A Fundamental Study on Static Strength Improvement of CFRP Bolted Joints by Increasing Friction Force

Journal of Solid Mechanicsand Materials

Engineering

Vol. 4, No. 6, 2010

711 711

A Fundamental Study on Static Strength Improvement of CFRP Bolted Joints

by Increasing Friction Force*

Tsukasa KATSUMATA**, Yoshihiro MIZUTANI***, Akira TODOROKI*** and Ryosuke MATSUZAKI***

** Graduate School of Tokyo Institute of Technology 1-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

E-mail: [email protected] *** Department of Mechanical Engineering, Tokyo Institute of Technology

1-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Abstract The bolted joint is a common assembling method for carbon fiber reinforced plastic (CFRP) members. However, cracks or plastic deformation can occur around bolt holes of CFRP members even when employing low fastening forces. As a result, CFRP failure occurs around bolt holes because of bearing forces, and the strength of a CFRP joint is reduced. To address this problem, we change the configurations of washers and insert a thin sheet having a high friction coefficient between CFRP members. In friction coefficient measurements, the insertion of sandpaper was found to be the most suitable method for increasing friction forces at the interface between CFRP members. Three-dimensional finite element method analyses were conducted to investigate the effect of washer configurations in terms of the maximum allowable fastening force and the joint strength. From these analyses, the cone washer was found to be the most effective design for increasing the joint strength. In the full model analyses, the failure strengths of proposed CFRP bolted joints were higher than those of normal CFRP bolted joints. The verification tests for the proposed joint were conducted according to ASTM D5961. It was observed that the load at failure for the proposed bolted joint was about 45 % greater than that for the conventional joint.

Key words: Composite Material, CFRP, Bolted Joint, Stress Concentration, Finite Element Method

1. Introduction

In modern aircraft, a large number of carbon fiber reinforced plastic (CFRP) members are used to decrease weight. One of the major assembling methods for the CFRP members is the bolted joint. However, to prevent damage to the CFRP around bolt holes during bolt fastening, bolt-fastening forces are controlled to be low (1). Because of the low bolt-fastening forces, friction forces at the interface between CFRP members are low and bearing forces acting on the sides of bolt holes are large (2). This results in stress concentrations around bolt holes when external loads are applied to the joints. For this reason, CFRP and bolt failure tend to occur around bolt holes. The strengths of conventional CFRP bolted joints are less than half the strength of the CFRP plates (1).

The objective of this study is to address these problems and increase the strengths of CFRP bolted joints with minimum weight gain.

*Received 16 Nov., 2009 (No. e134) [DOI: 10.1299/jmmp.4.711]

Copyright © 2010 by JSME

Journal of Solid Mechanics and Materials Engineering

712 712

Vol. 4, No. 6, 2010

2. Proposed Methods

To increase the strengths of CFRP bolted joints, high friction forces at the interfaces between CFRP members are required. There are two approaches for increasing friction forces. The first approach is increasing bolt-fastening forces without damaging the CFRP and the other is increasing the friction coefficient of CFRP contact surfaces. In this study, we consider changing the washer configuration as an example of the first approach and treating the CFRP surface or inserting a high-friction sheet between CFRP members as an example of the second approach.

3. Surface Treatment and Sheet Insertion

To increase the friction coefficient for CFRP contact surfaces, we examine both the treatment of CFRP surfaces and the insertion of sheets between CFRP members. We adopted sanding for the former and the use of sandpapers for the latter. Sanding roughens the CFRP surfaces without decreasing CFRP strength. Sandpaper has a high friction coefficient that does not depend on the normal stress on the contact area, unlike the coefficient of high-friction sheets currently available for joints, such as rubber sheets (3) and adhesive tapes (4). Measurements of the friction coefficients for these configurations are presented in Table 1, which shows that the insertion of sandpaper is the most effective method for increasing the friction coefficient.

4. Changing Washer Configurations

4.1. Normal washer Figure 1 shows the damage near a bolt hole when a bolt head is pressed into the bearing

Table 1 Measured friction coefficients of CFRP Surface Friction coefficient Untreated 0.36 Sanding (#100) 0.42 Sanding (#800) 0.39 Inserted (#40) 0.66 Inserted (#240) 0.68

Damage

Bearing surface

Strain gage Fig. 1 Damage of CFRP caused around a bolt hole.


713 713

Vol. 4, No. 6, 2010

surface of the CFRP plate. The diameter of the bolt was 6 mm and that of the washer was 13 mm. The stacking sequence of the CFRP was 16-layer quasi-isotropic [(45/0/-45/90)2]S. As seen in the figure, the edge of the hole was seriously damaged.

In the following, the distribution of the principal stress in CFRP is simulated by three-dimensional (3D) finite element method (FEM) analysis using the finite element code ANSYS 11.0. The finite element model is shown in Fig. 2. In this analysis, the elements of the CFRP were SOLID185 eight-node layered elements and those of the bolt heads were SOLID45 eight-node elements. The bottom surface of the CFRP was fixed and two bolts were pressed into the CFRP. The failure strengths of CFRP were taken from data sheets (5).

xzy

CFRP

Bolt

Washer

Compression

Fig. 2 FEM model used in bolt compression analysis.

Stress concentration

-28 -22 -17 -11 -5.6 MPa-28 -22 -17 -11 -5.6 MPa Fig. 3 Distribution of principal stress for a normal washer (thickness of 1 mm).

Diameter : 15mm

Diameter : 35mm

0 2 4 6 80

2

1

3

Washer thickness [mm]

Nor

mal

ized

max

imum

fa

sten

ing

forc

e

4

5

Fig. 4 Relationship between the thickness of the washer and maximum fastening force.


714 714

Vol. 4, No. 6, 2010

The results are presented in Fig. 3. The figure shows a stress concentration around the edge of the bolt hole, which corresponds to the experiment result shown in Fig. 1. 4.2. Effect of the washer thickness

The maximum bolt fastening forces that do not cause damage were simulated by the aforementioned 3D FEM analysis. The results for 15 and 35 mm diameter plain washers with changing varying washer thickness are shown in Fig. 4. In the figure, the horizontal axis represents the washer thickness and the vertical axis represents the maximum bolt fastening force normalized by that of a conventional plain washer with diameter of 15 mm and thickness of 1.0 mm. From these results, it is seen that the normalized maximum fastening force has an upper limit in the case of the 15 mm diameter washer but not in the case of the 35 mm diameter washer. Therefore, increasing the washer diameter allows the bolt fastening forces to be increased without CFRP failure. Therefore, increasing the washer diameter is an effective method of increasing the

Normal

Rib

Tapered

Cone

Fig. 5 Profiles of four types of washers.

PlainTaperedRibCone

20 40 60 80Weight [g]

00

2

1

3

Nor

mal

ized

max

imum

fa

sten

ing

forc

e 4

5

Fig. 6 Relationship between weight and normalized maximum fastening force for

various washer profiles.

Bolt

CFRP

Washer

36

270

3.2

Fig. 7 Finite element model of the CFRP bolted joint.


715 715

Vol. 4, No. 6, 2010

strength of a CFRP bolted joint. 4.3. Effect of the washer profile

The effect of changing the washer profile was investigated in the aforementioned 3D FEM analysis. Four washer configurations—a plain washer, tapered washer, ribbed washer and cone washer—were considered, as shown in Fig. 5. The washer diameter was fixed at 35 mm and the washer thicknesses varied. In Figure 6, the horizontal axis represents the total weight of two bolts, two nuts and four washers and the vertical axis represents the maximum bolt fastening force normalized by that of a conventional plain washer with a diameter of 15 mm and thickness of 1.0 mm. The results show that the cone washer is the most effective washer for increasing the strength of a CFRP bolted joint with the lowest weight gain. For this reason, the cone washer was adopted in this study.

-125 63 0 63 125 MPa188

xyz

xyz

Fig. 8 X-axis stress distribution in the CFRP for the conventional joint with plain washers

and no sheet inserted under a joint load.

-125 63 0 63 125 MPa188

xyz

Fig. 9 X-axis stress distribution in the CFRP for the proposed joint with cone washers

and sandpaper inserted under a joint load.

Table 2 Joint strengths determined from the full model FEM analyses Joint type Joint failure load [kN]

Conventional 6 Proposed 8


716 716

Vol. 4, No. 6, 2010

5. Full Model Analyses

To investigate the effects of inserting sandpaper and using the proposed cone washer, joint strengths and stress distributions when external loads are applied to the joints were investigated in 3-D FEM analyses. Figure 7 shows the finite element model for these analyses. In the model, CFRP was represented by SOLID46 eight-node layered elements, and the friction between CFRP and CFRP, between CFRP and washers, and between washers and bolts was considered. Hashin failure criteria (6) were used for CFRP and maximum stress failure criteria were used for the bolt and washers. The material properties and failure stress of CFRP were those of HTA/6376 (6).

The results of the full model analyses are presented in Figs. 8 and 9 and Table 2. Figure 8 shows the stress distribution of the joint with conventional plain washers when a 6 kN joint load and 1 kN bolt fastening forces were applied. The figure shows a stress concentration around the bolt holes. Figure 9 shows the stress distribution of the joint with the proposed cone washers when the same joint load and 5 kN bolt fastening forces were applied. The figure shows that the stress concentration improved around the bolt holes. Hence, the proposed cone washers reduce the contact forces between bolts and the sides of holes. Table 2 presents the joint strengths when using the conventional plain washers and proposed cone washers. The results show that use of the

72171

24

GFRP Tab

36

6φ3.6

Bolt hole

CFRP

Fig. 10 Configuration of the Fig. 11 Configuration of the specimen (unit : mm). cone washer (unit :mm).

Bolt No.2

Bolt No.1

Bolt No.2

Bolt No.1

Fig. 12 Specimen of the Fig. 13 Specimen of the conventional CFRP bolted joint. proposed CFRP bolted joint.


717 717

Vol. 4, No. 6, 2010

proposed cone washer increases the CFRP bolted joint strength. The effectiveness of the proposed cone washer was thus clarified by FEM analyses.

6. Experiments

To examine the proposed joining method of inserting sandpaper and using cone washers, single-lap joint tests were conducted for a conventional CFRP bolted joint and the proposed joint. These were conducted according to ASTM D5961 (7). The tensile speed was 1.0 mm/min. The CFRP specimens were made of PYROFIL380 prepreg from Mitsubishi Rayon Co. Ltd. and had a stacking sequence of 16-layer quasi-isotropic [(45/0/-45/90)2]S. The bolts were made of titanium alloy (Ti-6Al-4V) and had diameters of 6.0 mm. The configuration of the specimen is shown in Fig. 10. Strain gages (Kyowa KFG-

(a) Conventional joint

(b) Proposed joint

Fig. 14 Relationships between joint load, joint strain and bolt axial direction strain.


718 718

Vol. 4, No. 6, 2010

1.5-120-C20-11) were inserted for all bolts to measure the axial strain. The cone washers were made of aluminum alloy (A2017) and had the configurations are shown in Fig. 11.

For the conventional CFRP bolted joints, normal plane washers with a diameter of 14 mm and thickness of 1.6 mm were used and bolts were tightened by hand. For the proposed CFRP bolted joints, cone washers with a diameter of 35 mm and thickness of 10 mm were used and the fastening force was 5.5 kN. Figure 12 shows the specimen of the conventional CFRP bolted joint and Fig. 13 shows the specimen of the proposed CFRP bolted joint.

The relationships among joint load, joint strain and bolt axial strain for both joint types are shown in Fig. 14. The joint load increased with joint strain in a relatively linear manner in the first period (<1500 × 10–6 strain for the conventional joint, except during the very

Table 3 Results of the verification test Joint type Failure occurring load [kN] Conventional 11.5 Proposed 16.6

Bolt failure

CFRP failure

(a) Conventional joint

CFRP failure

No bolt failure

(b) Proposed joint

Fig. 15 Observed failures of both the conventional and the proposed joints.


719 719

Vol. 4, No. 6, 2010

initial stage, and <3000 × 10–6 strain for the proposed joint). The onsets of non-linear behavior were determined by drawing a line parallel to the linear portion of the load–strain curve but offset by 0.2% strain and investigating the crossing-point of the two curves as shown in Fig. 14. We took the loads at the crossing-points as the loads at failure.

The results of the tests are presented in Table 3. The load at failure for the proposed bolted joint was about 45% greater than that for the conventional joint. These results verify the effectiveness of the proposed joint.

Figure 15 shows the tested specimens. In the case of the conventional joint, CFRP bearing failure occurred first and then the bolt failed. This confirms the bolt axial strains presented in Fig. 14(a). On the other hand, in the case of the proposed joint, bolt failure did not occur because of the decrease in stress concentration around bolt holes due to the change in the manner of external force transmission; that is, the bearing forces were reduced and the friction forces were increased. As a result, the loads at failure were larger than those for the conventional joint as shown in Table 3.

7. Conclusions

In this study, methods for increasing the strength of the CFRP bolted joint were considered using friction measurement experiments and 3D FEM analyses. It was found that inserting sandpaper between CFRP surfaces and using a cone washer are the most effective methods for increasing the joint strength. 3D full model FEM analyses showed the proposed joint has a better failure load. Verification tests for the proposed joint were conducted according to ASTM D5961. From these experiments, the effectiveness of the proposed joint was verified.

References

(1) Niu, M.C.Y., Composite Airframe Structures, (1992), pp.285-354, Hong Kong Conmilit Press Ltd. (2) Straikov, R. and Schon, J., Quasi-static Behaviour of Composite Joints with Protruding-head bolts, Composite Structures, Vol.51 (2001), pp.411-425. (3) Yamada, S., Nakazawa, H-Y., Fukatsu, N. and Komiya, I., Experiments on the Connection Collapse of Pultruded Fiber Reinforced Polymeric Composites, IASS Symposium 2001: International Symposium on Theory, Design and Realization of Shell and Spatial Structures, (2001), pp.166-167. (4) Kishima, T., Effect of Clamping Force on the Strength of Bolted Joints in Pultruded GFRP Laminates, Journal of the Society of Materials Science, Japan, Vol.56, No. 11 (2007), pp.999-1004. (5) Mitsubishi Rayon Co., Ltd. ed. PYROFIL #380 Product Data Sheet, (2002), Mitsubishi Rayon Co., Ltd., Minato-ku, Tokyo, Japan. (6) Tserpes, K.I., Papanikos, P. and Kermanidis, T., A Three-dimensional Progressive Damage Model for Bolted Joints in Composite Laminates Subjected to Tensile Loading, Fatigue and Fracture of Engineering Materials & Structures, Vol.24 (2001), pp. 663-675. (7) American Society for Testing and Materials ed., Standard Test Method for Bearing Response of Polymer Matrix Composite Laminates, ASTM D5961/D5961M-01, American Society for Testing and Materials, West Conshohocken, PA, USA.

Documents

A Fundamental Study on Static Strength Improvement of CFRP Bolted Joints by Increasing Friction Force