African Fusion March 2017

UP peening to prevent fatigue

In this paper, presented at the IIW 2016 Conference in Melbourne, Australia last year, Jacob Kleiman and Yuri Kudryavtsev of Canada-based Structural Integrity Technologies (Sintec), present work about the use of ultrasonic impact treatment (UIT), also known as ultrasonic peening (UP), to prevent fatigue crack initiation and extend the service life of welded products. Extending the life of welded components through UP

T he formation and propagation of cracks in welded structures plays a critically important role in the total life cycle of welded components. Ultrasonic Impact reatment (UIT), also knownUltrasonic Peening (UP), was used in the rehabilitation and repair of welded elements with the goal of preventing possible fatigue crack initiation in existing welded elements and structures that are in service. A number of large-scalewelded specimens containing no- load carrying longitudinal attachments designed for fatigue testing were tested in the as-welded condition and after weld repair with and without the application of UP. The testing conditions were zero-to-tension stress cycles (R=0) with different levels of maximum stresses. The fatigue testing was stopped and the number of cycles was recorded when the lengthof fatigue crack on the surface reached 20mm. Then the fatigue crack was repaired by gouging and welding and the fatigue test was continued. After repair, the weld toe of the repair weld was UP treated. The fatigue testing of all specimens demonstrated that the repair of fatigue cracks by welding restores the fatigue strength of welded elements to the initial as-welded condi- tion. Repairing of fatigue cracks a second and a third time also practically restored the fatigue life of repaired welded

elements to their initial as-welded condition. However, when similar samples after the weld repairs were also treated by UIT/UP, the fatigue life of such samples was extended by about four times over the samples that were onlyweld repaired, thus extending the total life cycle of welded components many times. Introduction It is generally accepted that in the total life cycle of welded components the formation and propagation of cracks in welded structures plays a pivotal role. Many approaches exist today for the prevention of crack formation and their propagation [1, 2]. Thus, for instance, in a comparative study to evaluate the efficiency of traditional and advanced techniques for fatigue life improvement of structural elements with fatigue cracks, a number of large-scale specimens containing fatigue cracks were repaired using various techniques and subjected to fur- ther fatigue testing [2]. A number of techniques for restraining and repairing the fatigue cracks were evaluated and com- pared, such as: overloading, drilling of the crack tips, drilling of the crack tips with the installation of high strength bolts, local explosive treatment, local heat treatment, and welding with and without UP on the weld toe zones. The fatigue cyclic testing conditions were zero-to-tension stress cycle (R=0) with the maximum stresses at 155 MPa. It was found in this study that the repair of fatigue crack by welding with the subsequent UP treatment provided the lon- gest fatigue life in comparison with other ways to prolong the service life of structural elementswith fatigue cracks (Figure 1). As shown, UP treatment can be effectively applied for fa- tigue life improvement. It was also demonstrated that UP is a very efficient treatment during manufacturing, rehabilitation and repair of welded elements and structures [3-7]. Traditionally, UP treatment is considered inmost cases for high-cycle fatigue applications, where its effects are mostly pronounced. It is shown in this paper that in the total life cycle of welded components, inorder toachieve the longest possible service time, care must be taken of the welded structures at all stages of the cycle, that is, through manufacturing, main- tenance and rehabilitation. Use of Ultrasonic Peening (UP) Ultrasonic Peening (UP) is one of the efficientways of achieving fatigue life improvement of welded elements and structures [8, 9]. The effects of improvement treatments, particularly UP treatment, on the fatigue life of welded elements depend on

Figure 1: A comparison of the efficiency of different techniques of restraining and repairing fatigue cracks. 1: Initial condition; 2 and 3: Drilling of the crack tips with and without cold working; 4: Drilling of the crack tips with the installation of high strength bolts; 5: Overloading (yield strength); 5′ overloading (0.7 yield strength); 6: Local explosive treatment; 7: Local heat treatment; 8′: repair by welding with UP treatment of the weld toe zones; and 8′′: Repair by welding without UP.

14

March 2017

AFRICAN FUSION