The effect of porosity on the elastic constants of thin carbon fiber composite laminates

In recent years the use of carbon fiber reinforced organic composite materials for highly loaded military and civil aircraft components has continuously increased. It is becoming even more important to predict the material properties of composite laminates with different levels of material anomalies. One of the most common defects found in continuous carbon fiber composite laminates is porosity.; Analytical techniques to determine porous laminate stiffness and residual strength are not available, therefore a variety of expensive and time-consuming structural tests have to be performed to include different laminate thicknesses and stacking sequence effects for each composite material. An analytical model utilizing non-porous laminate material properties and based on a thermo-mechanical constitutive theory for elastic composites with distributed damage is proposed for determining the reduction in stiffness due to porosity in thin fiber reinforced organic composite laminates. The model utilizes second-order tensor internal state variables to represent porosity and takes the form of laminate macroanalysis equations modified by internal state variables.; State-of-the-art NDT (Non-Destructive Testing) techniques are utilized to evaluate the actual porosity size and distribution within the laminate and serve as input to semi-empirical equations. Numerous T300/3501-6 carbon/epoxy woven and AS4/3501-6 tape quasi-isotropic laminates with various levels of porosity are non-destructively inspected using ultrasonic through-transmission and pulse-echo techniques. The sound attenuation levels are correlated to the actual physical location and amount of entrapped voids/gas bubbles in the laminate. The results are verified for cloth and tape carbon fiber/toughened epoxy specimens.; The results are correlated with in-plane shear and open hole compression structural element tests performed for T300/3501-6 eight harness woven cloth, as well as for carbon/toughened epoxy tape and cloth specimens. A computer program based on the developed methodology is used to demonstrate the laminate thickness and layup effects on the engineering constants for laminates with different levels of porosity for a number of carbon/epoxy materials currently used throughout the aerospace industry.