Fabrics unidirectional laminates

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... 

Fiber reinforced composites, depending on the properties needed, can be fabricated in three different ways. Very short fibers can be used as filler, short fibers can be organized with random orientation and long fibers can be laid in one direction to form unidirectional composites. Short staple fibers may also be twisted together to form continuous yams to fabricate unidirectional composite laminates similar to those made using long fibers. Several unidirectional laminates may be combined by layering in different directions to form laminar composites. Yarns may also be woven or knitted into fabrics to form similar laminar composites. 

In-plane properties (e.g., modulus of elasticity and strength) of unidirectional laminates are highly anisotropic. Cross-, angle-, and multidirectional laminates are designed to increase the degree of in-plane isotropy multidirectional can be fabricated to be most isotropic degree of isotropy decreases with angle- and cross-ply materials. 

Using a combination of unidirectional and/or biaxial fabrics, the laminates provide strength in both longitudinal and transverse directions. The range of the tensile strength of PipeMedic laminates is 60—155 ksi (415—1070 MPa). 

The probabilistic analysis presented in this chapter is not restricted to a specific fabrication approach but rather presents results as a function of the fiber volume fraction and can be applied to a wide range of pipe rehabilitation methods. The time-dependent failure probability analysis is illustrated for specific fiber volume fractions and can be repeated for any fiber volume fraction depending on the manufacturing process of interest. Typical fiber volume fractions of 30% and 40% are selected conservatively for example probability analysis conducted in the chapter. Recent efforts to measure fiber volume fractions used in composite pipe applications report fiber volume fraction values ranging from 47.6% to 54% for filament-wound composite pipes (Abdalla et al., 2008), and Chin and Lee (2005) measured volume fractions of 47% for unidirectional laminates manufactured via resin transfer molding in a trenchless rehabilitation scheme. 

The Plate Constitutive equations can be used for curved plates provided the radius of curvature is large relative to the thickness (typically r/h > 50). They can also be used to analyse laminates made up of materials other than unidirectional fibres, eg layers which are isotropic or made from woven fabrics can be analysed by inserting the relevant properties for the local 1-2 directions. Sandwich panels can also be analysed by using a thickness and appropriate properties for the core material. These types of situation are considered in the following Examples. 

Unidirectional construction Refers to fibers that are oriented in the same direction, such as unidirectional fabric, tape, or laminate, often called UD. Such parallel alignment is included in pultrusion and filament winding applications. 

A thin molding of an acetylene-terminated phenylquinoxaline, fabricated by compression molding at 316°C for 26 hr and at 371°C for 5 hr, gave tensile strength of 103 MPa (15,000 psi), tensile modulus of 2.62 GPa (380,000 psi) and elongation of 5% (46). Preliminary unidirectional graphite fiber laminate properties are reported in Table VI. 

Quantification of residual stresses after manufacture. The build up of thermal stresses starts during fabrication of the laminate when it is cooled from the stress free temperature to room temperature. The stress free temperature in the case of an amorphous thermoplastic as used in this study is taken as the glass transition temperature [1] Tg of the Polyetherimide used is 215°C). On a fibre-matrix scale, the contraction of the matrix ( = 57 x 10 /°C) is constrained by the presence of the fibre (cif = -1 x 10 /°C for the carbon in the fibre direction). This results in residual stresses on a fibre-matrix scale (microscale). On a macroscopic scale, the properties of a unidirectional layer can be considered trans ersally isotropic. This means, in turn, that a multidirectional composite will not only contain stresses on a microscale, but also on a ply-to-ply (macroscopic) scale. 

For simplicity, it will be assumed that the stmcture consists of laminates made with unidirectional tape plies and linear stress—strain response. The equations derived are, in general, valid for other material forms such as fabrics, with some differences that will be pointed out in subsequent sections. 

The fabric s unidirectional fibers are constructed in a way that increases resin flow during molding, and wet-out is up to 40% faster than with other products. So that molders don not have to modify their existing laminate designs, the fabrics are based on traditional knitted fabric technology and have comparable properties. Channels are built into the fabric structure to ensure a fast, even resin distribution. The faster flow rate can lead to higher production and mold turnover and because there is no need for local resin distribution media, the fabric can potentially decrease molding costs. A continuous filament mat version of the fabric is said to offer even faster surface flow. 

Many combinations of resins and reinforcement types and weaves available for specific structural applications. Directional strength properties can be varied from unidirectional to orthotropic by choice of reinforcement type and laminate fabrication method. 

The properties of composite materials cannot be predicted adequately by considering the fibre and resin constituents one by one. An important mechanism of composite failure under stress is delamination caused by differences between the engineering properties of successive plies or layers. These differences arise from the fact that successive layers may have different fibre orientations [34] or, occasionally, different fibres. It is a feature of laminates made by stacking pre-impregnated layers of reinforcement and is not an issue with, for example, unidirectional pultrusions. The process of delamination has been reviewed by Davies [35]. The fabrication of three-dimensional composites is an important step towards reducing or eliminating unwanted delaminations. Such materials are at an advanced stage of development. 

The structure and orientation of the reinforcing fibers in the matrix system are essential for the mechanical properties of the work-piece. The choice of a particular architecture is dependent on multiple factors like drapeabiUty of the fabric, geometry/shape of the workpiece, mechanical requirements, and manufacturing process. Compraients made of unidirectional layers (laminates) are showing the best mechanical properties since the fibers are completely stretched (no undulation). Usually the single layers of a laminate are showing different fiber orientations. This causes anisotropic material behavior in the planar direction. The structure of a multiaxial layered laminate is shown in Fig. 2. 

M stands for cut fibres used as bound matts MW stands for a composite of matt and woven fabric FM stands for a composite of matt and unidirectional fibres). These laminates were tested in a laboratory before use. 

The unidirectional (UD) construction was first used in soft body armour for the chest and helmet by air crewmen in the Second World War and was reintroduced by AlliedSignal with the UHMWPE fibre Spectra in the mid-1980s. The unidirectional technology is based on the idea of combining the cross-plied filaments with an elastomeric matrix in a laminated systan. Fig. 6.7 shows a four-plied unidirectional system laminated by two films. Other companies, such as DSM and Park Technologies, also provide similar fabrics for personnel protection. 

Graphite fibers are available to the user in a variety of forms continuous filament for filament winding, braiding, or pultrusion chopped fiber for injection or compression molding impregnated woven fabrics and unidirectional tapes for lamination. 

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