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The residual stress in a roll formed product is generally very high and un-predictable.This is due to the occurrence of redundant plastic deformation in the roll forming process and it cancause various product defects.Although the residual stress of a roll formed product consists of longitudinal and transverse residual stress components,the longitudinal residual stressplays a key role in causingproduct defects of a roll formed product and therefore,the longitudinal residual stress is of mostconcern to the roll forming scholars and engineers.However,how to inspect the residual stress of a product quickly and economically as a routine operation is still a challenge.This paper introduces a new residual stress measurement method to study the longitudinal residual stressvariation through layers of the thickness of a roll formed product.The new method is also extendable to a product with similar processing such as a rolled sheet.The detailed measuring procedure is given and discussed.The residual stress variation through eachlayer can be derived based on the variation of curvature in different layers and steps.This new method has been explored and validated by experimental study on a roll formed square tube.The new method is expected to be a routine testing method to monitor the quality of a product that has been formed and that could have a great impact on the roll forming industry.