African Fusion March 2017

AFRICAN FUSION

March 2017

UP peening to prevent fatigue

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

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.

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.

Extending the life of welded

components

through UP