| Ceramics have received extensive attention in machinery manufacture, armorprotection, aerospace industry, and other fields due to their high hardness, high strength,good corrosive resistance, and excellent high-temperature stability. After several decades,structural ceramics, especially high performance ceramics, are still a hot research topic.The law of crack nucleation and extension in ceramics and the microscopic fracturemechanism are still not fully understood. How to locate cracks and determine the crackdevelopment states in real-time are the main difficulties in ceramics research. Acousticemission (AE) technique is a kind of detection method which can real-time and dynamictest damage process of materials and acoustic emission signals during the damage processof ceramics contains much information about the crack locations, crack scale aroundcrack extension and the development status of cracks.In order to comprehensively master those information, the acoustic emission testduring AD95alumina (95%alumina), pure alumina (99.9%alumina) and zirconiatoughening alumina (15%zirconia/alumina) disk compression failure process has beeninvestigated. Using AE characteristic parameters, AE location algorithm andsignal-processing techniques, the hidden crack information is effectively decoded. In thispaper, the main research contents are as follows:(1) We base an acoustic emission (AE) characteristic parameter study on fractureprocesses in alumina ceramics and zirconia toughened alumina ceramics. According toAE amplitude and hits characteristics, as well as energy and ringdown counts of AE, thefracture process is divided into four stages: crack closure, nucleation, development, andcritical failure. We established a corresponding relation between AE characteristics andthe damage degree of material through the analysis of change rules of AE amplitude, hits,energy and ringdown counts in the four stages for detecting the damage of ceramicmaterials.(2) The differences of ZTA ceramic and alumina ceramics were analyzed in thedamage process. Due to the toughening effect of zirconia particles in ZTA ceramics, cracks were blocked off. Therefore, only when bigger external force was applied to ZTAceramics, ZTA ceramic produced obvious acoustic emission signals. Cracks propagationin ZTA ceramic need to overcome the barrier of zirconia particles, therefore, the releaseof fracture surface energy in the ZTA ceramics is more than that in alumina ceramicswhen a crack propagates and it has more AE energy in the damage of ZTA ceramics thanthat in alumina ceramics. It is hard that cracks go straight down in the ZTA ceramic dueto the inhibition of zirconia particles, and with the external load increasing, cracknucleation and propagation will occur in another location. So it has more AE signals aredetected in ZTA ceramics than that in alumina in general. According to those acousticemission phenomenons the characterization of toughening effects of ZTA ceramics can bemore direct.(3) Ceramic material damage and failure is the result of development and evolutionof microcracks, and microcracks nucleation, propagation and converge in ceramics willcause acoustic emission phenomenon. Therefore, acoustic emission accumulativecharacteristic parameters can reflect the damage degree of ceramics. It is proposed thatthe damage degree of ceramics can be quantitatively evaluated and estimated using AEaccumulative events and AE accumulative energy. Damage evolution equations onceramics are given at a constant rate under uniaxial compression load.(4) A new position method was developed based on the attenuation rule for signalspropagation and AE energy. We can locate the precise crack source only by measuring theenergy value, and do not need to measure the attenuation characteristics of propagationmedium and the speed of sound. Experiments of lead break in hexagonal prisms and thinplates were did in order to verify the precision of energy location method, and theprecision are ensured.(5) Through analysis of AE signals, the evolutional states of cracks in aluminaceramics during fracture and failure under applied compression were quantitativelystudied. The spectrum of AE signal emitted by fractures in the ceramic sample underapplied compression during the whole fracture and failure was obtained by using Radix-2decimation in time fast Fourier transform (DIT-FFT). By analyzing the measured AEsignals and their related frequency spectra we find that the AE signals emitted from alumina ceramics during their fracture and failure are transient and random and consist ofa series of different frequencies distributed in the range of50~400KHz. Each AE signalwas decomposed by wavelet packet decomposition into different sub-bands, and theenergy eigenvalue of each sub-band was calculated, the frequency features of cracknucleation and the crack unstable propagation were given. Through the analysis offrequency characteristics of different damage stages, the inverse relationship between thefrequency of AE signals and the size of fracture source in the ceramic samples duringfracture and failure was proved.(6) Analyze the problem about initial value of Geiger algorithm. We can first get atemporary initial value by the least square method, and make it as the initial value ofGeiger algorithm. and this program is verified by2-dimensional and3-dimensionalexperiments and the positional accuracy is analyzed according to the experimental results.Lots of microcracks in a typical brittle material such as ceramics occur and grow duringloadings, which result in acoustic emission (AE).Through the compression test and3-dimensional locating detection of AD95(95%) alumina ceramic specimens, acousticemission characteristics are achieved and the positions of cracks were located by ouroptimized Geiger algorithm. The location result is in accordance with the actual fractureposition, and the formation and propagation of lots of cracks in ceramics can be analyzedby the method. |
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