To Determine The Behaviour Of A Specimen While Being Tested

OBJECT – To determine the behaviour of a specimen while being tested and to determine:1. 2. 3. 4. 5.

Upper & lower yield points. Ultimate strength. Breaking point. Percentage elongation of length. Percentage reduction of cross section.

Specification of the specimen – mild steel specimen.

10cm

Apparatus used – Universal Testing Machine ZD-20 (capacity 20 tonne), Micrometer, dividers steel rule, centres punch etc. Universal Testing Machine:Universal Testing Machine otherwise known as a materials testing machine/ test frame is used to test the tensile and compressive properties of materials. Such machines generally have two columns but single column types are also available. Load cells and extensometers measure the key parameters of force and deformation as the sample is tested. These machines are widely used and would be found in any materials testing laboratory. A tension test is a destructive test in the sense that the specimen is finally broken or fractured into two pieces. To perform the tensile test, the universal testing machine should be capable of applying that load which is required to break or fracture the material. The test piece or specimen of the material is generally a straight piece, uniform in the crosssection over the test length and often with enlarged ends which can be held in the machine holders. However, the machine can hold the specimen without enlarged ends also. Two fine marks are often made near the end of uniform test section of the

specimen and the distance between these points is termed "gauge length". The gauge length is that length which is under study or observation when the experiment on the specimen is performed. The gauge length of a specimen bears a constant standardized ratio to the cross-sectional dimension for certain reasons. The specimen is placed in the machine between the holders and any measuring device to record the change in length is fitted on to the specimen between the gauge points. If such a device, generally extensometer, is not fitted, the machine itself can record the displacement between its cross heads on which the specimen is held. Once the machine is started it begins to apply a slowly increasing load on specimen. At preset interval, the reading of the load and elongation of specimen are recorded. Finally, the specimen breaks in the form of cup and cone shape at the fracture point (for ductile metals).Before breaking, the area of cross section becomes very small, so a large stress is being produced. The maximum stress which the specimen can bear is the "ultimate stress”. We can also find the modulus of elasticity for the specimen. Ultimate strength The maximum stress a material can withstand when subjected to tension, compression or shearing. It is the maximum stress on the stress-strain curve. Yield strength The stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently. Proportionality limit Up to this amount of stress, stress is proportional to strain (Hooke's law), so the stress-strain graph is a straight line, and the gradient will be equal to the elastic modulus of the material. Breaking strength

The stress coordinates on the stress-strain curve at the point of rupture. Procedure 1. Measure the diameter of the specimen with the micrometer. Make the 10cm gauge length with centre punch in central zone of specimen. 2. Mount the appropriate jaws in the machine. 3. Place the fixed yoke in an appropriate position by moving the frame guide spindles. Start the pump to keep the movable yoke in a floating condition. 4. Bring the dial pointer to zero operating the zero adjuster 5. Bring the movable yoke in a suitable position and stop the pump. 6. Fix the specimen between the jaws taking care that the grip is perfect and full, and put the scale to zero position. 7. Start the plump. 8. Take reading up to destruction. 9. After failure, measure final diameter, and final gage length, and observe the character of the fracture. Precautions 1. After fixing the accessories cheek the zero position of the indicator. 2. During the test the rate of loading must be kept constant. Mechanism:The tensile test fixture suits to perform a tensile test for a specimen. The tensile test fixture includes a base, a pull bar and a forcing member. The pull bar includes a limiting member, a specimen-fixing member and a shaft member. Wherein, the shaft member is connected between the position limiting member and the specimen-fixing member, and the specimen is fixed between the base and the specimen-fixing member. Otherwise, the forcing member has a cavity, which includes an opening. The shaft member passes through the opening, and the position limiting member is located in the cavity. The

dimension of the limiting member is larger than the dimension of the opening so that the limiting member is restricted within the cavity. The forcing member is adopted to pull the limiting member to perform a tensile test for the specimen. Why commercial test is carried out? One of the most important concerns when selecting a material for a machine component is to ensure that the material properties are appropriate for the operating conditions of the component. Materials have both physical and mechanical properties and the most common way of distinguishing one material from another is by evaluating their physical properties. Mechanical properties are very important in design because the function and performance of a product depend on its capacity to resist deformation under the stresses encountered in service. Why percentage reduction of cross-section is measured? The percentage reduction of cross-section is the measure of the metrical is how much ductile. Fundamentals of Fracture Fracture is a form of failure where the material separates in pieces due to stress, at temperatures below the melting point. The fracture is termed ductile or brittle depending on whether the elongation is large or small. Steps in fracture (response to stress): • track formation • track propagation Ductile vs. brittle fracture Ductile

Brittle

deformation

extensive

little

track propagation

slow, needs stress

fast

type of

most metals (not

ceramics, ice, cold

materials

too cold)

metals

warning

permanent elongation

none

strain energy

higher

lower

fractured surface

rough

smoother

necking

yes

no

Stages of ductile fracture • Initial necking • small cavity formation (micro voids) • void growth (ellipsoid) by coalescence into a crack •

fast crack propagation around neck. Shear strain at 45o

• final shear fracture (cup and cone) Brittle Fracture There is no appreciable deformation, and crack propagation is very fast. In most brittle materials, crack propagation (by bond breaking) is along specific crystallographic planes (cleavage planes).

(A stress–strain curve typical of structural steel) Points 1. Ultimate Strength 2. Yield Strength 3. Rupture 4. Strain hardening region 5. Necking region A: Apparent stress (F/A0)

B: Actual stress (F/A) CONCLUSIONS A good knowledge of the materials properties is crucial in engineering as It is taking regard to these properties that the engineers would select the most suitable metal to build a bridge or an machine and if the metal is too stiff, too ductile or not hard enough the project of the engineers won’t fulfil their goals and thus it risks to put human lives in danger. It is the job of the engineers to make sure that such situation never happen and it is only by a better understanding of the materials properties and reactions to environmental variations that the engineers will complete their project according to the safety and financial limits.