Short beam shear stress.. Is it the right one for PCOMP card? 1

[Rinos]

I am doing simple buckling analysis on a carbon fiber tublar product subject to compressive load. I use NE nastran. My model contain PCOMP card and MAT8 card. I have the material property for the carbon prepreg from the supplier from which I am pluging in all the values in MAT8 card. In MAT8 card, there is a coloumn for shear strenth value in XY plane of fiber. The material data what I have contain short beam shear stress in 0 degree and 90 degree as per ASTM std. Which one is the right value to be pluged in for this shear stress value? I am interested in seeing failure critria of all the plies in the laminate.

Thanks

 

[SWComposites]

The 0 degree and 90 degree short beam shear data are (more or less) related to tauXZ and tauYZ interlaminar shear strength, not tauXY inplane shear strength. You will need actual XY shear strength data, which is usually obtained from a tensile loaded +/-45 laminate specimen. Note that you should only use the max strain failure criteria, as the others are generally much more inaccurate and unconservative when compared to actual laminate strength data. You can convert the ply strensgth values in terms of stress into strain values using the appropriate moduli. You should also reduce the vendor strength data to account for material scatter (published vendor data are typically average values which are often optimistically high).

 

[Rinos]

Thanks! In case if I do not have tauXY of lamina, will the avearge of tensile strength of fiber in 0 degree and 90 degree will be the good compromise for shear strength?

Regarding failure criteria, Am I required to use strain values for corresponding stress values (xt,yt,xc,yc,S) in MAT8 card? If I plugin allowable/failure strain values for material properties, Can I still use other failure theories like Tsai or Hill or Hoff or LARC02? Or I can only use max strain theory?

 

[SWComposites]

Absolutely not. The shear strength is much lower. If you have nothing else, use the short beam shear strength as it will be somewhat conservative. You can use whatever failure theory you want; the question is which will correlate best to actual structural strength. Based on my experience, max strain is the best approach, though even it doesn't always work well with lamina based strength values; its much better to use strain values from multi-directional laminate tests. If you want to dig deeper into the mess of failure theories, see the series of papers by M J Hinton in Composites Science and Technology a few years ago. Be aware that just because a FE code vendor puts a failure criteria into their software doesn't mean that it is any good.