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The effect of interference fit size on the fatigue life of bolted joints in composite laminates
Composites: Part B 53 (2013) 62–68
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of interference fit size on the fatigue life of bolted joints in composite laminates Jingchao Wei ⇑, Guiqiong Jiao, Purong Jia, Tao Huang Department of Engineering Mechanics, Northwestern Polytechnical University, Xi’an 710129, China
a r t i c l e
i n f o
Article history: Received 29 August 2012 Received in revised form 9 April 2013 Accepted 10 April 2013 Available online 25 April 2013 Keywords: A. Laminates B. Fatigue D. Mechanics testing E. Joints/joining Interference fit
a b s t r a c t An experimental investigation was conducted to determine the effect of interference fit size on fatigue life of double lap-type single bolted composite joints. Tension–compression reversed force/stress ratio, R = 1, was selected to evaluate the fatigue tests. The joints with blind bolts of four interference fit sizes, 0% (neat fit), 0.5%, 1.8%, 3% were tested separately. The appropriate levels of fatigue stress were determined by the ultimate bearing strength of the fastener structure obtained from the static tensile tests. The bearing stress and the fatigue life (S–N) date of different interference fit sizes specimen were presented and the relationship between interference fit sizes and fatigue life were obtained. The experimental results show that the interference fit can improve fatigue life of bolted joints compared to neat fit. For the composite material studied in this paper, the joints with 3% interference fit has the best fatigue performance in lower bearing stress (less than 540 MPa), however in higher bearing stress (more than 660 MPa), the best size of interference fit is 1.8%. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In advanced engineering fields such as aerospace engineering, composite materials have found wide applications because of their high strength-to-weight and stiffness-to-weight ratios. These applications require joining composites either to composites or to metals. The joints are commonly formed by mechanical fasteners. In composite structures, three types of joints are used, namely, mechanically fastened joints, adhesively bonded joints, and hybrid mechanically fastened/adhesively bonded joints. Bolted joints are still the dominant fastening mechanisms used in jointing of primary structural parts [1]. The strength of fastened joints depends on many factors, including lay-up sequence, geometric parameters such as e/d and w/d ratios, clearance or interference, washer size, initial preload and type of joints (single-lap or double-lap) [2–11]. Cooper and Turvey [2] investigated the effects of joint geometry (e/d and w/d ratios) on failure load and mode of failure of double-lap single bolted joints experimentally in tension, where e = edge distance, w = width of the specimen, and d = diameter of bolt–hole. The results show that the failure loads are directly affected by e/d ratios and w/d ratios, and increasing the e/d ratio changes the failure mode from shear-out (e/d < 3) to bearing (e/d > 3), and increasing the w/d ratio changes the failure mode from net-tension (w/d < 4) to bearing (w/d > 4). ⇑ Corresponding author. Tel.: 0086 029 88431027. E-mail address: [email protected] (J. Wei). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.04.048
Ascione et al. [3,4] investigated the effect of the fibre-to-load inclination, stacking sequence and bolt diameter on the bearing failure load of glass fibre/epoxy laminates. Results showed that the bearing failure load increased as the bolt diameter increased, and depended significantly on the fibre-to-load inclination, but was not sensitive to stacking sequence. Sun et al. [5] studied the effect of clamping area on the failure load of single bolt double-lap composite joint in tension by three configurations: a pin joint (without lateral support), a bolted joint without washer and a bolted joint with washer. Results indicate that the failure load of bolted joint with and without washer increased significantly than the pin joint without lateral support. Generally the failure load increases as clamping area increases. The effect of washer size on the strength of composite bolted joints made of glass fibre reinforced epoxy was investigated experimentally [6]. Furthermore, Feo et al. [7] found that the presence of washers decreased the stress distribution of muti-bolted joints. McCarthy et al. [8] have done extensive experimental investigation to determine the effects of bolt–hole clearance on the stiffness and bearing strength of composite joints. When the clearance was increased from 0% to 3%, the stiffness and 2% offset bearing strength decreased by 10% and 7.5%. Xiao et al. [9,10] conducted a detailed experimental investigation clarified the relationship between bearing strength and damage progression behavior in bolted composite joints, in a condition of neat fit between bolt and hole without considering the influence of clearance and interference [11]. Bearing fatigue responses of mechanically fastened composite joints are generally affected by the selection of R-ratio, fastener
J. Wei et al. / Composites: Part B 53 (2013) 62–68
type, fastener preload/torque, fastener bolt–hole clearance, and environmental conditions. Chen [12] investigated the bearing performance and the bolt torque (preload) relaxation of single fastener graphite epoxy bolted joints under hygrothermal cycles. Results show that high preload improves static failure strength and the fatigue life of specimen under room conditions. Counts [13] studied experimentally the effect of frequency, R-ratio, two composite materials, and thermal aging, and found that fatigue results were insensitive to frequency between 0.1 Hz and 10 Hz or 10,000 h thermal aging. Bolted or riveted joints have a significant effect on the failure of structures subjected to dynamical loading, and can cause local stiffness and damping altering. Starikov et al. [14] experimentally studied the fatigue performance of carbon fibre/epoxy material panels joined by countersunk titanium joints and composite fasteners. Ibrahim and Pettit [15] reviewed the role of joints parameter uncertainties and relaxation on the design and dynamic behavior of metallic and composite structure. Demelio [16] used shear and pull-out static and fatigue loading to study the performance of composite sandwich panels fastened by using blind fasteners and mechanical lock fasteners. Bond and Farrow [17] developed a phenomenological fatigue life model for angle-ply (matrix dominated) and quasi-isotropic (fibre dominated) composites incorporating a mechanical fastener. Although there have been considerable researches devoted to many influencing factors of bearing strength or fatigue behavior of mechanically fastened joint in composite laminates, there are few studies about the effect of interference fit on the static and fatigue behavior of double-lap single bolted composite joints. Interference fit can be generally used to lower the stress concentration factors and then reduce the magnitude of the local stress. In case of fatigue loading, the interference can greatly improve the fatigue life of composite joints. However, the current researches on interference fit are only limited to the pin/hole interference fit joints [18–20], and there are little researches on bolt/holt interference fit joints. Therefore, in the present paper, an experimental study was carried out to investigate the static and fatigue behavior of bolted joints in composite laminates with different interference fit sizes.
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and 5 mm diameter fastener hole is shown in Fig. 1. A double lap joint, with two 16-ply [45/0/45/90]2S laminates transferring load to a 20-ply [45/0/45/90/0/0/45/0/45/0]S main laminate, was used in all tests, since this provides the most stable configuration and minimizes the effect of bending in compression. The thickness of single ply is 0.15 mm. The geometrical configuration of the joint setup is shown in Fig. 1. For the clamping-up of experimental machine, a filler plate made of steel was used. All specimens were made with the same w/d = 5 ratio, e/d = 3 ratio, and specimen length of 210 mm. Taking into the account of the length of specimen, a metal fixture was assembled to avoid the bending of specimen in compression. For each specimen group, the load type, stacking sequence, interference fit size, and the fatigue bearing stress levels are identified in Table 1. 2.2. Interference fit and fastener Fig. 2 shows the assembly progress of interference-fit of blind bolt. The drive nut was screwed to impel the nut into the expansion sleeve. The expansion sleeve is used to keep uniform interference value to the hole. Expansion sleeve dimensions are for engineering reference only. When tightening torque of drive nut reached a setting value, the screw and drive nut would break off. Four sizes of interference-fit are considered in the tests: 0%, 0.5%, 1.8% and 3%. Interference fit size is given by the relation:
I¼
Dd 100% D
ð1Þ
where d is the bolt–hole diameter and D is the bolt diameter. 2.3. Failure definition
The same composite material, T700/BP9916 Carbon fibre Reinforced Plastic composite, was selected for comparison of different interference fit sizes. A specimen with dimensions of 90 25 mm
The failure definition under fatigue loading becomes rather complex for bolted joint. Applied fatigue stress level is best obtained from the static bearing strength, in order to get a reasonable S–N curve. ASTM D 5961/D 5961M-05e1 standard test method [21] for bearing response of polymer–matrix composite laminates was used to determine the ultimate bearing strength of polymer–matrix composite laminate specimens in double-shear tensile loading. Several certain percentages of the ultimate bearing strength were used to determine the fatigue stress level (in Table 1). Joint damage can be indicated through joint stiffness reduction or hole elongation (hole wear). A relatively simple way to monitor bearing damage during a fatigue test is needed. The method is from the hysteresis plot of stress versus test machine crosshead deflection data (Fig. 3). For each set of data at selected fatigue intervals, the
Fig. 1. Geometrical configuration of bolted joint specimen.
Fig. 2. Schematic of the assembly progress of blind bolt.
2. Experimental approach 2.1. Material and specimen preparation
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J. Wei et al. / Composites: Part B 53 (2013) 62–68
Table 1 Stress levels and interference fit sizes for specimen groups. Specimen number a
J1 J2a J3a P4-0 P4-1 P4-2 P4-3 P5-0 P5-1 P5-2 P5-3 P6-0 P6-1 P6-2 P6-3
Stress level
Interference fit size (%)
Static Static Static ±45%rultb ±45%rultb ±45%rultb ±45%rultb ±55%rultb ±55%rultb ±55%rultb ±55%rultb ±60%rultb ±60%rultb ±60%rultb ±60%rultb
0 0.5 3 0 0.5 1.8 3 0 0.5 1.8 3 0 0.5 1.8 3
a
Used only for static tests. Obtained from the approximate average value of static ultimate bearing strength.
Fig. 4. Schematic of experimental set-up using a infrared thermometer.
b
fastener translation after N fatigue cycles (dN) is calculated using the relation:
dN ¼ dNt dNc
ð2Þ
where dNt and dNc are crosshead or extensometer displacements at zero force after quasi-static tensile and compressive loading, respectively. The bolt–hole elongation at each prescribed fatigue interval may then be calculated using the relation:
DN ¼ dN di
ð3Þ
where di is the fastener translation prior to fatigue loading, mm. Testing was not generally terminated until enough bolt–hole elongation (hole wear) was produced (it may be 4% deformation of hole diameter) or 1,000,000 cycles were achieved.
cles, buckling of the joints was a strong possibility, especially in high bearing stress, so lateral supports must be used to prevent this. Fatigue tests were conducted according to the ASTM D 6873 standard [22] at a room temperature of 20 °C. A major factor to be considered was the potential for significant temperature increase, due to relative motion between the joint parts. If it became excessive, it would cause premature failure of the joint. To avoid this, frequencies between 2 Hz and 8 Hz were selected (higher frequencies were used for the joints tested at lower cycled loading). In addition, a cooling fan using compressed air directed onto the bolt was implemented, and the temperature of each bolt was monitored using infrared thermometer (Fig. 4). Bearing in mind that the temperature on the surface of the bolt is likely to be less than in the interior of the joint, the target maximum temperature for the bolt surface was set to 40 °C, and test frequency was adjusted to try to maintain temperatures below this.
2.4. Experimental procedures Static and fatigue bearing tests were performed using an INSTRON 8801 dynamic test machine. Static tests were conducted under room condition with 1 mm/min ramp speed. For the fatigue tests, the loading cycle applied to all specimens was a constant amplitude sine wave with a stress ratio of R = 1. This fully reversed cycle is considered the most severe type of constant amplitude fatigue loading, and leads to the shortest fatigue life. The test set-up is illustrated in Fig. 4. An extensometer was used in order to measure the bolt–hole elongation for the single-bolt double lap bolted joint specimen. On the compressive stroke of the fatigue cy-
Fig. 3. Hysteresis curve parameters for fastener translation calculation.
3. Results and discussions 3.1. Static tests The static ultimate bearing strength is the value of bearing stress, at the maximum load capability of a bearing specimen, using the following equation:
rult ¼
P ult td
ð4Þ
where t is the nominal thickness of the main laminates of bolted joints and d is the nominal bolt–hole diameter after interference fit between the bolt and the composite hole. Fig. 5 shows typical load–displacement curves of three types of specimens. J1 is the specimens with blind bolt of neat fit. J2 is the specimens with blind bolt of 0.5% interference-fit, while J3 is of 3% interference-fit. The elastic region and the onset of nonlinearity can be clearly observed from Fig. 5. The definition of static failure is defined as the maximum load achieved during the test. Test results show that the ultimate bearing strength of J2 group with 0.5% interference-fit is higher 7.6% than that of J1 group, and the J3 group’s ultimate bearing strength is lower 4.7% than J1 group’s. Proper interference fit can increase the ultimate bearing strength of composite joints and excessive interference fit can decrease the ultimate bearing strength. However, although the ultimate bearing strength of the specimen varies with the size of interference fit, we take the same nominal maximum bearing strength (rult = 1200 MPa) for getting better comparisons of fatigue results.
J. Wei et al. / Composites: Part B 53 (2013) 62–68
Fig. 5. Typical load–displacement curves for J1, J2 and J3 specimens.
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to limit specimen surface temperature below 40 °C. Fatigue lives of bolted joints with four interference fit sizes, 0% (neat fit), 1.5%, 1.8%, 3% were obtained. Several typical load–displacement hysteresis plots for neat fit and interference fit are shown in Fig. 6. It was clearly seen that the bolt–hole elongation increased as the number of fatigue cycles increased. Through the typical hysteresis load–displacement plots, the clearance would be observed after several fatigue cycles. After the gap between bolt and hole appeared, the bolt, impacting the hole wall under tension and compression cyclic loading, would cause the impact damage around the connecting hole in composite laminates. As the gap was larger, the impact damage would become more serious so that the gap reached to the failure value of specimens. For the joint with interference fit, the fastener would be made in close contact with the connecting hole after specimen preparation, so that largely delay the impact damage caused by dynamic loading. Same an exponential trend of the bolt–hole elongation with the number of cycles increasing can be observed from Fig. 7. Fatigue tests were not prevented until the bolt–hole elongation reached the requirement, 4% of the hole diameter.
3.2. Fatigue tests 3.3. Interference fit effect All tests were made with double lap joints under R-ratio = 1 fatigue loading. Tests containing three levels of maximum bearing strength amplitude (540 MPa, 660 MPa, 720 MPa,) were load-controlled in sinusoidal wave at a frequency range of 2–8 HZ adjusted
Specimens of interference fit sizes from 0% to 3% were tested to determine whether interference fit had improved bearing fatigue properties comparing to 0% neat fit. The results of fatigue tests
Fig. 6. Typical load–displacement plots with four sizes of interference fit of P6 group.
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J. Wei et al. / Composites: Part B 53 (2013) 62–68
Fig. 7. Bolt–hole elongation with interference effects, for P4, P5, P6 specimens.
with all interference fit sizes are shown in Fig. 8. The relationship between the bearing stress and the fatigue life of four interference fits sizes almost changes in linearity, but the S–N curves of bolted joints had a great change with the difference of interference fit sizes, and the fatigue life would be influenced directly by the size of interference fit. At different dynamic stress levels, the interference fit of the bolted joints which had the best fatigue life was not always invariable, such as, in lower bearing stress, the fatigue life of the joints with 3% interference fit reached the longest, however in higher bearing stress, the joints of the longest fatigue life was with 1.8% interference fit. At any of the stress level from the S–N curves, the fatigue life of the best interference and the worst interference (neat fit) would be a difference of 5–10 times. In particular, in the cyclic stress of 600 MPa, the neat fit of the fatigue life would be less an order of magnitude than the best interference fit. That showed that the bolted joint with interference fit does improve the fatigue life, and the important factors in designing composite bolted joint do not only include the static ultimate bearing strength of composite materials and the size of interference fit, but also the stress level at which the bolted joints generally applied. Selecting the appropriate size of interference fit can greatly improve the fatigue properties of composite bolted joints. To directly study the relationship between interference and fatigue life, Fig. 9 shows the plots of interference and fatigue life in different stress
levels. Average dotted line was gotten from each group with three specimens. In 540 MPa cyclic stress amplitude, the fatigue life increased with the increasing of the size of interference fit as shown in Fig. 9a, and the fatigue life of specimens with 3% interference fit reached more than 106. In higher stress levels, 660 MPa and
Fig. 8. S/N curves with four sizes of interference fit.
J. Wei et al. / Composites: Part B 53 (2013) 62–68
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Fig. 9. Effect of interference fit on the fatigue life of P4, P5 and P6 specimens.
720 MPa, it can be clearly seen from Fig. 9b and c that specimen with 1.8% interference fit had the best fatigue life, and whenever the size of interference fit was too high or too low, it would lead to the reduction of the fatigue life in varying extent. 4. Conclusion An experimental study has been carried out to investigate the effect of different sizes of interference fit on the fatigue behavior of bolted joints in composite laminates. The static ultimate bearing strengths of neat fit and interference fit were obtained through static tensile tests. The relationship between the load–displacement hysteresis curves, S–N curves, and failure mechanism were examined. Correlations between the sizes of interference fit and the dynamic stress levels were directly described by plots. Thus, the following results are concluded: (1) The results indicated that the fatigue life of composite bolted joints is associated with the size of interference fit and the dynamic stress levels. (2) Two extreme interference fits, 0.5% and 3%, were considered to evaluate the effect of interference on the ultimate bearing strength compared to neat fit. Results show that the ultimate bearing strength of J2 group with 0.5% interference-fit is higher 7.6% than that of J1 group with neat fit, and the J3 group’s ultimate bearing strength is lower 4.7% than J1 group’s.
(3) Compared to neat fit, proper interference fit can increase the ultimate bearing strength of composite joints and excessive interference fit can decrease the ultimate bearing strength. (4) The sizes of interference fit have a significant effect on the fatigue life of composite bolted joints. The fatigue life of the best interference and the worst interference (0%, neat fit) would be a difference of 5–10 times from the S–N curves. (5) The best size of interference fit of bolted joints is influenced by the cyclic stress levels. At different dynamic stress levels, the best interference fit of the bolted joints is not always invariable. For the material T700/BP9916 studied in this paper, the fatigue life of the joints with 3% interference fit is the longest in lower bearing stress (less than 540 MPa), however in higher bearing stress (more than 660 MPa), the joints of the longest fatigue life is with 1.8% interference fit.
Acknowledgement This work was supported by National Natural Science Foundation of China under Grant No.11102160. References [1] Thoppul Srinivasa D, Finegan Joana, Gibson Ronald F. Mechanics of mechanically fastened joints in polymer–matrix composite structure – a review. Compos Sci Technol 2009;69:301–29.
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[2] Cooper C, Turvey GJ. Effects of joints geometry and bolt torque on the structural performance of single bolt tension joints in pultruded GRP sheet material. Compos Struct 1995;32(1–4):217–26. [3] Ascione F, Feo L, Maceri F. An experimental investigation on the bearing failure load of glass fibre/epoxy laminates. Compos Part B: Eng 2009;40(3):197–205. [4] Ascione F, Feo L, Maceri F. On the pin-bearing failure load of GFRP bolted laminates: an experimental analysis on the influence of bolt diameter. Compos Part B: Eng 2010;41(6):482–90. [5] Sun HT, Qing X, Chang FK. The response of composite joints with bolt clamping load, part II: model verification. J Compos Mater 2002;36(1):69–91. [6] Khashaba UA, Sallam HEM, Al-Shorbagy AE, Seif MA. Effect of washer size and tightening torque on the performance of bolted joints in composite structures. Compos Struct 2006;73(3):310–7. [7] Luciano F, Gianfranco M, Ayman SM. Stress analysis of multi-bolted joints for FRP pultruded composite structures. Compos Struct 2012;94(12):3769–80. [8] McCarthy MA, Lawlor VP, Stanley WF, McCarthy CT. Bolt–hole clearance effects and strength criteria in single bolt, single lap, composite joints. Compos Sci Technol 2002;62(10–11):1415–31. [9] Yi Xiao. Takashi I. Bearing strength and failure of bolted composite joints (part I: experimental investigation). Compos Sci Technol 2005;65:1022–31. [10] Yi Xiao. Takashi I. Bearing strength and failure of bolted composite joints (part II: modeling and simulation). Compos Sci Technol 2005;65:1032–43. [11] Irisrri FX, Laurin F, Carrere N, Maire JF. Progressive damage and failure of mechanically fastened joints in CFRP laminates – Part I: Refined finite element modelling of single-fastener joints. Compos Struct 2012;94:2269–77.
[12] Chen HS. The static and fatigue strength of bolted joints in composites with hygrothermal cycling. Compos Struct 2001;52:295–306. [13] Counts WA, Johnson WS. Bolt bearing fatigue of polymer matrix composites at elevated temperature. Int J Fatigue 2002;24:197–204. [14] Starikov R, Schon J. Fatigue resistance of composite joints with countersunk composite and metal fasteners. Int J Fatigue 2002;24(1):39–47. [15] Ibrahim RA, Pettit CL. Uncertainties and dynamic problems of bolted joints and other fasteners. J Sound Vib 2005;279(3–5):857–936. [16] Demelio G. An experimental investigation of static and fatigue behavior of sandwich composite panels joints by fasteners. Compos B: Eng 2000;32(4):299–308. [17] Bond IP, Farrow IR. Fatigue life prediction under complex loading for XAS/914 CFRP incorporating a mechanical fastener. Int Fatigue 2000;32(8):633–44. [18] Lanza Di Scalea F, Cloud GL, Cappello F. On the effect of interference fit in composite pin joints. J Thermoplastic Compos Mater 1999;12(1):23–32. [19] Pradhan B, Ramesh BP. Assessment of beneficial effects of interference-fit in pin loaded FRP composite. J Reinforced Plastic Compos 2007;26(8):771–89. [20] Binnur GK. Effect of the clearance and interference-fit on failure of the pinloaded composites. Mater Des 2010;31:85–93. [21] D5961/D5961M-05e1. Standard test method for bearing response of polymer– matrix composite laminates. vol. 15.03. Composite Materials. ASTM International. West Conshohocken, PA: 2005. [22] D6873-03. Standard practice for bearing fatigue response of polymer–matrix composite laminates. Composite Materials. West Conshohocken (PA): ASTM International; 2003.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of interference fit size on the fatigue life of bolted joints in composite laminates Jingchao Wei ⇑, Guiqiong Jiao, Purong Jia, Tao Huang Department of Engineering Mechanics, Northwestern Polytechnical University, Xi’an 710129, China
a r t i c l e
i n f o
Article history: Received 29 August 2012 Received in revised form 9 April 2013 Accepted 10 April 2013 Available online 25 April 2013 Keywords: A. Laminates B. Fatigue D. Mechanics testing E. Joints/joining Interference fit
a b s t r a c t An experimental investigation was conducted to determine the effect of interference fit size on fatigue life of double lap-type single bolted composite joints. Tension–compression reversed force/stress ratio, R = 1, was selected to evaluate the fatigue tests. The joints with blind bolts of four interference fit sizes, 0% (neat fit), 0.5%, 1.8%, 3% were tested separately. The appropriate levels of fatigue stress were determined by the ultimate bearing strength of the fastener structure obtained from the static tensile tests. The bearing stress and the fatigue life (S–N) date of different interference fit sizes specimen were presented and the relationship between interference fit sizes and fatigue life were obtained. The experimental results show that the interference fit can improve fatigue life of bolted joints compared to neat fit. For the composite material studied in this paper, the joints with 3% interference fit has the best fatigue performance in lower bearing stress (less than 540 MPa), however in higher bearing stress (more than 660 MPa), the best size of interference fit is 1.8%. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In advanced engineering fields such as aerospace engineering, composite materials have found wide applications because of their high strength-to-weight and stiffness-to-weight ratios. These applications require joining composites either to composites or to metals. The joints are commonly formed by mechanical fasteners. In composite structures, three types of joints are used, namely, mechanically fastened joints, adhesively bonded joints, and hybrid mechanically fastened/adhesively bonded joints. Bolted joints are still the dominant fastening mechanisms used in jointing of primary structural parts [1]. The strength of fastened joints depends on many factors, including lay-up sequence, geometric parameters such as e/d and w/d ratios, clearance or interference, washer size, initial preload and type of joints (single-lap or double-lap) [2–11]. Cooper and Turvey [2] investigated the effects of joint geometry (e/d and w/d ratios) on failure load and mode of failure of double-lap single bolted joints experimentally in tension, where e = edge distance, w = width of the specimen, and d = diameter of bolt–hole. The results show that the failure loads are directly affected by e/d ratios and w/d ratios, and increasing the e/d ratio changes the failure mode from shear-out (e/d < 3) to bearing (e/d > 3), and increasing the w/d ratio changes the failure mode from net-tension (w/d < 4) to bearing (w/d > 4). ⇑ Corresponding author. Tel.: 0086 029 88431027. E-mail address: [email protected] (J. Wei). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.04.048
Ascione et al. [3,4] investigated the effect of the fibre-to-load inclination, stacking sequence and bolt diameter on the bearing failure load of glass fibre/epoxy laminates. Results showed that the bearing failure load increased as the bolt diameter increased, and depended significantly on the fibre-to-load inclination, but was not sensitive to stacking sequence. Sun et al. [5] studied the effect of clamping area on the failure load of single bolt double-lap composite joint in tension by three configurations: a pin joint (without lateral support), a bolted joint without washer and a bolted joint with washer. Results indicate that the failure load of bolted joint with and without washer increased significantly than the pin joint without lateral support. Generally the failure load increases as clamping area increases. The effect of washer size on the strength of composite bolted joints made of glass fibre reinforced epoxy was investigated experimentally [6]. Furthermore, Feo et al. [7] found that the presence of washers decreased the stress distribution of muti-bolted joints. McCarthy et al. [8] have done extensive experimental investigation to determine the effects of bolt–hole clearance on the stiffness and bearing strength of composite joints. When the clearance was increased from 0% to 3%, the stiffness and 2% offset bearing strength decreased by 10% and 7.5%. Xiao et al. [9,10] conducted a detailed experimental investigation clarified the relationship between bearing strength and damage progression behavior in bolted composite joints, in a condition of neat fit between bolt and hole without considering the influence of clearance and interference [11]. Bearing fatigue responses of mechanically fastened composite joints are generally affected by the selection of R-ratio, fastener
J. Wei et al. / Composites: Part B 53 (2013) 62–68
type, fastener preload/torque, fastener bolt–hole clearance, and environmental conditions. Chen [12] investigated the bearing performance and the bolt torque (preload) relaxation of single fastener graphite epoxy bolted joints under hygrothermal cycles. Results show that high preload improves static failure strength and the fatigue life of specimen under room conditions. Counts [13] studied experimentally the effect of frequency, R-ratio, two composite materials, and thermal aging, and found that fatigue results were insensitive to frequency between 0.1 Hz and 10 Hz or 10,000 h thermal aging. Bolted or riveted joints have a significant effect on the failure of structures subjected to dynamical loading, and can cause local stiffness and damping altering. Starikov et al. [14] experimentally studied the fatigue performance of carbon fibre/epoxy material panels joined by countersunk titanium joints and composite fasteners. Ibrahim and Pettit [15] reviewed the role of joints parameter uncertainties and relaxation on the design and dynamic behavior of metallic and composite structure. Demelio [16] used shear and pull-out static and fatigue loading to study the performance of composite sandwich panels fastened by using blind fasteners and mechanical lock fasteners. Bond and Farrow [17] developed a phenomenological fatigue life model for angle-ply (matrix dominated) and quasi-isotropic (fibre dominated) composites incorporating a mechanical fastener. Although there have been considerable researches devoted to many influencing factors of bearing strength or fatigue behavior of mechanically fastened joint in composite laminates, there are few studies about the effect of interference fit on the static and fatigue behavior of double-lap single bolted composite joints. Interference fit can be generally used to lower the stress concentration factors and then reduce the magnitude of the local stress. In case of fatigue loading, the interference can greatly improve the fatigue life of composite joints. However, the current researches on interference fit are only limited to the pin/hole interference fit joints [18–20], and there are little researches on bolt/holt interference fit joints. Therefore, in the present paper, an experimental study was carried out to investigate the static and fatigue behavior of bolted joints in composite laminates with different interference fit sizes.
63
and 5 mm diameter fastener hole is shown in Fig. 1. A double lap joint, with two 16-ply [45/0/45/90]2S laminates transferring load to a 20-ply [45/0/45/90/0/0/45/0/45/0]S main laminate, was used in all tests, since this provides the most stable configuration and minimizes the effect of bending in compression. The thickness of single ply is 0.15 mm. The geometrical configuration of the joint setup is shown in Fig. 1. For the clamping-up of experimental machine, a filler plate made of steel was used. All specimens were made with the same w/d = 5 ratio, e/d = 3 ratio, and specimen length of 210 mm. Taking into the account of the length of specimen, a metal fixture was assembled to avoid the bending of specimen in compression. For each specimen group, the load type, stacking sequence, interference fit size, and the fatigue bearing stress levels are identified in Table 1. 2.2. Interference fit and fastener Fig. 2 shows the assembly progress of interference-fit of blind bolt. The drive nut was screwed to impel the nut into the expansion sleeve. The expansion sleeve is used to keep uniform interference value to the hole. Expansion sleeve dimensions are for engineering reference only. When tightening torque of drive nut reached a setting value, the screw and drive nut would break off. Four sizes of interference-fit are considered in the tests: 0%, 0.5%, 1.8% and 3%. Interference fit size is given by the relation:
I¼
Dd 100% D
ð1Þ
where d is the bolt–hole diameter and D is the bolt diameter. 2.3. Failure definition
The same composite material, T700/BP9916 Carbon fibre Reinforced Plastic composite, was selected for comparison of different interference fit sizes. A specimen with dimensions of 90 25 mm
The failure definition under fatigue loading becomes rather complex for bolted joint. Applied fatigue stress level is best obtained from the static bearing strength, in order to get a reasonable S–N curve. ASTM D 5961/D 5961M-05e1 standard test method [21] for bearing response of polymer–matrix composite laminates was used to determine the ultimate bearing strength of polymer–matrix composite laminate specimens in double-shear tensile loading. Several certain percentages of the ultimate bearing strength were used to determine the fatigue stress level (in Table 1). Joint damage can be indicated through joint stiffness reduction or hole elongation (hole wear). A relatively simple way to monitor bearing damage during a fatigue test is needed. The method is from the hysteresis plot of stress versus test machine crosshead deflection data (Fig. 3). For each set of data at selected fatigue intervals, the
Fig. 1. Geometrical configuration of bolted joint specimen.
Fig. 2. Schematic of the assembly progress of blind bolt.
2. Experimental approach 2.1. Material and specimen preparation
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Table 1 Stress levels and interference fit sizes for specimen groups. Specimen number a
J1 J2a J3a P4-0 P4-1 P4-2 P4-3 P5-0 P5-1 P5-2 P5-3 P6-0 P6-1 P6-2 P6-3
Stress level
Interference fit size (%)
Static Static Static ±45%rultb ±45%rultb ±45%rultb ±45%rultb ±55%rultb ±55%rultb ±55%rultb ±55%rultb ±60%rultb ±60%rultb ±60%rultb ±60%rultb
0 0.5 3 0 0.5 1.8 3 0 0.5 1.8 3 0 0.5 1.8 3
a
Used only for static tests. Obtained from the approximate average value of static ultimate bearing strength.
Fig. 4. Schematic of experimental set-up using a infrared thermometer.
b
fastener translation after N fatigue cycles (dN) is calculated using the relation:
dN ¼ dNt dNc
ð2Þ
where dNt and dNc are crosshead or extensometer displacements at zero force after quasi-static tensile and compressive loading, respectively. The bolt–hole elongation at each prescribed fatigue interval may then be calculated using the relation:
DN ¼ dN di
ð3Þ
where di is the fastener translation prior to fatigue loading, mm. Testing was not generally terminated until enough bolt–hole elongation (hole wear) was produced (it may be 4% deformation of hole diameter) or 1,000,000 cycles were achieved.
cles, buckling of the joints was a strong possibility, especially in high bearing stress, so lateral supports must be used to prevent this. Fatigue tests were conducted according to the ASTM D 6873 standard [22] at a room temperature of 20 °C. A major factor to be considered was the potential for significant temperature increase, due to relative motion between the joint parts. If it became excessive, it would cause premature failure of the joint. To avoid this, frequencies between 2 Hz and 8 Hz were selected (higher frequencies were used for the joints tested at lower cycled loading). In addition, a cooling fan using compressed air directed onto the bolt was implemented, and the temperature of each bolt was monitored using infrared thermometer (Fig. 4). Bearing in mind that the temperature on the surface of the bolt is likely to be less than in the interior of the joint, the target maximum temperature for the bolt surface was set to 40 °C, and test frequency was adjusted to try to maintain temperatures below this.
2.4. Experimental procedures Static and fatigue bearing tests were performed using an INSTRON 8801 dynamic test machine. Static tests were conducted under room condition with 1 mm/min ramp speed. For the fatigue tests, the loading cycle applied to all specimens was a constant amplitude sine wave with a stress ratio of R = 1. This fully reversed cycle is considered the most severe type of constant amplitude fatigue loading, and leads to the shortest fatigue life. The test set-up is illustrated in Fig. 4. An extensometer was used in order to measure the bolt–hole elongation for the single-bolt double lap bolted joint specimen. On the compressive stroke of the fatigue cy-
Fig. 3. Hysteresis curve parameters for fastener translation calculation.
3. Results and discussions 3.1. Static tests The static ultimate bearing strength is the value of bearing stress, at the maximum load capability of a bearing specimen, using the following equation:
rult ¼
P ult td
ð4Þ
where t is the nominal thickness of the main laminates of bolted joints and d is the nominal bolt–hole diameter after interference fit between the bolt and the composite hole. Fig. 5 shows typical load–displacement curves of three types of specimens. J1 is the specimens with blind bolt of neat fit. J2 is the specimens with blind bolt of 0.5% interference-fit, while J3 is of 3% interference-fit. The elastic region and the onset of nonlinearity can be clearly observed from Fig. 5. The definition of static failure is defined as the maximum load achieved during the test. Test results show that the ultimate bearing strength of J2 group with 0.5% interference-fit is higher 7.6% than that of J1 group, and the J3 group’s ultimate bearing strength is lower 4.7% than J1 group’s. Proper interference fit can increase the ultimate bearing strength of composite joints and excessive interference fit can decrease the ultimate bearing strength. However, although the ultimate bearing strength of the specimen varies with the size of interference fit, we take the same nominal maximum bearing strength (rult = 1200 MPa) for getting better comparisons of fatigue results.
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Fig. 5. Typical load–displacement curves for J1, J2 and J3 specimens.
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to limit specimen surface temperature below 40 °C. Fatigue lives of bolted joints with four interference fit sizes, 0% (neat fit), 1.5%, 1.8%, 3% were obtained. Several typical load–displacement hysteresis plots for neat fit and interference fit are shown in Fig. 6. It was clearly seen that the bolt–hole elongation increased as the number of fatigue cycles increased. Through the typical hysteresis load–displacement plots, the clearance would be observed after several fatigue cycles. After the gap between bolt and hole appeared, the bolt, impacting the hole wall under tension and compression cyclic loading, would cause the impact damage around the connecting hole in composite laminates. As the gap was larger, the impact damage would become more serious so that the gap reached to the failure value of specimens. For the joint with interference fit, the fastener would be made in close contact with the connecting hole after specimen preparation, so that largely delay the impact damage caused by dynamic loading. Same an exponential trend of the bolt–hole elongation with the number of cycles increasing can be observed from Fig. 7. Fatigue tests were not prevented until the bolt–hole elongation reached the requirement, 4% of the hole diameter.
3.2. Fatigue tests 3.3. Interference fit effect All tests were made with double lap joints under R-ratio = 1 fatigue loading. Tests containing three levels of maximum bearing strength amplitude (540 MPa, 660 MPa, 720 MPa,) were load-controlled in sinusoidal wave at a frequency range of 2–8 HZ adjusted
Specimens of interference fit sizes from 0% to 3% were tested to determine whether interference fit had improved bearing fatigue properties comparing to 0% neat fit. The results of fatigue tests
Fig. 6. Typical load–displacement plots with four sizes of interference fit of P6 group.
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Fig. 7. Bolt–hole elongation with interference effects, for P4, P5, P6 specimens.
with all interference fit sizes are shown in Fig. 8. The relationship between the bearing stress and the fatigue life of four interference fits sizes almost changes in linearity, but the S–N curves of bolted joints had a great change with the difference of interference fit sizes, and the fatigue life would be influenced directly by the size of interference fit. At different dynamic stress levels, the interference fit of the bolted joints which had the best fatigue life was not always invariable, such as, in lower bearing stress, the fatigue life of the joints with 3% interference fit reached the longest, however in higher bearing stress, the joints of the longest fatigue life was with 1.8% interference fit. At any of the stress level from the S–N curves, the fatigue life of the best interference and the worst interference (neat fit) would be a difference of 5–10 times. In particular, in the cyclic stress of 600 MPa, the neat fit of the fatigue life would be less an order of magnitude than the best interference fit. That showed that the bolted joint with interference fit does improve the fatigue life, and the important factors in designing composite bolted joint do not only include the static ultimate bearing strength of composite materials and the size of interference fit, but also the stress level at which the bolted joints generally applied. Selecting the appropriate size of interference fit can greatly improve the fatigue properties of composite bolted joints. To directly study the relationship between interference and fatigue life, Fig. 9 shows the plots of interference and fatigue life in different stress
levels. Average dotted line was gotten from each group with three specimens. In 540 MPa cyclic stress amplitude, the fatigue life increased with the increasing of the size of interference fit as shown in Fig. 9a, and the fatigue life of specimens with 3% interference fit reached more than 106. In higher stress levels, 660 MPa and
Fig. 8. S/N curves with four sizes of interference fit.
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Fig. 9. Effect of interference fit on the fatigue life of P4, P5 and P6 specimens.
720 MPa, it can be clearly seen from Fig. 9b and c that specimen with 1.8% interference fit had the best fatigue life, and whenever the size of interference fit was too high or too low, it would lead to the reduction of the fatigue life in varying extent. 4. Conclusion An experimental study has been carried out to investigate the effect of different sizes of interference fit on the fatigue behavior of bolted joints in composite laminates. The static ultimate bearing strengths of neat fit and interference fit were obtained through static tensile tests. The relationship between the load–displacement hysteresis curves, S–N curves, and failure mechanism were examined. Correlations between the sizes of interference fit and the dynamic stress levels were directly described by plots. Thus, the following results are concluded: (1) The results indicated that the fatigue life of composite bolted joints is associated with the size of interference fit and the dynamic stress levels. (2) Two extreme interference fits, 0.5% and 3%, were considered to evaluate the effect of interference on the ultimate bearing strength compared to neat fit. Results show that the ultimate bearing strength of J2 group with 0.5% interference-fit is higher 7.6% than that of J1 group with neat fit, and the J3 group’s ultimate bearing strength is lower 4.7% than J1 group’s.
(3) Compared to neat fit, proper interference fit can increase the ultimate bearing strength of composite joints and excessive interference fit can decrease the ultimate bearing strength. (4) The sizes of interference fit have a significant effect on the fatigue life of composite bolted joints. The fatigue life of the best interference and the worst interference (0%, neat fit) would be a difference of 5–10 times from the S–N curves. (5) The best size of interference fit of bolted joints is influenced by the cyclic stress levels. At different dynamic stress levels, the best interference fit of the bolted joints is not always invariable. For the material T700/BP9916 studied in this paper, the fatigue life of the joints with 3% interference fit is the longest in lower bearing stress (less than 540 MPa), however in higher bearing stress (more than 660 MPa), the joints of the longest fatigue life is with 1.8% interference fit.
Acknowledgement This work was supported by National Natural Science Foundation of China under Grant No.11102160. References [1] Thoppul Srinivasa D, Finegan Joana, Gibson Ronald F. Mechanics of mechanically fastened joints in polymer–matrix composite structure – a review. Compos Sci Technol 2009;69:301–29.
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