![]() ![]() Holographic testing uses a laser to detect changes in the surface of the material which has been subjected to stress such as heat, pressure or vibration. Laser testing falls into three categories including holographic testing, laser profilometry and laser shearography. The waves are reflected or refracted when they encounter a buried object or material boundary with different electromagnetic properties. This geophysical NDT method sends radar pulses through the surface of a material or subsurface structure, such as rock, ice, water or soil. There are three types of electromagnetic testing, including eddy current testing, alternating current field measurement (ACFM) and remote field testing (RFT).Įddy current testing uses an alternating current coil to induce an electromagnetic field into the test piece, alternating current field measurement and remote field testing both use a probe to introduce a magnetic field, with RFT generally used to test pipes. This testing method uses an electric current or magnetic field which is passed through a conductive part. Leaks and active corrosion are detectable AE sources too. ![]() Frequently a method for use during proof tests of a pressure vessel, AE testing is also a continuous Structural Health Monitoring (SHM) method, for example on bridges. It is even possible to detect AE from plasticisation in highly stressed areas before a crack forms. Sensors dispersed over the surface the structure detect the AE. This is a passive NDT technique, which relies on detecting the short bursts of ultrasound emitted by active cracks under a load. National Structural Integrity Research CentreĬurrent NDT test methods include: Acoustic Emission Testing (AE).Structural Integrity Research Foundation.Comparison Fresnel Diffractionġ If the screen and the source of light is at a finite distance from the obstacle, then the diffraction is called Fresnel diffraction.ġ If the source of light and screen is at an infinite distance from the obstacle then the diffraction is called Fraunhofer diffraction.Ģ To obtain Fresnel diffraction, zone plates are used.Ģ To obtain Fraunhofer diffraction, the single-double plane diffraction grating is used.ģ The waterfronts falling on the obstacle in case of Fresnel diffraction are not plane.ģ The wavefronts falling on the obstacle in case of Fraunhofer diffraction are planes.Ĥ No convex lens is needed to converge spherical wavefronts.Ĥ Plane diffracting wavefronts are converged via a convex lens. The pattern observed is a fringed image of the source and in a particular direction. Fraunhofer Diffraction: Fraunhofer diffraction occurs with plane wave-fronts with the object situated at infinity. The waves are spherical and the pattern observed is a fringed image of the object.Ģ. Fresnel Diffraction: Fresnel diffraction occurs when light from a point source meets an obstacle. Whereas if the two are closer in size or equal, the amount of bending is noticeable and can be easily seen with the naked eye. ![]() So If the opening is greater than the wavelength of light, the bending will be almost negligible. But the proportion of bending depends on the relative size of the wavelength of light to the size of the opening. We can define Diffraction as bending of light when it passes around the edge of an object. When the light falls on the obstacle whose size is comparable with the wavelength of light then the light bends around the obstacle and enters the geometrical shadow. ![]()
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