) are in one long piece and all lie in the same direction which is the one in which the maximum mechanical characteristics of the materials— close to if not greater than metal—are found.

Joining is done using structural adhesives calculated on the basis of: (a) test procedure, (b) surface treatment of bonding surfaces, (c) effect of ageing, and (d) fatigue test.

For up to ten years primary structures of carbon fibre reinforced plastics (CFRP) were applied to prototypes of sailplanes. The starting of mass production two years ago required the definition of admissible service life. For gliders made of glass fibre reinforced plastics (GFRP) 3000 authorised flight hours have been the limit till now. Some of them reached this limit.

Because of its inherent capacities CFRP enables the permission of a sizeably higher stress level and longer servive life compared to GFRP.

This paper presents the results of investigations on two spars and one wing in CFRP design carried out at the Institut für Bauweisen- und Konstruktionsforschung of the DFVLR in Stuttgart. For the simulation of the service-loads we employed the loading programme commonly used in Germany.

After these dynamic loadings the spars and the wing were tested for residual strength. Periodic measurements allowed observation of stiffness behaviour during the simulated service life of 18 000 h.