UDK 539.44 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 41(5)249(2007) A PRELIMINARY S-N CURVE FOR THE TYPICAL STIFFENED-PLATE PANELS OF SHIPBUILDING STRUCTURES PRELIMINARNA KRIVULJA S-N ZA TOGE PLOŠČATE PANELE ZA LADJEDELNIŠKE STRUKTURE Luljeta Gusha1, Skender Lufi2, Marenglen Gjonaj2 technological University "I. Q. Vlora", Marine Faculty, Naval Engineering Department, Lagja: Pavaresia, Rruga: Sadik Zataj, Vlora-Albania 2Politechnical University of Tirana, Mechanical Faculty, Mechanical Department, Sheshi: Nene Tereza, Tirana-Albania gusha@aul.com.al Prejem rokopisa — received: 2006-05-17; sprejem za objavo - accepted for publication: 2007-07-10 This paper presents the results of a preliminary study focused on the structural behavior of typical stiffened plate panels used for shipbuilding structures and their fatigue strength under a lateral load. The investigated panels are thin plates, welded with longitudinal bulb stiffeners through alternate welding seams. This makes the panel a composite structural element with a complex strength behavior. The aim of the research was to obtain data about the failure conditions of the panels. Testing covers the bending tests carried out on the real-size panels of shipbuilding structures. A reliable definition of a fatigue design curve was not possible due to the limited number of specimens, although a tentative S-N curve was drawn on the basis of the test data. Key words: shipbuilding panels, fatigue, real-size testing, S-N data Članek predstavlja rezultate preliminarne študije ciljane na strukturno vedenje tipičnih togih ladijskih panelov in utrujenostno trdnost pri bočni obremenitvi. Paneli so tanke plošče zvarjene z podolžnimi rebri za preprečenje izbočenja z alternativnimi spoji. Taki paneli so kompozitni strukturni elementi s kompleksnim trdnostnim vedenjem. Cilj raziskave je bil opredeliti podatke o pogojih za nastanek preloma panelov. Preizkusi so obsegali upogib panelov realne velikosti za ladijske strukture. Zanesljiva opredelitev krivulje S-N ni bila mogoča zaradi omejenega števila preizkusnih panelov. Zato je bila določena le poizkusna krivulja S-N na podlagi rezultatov preizkusov. Ključne besede: ladjedelniški paneli, utrujenost, preizkušanje v realni velikosti, podatki S-N 1 INTRODUCTION Stiffened plate panels are the basic structural components of a ship's structure. Fatigue constitutes a major source of local damage in ships and other marine structures, since the most important loading on the structure, the wave-inducted loading, consists of large numbers of load cycles of alternating sign. The prevention of fatigue failure in ship structures is strongly dependent on proper attention to the design and fabrication of structural details. Much of the quantitative information on fatigue obtained by experiments and S-N fatigue design curves is drawn on the basis of test data. With the test results, and based on Wöhler's diagram, a tentative attempt is made to construct an S-N curve for the stiffened panels, where the thin plates are welded with longitudinal bulb stiffeners through alternate welding seams. According to IIW documents, more than 15 specimens are in general necessary to establish the fatigue limit and more than 25 for the S-N curve, using static analysis methods (e.g., the staircase method)8. 2 EXPERIMENTAL The experimental measurements were made at the DINAV of the Naval Structural Laboratory of Université degli Studi di Genova. 2.1 Data on the panel and model description As a model for the experimental test, a stiffened plate panel of real size, simply supported, was considered. The effects of stress and initial distortion were not considered. Panel-type stiffeners were welded in the span between the transversal T-beams, using alternate welding seams with a length of 50 mm and a step of 200 mm. It is worth pointing out that at a 50-mm interval the stiffeners are welded to the plate, alternatively on one or the other. The panels were built according to standard fabrication practice using semi-automatic arc welding. The welding parameters are as follows: Wire: FRO Fluxofil 19, d =1 mm Voltage: 23/24 V, Current: 140/150 A Welding speed: 50 cm/min Throat: 3.5 mm Table 1: Geometrical characteristics of the panel Tabela 1: Geometrijske značilnosti panelov Plate dimensions (1800 x 2600 x 5) mm Stiffeners(HP80X6) Ix = 39.0 cm4 Wmin = 8.15 cm3 Effective plate width included (s = 500 mm) Ix = 155 cm4 Wmin = 21 cm3 Transversal beams (180 x 90 x 5 x 8) mm (180 x 5; 90 x 8) mm Materiali in tehnologije / Materials and technology 41 (2007) 5, 231-236 231 L. GUSHA ET AL.: A PRELIMINARY S-N CURVE FOR THE TYPICAL STIFFENED-PLATE PANELS Reference standard: MM-042F-331 Welding on unpainted surfaces Table 2: Characteristics of materials Tabela 2: Lastnosti materiala Yield stress oy = 355 N/mm2 Yield load (s = 500 mm) Py = 49.7 MPa Young's modulus Ex = 2-105 MPa Shear modulus Gxy = 0.793-105 MPa Poisson's modulus f = 0.33 Density of the material d = 7.9-10-5 kg/mm3 All the panels were manufactured from the same material. The panel is modeled as shown in Figure 1, and is considered to be supported on two T-beams along both its longest sides (2600 mm) and along two other free sides (Figure 2). The assumed failure criteria are: • A crack propagation over the section of the bulb, • A number of cycles N =1.0 x 106 2.2 Procedure and experimental tests The bending was achieved with a transversal beam along the whole panel width. The loading beam had an Section B-B 1 ' ± v.- . 1 Section A-A I-v-i—v-i *_ 1 i ! ■ i : i „ 500 . 400 o o 00 i i i i ■ i i : i i OOS o o -3" 2600 _4j a) b) Figure 1: Sketch (a) and isometric (b) view of the tested panel Slika 1: Skica (a) in izometri~en (b) pogled preizkusnega panela "I" section with two large flanks of size (1800 x 200) mm and a thickness of 20 mm 112. Its inertia moment is large enough to ensure a constant distribution of the load along the beam. Between the flat and the panel surface, a thick rubber strip was interposed during the tests in order to prevent damage to the surface. The strip had a width of 200 mm and this should be regarded as the area of the 200 MPa load application. The load jack is hinged to a frame and acts vertically downward, as shown in Figures 2 and 3. It was selected on the basis of the predicted limit load of 10 t. A load cell was interposed between the jack and the loading beam 3,12. Seven linear strain gauges for each panel and one load cell (200 MPa max. load), as in Figure 3, were applied for each test. Two rows of linear strain gauges were placed near the loading "I" beam (Figure 3). These strain gauges measure the strain in the longitudinal direction of the bulb after each loading sequence. The panels were tested in the range of about 0.7 yield stress with a sinusoidal pulsating load. The maximum stress during the fatigue test did not exceed the yield stress 7S 910. The load and strain were continuously monitored. The tests were monitored with strain-gauge measurements and visual inspections. The signals were stored in the data files in millivolts and converted into the real physical quantities by means of a calibration (Figure 4). The displacement transducers were placed in their position and then calibrated "in situ" with mechanical gauges. The acquisition program was run for every test at a sampling rate of 1.4-1.5 Hz 18 for a predetermined period of time. For each acquisition the maximum, minimum, mean and range values were evaluated and subsequently converted into stress and the acting force. The range versus cycles and the maximum range versus cycle plots are presented in Figure 5 and 6. Figure 2: View of the testing device Slika 2: Preizkusna naprava 250 Materiali in tehnologije / Materials and technology 41 (2007) 5, 249-253 L. GUSHA ET AL.: A PRELIMINARY S-N CURVE FOR THE TYPICAL STIFFENED-PLATE PANELS Figure 3: View of the position of the strain gauges Slika 3: Položaji merilnih doz Figure 6: Test measurements plot: max vs. cycles Slika 6: Rezultati meritev obremenitev v odvisnosti od števila am- plitud Figure 4: Gauges and load-cell measurements at the end of the test after about 1.45 x 106 cycles Slika 4: Doze in meritve obremenitvenih celic pri koncu preizkusa pri 1,45 ■ 106 amplitudah 2.3 Results and discussion The results are shown in graphical form in Figures 5, 6 and 7. The panel was loaded with a pulsating sinusoidal loading wave with a range of about 41 MPa, at a mean level of about 25 MPa for 106 cycles (stress ratio R ~ 0.1). No cracks were found. The range load was then increased up to 52 MPa, at a mean level of 33 MPa (stress range R ~ 0.1) for 1.35 x 106 cycles. Then another 106 cycles were applied at 46.5 MPa, at the same mean level. No cracks were detected Figure 7: Cracks in the central stiffener, both sides and the lateral stiffener (after panel dismantling) Slika 7: Razpok v centralnem rebru, obe strani in bočna utrditev (po demontaži panela) Figure 5:. Test measurements plot: range vs. cycles. Slika 5: Rezultati meritev obremenitev v odvisnosti od števila amplitud Figure 8: Composite cross-section of beam Slika 8: Prečni prerez rebra with a visual examination and strain-gauges signal analysis. At about 1.45 x 106 cycles a crack started from the head of the central bulb stiffener about the span middle and propagated up to the plating, while a second crack started in the west side bulb stiffener in the same position and propagated up to the stiffener web. From the analysis of the gauge measurements, it is concluded that the crack propagated for about 104 cycles. The boundary conditions of the panel are considered as simply supported, the load is applied in the center of the panel and the panel is in a positive bending moment condition with the maximum of the bending moment in the center of the panel. 250 Materiali in tehnologije / Materials and technology 41 (2007) 5, 249-253 L. GUSHA ET AL.: A PRELIMINARY S-N CURVE FOR THE TYPICAL STIFFENED-PLATE PANELS According to the elastic beam theory, in this case, the neutral axis is near the plate, as in Figure 8, so the maximum stress is achieved at the head of the bulb. In the preliminary assessment using the finite-element method, for these panels 11 and in static loading, three critical areas of the panels' collapse were identified: a) The region including the plate area between the stiffeners (in compression); b) The region including the plate area around the stiffeners (in compression); c) The region including the head area of the stiffeners (in tension) The specific stress-strain situation in each of these regions defines the type of collapse that can occur in them. The expected collapse in the region (a) is generally of a static nature, while in the regions (b) and (c) it is generally of a fatigue type. In this case, since the region (b) is in compression, the region (c) is expected to collapse in fatigue 241011. The finite-element analysis showed the region (c) as the hottest stress area between the (b) and (c) regions. 3 PRELIMINARY S-N CURVE Due to the limited number of specimens it was not possible to obtain a curve. For this reason, a preliminary S-N curve was drawn on the basis of the test data. According to the IIW documents, more than 15 specimens are necessary to establish a reliable fatigue limit and more than 25 for the S-N curve, using static analysis methods (e.g., the staircase method) 8. To construct the S-N curve, we relied on Wohler's curve. The shape of this curve is shown in Figure 9 ^a4 5 610. Where: • S is the upper point, static resistance crr vs. 103 cycles. • G is the point of the fatigue limit