Which of the following is correct regarding the principal stresses and maximum in-plane shear stresses? a. principal stresses cannot be negative. b. the shear stress over principal stress planes is always zero. c. a principal stress plane can be reached by rotating the maximum in-plane shear stress plane 90° in the clockwise direction. d. the maximum in-plane shear stress is always smaller than principal stresses.

In a unit cell of fcc structure, indicate the position of one of the tetrahedral interstitial sites by plotting the corresponding tetrahedron. do the same for one of the octahedral interstitial sites (in a separate fcc unit cell). mark the positions of all the tetrahedral and octahedral interstitial sites in the unit cells. how many equivalent tetrahedral sites and how many equivalent octahedral sites are contained in a fcc unit cell, respectively. what is the coordination number for a tetrahedral site and what is the coordination number for an octahedral site?