IIW Recommendations for the HFMI Treatment For Improving the Fatigue Strength of Welded Joints Zuheir Barsoum

Transcription

1 IIW Recommendations for the HFMI Treatment For Improving the Fatigue Strength of Welded Joints Zuheir Barsoum KTH Royal Institute of Technology, Stockholm, Sweden Contact:

2 Improving the Fatigue Strength of Welded Structures Increased Service Life Good design practice Minimize fatigue loads, e.g. by avoiding resonance & vibration Use low SCF joints Avoid corrosion Place welds in areas with low stress High quality fabrication Good choice of plate and weld materials and process Good weld penetration, groove geometry Weld quality inspection w. correlation to fatigue life Improvement techniques Applied during fabrication Post fabrication treatment Geometry modification techniques Residual stress techniques Local stress peaks are reduced Lower SCF Surface quality is improved Tensile welding residual stresses are reduced High local compressive residual stresses are introduced Material work hardening Phase changes in weld material gives compressive stresses Surface quality is improved Lower SCF 2

3 Overview of Weld Improvement Techniques Green = covered by IIW recommendations Red = planned/ in progress (IIW) Blue = Current guideline 3

4 Improvement Techniques and their main effects 4

5 Current IIW Guidelines for Fatigue Strength Improvement Methods for Steel IIW Commission-XIII (2011) Geometry improvement techniques: burr-grinding and TIG re-melting (or TIG dressing) to reduce the size of the weld toe flaws to reduce the local stress concentration due to the weld profile up to 30% for steel Residual stress modification techniques: hammer peening and needle peening. to eliminate the high tensile residual stress in the weld toe region to induce compressive residual stresses at the weld toe up to 50% for s y > 355 MPa up to 30% for s y < 355 MPa Assumes an S-N slope of 3 for all joint types. 6

6 IIW HFMI Guidelines IIW Commission-XIII (2016) Published by Springer Ebook and hard copy 8

7 High Frequency Mechanical Impact - HFMI Example of HFMI devices available worldwide - ultrasonic impact treatment (UIT) - ultrasonic peening (UP) - ultrasonic peening treatment (UPT) - ultrasonic needle peening (UNP) - pneumatic impact treatment (PIT) - high frequency impact treatment (HiFiT) - Etc - > 90 Hz 9

8 High Frequency Mechanical Impact - HFMI Example of HFMI indenter sizes and configurations Typical weld toe profile in the as-welded condition and following HFMI treatment 10

9 Proposed Fatigue Strength Improvement using HFMI Some of Assumptions: The improvement method covered in these studies is applied to the weld toe All of fatigue design methods for HFMI improved welds are based on an assumed S-N slope of m = 5 and fatigue strength improvement factors are defined at N = cycles Examples of joints suitable for HFMI improvement Examples of joints NOT suitable for HFMI improvement 11

10 Literature Test Data Longitudinal non-load carrying welds Experimental R=0.1 constant amplitude axial fatigue data for HFMI treated longitudinal welds with stress concentration values obtained from finite element calculations based on structural and notch stresses Ref. steel type f y (MPa) treatment method plate thickness (mm) number of specimens in series K s K w 3 ( f =1mm) [23] S UP/ UIT [24] S690QL UIT [24] S690QL HiFIT [25] 16Mn UP/UPT [26] S UP/UPT [26] S UP/UPT [26] S TIG+UP [27] SS UP/UPT [27] 16Mn UP/UPT [27] Q235B UP/UPT [28] [29] S UIT [30] S PIT measured f y 2 nominal f y 12

11 Literature Test Data Cruciform welds Experimental R=0.1 constant amplitude axial fatigue data for HFMI treated cruciform welds with stress concentration values obtained from finite element calculations based on structural and notch stresses. Ref. steel type f y (MPa) treatment method plate thickness (mm) number of specimens in series K s 3 K w 3 ( f =1mm) [31] S355J UIT [31] S355J UIT [31] S460ML UIT [31] S460ML UIT [32] S355J PIT [32] S690QL PIT [33] AH UIT [34] 350W UIT measured f y 2 nominal f y 13

12 Literature Test Data Butt welds Experimental R=0.1 constant amplitude axial fatigue data for HFMI treated butt welds with stress concentration values obtained from finite element calculations based on structural and notch stresses. Ref steel type f y (MPa) treatment method plate thickness (mm) [19] S PIT 5 13 [20] S355J UIT number of specimens in series K 3 s K 3 w ( f =1mm) 1.12 (1.02) 1.61 (1.47) 1.18 (1.07) 1.79 (1.63) [20] S355J HiFIT (1.07) 1.79 (1.63) [19] S PIT (1.02) 1.61 (1.47) [20] S690QL UIT (1.07) 1.79 (1.63) [20] S690QL HiFIT (1.07) 1.79 (1.63) [21] E UP (1.06) 1.68 (1.53) [19] S PIT (1.02) 1.61 (1.47) 1 measured f y 2 nominal f y 14

13 maximum possible improvement, # of FAT classes Proposed Fatigue Strength Improvement using HFMI a design recommendation including one fatigue class increase in strength (about 12.5%) for every 200 MPa increase in static yield strength was proposed and shown to be conservative with respect to all available data Proposal for HFMI treated welds, m=5 IIW guideline for needle or hammer peening, m= over f y (MPa) 16

14 Nominal Stress Existing IIW FAT classes for as-welded and hammer or needle peened welded joints and the proposed FAT classes for HFMI treated joints as a function of f y f y (MPa) longitudinal welds cruciform welds butt welds as-welded, m = 3 all f y improved by hammer or needle peening, m = 3 f y < f y improved by HFMI, m = < f y * 140* 355 < f y < f y < f y * < f y New Guide line * no data available 17

15 Loading Effects the guideline states that the techniques are not suitable for R > 0.5 or when S max > 0.8 f y Minimum reduction in the number of FAT classes in fatigue strength improvement for HFMI treated welded joints as presented in previous Figure based on R ratio. R ratio Minimum FAT class reduction R 0.15 No reduction due to stress ratio < R 0.28 One FAT class reduction 0.28 < R 0.4 Two FAT classes reduction 0.4 < R 0.52 Three FAT classes reduction 0.52 < R No data available. The degree of improvement must be confirmed by testing 18

16 Loading Effects and Variable Amplitude Loading the guideline states that the techniques are not suitable for R > 0.5 or when S max > 0.8 f y Limitation on maximum constant amplitude stress range, Δσ, that can be applied to a weld in order to claim benefit from HFMI treatment (in MPa) 19

17 Structural Hot Spot Stress Proposed FAT values for SHSS approach as a function of f y. for non-load carrying welds for load carrying welds [4] Yildirim, H. C., Marquis, G. B. and Barsoum, Z., Fatigue Assessment of High Frequency Mechanical Impact (HFMI) Improved Fillet Welds by Local Approaches, International Journal of Fatigue, (accepted for publication). 22

18 Effective Notch Stress Proposed FAT values for ENS approach as a function of f y. 26

19 Effective Notch Stress Characteristic effective notch stress S-N curves for HFMI improved welded joints for R

20 Procedures Proposed Procedures and Quality Assurance Guidelines for HFMI Operator Training: days of operator training - identification of fatigue critical regions is also important to avoid extra costs and treatment Weld Preparation: - weld profile quality level B in ISO 5817 : Undercuts, Excessive overfill, Excessive concavity and Overlaps. - proper weld profile Safety Aspects: - less noise and vibration - eight hour work shift 28

21 Proposed Procedures and Quality Assurance Guidelines for HFMI Quality control (Qualitative Measures): Potential introduction of crack-like defect due to HFMI treatment of a weld with a steep angle or with too large of an indenter Resulting groove for a properly treated (left) and improperly treated weld toe (right) Micrographs of the induced crack-like defects due to improper HFMI treatment 29

22 Proposed Procedures and Quality Assurance Guidelines for HFMI Quality control (Qualitative Measures): - No thin line representing original fusion line should be visible the groove, No individual strikes visible thin crack-like defect which reduces or eliminates the effectiveness of the HFMI treatment defect-free groove but with individual indenter strike still visible indicating the need for additional passes 30

23 Proposed Procedures and Quality Assurance Guidelines for HFMI Quality control (Qualitative Measures): - The crack-like feature should be removed a) a) b) b) proper profile of an HFMI groove which has no sharp or crack-like features improper HFMI groove profile which shows the presence of a crack-like feature due to plastic deformation of the material. 31

24 Proposed Procedures and Quality Assurance Guidelines for HFMI Quality control (Quantitative Measures): - typically the optimum HFMI groove will be mm in depth and the width will be 3-6 mm gap Depth inspection using simple gauges. The gap between the base plate and the gauge in the right-hand picture indicates that 0.2 mm has not been achieved 32

25 Problem Example 1: Nominal stress design for a detail subjected to high R ratio Example: Consider the case of a welded detail which in the as-welded condition corresponds to FAT 63. The joint is fabricated from f y = 960 MPa steel and will be subjected to R = 0.5 loading. Question: What is the suitable characteristic line for design after HFMI treatment? Solution: An increase of eight fatigue classes can be claimed for R The resulting S-N curve is FAT 160 (63). The S-N curve for R=0.5 is reduced by three fatigue classes with respect to R 0.15 loading so the characteristic curve is considered to represent five FAT class improvement, i.e., FAT 112. With respect to the limitation on maximum stress range that can be applied to a weld in order to claim benefit from HFMI treatment in this example is Δσ = 340 MPa. This value would correspond to N 7760 cycles. The characteristic line is therefore considered to be valid over the entire life range shown. 34

26 Problem Example 1: Nominal stress design for a detail subjected to high R ratio 35

27 Demonstrator on Large Structures Bridge Components W-shape beams, f y = 690 MPa, R = 0.5 and -1 L = 4000 mm, FAT 80 (as-welded) Extracted HFMI-treated fatigue data for 550 < f y 750 MPa Solid lines are considered to represent 95% survival probability and dashed lines 5% survival probability. AW FAT FAT classes HFMI FAT 160 R- ratio penatly reduction - 3 FAT classes FAT 112 AW FAT FAT classes HFMI FAT 160 Kuhlmann, U., Dürr, A., Bergmann, J., and Thumser, R., Effizienter stahlbau aus höherfesten stählenunter ermüdungsbeanspruchung (fatigue strength improvement for welded high strength steel connections due to the application of post-weld treatment methods). Research report, Forschungsvorhaben P 620 FOSTA, Verlag und Vertriebsgesellschaft mbh., 36 Düsseldorf, Germany. 124

28 Demonstrator on Large Structures Civil/Infra Components Large-scale rolled and built-up ferrite steel beams f y = MPa and MPa R = Roy, S., and Fisher, J.W., Modified AASHTO design SN curves for post-weld treated welded details. Bridge Structures: Assessment, Design37and Construction, 2:4, pp

29 Demonstrator on Large Structures Civil/Infra Components (continued) HFMI-treated fatigue data for 355 < f y 550 MPa 0.15 R 0.28 < R < R 0.52 HFMI-treated fatigue data for 500 < f y 750 MPa 0.28 < R < R < R Roy, S., and Fisher, J.W., Modified AASHTO design SN curves for post-weld treated welded details. Bridge Structures: Assessment, Design and Construction, 2:4, pp

30 f y = 960 MPa, R =0.1, plate thickness = 8 mm Industrial Demonstrator Components in Cranes Finite Element Models and submodels using effective notch stress concept HFMItreatment at the weld toe 39

31 Industrial Demonstrator Components in Cranes (continued) Fatigue test results and evaluation according to the new proposals Major increase of fatigue strength by HFMI-treatment at the weld toe due to high notch effect at weld toe region (small, highly stressed volume) 40

32 Conclusions The design proposal is considered to apply to plate thickness 5 to 50 mm and for 235 MPa f y 960 MPa. Fatigue resistance curves for HFMI improved welds are based on an assumed S-N slope of m = 5 in the region N < cycles and, for variable amplitude loading, m = 9 for N. At least 4 FAT class increase in comparison to as-welded 1 FAT-class increase for every 200 MPa increase in yield strength Stress assessment may be based on nominal stress, structural hot spot stress or effective notch stress using stress analysis procedures as defined by the IIW. The design proposal includes proposals for the 1) effect of material strength, 2) special requirements for low stress concentration weld details, 3) high R-ratio loading conditions and 4) variable amplitude loading. Recommendation given for relevant equipment, proper procedures, material requirements, safety, training requirements for operators and inspectors, quality control measures and documentation has also been prepared and is published in this same issue. Succesful validation have been shown on larger industrial welded structures 42

33 Thank You for Your Kind Attention! Questions? 44