Composite materials and bonding structure have been widely used in aerospace, vehicles, chemical manufacturing fields with its light weight, high strength, easily processing forming and excellent heat-resistant, corrosion resistance. The composite material and bonding structure are extensive used in aircraft fuselage of modern aerospace. The nondestructive testing methods such as ultrasonic, acoustic impedance, sound frequency sweep are commonly used in composite materials and bonding structural. Due to the problems such as material, structural, production technology, there are lot of defects in composite material and bonding structure , therefore only a detection method is difficult to achieve a comprehensive test.
1.Aviation composite materials structure and its defects
The fiber reinforced resin laminated structure and sandwich structure are common composite material structures used in aircraft structure. Fiber reinforced resin layer plate structure is divided into carbon fiber reinforced resin structure (CFRP) and glass fiber reinforced resin structure (GFRP) according to different materials. Sandwich structure is mainly honeycomb sandwich structure, foam sandwich structure and a small amount of glass beads sandwich structure.
Composite components tend to generate damages and destructions in use process due to stress or environmental factors. The laws of produce and extension for composite material damages compare to metal structure have a larger difference, and the damages can rapidly expand and cause structural failure often after damage to a certain scale. So the test in use of composite materials is very important, also gets people's attention more and more.
1)The main defects in structure of fiber reinforced resin layer board
The defects such as mix, falsely layering should be produced in the process of forming of fiber reinforced resin layer plate structure due to process and random artificial operations. The defects such as porosity out of limit, layered, degumming will be produced by bad curing process. The layered defect of hole edges will be formed in the course of hole processing and assembling. The initial defects (layered, degumming) in use caused by combined actions of environmental factors such as load, vibration, damp and hot, acid and alkali will lead to expand, layer, degum, fracture and other new injury and damage.
2)The main defects existed in the sandwich structure
Some defects will be produced in the process of forming of sandwich structure due to process reasons. The defects such as deformed honeycomb core, dropped node are created by error operating, and weakly bonding is caused by too low honeycomb core. The defects such as local poor or rich adhesive, weakly bonding, styrofoam cavity are caused by bad curing process . The initial defect(such as weak degumming ) will expand and lead to the new damages and destructions such as degumming, water inflow and collapsed honeycomb core and so on in use. Foam sandwich structure will produce the defects such as degumming, cracked core, etc.
2.Ultrasonic testing
Ultrasonic testing technique is one of the major means for nondestructive detection, and used widely in detection of internal defects such as metal, non-metal, composite materials. For thicker composite materials such as multilayer metal bonding composite panels, the internal defects such as de-bonding, holes, porosity, poor bond quality[1] will be tested usually by adopting the methods such as ultrasonic pulse echo, pulse penetration, as shown in figure 1 and 2.
Fig 1. sketch map for ultrasonic pulse echo test method
Fig 2. testing sketch map for ultrasonic pulse penetration method
3.Acoustic impedance testing
Acoustic impedance testing, also known as mechanical impedance (MIA) test method, it converts the mechanical impedance reflected vibration characteristics of material to the load impedance of transducer. Due to certain relationship exists between the mechanical impedance and the material structure, therefore the impedance changes of material mechanics may be judged by measuring the characteristic of transducer, so as to achieve the aim of testing. The acoustic resistance method through bending vibration of the tested materials by point source excitation is generally used to detect bond quality, such as thin skin and honeycomb structure bond quality, etc. as shown in figure 3. Figure 4 is a sketch map for acoustic impedance testing spring model, the spring stiffness on good bonding structure is just contact stiffness, the spring stiffness at de-bonding top is the series of probe contact stiffness and discontinuous stiffness. Discontinuous stiffness depends on the size and buried depth of de-bonding. When the defects are smaller or buried deeply, the spring stiffness increases, the defect should be difficult to detect. The impedance changes with frequency, so selecting appropriate frequency is key to get good test results [2].
Acoustic impedance tester is a portable testing instrument developed for the integrity testing of composite material plate - plate bonding structure and honeycomb structure. Acoustic impedance tester realizes the detection aim through identifying the obvious differences of surface mechanical impedance between honeycomb bonding structure good area and bonding defect area. It is mainly used for the defect detection of plate core separation for aluminum single skin and honeycomb bonding structure of skin with base plate.
Fig 3. acoustic impedance test sketch map Fig 4. the sketch map with acoustic impedance testing de-bonding spring model
4.Sound frequency sweep test
The principle of acoustic frequency sweep test is similar to acoustic - ultrasonic testing, and using plate wave detection technology. Using low-frequency ultrasound probe with double chips, double contacts and without coupling agent, puts two voice contacts of probe on scanning line in reciprocating direction, exciting emission transducer with low frequency or radio frequency electrical signals, making ultrasonic into checked parts through contacts. Ultrasonic wave is horizontally transmitted as plate wave, the receiving transducer picks up acoustic signal transmitted by parts through another contact which is fixed distance intervals between receiving transducer and transmitting transducer, showing with amplitude and phase, representing the bonding quality on acoustic path, as shown in figure 5.
Fig 5. acoustic frequency sweep test schematic diagram
5.Acoustic resonance detection
Acoustic resonance test method is similar to conventional acoustic impedance test method, namely it excites detected objects through electro acoustic transducer, and measures the impedance changes caused by different parameters of detected objects to distinguish that if coupling agent is required as shown in figure 6. Acoustic resonance detection can be used for testing shear cohesion strength of cementing composite plates, tensile cohesion strength of honeycomb structure and the defects such as de-bonding , blowhole, layer whose cohesion strength is zero. Acoustic resonance test method can be roughly divided into two categories: the single frequency resonance test method which as a single frequency sound waves incidences detected part and frequency sweep resonance test method that incidences detected part as sound waves whose frequency changes with time [3].
Fig 6. acoustic resonance detection sketch map
1) Single frequency resonance test method
Single frequency resonance test is to incidence tested part using incentive sound wave with single frequency. Under the function of input frequency, tested structure as a load of transducer, its change of acoustic impedance shall change the amplitude, phase and resonant frequency, etc., of transducer output electric signal, and quality of the structure can be evaluated by measuring amplitude and phase of the transducer receiving signals.
2) Frequency sweep resonance test method
The incentive sound wave used by frequency sweep resonance test is continuous sound wave whose frequency changes with time. When natural frequency and resonant frequency of tested part is consistent with frequency of excitation signal, the load born by transducer is much heavier when compared to other frequency components, that is easy to detect the signal current changes caused by load changes. Using the method, thickness of plank, location and depth of the defects such as de-bonding, layering, blowhole of bonding structure and composite material can be reliably detected.
6.Fixed distance transmitting/receiving test
The principle of fixed distance transmitting /receiving test is similar to acoustic - ultrasonic testing, and using plate wave detection technology. Using low-frequency ultrasound probe with double chips, double contacts and without coupling agent, puts two voice contacts of probe on scanning line in reciprocating direction, exciting emission transducer with low frequency or radio frequency electrical signals, making ultrasonic into checked parts through contacts. Ultrasonic wave is horizontally transmitted with plate wave, the receiving transducer picks up acoustic signal transmitted by parts through another contact which is fixed distance intervals between receiving transducer and transmitting transducer, showing with amplitude and phase, representing the bond quality on acoustic path, as shown in figure 7.
Fig 7. fixed distance transmitting/receiving test schematic diagram
As shown in figure 8, SMART - 6000 composite integrated detector is comprehensive testing equipment used for a new generation of multifunctional composite materials and bonding structure. The detector is cooperatively developed by Eddysun (Xiamen) electronics co., LTD. and Air Force Equipment Department. The detector is based on NDT integrated detection technology theory [4], has the functions such as ultrasonic, acoustic impedance, sound frequency sweep, etc, and is a portable comprehensive testing equipment used for composite materials and bonding structure.
Fig 8. SMART - 6000 composite integrated detector
SMART - 6000 composite integrated detector is a portable comprehensive testing equipment used for composite materials and bonding structure. The detector has the functions such as ultrasonic, acoustic impedance, sound frequency sweep, etc, and a variety of detection models such as ultrasonic A scan, pulse, frequency sweep, mechanical impedance, etc, to adapt to the detections for different materials, different structure parts.
7.Probe
SMART - 6000 composite integrated detector can connect various ultrasonic probe, mechanical impedance detection probe, resonance detection probe, fixed distance transmitting/receiving frequency sweep test probe, fixed distance transmitting/receiving pulse detection probe, transmitting/receiving, the structure sketch map for part of the probes as shown in figure 9.
Fig 9. A schematic diagram of special probe for composite material and bonding structure detector
8.Application
SMART - 6000 composite integrated detector can be widely used in nondestructive testing in service of composite materials and bonding structure in the fields such as aerospace, automobile, chemical manufacturing. We should take nondestructive testing in service of composite materials and bonding structure used for aircraft fuselage for an example, introduce how to use the composite materials and bonding structure integrated detector.
In modern aviation manufacturing field, composite material has gradually replace conventional metal materials as the main materials in aircraft manufacturing. Boeing, for example, the amount of composite material in Boeing 787 aircraft has reached 50%, as shown in figure 10. The fuselage and wings parts adopt carbon fiber reinforced laminated composite plate instead of the aluminum alloy plate. Engine nacelle, tailplane and vertical tail, rudder surface areas, wing tips all use the carbon fiber reinforced sandwich board structure. The rectifier skins in joint of fuselage and wing adopt glass fiber reinforced composite materials [5]. Along with large application of composite materials in aircraft manufacturing, the nondestructive testing of composite material also gradually become the focus, traditional single detection method has been unable to meet the testing requirements increasingly complex for composite materials and composite bonding structure. The defect types used commonly composite materials and bonding structure as shown in figure 11 and 12. Table 1 shows the performance comparison of field inspection methods commonly used for honeycomb bonding structure, it describes that each method has its advantages and disadvantages, and a detection method cannot detect all defects.
Fig 10. Application of composite materials for Boeing 787 aircraft
Fig 11. A sketch map of defects produced possibly after composite honeycomb bonding plate is shocked
Fig 12. A sketch map of defects for composite material laminated bonding board
Table 1. A comparison for ultrasonic, acoustic impedance, acoustic resonance, fixed distance transmitting/receiving testing
The development of SMART - 6000 composite integrated detector solves successfully the test problem. It integrates almost all of the mature testing methods used for composite material and bonding structure, testing personnel can conveniently choice suitable detection methods according to specific detection object, or use different detecting methods for same testing object in detection.
Through data fusion. comprehensive analysis, mutual authentication for test results obtained by a variety of test method of, a more complete assessment conclusion can be obtained, and greatly improving detection efficiency and precision.
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