Fatigue Life Extension Peening of Welds in Hollow Section Connections
By Jeffrey A. Packer
Bahen/Tanenbaum Professor of Civil Engineering, University of Toronto, Ontario, Canada
It is well‐known, both internationally and in North America, that peening of welded joints produces an extension of the fatigue life of welded connections. The position of American and Canadian codes/standards on this topic is reviewed below, along with further international research evidence, with particular emphasis on material pertaining to welded tubular structures.
The writer is a member of both the AWS D1.1 code technical committee and the CSA W59 standard technical committee, which are cited below.
The current U.S. national welding standard produced by the American Welding Society (AWS 2010) contains a Section 8.4 on “Fatigue Life Enhancement” in which five methods for reconditioning weld details are listed: Profile Improvement; Toe Grinding; Peening (“Shot peening of weld surface, or hammer peening of weld toes”); TIG Dressing; Toe Grinding plus Hammer Peening. This section concludes with the general statement that … “The Engineer shall establish the appropriate increase in the allowable stress range”.
Elsewhere in this code, however, in Chapter 2 Part D – “Specific Requirements for Design of Tubular Connections (Statically or Cyclically Loaded)”, Clause 22.214.171.124 deals specifically with “Fatigue Behavior Improvement” of welded tubular connections. Herein, it states that … “For the purpose of enhanced fatigue behavior, and where specified in contract documents, the following profile improvements may be undertaken for welds in tubular T‐, Y‐, or K‐connections:
(3) The toe of the weld may be peened with a blunt instrument, so as to produce local plastic deformation which smooths the transition between weld and base metal, while inducing a compressive residual stress.” For regular welded tubular connections, with a diameter-to-thickness (D/t) ratio of the chord member not exceeding 48 and where appropriate inspection has been performed, Clause 126.96.36.199(3) supports a fatigue category improvement from X2 to X1 (if actual stress concentration factors are known), or from K2 to K1. These stress categories are described in Table 2.7 and refer to the S–N curves in Figure 2.13. Quantitatively, at 2 x 107 cycles for example, this implies an increase in permissible stress range by a factor of about 1.25 (or conversely, about 2.0 on cycle life).
The current Canadian national welding standard produced by the Canadian Standards Association (CSA 2003) contains a Clause 9.5–Fatigue Life Enhancement in which four methods for reconditioning welded details are listed: Toe Grinding; Peening (“Shot peening of weld surface, or hammer peening of weld toes”); TIG Dressing; Toe Grinding plus Hammer Peening. Thus, the mandatory part of CSA W59 endorses hammer peening for fatigue life extension.
CSA W59 also contains a non-mandatory Appendix R, in which Section R3.5–Hammer Peening gives detailed requirements for peening procedures and an applicable range of application (such as yield strengths ≤ 800 MPa [116 ksi], and thicknesses ≥ 10 mm). In CSA W59 Section R3.8–Stress Range Increase the standard states … “The allowable stress range for cyclically loaded connections may be increased by a factor of 1.3 along the S–N design curve, which is equivalent to a factor of 2.2 on cycle life, for an S–N curve slope of approximately 1/3, when toe grinding, hammer peening, or TIG dressing is used”. Thus, although in an “informative” rather than “normative” Appendix of the code, the permissible increase in fatigue life for such treatment is even conservatively quantified. Appendix R is supported by 12 references, many of which are documents from the International Institute of Welding (IIW) and The Welding Institute, U.K.
3. International Studies
A review of relatively recent research literature on peening treatments reveals that several modern methods of high frequency peening, all of which have European origins, are in current use:
(i) PIT – Pneumatic Impact Treatment
(ii) HiFIT – High Frequency Impact Treatment (also pneumatic)
(iii) UIT – Ultrasonic Impact Treatment
(iv) UP – Ultrasonic Peening
(v) UNP – Ultrasonic Needle Peening.
These all operate in a similar manner to ordinary hammer peening equipment, only at considerably higher frequency, which reduces vibration and noise and gives a high improvement in fatigue strength. Although they are different processes, their properties and resulting improvement in fatigue strength appear to be similar (Pedersen et al., 2009). Weich et al. (2008) have shown that even fatigue-damaged welds can be treated with the same success as new welds. The correct execution technique, operator training and quality control for such weld treatment has also been emphasized (e.g. Günther and Kuhlmann, 2009; Walbridge and Nussbaumer, 2008).
In the most recent comprehensive fatigue design guidance by the International Institute of Welding (Hobbacher, 2008), hammer peening is covered in Section 188.8.131.52. This document limits the application to steels with a yield strength ≤ 900 MPa [130.6 ksi], to 50 mm ≥ thickness ≥ 10 mm, and notes the influence of the applied stress ratio (R) on the fatigue life benefit. (For example, if R > 0.4 there is no benefit). The maximum stress-range benefit that can be claimed from hammer peening, for use with the nominal stress approach, is given by the Table below.
|Steel yield stress |
< 355 MPa [51.5 ksi]
|Steel yield stress |
≥ 355 MPa [51.5 ksi]
|Benefit for details classified in their as-welded condition as FAT* ≤ 90||1.3||1.6|
|Maximum possible fatigue class after improvement||FAT* 112||FAT* 125|
**FAT112 and FAT125 are also the applicable fatigue classes to be used, for load-carrying fillet welds improved by hammer peening, in conjunction with the hot-spot stress approach. (Note that a “thickness correction” to the FAT applies for t > 25 mm).
Amongst the many fatigue studies on high frequency hammer peening of welds, some have been specifically performed on welded hollow section connections (Kudryavtsev et al., 2006; Walbridge and Nussbaumer, 2008; Ummenhofer et al., 2011) and demonstrated the applicability to hollow section structures. Importantly, Ummenhofer et al. (2011) showed that the fatigue strength was increased by approximately 50% relative to untreated tubular connections (i.e. a stress range enhancement factor of 1.5), as well as 70–90% compared to CIDECT Design Guide No. 8 (Zhao et al., 2000) or ISO 14347 (2008) (i.e. a stress range enhancement factor of 1.7 to 1.9 beyond design levels). Furthermore, high frequency hammer-peened tests by Ummenhofer et al. (2011) indicated S–N curves with a slope of m=4, based on the mean of the test data. Thus, relative to typical S–N design curves utilizing m=3 the fatigue line is flatter, producing an even greater benefit from weld treatment at a very high number of load cycles.
Hammer peening – and high frequency hammer peening – of weld toes in welded joints is an internationally accepted method for fatigue life enhancement. The technique is recognized in Canadian and American codes/standards and has been used on railway and highway bridges in North America. When performed by properly trained operators, a fatigue stress range improvement factor of 1.3 (or alternatively a factor of 2.2 on fatigue life) can, in general, be conservatively assumed for welded tubular steel connections.
AWS. 2010. “Structural Welding Code – Steel”, AWS D1.1/D1.1M:2010, American Welding Society, Miami, Florida, U.S.A.
CSA. 2003. “Welded Steel Construction (Metal Arc Welding)”, CSA W59-03, Canadian Standards Association, Mississauga, Ontario, Canada.
Günther, H.P. and Kuhlmann, U. 2009. “Nachweiskonzepte zur Bemessung ermüdungs-beanspruchter Bauteile unter Berücksichtigung von Schweiβnahtnachbehandlungsverfahren durch höherfrequentes Hämmern”, Stahlbau, Vol. 78(9), pp. 613 – 621.
Hobbacher, A. (Ed.) 2008. “Recommendations for Fatigue Design of Welded Joints and Components”, IIW Doc. 1823-07, IIW Doc. XIII-1251-07, IIW Doc. XV-1254r4-07, International Institute of Welding, Paris, France.
ISO. 2008. “Fatigue – Design Procedure for Welded Hollow-Section Joints – Recommendations”, ISO 14347:2008(E), International Standards Organisation, Geneva, Switzerland.
Kudryavtsev, Y., Kleiman, J., Lugovskoy, A. and Prokopenko, G. 2006. “Fatigue Life Improvement of Tubular Welded Joints by Ultrasonic Peening”, IIW Doc. No. XIII-2117-06, International Institute of Welding, Paris, France.
Pedersen, M.M., Mouritsen, O.O., Hansen, M.R., Andersen, J.G. and Wenderby, J. 2009. “Comparison of Post Weld Treatment of High Strength Steel Welded Joints in Medium Cycle Fatigue”, IIW Doc. XIII-2272- 09, International Institute of Welding, Paris, France.
Ummenhofer, T., Herion, S., Weidner, P., Hrabowski, J., Schneider, M. and Josat, O. 2011. “Extension of the Fatigue Life of Existing and New Welded Hollow Section Joints”, CIDECT Report 7BY-7/11, Karlsruhe Institute of Technology, Germany.
Walbridge, S. and Nussbaumer, A. 2008. “A Probabilistic Assessment of the Effect of Post-Weld Treatment on the Fatigue Performance of Tubular Truss Bridges”, Engineering Structures, Vol. 30, pp. 247 – 257.
Weich, I., Ummenhofer, T., Nitschke-Pagel, Th., Dilger, K. and Eslami, H. 2008. “Influence of Treatment and Loading Conditions on the Fatigue Strength of Welds Improved by High Frequency Peening”, IIW Doc. XIII-2218-08, International Institute of Welding, Paris, France.
Zhao, X.L., Herion, S., Packer, J.A. Puthli, R.S., Sedlacek, G., Wardenier, J., Weynand, K., van Wingerde,
A.M. and Yeomans, N. 2000. “Design Guide for Circular and Rectangular Hollow Section Welded Joints under Fatigue Loading”, CIDECT Design Guide No. 8, TÜV, Köln, Germany.