Journal of Reinforced Plastics and Composites OnlineFirst, published on August 14, 2008 as doi: /

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1 Journal of Reinforced Plastics and Composites OnlineFirst, published on August 14, 28 as doi:1.1177/ Effect of T-Joint Geometry on the Performance of a GRP/PVC Sandwich System Subjected to Tension EGE ANIL D_ILER AND C _IC EK ÖZES Dokuz Eylu l University Engineering Faculty Department of Mechanical Engineering Bornova 31 _Izmir, Turkey GÖKDEN_IZ NES ER* Dokuz Eylul University Institute of Marine Sciences and Technology Baku B. 1 Inciraltı, 334 _Izmir, Turkey ABSTRACT: Mechanical performances of six sandwich type T-joints, used in marine applications subjected to tensile load, have been investigated both numerically and experimentally in this study. T-joints, each with different geometries, have been manufactured, Type A: continuous core in joint with right angle; Type B: core removed at joint; Type C: core with wedge fillet; Type D: core with 2 mm radius fillet; Type E: core with 7 mm radius fillet and Type F: DK-CND1 of Toftegaard and Lystrup with overlaminate. The skin was a mm thick orthophitalic polyester/glass laminated composite and the core was PVC (Divinycell H8). Due to absolute values of the maximum strain values of the T-joints, Type E shows promising performance under tension while Type B is the weakest. It is not recommended to use Type B in the structures subjected to tension. Grading from the strongest to the weakest of T-joints is Type C, D, A and F. Results of the numerical modelling and tests also affirm the utility of the 2D FE models for further studies of the strain distribution in such sandwich T-joints. KEY WORDS: composites in marine use, T-joints, PVC/GRP sandwich system. INTRODUCTION FOR COMPOSITE BOAT construction, hull and bulkhead are the primary structures in maintaining boat stiffness under various loadings. These loads are due to wave impact, structural and cargo weight as well as various actions, such as docking and berthing loads. Additionally, temperature induced stress should be included for MCMV applications [1]. *Author to whom correspondence should be addressed. gokdeniz.neser@deu.edu.tr Figures 2, 3 and 6 appear in color online: Journal of REINFORCED PLASTICS AND COMPOSITES, Vol., No. / /8/ 1 1 $1./ DOI: / ß SAGE Publications 28 Los Angeles, London, New Delhi and Singapore + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 1] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Copyright 28 by SAGE Publications. Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

2 2 E. A. D_ILER ET AL. The assembly of complex composite structures in marine use dictates the need for efficient joining methods. One type of joint found in structural applications is the composite T-joint. This consists of a flange, web, and filler. The flange interfaces with the skin, the web provides an interface for attachment to the substructure, and the filler provides continuity of load transfer between the web and flange. T-joints are found at sandwich bulkhead-to-skin, rib-to-skin, and spar-to-skin interfaces. The purpose of the T-joint is to transfer flexural, tension and shear loads to the skin [2]. Composite T-Joints are used extensively in the marine and aerospace industries. Shenoi et al. [3,4] studied structural composite T-joints in small boat applications. In their study, they evaluated the influence of joint geometry on the out-of-plane load transfer for hull bulkhead joints. Experimental testing was performed on T-joints with foam inserts and radius fillers. The reliability of the hull and bulkhead depends mainly on the T-joint as the connection between the structures. It has been found that the critical part of the joint is the overlaminate or the bonded interface between laminate and hull or bulkhead [1,]. Theotokoglou [6 8] reported experimental and numerical results of composite sandwich T-joints under pull of load. The behavior of the sandwich T-joint under tension was shown to be characterized by geometric non-linearities due to out-of-plane loads, and by material non-linearities due to local yielding [9]. TYPE OF T-JOINTS Samples of the joints were manufactured in six different geometries: Type A: continuous core in joint with right angle; Type B: core removed at joint; Type C: core with wedge fillet; Type D: core with 2 mm radius fillet; Type E: core with 7 mm radius fillet; and Type F: DK-CND1 of Toftegaard and Lystrup [1] with overlaminate. The geometries of the joints are shown in Figure 1. T-joints considered in this experimental study comprise of a 2 mm sandwich bulkhead joined at right angles to a 4 mm thick sandwich hull, by means of mm thick GRP skins and overlaminates. These GRP laminates were made from chopped strand mat with a density of 243 kg/m 3 and orthophitalic polyester (Dewester 196 from the Dewilux Inc.) widely used in boat building. A glass ratio of 3% by weight and 18% by volume was achieved. The core was structural PVC foam (Divinycell H 8) which is a general purpose grade suitable for most sandwich applications [11]. The same core material was used as the filler in joints C, D, E and F. Characteristics of the skin and core material can be found in Table 1. The bulkhead and hull sections were manufactured using hand lay-up in the workshop of a boat builder in Izmir. The overlaminate was hand lay-up, incorporating one layer of chopped strand mat in the interface. The same worker laid the glass fibers to overcome possible differences in workmanship practice. FE ANALYSIS The commercial FE code ANSYS.4 was used to make a 2-D plain strain model of the T-joint tensile specimen. The finite element model of a representative T-joint with + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 2] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

3 GRP/PVC Sandwich System Subjected to Tension A 1 B C D E F Figure 1. The geometries of the joints. boundary conditions and point of loading is shown in Figure 2. The set of boundary conditions fixed both ends in all directions but allowed the end to move in the lateral (x) direction. A tensile load of 2 kn was applied. Isotropic material properties were used for the overlaminate, hull, bulkhead and the filler. The material properties were determined experimentally. Elastic properties, Young modulus E and Poisson s ratio of the composite materials were determined by means of a tensile test on bulk samples, instrumented with an extensometer and strain gauges. + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 3] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

4 4 E. A. D_ILER ET AL. Table 1. Characteristics of the skin and core materials. Property GRP skin PVC core Density (kg/m 3 ) 14 8 Tensile modulus (MPa) 11 8 Shear modulus (MPa) Poisson s ratio Figure 2. The finite element model of a representative T-joint with boundary conditions and point of loading. Experimental Method EXPERIMENTS In the static tensile test of T-joints, samples were mounted to the load cell at the top of the joints, and the load was applied at both ends of the hull through pairs of steel plates with a distance of 36 mm. The dimension of these plates was in mm, and they were connected to each other by four steel rods of a diameter of 16 mm. Additionally, the specimens were fixed by steel plates of a dimension of mm and four M12 bolts. The test ring with a sample mounted in the testing machine is shown in Figure 3. Strain-gauges were placed at locations where significant strains were expected to occur according to the numerical model. The report of St John et al. [12] has indicated that failures occur under tensile loading, within the filler, between the filler and the hull, between the overlaminate and the hull, and between the overlaminate and the bulkhead. Table 2 explains the place and measurement characteristics of the strain-gauges readings. The tensile tests are performed on an Instron 1114 testing machine, with its data acquisition system at a constant cross-head speed of 1 mm/min. A tension load of 2 kn was + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 4] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

5 GRP/PVC Sandwich System Subjected to Tension Figure 3. The testing machine. Table 2. Strain-gauges readings type and place. Strain Strain-gauge readings Type Place SGA1 Axial Between the overlaminate and the hull SGA2 Between the hull s top skin and the core SGA3 Between the hull s bottom skin and the core SGA4 At the bottom surface of the hull SGP1 In plane At the top-side of the top surface of the overlaminate SGP2 At the bottom-side of the top surface of the overlaminate SGP3 At the bottom surface of the overlaminate SGT1 Through-thickness Between the overlaminate and the hull SGT2 Between the hull s top skin and the core SGT3 Between the hull s bottom skin and the core SGT4 At the bottom surface of the overlaminate applied through the web of joints. The hull was considered to be restrained near the two ends of the joint. Measurements of the strains were made with steps of 2 N, by interrupting the load applied in order to observe the strain distribution during tension. RESULTS Strain-gauge readings during the tests are shown in Figure 4. Regions of maximum axial and through-thickness strain-gauge readings are also summarized in Table 3. High strain + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. ] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

6 6 E. A. D_ILER ET AL. Type A Type B Type C Type D SGA1 SGA2 SGA3 SGA4 SGP1 SGP2 SGP3 SGT1 SGT2 SGT3 SGT Type E Figure 4. Strain-gauge readings. Type F Table 3. The strain-gauges which give maximum values. T-joint type Strain-gauge reading The maximum axial strain The maximum through-thickness strain A SGA4 SGT1 B SGA4 SGT2 C SGA1 SGT2 D SGA4 SGT2 E SGA4 SGT2 F SGA1 SGT1 + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 6] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

7 GRP/PVC Sandwich System Subjected to Tension 7 values between the overlaminate and hull in all of T-joints considered in this study were observed. From these weaknesses, it is certain that strengthening the mentioned regions is vital to manufacture the T-joints which have promising performance. Apart from this result the following apply:. For the type A joint, high strain values occurred at the top-side of the top surface of the overlaminate and at the bottom surface of the hull.. The top-side of the overlaminate of the type B joint tends to separate from the core of the bulkhead under the shear effect of the applied load. It is suggested that type B joints are much more suitable for work under compressive loads (i.e., deck loads in a boat) than under tensile loads. This type of joint, subjected to tensile loads, shows the weakest performance.. As for the type C joint, the highest strains all occured around the joint geometry. As there was a complex failure mechanism in this type joint, there is a need to strengthen all elements of it.. Weakness at the type D and type E joints occurred at the top surface of the overlaminate and between the overlaminate and the core of the bulkhead. There was also a decrease of performance around the hull core, close to the joint.. Under the fillet, there was no good performance in the type F joint. Optimization of the fillet size is a new subject for research. The results obtained from the numerical modeling and tests are shown in Figure. Failures occurred in some joints, also confirming the FE models (Figure 6). Type A SGT2 SGA3 SGI2 SGT3 SGA1 SGT1 SGI1 SGA4 SGA Type B Experimental Numerical SGI2 SGA2 SGT3 SGT2 SGI1 SGA1 SGT1 SGA4 SGA3 Type C SGI2 SGA4 SGA2 SGT2 SGA1 SGT4 SGT3 SGI1 SGT1 SGA3 SGA1 SGT2 SGT1 SGA4 SGI4 SGA2 SGT3 SGI2 SGA3 SGI3 Type D Type E Type F SGI2 SGA3 SGT3 SGI3 SGI1 SGA2 SGT1 SGT2 SGA4 SGA1 SGI2 SGA1 SGI1 SGA2 SGT3 SGI3 SGA3 SGT2 SGA4 SGT1 Figure. Results from the numerical modeling and tests. + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 7] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

8 8 E. A. D_ILER ET AL. Figure 6. Failures during the test. CONCLUSION The effects of geometry on the strain distribution in six types of T-joint in marine use were studied both numerically and experimentally. Due to absolute values of the maximum strain values of the T-joints, Type E shows promising performance under tension while Type B is the weakest. The use of Type B in + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 8] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

9 GRP/PVC Sandwich System Subjected to Tension Peak strain (µε) Type A Type B Type C Type D Type E Type F Type of joint Figure 7. Peak strains. the structures subjected to the tension is not suggested. Among the geometries tested, grading from the strongest to the weakest gives Type C, D, A and F. Peak strains are shown in Figure 7. The experimental results fell between the FE predictions for all cases, thereby validating the models. This affirms the utility of the 2D FE models for further studies of the strain distribution in such sandwich T-joints. It was observed that Type F has an advantage in manufacturing, as it has the shortest assembling time and less workmanship. ACKNOWLEDGMENTS The authors gratefully acknowledge Mr. Mehmet Emin Tacar from the Tacar Boat Company (_Izmir, Turkey) for his assistance in the experimental study, and the Tu rk Loydu Foundation, which is the most prominent supporter of research in ship technology in Turkey. REFERENCES 1. Hawkins, G. L. and Shenoi, R. A. (1993). A Parametric Study to Determine the Influence of Geometric Variations on the Performance of a Bulkhead to Shell Plating Joint. In: Proceedings of the 9th International Conference on Composite Materials, vol. 4, pp , July, Madrid. 2. Stickler, P. B. and Ramulu, M. (21). Investigation of Mechanical Behavior of Transverse Stitched T-joints with PR2 Resin in Flexure and Tension, Composite Structures, 2: Shenoi, R. A. and Violette, F. L. M. (199). A Study of Structural Composite Tee Joints in Small Boats, Journal of Composite Materials, 24: Shenoi, R. A. and Hawkins, G. L. (1992). Influence of Material and Geometry Variations on the Behavior of Bonded Tee Connections in FRP Ships, Composites, 23(): Dharmawan, F., Thomson, R. S., Li, H., Herszberg, I. and Gellert, E. (24). Geometry and Damage Effects in a Composite Marine T-joint, Composite Structures, 66: Theotokoglou, E. E. and Moan, T. (1996). Experimental and Numerical Study of Composite T-joints, Journal of Composite Materials, 3(2): Theotokoglou, E. E. (1997). Strength of Composite T-joints Under Pull-out Loads, Journal of Reinforced Plastics and Composites, 16(6): [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 9] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216

10 1 E. A. D_ILER ET AL. 8. Theotokoglou, E. E. (1999). Study of Numerical Fracture Mechanics Analysis of Composite T-joints, Journal of Reinforced Plastics and Composites, 18(3): Tada, Y., Ishikawa, T. (1989). Experimental Evaluation of the Effects of Stitching on CFRP Laminate Specimens with Various Shapes and Loading, Key Engineering Materials, 37: Toftegaard, H. and Lystrup, A. (2). Design and Test of Lightweight Sandwich T-joint for Naval Ships. Composites Part A: Applied Science and Manufacturing, 36: St John, N., Grabovac, I., Gellert, E., Shah Khan, Z., Mouritz, A. and Burchill, P. (2). Fiber-Resin Composite Research in Support of Current and Future Royal Australian Navy Vessels, In: International Conference on Lightweight Construction Latest Developments, London. + [Ver: A3B2 8.7r/W] [ :42pm] [1 1] [Page No. 1] REVISED PROOFS {SAGE_REV}Jrp/SOFA II/JRP d (JRP) Paper: JRP Keyword Downloaded from jrp.sagepub.com at PENNSYLVANIA STATE UNIV on September 2, 216