Nominal stress range fatigue of stainless steel fillet welds the effect of weld size

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1 Journal of Constructional Steel Research 54 (2000) Nominal stress range fatigue of stainless steel fillet welds the effect of weld size Kari E. Lahti a,*, Hannu Hänninen b, Erkki Niemi c a Oy Esab, Helsinki, Finland b Helsinki University of Technology, Helsinki, Finland c Lappeenranta University of Technology, Lappeenranta, Finland Abstract Three-point bending nominal stress range fatigue of stainless steel fillet welds was tested using two stainless steel grades: ferritic martensitic EN and austenitic EN The test results obtained were shown to be in good agreement with suggested fatigue classes in the Eurocode 3 design standard, derived from fatigue data on structural steels. However, if the size of the weld was increased, and the failure location could be moved to the weld toe instead of the weld root, a significant increase in fatigue strength was observed. Eurocode 3 was found to describe well the fatigue characteristics of the worst-case welds, i.e., welds prone to root failure Elsevier Science Ltd. All rights reserved. Keywords: Nominal stress; Fatigue; Stainless steel; Weld size; Throat thickness ; EN ; EN Introduction The fatigue strength of a material, as well as that of weldments, are key factors that are studied when selecting materials for applications which are prone to fatigue. The design of a structure under fluctuating loading needs careful consideration of applicable design guidelines and categorization of joint types of every detail of the structure. For example, in order to produce a fatigue-resistant transportation vehicle, the use of a suitable design standard such as Eurocode 3 (1992) [1] is needed. However, in the case of stainless steels, there is no specific design code dealing with fatigue at the moment. It has been customary to apply the same design guide- * Corresponding author. Tel.: ; fax: address: kari.lahti@esab.fi (K.E. Lahti) X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S X(99)

2 162 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) lines to stainless steel structures as for conventional carbon steel structures. In fact, the fatigue behaviour of stainless steel welds has indicated a broad similarity with that of carbon steel welded joints (e.g., [2,4,5,7]). Therefore, it is generally assumed that the same fatigue design rules apply for structural carbon manganese steels at temperatures up to 375 C, for austenitic stainless steels up to 430 C and for aluminium alloys up to 100 C. However, if the environment is corrosive, these upper limits for operating temperature are too high [4]. 2. Nominal stress approach by Eurocode 3 Fatigue data for stainless steels are very similar to those of carbon steels and, thus, the S N approach should be valid for stainless steels (S or s R =stress range, N=number of cycles to failure). In principle, the fatigue life for a given stress range is obtained from: log N log a m log( s R ), (1) alternatively expressed as: N a ( s R ) m. (2) The constants a and m are derived empirically from experimental data and are particular to different types of welded joint. For example, for the studied detail, if the slope m is 3, the corresponding value for log a is With a shallower slope of 5, log a is [1]. They include the effects of: local stress concentrations due to the weld geometry; the size and shape of acceptable discontinuities; the stress direction; residual stresses; metallurgical conditions; and the welding process and post-weld improvement procedures. Eqs. (1) and (2) relate to constant-amplitude loading. Welded structures are more commonly subjected to variable-amplitude loading caused by non-permanent actions like movement of loads along the structure or changes in loading directions [6]. For assessing the effects of variable-amplitude loading, use is made of the Palmgren Miner rule for cumulative damage: n i N i 1. (3) The scope of the design code Eurocode 3 [1] covers the design of stainless steel structures under non-corrosive conditions at ambient temperatures. The same classi-

3 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) fied details can be used for both carbon steels and stainless steels. Table 1 introduces some classified details for rectangular hollow sections (RHS). The detail used in this study is in the last row of the table with a fatigue class 36, which is the lowest of the Eurocode design curves. As can be seen, butt welds are graded higher than fillet welds, and also full-penetration welds are ranked higher than partial-penetration welds. This is in good agreement with the severity of stress concentration at the weld toe and root of the weld. 3. Test arrangement Two stainless steel grades were selected for study. EN is an austenitic stainless steel with 17% Cr and 7% Ni. It work hardens vigorously to high yield and tensile strength levels. The carbon content of this steel is typically low ( 0.03%) in order to decrease the susceptibility to sensitization during welding. The weldability of this steel is good, as it is with all austenitic stainless steels. EN is modified from conventional 12% Cr stainless steel by decreasing the carbon content to well below 0.03%, which is regarded as the limit for low-carbon steels. Also, the amount of titanium is limited because of the tendency of titanium to form brittle carbide Table 1 Classification of various weld details for rectangular hollow sections (Eurocode 3 [1], IIW document XIII , 1996) Fatigue class Detail Remarks EC3/11W 140/125 (90) Automated continuous longitudinal welds. No start of stop positions. Free from defects according to tolerance in the reference standard 9 - quality level 3. If start/stop positions are included, the class of 90 is used. 56/56 (45) Butt joint using butt welds. Height of the weld is less than 10% of the weld width, smooth joint. Welded in flat position, and found free from defects according to reference standard 9 - quality level 3. If not inspected by NDT, fatigue class of 45 must be used. 45/45 (50) Butt weld using intermittent plate. Load-carrying welds. Welds inspected and found free from defects according to reference standard 9 - quality level 3. If plate thickness is more than 8 mm, welds can be classified to one fatigue class higher, i.e., /36 (40) Fillet weld using intermittent plate. Load carrying welds. Plate thickness 8 mm. For plate thickness 8 mm, FAT 40 can be used.

4 164 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) phases in the heat-affected zone of a welded joint [3]. The mechanical properties and main chemical compositions of the studied RHS steels are shown in Table 2. Test welds were produced in many different locations by various welders, and also the welding equipment varied at different places. The consumables, shielding gases and materials remained unaltered between the different welding locations. Table 3 presents a summary of the welds produced at each location. Eurocode 3 design curves have two slopes. Down to cycles the slope is 3, and from cycles to the fatigue limit it is 5. The use of dual slopes is justified by assuming that, after crack initiation and some propagation, lower and lower stress cycles become effective. Therefore the fatigue limit is somewhat lower for variableamplitude-loaded structures than for purely constant-amplitude-loaded specimens. In this study tests were performed at constant-amplitude loading. In fatigue analysis, the nominal stress approach was used. Stresses were calculated by using linear elastic stress calculations according to: s M e, (4) I where M is the bending moment, I is the moment of inertia of the cross-section and e is the distance from centroid to the point considered. All fatigue tests were performed as three-point bending fatigue tests with a stress ratio (R) of 0.1. The stress was first calculated in the cross-section of the RHS wall, at a distance of 2.5 mm from the centerline of the intermittent plate. Depending on the actual fracture, the stress was recalculated after the test in the following way: 1. if fracture occurred from the weld root side through the weld throat, the calculated stress was multiplied by the ratio of the RHS wall thickness to the throat thickness at fracture; or 2. if fracture occurred at the weld toe, the stress was calculated at the weld toe taking into consideration the distance of the fracture from the centerline of the test specimen. 4. Results A total of 102 specimens was tested for three-point bending fatigue endurance. Specimens that lasted for more than cycles were not used in determining the slope of the fatigue curve. Also, specimens that did not fail by the dominating failure mode in each material/welding process combination, i.e., root or toe failure, were not included in the analysis. This left a total of 85 specimens to be used in the analysis of material/welding process combinations. When the specimens that lasted less than cycles but did not fail by the dominating failure mode were included in the analysis, 94 specimens qualified in total. Only those specimens that failed after cycles, or did not fail at all, were excluded in the latter analysis. Fatigue

5 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) Table 2 Chemical compositions and mechanical properties of the steel grades studied and the specific heats. Note the high R p0.2 values of RHS beams, representing a highly work-hardened condition Steel grade Chemical composition (%) Mechanical properties C Cr Ni Si Mn R p0.2 (N/mm 2 ) R m (N/mm 2 ) A5 (%) EN EN

6 166 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) Table 3 Summary of test series Welding performed at Material Process Consumable/shielding gas Carrus Oy, Vantaa, Finland EN Gas metal arc welding OK Autrod 16.52/Mison 2 Flux cored arc welding OK Tubrod 14.22/Robinon Simo Sainio Technics Ltd, Jämsänkoski, Finland EN Manual metal arc welding OK OK Plasma Modules Oy, Vantaa, Finland EN Powder plasma arc welding AISI 316L powder/aga MIX H 5 Helsinki University of Technology, Espoo, Finland EN Flux cored arc welding OK Tubrod 14.20/Mison 2 EN Flux cored arc welding Filarc PZ 6176/Robinon EN Gas tungsten arc welding OK Tigrod 16.10/Argon S EN Gas metal arc welding OK Autrod 16.52/Mison 2

7 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) test results for material/welding process combinations are summarized in Table 4 and Fig. 1. Test specimens can be divided into two separate classes depending on the fracture location: weld root failures and weld toe failures, Figs. 2 and 3. Figs. 4 and 5 show the corresponding S N data for each class. Slopes fitted to the S N data are 2.77 for the weld toe failure (37 observations) and 2.90 for the root failures (57 observations). Both curves are quite close to the slope of 3.0 suggested in Eurocode 3. The characteristic fatigue classes ( s c ) for the failure cases were 59 MPa for toe failures and 38 MPa for root failures, compared with s c =36 MPa suggested in Eurocode 3. Failure location and throat thickness (a) had a correlation as shown in Fig. 6 and Table 4. As the throat thickness increased, failure was more probable at the weld toe. On the basis of the average throat thickness values, it can be observed that from an approximate value of 4.5 mm upwards the dominant failure location was the weld toe. 5. Discussion If the failures are studied on the basis of location of failure, i.e., toe failure or root failure, there is a clear difference between the two groups (see Figs. 4 and 5). Weld root failures with a characteristic stress range at two million cycles, s c ( ), of 38 MPa are in good agreement with Eurocode 3, where the value s c ( )=36 MPa is given for the detail studied. Since Eurocode 3 does not set any specific requirements for the weld quality, it can be concluded that Eurocode 3 gives a reasonable design curve for the detail studied, if the failure occurs at the weld root. If the throat thickness is increased, failure is more likely to occur at the weld toe, Fig. 7. Weld toe failures have a s c ( ) of 59 MPa, which is fairly close to the Eurocode 3 class 56 for a butt-welded RHS joint. Therefore, it can be suggested that the severity of the weld root stress concentration decreases as the throat thickness increases. This results in weld toe failure, which is also a safer mode of failure because a crack can be detected before failure. In practice, this means that by increasing the throat thickness (i.e., the weld size) from 3 mm to 5 mm, the fatigue strength of the structure can be increased. For example, at the stress level s of 100 MPa, mean root failure life is approximately 500,000 cycles, but for mean toe failure the endurance is almost cycles! Therefore, slight overwelding may result in a prolonged life of a structure. 6. Summary A structural detail chosen from Eurocode 3 was studied in three-point bending fatigue tests. The stainless steels studied, EN and EN , were selected because of their increasing use in various structures. A total of eight different material/welding process conbinations was studied.

8 168 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) Table 4 Summary of S N test results Material/welding Number of Failure location Slope R 2a s c ( ) (MPa) Throat thickness process/consumable observations used (mm) [range and in slope median] determination EN /GMAW/OK 13 Weld toe Autrod b 4.4 EN /GMAW/OK 12 Weld toe Autrod c 5.2 EN /FCAW/PZ Weld toe EN /PPAW/AISI 316L 14 Weld root powder 1.8 EN /GTAW/OK 11 Weld root Tigrod EN /SMAW/OK Weld root EN /FCAW/OK 15 Weld root Tubrod a Value describing the goodness of the calculated slope (on a scale from 0 to +1, +1 being the best). b Manufacturing quality welded at Carrus. c Laboratory quality welded at HUT.

9 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) Fig. 1. Characteristic fatigue classes for various welding processes and materials. Corresponding average throat thickness is also presented on the secondary Y-axis. Fig. 2. Typical weld root failure. EN , GTAW, OK Tigrod 16.10, a=3.1 mm. The main factor dominating the fatigue endurance of the studied detail was shown to be the throat thickness of the weld. With increasing throat thickness, the stress concentration factor at the weld root decreases and the fatigue failure occurs at the weld toe. The fatigue class of 36 in Eurocode 3 agrees well with test results in the case of root failure. On the basis of this investigation, a higher fatigue class of 56 may be used for the studied detail if a sufficient weld size is guaranteed. Sufficient throat thickness is about twice the wall thickness of the RHS.

10 170 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) Fig. 3. Typical weld toe failure. EN , GMAW, OK Autrod 16.52, a=4.4 mm. Fig. 4. S N data for weld toe failures.

11 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) Fig. 5. S N data for weld root failures. Fig. 6. Effect of throat thickness on failure location. Endurance and throat thickness do not correlate.

12 172 K.E. Lahti et al. / Journal of Constructional Steel Research 54 (2000) Fig. 7. Schematic presentation of the effect of weld size on the failure location. References [1] Eurocode 3: Design of steel structures. Part 1-1: General rules and rules for buildings, ENV European Committee for Standardization, [2] Koskimäki M, Kyröläinen A. The fatigue endurance of stainless steels. In: Kähönen A, Sipilä S, editors. Väsymismitoitus (Fatigue Design), VTT Symposium 137, Espoo, Finland, 1993: [3] Lokka M. Properties of 12%Cr stainless steel welds. Ph.D. thesis. Espoo: Helsinki University of Technology, 1996:87 pp.+app. [4] Maddox SJ. Fatigue strength of welded structures. 2nd ed. Abington (UK): Abington Publishing, [5] Marquis G. High cycle spectrum fatigue of welded components, Appendix II. Long-life spectrum fatigue of carbon and stainless steel welds. Doctoral thesis. Espoo: Helsinki University of Technology, 1995:83 pp.+app. [6] Niemi E, editor. Stress determination for fatigue analysis of welded components, IIS/IIW Abington (UK): Abington Publishing, [7] Tubby PJ. The fatigue strengths of fillet welded joints in 3CR12 corrosion resisting steel (in air). In: Proceedings of Inaugural International 3CR12 Conference, Johannesburg, March, 1984: