Unidirectional plies

The fiber content in mat or random fiber RPs is usually somewhat lower than for an isotropic laminate which is comprised of a number of unidirectional plies. Both laminates may, for example, be planar-isotropic. The random criss-cross nature of chopped fibers in a mat does not permit close packing of the bundles, and thus the fiber content is usually lower. With a lay-up of unidirectional plies, the packing of fibers within a ply may be very close, and the fiber content can be very high. The higher fiber content made from individual plies tends to make it stiffer and stronger than the mat construction. 

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. 

Deviations of the delamination from the mid-plane imply mixed-mode conditions, i.e., a deviation from a pure Mode I opening load. The degree of mode mixity will depend on the relative stiffness of the two unequal beams. In the cross-ply laminates, this stiffness difference will be small , as long as the unidirectional plies that essentially provide the axial stiffness are equal in both beams. This is a rational justification for a validity criterion for cross-ply laminates that allows deviations from the mid-plane, as long as the delamination does not show a transition into the unidirectional plies. 

Fig. 7 shows that the values from the second laboratory tend to be lower than those from the first for both types of cross-ply lay-up, while those for the unidirectional lay-up agree fairly well. The scatter still seen in the R-curves for the cross-ply laminates with a single fracture surface topography (Fig. 7) can probably, at least in part, be attributed to different amounts of fibre-bridging (compare Fig. 1). Another factor is micro-cracking in front of the delamination that may make accurate determination of the delamination length difficult. This would also offer an explanation for the steep rise seen in the R-curves of those specimens for which the delamination does not deviate into the unidirectional plies. This is discussed in detail in [6]. Small (local and short-term) deviations of the delamination into the unidirectional plies not recognised in the visual inspection of the fracture surfaces might also contribute to the scatter by temporarily reducing Gic. Finally, the oscillating interlaminar - intralaminar type of delamination propagation could also account for some of the observed scatter. The analysi > presented in [4] concludes that the intralaminar G is considerably smaller than the...

Tsai—Hill The maximum stress and maximum strain failure criteria consider each stress component individually. This is a simplification. Test results show that if more than one stress is present in a ply, they can combine to give failure earlier (or later) than the maximum stress or maximum strain failure criterion would predict. One example that shows this effect is the case of a unidirectional ply under shear on which a tensile or a compressive stress is applied parallel to the fibers. The situation is shown in Eigure 6.7. 

Figure 6.7 Shear combined with (a) tension or (b) compression on a unidirectional ply.

Shokrieh MM, Lessard LB. Multiaxial fatigue behavior of unidirectional plies based on uniaxial fatigue experiments — IL Expeiimaital evaluation. Int J Fatigue 1997 19(3) 209—17. 

Therefore, moisture absorption has a larger effect on the transverse properties of a typical composite system. Despite this, the strength of a 0° composite is also affected by moisture ingress since the reloading of a broken fibre occurs through shear stress transfer Ifom the interphasal matrix. To achieve isotropy, unidirectional plies are stacked at a set of angles such as 0°, 45° and 90° to form a laminate. In this situation, moisture ingress will modify the residual stress state in the individual laminae. 

Small deviations from the intended fibre orientation within a nominally unidirectional ply can reduce the mechanical strength and stiffness of continuous fibre laminates considerably, especially with aramid reinforcement. Wisnom [6] has investigated the reduction in strength caused by misalignment of unidirectional carbon fibres in XAS/914 carbon/epoxy... 

As well as a reduction in Tg, water absorption results in a reduction in the modulus of the matrix. Since the matrix contributes significantly to the transverse modulus of an individual unidirectional ply, this is a significant limitation to the use of a composite in a structural capacity at elevated temperatures. 

Chapter 18 gives a brief introduction to the micromechanics of unidirectional composites and quotes some of the composite properties. Figure 17.30 shows the principal coordinate axes for unidirectional lamina. The unidirectional ply is characterized by ... 

Alternative conceptual designs consider different strut configurations and loading conditions. Some strut designs have braces below the strut to more efficiently carry the axial loading. One design to be examined, a modular, jointless strut of continuously wound glass fibers or unidirectional plies, is shown in Fig. 3. 

Notes PA=PA6 or PA12 YP HDPE-MAH copolymer commercially available from DSM as Yparex UDP MFC lamina obtained from continuous oriented cables arranged in the form of unidirectional ply CPC MFC laminate obtained from cross-ply arranged oriented cables MRB MFC obtained from middle-length randomly distributed bristles NOM composite obtained from non-oriented mixture... 

Figure 14.3. Preparation of the cross-ply laminates (CPC) (a) dimensions of composite plates, in mm (b) two unidirectional plies of oriented precursors, perpendicularly aligned (c) compression molding at temperature r=160°C, and pressure, p=1.5 MPa and (d) visual aspect of the resulting laminate plates used for flexural and impact resistance tests [69]...

Figure 14.4 shows some typical stress-strain curves of HDPE/PA6 unidirectional ply MFCs in the longitudinal direction. The 90/10/0 composition containing 10 wt% PA6 displays a ductile behavior similar to the HDPE matrix. In the two corresponding curves there exist clear yielding and necking, even though the strain at break of the composite (about 100%) is much smaller than the HDPE alone (about 800%). The other stress-strain... 

Figure 14.14. SEM images of surfaces (after cryogenic fracturing) of MFCs made from two HDPE/PA6/YP blends (la-3a 80/20/0 wt% lb-3b 70/20/10 wt%) UDP unidirectional ply fractured parallel to the fibril direction MRB composite from middle-length PA6 bristles with random distribution NOM material obtained from non-oriented granules of the two blends [63]. All SEM images are taken at the same magnification...

Table 7 shows that the highest reported mechanical properties for bidirectionally reinforced LCP-reinforced self-reinforced polymer composites was reported by Stellbrink et al. [187]. The authors describe the orientation of LCP fibres in stacked unidirectional plies that were then subjected to heat and pressure, resulting in thermal btaiding of adjacent fibres. This is assumed to be similar to a hot compaction-type process. It is not clear if these composites had a balanced lay-up, and so the values presented in Table 7 may overestimate the fine bidirectional mechanical performance of these composites. 

The anisotropy, or directional nature, of unidirectional fibre composites is mentioned in Chapter 1. To improve the modulus and strength for intermediate angles (i.e. between 0° and 90°) woven fabrics or multiple constructions are used. The latter is made up of a series of unidirectional plies laid up so that there is an angle, say, 10°, 20°, 30° or 45°, etc., between successive plies. To avoid the laminate distorting it is necessary to balance the construction about the centre plane — i.e. to have as many —0 plies as +0 ones. A typical balanced laminate is [0 45 0]s-Because there are now some plies in intermediate directions the modulus and strength of the laminate in these directions is increased. The exact values for thermoelastic properties can be calculated from classical laminate theory, see Jones (1975). It is more difficult to calculate the effect on strength because of interaction between failure modes and individual plies, etc. 

Baley, C., Perrot, Y., Busnel, R, Guezenoc, H., and Davies, R, Transverse Tensile Behaviour of Unidirectional Plies Reinforced with Flax Fibres ,... 

---