Strain, An Important Aspect of Durability Analysis
For structural durability engineers, handling strain data may become a daily routine. Deploying strain gauge sensors, collecting strain data, then running durability analysis are common activities performed to evaluate the durability of a product during prototype testing stage. This article will not cover the basic fundamental of stress and strain. This article will share more on the practical aspect of strain gauge sensor, data acquisition strategy, and data post-processing (durability analysis).
Strain gauge can be briefly described as sensor that can measure the strain value (unitless) of structure that is being subjected to specific loading. Strain gauge works based on change of sensor element resistance (resistive based sensor). By using Wheatstone bridge, this resistance change will be converted to voltage and read by data acquisition system.
Based on the measurement axis, the strain gauge sensor can be classified as per below:
1. Uniaxial Strain gauge
This type of strain gauge may become the simplest type available in the market. The size is relatively small, and it is relatively easier to deploy. But in order to have accurate results, we may need to understand the direction of maximum principal strain at a specific point in our product structure, before deploying this type of strain gauge. In some cases, having a Finite Element Analysis result may provide us with better guidance.
2. Biaxial Strain gauge
This type of strain gauge consists of two strain gauges that are perpendicularly aligned to each other. Since there are two strain gauges involved, there will be two channels at our data acquisition system to be occupied. Similar with Uniaxial type, we may need the direction of maximum or minimum principal strain at specific point in our product structure, before deploying this type of strain gauge. The difference is that by having this strain gauge, we will get both Maximum and Minimum Principal Strain values simultaneously. One of disadvantage of using this type of sensor, the size is relatively larger than Uniaxial type. In some cases, it may not be suitable for application where we want to measure strain at small area where the stress concentration is very high (example: corner radius area).
3. Strain Rosette
This type of strain gauge consists of three strain gauges that are aligned to each other at specific angle. 0-45-90 degrees and 0-60-120 degrees are the most common configurations available in the market. Since there are three strain gauges involved, there will be three channels at our data acquisition system to be occupied. By having this type of strain gauge, we will not need to know the direction of maximum or minimum principal strain at specific point in our product structure prior to data collection. After collecting data, we can post-process the data from those three channels and calculate the maximum principal strain, minimum principal strain, and maximum shear strain, based on Mohr Circle theory at specific point where we mount the strain gauges. We can also calculate other value such Von Mises Strain, etc. Similar with Biaxial type, due to its relative larger size compared to other type, makes it not suitable for some applications.
Other than above classifications, some manufacturers produce specific types of strain gauge for specific applications:
Strain gauge for shear strain measurement.
Strain gauge for low/high temperature application.
Strain gauge for measuring strain at different material (example: polymer, composite, concrete, etc).
Strain gauges are also produced in different size and also different resistance value for different applications. The most common resistance values available are 120 ohms and 350 ohms. Some manufactures also produce pre-wired strain gauge, which requires less effort and time to deploy.
Based on implementation strategy, there are three strain gauge configurations that we can use:
1. Quarter Bridge
This configuration requires only one single strain gauge to produce one single strain data. The other three resistances will be provided by data acquisition signal conditioner in order to have complete Wheatstone bridge. This configuration is considered relatively easier to deploy, and also more cost efficient since we just need to spend one strain gauge to produce one data. We may need to remember that strain gauge is typically considered as non-reusable sensor. Therefore, the type of selection and configuration strategy cam be important to make the data acquisition campaign to be efficient. The disadvantage of using this strain gauge comes from its tendency to be more vulnerable to ambient temperature changes compare to other configurations. Some manufacturer tries to counter this issue by producing strain gage with three output wires, instead of only two output wires. Other than that, quarter bridge configuration also has the lowest sensitivity compared to other configurations.
2. Half Bridge
This configuration requires two strain gauges to produce one single strain data. The other two resistances will be provided by data acquisition signal conditioner in order to have complete Wheatstone bridge. Compared to quarter bridge, this configuration requires more time and effort to deploy. But it has higher sensitivity and less vulnerable to ambient temperature changes. This configuration can be used to differentiate axial and bending stress at specific application.
3. Full Bridge
This configuration requires four strain gauges to produce one single strain data. Resistance from all four strain gauges will be used to form a complete Wheatstone bridge. Compared to others, this configuration requires the biggest effort to deploy. But it has highest sensitivity and less vulnerable to ambient temperature changes. This configuration can also be used to differentiate axial and bending stress at specific application. This configuration is typically used at strain gauge-based transducer, such as Strain Based Load Cell, Strain Based Accelerometer, etc.
Why do we need to measure Strain?
Strain can be briefly described as structural deformation at microscopic scale. Together with Stress value, Strain can be used to quantify the energy absorbed by the structure due to specific loading. By using Stress and Strain, we can also determine if some areas in our structure have reached plastic region or not.
Stress and strain are currently also playing important role in durability analysis. By using both data, we can quantify the fatigue damage have been subjected to certain area of structure. We may be able to predict if our structure is in infinite life region or not. We may be able to predict if our structure may fail after certain cycle of loading or not.
Do you need to run durability analysis? Do you have issue during product durability testing? With our expertise and experience in durability and vibration we are able to guide your team step by step, developing the systematic approach that applicable for most of the cases that you encounter.