What are Materials Properties?
A material’s property is an intensive property of some material, i.e., a physical property that does not depend on the amount of the material. These quantitative properties may be used as a metric by which the benefits of one material versus another can be compared, thereby aiding in materials selection.
A property may be a constant or maybe a function of one or more independent variables, such as temperature. Materials properties often vary to some degree according to the direction in the material in which they are measured, a condition referred to as anisotropy.
Materials properties that relate to different physical phenomena often behave linearly (or approximately so) in a given operating range. Modeling them as linear functions can significantly simplify the differential constitutive equations that are used to describe the property.
Equations describing relevant material properties are often used to predict the attributes of a system.
The properties are measured by standardized test methods. Many such methods have been documented by their respective user communities and published through the Internet; see ASTM International.
List of Mechanical properties of Materials
A description of some common mechanical and physical properties will provide information that product designers could consider in selecting materials for a given application.
- Corrosion Resistance
- 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 heat sinks.
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 electrochemical attack by the 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.
Ductility is the ability of a material to deform plastically (that is, stretch) without fracturing and retains 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 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.
The 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 on 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, the 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 the 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 depression by a harder material, including a steel ball, diamond, or another 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 the 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 loss for a given number of abrasion cycles at a given load.
Other Mechanical properties
- Brittleness: Ability of a material to break or shatter without significant deformation when under stress; opposite of plasticity, examples: glass, concrete, cast iron, ceramics etc.
- Bulk modulus: Ratio of pressure to volumetric compression (GPa) or ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume
- Coefficient of restitution: The ratio of the final to initial relative velocity between two objects after they collide. Range: 0-1, 1 for perfectly elastic collision.
- Compressive strength: Maximum stress a material can withstand before compressive failure (MPa)
- Creep: The slow and gradual deformation of an object with respect to time. If the s in a material exceeds the yield point, the strain caused in the material by the application of load does not disappear totally on the removal of load. The plastic deformation caused to the material is known as creep. At high temperatures, the strain due to creep is quite appreciable.
- Durability: Ability to withstand wear, pressure, or damage; hard-wearing
- Fatigue limit: Maximum stress a material can withstand under repeated loading (MPa)
- Flexibility: Ability of an object to bend or deform in response to an applied force; pliability; complementary to stiffness
- Flexural modulus
- Flexural strength: Maximum bending stress a material can withstand before failure (MPa)
- Friction coefficient: The amount of force normal to surface which converts to force resisting relative movement of contacting surfaces between material pair
- Mass diffusivity: Ability of one substance to diffuse through another
- Poisson’s ratio: Ratio of lateral strain to axial strain (no units)
- Resilience: Ability of a material to absorb energy when it is deformed elastically (MPa); combination of strength and elasticity
- Slip: A tendency of a material’s particles to undergo plastic deformation due to a dislocation motion within the material. Common in Crystals.
- Specific modulus: Modulus per unit volume (MPa/m^3)
- Specific strength: Strength per unit density (Nm/kg)
- Specific weight: Weight per unit volume (N/m^3)
- Stiffness: Ability of an object to resist deformation in response to an applied force; rigidity; complementary to flexibility
- Surface roughness: The deviations in the direction of the normal vector of a real surface from its ideal form
- Tensile strength: Maximum tensile stress of a material can withstands before failure (MPa)
- Viscosity: A fluid’s resistance to gradual deformation by tensile or shear stress; thickness
- Young’s modulus: Ratio of linear stress to linear strain (MPa)
- Acoustical absorption
- Speed of sound
- Sound reflection
- Sound transfer
- Third order elasticity (Acoustoelastic effect)
- Atomic mass: Applies to all elements. The average mass of the atoms of an element measured in atomic mass unit.
- Atomic number: Applies to pure elements only
- Atomic weight: Applies to individual isotopes or specific mixtures of isotopes of a given element
- Corrosion resistance
- Specific internal surface area
- Surface energy
- Surface tension
- Dielectric constant
- Dielectric strength
- Electrical resistivity and conductivity
- Electric susceptibility
- Electrocaloric coefficient
- Magnetoelectric polarizability
- Nernst coefficient (thermoelectric effect)
- Piezoelectric constants
- Seebeck coefficient
- Curie temperature
- Hall coefficient
- Magnetocaloric coefficient
- Magnetothermoelectric power (magneto-Seebeck effect coefficient)
- Pyromagnetic coefficient
- Spin Hall effect
- Castability: How easily a quality casting can be obtained from the material
- Machinability rating
- Machining speeds and feeds
- Absorbance: How strongly a chemical attenuates light
- Electro-optic effect
- Optical activity
- Refractive index
- Neutron cross-section
- Specific activity
- Half life
- Binary phase diagram
- Boiling point
- Coefficient of thermal expansion
- Critical temperature
- Curie point
- Ductile-to-brittle transition temperature
- Eutectic point
- Flash point
- Glass transition temperature
- Heat of vaporization
- Inversion temperature
- Melting point
- Thermal conductivity
- Thermal diffusivity
- Thermal expansion
- Triple point
- Vapor pressure
- Specific heat capacity