Structural Cracks and Classifications
A crack in a structural member introduces local flexibility that would affect vibration response of the structure. This property may be used to detect existence of a crack together its location and depth in the structural member. The presence of a crack in a structural member alters the local compliance that would affect the vibration response under external loads.
Classification of Structural Cracks
Based on geometries , cracks can be broadly classified as follows::
Transverse crack :
These are cracks perpendicular to beam axis. These are the most common and most serious as they reduces the cross section as by weaken the beam.They introduce a local flexibility in the stiffness of the beam due to strain energy concentration in the vicinity or crack tip.
Longitudinal cracks :
These are cracks parallel to beam axis. They are not that common but they pose danger when the tensile load is applied at right angles to the crack direction i.e perpendicular to beam axis.
Open cracks :
Breathing crack :
These are cracks those open when the affected part of material is subjected to tensile stress and close when the stress is reversed . The component is most influenced when under tension . The breathing of crack results in non‐linearity in the vibration behavior of the beam.
Most theoretical research efforts are concentrated on “transverse breathing” cracks due to their direct practical relevance.
Slant cracks :
These are cracks at an angle to the beam axis , but are not very common . There effect on lateral vibration is less than that of transverse cracks of comparable severity.
Surface cracks :
These are the cracks that open on the surface .They can normally be detected by dye‐penetrates or visual inspection.
Subsurface cracks :
Cracks that do not show on the surface are called subsurface cracks . Special techniques such as ultrasonic , magnetic particle , radiography or shaft voltage drop are needed to detect them .