Mechanical property is a material’s capacity to cope with a range of applied external forces such as shear stresses, load, weather conditions, and time. Mechanical engineers can measure the ability of the metal to resist shearing, stretching, twisting, compressing or breaking under a given set of conditions. Once established, these properties can be used to ascertain whether the material can withstand sudden loads and stresses, and its suitability for specific tasks. The following is a list of the most common tests carried out to determine the properties of metals.
If the metal returns to its original state after a given load or pressure is removed, it is said to be elastic. The absolute value, or modulus of elasticity, is the ratio between strain and stress and is a measure of the ‘stiffness of the material’.
Building elements are designed so the stresses in the materials are safely within the elastic regions of their stress/strain curves otherwise they would deform permanently.
Elastic limit = Point at which deformation ceases to be elastic.
Yield stress = Stress at which material yields and starts to deform plastically.
Proof stress = Stress at a particular strain (say 0.1% or 0.2%) where there is no clear yield point – indicates stress at the limit of elastic behaviour.
To find the yield strength of the material, an increasing load is applied up to the point where the elastic deformation no longer allows the material to return to its original shape when the load is removed. This is called the yield point and is usually given with the material’s specification.
Ultimate Tensile Strength
A material’s tensile strength indicates the maximum ability of a material to absorb energy and withstand stretching before it begins to crack or break. It has major significance when processing engineering materials and is a good indication of a material’s ability to resist fracture or breaking up. Because of the variety of materials, the engineering stress is calculated by dividing the weight (load) applied to the metal, by the original cross-sectional area. Engineering strain is the displacement, (the amount the material has stretched), divided by the material’s original length.
Measurement of tensile properties
Tensile testing machine
Load measured electronically in load cell
Load applied by
“Dogbone” shaped test samples are common for tensile testing.
Compressive strength is the ability of a material to withstand loads that apply in compression (opposite to tension). Compressive strength is a critical measurement for the design of structures. It is measured in MPa (as per the below example).
E.G. For concrete:
20MPa – common concrete
25-30MPa – industrial floor (harder)
40MPa – special engineering works
100MPa – very high strength concrete for columns in high rise buildings
Ductility is the property of a material (metal) that allows its shape to be changed without the metal breaking. It allows the metal to be pulled into a thin wire form and is resolved by the percentage reduction and elongation of the metal’s area.
Measures of Ductility
Ductility is a qualitative, subjective property of a material. It usually indicates the extent to which a metal can be deformed without fracture. There are two methods that can be used to obtain ductility from tension testing. These are:
- The engineering strain at fracture, ef, known as elongation where:
- The reduction in area at fracture, q, where:
Percentage Elongation & Reduction in Area
As mentioned above, both occurrences of these tests determine the metal’s ductility. After the test piece reaches its elastic limit and snaps, the parts are precisely reassembled. The minimum diameter and final length of the test piece are measured and the length and reduction in area, are then recorded as percentages.
The hardness of a metal is an important property that allows it to resist penetration and distortions such as buckling, warping, and twisting. Hard materials also have a higher resistance to machining and forming processes. There are a number of ways to test a metal’s hardness, with the Rockwell, Brinell, and Vickers tests being three of the most common. Hardness is also referred to as temper or stiffness.
The hardenability of a metal denotes the depth of metal that can be hardened to a specific hardness, by using a heat process. While hardness denotes resistance to penetration, hardenability shows how deep the metal can be penetrated by hardening, after quenching from high heat. The depth of hardening plays a big part in the strength of the steel part.
Brittle materials snap easily with little plastic deformation when subjected to stress. Even high strength brittle metals provide very low impact test values and soak up little energy before breaking. This can be shown by exposing a cast iron workpiece to an impact test. Possible reasons for a metal’s brittleness include impurities in the casting, coarse grain structure or incorrect heat treatment. A more accurate brittleness evaluation can be undertaken using the Izod or Charpy impact tests.
In this instance, toughness means the potential of the material to absorb sudden shock and impact loading by plastically deforming, but not fracturing. In metallurgy, it relates to the amount of absorbed energy per unit volume the metal can soak up before deformation permanently occurs by breaking or fracturing. To have the required level of toughness the metal needs adequate ductility, elasticity, and yield strength.
In certain circumstances, the toughness of a metal or metal part can be seriously affected by cracks, grooves, tool marks and other changes to the material’s cross-section. In toughness testing, the ‘notch effect’ is calculated using the Charpy or Izod impact and drop weight tests at various temperatures, to ascertain the effects on the test piece. A metal that tends to quickly lose its toughness at lower temperatures will fracture under a lighter loading.
Malleability, also known as plasticity, is a material’s ability to be moulded into something else without breaking. Modelling clay is one example of a material with excellent malleability. In metallurgy, mild steel (low-carbon steel), especially when heated, can be easily beaten into a variety of shapes. Cast iron, on the other hand, has virtually no malleability.
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