DELAMINATION CRACK GROWTH OF UNIDIRECTIONAL CFRPIN THERMO-MECHANICAL FATIGUE

Y. NAKAI, N. SAKATA, T. KADOWAKI and C.HIWADepartment of Mechanical Engineering, Kobe University

1-1, Rokkodai, Nada, Kobe 657, Japan

ABSTRACT

The delamination crack growth behavior under iso-thermaland thermo-mechanical fatigue was investigated withunidirectional CF/epoxy laminates. The crack growth tests wereconducted either in air or in water with double cantilever beamspecimens. They were conducted under constant ∆K conditionsat load ratio of 0.1 or 0.5 with a loading frequency of 1/120 Hzor 1/180 Hz. Test temperature was between 25℃and 70℃. Inair, the crack growth rates for the iso-thermal fatigue test werehigher than those for the thermo-mechanical fatigue test. Whenthe test temperature was changed from lower to highertemperature, the crack growth rate for the iso-thermal fatigue testwas unaltered. The crack growth rate for the out-of-phase testwere slightly higher than those for the in-phase test. In water, thecrack growth rate for the themo-machanical fatigue test wasalmost the same value for the rate for the iso-thermal test whosetest temperature was equal to that at the maximum load for thethemo-machanical test.

INTRODUCTION

Carbon fiber reinforced plastics (CFRP) laminates arewidely used in space and aircraft structures, and they are going toemploy in super conductivity magnet structures, which will beused for linear motor trains and nuclear fusion reactors. In thesestructures, not only load, temperature also changes during service.For example, surface temperature of wings in aircraft varies from-40 to 100 oC in a flight. The purpose of the present paper is toclarify the behavior of delamination fatigue crack growth undervariable temperature conditions. Since structures and machinecomponents are usually subjected to cyclic or variable loadingconditions, information about fatigue is particularly important.

In metals, the fatigue behavior under variable temperaturehave been known as “thermal fatigue” or “thermo-mechanicalfatigue”, and life for thermo/thermo-mechanical fatigue areusually shorter than that for iso-thermal fatigue. In CFRP,however, the behavior in thermo/thermo-mechanical fatigue hasnot studied. Since the dominant fracture mechanism of CFRPlaminates in most cases is delamination along the interface ofprepregs, fracture mechanics approaches to the delaminationcrack growth are required to assure the integrity of structures andmachine components made of CFRP(1)(2). Since delaminationcrack growth rate is not always a unique function of the stressintensity range, ∆K, constant ∆K tests were conducted(3)(4).

The authors have studied the effect of temperature change indelamination fatigue crack growth in CFRP laminates(4)(5), andfound that the interlaminar crack growth rate was unaltered whenthe test temperature is changed from lower to higher temperature.On the other hand, the interlaminar crack growth rate drasticallydecreases after the diminution of the temperature from higherthan 50 oC to lower than 50 oC. This phenomenon wasexplained by supposing that there is large difference in thestrength of the epoxy resin or of the interfaces around 50 oC.When the temperature is lower than 50 oC, the strength of theresin or of the interface is enough high, and therefore, thebridging hardly takes place. At high temperature, fibers can pull

out from the resin easily, but the resin cannot support high loadbecause its strength or that of the interface is low, and thebridging force by fibers is small for the temperature higher than50 oC. Hence, the effect of fiber bridging is small. When thetemperature decreases from higher than 50 oC to below than 50oC, fibers that pull out from the resin at high temperature bridgethe fracture surface at low temperature, and can support enoughload to reduce the true stress intensity factor at the crack tipbecause the strength of the resin or the interface is high enough atlow temperature. These findings indicate that the delaminationfatigue crack growth behavior in thermo/thermo-mechanicalfatigue can be different from that in iso-thermal fatigue.

EXPERIMENTAL PROCEDURE

Unidirectional CF/epoxy laminates used in the presentexperiments were made from prepregs of Toray P3060E-15 andP4052-12 (Table 1). Thickness of laminate after formation wasabout 8 mm. In the present paper, the laminate made from theformer prepregs are called as “Material A”, and the latter,“Material B”.

Elastic moduli of the laminate are shown in Table 2, wherethe coordinates 1 and 2 are the fiber direction and the thicknessdirection, respectively.

Double cantilever beam (DCB) specimens, shown in Fig. 1,were employed for the delamination crack growth tests. Twoholding-blocks with holes, which were made of an aluminumalloy, were bonded to the specimens. To introduce an initialnotch, Teflon film of 12 µm thick was inserted between prepregsin the midsection before processing. The initial crack length,which is the distance from loading line to initial crack tip, wasabout 35 mm.

Proceedings of International Conference on Materials and Mechanics ’97 (Tokyo, July 20-22, 1997), pp.653-658.

Table 1. Specification of prepregs.Material A Material B

Fiber T300 M40Matrix #3060 #2500Prepreg P3060-E-15 P4052-20Number of prepregs 54 40Curing temperature ( oC) 180 120Curing time (min) 120 60Volume fraction, Vf (%) 61.5 50.8

Table 2. Elastic moduli.Material A Material B

Young’s modules, E1 (GPa) 143 187E2 (GPa) 8.45 5.97

Shear modulus, G12 (GPa) 4.77 3.63Poisson’s ratio, ν12 0.30 0.34

Pump

Cold

fluid

in

Thermocontroller

Fluid outValve B

Specimen

Thermo couple

Tank

Hot fluid inValve A

Fig. 2 Thermo-mechanical fatigue equipment.

Teflon sheet

Al block

107.5mm

10.8

mm

22.5mm

ø6.2mm

8.0m

m

Fig. 1 DCB specimen (dimensions in mm).

Thermo-mechanical fatigue test were conducted by usingapparatus shown in Fig. 2. Heating and cooling of specimenwere accomplished by water, whose flow was controlled byelectro-magnetic valve, and test temperature was measured bychromel-alumel thermo-couple which was attached to thespecimen. Thermo-mechanical tests were conducted bycontrolling a electro-dynamic vibrator, electro-magnetic valves,and a flow pump with a personal computer to synchronizeloading and temperature waves. The loading frequency was1/120 Hz for Material A, and 1/180 Hz for Material B. Themaximum temperature for the thermo-mechanical tests was 70 oC,and the minimum temperature was 35 oC for Material A, and 20oC for Material B. Two types of thermo-mechanical tests wereconducted, i.e., in-phase tests and out-of-phase tests, shown inFig. 3.

Fatigue crack growth tests in air were conducted bywrapping specimens with small bags made of aluminum foil.

To conduct ∆K controlled tests, fatigue crack growth wasmonitored by using a compliance method, where the distance

between two loading grips was measured by a linear variabledifferential transformer (LVDT)(4)(6). Although the front ofactual cracks were curved in a thumbnail shape, theexperimentally obtained relation between the average cracklength and the compliance agreed with the relation obtained bythe finite element method(4).

A computer controlled electro-dynamic loading system(4)

was employed for the fatigue crack growth experiments. Thecrack growth behavior was examined under pseudo-constant ∆Kcondition by having the control computer automatically reducingthe load range after each 50 µm increment of crack growth. Testswere conducted from 20 oC to 70 oC in air. Fatigue crackgrowth tests were carried at a load ratio, R, of 0.1 or 0.5 (where Ris the ratio of minimum to maximum load during one fatigueloading cycle).

EXPERIMENTAL RESULTS AND DISCUSSION

Growth Behavior in Air(a) Material A

The delamination fatigue crack growth rates, da/dN, forMaterial A in thermo-mechanical fatigue tests as a function ofcrack extension from initial notch, ∆a, are shown in Fig. 4. Bothin in-phase test and out-of-phase test, the crack growth rates werealmost constant with crack extension under constant ∆Kcondition. The crack growth rates in out-of-phase were higherthan those in in-phase test. Under iso-thermal fatigue loading at25 oC or 70 oC, crack growth rates were too high to measure, i.e.,specimens fractured just after starting fatigue tests at ∆K of 0.52MPam1/2. It means that the crack growth rates in both types ofthermo-mechanical fatigue tests were lower than those in iso-thermal tests.

The crack growth rates under ∆K= 0.45 MPam1/2 in iso-thermal tests were shown in Fig. 5. The growth rates at 70 oC arehigher than those for 25 oC, but the difference is small. Figure 6shows the growth behavior under variable test temperature. Inthe test, iso-thermal tests were conducted, but the temperaturewas changed from 70 oC to 25 oC, and 25 oC to 70 oC during thetest. In this case, the growth rate at 70 oC was the same value asthat in completely constant temperature iso-thermal test (Fig. 5).Delamination crack, however, did not propagate at 25 oC, i.e.,the crack growth rate is unaltered when the test temperature ischanged from lower to higher temperature. On the other hand,the interlaminar crack growth rate drastically decreases after thediminution of the temperature from higher to lower temperature.These phenomena may come from the same mechanism as thatthe crack growth rates under thermo-mechanical fatigue werelower than those in iso-thermal fatigue, and the rates under in-phase thermo-mechanical fatigue were lower than those in out-of-phase thermo-mechanical fatigue.

At R=0.5, direct comparison between the growth rate underiso-thermal test and that under thermo-mechanical test could notbe achieved, tests were conducted at R=0.1. At R=0.1,delamination fatigue crack did not propagate at ∆K below 0.75MPam1/2. It indicates that the threshold stress intensity range,∆Kth, is higher for lower stress ratio.

Timeν / 20

Load

Pmin

max PTemp.(in-phase)Temp.(out-of-phase)

Load

max

min

Spec

imen

T

T

ν

tem

pera

ture

Fig. 3 Temperature change and load change.

0 1 2 3 4 510-8

10-7

10-6

10-5

Crack extension, Δa (mm)

Cra

ck g

row

th ra

te, d

a/dN

(m/c

ycle

)

ΔK=0.52MPam1/2

R=0.5, f=1/120Hz● In-phase○ Out-of-phase

(T300/3601)

Fig. 4 Crack growth behavior in thermo-mechanical fatigue in air at R = 0.5.

0 1 2 310-8

10-7

10-6

10-5

Crack extension,Δa (mm)

Cra

ck g

row

th ra

te,d

a/dN

(m

/cyc

le)

○ 25℃● 70℃

ΔK=0.45MPam1/2

(T300/3601)

R=0.5, f=1/120HZ

Fig. 5 Effect of test temperature on crack growthin iso-thermal fatigue in air at R = 0.5.

0 5000 1000036

37

38

39

40

Number of cycles, N (cycle)

Cra

ck le

ngth

,a (

mm

)

70℃ 70℃25℃ 25℃

(T300/3601)

ΔK=0.45MPam1/2

R=0.5, f=1/120Hz

Fig. 6 Effect of temperature change on crack growthin iso-thermal fatigue in air at R = 0.5.

The delamination fatigue crack growth rates for R = 0.1 areshown in Fig. 7. For this stress ratio, the growth rates were notconstant with crack extension. It indicates that the crack tipshielding by fiber bridging is larger for lower stress ratio. Asseen in Appendix, the fracture toughness value of this material isalmost independent of crack extension. Tanaka et al. reported forT800H/3631 laminates that the reduction of crack growth rateswith crack extension was larger for higher stress ratio(7). Thisdifference may come from the difference in strength of CF/epoxyinterface from present material.

The growth rates in iso-thermal fatigue tests are higher forhigher temperature. The growth rates in both types of thermo-mechanical fatigue are lower than the lowest growth rates for iso-thermal tests. The growth rates under in-phase thermo-mechanical fatigue are lower than under out-of-phase thermo-mechanical fatigue.(b) Material B

The delamination fatigue crack growth rates in air forMaterial B are shown in Fig. 8. For this material, the growthrates are not constant under constant ∆K condition. The growthrates decrease with crack extension. Crack tip shielding by fiberbridging may responsible for the diminution.

The effect of test temperature in iso-thermal test in air is notobserved. The growth rates at 70 oC are almost identical to thoseat 20 oC. The growth rates in-phase thermo-mechanical fatiguealso 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 inFig. 9. They are not constant value under constant ∆K fatiguecrack growth tests. On the contrary to the results in air, thegrowth rates for iso-thermal fatigue tests are lower for highertemperature. There is big difference in growth rates in in-phasethermo-mechanical fatigue and those in out-of-phase thermo-mechanical fatigue. The growth rates in in-phase thermo-mechanical fatigue are almost identical to those in iso-thermalfatigue at 70 oC, and the rates in out-of-phase are almost equal to

those in iso-thermal fatigue at 20 oC. Therefore, the growth ratesunder thermo-mechanical fatigue in water for Material A areconsidered to be controlled by the temperature at maximumloading.(b) Material B

The crack growth rates for Material B in water are shown inFig. 10. No conspicuous differences from the behavior ofMaterial A are observed, and overall response in crack growthbehavior are almost the same.

Effect of EnvironmentFor Material A, there was big difference in the crack growth

rates in air and in water, the crack growth tests could notconducted at the same value of ∆K. The threshold stress

0 0.5 1 1.5 210-7

10-6

Crack extension, Δa (mm)

Cra

ck g

row

th ra

te, d

a/dN

(m/c

ycle

)

● 70 ℃

◆ In-phase◇ Out-of-phase

(T300/3601)

ΔK=0.95MPam1/2

R=0.1, f=1/120Hz

○ 35 ℃

Fig. 9 Crack growth behavior in water at R = 0.1 forMaterial A.

0 2 4 6 8 1010-7

10-6

10-5

10-4

Crack extension, Δa (mm)　

Cra

ck g

row

th ra

te, d

a/dN

(m/c

ycle

)

● 70 ℃ ○ 20 ℃

◆ 20～70 ℃ (in-phase）◇ 20～70 ℃ (out-of-phase）

ΔK=0.47 MPam1/2

f=1/180 HzR=0.5

Iso-thermal fatigue

Thermo-machanical fatigue

(M40/2500)

Fig. 10 Crack growth behavior in water at R = 0.5for Material B.

0 1 2 3 4 5 6 7 8 9 1010-7

10-6

10-5

10-4

Crack extension, Δa (mm)　

Cra

ck g

row

th ra

te, d

a/dN

(m/c

ycle

)

● In air○ In water

ΔK=0.47 MPam1/2

f=1/180 Hz R=0.5, T=20 ℃

(M40/2500)

Fig. 11 Effect of environment on crack growth in iso-thermal fatigue at R= 0.5 for Material B.

0 1 2 3 4 5 610-7

10-6

10-5

Crack extension, Δa (mm)

Cra

ck g

row

th ra

te, d

a/dN

(m/c

ycle

)

● 70 ℃

◆ In-phase◇ Out-of-phase

(T300/3601)

ΔK=0.75MPam1/2

R=0.1, f=1/120Hz

○ 35 ℃

Fig. 7 Fatigue crack growth behavior in air (R=0.1).

0 2 4 6 8 1010-7

10-6

10-5

10-4

Crack extension, Δa (mm)　

Cra

ck g

row

th ra

te, d

a/dN

(m/c

ycle

)

● 70 ℃ ○ 20 ℃

◇ 20～70 ℃

ΔK=0.47 MPam1/2

f=1/180 Hz R=0.5

Iso-thermal fatigue

Thermo-mechanical fatigue(in-phase)

(M40/2500)

Fig. 8 Crack growth behavior in air at R=0.5 for Material B.

intensity range, ∆Κth in water is lower than that in air.Comparisons between crack growth behavior in air and in waterare made for Material B in Figs. 11 and 12, where Fig. 11 showsthe results in iso-thermal fatigue tests, and Fig. 12 shows those inin-phase fatigue tests. Since the out-of-phase fatigue tests wereconducted only in water, comparisons could not be made.

In iso-thermal fatigue tests, the growth rates are higher inwater. In thermo-mechanical fatigue tests, however, the growthrates are lower in water.

FractographyFigure 13 presents an example of side-surface of the

specimen. This figure was taken from a specimen made ofMaterial B and tested at 20 oC in air. In all specimens tested, this

kind of fiber bridging was observed, and no apparent differencecould be found between the test conditions.

To examine fracture mechanism, fractographic observationwas made by scanning electron microscopy. Figures 14 show thefracture surface for Material A in air. Roughness of the fracturesurface in iso-thermal fatigue test at 35 oC and both types ofthermo-mechanical fatigues was larger than that in iso-thermalfatigue test at 70 oC. This difference is considered to come fromthe difference in fiber bridging, which causes diminish of thecrack growth rate. Frequent occurrence of fiber bridging maybring the large roughness in fracture surface. This differencecorresponds to the crack growth rates.

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 theinterface between carbon fiber and epoxy resin. For test

Fig. 13 side surface of Material B tested in air at 20 oC.

0 2 4 6 8 1010-7

10-6

10-5

10-4

Crack extension, Δa (mm)　

Cra

ck g

row

th ra

te, d

a/dN

(m/c

ycle

)

● In air○ In water

ΔK=0.47 MPam1/2

f=1/180 Hz, R=0.5Tmax=70 ℃, Tmin=20 ℃

(M40/2500)

Fig. 12 Effect of environment on crack growth in in -phasethermal fatigue at R= 0.5 for Material B.

　　

(a) 35 oC (b) 70 oC

　　

(c) In-phase fatigue (d) Out-of-phase fatigueFig. 14 Fracture surface for Material A in air.

　　

（ａ） 20 oC (b) 70 oC

（ｃ） In-phase fatigueFig．15 Fracture surface for Material B in air.

　　

(a) 20 oC (b) 70 oC

　　

(c) In-phase fatigue (d) Out-of-phase fatigueFig. 16 Fracture surface for Material B in water.

conditions which gave similar crack growth behavior, similarfracture surfaces are observed. The fracture surfaces formed inwater show more ductile than in air. This ductility may cause thediminution of crack growth rates. Fracture surfaces formed in in-phase thermo-mechanical fatigue test in water and iso-thermalfatigue test at 70 oC in water show most ductile appearance, andthe lowest crack growth rates were obtained under these testconditions.

CONCLUSIONS

The delamination crack growth in thermo-mechanicalfatigue for two kind of CF/epoxy laminates were examined withdouble cantilever beam (DCB) specimens under constant ∆Kcondition. The following results were obtained.(1) In AirMaterial A (180 oC cured type) The crack growth rates inthermo-mechanical fatigue tests were lower than those in iso-thermal fatigue tests which were conducted at highest or lowesttemperature for the thermo-mechanical fatigue tests. The fatiguecrack growth rates in in-phase fatigue tests were lower than thosein out-of-phase thermo-mechanical fatigue tests.Material B (130 oC cured type) The effect of test temperature oncrack growth rates in iso-thermal tests were not observed. Thegrowth rates under in-phase thermo-mechanical fatigue also tookthe same value as those in the iso-thermal fatigue tests.(2) In Water

In both materials, the growth rates under thermo-mechanicalfatigue tests were controlled by the temperature at maximumloading. The growth rates under in-phase thermo-mechanicalfatigue are almost identical to those under iso-thermal fatigue at70 oC, and the rates in out-of-phase fatigue are almost equal tothose in iso-thermal fatigue at 20 oC.

Acknowledgment Support of this work by Grant-in-Aid forScientific Research (C) (Project No. 08650112) by the Ministryof Education, Science, Sports and Culture is gratefullyacknowledged.

APPENDIX

Fracture Toughness TestFor the limit case of fatigue test, fracture toughness test was

conducted for Material A. It is considered as iso-thermal fatiguetest at R = 1. A specimen was loaded by a stepping motor driventesting 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, andautomatically controlled fracture toughness test could beachieved. The crack extension was monitored by unloadingelastic compliance method. Figure 17 shows load-displacementcurve for Material A at 20 oC. Resistance curve (R-curve),shown in Fig. 18, could be made from the load-displacementcurve. The value of fracture toughness is almost constantindependent of crack extension. Todoroki et al. also reported thesame trend for CF/epoxy (T300/934) laminates(8) and CF/PEEKlaminates(9).

REFERENCES

1. Nakai, Y, Yamamori, H., Nakamura, M., and Ohji, K.,Effects of frequency and temperature on delamination crackgrowth of unidirectional CFRP under cyclic loading, J. Mat.Sci., Japan, 42, 384 (1993).

2. Nakai, Y. and Ohji, K., Effects of frequency and temperatureon delamination fatigue crack growth in unidirectionalCFRP, 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., Deformationand propagation of mode I fatigue cracks with crack-bridging in unidirectional CFRP, Preprint of the 70th JSMEFall Annual Meeting, No.920-78, Japanese Soc. Mech.Eng. , 139 (1992).

4. Nakai, Y. and Yamashita, M., Effects of frequency andtemperature on delamination fatigue crack growth ofunidirectional CFRP under constant �K condition, J. Mat.Sci., Japan, 43, 1258 (1994).

5. Nakai, Y., Effect of Temperature Change on DelaminationCrack 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 FiberOrientation and specimen width on delamination fatiguecrack growth in CFRP laminates, To be published in J. Mat.Sci., Japan, Vol. 46, No. 10 (1997).

7. Tanaka, K., Tanaka, H., Tsuji, T., and Yamagishi, K., Effectof stress ratio on Mode I propagation of interlaminar fatiguecracks in CFRP, J. Soc. Mat. Sci., Japan, 44, 960 (1995).

8. Todoroki, A., Evaluation of delamination fracture toughnessof high-strength CFRP and micromechanism, Trans. JSME,57A, 1648 (1991).

9. Todoroki, A., Micromechanism and delamination resistanceof CF/PEEK, Trans. JSME, 60A, 1272 (1994).

0.5 1 1.5 2

20

40

60

80

100

0Displacement, δ (mm)

Load

, P (N

)(T300/3601)

1.5 mm/min

Fig. 17. Load-displacement curve.

10 20 30

0.5

1

1.5

2

0Crack extension, Δa (mm)

Frac

ture

toug

hnes

s,K

c (M

Pam1/

2 )

(T300/3601)

1.5 mm/min

Fig. 18. Resistance curve.