Abstract:The need for a static test method for modeling low-velocity foreign object impact events to composites would prove to be very beneficial to researchers since much more data can be obtained from a static test than from an impact test. In order to examine if this is feasible, a series of static indentation and low velocity impact tests were carried out and compared. It would be very beneficial to simulate an impact event using a "quasi-static" loading test. By using this test, damage initiation and propagation can be more easily detected,

Composite material usage is necessary on NASA’s future launch vehicles in order to obtain a low mass vehicle. While aircraft and launch vehicles that utilize load-bearing composite components have many similar damage tolerance requirements, the distinct differences between a part that has a lifetime of ∼500 s (one launch) and can be inspected in detail before use and one that has a lifetime of many tens of thousands of flight hours and can only undergo a ‘walk around’ inspection before each flight (commercial transport) needs to be taken into account. This article presents these differences and uses data from the ARES I composite interstage as an example of how to arrive at preliminary compression after impact strength values for the sandwich structure in the acerage of this part using residual strength curves. Results show that if severity of damage can be quantified by a nondestructive method (other than dent depth), the mass of the structure can be reduced due to better characterization of the damage.

A method of measuring the mode I (peeling) fracture toughness of core/face sheet bonds in sandwich structures is desired, particularly with the widespread use of models that need this data as input. This study examined if a mode I critical strain energy release rate, G IC , can be obtained from the climbing drum peel (CDP) test. The CDP test is relatively simple to perform and does not rely on measuring small crack lengths such as required by the more commonly used double cantilever beam (DCB) test. Simple energy methods were used to calculate G IC from CDP test data on composite face sheets bonded to a honeycomb core. Face sheet thicknesses from 2 to 5 plies (0.51-1.27 mm) were tested to examine the upper and lower bounds on face sheet thickness requirements. Suggestions on conducting the test and on modifying the CDP apparatus to test composite face sheets (as opposed to metallic) are also presented. Results from the study suggest that the CDP test, with certain provisions, can be used to find the G IC value of a core/face sheet bond.

Low-velocity instrumented dropweight impact tests were performed on carbon/ epoxy laminates. The composite plates were 8-ply (+45, 0, -45, 90)s laminates supported in a clamped-clamped/free-free configuration with varying amounts of in-plane load, Nx applied. The amount of damage induced into the specimen was evaluated using the instrumented impact data and X-ray inspection. Results showed that for a given impact energy level, more damage was induced into the specimen as the external in-plane load, Nx, was increased. The majorityof damage observed consisted of backface splitting of the matrix parallel to the fibers in that ply, associated with delaminations emanating from these splits. A free-edge delamination model was used to explain the type and extent of the major delaminations caused by the preload/impact combinations.

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