Fracture surface topography

The mechanical properties of rapidly polymerizing acrylic dispersions, in simulated bioconditions, were directly related to microstructural characteristics. The volume fraction of matrix, the crosslinker volume in the matrix, the particle size distribution of the dispersed phase, and polymeric additives in the matrix or dispersed phase were important microstructural factors. The mechanical properties were most sensitive to volume fraction of crosslinker. Ten percent (vol) of ethylene dimethacrylate produced a significant improvement in flexural strength and impact resistance. Qualitative dynamic impact studies provided some insight into the fracture mechanics of the system. A time scale for the elastic, plastic, and failure phenomena in Izod impact specimens was qualitatively established. The time scale and rate sensitivity of the phenomena were correlated with the fracture surface topography and fracture geometry in impact and flexural samples. 

Fig. 5 R-curves for unidirectional [0°]2 (light grey symbols), symmetric [0°/90°]ss (black symbols) and non-symmetric lay-up [0°/90°]n (dark grey symbols), tested in one laboratory, filled/open symbols indicate specimens with a single/mixed fracture surface topography (open symbols are connected by dashed lines to guide the eye).

Fig. 7 shows that the values from the second laboratory tend to be lower than those from the first for both types of cross-ply lay-up, while those for the unidirectional lay-up agree fairly well. The scatter still seen in the R-curves for the cross-ply laminates with a single fracture surface topography (Fig. 7) can probably, at least in part, be attributed to different amounts of fibre-bridging (compare Fig. 1). Another factor is micro-cracking in front of the delamination that may make accurate determination of the delamination length difficult. This would also offer an explanation for the steep rise seen in the R-curves of those specimens for which the delamination does not deviate into the unidirectional plies. This is discussed in detail in [6]. Small (local and short-term) deviations of the delamination into the unidirectional plies not recognised in the visual inspection of the fracture surfaces might also contribute to the scatter by temporarily reducing Gic. Finally, the oscillating interlaminar - intralaminar type of delamination propagation could also account for some of the observed scatter. The analysi > presented in [4] concludes that the intralaminar G is considerably smaller than the...

Macroscopic Appearance of Fracture Surfaces. The most striking observations (Table III) of fracture surface topography are the occurrences of stress-whitening and extensive plastic deformation at the higher water contents, and the occurrence of arrest lines at high values of AK. 

Figure 5 shows the SEM micrographs, operating with secondary electron imaging, which show the surface topography of cold fractured film edges. The films were cryro-fractured in liquid N2 to obtain an undistorted view representitve of the bulk material. 

Fig. 3 Photographs of the fracture surface of two types of specimens with non-symmetric lay-up after the test, specimen number is indicated beside the lay-up. Note the shorter wavelength of the topography compared with the symmetric lay-up (Figure 2), specimen width 20 mm each.

TEM observations were performed in the as-received and deformed samples in order to reveal the effects of microstructure on the fatigue response of the studied alloy. Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope (JEOL JSM6500F) equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture. 

Fractograms of fractured surfaces of notched specimens at 293 K (Fig. 2) and at 77 K (Fig. 3) indicate that the two alloys have practically the same fracture topography at these temperatures. In all the cases, indications of a very tough fracture appearance of the plate specimens were observed for both longitudinal and transverse directions. 

The test specimen, which may be smooth or notched, is fixed in a tensile testing machine permitting to program the strain rate. A slow constant strain rate is applied and maintained throughout the experiment until the specimen fractures. During the test one measures the stress strain curve, a = f(e) and after the test one studies the cross sectional area and the topography of the fracture surface. 

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