MEASUREMENT OF SILICON-CARBIDE FIBER STRENGTH IN COMPOSITES FROM STUDIES OF THEIR FRACTURE SURFACES

The strength of composites of glass and glass ceramics reinforced with ceramic fibre depends to a large extent on the strength of the fibre. It is therefore of considerable use to be able to estimate the streng ths of fibres in fractured samples. Measurement of the 'mirror zone' o n the fracture surface of the fibre can be used to estimate its streng th at the time of failure of the composite, given that the relationshi p between 'mirror zone' and fibre strength has been established. The s trengths of single Nicalon silicon carbide fibres were measured on a t ensile test rig and the radii of the mirror zones on a scanning electr on microscope. The fibre diameters were typically 12-18 mum and streng ths approximately 3 GN m-2. To provide a range of strengths and mirror zone sizes various methods were used to weaken the fibres. Mechanical damage proved to be the most acceptable and strengths down to 1 GN m- 2 were measured. Whiplash in the fibres at failure tended to cause los s of primary fracture surfaces so a method was developed to damp the v ibrations in the fibre at failure so that matching fibre ends were obt ained. The data were fitted successfully to the equation (s + a)r1/2=c onstant, where s=fibre strength, r=mirror zone radius, and a is a cons tant for a particular fibre type. Some preliminary measurements of fib re mirror zones on Pyrex composites reinforced with Nicalon silicon ca rbide fibre have been made. (C) 1993. The Institute of Materials.