DELAMINATION CRACK GROWTH OF UNIDIRECTIONAL CFRP IN THERMO-MECHANICAL FATIGUE

Transcription

1 Proceedings of International Conference on Materials and Mechanics 97 (Tokyo, July 2-22, 1997), pp DELAMINATION CRACK GROWTH OF UNIDIRECTIONAL CFRP IN THERMO-MECHANICAL FATIGUE Y. NAKAI, N. SAKATA, T. KADOWAKI and C.HIWA Department of Mechanical Engineering, Kobe University 1-1, Rokkodai, Nada, Kobe 657, Japan ABSTRACT The delamination crack growth behavior under iso-thermal and thermo-mechanical fatigue was investigated with unidirectional CF/epoxy laminates. The crack growth tests were conducted either in air or in water with double cantilever beam specimens. They were conducted under constant K conditions at load ratio of.1 or.5 with a loading frequency of 1/12 Hz or 1/18 Hz. Test temperature was between 25 and 7. In air, the crack growth rates for the iso-thermal fatigue test were higher than those for the thermo-mechanical fatigue test. When the test temperature was changed from lower to higher temperature, the crack growth rate for the iso-thermal fatigue test was unaltered. The crack growth rate for the out-of-phase test were slightly higher than those for the in-phase test. In water, the crack growth rate for the themo-machanical fatigue test was almost the same value for the rate for the iso-thermal test whose test temperature was equal to that at the maximum load for the themo-machanical test. INTRODUCTION Carbon fiber reinforced plastics (CFRP) laminates are widely used in space and aircraft structures, and they are going to employ in super conductivity magnet structures, which will be used for linear motor trains and nuclear fusion reactors. In these structures, not only load, temperature also changes during service. For example, surface temperature of wings in aircraft varies from -4 to 1 o C in a flight. The purpose of the present paper is to clarify the behavior of delamination fatigue crack growth under variable temperature conditions. Since structures and machine components are usually subjected to cyclic or variable loading conditions, information about fatigue is particularly important. In metals, the fatigue behavior under variable temperature have been known as thermal fatigue or thermo-mechanical fatigue, and life for thermo/thermo-mechanical fatigue are usually shorter than that for iso-thermal fatigue. In CFRP, however, the behavior in thermo/thermo-mechanical fatigue has not studied. Since the dominant fracture mechanism of CFRP laminates in most cases is delamination along the interface of prepregs, fracture mechanics approaches to the delamination crack growth are required to assure the integrity of structures and machine components made of CFRP (1)(2). Since delamination crack growth rate is not always a unique function of the stress intensity range, K, constant K tests were conducted (3)(4). The authors have studied the effect of temperature change in delamination fatigue crack growth in CFRP laminates (4)(5), and found that the interlaminar crack growth rate was unaltered when the test temperature is changed from lower to higher temperature. On the other hand, the interlaminar crack growth rate drastically decreases after the diminution of the temperature from higher than 5 o C to lower than 5 o C. This phenomenon was explained by supposing that there is large difference in the strength of the epoxy resin or of the interfaces around 5 o C. When the temperature is lower than 5 o C, the strength of the resin or of the interface is enough high, and therefore, the bridging hardly takes place. At high temperature, fibers can pull out from the resin easily, but the resin cannot support high load because its strength or that of the interface is low, and the bridging force by fibers is small for the temperature higher than 5 o C. Hence, the effect of fiber bridging is small. When the temperature decreases from higher than 5 o C to below than 5 o C, fibers that pull out from the resin at high temperature bridge the fracture surface at low temperature, and can support enough load to reduce the true stress intensity factor at the crack tip because the strength of the resin or the interface is high enough at low temperature. These findings indicate that the delamination fatigue crack growth behavior in thermo/thermo-mechanical fatigue can be different from that in iso-thermal fatigue. EXPERIMENTAL PROCEDURE Unidirectional CF/epoxy laminates used in the present experiments were made from prepregs of Toray P36E-15 and P (Table 1). Thickness of laminate after formation was about 8 mm. In the present paper, the laminate made from the former prepregs are called as Material A, and the latter, Material B. Elastic moduli of the laminate are shown in Table 2, where the coordinates 1 and 2 are the fiber direction and the thickness direction, respectively. Double cantilever beam (DCB) specimens, shown in Fig. 1, were employed for the delamination crack growth tests. Two holding-blocks with holes, which were made of an aluminum alloy, were bonded to the specimens. To introduce an initial notch, Teflon film of 12 µm thick was inserted between prepregs in the midsection before processing. The initial crack length, which is the distance from loading line to initial crack tip, was about 35 mm. Table 1. Specification of prepregs. Material A Material B Fiber T3 M4 Matrix #36 #25 Prepreg P36-E-15 P452-2 Number of prepregs 54 4 Curing temperature ( o C) Curing time (min) 12 6 Volume fraction, V f (%) Table 2. Elastic moduli. Material A Material B Young s modules, E 1 (GPa) E 2 (GPa) Shear modulus, G 12 (GPa) Poisson s ratio, ν Thermo controller 1.8mm ø6.2mm Al block 8.mm Pump Hot fluid in Valve A Thermo couple 22.5mm Teflon sheet Cold fluid in Specimen Tank 17.5mm Fig. 1 DCB specimen (dimensions in mm). Valve B Fluid out Fig. 2 Thermo-mechanical fatigue equipment.

2 Thermo-mechanical fatigue test were conducted by using apparatus shown in Fig. 2. Heating and cooling of specimen were accomplished by water, whose flow was controlled by electro-magnetic valve, and test temperature was measured by chromel-alumel thermo-couple which was attached to the specimen. Thermo-mechanical tests were conducted by controlling a electro-dynamic vibrator, electro-magnetic valves, and a flow pump with a personal computer to synchronize loading and temperature waves. The loading frequency was 1/12 Hz for Material A, and 1/18 Hz for Material B. The maximum temperature for the thermo-mechanical tests was 7 o C, and the minimum temperature was 35 o C for Material A, and 2 o C for Material B. Two types of thermo-mechanical tests were conducted, i.e., in-phase tests and out-of-phase tests, shown in Fig. 3. Fatigue crack growth tests in air were conducted by wrapping specimens with small bags made of aluminum foil. To conduct K controlled tests, fatigue crack growth was monitored by using a compliance method, where the distance Pmax Load Pmin Load Temp.(in-phase) Temp.(out-of-phase) ν / 2 Time Fig. 3 Temperature change and load change. 1-8 ν T max Specimen temperature T min K=.52MPam 1/2 R=.5, f=1/12hz In-phase Out-of-phase Fig. 4 Crack growth behavior in thermo-mechanical fatigue in air at R =.5. between two loading grips was measured by a linear variable differential transformer (LVDT) (4)(6). Although the front of actual cracks were curved in a thumbnail shape, the experimentally obtained relation between the average crack length and the compliance agreed with the relation obtained by the finite element method (4). A computer controlled electro-dynamic loading system (4) was employed for the fatigue crack growth experiments. The crack growth behavior was examined under pseudo-constant K condition by having the control computer automatically reducing the load range after each 5 µm increment of crack growth. Tests were conducted from 2 o C to 7 o C in air. Fatigue crack growth tests were carried at a load ratio, R, of.1 or.5 (where R is the ratio of minimum to maximum load during one fatigue loading cycle). EXPERIMENTAL RESULTS AND DISCUSSION Growth Behavior in Air (a) Material A The delamination fatigue crack growth rates, da/dn, for Material A in thermo-mechanical fatigue tests as a function of crack extension from initial notch, a, are shown in Fig. 4. Both in in-phase test and out-of-phase test, the crack growth rates were almost constant with crack extension under constant K condition. The crack growth rates in out-of-phase were higher than those in in-phase test. Under iso-thermal fatigue loading at 25 o C or 7 o C, crack growth rates were too high to measure, i.e., specimens fractured just after starting fatigue tests at K of.52 MPam 1/2. It means that the crack growth rates in both types of thermo-mechanical fatigue tests were lower than those in isothermal tests. The crack growth rates under K=.45 MPam 1/2 in isothermal tests were shown in Fig. 5. The growth rates at 7 o C are higher than those for 25 o C, but the difference is small. Figure 6 shows the growth behavior under variable test temperature. In the test, iso-thermal tests were conducted, but the temperature was changed from 7 o C to 25 o C, and 25 o C to 7 o C during the test. In this case, the growth rate at 7 o C was the same value as that in completely constant temperature iso-thermal test (Fig. 5). Delamination crack, however, did not propagate at 25 o C, i.e., the crack growth rate is unaltered when the test temperature is changed from lower to higher temperature. On the other hand, the interlaminar crack growth rate drastically decreases after the diminution of the temperature from higher to lower temperature. These phenomena may come from the same mechanism as that the crack growth rates under thermo-mechanical fatigue were lower than those in iso-thermal fatigue, and the rates under inphase thermo-mechanical fatigue were lower than those in outof-phase thermo-mechanical fatigue. At R=.5, direct comparison between the growth rate under iso-thermal test and that under thermo-mechanical test could not be achieved, tests were conducted at R=.1. At R=.1, delamination fatigue crack did not propagate at K below.75 MPam 1/2. It indicates that the threshold stress intensity range, K th, is higher for lower stress ratio. Crack growth rate,da/dn (m/cycle) 1-8 K=.45MPam 1/2 R=.5, f=1/12hz Crack length, a (mm) K=.45MPam 1/2 R=.5, f=1/12hz Number of cycles, N (cycle) 25 Fig. 5 Effect of test temperature on crack growth in iso-thermal fatigue in air at R =.5. Fig. 6 Effect of temperature change on crack growth in iso-thermal fatigue in air at R =.5.

3 K=.75MPam 1/2 R=.1, f=1/12hz 7 35 In-phase Out-of-phase K=.95MPam 1/2 R=.1, f=1/12hz 7 35 In-phase Out-of-phase Fig. 7 Fatigue crack growth behavior in air (R=.1). 1-4 Iso-thermal fatigue 7 2 Thermo-mechanical fatigue 2 7 (in-phase) K=.47 MPam 1/2 f=1/18 Hz R= Fig. 8 Crack growth behavior in air at R=.5 for Material B. The delamination fatigue crack growth rates for R =.1 are shown in Fig. 7. For this stress ratio, the growth rates were not constant with crack extension. It indicates that the crack tip shielding by fiber bridging is larger for lower stress ratio. As seen in Appendix, the fracture toughness value of this material is almost independent of crack extension. Tanaka et al. reported for T8H/3631 laminates that the reduction of crack growth rates with crack extension was larger for higher stress ratio (7). This difference may come from the difference in strength of CF/epoxy interface from present material. The growth rates in iso-thermal fatigue tests are higher for higher temperature. The growth rates in both types of thermomechanical fatigue are lower than the lowest growth rates for isothermal tests. The growth rates under in-phase thermomechanical fatigue are lower than under out-of-phase thermomechanical fatigue. (b) Material B The delamination fatigue crack growth rates in air for Material B are shown in Fig. 8. For this material, the growth rates are not constant under constant K condition. The growth rates decrease with crack extension. Crack tip shielding by fiber bridging may responsible for the diminution. The effect of test temperature in iso-thermal test in air is not observed. The growth rates at 7 o C are almost identical to those at 2 o C. The growth rates in-phase thermo-mechanical fatigue also take the same value as those in iso-thermal fatigue. Growth Behavior in Water (a) Material A The crack growth rates for Material A in water are shown in Fig. 9. They are not constant value under constant K fatigue crack growth tests. On the contrary to the results in air, the growth rates for iso-thermal fatigue tests are lower for higher temperature. There is big difference in growth rates in in-phase thermo-mechanical fatigue and those in out-of-phase thermomechanical fatigue. The growth rates in in-phase thermomechanical fatigue are almost identical to those in iso-thermal fatigue at 7 o C, and the rates in out-of-phase are almost equal to Fig. 9 Crack growth behavior in water at R =.1 for Material A. 1-4 K=.47 MPam 1/2 f=1/18 Hz R=.5 Iso-thermal fatigue 7 2 Thermo-machanical fatigue 2 7 (in-phase 2 7 (out-of-phase Fig. 1 Crack growth behavior in water at R =.5 for Material B. 1-4 In air In water K=.47 MPam 1/2 f=1/18 Hz R=.5, T= Fig. 11 Effect of environment on crack growth in isothermal fatigue at R=.5 for Material B. those in iso-thermal fatigue at 2 o C. Therefore, the growth rates under thermo-mechanical fatigue in water for Material A are considered to be controlled by the temperature at maximum loading. (b) Material B The crack growth rates for Material B in water are shown in Fig. 1. No conspicuous differences from the behavior of Material A are observed, and overall response in crack growth behavior are almost the same. Effect of Environment For Material A, there was big difference in the crack growth rates in air and in water, the crack growth tests could not conducted at the same value of K. The threshold stress

4 1-4 ΔK=.47 MPam 1/2 f=1/18 Hz, R=.5 Tmax=7, Tmin= In air In water Crack extension, Δa (mm) 1 Fig. 13 side surface of Material B tested in air at 2 oc. Fig. 12 Effect of environment on crack growth in in -phase thermal fatigue at R=.5 for Material B. intensity range, Κth in water is lower than that in air. Comparisons between crack growth behavior in air and in water are made for Material B in Figs. 11 and 12, where Fig. 11 shows the results in iso-thermal fatigue tests, and Fig. 12 shows those in in-phase fatigue tests. Since the out-of-phase fatigue tests were conducted only in water, comparisons could not be made. In iso-thermal fatigue tests, the growth rates are higher in water. In thermo-mechanical fatigue tests, however, the growth rates are lower in water. kind of fiber bridging was observed, and no apparent difference could be found between the test conditions. To examine fracture mechanism, fractographic observation was made by scanning electron microscopy. Figures 14 show the fracture surface for Material A in air. Roughness of the fracture surface in iso-thermal fatigue test at 35 oc and both types of thermo-mechanical fatigues was larger than that in iso-thermal fatigue test at 7 oc. This difference is considered to come from the difference in fiber bridging, which causes diminish of the crack growth rate. Frequent occurrence of fiber bridging may bring the large roughness in fracture surface. This difference corresponds to the crack growth rates. Fractography Figure 13 presents an example of side-surface of the specimen. This figure was taken from a specimen made of Material B and tested at 2 oc in air. In all specimens tested, this Figures 15 and 16 show the fracture surface of Material B, where Figs. 15 are the surface of specimen tested in air, and Figs. 16 are those in water. Fracture surfaces formed along the interface between carbon fiber and epoxy resin. For test (a) 35 oc (b) 7 oc (c) In-phase fatigue (d) Out-of-phase fatigue Fig. 14 Fracture surface for Material A in air.

5 ａ 2 oc (b) 7 oc ｃ In-phase fatigue Fig 15 Fracture surface for Material B in air. (a) 2 oc (b) 7 oc (c) In-phase fatigue (d) Out-of-phase fatigue Fig. 16 Fracture surface for Material B in water.

6 Load, P (N) mm/min Fracture toughness, K c (MPam 1/2 ) mm/min Displacement, (mm) Fig. 17. Load-displacement curve. Fig. 18. Resistance curve. conditions which gave similar crack growth behavior, similar fracture surfaces are observed. The fracture surfaces formed in water show more ductile than in air. This ductility may cause the diminution of crack growth rates. Fracture surfaces formed in inphase thermo-mechanical fatigue test in water and iso-thermal fatigue test at 7 o C in water show most ductile appearance, and the lowest crack growth rates were obtained under these test conditions. CONCLUSIONS The delamination crack growth in thermo-mechanical fatigue for two kind of CF/epoxy laminates were examined with double cantilever beam (DCB) specimens under constant K condition. The following results were obtained. (1) In Air Material A (18 o C cured type) The crack growth rates in thermo-mechanical fatigue tests were lower than those in isothermal fatigue tests which were conducted at highest or lowest temperature for the thermo-mechanical fatigue tests. The fatigue crack growth rates in in-phase fatigue tests were lower than those in out-of-phase thermo-mechanical fatigue tests. Material B (13 o C cured type) The effect of test temperature on crack growth rates in iso-thermal tests were not observed. The growth rates under in-phase thermo-mechanical fatigue also took the same value as those in the iso-thermal fatigue tests. (2) In Water In both materials, the growth rates under thermo-mechanical fatigue tests were controlled by the temperature at maximum loading. The growth rates under in-phase thermo-mechanical fatigue are almost identical to those under iso-thermal fatigue at 7 o C, and the rates in out-of-phase fatigue are almost equal to those in iso-thermal fatigue at 2 o C. Acknowledgment Support of this work by Grant-in-Aid for Scientific Research (C) (Project No ) by the Ministry of Education, Science, Sports and Culture is gratefully acknowledged. APPENDIX Fracture Toughness Test For the limit case of fatigue test, fracture toughness test was conducted for Material A. It is considered as iso-thermal fatigue test at R = 1. A specimen was loaded by a stepping motor driven testing machine, which was designed by the present authors. This machine was controlled by a personal computer, which had a stepping motor driving unit and a A-D converter, and automatically controlled fracture toughness test could be achieved. The crack extension was monitored by unloading elastic compliance method. Figure 17 shows load-displacement curve for Material A at 2 o C. Resistance curve (R-curve), shown in Fig. 18, could be made from the load-displacement curve. The value of fracture toughness is almost constant independent of crack extension. Todoroki et al. also reported the same trend for CF/epoxy (T3/934) laminates (8) and CF/PEEK laminates (9). REFERENCES 1. Nakai, Y, Yamamori, H., Nakamura, M., and Ohji, K., Effects of frequency and temperature on delamination crack growth of unidirectional CFRP under cyclic loading, J. Mat. Sci., Japan, 42, 384 (1993). 2. Nakai, Y. and Ohji, K., Effects of frequency and temperature on delamination fatigue crack growth in unidirectional CFRP, in Bailon, J.-P. and Dickson, J.I. (eds.), Fatigue 93, Engineering Materials Advisory Service, U.K., 1379 (1993). 3. Tanaka, K., Tanaka, H., and Yamagishi, K., Deformation and propagation of mode I fatigue cracks with crackbridging in unidirectional CFRP, Preprint of the 7th JSME Fall Annual Meeting, No.92-78, Japanese Soc. Mech. Eng., 139 (1992). 4. Nakai, Y. and Yamashita, M., Effects of frequency and temperature on delamination fatigue crack growth of unidirectional CFRP under constant K condition, J. Mat. Sci., Japan, 43, 1258 (1994). 5. Nakai, Y., Effect of Temperature Change on Delamination Crack Growth of unidirectional CFRP under Cyclic Loading, Fatigue under Thermal and Mechanical Loading, Mechanism, Mechanics and Modelling, Edited by J. Bressers and L. Remy, 279 (1996). 6. Nakai, Y., Fujiwara, M., and Han, J., Effects of Fiber Orientation and specimen width on delamination fatigue crack growth in CFRP laminates, To be published in J. Mat. Sci., Japan, Vol. 46, No. 1 (1997). 7. Tanaka, K., Tanaka, H., Tsuji, T., and Yamagishi, K., Effect of stress ratio on Mode I propagation of interlaminar fatigue cracks in CFRP, J. Soc. Mat. Sci., Japan, 44, 96 (1995). 8. Todoroki, A., Evaluation of delamination fracture toughness of high-strength CFRP and micromechanism, Trans. JSME, 57A, 1648 (1991). 9. Todoroki, A., Micromechanism and delamination resistance of CF/PEEK, Trans. JSME, 6A, 1272 (1994).