Tensile Tests and Strain Estimation

Black paint spraying was applied to generate the optically active random structure. The CCD camera used had a resolution of 1280x 1024 pixels. A grating was used with mesh width of normally 10x10 pixels, and each point was identified within a cell of 13 x 13 pixels. 

In addition to the graphical display of 2D-displacements, also the displacement and tensile strain along the y-direction in the middle of the sample was investigated. This allows to trace the failure at the pole position of the interface. 

The loading of a two-material composite petpendicular to the interface should show a clear step in the strain because of the different stiffness of the materials. A failed interface would exhibit a pole between the relatively moderate-strained components (i.e., an offset of the borderlines of the two components). However, if a crack propagates from the surface of the specimen in the pole region into the specimen along the interface (through-thickness direction), one observes a large displacement of the cells close to the interface and a decreased strain 

With increasing load, the strain level grows steadily until failure. An additional strain tip establishes directly at the interface. It is relatively difficult to detect the exact moment of the crack initiation rumiing from the surface to the specimen middle at the pole. After the interface failed, the total strain curve drops down. 

The measurement allows us to draw the conclusion that failure starts at the surface in the pole region of the sample. This can be understood if the throughthickness (z-direction) geometry at the pole is considered. In this direction. Material 1 meets Material 2 under angles 0j = = 90°. In such cases, a stress singularity 

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