Unidirectional laminate

ICl Development polymide Resin QX-13 and Morganite Modmor Type I (treated) carbon fibre. Unidirectional laminate (S2% v/v fibre content)]. vSource of data ICl Trade Lilerature... 

The short beam shear test designated in ASTM D 2344 (1989) involves loading a beam fabricated from unidirectional laminate composites in three-point bending as... 

The [10°] off axis tension specimen shown in Fig 3.23 is another simple specimen similar in geometry to that of the [ 45 ]s tensile test. This test uses a unidirectional laminate with fibers oriented at 10° to the loading direction and the biaxial stress state (i.e. longitudinal, transverse and in-plane shear stresses on the 10° plane) occurs when it is subjected to a uniaxial tension. When this specimen fails under tension, the in-plane shear stress, which is almost uniform through the thickness, is near its critical value and gives the shear strength of the unidirectional fiber composites based on a procedure (Chamis and Sinclair, 1977) similar to the [ 45°]s tensile test. 

Outwater, J.D. and Murphy, M.C. (1969). On the fracture energy of unidirectional laminates. In 24th Annual Tech. Conf. Reinforced Plast. Composites Inst. SPI, New York, Paper IIC. 

Bowles, D.E. and Griffin, O.H. (1991a). Micromechanics analysis of space simulated thermal stresses in composites, part I Theory and unidirectional laminates. J. Reinforced Plast. Composites 10, 504-521. 

Two types of composite physical property tests were conducted to measure properties which are sensitive to the degree of adhesion and failure mode of the fiber-matrix interphase. Short beam shear tests (ASTM D2344-84) were conducted on 18 ply unidirectional laminates. The support span-to-thickness ratio... 

Flexure testing (ASTM D790-76) was performed on 12 ply unidirectional laminates with the fibers oriented at 0° and 90° to the support span. Specimens were 25 mm wide, nominally 100 mm long and tested at a span-to-thickness ratio of 32 1. The specimens were three point loaded and the rate of strain in the outer fibers was kept constant at 0.01 (mm/min) per min. Load vs. deflection curves were recorded for determination of the modulus and yield properties. 

A unidirectional laminate consists of continuous glass fibers in an epoxy matrix and has a 60% fiber volume fraction. Calculate the strains (a) when a tensile stress of 100 MPa is applied in the fiber direction and (b) when a stress of 20 MPa is applied in the direction transverse to the fibers. The tensile moduli and Poisson s ratios for the glass and epoxy are Ef = 80 GPa, Em = 2.5 GPa, Vf = 0.2 and = 0.35, respectively. 

If, as in the fracture surface type shown in Fig. 3 on the left hand side, the width of the featureless delamination is increasing with delamination length this is also reflected in the R-curve (Fig. 5). There, values are dropping from relatively high cross-ply -values to a value typical of the unidirectional laminate. This could be a hint that energy release rates determined for the cross-ply laminates using the analysis developed for the unidirectional material are valid in the sense that relative proportions are conserved. 

It could be argued that even in the unidirectional lay-up the delamination is not strictly miming along the mid-plane, at least on a microscopic scale (Fig. 6). Deviations are on the order of 0.05 mm. In the case of the cross-ply material, the maximum deviation is limited by the distance between the 0°-plies, i.e. about 0.17 and 0.38 mm, respectively, as long as the delamination does not further deviate into the unidirectional plies. This is about 3 and 7 times, respectively, more than in the unidirectional laminates. Sufficiently small , limited deviations will not affect the data analysis in a way that renders the values meaningless. In the following, it is proposed that the validity of cross-ply data can be defined by requiring the delamination not to deviate into the adjacent unidirectional plies. This criterion could easily be verified by inspection of the fracture surface. 

The comparison of the different initiation points in the three laminate types (Tables 2-4) raises the question which definition shall be used for initiation in the cross-ply laminates. Since visual initiation (VTS-point) and probably also non-linearity of the load-displacement plot (NL-point) yield values similar to initiation values in the corresponding unidirectional laminate, the maximum load or 5% offset in compliance (MAX/5%-point) seems to reflect the higher delamination resistance of cross-ply compared with unidirectional laminates better. Further analysis of additional data from the 3" round robin may allow a better assessment of this question. 

The primary purpose of the meso-damage modelling is to describe failure as processes occurring at the yam scale. Compared to unidirectional laminated composites, damage accumulation in textile composite has some specific features (1) the early initiation of intra-yam cracking (0.1—0.3% of applied deformation), (2) the concurrent accumulation of crack density and incremental crack length growth. 

The criterion is, in general, rather easy to apply, since it requires only two fatigue curves for its calibration this is of great help in overcoming the difficulty in considering, explicitly, variations of the load ratio. A limitation, however, is that the model, in the present form, cannot be applied to life prediction of unidirectional laminates due to their anisotropic response resulting in different limits for the strain energy density in the fiber direction and normal to it. 

Currently available data indicate that 3D reinforcements typically result in high values of Gc compared with the unidirectional laminate made from the same fibre and matrix. Whether the high delamination resistance of 3D FRP laminates does represent material values or apparent values, for example depending on specific test and specimen parameters, is still debated (see e.g. Ref. [92]). More generally, the applicability of linear elastic fracture mechanics to through-thickness reinforced FRP composites is questionable [98]. 

Figure 12.9 Illustration of the effect of thermal spiking to 140 °C on the moisture absorption of a Fibredux 927C unidirectional laminate in 96% RH at 50 °C. The individual points represent the actual moisture content immediately before and after a thermal spike. The continuous line is for the isothermal control under identical humid conditions.

The strength and modulus of the reinforced material falls rapidly from those predicted by the law of mixtures for unidirectional laminates, as the angle between the fibre direction and the stress is increased from zero towards 90°. We assumed above that the fibres are all parallel to the applied uniaxial stress, but if they are lying normal to the stress, the equation below applies and gives a much lower value for the composite modulus ... 

The elastic mechanical properties of carbon fibre-reinforced laminates are highly dependent on the properties of the fibres and the matrix chosen and on the direction of loading relative to the fibre orientation. In a unidirectional laminate, with all fibres orientated in one direction and loaded parallel to the fibres, the properties are mainly dependent on those of the fibres and can be estimated by the rule of mixtures, taking into account the volume fractions of the fibres and the matrix. Equation [5.1] shows that E, the Young s modulus parallel to the fibres is simply given by... 

For a unidirectional laminate the elastic stress-strain relations define an orthotropic material for which the generalized form of Hooke s Law, relating the stress o to the strain e,... 

---