Proceedings e report - Firenze University Press

MEASUREMENT AND SIMULATION OF DIMENSIONAL CHANGES DUE TO
FLOWS OF HEAT AND MOISTURE IN WOOD AND WOOD-BASED MATERIALS:
HOW OBJECTS OF CULTURAL HERITAGE CAN BE STUDIED
BY METHODS DEVELOPED FOR INDUSTRIAL APPLICATIONS
Dr. Jochen Aderhold*, Dr. Hiltrud Brocke
Fraunhofer Wilhelm-Klauditz-Institute for Wood Research (WKI), Braunschweig, Germany
Abstract
The response of wood and wood based materials to environmental factors such as temperature and moisture is
significant for technical applications as well as for works of art. Whereas climatic conditions for works of art are
generally less severe and more stable than in technical applications, damages to objects of cultural heritage
would imply more than financial loss. Consequently, the study of dimensional stability with respect to flows of
heat and moisture is equally important in both fields. The aim of this contribution is to outline the state of the art
from a technical point of view and to illustrate applications to culture heritage.
1. Techniques for the measurement of dimensional changes
A variety of techniques to measure dimensional changes due to climatic variations has been developed
over the years. Besides conventional strain gauges, optical methods are often preferred since they are
strictly non-contact and can cover large areas in short time. One has to differentiate between
techniques which measure relative and absolute dimensional changes, respectively. Due to space
restrictions, this paper will only discuss relative techniques. Relative techniques are generally more
sensitive than absolute techniques and include sophisticated laser methods such as Electronic Speckle
Pattern Interferometry (ESPI) as well as the rather simple but powerful Digital Image Correlation
Technique (DICT). Both ESPI and DICT can measure displacements in all three spatial directions if
the right set-up is used.
1.1. ESPI
If a rough surface is illuminated with a coherent laser beam, the reflected light rays can interfere and
form a so-called speckle pattern which looks like an arrangement of dark and bright dots (Fig. left).
Speckle patterns are fingerprints for the surface under study. In ESPI, the speckle pattern is
superimposed with a reference beam derived from the same laser [1-3]. The result is recorded with a
camera and taken as the reference image (Fig.1 right).
Fig. 1: Surface of a sandstone piece and its speckle pattern (left) and set-up for ESPI (right)
* E-mail: Jochen.aderhold@wki.fraunhofer.de
Object
Illumination
Camera
Reference beam
Image
processing
Screen with
correlation
fringes
Joseph Gril (edited by), Wood Science for Conservation of Cultural Heritage –Braga 2008: Proceedings of the International
Conference held by COST Action IE0601 (Braga - Portugal, 5-7 November 2008, ISBN 978-88-6453-157-1 (print)
ISBN 978-88-6453-165-6 (online) © 2010 Firenze University Press