Acoustic emission measurements use microphones, amplifiers, and computers to detect and record the release of elastic waves during stress relaxation processes within materials, such as crack propagation at both the macro and micro scales. This talk discussed how acoustic emission (AE) has been used to track these processes in wooden and stone objects under varying levels of relative humidity and outlined how such studies have been used to generate and validate RH guidelines.
During AE experiments, the microphones are positioned against objects mechanically, without the need for glues or clamps, and environmental noise can be determined when there is sufficient distance between two sensors. It is important to note that recording AE monitors internal stress-relieving processes in real-time but is not able to predict when damage may occur.
The AE from wooden cylinders was found to depend on both the change in RH and the rate at which this change occurred. Mild changes in RH applied over 48 hours, for example, did not lead to detectable acoustic emissions. Monitoring the AE of a wooden altarpiece in a church lead to the establishment of expanded RH guidelines of 35 – 60% RH.
AE studies have also been performed on clay-containing sandstones similar to those found in medieval cathedrals. The studies monitored the sandstone’s response to damaging wetting-drying cycles and detected a linear AE increase with the number of cycles. The results of this study, in conjunction with predicted climate data, were used to anticipate areas of Europe in which clay-containing sandstones may be at particular risk for damage.