CC BY-NC-ND
0Avail able online at mwv.sciencedirect.com
Structural Integrity
* * __.
CrossMark
Procedia Structural Integrity 3 (2017) 77-84
■_>LI U^LUI Ul 11 Pl-cyi I Ly
ScienceDirect PrOCed ¡0
www.elsevier.com/locate/procedia
XXIV Italian Group of Fracture Conference, 1-3 March 2017, Urbino, Italy
Mechanical behaviour of hot dip galvanized steel connection under
cyclic loading
F. Bertoi*, S.M.J. Razavia, M.R. Ayatollahib F. Mutignania
a Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology (NTNU), Richard Birkelands vei 2b,
7491, Trondheim, Norway.
b Department of Mechanical Engineering, Iran University of Science and Technology, Narmak, 16846, Tehran, Iran.
Abstract
This short technical note; summarizes some recent data from hot dip galvanized steel bolted connections tinder fatigue loading;. In particular the effect of a galvanizing coating on the fatigue strength of S355 structural steel is analyzed in detail showing that the decrease of the fatigue life is very limited if compared with that of uncoated joints and the results are in good agreement with Eurocode detail category, without substantial reductions. The procedure for the preparation of the specimens is systematically described in this note providing a useful tool for engineers involved in similar practical applications. The results are compared with previous data from notched galvanized specimens weakened by a central hole and not treated specimens characterized by the same geometry.
CopyriSit © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creati vecommens.otbiliceтs es/by-nc-ndM.O/).
Peec-review under responsibilityof the Scientific Commitee of IGF Ex-Co.
Keywords: ga^n^ed steel, high cycle fatiguw, notch effect, stress coecceOratioe factor
1. Introduction
Different kinds of structural joints such as welded joints, bolted joints, rivet joints and adhesive joints are widely used in various industries, however, according to specific geometry of these joints, they are commonly considered as the most critical components in structures. (Khoramishad and Razavi 2014; Ayatollahi et al. (in press); Razavi et al.
* Corresponding author. Tel.: +47-735-93831. E-mail address: filippo.berto@ntnu.no
Copyright © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the Scientific Committee of IGF Ex-Co.
10.1016/j.prostr.2017.04.011
2017; Ayatollahi et al. (in press); Esmaeili et al. (in press)). Dealing with structural steel members, bolted connections are one of the most widely used methods for joining. It is advantageous for its versatility and reliability. Among the main advantages of bolted connections over those obtained by welding and riveting the following can be summarised: the economy of the process, the speed and ease of assembly, the reliability of service; the easiness of inspection, the performance under variable applied stresses. It is also worth of mentioning that for bolted connections pre-heating and heat damage to the coating on hot dip galvanized can be avoided as well as weld cracking or induced internal stresses.
Nomenclature
Act Nominal stress range due to tensile loading
Aoo Reference value of the fatigue strength at Nc = 2 million cycles
k Inverse slope of the fatigue curves
N Number of loading cycles to failure
R Nominal load ratio
Ta Normal stress-based scatter index (for 10-90% probabilities of survival)
HDG Hot Dip Galvanized
Ps Probability of survival
It is well known that the durability of structural components is strongly influenced by the degree of corrosion encountered in service operative conditions and due to the external environment or aggressive factors (Espallargas et al. (2013a), Espallargas et al. (2013b), Espallargas et al. (2015), Haugan et al. (2017)). Deterioration due to corrosion usually leads to the seizure of fasteners and premature failures, in the form of corrosion fatigue. A proper protection of bolted connections is, therefore of paramount importance if the overall integrity of a structure is considered a key point in the design. Hot-dip galvanizing is a surface treatment that allows the protection from corrosion and environmental aggressive agents. It can be successfully used in a large range of applications. Among them steel wires for bridges (Jiang et al. (2009), Yang et al. (2012)), automotive industries (Berchem and Hocking (2006a), Berchem and Hocking (2006b)), steel structures (James (2009)) can be mentioned.
Some authors correlated the fatigue strength to the coating thickness of the zinc layer (Bergengren and Melander (1992)) while other authors did not support any specific correlation of loss in the fatigue properties due to the coating thickness (Nilsson et al. (1989), Browne et al. (1975)). Vogt et al. (2000), by appropriately employing the Kitagawa-Takahashi diagram were able to identify a threshold value of the coating thickness not affecting the fatigue behaviour of unnotched components made of structural steels. Dealing with hot dip galvanized structural components it is worth of mentioning a recent contribution by the same authors (Berto et al. (2016)). The only preliminary study carried out on hot dip galvanized bolted connections was published by Huhn and Valtinat in a conference held in 2004. The available data on this topic are the very few and the present technical note is aimed to partially fill this gap of the recent and past literature providing also a clear explanation for the preparation and final assembly of the specimens. The paper is structured in the following way: in section 2 the geometry of specimens is briefly described, in section 3 the procedure for fabrication and assembly of the specimens is clearly provided and in section 4 the new fatigue data are summarized and compared with those taken from the standard in force for the same detail category. Finally a comparison between the present data and some recent data by the same authors from notched galvanized specimens weakened by a central hole is carried out.
2. Material and geometry of the specimens
The test specimens, made of S355 structural steel, for the bolted connection are shown in Fig. 1. Preloaded M12 bolts of class 10.9, system HR, were used in drilled holes. Hot dip galvanized coatings of fasteners according to UNI EN ISO 10684. The dimensions of the test samples were designed primarily to produce a net section fatigue failure of the middle main plate, and not in the bolts or cover plates (EN 1993-1-8). All the samples were hot-dip galvanized for an immersion time of 14 minutes which is typical in the application. The result was a zinc layer of about 400 ^m. This layer is commonly employed in practise in large structures.
Subsequently, the joint surfaces were treated according to a light sandblasting process (sweep blasting). In the
absence of precise information by the national and European regulations it is proposed a specific procedure which relates to indications in the literature (ISO 8503). In fact, the hot-dip galvanized surface of structural steels requires special handling. The aim was to remove the outer layer consisting of pure zinc notoriously soft and malleable thereby making the surface rough. Moreover, this type of blasting does not severely damage the existing coating obtained by hot dip galvanizing. A specified torque was applied to the nut in two steps using a calibrated wrench capable of an accuracy of ± 4 % according to ISO 6789. Most of these connections are used in steel structures frequently submitted to cyclic loading such as splice joints used in steel and composite bridges.
Fig. 1. Geometry of the test specimen employed in the research program.
3. Detailed procedure for specimen fabrication
After the hot dip galvanization process of steel plates, an adequate surface preparation was performed. The features of the sweep blasting procedure utilized are shown in Table 1. All above mentioned data were certified by the sand blaster firm inspection procedure. The results were a roughness equal to 32 ^m and a reduction of the zinc layer measured in 12 ^m (max. value). The surface preparation grade corresponded to SA1 (light blast cleaning). Fig. 2 illustrates hot-dip galvanized bolted connection before the test. A comparison before and after sweep blasting procedure has been shown in Figs. 3a and 3b, respectively.
According to prescriptions of UNI EN 1090-2 the assembly of the joints were carried out: the high strength bolts, class 10.9, system HR (UNI EN 14399-3) were tightened by the torque control method in two steps. The final torque applied to the fasteners (equal to 1.1 Mr.2) corresponded to 91 Nm as defined and declared by the fastener manufacturer in the box label. Before the fatigue tests all the joint bolt torques were checked.
Table 1. Sweep blasting procedure.
Adopted values Suggested values (*)
Abrasive garnet garnet or ilmenite
Mesh 80 80-100
Orifice diameter (mm) 10 10^13
Distance from the surface (mm) 400 350-^400
Angle to the surface 45° <45°
Blast pressure(kPa) 200 <275
Venturi nozzle
(*) with reference to well estiblished internitionil specificitions reliting to sarfice treitment for piintings (ISO 8573).
4. Results from fatigue tests
The fatigue tests were performed by using a servo-hydraulic MTS810 test system with a load cell capacity of 250 kN. All tensile stress-controlled fatigue tests were carried out over a range frequency varying from 5 to 10 Hz depending on the level of the applied load. A constant value of the load ratio, ^=0, was employed in all tests.
After the tests the specimens were examined and the fracture surfaces were analysed to get information about the crack initiation and propagation. Failure always happened, as expected, in correspondence of the first bolt of the connection, as visible in Fig. 4. In particular Fig. 4a shows a lateral view and Fig. 4b an upper view of the specimen. Fig. 5 shows two broken parts of a specimen. In particular Fig. 5a shows the holed plate and Fig. 5b the fracture surface in proximity of the bolted connection.
Fig. 2. Hot-dip galvanized bolted connection before the test
Fig. 3. Comparison between specimens before and after sweep blasting.
The representative failures shown in the figure always started from the net section in correspondence of the hole. From there the crack propagated through the material until the net section was so much weakened that finally a static crack caused the final failure. Multiple initiation points are well visible with a regular propagation until the final failure. This in agreement with (Valtinat and Huhn (2004)) which shows that in members with drilled holes a surface crack at the wall of hole was found always predominant. According to the fracture surfaces it can be observed that the crack has not a constant configuration in thickness. Thus, it is verified that the preloading applied to bolts influence crack initiation phase. Fig. 6 shows an example of the zinc layer from a SEM image.
Fig. 7 summarizes the results from fatigue tests of hot-dip galvanized bolted connections subjected to a nominal load ratio ^=0. The stress range over load cycles N is plotted in a log-log diagram. The unbroken specimens (run-out samples), over three million cycles, were not considered in the statistical analysis. They are marked with an arrow in Fig. 7. The mean curve corresponding to a probability of survival, Ps, equal to 50% is reported in the figure as well as the scatter band defined by lines with Ps 10%-90%. The dashed line refers, instead to a Ps of 97.7% for a direct
comparison with the Eurocode detail (EN 1993-1-9). The typical expression for the S-N curve in EC3 is reported below:
ActRNr = ACTkc 2 x!06
The inverse slope k value of the S-N curve and the scatter index T referred to Ps 10%-90% are also shown. The complete list of data related to hot-dip galvanized bolted connections is summarised in Table 2 properly marking the run-out specimens. The results from the statistical re-analysis is provided in Table 3. From the re-analyses of the data it is clear that considering a Ps of 97.7% at two million cycles Act is equal to 100 MPa which is slightly lower than the corresponding classified category Actc=112 MPa derived from EC3 for the considered uncoated bearing-type connection. In contrast to (Valtinat and Huhn (2004)), the inverse slope of the curve, k, is very close to that suggested by EC3. The data from hot dip galvanized specimens are plotted together with data from uncoated specimens characterized by the same geometry and tested in this research program. It is possible to observe that all data fall inside a narrow scatterband and that the reference value at two million of cycles and corresponding to a Ps of 97.7% remains almost the same (101 MPa) with almost no significant differences between galvanized and not-galvanized specimens (see Fig. 8). This result is in agreement with that reported in (Valtinat and Huhn (2004)) where it was shown that the use of preloaded high strength bolts gave a remarkable positive influence on the achieved fatigue life and that the detrimental effect of hot dip galvanizing can be easily neutralized. The advantage of this method is the easiness of handling with the maximum of efficiency of the bolted connection under fatigue loading. In this optic the accurate procedure described in section 3 for specimens preparation and assembly is surely necessary to guarantee a good repeatability of the connections in the different specimens. This procedure permits to allow beneficial compressive stresses in the neighbouring of the holes which are advantageous for the fatigue behaviour.
In fact, as described in (Berto et al. (2016)), the difference between galvanized and non-galvanized simple plates weakened by a central hole is much higher than that reported in the present paper for bolted connections. In the research conducted by Berto et al. (2016) a non-negligible deviation approximately equal to 30% has been found between coated and uncoated specimens with an insignificant reduction of the fatigue life due to influence of galvanization process. In that case two well different scatter bands were given without the possibility of providing a unified band for coated and uncoated specimens which is instead possible in the present investigation dealing with bolted connections.
Fig. 4. Typical failure from the hole corresponding to the first bolt of the connection. (a) lateral view and (b) upper view.
Fig. 5. Typical fracture surface after cyclic loading. (a) Plate and (b) bolted part.
Fig. 6. Layer thickness from a SEM image.
1.00E+04 1.00E+05 1.00E+06
Number of cycles to failure Fig. 7. S-N-curves of hot dip galvanized connections.
Scatter index T„ = 1.409 ! Ps = 2.3 % ■ ^^ Aos0.% = 120 MPa (N = 2 • 106)
ps = 97.7 %
101 MPa
89 MPa
A Hot dip galvanized specimens
o Uncoated specimens
1.00E+04 1.00E+05 1.00E+06
Number of cycles to failure
1.00E+07
Fig. 8. Unified scatterband considering uncoated and hot dip galvanized connections.
Table 2. Summary of all fatigue data from hot-dip galvanized bolted joints.
Act [MPa] N Notes
220 176000
120 3000000 Run-out
160 736700
160 597000
280 85000
100 3000000 Run-out
280 95000
140 3000000 Run-out
140 3200000 Run-out
160 3000000 Run-out
220 253000
220 167000
220 187000
200 251000
180 432807
180 393000
180 395000
140 1324000
Table 3. Summary of the statistical re-analysis for the hot-dip galvanized series.
Ps Act N
10% 529 10000
10% 130 2000000
50% 489 10000
50% 120 2000000
90% 452 10000
90% 111 2000000
97.70% 100 2000000
5. Conclusions
This short technical note reports some new data from hot dip galvanized steel bolted connections under fatigue loading. In particular the effect of a galvanizing coating on the fatigue strength of S355 structural steel has been accurately investigated showing that the reduction in the fatigue life is very limited if compared with that of uncoated joints. A detailed procedure for the accurate preparation of the specimens has been also systematically provided, showing a good repeatability in the assembly of the specimens and then in the final fatigue behavior.
References
Ayatollahi, M.R., Nemati Giv, A., Razavi, S.M.J., Khoramishad, H., (in Press). Mechanical properties of adhesively single lap bonded joints reinforced with multi-walled carbon nanotubes and silica nanoparticles. The Journal of Adhesion.
Ayatollahi, M.R., Samari, M., Razavi, S.M.J., da Silva, L.F.M., (in press). Fatigue performance of adhesively bonded single lap joints with non-flat sinusoidal interfaces, Fatigue & Fracture of Engineering Materials & Structures.
Berchem, K., Hocking, M.G., 2006a. The influence of pre-straining on the corrosion fatigue performance of two hot-dip galvanised steels. Corrosion Science 48; 4094-4112.
Berchem, K., Hocking, M.G., 2006b. The influence of pre-straining on the high-cycle fatigue performance of two hot-dip galvanised car body steels. Materials Characterisation 58; 593-602.
Bergengren, Y., Melander, A., 1992. An experimental and theoretical study of the fatigue properties of hot dip-galvanized high strength sheet steel. International Journal of Fatigue 14; 154-162.
Berto, F., Mutignani, F., Tisalvi, M., Laurenti, A., 2016. Effect of hot-dip galvanization on the fatigue behavior of notched and welded structural steel. Ingegneria ferroviaria 2, 105-118.
Browne, R.S., Gregory, E.N., Harper, S., 1975. The effects of galvanizing on the fatigue strengths of steels and welded joints, In: Proc. Seminar on Galvanizing of Silicon Containing Steels, ILZRO Publishers, Liege, 246-264.
EN 1993-1-8: Eurocode 3: Design of steel structures - Part 1-8: Design of joints.
EN 1993-1-9 : Eurocode 3: Design of steel structures - Part 1-9: Fatigue.
Esmaeili, E., Razavi, S.M.J., Bayat, M., Berto, F., (in press). On the flexural behavior of metallic fiber reinforced adhesively bonded single lap joints, Journal of Adhesion.
Espallargas, N., Aune, R.E., Torres, C., Papageorgiou, N., Munoz, A.I., 2013a. Bulk metallic glasses (BMG) for biomedical applications—A tribocorrosion investigation of Zr55Cu30Ni5Al10 in simulated body fluid. Wear 301(1-2), 271-279.
Espallargas, N., Johnsen, R., Torres, C., Munoz, A.I., 2013b. A new experimental technique for quantifying the galvanic coupling effects on stainless steel during tribocorrosion under equilibrium conditions. Wear 307(1-2), 190-197.
Espallargas, N., Torres, C., Munoz, A.I., 2015. A metal ion release study of CoCrMo exposed to corrosion and tribocorrosion conditions in simulated body fluids. Wear 332-333, 669-678.
Haugan, E.B., N«ss, M., Torres Rodriguez, C., Johnsen, R., Iannuzzi, M., 2017. Effect of Tungsten on the Pitting and Crevice Corrosion Resistance of Type 25Cr Super Duplex Stainless Steels, Corrosion 73(1), 53-67.
ISO 8503 - Preparation of steel substrates before application of paints and related products - Surface roughness characteristics of blast cleaned substrates.
ISO 6789 - Assembly tools for screws and nuts - Hand torque tools - Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure.
James, M.N., 2009. Designing against LMAC in galvanised steel structures. Engineering Failure Analysis 16; 1051-1061.
Jiang, J.H., Ma, A.B., Weng, W.F., Fu, G.H., Zhang, Y.F., Liu, G.G., Lu, F.M., 2009. Corrosion fatigue performance of pre-split steel wires for high strength bridge cables. Fatigue and Fracture of Engineering Materials and Structures 32; 769-779.
Khoramishad, H., Razavi, S.M.J., 2014. Metallic Fiber-Reinforced Adhesively Bonded Joints. International Journal of Adhesion and Adhesives 55, 114-122.
Nilsson, T., Engberg, G., Trogen, H., 1989. Fatigue properties of hot-dip galvanized steels. Scandinavian journal of metallurgy 18; 166-175.
Razavi, S.M.J., Esmaeili, E., Samari, M., Razavi, S.M.R., (in press). Stress analysis on a non-flat interface bonded joint, Journal of Adhesion.
UNI EN 1090-2 - Execution of steel structures and aluminium structures, Part 2: Technical requirements for steel structures.
UNI EN 14399-3 - High-strength structural bolting assemblies for preloading, Part 3: System HR - Hexagon bolt and nut assemblies.
UNI EN ISO 10684, Fasteners - Hot dip galvanized coatings.
Valtinat, G., Huhn, H., 2004. Bolted connections with hot dip galvanized steel members with punched holes. Connections in Steel Structures V, Amsterdam, pp. 297-309.
Vogt, J.B., Boussac, O., Foct, J., 2000. Prediction of fatigue resistance of a hot-dip galvanized steel. Fatigue and Fracture of Engineering Materials and Structures 23; 33-39.
Yang, W.J., Yang, P., Li, X.M., Feng, W.L., 2012. Influence of tensile stress on corrosion behaviour of high strength galvanized steel bridge wires in simulated acid rain. Materials and Corrosion 63; 401-407.
