THINK ABOUT THE FINISH LINE, ACT AS IN THE STARTING LINE

If you have a sensor installation coming up – be it your first strain gage bonding or your 100^th installation – you might want to follow the same good advice that our experts at Micro-Measurements offer to stress analysts. Or, you can decide to make you own specific bucket list as you get closer to your B(Bonding)-Day.

The question of what our measurement goals are is not always an easy one to answer. But it’s important to figure out because different goals (tension, compression, torque, bending, pressure etc.) can lead to very different Data Acquisition results. Hey, I wish someone had told me that the end of a measurement is a mathematical equation.

Drawing from the old saying that a balanced stress analyst needs three positive parts – body, mind and spirit – we divided the test and measurement process into three parts:

Part One:

Take your mark and create a (check) list of small steps or little goals – We always ask our stress analysts to set a number of goals. Using a list of goals can seem tedious and boring, but the practice can save your test by improving efficiency and minimizing mistakes. This includes selecting the right strain gage for the application in terms of type and environmental compatibility. Stop making choices based only on price — cost of ownership is more important, as is setting a series of goals based on how you feel about the location during the first hour of the installation.

By dividing your measurement journey into logical steps, you can avoid FOMO (fear of missing out a step) and you can ensure that everything starts with your first step, applying a sound technical understanding to the problem will simplify the use of any sensor such as a strain gage.

Step 1: Gear Check = a bag of items that you’d like available at any phase of the strain gage installation. Usually placed in a designated bag or toolbox that accompanies you from the starting line to the finish line.

Step 2: Define the test conditions: Don't Show Up at the strain gage installation site Last Minute

Step 3: Ensure appropriate surface preparation materials are on hand: Don't Skip Your Warm-Up.

Step 4: Select the right strain gage sensor.

Step 5: Select the right adhesive

Step 6: Select a protective coating

Step 7: Select the measurement instrumentation: Finish Strong

Part Two:

Go through bonding scenarios – Use imagery visualization to go through what could go wrong, and start building what we call a stress analyst toolbox of responses. What will you do if you get resistance to ground problems? What if your sensors fall off pace early or are “popping off” during the measurement?

What if there is a poor return to zero after loading? What will happen when I apply strain levels that are beyond the capability of the strain gage or bonding adhesive, which causes unintended failure? When those situations arise, have a few responses ready. Stop briefly to observe, practicing positive approach, take a sip of water, repeat a mantra – whatever tools you find helpful. If you’ve worked through scenarios before you start a test, tackling them head-on becomes more manageable. We believe that a successful strain gage measurement can almost always be accomplished when we engage with the environment and use a common sense approach.

Ten incredibly common measurement scenarios:

I forgot to remove soldering flux (resistance drift will occur).
I skipped a few steps in the bonding adhesive instructions (gage un-bonding can occur).
I made the wrong wire connections to the instrument or half/full bridge strain gage (cannot balance).
I used the incorrect gage factor value (measurement result is incorrect).
I chose the wrong shunt calibration resistor value (measurement result is incorrect).
I applied excessive bridge excitation (causes drift).
I applied strain levels that produce yield in the structure (produces a zero shift in the strain gages).
I applied strain levels that are beyond the capability of the strain gage or bonding adhesive (causes a zero shift in the strain gages).
I applied strain gages to bolted assemblies (bolted joints typically slip during loading, resulting in the strain gage’s poor return to zero after loading).
I’m not using the best strain gage protective coating (corrosive attack on the gage foil causes gage resistance drift over time).

Part Three:

Think about the finish line and your technical support team – We encourage stress analysts to ask themselves, “What would you do if you do really well with your strain gage installation, and what would you do if you don’t do really well?” If you are someone who may need to measure a tension (pronounced attention) before sharing your progress with your colleagues or Boss, that’s great too! Think about the people you want at that finish line when you analyze the data from the DAQ, or at the other end of the smart phone, or laptop to support you in either scenario. One of our recommendations once you already see the finish line is to check the strain gage electrically using our Model 1300 Gage Installation Tester a Model P3 Strain Indicator / Recorder, one of our Signal Conditioning Amplifiers or StrainSmart data acquisition systems to ensure the installed gages meet the required insulation resistance to ground, zero stability and zero return characteristics prior to taking any test data.

Five (5) signs that you are at the finish line:

The strain gage readings are stable from power-up and with no load (force) applied.
The strain gage circuits have nearly infinite resistance to ground (>10k MΩ).
Under elastic loading conditions, the strain gage returns to zero when unloaded.
Shunt calibration resistors have been used to verify that strain sensor gage factor settings are correct and that lead-wire desensitization has been accounted for.
The strain gage has been properly protected from the weather (environmental protection)

And last but not least did you remember to measure your “Pace” (= a measurement of speed of strain gage installation, usually measured as how many minutes it takes you to bond one strain gage).