i have a plate subjected F(x,y,z) in global coordinates. the normal x stress plot gives a max and min value - please refer the screenshot. its shows a max=184MPa & min = -246.7MPa... how do i know the compressive stress in the model.
The answer is in your plot, positive results is tension, negative compression. But instead to use uniaxial normal stress SX better use PRINCIPAL STRESSES both MAJOR AND MINOR that will give you the maximum principal stress at tension & compression.
You should google "Mohrs Circle" and do a quick review of solid mechanics. Any "element" will have a stress condition which could be decribed in a few ways. For a plate, there will be X stresses, Y stresses and XY shear stresses. Note that "X" and "Y" element local stresses in Femap are unlikely to be oriented in the global X or Y directions. Femap allows you to transform your results from the local element directions to a direction of your choice, to see what the direct stress in your chosen direction would be. Note that for a 2D (eg plate) stress condition there is an in-plane orientation where the shear stresses will be zero. That orientation also corresponds with the highest and lowest direct stresses in the perpendicular directions of the orientation. The highest stress is the "Maximum Principal Stress". The perpendicular stress (the lowest direct stress) is the "Minimum Principal Stress". Note that the Max can be negative, and the Min can be positive (but not at the same time / location / result case). So some feasible Max / Min example pairs could be (140, -50) or (290, 120) or (-50, -200). Note that in the -50, -200 combo, the Max is -50 and the Min is -200 despite the magnitudes.
For your Femap model, you can show Max and Min Principal stress vectors simultaneously, but to keep it easy, you are most likely interested in the Min Principal Stresses across the model. Even though this may range from negative to positive in your stress range, these stresses are the most negative (most compressive) stresses present in each element in the model, for the result case.