Tensile testing - A range lab report containing detailed analysis for reference. The format and PDF

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A range lab report containing detailed analysis for reference. The format and layout are great as well....

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THE UNIVERSITY OF HONG KONG Department of Civil Engineering Engineering Mechanics and Materials CIVL1113 Tensile Test of Metals

Objectives: 1. To study the tensile stress-strain behaviour of ductile (represented by mild or high-yield steel) and non-ductile (represented by cast iron) steel bars. 2. To determine: (a) Modulus of elasticity. (b) Stress at elastic limit or proportionality limit. (c) Yield stress for steel and 0.2% proof stress for cast iron. (d) Peak/ultimate point and breaking point. (e) Percentage of elongation. Apparatus: Instron model 4469 Material Testing Machine, data logger, digital callipers, extensometer. Cross head

Hand wheel

Control panel

Specimen Emergency stop button

Figure - Instron Model 4469 Material Testing Instrument

Background theory: Experimental tensile tests on prismatic bars of various metals have shown that within the range of elastic behaviour of the material, the elongation δ is proportional to both the tensile force P and the length L of the bar. It is also observed that δ is inversely proportional to the cross-sectional area A of the bar. 𝛿∝

𝑃𝐿 𝐴

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𝑃𝐿

Hooke’s Law: 𝛿 = 𝐴𝐸 where E is a constant and defined as modulus of elasticity. 𝑃 Stress: 𝜎 = Strain: 𝜀 = 𝛿

𝐴 𝛿

𝐿 𝑃𝐿

∴ 𝜀 = 𝐿 = 𝐴𝐸𝐿 = ∴𝐸=

𝜎 𝜀

𝑃 𝐴𝐸

=

𝜎 𝐸

 which is the slope of the stress-strain curve up to elastic limit.

References: 1. Nash W.A., “Strength of materials”, Chapter 1, 4th edition, 1998. 2. Subramanian R., “Strength of materials”, Chapter 3, 2nd edition, 2010. 3. Shukla K.K., Jain A. and Pandey R., “Strength of materials”, Chapter 1, 2014.

Procedure of test: 1. Measure the overall length and the diameter of each type of specimen. 2. Record the gauge length of the extensometer. 3. Mark the original gauge length (Lo) at an interval equal to 5 times the diameter along the length of the specimen. 4. Switch on the testing machine, install the specimen onto the testing machine and attach the extensometer at around the middle length of the specimen. At this stage the reading of load and extensometer may not be zero. If so, set the readings (both load and extension) to be zero. 5. For the mild (or high-yield) steel bar set the displacement speed to 4mm/min (or 2.5mm/min) and the cast iron bar to 0.2mm/min throughout the test. 6. Start the test. The readings of Load and its corresponding Extension will be recorded automatically by the data logger at a suitable interval. 7. During the test until rupture of the specimen, removal of the extensometer is not necessary. 8. Record the final gauge length (Lu) of the specimen by carefully fitting together the ruptured specimen, so that its axis is aligned.

Test results: 1. For each type of specimen, plot a graph of Stress against Strain and clearly indicate the elastic limit, yield point or 0.2% proof stress, peak (ultimate) point, and breaking point. Plot Stress against Strain for both types of specimens on the same graph paper for comparison. 2. Determine the properties of the specimens as mentioned in articles 2(a) to 2(e) of Objectives.

Discussion: 1. State and comment upon the values obtained from the tests. 2. Compare and comment upon the test results of steel and cast iron specimens in respect of tensile strength, modulus of elasticity, ductility, etc.

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APPENDIX: Some of the definitions relating to the tensile test are briefly explained below: Elastic limit: The point beyond which the stress is no longer proportional to strain. Modulus of elasticity: The ratio of the unit stress to the unit strain. It is also defined as Young’s Modulus. It has the same unit as the stress. Yield point: The point at which there is an appreciable elongation of the material without any increase of load. Sometimes, the load may actually decrease while the yielding occurs. That is why some materials exhibit two yield points, e.g. upper and lower yield points. Tensile strength: The stress corresponding to the maximum ordinate of the curve (at peak point). It is also called ultimate strength. Ductile material: Material which has a relatively large tensile strain up to the point of rupture. Brittle material: Material which has a relatively small tensile strain up to the point of rupture. An arbitrary strain of 0.05 is frequently taken as the dividing line between ductile and brittle materials. Percentage of elongation: The increase in length from the initial length upon fracture divided by the initial length and multiplied by 100 percent. Note: Stress at elastic limit, yield point, 0.2% proof stress, peak point and breaking point are all calculated on the basis of the original cross-sectional area of the specimen.

PHJ 4/9/2014

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