| Popis: |
Glass specimens tested in simple tension, bending, or torsion, with breaking strengths up to approximately 100 Kg/mm2, develop a characteristic morphology of the fracture face which has become known as "mirror, mist and hackle". This general pattern and some variants have been observed in the fracture of other materials. Smekal (1) has defined a quantity called the "reduced tensile strength" based upon this fracture pattern. During an unsuccessful experimental reexamination of Smekal's parameter, an empirical relationship between breaking strength and the "mirror" depth was observed. This relationship, arising from an independent observation, was later found to have been used by several workers (Levengood (2) Shand (3), (4), but had not been examined in any detail by them. The relationship may be expressed in the form Zc 1/2 = constant where Z is the breaking strength, and c is the "mirror" depth (a term fully defined later). The experiments described in this thesis have yielded information on the numerical value of the constant in the above relationship and on the significance of the characteristic morphological fracture pattern itself. Chapter 1 describes the gross features of the mirror, mist and hackle pattern and defines some of the terms in constant use throughout the work. The second chapter describes the apparatus and techniques used to measure the numerical value of Zc1/2 for some common glasses under various experimental conditions. The results are reviewed briefly in Chapter 3. The main conclusions are that at a given temperature of test the value of Zc1/2 for any one glass is constant, but different for different glasses. The constant appears to be unaffected by the surrounding media and is relatively insensitive to the fibre drawing method. For one glass a variation in the value of Zc1/2 with temperature seems likely. Particular attention is paid to the results of tensile and bend tests on "as received" soda glass rods which suggest a method of determining the mirror shape to be expected from tests conducted with different stressing systems. Also in this chapter is a review of previous observations of the empirical fact that Zc1/2 = constant. Chapter 4 describes the qualitative examination of detail in the mist zone with the aid of optical microscopy. This examination showed the presence of stopped sub-surface micro-cracks in the mist zone which correlate with surface features. It is proposed that the morphological elements making up the mist zone can be classified by introducing two generalised types of mist feature. A discussion concerning the possibility of explaining the formation of these features in terms of micro-crack interaction follows. Finally this chapter closes with a discussion of possible mechanisms of micro-crack formation during crack growth and the acceptance of secondary nucleation as a working hypothesis. Chapter 5 contains a review of theoretical work having a bearing on the problem of the formation of the mirror, mist and hackle morphology. Chapter 6 discusses the energetics of brittle fracture in real materials with reference to the work of Griffith (5) and Mott (6). Using a critical energy criterion for the bifurcation condition and utilising the work of Gulf (7), a form of Mott's equation is proposed which gives theoretical Zc1/2 values for glass approaching those found in practice. This criterion associates the value of Zc1/2 with the fracture energy of the material. Further discussion is given to the possibility of extenndng these principles to include other materials. The theoretical value for Zc1/2 in perspex is predicted on this basis. The postscript gives a brief account of further experiments performed to check this latter value. Preliminary results show encouraging agreement between theory and experiment even for this material. |
