Basics of core measurement of force, tension and strain

Force measurement is fundamental in various engineering disciplines, from the design of complex mechanical systems to the construction of robust structures.

For users of Interface load cells and other precision force measuring instruments, a solid understanding of the underlying principles of kraft, tension and strain essential for successful projects, seamless integration and accurate test lab programs.

What is power?

A force, often called a load, is an external influence acting on an object. It has both a magnitude and a direction, and its action can cause a body to change its state of motion (by accelerating, decelerating, or changing its direction) or its shape.

In mechanical engineering, forces are often used to achieve a desired movement, while in civil engineering, the primary goal is usually to counteract or minimize unwanted movement and deformation to maintain integrity. Visit our Test and measurement solutionsto see different applications for force measurement.

Stress is inner resistance

Stress is the internal resistance per unit area that an object develops in response to an external force applied to it. It quantifies the distribution of internal forces within a material. Understanding stress is critical for users of interface products. When you apply a load to a structure or component, that load induces internal stress. Your interface load cell accurately measures the applied force and provides the data needed to calculate these stresses. This ensures that a material remains within its safe operating limits.

As in our solutions for Stress testing As described above, Interface fatigue-tested load cells are widely used in testing machines and test benches for stress testing. These precision load cells are designed to withstand up to 100 million fully reversed load cycles and provide overload capability of up to 300% in both tension and compression modes.

Strain is the measure of deformation

Strain is the measure of the deformation of a material in response to an applied stress. Unlike stress, which is an internal force per unit area, strain is a dimensionless quantity that represents the relative change in shape or size. It is usually expressed as the ratio of the dimensional change to the original dimension. As shown in our solutions for Tensile tests As explained, understanding yield strength and other key metrics enables engineers to design products that can withstand real-world stresses and prevent potential failures.

The relationship between stress and strain is crucial. Within a material's elastic limit, stress is directly proportional to strain, a concept described by Hooke's law. This relationship involves a material property known as the elastic modulus (or Young's modulus).

Strain is particularly relevant for interface users, as many force measuring devices, including load cells, operate on the principle of strain measurement. Strain gauges, the basic sensor elements in interface load cells, are precisely connected to a carefully designed spring element (Bending element). When an external force deforms this element (strains), the electrical resistance of the strain gauge changes proportionally. This resistance change is then converted into a precise electrical signal, which your interface measuring device interprets as a precise force value.

How objects react to force

When a force acts on an object, it can lead to various outcomes, including different types of deformation and even failure. These responses can be categorized as follows:

#1 – Elastic deformation refers to a temporary change in the shape or size of an object that completely disappears once the applied force is removed. Materials behave elastically up to a certain point, known as the elastic limit. Specific types of elastic deformation include

• Tensile strength: An elongation or extension caused by tensile forces. See "How does a tensile test work?".
• Compression: A reduction in length or volume due to compressive forces. See: Compression testing of concrete
• Shear: Deformation in which parallel layers of a material slide past each other, often resulting in a distortion of an object's angles. Warping is a special case of shear deformation that typically occurs in frame structures. See: Comparison between shear force and compressive force measurement.
• Flexural: A combination of tensile and compressive stresses that causes an object to bend or bulge. The resulting displacement from its original position is called deflection. Read: Basics of bending beam load cells.
• Torsion: A rotational deformation caused by an applied twisting force or torque. Use this great tool: Torque transducer cheat sheet.

#2 – Plastic deformation: A permanent change in shape that persists even after the force is removed. This occurs when the applied stress exceeds the elastic limit of the material and causes irreversible internal structural changes.

#3 – Fracture: The complete breaking or separation of a material due to excessive force or stress. Unlike elastic or plastic deformation, fracture represents a failure of the material rather than a reversible change in shape. Discover Applications for fatigue testingto learn more about this type of exam.

Load cells for measuring force, stress and strain

Interface load cells are sophisticated devices designed to accurately measure applied force by converting mechanical force into a measurable electrical signal. They are used worldwide in numerous force measurement projects, integration systems, and test laboratories.

At their core, interface load cells achieve this precision through the use of precisely engineered strain gauges. These tiny sensors, typically arranged in a Wheatstone bridge circuit, are connected to a carefully machined metal element within the load cell, often referred to as a "flexure" or "spring element."

When an external force is applied to the load cell, the flexure plate deforms slightly (strain) proportional to the applied force. This slight deformation causes the strain gauges to stretch or contract, changing their electrical resistance. The Wheatstone bridge circuit then converts these resistance changes into a highly precise and repeatable electrical voltage signal proportional to the applied force. This voltage signal is applied to devices such as amplifiers, display devices and Data acquisition systems which process and display the force value, often in units such as pound-force (lbf), newton (N) or kilogram-force (kgf).

Interface: The world's leading supplier of force measuring instruments

For users of interface products in test labs, manufacturing, aerospace, automotive, and many other industries, precise force measurement is essential. It ensures product quality and safety, ensures consistent process control, supports research and development in characterizing material properties, ensures regulatory compliance, and ultimately increases efficiency and reduces costs by optimizing designs and avoiding costly failures.

By understanding the basic principles of force, stress, and strain, and knowing how Interface's advanced load cells and measurement devices accurately measure these phenomena, you'll be better prepared to maximize the potential of your force measurement solutions for all your critical applications. Use Interface's new resource hub, ForceEDU, to find more articles, videos, and materials on force measurement.

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