Introduction

The mechanical properties of materials used in engineering are determined by experiments conducted on small samples. These experiments are carried out in laboratories equipped with testing machines that are capable of carrying out loading in tension or compression. The American Society for Testing and Materials (ASTM) has published guidelines for conducting tests. Check the basic DBefore delving into the intricacies of the stress-strain curve, it’s essential to grasp the fundamental definitions of stress and strain By clicking here.

Understanding the Tension Test

The tension is conducted on uniaxial testing machine (UTM). Inside the controlled environment of the laboratory, these small but strong specimens, particularly mild steel, undergo stress tests that highlight their mechanical behaviour. The resulting stress-strain curve tells a compelling story, beginning with linear elasticity, transitioning smoothly into plasticity at the post-yield point, and reaching a climax at ultimate tensile strength (UTS). The testing ultimately highlights the flexibility of the material, providing valuable insights important for engineering applications ||

In this brief exploration, we have highlighted the important role of small samples and their stress tests in shaping our understanding of material properties while seamlessly aligning them with the stringent testing standards of ASTM |||

Preparing for the Tension Test/ Specification of specimen

Specification of specimen
Specimen is solid cylindrical Rod
Dia of middle section 0.5″
Gauge length 2.0″
L/D Ratio = 4.0

Objective of Tensile test

Tensile tests on mild steel are important for developing new materials, enhancing product quality, and designing safer structures.

The Testing Procedure

Tensile tests involve stretching a steel sample until it breaks, recording the force required and the elongation of the sample. This data is used to calculate the stresses and strains at various points during the test.

The stress-strain curves prepared from these tests indicate stress-strain relationships aiding in material development and structural safety and product improvement

Step 1; Preparation

Calibrate the Universal Testing Machine (UTM).

Step 2; Sample setup:

Securely mount mild steel cylindrical rods in UTM grips.
Ensure proper alignment and mark the gauge length.

Step 3; Zeroing and calibration:

Perform zero load calibration.
Set the initial grip separation to the gauge length.

Step 4; Applying load:

Gradually increase the load using UTM.
Record the stress (load divided by the original cross-sectional area) and strain (change in gauge length divided by the original gauge length).

Step 5; Data monitoring and recording:

Constantly monitor the load and associated stress.
Record the stress and strain at specific intervals or load points.

Step 6; Observation of Behaviour:

Observe the behaviour of the specimen for yielding, strangulation, etc.

Step 7; Accessing UTS:

Continue testing until the neck cracks and fractures.
Record the maximum load and corresponding stress (Ultimate Tensile Strength – UTS).

step 8; Post Test Analysis:

Analyse the stress-strain curve to identify key points such as yield, UTS and fracture.
Calculate mechanical properties such as Young’s modulus, yield strength and elongation at fracture.

Analysing the Stress-Strain Curve for Tension

A - Limit of Proportionality (Hook's Law Zone)

A as the limit of proportionality and the range where Hook’s Law is valid.

Mentioning its significance in the linear variation of stress and strain.

Section 2: B - Elastic Limit

B as the elastic limit, where the specimen regains its original length upon unloading.

Highlighting that B may vary for different materials, sometimes greater than A.

C - Lower Yield Point

C is Lower Yield point; This is also called actual yield point. The stress at C is the Yield Stress (ϭ_y)

with a typical Value ϭ =250N/mm2 (for Mild steel). The yielding begins at this stress.

CD - Strain Hardening

CD as the strain hardening range, detailing how stress further strains the material.

Clarifying that this portion is not used for structural design

E – Ultimate Point

E as the ultimate point, the stress corresponding to this point is ultimate stress (ϭ_u) and corresponding strain is about 20% for mild steel.

F - Necking Region

Stress corresponding to this is called breaking stress and strain is called strain it is about 20% for mild steel

Describing E and F as the necking region, where the cross-sectional area drastically decreases.

Important Note

Strain that occurs before the yield point is called elastic strain that which occurs after yield point with no increase in stress is called plastic strain. For Mild Steel, Plastic strain is 10 to 15 time of elastic strain.

Ideal curve of tension in shown in above fig. However, actual behaviour is different and indicates apparently reduced yield stress in compression for mild steel. The divergence between tension and compression results is explained by Bauschinger and is called Bauschinger effect.

Stress Strain Curve Real Life Applications

Design of Structures and Machines

Engineers use stress-strain curves to design strong structures such as bridges, buildings, aeroplanes, and cars to ensure that they can withstand expected loads without failure.

Material Selection

Stress-strain curves assist in selecting appropriate materials for specific applications, such as selecting high-strength steel in bridges to support heavy loads.

Quality control

Manufacturers use stress-strain curves to verify the quality of a material, ensuring that it meets specified strength and ductility requirements.

Failure Analysis

Another aspect to use stress strain curve is to determine the cause, whether overloading fatigue or other factors, by analysing stress – strain curves after failure., which aids in future designs

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Typical Applications

Bridge

Designing bridges to bear vehicle and pedestrian weight, wind and seismic forces using stress-strain curves.

Buildings

Using stress-strain curves to design buildings that can support their own weight, occupants, and furnishings.

Aeroplane

Designing airplanes able to withstand take-off and landing forces using stress-strain curves.

Cars

Using stress-strain curves to design cars that can withstand forces from acceleration, braking, and collisions.

Line pipe

Designing pipelines capable of withstanding fluid pressure during transportation using stress-strain curves.

Pressure vessels

Using stress-strain curves to design pressure vessels that can withstand the internal pressure of the gases or liquids contained.

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Quiz : Stress Strain Curve

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