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.

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 work, due in Module 3, is to write a report of a minimum of 2,000 words.

Answer 1

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. Brittle materials include most ceramics and glasses (which do not deform plastically) and some polymers, such as PMMA and polystyrene. Many steels become brittle at low temperatures (see ductile-brittle transition temperature), depending on their composition and processing.

why are ceramics so much more brittle than metals? It has to do with the bonding. In metals, their metallic bonds allow the atoms to slide past each other easily. In ceramics, due to their ionic bonds, there is a resistance to the sliding. Since in ionic bonding every other atom is of opposite charge when a row of atoms attempts to slide past another row, positive atoms encounter positive atoms and negative atoms encounter negative atoms. This results in a huge electrodynamic repulsion which inhibits rows of ceramic atoms from sliding past other rows. In metals, the sliding of rows of atoms results in slip, which allows the metal to deform plastically instead of fracturing. Since in ceramics the rows cannot slide, the ceramic cannot plastically deform. Instead, it fractures, which makes it a brittle material.

When used in materials science, it is generally applied to materials that fail when there is little or no plastic deformation before failure. One proof is to match the broken halves, which should fit exactly since no plastic deformation has occurred.

When a material has reached the limit of its strength, it usually has the option of either deformation or fracture. A naturally malleable metal can be made stronger by impeding the mechanisms of plastic deformation (reducing grain size, precipitation hardening, work hardening, etc.), but if this is taken to an extreme, fracture becomes the more likely outcome, and the material can become brittle. Improving material toughness is therefore a balancing act.

Brittle material breaks while little to no energy is absorbed when stressed. The material fractures with no plastic deformation. The material in the figure below marked with (a) shows what a brittle material will look like after pulling on a cylinder of that material. Typically, there will be a large audible snap sound when the brittle material breaks. A brittle material is also known as a material having low ductility. A stress-strain curve for brittle and ductile materials is shown in the figure below. We will talk more about ductile materials in the next section

How Brittleness is calculated and measured

In other word brittleness is also called toughness. one way to measure toughness is by calculating the area under the stress strain curve from a tensile test. This value is simply called “material toughness” and it has units of energy per volume.

one can do a uniaxial tensile test. The strain to fracture is a measure of ductility of the material. Brittleness, being opposite of ductility, will be higher smaller is this strain to fracture.

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.