Little research has been done on how UP-treated welds behave when they are subjected to real world conditions such as compressive overload or variable amplitude loading. Lloyd’s Register provides quality assurance to the marine industry, and they have been using ENGIN-X to investigate UP welded joints in these conditions. Understanding the process and its benefits will allow improved control of fatigue cracking, lower maintenance costs, and extending the life of welded connections in marine and other industries.
Ultrasonic peening has been used since the 1970s to improve the fatigue strength of welded joints. Lloyd’s Register has accepted the benefits of the technique in welded joints, with mild steels seeing a fatigue improvement factor of 2.5, and up to 3.5 in high tensile steels. The challenge now is to assess what happens when the welds are subject to compressive overload or variable amplitude loading.
STFC’s Harwell Imaging Partnership (HIP) has aided LLoyd's Register with direct support and by providing access to the ISIS neutron and muon facility. Numerical models have been developed to predict the fatigue performance of UP weld connections, but few tests have been made to validate these. Neutrons provide a unique tool for measuring residual stress in metals and so validate the models. Lloyd’s Register worked with Professor Mike Fitzpatrick, now at Coventry University, to carry out experiments on ENGIN-X to evaluate the performance of welds with different stress histories and loading conditions.
The role of ISIS
ENGIN-X is a powerful tool for studying residual stress in engineering components. Neutrons can penetrate several cm into steel and determine the residual stress with an accuracy of 20 MPa or less, which compares well to the measured peak stresses in these materials that can exceed 400 MPa. ENGIN-X has the capacity to measure large samples non-destructively.
Showing that ultrasonic peening works even when welded connections experience compressive load or variable amplitude loading allowed Lloyd’s Register to determine how widely the technique can be applied to ships and marine infrastructure. UP tools are portable, easy to use, and environmentally-friendly. In addition the parameters are computer controlled, reducing operator error and improving quality control. Ultimately using UP will reduce fatigue cracking, lower maintenance costs and extend the lives of structures in the marine and other industries.
Professor Fitzpatrick says, “We found that ultrasonic peening is good at inducing beneficial compressive residual stress at the surface of the weld, which is just as intended and counteracts the detrimental tensile stresses that are associated with welding. However, a high compressive overload effectively removed the effect of the peening and re-generated tensile stresses. This clearly has implications for the application and effectiveness of ultrasonic peening in structures where high compressive loads may occur.”
David Howarth of Lloyd’s Register adds, “For many years the advantages of adding a compressive stress in the area of the weld toe have been known but could not be practically applied to large ship structures. The advent of ultrasonic peening provides the ship builder with the freedom to do this without affecting the speed of construction seen in modern shipyards. The ability to understand how residual stresses perform under realistic conditions has been a great help to the industry in pushing the process forward.”
Research date: July 2014
For further information please contact Sara Fletcher