Buckling and fracture behavior of tapered composite panels containing ply drops13;

In typical applications like in the top skin of the wing or in control surfaces of an airframe, laminated composites are often found to be buckling-critical. Under excessive compression loading composite panels may undergo buckling and failure mechanisms would generally involve delamination or fiber breakage. Tapered composite panels with ply drops, when employed in wing skin, are expected to be more prone to the above mode of failure This is one important area of concern to the designers of composite structures and both analysis and experimental verification of the buckling behavior is recommended as a part of design validation effort. However, data from such analysis are scarce in open literature. In this paper, results of a study on buckling and fracture behavior of laminated Carbon Fiber Composite (CFC) panels containing ply drops are presented. CFC panels of varying thickness with normal and inclined ply drop were tested under compression loading until buckling and ultimate failure. Nearly simply supported boundary conditions at the ends and along the edges were simulated with the help of specially designed test fixtures. Tests were conducted both under room temperature and hot-wet (100amp;deg;C amp;plusmn; 3amp;deg;C and greater than or equal 85% RH) environmental conditions. Back-to-back strain gage output and out-of-plane deformation data were recorded and analyzed to determine the critical buckling loads and corresponding mode shapes. Finite element analysis using MSC NASTRAN was also made in order to predict critical buckling loads and corresponding mode shapes. These results were compared with those obtained from experiments and the agreement was found to be good in most cases. It was also found that under hot-wet conditions, both critical buckling loads and failure loads were lower compared to the room temperature values.13

Influence of environmental and geometric parameters on the behavior of fastener joints in advanced composites

Single- and double-shear bearing strength of mechanically fastened joints in carbon fiber composite (CFC) systems have been studied and the analysis of results presented. CFC laminates made from unidirectional prepreg tapes as well as bidirectional fabric prepregs were tested in as-received condition at room temperature and under hot/wet environmental conditions after hygrothermal aging. The influence of geometric parameters like the specimen width-to-hole diameter ratio (w/d) and the specimen thickness-to-hole diameter ratio (t/d) on the bearing strength has been investigated. Bearing strength tests were performed with three different fastener bolt materials, and, in all, over 300 specimens were tested. Bearing stresses were evaluated at ultimate failure, at two-percent hole deformation, at onset of nonlinearity, and at first load drop; and two percent offset bearing strength has been selected for the purpose of comparison of data. The influence of lamina configuration, mode of loading, hole tolerance, and fastener bolt material on bearing strength has been investigated. The degradation of bearing strength in hygrothermally aged CFC specimens under hot/wet environmental conditions has been found to be around 25-30 percent, compared to the room temperature values

Compression buckling behavior of laminated composite panels

Compression buckling tests on 6.0-mm-thick carbon fiber composite (CFC) panels are described for which a reasonably good simulation of the boundary condition was achieved, especially under the simple supported condition. Carefully designed test fixtures that provided excellent alignment of the panel with the loading axis and adequate lateral stiffness for edge supports during buckling of the CFC panels were used to achieve this goal. Both simply supported and clamped boundary conditions are studied. Tests in room-temperature-as-received (RT/AR) and hot-wet (H/W) conditions are described. A buckling test on a simply supported panel with prior moisture conditioning was performed under the H/W condition with an environmental test chamber mounted on the test rig to prevent any loss of moisture during the test at 100 C and not less than 85 percent relative humidity. Based on these results, a test procedure was developed to identify buckling loads through measurement of out-of-plane displacements rather than relying solely upon longitudinal strata measurement