Optical Interferometry on Crazes in Solvent Vapours at Very Low Pressure in Polymethylmethacrylate.
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OPTICAL INTERFEROMETRY ON CRAZES IN SOLVENT VAPOURS AT VERY LOW PRESSURE IN POLYMETHYLMETHACRYLATE. by R. Schirrer and G. Galleron I.C.S, 4, rue Boussingault, F-67000 STRASBOURG. ABSTRACT The fracture toughness and the craze shape of a propagating single crack/craze in polymethylmethacrylate has been measured in a very low pressure toluene gas environment by means of optical interferometry. Below a velocity of 0.01 mm/s the craze size increases up to 4 times the craze size in air, whereas the fracture toughness remains identical to the toughness in air. Above 0.01 mm/s the crack/craze behaves like a crack/craze in air. INTRODUCTION The influence of the environment on the crazing and fracture properties of polymers is of major importance in the practical uses of the material. Therefore, it has been widely studied, specially from the experimental point of view. Most works have been done with liquid environments (solvents and non solvents of the material), or high pressure gas environment, near the condensation pressure (liquid nitrogen, argon, etc...). There are several categories of problems related to environmental effects, namely: (i) the influence of the environment on craze nucleation, (ii) on craze growth, (iii) on craze fibrils rupture. The crazing/rupture mechanism can be governed by the environment, depending on some parameters like the solubility coefficient of the liquid environment, its viscosity, the stress level applied on the sample and the crack/craze growth velocity. Several experimental techniques have been used: the most common one uses simple standard fracture samples, with or without notches, and performs standard fracture tests in the studied environment. No measurements are made at the very crack tip, and only fracture toughnesses are recorded. More sophisticated techniques are required to study isolated crazes or the craze at a crack tip. Craze initiation has been followed optically by light reflexion on the craze surface (1), the stress concentration at craze tip and crack tip has been measured by shadow optics (2), whereas the craze shape and structure have been measured by means of SAXS (3), TEM (4), and even holography (5). Each of these technique has some limitations: SAXS applies only on multiple crazes or on very large single crazes, TEM and LAED must be performed under vacuum, and holography applies only to very large crazes. Reviews may be found in (6) and (7). EXPERIMENTS: In this work we used the well known optical interferometry to measure in real time the craze shape in a controlled the advantages Compared to the other techniques, environment. (ii)its (i)the craze can be very short are the following: the (Iii), as 0.1 mm/s, growth velocity can be as high Mat. Res. Soc. Symp. Proc. Vol. 79.
1987 Materials Research Society
288
measurement is done on a single crack-craze system (easy micromechanics) in the environment. The weakness of the method lies in its poor resolution: the inner structure of the craze is not directly visible. The general principles of the methods have been d
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