We have three main forces that are at the core of how materials respond to external factors: compression, tension, and shear. These forces come into play when an external force is applied to an object. How the object reacts to these forces depends mainly on how the force is applied in relation to the object.

However, when we dig deeper, we discover that there are six categories of stress identified: compression, tension, shear, bending, torsion, and fatigue. For example, when we talk about bending, it's essentially a combination of compression and tension forces working within an object. Similarly, torsion happens when there's a twist, which is like shear with a twist, often caused by a rotational force known as a moment. It's important to note that fatigue doesn't belong to the force category. Instead, it's a material property related to how well an object can endure repeated stress over time.

Before we dive into a detailed exploration of each stress type for a deeper understanding, let's take a moment to distinguish between force and stress. Here's the key differences between them:



To put it briefly, forces are the external loads or influences applied to a structure, while stress is the internal response of the material or structure to those forces. Stress is a measure of how the material resists deformation and is influenced by both the applied forces and the structural characteristics of the material. Understanding both concepts is crucial in structural analysis to ensure the safe and efficient design of structures.


Compression Force


Compression occurs when external forces act on an object or material in a manner that pushes or squeezes it, causing it to contract or become shorter in the direction of the applied force. This type of stress is characterized by the compression forces tending to push the particles or molecules within the material closer together. Compression stress is prevalent in various real-world scenarios, such as the weight of a book resting on a table's surface, where the table's material experiences compressive forces that counteract the gravitational force. Engineers and designers must consider compression stress when designing structures to ensure that materials can withstand the downward forces they will encounter, preventing deformation or structural failure.

Tension Force


Tension occurs when external forces act on an object or material in a manner that stretches or elongates it, causing it to become longer in the direction of the applied force. This type of stress involves forces pulling the particles or molecules within the material apart. A common real-world example of tension stress is a rope or cable bearing a heavy load, where the material experiences tension forces that attempt to elongate and ultimately weaken it. Engineers and designers must carefully consider tension stress in various applications, such as bridge construction and the design of cables and wires, to ensure that materials can withstand these stretching forces without breaking or failing.

It's important to recognize that compression and tension involve forces that directly oppose each other. When there isn't an opposing force aligned with the initial force, it leads to a different type of force known as shearing. If we examine this phenomenon at the molecular level, we can see that because these forces are aligned, the molecular bonds do not allow the molecules to slide past each other until they eventually break. Typically, this opposing force is referred to as the normal force, which represents the resistance exerted by surfaces to prevent solid objects from passing through each other.


Shear force occurs when two forces are applied parallel to each other but in opposite directions, causing one part of a material or structure to slide or deform relative to the other. Unlike compression and tension, which act along the axis of an object, shear forces act in a plane parallel to the surface of the material. Think of it as trying to slide one layer of a material past another. Shear forces are responsible for effects like cutting, tearing, or bending materials without changing their overall shape or size. In engineering and construction, understanding shear forces is crucial for designing stable and safe structures, as they can influence the structural integrity of beams, bridges, and various other components.

Shear Force
Bending Force


Bending force, also known as flexural stress, occurs when external forces act on an object in such a way that it induces a curving or bending deformation. This force is typically applied perpendicular to the longitudinal axis of an object, resulting in the material or structure curving or flexing around a central point. Think of bending a thin piece of wood or a sheet of paper—the application of force causes it to bow or take on a curved shape. Bending forces are encountered in a wide range of applications, from beams and bridges supporting heavy loads to everyday objects like diving boards and springy floorboards. Understanding the principles of bending is crucial for engineers and architects when designing structures to ensure they can withstand and distribute these forces effectively while maintaining structural integrity.

When an object undergoes bending, it experiences both compression and tension forces simultaneously. Imagine a simple wooden beam being subjected to a bending force in the middle. The upper part of the beam, known as the 'compression side,' is compressed as it's pushed together, while the lower part, known as the 'tension side,' is stretched apart. This combination of compression and tension is a direct consequence of the bending action. The material on the compression side tries to resist being squished, while the material on the tension side resists being pulled apart. Engineers and architects carefully consider these forces when designing structures like bridges or beams to ensure that materials used can withstand the resulting compression and tension stresses effectively, preventing deformation or structural failure.


Torque stress, also known as torsional stress, occurs when you twist or turn an object, like when you're opening a jar. This twisting force causes the material in the object to experience both torque stress and shear force.  Similarly, when you turn a steering wheel, ride a bicycle, or use a wrench to tighten a bolt, you're dealing with torque. It's this twisting or rotational force that makes things spin or rotate around an axis. Torque stress is like the internal squeezing or twisting, while shear force is a sideways push within the material. Understanding torque is crucial in various situations, whether it's designing machinery, ensuring safety in vehicles, or even enjoying the simple act of turning a key.


Fatigue stress is a unique aspect to consider when discussing the various forces that act on materials. Unlike compression, tension, shear, bending, and torsion, which represent immediate forces on an object, fatigue stress deals with the long-term effects of repeated stress cycles. It's like the wear and tear a material experiences over time due to constant or cyclic loading. Imagine bending a paperclip back and forth repeatedly until it breaks; that's a simple example of fatigue stress. Over time, even though a material may withstand individual forces, it can weaken and eventually fail when subjected to repeated stresses. Engineers and scientists study fatigue to design materials and structures that can endure for the long haul, ensuring safety and reliability in a wide range of applications, from bridges and aircraft to everyday items like springs and bicycle frames.