Chandra Sekher Yerramalli

Scaling Studies in Modeling for Compressive Strength of Thick Composite Structures

Composite material usage in primary load bearing structures has continued to expand in aerospace, auto and wind energy industries. Large composite part thicknesses in some load bearing applications lead to defects during manufacturing. Typically, these defects are in the form of fiber waves, voids and delaminations. It is well known in the composite literature that composite compressive strength is a strong function of fiber alignment, and fiber waviness can cause failure due to fiber microbuckling and kinking or failure by splitting at the fiber/resin interface. A detailed micromechanical analysis of these wavy defects is needed to estimate the strength reductions due to presence of wavy defects in thick uni-directional (UD) laminates. For example, real composite part thicknesses in industrial applications are in the range of 40 mm-60 mm while individual fiber and resin layers are only a few microns in thickness. Hence, micromechanics finite element (FE) models involving individual layers require an enormous number of elements, which, in addition, scales poorly with the part thickness. Earlier studies on the effect of fiber waviness have focused on simplified homogenized models to study the effect of fiber waviness. However, such models cannot resolve local details such as inter-layer stresses that initiate resin yielding. In the present work, two modeling approaches are investigated — (i) a micromechanics approach in which individual fiber and resin layers are explicitly modeled, and (ii) a tow-level approach in which the fiber and resin properties are homogenized to generate effective properties of a tow. It is demonstrated that the two approaches lead to identical predictions of peak load for identical coupon dimensions. It is also shown that the peak compressive load plateaus beyond a certain value of coupon thickness. This information enables the modeling and testing of an actual thick part using a coupon of greatly reduced thickness and hence smaller number of elements in the computational model without compromising on the details afforded by a micromechanical model.Copyright