AbstractThis essayintroduces the typical damage modes, the principle of acoustic emissiondetection, the characteristics of acoustic emission signals, the signalanalysis and processing techniques, and the advantages and disadvantages ofacoustic emission detection.                                     Contents 1.    Introduction 1 2.

    Literature review 2 2.1 Acoustic emission detection 2 2.2 Acoustic emission signal generation 2 2.3 Acoustic emission signal characteristics 2 2.4 Acoustic emission signal analysis and processing technology 3 2.5 Carbon fibre composite materials testing 3 2.6 The advantages and disadvantages of acoustic emission detection 3 3.

    Conclusions 4 Reference 5                          1.IntroductionCarbon fibrecomposite material refers to the use of carbon fibre reinforcedhigh-performance reinforced composite materials, as well as matrix is mostlyadvanced resin-based. Its overall performance with the aluminum alloy, but thestiffness and strength higher than the aluminum alloy (Mair, 2002).Carbon fibre is mainly composed of a special type of carbon element. The carboncontent varies with different species, the general mass fraction of 90%.

Carbonfibre with the general characteristics of carbon materials, such as hightemperature, abrasion resistance, electrical conductivity, thermal conductivityand corrosion resistance, but with the general carbon material differences arethat carbon fibre’s shape has a significant anisotropy, soft, can be processedinto kind of fabric and high strength along the fibre axis. Inthe fields of density, stiffness, weight, fatigue properties and otherdemanding requirements, as well as in demanding high temperature, chemicalstability of the occasion, carbon fibre composite materials are veryadvantageous. Carbon fibre composite materials in the construction,transportation, aerospace industry has been widely used (Huang and Zhao, 2017).Although carbon fibre composites have been widely used as a new material, butdue to process instability, defects such as voids and inclusions cannot becompletely avoided in the production process, and their transverse bearing andshear resistance are low. In the impact or fatigue Under the action of otherloads easily damaged until destroyed.When theparts of carbon fibre composite materials are assembled and connected withother parts and components, a great deal of holes are inevitably processed, anddefects such as debonding of the composites are easily caused during theprocessing of the holes.

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Zhang Yunfeng and other studies have found that thedirection of the fibre has a serious impact on the formation of drillingdefects; the greater the axial force, the more severe delamination defects, whilerapid increase in tear defects. Wang Hongfei and other defects in the carbon fibrecomposite drilling holes were analyzed and classified, found that processingwill appear glitches and delamination defects.Composite materialdamage patterns varied. Basic damage patterns can be divided into twocategories, namely, in-plane and out-of-plane damage. In-plane damage includesmatrix cracking, debonding at the fibre interface, fibre buckling and fibrebreakage; and out-of-plane damage occurs in the laminate, exhibitingdelamination.Figure. 1. Four types ofdamage in fibre reinforced composites: (a) matrix cracking; (b)fibre buckling; (c) fibre breakage; (d) delamination.

(Wen, Xia and Choy,2011) In summary,the carbon fibre reinforced composite material damage detection and real-timemonitoring is particularly important. In order to ensure the safe applicationof composite materials, the detection and research of composite materials arewidely regarded by people. There are a variety of methods available for thetesting of carbon fibre composites, including acoustic emission testing,ultrasonic testing, laser testing, penetration testing, optical fiber testing,X-ray testing and magnetic particle testing. Various testing methods play arole in their fields of application their own advantages, but also have theirown shortcomings. The following analysis of the acoustic emission detection ofcarbon fibre composite materials in the application of the status quo. 2.

Literature review2.1 Acousticemission detectionAcousticemission, also known as stress wave emission, is a phenomenon in whichdeformation or fracture or internal stress exceeds the yield limit and entersthe stage of irreversible plastic deformation when material or component issubjected to external force and releases strain energy in the form of transientelastic wave. 2.2Acoustic emission signal generationMaterials orstructures that deform and break under stress can cause acoustic emissionsignals. For composite materials, there are many types of damage and damagethat can produce acoustic emission signals such as commonly encountered fibrebreakage, cracking of the substrate, layered expansion and interface separationdamage are all sources of acoustic emission signals.

(1)Plastic deformationForcrystalline materials, the deformation or rupture can produce acoustic emissionsignals, and plastic deformation is one of the important mechanisms to generateacoustic emission signals. At the yield point, the count rate of acousticemission signal reaches the peak (Jemielniak, 2001).(2)Crack formation and expansionThe materialwill undergo deformation and fracture under the stress, and the fracturefailure is just the result of crack formation and expansion.

It is an importantacoustic emission source when the crack is formed and expanded.Crackformation, crack propagation and final fracture are three parts of the materialfracture process. All three phases release energy to form a strong acousticemission signal.

(3)Generation of acoustic emission signals of fibre reinforced compositesFibre-reinforcedcomposites are formed by the combination of matrix and fibres. The combinationof the matrix and the fibres enables the two completely different materials toexert their respective excellent performances. The interlaced fibre laminationmakes the composite material as a whole, which causes a lot of acousticemission signals during stress destruction. 2.3Acoustic emission signal characteristicsAccording tothe characteristics of acoustic emission signals, acoustic emission signals canbe classified, which can be divided into burst type and continuous type fromthe waveform characteristics.

Burst signals are waveform signals that are notconsecutive and separate in the time domain. For fibre-reinforced composites,in the case of fibre breakage, crack propagation, and inclusions fragmentation,a burst type acoustic emission signal is generated. For continuous acousticemission signals, it is mainly due to the frequency of acoustic emission in thetime domain to achieve the degree of inseparability and connected together.

 2.4Acoustic emission signal analysis and processing technologyAcousticemission detection refers to the use of professional instruments to be testedunder the external force deformation or rupture when issued by the acousticemission signal acquisition, and then use advanced acoustic emission signalanalysis and processing of the collected signal Analysis and processing,through the analysis and processing technology to collect the signal analysisand processing. Through the analysis and processing of acoustic emissionsources can be positioned to identify the type of acoustic emission source toidentify the severity of damage assessment and predict the future developmentof damage and other functions. Currently, the commonly used acoustic emissionsignal analysis and processing techniques are time domain analysis, frequencydomain analysis, parametric analysis, wavelet analysis and so on. 2.5Carbon fibre composite materials testingThe researchon the acoustic emission characteristics of composites damage has become acommon method used to study the fracture mechanism of composites.

At present,the acoustic emission detection technology can not only detect and analyze theload distribution of a single carbon fibre filament or a tow when the fractureoccurs, but also judge the quality of the carbon fibre filament or the carbon fibrefilament. At the same time, it is also possible to identify the type of damagethat the carbon fibre composite has in each stage of the damage process, suchas fibre breakage, crack initiation, crack formation and propagation, and matrixdelamination, etc. 2.6 Theadvantages and disadvantages of acoustic emission detectionAcousticemission detection is an online dynamic detection method. Acoustic emissiondetection can collect acoustic emission signals through professionalinstruments.

After analyzing and processing the signals, an online assessmentof the damage state of the tested object can be achieved. Acoustic emissiondetection has the following advantages:(1) Acousticemission detection belongs to passive detection. It detects the stress wavesreleased when the test object is damaged, while the signal source ofnon-destructive testing methods such as ray detection or ultrasonic inspectionis some professional testing instruments. Therefore, Ray detection andultrasonic testing belong to the active detection.(2) Acousticemission source can be achieved qualitative analysis, positioning and otheranalysis.

(3) Sinceacoustic emission detection detects and records the whole process of thedamage, acoustic emission detection can obtain complete acoustic emissioninformation about the occurrence and spread of the damage. By processing theseinformation by means of modern signal analysis and processing, not only canKnowing the status quo of the damage to the test piece and predicting thefuture trend of the damage can make it possible to assess the degree of damagedone, the useful life of the product and the structural integrity.(4) Acousticemission technology detection area is very wide.

As long as the surface of themember to be detected coupling fixed enough sensors, it can detect componentsfrom all directions of the defect acoustic emission signal, do not need tofrequently move the sensor position to do the scan operation.(5) Acousticemission detection technology can be applied to almost all materials on-linedefect detection. At the same time the technology will not be influenced by theshape and size of components, the application is very extensive.(6) Since acousticemission detection does not require direct contact with the material to beinspected, it is suitable for environments with strong radiation, flammable,explosive, poisonous and extreme temperatures that other non-destructivetesting methods cannot achieve.Acousticemission testing has wide range of applications, which can predict and evaluatethe development trend of damage, and can realize the dynamic monitoring ofdamage and many other advantages. This method is gradually accepted by thevarious walks of life and widely used.

However, acoustic emission detection inpractical applications also has some limitations:(1) Acousticemission detection cannot detect the static defect because acoustic emissiondetection records stress wave release when the object is damaged.(2) In thecomposite material testing, due to irreversible damage to the compositematerial, if the material itself has been damaged under the test, then theapplied load does not exceed the required load of the existing damage,Therefore, in order to obtain accurate acoustic emission detection results, itis necessary to understand the current status of material damage beforeperforming acoustic emission testing. At this time, other damage-free materialssuch as ultrasonic testing or radiation testing are needed Detection method.(3) Due tothe particularity of acoustic emission detection, the detected signal resultsare often disturbed by noise. Therefore, if the filtering process is notperformed on the obtained signal by using the correct filtering method, theanalysis result will be greatly affected.(4) Onlythe detection of acoustic emission signals can not determine the type andextent of damage to materials or components, and a number of signal analysismethods are also needed to assist with the assessment.

 3. ConclusionsAcousticemission detection technology compared with other conventional non-destructivetesting methods, has the following advantages: (1) Acousticemission technology detection area is very wide. As long as the surface of themember to be detected coupling fixed enough sensors, it can detect componentsfrom all directions of the defect acoustic emission signal, do not need tofrequently move the sensor position to do the scan operation. (2) Acousticemission detection technology can be applied to almost all materials on-linedefect detection. At the same time the technology will not be influenced by theshape and size of components, the application is very extensive. (3) Sinceacoustic emission detection detects and records the whole process of thedamage, acoustic emission detection can obtain complete acoustic emissioninformation about the occurrence and spread of the damage. By processing theseinformation by means of modern signal analysis and processing, not only canKnowing the status quo of the damage to the test piece and predicting thefuture trend of the damage can make it possible to assess the degree of damagedone, the useful life of the product and the structural integrity. (4) Since acousticemission detection does not require direct contact with the material to beinspected, it is suitable for environments with strong radiation, flammable,explosive, poisonous and extreme temperatures that other non-destructivetesting methods cannot achieve.

In practicalapplications, often need to use a variety of non-destructive technology inorder to achieve the most comprehensive test results. Acoustic emissiondetection is suitable for detecting dynamic crack growth, crack initiation andcrack growth; ultrasonic testing and X-ray testing are suitable for detectingdefects such as cracks, delamination, inclusions, pores and slag inclusions inmaterials or components, that laser detection is only suitable for detectingnear-surface defects; for minor changes in materials or components, defectssuch as splice quality and cracks in plywood honeycomb structures can bedetected using laser testing; for magnetic materials, magnetic particle testingcan be used to detect defects such as cracks, folds, interlayers, and slaginclusions on the surface or near the surface of the material or component; opticalfibre detection is suitable for detecting defects in the pump body, castings,boilers, pressure vessels and pipe surfaces as well as defects such as weldquality and fatigue cracks; for non-porous metal and non-metallic materials,penetration testing can be used to detect defects such as cracking, folding andloosening, and at the same time, the location, size and shape of defects can beobtained. ReferencesMair,R.

(2002). Advanced composite structures research in Australia. Composite Structures,57(1-4), pp.3-10. Huang,X.

and Zhao, S. (2017). Damage tolerance characterization of carbon fibrecomposites at a component level: A thermoset carbon fibre composite. Journal ofComposite Materials, 52(1), pp.37-46. Zhang,Y.

, Luo, R., Zhang, J. and Xiang, Q. (2011).

The reinforcing mechanism ofcarbon fibre in composite adhesive for bonding carbon/carbon composites. Journalof Materials Processing Technology, 211(2), pp.167-173. Wang,H., Li, H., Lu, L., Xie, Y. and Xiao, Y.

(2014). Delamination Analysis inDrilling Carbon Fibre-Reinforced Composites. Applied Mechanics and Materials,697, pp.62-66. Jemielniak,K. (2001). Some aspects of acoustic emission signal pre-processing.

Journal ofMaterials Processing Technology, 109(3), pp.242-247. Wen,J., Xia, Z. and Choy, F. (2011). Damage detection of carbon fibre reinforcedpolymer composites via electrical resistance measurement. Composites Part B:Engineering, 42(1), pp.77-86.