The Kaiser effect in wood – does historic wood have a stress memory?

Acoustic emission (AE) signals generated under loading provide valuable information about structural changes undergoing in a material. For materials which have ability to remember load history, the AE activity is observed only above a load level that has been previously exerted on the same material. This ability to memorize the highest level of stress which the material has experienced in the past is known as the Kaiser effect. The persistence of the Kaiser effect in time for metals and mineral materials has been observed by many researchers. In case of organic materials such as wood, the evidence for the phenomenon is very limited. Earlier experimental evidence has shown that the Kaiser effect fades over the time for new seasoned wood. The process is accelerated by temperature and changes of moisture content in the tested material. In this article, a systematic study of the Kaiser effect in historic wood is presented. Measurements were performed in the laboratory on spruce beams from a roof timber structure of a historic palace in Northern Italy. The wood is almost 100 years old. The results clearly show that the Kaiser effect is observed even after one year between subsequent loadings. The observed long-time memory of historic wood can be explained by much higher ratio between the crystalline and amorphous phases of the material when compared to new wood, reported in the literature. The observation opens a new perspective for the determination of load history of wooden works of art and constructions, which is essential for assessing risk of their physical damage.