| Surface-breaking cracks on structural elements are the effects of structural loads, environmental conditions or material volume changes and thus play an important role in characterizations of material properties and integrity of structural members. Several researchers have worked on the evaluation of the depth of surface-breaking cracks in concrete using mechanical-wave-based nondestructive testing techniques. Most of previous studies involve the use of diffracted compression waves (P-waves), where the arrival time or amplitude in time domain is measured. At the same time, surface waves can be used for crack detection, because they exhibit unique properties suitable for the detection of near surface defects. In comparison with compression waves, surface waves dominate in the signal along surface. Most of surface wave energy concentrates in a depth equal to one third of the wavelength; therefore in the presence of a surface-breaking crack, the propagation of surface waves is affected differently depending on the frequency (wavelength). (Abstract shortened by UMI.)... |
