Carbon fiber-reinforced plastic (CFRP) laminates with initially cut fibers (ICFs) have good formability without large degradation of static strength; however, their fatigue behavior has not been investigated thus far. In this paper, we investigated fatigue behavior and damage progress of open-holed CFRP laminates with ICFs having interlayers. Three types of CFRP laminates were employed: a laminate without ICF fabricated using an autoclave (Continuous-A), a laminate with ICF fabricated using an autoclave (ICF-A) and a laminate with ICF fabricated using press molding (ICF-P). First, fatigue test was conducted to obtain S (maximum stress)-N (the number of cycles to failure) curves in order to reveal fatigue strength. The fatigue tests for several specimens were interrupted at three prescribed numbers of cycles to observe damage progress. It is found that the Continuous-A laminate shows little strength degradation in the S-N curve while fatigue strength in both ICF laminates is decreased by approximately 30% at N of 106. In contrast, the damage progress of the ICF-P laminate is the least among the three laminates while the delamination progress at both edges and around the hole in the Continuous-A laminate is the most prominent.
Recently, carbon fiber-reinforced plastics (CFRPs) have been widely applied to primary structural components of aircraft and automobiles because of their contribution to higher fuel efficiency and lower emission of CO2. Such structural components often have circular holes for several technical reasons such as joining, weight reduction and functional needs. In addition, these components are frequently subjected to cyclic loads and vibrations, which may cause degradation of structural integrity because of fatigue damage.
In particular, for design requirements and functional needs, an automobile is composed of many complexly shaped components. It is quite difficult to fabricate such components using only conventional CFRP prepreg with continuous fiber owing to its poor formability. In contrast, discontinuous fiber-reinforced plastics fabricated by sheet molding compound (SMC) or injection molding have already been used as automotive parts since they have good molding flowability. However, their strength is much lower than that of composites reinforced with continuous fiber. Therefore, a new material, called unidirectionally arrayed chopped strands, was developed by Taketa et al. [
Several experimental results have been reported on ICF laminates thus far. In the subsequent paper by Taketa et al. [
Meanwhile, numerous studies have been performed on fatigue behavior of various kinds of open-holed CFRP composites under a variety of loading conditions. O’Higgins et al. [
The objective of the present study is to investigate fatigue behavior and damage progress of open-holed ICF CFRP laminates with toughened interlayers. Two kinds ofQI ICF laminates were fabricated by autoclave and hot press molding methods. For comparison, a QI laminate made of continuous prepreg is also manufactured by the autoclave method. First, S (maximum stress)-N (the number of cycles to failure) curves were measured for the above three laminates. Second, the internal fatigue damage progress was observed using optical microscopy and radiography. Finally, the effect of interlayers on fatigue progress was discussed based on the microscopic observation of the damages.
The prepreg used in this study is made of carbon fiber (volume fraction 0.58) and epoxy resin with interlayers, (T800S/#3900-2B, Toray Industries). This prepreg system contains about 30 mm thick interlayers including tough thermoplastic particles on the 150 μm thick base CFRP layer. A micrograph on the cross-section of the laminate is presented in
In this study, we made ICF sheets as proposed by Taketa et al. [
ICF-P laminates, respectively.
An ICF-A QI laminate was built by stacking ICF plies in a sequence of [45/0/-45/90]s and then cured in the autoclave. The schematic of the press molding process is illustrated in
OHT specimens were then cut out from laminates according to the ASTM standard (D5766). The dimensions and geometry of OHT specimens are shown in
The tensile fatigue tests were conducted at a room temperature with a sinusoidal waveform under a load-control condition using a hydraulic testing machine (8516 INSTRON). All the tests were performed at a stress ratio of 0.1 and at a frequency of 3 Hz. The maximum stress σmax was varied between 60% and 95% of the static tensile
Laminate | Stacking sequence | Thickness | Static tensile strength (MPa) | |
---|---|---|---|---|
Laminate (mm) | Ply (mm) | |||
Continuous-A | [+45/0/−45/90]s | 1.55 | 0.194 | 531.2 |
ICF-A | [+45/0/−45/90]s | 1.56 | 0.195 | 432.7 |
ICF-P | [+45/0/−45/90]2s | 2.16 | 0.135 | 531.6 |
strength σOHT of each ICF OHT specimen. The tests were terminated at N of 106 cycles even if no failure occurred. In order to quantitatively evaluate the magnitude of damage, fatigue tests (σmax = 0.73 σOHT) of one specimen for each laminate was interrupted at the prescribed numbers of cycles (N = 103, 104, 105) before final failure. The internal damage progress during the fatigue test was observed using transmissive radiography (SOFTEX M-100) with the aid of zinc iodide as a contrast medium. The projected internal damage area was then quantitatively measured using commercial imaging software.
First, at N of 103, transverse cracks in the ±45˚ and 90˚ plies and splitting cracks in the 0˚ plies are initiated from the hole roots (arrows A) as well as from the both edges in all the laminates. The crack density, which is defined as the sum of the crack length per unit area around the hole, in the Continuous-A laminate is the largest among the three laminates as shown in the
Second, at N of 104, the transverse cracks from the both free edges propagated and were connected with each other across the width of the specimens. In the ICF-A laminate (
Third, at N of 105, the density and length of cracks increase rapidly, especially in the Continuous-A and ICF-A laminates (
width. The delaminations are mainly caused by the interlaminar shear stress at the both free edges (
From the above schematics, it is found that the delamination in the Continuous-A laminate is more prominent than that in the ICF-A laminate. Contrarily, relatively small delamination locally extends around the hole in the ICF-P laminate (
The final fracture behavior for the three types of laminates are schematically shown in
final failure. The splitting cracks generated from the hole roots in the 0˚ plies propagate along the specimen length to induce the delamination, which causes full peel-off of the plies at the 0˚/−45˚ interfaces. The delamination starting from the hole propagates in the loading (longitudinal) direction and reaches both ends of the specimen at the final failure stage. In contrast, in the both ICF laminates, the final fracture mode is fiber breakage around the hole, although free-edge delamination is more remarkable in the ICF-A laminate. In addition, the damages including delamination and fiber breakage in the both ICF laminates tend to extend in the transverse direction rather than in the longitudinal direction. The propagation behavior of the splitting cracks in the 0˚ plies in the Continuous-A, ICF-A, and ICF-P laminates is illustrated in
where the intact ligament width becomes smaller by the connection of ICFs. In addition, comparing the present result with the result of static tensile test for the similar OHT specimens [
The fatigue behavior and damage progress in open-holed CFRP laminates with ICF toughened with interlayers are presented. From the experimental results, the following conclusions are derived:
1) The fatigue strength of the Continuous-A laminate is the highest among the three laminates. The decrease ratio of fatigue strength at N of 106 is only 5% of its static strength.
2) The fatigue strength of the ICF-P laminate is higher than that of the ICF-A laminate. However, the decrease ratio of fatigue strength at N of 106 is about 30% of its static strength, which is almost the same in the both ICF laminates.
3) The degree of damage in the ICF-P laminate is the smallest among the three laminates while the increase rate of the crack density is the greatest in the ICF-A laminate with thicker plies.
4) The damage propagation direction around the hole in the Continuous-A laminate is rather longitudinal while that in the both ICF laminates is transverse.
5) The interlayer acts as a crack arrestor for fatigue crack propagation even near the
hole.
6) Further work is still required to quantitatively predict fatigue damage development in both ICF laminates by finite element analysis.
Sudarsono and Ogi, K. (2017) Fatigue Behavior of Open- Holed CFRP Laminates with Initially Cut Fibers. Open Journal of Composite Materials, 7, 49-62. http://dx.doi.org/10.4236/ojcm.2017.71003