The difference in force measurement: strain versus tension

In materials science and structural testing, stress and strain are often treated together, even though they describe fundamentally different physical processes. For engineers and testing laboratories that require high-precision testing, this distinction is crucial. Load cells and Torque transducer When using this technology, the distinction between the two is crucial – for defining test parameters, selecting suitable sensor capacities, and interpreting data from the elastic and plastic range of a material.

Internal force compared to geometric deformation

Basically is tension An internal force distribution. When an external load acts on a solid body, internal resistance forces arise that counteract this load. Mathematically expressed, it is the applied force per unit area. In a testing environment, the load cell measures the total force, while the stress is calculated from the cross-sectional geometry of the test specimen.

strain In contrast, strain is a dimensionless measure of deformation. It describes the displacement between particles within the material relative to a reference length. Technical strain is defined as the change in length divided by the original length. While stress describes what the material "feels" internally, strain describes how it physically reacts or moves. In this Interface Tech Talk, we examine the relationship and differences in testing.

Measurement technology and sensor integration

The relationship between stress and strain is fundamental for sensor selection. Voltage measurement It is based on the direct output signal of a load cell. The accuracy of the voltage depends on the linearity of the sensor and the precision of the known dimensions of the test specimen. Load cells use internally Strain gauges, which are based on a spring body are glued on. The sensor uses its own controlled internal strain to generate an electrical signal that represents the external voltage on the test object.

The relationship between stress and strain is determined by the constitutive equations of the material, in particular by the Young's modulus (modulus of elasticity) within the elastic limit.

Determining a material's modulus of elasticity—the ratio of stress to strain—is where sensor precision becomes crucial. For this, the load cell must deliver a highly synchronized force measurement precisely at the moment the displacement (strain) is detected. Any delay or non-linearity The output signal of the load cell leads to an incorrect module calculation and thus to distorted data about the stiffness of the material.

Material behavior and testing methods under stress and strain

The difference is in the Testing and measurement technology This is particularly important when evaluating different material classes. Brittle materials such as ceramics or high-carbon steels exhibit high stress at minimal strain before catastrophic failure occurs. Ductile materials such as aluminum or polymers show significant strain and often a necking phase, during which the material continues to deform even though the stress values ​​fluctuate.

Voltage-controlled testing

Voltage tests They assess the maximum strength of an object and help identify weaknesses or design flaws. In these scenarios, the testing system maintains a specific load or pressure, regardless of how much the material deforms. This is used in creep tests or Fatigue analyses This is common in applications where a component must withstand constant or changing internal pressure over long periods. The load cell serves as the primary control variable for the actuator and ensures that the force remains constant even if the material begins to fail or stretch.

Selecting load cells with verifiable calibration increases the reliability of the stress test results. Four Specifications The following must be considered before selecting a load cell for the voltage testing application:

• Mechanically: Dimensions and assembly
• Electrical: Output signal and power supply
• Environment: Temperature and humidity
• Power: Accuracy and thermal behavior

TIPP: Read our 101 series article Stress Testing 101.

Strain-controlled testing

Load cells that are used in Tensile tests These systems provide valuable data for material selection, design optimization, and ensuring product safety and performance across various industries. This requires a system that operates at a constant strain rate. It is the standard for tensile and compression testing to determine yield strength and tensile strength. By controlling the strain rate, laboratories can observe how internal stress evolves through the elastic range, the yield point, and into the plastic range, where permanent deformation occurs.

By measuring the strain under controlled loading conditions, engineers can determine:

• Stretch limit – the point at which the material becomes permanently deformed.
• Tensile strength – the maximum stress a material can withstand before it breaks.
• elastic limit – the stress limit above which the material no longer returns to its original shape after being relieved of stress.
• fatigue strength – how well the material tolerates repeated loading and unloading cycles.

TIPP: Read our 101 series article Strain Testing 101.

Application examples for stress and strain measurements

Stress measurement on composite fasteners in the aerospace industry – This application will contain a Through-hole load plate load cell It is used to monitor the stress applied to a composite joint during assembly. The goal is to achieve a specific preload (stress) without damaging the composite fibers. The load cell provides real-time force data, which is divided by the bearing surface of the bolt head to calculate the compressive stress. This ensures that the joint is strong enough to withstand vibrations but remains below the stress limit that would cause material delamination. Here is another A concrete example is an aircraft manufacturer that offers an interface solution for torque control. used it when tightening screws on his model aircraft to avoid material damage and excessive torque.

Strain measurement during elasticity tests on medical hoses – To determine the burst point and elasticity of medical silicone tubing, a MB Miniature Beam Load Cell Integrated into a motorized test rig with low capacity, the load cell measures the resistance while the test rig pulls the tubing at a constant speed, and an extensometer tracks the elongation. This data allows medical device testing laboratories and quality engineers to plot the strain—the percentage of the tubing stretched relative to its original length—against the force. This determines the maximum strain the tubing can withstand before losing its ability to return to its original shape—a critical safety factor in fluid delivery systems.

Considerations for the testing laboratory

Understanding the interplay between force (stress) and displacement (strain) enables precise calibration of the test limits. Engineers must ensure that the cell capacity is high enough to capture the peak stress of the material without entering the plastic deformation range of the sensor itself – and at the same time, the data acquisition system must have the resolution to capture the smallest strain increments that define the structural integrity of the material.

If you have any questions about the right sensor for your specific stress or strain testing application, please contact our experienced application engineers.