What is the main principle of bending-shear oblique cracks without web reinforcement in beams?

Under the load, there is only normal stress (tensile stress) in the lower part of the mid-span section of the beam. When its value exceeds the tensile strength limit of the material, it will crack and the crack is vertical. There are normal stress σ and shear stress τ at the lower part of the section near the bearing, which synthesize principal tensile stress and principal compressive stress. The direction of principal tensile stress is close to 45, and it inclines from top to bottom to mid-span. When its value exceeds the tensile strength limit of the material, the crack direction is close to 45, and it is supported obliquely from top to bottom (the whole beam is splayed). At this point, the principal compressive stress may force the crack to move to Zhang Kuan. If there is no web reinforcement or insufficient web reinforcement, the beam cannot resist the principal tensile stress.