L. E. Malvern

Matrix Cracking in Impacted Glass/Epoxy Plates

In addition to delamination, which has been found to be the major failure mode in a fiber-reinforced laminated composite plate subjected to subperforation impact, matrix cracking and fiber breakage have also been observed. Each failure mode, however, does not generate independently. On the contrary, the different modes interact with one another. Therefore, in order to have an overview of the failure mechanism in an impacted com posite plate, it is necessary to find out the mechanism of every failure mode and the in teractions among them. Microscopic observations of the cross sections of impacted com posite materials have revealed the interaction between matrix cracking and delamination. In this study, however, several techniques have been used to present the matrix cracking patterns in impacted glass/epoxy plates with different thickness, geometries, boundary conditions, stacking sequences, and types of loading. The matrix cracking patterns on the surfaces of impacted plates have been enhanced by dye penetrant while those of the in ternal laminae have been investigated by edge replication. Results have shown that the delamination area on the second interface of a three-lamina glass/epoxy plate coincides with both the central matrix cracking zone of the impacted lamina and the middle crack- free zone of the non-impacted lamina. The matrix cracks in the internal lamina are also restricted by the projected delamination area. The association between delamination and matrix cracking has been discussed and also the effect on matrix cracking of the tensile stresses introduced by the flexural wave.

Delamination-crack propagation in ballistically impacted glass/epoxy composite laminates

For ballistically impacted glass/epoxy cross-ply laminated plates with three five-layer unidirectional laminas, high-speed photos were taken from the back of plates, illuminated from the front side. The semitransparency of the plates enabled a Nova high-speed camera to record delamination-crack propagations at speeds of up to 40,000 frames/s. The delamination crack in the fiber direction of the first (second) lamina at the first (second) interface, propagated initially at 300–400 (400–500) m/s which decreased to 200–300 (270–400) m/s during the period of observation, and decelerated to a stop within 100 (300) μs. This last velocity range (270–400 m/s) agreed well with the largest-amplitude flexural-wave velocity measured by strain gages. This is a documentation that delamination is associated with the flexural wave.A velocity gage consisting of a silver conductive paint was modified to measure propagation velocities of the generator strip which was cut from the first lamina by two through-the-thickness cracks and which initiated a sequential delamination. This generator-strip-formation velocity was higher than the measured delamination-crack-propagation velocity. This fact is consistent with the assumption that the generator strip initiates delamination cracks.

Wave Propagation Experiments On Ballistically Impacted Composite Laminates

The details of the time history of the elastic waves in ballistically im pacted composite laminates have been obtained by using surface and em bedded strain gages. Records show that a very low-amplitude in-plane tensile wave arrives first followed by a medium-amplitude flexural wave and then a high-amplitude flexural wave. Measured and calculated values of wave speeds agree well.

Passively confined tests of axial dynamic compressive strength of concrete

Passively confined dynamic impact experiments on plain concrete specimens were performed employing steel and aluminum jackets with a 76.2-mm-diameter split-Hopkinson pressure bar system. The specific requirements of the specimens for jacketed tests were achieved with metallicmold casting. The confining pressures were determined from foil strain gages mounted on the outer wall surface of the jackets. Permanent deformation and residual static strength were measured and studied in the characterization of the behavior of plain concrete specimens under dynamic multiaxial loads.

Impact Failure Mechanisms in Fiber-Reinforced Composite Plates

Experimental studies of centrally impacted (0–90° lay-up) fiberglass-epoxy plates have revealed a sequential delamination mechanism, which appears to be a significant factor in the energy absorption at impact speeds below the speed required for perforation of the plate by a small rigid impactor. A linear relationship is found between impactor kinetic energy and total delamination area in this impact speed range (above a threshold speed) when the delamination does not extend to the edge of the plate.