90°折弯试验　
W-shaped bending test

将不会出现裂纹的折弯半径R除以材料板厚t所得的值定义为安全折弯半径（MBR/t）。可以说MBR/t的数值越小，材料的折弯加工性越好。
例：如果板厚=0.2mm，折弯半径=0.2mm，并且没有观察到裂纹，则折弯半径为R/t = 0.2/0.2 = 1.0。

The minimum bend radius (MBR/t) is the radius below which a test piece can be bent without a crack at the outer surface of the curve. The smaller the value of MBR/t is, the better the bend formability of the material is.
Ex: When the thickness is 0.2mm and the bend radius is 0.2mm, the value of R/t equals 1.0.