. 682 Downloads. Abstract The aim of this study was to investigate the applicability of acoustic emission (AE) technique to evaluate delamination crack in glass/epoxy composite laminates under quasi-static and fatigue loading. To this aim, double cantilever beam specimens were subjected to mode I quasi-static and fatigue loading conditions and the generated AE signals were recorded during the tests.
By analyzing the mechanical and AE results, an analytical correlation between the AE energy with the released strain energy and the crack growth was established. It was found that there is a 3rd degree polynomial correlation between the crack growth and the cumulative AE energy.
Using this correlation the delamination crack growth was predicted under both the static and fatigue loading conditions. The predicted crack growth values was were in a good agreement with the visually recorded data during the tests. The results indicated that the proposed AE-based method has good applicability to evaluate the delamination crack growth under quasi-static and fatigue loading conditions, especially when the crack is embedded within the structure and could not be seen visually. Fiber reinforced plastic composites (FRP) have many advantages such as high specific strength, specific stiffness, etc. However, these materials suffer from different damage mechanisms, such as matrix cracking, fiber breakage and delamination ,. The principal mode of failure in laminated composites is the separation along the interfaces of the layers, viz, delamination ,.
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This failure results in dramatic reduction of residual strength and stiffness of the structure. Delamination occurs under different loading conditions, i.e. Mode I, mode II and mode III. However, mode I delamination is the most common mode of failure occurred in the structures. This is due to lower energy that is required for the initiation of mode I delamination ,. Due to complexity of laminated composites, prediction of fatigue behavior in these materials is not straight forward. Accurate measurement of fatigue crack growth has become a challenging issue in fracture mechanics analyses.
Fatigue crack growth monitoring is a difficult and time-consuming test. In addition, work gets harder when the crack is embedded within the structure and could not be seen visually. Acoustic emission (AE) is a naturally occurring phenomenon, which is the result of transient elastic wave propagation caused by a sudden release of energy inside the material. There are various sources of AE events in composite materials such as matrix cracking, fiber/matrix debonding, fiber breakage, etc. Recently, AE has been utilized as an applicable technique to detect in-situ information from the damages that occur in laminated composites ,. Some studies have used AE technique to investigate the delamination behavior under quasi-static loading condition ,. Fotouhi and Ahmadi investigate initiation of delamination in laminated composites under mixed-mode loading condition using AE technique.
Arumugam et al. investigated damage mechanisms in glass/epoxy composite specimens under mode I delamination using AE and fast Fourier transform (FFT) analysis. Saeedifar et al. determined interlaminar fracture toughness of glass/ epoxy composites under mode I, II and mixed-mode I&II loading using AE and finite element (FE) methods. The literature review shows that most AE based studies were focused on delamination initiation and there is a lack in the investigation of delamination propagation behavior using AE technique.
Due to the complexity of the fatigue phenomenon in composite materials, little work has been done on the behavior of delamination in laminated composites under cyclic loading using AE technique. Silversides et al. studied delamination initiation in carbon/epoxy specimens under mixed-mode cyclic loading conditions. Romhany et al.
offered an algorithm to predict delamination crack in carbon/epoxy specimens subjected to cyclic loading. Romhany’s method has two disadvantages: (a) to predict the fatigue crack growth at least two AE sensors must be utilized, and (b) the accurate AE wave propagation speeds in the specimens must first be calculated. The aim of this paper is to investigate the delamination propagation in glass/epoxy composites under mode I quasi-static and fatigue loading conditions. The article is composed of two sections.
In the first section, the delamination behavior under quasi-static loading condition is investigated using the mechanical and AE data. Then, the correlations between AE energy, the released strain energy and the crack growth are established theoretically and it is found that there is a 3rd degree polynomial correlation between the crack growth and the cumulative AE energy. Then, quasi-static delamination growth was predicted using the obtained correlation and the obtained AE signals.
In the second section, the delamination crack growth under fatigue loading is predicted using the same concept as the static loading. The advantage of the proposed method is prediction of delamination crack growth using only one AE sensor without any need to determine AE wave propagation speed in the specimens. Consistency of the AE-based evaluated results and visually recorded values illustrates that the proposed AE method is more suitable than the conventional methods for detection of delamination crack growth in the laminated composites under quasi-static and fatigue loading conditions. 2 Experimental Procedures. 1 The specimens geometry and dimensions 2.2 Test Procedure 2.2.1 Quasi-Static and Cyclic Loadings The specimens were subjected to quasi-static loading according to ASTM D5528 standard.
A properly calibrated tensile test machine (HIWA), in the range of 0.5–500 mm/min, and the displacement control mode were used for the tests. The tests were carried out at room temperature and at a constant displacement rate of 3 mm/min. The load and displacement were continuously recorded by the tensile machine and the crack length was recorded using a digital video camera (SONY HDR-XR150) with 25X optical zoom and 300X digital zoom. AE events were recorded using AE software AEWin and a data acquisition system Physical Acoustics Corporation (PAC) PCI-2 with a maximum sampling rate of 40 MHz. PICO which is a broadband, resonant-type, single-crystal piezoelectric transducer from PAC, was used as the AE sensor.
The sensor has the resonance frequency of 513.28 kHz and an optimum operating range of 100–750 kHz. In order to provide good acoustic coupling between the specimen and the sensor, the surface of the sensor was covered with grease. The signal was detected by the sensor and enhanced by a 2/4/6-AST preamplifier. The gain selector of the preamplifier was set to 37 dB. The test sampling rate was 1 MHz with 16 bits of resolution between 10 and 100 dB. The AE sensor was placed on the surface of the sample with 80 mm distance from the delamination tip.
The aim of this study was to investigate the delamination propagation in glass/epoxy composites under mode I quasi-static and fatigue loading conditions. The results are represented in two sections.
In the first section, correlations among the AE energy with the released strain energy and the crack growth were established based on the theoretical fundamentals. It was shown that there is a 3rd degree polynomial correlation between the cumulative AE energy of delamination and the cumulative crack growth and accordingly the quasi-static delamination crack growth was predicted using the AE method. In the second section, the delamination crack growth under the fatigue loading is predicted using the proposed AE method. The proposed AE method has some advantages such as predicting delamination growth using only one AE sensor without a need to determine AE wave propagation velocity in the specimens. This method is a robust technique for detecting and measuring the crack length, especially when the crack is hidden and could not be seen visually.