Alvin Lin
Student
- Aug 8, 2024
- 2
Hi so I've been looking into wheatstone bridges, strain gauges, and the various configurations and the logic behind everything since I've just left my almost year long co-op at a aerospace company where I did a lot of strain gauge testing.
For the most part I've gotten a very good understand (I think) on strain gauges but I'm very confused on the purpose of poissons ratio strain gauge configurations and a few other questions. At first I thought they that the purpose was to get a output from the the strain gauge that is in the direction of axial strain and add on the lateral strain from poisson strain gauge which is positioned transverse to the other strain gauge. Hence also why its 1+v more sensitive than a simple quarter bridge. The logic of the resistance response for a half bridge poisson configuration would be:
(∆R4,LOAD+∆R4,TEMPERATURE)-(-∆R3,POISSION LATERAL+∆R3,TEMPERATURE)
=> ∆R4,LOAD + ∆R3,POISSION LATERAL, with R4 being at the top half of the wheatstone with R3 being at the bottom half.
But I've dug around online and this website:
Which says that the poisson gauge "Compensates for the aggregate effects on the principle strain measurement due to the poissons ratio of the specimen material". This makes it sound like to me that the purpose of the poisson gauge is to cancel out any strain caused in the axis of the load by the strain caused in the perpendicular direction? That doesn't really make sense to me and I fail to see the point of this.
So:
Was I originally correct and I'm misunderstanding that quote from that website? If not please explain in detail what is going on.
Further questions I have in mind:
2. For full bridge configurations, it states in the website above that lead resistances are canceled out. I know that wire resistances can me remediated by using 3 wire strain gauges instead of 2 and keeping the wires as short as possible if you have a quarter bridge setup. Or if you have a half bridge or full bridge setup, you make sure that the wires connecting the strain gauges to the bridge are the same length. But how does using 4 strain gauges specifically stand out on this?
3. In the website above, on Full-bridge Type 3 bridge, why does it reject bending? Its essentially just a double poisson half-bridge set up. If you apply a bending load, wouldn't R2 experience tension strain, R4 compression strain, R1 compressive poisson effect, R2 tensile poisson effect?
On the other hand why is Full-bridge type 2 bridge set up the way it is? With the strain gauges paired the way they are with R4 and R3 being a pair that only goes along the axis of load while R1 AND R2 are a pair that only go along the transverse direction? And how does it being set up this way reject bending?
4. Also related to the question above, how does Half-bridge type 2 and full bridge type 1 (also from website above) reject axial strain? I don't see how I wouldn't get a reading if I applied a axial load to those beam.
I'm not sure if this is the right place to post but I couldn't in the other engineering related subreddits I found so I thought I'll post here and I'm thinking about using this in a future project in my final year.
Please do explain to me everything with as simple language as possible since I'm still new to strain gauges and let me know if I made anything unclear. Thanks!
For the most part I've gotten a very good understand (I think) on strain gauges but I'm very confused on the purpose of poissons ratio strain gauge configurations and a few other questions. At first I thought they that the purpose was to get a output from the the strain gauge that is in the direction of axial strain and add on the lateral strain from poisson strain gauge which is positioned transverse to the other strain gauge. Hence also why its 1+v more sensitive than a simple quarter bridge. The logic of the resistance response for a half bridge poisson configuration would be:
(∆R4,LOAD+∆R4,TEMPERATURE)-(-∆R3,POISSION LATERAL+∆R3,TEMPERATURE)
=> ∆R4,LOAD + ∆R3,POISSION LATERAL, with R4 being at the top half of the wheatstone with R3 being at the bottom half.
But I've dug around online and this website:
Which says that the poisson gauge "Compensates for the aggregate effects on the principle strain measurement due to the poissons ratio of the specimen material". This makes it sound like to me that the purpose of the poisson gauge is to cancel out any strain caused in the axis of the load by the strain caused in the perpendicular direction? That doesn't really make sense to me and I fail to see the point of this.
So:
Was I originally correct and I'm misunderstanding that quote from that website? If not please explain in detail what is going on.
Further questions I have in mind:
2. For full bridge configurations, it states in the website above that lead resistances are canceled out. I know that wire resistances can me remediated by using 3 wire strain gauges instead of 2 and keeping the wires as short as possible if you have a quarter bridge setup. Or if you have a half bridge or full bridge setup, you make sure that the wires connecting the strain gauges to the bridge are the same length. But how does using 4 strain gauges specifically stand out on this?
3. In the website above, on Full-bridge Type 3 bridge, why does it reject bending? Its essentially just a double poisson half-bridge set up. If you apply a bending load, wouldn't R2 experience tension strain, R4 compression strain, R1 compressive poisson effect, R2 tensile poisson effect?
On the other hand why is Full-bridge type 2 bridge set up the way it is? With the strain gauges paired the way they are with R4 and R3 being a pair that only goes along the axis of load while R1 AND R2 are a pair that only go along the transverse direction? And how does it being set up this way reject bending?
4. Also related to the question above, how does Half-bridge type 2 and full bridge type 1 (also from website above) reject axial strain? I don't see how I wouldn't get a reading if I applied a axial load to those beam.
I'm not sure if this is the right place to post but I couldn't in the other engineering related subreddits I found so I thought I'll post here and I'm thinking about using this in a future project in my final year.
Please do explain to me everything with as simple language as possible since I'm still new to strain gauges and let me know if I made anything unclear. Thanks!
