There is a difference between mechanical properties and physical properties of an alloy.
- Physical properties are things that are measureable. Those are things like density, melting point, conductivity, coefficient of expansion, etc.
- Mechanical properties are how the metal performs when different forces are applied to them. That includes things like strength, ductility, wear resistance, etc.
The mechanical and physical properties of materials are determined by their chemical composition and their internal structure, like grain size or crystal structure. Mechanical properties may be greatly affected by processing due to rearrangement of the internal structure. Metalworking processes or heat treatment might play a role in affecting some physical properties like density and electrical conductivity, but those effects are usually insignificant.
Mechanical and physical properties are a key determinant for which alloy is considered suitable for a given application when multiple alloys satisfy the service conditions. In almost every instance, the engineer designs the part to perform within a given range of properties. Many of the mechanical properties are interdependent – high performance in one category may be coupled with lower performance in another. Higher strength, as an example, may be achieved at the expense of lower ductility. So a broad understanding of the product’s environment will lead to the selection of the best material for the application.
A description of some common physical and mechanical properties will provide information that product designers could consider in selecting materials for a given application.
- Corrosion Resistance
- Ductility / Malleability
- Elasticity / Stiffness
- Fracture Toughness
- Strength, Fatigue
- Strength, Shear
- Strength, Tensile
- Strength, Yield
- Wear Resistance
Expanding on those definitions:
Thermal conductivity is a measure of the quantity of heat that flows through a material. It is measured as one degree per unit of time, per unit of cross-sectioned area, per unit of length. Materials with low thermal conductivity may be used as insulators, those with high thermal conductivity may be a heat sink. Metals that exhibit high thermal conductivity would be candidates for use in applications like heat exchangers or refrigeration. Low thermal conductivity materials may be used in high temperature applications, but often high temperature components require high thermal conductivity, so it is important to understand the environment. Electrical conductivity is similar, measuring the quantity of electricity that is transferred through a material of known cross-section and length.2. Corrosion resistance
Corrosion resistance describes a material’s ability to prevent natural chemical or electro-chemical attack by atmosphere, moisture or other agents. Corrosion takes many forms including pitting, galvanic reaction, stress corrosion, parting, inter-granular, and others (many of which will be discussed in other newsletter editions). Corrosion resistance may be expressed as the maximum depth in mils to which corrosion would penetrate in one year; it is based on a linear extrapolation of penetration occurring during the lifetime of a given test or service. Some materials are intrinsically corrosion resistant, while others benefit from the addition of plating or coatings. Many metals that belong to families that resist corrosion are not totally safe from it, and are still subject to the specific environmental conditions where they operate.
Density, often expressed as pounds per cubic inch, or grams per cubic centimeter, etc., describes the mass of the alloy per unit volume. The density of the alloy will determine how much a component of a certain size will weigh. This factor is important in applications like aerospace or automotive where weight is important. Engineers looking for lower weight components may seek alloys that are less dense, but must then consider the strength to weight ratio. A higher density material like steel might be chosen, for example, if it provides higher strength than a lower density material. Such a part could be made thinner so that less material could help compensate for the higher density.
4. Ductility / Malleability
Ductility is the ability of a material to deform plastically (that is, stretch) without fracturing and retain the new shape when the load is removed. Think of it as the ability to stretch a given metal into a wire. Ductility is often measured using a tensile test as a percentage of elongation, or the reduction in the cross sectional area of the sample before failure. A tensile test can also be used to determine the Young’s Modulus or modulus of elasticity, an important stress/strain ratio used in many design calculations. The tendency of a material to resist cracking or breaking under stress makes ductile materials appropriate for other metalworking processes including rolling or drawing. Certain other processes like cold-working tend to make a metal less ductile.
Malleability, a physical property, describes a metal’s ability to be formed without breaking. Pressure, or compressive stress, is used to press or roll the material into thinner sheets. A material with high malleability will be able to withstand higher pressure without breaking.
5. Elasticity, Stiffness
Elasticity describes a material’s tendency to return to its original size and shape when a distorting force is removed. As opposed to materials that exhibit plasticity (where the change in shape is not reversible), an elastic material will return to its previous configuration when the stress is removed.
The stiffness of a metal is often measured by the Young’s Modulus, which compares the relationship between stress (the force applied) and strain (the resulting deformation). The higher the Modulus – meaning greater stress results in proportionally lesser deformation – the stiffer the material. Glass would be an example of a stiff/high Modulus material, where rubber would be a material that exhibits low stiffness/low Modulus. This is an important design consideration for applications where stiffness is required under load.
6. Fracture Toughness
Impact resistance is a measure of a material’s ability to withstand a shock. The effect of impact – a collision that occurs in a short period of time – is typically greater than the effect of a weaker force delivered over a longer period. So a consideration of impact resistance should be included when the application includes an elevated risk of impact. Certain metals may perform acceptably under static load but fail under dynamic loads or when subjected to a collision. In the lab, impact is often measured through a common Charpy test, where a weighted pendulum strikes a sample opposite of machined V-notch.
Hardness is defined as a material’s ability to resist permanent indentation (that is plastic deformation). Typically, the harder the material, the better it resists wear or deformation. The term hardness, thus, also refers to local surface stiffness of a material or its resistance to scratching, abrasion, or cutting. Hardness is measured by employing such methods as Brinell, Rockwell, and Vickers, which measure the depth and area of a depression by a harder material, including a steel ball, diamond, or other indenter.
Plasticity, the converse of elasticity, describes the tendency of a certain solid material to hold its new shape when subjected to forming forces. It is the quality that allows materials to be bent or worked into a permanent new shape. Materials transition from elastic behavior to plastic at the yield point.
9. Strength – Fatigue
Fatigue can lead to fracture under repeated or fluctuating stresses (for example loading or unloading) that have a maximum value less than the tensile strength of the material. Higher stresses will accelerate the time to failure, and vice versa, so there is a relationship between the stress and cycles to failure. Fatigue limit, then, refers to the maximum stress the metal can withstand (the variable) in a given number of cycles. Conversely, the fatigue life measure holds the load fixed and measures how many load cycles the material can withstand before failure. Fatigue strength is an important consideration when designing components subjected to repetitive load conditions.
10. Strength – Shear
Shear strength is a consideration in applications like bolts or beams where the direction as well as the magnitude of the stress is important. Shear occurs when directional forces cause the internal structure of the metal to slide against itself, at the granular level.
11. Strength – Tensile
One of the most common metal property measures is Tensile, or Ultimate, Strength. Tensile strength refers to the amount of load a section of metal can withstand before it breaks. In lab testing, the metal will elongate but return to its original shape through the area of elastic deformation. When it reaches the point of permanent or plastic deformation (measured as Yield), it retains the elongated shape even when load is removed. At the Tensile point, the load causes the metal to ultimately fracture. This measure helps differentiate between materials that are brittle from those that are more ductile. Tensile or ultimate tensile strength is measured in Newtons per square millimeter (Mega Pascals or MPa) or pounds per square inch.
12. Strength – Yield
Similar in concept and measure to Tensile Strength, Yield Strength describes the point after which the material under load will no longer return to its original position or shape. Deformation moves from elastic to plastic. Design calculations include the Yield Point to understand the limits of dimensional integrity under load. Like Tensile strength, Yield strength is measured in Newtons per square millimeter (Mega Pascals or MPa) or pounds per square inch.
Measured using the Charpy impact test similar to Impact Resistance, toughness represents a material’s ability to absorb impact without fracturing at a given temperature. Since impact resistance is often lower at low temperatures, materials may become more brittle. Charpy values are commonly prescribed in ferrous alloys where the possibilities of low temperatures exist in the application (e.g. offshore oil platforms, oil pipelines, etc.) or where instantaneous loading is a consideration (e.g. ballistic containment in military or aircraft applications).
14. Wear resistance
Wear resistance is a measure of a material’s ability to withstand the effect of two materials rubbing against each other. This can take many forms including adhesion, abrasion, scratching, gouging, galling, and others. When the materials are of different hardness, the softer metal can begin to show the effects first, and management of that may be part of the design. Even rolling can cause abrasion because of the presence of foreign materials. Wear resistance may be measured as the amount of mass lost for a given number of abrasion cycles at a given load.
Considering this information can promote an optimized metal selection for a given application. Because of the multitude of materials available – and the ability to modify properties through alloying and often through heat treatment efforts – it can be time well spent to consult with metallurgical experts to select the material that provides the needed performance balanced with cost effectiveness.