That's phi is the angle downwards from the z axis.
这个角度是φ，它是从z轴偏转的角度。
2
That measures the angle down from the positive z-axis.
这是与z正轴的夹角。
3
By observing the initial Angle of crack arid its propagating path, a simple model to predict the retardation effect of Z-shaped fatigue crack was proposed.