FAILURE OF SINGLE-LAP SINGLE-BOLT TENSION JOINTS IN PULTRUDED GLASS FIBRE REINFORCED PLATE

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1 FAILURE OF SINGLE-LAP SINGLE-BOLT TENSION JOINTS IN PULTRUDED GLASS FIBRE REINFORCED PLATE Geoffrey TURVEY Senior Lecturer Lancaster University Bailrigg, Lancaster, LA1 4YR, UK Abstract An experimental investigation of the ultimate load and strength of pultruded glass reinforced polymer (GFRP) composite single-lap single-bolt joints subjected to tension is described. Details are presented of the mechanical properties of the GFRP composite, the joint geometry and test matrix (defined in terms of the end distance to hole diameter E/D and joint width to hole diameter W/D ratios), the joint fabrication process, the test setup and the test procedure. Average ultimate loads, average ultimate strengths and overall axial extensions are tabulated for the 45 joints tested. Examples of failure modes are also illustrated. The average ultimate load and strength data are presented graphically as functions of the E/D and W/D ratios, i.e. in a format which may be useful for preliminary joint design. It is concluded that average ultimate loads and strengths increase up to a threshold value of the E/D ratio and remain reasonably constant thereafter. Moreover, the ultimate loads increase as W/D increases, whereas the ultimate strengths reduce as W/D increases. Keywords: Bolted joints, glass fibre reinforced polymer, pultrusion 1. Introduction Interest in the use of pultruded glass fibre reinforced polymer (GFRP) composite materials in primary and secondary infrastructure has been growing steadily during the past few decades. During this period knowledge and understanding of material properties, behaviour of structural components, full-scale structures and joints has increased steadily. In pultruded all-gfrp frameworks bolted joints appear to be the preferred means of connecting members. These joints are potentially the weakest parts of the structure because bolt holes cause stress concentrations in the components forming the joints. Furthermore, as GFRP is an elastic-brittle material, the stress concentrations cannot be relieved by yielding. Consequently, there has been a considerable effort over the past two decades to develop an understanding of the behaviour of bolted joints in pultruded GFRP composite materials. Summaries of research progress up to 2004 are given in [1] [3] and detailed investigations of bolted tension joints at ambient temperature are reported in [4] [8]. More recent studies of the effects of environmental conditions on tension joint failure are given in [9] [12]. Experimental research on pultruded GFRP plate-to-plate bolted tension joints has been concerned predominantly with symmetric double-lap configurations. The majority of tests (circa 700) have been on single-bolt joints. These have been complemented with a much smaller number of tests on multi-bolt joints. Somewhat surprisingly few, if any, tests have been reported on single-lap joints, even for the single-bolt configuration. Single-lap bolted tension joints are, on occasions, unavoidable in frameworks, but, as far as the author is aware, there is currently no quantitative data on their ultimate loads and strengths. Recognition of Page 1 of 8

2 this situation has provided the catalyst for the present investigation of the failure of pultruded GFRP single-lap, single-bolt tension joints. First, brief details of the constituents of the pultruded GFRP material are given together with specific values of their mechanical properties, derived from tension coupon tests carried out earlier on the same material. The three parameters that define the geometry of a single-bolt joint are introduced and combined into two dimensionless ratios. The joint test matrix, defined in terms of specific values of these ratios, is then presented. Thereafter, several steps of the joint fabrication procedure are outlined. This is followed by a very brief description of the tension test procedure. Examples of load extension plots and failure modes of joints are also presented. The test results are then given in tabular format. Graphical design charts are then presented for average ultimate loads and strengths as functions of the joints geometric ratios. The paper is concluded with a summary of the principal findings deduced from the single-lap single-bolt joint tests. 2. Material Properties and Joint Geometry 2.1 Elastic Modulus of Pultruded GFRP Composite Material The pultruded GFRP material used to fabricate the joints was EXTREN 500 series flat plate with a nominal thickness of 6.35 mm [Note: Reference to a trade name is solely for the purposes of factual accuracy; no endorsement of the product is implied]. The glass reinforcement is in two forms, namely rovings (uni-directional fibre bundles) and continuous filament mat (CFM). The matrix material is mainly polyester resin with a small quantity of filler, usually chalk or kaolin. Typical ranges for the volume percentages of fibre, resin and filler are 30 40%, 50 60% and 5 10% respectively. Strain gauged tension coupon tests have been carried out on rectangular coupons cut out of the pultruded GFRP plate with the rovings at various angles to the tension direction in order to determine the effects of roving orientation on the mechanical properties. Of relevance to the current investigation are the average values of the longitudinal elastic modulus and ultimate strength when the rovings are parallel to the tension direction. These average values are reported in [9] and compared with the manufacturer s minimum values given in [13]. For the sake of completeness both sets of values are reproduced in Table 1. It is evident that the former values are 20-30% greater than the latter values. Table 1. Average longitudinal stiffness and strength of pultruded GFRP plate DATA SOURCE LONGITUDINAL ELASTIC MODULUS (GPa) LONGITUDINAL TENSILE STRENGTH (MPa) Coupon Tests [9] Design Manual [13] Single-Lap Single-Bolt Joint Configuration A side elevation of a single-lap single-bolt tension joint is shown in Figure 1(a). Because the half laps of the joint are eccentric to each other, a packing piece (equal to the thickness of a half lap) is bonded to the end of each half lap remote from the joint. This ensures that the joint remains initially straight when it is clamped between the grips of the test machine. As the grips of the test machine move apart the joint is subjected to tension and bending with the latter being equal (initially) to the tensile load multiplied by the thickness of a half lap. Page 2 of 8

3 Figure 1(b) shows a plan view of the bolt hole end of part of one half lap. The geometry is defined by three dimensions, i.e. the end distance E, the width W and the hole diameter D (also equal to the bolt diameter for zero hole clearance), which are usually expressed as the two dimensionless ratios, E/D and W/D. Bolt Head Washer Tensile Load Tensile Load Half Lap Nut Packing Block (a) Axial Tension W E Bolt Hole D (b) Figure 1. (a) Side elevation of a single lap single-bolt tension joint and (b) variables defining the geometry at the joint end of a half lap 3. Joint Fabrication The joint fabrication procedure was relatively straightforward. Rectangular strips were cut out of 6.35 mm thick GFRP plate such that the rovings were parallel to their long sides. Each strip was cut transversely to form two half laps and two packing blocks. One face of each packing block was abraded to remove its surface veil. Likewise, one face at one end of the half laps was similarly abraded over an area slightly greater than the area of the block face. All of the abraded areas were then cleaned. Araldite 2015 epoxy adhesive was applied to the prepared surface of one of the packing blocks and one of the half laps. Four short lengths (2 4 mm) of copper wire with a nominal diameter of 1 mm were placed in the adhesive at the end of the half lap. Their function was to ensure a uniform thickness of adhesive over the bond area. The block was then clamped in position at the end of the half lap and left for 24 hours whilst the adhesive cured. The process was repeated with the other half lap and packing block. During each fabrication session the packing blocks were bonded to the half laps in batches of six, i.e. enough for three nominally identical single-lap joints. After the adhesive had cured, the clamps were removed. A pair of half laps was overlapped to the required length, placed on top of a block of wood and then clamped to the base plate of a pillar drill. The drill was positioned at the centre of the overlap length and the two holes for the bolt were drilled through the laps in one operation. The purpose of the wooden block was to minimise delamination at the periphery of the hole due to drill break-through at the lower surface of the lower half lap. The same procedure was followed to create the bolt holes in the other two pairs of half laps. Each pair of half laps was connected by means of a single bolt to form a single-lap joint, as shown in Figure 1(a). Mild steel bolts with smooth shanks were used. The smooth parts of the shanks were slightly longer than the total thickness of the two half laps so that threads on the shanks did not contact the GFRP material and promote internal delamination at the edge of Page 3 of 8

4 the hole as the load on the joint increased. One or more standard washers were used under the bolt head and nut. 4. Test Matrix for Single-Lap Bolted Tension Joints In order that the single-lap single-bolt tension joint test data might complement previously reported test data on double-lap joints [8], it was decided to use 10 mm diameter mild steel bolts torqued to 3 Nm (finger tight condition) in 10 mm diameter holes in the GFRP half laps. Hence, the bolts were nominally snug fitting. However, previous measurements of the diameters of the smooth parts of the bolt shanks revealed that they were about 0.2 mm smaller than the nominal bolt diameter. Hence, the nominal hole clearance was about 0.2 mm. In fact, this probably is probably a conservative estimate, because measurements of 10 mm nominal diameter holes drilled through the GFRP half laps have shown them to be between 0.1 and 0.5 mm oversize. The test matrix was defined solely in terms of the geometry of the joints. For a single-bolt joint (see Figure 1(b)) its geometry may be defined in terms of specific values of the E/D and W/D ratios. It was decided to use five values of the former and three values of the latter ratio in order to span the likely practical range of joint geometries. In addition, for each pair of E/D and W/D ratios, three nominally identical joints were fabricated for testing. Thus, the scatter of the ultimate loads of nominally identical joints could be observed and average ultimate loads could be estimated. The joint test matrix is shown in Table 2. Table 2. Test matrix of single-lap single-bolt tension joint geometries (D = 10 mm) JOINT WIDTH TO HOLE DIAMETER RATIO (W/D) JOINT END DISTANCE TO HOLE DIAMETER RATIO(E/D) Joint Test Setup and Outline of Test Procedure Three nominally identical joints for each of the fifteen joint geometries were tested in tension in a servo-hydraulically controlled test machine. The tensile load was applied at a constant rate and the load and overall extension were recorded at frequent intervals during each test. Typical load versus extension plots are shown in Figure 2 for three single-lap single-bolt joints, two of which are nominally identical. The general shapes of the plots are similar. There is a plateau in the response over the extension range 0.2 mm to 0.5 mm. It is believed that this is associated with the onset of visible joint rotation. It is also noticeable that the responses and ultimate loads of the two nominally identical joints are in reasonable agreement. However, their ultimate loads and extensions to failure are significantly smaller than that of the third joint which has a larger E/D ratio. Images of a single-lap single-bolt joint set up for testing and after failure are shown in Figure 3. It is evident that the bolt undergoes a large rotation (due to the eccentricity of the load path through the joint) as the joint fails and there is significant delamination within the joint and at the ends of the half laps. After each joint had been tested it was removed from the test machine and dismantled so that the failure mode could be inspected visually and photographed. An example of the failure mode of a single-lap single-bolt joint is shown in Figure 4. The inner face of left half lap appears to exhibit cracking that is typical of the cleavage failure mode. This also appears to be the case for the inner face of the right half lap. However, the outer faces of the two half laps exhibit much less cracking. There is also evidence of surface damage on the left half lap caused by the washer under the bolt head. In general, because of the effect of bending within Page 4 of 8

Load - extension plots of three single-lap single-bolt joints (a) (b) Figure 3.

5 Load [kn] the joint, failure modes of the single-lap joints tend to be more complicated than symmetric double-lap joints Joint 3 E/D=1.5,W/D=4 Joint 1 E/D=1.5,W/D=4 Joint 1 E/D=4,W/D= Extension [mm] Figure 2. Load - extension plots of three single-lap single-bolt joints (a) (b) Figure 3. Single-lap single-bolt joints: (a) ready for testing and (b) after failure Figure 4. Crack patterns on faces of each half-lap of a single-lap single-bolt joint (E/D = 2.5, W/D = 4) Page 5 of 8

6 6. Joint Test Results and Design Guidance The average ultimate load, average ultimate strength (stress) and overall axial extension of each group of three nominally identical joints are given in Table 3. Table 3. Average ultimate loads, ultimate stresses and axial extensions of single-lap single-bolt joints W/D E/D LOAD (kn) STRENGTH (MPa) EXTENSION (mm) The data in Table 3 have been used to plot graphs of average ultimate load and average ultimate strength versus E/D for each value of W/D in order to illustrate the effects that changes in geometry have on the structural performance of single-lap single-bolt tension joints. Figure 4 shows the effect of joint geometry on the average ultimate load. There is a rapid increase in ultimate load as the E/D ratio increases from 1.5 to about 2.5. For E/D ratios greater than 3, the average ultimate loads appear to be nearly constant for each W/D ratio. The average ultimate loads also increase with increasing joint width. For E/D ratios less than 2.5 the average ultimate loads of joints with W/D ratios of 4 and 5 are nearly equal and are significantly greater than those of joints with a W/D ratio of 3. On the other hand, for E/D ratios greater than 3 the increase in average ultimate load is significantly larger as W/D changes from 3 to 4 than from 4 to 5. The effect of joint geometry on average ultimate strength is shown in Figure 5. In this case, it is evident that for E/D ratios less than 2.5 the average ultimate strength of single-lap singlebolt tension joints increases as the W/D ratio decreases from 5 to 3. Moreover, the increase in strength is roughly proportional to the decrease in W/D ratio. However, for E/D ratios greater than 3 it appears that the average ultimate strength reaches the same constant value for W/D ratios of 3 and 4. The average ultimate strength of joints with W/D of 5 is also constant for E/D ratios greater than 3 but is significantly lower than that of joints with smaller W/D ratios. Page 6 of 8

7 Ultimate Strength [MPa] Ultimate Load [kn] W/D = 5 W/D = 4 W/D = End Distance to Hole Diameter Ratio [E/D] Figure 4. Average ultimate load versus E/D for single-lap single-bolt tension joints W/D = 5 W/D = 4 W/D = End Distance to Hole Diameter Ratio [E/D] Figure 5. Average ultimate strength versus E/D for single-lap single-bolt tension joints 7. Concluding Remarks A total of 45 single-lap single-bolt joints have been fabricated from 6.35 mm thick pultruded GFRP composite plate. The rovings in the half laps of the joints were parallel to their longitudinal axes. Pairs of half laps were connected to form a single-lap joint by a snug fitting single mild steel bolt of nominal diameter 10 mm that was tightened to a torque of 3Nm. Each joint was loaded in tension until it failed and its ultimate load and overall axial extension was recorded. After testing each joint was dismantled and a photographic record was made of the crack patterns surrounding the bolt hole on both faces of the laps. In general, the crack patterns were more complicated than those observed in double-lap single-bolt tension joints. The data from the joint tests was used to compile preliminary design guidance in the form of graphs of average ultimate load and strength versus E/D for three values of W/D. From the graphs it appears that there is a threshold value of E/D (approximately equal to 3) above which the average ultimate load and strength remains constant for each value of W/D. It is also evident that below the threshold value average ultimate loads increase with increasing E/D and W/D, whereas average ultimate strengths decrease as the latter ratio increases. Page 7 of 8

8 8. Acknowledgements The author wishes to acknowledge the contribution of Mr. J. Godé, a Visiting French Student, who carried out the joint tests under the author s direction during his Summer Internship in the Engineering Department. The assistance of the Engineering Department s Technicians with the experimental work is also gratefully acknowledged. 9. References [1] TURVEY, G.J., Bolted Connections in PFRP Structures, Progress in Structural Engineering and Materials, Vol. 2, No. 2, April/June 2000, pp [2] MOTTRAM, J.T., TURVEY, G.J., Physical Test Data for the Appraisal of Design Procedures for Bolted Joints in Pultruded FRP Structural Shapes and Systems, Progress in Structural Engineering and Materials, Vol. 5, No. 4, October/December 2003, pp [3] TURVEY, G.J., COOPER, C., Review of Tests on Bolted Joints Between Pultruded GRP Profiles, Proceedings of the Institution of Civil Engineers: Structures and Buildings, Vol. 157, No. 3, June 2004, pp [4] ABD-EL-NABY, S.F.M., HOLLAWAY, L., The Experimental Behaviour of Bolted Joints in Pultruded Glass/Polyester Material. Part 1: Single-Bolt Joints, Composites, Vol. 24, No. 7, October 1993, pp [5] ROSNER, C.N., RISKALLA, S.H., Bolted Connections for Fiber-Reinforced Composite Structural Members: Experimental Program, Journal of Materials in Civil Engineering, Vol. 7, No. 4, November 1995, pp [6] ABD-EL-NABY, S.F.M., HOLLAWAY, L., The Experimental Behaviour of Bolted Joints in Pultruded Glass/Polyester Material. Part 2: Two-Bolt Joints, Composites, Vol. 24, No. 7, October 1993, pp [7] HASSAN, N.K., MOHAMEDIEN, M.A., RISKALLA, S.H., Multibolted Joints for GFRP Structural Members, Journal of Composites for Construction, Vol. 1, No. 1, February 1997, pp [8] COOPER, C., TURVEY, G.J., Effects of Joint Geometry and Bolt Torque on the Structural Performance of Single Bolt Tension Joints in Pultruded GRP Sheet Material, Composite Structures, Vol. 32, Nos. 1-4, 1995, pp [9] TURVEY, G.J., WANG, P., Effects of Elevated Temperature on the Structural Integrity of Bolted Joints in Pultruded Glass Fibre Reinforced Plate, Proceedings of the International Conference on Composites in Construction CCC2001, Porto, Portugal, October 2001, pp [10] TURVEY, G.J., WANG, P., Failure of PFRP Single-Bolt Tension Joints Under Hot-Wet Conditions, Composite Structures, Vol. 77, No. 4, February 2007, pp [11] TURVEY, G.J., WANG, P., Failure of Pultruded GRP Bolted Joints: a Taguchi Analysis, Proceedings of the Institution of Civil Engineers: Engineering and Computational Mechanics, Vol. 162, No. EM3, September 2009, pp [12] TURVEY, G.J., WANG, P., Environmental Effects on the Failure of GRP Multi-Bolt Joints, Proceedings of the Institution of Civil Engineers: Structures and Buildings, Vol. 162, No. SB4, August 2009, pp [13] ANON., EXTREN Fiberglass Structural Shapes Design Manual, Strongwell, Bristol, VA, USA, Page 8 of 8

FAILURE OF PULTRUDED GRP ANGLE-LEG JUNCTIONS IN TENSION

FAILURE OF PULTRUDED GRP ANGLE-LEG JUNCTIONS IN TENSION G.J. Turvey* and P. Wang** * Engineering Department, Lancaster University, Bailrigg, Lancaster, LA1 4YR, UK g.turvey@lancaster.ac.uk ** Schlumberger,