Material Testing is an important aspect of engineering. Quality control requirements call for material integrity and reliability to prevent possible disasters. Specimens are tested for its compressive, tensile and flexural strength as a standard for inspection. Construction materials are examined using different testing techniques to determine and prevent failures that may cause safety hazards.
Strength properties of construction materials are usually measured by the destructive stresses acting on the material. But several non-destructive methods are now developed that can assess the material’s integrity without damaging it.
Acoustic Emission Testing is one non-destructive testing method that plays an important role in monitoring processing equipment and construction materials. It enables early defect detection to prevent material failure with the generation of transitory elastic waves via mechanical loading.. It has numerous applications, from testing materials in the laboratory, to testing field equipment, to testing foundations for pile installations and construction.
Acoustic Emission Testing (AET)
According to the American Society for Testing and Materials (ASTM), Acoustic Emission (AE) is the “generation of transient elastic waves during the rapid release of energy from localized sources within a material”[1] It is a non-destructive testing wherein energy is forced from a material by means of mechanical loading. This test is based on the wave propagation theory.
Mechanical loading produces mechanical tension that releases elastic energy. Acoustic emissions come from fatigue cracks, fiber breakage, stress build-up, and deformations like melting, thermal expansion and phase transformation.
AE sensors detect the waves and then convert it to AE signals. There are two types of acoustic signals, namely the burst and the continuous signal. Continuous signals are mostly noise. On the other hand, burst type or transient signals are short duration pulses that have distinct waves and can be differentiated from its background noise.
Bursts are generated by twinning, micro yielding, and development of cracks.[2] This is generally the type of signal that is used in the AE test, although to get significant data, background noise should be particularly minimized.
[1] Envirocousics S.A., “Acoustic Emission Theory,” Envirocoustics Non Destructive Testing, retrieved 23 October 2007, <http://www.envirocoustics.gr/acoustic_emission_theory_eng.htm>.
[2] B. Purna Chandra Rao, “An Introduction to Acoustic Emission,” Non-destructive Evaluation (NDE), retrieved 23 October 2007, <http://www.geocities.com/raobpc/AET.html>.
