Acoustic emission as a non-destructive method for tracing damage: from laboratory testing to monitoring historic structures

Acoustic emission (AE) is defined as a transient elastic wave produced by the rapid release of energy stored in material under stress. The energy propagates itself as ultrasound and sound waves, and can be detected at the surface using a piezoelectric sensor which converts the vibration of the surface into an electric signal. The AE technique has recently become an important non-destructive tool to gain insight into the evolution of damage in materials. It is widely used as a laboratory method in material science and civil engineering. However, it has not been applied sufficiently in the field of cultural heritage, mainly due to the great variety of historic materials and limited knowledge about their physical properties. In comparison with other non-destructive techniques AE possesses some very important advantages – it is simple in application and does not require any external stimulation of the object. Due to its high sensitivity (which is both an advantage and disadvantage) complications in interpretation of the recorded signals are unavoidable, especially during long-term monitoring in noisy environmental conditions. The main problem is how to distinguish AE signals related to damage from signals which originate from other non-destructive processes as well as from ambient noise. In this paper a comparison of AE signals measured for different materials (stone samples, timber joist specimens) and different destructive and non-destructive processes, including mechanical tests, is presented. The proper evaluation of the results obtained in the laboratory can help to distinguish between AE signals related to material damaging and non-damaging processes. The presented work demonstrates the potential of AE as a practical tool to assess the risk of mechanical damage to historic building materials also at different moisture contents or in varying state of decay.