The Divergence in Force Measurement of Strain Versus Stress
In material science and structural testing, stress and strain are often discussed in tandem, yet they represent fundamentally different physical events. For engineers and test labs utilizing high-precision load cells and torque transducers, distinguishing between the two is critical for defining test parameters, selecting sensor capacities, and interpreting data from a material’s elastic and plastic regions.
Internal Force Contrasted with Geometric Deformation
At the most basic level, stress is an internal distribution of force. When an external load is applied to a solid body, internal resistive forces arise to counteract that load. Mathematically, it is the force applied per unit area. In a testing environment, the load cell measures the total force, while the stress is calculated from the specimen’s cross-sectional geometry.
Strain, conversely, is a dimensionless measure of deformation. It represents the displacement between particles in the material body relative to a reference length. Engineering 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 responds or moves. In this Interface Tech Talk, we explore the relationship and testing differences.
Physics of Measurement and Sensor Integration
The relationship between stress and strain underpins sensor selection. Stress measurement relies on the direct output of a load cell. The accuracy of stress depends on the sensor’s linearity and the precision of the specimen’s known dimensions. Load cells utilize internal strain gages bonded to a flexure. The sensor uses its own controlled internal strain to generate an electronic signal that represents the external stress on the test subject.
The relationship between stress and strain is governed by the constitutive equations of the material, most notably Young’s Modulus within the elastic limit.
Determining the material’s Young’s Modulus, the ratio of stress to strain, is where the sensor’s precision becomes paramount. To calculate this, the load cell must provide a highly synchronized force reading at the exact moment the displacement (strain) is recorded. Any lag or nonlinearity in the load cell output will result in an incorrect modulus calculation, leading to flawed data regarding the material’s stiffness.
Stress Versus Strain Material Behavior and Testing Modalities
The distinction becomes vital in test and measurement when evaluating different material classes. Brittle materials, such as ceramics or high-carbon steels, exhibit high stress with minimal strain before catastrophic failure. Ductile materials, such as aluminum or polymers, exhibit significant strain, often showing a necking phase in which the material continues to deform even as stress levels fluctuate.
Stress-Controlled Testing
Stress testing evaluates an object’s maximum strength, helping to pinpoint any weaknesses or flaws in its design. In these scenarios, the test system maintains a specific load or pressure, regardless of how much the material deforms. This is common in creep testing or fatigue analysis, where a component must withstand constant or cycling internal pressure over long durations. The load cell serves as the primary feedback loop for the actuator, ensuring the force remains constant even as the material begins to fail or stretch.
Choosing load cells with verifiable calibration reinforces the reliability of the stress test results. Four specifications must be addressed for the stress test application before choosing a load cell:
- Mechanical dimensions and mounting
- Electrical output and excitation
- Environmental temperature and moisture
- Performance accuracy and thermals
TIP: Review our 101 Series article Stress Testing 101.
Strain-Controlled Testing
Load cells used in strain testing offer instrumental data that aid in material selection, optimize designs, and ensure the safety and performance of products across multiple industries. This requires a system that moves at a constant rate of deformation. This is the standard for tensile and compression testing used to determine yield strength and ultimate tensile strength. By controlling the strain rate, labs can observe how the internal stress evolves through the elastic region, the yield point, and into the plastic region where permanent deformation occurs.
By measuring strain under controlled loading conditions, engineers can determine:
- Yield strength, which is the point at which the material permanently deforms.
- Ultimate tensile strength to classify the maximum stress a material can withstand before breaking.
- Elastic limit to determine the stress level beyond which the material will not return to its original shape after unloading.
- Fatigue resistance to know how well the material can withstand repeated loading and unloading cycles.
TIP: Review our 101 Series article Strain Testing 101.
Stress Versus Strain Application Notes
Measuring Stress in Aerospace Composite Fasteners – In this application, a through-hole load washer load cell is used to monitor the stress applied to a composite joint during assembly. The objective is to reach a specific pre-load (stress) without crushing the composite fibers. The load cell provides real-time force data, which is divided by the fastener head surface area to calculate compressive stress. This ensures the joint is tight enough to resist vibration but remains below the stress threshold that would cause material delamination. Here is another specific example of an airplane manufacturer that used an Interface solution to control torque when fastening screws on their aircraft models. They do not want to cause any damage to materials or apply excessive torque when fastening plane components together.
Measuring Strain in Medical Tubing Elasticity Tests – To determine the bursting point and elasticity of medical-grade silicone tubing, a low-capacity MB Miniature Beam Load Cell is integrated into a motorized test stand. As the stand pulls the tubing at a constant velocity, the load cell captures the resistance force while an extensometer tracks the elongation. The data allows medical device testing labs and quality engineers to plot strain, the percentage stretch relative to the original length, against force. This determines the maximum strain the tubing can handle before it loses its ability to return to its original shape, a critical factor for safety in fluid delivery systems.
Test Lab Considerations
Understanding the interplay between force (stress) and displacement (strain) enables proper calibration of testing limits. Engineers must ensure that the cell capacity is high enough to capture the peak stress of the material without entering the sensor’s own plastic deformation zone, while simultaneously ensuring the data acquisition system has the resolution to capture the minute increments of strain that define the material’s structural integrity.
If you have questions about the right sensor for your specific stress testing or strain testing application, please reach out to our experienced application engineers.
ADDITIONAL RESOURCES
Core Measurement Fundamentals of Force, Stress, and Strain
Airplane Static Stress Testing
Load Cells Built for Stress Testing
