Strength

Strength is one of the most fundamental properties of materials, playing a decisive role in determining their load-bearing capacity. Whether in mechanical engineering, the automotive industry, or aerospace, strength is a key factor across all sectors. It defines how well a material can withstand various types of stress, thereby contributing to the safety, durability, and performance of components.

What Is Strength? Fundamentals and Definition

Strength is a material property that describes a material’s ability to withstand loads without undergoing permanent deformation or failure. It is a measure of resistance to various types of mechanical stress. Material failure may involve plastic deformation or fracture. The higher the strength, the greater the mechanical load the material can endure. 

What Types of Strength Are There?

Strength can be divided into several types, depending on the kind of load applied to the material. Each type of strength describes a material’s specific ability to resist a particular form of force.

Tensile strength and compressive strength

Tensile and compressive strength describe a material’s resistance to axial forces. While tensile strength measures how well a material withstands tensile forces—that is, forces that pull it apart—compressive strength indicates how resistant a material is to compressive forces that push it together. The so-called tensile test, used to determine tensile strength, is the most common method for testing material strength.

Flexural Strength and Buckling Strength

Flexural and buckling strength describe how materials behave under loads that cause deformation along their longitudinal axis. Both are highly dependent on the geometry and length of the component.

Flexural strength refers to a material’s ability to resist bending under a load applied perpendicular to its longitudinal axis. This is especially important for components such as beams, shafts, and axles, which require high flexural strength to avoid bending under stress.

In contrast, buckling strength describes the compressive load a component can withstand along its longitudinal axis before buckling occurs. This property is also influenced by the component’s support conditions (boundary conditions).

Shear Strength and Torsional Strength

Shear strength refers to a material’s ability to resist opposing lateral forces. It describes how well a material can withstand the sliding of internal layers against each other before failure or fracture occurs. A common example is bolts and screws, which are often subjected to high shear forces in order to securely connect components.

Torsional strength, on the other hand, describes a material’s ability to remain stable under twisting loads. Such torsional moments occur in shaft-hub connections, as found in automotive drivetrains, where torque is transmitted from the engine to the wheels.

Fatigue Strength

Another type of strength is fatigue strength, which refers to a material’s resistance to fluctuating or cyclic loads. It is particularly important for components that are continuously exposed to alternating tensile, compressive, bending, or torsional stresses — such as bridges, aircraft wings, or rotating machine parts. Since such loads can lead to material fatigue and fatigue fracture over time, fatigue strength is a critical factor in ensuring the longevity and safety of these components.

Measuring Strength: An Overview

The strength of a material is determined using standardized mechanical testing methods that simulate specific types of loading, such as tensile, compressive, bending, or torsional forces. One of the most fundamental tests for assessing material properties is the tensile test. In this procedure, a specimen with a standardized shape is stretched—typically until it fractures or necking occurs. During the test, a stress-strain diagram is recorded, showing the relationship between the applied force (relative to the specimen’s cross-sectional area) and the resulting deformation (strain).

Stress-Strain Curve

Several characteristic values can be determined from the stress-strain curve:

• Elastic Modulus (Young’s Modulus): The elastic modulus describes a material’s stiffness. It indicates how much elastic deformation occurs under a given force.
• Yield Strength: The yield strength is the critical point at which a material begins to deform permanently. Any stress below this value allows the material to fully return to its original shape.
• Tensile Strength: The peak point in the diagram represents the tensile strength — the maximum stress the material can withstand before failure.

The stress–strain curve provides valuable insights into a material’s behavior, such as its elastic deformation characteristics and key mechanical properties. However, it does not allow for exact calculations, since the tensile test is based on idealized conditions — specifically, uniaxial and purely tensile loading. In reality, components are often subjected to a combination of tensile, compressive, and bending forces. As a result, the findings from a tensile test cannot be directly applied to real-world loading scenarios. Instead, the obtained values serve as comparative reference points and are supplemented by safety factors to account for the differences between theoretical conditions and practical applications.

Strength, Hardness, and Stiffness at a Glance

The terms strength, hardness, and stiffness are often confused or used interchangeably, even though they describe important but distinct properties of a material. Understanding these differences is essential, as each property reflects different performance requirements in practical applications.

What Is the Difference Between Strength and Hardness?

Strength refers to a material’s ability to resist mechanical loads without breaking or undergoing permanent deformation. It indicates how much force a material can withstand before failure. A high-strength material remains stable even under heavy loading conditions.

Hardness, on the other hand, measures a material’s resistance to surface penetration by another object. It is often associated with wear resistance. A material can be very hard and scratch-resistant yet still be brittle and prone to fracture — as is the case with ceramics. Conversely, a material may exhibit high strength but lower hardness and still perform reliably under stress.

What Is the Difference Between Strength and Stiffness?

Stiffness refers to a material’s resistance to elastic deformation — that is, deformation that is fully reversible once the load is removed. As long as the applied stress remains below the yield strength, deformation is purely elastic. If the yield point is exceeded, plastic deformation occurs in addition to the elastic response. Upon unloading, only the elastic portion recovers, while the plastic portion results in permanent deformation. A stiff material deforms very little under load, meaning it is less flexible and better at maintaining its shape.

In contrast, strength describes the overall amount of load a material can withstand before it fails or undergoes permanent (plastic) deformation — beyond uniform elongation, such as when a tensile specimen fails due to necking.

Factors Influencing Material Strength

A material’s strength depends on several fundamental factors. Among the most important are its atomic structure and microstructure. In metallic materials, characteristics such as grain size and dislocation density have a significant impact on strength.

Finer grain structures and higher dislocation densities increase strength because they hinder the movement of dislocations — small defects within the crystal lattice — thereby enhancing resistance to deformation. Alloying elements and heat treatment also play a key role, as they can be used to deliberately modify the material’s structure and optimize its strength.

Increased Strength Through Cold Forming: Advanced Technologies from Felss

Cold forming offers significant advantages in the production of components, especially when it comes to increased strength and lightweight construction. At Felss, we use specialized processes such as axial forming, rotary swaging, and flow forming to manufacture components with high precision and strength. Unlike machining processes, which remove material, cold forming strengthens the material structure. The high compressive forces applied during forming increase the dislocation density within the material, resulting in a localized increase in strength. This effect significantly enhances the mechanical properties of the component.

One of the key advantages of cold forming is the ability to tailor material distribution to load paths and leverage strength-enhancing effects for especially lightweight parts. Additionally, cold forming requires less energy compared to forging or hot forming, as no external heating is needed. This not only saves energy but also enables the production of complex, high-precision components — ideal for the demands of the automotive industry and other technology-driven sectors.

Applications: Where Strength Matters Most

Material strength is critical across numerous industries, as it directly affects the load-bearing capacity, safety, and durability of components. Strength plays a particularly vital role wherever human lives may be at risk — for example, due to heavy loads, high speeds, or extreme rotational forces. Below are key application areas:

• Transportation: In the automotive, rail, and aerospace sectors, high strength is essential. Components such as axles, drive shafts, and structural parts must withstand extreme forces to ensure passenger safety. At the same time, material strength enables lightweight construction, which improves efficiency, extends service life, and reduces fuel or energy consumption.
• Aerospace: Here, the combination of high strength and low weight is especially important. Aircraft and spacecraft require components that can endure enormous forces while being light enough to optimize performance and minimize fuel usage.
• Mechanical Engineering: Production equipment and machine tools are exposed to extreme conditions and high loads on a daily basis. High-strength components are essential to ensure reliable, long-term operation with minimal downtime.
• Precision engineering: Whether for watches or in sensor technology: precision mechanical components must remain resilient despite their tiny dimensions.
• Construction: In the building sector, human safety is paramount. Structures such as buildings, bridges, and transmission towers must withstand heavy loads and harsh environmental conditions to ensure stability and prevent structural failure or collapse.

In Summary: Strength

Strength is a fundamental concept in materials engineering, describing a material’s ability to withstand mechanical loads. Different types of strength — such as tensile, compressive, flexural, and torsional — are critical to the function and safety of components. Processes like cold forming can locally enhance strength, which can be used advantageously in many cases — for example, by enabling the use of less expensive or reduced material volumes, or by eliminating the need for additional hardening steps. This highlights the importance of tailoring material properties to the specific application — whether in the automotive industry, aerospace, or other sectors.