Kaiser effect in historic timber elements

M Strojecki,C Colla,E Gabrielli,M Łukomski

doi:10.1007/s00107-013-0738-8

Abstract

Timber elements are major structural and architectural components in historic buildings and at the same time belong to the category of materials vulnerable to degradation. The recovery of 150-year old timber beams from a roof of a historic building made possible the non-destructive investigation of their response to cyclic loading. The experimental study carried out using the acoustic emission technique provided evidence that historic wood shows the load memory known as the Kaiser effect. The effect was observed for different loading and unloading time windows. The observations open up a new perspective for the determination of defects in wooden objects and constructions, important for assessing their possible structural instability.

Highlights

The Acoustic Emission (AE) method is known to be an excellent monitoring technique for tracing damage development in materials
The results presented clearly point to an almost perfect shorttime Kaiser effect that occurs in the tested historic timbers
Bending tests conducted on specimens obtained from a 150-year old spruce timber beam recovered from the roof structure of a historic building in Ferrara, Italy, combined with AE monitoring revealed stress memory in naturally aged wood

Summary

Introduction

The Acoustic Emission (AE) method is known to be an excellent monitoring technique for tracing damage development in materials. It is defined as energy released due to micro-displacements in a structure undergoing deformation. The AE technique is suitable for monitoring structural elements made of metals, due to crystalline structure as well as their isotropy and high homogeneity. AE is not observed during the reloading of a structure until the stress exceeds its previous value. This ability to memorize the highest stress level already experienced was discovered in the early 1950s and was termed the Kaiser effect (Kaiser 1953). Studies by Li and Nordlund (1993); Lavrov (2001) and Ko and Yu (2008)

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