An overview of the mechanical properties of metals

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.

 

Elasticity

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’.

Elastic Limit

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.

stress-strain-curve

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.

 

Yield Strength

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.

yield-point

 

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

Tensile testing machine

Load measured electronically in load cell

Load applied by

  • Hydraulics
  • Screw

“Dogbone” shaped test samples are common for tensile testing.

 

 

 

 

 

ultimate-tensile-strength
 

Compressive Strength

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).

 

Compressive Properties

compressive-properties

compressive-load

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

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:

ductility formula 1

  • The reduction in area at fracture, q, where:

Ductility formula 2

 

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.

 

Hardness

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.

hardness visual                   hardness example

 

Hardenability

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.

 

Brittleness

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.

charpy-tester charpy-impact-test

 

Toughness

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

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.

Reach us here at Technoweld for training, inspection, consultancy, and supervision of the welding procedures. We can also research and document welding procedures for your specific welding processes as well as run the procedures.

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