Written by Grant Maloy Smith, the data acquisition expert
In this article you will learn how strain gages are used, with enough detail that you will:
- Understand what strain gages are at the top level
- Learn why engineers and technicians use strain gages
- See the industries and applications where strain gages are found
The measurement of strain (and stress) is vitally important for product design and public safety alike. There are countless strain gage measurements conducted across all major industries, including aerospace, automotive, manufacturing, civil engineering, and more. Strain gages can be found in unexpected places, even in our homes. In this article, we will outline the most common and important strain gage (aka “strain gauge”) applications.
Major Strain Gauge Applications:
- Industrial Strain Gage Applications
- Aerospace Strain Gage Applications
- Automotive Strain Gage Applications
- Rail Strain Gage Applications
- Maritime Strain Gage Applications
- Civil Engineering Strain Gage Applications
- Medical and Healthcare Strain Gage Applications
- Household Strain Gage Applications
What is a Strain Gage?
A strain gage is a sensor that measures strain. Lord Kelvin discovered centuries ago that a wire’s resistance changed when it was stretched or compressed. Today’s strain gage is simply an electrical conductor arranged in a zigzag pattern and mounted to a flexible foil backing material.
The gage is directly bonded (glued) to the object under test. When the object bends, it outputs a change in resistance proportional to the amount of strain. Each strain gage senses strain in only one direction, so multiple gages are often used, each facing a different direction in order to capture the bending in multiple directions.
Typical foil strain gage
The output of a strain gage is routed to a specialized signal conditioner that powers it, provides controls for “balancing” it before each test, and amplifies its small voltage output to one more suitable for digitizing. The DAQ system digitizes the conditioned signal, displays it on a screen, and records it to disk.
DAQ systems designed to handle strain gages often have at least eight channels, and sometimes into the hundreds of channels in large-scale testing applications. More about that later in this article.
We need to know how much and when a structure will bend so that we can ensure that it will work properly under real conditions. It’s a fundamental requirement of product testing and is crucial not just for product reliability, but for public safety. Strain gages are also found at the heart of load, force, and pressure transducers, such as load cells and force transducers.
The proper application of strain gauge, i.e. the physical gluing or bonding it to a surface, is critical to getting a good measurement. Whole books have been written about this subject.
Learn more about strain gages:
Industrial Strain Gage Applications
- Truck weighing systems on the highways
- Weighing systems in manufacturing environments
- Silos that empty their contents into trucks or train cars use strain gage based sensors on the silo itself that measure the weight of the contents
- Crushing machines use strain gages to detect stress spikes and shut down the machine if a jam has happened before the machine becomes damaged
- Items with identical weights such as screws, nails, et al, are often counted by their weight. They are loaded into a hopper which weighs them and divides the overall weight by the item’s per unit weight in order to calculate the quantity.
Rotating parts testing:
- Torque and power measurements on drive shafts of all kinds, in power plants, refineries, fans, turbines, and more.
Wind Turbine testing:
- Measuring the stresses on wind turbine blades, nacelles, and the towers themselves.
- Torque measurement testing on shafts and other rotating equipment
Critical components testing:
- Very small strain gages are used to measure stresses on printed circuit boards
- Residual stress testing on the cast and welded parts
- Strain gages are used to test critical valves in nuclear power plants
Factory systems testing:
- Conveyor belt and conveyor track system monitoring.
- Proper tensioning in paper mills is critical to paper production and minimizing downtime and production loss
- Printing presses use strain gage systems to ensure paper alignment
Aerospace Strain Gage Applications
Of course, airplanes must be made as safe as possible. Rigorous testing is required during development and certification. Aircraft are also tested at regular intervals to ensure that they are structurally sound.
People think first about the engines when it comes to flight safety, but there are numerous other components that must also perform flawlessly, day in and day out. These include control surfaces like flaps that allow the pilots to steer the airplane in three dimensions and actuators that control things like the landing gear.
Every load-bearing structure in an aircraft must be tested for stress and strain, including:
- Wings - much as tall buildings and bridges are meant to flex when subjected to external forces, aircraft wings are designed to bend as well. Strain gage testing ensures that wing flexure is within critical specifications.
- Fuselage stress and strain due to both mechanical forces and changes in outside and inside pressure
- Critical structures like engine mounts are tested to ensure against critical failures.
- Torque and power measurements on rotating/spinning components such as fans, turbines, and more.
- Testing actuators that move control surfaces like wing flaps, deploy landing gear, and more.
Automotive Strain Gage Applications
Every kind of vehicle undergoes extensive testing during development. This includes cars, trucks, buses, and motorcycles.
- Body panels are often instrumented with dozens of strain gages. This allows engineers to see how the panels move when subjected to a variety of road conditions at a variety of speeds and on different driving surfaces
- Suspension systems experience constant dynamic and static loading during vehicle operation and are tested to ensure that they are bearing these loads according to their design criteria
- Bumper and crumple zone testing during crash tests involve strain gages, in addition to accelerometers and high-speed cameras
- Torque and power measurements on the drive shaft
- Crash test dummies are equipped with strain gages and other sensors to measure the forces on vehicle occupants during a variety of standardized safety tests
- Strain gages are used in passenger seats to determine whether the seat is occupied or not. This determines whether the airbags at this seat should deploy in the event of an accident, or not. They can also be used to deploy the airbag at a reduced speed in the event that a child is in the seat as opposed to a heavier adult.
Rail Strain Gage Applications
Example test output using a Dewesoft DAQ system
It’s not just the train that must be checked, but the rail tracks themselves because they are subjected to heavy stress when heavy trains pass over them at high speeds. Tracks are also subject to stress and strain due to shifting earth below them, potentially caused by tectonic activity, frost heaves, sinkholes, the possible presence of mines below them.
The consequences of a train filled with passengers hitting a damaged or out-of-alignment section of track would be devastating. Train tracks are built in sections, with gaps called “expansion joints” deliberately left between them. These gaps allow for track expansion and contraction across a wide range of ambient temperatures. Heat and strain sensors are often mounted on these expansion joints and connected to a warning system that alerts train operators if a section of track has experienced an extreme condition.
Train Car Testing
Trains themselves are also the subject of strain and stress testing. Basically, any large mechanical structure requires a battery of testing to ensure that it is inherently safe, especially those involving public safety. This goes for both freight and commuter rail systems.
Everything from the train car body itself to the “truck” below, and moving parts like the pantograph that is raised to connect to the catenary power lines, or the shoe below that is extended to make contact with the third rail power system are subject to strain gage testing.
Maritime Strain Gage Applications
Whether it’s a small boat on a lake or an aircraft carrier with 5000 sailors on board, it has to be tested.
Submarine fuselage stress and strain testing - imagine the pressures that the hull is subjected to at increasing depths. Boat and ship hulls require testing as well to ensure that they can handle the stresses of crashing through waves and remain structurally sound.
Propellers are also tested, as well as stress, strain, and torque measurements on drive shafts and other rotating parts in the propulsion system.
- Ship hulls and load-bearing structures must be tested to ensure proper rigidity and ability to handle required stresses
- Torque and power measurements on the driveshaft and propellers
Civil Engineering Strain Gage Applications
Most civil engineered structures are designed to last 100 years. But this requires regular testing since most failures happen in the later years of a structure’s lifespan. Today, many structures are being built with strain gage and other sensors in them right from the beginning.
Strain gages can be applied on the exterior of concrete walls and footings. But they can also be embedded inside these same structures, and the wires are brought out through special tubes. In this case, strains inside the concrete can be monitored in real-time by connecting a DAQ system outside.
Strain, stress, load, and force measurements are commonly made on critical load-bearing structures, including:
- Buildings, especially public buildings, hospitals, skyscrapers, etc.
- Roadways, especially high traffic ones, highways, etc.
- Bridges - steel beams, concrete footings, cables, gusset plates and trusses must be tested at regular intervals. Dynamic load testing is performed with vehicles using the bridge. Moving elements (such as drawbridges) are tested for stress and strain during raising and lowering operations.
- Overpasses and walkways
- Dams - imagine the enormous pressures that they are designed to withstand, essentially forever. Imagine also the terrible consequences if a dam were to fail.
- Oil rigs at sea, including the sea bed on which they are sitting
- Airport taxiways
- Rock fall tests - steel nets are used along mountain roadways to protect against rocks falling down, threatening drivers, and the road itself. Strain gages are used to test these steel nets for their load-bearing capabilities.
- Residual stress testing on structures and materials (see below for more details)
What is Residual Stress?
Grinding is one possible cause of residual stress
It may seem counterintuitive, but structures and materials can have stress within them, even without any external forces being applied. This is known as “residual stress.”
What causes this? Residual stress is the byproduct of one or more manufacturing processes, including welding, machining, heat treatments, grinding, and more. Testing a structure or material’s residual stress is the best way to ensure that it can withstand the loads that it will experience during its operational life.
Residual stress testing is performed on a huge variety of structures and materials, from crankshafts to bridge eyebars and cables. There are optical and X-ray diffraction methods of testing for residual stress, but the most popular method remains to drill a hole in the material and apply strain gages on the surrounding surface.
Medical and Healthcare Strain Gage Applications
There are numerous strain gages applications in the medical and healthcare industry. For example:
- They are often used in medical instruments like kidney dialysis machines and syringe pumps to help monitor fluid flow rates.
- Strain gages are also used in patient weighing and patient lift systems.
- Wireless strain gages can be found in CT scanners and mammography machines.
- Patient positioning systems used during radiation treatments
- In physical therapy applications, strain gage based force sensors are used to measure forces on joints (shoulders, hips, knees, et al).
- Force feedback crutches aid patients undergoing orthopedic therapy by detecting the amount of weight is being borne by the crutch
Household Strain Gage Applications
Robot vacuum cleaners use strain gages as a sensor
You may not realize it, but strain gages are all around your house. Your bathroom scale has several of them inside, configured as load cells. When you step on the scale, you apply strain to the scale, which converts it to a measurement of your weight in kg or lbs.
Gaming controllers also use strain gage based force transducers, as do some home appliances such as refrigerators. The trend toward more and more household appliances being “connected” is driving more and more sensors into them. Robot vacuum cleaners are another appliance where strain gages are commonly used.
You can see that strain gages are everywhere around us. They’re not just abstract tools used by scientists or engineers but are in our homes, appliances, phones, games, cars, where we work, and just about everywhere else. The strain that they measure, with its attendant quantities like stress, weight, load, and force is critically important to measure for countless applications, in every major industry, everywhere in the world.