2.3.5 Intrinsic lamina properties

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Lamina properties include those derived from the fiber and matrix plus a series of properties that are intrinsic to the lamina (they cannot be derived). These are:

$V_ f$ is the fiber volume fraction.

$\alpha _\sigma $ is the standard deviation of fiber misalignment. This property is used to calculate the longitudinal compressive strength $F_{1c}$ of the unidirectional lamina. Longitudinal means fiber direction. If you do not know the value, a carbon/epoxy prepreg processed by autoclave should have $\alpha _\sigma $ in the range 1.1–1.3 deg. Anything more than that is an indication of lesser quality of the prepreg, the fabrication, or both. CADEC provides you with a default value.

$\alpha _0$ is the angle of the fracture plane. For carbon/epoxy laminates, it is approximately 54 degrees. This angle is used to calculate $F_4$, discussed below.

$G_{Ic}$ is the critical energy release rate (ERR) in mode I (opening) fracture. It is also (loosely) called fracture toughness. We say loosely because in the fracture mechanics literature the term fracture toughness has been coined as the critical value of stress intensity. Anyway, ERR and stress intensity are related by a constant. This is explained in [2, §1.5]. This value is used to predict transverse and shear damage in Chapter Damage. If you don’t find values of critical ERR, you could approximate them by using [2, Eq. 7.39].

$G_{IIc}$ is the critical energy release rate (ERR) in mode II (shear opening) fracture. The third mode, that is III (tearing) does not happen in laminated composites because adjacent laminas prevent that mode of deformation; see mode III in [2, Fig. 1.8]. This value is used to predict shear damage in Chapter Damage. If you don’t find values of critical ERR, you could approximate them by using [2, Eq. 7.39].

$t_ t$ is the transition thickness, explained in [2, §7.2.1]. This value is used to correct the strength predictions by the so called in-situ strength [2, §7.2.1]. Default values of transition thickness are approximately 0.6 mm for Glass-Epoxy and 0.8 mm for Carbon/Epoxy.

Also, the transition thickness can be used (outside CADEC) to predict values for $G_{IIc}, G_{IIc}$ in terms of $F_{2t}, F_6$. The computation is not done internally in CADEC to allow users that wish to use separate experimental values of fracture toughness and strength. Values of $G_{IIc}, G_{IIc}$ are very important and necessary to predict transverse/shear damage in Chapter Damage.

A Lamina is used to build Laminates. However, you can do some analysis for a just Lamina, if you go to Chapters/Micromechanics.