Mechanical Testing

7 Mechanical Properties

Intro

• Materials are tested to determine their basic properties

• Determine numerical values for the properties. • To evaluate the material’s ability to carry or

resist mechanical forces such as tension, compression, shear, torsion and impact.

• Testing usually results in permanent damage to the specimen.

7 Mechanical Properties

• Mechanical properties: • Strength• Hardness• Elasticity• Plasticity• Ductility• Toughness• Brittleness

7 Mechanical Properties : Strength

• The ability of material to withstand compressive(a), tensile(b) and shear loads(c) without breaking.

100 kN

Fixed beam

Rod is being stretched by the load

7 Mechanical Properties : Hardness• The ability of a materials to

withstand scratching or indentation by another hard body.

• Indicates the wear resistance of a material.

• Press a hardened steel ball into a hard material and then into a soft material by the same load.

• Small indentation in the hard material.

• Deeper impression in the softer material.

Constant Load

Hard steel ball

Hard material Soft material

7 Mechanical Properties : Elasticity• The ability of a material to deform

under load and return to its original size and shape when the load is removed.

• An elastic will be the same length before and after the load is applied.

• All material has its own elastic limit. • Stress beyond this limit, permanent

deformation (plastic deformation), and ultimately fracture, occurs.

• Materials are only stressed within the elastic range under normal service conditions.

Elastic extension

Load

Load

Before and after

loading

7 Mechanical Properties : Plasticcity

• Opposite to elasticity. • Material been loaded beyond its elastic limit causing

the material to reform permanently. • Materials takes a permanent set and were not return to

its original sizes and shape when the load is removed.

Punch

Die

Strip before bending force is applied

Bending force

End Product

7 Mechanical Properties : Ductility

• Plastic deformation occurs as the result of applying a tensile load.

• A ductile material allows an amount of plastic deformation to occur under tensile loading before fracture occurs.

• Processes as wire drawing , tube drawing and cold pressing low carbon steel sheets into motor car body panels.

Rod being drawn

Die

Direction of drawn

7 Mechanical Properties : Toughness

• The ability of material to withstand shatter (e.g. Glass).

• If the rod is made from a piece of high carbon steel – for example, silver steel in the annealed (soft) condition – it will have only a moderate tensile strength.

• But under the impact of the hammer it will bent without breaking, therefore it is tough.

7 Mechanical Properties : Brittleness

• Opposite of ductility and malleability. • It is the property of a material that shows little

or no plastic deformation before fracture when a force is applied.

• For example, a steel rod can be bent but a grey cast iron rod snaps when you try to bend it.

• Therefore grey cast iron is a brittle material.

Destructive Testing

Destructive Testing

• Part or product tested no longer maintains its original shape or surface texture.

• Mechanical test methods are all destructive. • Other destructive tests include– speed testing of grinding wheels to determine

their bursting speed – high pressure testing of pressure vessels to

determine their bursting pressure.

Destructive Testing—Hardness Tests

• Hardness tests with large indentations may be regarded as destructive testing.

• Micro hardness tests may be regarded as non-destructive because of the very small permanent indentations.

Destructive Testing : Brinell

• Hardness is measured by pressing a hard steel ball into the surface to the test piece, using a known load.

• To ensure consistent result : – Thickness of the specimen should be at least seven times

the depth of the indention– The edge of the indentation should be at least three times

the diameter of the indentation from the edge of the test piece.

– The test is unsuitable for materials whose hardness exceeds 500 HB, as the ball indenter tend to flattened.

Destructive Testing : Vickers

• Preferable to the Brinell test for hard materials.

• It use a diamond intender. (Diamond is the hardest materials known – approximately 6000HB).

• Figure shows a universal hardness testing machine suitable for performing both Brinell and Vickers hardness test.

Destructive Testing : Shore

• The shore hardness is determined by using selroscope. • Works based on deference principles, and the

hardness is measured as a function of resilience. • Scelroscope can be carried to the work piece; useful

for the testing large surfaces such as the slideways in machine tools.

• A diamond-tipped hammer of mass 2.5g drops through a height of 250mm.

• The height of the first rebound indicates the hardness in a 140-divition scale.

Destructive Testing : Shore

Destructive Testing : Shore

Destructive Testing : Rockwell• Not as reliable as the Brinnel and Vikers hardness tests.• Rockwell test is widely used in industry as it quick, simple and

direct reading. • Principles—Rockwell compares the difference in depth of

penetration of the intender when using forces of two deferent values.

• A minor is first applied (to take up the backlash and pierce the skin of the component) and the scales are set to read zero.

• Then a major force is applied over and above the minor force and the increase depth of penetration is shown on the scales as a direct reading of hardness.

• No the need of calculation or conversion table.

Destructive Testing : Rockwell

Destructive Testing : Izod Test

• A 10mm square, notch specimen is used. • The striker of the pendulum hits the specimen

with a kinetic energy of 162.72J at a velocity of 3.8m/s.

• Figure shows the standard impact test machine for both Izod and Charpy testing occasions.

Destructive Testing : Izod Test

2

22 ½0 22 ½0

Root radius 0.25

Vice

100

50

Striker

70 28 22

Test Piece

Destructive Testing : Charpy Test

• In the Izod test the specimen is supported as a cantilever.

• But in the Charpy test it is supported as a beam.

• It is struck with a kinetic energy of 298.3J at a velocity of 5 m/s.

• Figure shows details of Charpy test specimen and the manner in which it is supported.

Destructive Testing : Charpy Test

Non-destructive Testing

NDT : Intro

• Non-destructive testing (NDT) defines a discontinuity as an interruption in the normal physical structure or configuration of a part, such as a crack or porosity.

• A discontinuity may or may not be detrimental to the usefulness of a part.

• A defect is a discontinuity whose size, shape, location or properties adversely affect the usefulness of the part or exceed the design criteria for the part.

• The purpose of NDT can serve to analyze an existing failure or be used to prevent future failures.

NDT : Penetration Test

• Penetration techniques are used to make surface cracks visibl.

• Immerse the casting in a bath of hot paraffin. • Heating the paraffin reduces its viscosity so that, combined

with its already high surface tension, it is easily drawn into the finest cracks and porosity by capillary attraction.

• The casting is removed and wiped thoroughly clean, after which it is painted with whitewash.

• Paraffin seeping out from the cracks, will discolour the whitewash and reveal the presence of surface cracks and porosity.

NDT : Penetration Test

NDT : Magnetic Test• Can only be applied to ferromagnetic materials. • Only appropriate for surface cracks and discontinuities not more

than 10 mm below the surface of the component.• Based upon the fact that the magnetic resistance in the region of

a discontinuity is greater than the surrounding metal.• It distorts the magnetic flux distribution. • The resulting distortion of the flux field is usually detected by

means of magnetic powder in suspension in light machine oil or paraffin.

• The suspension is spread thinly over the surface of the surface of the component and the magnetic powder ‘bunches’ in the vicinity of the fault.

NDT : Magnetic Test

NDT : Ultrasonic

• Pulse of high-frequency oscillations are driven into the component by the transducer.

• When the sound waves meet any discontinuity, such as crack, the waves are reflected back into the transducer where they are converted into electrical pulse which can be display on the screen of the computer.

NDT : Ultrasonic

NDT : X-ray• This is a photographic process in which the ‘illumination’ of the

component is by X-rays of the even more penetrating gamma-rays. • These are electromagnetic radiations exactly the same as radio

waves and light waves, except they have very much shorter wavelength.

• This enables X-ray and gamma-rays to penetrate solid objects. • When photographic film is exposed to X-ray or gamma-ray and then

develop, the film become dark. • If a solid object is placed between the source of radiation and the

film so that it casts a shadow on the film, the level of radiation reaching the film in the shadow area will be reduce and the shadow will appear on the film.

• After development, as a less dark area.

NDT : X-ray