Quasi-isotropic laminates

Acoustic Emission to Model the Fatigue Behaviour of Quasi-Isotropic Carbon-Epoxy Laminate Composites. 

In this study, the cumulative AE counts have been used to quantify the fatigue damage development in quasi-isotropic CFRP laminates and a model based on AE data has been formulated. 

The results presented below were obtained using a 2 mm thick carbon fiber reinforced epoxy composite laminate with 16 layers. The laminate was quasi isotropic with fiber orientations 0°, 90° and 45°. The laminate had an average porosity content of approximately 1.7%. The object was divided in a training area and an evaluation area. The model parameters were determined by data solely from the training area. Both ultrasound tranducers used in the experiment had a center frequency of 21 MHz and a 6 dB bandwidth of 70%. 

Quasi-isotropic laminates have the same ia-plane stiffness properties ia all directions (1), which are defined ia terms of the [A] matrix of the laminate. For the laminate to be quasi-isotropic. 

It is also important to note that although the laminae [ 45] indicates that Ex = Ey =. GN/m, this laminate is not isotropic or even quasi-isotropic. As shown in Chapter 2, in an isotropic material, the shear modulus is linked to the other elastic properties by the following equation... 

Some laminates do exhibit quasi-isotropic behaviour. The simplest one is [0, 60]j. For this laminate... 

First, we need a common, unambiguous manner of writing how a laminate is specified to be laid-up, i.e., stacking-sequence notation. Then, quasi-isotropic, balanced, and hybrid laminates are defined. 

The term quasi-isotropic iaminate is used to describe laminates that have isotropic extensionai stiffnesses (the same in all directions in the plane of the laminate). As background to the definition, recall that the term isotropy is a material property whereas laminate stiffnesses are a function of both material properties and geometry. Note also that the prefix quasi means in a sense or manner. Thus, a quasi-isotropic laminate must mean a laminate that, in some sense, appears isotropic, but is not actually isotropic in all senses. In this case, a quasi-isotropic... 

Figure 4-21 Unbonded View of Simple Quasi-isotropic Laminates Balanced Laminates...

Quasi-isotroplc laminates do not behave like Isotropic homogeneous materials. Discuss why not, and describe how they do behave. Why is a two-ply laminate with a [0°/90°] sacking sequerx and equat-thickness layers not a quasi-isotropic laminate Determine whether the extensional stiffnesses are the same irrespective of the laminate axes for the two-ply and three-ply cases. Hint use the invariant properties In Equation (2.93). 

Fig. 6.23. R-curve prediction ( — ) and experimental data (O) for a carbon fiber-epoxy matrix quasi-isotropic [C/ 45°/90 ), laminate. After Wells and Beaumont (1987).

One of the most important properties which control the damage tolerance under impact loading and the CAI is the failure strain of the matrix resin (see Fig. 8.8). The matrix failure strain influences the critical transverse strain level at which transverse cracks initiate in shear mode under impact loading, and the resistance to further delamination in predominantly opening mode under subsequent compressive loading (Hirschbuehler, 1987 Evans and Masters, 1987 Masters, 1987a, b Recker et al., 1990). The CAI of near quasi-isotropic composite laminates which are reinforced with AS-4 carbon fibers of volume fractions in the range of 65-69% has... 

Fig. 8.7. Damage area of 24 ply quasi-isotropic carbon fiber reinforced composite laminates containing different resin matrices. After Srinivasan et al. (1992).

Sohi. M.M., Hahn, H.T. and Williams, J.G. (1987). The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates. In Toughened Composites. ASTM STP 937 (N.J. Johnston cd.). ASTM. Philadelphia, PA. pp 37 60. 

Figure 14.10 Photograph of a quasi-isotropic Gr/PEEK laminate in which a bend was propagated further into the workpiece, showing overcurvature...

The maximum strengths can be quite high, fig. 4. What is particularly notable is that the carbon fibre angle ply with quasi isotropic laminate. 

Fig. 5 Photoelastic patterns, from left to right quasi isotropic laminate 45° 15° and 30° angle ply laminates stresses (GPa) and notch sizes (mm) as indicated.

The fracture toughness should be independent of notch depth. Figs. 6-9 show that this is approximately the case for a > 5mm, for cp = 15°and 30° for carbon-epoxy, 60° and 75° for carbon-epoxy and glass-epoxy, and for the quasi isotropic laminate, but not for the lower angle glass-epoxies or the 45° carbon-epoxy. 

In the case of the quasi isotropic laminate, for the 10mm and 12.5mm notches the fracture toughness is in agreement, within experimental error, of the value given for a... 

Carbon fibers show negative thermal expansion behavior at temperatures between 20 and around 500°C as shown in figure 4. This behavior depends on each fiber s grade. By utilization of this negative behavior, materials whose coefficient of thermal expansion is zero can be created when quasi-isotropic laminates are controlled with an optimum fiber volume fraction and are incorporated with matrices which have positive coefficient of thermal expansion. In practice, those materials can be used in satellites or space telescopes which demand severe thermal environment resistance. 

Figure 1. Specific ultimate tensile strength vs. specific stiffness of current and developmental aerospace structural materials. Data are displayed on a log-log plot in (a), where P signifies PAN-based reinforcements Gr represents graphite fibers 0 0 and 90 0 indicate data collected parallel to and transverse to the fiber direction in uniaxial composites, respectively and Q/I represents quasi-isotropic laminates. The (f represents fiber reinforcements in MMCs. The dashed line in (b) represents the combinations of specific strength and stiffness that are double those of conventional metal alloys.

Quasi-isotropic laminate Specially orthotropic laminate... 

A recent study [17] subjected a population of samples of quasi-isotropic (QI) CFRP laminate to identical impacts and then to residual strength tests to measure the CAI. Significant variability in CAI values was found. The coefficient of variation (CoV — the standard deviation/mean) of the compression strength increased from 1.8% before impact to 5.4% after impact. Fitting the distribution of strength to a... 

Davies GAO, Hitchings D, Wang J. Prediction of threshold impact energy in quasi-isotropic carbon/epoxy composite laminates under low-velocity impact. Compos. Sci Technol 2000 60(l) l-7. 

Isa MD, Feih S, Mouritz AP. Compression fatigue properties of z-pinned quasi isotropic carbon epoxy laminate with barely visible impact damage. Compos Struct 2011 93 2269-76. 

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