Tensile testing lab report (Lab Report Sample)

Tensile testing Name Institution Introduction Tension test plays an important role in assessing fundamental properties of engineering materials as well as in manufacturing of new materials and in maintaining the quality of materials for use in design and construction. Metallic engineering materials can be classified as either brittle or ductile materials. Ductile materials have relatively higher tensile strains up to the point of rapture like aluminium and structural steel. Brittle materials on the other hand have relatively lower tensile strain up to point of rapture like concrete and cast iron. Strain Є = Where = extension and l0 = original length. Stress σ = force per unit area (HYPER13 EMBED Equation.3 
) A typical stress-strain curve is shown below.
Hooke’s law From point A to point B the stress-strain curve is a straight line. This relation between extension and force causing it was first discovered by Robert Hooke and named after him. The law states that within the elastic region (between A &B) stress is directly proportional to strain. σ α Є or σ = kЄ The constant of proportionality k, is the Young's Modulus and is equal to the gradient of the stress- strain curve within the elastic region. On the curve beyond point B (yield point), the material will no longer retain its original shape due to permanent deformation. Yield stress is the level of stress at which a material is permanently deformed. The region between point B&C is known as the plastic region. Ultimate Strength is the maximum value of stress in the stress-strain curve. The magnitude and shape of the stress-strain diagram of a metal depends on heat treatment, its composition, the strain rate, plastic deformation history and state of stress imposed during the testing. Method In this experiment, a material is prepared to be gripped into the jaws of the testing machine. The material used is uniform over a gage length. The gage portion (Central part) of the length is smaller than the end portions to ensure the failure occurs at a portion where the stresses are not affected by the gripping device.
The specimen is then marked with gage marks and the initial gage length and diameter measured and recorded. A suitable load scale that can deform and fracture the specimen is selected. (Tensile strength of the material used has to be known). During the test Record the maximum load and conduct the test while increasing the load until fracture. After the test measure the final diameter and gage length of the specimen. Diameter to be measured from the neck Results Lab data from each test was recorded in the form of Force (N) vs. extension (mm). Polycarbonate This is an example of a polymer and its results to tension test are unique. The polycarbonate specimen obeys Hooke’s law in the elastic region as the gradient of the slope remains constant. It began to yield as the graph reaches non-linearity. It began to neck during the time period represented by the download slope after yield. As the applied force is increased further the neck propagates outward from the center and the polycarbonate begins to strain hard. This phenomenon is due to the carbon chains re-orienting themselves as the strain increases. In the long run, the stress becomes unbearable and the carbon chains can no longer support the specimen resulting to fracture. Ultimate Stress = 150.715 MPa Yield stress = 138.456 MPa Young’s modulus =  EMBED Equation.3 
 = 114.2 MPa Aluminium The aluminium specimen also obeyed Hooke’s law in the elastic region. However at around 1384 N the relation is no longer linear as the bonds are beginning to separate at molecular level. It begins t...