Thursday, February 17, 2011

Material Testing

Functions of material Testing

Material testing consists of ascertaining the strength of materials and of determining the effects of external influences upon them. Technological tests are carried out to determine the characteristics of materials when being worked (cut, reshaped).

The major function of mechanical tests is to evaluate the resistance of the material to deformation and breaking. In terms of the nature of the force exerted, one distinguishes between tensile, compressive, bending and shearing stresses. For each of these types of stress, a distinction is made between the strength value relating to gradually applied force and the value for sudden stress.

Testing In the Factory

These tests do not yield numerically quantified results, but only an idea of how the material behaves when processes.

Forge steel ability: After heating, a steel flat is forged out just to the point when cracks begin to appear on the edges. The flat should broaden to twice or thrice the original dimensions before cracks appear.

Cold reshaping
  1. A steel flat (3 mm thick) is bent in a bench vise until cracks appear. A steel which deform well can be folded through 180o.
  2. Alternate bending test. A rod/wire is clamped in a vise and bent back and forth several times through 180o. Steels of high strength are difficult to bend and can stand only few flexure. The number of flexure necessary to break the piece in an indication of the strength, malleability and deformability of the material.
Filling Test

The chip removal is slower at the same force for harder steels.

Deep drawing test

A clamped plate is indented by a rounded pin, with gradually increasing force. The identification is observed until the first crack appears.

Sparking test

The pattern of sparks formed against a grinding wheel indicates the type of steel being ground.

Ringing Test

The sample (generally a casting) is suspended freely and lightly struck. Cast steel (bright sound) and grey cast iron (dull sound), for instance, can be identified in this way, as well as fissured pieces or pieces containing shrink holes.

Evaluation by appearance

Checking the surface quality and detecting external fault e.g. pores, cracks, indentation.

Drift test and flanging tests for tubes

These reveal whether pipes split when their ends are reamed up. In the flagging test, the pipes are flanged to the point where cracks appear.

Measuring hardness by comparing Indentations

This method is based upon the plastic properties of materials. Two plates of differing e.g. steel and aluminum, are clamped between the jaws of a vise, with all between them. A comparative value for hardness is obtained by measuring the diameters of the indentations on each plate.

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