# Homework Help: T-delta rosette strain gauge

1. Jul 23, 2009

### jcurtis

1. The problem statement, all variables and given/known data For experimental validation of a design based on FEA, a prototype was tested using strain gauge rosettes at critical locations in a typical t-delta configurations, consisting of 3 gauges: A, B, C and fourth (extra) gauge D. D normally serves as a check on the strains recorded by the rosette arms. In this instance due to a fault in the measuring circuit, the strain response in gauge A could not be recorded the measured strains in the other three gauges were: eB = (-)135x10-6, eC=227x10-6 and eD= (-)63 x 10-6.
What would have been the strain recorded by gauge A?

2. Relevant equations
I found that J=eXX + eYY = eA + eC = eB + eD

3. The attempt at a solution I've looked in my notes that my lecturers put online and I've been searching on google all day and I can't find anything. I know it has something to do with the fourth gauge, but there's no information on the web about it that I could find. The equation I found seemed too simple as the question is worth 8 marks. Or is it more simple than I'm making out? Can anyone help please?

2. Jul 24, 2009

### nvn

jcurtis: As I currently see it, you have four equations and four unknowns. Your relevant equation is currently incorrect; eB and eD are not mutually perpendicular. Try it again. And, you need to list three more equations, for strain in a strain gauge leg. You must list relevant equations yourself. The angle from gauge A to gauge D is 90 deg; but I think you knew that, right? Hint: thetaA = 0 deg.

3. Jul 24, 2009

### jcurtis

Thanks for replying. So you're saying I should swap eB and eA around because a and d are perpendicular?
I can't find any other strain equations that aren't to do with resistance. other than principle strain equations which I need the value of A for, however I know it has to just do with the values of B, C and D as that's all they've given me.. Sorry I'm being a bit dense here, I'm just very confused.

4. Jul 24, 2009

### nvn

Yes, swap eB and eA, then omit eB + eC from your above equation, because gauges B and C are not mutually perpendicular.

5. Jul 25, 2009

### nvn

Hint 2: thetaYY = 90 deg. Do you have anything else at 90 deg? What does that tell you about the value of eYY?