either case this will cause the material sections to begin bending and lead to ultimate failure. Microcracking Microcracks are a leading cause of failure under compression for brittle and quasi-brittle materials. Sliding along crack tips leads to tensile forces along the tip of the crack. Microcracks tend to form around any pre-existing crack tips. In all cases it is the overall global compressive stress interacting with local microstructural anomalies to create local areas of tension. Microcracks can stem from a few factors. Porosity is the controlling factor for compressive strength in many materials. Microcracks can form around pores, until about they reach approximately the same size as their parent pores. (a) Stiff inclusions within a material such as a precipitate can cause localized areas of tension. (b) When inclusions are grouped up or larger, this effect can be amplified. Even without pores or stiff inclusions, a material can develop microcracks between weak inclined (relative to applied stress) interfaces. These interfaces can slip and create a secondary crack. These secondary cracks can continue opening, as the slip of the original interfaces keeps opening the secondary crack (c). The slipping of interfaces alone is not solely responsible for secondary crack growth as inhomogeneities in the material's Young's modulus can lead to an increase in effective misfit strain. Cracks that grow this way are known as wingtip microcracks. It is important to emphasize that the growth of microcracks is not the growth of the original crack/imperfection. The cracks that nucleate do so perpendicular to the original crack and are known as secondary cracks. The figure below emphasizes this point for wingtip cracks. These secondary cracks can grow to as long as 10-15 times the length of the original cracks in simple (uniaxial) compression. However, if a transverse compressive load is applied. The growth is limited to a few integer multiples of the original crack's length. Shear bands If the sample size is large enough such that the worse defect's secondary cracks cannot grow large enough to break the sample, other defects within the sample will begin to grow secondary cracks as well. This will occur homogeneously over the entire sample. These micro-cracks form an echelon that can form an “intrinsic” fracture behavior, the nucleus of a shear fault instability. Shown right: Eventually this leads the material deforming non-homogeneously. That is the strain caused by the material will no longer vary linearly with the