1. STRAIN GAGE CHARACTERISTICS
11/20/2018
Technical Committee on Strain Gages Tutorial
SEM Spring Conference 2011
STRAIN GAGE CHARACTERISTICS
Becky Showalter, Development Engineer
Micro-Measurements, Raleigh, NC
Welcome to the Strain Gage Tutorial offered by the Technical Committee on Strain Gages.
Technical Committee on Strain Gages Tutorial, SEM Spring Conference 2011
11/20/2018

2. What is a Strain Gage? Resistive grid
                    
Resistive grid
Intimately bonded to surface for strain transfer
Strain cause resistance change
Let’s tackle the obvious question first. What is a strain gage? As I’m sure that most of you are aware, a strain gage is a resistor that when intimately bonded to the surface of a substrate changes resistance in response to strain.
The most common strain gage type uses thin foil laminated to an insulating backing.
Gages can come is many different geometries and constructions – often specifically produced for certain test conditions or strain fields. A uniaxial gage is shown here.
11/20/2018
Technical Committee on Strain Gages Tutorial, SEM Spring Conference 2011

3. Technical Committee on Strain Gages Tutorial, SEM Spring Conference
11/20/2018
Strain Gage System Basics - Construction
So let’s talk first about how the gage itself is made. (1) We start with an insulating layer like polyimideor epoxy-phenolic. (2) A layer of epoxy is then applied to the top surface of the insulating layer. (3) A piece of ultra-thin foil is laminated to the insulating layer with temperature and pressure. (4) Using photolithography, photoresist images are produced on the foil surface. (5) The surface is then wet etched to remove all foil that is not part of the gage geometry. (6) The plate is then developed to remove the remaining photoresist and you are left with an array of the appropriate geometry.
The plate is now ready for secondary operations. These can include (but are not limited to) being adjusted to the correct resistance within a tighter tolerance, having encapsulation or wires added, and being cut.
Before we leave this slide, I do want to point out that every part of this process will effect how this gage behaves and the results that will be obtained from it. Backing decisions affect the stiffness of the gage, the laminating adhesive will affect temperature range and flexibility, and each foil type has its own specific set of thermal, resistive and mechanical characteristics. Due to that, when purchasing strain gages, it is important to understand the properties of each construction and what type of test conditions are most appropriate for each.
11/20/2018
Technical Committee on Strain Gages Tutorial, SEM Spring Conference

4. Technical Committee on Strain Gages Tutorial, SEM Spring Conference
11/20/2018
Strain Gage System Basics - Geometry
And as if that wasn’t enough, geometry will also play a big part in how a gage responds.
First, let’s go over the terminology that goes along with gage geometry. In this case we’re looking at a linear gage that will be measuring strain in the directions of the grid lines – shown here. The gage length and the grid width define the main sensing area of the gage. The solder tabs provide a larger foil surface to attach wires or cable.
One thing to be aware of is that strain gages are averaging devices and will average the strain in the area covered by the grid. That is important to know especially if you are expecting a short spike in your strain field that might be lost if averaged with lower strain areas, or if you are trying to take data on a non-uniform substrate and WANT to average the strain over some distance (as can be the case with certain composites).
Line and space sizes as well as solder tab designs can affect fatigue life, and even the endloops (or turn-arounds) can affect certain measurements. All this is said, not to frighten the novice user, but to stress that the strain gage system is a complex system and some thought needs to be given to both construction and geometry when beginning a test.
11/20/2018
Technical Committee on Strain Gages Tutorial, SEM Spring Conference

5. Technical Committee on Strain Gages Tutorial, SEM Spring Conference
11/20/2018
Strain Gage System Basics – Gage, Installation and Protection
Once the gage geometry and construction are set, it’s time to adhere the gage to the substrate. In this slide, I’ve broken out all the different layers that make up the strain gage system up to this point. You have the overlay, the adhesive holding the overlay onto the gage, the metal foil resistor, the adhesive holding the foil to the insulating layer, the insulating layer itself, the adhesive holding the gage to the specimen and finally the specimen itself.
Notice that the measurement layer (the foil) is not directly on the substrates surface – poor adhesive or a poor gage can make strain transfer through these two layers difficult. Also notice that there are 3 adhesive layers – that’s a lot of glue, each with its own thermal and mechanical properties that can and do affect the behavior of the gage.
Once the gage is bonded, solder is applied to the tabs of the gage to electrically connect the gage to the wire or cable, and then environmental protection is often added. Even when it is a lab test, gages are often protected with RTV or polyurethane. All these things together can make the system relatively stiff, and reinforcement of your specimen can be an issue. If the gage is thick enough to reinforce you specimen, then the strain reading you get will be lower than that of the actual part without a strain gage on it.
11/20/2018
Technical Committee on Strain Gages Tutorial, SEM Spring Conference