Acoustic emission for tracing the evolution of damage in wooden objects

The monitoring of acoustic emission (AE) has allowed direct tracing of the fracturing intensity in wooden cultural objects exposed to variations in temperature and relative humidity (RH). High-frequency components produced by the mechanical fracturing were extracted from the raw AE signals using wavelet transforms. The accumulated energy of these components depended on the magnitude and rate of RH variations. The AE activity became negligible below the allowable magnitude for rapid RH variation established by numerical modelling, or when the time interval allowed for the RH variation was long enough. On-site AE monitoring of a wooden altarpiece in an historic church further confirmed the usefulness of the technique in tracing climate-induced stress in wood. The development of practical AE sensors to indicate risk to wooden objects in museums and at historic sites, or during their transportation, is discussed.