FAILURE ANALYSIS OF QUASI-ISOTROPIC COMPOSITE LAMINATES WITH FREE EDGES UNDER OFF-AXIS LOADING
Abstract
Composite materials with traction free edges are known to develop interlaminar stress concentrations near the edge region. Such stresses play a vital role in failure of composite laminates. Experiments showed that a stiffness-quasi-isotropic laminate was not quasi-isotropic in strength. To interpret this phenomenon, an extensive experimental program was carried out on quasi-isotropic laminates. A typical matrix-dominated crack and delamination mode was found near the free edge of the specimen for all the off-axis cases. Such a through-the-thickness-matrix-crack would result in complete loss of load-carrying capability in the samll failure zone and would trigger an unstable failure progression into the interior of the laminate ending in total laminate failure. Classical failure criteria were found inadequate in predicting laminate strength. To predict those failures, a new criterion based on interlaminar free edge stresses and average stress method was introduced. The theoretical results obtained using this new approach were shown to agree very well with experimental results. Extensional predictions for the angle-ply laminate and ($\pm\Theta/90$) $\sb{\rm s}$ laminate were also achieved. Laminates cured with adhesive films were found capable of reducing edge delamination and thereby enhancing the tensile strength of the laminate.
Degree
Ph.D.
Subject Area
Aerospace materials
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