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Compressive fatigue behavior of low velocity impacted and quasi-static indented CFRP laminates
Composite Structures 133 (2015) 1009–1015
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Compressive fatigue behavior of low velocity impacted and quasi-static indented CFRP laminates Jianyu Zhang a, Libin Zhao b,⇑, Ming Li c, Yuli Chen c a
College of Aerospace Engineering, Chongqing University, Chongqing 400044, PR China School of Astronautics, Beihang University, Beijing 100191, PR China c Institute of Solid Mechanics, Beihang University, Beijing 100191, PR China b
a r t i c l e
i n f o
Article history: Available online 10 August 2015 Keywords: Composite laminates Fatigue behavior Impact behavior Mechanical testing
a b s t r a c t The compression fatigue behaviors of CFRP laminates with impacted or quasi-static indented damage for two material systems, CCF300/QY9511 and CCF300/5428, were compared in this paper. The same surface damages, characterized by dent depth, were induced by low velocity impact (LVI) or quasi-static indentation (QSI) tests for the CFRP laminates. Using visual observation, C-Scan and thermal de-ply experimental measure methods, the surface damage and internal delamination of the specimens are described in detail to provide more information to understand the mechanical behaviors of impacted or quasi-static indented CFRP laminates. Static tests and staircase fatigue tests were performed to obtain the static compressive strength and compressive fatigue strength. The experimental outcomes show that the compressive fatigue strength of the quasi-static indented specimens is obviously greater than that of impacted specimens, while the specimens with quasi-static indented damages have a similar static compressive strength as those with impacted damages, for both material systems. Using QSI-induced damage to replace LVI-induced damage provides a roughly equivalent strength evaluation in static compression tests but exaggerates fatigue strength estimation in compression fatigue tests. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In last few decades, composite materials have attracted an increasing and particular interest for aviation structures due to their high in-plane specific strength and stiffness. However, as a result of weak interlaminar strength, most of the aircraft composite structures stacked by laminae will suffer from strict limitations under out-of-plane impact loads, such as those delivered by dropped tools or runway debris [1–3]. Such a damage mode, which is dominated by internal delamination and matrix cracking (driven largely by interlaminar shear and tension or back surface tension driven by specimen bending, but completely invisible when viewed from the external impacted face), leads to a drastic reduction up to 60% of the static compressive strength and further fatigue strength when cyclic loads are applied [4–7]. The fatigue resistance of composite is a very complex problem, as it appears in multiple mechanisms of failure throughout the material due to microcracking, splitting and delamination, etc. [8,9], and thus has attracted great interest in the past two decades
⇑ Corresponding author. Tel./fax: +86 (0)10 82339228. E-mail address: [email protected] (L. Zhao). http://dx.doi.org/10.1016/j.compstruct.2015.08.046 0263-8223/Ó 2015 Elsevier Ltd. All rights reserved.
[10–13]. Generally, two types of experimental methods are used regarding the understanding of the fatigue resistance. One method is the fatigue life test provided with certain load levels, and the other is the fatigue strength test (or the staircase method) under certain load cycles. Most researchers are inclined to conduct the fatigue life test of laminates after impact [14–17], in which a relatively simple test procedure is required and more information about the damage propagation during the cycling loading can be obtained. However, the experimental outcomes of the fatigue lives express a rather large scatter. In contrast to the fatigue life test, the staircase method for fatigue strength could provide a considerably smaller amount of scatter, which is required to obtain satisfactory results in fatigue comparison tests. Moreover, two damage-induced methods, the low velocity impact (LVI) and quasi-static indentation (QSI) tests, are commonly used to simulate the actual impact status in service environments; the LVI is deemed to be better because it can provide more similarity due to its dynamic impact process. However, because the instantaneous impact process is not controlled and the various damage modes of composite materials propagate without certain rules and interconnect with each other, which express more sensitivity to the dynamic impact process, the same impact energy frequently leads to varied damage. In contrast with the LVI, the QSI
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method undoubtedly benefits from its ease of measurement and controllability [18]. In the QSI test, displacement loadings can be applied to specimens step by step until the appropriate surface damage is obtained, which could conveniently result in a series of specimens with the same damage pattern. Thus, using the QSI test instead of the LVI test to simulate the damage due to dropped tools or runway debris for CAI or compressive fatigue strength after impact is recommended [18–20]. In this paper, experimental studies on the compression fatigue response of impacted and quasi-static indented composite laminates are reported. The compressive fatigue strength and static compressive strength of composite laminates with impacted and quasi-static indented damage are compared. To deeply understand the compression fatigue behavior, information about the damage patterns of laminates originated in LVI and QSI tests is elaborated and compared. Two materials systems were used to study the dependence of failure behavior on the material properties. 2. Specimens and test procedures The laminates selected to investigate compression–compression fatigue behaviors were made of carbon/bismaleimide composite materials (CCF300/QY9511 and CCF300/5428), in which the former has a higher mode II fracture toughness, according to the material behaviors provided by the material producer: GIC = 210 J/m2 and GIIC = 1123 J/m2 for CCF300/QY9511 and GIC = 226 J/m2 and GIIC = 939 J/m2 for CCF300/5428. All the laminates provided with lay-ups [45/0/ 45/90]2s were cut into specimens with dimensions of 150 mm by 100 mm by 2 mm. Non-destructive inspection was performed on each specimen before the LVI and QSI tests, and only those without defects were selected for the tests. Similar damage scaled by the same depth of dent were induced on the surfaces of the specimens in the LVI or QSI tests and measured using visual observation, ultrasonic C-Scan inspection and thermal de-ply tests. Staircase tests were performed to determine the fatigue strength of the specimens, in which the initial load level was determined using static compressive tests of the impacted or quasi-static indented specimens. The experimental details are described below. 2.1. LVI and QSI tests The LVI tests were performed according to ASTM D7136-05 [21]. A drop weight low velocity impact testing machine was used in the tests. A hemispherical tip indenter with a mass of 5.5 kg and a diameter of 12.7 mm impacted the center of the specimen, which was simply supported by a steel fixture with a 125 mm 75 mm opening window. For different material systems, the drop height of the indenter was adjusted to achieve the same depth of dents on the surface of the specimen. To reduce the error caused by the relaxation of dents, the depth was measured immediately after the LVI tests using digital calipers. A series of LVI tests were performed to obtain a depth of dent of 1 mm on the specimen surface, and the impact energies of 11.5 J and 10.25 J were selected for the CCF300/QY9511 and CCF300/5428 specimens, respectively. The QSI tests were conducted following ASTM D6263-98 [22]. A micro-control electronic universal testing machine WDW-100 (E/A/S) with a range of 100 kN was used in the tests. The loading speed was set to 0.5 mm/min. For comparison with the induced damages stemmed from impact, the same indenter was used to induce the quasi-static indented damage on the specimen, which was also simply supported in the same manner as in the LVI test. The dent depth was also measured immediately using the digital calipers after the tests to eliminate the influence of dent relaxation. Based on a series of QSI tests, the indenter displacements
of 6.15 mm and 5.93 mm were determined for the CCF300/ QY9511 and CCF300/5428 specimens, respectively, to induce a depth of dent of 1 mm on the specimen surface. 2.2. Damage measurement In addition to the visual inspection of direct observation and the measurement using the aforementioned digital calipers, nondestructive ultrasonic C-Scan detection and a destructive thermal de-ply technique were also used to characterize the damage. The ultrasound C-Scan equipment (Olympus NDT MZ-03, Japan) was used to detect the internal delamination patterns, in which the detection frequency was 20 MHz and the step length was 0.5 mm. The thermal de-ply technique was used for detailed observation of the delaminations at each of the interfaces and further comparison of the damage sizes between the impacted and quasi-static indented specimens. A contrast solution of gold chloride–ether was used in the thermal de-ply process. The damage zones of the specimens were first immersed in the contrast solution for at least 30 min, and then the soaked specimens were placed into the high temperature chamber at 475 °C for 70 min to remove the resin between layers. Thus, the delamination between adjacent layers could be observed clearly. 2.3. Static and fatigue tests Static compressive tests were performed as per ASTM/D/ 7137M-07 [23] in advance to obtain the compressive strength of each of the impacted and quasi-static indented specimens, which was used to determine the fatigue initial stress level of each specimen. The loading rate in static test was set to 1 mm/min. The staircase method [24–26] was used to obtain the compression fatigue strength under a specified fatigue life. The compression–compression fatigue tests were conducted with the stress ratio of R = 10. In the current work, the fatigue strength for 105 cycles was studied. In the tests, if a specimen survives after 105 cycles, then the next specimen will be tested under the next higher load level. Conversely, if a specimen fails within 105 cycles, then the next specimen will be tested under the next lower load level. The fatigue tests stops until at least three pairs of valid data appear, and the data close in the staircase map. All the specimens were tested in a unified load spectrum with the same stress increment. To ensure the correctness of the test results, the specimen for each stress level was selected randomly. In both static compression tests and compression–compression fatigue tests, an Instron-8803 hydraulic pressure servo material testing machine with a range of 250 kN was used. To guarantee the centering and stability of the specimens during the compression process, a compression fixture was built based on the experiments of Uda et al. [27], as shown in Fig. 1. The specimen was placed in the fixture to avoid out-of-plane directional movements. 3. Experimental results and discussion 3.1. Induced damage The damage patterns resulting from the LVI and QSI tests appeared to be similar by visual observation. For both material systems, there was an induced dent on the front surface of the laminates and a clearly matrix uplift and matrix fracture on the back surface, which appears as a narrow band along the fiber direction of the back surface lamina, as shown in Fig. 2. To further characterize the types of damage, a schematic diagram of the damage dimensions is provided in Fig. 3. The types of damage on the back surface are characterized by the length ‘‘l” of the matrix fracture
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Table 1 Damage dimensions of impacted and quasi-static indented specimens measured before and after fatigue test. Material system
CCF300/QY9511
Damage-induced method
LVI
QSI
CCF300/5428 LVI
QSI
Before fatigue test
l/mm h/mm d/mm
108.1 0.7 24.3
118.9 0.9 27.9
94.3 0.8 26.1
99.4 1.0 29.0
After fatigue test
l/mm h/mm d/mm
108.2 0.7 24.3
118.9 1.0 28.0
94.3 0.8 26.1
99.5 1.0 29.1
150mm
l Fig. 1. Compression fixture.
band and the height ‘‘h” of the matrix uplift, and the corresponding data of typical specimen for each type test are listed in Table 1. A longer matrix fracture band and a higher matrix uplift on the back surface of the quasi-static indented specimen could be obtained compared to those of the impacted specimen for both material systems, which indicates that the QSI test can cause slightly more severe surface damage than the LVI test. In addition, the internal delamination patterns detected by the ultrasound C-Scan system for the same specimen listed in Table 1 are shown in Fig. 4. The overlapped delamination damage occurred in the center of the specimen, with appearance similar to a round disk. The matrix fracture was also observed clearly in the C-Scan images. The fracture was along the fiber direction of layer on the back of the specimen. The largest diameter of the round overlapped delamination damage at the center, characterized by d in Fig. 3, as well as the longest length of the matrix fracture were measured. The former is listed in Table 1, and the latter agrees well with the results from the direct measurement method, which is also listed in Table 1. The C-Scan results further illustrate that the largest damage originated from the QSI test is more severe than that caused by the LVI test for both material systems. More detailed information of the delamination at each interface between adjacent layers was obtained via thermal de-ply techniques. Fig. 5 shows a representative thermal de-ply result of the CCF300/5428 specimen with quasi-static indented damage, in which the typical shapes of delaminations and the changing tendency through thickness direction are illustrated. The number on the top left corner of the lamina denotes the ply number, as counted from the front surface of specimen. The delamination damage is found to be distributed like a dumbbell, in which the long axis is along the fiber direction of the adjacent layer closer
Fig. 2. Back surface of the CCF300/5428 laminates with impacted and quasi-static indented damage.
d
(a)
h
(b)
Fig. 3. Schematic diagram of damage dimensions (a) top view (b) front view.
to the back of the specimen. Similar delamination shapes and changing tendencies were observed in the impacted specimen for both material systems according to the thermal de-ply analysis. To further analyze the delamination damage quantitatively, the length of long axis, a, of the dumbbell-shaped delamination was measured one by one for the impacted and quasi-static indented specimens and plotted in Fig. 6 for both material systems. The delaminations through the thickness direction were found to be divided into four stages. Stage I includes four interfaces counted from the front of the specimen exhibiting an approximately linear increase in delamination size. Stage II considers the next three interfaces; an obvious decrease in the delamination size at the beginning of stage II could be observed, followed by an approximately linear increase. Next, another decrease in the delamination size is observed at the beginning of stage III, which is almost the same size as that of stage II, and a nonlinear enlargement of the delamination size is observed in the following interfaces until the last one. Finally, in stage IV, a remarkable delamination is observed in the last interface. The delamination size of the last interface measured by thermal de-ply technique is the same as that obtained from the C-Scan image (see Table 1), which indicates the consistency between the two inspection approaches. Note that in the middle interface of specimens, denoted by 90°/90°, no delamination information was obtained for both of the material systems, either from the LVI test or from the QSI test, because the adjacent layers possess the same fiber direction. Comparing the delamination damage of the impacted and the quasi-static indented specimens, the delaminations either from the LVI test or the QSI test are similar in the first stage. In the second and third stages, the delaminations resulted from LVI are obviously greater than those from QSI. However, in the last stage, the
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(a)
(b)
(c)
(d)
Fig. 4. Ultrasound C-Scan images of specimens with induced damage. (a) Damage of CCF300/QY9511 laminates by LVI. (b) Damage of CCF300/QY9511 laminates by QSI. (c) Damage of CCF300/5428 laminates by LVI. (d) Damage of CCF300/5428 laminates by QSI.
Fig. 5. Thermal de-ply images of specimens with induced damage.
delamination caused by QSI is significantly larger than that by LVI. The results indicate that when the same depth dents on the specimens are induced, the internal delamination caused by LVI is more severe than that originated from QSI, except for the last interface, for which the opposite results were obtained. In addition, an interesting observation is that the differences obtained from LVI and QSI are independent of the material systems. Fig. 6 also compares the internal delamination sizes of two material systems. In the first stage, the internal delamination damage of laminates resulting from LVI or QSI is similar for both material systems. For the case of the impacted specimen, except for the delamination sizes in the fifth and ninth interfaces, which are located at the beginning of stages II and III, respectively, the delamination located at the interfaces of stages II, III and IV of CCF300/QY9511 laminates is slightly smaller than that of CCF300/5428. Regarding the quasi-static indented specimens, the delaminations located at the interfaces of stages II, III and IV are approximately equivalent, except for those at the last two
interfaces, at which the delamination of CCF300/QY9511 is obviously smaller than that of CCF300/5428. Thus, a similar changing tendency for the internal delamination damage from the front surface to the back surface of the impacted or quasi-static indented specimens for both material systems could be concluded. In the majority of the specimens, the internal delamination of CCF300/QY9511 is slightly smaller than that of CCF300/5428. Further, for the last two interfaces, the difference between CCF300/QY9511 and CCF300/5428 is enlarged. This behavior occurs because CCF300/QY9511 has higher mode II fracture toughness than CCF300/5428. 3.2. Static compressive strength The results of the static compressive strength tests are pre^ st are sented in Table 2. The average compressive strengths l 164.0 MPa and 160.9 MPa for CCF300/QY9511 specimens with impacted and quasi-static indented damages, respectively, and
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Fig. 6. Delamination size at interfaces.
Table 2 Static compressive strengths. Material system
Damage
Spec. No.
Static compressive strength / MPa 1
2
3
l^ st /
r^ st /
MPa
MPa
Cv/%
CCF300/ QY9511
LVI QSI
170.4 153.8
161.0 167.6
160.5 161.6
164.0 161.0
5.6 6.9
3.40 4.30
CCF300/5428
LVI QSI
121.4 130.1
141.1 137.7
120.5 147.2
127.7 138.3
11.6 8.6
9.12 6.19
could be observed between the impacted and quasi-static indented laminates, the difference could be ignored because of the relative large scatters existing in the experimental outcomes. Comparing the static compressive strengths of the two material systems, the static compressive strength of CCF300/QY9511 laminates is greater than that of CCF300/5428 because the former has a higher mode II fracture toughness. 3.3. Compression fatigue behavior
127.6 MPa and 138.3 MPa for CCF300/5428 specimens with impacted and quasi-static indented damages, respectively. Fig. 7 shows the experimental static compressive strengths and the corresponding scatter degrees of the impacted and quasi-static indented laminates for the two material systems. For CCF300/ QY9511, the static compressive strengths of impacted and quasistatic indented laminates have similar average values and relatively small scatters. For CCF300/5428, although a slight difference
LVI QSI
160
Static fatigue
180
160
140
140
120
120
100
100
80
CCF300/QY9511
CCF300/5428
Compressive fatigue strength (MPa)
Static compressive strength (MPa)
180
80
Material type Fig. 7. Static and fatigue compressive strengths for CFRP laminates after LVI and QSI tests.
A total of 11 stress levels, with the minimum stress in the fatigue test rmin from 156.70 MPa to 109.20 MPa and the stress increment of Dr = 4.75 MPa, were established for all the staircase fatigue tests, as listed in Table 3. Table 3 also shows the fatigue life of the LVI and QSI specimens for both material systems obtained from the compression fatigue tests. The stress level of the staircase map for the QSI specimen is higher than that of the LVI specimen, for both material systems. In addition, the stress level of the staircase map of the CCF300/QY9511 specimen is higher than that of the CCF300/5428 specimen, for both LVI and QSI damages. Further, according to the staircase method, the average compression fatigue strength and the standard deviation for 105 cycles were calculated, as listed in Table 4, in which the static compressive strengths of the impacted and the quasi-static indented specimens are listed in Table 2, and the ratios of the fatigue compressive strength and static compressive strength are also provided. The high ratios larger than 85% provide evidence that the specimens containing damage have superior fatigue resistance. Fig. 7 also shows the experimental fatigue compressive strengths and the corresponding scatter degrees of the impacted and quasi-static indented laminates for both material systems. The fatigue compressive strengths of quasi-static indented laminates have a higher average value than that of the impacted laminates, considering relative small scatters in all of the experimental compressive fatigue strengths, for both material systems. In addition, the fatigue compressive strength of the CCF300/ QY9511 laminates is greater than that of the CCF300/5428 laminates because CCF300/QY9511 has higher mode II fracture
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Table 3 Fatigue life obtained by staircase method with stress ratio R = 10 (unit: cycles). Stress level
No.
1
rmin/MPa
CCF300/QY9511 laminates with LVI damage
1 2 3 4 5 6 7 8 9 10 11
156.70 151.95 147.20 142.45 137.70 132.95 128.20 123.45 118.70 113.95 109.20
2
3
4
5
6
7
8
1
2
3
4
7
8
8132 >105
15,364
>105
33,951
5
9949 >105
6
CCF300/ QY9511 laminates with QSI damage 2867
845
5
>105
>10 >105
39,599 >105
CCF300/5428 laminates with QSI damage 923 13,271 >105 5127 >105
CCF300/5428 laminates with LVI damage 15,178 1838 >105 4923 854
>105
>105 >105 >105
Table 4 Compressive fatigue strength. Material system
Damage
l^ 105 /MPa
r^ 105 /MPa
Cv/%
l^ 105 =l^ st
CCF300/QY9511
LVI QSI
143.2 152.0
2.7 2.7
1.89 1.78
0.87 0.94
CCF300/5428
LVI QSI
117.5 129.0
4.5 2.7
3.83 2.09
0.92 0.93
toughness. Thus, in the fatigue loading process, the delamination in the CCF300/5428 laminates was easier to propagate than that in the CCF300/QY9511 laminates. Along with the results of Section 3.2, higher compressive fatigue strengths with similar static compressive strengths of quasi-static indented specimens compared with impacted specimens could be obtained for both material systems. In addition, the scatters of the compressive fatigue strengths are obviously smaller than those of static compressive strengths. According to the damage patterns shown in Section 3.1, the fatigue compressive strength is dominated by all the delaminations present in all the interfaces, while static compressive strength is mainly dependent on the maximum delamination and the matrix fracture.
The damage parameters of typical survival specimens after the fatigue test, which are the same as those of the forenamed specimens, were measured using digital calipers and C-Scan, with the values listed in Table 1. Almost no differences for the length of the matrix fracture band, the height of the matrix uplift and the largest diameter of overlapped delamination of laminates could be found before and after the fatigue test, for any damageinducing method or material system. Fig. 8 shows the C-Scan images of the unbroken specimens (which are the same as those in Fig. 4) after 105 fatigue cycles. Compared with Fig. 4, the damage patterns after the fatigue tests are in good agreement with those before fatigue loading. All these experimental outcomes prove that the damage propagation originated in fatigue loading is incredibly slow, either in impacted or in quasi-static indented specimens, for both material systems. 4. Conclusions The compressive fatigue behaviors of CCF300/QY9511 and CCF300/5428 laminates with the same initial damages, which were characterized by depth dent and induced by LVI and QSI tests, were studied. Based on the results, the following conclusions can be drawn.
(a)
(b)
(c)
(d)
Fig. 8. Ultrasound C-Scan images of unbroken specimens after 105 fatigue cycles. (a) Damage of CCF300/QY9511 laminates by LVI. (b) Damage of CCF300/QY9511 laminates by QSI. (c) Damage of CCF300/5428 laminates by LVI. (d) Damage of CCF300/5428 laminates by QSI.
J. Zhang et al. / Composite Structures 133 (2015) 1009–1015
The LVI and QSI tests led to front surface dents and back surface matrix fracture and matrix uplift of the specimens. For the same dent depth damage, the QSI-induced matrix damage on the back surface was slightly more severe than the LVI-induced damage for both material systems. Furthermore, the LVI and QSI tests resulted in a similar delamination shape and similar general trend of delamination size throughout the thickness direction. For the case of same front surface induced dent, the LVI-induced delamination size was slightly greater than that from the QSI test, except for the last interface, at which the QSI-induced delamination size was much larger. Similar static compressive strengths for impacted and quasistatic indented specimens were obtained if dents of the same depths were induced, which is suitable for both material systems. However, the fatigue compressive strength of the impacted laminates was obviously lower than that of quasi-static indented laminates, for both material systems. Therefore, using QSIinduced damage to replace LVI-induced damage provides roughly equivalent strength evaluations in static compression tests but exaggerated fatigue strength estimations in compression fatigue tests. As a result, the use of QSI-induced damage instead of LVI-induced damage is not recommended in studies on the compression fatigue failure behaviors of CFRP laminates. Acknowledgments The research work is supported by the National High Technology Research and Development Program of China (2012AA040209) and the National Science Foundation of China (11372020). References [1] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44:155–90. [2] Cantwell WJ, Morton J. The impact resistance of composite materials – a review. Composites 1991;22:347–62. [3] Quaresimin M, Ricotta M, Martello L, Mian S. Energy absorption in composite laminates under impact loading. Compos Part B 2013;44:133–40. [4] Moura MFSF, Marques AT. Prediction of low velocity impact damage in carbon–epoxy laminates. Compos Part A 2002;33:361–8. [5] Icten BM, Kiral BG, Deniz ME. Impactor diameter effect on low velocity impact response of woven glass epoxy composite plates. Compos Part B 2013;50:325–32.
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Compressive fatigue behavior of low velocity impacted and quasi-static indented CFRP laminates Jianyu Zhang a, Libin Zhao b,⇑, Ming Li c, Yuli Chen c a
College of Aerospace Engineering, Chongqing University, Chongqing 400044, PR China School of Astronautics, Beihang University, Beijing 100191, PR China c Institute of Solid Mechanics, Beihang University, Beijing 100191, PR China b
a r t i c l e
i n f o
Article history: Available online 10 August 2015 Keywords: Composite laminates Fatigue behavior Impact behavior Mechanical testing
a b s t r a c t The compression fatigue behaviors of CFRP laminates with impacted or quasi-static indented damage for two material systems, CCF300/QY9511 and CCF300/5428, were compared in this paper. The same surface damages, characterized by dent depth, were induced by low velocity impact (LVI) or quasi-static indentation (QSI) tests for the CFRP laminates. Using visual observation, C-Scan and thermal de-ply experimental measure methods, the surface damage and internal delamination of the specimens are described in detail to provide more information to understand the mechanical behaviors of impacted or quasi-static indented CFRP laminates. Static tests and staircase fatigue tests were performed to obtain the static compressive strength and compressive fatigue strength. The experimental outcomes show that the compressive fatigue strength of the quasi-static indented specimens is obviously greater than that of impacted specimens, while the specimens with quasi-static indented damages have a similar static compressive strength as those with impacted damages, for both material systems. Using QSI-induced damage to replace LVI-induced damage provides a roughly equivalent strength evaluation in static compression tests but exaggerates fatigue strength estimation in compression fatigue tests. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In last few decades, composite materials have attracted an increasing and particular interest for aviation structures due to their high in-plane specific strength and stiffness. However, as a result of weak interlaminar strength, most of the aircraft composite structures stacked by laminae will suffer from strict limitations under out-of-plane impact loads, such as those delivered by dropped tools or runway debris [1–3]. Such a damage mode, which is dominated by internal delamination and matrix cracking (driven largely by interlaminar shear and tension or back surface tension driven by specimen bending, but completely invisible when viewed from the external impacted face), leads to a drastic reduction up to 60% of the static compressive strength and further fatigue strength when cyclic loads are applied [4–7]. The fatigue resistance of composite is a very complex problem, as it appears in multiple mechanisms of failure throughout the material due to microcracking, splitting and delamination, etc. [8,9], and thus has attracted great interest in the past two decades
⇑ Corresponding author. Tel./fax: +86 (0)10 82339228. E-mail address: [email protected] (L. Zhao). http://dx.doi.org/10.1016/j.compstruct.2015.08.046 0263-8223/Ó 2015 Elsevier Ltd. All rights reserved.
[10–13]. Generally, two types of experimental methods are used regarding the understanding of the fatigue resistance. One method is the fatigue life test provided with certain load levels, and the other is the fatigue strength test (or the staircase method) under certain load cycles. Most researchers are inclined to conduct the fatigue life test of laminates after impact [14–17], in which a relatively simple test procedure is required and more information about the damage propagation during the cycling loading can be obtained. However, the experimental outcomes of the fatigue lives express a rather large scatter. In contrast to the fatigue life test, the staircase method for fatigue strength could provide a considerably smaller amount of scatter, which is required to obtain satisfactory results in fatigue comparison tests. Moreover, two damage-induced methods, the low velocity impact (LVI) and quasi-static indentation (QSI) tests, are commonly used to simulate the actual impact status in service environments; the LVI is deemed to be better because it can provide more similarity due to its dynamic impact process. However, because the instantaneous impact process is not controlled and the various damage modes of composite materials propagate without certain rules and interconnect with each other, which express more sensitivity to the dynamic impact process, the same impact energy frequently leads to varied damage. In contrast with the LVI, the QSI
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method undoubtedly benefits from its ease of measurement and controllability [18]. In the QSI test, displacement loadings can be applied to specimens step by step until the appropriate surface damage is obtained, which could conveniently result in a series of specimens with the same damage pattern. Thus, using the QSI test instead of the LVI test to simulate the damage due to dropped tools or runway debris for CAI or compressive fatigue strength after impact is recommended [18–20]. In this paper, experimental studies on the compression fatigue response of impacted and quasi-static indented composite laminates are reported. The compressive fatigue strength and static compressive strength of composite laminates with impacted and quasi-static indented damage are compared. To deeply understand the compression fatigue behavior, information about the damage patterns of laminates originated in LVI and QSI tests is elaborated and compared. Two materials systems were used to study the dependence of failure behavior on the material properties. 2. Specimens and test procedures The laminates selected to investigate compression–compression fatigue behaviors were made of carbon/bismaleimide composite materials (CCF300/QY9511 and CCF300/5428), in which the former has a higher mode II fracture toughness, according to the material behaviors provided by the material producer: GIC = 210 J/m2 and GIIC = 1123 J/m2 for CCF300/QY9511 and GIC = 226 J/m2 and GIIC = 939 J/m2 for CCF300/5428. All the laminates provided with lay-ups [45/0/ 45/90]2s were cut into specimens with dimensions of 150 mm by 100 mm by 2 mm. Non-destructive inspection was performed on each specimen before the LVI and QSI tests, and only those without defects were selected for the tests. Similar damage scaled by the same depth of dent were induced on the surfaces of the specimens in the LVI or QSI tests and measured using visual observation, ultrasonic C-Scan inspection and thermal de-ply tests. Staircase tests were performed to determine the fatigue strength of the specimens, in which the initial load level was determined using static compressive tests of the impacted or quasi-static indented specimens. The experimental details are described below. 2.1. LVI and QSI tests The LVI tests were performed according to ASTM D7136-05 [21]. A drop weight low velocity impact testing machine was used in the tests. A hemispherical tip indenter with a mass of 5.5 kg and a diameter of 12.7 mm impacted the center of the specimen, which was simply supported by a steel fixture with a 125 mm 75 mm opening window. For different material systems, the drop height of the indenter was adjusted to achieve the same depth of dents on the surface of the specimen. To reduce the error caused by the relaxation of dents, the depth was measured immediately after the LVI tests using digital calipers. A series of LVI tests were performed to obtain a depth of dent of 1 mm on the specimen surface, and the impact energies of 11.5 J and 10.25 J were selected for the CCF300/QY9511 and CCF300/5428 specimens, respectively. The QSI tests were conducted following ASTM D6263-98 [22]. A micro-control electronic universal testing machine WDW-100 (E/A/S) with a range of 100 kN was used in the tests. The loading speed was set to 0.5 mm/min. For comparison with the induced damages stemmed from impact, the same indenter was used to induce the quasi-static indented damage on the specimen, which was also simply supported in the same manner as in the LVI test. The dent depth was also measured immediately using the digital calipers after the tests to eliminate the influence of dent relaxation. Based on a series of QSI tests, the indenter displacements
of 6.15 mm and 5.93 mm were determined for the CCF300/ QY9511 and CCF300/5428 specimens, respectively, to induce a depth of dent of 1 mm on the specimen surface. 2.2. Damage measurement In addition to the visual inspection of direct observation and the measurement using the aforementioned digital calipers, nondestructive ultrasonic C-Scan detection and a destructive thermal de-ply technique were also used to characterize the damage. The ultrasound C-Scan equipment (Olympus NDT MZ-03, Japan) was used to detect the internal delamination patterns, in which the detection frequency was 20 MHz and the step length was 0.5 mm. The thermal de-ply technique was used for detailed observation of the delaminations at each of the interfaces and further comparison of the damage sizes between the impacted and quasi-static indented specimens. A contrast solution of gold chloride–ether was used in the thermal de-ply process. The damage zones of the specimens were first immersed in the contrast solution for at least 30 min, and then the soaked specimens were placed into the high temperature chamber at 475 °C for 70 min to remove the resin between layers. Thus, the delamination between adjacent layers could be observed clearly. 2.3. Static and fatigue tests Static compressive tests were performed as per ASTM/D/ 7137M-07 [23] in advance to obtain the compressive strength of each of the impacted and quasi-static indented specimens, which was used to determine the fatigue initial stress level of each specimen. The loading rate in static test was set to 1 mm/min. The staircase method [24–26] was used to obtain the compression fatigue strength under a specified fatigue life. The compression–compression fatigue tests were conducted with the stress ratio of R = 10. In the current work, the fatigue strength for 105 cycles was studied. In the tests, if a specimen survives after 105 cycles, then the next specimen will be tested under the next higher load level. Conversely, if a specimen fails within 105 cycles, then the next specimen will be tested under the next lower load level. The fatigue tests stops until at least three pairs of valid data appear, and the data close in the staircase map. All the specimens were tested in a unified load spectrum with the same stress increment. To ensure the correctness of the test results, the specimen for each stress level was selected randomly. In both static compression tests and compression–compression fatigue tests, an Instron-8803 hydraulic pressure servo material testing machine with a range of 250 kN was used. To guarantee the centering and stability of the specimens during the compression process, a compression fixture was built based on the experiments of Uda et al. [27], as shown in Fig. 1. The specimen was placed in the fixture to avoid out-of-plane directional movements. 3. Experimental results and discussion 3.1. Induced damage The damage patterns resulting from the LVI and QSI tests appeared to be similar by visual observation. For both material systems, there was an induced dent on the front surface of the laminates and a clearly matrix uplift and matrix fracture on the back surface, which appears as a narrow band along the fiber direction of the back surface lamina, as shown in Fig. 2. To further characterize the types of damage, a schematic diagram of the damage dimensions is provided in Fig. 3. The types of damage on the back surface are characterized by the length ‘‘l” of the matrix fracture
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Table 1 Damage dimensions of impacted and quasi-static indented specimens measured before and after fatigue test. Material system
CCF300/QY9511
Damage-induced method
LVI
QSI
CCF300/5428 LVI
QSI
Before fatigue test
l/mm h/mm d/mm
108.1 0.7 24.3
118.9 0.9 27.9
94.3 0.8 26.1
99.4 1.0 29.0
After fatigue test
l/mm h/mm d/mm
108.2 0.7 24.3
118.9 1.0 28.0
94.3 0.8 26.1
99.5 1.0 29.1
150mm
l Fig. 1. Compression fixture.
band and the height ‘‘h” of the matrix uplift, and the corresponding data of typical specimen for each type test are listed in Table 1. A longer matrix fracture band and a higher matrix uplift on the back surface of the quasi-static indented specimen could be obtained compared to those of the impacted specimen for both material systems, which indicates that the QSI test can cause slightly more severe surface damage than the LVI test. In addition, the internal delamination patterns detected by the ultrasound C-Scan system for the same specimen listed in Table 1 are shown in Fig. 4. The overlapped delamination damage occurred in the center of the specimen, with appearance similar to a round disk. The matrix fracture was also observed clearly in the C-Scan images. The fracture was along the fiber direction of layer on the back of the specimen. The largest diameter of the round overlapped delamination damage at the center, characterized by d in Fig. 3, as well as the longest length of the matrix fracture were measured. The former is listed in Table 1, and the latter agrees well with the results from the direct measurement method, which is also listed in Table 1. The C-Scan results further illustrate that the largest damage originated from the QSI test is more severe than that caused by the LVI test for both material systems. More detailed information of the delamination at each interface between adjacent layers was obtained via thermal de-ply techniques. Fig. 5 shows a representative thermal de-ply result of the CCF300/5428 specimen with quasi-static indented damage, in which the typical shapes of delaminations and the changing tendency through thickness direction are illustrated. The number on the top left corner of the lamina denotes the ply number, as counted from the front surface of specimen. The delamination damage is found to be distributed like a dumbbell, in which the long axis is along the fiber direction of the adjacent layer closer
Fig. 2. Back surface of the CCF300/5428 laminates with impacted and quasi-static indented damage.
d
(a)
h
(b)
Fig. 3. Schematic diagram of damage dimensions (a) top view (b) front view.
to the back of the specimen. Similar delamination shapes and changing tendencies were observed in the impacted specimen for both material systems according to the thermal de-ply analysis. To further analyze the delamination damage quantitatively, the length of long axis, a, of the dumbbell-shaped delamination was measured one by one for the impacted and quasi-static indented specimens and plotted in Fig. 6 for both material systems. The delaminations through the thickness direction were found to be divided into four stages. Stage I includes four interfaces counted from the front of the specimen exhibiting an approximately linear increase in delamination size. Stage II considers the next three interfaces; an obvious decrease in the delamination size at the beginning of stage II could be observed, followed by an approximately linear increase. Next, another decrease in the delamination size is observed at the beginning of stage III, which is almost the same size as that of stage II, and a nonlinear enlargement of the delamination size is observed in the following interfaces until the last one. Finally, in stage IV, a remarkable delamination is observed in the last interface. The delamination size of the last interface measured by thermal de-ply technique is the same as that obtained from the C-Scan image (see Table 1), which indicates the consistency between the two inspection approaches. Note that in the middle interface of specimens, denoted by 90°/90°, no delamination information was obtained for both of the material systems, either from the LVI test or from the QSI test, because the adjacent layers possess the same fiber direction. Comparing the delamination damage of the impacted and the quasi-static indented specimens, the delaminations either from the LVI test or the QSI test are similar in the first stage. In the second and third stages, the delaminations resulted from LVI are obviously greater than those from QSI. However, in the last stage, the
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(a)
(b)
(c)
(d)
Fig. 4. Ultrasound C-Scan images of specimens with induced damage. (a) Damage of CCF300/QY9511 laminates by LVI. (b) Damage of CCF300/QY9511 laminates by QSI. (c) Damage of CCF300/5428 laminates by LVI. (d) Damage of CCF300/5428 laminates by QSI.
Fig. 5. Thermal de-ply images of specimens with induced damage.
delamination caused by QSI is significantly larger than that by LVI. The results indicate that when the same depth dents on the specimens are induced, the internal delamination caused by LVI is more severe than that originated from QSI, except for the last interface, for which the opposite results were obtained. In addition, an interesting observation is that the differences obtained from LVI and QSI are independent of the material systems. Fig. 6 also compares the internal delamination sizes of two material systems. In the first stage, the internal delamination damage of laminates resulting from LVI or QSI is similar for both material systems. For the case of the impacted specimen, except for the delamination sizes in the fifth and ninth interfaces, which are located at the beginning of stages II and III, respectively, the delamination located at the interfaces of stages II, III and IV of CCF300/QY9511 laminates is slightly smaller than that of CCF300/5428. Regarding the quasi-static indented specimens, the delaminations located at the interfaces of stages II, III and IV are approximately equivalent, except for those at the last two
interfaces, at which the delamination of CCF300/QY9511 is obviously smaller than that of CCF300/5428. Thus, a similar changing tendency for the internal delamination damage from the front surface to the back surface of the impacted or quasi-static indented specimens for both material systems could be concluded. In the majority of the specimens, the internal delamination of CCF300/QY9511 is slightly smaller than that of CCF300/5428. Further, for the last two interfaces, the difference between CCF300/QY9511 and CCF300/5428 is enlarged. This behavior occurs because CCF300/QY9511 has higher mode II fracture toughness than CCF300/5428. 3.2. Static compressive strength The results of the static compressive strength tests are pre^ st are sented in Table 2. The average compressive strengths l 164.0 MPa and 160.9 MPa for CCF300/QY9511 specimens with impacted and quasi-static indented damages, respectively, and
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Fig. 6. Delamination size at interfaces.
Table 2 Static compressive strengths. Material system
Damage
Spec. No.
Static compressive strength / MPa 1
2
3
l^ st /
r^ st /
MPa
MPa
Cv/%
CCF300/ QY9511
LVI QSI
170.4 153.8
161.0 167.6
160.5 161.6
164.0 161.0
5.6 6.9
3.40 4.30
CCF300/5428
LVI QSI
121.4 130.1
141.1 137.7
120.5 147.2
127.7 138.3
11.6 8.6
9.12 6.19
could be observed between the impacted and quasi-static indented laminates, the difference could be ignored because of the relative large scatters existing in the experimental outcomes. Comparing the static compressive strengths of the two material systems, the static compressive strength of CCF300/QY9511 laminates is greater than that of CCF300/5428 because the former has a higher mode II fracture toughness. 3.3. Compression fatigue behavior
127.6 MPa and 138.3 MPa for CCF300/5428 specimens with impacted and quasi-static indented damages, respectively. Fig. 7 shows the experimental static compressive strengths and the corresponding scatter degrees of the impacted and quasi-static indented laminates for the two material systems. For CCF300/ QY9511, the static compressive strengths of impacted and quasistatic indented laminates have similar average values and relatively small scatters. For CCF300/5428, although a slight difference
LVI QSI
160
Static fatigue
180
160
140
140
120
120
100
100
80
CCF300/QY9511
CCF300/5428
Compressive fatigue strength (MPa)
Static compressive strength (MPa)
180
80
Material type Fig. 7. Static and fatigue compressive strengths for CFRP laminates after LVI and QSI tests.
A total of 11 stress levels, with the minimum stress in the fatigue test rmin from 156.70 MPa to 109.20 MPa and the stress increment of Dr = 4.75 MPa, were established for all the staircase fatigue tests, as listed in Table 3. Table 3 also shows the fatigue life of the LVI and QSI specimens for both material systems obtained from the compression fatigue tests. The stress level of the staircase map for the QSI specimen is higher than that of the LVI specimen, for both material systems. In addition, the stress level of the staircase map of the CCF300/QY9511 specimen is higher than that of the CCF300/5428 specimen, for both LVI and QSI damages. Further, according to the staircase method, the average compression fatigue strength and the standard deviation for 105 cycles were calculated, as listed in Table 4, in which the static compressive strengths of the impacted and the quasi-static indented specimens are listed in Table 2, and the ratios of the fatigue compressive strength and static compressive strength are also provided. The high ratios larger than 85% provide evidence that the specimens containing damage have superior fatigue resistance. Fig. 7 also shows the experimental fatigue compressive strengths and the corresponding scatter degrees of the impacted and quasi-static indented laminates for both material systems. The fatigue compressive strengths of quasi-static indented laminates have a higher average value than that of the impacted laminates, considering relative small scatters in all of the experimental compressive fatigue strengths, for both material systems. In addition, the fatigue compressive strength of the CCF300/ QY9511 laminates is greater than that of the CCF300/5428 laminates because CCF300/QY9511 has higher mode II fracture
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Table 3 Fatigue life obtained by staircase method with stress ratio R = 10 (unit: cycles). Stress level
No.
1
rmin/MPa
CCF300/QY9511 laminates with LVI damage
1 2 3 4 5 6 7 8 9 10 11
156.70 151.95 147.20 142.45 137.70 132.95 128.20 123.45 118.70 113.95 109.20
2
3
4
5
6
7
8
1
2
3
4
7
8
8132 >105
15,364
>105
33,951
5
9949 >105
6
CCF300/ QY9511 laminates with QSI damage 2867
845
5
>105
>10 >105
39,599 >105
CCF300/5428 laminates with QSI damage 923 13,271 >105 5127 >105
CCF300/5428 laminates with LVI damage 15,178 1838 >105 4923 854
>105
>105 >105 >105
Table 4 Compressive fatigue strength. Material system
Damage
l^ 105 /MPa
r^ 105 /MPa
Cv/%
l^ 105 =l^ st
CCF300/QY9511
LVI QSI
143.2 152.0
2.7 2.7
1.89 1.78
0.87 0.94
CCF300/5428
LVI QSI
117.5 129.0
4.5 2.7
3.83 2.09
0.92 0.93
toughness. Thus, in the fatigue loading process, the delamination in the CCF300/5428 laminates was easier to propagate than that in the CCF300/QY9511 laminates. Along with the results of Section 3.2, higher compressive fatigue strengths with similar static compressive strengths of quasi-static indented specimens compared with impacted specimens could be obtained for both material systems. In addition, the scatters of the compressive fatigue strengths are obviously smaller than those of static compressive strengths. According to the damage patterns shown in Section 3.1, the fatigue compressive strength is dominated by all the delaminations present in all the interfaces, while static compressive strength is mainly dependent on the maximum delamination and the matrix fracture.
The damage parameters of typical survival specimens after the fatigue test, which are the same as those of the forenamed specimens, were measured using digital calipers and C-Scan, with the values listed in Table 1. Almost no differences for the length of the matrix fracture band, the height of the matrix uplift and the largest diameter of overlapped delamination of laminates could be found before and after the fatigue test, for any damageinducing method or material system. Fig. 8 shows the C-Scan images of the unbroken specimens (which are the same as those in Fig. 4) after 105 fatigue cycles. Compared with Fig. 4, the damage patterns after the fatigue tests are in good agreement with those before fatigue loading. All these experimental outcomes prove that the damage propagation originated in fatigue loading is incredibly slow, either in impacted or in quasi-static indented specimens, for both material systems. 4. Conclusions The compressive fatigue behaviors of CCF300/QY9511 and CCF300/5428 laminates with the same initial damages, which were characterized by depth dent and induced by LVI and QSI tests, were studied. Based on the results, the following conclusions can be drawn.
(a)
(b)
(c)
(d)
Fig. 8. Ultrasound C-Scan images of unbroken specimens after 105 fatigue cycles. (a) Damage of CCF300/QY9511 laminates by LVI. (b) Damage of CCF300/QY9511 laminates by QSI. (c) Damage of CCF300/5428 laminates by LVI. (d) Damage of CCF300/5428 laminates by QSI.
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The LVI and QSI tests led to front surface dents and back surface matrix fracture and matrix uplift of the specimens. For the same dent depth damage, the QSI-induced matrix damage on the back surface was slightly more severe than the LVI-induced damage for both material systems. Furthermore, the LVI and QSI tests resulted in a similar delamination shape and similar general trend of delamination size throughout the thickness direction. For the case of same front surface induced dent, the LVI-induced delamination size was slightly greater than that from the QSI test, except for the last interface, at which the QSI-induced delamination size was much larger. Similar static compressive strengths for impacted and quasistatic indented specimens were obtained if dents of the same depths were induced, which is suitable for both material systems. However, the fatigue compressive strength of the impacted laminates was obviously lower than that of quasi-static indented laminates, for both material systems. Therefore, using QSIinduced damage to replace LVI-induced damage provides roughly equivalent strength evaluations in static compression tests but exaggerated fatigue strength estimations in compression fatigue tests. As a result, the use of QSI-induced damage instead of LVI-induced damage is not recommended in studies on the compression fatigue failure behaviors of CFRP laminates. Acknowledgments The research work is supported by the National High Technology Research and Development Program of China (2012AA040209) and the National Science Foundation of China (11372020). References [1] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44:155–90. [2] Cantwell WJ, Morton J. The impact resistance of composite materials – a review. Composites 1991;22:347–62. [3] Quaresimin M, Ricotta M, Martello L, Mian S. Energy absorption in composite laminates under impact loading. Compos Part B 2013;44:133–40. [4] Moura MFSF, Marques AT. Prediction of low velocity impact damage in carbon–epoxy laminates. Compos Part A 2002;33:361–8. [5] Icten BM, Kiral BG, Deniz ME. Impactor diameter effect on low velocity impact response of woven glass epoxy composite plates. Compos Part B 2013;50:325–32.
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