Posts

How Does Tensile Testing Work?

Tensile testing, also known as tension testing, is a type of mechanical test used to determine how a material responds to a stretching force. This test helps evaluate the mechanical properties of an object, such as metals, polymers, composites, and various other materials.

Performing a tensile test applies a load to specimen, and gradually increasing the load sometimes until failure or destruction. The tensile data is analyzed by using a stress-strain curve.

Interface stain gage load cells are commonly used in tensile testing due to their high precision and sensitivity. They work by measuring the strain in a material, which is directly related to the applied force. This strain data is then converted into force measurements. Learn more in Tension Load Cells 101.

Tensile testing is fundamental in test and measurement. It is used by researchers, testing labs, and engineers across industries including infrastructure, medical, manufacturing, aerospace, consumer goods, automotive, energy, and construction.

How Tensile Testing Works

Tensile testing is essential in materials science and engineering to understand the material’s behavior under tension and to ensure its suitability for specific applications.

First, a specimen of the material is prepared with a specific shape and dimensions. This sample is carefully controlled to meet testing standards based on the test plan.

Interface supplies a variety of load cells for these tests. The load cell is typically mounted in a tensile testing machine. The tensile test machine has two separate jaws, one of which will move away from the other at a controlled rate during the test. As it moves away, it is pulling on the material, stretching it until it the test is complete, or it breaks. This is also referred to as testing to failure or destruction. The controlled rate is called the strain rate, and materials will behave differently under different strain rates.

The specimen is then securely mounted in a testing machine, which is usually called a tensile testing machine or universal testing machine. The load cell is positioned in such a way that it bears the load applied to the specimen during the test.

Load cells are commonly used in tensile testing to measure and record the force or load applied to a specimen during the test. These sensor devices are crucial for accurately determining the mechanical properties of materials under tension.

The testing machine applies a pulling force (tensile force) to the specimen along its longitudinal axis. The force is gradually increased at a constant rate, causing the specimen to elongate.

As the tensile testing machine applies a pulling force to the specimen, the load cell measures the force in real-time. This force measurement is typically displayed on a digital instrumentation device or recorded by a data acquisition system.

The recorded data, including the applied force and the corresponding elongation or deformation of the specimen is usually plotted on a stress-strain curve for analysis. The stress-strain curve provides valuable information about the material’s mechanical properties, including its ultimate tensile strength, yield strength, Young’s modulus, and elongation at break.

Engineering Checklist for Tensile Test Plans

  • Identify the Purpose of the Tensile Test
  • Select the Material and Test Standard
  • Define the Mechanical Properties
  • Determine the Specific Mechanical Properties for Evaluation
    • Common properties include tensile strength, yield strength, modulus of elasticity (Young’s modulus), elongation, reduction in area, stress-strain curve characteristics
  • Establish Test Conditions
    • Include temperature, strain rate and testing environment
  • Define Sample and Specimen Requirements
  • Determine Measurement Accuracy Requirements
  • Prepare Instrumentation and Equipment
  • Plan for Data Recording and Reporting
  • Review Compliance Requirements and Safety Standards
  • Document Test Plan
  • Publish Verification and Validation Processes
  • Report Results

Defining measurement requirements for tensile tests by specifications is a crucial step in ensuring that the tests accurately and reliably assess the mechanical properties of materials.

Tensile Testing Terms to Know

Stress: Stress is the force applied per unit cross-sectional area of the specimen and is usually denoted in units of pressure. Stress is calculated by dividing the measured force by the cross-sectional area of the specimen. The load cell’s force measurement ensures that the stress values are accurate and precise. Simply, stress is the amount of force applied over a cross-cross-section.

Strain: Strain represents the relative deformation of the material and is the change in length (elongation) divided by the original length of the specimen. Strain is the amount of elongation in a sample as it is stretched or squashed.

Elastic Region: In the stress-strain curve, the initial linear region where stress is directly proportional to strain is known as the elastic region. Here, the material returns to its original shape when the load is removed.  As soon as a material is placed under any load at all, it deforms. Visually, the deformation may not be noticeable, but right away, the material is deforming. There are two types of deformation: elastic (not permanent) and plastic (permanent).

Yield Point: The yield point is the stress at which the material begins to exhibit permanent deformation without an increase in load. It marks the transition from elastic to plastic deformation.

Ultimate Tensile Strength (UTS): UTS is the maximum stress the material can withstand before breaking. It is the highest point on the stress-strain curve. If the material is loaded to its UTS, it will never return to its original shape, but it can be useful in engineering calculations, as it shows the maximum, one-time stress a material can withstand.  Load cells can detect the exact moment of specimen failure, such as fracture or breakage. This information is crucial for determining the ultimate tensile strength and other mechanical properties of the material.

Elongation at Break: Elongation at break is the amount the specimen stretches before it breaks, expressed as a percentage of the original length.

Load cells can also be used for real-time monitoring and control during the test. Test operators can set specific load or strain rate parameters to control the testing machine’s operation and ensure the test is conducted within specified conditions.

Load cells play a safety role by providing feedback to the testing machine’s control system. If the load exceeds a certain threshold or if the load cell detects an anomaly, the testing machine can be programmed to stop or take corrective actions to prevent damage to the equipment or ensure operator safety.

To discuss Interface products and experience in tensile testing, be sure to reach out to our global representatives in the field or contact us. We are always here to help!

How Load Cells Can Go Bad

Load cells are electronic devices that measure the force applied to them. Interface products are made to last, in fact we have many load cells that are in-market and being used for high-accuracy testing that were manufactured decades ago. Why do they last? Quality of design, material construction, build process, calibration, and regular maintenance prolong the life of a load cell.

Like any electronic device, load cells can go bad for a few reasons. It is also important to know that load cells can be repaired. Outside of complete destructive testing, the following issues are most common for how load cell can go bad.

Overloading: Load cells have a maximum capacity, and if they are subjected to a force beyond that limit, they can get damaged. Overloading can cause the load cell to deform or break, resulting in inaccurate readings or complete failure. Preventative options are to use overload protected load cells.

Mechanical and physical damage: Load cells are sensitive devices and can be damaged by impact, vibration, or shock. Mechanical damage can cause the load cell to deform or lose its calibration, resulting in inaccurate readings. Physical damage to devices is often because the load cells are dropped or mishandled during use.

Moisture: Load cells are often used in damp or wet environments, and prolonged exposure to moisture can cause corrosion or damage to the internal circuitry. Environmental exposure to moisture can also cause electrical shorts or create a conductive path between the components, resulting in inaccurate readings or complete failure. Review submersible options if testing in these environments is common.

Temperature: Load cells can be sensitive to temperature changes, and extreme temperatures can cause damage to the internal components. Thermal expansion or contraction can cause mechanical stress, resulting in deformation or damage to the load cell. Interface offers high-temperature and low-temperature load cells options.

Electrical noise: Load cells are susceptible to electrical noise, which can cause interference in the signals and result in inaccurate readings. Electrical noise can be caused by electromagnetic interference (EMI), radio-frequency interference (RFI), or other sources of electrical interference.

Aging: Not all load cells are made the same way. Interface load cells are designed to outlast any testing use for long-periods, we are talking millions of cycles. However, some load cells can wear out over time due to repeated use, exposure to the environment, or other factors. Aging can cause a decrease in sensitivity, accuracy, or stability, resulting in inaccurate readings or complete failure. All load cells need good health checks to stay working at optimal performance.

To avoid load cell failures, it is important to use them within their rated capacity, protect them from mechanical damage, and provide adequate protection from moisture, temperature, and electrical noise. Regular maintenance and calibration services, preferably every year, can also help ensure accurate and reliable performance over time.

What is the best way to determine if a load cell is bad or not working?

There are several ways to determine if a load cell is bad or not working. Here is a reminder of five quick checks:

#1 Visual Inspection: Start by visually inspecting the load cell for any signs of physical damage, such as cracks, deformations, or loose connections. Check for any corrosion or signs of moisture, as well as any visible wear and tear.

#2 Zero Balance Testing: A zero balance test is a quick and straightforward way to check if a load cell is functioning properly. With no weight applied, the load cell should read zero. If it does not, there may be an issue with the load cell or its connections.

#3 Load Testing: Load testing involves applying a known weight to the load cell and checking the reading. If the load cell is accurate, the reading should match the known weight. If there is a significant discrepancy, the load cell may be faulty.

#4 Bridge Resistance Tests: Load cells are typically constructed with a Wheatstone bridge circuit, which can be assessed for proper resistance values. If there is a significant deviation from the expected resistance values, there may be an issue with the load cell or its connections.

#5 Temperature Tests: Load cells can be sensitive to temperature changes, and extreme temperatures can cause damage to the internal components. Evaluating the load cell at different temperatures can help to identify any issues with temperature sensitivity.

Interface provides complete evaluations of any product we manufacture, to determine if the load cell is working properly. To request services, go here.

How does calibration help load cells from going bad?

Calibration is the process of adjusting a load cell to ensure its accuracy and reliability in measuring weight or force. Regular calibration is essential for maintaining the accuracy and reliability of load cells. Interface recommends annual calibration services as a preventative measure and for good maintenance of your force measurement devices.

Calibration helps to ensure that a load cell provides accurate and consistent readings. Over time, load cells can drift from their initial calibration due to environmental factors, wear and tear, and other factors. Regular calibration ensures that any deviations from the standard are detected and corrected, preventing inaccurate readings that can lead to errors in weighing and other measurements.

Load cells that are not calibrated regularly may experience premature wear and tear due to repeated use, leading to damage or failure. Calibration helps to identify any issues early on and prevent further damage, extending the lifespan of the load cell and saving on replacement costs.

Many industries and applications have strict standards and regulations for measuring weight and force. Regular calibration helps to ensure that load cells meet these standards and regulations.

Regular calibration can help load cells from going bad in multiple ways. It can help to prevent inaccurate readings, extend the lifespan of load cells, improve efficiency, and ensure compliance with standards and regulations. Accurate measurements are critical, and calibration helps to ensure that load cells is working properly. Request a repair or calibration service online.

ADDITIONAL SERVICES

Load Cell 101 and What You Need to Know

Load Cell Sensitivity 101

Can Load Cells Be Repaired?

Services & Repair

Mechanical Installation Load Cell Troubleshooting 101

How Do Load Cells Work?

Regular Calibration Service Maintains Load Cell Accuracy