Stress In Material Science

Introduction

In materials science, stress refers to the force applied per unit area of a material. It is a fundamental concept used to understand material behaviour under external loading. Stress plays a vital role in determining a material’s ability to withstand various loading conditions without failure. Engineers use stress calculations to design materials and structures that maintain strength, durability and performance under different conditions.

Definition of Stress

Stress is mathematically defined as the force F applied to an object divided by the cross-sectional area A that receives the force:

Stress(σ)=F/A

Where:

• F is the applied force (in Newton, N)
• A is the cross-sectional area (in square metres, m²)
• σ is the stress, measured in Pascal (Pa) or Newton per square metre (N/m²).

In materials science, there are two primary types of stress: normal stress and shear stress.

1. Normal stress: Occurs when the force is applied perpendicular to the surface, resulting either in tension or compression.

• Tensile stress: Occurs when a material is pulled (for instance, when stretching a wire).
• Compressive stress: Occurs when a material is pressed (for example, compressing a column).

2) Shear stress: Occurs when a force is applied parallel to the surface, causing adjacent layers of the material to slide relative to one another. For instance, this is observed when cutting a metal piece with a shear force.

Types of Stress in Materials

Tensile Stress:

Tensile stresses occur when a material is subjected to pulling forces, resulting in elongation.

Example: A rubber band is stretched.

Compressive Stress:

Compressive stress arises when a material is subjected to compressive forces, leading to contraction or compaction.

Example: A column supporting the weight of a structure.

Shear Stress:

Shear stress occurs when two opposing forces act parallel to a surface, causing a sliding deformation.

Example: Scissors exerting a shear force to cut paper.

Bending Stress:

Bending stress is a combination of tensile and compressive stresses that occurs when a material is bent.

Example: A beam loaded at its centre.

Factors Influencing Stress

Several factors may affect the stress in a material, for example:

-Material properties: The strength, ductility and elasticity of a material determine its response to stress. Metals, for instance, tend to exhibit greater ductility under tensile stress, whereas ceramics may fail at lower stress levels. See also Metals.

-Temperature: Elevated temperatures can reduce a material’s strength and its load-bearing capacity. Certain materials may become more ductile and deform with less resistance, while others may become brittle with a higher chance of failure. Further details are available at High Melting Point Materials.

-Loading conditions: The rate and duration of the applied load influence the material’s response. Consequently, a material may fail if a high load is applied rapidly, whereas the same load applied gradually over an extended period might be sustained.

Relationship between Stress and Strain

Stress and strain are directly related through the elastic modulus of a material. The stress–strain curve illustrates how a material deforms under different stress levels. The principal regions of the stress–strain curve are:

1. Elastic region: In this region, the material returns to its original shape once the stress is removed. The relationship between stress and strain is linear.

2. Plastic region: Beyond the elastic limit, the material deforms permanently after reaching its yield point.

3. Fracture point: When the material is subjected to stresses beyond its maximum capacity, it ultimately fractures.

Applications of Stress in Materials Science

-Civil engineering: Understanding stress is necessary for designing buildings, bridges and other structures so that they can withstand forces such as weight, wind and seismic loads without structural failure.

-Manufacturing: In processes such as casting, forging and welding, engineers consider stress to prevent deformation or failure of materials during production.

-Material selection: Different materials possess varying load-bearing capacities. For this reason, materials like steel are chosen in civil engineering because they can withstand high tensile and compressive stresses.

-Fatigue and failure analysis: Repeated or cyclic loads may gradually weaken a material. An understanding of stress allows engineers to predict material fatigue and prevent failures in components such as aircraft wings and engine parts.

- Further information can be found at Stanford Advanced Materials (SAM).

Frequently Asked Questions

What is the difference between tensile stress and compressive stress?

Tensile stress occurs when a material is pulled, resulting in elongation. Compressive stress occurs when a material is pressed, leading to contraction or crushing.

What is the effect of temperature on stress in materials?

Higher temperatures generally reduce a material’s strength and load-bearing capacity. Given that some materials become more ductile and deform more readily, others become brittle and may fail sooner.

What is the yield point in the stress–strain curve?

The yield point is the stress level at which a material begins to deform plastically and cannot fully recover its original shape once the load is removed.

Why is an understanding of stress important in civil engineering?

Engineers must understand stress to ensure that structures such as buildings and bridges withstand forces including weight, wind and seismic activity without collapsing.

What is fatigue in materials, and how is it related to stress?

Fatigue refers to the progressive weakening of a material due to repeated or cyclic loading. Even when the applied stress is below the ultimate strength of the material, continuous or cyclic loading can lead to failure.