Thermomechanical behaviour of CFRP tubes for space structures

Acta Astronautica Vol. 12, No. 5, pp. 323-333, 1985

0094-5765/85 $3.00+ .00 © 1985 Pergamon Press Ltd.

Printed in Great Britain.

THERMOMECHANICAL BEHAVIOUR OF CFRP TUBES FOR SPACE STRUCTURESt G. G. REIBALDI* Mechanical Systems Division, European Space Agency/ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands (Received 31 January 1984; revised version received 13 June 1984)

Abstract--Carbon fiber reinforced plastic (CFRP) tubes, with the increasing dimensions and performances requested for space structures, are becoming a basic building element of boom-type structures for large precision reflectors, towers and payload support structures such as the Modular Payload Support Structure, the Shuttle Pallet Satellite or the European Retrievable Carrier. It is very important for such applications that the CFRP tubes have minimum thermal distortions and very high stiffness. An extensive test program was performed to characterise the CFRP tubes that are used for such applications. Measurements of coefficient of thermal expansions, thermal conductivity, thermal cycling, microstructure behaviour, as well as mechanical tests and outgassing tests were performed. The main purpose was to correlate the microcracking with the thermal cycling and the coefficient of thermal expansion and thermal conductivity. These types of activities for the CFRP tubes were performed for the first time in Europe and important results were found, especially in the area of microcracking generation and correlation with engineering parameters. The influence of the thermal cycling speed on the microcracking was also studied. Most of the tests were conducted at ESTEC (European Space Research & Technology Centre, Holland) by the European Space Agency in the frame of the technology research activities. I. INTRODUCTION

2.

Carbon fiber reinforced plastic tubes are important building elements for many applications such as optical telescopes, telecommunications antenna support structures, towers, experiment platforms (see Figs. 1-3) because of their low mass, high stiffness and thermal stability. Because of the commonality of these tubes to many space programs[l, 2], ESA decided to perform a test program to characterise these tubes in the frame of the technology activities. Special emphasis was given to studying the influence of the space environment on the thermomechanical properties in the light of the ever-increasing mission durations envisaged. In particular, the effect of the thermal cycling on the material properties has been investigated and correlated with special attention paid to the coefficient of thermal expansion. The coefficient of thermal expansion is dependent on the operating temperature of the material[2], and for very stable structures like antenna boom, support structure and tower it is important to know its variation with the temperature and with thermal cycling. It is also important to correlate the coefficient of thermal conductivity with thermal cycling because of its influence on the thermal balance. Continued emphasis is placed, in fact, on weight, reliability and materials with long space survivability and low contamination. Results of the tests performed and recommendations on the need to perform aging cycles on the hardware are given.

TESTING PROGRAM

In Table 1, the test sequence is shown. Because the real space structures have long tubes, the tests have been performed with the longest possible components in compatibility with the test facilities and with the type of test.

3. MECHANICAL TESTS The results of the mechanical and physical tests of the material used are indicated in Table 2. The fiber content measured on the batch is about 62%. The tube is composed of 80% longitudinal HMS fibers and by two transverse layers internal and external of T-300 (high strength) fibers. The resin system is CY203/HT872. The diameter is 40 m m --- 0.2 and the thickness is 1.2 mm --- 0.1; the layup is (90 °, 08° 90°). 4. OUTGASSlNG TEST The outgassing test gave the following results: TML%: 0.33% < 1.0% (total mass loss); CVCM%: 0.01% < 0.1% (condensed volatile condensible materials). The material does not present any problem of contamination. 5. THERMAL CONDUCTIVITY TESTS The test objective was to measure the longitudinal thermal conductivity coefficient for a defined range of average tube temperatures. The test philosophy was to establish stable steadystate longitudinal temperature gradients along the tube

tPaper presented at the 34th Congress of the International Astronautical Federation, Budapest, Hungary, 10-15 October 1983. *Member A.I.A.A., B.1.S. 323

324

G. G. REIBALDI

AT/Ax : longitudinal temperature gradient; x = K : S = ,;~ =

length of thermal path; conductivity (X) of total tube section; total section of tube; longitudinal conductivity coefficient of material. The same test was carried out after 3000 thermal cycles. The results of the test are shown before and after thermal cycling in Fig. 4.

"'":t

6. COEFFICIENT OF THERMAL EXPANSION TEST

Fig. 1. 20 GHz multibeam antenna. specimen, by heating one extreme and cooling the opposite one, in such a way that measuring the stabilized temperatures and the amount of heat power passing along the tube, the conductance (K) or the longitudinal conductivity coefficient could be determined by

AT Q = K-~,

K =)~S,

where Q = heat power crossing along the tube main axis (X);

A test setup has been prepared in ESTEC to measure the coefficient of thermal expansion (CTE) of up to 1 mlong CFRP tubes and for sandwiches. An important goal was to investigate the possible influence of thermal cycling tests on the CTE results. The method of measurement of the CTE selected is based on a laser interferometer system. In Fig. 5 the test setup used is shown. A vacuum chamber of 550 mm diameter and 1300 mm long was used. Part of the optics, two retroreflectors, were positioned inside the shroud unit. The shroud is used to change the temperature of the tested tube or sandwich. The remote interferometer block was outside the shroud and was kept at a constant temperature of 54°C during the test to avoid change in the optical path (see Fig. 6). It was monitored by a thermocouple. The interferometer and the retroreflectors were especially prepared for vacuum use. Venting holes were drilled into the interferometer housing and the rubber parts of the retroreflectors were replaced by stainless steel springs. To perform a continuous monitoring of the test parameters, length variation and tube temperature, a video recording system was used (see Fig. 7).

Fig. 2. Shuttle pallet satellite, SPAS.

CFRP tubes for space structures

325

Because of the high number of cycles an accelerated test was considered. The accelerated thermal cycling chamber available in ESA/ESTEC was utilised (ATC2). A description of the system is given in [4]. In Fig. 9 a typical cycle of the accelerated thermal cycling test is shown. The useful dimension for the test sample was 500 x 500 x 250 mm. Six tubes were then loaded in this chamber (see Fig. 10). Their acceleration factor is 50 compared to low orbit condition. The atmosphere in which the samples are surrounded during the test is dry nitrogen. The test was interrupted every 1000 cycles, and more often, to allow the possibility of cutting sections of tubes in order to monitor the microcracks. Fig. 3. European retrievable carrier, EURECA. The test starts pumping down the facility, in order to create a vacuum lower than 10 -5 Tort. In order to " d r y " and de-gas the CFRP tube, the temperature was raised to + 70°C for at least 48 h. Following the heating period at + 70°C, the laser system was adjusted to maximum beam alignment and the laser reading was set to L = 0.0. At the lowest temperature, the nitrogen supply was stopped and a temperature rise was obtained by the parasitic heat leaks of the equipment up to about 20°C. At this temperature, the heating system was activated in order to allow the tube to reach + 70°C. In Fig. 8, the average CTE results with its standard deviation are shown before and after thermal cycling. The coefficients of thermal expansion quoted are always longitudinal, that is in the direction of the symmetry axis of the tubes. The CTE is always positive. The influence of the thermal cycling on the CTE results is quite evident. The results after thermal cycling are lower and more uniform with lower standard deviation, in other words, the material behaves in a more homogenous way after thermal cycling.

7. FAST THERMAL CYCLING The purpose of this test was to simulate a life of 10 yr for antenna components (CFRP tubes) of a typical telecommunication satellite. It was decided to perform thermal cycling tests on the tubes with 3000 cycles between - 180 ° to + 100°C. This temperature variation is due to satellite body shadowing. Table 1. Test sequence 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Mechanical tests Outgassing Thermal conductivity tests Coefficient of thermal expansion (CTE) test Fast thermal cycling ( - 178/+ 95°C), 3000 cycles Thermal conductivity tests after 3000 thermal cycles CTE after 3000 thermal cycles Mechanical testing after 3000 thermal cycles Slow thermal cycling ( - 178/+85°C) 100 cycles Microsections at 0/5/30/60/100/200/700/1000/2000/ 3000 cycles

8. MECHANICAL TESTING AFTER THERMAL CYCLING Besides all the mechanical testing performed before thermal cycling, and shown in Table 2, a set of short compression tests was performed on the tubes after 3000 thermal cycles as described in the next paragraph. The results are presented in Table 3. There is hardly any influence of the thermal cycling on the short compression test results. Only the coefficient of variation is lower after thermal cycling, as it has been shown also for the CTE, due to the microcracking present in the material. 9. SLOW THERMAL CYCLING Slow thermal cycles (100), cycles from - 1 5 0 ° C to +80°C every 6 h in vacuum, were performed on the same type of tubes to see the influence of the cycle's speed on the tube microstructure.

I0. MICROSECTIONS The objective of this inspection was mainly to observe change in the microstructure of the tubes before, inbetween and after thermal cycling. It is known that in carbon fiber reinforced plastic components there is a big difference in coefficient of thermal expansion between the matrix and the fiber and due to thermal cycling, microcracking will originate in the matrixl5, 6]. These cracks occur when the internal stresses exceed the transverse strength of an individual lamina. Samples examined were potted in a low-exotherm plastic in both the transverse and longitudinal direction to the tube axis (see Fig. 11). Crack density has been computed from the microstructure observation and all findings are presented in Table 4. The crack propagation direction is noted to be from the outer edge of each layer toward the tube inside surface; crack opening displacement is generally wider towards the outer tube surface. After 3000 cycles the average width (at room temperature) is approximately 5 microns. The crack path usually runs radially to the tube axis and passes in the resin matrix so as to separate the graphite fibers. After extended thermal cycling (between 2000 and 3000 cycles) some delamination is observed

326

G. G. REIBALDI Table 2. Mechanical properties of HMS/T300 filament/wound tube Ultimate Tensile Strength [N/MML]

1032

Tensile Compression Compression Modulus Strength Modulus

Flexural Strength

Flexural Modulus

[N/MM2]

[N/MM2]

[N/MM2]

[N/MM2]

166800

740

213470

[N/MM2] 585

on all specimens between the outer 90 ° layer and the thick 0 ° layer. The radial crack density in each layer increases rapidly during the initial 100 thermal cycles; after this, further cycling has little effect on such cracking, but at around 3000 cycles a different mode of degradation is observed, that is delamination between layers (see Figs. 12, 13). It is important to note that the larger resin-rich volumes at the tube surfaces always contain cracks after thermal cycling; it is likely that these cracks will propagate into the tube wall thickness. The phenomenology of crack generation was found to be the same after 100 cycles. Slow thermal cycling (6 h) and in vacuum was performed for 100 cycles in order to compare the effect of the thermal cycling speed on the microcracking formation. Comparable numbers of microcracks were found in both cases. Further microsection was performed also with tubes proof-loaded at 46 and 85% of its ultimate tensile strength. No microcracking was observed, due to the fact that 80% of the fiber is at 0 °. II. CONCLUSIONS The following relations between coefficient of thermal expansion, thermal cycling and microcracking can be identified. 1. Coefficient of thermal expansion (CTE) is decreasing with the decreasing of the temperature and is always positive. 2. CTE is decreasing with the increasing of thermal cycling and CTE coefficient of variation is decreasing with the increasing of thermal cycling. (50% coefficient of variation before thermal cycling; t ~,oc h e 38% r m after a l ~

155200

cycling.) The material seems more homogenous after thermal cycling. 3. Microcracking versus thermal cycling in the tubes can be seen in "Fable 5. There are no microcracks before thermal cycling. The crack density increases with the number of thermal cycles, up to 2000 cycles. Between 2000 and 3000 cycles the total microcracking is comparable and at 3000 cycles the microcracking becomes debonding between layers. Microcracking is then mainly due to thermal fatigue. The results of the microsections obtained from different tubes seem quite consistent with the exception of the transversal microsection at 1000 cycles where a large scatter in microcracking is observed. Microcracking increases with the number of thermal cycles up to 2000 cycles, where it stabilises, and at 3000 cycles debonding between layers starts. Very few thermal aging cycles (5) performed on the hardware will generate a microcracking density comparable with many more thermal cycles. There are indications showing that thermal aging tests to stabilize the tubes needs only very few thermal cycles. 4. The influence of thermal cycles' speed on the microcracking generation was considered negligible comparing the microcrack density after 100 cycles. 100 cycles slow (6 h) in vacuum 0.6 microcracks/cm

v a ~

] .......... ~ J i

100 cycles fast (4 min) in nitrogen 0.5 microcracks/cm

char.

~', ° fXq "

I..............

l j lJ i ~

"~t" .....

Re~rore f lector.

.

.

~?.,, - ~ :

Fig. 4. Influence of thermal cycling on thermal conductivity.

.

.

:-

.

.

" _~; ' . . P .

_z~_

ssoo c .

Fig. 5. Coefficient of thermal expansion test setup.

_

r.

CFRP tubes for space structures

327

Fig. 6. Shroud and remote interferometer.

Table 3. Effect of thermal cycling on mechanical properties

Before thermal cycling El [N/MM2] S%

S% AA12:5-D

166800

After thermal cycling 165100

4.18

3.43

0.072

0~069

34.5

29.

This result indicates that performing accelerated thermal cycles are representative of longer thermal cycles in vacuum for these tubes. 5. CTE is influenced by microcracking and decreases with the increases of microcracks especially at room temperature. At low temperature the difference is negligible. 6. Coefficient of thermal expansion is influenced by microcracking and decreases with the increases of microcracks especially at room temperature. The coefficient of variation is lower after the 3000 thermal cycles. Also this material property is more uniform due to microcracking.

328

G . G . REIBALDI

Fig. 7. Coefficient of thermal expansion measurement facility.

Table 4. Influence of thermal cycling on initiation of cracks Plane of microsection to tube axis

NUMBERCYCLES (BETWEEN -175 ° and 90°C) AND EFFECT ON MICROSTRUCTURE O(as received)

I000 cycles 2000 cycles 3000 cycles

TRANSVERSE

No cracks (fig.12a)

6 cracks per cm. in middle l a y , 0• (fig.12b)

6 cracks per cm. in middle l a y , O° , wider, (fig.12c)

8 cracks per cm. in middle lay, 5 cm. wide w i t h delamination ( f i g . Z2d)

LONGITUDINAL

No cracks

5 cracks per cm. in periphery lays, 90 °

14 cracks per cm. in periphery lays, 90", wider

13 cracks per cm. in periphery l a y s , 90", wider with del amina-

[ ion

CFRP tubes for space structures

329

CTE (10-6 )°E-1

. ~ RESULTS

BEFORE

÷ ~ RESULTS AFTER

THERMAL CYCLING 3000 (-1751+95"[) THERMAL CYCLING

f

07-

0,5 ¸

I 0.3

'->x.

"

T

"",,\I -. ; "

~ "r

T __

- - - t

01

__T~ *I

. . . .

.L I

÷20

I

-I0

I

-l~0

I

-70

-130

-I00

T( o C )

Fig. 8. Influence of thermal cycling on the CTE.

7. Mechanical properties do not vary significantly with the thermal cycling. 8. In the thermal distortion analysis, the mechanical and thermal properties derived after thermal cycling should be considered because they are more representative of the orbital condition. 9. Because of the CFRP material anisotropy, local effects are very important for the CTE measurements. It is suggested to use components as big as possible for the CTE measurements in order to have global values, and not local values. 10. Even with extreme care, wide scatter has been observed in the coupon data and the variability is attributed to inconsistencies in material's response created by

time and environment-related change. Despite the wide variability of CTE (with temperature) on and between individual specimen data, the mean CTE does not vary appreciably. 11. More work is needed to study the effect of thermal cycling on CFRP tubes occurring at low orbit (500-600 km) with very high numbers of thermal cycles, 80 000, on the mechanical/thermal properties and on the dimensional stability, in view of the future free-flyer reusable structures like EURECA.

Acknowledgements--The author is grateful to CASA Space Division who supplied all the CFRP tubes tested and to the Metrology Lab, Thermal Control Lab, Metallurgy Section, Solar Array Section, Testing Division in ESA/ESTEC which collaborated in this work. All the work was performed by the European Space Agency in the frame of the technology research activities.

- - ,95-,50C

Table 5. Microcracking versus thermal cycling

,I L

,on.' I /

- 178 _* I ° C

Fig. 9. Typical cycle of the accelerated thermal cycling test.

Microcracks/cm. Long.

Trans.

0.0 0.0 0.0 0.0 0.0 0.5 1.3 6.0 14.0 12.0

0.0 0.4 0.4 0.4 0.7 1.3 1.1 4.0 7.0 8.9

N. cycles

5 30 60 100 200 700 1000 2000 3000

330

G . G . REIBAt,DI

Fig. 10. Accelerated thermal cycling facility.

\

x,x,,x~, f ,,~)¢I.~

\

Fig. I 1. Orientation of samples taken for microsections.

CFRPtubesfor spacestructures

331

o

e~

"O |

§

~× ca.¢~

O4"

U

/

oo

L

,i /

z~ ¢q k~

332

G. G. RE1BALDI

external periphery of tube 90o

middle part of tube 0°

,L

crack direction

Fig. 13. Detail of Fig. 12(d) showing microcrack m resin in the middle part of the tube after 3000 cycle Crack width approximately 5 ,um( × 600).

CFRP tubes for space structures REFERENCES

1. CASA ASTP High stability large shaped reflector, Phase 1I, ESA CR-4511 (1982). 2. Selenia ASTP 20/30 GHz muhibeam antenna, Phase II, ESA CR-4550 (1984). 3. G. T. Hayes, Design of the antenna module structure for lntelsat V Spacecraft, Ford Aerospace. A1AA paper 78-592,

333

19th Structural and Structural Dynamic Conf., San Diego, CA, U.S.A. (1978). 4. J. C. Larue, Accelerated thermal cycling of spacecraft structures, ESA Bulletin 32, 27-34 (1982). 5. L. Camahart, Effects of thermal cycling environment on graphite/epoxy composites. ASTM STP 602 (1981). 6. E. Bowles, Effects of micro-cracking on the thermal expansion of graphite epoxy composites. In Proc. Large Space Syst. Technol. Conf., Hampton, VA, NASA CP-2193 (1981).