Static vs. Dynamic Strain Gage Testing
Moving Measurement Efficiency from Controlled Environments to the Field
Micro-Measurements efficient and highly-sensitive strain gages measure the effect of external forces on an object. These measurements, essential in a variety of industry applications, from civil engineering to aerospace—can be taken in both controlled laboratory environments, and in less predictable field environments.
It should be noted that strain gages are capable of incredibly high stability for long-term monitoring of structural movements and creep, as well as very high-speed transients in excess of 500kHz. The response of well-specified and installed gauges are limited only by the chosen instrumentation, so it’s essential to choose the correct instrument.
In this blog, we’ll look at the factors that determine how to approach strain gauge testing in static vs dynamic situations.
WHAT IS STATIC STRAIN GAUGE TESTING?
In a static test, the operator is able to read the value displayed on their instrument and then write that value down without it changing. In this case, the operator is controlling the environment. If there’s relative motion in either the test article or the loading sequence acting upon it, the environment becomes dynamic. But if the engineer loads the structure, takes a reading, increases the load, and then repeats, they’re conducting a repeated static test.
In general terms, static tests are often conducted at the component level in incremental steps to determine structural integrity, or to validate Finite Element Analysis (FEA) models. This allows engineers to predict how an object will react to heat, vibration, and other real-world elements.
In static testing, it’s common to take the specimen to its load limits in order to correctly determine a safety factor. Static testing is considered safer to conduct when product performance is unknown. Therefore, static testing can be used for preliminary testing when trying to determine whether an object will wear out or work as designed when presented with varying degrees of strain.
Ideal instruments for this type of testing are our Model P3 and D4. The P3 is a stand-alone battery-powered unit with a membrane switch panel, LCD, and lever clamp connectors for fast connection. The D4 is a small box requiring USB connection to a PC for power and control, with 4 channels using RJ45 input connectors. Both units can be connected and communicated with through a free software app. They offer excellent stability and flexibility where the test parameters are static or change slowly.
WHAT IS DYNAMIC STRAIN GAGE TESTING?
In a dynamic test, the load or test specimen is moving or being acted upon in its intended environment. Dynamic tests are usually functional, fatigue, or life cycle tests of an entire assembly under anticipated operating conditions.
Because dynamic tests are often conducted outside of the laboratory environment in real-world conditions, the preparation for acquiring precise strain gauge measurements will be more labor-intensive than in a more easily controlled, static test.
Micro-Measurements offers a range of instruments for faster-moving signals, from cyclic testing in automotive, aerospace and civil engineering all the way up to transient recording for high-energy tests such as impact testing and ballistics. Sample rates of our digital systems are from 1000 samples/second up to 10M samples/second and include StrainSmart data systems and Pacific Instruments Series 6000 and 5800. For analogue systems we have our 2000-series amplifiers such as the 2310, a widely-used amplifier for energetics, and PI Series 6100 digitally-controlled amplifiers.
STATIC VS DYNAMIC STRAIN GAUGE TESTING
In most situations, the type of testing doesn’t influence the choice of strain gage or adhesive. However, whether a strain gauge test is dynamic or static will affect the type of environmental protection chosen for the strain gauge. It will also affect the method of lead wire attachment and strain relief. Additionally, it will have a significant impact on hardware requirements. For example, in dynamic testing situations, it may be necessary to use the Micro-Measurements weldable strain gauges to withstand real-world elements like wind and rain.
While strain indicators and data acquisition systems can easily capture the signal from a static test, objects in motion are harder to evaluate. Dynamic tests require the consideration of several more factors, from the type of environmental protection used for the gauge to the scanning rate and number of active channels.
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