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通常认为,弯曲变形时的应力中性层和应变中性层是一致的,并与弯曲件的重心相重合。实际上,弯曲件长度方向的应力和应变沿厚度分布是不均匀的(图1),而且,只有毛料原始曲率等于零,弯曲变形不大时,两个中性层才与重心重合。有原始曲率的毛坯弯曲时,即使变形较小,中性层也不在它们的重心部位。弯曲变形比较大时,无论原始毛坯有无曲率,应力中性层和应变中性层都不在它的重心部位,而且由于塑性变形的不可逆性,它们二者也是不重合的,通常都要内移一些。且应力中性层总是比应变中性层内移得多些。应力中性层移动的原因,主要是随着弯曲过程的进行,弯曲件厚度方向的应力(O_3)逐渐增大的缘故。
It is generally accepted that the stress-neutral layer and the strain-neutral layer during bending deformation are identical and coincide with the center of gravity of the bending member. In fact, the distribution of stress and strain along the length of the bending member is not uniform along the thickness (Figure 1). Moreover, only when the original curvature of the wool is equal to zero and the bending deformation is not large, the two neutral layers coincide with the center of gravity. When the original curvature of the curved surface of the blank, even if the deformation is small, the neutral layer is not in their center of gravity. When the bending deformation is relatively large, the stress neutral layer and the strain neutral layer are not located in the center of gravity of the blank regardless of the curvature of the original blank, and because the irreversibility of the plastic deformation is not coincident with each other, the inner blank usually moves inward some. And the stress-neutral layer always moves more than the strain-neutral layer. The reason why the stress neutral layer moves is mainly that as the bending process proceeds, the stress (O 3) in the thickness direction of the bending part gradually increases.