Two Uncoating Techniques for Measuring Cold-Formed Steel Residual Stress Using Cos-α X-ray Diffraction Method

The cold-bending effect during the roll-forming process may affect the material's mechanical properties and induce residual stress in the cold-formed steel sections. Cos-α X-ray Diffraction is an appropriate method for measuring residual stress in cold-formed steel due to the materials’ thinness. This method also offers excellent precision and simplicity. However, the limited penetrating ability of X-rays, which extend only a few microns, significantly hinders the measurement of residual stresses in cold-formed steel when coatings are present. Therefore, this study will implement two uncoating or de-coating techniques for measuring residual stress using the cos-α X-ray Diffraction method on the surface of cold-formed steel with a 50 μm layer of aluminum-zinc coating. These techniques include water sanding and chemical solutions. Two procedures are performed for the chemical solution: the first procedure combines a 25% hydrochloric acid (HCl) solution with a 25% ammonium hydroxide (NH4OH) solution, while the second procedure uses only a 25% hydrochloric acid (HCl) solution. This study demonstrates that the second procedure effectively removes the surface coating from cold-formed steel and provides a good classification of cos-α X-ray Diffraction intensity data related to the Debye-Scherrer ring. A combination of 25% hydrochloric acid (HCl) and 25% ammonium hydroxide (NH4OH) solution results in a mediocre classification. On the other hand, the water sanding technique produced poor classifications. Furthermore, the key to the success of the cos-α X-ray Diffraction method is removing the coating from the cold-formed steel.