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uses precision instruments there are not that many companies worldwide that manufacture strain gauges these are devices fabricated using similar technology to integrated circuits there's a layer of photoresist that is deposited on the foil on top of a very very thin foil of constant time so let me start let me back up the process starts with a foil the packing foil and then the backing foil which is polymeric very thin and insulating on top of it you get the continuous layer of constant and very thin layer of metal on top of that you have a layer

of this light-sensitive paint which is called photoresist that is all performed all that is deposited in the dark and the park is maintained in the dark then the film with that photoresist on it is subject to the photographic process of projecting an image of the part that needs to be cut that means to be preserved and where light hits the the photoresist the photoresist undergoes a chemical reaction and afterwards one can provide the chemical of that that removes the constant and from everywhere else except where where the photoresist has reacted because of light and then

ultimately the layer of photoresist is washed away so it's a delicate process is a complicated process it's a it's a again integrated circuit type manufacturing process and the result are strain gauges like this think of them as small stamps with a wire that is created on the stamp and you have these paths where you can connect wires where you can solder wires in order to have an electrical connection to the strain gauge now as I mentioned there are worldwide the limited number of companies that manufacture such precision instruments one of them is located in North

Carolina it's called fisheye it has changed hands a number of times but the name of the company I think stayed the same its measurements group I believe is part of measurements group and it's a fisheye company and it makes strain gauges now when you purchase some strain gauges they come second they come typically in a package that looks like so on the back of the package you have information and will provide you with a scanned image of this that defines the properties of the strain gauges inside now those train gauges did not have their properties

individually measured however the properties have been measured for the batch so as batches of strain gauges are being manufactured just the way integrated circuits are manufactured the properties of samples from deathmatch are measured and they provide the information that is printed on this label so whenever using a strain gauge one has to look up on this table to propane the value the precise value of the resistance and the gauge factor so in this case the resistance of the strain gauges in this package was 120 point point zero ohms plus or minus zero point three percent

and then the gauge factor at 24 degrees centigrade is two point one zero zero plus or minus 0.5 percent I want you to appreciate that 0.5 percent is a very small amount of variable and then there's a table here that does that provides the thermal output coefficient for 2024 T for aluminum in other words if you bond the strain gauge on aluminum 2024 alloy then you will have a number of you have a change as a function of temperature of the resistance so you need to know how to compensate for that and there are a

number of coefficients and values provided here so again all this information is extremely useful and over here you have a diagram that provides the calculation of thermal output for strain gauges in other words what change in resistance is a result of temperature change as opposed to strain so again there are a number of details very important details in using strain gauges in order to perform accurately measurements however the benefit is that they are very very sensitive devices and they allow you to measure very very small strains very accurate now inside there is a package that

you're gonna see when you come to the lab but this is generally what strain gauges look like as they arrive inside the package so you will find a number of strain gauges each of them inside this package and there's our particularly handling precautions that I'm going to point you to three videos that are on YouTube from the cup one come one such company manufacturing strain gauges which is v-shape and they're going to show you a typical preparation of the specimen in order to install a strain gauge installation of the strain gauge and the electrical connections

provided to a strain gauge making it ready for use it's about fifteen twenty minutes of video watching on YouTube and I count on the fact that you're going to do it now one once everything is done here's what an installed strain gauge on the steel specimen dog-bone specimen looks like and you're gonna have an opportunity to see this for yourself when you come to the lab now the two tabs of this of the strain gauge have been connected with some wires and the wires have been routed to a strain indicator and I'm going to point

the strain indicator I'm gonna set it as such I'll hold it as such that you can see you're going to see this train indicator unit and we're gonna explain in more detail how exactly it works but in principle you have a number of connectors at which you can connect one strain gauge or two strain gauges as a so one strain gauge as a quarter bridge connection two strain gauges as a half bridge or four strain gauges as a full bridge and the explanation for the specific way to connect a quarter bridge a half bridge and

the full bridge to the connectors of the box are provided now we're going to provide you with more details when you come to the lab but the important thing I want to demonstrate here is how sensitive and you're going to experience this for yourself how sensitive a strain gauge can be I want you to pay attention to the needle here that is set to zero to begin with and what I'm going to do is I'm going to use my fingers to very gently apply some bending strain bending deformation to the steel specimen and again I'm

barely applying a bending moment and you see the needle moving as a result I want you when you come to the lab to develop an intuitive sense for how sensitive strain measurement using strain electrical resistant strain gauges can be again each of you is gonna have an opportunity to perform this test and see for yourselves how quickly and how sensitive how keeping the needle moves and how sensitive the needle is and then this box can be used to measure strain and again can measure strain down to units of strength micro strain so parts per million

deformation so that's a measure of how sensitive the method is you can measure correctly accurately precisely with this method strains down to parts per million all right that's the story with regard to strain gauges and the use of strain gauges to measure measure stress strength the extensible meter that was used for the testing experiment numbers - in lab number two is based on strain gauge technology and in fact now I can tell you that the extensometer inside contains a full bridge for strain gauges that are positioned in a particular way such that the relative deformation

the relative movement of the two arms of the extensometer causes a bending deformation in one of the arms in an elastic part - one of the arms which is sensed by the four strain gauges and the indication is sent to the computer I will stop here for a second this is the end of the discussion of strain gauges again between listening to the lecture hopefully understanding all the details of what I explained coming to the lab and experiencing for yourselves how sensitive the method is and watching the videos about how you install a strain gauge

on a specimen we have covered this space up here