Core Measurement Fundamentals of Force, Stress, and Strain

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

For users of Interface load cells and other precision force measurement devices, a solid understanding of the underlying principles of force, stress, and strain is essential for successful projects, seamless integration, and accurate testing lab programs.

What is Force?

A force, often referred to as a load, is an external influence that acts upon an object. It has both magnitude and direction, and its application can cause a body to change its state of motion (by accelerating, decelerating, or changing direction) or to alter its shape.

In mechanical engineering, forces are often applied to achieve desired movement, while in structural engineering, the primary objective is typically to counteract or minimize unwanted movement and deformation to maintain integrity. Visit our Test and Measurement solutions to review various applications for measuring force.

Stress is Internal Resistance

Stress is the internal resistance per unit area that an object develops in response to an external force acting on it. It quantifies the distribution of internal forces within a material. Understanding stress is essential for Interface product users. When you apply a load to a structure or component, that load causes internal stress. Your Interface load cell, by accurately measuring the applied force, provides the data needed to calculate these stresses and ensures that a material stays within its safe operating limits.

As outlined in our stress testing solutions, Interface fatigue-rated load cells are commonly found in testing machines and stands used in stress testing. These precision load cells are engineered to withstand up to 100 million fully reversed load cycles and have overload ratings of up to 300% in both tension and compression modes.

Strain is The Measurement of Deformation

Strain is the measure of the deformation of a material in response to 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 typically expressed as a ratio of the change in dimension to the original dimension. As outlined in our solutions for strain testing, understanding yield strength and other key metrics enables engineers to design products that withstand real-world stresses and prevent potential failures.

The relationship between stress and strain is critical. 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 Young’s Modulus (or modulus of elasticity).

For Interface users, strain is particularly relevant because many force measurement devices, including load cells, operate on the principle of strain measurement. Strain gages, the fundamental sensing elements within Interface load cells, are precisely bonded to a carefully designed spring element (flexure). As an external force causes this element to deform (strain), the electrical resistance of the strain gage changes proportionally. This change in resistance is then converted into an accurate electrical signal, which your Interface instrumentation interprets as a precise force reading.

 How Objects React with Application of Force

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

#1 – Elastic Deformation refers to a temporary change in an object’s shape or size that is fully recovered 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: An increase in length or stretching caused by pulling forces. Refer to How Does Tensile Testing Work?.
• Compressive: A decrease in length or volume caused by pushing forces. Reference: Concrete Compression Testing
• Shear: Deformation where parallel layers of a material slide past each other, often leading to a distortion of an object’s angles. Racking is a specific instance of shear deformation, typically seen in framed structures. Check out Comparing Shear Versus Compaction Force Measurement.
• Bending (Flexural): A combination of tensile and compressive stresses that causes an object to curve or arc. The resulting displacement from the original position is referred to as deflection. Read: Bending Beam Load Cell Basics.
• Torsion (Twisting): 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 remains even after the force is removed. This occurs when the applied stress exceeds the material’s elastic limit, causing irreversible internal structural changes.

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

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 in numerous force measurement projects, integration systems, and testing laboratories worldwide.

At their core, Interface load cells achieve this precision by employing precisely engineered strain gages. These tiny sensors, typically arranged in a Wheatstone bridge circuit, are bonded to a carefully machined metallic element within the load cell, often referred to as the “flexure” or “spring element.”

When an external force is applied to the load cell, the flexure slightly deforms (strains) in proportion to the applied load. This small deformation causes the strain gages to stretch or compress, which in turn changes their electrical resistance. The Wheatstone bridge circuit then converts these resistance changes into a highly accurate and repeatable electrical voltage signal, proportional to the applied force. This voltage signal is transmitted to Interface instrumentation, including amplifiers, indicators, and data acquisition systems, which process and display the force reading, often in units like pounds-force (lbf), Newtons (N), or kilograms-force (kgf).

Interface: The World Leader in Force Measurement

For users of Interface products in testing laboratories, manufacturing, aerospace, automotive, and many other industries, precise force measurement is essential. It ensures product quality and safety, maintains consistent process control, supports research and development to characterize material properties, ensures regulatory compliance, and ultimately enhances efficiency and reduces costs by optimizing designs and avoiding costly failures.

By understanding the basic principles of force, stress, and strain, and recognizing how Interface’s advanced load cells and instrumentation accurately measure these phenomena, you are 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 additional articles, videos, and materials related to force measurement.

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