Mechanical Testing and PropertiesThe selection of materials for many applications is performed by matching the service requirements to the mechanical properties of the material. The steps;1. Analyse the application to determine the most important characteristics that the material must possess.2. Select material which suits the requirements from the data list

a material that can develop the desired physical and mechanical properties

a material that can be processed or manufactured into the desired shape

a material and process that are economical

Brain Storming Exercise

What properties are required for an exhaust pipe to be able for it to be used in service?

Tensile Test

The tensile test measure the resistance of a material to a static or slowly applied force. Information that can be obtained from the tensile test includes; Yield strength Tensile strength Modulus of elasticity Ductility

Yield Strength

It is the stress at which slip becomes noticeable and significant, which divides the elastic and plastic behaviour of a material. For designing of component, selection of material must ensure that it has high yield strength whilst in deforming process the applied stress must exceed the yield strength to produce a permanent change in the shape of the material.

The Stress-Strain Curve

Fig 1: The stress-strain curve for an aluminium alloy

Tensile Strength

It is the stress obtained at the highest applied force and thus is the maximum stress on the engineering stress-strain curve

Proof Stress

For some materials, the yield stress is difficult to determine. It can be determined through offsetting yield strength of proof stress which taken into 0.2% allowable permanent deformation.

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Elastic DeformationThe degree to which a structure deforms or strains depends on the magnitude of an imposed stress. For most metals that are stressed, stress and strain are proportional.

= Hooke’s LawThe constant proportionality E (GPa) is the modulus of elasticity / Young’s ModulusDeformation in which stress and strain are proportional is called elastic deformation

Elastic Deformation (con’t)

Elastic Deformation (con’t)

Elastic Deformation (con’t)

Elastic Deformation (con’t)

Strain Hardening

Is a phenomenon whereby a ductile metal becomes harder and stronger as it is platically deformed.The strengthening obtained during cold working, which is brought about by increasing the number of dislocations is called strain hardening or work hardening.

Cold Working relationship to the stress-strain curve

As long as the stress does not exceed the yield strength y, no permanent plastic deformation occurs and the elastic deformation is recovered. This condition is required when the component is put in service.

Cold Working relationship to the stress-strain curve (con’t)

For manufacturing of component by deformation processing, the applied stress must exceed the yield strength, causing the metal to deformed into a useful shape.

Cold Working relationship to the stress-strain curve (con’t)

If a stress 1 is applied exceeding the original yield strength y, of the metal, and then removed, a permanent deformation or strain 1, occurs. If retest, a different stress-strain curve is obtained, and would have a higher yield strength but lower ductility. If the process is repeated, tensile and yield strength increase while the ductility decreases. This can be done until the yield, tensile and the breaking strengths are equal and there is no ductility, where the metal cannot be deformed further.

Cold Working relationship to the stress-strain curve (con’t)

By applying a stress that exceeds the original yield strength of the metal, strain hardened or cold work the metal is achieved and simultaneously deforming the metal into a more useable shape.

Cold Working relationship to strain hardening

The strain hardening develops through the increase of dislocation density in metal by deformation or work hardening. This is due to dislocation multiplication or formation of new dislocations. Consequently, the average distance of separation between dislocations decreases i.e the dislocation are positioned closer together. The motion of a dislocation is hindered by the presence of other dislocation. As the dislocation density increases, this resistance to dislocation motion by other dislocations becomes more pronounced, thus the imposed stress necessary to deform a metal increases with increasing cold work.

Deformation Processing Techniques

Drop Forging

Upsetting

Direct Extrusion

Metal Flow During Extrusion

Tempaan upset

Wire Drawing

Drawing Die

Shearing Process

Penghasilan cawan tirus yg dalam memerlukan beberapa peringkat proses penarikan semula

Properties versus percent cold work

By controlling the amount of deformation the amount of strain hardening can be controlled. The measurement of the amount of deformation can be made by defining the % cold work

% cold work = A0 – Af X 100A0

Properties versus percent cold working (con’t)

The effect of cold work on the mechanical properties of commercially pure copper

As the % cold work increases both the yield and the tensile strength increase. However, the ductility decreases and approaches zero. The metal will break if more cold work is attempted

Structure of Cold Work Material

Fig: The fibrous grain structure of copper after cold working (a) 0% cold work (b) 31% cold work (c) 67% cold work (d) 82% cold work

(a)

(b)

(c) (d)

Structure of Cold Work Material (con’t)

Use of Cold-Working Relationship

To determine the number of resizing of a material to the final dimensions To predict the properties of a metal or alloy provided that amount of cold-work achieved is known

Study Example

1. Determine if a 10mm x 60mm x 200mm copper plate can be cold rolled in a single pass to a plate of 1mm x 60mm x 2000mm.

2. A copper rod 12 mm in diameter is to be reduced by bar drawing process to a 7mm diameter. Determine the expected mechanical properties.

3. If a copper bar that has at least 415MPa tensile strength, 380MPa yield strength, and 5% elongation. Determine the range of cold work required.

Characteristics of Cold Working

Simultaneously strengthen the metal while producing a desired final shapeobtain excellent dimensional tolerances and surface finishes by the cold-working processcold-working process is an inexpensive method for producing large numbers of small parts. For larger part, too much deformation the metal may fail during processing and requires large and expensive forming equipment.

Characteristics of Cold Working (con’t)

Some metal such as HCP magnesium, are rather brittle at room temperature. Only a small degree of cold working can be accomplished without causing the part to embrittle and failDuctility, electrical conductivity, and corrosion resistance are impaired by the cold working process. However, comparatively cold working process reduces electrical conductivity less than many other strengthening processes, such as solid solution strengthening. This makes cold working a more satisfactory way to strengthen conductor materials such as copper wires used for transmission of electrical power.

Characteristics of Cold Working (cont)

Characteristics of Cold Working (cont)

Residual stresses and anistropic behaviour may be introduced during cold working. These characteristics may be either harmful or beneficial, depending on how they can be controlled.Some deformation processing technique can only be accomplished if cold working occurs eg. wire drawing

Characteristics of Cold Working (con’t)

Hot Working

Hot working is defined as plastically deforming the metal at a temperature above recrystallisation temperature. During hot working, the metal is continually recrystallised.

Characteristics of the Hot Working Processess

Lack of strengthening – No strengthening occurs during deformation by hot working. The amount of plastic deformation is almost unlimited. It is suited for forming of large partsElimination of imperfections – some imperfections in the original metal may be eliminated or their effects minimisedSurface finish and dimensional accuracy