Tensile Lab Report Example 2

Title

Tensile Testing of Metals

Objective

·         To determine the tensile strength of metal

Introduction

Tensile testing is an important test for getting information about materials and their properties. Some of the properties are ultimate tensile strength, maximum elongation, yield strength, Young’s modulus and Poisson’s Ratio. The results can be used when choosing suitable materials for production and construction.

The test is done by applying an increasing axial force to a standard tensile specimen until failure. The specimen has two large ‘shoulders’ for gripping and a thinner middle section for deformation and fracture. The dimensions differ in various standardised tests. During the test the tensile force and the extension are recorded. The information is used to calculate the stress and strain. First the sample would undergo elastic deformation until the yield point. After that the plastic deformation takes place until failure.

Figure 1: Standard Specimen (Weiler, 2013)

Theory

In the test the elongation is measured against the tensile force. This is used to calculate the strain.

                         

The force is used to calculate the engineering stress.

·         ∆L – Elongation

·         L₀ – Initial gauge length

·         F – Tensile force

·         A₀ – Initial cross section

The following image shows the stress-strain curve and the condition of the specimen at significant moments.

Figure 2: Stress-Strain curve of ductile material (Shah, 2011)

Materials and Apparatus

·         Manual Tensile testing machine

·         Medium carbon steel specimen

·         Vernier calliper

·         Computer

Procedure

·         First the initial gauge length and diameter of the specimen were measured using the Vernier calliper.

·         Secondly, the specimen was attached to the testing machine.

·         Next the machine was turned on and a tensile force was given to the specimen manually.

·         The force was gradually increased until the specimen went through necking and finally fractured.

·         After that, the results from the machine were imported to the computer.

·         Then the Young’s modulus, upper and lower yield stress, and the ultimate tensile strength were observed from the stress-strain diagram.

·         Finally the final gauge length and diameter of the fractured specimen was measured using the Vernier calliper.

Discussion

Macroscopic inspection of a fracture surface may reveal important information on tensile failure. Visible distortion indicates plastic deformation and possible necking. Visible cracks or gouges and radial marks may designate the fracture initiation site. Flat fracture and sheer lips on fracture surface shows that the crack propagation direction is parallel to the sheer lips. A tightly closed crack may signify a possible processing imperfection. Fracture surface discoloration might mean corrosion or temperature difference. The roughness of the surface can increase in the direction of crack growth. (Becker, 2009)

Stress-strain curves can be used analyse the behaviour of materials under a tensile force. Figure 2 is the stress-strain curve for most ductile materials. Steel has two additional points called upper and lower yield points. The upper yield point corresponds to the load required to initiate plastic deformation, while the lower yield point relates to the minimal load required to maintain it (Abraham, 2015). The curve for Aluminium resembles the one in Figure 2. The stress-strain curve for rubber is nonlinear, has large deformation and high elongation at break. This is typical of most elastomers (Bauman, 2010). Glass being extremely brittle does not go through plastic deformation. It fractures almost immediately after elastic deformation.

Tensile testing is imperative for ensuring a safe, high quality material for construction of structures and manufacturing products. This will dramatically reduce the chance of failure. Also because the tests are done according to a standard the results can be compared. The tests provide much information on the properties of material which can lead to the development of advanced material.

Conclusion

The tensile test is an important test in engineering which ensures safe, high quality and suitable material are selected for any project.

Reference

 

Abraham, P. (2015) Upper and Lower Yield Points, 14 October, [Online], Available: https://www.quora.com/What-is-the-difference-between-upper-yield-point-and-lower-yield-point-of-a-stress-strain-curve-for-mild-steel [6 August 2016].

Bauman, J. (2010) Rubber Stress Strain Behaviour, [Online], Available: http://files.hanser.de/hanser/docs/20081113_281113144257-93_978-3-446-41681-9_extract.pdf [6 August 2016].

Becker, T. (2009) Fracture Appearance and Mechanisms of Deformation of Fracture, [Online], Available: http://jpkc.fudan.edu.cn/picture/article/348/1b/ee/6dce0ae740cf8673b53e4e96abb8/6ad0c8ee-53c3-4790-a931-211df202df69.pdf [6 August 2016].

Shah, P. (2011) Tensile Properties, [Online], Available: http://practicalmaintenance.net/?p=948 [6 August 2016].

Weiler, K. (2013) Tensile Testing, [Online], Available: http://www.southerninspection.com/PageDisplay.asp?p1=13507 [6 August 2016].