ISO 5628 specifies the general principles for determining the flexural stiffness of paper and Cardboard, covering two-, three- and four-point test methods. Bending stiffness is an important indicator to measure the ability of a material to resist deformation when bent under force, affecting its performance in applications such as printing, Encasement, etc. The standard describes in detail the basic principles, test conditions and data calculation requirements of each test method to ensure the comparability and Reliability of test results. The standard applies to different types of paper and Cardboard, providing technical support for product quality control and performance evaluation.