schopenhauer
Marine/Ocean
Hi,
Recently I have been reading about the details of Puck's failure theory, and I reached a point where now I'm more confused than when I started, and some stuff that I thought I understood now is not so clear. So, I would like to share these basic questions to see if I can get a different opinion from the community.
My first question is very simple, how do you carry a tensile test if you want to get the tensile strength of your composite material (let's assume carbon fibre prepreg)? I used to think that you simply layup a bunch of 0° and put the samples in your Instron machine (with appropriate tabbing). After reading Puck's book, wouldn't this sample fail in shear much earlier than the real longitudinal strength (S11) of the material? I know that something similar happens with wood, so I don't know is this issue happens in real life testing, and how is solved.
Next, suppose that you have a hydrostatic state of stress in a sample, what laminate would be strong enough to support this load? What I mean is that all directions are principal stress directions, all failure theories at some point compare the strength of the material along and perpendicular to the fibres with the stresses in those directions. In principle, a cross-ply laminate should do the work, but how do we know that the laminate is strong enough to take the stress at 45°(no fibres are aligned in that direction, and the stress is by defintion hydrostatic)?
Related with the previous question, I have seen examples of parts where the laminate is designed by laying-up material along the principal stress directions (two fibre orientations), and where the laminate is designed by rotating the stress element to the position of maximum shear stress and laying-up three fibre orientations (maximum stress, minimum stress, and +-45° to take the shear). Of course, the result is completely different, and what I (think) I know is that principal stress design should be avoided because is too risky because all your material is only along two directions, the laminate is extremely weak in almost any other direction. Thus, that this mean that we should always lay-up the material where the state of stress manifest both tensile and shear stresses? Is this related with the 10% rule for laminates? I suppose that a practical case would be a composite pressure vessel, in the cylindrical portion of the vessel, do we layup the material only in the hoop and axial directions? Or should we rotate that stress element to the position where shear is maximum and layup three angles? Is there any way to quantify the risk/benefit of single vs multiple angle laminates?
I hope these questions are clear enough, but please let me know if I should clarify some of the points exposed. I'm really looking forward to reading your answers!
Thanks!!
Recently I have been reading about the details of Puck's failure theory, and I reached a point where now I'm more confused than when I started, and some stuff that I thought I understood now is not so clear. So, I would like to share these basic questions to see if I can get a different opinion from the community.
My first question is very simple, how do you carry a tensile test if you want to get the tensile strength of your composite material (let's assume carbon fibre prepreg)? I used to think that you simply layup a bunch of 0° and put the samples in your Instron machine (with appropriate tabbing). After reading Puck's book, wouldn't this sample fail in shear much earlier than the real longitudinal strength (S11) of the material? I know that something similar happens with wood, so I don't know is this issue happens in real life testing, and how is solved.
Next, suppose that you have a hydrostatic state of stress in a sample, what laminate would be strong enough to support this load? What I mean is that all directions are principal stress directions, all failure theories at some point compare the strength of the material along and perpendicular to the fibres with the stresses in those directions. In principle, a cross-ply laminate should do the work, but how do we know that the laminate is strong enough to take the stress at 45°(no fibres are aligned in that direction, and the stress is by defintion hydrostatic)?
Related with the previous question, I have seen examples of parts where the laminate is designed by laying-up material along the principal stress directions (two fibre orientations), and where the laminate is designed by rotating the stress element to the position of maximum shear stress and laying-up three fibre orientations (maximum stress, minimum stress, and +-45° to take the shear). Of course, the result is completely different, and what I (think) I know is that principal stress design should be avoided because is too risky because all your material is only along two directions, the laminate is extremely weak in almost any other direction. Thus, that this mean that we should always lay-up the material where the state of stress manifest both tensile and shear stresses? Is this related with the 10% rule for laminates? I suppose that a practical case would be a composite pressure vessel, in the cylindrical portion of the vessel, do we layup the material only in the hoop and axial directions? Or should we rotate that stress element to the position where shear is maximum and layup three angles? Is there any way to quantify the risk/benefit of single vs multiple angle laminates?
I hope these questions are clear enough, but please let me know if I should clarify some of the points exposed. I'm really looking forward to reading your answers!
Thanks!!
