Concept:
Normal stress at a plane with an ∠θ from plane under the action of σx,
τxx=(2σx+σy)+(2σx−σy)cos2θ+σxysin2θ;;;;;…(1)
The shear stress at a plane with an ∠θ from plane under the action of σx,
τxy=−(2σx−σy)sin2θ+σxycos2θ;;;;;…(2)
(θ = - 45° as the plane under +P is rotated by 45° anti-clockwise to get the plane under +τxx)
Also,
τxx + τyy = σx + σy ...(3)
Calculation:
Given:
Since the given plane is the principal plane ⇒ σxy = 0**,** σx = P, σy = - P
By using above equation (1),
τxx=(2P−P)+(2P−(−P))cos(−90∘)+0=0
By using above equation (2),
τxy=−[2P−(−P)]sin(−90∘)−0=P
By using equation (3),
⇒ τyy = (σx + σy) - τxx = 0
∴ τxx = τyy = 0, τxy = P
Alternate method:
The given plane is the principal plane because σxy = 0
At 45° from the principal plane, the plane of maximum shear occurs on the plane of maximum shear.
τxx=τyy=2σ1+σ2=2P−P=0
τxy=2σ1−σ2=2P−(−P)=P