Engineering Materials

 A knowledge of the properties of a material is essential to every engineer. This enables suitable material choice for a particular application, appropriate design of the components or parts, and their protection, where necessary, from corrosion or damage. 

Material properties 

The behaviour of a metal under various conditions of loading is often described by the use of certain terms: Tensile strength. This is the main single criterion with reference to metals. It is a measure of the material's ability to withstand the loads upon it in service. 

Terms such as 'stress', 'strain', 'ultimate tensile strength*, 'yield stress' and 'proof stress' are all different methods of quantifying the tensile strength of the material. Ductility, This is the ability of a material to undergo permanent change in shape without rupture or loss of strength. 

Brittleness, A material that is liable to fracture rather than deform when absorbing energy (such as impact) is said to be brittle. Strong materials may also be brittle. Malleability. A material that can be shaped by beating or rolling is said to be malleable. A similar property to ductility. Plasticity. The ability to deform permanently when load is applied. Elasticity. 

The ability to return to the original shape or size after having been deformed or loaded. Toughness. A combination of strength and the ability to absorb energy or deform plastically. A condition between brittleness and softness. Hardness. A material's ability to resist plastic deformation usually by indentation. 

Testing of materials 

Various tests are performed on materials in order to quantify their properties and determine their suitability for various engineering applications. For measurement purposes a number of terms are used, with 'stress' and 'strain' being the most common. Stress, or more correctly "intensity of stress', is the force acting on a unit area of the material. 

Strain is the deforming of a material due to stress. When a force is applied to a material which tends to shorten or compress it, the stress is termed 'compressive stress'. When the force applied tends to lengthen the material it is termed 'tensile stress'. When the force tends to cause the various parts of the material to slide over one another the stress is termed 'shear stress'. 

Tensile test 

A tensile test measures a material's strength and ductility. A specially shaped specimen of standard size is gripped in the jaws of a testing machine, and a load gradually applied to draw the ends of the specimen apart such that it is subject to tensile stress. 

The original test length of the specimen, LI, is known and for each applied load the new length, L-2, can be measured. The specimen will be found to have extended by some small amount, Lg—LI. This deformation, expressed as 

(extension /original length) this known as the linear strain.

Additional loading of the specimen will produce results which show a uniform increase of extension until the yield point is reached. Up to the yield point or elastic limit, the removal of load would have resulted in the specimen returning to its original size.

continues beyond the yield point the specimen will 'neck' or reduce in cross section. The load values divided by the original cross section would give the shape shown. The highest stress value is known as the 'ultimate tensile stress' (UTS) of the material. Within the elastic limit, stress is proportional to strain, and therefore 

stress /strain = constant 

This constant is known as the 'modulus of elasticity* (E) of the material. The yield stress is the value of stress at the yield point. Where a clearly defined yield point is not obtained, a proof stress value is given. 

This is obtained by drawing a line parallel to the stress—strain line at a value of strain, usually 0.1%. The intersection of the two lines is considered the proof stress. 

A 'factor of safety' is often specified for materials where this is the ratio of ultimate tensile strength to working stress, and is always a value greater than unity.

factor of safety =  UTS / working stres

Impact test

This test measures the energy absorbed by a material when it is fractured. There are a number of impact tests available; the Charpy vee-notch test is usually specified. The test specimen is a square sectionbar with a vee-notch cut in the centre of one face. The specimen is mounted horizontally with the notch axis vertical. 

The test involves the specimen being struck opposite the notch and fractured. A striker or hammer on the end of a swinging pendulum provides the blow which breaks the specimen. The energy absorbed by the material in fracturing is measured by the machine.

HardnessTest

The hardness test measures a material's resistance to indentation. A hardened steel ball or a diamond point is pressed onto the material surface for a given time with a given load. The hardness number is a function of the load and the area of the indentation. The value may be given as a Brinell number or Vickers Pyramid number, depending upon the machine used. 

Creep test 

Creep is the slow plastic deformation of a material under a constant stress. The test uses a specimen similar to that for a tensile test. A constant load is applied and the temperature is maintained constant. Accurate measurements of the increase in length are taken often over very long periods. The test is repeated for various loads and the material tested at what will be its temperature in service. Creep rate and limiting stress values can thus be found. 

Fatigue test 

Fatigue failure results from a repeatedly applied fluctuating stress which may be a lower value than the tensile strength of the material. A specially shaped specimen is gripped at one end and rotated by a fast revolving electric motor. The free end has a load suspended from it and a ball race is fitted to prevent the load from turning. 

The specimen, as it turns, is therefore subjected to an alternating tensile and compressive stress. The stress reversals are counted and the machine is run until the specimen breaks. The load and the number of reversals are noted and the procedure repeated. The results will provide a limiting fatigue stress or fatigue limit for the material. 

Bend test 

The bend test determines the ductility of a material. A piece of material is bent through 180° around a former. No cracks should appear on the material surface. 

Non-destructive testing 

A number of tests are available that do not damage the material under test and can therefore be used on the finished item if required. These tests are mainly examinations of the material to ensure that it is defect free and they do not, as such, measure properties. 

Various penetrant liquids can be used to detect surface cracks. The penetrant liquid will be chosen for its ability to enter the smallest of cracks and remain there. A means of detecting the penetrant is then required which may be an ultra-violet light where a fluorescent penetrant is used. Alternatively a red dye penetrant may be used and after the surface is wiped clean, a white developer is applied.

Radiography, the use of X-rays or -y-rays to darken a photographic plate, can be used to detect internal flaws in materials. The shadow image produced will show any variations in material density, gas or solid inclusions, etc. Ultrasonic testing is the use of high-frequency sound waves which reflect from the far side of the material.

The reflected waves can be displayed on a cathode ray oscilloscope. Any defects will also result in reflected waves. The defect can be detected in size and location within the material. Iron and steel production