A free body diagram is necessary for determining the external moments and forces:
The angle can be determined from the dimensions (see Figure P8.31 in the text):
The external forces and moments are easily computed...
I'll also compute the properties of the cross section, so that they are available later when I need them:
The area AQ is that of half of the cross section - the part above the N.A. Here's a free body of the short section from A to a-c-b, exposing the internal forces and moment:
At the top and bottom of the cross section (a and b), the normal stress arises from two terms:
At the center of the cross section (point c) only the P/A term applies: - 0.320 MPa.
The shearing stress at c is:
Can you see why this stress is negative? There is no shearing stress at positions a or b. Can you see why?