Strength Of Materials 1 EXPERIMENT NO.: 01 Title: Tension test AIM:To conduct a tension test on ductile material using extensometer. LEARNING OBJECTIVES: 1. To learn how to conduct tension test on a material. 2. To learn how to determine various properties of metal, observe deformation of material under axial loading and plotting of stress strain diagram. EQUIPMENTS REQUIRED: 1. Universal Testing Machine (U.T.M.) 2. Standard specimen. THEORY: Stress – strain diagram provides us the information about mechanical properties and behavior of material under the action of external load and to plot this stress strain diagram a material is to be tested in materials testing laboratories. The most common materials test is the tension test, in which a test specimen of circular, square or rectangular cross- section of a suitable size is prepared from the material to be tested. The specimen is held by suitable means between the two heads of a testing machine subjected to a progressively increasing tensile load until it fractures. A record of load on the specimen with progressive extension of the specimen is obtained. Tensile tests are done on fixed lengths called as gauge lengths. Before commencement of the test, two permanent marks are made on the specimen at appropriate distance called as the original gauge length. ENGINEERING (i.e. Nominal or Conventional) STRESS-STRAIN CURVES These curves are obtained from load extension curves on the assumption that dimensions of sample does not change during testing. In fact during testing of a sample length increases and cross-sectional area decreases; but still as a matter of convenience it is assumed that they remain unchanged. TRUE STRESS-STRAIN CURVES These curves are plotted by using the values of true stress and true strain from instant to instant during tensile testing. As the test proceeds, the length of the sample increases and correspondingly the area of the sample decreases. The value of the true stress gradually increases because of continuous decrease in cross-sectional area and true strain decreases due to continuous increase in length. PREC,LONI (MED) Pepared by Parkhe R. A.
2 Strength Of Materials EVALUATION OF PROPERTIES The test is used to evaluate the following properties of the materials. (i) Proportional Stress (P .S.) : It is the highest value of the stress up to which stress is proportional to strain. Above this stress, proportionality between stress and strain ceases and the stress-strain curve diverts from the linearity. This can be found out by extending the linear part of the curve and note down the stress value at the diversion point. (ii) Elastic Limit or Stress (E.L.) : It is the highest value of the stress up to which the deformations are elastic or temporary and beyond which they are plastic: or permanent. This stress is slightly higher than the proportional stress. This can not be accurately found out from the stress-strain curve. For accurate determination of this value, the specimen should be loaded slightly above the proportional stress and unloaded. This should be done several times with slightly increasing load at every time, till the sample does not show permanent deformation. (iii) Ultimate Tensile Stress (U.T.S.) : It is the highest value of the stress that the material can bear or sustain without fracture. (iv) Breaking Stress ( B.S.) or Fracture Stress (F .S.) : It is the stress value at the point of fracture or failure. (v) Yield Stress (Y.S.) : It is the stress at which the material Yields shows means appreciable plastic deformation at almost constant stress without any strain hardening. The lower yield stress almost remains unaffected whereas upper yield stress is likely to vary with the condition of steel. Due to this, the lower yield stress is used in the design of components. (vi) Proof Stress: Proof stress is reported for those materials which do not show yield point or well defined straight line portion on their stress-strain diagrams and is an equivalent term to the yield stress or elastic limit stress. (vii) Resilience: It is the total energy absorbed by the material during its elastic deformation. It is the area up to the elastic load in a load-extension diagram (viii) Modulus of resilience: It is the energy absorbed by the material in the elastic region per unit volume PREC,LONI (MED) Pepared by Parkhe R. A.
Strength Of Materials 3 (ix) Toughness: It is the total energy absorbed by the material prior to its fracture; this energy is the sum of elastic energy and plastic energy. It is the total area under the Load extension diagram (x) Modulus of toughness: It is the energy absorbed by the material prior to its fracture per unit volume. (xi) Ductility: It is the ability of a material to exhibit large plastic deformation prior to fracture under tensile loading conditions. It is also defined as the ability of a material to drawn into fine wire. Ductility is expressed by percent elongation. Specimen preparation: ASTM standard specimen: Diameter – 0.5 in (12.5 mm) Gauge length – 2 in (50 mm) Prepare the specimen of a material to be tested as a gauge length is equal to five times the gauge diameter. PROCEDURE: 1) Measure the diameter of specimen. 2) Mark the Gauge length as 5 times the diameter of the specimen. 3) Attach extensometer firmly to tension testing machine taking care that knife edges of extensometer are in contact with portion of specimen marked at gauge length. 4) Fix the whole assembly to the fixed cross head and upper cross head of UTM and tighten the grip to protect slipping of specimen. 5) Bring pointer of UTM to its zero reading and close the press release valve. 6) Start the UTM turning the load regulating valve gradually and apply the load. 7) Note the readings of the yield point. 8) Observe the phenomenon of neck forming just before failure and at failure note the corresponding readings. 9) Remove the broken pieces from the machine and note down the diameter at fracture. Also note down the length between gauge marks which will be more than original. 10) Do the calculation for stress and strains. 11) Plot the graph of Stress Vs Strains and Find Young's Modulus from it. OBSERVATIONS: 1) Original diameter (do) = 2) Gauge length ( Lo ) = 3) Original C/ S Area ( Ao ) = 4) Load at yield point = 5) Stress at yield point = load at yield Pt. /Ao PREC,LONI (MED) Pepared by Parkhe R. A.
Strength Of Materials 4 6) Maximum load = 7) Ultimate tensile strength = maximum load /Ao 8) Breaking load = 9) Final length (Lf) = 10) Diameter of fracture (df) = 11) Area of fracture (Af) = 12) % Elongation = (Lf- Lo) X 100 / Lo = 13) % Reduction in area = (Ao- Af) X 100 / Ao = 14) Nominal braking stress = Braking load/ Ao RESULT: 1) Stress at yield point = 2) Ultimate tensile strength = 3) Nominal breaking stress = . 4) Actual breaking stress = . 5) % Elongation = . 6) % Reduction in area = 7) Young's modulus =( From graph.) DIAGRAM: Figure 1.1 Tension testing machine PREC,LONI (MED) Pepared by Parkhe R. A.