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The effect of shimming material on flexural behavior for composite joints with assembly gap
Composite Structures 209 (2019) 375–382
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
The effect of shimming material on flexural behavior for composite joints with assembly gap
T
⁎
Yuxing Yanga, Yi-Qi Wanga, Xueshu Liub, , Hang Gaoa, Yongjie Baoa a
Key Lab. for Precision and Non-traditional Machining Technology of Ministry of Education, School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China b School of Automotive Engineering, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite joints Assembly gap Shimming material Flexural behavior
Shimming material, like liquid shim, is generally employed to compensate the assembly gap between composite components in structure assembly process of aircraft. To investigate the effect of shimming material on flexural behavior for composite joints, specimens with predesigned assembly gaps were manufactured and three-point bending tests were conducted with different initial gap sizes, shimming materials, shimming percentages as well as bolt-hole clearances. The major conclusions from experiments are: (i) the presence of assembly gap decreases the strength of the assembly, while the shimming material can make up for the loss; (ii) mechanical properties of shimming material have a positive effect on flexural behavior; (iii) the higher the shimming percentage the better the flexural behavior; (iv) bolt-hole clearance influences the load take-up stiffness.
1. Introduction Composite materials have been increasingly used in structural components of commercial aircraft due to the inherent advantages of lightweight, high specific strength and high specific stiffness. It is increasingly necessary to manufacture more and more complex composite structures, such as the wing-box, the fuselage and tail assembly of the aircraft. However, it is a tough task challenging to meet the demands of complex shaped structures and extreme tight tolerances of the composite parts due to unavoidable defects that may occur during composite manufacturing. Shape distortion is one of the major defects associated with the manufacturing process of large composite parts, which causes out-of-tolerance and poor mating surface conditions of the assembly parts [1–3]. As a result, assembly gaps may appeared between two composite parts, and delamination may occur and/or propagate around the fastener holes in composite structure [4–6]. For better load transfer and less assembly-induced-damage, shimming is a much more common strategy used to compensate the gaps in aircraft industries [7–10]. There are different categories of shimming materials used in assembly process, such as liquid shim, laminated shim, solid shim and their hybrid forms. Epoxy-based liquid shims, like EA 9394 and EA 9377, are most widely employed because of their conformable characteristics. Much research has been carried out concerning the effect of the assembly gap and liquid shim on bearing behaviors by both ⁎
experimental and numerical methods. Hühne et al. [11] investigated the effect of shim thickness on the shear behavior of single-lap singlebolt composite bolted joints with liquid shim layers using numerical methods. Both a progressive damage model and a continuous degradation model were developed, which regard the shim as an isotropic material. The progressive damage model combining Hashin’s three-dimensional failure criterion and a constant degradation model yielded very conservative results, while the continuous degradation model showed very good correlation with the experimental data. The structural performances of the liquid shim in multi-fastener single-lap composite-titanium bolted joints were evaluated mainly by numerical methods in literature [12,13]. The results showed that the maximum load, joint stiffness and design load of the joints decreased as the shim layer’s thickness increased; furthermore, it was found that shimming material could improve the load capacities of the joints. Thermo-mechanical fatigue tests of double-fastener composite-aluminum alloy hybrid joints with two types of liquid shims (EA 9394 and EA 9377) were conducted by Comer et al. [9]. The conclusions were: degradation of the liquid shim was not apparent; presence of the liquid shim caused a reduction in joint stiffness and the reduction was dependent on the thickness of the liquid shim employed; the joints with the third-generation liquid shim (EA 9377) showed higher stiffness than that with the second-generation liquid shim (EA 9394). The similar double-fastener hybrid joints and single-bolt composite joints considering the shimming materials were studied in literature [10] in terms of the effect
Corresponding author. E-mail address: [email protected] (X. Liu).
https://doi.org/10.1016/j.compstruct.2018.10.086 Received 15 March 2017; Received in revised form 19 September 2018; Accepted 23 October 2018 Available online 25 October 2018 0263-8223/ © 2018 Elsevier Ltd. All rights reserved.
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2. Experiments
of liquid shim on in-plane strain and out-of-plane deformation using 3D Digital Image Correlation. It was found that the shim layer magnified the secondary bending effect and led to higher tensile strains in the laminates because the shim increased the geometric eccentricity of the load path and induced a higher magnitude of bolt tilting. The compressive properties of the liquid shim under different exposure conditions were investigated in [14] and an optical exposure temperature (+85 °C) was found that can increase mechanical properties. Zhai et al. [15] stated that the shape of gap can be approximated to taper, which can be characterized by thickness and span. The gap was represented by machining a small slope annulus on the surface of the aluminum plate for the composite-metal hybrid joints. And experimental study on bearing behavior of single-lap composite-metal hybrid joints with predesigned gap was conducted. The results showed that the presence of the assembly gap significantly weakened the bearing performance of single-lap hybrid joints; specimen with solid shim gained a little better bearing strength than specimen with liquid shim; increasing the gap span had little effect on bearing performance. In addition, there were many patents related with shimming process, such as automation shimming systems[16], shimming methods [17–20] and new shimming materials [17,20,21]. As mentioned above, there are two ways to model the gaps appeared in the composite bolted structures. One is to simplify the gap as a rectangular shape between two plates, which are shimmed by sheet metals or other materials at both sides, named as the Model I (Fig. 1(a)), as reported in [22]. The other one is to create the gap by machining one of the assembly parts, usually the metal part, as presented in [15]. By this way, the gap can be designed as any shape as Model II (Fig. 1(b)). However, it can be seen that the created gaps are different from those appeared in actual composite structures (Fig. 1(c)). As shown by the deformation diagram of the upper plate in Fig. 1, when subjected to the flexural loading, the composite laminate close to the loading nose changes from flat shape to concave shape for both Model I and Model II, while it changes from convex shape to concave shape for the actual structure. Accordingly, bearing behaviors are different between the created gap models and the actual ones due to not only the gap itself but also geometrical deviation of the component and the interactions between the assembly components especially when a flexural load is imposed. Therefore, the gaps should be naturally created to gain a better understanding of their influences on composite structures. This study aims to investigate the effect of shimming material on the flexural behavior of composite joints with assembly gaps subjected to three-point bending by experimental methods. A double-fastener composite shimmed assembly specimen with predesigned assembly gaps, as a reduced-scale model of the large composite assembly with gaps between faying surfaces, was proposed and tested; meanwhile, a type of hybrid shim was proposed, which was made by EA 9394 and solid peelable fiberglass cloth; and the shimming-percentage-control method was developed to study the effect of shimming percentage on flexural behavior of the structure. The major research parameters in this paper are gap size, shimming materials, shimming percentage as well as bolthole clearance.
2.1. Geometry and material The specimen consists of an arched composite laminate, a flat composite laminate and two fasteners as illustrated in Fig. 2. The shimming material was only cured on the arched laminate, while the flat one was free from the shimming material by the release agent PART # 67 PVA GREEN (Finish Kare, America). The arched composite laminate included two flat sections at both sides and an arched section with radius R in the middle. The supports span was 96.0 mm. The loading nose radius and supports radius are 3.0 mm. The arch span A is 50 mm. As shown in Fig. 3, the arch radius R is associated with the predesigned gap size δ , R = δ /2 + A2 /8δ . In this study, the gap size ranges from 1.0 mm to 5.0 mm. The laminates were manufactured from T700/YPH-25 carbon fiber/ epoxy prepreg tapes (nominal ply thickness is 0.125 mm). Mean volume percentage of the carbon fiber was 60% ± 3%. The lay-up of the arched laminate was [0/90/45/−45]5, while that of the flat laminate was [0/90/45/−45]6. Thickness of the arched laminate and flat laminate were 2.5 mm and 3.0 mm, respectively. Two types of shimming materials were investigated, liquid shim and hybrid shim. The liquid shim was a two-part structural epoxy paste adhesive, Hysol EA 9394, which was mixed by part A and part B at the weight ratio 100:17 [23]. As Fig. 4 shows, the hybrid shim was made by combining solid peelable fiberglass cloth EA 9394 epoxy together, in which the fiberglass cloth was the major part. Considering the possible engineering condition that assembly gap may be not fully filled up, shimming percentage for liquid shimming configuration was considered in this study with volume percentage values of 0%, 50% and 100%. The shimming percentage was represented by the ratio of shimming material volume to initial gap volume, and it was controlled by the bespoke tool, which was made by the solid peelable fiberglass cloth (nominal thickness is 0.05 mm per layer) and scotch tape (nominal thickness is 0.01 mm) as shown in Fig. 5. The non-shimming region was filled by the fiberglass cloth, and the scotch tape was used to keep the non-shimming region away from the EA 9394 epoxy in order to take out the bespoke tool after curing. The shimming percentage can be calculated by Eqs. (1) and (2), where Vshim is shimming material volume, Vgap is initial gap volume, w is the width of the shim, s is the thickness of the shim, α and β are central angle of the sectors for the gap section and the shim section, respectively.
Shim% =
2R2β − w (R − s ) Vshim = 2R2α − A (R − δ ) Vgap
α = 2 × arccos(
R−δ ) , R
β = 2 × arccos(
(1)
R−s ) R
(2)
2.2. Configuration and procedure Twelve test groups are labelled in Table 1, which consist of different gap sizes, shimming materials, shimming percentages and bolt-hole clearances. The holes were all drilled by the numerical control machine
Fig. 1. Gap models and their deformation diagrams for: (a) Model I; (b) Model II; (c) Actual gap. 376
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Fig. 2. The specimen geometry and the loading configuration of the three-point bending test. Table 1 Groups of the three-point bending tests.
Fig. 3. The relationship between the arch radius and the predesigned gap size.
Fig. 4. Hybrid shim.
Label
Gap size (mm)
Shimming material
Shimming percentage
Bolt-hole clearance
G1-N G1-L100 G2-N G2-L50 G2-L100 G2-H100 G4-N G4-L100 G4-H100 G5-N G5-L100 G2-N-C2
1.0 1.0 2.0 2.0 2.0 2.0 4.0 4.0 4.0 5.0 5.0 2.0
No shim Liquid shim No shim Liquid shim Liquid shim Hybrid shim No shim Liquid shim Hybrid shim No shim Liquid shim No shim
0% 100% 0% 50% 100% 100% 0% 100% 100% 0% 100% 0%
C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C2
3. Results and discussion 3.1. Load-displacement response
with tolerance of 0.018 mm. The half thread hexagon socket head cap screws and common nuts with grade of 12.9 (DIN 912) were used. Diameters of the bolts range from 5.94 mm to 5.99 mm. Thus, the bolthole clearance was between 0.028 mm and 0.078 mm for hole diameter of 6.0 mm (C1), while that was between 0.128 mm and 0.178 mm for hole diameter of 6.1 mm (C2). Each configuration was repeated with 5 specimens to evaluate the variation of the results. Three-point bending test configurations with different shimming materials were shown in Fig. 6. The test began with the fastening procedure, during which the specimen was fastened by two protruding head bolts using torque wrench with preload torque of 9.3 N·m (about 6410 N in force). Then three-point bending procedure was conducted on a WDW-100 electronic universal testing machine (maximum load 100 kN) in displacement control at a rate of 1.0 mm/min according to ASTM D7264 [24]. The three-point bending procedure stops as soon as either load decreases to about 60% of the ultimate load or displacement reaches to 13.0 mm. The load-displacement curves were recorded by the testing machine. The damage profile micrographs through the thickness of the specimens were obtained by the Digital Microscope VHX-600E (KEYENCE, Japan).
From experimental results, it can be found that typical flexural behavior of the specimen with predesigned assembly gap can be approximated into five stages of loading as: contact, gap closure, load take-up, damage accumulation, and failure as illustrated in Fig. 7(a) and (b). The former three stages, which belong to the undamaged stages, are enlarged in Fig. 7(b). The first stage, from the original point to the stable contact load PC, is regarded as contact establishment process. In this stage, it starts to establish the interactions among the arched laminate, flat laminate and fasteners. The load-displacement curve in the first stage is assumed to be quasi-linear with initial stiffness KI, which mainly represents the stiffness of the arched laminate during stable contact establishment process. Once the stable contact load PC is exceeded, the assembly gap between two laminates begins to close as the load increases After the gap closure load PG is reached, the assembly gap is completely closed. Then the load is taken up with slope KII, which reflects the stiffness of the whole assembly. The latter two stages belong to the damaged stages. When the initial Fig. 5. Schematic of the shimming-percentage-control method.
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Fig. 6. Three-point bending test configurations with different shimming materials.
damage load PD is exceeded, damage of the specimen will propagate as the load increases, which results in non-linear load-displacement response. Once the ultimate load (PF) is reached, any displacement increase will lead to greater damage until the specimen cannot carry any load. Taking G2-N configuration as an example (Fig. 8), the first linear stage of loading is from 0 N to about 150 N with initial stiffness KI of 293.25 N/mm with standard deviation of 10.68 N/mm. The second linear stage of loading is from about 450 N to about 1400 N with load take-up stiffness KII of 178.74 N/mm with standard deviation of 6.01 N/ mm. Between two linear stages, there is a gap closure stage. At Point 1, the gap between the arched laminate and the flat laminate is completely closed. At Point 2, obvious damage on the top surface of the specimen can be observed with the named first peak load of 1490.5 N with standard deviation of 10.5 N. Point 3 represents the ultimate load of 1668.0 N with standard deviation of 14.08 N. At Point 4, the specimen can be regarded to be totally broken and cannot carry load. Fig. 8. Typical load-displacement response of G2-N configuration.
3.2. Effect of gap size shimming material can significantly improve it. In Fig. 10, both the stiffness KI and KII of 100% liquid shimming configurations are higher than that of non-shimming configurations. The initial stiffness KI increases as the gap increases for both nonshimming and 100% liquid shimming configurations, while the load take-up stiffness KII may have an inflection point between gap 2.0 mm to 4.0 mm as shown in Fig. 11. Here, KII for G5-N configuration was missed due to that the specimens of this configuration were totally broken before the gap was closed Taking G1.0-N and G5-N in Fig. 12 as examples. When the displacement of the testing machine reached 5.0 mm, the gap for G1-N has already been closed and there was no visible damage, while the gap for G5-N was still not closed and there was obvious damage. It indicates that the initial gap size influences not only the strength and stiffness but also the failure process of the specimen. The stiffness KI and KII of 100% liquid shimming configuration
Figs. 9 and 10 show the relationships between gap size and bearing properties of the specimen for both non-shimming configuration (N) and 100% liquid shimming configuration (L100). In Fig. 9, for non-shimming configurations, ultimate load decreases about 15.23% as the gap size increases from 1.0 mm to 5.0 mm. The configuration with a bigger gap between two parts means that the arched laminate gets less support from the flat laminate, which is easier to be broken than that with a smaller gap. However, for 100% liquid shimming configurations, when initial gap size increases from 1.0 mm to 5.0 mm, ultimate load increases about 52.51%. It is mainly due to the fact that the shimming material increases the real thickness of the specimen. The thicker the specimen is the higher the strength. Thus, ultimate load of the shimming configuration with bigger initial gap size is higher than that with a smaller one. The results indicate that the presence of the gap decreases the strength of the assembly, while the
Fig. 7. Typical load-displacement curve of three-point bending tests for specimen with assembly gap: (a) load-displacement response; (b) enlarged view of the undamaged stages. 378
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Fig. 9. The effect of gap size on ultimate load of the specimen for both nonshimming and 100% liquid shimming configurations.
Fig. 12. The effect of shimming material on ultimate load of the specimen.
point bending as a whole structure for 100% liquid shimming configuration, which has better contact conditions at beginning than nonshimming configuration. 3.3. Effect of shimming material In Fig. 12 and Fig. 13, the liquid shimming configuration (L100) and hybrid shimming configuration (H100) are compared with the nonshimming configuration (N). The behaviors of shimming materials are evaluated by the ratio of strength difference to initial gap size or the ratio of stiffness difference to initial gap size, as given in Eq. (3), where S% means behavior difference (strength difference or stiffness difference) between shimming configuration and non-shimming configuration, Sshim is behavior of the shimming configuration, and Sno is behavior of the non-shimming configuration. As for ultimate load in Fig. 12, non-shimming configurations are the lowest, liquid shimming configurations are about 15% higher than non-shimming configurations, and hybrid shimming configurations are more than 17% higher than nonshimming configurations. It is because the strength of the hybrid shim is higher than that of the liquid shim, which means that the mechanical properties of the shimming material have a positive effect on bearing strength of the specimen. In Fig. 13(a) and (b), for configurations with initial gap size of 2.0 mm, the trend is almost same as that in Fig. 12. However, for configurations with initial gap size of 4.0 mm, stiffness differences of the liquid shimming and hybrid shimming configurations are so small (|SL% − SH%| < 0.4%), and strength differences of which are only 2.19%. It indicates that there will be little differences for different shimming materials when initial gap size is large enough.
Fig. 10. The effect of gap size on stiffness of the specimen for both non-shimming and 100% liquid shimming configurations.
S% =
(Sshim − Sno ) × 100% δ
(3)
3.4. Effect of shimming percentage In order to evaluate the effect of non-full shimming and full shimming conditions on flexural behavior, three shimming percentages, 0%, 50% and 100%, were tested for liquid shimming configurations with initial gap size of 2.0 mm as shown in Figs. 14 and 15. Both strength and stiffness increase quasi-linearly as the shimming percentage increases. As shimming percentage increases from 50% to 100%, ultimate load increases about 9.70%, KI increases about 33.87%, KII increases about 15.05%. It can be concluded that non-full shimming influences the flexural behaviors of the composite joints, especially the initial stiffness KI. Hence, full shimming condition is recommended in engineering applications within affordable cost and weight.
Fig. 11. The comparison of the load-displacement curves between configurations with small gap and big gap.
are higher than that of non-shimming configuration, which means the shimming material can make the structure harder to bend. It is because that both the arched laminate and flat laminate are subjected to three379
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Fig. 13. The effect of shimming material on stiffness of the specimen: (a) KI; (b) KII.
Fig. 16. The effect of bolt-hole clearance on bearing behavior of the specimen.
Fig. 14. The effect of shimming percentage on ultimate load of the specimen.
indicates that the bolt-hole clearance mainly influences the load takeup stiffness KII. It may be because that both the arched laminate and flat laminate subject to three-point bending after the gap is completely closed, and bigger bolt-hole clearance gives more space for the arched laminate to move so that the load take-up stiffness KII decreases.
3.5. Effect of bolt-hole clearance As Fig. 16 shows, two kinds of bolt-hole clearances, C1 (0.028 mm–0.078 mm) and C2 (0.128 mm–0.178 mm), were compared for non-shimming configurations with initial gap size of 2.0 mm. When bolt-hole clearance increases about 0.1 mm, stiffness KI changes only 0.34%, which is not sensitive to bolt-hole clearance; however, stiffness KII and ultimate load decrease about 6.11% and 3.42%, respectively. It
3.6. Failure mechanism Fig. 17(a)–(c) show the failure micrographs of the specimens with
Fig. 15. The effect of shimming percentage on stiffness of the specimen: (a) KI; (b) KII. 380
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Table 2 Debonding load and delamination load of configurations with different shimming materials. Shimming material
Debonding load (N)
Delamination load (N)
No shim Liquid shim Hybrid shim
– 1509.63 ± 162.91 612.90 ± 81.23
1490.50 ± 10.50 2120.56 ± 86.38 2336.60 ± 78.00
shim was firstly observed. It can be described by debonding load that corresponds to the moment when the adhesive interface of the arched laminate and shim was debonded. Table 2 shows the debonding load and delamination load of the configurations with different shimming materials. Considering the variance of the experiments (different porosities of the liquid shims, different bearing properties of the hybrid shims and laminates), it can be concluded that the presence of shimming material improves the delamination load. It is because that the debonding of adhesive interface occurs before delamination initiation of the laminates for shimming configurations, which releases partial energy of the specimen subjected to three-point bending so that the laminates are protected. 4. Conclusion In this paper, an experimental study was conducted to investigate the flexural behavior of composite shimming joints with predesigned assembly gap subjected to three-point bending. A type of hybrid shim was proposed, which was made by the epoxy and certain thickness solid peelable fiberglass cloth, and it shows better properties than the liquid shim. Furthermore, a method to control the shimming percentage was developed using a bespoke tool (Fig. 5) made by solid peelable fiberglass cloth (guarantee the shim thickness) and scotch tape (protect the fiberglass cloth from the epoxy in order to remove the bespoke tool). The experimental parameters contain gap size, shimming material, shimming percentage and bolt-hole clearance. The conclusions from experiments are as follows: 1) The typical flexural behavior of the composite shimming joints can be approximated into five stages of loading as: contact, gap closure, load take-up, damage accumulation and failure. 2) The presence of assembly gap decreases the strength of the joints, while the shimming material can make up for the loss. As the initial gap size increases from 1.0 mm to 5.0 mm, ultimate load of nonshimming configuration decreased by 15.23%, while that of liquid shimming configuration increased by 52.51%. Both the stiffness KI and KII of 100% liquid shimming configuration are higher than that of non-shimming configuration. 3) Mechanical properties of the shimming material have a positive effect on flexural behavior. The strength and stiffness of non-shimming configurations are the lowest, those of liquid shimming configurations are medium, and those of hybrid shimming configurations are the highest among three types of shimming conditions. 4) The higher the shimming percentage, the better the flexural behavior. The strength and stiffness increase quasi-linearly as the shimming percentage increases. Therefore, full shimming condition is recommended in engineering applications within affordable cost and weight. 5) Bolt-hole clearance mainly influences the load take-up stiffness KII, both ultimate load and initial stiffness KI are not so sensitive to bolthole clearance.
Fig. 17. Failure micrographs of the specimens with different shimming conditions: (a) no shim; (b) liquid shim; (c) hybrid shim.
initial gap size of 2.0 mm at same load conditions corresponding to nonshimming, liquid shimming and hybrid shimming configurations, respectively. Delamination was one of the major damages for all three configurations. The arched laminate was obviously broken by delamination around the central layers and delamination between first two plies close to the top surface due to strong compressive stress, while the flat one was damaged with serious ply fractures close to the bottom surface due to strong tensile stress. For non-shimming configuration, the specimen was firstly damaged by delamination at the top surface of the arched laminate, which can be described by delamination load that corresponds to Point 2 in Fig. 8. For shimming configurations, debonding of the adhesive interface between the arched laminate and
Acknowledgements This work was supported by the National Key Basic Research Program of China (973 Project) [Grant No. 2014CB046504], the 381
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National Natural Science Foundation of China [Grant No. 51375068 & Grant No. 51475073] and the Fundamental Research Funds for the Central Universities [Grant No. DUT15RC(3)089]. The authors would like to acknowledge the above financial supports.
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The effect of shimming material on flexural behavior for composite joints with assembly gap
T
⁎
Yuxing Yanga, Yi-Qi Wanga, Xueshu Liub, , Hang Gaoa, Yongjie Baoa a
Key Lab. for Precision and Non-traditional Machining Technology of Ministry of Education, School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China b School of Automotive Engineering, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite joints Assembly gap Shimming material Flexural behavior
Shimming material, like liquid shim, is generally employed to compensate the assembly gap between composite components in structure assembly process of aircraft. To investigate the effect of shimming material on flexural behavior for composite joints, specimens with predesigned assembly gaps were manufactured and three-point bending tests were conducted with different initial gap sizes, shimming materials, shimming percentages as well as bolt-hole clearances. The major conclusions from experiments are: (i) the presence of assembly gap decreases the strength of the assembly, while the shimming material can make up for the loss; (ii) mechanical properties of shimming material have a positive effect on flexural behavior; (iii) the higher the shimming percentage the better the flexural behavior; (iv) bolt-hole clearance influences the load take-up stiffness.
1. Introduction Composite materials have been increasingly used in structural components of commercial aircraft due to the inherent advantages of lightweight, high specific strength and high specific stiffness. It is increasingly necessary to manufacture more and more complex composite structures, such as the wing-box, the fuselage and tail assembly of the aircraft. However, it is a tough task challenging to meet the demands of complex shaped structures and extreme tight tolerances of the composite parts due to unavoidable defects that may occur during composite manufacturing. Shape distortion is one of the major defects associated with the manufacturing process of large composite parts, which causes out-of-tolerance and poor mating surface conditions of the assembly parts [1–3]. As a result, assembly gaps may appeared between two composite parts, and delamination may occur and/or propagate around the fastener holes in composite structure [4–6]. For better load transfer and less assembly-induced-damage, shimming is a much more common strategy used to compensate the gaps in aircraft industries [7–10]. There are different categories of shimming materials used in assembly process, such as liquid shim, laminated shim, solid shim and their hybrid forms. Epoxy-based liquid shims, like EA 9394 and EA 9377, are most widely employed because of their conformable characteristics. Much research has been carried out concerning the effect of the assembly gap and liquid shim on bearing behaviors by both ⁎
experimental and numerical methods. Hühne et al. [11] investigated the effect of shim thickness on the shear behavior of single-lap singlebolt composite bolted joints with liquid shim layers using numerical methods. Both a progressive damage model and a continuous degradation model were developed, which regard the shim as an isotropic material. The progressive damage model combining Hashin’s three-dimensional failure criterion and a constant degradation model yielded very conservative results, while the continuous degradation model showed very good correlation with the experimental data. The structural performances of the liquid shim in multi-fastener single-lap composite-titanium bolted joints were evaluated mainly by numerical methods in literature [12,13]. The results showed that the maximum load, joint stiffness and design load of the joints decreased as the shim layer’s thickness increased; furthermore, it was found that shimming material could improve the load capacities of the joints. Thermo-mechanical fatigue tests of double-fastener composite-aluminum alloy hybrid joints with two types of liquid shims (EA 9394 and EA 9377) were conducted by Comer et al. [9]. The conclusions were: degradation of the liquid shim was not apparent; presence of the liquid shim caused a reduction in joint stiffness and the reduction was dependent on the thickness of the liquid shim employed; the joints with the third-generation liquid shim (EA 9377) showed higher stiffness than that with the second-generation liquid shim (EA 9394). The similar double-fastener hybrid joints and single-bolt composite joints considering the shimming materials were studied in literature [10] in terms of the effect
Corresponding author. E-mail address: [email protected] (X. Liu).
https://doi.org/10.1016/j.compstruct.2018.10.086 Received 15 March 2017; Received in revised form 19 September 2018; Accepted 23 October 2018 Available online 25 October 2018 0263-8223/ © 2018 Elsevier Ltd. All rights reserved.
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2. Experiments
of liquid shim on in-plane strain and out-of-plane deformation using 3D Digital Image Correlation. It was found that the shim layer magnified the secondary bending effect and led to higher tensile strains in the laminates because the shim increased the geometric eccentricity of the load path and induced a higher magnitude of bolt tilting. The compressive properties of the liquid shim under different exposure conditions were investigated in [14] and an optical exposure temperature (+85 °C) was found that can increase mechanical properties. Zhai et al. [15] stated that the shape of gap can be approximated to taper, which can be characterized by thickness and span. The gap was represented by machining a small slope annulus on the surface of the aluminum plate for the composite-metal hybrid joints. And experimental study on bearing behavior of single-lap composite-metal hybrid joints with predesigned gap was conducted. The results showed that the presence of the assembly gap significantly weakened the bearing performance of single-lap hybrid joints; specimen with solid shim gained a little better bearing strength than specimen with liquid shim; increasing the gap span had little effect on bearing performance. In addition, there were many patents related with shimming process, such as automation shimming systems[16], shimming methods [17–20] and new shimming materials [17,20,21]. As mentioned above, there are two ways to model the gaps appeared in the composite bolted structures. One is to simplify the gap as a rectangular shape between two plates, which are shimmed by sheet metals or other materials at both sides, named as the Model I (Fig. 1(a)), as reported in [22]. The other one is to create the gap by machining one of the assembly parts, usually the metal part, as presented in [15]. By this way, the gap can be designed as any shape as Model II (Fig. 1(b)). However, it can be seen that the created gaps are different from those appeared in actual composite structures (Fig. 1(c)). As shown by the deformation diagram of the upper plate in Fig. 1, when subjected to the flexural loading, the composite laminate close to the loading nose changes from flat shape to concave shape for both Model I and Model II, while it changes from convex shape to concave shape for the actual structure. Accordingly, bearing behaviors are different between the created gap models and the actual ones due to not only the gap itself but also geometrical deviation of the component and the interactions between the assembly components especially when a flexural load is imposed. Therefore, the gaps should be naturally created to gain a better understanding of their influences on composite structures. This study aims to investigate the effect of shimming material on the flexural behavior of composite joints with assembly gaps subjected to three-point bending by experimental methods. A double-fastener composite shimmed assembly specimen with predesigned assembly gaps, as a reduced-scale model of the large composite assembly with gaps between faying surfaces, was proposed and tested; meanwhile, a type of hybrid shim was proposed, which was made by EA 9394 and solid peelable fiberglass cloth; and the shimming-percentage-control method was developed to study the effect of shimming percentage on flexural behavior of the structure. The major research parameters in this paper are gap size, shimming materials, shimming percentage as well as bolthole clearance.
2.1. Geometry and material The specimen consists of an arched composite laminate, a flat composite laminate and two fasteners as illustrated in Fig. 2. The shimming material was only cured on the arched laminate, while the flat one was free from the shimming material by the release agent PART # 67 PVA GREEN (Finish Kare, America). The arched composite laminate included two flat sections at both sides and an arched section with radius R in the middle. The supports span was 96.0 mm. The loading nose radius and supports radius are 3.0 mm. The arch span A is 50 mm. As shown in Fig. 3, the arch radius R is associated with the predesigned gap size δ , R = δ /2 + A2 /8δ . In this study, the gap size ranges from 1.0 mm to 5.0 mm. The laminates were manufactured from T700/YPH-25 carbon fiber/ epoxy prepreg tapes (nominal ply thickness is 0.125 mm). Mean volume percentage of the carbon fiber was 60% ± 3%. The lay-up of the arched laminate was [0/90/45/−45]5, while that of the flat laminate was [0/90/45/−45]6. Thickness of the arched laminate and flat laminate were 2.5 mm and 3.0 mm, respectively. Two types of shimming materials were investigated, liquid shim and hybrid shim. The liquid shim was a two-part structural epoxy paste adhesive, Hysol EA 9394, which was mixed by part A and part B at the weight ratio 100:17 [23]. As Fig. 4 shows, the hybrid shim was made by combining solid peelable fiberglass cloth EA 9394 epoxy together, in which the fiberglass cloth was the major part. Considering the possible engineering condition that assembly gap may be not fully filled up, shimming percentage for liquid shimming configuration was considered in this study with volume percentage values of 0%, 50% and 100%. The shimming percentage was represented by the ratio of shimming material volume to initial gap volume, and it was controlled by the bespoke tool, which was made by the solid peelable fiberglass cloth (nominal thickness is 0.05 mm per layer) and scotch tape (nominal thickness is 0.01 mm) as shown in Fig. 5. The non-shimming region was filled by the fiberglass cloth, and the scotch tape was used to keep the non-shimming region away from the EA 9394 epoxy in order to take out the bespoke tool after curing. The shimming percentage can be calculated by Eqs. (1) and (2), where Vshim is shimming material volume, Vgap is initial gap volume, w is the width of the shim, s is the thickness of the shim, α and β are central angle of the sectors for the gap section and the shim section, respectively.
Shim% =
2R2β − w (R − s ) Vshim = 2R2α − A (R − δ ) Vgap
α = 2 × arccos(
R−δ ) , R
β = 2 × arccos(
(1)
R−s ) R
(2)
2.2. Configuration and procedure Twelve test groups are labelled in Table 1, which consist of different gap sizes, shimming materials, shimming percentages and bolt-hole clearances. The holes were all drilled by the numerical control machine
Fig. 1. Gap models and their deformation diagrams for: (a) Model I; (b) Model II; (c) Actual gap. 376
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Fig. 2. The specimen geometry and the loading configuration of the three-point bending test. Table 1 Groups of the three-point bending tests.
Fig. 3. The relationship between the arch radius and the predesigned gap size.
Fig. 4. Hybrid shim.
Label
Gap size (mm)
Shimming material
Shimming percentage
Bolt-hole clearance
G1-N G1-L100 G2-N G2-L50 G2-L100 G2-H100 G4-N G4-L100 G4-H100 G5-N G5-L100 G2-N-C2
1.0 1.0 2.0 2.0 2.0 2.0 4.0 4.0 4.0 5.0 5.0 2.0
No shim Liquid shim No shim Liquid shim Liquid shim Hybrid shim No shim Liquid shim Hybrid shim No shim Liquid shim No shim
0% 100% 0% 50% 100% 100% 0% 100% 100% 0% 100% 0%
C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C2
3. Results and discussion 3.1. Load-displacement response
with tolerance of 0.018 mm. The half thread hexagon socket head cap screws and common nuts with grade of 12.9 (DIN 912) were used. Diameters of the bolts range from 5.94 mm to 5.99 mm. Thus, the bolthole clearance was between 0.028 mm and 0.078 mm for hole diameter of 6.0 mm (C1), while that was between 0.128 mm and 0.178 mm for hole diameter of 6.1 mm (C2). Each configuration was repeated with 5 specimens to evaluate the variation of the results. Three-point bending test configurations with different shimming materials were shown in Fig. 6. The test began with the fastening procedure, during which the specimen was fastened by two protruding head bolts using torque wrench with preload torque of 9.3 N·m (about 6410 N in force). Then three-point bending procedure was conducted on a WDW-100 electronic universal testing machine (maximum load 100 kN) in displacement control at a rate of 1.0 mm/min according to ASTM D7264 [24]. The three-point bending procedure stops as soon as either load decreases to about 60% of the ultimate load or displacement reaches to 13.0 mm. The load-displacement curves were recorded by the testing machine. The damage profile micrographs through the thickness of the specimens were obtained by the Digital Microscope VHX-600E (KEYENCE, Japan).
From experimental results, it can be found that typical flexural behavior of the specimen with predesigned assembly gap can be approximated into five stages of loading as: contact, gap closure, load take-up, damage accumulation, and failure as illustrated in Fig. 7(a) and (b). The former three stages, which belong to the undamaged stages, are enlarged in Fig. 7(b). The first stage, from the original point to the stable contact load PC, is regarded as contact establishment process. In this stage, it starts to establish the interactions among the arched laminate, flat laminate and fasteners. The load-displacement curve in the first stage is assumed to be quasi-linear with initial stiffness KI, which mainly represents the stiffness of the arched laminate during stable contact establishment process. Once the stable contact load PC is exceeded, the assembly gap between two laminates begins to close as the load increases After the gap closure load PG is reached, the assembly gap is completely closed. Then the load is taken up with slope KII, which reflects the stiffness of the whole assembly. The latter two stages belong to the damaged stages. When the initial Fig. 5. Schematic of the shimming-percentage-control method.
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Fig. 6. Three-point bending test configurations with different shimming materials.
damage load PD is exceeded, damage of the specimen will propagate as the load increases, which results in non-linear load-displacement response. Once the ultimate load (PF) is reached, any displacement increase will lead to greater damage until the specimen cannot carry any load. Taking G2-N configuration as an example (Fig. 8), the first linear stage of loading is from 0 N to about 150 N with initial stiffness KI of 293.25 N/mm with standard deviation of 10.68 N/mm. The second linear stage of loading is from about 450 N to about 1400 N with load take-up stiffness KII of 178.74 N/mm with standard deviation of 6.01 N/ mm. Between two linear stages, there is a gap closure stage. At Point 1, the gap between the arched laminate and the flat laminate is completely closed. At Point 2, obvious damage on the top surface of the specimen can be observed with the named first peak load of 1490.5 N with standard deviation of 10.5 N. Point 3 represents the ultimate load of 1668.0 N with standard deviation of 14.08 N. At Point 4, the specimen can be regarded to be totally broken and cannot carry load. Fig. 8. Typical load-displacement response of G2-N configuration.
3.2. Effect of gap size shimming material can significantly improve it. In Fig. 10, both the stiffness KI and KII of 100% liquid shimming configurations are higher than that of non-shimming configurations. The initial stiffness KI increases as the gap increases for both nonshimming and 100% liquid shimming configurations, while the load take-up stiffness KII may have an inflection point between gap 2.0 mm to 4.0 mm as shown in Fig. 11. Here, KII for G5-N configuration was missed due to that the specimens of this configuration were totally broken before the gap was closed Taking G1.0-N and G5-N in Fig. 12 as examples. When the displacement of the testing machine reached 5.0 mm, the gap for G1-N has already been closed and there was no visible damage, while the gap for G5-N was still not closed and there was obvious damage. It indicates that the initial gap size influences not only the strength and stiffness but also the failure process of the specimen. The stiffness KI and KII of 100% liquid shimming configuration
Figs. 9 and 10 show the relationships between gap size and bearing properties of the specimen for both non-shimming configuration (N) and 100% liquid shimming configuration (L100). In Fig. 9, for non-shimming configurations, ultimate load decreases about 15.23% as the gap size increases from 1.0 mm to 5.0 mm. The configuration with a bigger gap between two parts means that the arched laminate gets less support from the flat laminate, which is easier to be broken than that with a smaller gap. However, for 100% liquid shimming configurations, when initial gap size increases from 1.0 mm to 5.0 mm, ultimate load increases about 52.51%. It is mainly due to the fact that the shimming material increases the real thickness of the specimen. The thicker the specimen is the higher the strength. Thus, ultimate load of the shimming configuration with bigger initial gap size is higher than that with a smaller one. The results indicate that the presence of the gap decreases the strength of the assembly, while the
Fig. 7. Typical load-displacement curve of three-point bending tests for specimen with assembly gap: (a) load-displacement response; (b) enlarged view of the undamaged stages. 378
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Fig. 9. The effect of gap size on ultimate load of the specimen for both nonshimming and 100% liquid shimming configurations.
Fig. 12. The effect of shimming material on ultimate load of the specimen.
point bending as a whole structure for 100% liquid shimming configuration, which has better contact conditions at beginning than nonshimming configuration. 3.3. Effect of shimming material In Fig. 12 and Fig. 13, the liquid shimming configuration (L100) and hybrid shimming configuration (H100) are compared with the nonshimming configuration (N). The behaviors of shimming materials are evaluated by the ratio of strength difference to initial gap size or the ratio of stiffness difference to initial gap size, as given in Eq. (3), where S% means behavior difference (strength difference or stiffness difference) between shimming configuration and non-shimming configuration, Sshim is behavior of the shimming configuration, and Sno is behavior of the non-shimming configuration. As for ultimate load in Fig. 12, non-shimming configurations are the lowest, liquid shimming configurations are about 15% higher than non-shimming configurations, and hybrid shimming configurations are more than 17% higher than nonshimming configurations. It is because the strength of the hybrid shim is higher than that of the liquid shim, which means that the mechanical properties of the shimming material have a positive effect on bearing strength of the specimen. In Fig. 13(a) and (b), for configurations with initial gap size of 2.0 mm, the trend is almost same as that in Fig. 12. However, for configurations with initial gap size of 4.0 mm, stiffness differences of the liquid shimming and hybrid shimming configurations are so small (|SL% − SH%| < 0.4%), and strength differences of which are only 2.19%. It indicates that there will be little differences for different shimming materials when initial gap size is large enough.
Fig. 10. The effect of gap size on stiffness of the specimen for both non-shimming and 100% liquid shimming configurations.
S% =
(Sshim − Sno ) × 100% δ
(3)
3.4. Effect of shimming percentage In order to evaluate the effect of non-full shimming and full shimming conditions on flexural behavior, three shimming percentages, 0%, 50% and 100%, were tested for liquid shimming configurations with initial gap size of 2.0 mm as shown in Figs. 14 and 15. Both strength and stiffness increase quasi-linearly as the shimming percentage increases. As shimming percentage increases from 50% to 100%, ultimate load increases about 9.70%, KI increases about 33.87%, KII increases about 15.05%. It can be concluded that non-full shimming influences the flexural behaviors of the composite joints, especially the initial stiffness KI. Hence, full shimming condition is recommended in engineering applications within affordable cost and weight.
Fig. 11. The comparison of the load-displacement curves between configurations with small gap and big gap.
are higher than that of non-shimming configuration, which means the shimming material can make the structure harder to bend. It is because that both the arched laminate and flat laminate are subjected to three379
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Fig. 13. The effect of shimming material on stiffness of the specimen: (a) KI; (b) KII.
Fig. 16. The effect of bolt-hole clearance on bearing behavior of the specimen.
Fig. 14. The effect of shimming percentage on ultimate load of the specimen.
indicates that the bolt-hole clearance mainly influences the load takeup stiffness KII. It may be because that both the arched laminate and flat laminate subject to three-point bending after the gap is completely closed, and bigger bolt-hole clearance gives more space for the arched laminate to move so that the load take-up stiffness KII decreases.
3.5. Effect of bolt-hole clearance As Fig. 16 shows, two kinds of bolt-hole clearances, C1 (0.028 mm–0.078 mm) and C2 (0.128 mm–0.178 mm), were compared for non-shimming configurations with initial gap size of 2.0 mm. When bolt-hole clearance increases about 0.1 mm, stiffness KI changes only 0.34%, which is not sensitive to bolt-hole clearance; however, stiffness KII and ultimate load decrease about 6.11% and 3.42%, respectively. It
3.6. Failure mechanism Fig. 17(a)–(c) show the failure micrographs of the specimens with
Fig. 15. The effect of shimming percentage on stiffness of the specimen: (a) KI; (b) KII. 380
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Table 2 Debonding load and delamination load of configurations with different shimming materials. Shimming material
Debonding load (N)
Delamination load (N)
No shim Liquid shim Hybrid shim
– 1509.63 ± 162.91 612.90 ± 81.23
1490.50 ± 10.50 2120.56 ± 86.38 2336.60 ± 78.00
shim was firstly observed. It can be described by debonding load that corresponds to the moment when the adhesive interface of the arched laminate and shim was debonded. Table 2 shows the debonding load and delamination load of the configurations with different shimming materials. Considering the variance of the experiments (different porosities of the liquid shims, different bearing properties of the hybrid shims and laminates), it can be concluded that the presence of shimming material improves the delamination load. It is because that the debonding of adhesive interface occurs before delamination initiation of the laminates for shimming configurations, which releases partial energy of the specimen subjected to three-point bending so that the laminates are protected. 4. Conclusion In this paper, an experimental study was conducted to investigate the flexural behavior of composite shimming joints with predesigned assembly gap subjected to three-point bending. A type of hybrid shim was proposed, which was made by the epoxy and certain thickness solid peelable fiberglass cloth, and it shows better properties than the liquid shim. Furthermore, a method to control the shimming percentage was developed using a bespoke tool (Fig. 5) made by solid peelable fiberglass cloth (guarantee the shim thickness) and scotch tape (protect the fiberglass cloth from the epoxy in order to remove the bespoke tool). The experimental parameters contain gap size, shimming material, shimming percentage and bolt-hole clearance. The conclusions from experiments are as follows: 1) The typical flexural behavior of the composite shimming joints can be approximated into five stages of loading as: contact, gap closure, load take-up, damage accumulation and failure. 2) The presence of assembly gap decreases the strength of the joints, while the shimming material can make up for the loss. As the initial gap size increases from 1.0 mm to 5.0 mm, ultimate load of nonshimming configuration decreased by 15.23%, while that of liquid shimming configuration increased by 52.51%. Both the stiffness KI and KII of 100% liquid shimming configuration are higher than that of non-shimming configuration. 3) Mechanical properties of the shimming material have a positive effect on flexural behavior. The strength and stiffness of non-shimming configurations are the lowest, those of liquid shimming configurations are medium, and those of hybrid shimming configurations are the highest among three types of shimming conditions. 4) The higher the shimming percentage, the better the flexural behavior. The strength and stiffness increase quasi-linearly as the shimming percentage increases. Therefore, full shimming condition is recommended in engineering applications within affordable cost and weight. 5) Bolt-hole clearance mainly influences the load take-up stiffness KII, both ultimate load and initial stiffness KI are not so sensitive to bolthole clearance.
Fig. 17. Failure micrographs of the specimens with different shimming conditions: (a) no shim; (b) liquid shim; (c) hybrid shim.
initial gap size of 2.0 mm at same load conditions corresponding to nonshimming, liquid shimming and hybrid shimming configurations, respectively. Delamination was one of the major damages for all three configurations. The arched laminate was obviously broken by delamination around the central layers and delamination between first two plies close to the top surface due to strong compressive stress, while the flat one was damaged with serious ply fractures close to the bottom surface due to strong tensile stress. For non-shimming configuration, the specimen was firstly damaged by delamination at the top surface of the arched laminate, which can be described by delamination load that corresponds to Point 2 in Fig. 8. For shimming configurations, debonding of the adhesive interface between the arched laminate and
Acknowledgements This work was supported by the National Key Basic Research Program of China (973 Project) [Grant No. 2014CB046504], the 381
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National Natural Science Foundation of China [Grant No. 51375068 & Grant No. 51475073] and the Fundamental Research Funds for the Central Universities [Grant No. DUT15RC(3)089]. The authors would like to acknowledge the above financial supports.
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