DETERIORATION OF SURFACES EXPOSED TO ENVIRONMENTAL CHANGES
GUSTAV A. BERGER, & WILLIAM H. RUSSELL
5 STIFFNESS, RESISTANCE TO DEFORMATION (MODULUS “E”)
WHEN A fabric loses its tension, it also loses its elasticity and its resistance to deformation. Canvas under tension must be straight unless it is deformed by a stronger force. The reverse of aged canvas paintings provides clear evidence that the paint film deforms the canvas, not the other way around, as was widely believed.
This and other considerations led us to test the stiffness, or resistance to deformation, of stretched canvas (Modulus of Elasticity, E) (fig. 4). It was measured in the same range of temperature and RH as generally used in our tests, that is, from 4% RH to 100% RH and from freezing to 50°C. These tests showed that within the above limits, temperature and RH had only very little influence on the resistance of canvas to deformation. Instead, tension was always found to be the decisive factor that determined how strongly a canvas resisted displacement, such as that caused by an expanding paint film. For instance, it was found that if a particular fabric at standard tension (MST=100 N/m) has a resistance to displacement of E=30,000 N/m, it might display a resistance of only 17% (E=5,000 N/m) at 50% of this tension (50 N/m). When this happens, the canvas no longer behaves like an elastic solid, and the paint film it carries is attached to a very soft, slack “support” that is unable to restrain its movements. However, a continuous paint film is still restrained from expanding by its adjacent paint. Thus, with every expansion of the paint film it is compressed. But when it contracts it is no longer restrained, except by its own weak cohesion. With every cycle of expansion and contraction, the paint film is compressed, its virtual lateral expansion reduced, and the tension within it increased. In fact, we have noticed here a mechanism that, with every cycling change, reduces the size of the paint film and creates tensions within it.