Unidirectional plies composites

The previous analysis has shown that the properties of unidirectional fibre composites are highly anisotropic. To alleviate this problem, it is common to build up laminates consisting of stacks of unidirectional lamina arranged at different orientations. Clearly many permutations are possible in terms of the numbers of layers (or plies) and the relative orientation of the fibres in each... 

In cross-ply laminates, the stress-strain behavior is slightly nonlinear, as illustrated in Figure 5.123. The stress-strain behavior of a unidirectional lamina along the fiber axis is shown in the top curve, while the stress-strain behavior for transverse loading is illustrated in the bottom curve. The stress-strain curve of the cross-ply composite, in the middle, exhibits a knee, indicated by strength ajc, which corresponds to the rupture of the fibers in the 90° ply. The 0° ply then bears the load, until it too ruptures at a composite fracture strength of ct/. 

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. 

The failure strain of a unidirectional fibre composite in the transverse direction is normally low because the matrix resins have relatively low failure strains while the large difference between the moduli of the components magnifies the strain in the matrix under stress. Thus, in an angle ply laminate, the first failure event occurs in the transverse ply or phes. Reloading of the transverse ply via shear stress transfer at the ply interfaces leads to multiple cracking before the fibres reach their failure... 

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. 

In the course of the recent revival of bio-based polymers [58], cellulose esters have gained attention as matrix materials both for macroscopic and nano composites. In a series of papers, Seavey, Glasser, et al. have investigated continuous cellulose fibre reinforced cellulose ester composites of which the last is dealing with commercial matrix and fibre options [59]. A sort of hand lay-up with acetone solutions was used as the manufacturing method. For various commercial CABs and Lyocell fibres moduli between 15 GPa and 21 GPa were obtained in unidirectional (UD) composites while for cross-ply (CP) architectures, the values were between 10 and 15 GPa. Strengths go up to 310 MPa for UD composites and to 210 MPa for CP materials. 

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 ... 

Wang ASD, Pipes RB, Ahmadi A, Thermoelastic expansion of graphite/epoxy unidirectional and angle-ply composites. Composite Reliability ASTM STP580, American Society for Testing and Materials, Philadelphia, 574-585, 1975. 

An area where nanocomposites could achieve a dramatic commercial prominence is in the advanced polymer composites. CFRP composites have a limit on the achievable properties, particularly in cross-ply composites due to the low modulus and strength of the matrix phase. Modification of the matrix with carbon nanotubes at the lower scale of dimensions and with carbon nanofibres at a higher dimensional scale would allow for significant increase in the modulus and strength contributions of the matrix to the overall composite properties. Whilst this would offer some improvement in unidirectional composites, it could be dramatic in the case of cross-ply composites which are the major type of composite stmcture utilised in some advanced composite applications. 

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. 

For over three decades, there has been a continuous effort to develop a more universal failure criterion for unidirectional fiber composites and their laminates. A recent FAA publication lists 21 of these theories. The simplest choices for failure criteria are maximum stress or maximum strain. With the maximum stress theory, the ply stresses, in-plane tensile, out-of-plane tensile, and shear are calculated for each individual ply using lamination theory and compared with the allowables. When one of these stresses equals the allowable stress, the ply is considered to have failed. Other theories use more complicated (e.g., quadratic) parameters, which allow for interaction of these stresses in the failure process. 

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. 

The calculations were performed for laminates made up of orthotropic and isotropic layers In the first case, the two-dimensional model imitates the X, section of the cross-ply composite plate with unidirectionally reinforced monolayers The material of a monolayer is graphite-epoxy composite having the follov/ing characteristics Ej=I 94 I0 N/ra E =7.72.10 N/m 0.3 C 3=4.21.10 N/m ... 

A single ply unidirectional carbon fibre reinforced PEEK material has a volume fraction of fibres of 0.58. Use the data given below to calculate the Poisson s Ratio for the composite in the fibre and transverse directions. 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

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