Fibre alignment

Example 3.6 A unidirectional composite consisting of carbon fibres in a PEEK matrix has the fibres aligned at 25° to the loading axis. If the fibres and matrix have the properties indicated below, calculate Ex, Ey, Gxy, Vxy, and Vyx. 

Consider a unidirectional sheet of material with the fibres aligned in the x-direction and subjected to a stress, If the sheet has thickness, h, as shown in Fig. 3.15 and we consider unit width, then the normal axial force Nx is given by... 

The initial studies described above were conducted on Raman microspectrometers on extracted human teeth. In order to translate this research from the laboratory bench to the dental chair side, the next step it is to develop dedicated systems for clinical use. A key element to this development is the use of fibre-optic probes to allow measurements in vivo. We recently reported a study in which optical fibres were used for PRS measurements [48], Although not yet fully realized into a dental probe, this study demonstrated the design and feasibility of acquiring parallel- and cross-polarized Raman spectra via a bifurcated optical fibre whose distal terminal has the two fibres aligned vertically for simultaneously collecting spectra from the two polarization channels on a 2D CCD array. Simultaneous data acquisition will allow for more rapid measurement times in vivo. 

Shang SH et al (2010) The effect of electrospun fibre alignment on the behaviour of rat periodontal ligament cells. Eur Cells Mater 19 180-192... 

There are no spots on the meridian for perfect fibre alignment spots occur in pairs symmetrically on either side, but it is easy to determine the c-axis repeat distance c from a knowledge of where the layer lines cut the meridian, using equation (4.1). For imperfect alignment, meridional spots... 

Although off-angle and some multidirectionally reinforced FRP laminates may still have fibres aligned in one single direction or orientation in each ply (and just have different fibre orientations from ply to ply), there are more complex fibre arrangements that also need to be considered. 

In short fibre reinforced composites with aligned or randomly distributed fibres, cracks initiate at flaws, such as pores or in resin-rich areas with local strain inhomogeneities caused by improper fibre alignment or at fibre ends. 

Because of the virtually unlimited variations in fibre arrangements (short and continuous fibres, aligned and randomly distributed, laminated, woven, knitted or braided) there are too many materials available to discuss fully all the details of their fatigue behaviour. This chapter will therefore concentrate on some selected composites for demonstration purposes. We shall discuss both tension fatigue, which is the most important load situation in metals, and compression fatigue. The influence of harsh environments will be mentioned as well. 

S.H. Aziz, M.P. Ansell, The effect of alkalization and fibre alignment on the mechanieal and thermal properties of kenaf and hemp bast fibre composites part 1-polyester resin matrix. Compos. Sci. Technol. 64(9), 1219-1230 (2004)... 

For a material such as a unidirectional lamina that contains all of its fibres aligned in one direction, the conditions of transverse anisotropy apply. The stress/strain relationship can be written as (Eckold 1994, Herakovich 1998) ... 

One common characteristic of all C/SiC composites is their distinct anisotropy in the mechanical as well as thermophysical properties. Considerable lower values of the tensile strength and the strain to failure have to be considered for an appropriate design if the load direction and the fibre alignment are not congment. As the carbon fibres show a different physical behaviour in longitudinal and radial direction, the composite s properties like thermal conductivity and coefficient of thermal expansion differ widely with respect to the in-plane or transverse direction. 

The properties of the composite are affected by fibre and matrix type, the relative quantity of each in the composite and the directionality of the fibres. A composite which has all the fibres aligned in one direction, i.e. is uni-directional, is stiff and strong in that direction, but in the transverse direction it will have low modulus and low strength. 

Figure 12.7 Schematic of the typical fibre orientations found in cell walls. Bold lines indicate the fibre alignment.

For example, in an ideal PA 66 compound, reinforced with 50% glass fibre and with all fibres aligned along the length of the moulding, the flexural and tensile moduli increase rapidly as the fibre length is increased from 0.1 to 1.0 mm. 

Jensen (1989) attributed the popularity of MSM for use in meat products mainly to its low cost. He pointed out that the separation method used to produce MSM was such that the resultant material had far fewer intact meat fibres than hand-boned meat. This led to poor cohesiveness and little fibre alignment. With MSM finding use in recipes where it is in direct competition with cheaper cuts of meat, which tend to be high in fat and connective tissue, it should be seen as a cheap ingredient that is comparatively low in fat and of high nutritional value. 

Thermoplastic sheet forming turns a flat composite laminate, which contains reinforcing fibres aligned in one or more directions, into a shaped structure with the characteristics a design needs. Sheet forming can be the final process for the component however, thermoplastics allow for sheet forming steps and additional processing. 

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