For example, tests of old concrete shows that it could contain animal remains such as blood or entire bits and pieces. This shows an intuitive understanding of fracture mechanics. The stress field is rather uniformly distributed within the block. That is, no part of the block is being stretched or squeezed more than any other part. However, assume that the material the block is made of is not uniform.
Some areas are relatively strong, while others are relatively weak.
When the stress field that is being applied becomes strong enough, microcracks will develop in the areas of the block where the material is weak. Once the microcracks form, the distribution of stress within the object changes - the stresses have to go around the microcracks. This leads to there being a stress-buildup at the tips of the microcracks. Stand at the shore of a river. Hold a plate into the water at a right angle to the direction of the stream.