In this blog post, we’ll explore the difference between the term “stress” as used in everyday language and the engineering concept of “stress,” the types and characteristics of stress, and how stress acts on structures and machines around us.
Definition and Classification of Stress
While the term “stress” used in everyday language refers to psychological pressure or tension, the term “stress” used in engineering refers to mechanical stress. Mechanical stress is a physical quantity expressed per unit area that represents the internal resistance generated within an object when external forces such as compression, tension, bending, or torsion act upon it. In other words, it is defined as the force applied to an object divided by the cross-sectional area over which that force acts. Although the same word “stress” is used, it is a concept entirely different from the psychological meaning and is a crucial concept for describing the physical forces that push, pull, bend, or twist an object.
Stress is broadly classified into two types depending on the direction in which the force acts. One is normal stress, which acts perpendicular to the cross-section, and the other is shear stress, which acts parallel to the cross-section. Compression, tension, and bending are primarily related to normal stress, while torsion is a typical example of shear stress in action.
Phenomena Caused by Compression, Tension, Bending, and Torsion
First, in structures subjected to significant compressive stress, a phenomenon called buckling can occur. Buckling is a phenomenon in which a long, slender structure—such as a column—suddenly bends sideways when the compressive load exceeds a certain limit. Even if the material itself does not break, the structure can suddenly deform and lose its functionality; therefore, it is a critical factor that must be considered in the design of buildings, bridges, towers, and other structures.
A material’s response to tensile stress is broadly classified as ductile or brittle, depending on its properties. Ductile materials exhibit a phenomenon called necking, in which the center gradually thins as the material stretches. This is easy to understand if you imagine pulling on clay or another highly ductile material with both hands and watching the center gradually become thinner. In contrast, brittle materials exhibit a characteristic where they suddenly fracture the moment a certain limit is exceeded, without any noticeable deformation. Glass, ceramics, and concrete are representative examples of brittle materials, and paper also demonstrates this characteristic to some extent, as it tears suddenly without stretching significantly.
Bending and torsion refer, quite literally, to the forces that bend or twist an object. If these forces act continuously or exceed the allowable range, the material will eventually fracture. In particular, when tensile, bending, and torsional forces act together, small surface scratches or cracks become points of stress concentration, causing the cracks to grow rapidly and significantly increasing the likelihood of eventual failure.
Fatigue and Everyday/Engineering Examples
Material failure does not necessarily occur solely due to a single large force. If you repeatedly bend a spoon or a piece of wire back and forth, you may find that it eventually breaks even under relatively small forces. This type of failure, caused by the gradual accumulation of internal damage within the material due to repeated tension and compression, or repeated bending, is called fatigue.
The S-N curve is a representative graph illustrating fatigue characteristics. This graph shows the relationship between the number of cycles (N) and the magnitude of stress (S), demonstrating that as the number of cycles increases, the stress required to cause failure gradually decreases.
While some metallic materials have a fatigue limit—a stress level below which fatigue failure does not occur even under very high numbers of cycles—many materials, such as aluminum alloys, do not have a clearly defined fatigue limit. Therefore, when designing actual structures and machinery, the operating environment and cyclic loading conditions must be thoroughly considered.
Furthermore, fatigue is influenced by various factors, including temperature, the rate of load cycling, corrosion caused by salt or moisture in the air, the material’s microstructure and grain size, average stress, and surface condition. In particular, corrosion fatigue—where fatigue and corrosion occur simultaneously—can cause damage to progress much faster than typical fatigue; therefore, it is considered a critical design factor in marine structures, bridges, ships, and offshore wind power facilities. Failure to adequately account for stress can have significant consequences. In buildings and bridges, excessive or repetitive loads can cause cracks or structural damage, while in machinery that is continuously subjected to repetitive loads—such as automobiles, rail vehicles, and aircraft—fatigue failure can lead to serious accidents. Furthermore, everyday items such as chairs, bookshelves, bicycle frames, and the metal frames of smartphones are also more likely to deform or break if they are designed or manufactured without sufficient consideration of stress. Ultimately, stress is one of the most fundamental and important concepts in the design of machines and structures. While we often use the word “stress” in everyday life solely in a psychological sense, the concept of stress in engineering is also a key factor determining the safety and performance of products, buildings, transportation systems, and industrial facilities throughout our lives. Understanding this engineering concept of stress allows us to view the meaning of the word “stress” from a much broader perspective.
