Page 52 - SSAB Welding Handbook Edition 2

©2009-2019 by SSAB Group of companies (SSAB). All rights reserved. Only digital PDF file. No distribution. No printing allowed!
No part of this handbook may be reproduced in any form or by any means without permission in writing from SSAB.
8.0 Fatique in welded joints Welding handbook

The FAT is 80 MPa taken from fig. 8.10, no. 213. 8.6 Improvement methods
The corrected FAT for different thicknesses is shown
in fig. 8.12. The amount of misalignment already in In general, improvement methods are divided in to
the FAT can also be seen in fig. 8.10, for a 10 mm two types: geometric and residual stress. Geometric
plate this misalignment is 0.5 mm and for a 20 mm it improvement methods include burr grinding and
is 1 mm. For a 20 mm plate with a misalignment of TIG-dressing. Residual stress improvement methods
4 mm, the FAT is about 58 MPa instead of 80 MPa; include post-weld heat treatment (PWHT), hammer
see the red line in fig. 8.12. This gives a reduction peening, blast peening and ultrasonic methods. It
in lifetime of about 2.6 times. Chapter 13 explains is very important to know that if improvement
how to minimize the misalignment. methods are needed, they should only be applied
locally to the problem areas, see fig. 8.13, otherwise
using improvement methods becomes very costly.

90
80
70
60
FAT/MPa 50 Approximately 4xt Stress
40
30
Plate thickness
20
10 mm ©SSAB
10 20 mm
0
0 2 4 6 8 10
Misalignment/mm
Fig. 8.13: Example of where to use improvement methods.

Fig. 8.12: Corrected FAT due to misalignment.
For an improvement method to be successful, it
must be performed with the correct parameters.
Two other fatigue assessment methods have been For more information about the process parameters
developed from the nominal stress method: the and gains in fatigue strength, it is recommended to
structural stress method Hot Spot Stress, HSS in consult IIW “Recommendations on Post Weld
IIW, and the effective notch stress method. One Improvement of Steel and Aluminium Structures”.
other method to access fatigue is to use linear elastic
fracture mechanics, mostly used with an existing
crack or at the root side. All of these methods are
covered in IIW “Recommendation for fatigue design
of welded joints and components”.

52 This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.