18 Investigation of Flexural Progressive …
201
et al. [21] studied the effect of bearing direction on the progressive damage of 3D
five-directional braided composites by three-point bending test. AE and micro-CT
techniques were used to monitor the damage evolution behavior of the composites
and the identification of internal damage initiation/evolution and distribution characteristics. The experimental results showed that the microscopic damage morphology
observed by micro-CT can be well correlated with the variation characteristics of the
collected AE parameters.
This paper is motivated by exploring the bending progressive damage mechanism
of 3D five-directional braided composites. The novelty of the work presented stems
from the combined use of multiple complementary methods including AE which is
damage monitoring, DIC which provides full field and surface deformation information, and micro-CT which can provide the internal damage initiation/evolution.
The mechanical curves, maximum stress value is recorded and discussed under flexural loadings. The progressive damage behaviors in composites are observed and
evaluated. Furthermore, the failure mechanisms are analyzed.
18.2 Materials and Experimental Details
18.2.1 Specimens Preparation
In the present paper, Specimens provided by the Institute of Composite Materials of
Jiangnan University (China) are fabricated by carbon fibers of Toray T700-12k as
reinforcement and consolidated by TDE-85 epoxy resin using RTM (resin transfer
molding) process [22, 23]. 3D five-directional braided preforms with the thickness
of 4.7 ± 0.2 mm are cut into the size of 60 mm × 20 mm by cutting machine.
Then, black/white paint is randomly sprayed on the side of the composite specimens
to measure the displacement and strain fields of the composites. A total of three
samples are employed.
18.2.2 Method
The three-point bending tests are conducted by a LD24 machine at a displacementcontrol velocity of 2 mm/min according to ASTM standard D790 partly. The span
of two supporting rollers is set as 48 mm as shown in Fig. 18.1a. Meanwhile, AE
signals and the speckle images are collected by a DS2A instrument and CMOS
(MER-500-7UM-L) in real time, as shown in Fig. 18.1b–c. Two RS-54A AE sensors
(the frequency range is 100–900 kHz) are used and their distance is 40 mm and
preamplifier with the output of 40 dB. The threshold and the sampling frequency are
fixed to 10 mV (40 dB) and 3 MHz by repeated tests. In addition, speckle images
are scanned at 30.45 pixels/mm and collected twice per second by a CMOS camera
201
et al. [21] studied the effect of bearing direction on the progressive damage of 3D
five-directional braided composites by three-point bending test. AE and micro-CT
techniques were used to monitor the damage evolution behavior of the composites
and the identification of internal damage initiation/evolution and distribution characteristics. The experimental results showed that the microscopic damage morphology
observed by micro-CT can be well correlated with the variation characteristics of the
collected AE parameters.
This paper is motivated by exploring the bending progressive damage mechanism
of 3D five-directional braided composites. The novelty of the work presented stems
from the combined use of multiple complementary methods including AE which is
damage monitoring, DIC which provides full field and surface deformation information, and micro-CT which can provide the internal damage initiation/evolution.
The mechanical curves, maximum stress value is recorded and discussed under flexural loadings. The progressive damage behaviors in composites are observed and
evaluated. Furthermore, the failure mechanisms are analyzed.
18.2 Materials and Experimental Details
18.2.1 Specimens Preparation
In the present paper, Specimens provided by the Institute of Composite Materials of
Jiangnan University (China) are fabricated by carbon fibers of Toray T700-12k as
reinforcement and consolidated by TDE-85 epoxy resin using RTM (resin transfer
molding) process [22, 23]. 3D five-directional braided preforms with the thickness
of 4.7 ± 0.2 mm are cut into the size of 60 mm × 20 mm by cutting machine.
Then, black/white paint is randomly sprayed on the side of the composite specimens
to measure the displacement and strain fields of the composites. A total of three
samples are employed.
18.2.2 Method
The three-point bending tests are conducted by a LD24 machine at a displacementcontrol velocity of 2 mm/min according to ASTM standard D790 partly. The span
of two supporting rollers is set as 48 mm as shown in Fig. 18.1a. Meanwhile, AE
signals and the speckle images are collected by a DS2A instrument and CMOS
(MER-500-7UM-L) in real time, as shown in Fig. 18.1b–c. Two RS-54A AE sensors
(the frequency range is 100–900 kHz) are used and their distance is 40 mm and
preamplifier with the output of 40 dB. The threshold and the sampling frequency are
fixed to 10 mV (40 dB) and 3 MHz by repeated tests. In addition, speckle images
are scanned at 30.45 pixels/mm and collected twice per second by a CMOS camera