Physical Properties
Materials are selected for various applications based on their physical and chemical properties. This article discusses the various physical properties of materials.
Strength
Strength is the ability of a material to resist deformation. The strength of a component is usually considered based on the maximum load that can be borne before failure is apparent. Under simple compression, the load at fracture will be the maximum applicable over a significantly enlarged area compared with the cross-sectional area under no load.
This obscurity can be overcome by utilising a nominal stress image for tension and shear. This is found by dividing the relevant maximum load by the original area of cross section of the component. Thus, the strength of a material is the maximum nominal stress it can sustain. The nominal stress is referred to in quoting the "strength" of a material and is always qualified by the type of stress, such as tensile strength, compressive strength, or shear strength.
For most structural materials, the difficulty in finding compressive strength can be overcome by substituting the tensile strength value for compressive strength. This substitution is a safe assumption since the nominal compression strength is always greater than the nominal tensile strength, because the effective cross section increases in compression and decreases in tension.
When a force is applied to a metal, layers of atoms within the crystal structure move in relation to adjacent layers of atoms. This process is referred to as slip. Grain boundaries tend to prevent slip. The smaller the grain size, the larger the grain boundary area. Decreasing the grain size through cold working or hot working of the metal tends to retard slip and thus increase the strength of the metal. Cold and hot working are discussed in the next section.
Ultimate Tensile Strength
The ultimate tensile strength (UTS) is the maximum resistance to fracture. It is equivalent to the maximum load that can be carried by one square inch of cross-sectional area (or one square metre) when the load is applied as simple tension. It is expressed in pounds per square inch or Newtons per metre squared.
psi or N/m2
If the complete engineering stress-strain curve is available, the ultimate tensile strength appears as the stress coordinate value of the highest point on the curve. Materials that elongate greatly before breaking undergo such a large reduction of cross-sectional area that the material will carry less load in the final stages of the test. A marked decrease in cross-section is called "necking". Ultimate tensile strength is often shortened to "tensile strength" or even to "the ultimate." "Ultimate strength" is sometimes used but can be misleading and, therefore, is not used in some disciplines.
Yield Strength
A number of terms have been defined for the purpose of identifying the stress at which plastic deformation begins. The value most commonly used for this purpose is the yield strength. The yield strength is defined as the stress at which a predetermined amount of permanent deformation occurs. The graphical portion of the early stages of a tension test is used to evaluate yield strength. To find yield strength, the predetermined amount of permanent strain is set along the strain axis of the graph, to the right of the origin (zero). It is indicated in the image below as Point (D).
A straight line is drawn through Point (D) at the same slope as the initial portion of the stress-strain curve. The point of intersection of the new line and the stress strain curve is projected to the stress axis. The stress value, in pounds per square inch or Newtons per metre squared, is the yield strength. It is indicated in the image below as Point 3. This method of plotting is done for the purpose of subtracting the elastic strain from the total strain, leaving the predetermined "permanent offset" as a remainder. When yield strength is reported, the amount of offset used in the determination should be stated. For example, "Yield Strength (at 0.2% offset) = 51,200 psi."
Typical Brittle Material Stress-Strain Curve
Examples of Yield Strength
Some examples of yield strength for metals are as follows.
Aluminium 3.5 x 104 to 4.5 x 104 psi
Stainless steel 4.0 x 104 to 5.0 x 104 psi
Carbon steel 3.0 x 104 to 4.0 x 104 psi
Alternate Values
Alternate values are sometimes used instead of yield strength. Several of these are briefly described below.
- The yield point, determined by the divider method, involves an observer with a pair of dividers watching for visible elongation between two gage marks on the specimen. When visible stretch occurs, the load at that instant is recorded, and the stress corresponding to that load is calculated.
- Soft steel, when tested in tension, frequently displays a peculiar characteristic, known as a yield point. If the stress-strain curve is plotted, a drop in the load (or sometimes a constant load) is observed although the strain continues to increase. Eventually, the metal is strengthened by the deformation, and the load increases with further straining. The high point on the S-shaped portion of the curve, where yielding began, is known as the upper yield point, and the minimum point is the lower yield point. This phenomenon is very troublesome in certain deep drawing operations of sheet steel. The steel continues to elongate and to become thinner at local areas where the plastic strain initiates, leaving unsightly depressions called stretcher strains or "worms."
- The proportional limit is defined as the stress at which the stress-strain curve first deviates from a straight line. Below this limiting value of stress, the ratio of stress to strain is constant, and the material is said to obey Hooke's Law (stress is proportional to strain). The proportional limit usually is not used in specifications because the deviation begins so gradually that controversies are sure to arise as to the exact stress at which the line begins to curve.
- The elastic limit has previously been defined as the stress at which plastic deformation begins. This limit cannot be determined from the stress-strain curve. The method of determining the limit would have to include a succession of slightly increasing loads with intervening complete unloading for the detection of the first plastic deformation or "permanent set." Like the proportional limit, its determination would result in controversy. Elastic limit is used, however, as a descriptive, qualitative term.
In many situations, the yield strength is used to identify the allowable stress to which a material can be subjected. However, for components that have to withstand high pressures, this criterion is not fully adequate and other factors must be considered (topics beyond the scope of this text).
Ductility
The percent elongation reported in a tensile test is defined as the maximum elongation of the gage length divided by the original gage length. The measurement is determined as shown in the next image.
Measuring Elongation After Fracture
Reduction of area is the proportional reduction of the cross-sectional area of a tensile test piece, at the plane of fracture, measured after fracture.
The reduction of area is reported as additional information (to the percent elongation) on the deformational characteristics of the material. The two are used as indicators of ductility, the ability of a material to be elongated in tension. Because the elongation is not uniform over the entire gage length and is greatest at the center of the neck, the percent elongation is not an absolute measure of ductility. Because of this, the gage length must always be stated when the percent elongation is reported. The reduction of area, being measured at the minimum diameter of the neck, is a better indicator of ductility.
Ductility is more commonly defined as the ability of a material to deform easily upon the application of a tensile force, or as the ability of a material to withstand plastic deformation without rupture. Ductility may also be thought of in terms of bendability and crushability. Ductile materials show large deformation before fracture. The lack of ductility is often termed brittleness. Usually, if two materials have the same strength and hardness, the one that has the higher ductility is more desirable. The ductility of many metals can change if conditions are altered. An increase in temperature will increase ductility. A decrease in temperature will cause a decrease in ductility and a change from ductile to brittle behaviour.
Cold-working also tends to make metals less ductile. Cold-working is performed in a temperature region and over a time interval to obtain plastic deformation, but not relieving the strain hardening. Minor additions of impurities to metals, either deliberate or unintentional, can have a marked effect on the change from ductile to brittle behaviour. The heating of a cold-worked metal to, or above, the temperature at which metal atoms return to their equilibrium positions will increase the ductility of that metal; this process is called annealing.
Ductility is desirable in high temperature and high-pressure applications because of the added stresses on the metals; high ductility helps prevent brittle fracture.
Malleability
Where ductility is the ability of a material to deform easily upon the application of a tensile force, malleability is the ability of a metal to exhibit large deformation or plastic response when being subjected to compressive force. Uniform compressive force causes deformation in the manner shown in the image below. The material contracts axially with the force and expands laterally. Restraint due to friction at the contact faces induces axial tension on the outside. Tensile forces operate around the circumference with the lateral expansion or increasing girth. Plastic flow at the center of the material also induces tension.
Malleable Deformation of a Cylinder Under Uniform Axial Compression
Therefore, the criterion of fracture (that is, the limit of plastic deformation) for a plastic material is likely to depend on tensile rather than compressive stress. Temperature change may modify both the plastic flow mode and the fracture mode.
Toughness
The quality known as toughness describes the way a material reacts under sudden impacts. It is defined as the work required to deform one cubic inch (metric units are joule per cubic metre) of metal until it fractures. Toughness is measured by the Charpy test or the Izod test.
Both of these tests use a notched sample. The location and shape of the notch are standard. The points of support of the sample, as well as the impact of the hammer, must bear a constant relationship to the location of the notch.
Charpy Test Equipment
The tests are conducted by mounting the samples as shown in the image above and allowing a pendulum of a known weight to fall from a set height. The maximum energy developed by the hammer is 120 ft-lb (163 N/m) in the Izod test and 240 ft-lb (325 N/m) in the Charpy test. By properly calibrating the machine, the energy absorbed by the specimen may be measured from the upward swing of the pendulum after it has fractured the material specimen as shown in the image below. The greater the amount of energy absorbed by the specimen, the smaller the upward swing of the pendulum will be, and the tougher the material is.
Material Toughness Test
Indication of toughness is relative and applicable only to cases involving exactly this type of sample and method of loading. A sample of a different shape will yield an entirely different result. Notches are used to confine the deformation to a small volume of metal. In effect, it is the shape of the metal in addition to the material composition that determines the toughness of the material.
Hardness
Hardness is the property of a material that enables it to resist plastic deformation, penetration, indentation, and scratching. Therefore, hardness is important from an engineering standpoint because resistance to wear by either friction or erosion, from steam, oil, and water, generally increases with hardness.
Hardness tests serve an important need in industry even though they do not measure a unique quality that can be termed hardness. The tests are empirical, based on experiments and observation, rather than fundamental theory. Its chief value is as an inspection device, able to detect certain differences in material when they arise even though these differences may be undefinable. For example, two lots of material that have the same hardness may or may not be alike, but if their hardness is different, the materials certainly are not alike.
Several methods have been developed for hardness testing. Those most often used are Brinell, Rockwell, Vickers, Tukon, Sclerscope, and the files test. The first four are based on indentation tests and the fifth on the rebound height of a diamond-tipped metallic hammer. The file test establishes the characteristics of how well a file takes a bite on the material.
As a result of many tests, comparisons have been prepared using formulas, tables, and graphs, that show the relationships between the results of various hardness tests of specific alloys. There is, however, no exact mathematical relationship between any two of the methods. For this reason, the result of one type of hardness test converted to readings of another type should carry the notation "______ converted from ______" (for example "352 Brinell converted from Rockwell C-38").
How Alloys Affect Physical Properties
Nickel is an important alloying element. In concentrations of less than 5%, nickel will raise the toughness and ductility of steel without raising the hardness. It will not raise the hardness when added in these small quantities because it does not form carbides, solid compounds with carbon.
Chromium in steel forms a carbide that hardens the metal. The chromium atoms may also occupy locations in the crystal lattice, which will have the effect of increasing hardness without affecting ductility. The addition of nickel intensifies the effects of chromium, producing a steel with increased hardness and ductility.
Copper is quite similar to nickel in its effects on steel. Copper does not form a carbide, but increases hardness by retarding dislocation movement.
Molybdenum forms a complex carbide when added to steel. Because of the structure of the carbide, it hardens steel substantially, but also minimises grain enlargement. Molybdenum tends to augment the desirable properties of both nickel and chromium.
Stainless steels are alloy steels containing at least 12% chromium. An important characteristic of these steels is their resistance to many corrosive conditions.
Summary
The important information in this section is summarised below.
Physical Properties Summary
- Strength is the ability of a material to resist deformation. An increase in slip will decrease the strength of a material.
- Ultimate tensile strength (UTS) is the maximum resistance to fracture.
- Yield strength is the stress at which a predetermined amount of permanent deformation occurs.
- Ductility is the ability of a material to deform easily upon the application of a tensile force, or the ability of a material to withstand plastic deformation without rupture. An increase in temperature will increase ductility. Ductility decreases with lower temperatures, cold working, and irradiation. Ductility is desirable in high temperature and high-pressure applications.
- Malleability is the ability of a metal to exhibit large deformation or plastic response when being subjected to compressive force.
- Toughness describes how a material reacts under sudden impacts. It is defined as the work required to deform one cubic inch (or one cubic metre metric) of metal until it fractures.
- Hardness is the property of a material that enables it to resist plastic deformation, penetration, indentation, and scratching.