Gary Marquis

Fitness for Service

Assessment of welds not meeting “standard” requirements may sometimes be of interest to investigate. An example is to determine the fatigue life from a found crack-like defect to failure. It is recommended to use local-based methods, such as the effective notch method or the fracture mechanics method with the guidance of well-established recommendations as, e.g., BS 7910 or comparable ones.

Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed fatigue assessment guidelines

In the past decade, high-frequency mechanical impact (HFMI) has significantly developed as a reliable, effective, and user-friendly method for post-weld fatigue strength improvement technique for welded structures. During this time, period 46 documents on HFMI technology or fatigue improvements have been presented within Commission XIII of the International Institute of Welding. This paper presents one possible approach to fatigue assessment for HFMI-improved joints. Stress analysis methods based on nominal stress, structural hot spot stress, and effective notch stress are all discussed. The document considered the observed extra benefit that has been experimentally observed for HFMI-treated high-strength steels. Some observations and proposals on the effect of loading conditions like high mean stress fatigue cycles, variable amplitude loading, and large amplitude/low cycle fatigue cycles are given. Special considerations for low stress concentration details are also given. Several fatigue assessment examples are provided in an appendix. A companion paper has also been prepared concerning HFMI equipment, proper procedures, safety, training, quality control measures, and documentation has also been prepared. It is hoped that these guidelines will provide stimulus to researchers working in the field to test and constructively criticize the proposals made with the goal of developing international guidelines relevant to a variety of HFMI technologies and applicable to many industrial sectors. The proposal can also be used as a means of verifying the effectiveness of new equipment as it comes to the market.

A round robin study of high-frequency mechanical impact (HFMI)-treated welded joints subjected to variable amplitude loading

High-frequency mechanical impact (HFMI) treatment has been significantly developed as a reliable, effective and user-friendly method for post-weld fatigue strength improvement technique for welded structures. The development of an International Institute of Welding best practice guideline for implementing HFMI has been hindered by the lack of directly comparable experimental data for numerous HFMI methods. In this study, nominally identical longitudinal attachments in high-strength steel were manufactured in one welding workshop and distributed to four HFMI equipment manufacturers for treatment. Specimens were fatigue tested on a machine using identical variable amplitude loading histories. HFMI groove measurements were done for each specimen and X-ray diffraction-based residual stress measurements were performed on 10 specimens. The HFMI groove dimensions and the residual stress states showed similarity in general, however small changes were observed. Experimental results indicate that all of the HFMI-improved welds from the HFMI equipment manufacturers satisfied the previously proposed characteristic S–N line based on both the yield strength and the specimen geometry. Results of the study are valuable and promising with respect to the development of a future guideline. The goal of the study has not been to compare treatments, so specific data points are not associated specific HFMI equipment manufacturers.

Overview of Fatigue Data for High Frequency Mechanical Impact Treated Welded Joints

Abstractpaper provides an overview of published experimental data on the fatigue strength of welded joints by high frequency mechanical impact (HFMI) treatment methods, In total, 414 data points from four specimen types are available,tests were performed using constant amplitude R = 0.1 axial tension fatigue, but some data for other R rations, variable amplitude testing and bending fatigue are also reported. An S-N slope of m = 5 gives a very good description of both individual data sets and of the composite data Design curve recommendations for the four joint types and for the structural stress-based design curve are given. HFMI treated specimens generally follow the same trend as experimental data for hammer peened specimens, but the degree of improvement is better. Data for large structures, at stress ratios other than R=0.1 and for variable amplitude loading are still needed in order to update the IIW guideline for post-weld improvement. There is a general trend for increasing fatigue strength improvement as a function of steel yield strength but this influence needs further study in order to develop guidelines. Quality assurance measures for HFMI treatment methods must also be defined.

Fatigue Strength of a Longitudinal Attachment Improved by Ultrasonic Impact Treatment

Improvement methods can be divided into two main groups: weld geometry modification and residual stress modification. The former remove weld toe defects and/or reduce the stress concentration while the latter introduce compressive stress fields in the area where fatigue cracks are likely to initiate. Ultrasonic impact treatment belongs to residual stress improvement methods. It makes use of an ultrasonic carrier frequency to accelerate hardened tools that, in turn, impact the weld toe. The fatigue strength of non-load carrying attachments in the as-welded condition has been experimentally compared to the fatigue strength of ultrasonic impact treated welds. Longitudinal attachment specimens made of two thicknesses of steel S355 J0 have been tested for determining the efficiency of ultrasonic impact treatment. Treated welds were found to have about 50% greater fatigue strength, when the slope of the S-N-curve is three. High mean stress fatigue testing based on the Ohta-method did not decrease the degree of weld improvement due to UIT. This indicated that the method could be also applied for large fabricated structures operating under high reactive residual stresses equilibrated within the volume of the structure.

Development of Weld Quality Criteria Based on Fatigue Performance

Historically, production technology researchers and structural design researchers have had only limited dialogue and each group has focused on their own narrow field of interest. This has led to inconsistencies in the definition of so-called “weld class systems” which have been primarily developed based on concepts related to good workmanship but have little or no relation to the actual performance of the welded structure. Additionally, some of the quality measures in existing systems are qualitative and, thus, subjective. In recent years, Commission XIII of the International Institute of Welding (IIW) has been promoting research and developing the technical background needed to develop a weld quality guideline which quantitatively relates weld acceptance criteria to the expected structural performance (primarily fatigue strength). A new weld class system with this same objective has recently been developed as a Volvo Group Standard. This system is described in this paper. The new standard has three quality levels for fatigue; as-welded normal quality, as-welded high quality and post-weld treated quality. It contains acceptance limits which are consistent with the expected fatigue strength and which can more objectively handle revisions. This new system will help in the development of new structures with lower weight and increased reliability.

The Need for a Weld Quality System for Fatigue Loaded Structures

Historically, production technology researchers and structural design researchers have had only limited dialogue and each group has focused on their own narrow field of interest. However, the drive to increase welding speed and the desire to increase the use of higher strength steel for fatigue-loaded structures have led to the development of an integrated research approach that includes joint efforts in several key technologies: high-speed welding processes, high strength materials, cost-effective NDE, post-weld treatments and FE-based design assessment tools. The current paper focuses on the need to develop a suitable classification system for high quality welds. Current weld quality systems for welds were developed primarily with static loading in mind and are not directly suitable for fatigue loaded components. The need for a multi-technical approach and implementation of a design for purpose philosophy are emphasised.

Behavior of compressive residual stresses in high strength steel welds induced by high frequency mechanical impact treatment

Residual stress state plays an important role in the fatigue life of welded structures. The effect can be beneficial or detrimental, depending on the nature of residual stresses. High frequency mec ...

MIG Brazing as a Means of Fatigue Life Improvement

MIG-brazing is examined as a potential fatigue strength improvement method for welded structures. The fatigue strength of joints produced using MIG-brazing as the primary joining technique and for the improvement of joints welded using traditional welding was studied experimentally. The effect of the improvement for a typical structural steel weld is achieved by a final pass along the weld toe using MIG-brazing. In total, 31 fatigue test results are presented. These include four pilot cases of welded cruciform joints and two main cases of plates with transverse attachments. The results of the fatigue tests were compared to IIW recommendations for as-welded and improved welds. The fatigue strength of MIG-brazed joints was good. For transverse non-load-carrying attachments, an improvement of 70–80 % measured as local nominal stress was gained with respect to IIW recommendations. The fatigue strength of solely MIG-brazed joints and joints improved by MIG-brazing was almost equal. As an alternative joining method, MIG-brazing is used in some industrial applications. This study has given clear indications that this method could also potentially be used as a weld improvement method for new or repaired structures. However, additional research is needed in order to gain more confidence that the degree of improvement can reliably be obtained in production.

Development of Data Sheets for Statistical Evaluation of Fatigue Data

Instructions were developed for statistical analysis of the laboratory fatigue data step-by-step. Practical cases for solving several questions raised in the treatment of test data, which involved estimation of necessary sample size, verification of the statistical equivalence of the collated sets of data, and determination of characteristic curves in different cases, were given by using the methods and formulae in the document IIW-XIII-2138-06 (best practice guide on the statistical analysis of fatigue data) as a demonstration of various statistical methods of developing a sound procedure for creating reliable calculation rules for the fatigue analysis.