Question

Materials are subject to changes from temperature. We know there are several states of matter: solid, liquid, gas, and plasma. However, within a solid-state, materials have their properties affected, in particular brittleness. The Liberty ships of the Second World War were often breaking from cold weather.

These failures highlighted a material fracture problem.

What causes these failures, what materials are affected, and are there any that are not? How is brittleness calculated and measured (types of measurement) and what does this means for aircraft material selection and design considerations? You may include the influence of this problem from a maintenance perspective.

Your report of a minimum of 2,000 words is due for this assignment.

Answer 1

Calculation Method:

Brittleness calculation includes the following, based on an energy conservation model, the strength offered by a number of brittle materials has been calculated from depth-of-penetration (DOP) test results. Each material was completely penetrated by a tungsten carbide cored projectile of known kinetic energy and the residual penetration into a ductile aluminium alloy backing material was measured.

The energy transferred to the tile by the projectile has been calculated and has been shown to vary linearly with the tile thickness. From the energy transferred to the armour tile, the mean resisting stress that was offered to the penetrator was calculated and for the materials tested, scaled with the material hardness. This work shows that for DOP testing, where the projectile remains intact, the measured DOP is merely a facet of the ceramic’s hardness and not its true ballistic performance. The possibility of using this method to measure the strength of damaged ceramic is also discussed.

Measurement of Brittleness:

Here is the way to measure brittleness. A material is brittle if, when subjected to stress, it breaks without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound.

Three alternative methods for measuring the tensile strengths of brittle materials are investigated and compared with the conventional pull method. These are a bending test, an indentation test and a test in which disks are compressed diametrally. Experiments on plaster of Paris, coal and cement show that, apart from the bending test, the methods give results in reasonable agreement. The bending test, however, is liable to give considerably different results, and it is shown that this can be attributed to its sensitivity to surface conditions.