Irwin was the first to observe that if the size of the plastic zone around a crack is small compared to the size of the crack, the energy required to grow the crack will not be critically dependent on the state of stress ( the plastic zone ) at the crack tip.
22.
Assuming we know the stress components \ sigma _ x, \ sigma _ y, and \ tau _ { xy } at a point P in the object under study, as shown in Figure 4, the following are the steps to construct the Mohr circle for the state of stresses at P:
23.
This is the weightlessness in " free fall in a uniform gravitational field " . ( The situation is more complicated when the gravitational field is not uniform, or, when a body is subject to multiple forces which may, for instance, cancel each other and produce a state of stress albeit weight 2 being zero.
