Objective

This lab investigates the tensile properties of a material sample, such as Young's modulus, yield strength, ultimate tensile strength, and elongation at break. These properties will be determined through a uniaxial tensile test using a universal testing machine. Tensile test is one of mechanical test used in materials science and engineering, in which controlled pulling force is applied on the opposite ends of the material until the breaking point of the material has been reached to determine the material’s response to applied tensile force. In material analysis, the primary focus lies on strength, measured by either the stress causing significant plastic deformation or the maximum stress the material can endure. These strength indicators are cautiously applied in engineering design with safety factors.

1. Theory Overview

• Young's Modulus (Modulus of Elasticity, E):

Young modulus is defined as the slope of the stress-strain curve (Figure 1) in the linear region that is calculated with the Equation 1. Value of the Young's modulus is a constant for a given material.

𝜎𝑎𝑥𝑖𝑎𝑙 is engineering stress along the loading axis (Equation 2), 𝜀𝑎𝑥𝑖𝑎𝑙 is engineering strain where 𝐹 is the tensile force (Equation 3) and 𝐴0 is the initial cross-sectional area of the gage section. ∆𝐿 is the change in gage length demonstrated in the Equation 4.

• Toughness (W):

Toughness is required energy to sample breakage, it can be found from the area of the under the stress–strain curve (Figure 2). In order to be tough, a material must be both strong and ductile. For example, materials (like ceramics) that are strong but with limited ductility are not tough; conversely, very ductile materials with low strengths are also not tough.

UT = Area underneath the stress–strain (σ–ε) curve = σ × ε

• Fracture Stress:

Fracture stress is the amount of stress required to cause a material to fail by fracturing or breaking apart. In simpler terms, fracture stress is the maximum stress a material can withstand before it breaks. Its formula is demonstrated in the Figue 3. The surface formed when a material fails or fractures is referred to as a "fracture surface" in materials science. The observation of fracture helps to understand the mechanical behavior and failure mechanisms of materials (Figure 4). Brittle fractures are characterized as having little or no plastic deformation prior to failure.

• True Stress and Strain:

The true stress of a material is calculated (Equation 5) by dividing its applied load by its actual cross-sectional area, which is its changing area over time (Figure 5). The applied force divided by the material's initial cross-sectional area yields engineering stress. Likewise referred to as nominal stress.

• Poisson Ratio:

Poisson's ratio is the ratio of expansion along one axis to contraction along the opposite axis when a material is subjected to tensile or compressive forces (Equation 6).

• Suggested Reading:

1. Ashby, M. F. and D. R. H. Jones, D. R. H. (1996) Engineering Materials 1: An Introduction to  Their Properties and Applications. 2nd ed. Butterworth Heinemann, Oxford, UK.

2. Courtney, T. H. (1990) Mechanical Behavior of Materials, McGraw-Hill, New York. 

3. Dieter, G. E. (1986) Mechanical Metallurgy. 3rd ed. McGraw-Hill, New York.

4. Dowling, N. (1993) Mechanical Behavior of Materials, Prentice Hall, Englewood Cliffs, NJ. 

5. Hearn, E. J. (1985) Mechanics of Materials. vols. 1 and 2, 2nd ed. Pergamon Press, New York. 

6. Hertzberg, R. W. (1996) Deformation and Fracture Mechanics of Engineering Materials. 4th ed. Wiley, New York.

7. McClintock, F. A. and A. S. Argon, A. S. (eds) (1965) Mechanical Behavior of Materials. Tech Books, Fairfax, VA

2. Pre-Lab Quiz:

• Quiz Date:11-18-2024

• The quiz will cover the theory of mechanical properties of materials.

3. Experimental Procedure:

• Hardware:

• The ElectroPuls® E3000 LINEAR ALL-ELECTRIC DYNAMIC TEST INSTRUMENT (Figure 6) is designed for dynamic and static testing on a wide range of materials and components. It includes Instron advanced digital control electronics, bi-axial Dynacell load cell, Console software, and the very latest in testing technology – hassle-free tuning based on specimen stiffness, electrically operated crosshead lifts, a T-slot table for flexible test set ups and a host of other user-orientated features. Powered from a single-phase supply it requires no additional utilities for basic machine operation (for example, pneumatic air, hydraulics, or water).

• Linkam Modular Force Stage (MFS) (Figure 7) is a modular system designed to characterise the mechanical properties of samples with temperature and environmental control modules. It can be used to test tension, compression and multi-point bending of various materials, such as polymers, metals, ceramics, biomaterials and nanomaterials. The Linkam MFS allows researchers and engineers to explore and enhance how materials behave and last for different applications across a wide range of temperatures and sample types.

• Experiments Details:

1. Instron Experiment:

• Materials and Equipment:

 Instron ElectroPuls® E3000 dynamic mechanical test machine

 Ruler

 Different polymer composite samples,exact composition is not known (Figure 8)

• Experimental Procedure:

I. By use of ruler or caliper, the thickness and width of each polymer samples are measured.

II. Samples are put in between jaws of the Instron ElectroPuls® E3000 dynamic mechanical test machine.

III. After samples are placed in between jaws, appropriate test method is activated and test is started according to the related standard.

IV. The data is later retrieved for calculation and plotting.

2. Linkam Experiment

• Materials and Equipment

 Linkam MFS

 Thin polymer films

• Experimental Procedure

I. The polymer film is clamped in the microtensile test stage. Conditions like as humidity and an inernt atmosphere are realized if the experiment is conducted in a controlled environment.

II. Film is stretched at constant speed.

III. The data is later retrieved for calculation and plotting.

4. Data Collection

• During the lab, collect the following:

  • Stress-strain curve

  • Fracture surface image

5. Post-Lab Report:

• Due Date: 11-18-2024

•  Abstract and Introduction (What is a tensile test, and why is that important for mechanical engineering? Define terms that we learned in the lesson.)

•  Procedures and Steps (What are the experiment’s steps and what is the output of the experiment?)

•  Experiment Related Plots & Calculations (Draw a stress-strain curve of below data, calculate Young’s modulus.)

Additional Resources:

https://www.youtube.com/watch?v=D8U4G5kcpcMhttps://www.youtube.com/watch?v=uXKU0wySTa4&list=PLFm0Px6LPHr2NVEWuRvtcQGizAsdy6Tl3

Responsible TAs:

Maide Miray Albay, malbay23@ku.edu.tr