Tensile Stress vs. Compressive Stress: The Key Differences

In the fields of engineering and materials science, stress is defined as the force per unit area experienced by a material. While there are several different types of stresses, tensile stress, and compressive stress are the easiest to understand. Tensile stress occurs when a force pulls or elongates the material, while compressive stress occurs when a force compacts the material. Figure 1 below illustrates these phenomena:

The effects of tensile and compressive stresses can vary depending on the type of material experiencing the load, its properties, and the environmental conditions under which the stress occurs. Understanding both tensile and compressive stresses — from what causes them and how they affect a material’s properties to how they are calculated and determined — is crucial to the successful design and safe implementation of products and structures. This article will review the causes, mechanics, benefits, and implications of tensile and compressive stresses. 

What Is a Tensile Stress?

The term “tensile stress” describes the state in which an applied force tends to elongate the material along the axis of the applied force. It can be numerically represented as the ratio of the magnitude of the applied force to the cross-sectional area on which the force is applied: 

stress = F/A

Measurements of tensile stress, as well as those for all other types of stress, are reported in units of Pascals (Pa) or pounds per square inch (psi). Understanding how tensile stresses impact a material’s behavior is essential to designing safe products that function as expected.

How Does Tensile Stress Work?

When a force is applied to a material that tends to pull the material's atoms apart, the material resists, because the bonds between atoms are so strong. The total stretching force is spread across all the atoms on a cross-sectional plane, all of which are resisting the applied force. The stretching force per unit of cross-sectional area is what we call "tensile stress.

The level of tensile stress at which plastic deformation begins to occur varies depending on the material’s chemical composition, microstructural details, and environmental conditions (such as temperature). These factors largely control a material’s mechanical properties. Stress concentrations due to part geometry can also impact the deformation mechanics of a material.

What Causes Tensile Stresses?

Tensile stresses are caused by the dislocation motion of atoms in a material’s microstructure when a force is applied. Dislocation typically happens in the same direction as the applied force. 

What is Tensile Stress Use For?

Tensile stress has many practical applications that enable the proper function of parts and structures. For example, the cables on a suspension bridge experience tensile stresses as vehicles pass across the bridge decking. The cables must be made of materials able to withstand these planned tensile stresses.

Another example where tensile stress is used for a practical purpose is in the ropes or steel cables in pulleys and cranes. As with the cables of a suspension bridge, these lines must be able to withstand the tensile stresses developed while an object is suspended from it. In both cases, the parts must withstand the applied tensile stress for safe and effective use.

What Is the Importance of Tensile Stress Testing?

It is essential for engineers and designers to understand tensile stress and its effects on different materials so that parts and structures can be properly designed for safe and functional use. Conducting a tensile test in a laboratory to determine the yield stress and ultimate tensile strength of a material is one way engineers and designers can do this. From these values, many other mechanical properties such as: elastic modulus, Poisson’s ratio, and strain hardening characteristics can be determined.

What Are the Benefits of Tensile Stress Testing? 

The benefits of testing materials under conditions of tensile stress are listed and described below:

1. Determine Tensile Strength: Some materials are preferred over others for a particular application due to their higher tensile strengths. Tensile strength describes the stress at which a material will rupture while yield strength is the stress value at which plastic deformation occurs. High tensile strength means that the material is strong and can be used in structural applications where applied forces tend to elongate or stretch materials. 
2. Measure Ductility: Ductility can be determined from tensile strength by calculating the slope of the linear portion of a stress-strain curve. High ductility means materials can experience significant deformation before failure. This is highly desirable in numerous applications such as the cables in suspension bridges, structural beams, and more.
3. Evaluate Fatigue Resistance: Fatigue resistance is a material’s ability to withstand cyclic or repeated loading. Materials with higher tensile strength and ductility have higher fatigue resistance. Fatigue resistance is desirable in applications with oscillating loads such as bridge crossmembers and aircraft components.

What Are Examples of Tensile Stresses? 

There are countless examples of tensile stress in everyday life and in different applications and industries. Some examples of tensile stresses are listed below:

1. A rubber band being pulled apart.
2. Two people on opposite ends of a rope, pulling the rope away from each other.
3. Developed in the cable of a construction crane when an object is suspended.
4. Developed in the cables of a suspension bridge as cars and other vehicles pass over the bridge decking.

What Is a Compressive Stress?

In contrast to tensile stress, compressive stress is the state in which an applied force tends to compact or compress the material along the axis of the applied force. The units used to express the value of compressive stress are Pascals (Pa) or pounds per square inch (psi).

To learn more, see our guide on What is Compressive Stress.

How Does a Compressive Stress Work? 

Compressive stress is caused by forces that compress or compact a material. The dimensions of a material tend to shrink in the direction parallel to the direction of the compressive force and grow in the directions perpendicular to the application of the compressive load.

As compressive stress is applied, the atoms in the material’s crystal structure are pushed closer together. As atoms in the crystal structure move, they push against other atoms. Since atoms cannot occupy the same space, the displaced atoms tend to move in directions perpendicular to the applied force. This causes the material to bulge, or become thicker, in those directions. As compressive stress increases, buckling failure or the sudden bending or twisting of the material occurs. As with tensile stress, the level of compressive stress at which plastic deformation occurs is the yield stress while the level of stress at failure is the compressive strength.

What Causes Compressive Stress? 

Compressive stress is caused by atomic dislocations. The dimensions of a material tend to grow in the directions perpendicular to the direction at which the compressive force is applied. When a compressive force is applied, atoms dislocate around a slip plane or an imaginary plane through the material’s microstructure. The atoms above the slip plane compress while the atoms below the slip plane go into tension. This phenomenon is what causes the “widening” effect when materials are in compression.

Where is Compressive Stress Use For?

Compressive stress has many practical applications that allow engineers to design structures and parts for safe and effective use. For instance, when it comes to designing structural columns, knowing the compressive strength of a material is essential. With this knowledge, engineers can design structures based on the expected loads to avoid buckling of columns.

What Is the Importance of Compressive Stress?

It is critical for engineers to understand compressive stress and how different materials respond to compressive stresses. The safety and functionality of large structures such as bridges, high-rises, and other buildings are highly dependent on their structural members’ ability to withstand compressive stresses. Conducting a compression test in a laboratory can help determine the yield strength of a material in compression, as well as its ultimate compressive strength, and its buckling characteristics such as Euler’s critical load. These properties then enable engineers to design structures that satisfy an application’s loading requirements.

What Are the Benefits of Compressive Stress Testing? 

Some benefits of compressive stress testing are listed below:

1. Determine Compressive Strength: Some materials are stronger in compression than in tension.Materials such as concrete and steel, have high compressive strengths, which make them great for applications such as roads, bridges, high-rise buildings, and roller mills used for compressing steel sheets or crushing rocks into small pieces, where compression stresses are common.
2. Improved Stiffness: Materials that are stronger in compression than in tension tend to also be stiffer. This makes them suitable for applications where high stiffness is desired, such as in road, bridge, and high-rise construction, but also in furniture.

What Are Examples of Compressive Stresses?

Numerous examples of compressive stress environments are encountered in everyday life and in various industries and applications. Listed below are some examples of compressive stress conditions:

1. Concrete slabs on the floors of buildings or on sidewalks experience compressive stresses as people, animals, and machinery move atop them.
2. Structural columns that run the full height of  the building support the weight of all the floors and people above it.
3. Pieces of furniture such as chairs, couches, tables, and more must be able to sustain the weight of the people and items placed on top of them.

What Is the Significance of Tensile Stress vs. Compressive Stress in 3D Printing?

Both tensile and compressive stresses are important in 3D printing. However, tensile stresses tend to have a larger impact on 3D-printed parts than compressive stresses. 3D-printed plastic parts are often weaker than plastic parts produced by traditional plastic manufacturing techniques such as injection molding or extrusion. This is due to the nature of the 3D printing process, in which parts are constructed layer by layer. 3D-printed parts are weakest in the direction in which the layers build up — the Z-axis. For load-bearing 3D-printed parts, it is a good practice to 3D-print parts in such a way that loads are applied perpendicularly to the Z-axis rather than parallel to it to maximize the amount of tensile stress the part can withstand.

To learn more, see our guide on What is 3D Printing.

How Are Tensile and Compressive Stresses Calculated?

Tensile and compressive stresses can be determined using the same equation: the general equation for engineering stress. The equation for engineering stress is shown below:

=FA

Where: 

1. σ = engineering stress (Pa or psi)
2. F = applied force (N or lbf)
3. A = cross-sectional area on which the force is applied 

Stress is commonly expressed in units of Pascals (Pa) or pounds per square inch (psi). If both the force and cross-sectional area are known, it is easy to calculate  the tensile or compressive stress. 

How Do Tensile and Compressive Stresses Affect Material Behavior?

Both tensile and compressive stresses affect material behavior. The atoms making up the material attempt to resist attempts to either pull them apart or push them closer together. Tensile and compressive stresses cause atoms in the material’s microstructure to deform and dislocate. As this response occurs, atoms in the microstructure attempt to maintain their interrelationships (elastic deformation). After the stress becomes too great, the atoms can no longer maintain their relationship, and plastic deformation begins. Once plastic deformation begins, elongation or compression due to the applied force becomes permanent. Beyond the physical changes in the material’s microstructure due to excess tensile or compressive stresses, increased susceptibility to corrosion is possible due to the increase in lattice and surface energy.

Can Tensile and Compressive Stresses Occur Together? 

Yes, tensile and compressive stress can occur together. This is commonly observed in parts that are subjected to bending. Bending a pencil until it breaks is a great example to illustrate these phenomena. Imagine an imaginary line that runs along the entire length of the pencil. When held at both ends and pushed down at a point in the middle, the pencil forms a U-shape, with the middle of the “U” being forced below its original position. Near the bottom of the U, the upper surface of the pencil experiences compressive stresses, while the bottom surface of the pencil experiences tensile stresses.

Is Tensile Stress More Important Than Compressive Stress? 

It depends on the stress environment in service. For many applications, only tensile stresses need to be considered, while for others, only compressive stresses are important. However, most products and structures operate in complex stress environments, requiring consideration of the effects of both tensile and compressive stresses. When it comes to tensile stress vs. compressive stress, engineers and designers should carefully consider the effects of both. By calculating and analyzing the effects of both types of stresses, engineers can ensure that they produce parts and structures that can tolerate the stresses that will be encountered in service and will not fail or collapse.

Is Compressive Stress Higher than Tensile Stress?

Yes, applied compressive stress levels can often be higher than applied tensile stresses, since some materials (such as concrete) tend to be stronger in compression than in tension. However, applied compressive stresses are not inherently larger than applied tensile stresses. Various factors such as loading, part geometry, and material properties can affect the allowable magnitudes of applied tensile and compressive stresses. For instance, concrete can withstand high compressive stresses, but is weak when tensile stress is applied.

Summary

This article presented tensile stress vs. compressive stress, explained each of them, and discussed their key differences. To learn more about tensile stress and compressive stress, contact a Xometry representative.

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