Abstract
The failure behaviours of unidirectional pultruded carbon fiber reinforced polymer (CFRP) composites were monitored by the electrical resistance measurement during tensile loading, three-point-bending, interlaminar shear loading. The tensile failure behaviour of carbon fiber tows was also investigated by the electrical resistance measurement. Infrared thermography non-destructive evaluation was performed in real time during tensile test of CFRP composites to validate the change of microdamage in the materials. Experiment results demonstrated that the CFRP composites and carbon fiber tows were damaged by different damage mechinsms during tensile loading, for the CFRP composites, mainly being in the forms of matrix damage and the debonding between matrix and fibers, while for the carbon fiber tows, mainly being in the forms of fiber fracture. The correlation between the infrared thermographs and the change in the electrical resistance could be regarded as an evidence of the damage mechanisms of the CFRP composites. During three-point-bending loading, the main damage forms were the simultaneity fracture of matrix and fibers firstly, then matrix cracking and the debonding between matrix and fiber were carried out. This results can be shown in Fig. 9(a) and (b). During interlaminar shear loading, the change in the electrical resistance was related to the damage degree of interlaminar structure. Electrical resistance measurement was more sensitive to the damage behaviour of the CFRP composites than the stress/time curve.
Keywords : CFRP (carbon fiber reinforced polymer), electrical resistance measurement, damage mechanism, matrix cracking, debonding
1. Introduction
CFRP composites are important structural materials due to their high tensile strength, high tensile modulus and low den- sity, and therefore attractive for aircraft applications. With increasing use in primary parts, it becomes urgently necessary to detect, evaluate and understand the development of damage in CFRP components in order to guarantee safe use over their entire lifetime. Structure health monitoring refers to the monitoring of the integrity of a structure for the purpose of hazard mitigation. Recent attention on structure health monitor- ing has been centered on the use of embedded or attached sensors, such as optical fiber, piezoelectrical, microelectro- mechanical, acoustic, dynamic response, phase transformation, and other sensors.
The CFRP composites have conductivity due to containing conductive carbon fibers. The conductivity of CFRP composites depends on the directions, the orientation and the volume fraction of conducting carbon fibers. As for unidirectional CFRP composites, with the increasing of volume fraction of carbon fiber, the conductivity of the materials increases.
When the fiber volume fraction reaches to a threshold (about 40%~60%), a strongly connected electrical network with anisotropy forms. if the unidirectional CFRP composites
subject to damage, the conductivity of the materials will change. So through monitoring the variation of the electrical resistance, the damage degree in the unidirectional CFRP composites can be detected so as to avoid fateful mar. The electrical resistance measurement is a particularly effective method for detecting small and subtle defects in CFRP composite materials. Previous work has focused on the setup of electrical resistance experiments and probing into damage mechanism of CFRP laminates [1-5].
In this work, the changes in the electrical resistance of the unidirectional pultruded CFRP composites during tensile, three-pointbending, interlaminar shear loadings were monitored in order to analyze damage mechanisms of the CFRP com- posites.
2. Experimental
The unidirectional pultruded CFRP composites used in this work were manufactured via the continuous pultrusion process as reported in reference [6]. The tensile test speci- mens, with a cross section of 6×2 mm2, were cut from the pultruded CFRP composites. The specimens for three-point bending and interlaminar shear testing had the same cross
section of 25×2 mm2. In order to perform the electrical resistance measurement, copper electrodes were adhered to the polished specimen surfaces as shown in Fig. 1(a), with the type SY-73 conductive silver adhesive [7] from Beijing Institute of Aeronautical Materials. The silver adhesive, cured at 120oC for 2 hours, has an electrical resistance of 0.06Ω(≤0.1Ω), so the contact electrical resistance between the copper electrodes and carbon fibers can be neglected, and the electrical resistance values measured in this experi- ment are stable.
The electrical resistance was measured by four-wire technique with digital multimeter (Agilent 34401A) to ensure a quasi DC resistance value. An infrared thermography apparatus was used in the tensile test for non-destructive evaluation. The Schematics of the experimental setup were shown in Fig 2. Three-point bending and interlaminar shear tests were carried out with Instron-1185 universal materials testing machine according to ASTM 02344-84 and ASTM- 790-00.
3. Results and Discussion
3.1. Tensile loading test
Fig. 3(a) and Fig. 3(b) illustrated the results of the tensile tests on the unidirectional pultruded CFRP composite sample (how about to change to specimen ?? they are two tests respectively.) and carbon fiber tow. The stress-time and elec- Fig. 1. Polarized photographys of polished surfaces of the
CFRP composites (10×50) (a) profile and (b) cross section.
Fig. 2. Schematics of the experimental setup; the electrical resistance measurement during (a) tensile test, (b) three-point bending and interlaminar shear test, and (c) Infrared thermgro- phy tracking during tensile test.
trical resistance-time curves of the samples were simultane- ously measured.
The longitudinal electrical resistance was constant as increasing the tensile stress up to the 0.7 Gpa as shown in Fig. 3(a). This was due to the less elongation quantity of sample. However, the tensile stress was over the 0.7 GPa, the electrical resistance suddenly dropped, which was related to the increasing contact degree of 00 fiber alignment in the unidirectional composites. The improved 00 fiber contact degree can be explained as below: above 0.7 GPa of a stress, the fact that the elastical modulus (Ef) of carbon fibers is far higher than that of the matrix (Em), caused the shear stress between matrix and fibers, resulting in matrix damage and debonding between matrix and fibers, as shown in Fig. 4 [8].
Having some spaces to move in, the carbon fibers surround- ed by insulating matrix were unwinded, resulting in the compactness alignment of 00 fibers and the subsequent decrease in the volume resistance along the fiber directions.
With the more increasing in tensile stress, some filaments
with defects fractured, causing a series of electrical resistance peaks. The fracture fiber ends touched to the neighborhood fibers, froming a conductive network as shown in Fig. 5, therefore, the whole change trend of the electrical resistance was ever decreasing with a series of electrical resistance peaks. Once residual intact fibers couldnt support the increasing stress, the sample completely ruptured. The electrical resistance increased in a stepwise manner to a higher value due to the destroying of the conductive network.
In order to analyze the damage mechanism of the CFRP composites by the electrical resistance method, a tensile test at 2 mm/min was performed on a single fiber tow (T-300, 3 K). It was found from the result, as shown in Fig. 3(b), that before rupture, the electrical resistanceof the fiber tow always increased with increasing stress, with a stress above 2.3 GPa, the electrical resistances rose suddenly to a higher value because of fiber rupture, accompanying with a much Fig. 3. Longitudinal electrical resistance and tension stress vs time curve (a) CFRP specimen with 62 vol.% of carbon fibers and (b) T- 300 carbon fiber tow (3 K).
Fig. 4. Matrix damage and the debonding between matrix and fiber caused by straightening of fibers under tension.
Fig. 5. Conductive network form.
obvious decline in the stress/time curve.
3.2. Infrared thermography non-destructive evaluation Infrared thermography non-destructive evaluation was used to validate the damage in the materials. Fig. 6 shows the simultaneously obtained curves of the electrical resistance, stress, and temperature of the middle section of specimens during tensile testing at 1mm/min (tensile speed). From A to B, the electrical resistance and the temperature of the specimens were hardly change with increasing the stress, indicating that there was no clear microstructure change occurred in the materials. From B to C, the electrical resistance decreased, while the temperature of the specimen rose with increasing the stress and remarkable microstructure changes were occurred in the materials. From C to D, the electrical resistance reduced to the lowest, a peak at M
moment appeared in the temperature vs time curve, with an indicator of great changes in the microstructure. At D point, the electrical resistance of the specimens was suddenly risen to the highest value and the corresponding temperature was dropped from the peak value [the peak point is correspond- ing to the M point ?? yes], due to the specimen breakage and release the accumulated heat into surroundings. Infrared thermograph at M moment was shown in Fig. 7. There are numerous mechanisms in the CFRP composites which produce the temperature changes of sample as shown in Fig. 6, e.g:
matrix deformation and cracking, fiber fracture, especially the debonding and delamination of composites. The correlation between the infrared thermographs and the change in the Fig. 6. Curves of electrical resistance, stress, and temperature of
specimens.
Fig. 7. Infrared thermographs at M moment.
Fig. 8. Stress and transverse resistance vs time curve (a) three-point bending test (2 mm/min) (b) interlaminar shear test (1 mm/min).
electrical resistance is useful for analyzing the damage mechanisms of the CFRP composites.
3.3. Three-point bending and interlaminar shear loading test Fig. 8 shows the changes of transverse electrical resistance on the CFRP composite during three-point bending and interlaminar shear loadings.
Two transitions of the electrical resistance were occurred in Fig. 8(a). According to the bending damage theory for CFRP composites [9], a premature failure appeared at the compressive side beneath the loading nose. The main damage forms were the simultaneous fracture of matrix and fibers.
Meanwhile, matrix cracking and debonding between matrix and fibers were followed, as shown in Fig. 9(a), which leaded to the first transition of the electrical resistance. Then the crack propagated from the compressive layer to the tensile layer, causing progressive debonding, and resultant delaminationin in the tensile layer. The fact that the current may flow in every laminar resulted in the increasing in the electrical resistance. While the tensile layer completely fractured, the broken fibers pulled out from the matrix, as shown in Fig. 9(b), leading to the second transition of the electrical resistance. The eletrical resistance change resulted from the microstructure changes in the materials.
Fig. 8(b) shown that the transverse electrical resistance decreased with increasing shear stress. While the specimen was subjected to completely shear destroy, the transverse electrical resistance dropped to the lowest. The shear stress destroyed the interface structure between matrix and fibers, causing the carbon fibers surrounded by insulating matrix have some spaces to move in, and touch to adjacent carbon fibers. The imprived transverse contacts between carbon fibers resulted in the dropping in the transverse electrical resistance. The change in the transverse electrical resistance can obviously indicate the damage appearance, but with a much lower resolution in stress/time curve.
debonding between matrix and fibers occurred, all this damages resulted in two transitions of the electricla resistance. During interlaminar shear loading test, the dropping in the electrical resistance relected the damage degree of the interlaminar structure. The electrical resistance measurement was more sensitive to damage behaviours of CFRP composites than the stress/time curve.
Acknowledgements
The authors wish to thank to the Center of Instron test machine lab., Beijing University of Chemical Engineering and Non-destructive detecting lab, China Research Institute of Architecture Science, Beijing for supporting this work.
This work was supported by Hi-Tech Research and Development Program of China (2001A335030).
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