Failure analysis of composite single bolted-joints subjected to bolt pretension

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1 Indian Journal of Engineering & Materials Sciences Vol. 16, April 009, pp Failure analysis of composite single bolted-joints subjected to bolt pretension Murat Pakdil* Department of Mechanical Engineering, Abant Izzet Baysal University, 180, Bolu, Turkey Received 11 September 007; accepted 17 February 009 In this study, failure analysis of composite single bolted joints has been performed. To determine effects of joint geometry and stacking sequences of laminated composite plates on bearing strength and failure mode, experimental studies have been carried out by which edge distance-to-hole diameter ratio (), and plate width-to-hole diameter ratio () are varied from 1 to and to, respectively. In addition, bolt pretensions of 0, and 6 Nm are applied. The composite laminated plates are stacked with the following four different orientations: [0 o /0 o /0 o /0 o ] s, [0 o /0 o / o / o ] s, [0 o /0 o /60 o /60 o ] s, and [0 o /0 o /90 o /90 o ] s. Experimental results show that failure mode and bearing strength are closely related to by stacking sequence of plates, geometrical parameters, and bold tension. An increase in bolt pretension positively affected failure behaviour of bolted joints. Keywords: Bearing strength, Bolted composites, Composite plates, Failure analysis, Failure mode Fiber reinforced composite materials have been widely used in aircraft and space structures because of their high specific modulus and high specific strength 1. Bolts and pins have been preferred widely in these applications for transferring load between the structural components owing to their low cost, simplicity and ease of disassembly for repairs. Contrary to many metallic structural members, for which the strength of the joints is mostly governed by the shear and tensile strengths of the pins or bolts, composite joints present specific failure modes owing to their heterogeneity and anisotropy. Meola et al. carried out an experiment on an innovative glass fiber reinforced metal laminate (FRML) with the aim of determining its strength and behaviour in the case of mechanical joints. Tested specimens, after undergoing bearing stress, were analyzed in both non-destructive and destructive ways. Tong 6 conducted an experimental study on effect of non-uniform bolt-to-washer radial clearance on bearing failure of bolted joints under different clamping forces with various lateral constraints. Experimental results were also used to validate an existing model. Two extreme diametric fit positions, with a positive or negative bolt hole-to-washer clearance, were also investigated. Pakdil et al. 7 investigated effect of bolt pretension on failure response of glass-epoxy laminated composite bolted-joints and performed with a clearance for the diameters of the bolt and circular * pakdilm@ibu.edu.tr holes for and 6 mm, respectively, unlike this study without a clearance. Kovacs et al. 8 determined moment-rotation diagrams and cyclic parameters, which characterize the failure modes. The details of the specimens, the loading history, and the testing procedure were evaluated. Hamada and Maekawa 9 studied failure analysis of quasi-isotropic carbon epoxy laminates both numerically and experimentally. Icten and Sayman 10 obtained failure loads and failure modes experimentally in an aluminum-glass-epoxy sandwich composite plate, with a circular hole, which was subjected to a traction force by a pin. Okutan 11 carried out a numerical and experimental study to observe failure behaviour of mechanically fastened fiber-reinforced laminated composite joints. Tests were performed on single pinned joints in [0/90/0] s and [90/0/90] s laminated composites. Aktas and Dirikolu 1 experimentally studied a pin loaded carbon epoxy composite laminate with different stacking sequences in order to obtain safe and maximum bearing strengths. The laminate with [90 o / o /- o /0 o ] s and [0 o / o /- o /90 o ] s orientations had a 6% fiber volume fraction. Lin and Lin 1 investigated stresses around a pin-loaded hole in symmetrically stacked composite laminates of finite size using a two-dimensional direct boundary element method. Effects of friction and clearance between the pin and hole edge on the stresses were also obtained for geometric compatibility, and force constraints between the pin and the plate were enforced.

2 80 INDIAN J. ENG. MATER. SCI., APRIL 009 Failure modes of bolted joints in laminated composite plates under tensile loads usually occur in four basic modes: cleavage, net-tension, shear-out and bearing modes (Fig. 1) 10,1-1. Nevertheless, combinations of these failure modes are possible in practical applications. In this study, an experimental failure analysis was carried out to observe failure mechanism in glass fiber reinforced-epoxy laminated composite single boltedjoints. In order to observe effects of bolt pretension, ply-orientations, and different geometrical parameters of specimens on failure behaviour, experiments were carried out both with and without any bolt pretensions. Materials and Methods Let us consider a composite rectangular specimen of length L+E and width W with a circular hole of Fig. 1 Failure modes in bolted composite plates Fig. Dimensions of single bolted composite specimen Table Mechanical properties of laminated composite material diameter D, as shown in Fig.. The hole was at a distance E, from the free edge of the specimen. A bolt was positioned at the center of the hole, and a uniform tensile load P was applied to the specimen. The tensile load was parallel to the specimen plane and was symmetric with respect to the centerline and serves to attain bearing strength and damage mode. To determine influences of joint geometry and plyorientations on bearing strength and failure mode, empirical studies were carried out systematically changing some important geometric and material parameters for single bolted-joint. Thus, two different geometrical parameters were examined in this study in that edge distance-to-hole diameter ratio () was chosen from 1 to, while the plate width-to-hole diameter ratio () was selected from to. In addition, laminated composite plates were oriented in the following four ply-orientations: [0 o /0 o /0 o /0 o ] s, [0 o /0 o / o / o ] s, [0 o /0 o /60 o /60 o ] s and [0 o /0 o /90 o /90 o ] s. These orientations were also expressed with group numbers as given in Table 1. For each specimen type, the following bolt pretensions of 0, and 6 Nm were applied during the experiments to determine effects of applied bolt pretensions on failure behaviour of composite joint. Laminated composite plates were produced by Izoreel Firm in Izmir. Each laminated composite plate was manufactured as composed of eight laminas under press and heat. Furthermore, each composite plate had a nominal thickness of mm at a volume fraction of 60%. Some mechanical tests were carried out to measure mechanical properties of glass fiber reinforced-epoxy laminated composite material using standard test methods in the literature related to composite science and technology Mechanical properties of the laminated composite material are presented in Table 7. Experiments were carried out in Table 1 Ply-orientations of laminated composite plates Group number Stacking sequence Average thickness (mm) Total number of lamina 1 [0 o /0 o /0 o /0 o ] s 8 [0 o /0 o / o / o ] s 8 [0 o /0 o /60 o /60 o ] s 8 [0 o /0 o /90 o /90 o ] s 8 E 1 E G 1 ν 1 X t Y t X c Y c S V f (%)

3 PAKDIL et al.: FAILURE ANALYSIS OF COMPOSITE SINGLE BOLTED-JOINTS 81 tension mode of the Instron-111 Tensile Test Machine at a crosshead speed of 0. mm/min. The single bolted-joint test fixture is shown in Fig.. The lower edge of the specimen is clamped and loaded from the steel bolt by stretching the specimens. The load versus bolt displacement curves for all composite configurations were drawn by using a computer linked to the test machine. Statistical analysis of data from the experiment was performed using Minitab 1.1. There are four sources of variability in this experiment: moment (Nm), E/W (mm/mm), D/W (mm/mm), and four specimen types. The four explanatory variables have been chosen to systematically vary during the experiment to determine their effects on the response variable of bearing strength of composite single bolted joints. Multiple linear regression (MLR) model was built to relate bearing strength to the four explanatory variables (three quantitative and one categorical variable with four levels). Statistical significance of mean differences at P < 0.0 was tested using general linear model (GLM) with Tukey s multiple comparison. Results and Discussion Only two tests were conducted for every type of composite specimens since measurements repeated were very close to one another. In addition, each bolted-joint was loaded up to a bolt displacement of 7 or 8 mm from the initial position. Failure modes of the tested specimens are presented in Tables -6 for each group of specimens. As can be seen in Tables - 6, four failure modes were observed as cleavage, nettension- shear-out and bearing (Fig. 1). Mixed failure modes also occurred in some specimens. The mixed failure modes were the combination of bearing plus net-tension, or shear-out plus net-tension. For the mixed modes, failure was first initiated as bearing or shear out, and then, the net tension mode occurred. Fig. Single bolted-joint test fixture Table Failure modes of Group 1 specimens, [0 o /0 o /0 o /0 o ] s 0 Nm Nm 6 Nm 1 S+N C S B+C C C B N N B B+N B+N B B+N B+N B+C B+C B+C B B+C B+C B B-C B+N B+C S B+C B B+C C B B+C B+C B+C S B+C B B+C C B B+C B+C Table Failure modes of Group specimens, [0 o /0 o / o / o ] s 0 Nm Nm 6 Nm 1 S S+C S S+N S+N C B+N S+N B+C B+N N B B+N N N S S C B S B+C B B B+N S S S B S B+C B B B+N S S S B S B+C B B B+N

4 8 INDIAN J. ENG. MATER. SCI., APRIL 009 Table Failure modes of Group specimens, [0 o /0 o /60 o /60 o ] s 0 Nm Nm 6 Nm 1 N N N N N N N N N N N B+N N N N 1 C+N N N N N N N N N N N B+N N N B+N 1 C+N N N C+N N N B+N N N B+N N B+N B+N N B+N 1 C+N N N C+N N N B+N N N B+N N B+N B+N N B+N Table 6 Failure modes of Group specimens, [0 o /0 o /90 o /90 o ] s 0 Nm Nm 6 Nm 1 N S+N N C N N B+N N N B+N N N B+N N N 1 N C+N S+N N S+N N B+N N N B+N S+N B+N B+N B+N B+N 1 S+N S+N S+N S+N S+N S+N B+N S+N B+N B+N S+N B+N B+N B+N B+N 1 S+N S+N S+N S+N S+N S+N B+N S+N B+N B+N S B+N B B+N B+N Failure modes changed in relation to variations in the geometric parameters of and ratios, and increases in the applied bolt pretensions. Three basic failure modes consisting of cleavage, net tension and shear-out were created with small and ratios. For the other specimens, bearing mode or mixed mode occurred. The full bearing failure mode was seen for =, and, especially. In other words, when = 1, failure mode changed from net tension, cleavage or shear-out to bearing or mixed mode with an increase in ratio. In addition, an increase in the bolt pretensions caused a change in the modes. A bearing damage is known to be more advantageous than either net tension, cleavage or shear-out damage modes for a safe bolted joint. Thus, increasing bolt pretensions can be very appropriate for ensuring bearing damage mode of bolted composite joint. In the meantime, even if the bolt joints carry on bearing load later than the first peak, the constructer must guarantee that the boltedjoint configuration never reaches the first peak, as bolted joint properties cannot be expected to have higher values except near this peak. The influences of ratio on the bearing strength for all groups are shown in Fig. depending on ratio and applied bolt pretensions. The values of bearing strengths increased with an increase in both and ratios. For this reason, the highest values of bearing strengths were obtained for = and =. When = 1, the laminate was the weakest since bearing strength was found to be smaller than the other ratios. When the bolt pretensions were increased, the values of bearing strengths increased. The bearing strengths were found Table 7 Multiple linear regression (MLR) model relating bearing strength of composite single bolted joints to four explanatory variables Explanatory variable Coefficient SE of coefficient T-value P-value Intercept (mm/mm) < (mm/mm) < Moment (Nm) < Specimen type < [0 o /0 o /0 o /0 o ] s Specimen type < [0 o /0 o / o / o ] s Specimen type- [0 o /0 o /60 o /60 o ] s < Adjusted R 7.% Standard error (SE) 8.97

5 PAKDIL et al.: FAILURE ANALYSIS OF COMPOSITE SINGLE BOLTED-JOINTS 8 Table 8 Tukey s multiple comparisons of mean bearing strengths using general linear model (GLM) Variables Levels P-value Moment (Nm) 0 6 Bearing strength a b c < (mm/mm) 1 Bearing strength a b bc c c < 0.0 Specimen type [0 o /0 o /0 o /0 o ] s [0 o /0 o / o / o ] s [0 o /0 o /60 o /60 o ] s [0 o /0 o /90 o /90 o ] s Bearing strength a b c d < (mm/mm) Bearing strength a b b b < 0.0 The different letters in rows denote statistically significant differences among means.

6 8 INDIAN J. ENG. MATER. SCI., APRIL 009 Fig. Effect of ratio on bearing strength for Groups 1,, and [( ) Group 1,0 Nm; ( ) Group,0 Nm; ( ) Group,0 Nm; ( ) Group,0 Nm; ( ) Group 1, Nm; ( ) Group, Nm; ( ) Group, Nm; ( ) Group, Nm; ( ) Group 1,6 Nm; ( ) Group,6 Nm; ( ) Group,6 Nm; ( ) Group,6 Nm] higher with the applied bolt pretensions of and 6 Nm than without any bolt pretension (0 Nm). The highest values of bearing strengths were found for Group specimens, while the lowest values were usually obtained for Group specimens both with and without bolt pretension. The maximum value of bearing strength was estimated at MPa for Group specimens with a bolt pretension of 6 Nm, when and =. Thus, the bearing strengths and failure modes were positively affected by the ply orientation of laminated plates. The best oriented specimens were seen as Group, [0 o /0 o / o / o ] s, according to the ply orientations investigated in this study. Based on the MLR model constructed, each coefficient estimates the rate of change in the mean response per unit increase in one explanatory variable when all other predictors are held constant (Table 7). The MLR model accounted for 7% of variation in bearing strength of the composite specimens as a function of moment,,, and specimen types (P < 0.001). Tukey s multiple comparisons following GLM revealed that the three applied moments, and the four specimen types differed from one another in their mean responses (P < 0.001) (Table 8). Mean responses of bearing strength did not vary from one another when, or = to (Table 8).

7 PAKDIL et al.: FAILURE ANALYSIS OF COMPOSITE SINGLE BOLTED-JOINTS 8 Conclusions The following conclusions can be drawn from the present study: (i) Bearing strength increased with an increase in edge distance to diameter ratio () and/or in width to diameter ratio (). (ii) When ratio is, bearing or mixed failure modes usually occurred. Bearing failure was the best suitable mode due to its load carrying capacity. (iii) When = 1, the laminate was the weakest. (iv) The best ply orientation was seen as Group, [0 o /0 o / o / o ] s for the orientations selected in this study. (v) The weakest stacking sequence was observed as Group, [0 o /0 o /60 o /60 o ] s in the plates selected and investigated. (vi) Bearing strengths without any bolt pretension were smaller than the applied bolt pretensions of and 6 Nm. Increasing the applied bolt pretensions increased bearing strengths. (vii) An increase in bolt pretensions appeared to be rather suitable for a safe bolted-joint owing to its provision of bearing mode and high bearing strengths. Acknowledgments The author gratefully acknowledges the testing support provided by Dr O Sayman, Dr F Sen, and personnel of the Mechanical Laboratory of Dokuz Eylul University. References 1 Choi J-H & Chun Y-J, J Compos Mater, 7 (00) Wu T J & Hahn H T, Compos Sci Technol, 8 (1997) Scalea F L D, Cappello F & Cloud G L, J Thermoplast Compos Mater, 1 (1999) 1-. Pierron F, Cerisier F & Grediac M A, J Compos Mater, (000) Meola C, Squillace A, Giorleo G & Nele L, J Compos Mater, 7 (00) Tong L, Compos Pt A: Appl Sci Manufact, 1 (000) Pakdil M, Sen F, Sayman O & Benli S, J Reinforced Plast Compos, 6 (007) Kovacs N, Calado L & Dunai L, J Construct Steel Res, 60 (00) Hamada H, Maekawa Z I, J Compos Mater, 0 (1996) İçten B M & Sayman O, Compos Sci Technol, 6 (00) Okutan B, Compos Pt B: Eng, (00) Aktas A & Dirikolu M H, Compos Struct, 6 (00) Lin C-C & Lin C-H, Int J Solids Struct, 6 (1999) Mallick P K, Fiber-reinforced composites materials, manufacturing and design, nd ed (Marcel Decker), Jones R M, Mech Compos Mater (Taylor & Francis), Gibson R F, Principals of composite material mechanics (Mc Graw-Hill), 199.

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET)

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 ISSN 0976-6480 (Print) ISSN