Silas Mallon

Experimental Investigation on Material Dynamic Behaviors Using Ultra-high-speed Cameras

The full-field dynamic response of a material undergoing impact loading, especially the early stage of a single impact pulse event, is difficult to measure considering short time duration of the pulse and lack of measurement methods capable of acquiring data at sufficiently high rate. In this study, the investigators employed a recently developed ultra-high-speed HPV-X camera with framing rates up to 5,000,000 fps and a 400 × 250 pixel array to acquire whole field image data, including deformations, velocities, accelerations, strains and strain rates, which occur in the first 100 μs of initial impact of a three-point bend copper beam specimen subjected to centerline impact by a high strength aluminum bar. In addition to 2D image data, strain gauges and load cells are used to (a) provide offset trigger to the flash unit and the camera to ensure uniform illumination and (b) record the time history of force reactions at the two supports for a three-point bend specimen, respectively. Results clearly show that it is possible to quantify the full-field mechanical response of the specimen for framing rates from 1,000,000 to 5,000,000 over a range of input impact amplitudes, providing essential early-stage data that can be used to determine various properties of interest.

Fracture of Pre-stressed Woven Glass Fiber Composite Exposed to Shock Loading

Fracture study of an orthogonally woven glass fiber-reinforced epoxy composite under shock loading was performed. A shock tube apparatus was utilized to produce dynamic loading, applied to pre-stressed composite specimens. The displacement and strain fields present in the material were observed using the 3D digital image correlation technique. Displacement components obtained from the DIC were analyzed with the use of an over deterministic numerical approach to determine the stress intensity factor present in the composite specimens. The effect of the orientation of the reinforcing fiber with respect to the crack faces was observed via the use of several specimen types, having various angles of fiber orientation. The effect of the value of prestress is also examined with the use of two distinct magnitudes of prestress. Results indicate that the value of applied pre-stress might significantly influence the velocity of propagating crack, as well as the out-of-plane displacement component, while the stress intensity factor present in the specimen at the time of fracture will not undergo a considerable change with varying the magnitude of the applied pre-stress.

Through Thickness Fracture Behavior of Transversely Graded Ti/TiB Material

Fracture behavior of a Ti/TiB graded material with a crack perpendicular to the gradient direction was investigated. Three-point bending experiment was conducted and full-field displacement fields were measured on both faces of the specimen, metallic and ceramic-rich surfaces, using 2D digital image correlation technique. Stress intensity factors were calculated using the displacement fields obtained on both faces of the specimen, and were compared to the effective fracture toughness determined from fracture load data. The overall fracture toughness was found to be very close to the stress intensity factor determined using the displacement field on ceramic-rich side. However, the stress intensity factors calculated on both surfaces using the displacement fields, showed different values. In addition, scanning electron microscope observations were performed to examine the fracture surface of the material. It was observed that the crack front is inclined, indicating that the fracture may have initiated in the ceramic side and followed by a very fast propagation to the metallic side.

Full-Field Deformation Observation of Polymer Foam Subjected to Shock Loading

Dynamic material response of polymer foam subjected to shock loading has been investigated, and the effects of density and loading rate have been studied. Cubic specimens 20 and 30 lb/ft3 nominal densities were subjected to shock loading using a single diaphragm shock tube apparatus. Full field displacements and strain fields are obtained with the use of stereo high speed cameras in conjunction with the 3D digital image correlation technique. Simultaneously, load data is obtained with the use of piezotronic load cells. Material responses of the two different mass densities of polymer foam have been examined, while the effect of loading rate is also examined by comparisons with results from quasi-static compression testing. The material response as a function of loading rate was recovered using load cell data and digital image correlation strain fields. Failure mechanisms are observed to differ in specimens of different density. Failure stress is seen to increase significantly with increasing foam density. Both 20 and 30 lb/ft3 density foams exhibit substantial strain rate dependence, with large increases in stress at failure observed at an elevated strain rate.