Material Properties: There are an endless number of materials that are used in modern manufacturing. Here are some basic kinds:
Ferrous metals (iron-alloys): carbon-, alloy-, stainless-, tool-and-die steels
Non-ferrous metals: aluminum, magnesium, copper, nickel, titanium, superalloys, refractory metals, beryllium, zirconium, low-melting alloys, gold, silver, platinum, …
Plastics: thermoplastics (acrylic, nylon, polyethylene, ABS,…), thermosets (epoxies, Polymides, Phenolics, …), elastomers (rubbers, silicones, polyurethanes,)
Ceramics, Glasses, Graphite, Diamond, Cubic Boron Nitride, …
Composites: reinforced plastics, metal matrix and ceramic matrix composites Nanomaterials, shape-memory alloys, superconductors, …
Properties of materials We shall concern ourselves with three types of issues:
(a) Mechanical properties of materials (strength, toughness, hardness, ductility, elasticity, fatigue and creep).
(b) Physical properties (density, specific heat, melting and boiling point, thermal expansion and conductivity, electrical and magnetic properties)
(c) Chemical properties (Oxidation, corrosion, flammability, toxicity, etc.)
Mechanical properties Mechanical properties are useful to estimate how parts will behave when they are subjected to mechanical loads (forces, moments etc.). In particular, we are interested to know when the part will fail (i.e. break, or otherwise change shape/size to go out-of-specification), under different conditions. These include loading under: tension, compression, torsion, bending, repeated cyclic loading, constant loading over long time, impact, etc. We are interested in their hardness, and how these properties change with temperature. We are sometimes interested in their conductivity (thermal, electrical) and magnetic properties. Let’s look at how these properties are defined, and how they are tested.
Basics of Stress Analysis We briefly study the basics of solid mechanics, which are essential to understand when materials break (this is important in product design, where we usually do not want the material to break; it is important in manufacturing, where most operations, e.g. cutting, are done by essentially ‘breaking’ the material).
Essentially, any load applied to a solid will induce stress throughout the solid. There are two types of stresses: shear and tensile/compressive, as shown in the figure below. Consider that some force(s) are applied to a solid such that it is experiencing stress but is in stable equilibrium. We consider an infinitesimal element inside the solid under such stresses.
Figure 1. Tensile, compressive and shear stresses; stresses in an infinitesimal element of a beam
The question we need to answer is: under some given set of stresses as shown, will the material fail? To simplify matters, let’s look at the 2D situation (XY plane only). To answer our question, we first find the resultant stresses, σ and τ, along some arbitrary direction inclined at angle φ to the y-axis (see figure below). Since the element is at equilibrium, the resultant of all forces must balance. Also, by definition, stress = force/area. From this, we get the following relation:
Figure 2. Computing the principal stresses (2D case)