A S.M.A.R.T way to throw spaghetti against the wall.

I’m sure you’ve heard the saying, “throw spaghetti against the wall and see what sticks.” Or as the stress analyst will rephrase “throw a strain gage against the structure and see what sticks”. Indeed, it’s a common description of many strain measurement strategies, to identify what bonds (sticks) and what doesn’t work (peeled). It’s a good analogy to visualize the process of testing. And, if you have a baby at home, then I’m sure you will get a live demonstration of the spaghetti test sooner or later!

However, there’s one problem with this approach. What happens if you try to throw (or bond) a strain gage against the wall, using the wrong adhesive, that you just bought in the local hardware store? None of it will stick! You’ll just end up with the opposite phenomena of a real en-gage-ment. In other words, if you’re trying to test multiple unapproved adhesives at the same time to see what “sticks,” then you’re going to be disappointed by the results.

So what does a fully “cooked” strain gage system tactic look like? Let’s take a look. (Note, when we say a cooked strain gage, we refer to the complete system of the strain gage element, adhesive and the surface that you want it to stick to). Before any stress analyst launches a strain measurement campaign, he or she also needs to have a clear description of what success looks like at the end. In other words, what is the goal? You could say that the whole stress analyst endeavor is geared towards setting and achieving goals.

Questions you may ask yourself when setting your stress/strain measurement goals and objectives:

What exactly do I want to achieve by bonding a strain gage?
Where should I position the strain gage?
How well it will stick to the surface?
When should I apply pressure to cure it?
What are the conditions and limitations of the adhesive?
What are possible alternative adhesives of achieving the same goal?
How will you know the data meets expectations?

But we can’t just set a broad target; it needs to be a S.M.A.R.T. goal. To make your goal S.M.A.R.T., it needs to obey to the following criteria: Specific, Measurable, Attainable, Relevant and Timely.

The establishment of all your strain or stress objectives should be created using the S.M.A.R.T. philosophy.

Specific: Be specific about the environment and the specimen that you want to bond the gage on and "what is to be done?"

Measurable: Ensure to use the correct strain gage and adhesive for high-elongation strain measurements, it means that you identify exactly what it is you expect to see when you use a strain indicator or a DAQ. You'll need concrete evidence about your measurement.

Attainable: Aim high to the yield point; ask yourself can I do it? But don’t be unrealistic to cross to the plastic zone. Use research and historical data to project the strain you want to measure. Can the measurement be done giving the time frame, adhesive and resources?

Relevant: Make sure your adhesive will transfer an accurate strain, what will be the impact of using a different adhesive?

Time-related: Remember, Time is money! When is the deadline to hit your strain measurement? How much time you need to cure it?

In stress analysis, SMART goal setting is one of the most effective and yet least used tools for achieving your measurements goals. If you're lacking certain skills in using a strain gage, you can get training. If you lack certain resources or materials, you can look for ways of getting them. Training Programs: http://www.vishaypg.com/micro-measurements/training-programs/ / StrainTalks™: https://straintalks.com/

Since different adhesives are intended for different types of applications and different environmental conditions, it is obviously important to select the most appropriate adhesive for each strain measurement task.

M-BOND 200: Most widely used general-purpose adhesive. Easiest to handle. Fast room-temperature curing.
M-BOND AE-10: General-purpose adhesive highly resistant to moisture and most chemicals. Room-temperature curing.
M-BOND GA-2: Special-purpose adhesive primarily used on very rough and irregular surfaces. Room-temperature curing.
RTC-2 EPOXY: General-purpose, room-temperature curing adhesive for lab and field applications with high-elongation strain gages. Also excellent for strain measurement at cryogenic temperatures.
M-BOND A-12: Special-purpose, very high-elongation adhesive. Used only when other adhesives cannot meet elongation requirements. Elevated-temperature curing.
M-BOND AE-15: Similar to AE-10. Recommended for more critical applications, including transducer gaging. Moderately elevated-temperature curing.
M-BOND GA-61: Special-purpose adhesive with a higher operating temperature range than GA-2, and more viscous. Also used to fill irregular surfaces and to anchor leadwires. Elevated-temperature curing.
EPOXYLITE 813: Used for long term, high temperature applications requiring a filled glueline. Wider temperature range than GA-61.
EPY-500: Two-part, heat-curing, filled epoxy system with a wide temperature range.
QA-500: Two-component, clear liquid and powder adhesive for use with strain gages. Has excellent moisture and chemical resistance.
M-BOND 610: Used primarily in stress analysis applications over a wide temperature range, and in precision transducers. Elevated-temperature curing.
M-BOND 600: Similar to 610, but faster reacting. Can be cured at somewhat lower temperatures than 610.
M-BOND 43-B: Normally used in precision transducers. Highly resistant to moisture and chemical attack. Elevated temperature curing.
M-BOND 450: Special-purpose, high-performance epoxy for higher-temperature transducer applications.
DENEX #3: One-part epoxy for lab and transducer work requiring minimal creep. Elevated temperature curing.
P: Single-part solvent thinned polyimide adhesive. Excellent for long-term high temperature applications.
M-BOND 300: Special-purpose polyester adhesive used primarily when low-temperature curing is required. Sensitive to solvents. Not recommended as a general-purpose adhesive.
NCC-3: Ceramic cement for bonding free-filament strain gages. Has superior bond strength to super-alloy materials, stainless steel, and titanium. Not for use on mild steel.
WC-16: Ceramic cement for bonding free-filament strain gages to materials with low thermal expansion coefficients. Not for use on iron-based alloys.
HG-1: Ceramic cement for bonding free-filament strain gages to most metals. Thermal expansion coefficient closely matches that of steel.
GC: Single-part ceramic cement used for free-filament gages. Recommended for use on low TCE materials, such as carbon.
H CEMENT: One-part ceramic cement/coating used for free filament strain gages. Good adhesion to most metals.
PBX: Two-part ceramic cement/coating used for free-filament strain gages. Good adhesion to most metals.
SAUEREISEN DKS-8: Single-part chemical setting zircon-based cement used for free-filament strain gages. High electrical insulation and thermal conductivity.

Warning: Note: It is usually misguided economy to attempt installing strain gages with outdated adhesive, or adhesive that has not been stored as recommended. It should also be noted that conventional industrial and consumer adhesives are not generally suitable for bonding strain gages.