Applied Mechanics of Materials in Conservation Research
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APPLIED MECHANICS OF MATERIALS IN CONSERVATION RESEARCH Marion F. Mecklenburg Conservation Analytical Washington, D.C. 20560
Laboratory,
Smithsonian
Institution
ABSTRACT Much of the damage found in cultural and artistic objects is not chemical in nature but results from mechanical responses to stimuli such as changes in temperature, relative humidity, impact, and vibration. Analytical tools of engineering mechanics are available that allow us both to diagnose existing problems as well as to predict the effects of future potential hazardous conditions for many objects. A systematic approach of applying engineering principles to cultural objects requires two fundamental steps: 1, determining the mechanical properties of the constitutive material found in objects, and 2, developing analytical procedures that determine the overall effect of the individual material responses to various stimuli on the object as a whole. The individual material properties are typically defined as the yield and ultimate strengths, the elastic modulus, and the strains to yield and failure. For the vast proportion of cultural objects, the materials are organic, and their mechanical properties are dramatically altered by environmental factors such as changes in temperature and relative humidity. One of the most successful analytical techniques is Finite Element Analysis (FEA) using the digital computer. This method will allow one to numerically model an object, to mathematically induce environmental changes as well as determine the mechanical effects of these changes on the object modeled. INTRODUCTION There are four basic conditions that represent potential hazards to the structural integrity of cultural and artistic works. These conditions include exposure of an object to: 1, changes in relative humidity (RH); 2, temperature variations; 3, vibration, and 4, shock. Of the four conditions, the effects of RH on objects have received the most attention. Additionally, changes in temperature have been traditionally viewed as a parameter that affects RH and not necessarily as a potential hazard independent of RH considerations. Research into vibration and shock has never been conducted to the degree that would allow a clear description of the adverse effects of this type of stimuli on cultural objects. Control of temperature and relative humidity Current museum practice is that objects should be maintained in an environment of constant temperature at 20' C and a RH of 50%. This recommendation is, in fact, expressing the need to keep the moisture content of the object constant, thereby keeping it dimensionally stable (1,2,3,4,5). These values seem to work well within buildings for paintings on canvas as well as those painted on wood panels, and in general for those objects composed of hygroscopic materials. What is not clearly defined is the acceptable range of deviation from the recommended values that objects might safely endure. This consideration is important, Mat. Res. Soc. Symp. Proc. Vol. 185. c 1991 Materials Research Society
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