An experimental investigation of static and fatigue behaviour of sandwich composite panels joined by fasteners

Composites: Part B 32 (2001) 299±308

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An experimental investigation of static and fatigue behaviour of sandwich composite panels joined by fasteners Giuseppe Demelio, Katia Genovese, Carmine Pappalettere* Dipartimento di Progettazione e Produzione Industriale, Politecnico di Bari, Viale Japigia 182, 71026 Bari, Italy Received 5 June 2000; accepted 21 February 2001

Abstract An experimental investigation has been carried out to estimate the static and fatigue behaviour of specimens made up of steel plates and sandwich composite panels joined together by either blind or mechanical lock fasteners. A preliminary study was carried out in order to analyse the drilling operation of sandwich panels to determine the best values of parameters to use for fastener installation. A ®rst set of pull-out and shear static tests was performed in 1992, using sandwich panels composed of a nomex honeycomb core between two laminates of glass/graphite/kevlar ®bres in epoxy matrix. The investigation was completed in 1998. It consisted of performing a set of pull-out and shear fatigue tests on joints with blind fasteners, and of performing a new set of static tests on identical specimens with the same loading conditions as in 1992 so as to evaluate the possible ageing effect on mechanical proprieties of sandwich panels tested. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Fibres; A. Honeycomb; B. Fatigue; Sandwich panels

1. Introduction Material of the experimental investigation reported in this paper are joints composed of steel plates and sandwich composite panels fastened with blind or mechanical lock rivets. The tested composite sandwich panels have been fabricated by bonding two laminates (made up of different combinations of layers of carbon, glass and kevlar ®bres in a epoxy matrix) to a thick, lightweight nomex honeycomb core (Fig. 1). The peculiar structure of a sandwich composite panel makes it ideal for both structural (load bearing) and non-structural (not load bearing) applications [1±5]. In fact, the facings of a sandwich panel act similarly to the ¯anges of an I-beam because they resist the bending loads and increase the bending stiffness of the section by spreading the facings apart. This shape gives excellent stiffness/ weight and strength/weight ratios to the structure. Furthermore, good thermal and acoustic insulation properties exist due to the core, whose tasks are to bear shear loads, to separate the facings and to carry loads from one facing to the other. Unfortunately, sandwich panels are weak at bearing * Corresponding author. Tel.: 139-080-5962700; fax: 139-0805962777. E-mail address: [email protected] (C. Pappalettere).

concentrate loads; hence they could be dif®cult to join. The recognised methods for joining composite structures to other composite or to metallic parts are adhesive bonding, mechanical fastening with rivets and joining by specially designed pieces. Fewer pieces, lower weight, good load distribution and lower cost are advantages offered by adhesive joints [6]; however, in many cases, mechanical fastened joints must be used because of requirements for disassembling the joint to replace damaged structure or to achieve access to underlying structure. By combining adhesive bonding with mechanical fastening it is possible to develop surface-mounted fasteners with the twofold bene®t of preserving the structural integrity of the material by the elimination of holes, and by making the disassembly of the joint possible. However their use often results in higher cost and increased weight. Finally, mechanical fastening tends to be preferred because of high tolerance to repeated loads, good resistance to most environments, ease of inspection and high reliability. However, using this joining method requires special considerations about fastener material and design, hole preparation and fastener installation, so as to ensure joint quality and long fatigue life. A ®rst set of decisions concerns the fastener type and fastener installing conditions [7±11]. Since composite

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Fig. 1. Tested sandwich composite panels.

panels have low compression strength, the fastener head should expand enough to ensure a suf®cient clamp area. This prevents possible crushing or delamination around the hole due to an excessive or insuf®cient load on the bearing area. Generally, to increase fatigue joint performance and to prevent fasteners cocking under shear loads, the fastener is installed in an interference-®t hole, or a hole®lling fastener that expands radially during installation is used. Finally, particular care must be used in the drilling operation to prevent skin delamination and fraying, and resin damage due to local overheating. The goal of the investigation reported in this paper is to test a set of different combinations of sandwich panels and fasteners under shear and pull-out static and fatigue loading.

The investigation has also included a preliminary study looking for the best set-up of parameters for drilling composite panels. 2. Experimental A In 1992 a ®rst set of tests was performed to evaluate static pull-out and shear strength of different types of specimens made up of sandwich composite panels and steel plates joined by both blind and mechanical lock fasteners [12]. Specimens used for the pull-out tests (Fig. 2(a)) have been created by ®xing a 50 mm square steel plate on a 150 mm square sandwich panel with the fastener. The

Fig. 2. (a) Pull-out test specimen, (b) ®xture used for load application in pull-out test (blind fastener), (c) pull-out test specimen under loading.

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Fig. 3. (a) Shear test specimen, (b) ®xture used for load application in shear test (mechanical lock fastener), (c) shear test specimen under loading.

steel plate is joined with another plate with a central dowel screw to allow the clamping to the testing machine. To restrict the load to only the joint during the test, a special ®xture that clamps the panel is used to apply the force to the specimen (Fig. 2(b)). The specimens used in shear tests are made up of 200 £ 100 mm 2 rectangular panels with two steel plates Table 1 Geometric properties and conventional designation of skin lay-up of specimens Specimen

Base (mm)

Height (mm)

Thickness (mm)

Skin denomination

101 103 105 107 109 111

150 150 150 100 100 100

150 150 150 200 200 200

19.05 19.05 40 19.05 19.05 40

121 125 125 123 127 127

®xed in place by fasteners at a proper distance from the edges (Fig. 3(a)). The load has been applied directly to the plates and the displacement between fasteners was measured during the test with a strain gauge (Fig. 3(b)). The sandwich panels used have core thickness of 19.05 or 40 mm and skins with four different lay-ups. The characteristics of panels are detailed in Tables 1 and 2. In particular, the ®rst table details geometric proprieties and the conventional designation of skin lay-up; the second table shows the type and the orientation of reinforcement ®bres for each designation. For the tested joints, three different types of blind fasteners (UBP-MV, NAS 1919C, NAS 1919M) and the mechanical lock fastener 2Asp509P from Huck International production have been selected. All fasteners used have protruding heads because it was impossible to countersink the hole due to the thickness of the laminate (<1 mm). To install the mechanical lock fastener it was necessary to counter-sink the steel plate due to the ¯ush

Table 2 Skin lay-up (GR/EP Ð graphite ®bres in epoxy matrix, KEV/EP Ð kevlar ®bres in epoxy matrix, GL/EP Ð glass ®bres in epoxy matrix) Skin denomination

GR/EP (8)

GR/EP (8)

GR/EP (8)

GR/EP (8)

KEV/EP (8)

KEV/EP (8)

GL/EP (8)

121 123 125 127

0 45 0 45

0 45 0 45

90 245 90 245

90 245 90 245

0 0 ± ±

0 0 ± ±

± ± 45 45

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Table 3 Characteristics of fasteners used for tested joints Fastener

Material

Nom. Diameter (mm)

Grip

Weight (gr)

Cost ($)

2Asp 509 P (Huck international)

6Al-4V Titanium

5

06

1.17

1.8/2

UBP-MV (Huck International)

Titanium

5

06

1.17

1.8/2

NAS 1919 C (Huck International)

Stainless Steel A-286 (Cres)

5

06

2.80

0.8/1

NAS 1919 M (Huck International)

Monel 400

5

06

2.80

0.8/1

sleeve. Characteristics of the fasteners used are detailed in Table 3. The blind and mechanical lock fasteners installation procedure steps and the special installation tool used are respectively shown in Fig. 4(a)±(c). Particular care was used in drilling and machining the panels so as to choose the better speed to prevent delamination and fuzzing of skins and resin damage due to local overheating. For all the panels a common electric drill of 3000 rpm with an automatic drilling rate of 4 mm/min and 27.5 mm/min was used for GR/GL ®bres and KEV ®bres reinforced panels respectively. Vidia conventional drills have been successfully used in drilling panels even though their use is not commonly recommended for composites. This result has to be ascribed to the stiffening effect of the adhesive bonding core and skin that eliminates fuzzing and delamination around the hole. Generally, skins with woven ®bres on the top and bottom surfaces of the panels produce better holes, for they leave clean entrance and exit surfaces. Hence, in dealing with laminates with unidirectional ®bres, a backup plate of steel

was used during drilling to reduce surface delamination and to keep the ®bres from fraying. 3. Experimental B In 1998 new static tests were performed to determine the ageing effect on mechanical properties of sandwich panels tested in 1992. As well as having to withstand loading extremes, in fact, composite components must survive in a range of different environments of moisture and temperature. Relative humidities can vary from 0 to 100% and temperatures for most uses range from 240 to 708C. Water acts as a plasticizer when absorbed by most organic resins, thereby softening the matrix and reducing some properties of the laminate. Epoxy resins can absorb between 1 and 10% by weight of moisture. Moisture may also migrate along the ®bres±matrix interface, affecting the adhesion. Unbinding can occur due to formation of blisters and cracking in the matrix [13,14]. For these reasons, the

Fig. 4. Installation steps for (a) blind fasteners, (b) mechanical lock fasteners, (c) special installing tool.

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Fig. 5. Load±displacement curves of 1992 static tests.

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Fig. 6. Different cracking behaviour of (a) carbon/glass, (b) carbon/kevlar laminates under pull-out loading.

effect of environment on composite properties needs to be known and, ideally, predicted. In the period ranging from 1992 to 1998 sandwich panels under investigation were stored in a cardboard box in a dark but humid place in our department storeroom, and at the moment of the testing, they appeared to be in a good state of conservation. Reliable results to compare with those of 1992 were obtained because the same loading conditions and identical specimens were selected. Only joints with blind fasteners NAS 1919M Huck have been tested because of their lower cost and temporary availability. To complete the evaluation of the performances of the joints under investigation, a set of pull-out and shear fatigue tests was performed. The same type of specimens and fasteners of 1998 static tests were used. Fatigue tests were run on System Hydroplus SCHENCK machine of 250 kN of load capacity with an INSTRON 8500 PLUS controller. Shear fatigue tests were performed in load control applying a cyclic stress variable with a sine waveform of Smin ˆ 0:1Smax ;

…1†

Amplitude ˆ …Smax 2 Smin †=2;

…2†

Preload ˆ …Smax 1 Smin †=2:

…3†

Tension fatigue tests were carried out in displacement control because of the dif®culty in controlling the load for

the low values of forces at stake due to the poor strength of these kinds of joints to pull-out loading. Documenting the progressive damage of the joint was done by carrying out preliminary tests for every selected amplitude of cyclic stress to apply, to estimate the corresponding range of displacement and then by running the test in displacement control until the load dropped down. Then the test was stopped and a new amplitude of displacement was selected to obtain the desired value of load amplitude. 4. Results A Obtained data from 1992 pull-out and shear static tests on different types of joints under investigation are illustrated in Fig. 5 in form of load-displacement curves. In shear tests, joints with blind fasteners showed better performances than those with mechanical lock fasteners because of the state of compressive stress that arose between head and shop head that reduces the oval forming effect of the hole. From observation of pull-out test curves of blind-fasteners joints, the following can be noted: ² a ®rst short line with a slight slope, probably due to a settling between the structure and the load ®xture, ² a second line, when the load increases rapidly with deformation, ² a third line, when the slope bluntly changes before reaching the maximum value of load. It happens around values

Fig. 7. Different cracking behaviour of (a) carbon/glass, (b) carbon/kevlar laminates under shear loading.

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Fig. 8. Comparison of load±displacement curves related to 1992 (dashed lines) and 1998 (solid lines) static tests.

305

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Fig. 9. S±N curves related to 111, 109 e 107 specimen types.

of 350±400 N and it could be due to the ungluing of skin and core of the panel and to a consequent change of the stiffness of the structure, ² a ®nal line related to the protrusion of the blind fastener out of the skin. For shear test curves of blind-fasteners joints a common trend can also be observed: ² a ®rst line related to the structure's settling over the load, ² a second line in which the slope rapidly increases until it reaches the maximum value of joint strength, ² a third line, when the load drops down as the fastener ®rst widens the hole, crushes the skin and then exits the panel. For both pull-out and shear curves of mechanical lock fastener joints no evident change of slope was observed. In pull-out tests, specimens with kevlar reinforced panels exhibit the better behaviour, even if the highest values of loads reached are modest. In Fig. 6 the different cracking behaviour of carbon/glass and carbon/kevlar laminates is showed. Kevlar ®bres are

more elastic than carbon and glass ®bres; hence the blind head of the fastener exits through the panel by breaking the skin in the former case and widening the hole in the latter case. In Fig. 7, the different behaviour of the two different laminates when shear loaded can be seen. For kevlar reinforced skins the fuzzing of the laminate is more evident. 5. Results B: static tests Static tests data obtained from the joints investigated in 1998 are illustrated in Fig. 8 by a solid line. In a comparison of load±displacement curves related to pull-out tests, it can be seen that specimens with kevlar reinforced skins have shown the best results, which reached a breaking load of almost 1000 N. Joints have performed similarly to the 1992 joints. It can be concluded that the decay of composite panels was slight. This was probably due to the fact that absorption of moisture from humid environments at room temperature is usually a reversible process, and with most advanced composite there is no degradation in room temperature properties.

Fig. 10. S±N curves related to shear fatigue tests.

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Fig. 11. Load±displacement curves related to pull-out fatigue tests on specimens 101.

Fig. 12. Load±displacement curves related to pullout fatigue tests on specimens 103.

Fatigue behaviour of this kind of joint is affected by several factors such as the composite reinforcement material, the number of layers of the skin, the core thickness, the quality of drilling and the installing fastener operations [15±18]. Obtained data from 1998 shear fatigue tests on different types of joints under investigation are reported in Fig. 9 and a line of best ®t is drawn through the data points. Despite the small number of tests and the scattering of results, it was possible to mark up the linear trend of the S± N curves and the effect of the reinforcement (compare 107GR/KEVL- with 109 and 111-GR/GL-) and of the core thickness (compare 109 with 111) on fatigue strength of

the joints. The slight in¯uence of the core thickness on the fatigue strength of the joint could be explained by observing the way in which the specimens have been constructed: shear stress affects mostly the skin of the sandwich panel and the core thickness in¯uences only the global stiffness of the structure. If all shear fatigue test results are grouped into a single plot (Fig. 10) it is possible to point out a rather narrow band with a scatter band height of 20%, which could be useful for dealing with fatigue design analyses of these kinds of joints. Tables 4±6 and Figs. 11±13 show the pull-out fatigue strength data obtained with 7/10 Hz load controlled sine waveform. The way in which the test was carried out made it possible to mark up the considerable damage of the panel already incurred from the early cycles and to

Table 4 Results of pull-out fatigue tests on specimens 101

Table 5 Results of pull-out fatigue tests on specimens 103

6. Results B: fatigue tests

Specimen

Maximum load (N)

Minimum load (N)

Frequency

Cycles to failure

Specimen

Maximum load (N)

Minimum load (N)

Frequency

Cycles to failure

27 26 28 29

637.6 598.6 540.2 470.9

147.2 117.7 98.1±117.2 88.3±117.2

7 7 7±8 7±8

4347 16,634 18,342 70,459

31 33 32 34

470.9 412 353.7 333.6

98.1±121 106 59±88.3 59

7 7±8 7±8 10

40,959 4963 56,654 20,0000

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Fig. 13. Load±displacement curves related to pull-out fatigue tests on specimens 105. Table 6 Results of pull-out fatigue tests on specimens 105

References

Specimen

Maximum load (N)

Minimum load (N)

Frequency

Cycles to failure

37 12 11 38

589.6 549.4 510 470.9

127.5±176.6 117.8±137.3 117.8±127.5 88.3±117.8

7±8 7±8 7±8 7±8

8719 38,915 81,849 74,278

illustrate the progress of force±displacement curves during subsequent cycles. The lack of superposition between ascending and descending curves reveals a probable failure between core and skin; it becomes more distinct if the 300/320 N threshold is crossed.

7. Conclusions From static and fatigue test data obtained, it is possible to draw some considerations about the use of sandwich composite structures joined by fasteners. Drilling process, that is usually critical and requires special tools and re®ned technological solutions, was less critical than expected, probably due to the stiffening effect of the adhesive between core and skin. Concerning static tests, joints performed better when shear loaded, even if GR/KEV reinforced panels showed breaking loads near 1000 N under pull-out loading. The ageing of sandwich composite panels resulted only in a slight decay of the mechanical characteristics of the joints. Both skin reinforcement ®bre material and core thickness of the sandwich panels affect the fatigue strength of tested joints. Despite the scattered data of the shear fatigue tests, it was possible to point out a rather narrow band with a scatter band height of 20%, which is useful for dealing with fatigue design analyses of these kinds of joints.

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