Hole-Drilling Method for Deep Residual Stress Measure in Tensile Armour Wires

Background Incremental center hole drilling method (ICHD) emerges as a reliable and practical method to characterize residual stresses (RS) in tensile armor wires of flexible pipelines. However, geometric restrictions imposed by the standard ASTM E837 limit RS measurements in these wires to shallow depths, around 0.8 mm. Objective The objective of this work is to develop a methodology for extend the ICHD measurement depth in tensile armor wires up to half their thickness. Methods Numerical and experimental studies were carried out to evaluate specific calibration coefficients for the geometry of the tensile armor wires. The validation of the results was carried out on samples of wires, RS free, submitted to different loading type conditions. The work also produced wire samples with permanent RS that allow one to compare the results of the extended ICHD (eICHD) with conventional ICHD (within the limits recommended by standard E-837), and by layer-removal X-ray diffraction (XRD). At last, the eICDH and ICDH methods have been applied to a tensile wire sample that was taken out of service. Results Residual stress profile measured by eICDH shows agreement with analytical results in elastic load samples. In the permanent RS sample, the comparative results between eICDH, ICDH, and XRD showed an excellent correlation. The tensile wire taken out of service showed a complex RS state near-surface with the shear stress influencing RS more deeply than longitudinal and transverse stress. Conclusions Experimental and numerical tests was carried out to determine specific calibrations coefficients for ICHD to extend measure depth in tensile armors wires. The develop methodology for eICHD allow measure RS profile in tensile armor wires from 0.08 mm until 2.4 mm, approximately half-thickness.