Weld Residual Stress Measurement Using Portable XRD Equipment in a Shipyard Environment
Authors: Yang Yu-Ping , Huang T. D. , Rucker Harry J. , Fisher Charles R. , Zhang Wei , Harbison Michael , Scholler Steven T. , Semple Jennifer K. , Dull Randal 
Published: July 2018

Weld residual stress plays an important role in the production and operating performance of ship structures. Various factors such as background noise, vibration, movement during ship construction, a layer of primer on the plate surface, and a layer of paint after ship construction bring challenges to measure weld residual stress in a shipyard. Three large test panels made of DH-36, High-strength low-alloy steel (HSLA), HSLA-65, and HSLA-80 steels were fabricated to examine the feasibility of using commercially available portable x-ray diffraction (XRD) equipment to measure residual stress in a shipyard environment. The measured results show that portable XRD equipment provided reliable measurements, with the shipyard environment effects, on the panels made of DH-36 and HSLA-65. On the other hand, the primer affected the accuracy of measured residual stress on the panel made of HSLA-80, but electropolishing could have been used to remove the primer to achieve a good measurement.

1. Introduction
Welding is one of the most important manufacturing processes in shipbuilding and inevitably induces residual stress and distortion on ship structures. In addition, flame straightening, often used to remove distortion in the final stage of shipbuilding, can result in even higher residual stress because of higher constraints after ship structures are assembled. It is well known that residual stress affects the buckling strength, fatigue performance, corrosion resistance, and dimensional stability of ship structures. As shipbuilding has been increasingly using thinner and higher strength materials such as HSLA-80 and HSLA-100 to reduce weight and increase mobility, residual stress plays an even more important role in the operating performance of ship structures. Understanding the residual stress evolution from raw material to a completed ship during service is critical to improve the ship's performance.
Multiple methods have been developed to measure residual stress which can be classified into three categories: nondestructive techniques, semidestructive techniques, and destructive techniques. The common nondestructive techniques include x-ray diffraction (XRD) (Gou et al. 2015), neutron diffraction (Kartal et al. 2006; Palkowski et al. 2013), magnetic method, ultrasonic methods (Bray & Junghans 1995), and impact-indentation method (Lin et al. 2005; Choi et al. 2010; Zhu et al. 2015). The semidestructive techniques include holedrilling and ring-core methods, and the destructive techniques include block removal, splitting, layering, and contour methods (Tebedge et al. 1973; Leggatt et al. 1996). The U.S. Nuclear Regulatory Commission and the Electric Power Research Institute organized an international round robin program to measure weld residual stress in pressurized water reactor primary cooling loop components containing dissimilar metal welds (Fredette et al. 2011; Rathbun et al. 2011). Neutron diffraction, deep-hole drilling, XRD, surface-hole drilling, ring-core method, and contour method were used to measure residual stress in this program. The measured results between different measurement techniques were compared and validated against each other. In addition, a round robin study in Europe was conducted to investigate the accuracy of the XRD method from March 2012 to December 2013 (GKN & DAkkS 2014). Thirty laboratories and companies determined the residual stresses in the surface of two reference samples. Statistical evaluation of all results found that the XRD method has a good measurement accuracy. The robust means are between 4.7 and 6.3 MPa and the robust deviations are between 3.1 and 4.0 MPa. These studies have greatly improved the residual stress measurement techniques.