Assignment Task

Aim

The aim of this part is to characterize various specimens by means of the tensile test.

Introduction

There are various tests to characterise the properties of a material and they are classified in two main categories: destructive and non-destructive tests. The latter are mainly utilized to check for defects whereas the former is used to obtain the material strength and stiffness. Amongst the destructive material tests , the uniaxial tensile test is the most important of them all (see Figure 1).

A tensile test is conducted by means of a specialised machine, called the tensile testing machine. In such machines, a controlled loading of the sample is possible. The deformation of the sample is its response to the applied load. During the test, the values of the force and deformation are continuously recorded. Engineering stress (σ) can be calculated using the recorded forces:

σ=F/Ao

where F is the instantaneous load applied perpendicular to the specimen cross section in units of newtons, A0 is the original cross-sectional area before any load is applied (m²). The units of engineering stress is pascal (Pa).

Engineering strain (ε) is the change in dimension per unit length. Engineering strain can be calculated by knowing the deformation at every instance.

ε=∆l/lo

where lo is the original length, i.e., the gauge length, before any loading, l is the instantaneous length, l − lo is the deformation elongation or change in length at some instant, as referenced to the original length. The percentage of strain is the engineering strain times 100.

During the test, the applied force and the displacement, which is the response of the sample to the applied load, is constantly measured. The tensile testing machine to be used is depicted in Figure 2.

A tensile sample, or sometimes called a coupon, has a plain geometry (see Figure 3). It consists of clamping areas at the two ends of the sample and a prismatic section in the middle. The prismatic section is the area of interest which incorporates the gauge length. This is the same area in which failure will happen. The two ends serve the sole purpose of fixating the sample in the apparatus with a better grip. Nevertheless, other geometries are also possible for the sample.

Scenario

In this project, your group should provide the answer for the following scenario through results of some tensile tests.

You are employed as the designer engineer in Byke Corp. which is a manufacturer of bicycle frames. There are a few options available for the frames and you need to compare them in terms of their mechanical properties. The material options are two types of steel, copper, two polymers, and a composite. Discuss the mechanical properties of the available materials and choose a suitable material based on the results of the tensile test.

From the tensile tests you must provide the following values/graphs.

1. Young’s modulus
2. Yield stress and strain
3. Tensile stress and strain
4. Failure stress and strain
5. Stress/strain graph
6. 02% offset line

Equipment

The following equipment are required to conduct the tensile tests:

• a digital calliper,
• an extensometer,
• a data acquisition unit,
• the WP300 Universal Material Tester (20 kN), and
• 6 tensile samples:
  • Tempered steel (1 samples),
  • Annealed steel (1 sample),
  • Copper (1 sample),
  • PMMA (Acrylic) (1 sample),
  • PVC (1 sample),
  • Plywood (1 sample).

How a tensile test works

The force is measured by the load cell, located at the lower part of the machine whereas the deformation of the gauge length is measured by an extensometer (see Figure 2a). Namely, the deformation is measured along a specific length in the middle of the tensile sample. The measured load is displayed by the force gauge. The load cell is integrated into the machine but the extensometer must be attached to the specimen exactly on the marked gauge length. Once the extensometer is installed, the setup is ready to be used (see Figure 4).