Crack Definition
A crack is a type of damage that occurs when a material fractures, usually along a relatively straight line, into two or more parts. Cracks may occur in most materials: metals, ceramics, composites, and inorganic materials such as concrete and asphalt.
In mechanical engineering, cracks can be seen as the result of poor cycle time management or incorrect material selection. Cracks often occur in metals due to fatigue, plastic deformation, heat treatment, and corrosion. In brittle materials, such as ceramics, cracking may result from thermal stress.
Cracks in materials and objects can be caused by a variety of reasons including: impacts, thermal expansions, stress from static or dynamic loads, fatigue, impact loading, chemical and environmental attack, and corrosion. They can manifest in a variety of forms and sizes, from very small hairline fractures to large breaks in a material.
A crack can weaken the structure of an object, impacting its ability to hold up under loads or resist environmental attack, and cause an object to catastrophically fail. It is important to detect and assess cracks early, as their size and shape can change over time from a variety of factors, including changes in temperature or pressure, further impacts, chemical reactions, vibration and internal stresses.
Crack detection and assessment can be done through a variety of methods including observation through visual inspection, imaging, and x-ray testing. If a crack is found, its shape and characteristics, such as orientation, size, location, and depth, need to be properly identified and assessed for potential significance. Early identification, assessment, and repair can prevent further propagation of the crack and potential failure of the structure.
Cracks are classified according to the shape and size, common types being longitudinal, transverse, surface, subsurface and through-thickness cracks. Longitudinal cracks run parallel to the direction of loading, and are the result of tensile stress applied in a longitudinal direction. Transverse cracks occur perpendicular to the intended direction of loading, and result from stress applied in a transverse direction. Surface cracks run along the surface of the material in any direction, and are usually caused by fatigue, pores, or stress concentration. Subsurface or hidden cracks also occur under the surface layers, and are usually the result of thermal stress or shock loads. Through-thickness cracks extend throughout the thickness of the material, and are usually caused by creep, impact, fatigue, and corrosion.
In conclusion, it is important to understand the properties of cracks in order to assess and detect them early and accurately. The shape, size, and category of the crack provide valuable information about the extent of the damage, the likely cause and potential consequences. Early detection and assessment of cracks can prevent catastrophic failure of structures and materials.
