What are Cracks?
Cracks are linear opening that form in materials of all kind and their function is to relieve stress. When an elastic material reaches its breaking point due to stretching or shrinking uniformly, cracks form and they usually meet at a node. Conversely, when inelastic material fails, straight cracks form to relieve its stress. Further stress in the same direction would make the cracks open up the existing cracks even further, while stress at right angles would create new cracks, at 90 degrees to the old ones. Angles of applied stress would influence the angle of the new crack compared to the old one.
Classification of Cracks
As in nature and in constructions, cracks can be classified by width and shape.
Based on width cracks can be:
- Thin Crack – Less than 1mm in width
- Medium Crack – 1 to 2 mm in width
- Wide Crack – More than 2mm in width
- Crazing – Closely spaced fine cracks at the surface of a material
Based on shape cracks can be:
- Map Pattern or of Random Type
- Vertical, Horizontal or Diagonal
Dynamics of Cracks
In nature, crack patterns are observed in many situations ranging from cracks in garden mud, to the snouts of Nile crocodiles or the vast polygonal networks that stretch across the polar deserts of Earth and Mars. The physics and behavior of these patterns is captured by an energy balance as the cracks grow. Since release of energy happens by the widening of a crack anywhere along its path, whereas to a decent approximation energy is only spent in a small area immediately around a growing crack tip, the entire lifetime of the crack affects how it will grow at any instant. This means that even through the highly practical and important question of whether a crack will grow or not (whether a bridge will collapse, or an airplane wing snap off) is solved, nobody can accurately predict the direction that a crack will grow in any but the simplest cases.
Fractography is the study of the fracture surfaces of materials. Fractographic methods are mainly used to determine the cause and reason for failure in engineering structures, especially in product failure and the practice of forensic engineering or failure analysis. The usage of fractography in material science, is to develop and evaluate theoretical models of crack growth behavior.
One of the many aims of fractographic examination is to deduce the cause of failure in a fractured surface by studying its characteristics. Different types of crack growth (e.g. fatigue, stress corrosion cracking, hydrogen embrittlement) produce different characteristic features on the surface, using which can help identify the failure mode. A single crack could be less important than the overall pattern of cracking, however, especially in the case of brittle materials like glasses and ceramics.