Mechanical properties and failure characteristics of a recycled CFRP under tensile and cyclic loading

International Journal of Fatigue 55 (2013) 257–267

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International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue

Mechanical properties and failure characteristics of a recycled CFRP under tensile and cyclic loading Mitsuhiro Okayasu ⇑, Tomohiro Yamazaki, Kohei Ota, Keiji Ogi, Tetsuro Shiraishi Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan

a r t i c l e

i n f o

Article history: Received 5 December 2012 Received in revised form 24 June 2013 Accepted 4 July 2013 Available online 12 July 2013 Keywords: Recycled CFRP Carbon fiber Mechanical property Crack growth Failure mechanism

a b s t r a c t An examination has been made of the mechanical and failure properties of a recycled short carbon fiber reinforced plastic (rCFRP). The rCFRP samples were fabricated by the following process: the CFRP, consisting of epoxy resin with carbon fiber, is ground before mixing with acrylonitrile butadiene styrene resin with different weight fractions of CFRP. The tensile strength (rUTS) increased with increasing CFRP content, but dropped considerably for the sample with higher fiber content. From in situ measurement of localized failure in rCFRP, it appeared that material failure occurs even if a low tensile stress of 30% rUTS is applied. The localized damage was related to the pull-out (or debonding) of the fibers from the matrix. The fatigue strength increased with increasing the content of the recycled carbon fiber even for the samples with low tensile strength. This was attributed to the low crack driving force arising from severe crack closure. Details of the crack growth behavior were discussed using various crack growth models proposed in previous studies. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, a number of carbon fiber reinforced plastics (CFRP) have received special attention due to their high specific strength, high specific rigidity and low specific weight [1]. In fact, it is considered that CFRPs have come into practical use for the aerospace and automotive industries, instead of iron and aluminum alloys [2], because of their contribution to higher fuel efficiency and lower levels of emissions. The demand for CFRPs has dramatically increased in recent years [3]. Although CFRPs have been employed in various industries, the recycling issue for CFRPs has not perfectly been solved yet. In fact, post-use CFRPs seem to be thrown away into landfill without any consideration of environmental problems [4]. This occurrence will be a significant issue in the future, since the amount of waste CFRPs will increase dramatically [5]. Several researchers have studied recycling techniques for CFRPs. Uzawa et al. [6] have investigated the recyclability of CFRP, focusing their attention on recycling crushed CFRTP (carbon fiber reinforced thermoplastics). This would be one of the appropriate techniques for mass production, since these are cheap and easily recycled. It is required that CFRP manufacturers as well as the related product users are liable to reduce the deposited waste to a minimum in Europe [7]. Ogi et al. [4] have also reported a useful recycling technique by a crushing and classification technique, and they have examined the tensile properties of their recycled ⇑ Corresponding author. Tel./fax: +81 89 927 9811. E-mail address: [email protected] (M. Okayasu). 0142-1123/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2013.07.005

CFRP samples (rCFRP). Takahashi et al. [8] have developed CFRTP using waste CFRP, and the target mechanical properties of the recycled CFRP are higher than those of the secondary structural parts of current automobiles. Although several researchers have investigated the tensile properties of rCFRPs, the information available appears to be insufficient. In addition, even though the fatigue properties for CFRPs have been investigated [9,10], there are still no clear conclusions concerning the fatigue properties of rCFRPs. In general, the experimental approach to fatigue has involved the characterization of (i) cyclic number to failure, i.e., the S–N approach, and (ii) crack growth rate to failure, i.e., the da/dN  DK approach. The fatigue strength is one of the significant design parameters for engineering applications. Fatigue failures generally take place under the influence of cyclic loads whose peak values are considerably smaller than the safe load values, estimated on the basis of static fracture analysis [11]. The aim of this study is, therefore, to investigate the fatigue properties of a recently proposed recycled CFRP (rCFRP) [4] via the S–N and da/dN  DK approaches. In addition, the fundamental aspects of tensile properties and the failure characteristics of the rCFRP samples are investigated. 2. Materials and experimental procedure 2.1. Material preparation In the present work, recycled CFRP samples, prepared in Ref. [4], were employed. Those rCFRP samples were made by the following

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CFRP 10%

CFRP 30%

Flow direction Carbon fibers

Matrix 0.1mm

CFRP 50%

CFRP 70%

Fig. 1. Optical micrographs of the rCFRP samples showing the carbon fiber and matrix.

(a) Type T 150 80 T axis Rectangular specimens

CT specimens Loading direction

L axis

150 80

Flow direction

t=3

Gate Matrix

Fibers

(b) Type L

Rectangular specimens

CT specimens

Parallel area (7 × 3×1mm)

Fig. 2. Schematic diagrams showing the test specimens in the rCFRP samples made by injection procedure.

process. The precursor CFRP, consisting of epoxy resin with a volume fraction of 60% carbon fiber (T700S, Toray), was first crushed using a rotating blade to make small fragments for which the average length by width is 3.4 mm  0.4 mm. The crashed CFRP pieces were then separated individually, as far as possible, into fiber and epoxy resin (thermosetting high polymer) after the ball milling process. Most part of the surface of separated carbon fibers is not already coated by epoxy resin, while some fiber bundles were present that contained epoxy resin [4]. It should be pointed out that the fiber bundles could make reduction in the mechanical properties of the rCFRP samples, which will be discussed in the later section of this paper. The average length of the carbon fibers after the grinding process was about 200 lm. The recycled CFRP samples,

consisting of acrylonitrile butadiene styrene (ABS) resin (thermoplastic resin) and CFRP pieces, were fabricated using standard mixing, grinding and injection molding procedures. In this case, the CFRP pieces were added to the ABS resin before the injection process with five different weight fractions of 0 (i.e., pure ABS), 10, 30, 50 and 70 wt.%. Details of this recycling process can be found elsewhere [4]. Fig. 1 displays the optical micrographs of the rCFRP samples showing the carbon fibers and matrix. As seen, fiber content is different depending on the sample. Moreover, the injection process produces alignment of fibers largely parallel to the flow direction. The injection molding process was used to prepare simple rectangular plates 150 mm  150 mm  3 mm. The test specimens (dumbbell-shaped specimen and compact tension (CT) specimen) were obtained from the center area of the rectangular plate, in which the plate cut in two different directions from the mid-section of the rectangular plates, either with the loading direction (longitudinal axis of the specimen) in the direction perpendicular (Type T) or parallel (Type L) to the flow (or carbon fiber) direction (see Fig. 2). The dimension of the parallel area in the dumbbellshaped specimen is 7 mm (l)  3 mm (w)  1 mm (t), and that of CT specimen is W = 24.5 mm and B = 3 mm. The CT specimen was designed based upon the ASTM standard E399 [12]. In the mid-section of the CT specimens, a through-slit (15 mm in length with a Vnotch root angle of 45°) was machined. 2.2. Tensile and fatigue tests Tensile and fatigue tests were performed at room temperature using an electro-servo-hydraulic system with 50 kN capacity. In the tensile tests, dumbbell-shaped specimens (Fig. 2) were used. The loading speed for the tensile test was fixed at 1 mm/min up to the fracture point. The tensile strain was measured continuously using strain gauge 5 mm long. The fatigue strength was examined using two different approaches, namely S–N and da/dN  DK. In the S–N approach, the relationship between the applied stress and cycle number to final failure was investigated using the dumbbell-shaped specimens. The cyclic loading was applied under load control with a sinusoidal waveform at a frequency of 30 Hz and load (R) ratio of 0.1 up to 107 cycles. The maximum applied loads were determined based upon the tensile fracture stress of the appropriate sample, e.g.

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80

50%

60

30% 10% 0% (Pure ABS)

70%

40

20

0 0

1

2

3

Strain, %

(b) Type L 100 50% 30%

80

ð1Þ

To understand the crack growth behavior in detail, the load vs. CMOD (crack mouth opening displacement) was examined at several stages during the da/dN  DK measurements. The load–CMOD curves, measured by a standard load cell and an extensometer, were obtained using a data acquisition system in conjunction with a computer through the electro-servo-hydraulic system. 2.3. Failure characteristics

Tensile stress, MPa

 a  a 2 DP 2 þ a=W DK ¼ pffiffiffiffiffiffi 0:866 þ 4:64  13:32 3=2 W W B W ð1  a=WÞ  a 3  a 4   5:6 þ14:72 W W

(a) Type-T 100

Tensile stress, MPa

50–90% of rUTS. It should be pointed out that the sample temperature may affect the fatigue properties. This is because that temperature could increase during the fatigue test especially at the higher frequency due to the great energy accumulation. We have examined the variation of the sample temperature during the cyclic loading at 30 Hz using CFRP 50%-Type L. From our experiments, it was clarified that the sample temperature increased as cyclically loaded for more than 106 cycles, but that is only a few degrees. From this result, it might be considered that the temperature of our rCFRPs is not so significant to alter the fatigue properties. In the da/dN  DK approach, the crack growth parameters, such as DKth (threshold stress intensity factor range) and DKeff (effective stress intensity factor range), were examined using the CT specimens (Fig. 2). A pre-crack of approximately 0.5 mm was made in the specimen through cyclic loading under small-scale yielding conditions. The fatigue crack length during the da/dN  DK approach was continuously monitored using a traveling light microscope with a resolution of 0.01 mm. From the ASTM standard, DK value was calculated from Eq. (1) using the measured parameters of crack length (a) and applied cyclic load range (DP):

60 10% 0%

70% 40

20

0 0

1

2

3

Strain, % The failure characteristics of the rCFRP samples were investigated during the tensile test. To examine them, an experimental technique was proposed based upon acoustic emission method. In this approach, a piezoelectric ceramic was attached to the dumbbell-shaped specimen. The essence of this approach is to detect small vibration in the sample as localized failure occurs [13,14,15]. Commercial lead zirconate titanate (PZT) ceramics were used in the form of a thin round plate £9.0 mm  0.12 mm (t). The electric voltage was continuously monitored using a digital multi meter (8846A, Fluke). 3. Results and discussion 3.1. Tensile properties Fig. 3 shows representative stress–strain curves for all samples, and based upon these stress–strain curves, the ultimate tensile strength (rUTS) and strain to failure (ef) for all the samples are summarized in Fig. 4. As seen in Figs. 3 and 4, different tensile properties are obtained depending on the CFRP content and type. There is no clear anisotropic effect on the tensile properties for the ABS samples, and their tensile properties are as follows: rUTS = 38.8 MPa, ef = 1.95% for Type T and rUTS = 40.2 MPa, ef = 2.16% for Type L. Note, the tensile properties for the ABS samples are at a similar level to those for the related material reported previously [16]. For both samples Type T and L, the tensile strength increases with increasing CFRP content, but a considerable drop in the tensile strength was detected for CFRP 70%. The overall tensile strength for Type L is higher than that for Type T, particularly CFRP 30%- and CFRP 50%-Type L, e.g., the mean rUTS value for CFRP 50%Type L is more than 1.6 times higher than the CFRP 50%-Type T one.

Fig. 3. Representative stress–strain curves of ABS and rCFRPs: (a) Type T and (b) Type L.

This corresponds to the anisotropic effect in the sample, where the fiber direction prevails for the strength although the fiber length is as short as about 200 lm. In previous work by Pimenta et al. [17], the similar mechanical behavior of rCFRP was investigated using the samples produced by a papermaking technique (T300, Toray). Their tensile and compressive properties depend on the loading direction, i.e., a higher tensile strength is obtained in the specimen loaded in the longitudinal direction [17]. It is also seen in Figs. 3 and 4 that tensile strain is different depending on the sample. As in Fig. 4, the strain value decreases with increasing CFRP content for both Type T and L. The strain for ABS is about 2%, and that reduced by factor of 4 for the CFRP 70% samples. There are almost linear correlations between ef and CFRP content with high correlation rate (R2) of more than 0.9: ef = 0.024 CFRP + 2.34 (for Type T) and ef = 0.021 CFRP + 2.09 (for Type L). From this approach, it is apparent that the higher rUTS and the lower ef are obtained for the samples with the higher CFRP content, but the rUTS as well as ef values decrease for the CFRP 70% sample. This would be attributed to severe brittleness, and this result suggests that there is weak reinforcement effect for the CFRP 70% samples. The reason behind this in detail will be discussed in the next section of their failure characteristics. 3.2. Failure characteristics It has been reported by Ogihara and Tanaka [18] that failure characteristics (delaminations) in a toughed CFRP laminate

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(a) Type-T

(b) Type-L 100

0% 10% 30% 50% 70%

80

Ultimate tensile strength, MPa

Ultimate tensile strength, MPa

100

60

40

20

0 0

10

20

30

40

50

60

70

0% 10% 30% 50% 70%

80

60

40

20

0

80

0

10

20

CFRP content, % 5

40

50

60

70

80

70

80

5 0% 10% 30% 50% 70%

0% 10% 30% 50% 70%

4

Strain to failure, %

4

Strain to failure, %

30

CFRP content, %

3

f=

2

-0.024 CFRP + 2.34

1

3

2

f

= -0.021 CFRP + 2.09

1

0 0

10

20

30

40

50

60

70

80

0 0

10

20

CFRP content, %

30

40

50

60

CFRP content, %

Fig. 4. Ultimate tensile strength (rUTS) and strain to failure (ef) of ABS and rCFRPs: (a) Type T and (b) Type L.

adjacent to the crack tip are observed at a higher strain level, and the delamination grows mainly as a consequence of fiber/matrix interfacial debonding. To examine the failure characteristics of our rCFRP samples, the electric voltage arising from the PZT ceramics was examined continuously during tensile loading. The obtained result is shown in Fig. 5, in which the associated tensile stress vs. strain curve (Fig. 3) is also indicated to understand clearly their failure characteristics. As seen small and high electric voltage peaks are obtained. A high electric peak is detected at the fracture point for all the samples. This is attributed to severe damage in the specimen. It is interesting to mention that there are several small electric voltage peaks before the final failure. Such small peaks are affected by localized material failure, where the pull-out (or debonding) of the fibers from the matrix may occur although this is speculative incidence. Details of failure characteristics will be analyzed in a later section of this paper. It is also clear that the number of small peaks depends on the sample. The number of peaks increases with increasing CFRP content, especially for the CFRP 70% samples. From the results of both CFRP 70% samples, it is clear that localized failure occurs even if a low tensile load is applied, e.g., 30 MPa, and this occurrence leads to an acceleration of the reduction in their tensile properties, where dense short-length fiber and fiber bundles are affected. With the fracture surface observation using a scanning electron microscope, the fiber bundles increases with increasing the CFRP content. Fig. 6 shows the representative fiber bundle in the CFRP 70% sample. Because of the weak bonding force between the fiber bundle (including epoxy) and ABS, such bundles make a reduction in the mechanical

properties of rCFRP. It should be noted that, in this approach, such peaks of electric voltage are not a method for the evaluation of quantitative damage level. 3.3. Fatigue properties Fig. 7 shows the relationship between the stress amplitude and cycle number to failure (Sa–Nf curve). It should be noted first that the arrows in this figure indicate the specimens which did not fail within 107 cycles, and their endurance limits (ren) are summarized in Table 1. Moreover, because there is no clear anisotropic effect on tensile properties for the ABS samples as mentioned in Fig. 4, all the ABS specimens (i.e., both CFRP 0%-Type T and CFRP 0%-Type L) mixed up were used in this fatigue test. From Fig. 7(a), the Sa– Nf relationships, including the endurance limit (ren), seem to be similar level for all Type T samples, while the slope of their Sa–Nf relationships is slightly different depending on the CFRP content. For example, the higher the CFRP content (e.g., CFRP 70%), the lower the slope of Sa–Nf relations, in which Sa vs. Nf for CFRP 70%-Type T crosses those for the other Type T samples around 103–104 cycles as indicated in Fig. 7(a). As for CFRP 70%-Type T, the lowest slope of the Sa vs. Nf curve is obtained for CFRP 70%-Type L (Fig. 7(b)), which also crosses the other ones at around 103 cycles but only for ABS and 10%-Type L. Interestingly, the endurance limit for both CFRP 70% is the same level of about 15.4 MPa. The Sa–Nf curves for CFRP 30%- and 50%-Type L are located at a higher level compared to the others, even though the endurance limits for CFRP 30%- and 50%Type L are close to that for CFRP 70%-Type L. An important

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(a) CFRP0% (pure ABS)

Tensile stress, MPa

100

Fracture point

80 60 40

Stress -strain curve Electric voltage

20 0 0

0.5

1

1.5

2

2.5

3

Strain, %

(b) Type T CFRP 10%

CFRP 30%

100

80 60

Tensile stress, MPa

Tensile stress, MPa

100

High peak

40 Small peaks 20 0

80 60 40 20 0

0

0.5

1

1.5

2

2.5

3

0

0.5

1

Strain, % CFRP 50%

2

2.5

3

2.5

3

CFRP 70%

100

80

Tensile stress, MPa

Tensile stress, MPa

100

1.5

Strain, %

60 40 20 0

80 60 40 20 0

0

0.5

1

1.5

2

2.5

3

0

0.5

1

1.5

2

Strain, %

Strain, %

Fig. 5. Variation of electric voltage arising from the PZT ceramics during the tensile loading: (a) ABS, (b) Type T and (c) Type L.

observation from Fig. 7(a) and (b) is that relatively high endurance limit was obtained for both CFRP 70% in spite of the low tensile properties (Fig. 4). Such fatigue properties for CFRP 70% are associated with different crack growth rates. For instance, a rough fracture surface makes low crack growth rate [19]. Details of this will be discussed in a later section. To understand clearly the fatigue behavior of the rCFRP samples, the Sa–Nf relationships were quantitatively evaluated by a power law dependence of cyclic stresses and cycles to failure:

ra ¼ rf Nb f ; MPa

ð2Þ

where ra is the stress amplitude, Nf represents the cycle number to final fracture, rf is the fatigue strength coefficient and b is the fatigue exponent. The values of rf and b for some samples, obtained by least square analysis, are listed in Table 1. An increased fatigue life is expected for a decreasing fatigue strength exponent b and increasing fatigue strength coefficient rf. In the present case, the rf and b for the CFRP 50%-Type L sample may show high

fatigue strength. On the other hand, different fatigue properties were obtained for both CFRP 70% samples with lower b and lower rf values, which caused by low slope of Sa vs. Nf curves as described above. To understand the failure characteristics of rCFRPs, fracture surface observations after the fatigue test (more than 105 cycles) were made using a scanning electron micrograph (SEM). Fig. 8 presents SEM images of the fracture surfaces. For the ABS sample, a smooth fracture surface including crazing can be seen, where the large local strains, stemmed from the fatigue loading, generate the discontinuous crack growth bands. This fracture mode is similarly observed in the related work [20]. In contrast, rougher fracture surfaces were observed in rCFRPs, in which the debonded fibers and the fibers pulled out from the matrix are detected in the samples of Type L and Type T, respectively. This occurrence may affect the main failure mode of the rCFRP samples, as pointed in Section 3.2. From this, it is considered that a tortuous fatigue crack growth occurs on the fracture surface. The extent of the surface roughness seems to be different depending on the sample, where

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100 80 60

High peak

40 Small peaks

20 0

CFRP 30%

100

Tensile stress, MPa

Tensile stress, MPa

(c) Type L

CFRP 10%

80 60 40 20 0

0

0.5

1

1.5

2

2.5

0

3

0.5

1

CFRP 50%

80 60 40 20 0 0

0.5

1

1.5

2

2.5

3

2

2.5

3

CFRP 70%

100

Tensile stress, MPa

Tensile stress, MPa

100

1.5

Strain, %

Strain, %

2.5

3

80 60 40 20 0 0

Strain, %

0.5

1

1.5

2

Strain, % Fig. 5 (continued)

(a) Optical micrograph

(b) SEM micrograph

0.1mm

0.04mm

Fig. 6. (a) Optical micrograph and (b) SEM micrograph of CFRP 70% showing fiber bundle.

the higher the CFRP content, the rougher the fracture surface. Thus, roughness-induced crack closure might occur particularly in samples with the higher CFRP content. Because the fiber bundles contain epoxy resin as mention in Section 2.1, those make an increment of the extent of the crack closure. 3.4. Crack growth properties Fig. 9(a) represents the relationship between crack growth rate and the stress intensity factor range for all samples (da/dN  DK). Because there are many experimental data plotted in the same chart of Fig. 9(a), those were divided with several patterns: (i) the specimen type (Type T or Type L) in Fig. 9(b) and (ii) CFRP content (CFRP 10%, 30%, 50% or 70%) in Fig. 9(c). From Fig. 9(a), two distinct regions of fatigue crack growth can be identified. Region I is the range of crack growth rate above the threshold stress intensity, in which the crack growth rate is very low. The crack either remains dormant or grows at undetectably slow rates below the

threshold. With increasing da/dN as a function of DK, there is a linear variation of log da/dN versus log DK in Region II, i.e., da/ dN = A(DK)m, defined on the basis of linear elastic fracture interpretations [21]. From Fig. 9(a), different crack growth characteristics are observed, although the data is relatively scattered. The resistance to fatigue crack propagation (or crack growth thresholds, DKth) appears to be substantially lower for the ABS sample. With increasing CFRP content, the crack growth rate decreases as shown in Fig. 9(b). However, relatively high crack growth rates are similarly observed for Type L of CFRP 30%, CFRP 50% and CFRP 70%. This trend is associated with their high endurance limits (Fig. 7). A comparison of samples of Type T and L in Fig. 9(c) shows that the da/dN vs. DK for the Type L samples is shifted slightly to the right with respect to the Type T curves, but similar trend is observed for both CFRP 70% samples. This is also attributed to the results of endurance limit. The DKth values for all samples approximated are summarized in Table 1. Fig. 10 depicts the relationship between ren and DKth for all the samples. As seen, there is

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(a) Type T 50

Stress amplitude, MPa

0%

10%

40

30%

103 cycles

104 cycles

50%

30

70%

20

10

0

10 2

1

10 4

10 6

10 8

Number of cycles to failure

(b) Type L 50 0% 10%

Stress amplitude, MPa

40

30% 50%

30

70%

20

10 103 cycles

0

10 2

1

10 4

10 6

10 8

Number of cycles to failure Fig. 7. Relationship between the stress amplitude and cyclic number to failure (Sa–Nf curve) for ABS and rCFRPs: (a) Type T and (b) Type L.

Table 1 Fatigue properties of ABS and rCFRPs (ren, rf, b and DKth). Sample CFRP0% (ABS) CFRP10%-Type CFRP30%-Type CFRP50%-Type CFRP70%-Type CFRP10%-Type CFRP30%-Type CFRP50%-Type CFRP70%-Type

T T T T L L L L

ren (MPa)

rf (MPa)

b

DKth (MPa m0.5)

4.3 6.4 6.9 10.1 15.4 5.6 16.1 16.6 15.4

65.9 44.6 47.7 36.8 21.3 53.4 42.3 51.3 21.3

0.17 0.12 0.12 0.08 0.02 0.14 0.06 0.07 0.02

0.7 0.9 1.3 2.0 2.4 1.2 2.1 2.5 2.6

a clear correlation between them with R2 = 0.90. As mentioned, the higher CFRP content gives rise to higher fatigue strength, e.g., the high DKth value of about 2.6 MPa m0.5 for CFRP 70%-Type L. In previous works, a delamination fatigue crack growth behavior has been investigated experimentally using alumina fiber/epoxy laminates [22], and their DKth value is found to be lower than that for the CFRP 70%-Type L sample. Note due to the lack of experimental data of the DKth value for CFRP, we could not make comparison of the DKth value for our rCFRP and the other related ones. The DKth values of the rCFRP samples were further analyzed with different fatigue parameters: rf and b as indicated in Table 1. From this approach, it is found that a no strong correlation was detected between DKth and rf with R2 = 0.52, whereas there is a clear correlation between DKth and b with R2 = 0.87: DKth = 0.07 b + 0.21 (MPa m0.5).

Fig. 11 depicts the relationship between the endurance limit and crack growth threshold for various engineering materials, as reported in this and previous work [23]. It should be pointed out that the endurance limits for previous study in Fig. 11 were obtained under cyclic loading at R = 1, which is different with ours (R = 0.1). Although R-ratio is different, we believe that our experimental data (ren) can be plotted in the same chart. This is because the endurance limit for previous data in Fig. 11 is assessed by the stress amplitude (Sa). In our preliminary study, an attempt was made to examine the R-ratio effect on the fatigue life. In this approach, fatigue tests were carried out at different R-ratio, 0.1, 0.5 and 1; and it appears that there is similar trend of the Sa–Nf relationships for all the samples [24], whereas dissimilar relations were obtained in Smax–Nf. From Fig. 11, it is clear that there is an almost linear correlation between these parameters for all engineering materials. Furthermore, it is obvious that our rCFRP samples are better than the engineering polymers but poorer than the engineering alloys. Also confirmed is the important fact as to whether or not the fatigue strength is affected by the yield strength. Fig. 12 displays the endurance limits (ren) plotted against the yield strengths (ry) for various engineering materials [23,25–27]. Note that the ry values for our rCFRPs are defined as the specific stress level, at which a 0.05% plastic strain is measured. Because the ry values cannot be detected clearly in some samples due to low permanent deformation, the ry values for CFRP 30% and CFRP 50% are only indicated in Fig. 12. As with the results of Fig. 11, there is an almost linear relationship between ren and ry, and our

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CFRP 0% (Pure ABS)

40µm

0.5mm

CFRP 10%-Type T

CFRP 10%-Type L

Carbon fibers

0.5mm

Matrix

40µm

40µm

CFRP 30%-Type T

CFRP 30%-Type L

CFRP 50%-Type T

CFRP 50%-Type L

CFRP 70%-Type T

CFRP 70%-Type L

Fig. 8. SEM images of the fracture surfaces after the fatigue tests (more than 105 cycles).

rCFRP data appear to be located in the lower engineering alloys and in the middle of engineering polymers. From this, it is considered that there is no strong effect of the CFRP pieces on the tensile strength. Fig. 13 shows the relationship between ren and rUTS for our samples. Although the rUTS level increases with increasing ren, data for the CFRP 70% samples strayed from this linear relationship. Such a change may be explained by the different material properties for the CFRP 70% samples, caused by the large fiber content. In general, large fiber (more than limitation) makes material brittleness leading to the low tensile properties, but that could make rough fracture surface, resulting in severe crack closure (i.e., high fatigue strength). Even though linear elastic fracture mechanics (LEFM) has been accepted for the description of cracked solids, the LEFM concepts of small crack growth and rough crack surface has proved questionable, due to the reduction in crack driving force caused by crack closure. Crack closure resulting from fracture surface contact has

been modeled subsequently by several researchers [28]. Shiraishi et al. [29] have examined the crack growth characteristics in various engineering polymers (PVC, PMMA and PA) considering plasticity-induced crack closure. The maximum crack opening displacement in a fatigue cycle is obtained from the small-scale plasticity solution [30], which is of the form [31]:

CODmax ¼

K 2max ð1  m2 Þ E ry

ð3Þ

where E is the elastic constant and m is Poisson’s ratio. To investigate the crack closing behavior, the K vs. COD relations and the effective crack driving force (DKeff) values were examined. Fig. 14 shows the representative K vs. CMOD (crack mouth opening displacement) curves obtained in Region II. It is seen that the load–CMOD relationship exhibits a concave shape, signifying an acceleration in the reduction of the measured CMOD value at the minimum stress intensity factor (Kmin). Such a concave curve as well as the

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Crack growth rate (da/dN), mm/cycle

(a)

10-2

Region II

10-3 10-4

σen = 6.7 ΔKth - 0.86 Region I

10-5 10-6 10-7 10-8 10-9 0.5

5

Stress intensity factor range (Δ Δ K), MPa m0.5 Type T

10-2

Type L

Fig. 10. Relationship between endurance limit and crack growth threshold (ren vs. DKth) for ABS and rCFRPs.

10-3 102

10-4 10-5 10-6

Fatigue threshold (Δ Δ Kth), MPa m0.5

Crack growth rate (da/dN), mm/cycle

(b)

10-7 10-8 10-9 0.5

5

0.5

5

Stress intensity factor range (ΔK), MPa m0.5

Crack growth rate (da/dN), mm/cycle

(c) 10-2

10%

30%

50%

70%

10

1

10-1

10-2

10-3 10-4

10-3 -1 10

10-5

10

10 2

10 3

10 4

Endurance limit (σen ), MPa

10-6

Fig. 11. Relationship between endurance limit and crack growth threshold (ren vs. DKth) for various engineering materials.

10-7 10-8 10-9 0.5

1

DK eff ¼ 5

0.5

5

0.5

5

0.5



 K max  K op DK K max  K min

ð4Þ

5

Stress intensity factor range (ΔK), MPa m0.5 Fig. 9. Relationship between crack growth rate and stress intensity factor range (da/dN  DK) for ABS and rCFRPs.

minimum CMOD value (CMODmin) might be associated with the contact action, arising from the rough fracture surface [32,33]. It is also found that the slope of K vs. CMOD and the Kop value depend on the sample. The incorporation of crack closure effects in terms of the effective stress intensity factor range involves the maximum stress intensity factor and the minimum stress intensity factor. Based upon this, the DKeff value, defined as DKeff = Kmax  Kmin, can be obtained by the following formula.

where Kop is stress intensity factor at crack opening. Such an approach is in accordance with the study of Elber [32]. In this study, Elber model is employed as a first approximation even though there are various crack closure models [32–37]. Based on the DKeff values assessed by Eq. (4), the relationship between DKeff and da/dN for all the samples was examined, and the results obtained are shown in Fig. 15(a). To understand the crack closure effect clearly, DK vs. da/dN curves (Fig. 9(a)) were also shown in Fig. 15(b). It is clear that the DKeff – da/dN relations, especially slow growth rate regime, e.g., da/dN > 105 m/cycle, fall within quite a narrow scatter-band, indicating the crack closure effect. This trend is similarly observed in the published paper reported by Gloanec et al. [38]. From this, it is considered that crack closure occurs severely in the rCFRP sample, and this would be affected by rough fracture surface.

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M. Okayasu et al. / International Journal of Fatigue 55 (2013) 257–267

104

Endurance limit (σen ), MPa

103

102

10

1

10-1

10-2 10-1

1

10

2

10

103

4

10

Yield strength (σ y), MPa Fig. 12. Relationship between endurance limit and yield strength (ren vs. ry) for various engineering materials.

Fig. 13. Relationship between endurance limit and ultimate tensile strength (ren vs. r UTS) for rCFRPs.

It is generally considered that the DKth value is an ever decreasing threshold as the crack length a becomes shorter, as shown in a Kitagawa–Takahashi diagram, ultimately giving the EI Haddad parameter (a0) [39]. This is because, in the permanent deformation zone, the head of the crack tip is comparable to the crack size. This diagram illustrates the dependence of the crack propagation threshold on the crack size [40]. It is reported by Liu et al. [41] that El Haddad et al. [42] proposed a model for a crack in the center of an infinite plate to express the fatigue endurance limit range Dren using the fatigue threshold stress intensity factor range DKth and the crack length.

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DK ¼ Dr pða þ a0 Þ

DK th

pffiffiffiffiffiffiffiffi ¼ Dren pa0

ð5Þ

Crack growth rate (da/dN), mm/cycle

Fig. 14. Stress intensity factor (K) vs. crack mouth opening displacement (CMOD) showing the stress intensity factor at crack opening (Kop), the maximum (Kmax) and the minimum stress intensity factors (Kmin) for rCFRPs: (a) Type T and (b) Type L.

10-3 10-4 10-5 10-6 10-7 10-8 10-9

0

Because the original El Haddad model is for a crack in the center of an infinite plate, the above relationship has been modified by Liu et al. [41] for an edge crack with surface correction.

2

4

6 0

2

4

6

Stress intensity factor range, MPa m0.5 Fig. 15. Relationship between crack growth rate and stress intensity factor range of rCFRPs: (a) da/dN  DKeff and (b) da/dN  DK.

DK th ¼ 1:122Dren ð5aÞ

(b) K

(a) Keff

10-2

pffiffiffiffiffiffiffiffi pa0

ð6Þ

From Eq. (4), a0 can be taken as

a0 ¼

1

p



DK th 1:122Dren

2 ð7Þ

M. Okayasu et al. / International Journal of Fatigue 55 (2013) 257–267

From DKth and Dren, as indicated in Table 1, the a0 values can be calculated by Eq. (7). The mean a0 values obtained for all the rCFRP samples are about 0.0081 m with standard deviation of 0.0021 m. The a0 values for engineering alloys (A533B and Ti alloy) have been reported by several researchers [39,43], and their a0 values were found to be smaller than ours. This occurrence would be related to the weak permanent deformation in the rCFRP samples.

4. Conclusions An examination has been made of the mechanical and failure properties for a recycled carbon fiber reinforced plastic (rCFRP) material. The results have yielded the following conclusions. (1) The tensile strength is found to depend not only on the CFRP content, but also on the fiber direction. The tensile strength increases with increasing CFRP content, but drops suddenly for rCFRP with higher fiber content. In addition, under the same CFRP content, the higher tensile strength is detected as the fiber direction is parallel to the loading direction. (2) Material failure occurs in the rCFRP samples with higher fiber content during tensile loading before final fracture, even if a low value of tensile stress of 30%UTS applied. A greater degree of the material damage gives rise to a lower tensile strength. The failure is related to the pull-out (or debonding) of the fibers from the matrix. (3) The fatigue strength and crack growth resistance increase with increasing CFRP content. In spite of the lower tensile properties for the rCFRP due to higher fiber content, their fatigue properties are high level compared to that for the lower fiber content one. The high fatigue properties are attributed to the reduction in the crack driving force arising from severe crack closure. The DKeff – da/dN relations, especially slow growth rate regime, are especially affected by severer crack closure. (4) Mechanical properties of some rCFRP samples show relatively high level. Those are higher than those for the engineering polymer samples. Because of the recycled material with relatively high mechanical properties, the rCFRP samples may be useful as new material in the future.

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