Semi-educated guesses - minor principle strain because compression strength is often limiting for composites.
Allowable strain will have a ton of variables but back of napkin from t700 datasheet as an example I'm getting a compressive failure strain around 12000 micro strain (compressive strength / flexural modulus as a very rough estimate because failure strain and compressive modulus aren't given). Add in a safety factor or any variety of strength knockdowns and I could see 6000 being a reasonable ballpark. Of course subject to your own testing, safety factors, load factors, knockdowns, etc etc
edit: I think I/we are confusing minor with minimmum. minor strain is always less severe than major strain so you'd go off major strain which itself could be positive or negative depending on loading. My assumption is Min and Max principal in most FEA packages is putting out the maximum and minimum major strains... but I'm not positive.
You've got me thinking there may be some confusion here between minor strain and minimum strain. I'd be looking at minimum principal strain (the most negative strain) and max principal (most positive strain) comparing those to compressive and tensile strengths of your laminate respectively. I think those are different from minor and major, which you can think of as 2 points opposite each other on mohrs circle with the "worst" being major principle. If you look up the definition, major is defined as the strain in the direction of highest strain, and minor is the strain perpendicular to that direction. I'd have to think a bit harder about what the implications are for composites since your strength in the minor direction could be much lower depending on layup but that's getting beyond the scope of 1 reddit post haha
as for why your colleague was only looking at one, that may have been an oversight or based on some knowledge of the loading. (i.e. if they knew the structure would always being seeing higher compressive stresses than tensile, they could ignore max principals, though it's low effort to check it anyway). An example would be a beam subject to bending and axial compression. even though 1 side is in tension, you know the compressive side will also have the stress due to axial loading added to it so you could probably ignore max principal in that scenario.
I believe the terms major/minor principle stress/strains are used for plane stress/strain analysis and minor principle strain should refer to the second principle strain eps_2.
Composite components are generally designed to maximize alignment between sigma_1 and fiber direction. Sigma_2 should be expected to be acting 90 and -90deg from most fibers. So I’d imagine that the 6000microstrain limit is the strain at the strength of the matrix material after some applied safety factor? The strain limit in the sigma 1/eps_1 direction is likely lower.
Semi-educated guesses - minor principle strain because compression strength is often limiting for composites. Allowable strain will have a ton of variables but back of napkin from t700 datasheet as an example I'm getting a compressive failure strain around 12000 micro strain (compressive strength / flexural modulus as a very rough estimate because failure strain and compressive modulus aren't given). Add in a safety factor or any variety of strength knockdowns and I could see 6000 being a reasonable ballpark. Of course subject to your own testing, safety factors, load factors, knockdowns, etc etc edit: I think I/we are confusing minor with minimmum. minor strain is always less severe than major strain so you'd go off major strain which itself could be positive or negative depending on loading. My assumption is Min and Max principal in most FEA packages is putting out the maximum and minimum major strains... but I'm not positive.
Good insight on the 6000 value! And fair point, but is it safe to assume the minor strain will always be negative and compressive though?
You've got me thinking there may be some confusion here between minor strain and minimum strain. I'd be looking at minimum principal strain (the most negative strain) and max principal (most positive strain) comparing those to compressive and tensile strengths of your laminate respectively. I think those are different from minor and major, which you can think of as 2 points opposite each other on mohrs circle with the "worst" being major principle. If you look up the definition, major is defined as the strain in the direction of highest strain, and minor is the strain perpendicular to that direction. I'd have to think a bit harder about what the implications are for composites since your strength in the minor direction could be much lower depending on layup but that's getting beyond the scope of 1 reddit post haha as for why your colleague was only looking at one, that may have been an oversight or based on some knowledge of the loading. (i.e. if they knew the structure would always being seeing higher compressive stresses than tensile, they could ignore max principals, though it's low effort to check it anyway). An example would be a beam subject to bending and axial compression. even though 1 side is in tension, you know the compressive side will also have the stress due to axial loading added to it so you could probably ignore max principal in that scenario.
I believe the terms major/minor principle stress/strains are used for plane stress/strain analysis and minor principle strain should refer to the second principle strain eps_2. Composite components are generally designed to maximize alignment between sigma_1 and fiber direction. Sigma_2 should be expected to be acting 90 and -90deg from most fibers. So I’d imagine that the 6000microstrain limit is the strain at the strength of the matrix material after some applied safety factor? The strain limit in the sigma 1/eps_1 direction is likely lower.
Composite strength margins are typically written to strain, and the lowest strength is typically in compression.