Acoustic-emission study of the possibilities of localizing flaws of a welded joint during cooling

A method of localizing acoustic-emission signals from flaws of a welded joint during its cooling is considered. Aluminum and titanium inserts were used to simulate flaws of the welded joint. The developed technique of clustering made it possible to exclude noise accompanying the processes of welding and cooling the welded joint. The welded specimens were tested on an MTS-50 loading machine. At the finishing stage, specimens with double-sided welded joints containing titanium inserts were subjected to metallographic analysis.     

Nondestructive testing of the quality of blanks for the fabrication of hot-rolled strips using the acoustic-emission method

A method for predicting the quality of hot-rolled strips based on the results of recording acoustic-emission (AE) signals is proposed. Considering the development of flaws in a slab under the effect of technological loads during rolling as a process of the loss of the integrity of the slab material, the AE signals recorded in the course of diagnostic loading of experimental specimens are interpreted from the standpoint of a micromechanical model and the developed methodology for nondestructive testing of the strength of a material and quality estimation. The most informative diagnostic parameters and diagnostic features of the method are determined by comparing the results of the interpretation with the defect level of the obtained strip.     

Studying sources of acoustic emission during the cooling of a weld seam with the use of cluster analysis

Acoustic-emission (AE) signals localized during cooling of a weld seam were analyzed. The hazard degree of flaws (faulty fusions, cracks in the root of a weld seam, etc.) was studied using cluster analysis in the parameters of AE signals. Flaws were simulated via introduction of titanium and duralumin inserts into a weld seam. The results of metallographic investigations of specimens with artificial and actual flaws were analyzed. Localization of AE signals was performed for the specimens studied, and a time distribution taking into account the clustering of the total count and energy of AE signals was constructed.     

Classification of acoustic emission signals collected during tensile tests on unidirectional ultra high molecular weight polypropylene fiber reinforced epoxy composites using principal component analysis

The objective of this study is to utilize a mathematical procedure for the evaluation of damage mechanism in ultra high molecular weight polypropylene/epoxy composite (UHMWPP/epoxy) specimen with different configuration based on the analysis of the AE signals of presented. Principal component analysis (PCA) is powerful tool, which utilized for the classification of the monitored AE transients. We worked on epoxy L160 resin, UHMWPP fiber bundle and UHMWPP/epoxy unidirectional specimens, subjected to tensile loading. Using model specimens exhibiting a predominant damage mechanism, correlations were established between the observed damage growth mechanisms and the AE parameter variances result in PCA. Outputs from this study revealed that the PCA is an effective tool for identifying damage modes such as matrix cracking, fibre/matrix debonding, fibre breakage and fibre pull-out in the UHMWPP/epoxy composites. The presence of damage modes in UHMWPP/epoxy composites was proven with scanning electron microscopy (SEM) images.     

Acoustic emission used for detection of crack generation in propellers of turbine-pump units

The problem of applying acoustic emission (AE) for detection of crack generation in heavily loaded elements of rotor systems, in particular, in propellers of turbine-pump units, in which breakdown of propellers during the operation is possible because of fatigue-crack generation, is studied. The data on AE signals radiated from some materials of the propellers exposed to monotonic and cyclic loading are presented, and the possibilities of emission and detection of AE signals with setups built in the rotor system construction are determined. Two developed setups are compared, one of which is characterized by high reliability, sufficient sensitivity to crack generation, and the absence of electronic amplifiers and power supply units. The design and results of the tests performed with this setup for AE-technique-based monitoring of the crack formation are presented.     

Acoustic Emission and Destruction of Inelastically Strained Metal

The results of experimental acoustic emission (AE) studies on the stepwise character of deformation and fracture in the uniaxial tensioning of steel specimens are presented. True stress–residual strain plots and parameters of the accompanying AE are analyzed in order to determine the characteristic points of the transition of the defect accumulation process from one stage to another. Based on the integral characteristics of the flow of AE events, a method for the detection and quantitative assessment of metal fracture (loosening) under mechanical loading is proposed; this method can be directly used in diagnostics of the prefailure condition of materials.     

Dependence of the number of acoustic-emission pulses on the mechanical testing load of bearing races in an axle unit

Acoustic-emission tests of flawless bearing races in axle units are performed on a special loading stand. Dependences of the number of AE pulses on the loading force are established for objects with and without flaws. The spatial distribution of AE sources in the flawless bearing races and a change in activity during reloading of the races are studied. Numerical parameters of the local-dynamic rejection criterion are determined on the basis of the analysis of the experimental data on races containing fatigue cracks.     

Studying the deformation and temperature distributions in a specimen during acoustic-emission testing of the weld-seam welding and cooling process

The results of experimental studies of the weld-seam welding and cooling process in steel specimens using acoustic emission (AE) and strain gauging are presented. Deformation was measured by high-temperature strain gauges welded to specimens using spot welding. Temperature was measured by chromel-alumel thermocouples. Measurements were performed with measured with an MMTC-64.01 microprocessor strain-gauging system and a C??a?? ?16.10 AE system. Computers of these two systems exchanged information. Residual strain deformations were recorded on a specimen after cooling of a weld seam. Metallographic studies of transverse sections, which were were cut out from the clusterization zones of a weld seam, confirmed the presence of cracks, pores, and faulty fusions.     

Fatigue crack propagation behavior at a notch for needled C/SiC composite under tension-tension loading

Fatigue test of a needled C/SiC composite with a notch under tension-tension cyclic loading was completed, and the main fatigue crack propagation curve of the needled composite was obtained by the in situ observation of the fatigue process. By analyzing the influence of the failure number and distribution on the tensile loading subjected by 0° fiber bundles, the relationship between the main fatigue crack propagation and the distribution of 0° fiber bundles in the needled composite was established. By observing the fracture microstructure (especially the distribution of 0° fiber bundles) of the needled composite through scanning electron microscopy, the reasons for the varying fatigue resistance of different notched specimens were also explained. In addition, acoustic emission (AE) was also used to analyze the AE energy characteristics during the fatigue crack propagation process of the needled composite.

     

Artificial neural network a tool for predicting failure strength of composite tensile coupons using acoustic emission technique

A series of 18 tensile coupons were monitored with an acoustic emission (AE) system, while loading them up to failure. AE signals emitted due to different failure modes in tensile coupons were recorded. Amplitude, duration, energy, counts, etc., are the effective parameters to classify the different failure modes in composites, viz., matrix crazing, fiber cut, and delamination, with several subcategories such as matrix splitting, fiber/matrix debonding, fiber pullout, etc. Back propagation neural network was generated to predict the failure load of tensile specimens. Three different networks were developed with the amplitude distribution data of AE collected up to 30%, 40%, and 50% of the failure loads, respectively. Amplitude frequencies of 12 specimens in the training set and the corresponding failure loads were used to train the network. Only amplitude frequencies of six remaining specimens were given as input to get the output failure load from the trained network. The results of three independent networks were compared, and we found that the network trained with more data was having better prediction performance.     

Evaluation of fracture behavior of SA-516 steel welds using acoustic emission analysis

The purpose of this study is to evaluate the AE characteristics for the basemetal, PWHT (post-weld heat treatment) and weldment specimens of SA-516 steel during fracture testing. Four-point bending and AE tests were conducted simultaneously. AE signals were emitted in the process of plastic deformation. AE signal strength and amplitude of the weldment was the strongest, followed by PWHT specimen and basemetal. More AE signals were emitted from the weldment samples because of the oxides, and discontinuous mechanical properties. AE signal strength and amplitude for the basemetal or PWHT specimen decreased remarkably compared to the weldment because of lower strength. Pre-cracked specimens emitted even lower event counts than the corresponding blunt notched specimens. Dimple fracture from void coalescence mechanism is associated with low-level AE signal strength for the basemetal or PWHT. Tearing mode and dimple formation were shown on the fracture surfaces of the weldment, but only a small fraction produced detectable AE.     

A correlation analysis and invariant method of acoustic-emission signals in the diagnostics of predestruction states of materials

The results of the application of correlation analysis and invariant method of acoustic-emission (AE) signals during the mechanical loading of 95X18 steel samples are considered. The circuit of a controller that is intended for calculation of statistical characteristics of AE signals is given. A method for diagnostics of predestruction states based on the appearance of stably positive values of the amplitude-correlation coefficient and AE pulse-repetition intervals and deviations of invariant AE relationships from their stable values is proposed.     

On the Information Value of the Method of Invariants of Acoustic-Emission Signals in the Diagnostics of Pre-Failure State in Materials

The specific features of using the method of the invariants of acoustic emission (AE) signals for the diagnostics of a pre-failure state are considered. Results of the diagnostics of pre-failure by the amplitude and temporal invariants of AE event stream under mechanic loading of steel specimen are presented. It has been established that the measurement and analysis of amplitude and temporal informative parameters is more pictorial and should be preferred to the direct measurement of invariant relationships. Incorrect application of the method of invariants has been analyzed.     

Relation between the amplitude of acoustic emission of an ensemble of microflaws and the deformation rate: Reconstruction of the number of AE events during a change in the dynamic range of detected signals

A model of acoustic emission from a dislocation cluster during its passage through a boundary is considered. A dependence of the amplitude of an acoustic-emission (AE) event on the deformation rate and the loading frequency has been obtained. Algorithms for reconstructing the number of lost AE events below the discrimination threshold of the instrumentation are obtained for the case of a power-law amplitude distribution. A method for correcting the parameters of the detected stream of events at various discrimination thresholds, deformation rates, and loading frequencies is described. A method for estimating the dynamic range on the basis of the average value of the detected AE-signal amplitudes is proposed. We propose using the results of this study for increasing the reliability of the AE method for diagnostics of a prebreakdown state.     

Acoustic sources from damage propagation in Ti grade 5

In this paper fatigue test results are presented for titanium grade 5 specimens subjected to uniaxial cyclic loads. The material behavior of titanium was studied by comparing the Acoustic Emission (AE) detection technique with the Infrared Thermography (IT). The AE technique consists on attaching a couple of piezoelectric sensors on the surface of the sample in order to allow real-time recording of acoustic activity occurring in the material during the test and to localize the acoustic source, based on the assumption that part of the acoustic activity depends on the crack propagation process. All typical data of AE were collected during the tests and some of them were properly post-processed by using filters or derivative functions in order to better understand the crack propagation phenomenon. At the same time, thermographic analysis was also carried out during the experiments by continuous monitoring of surface temperature of the sample. Results of the fatigue behavior of the analyzed material beside the acoustic emission track and the thermal images of the sample are analyzed and critically discussed, in order to assess the capability of each technique in predicting the imminent failure of material.     

Analysis of variations of the dynamic characteristics of acoustic-emission sources under static loading of metal specimens

Changes in the energy of acoustic-emission (AE) signals and their cross-correlation coefficient for sources of different types have been experimentally studied for statically loaded specimens. The results allow separation of AE signals appearing during irreversible changes in the region of stress concentrators and signals of other types.     

夹套式反应罐事故压力分析

为了得到某夹套式反应罐事故压力,提出了一种基于有限元方法的封闭空间内可压缩流体压力的迭代分析方法,得到了夹套压力与内筒内压力之间的关联曲线。发现加载曲线具有明显的阶段性特征。基于设备事故后实测残余变形和人孔盖脱落模拟,分析得到了发生事故时罐内压力为7.44 MPa,为设计压力的12.4倍。     

Studying the failure of a CFRP sample under static loading by the acoustic-emission and fractography methods

Relationship between the main informative parameters of acoustic-emission (AE) signals recorded during static loading of a CFRP sample and the process of material fracture has been investigated. Clusterization of AE signals by digitized shape has made it possible to locate a zone they originate from. Material was then cut out from this zone to prepare thin sections that were subjected to fractography. Fractography revealed fiber fracture, brittle cracking of the matrix, its exfoliation from the carbon fibers, etc.     

The relationship between AE and dissipation energy for fretting wear

This paper describes the behavior of acoustic emission (AE), especially the correlation between the AE output and the dissipation energy under the fretting conditions. Fretting tests are conducted with a ball contacting with a flat disc in air. The specimens used are a bearing steel for a ball, and a bearing steel or an aluminum alloy for a flat disc. The results show that the behavior of the AE output and the dissipation energy during each fretting cycle is not so similar to each other throughout the test, but the total AE root-mean-square and the total dissipated energy during the test have a good correlation between them.     

Acoustic emission during run-up and run-down of a power generation turbine

The measurement of acoustic emission (AE) activity at the bearings of power generation turbines is evolving into a viable complementary diagnostic technique especially adept at indicating the early stages of shaft-seal rubbing. This paper reports on an ongoing investigation on the application of acoustic emission for shaft-seal rub detection in power generation turbines. A pre-requisite in developing such a technique requires an understanding of the operational AE background noise.The detection of shaft-seal, and blade-case, rubbing with AE has been reported [Electrical Eng. Japan 110 (2) (1990); Joint ASME/IEEE Power Generation Conference, October 4–8, 1981, St. Louis, MI, USA; International Conference on Condition Monitoring. Oxford, UK, July 2–4, 2003]. In these instances, the observations of AE activity were associated with particular dynamic and process conditions known to cause such defects. However, the effect of variations of turbine loading on AE activity is unknown. This report presents observations of AE activity during the run-up and run-down of a 500 MW steam turbine and is the first known document detailing correlations between AE, vibration and turbine loading.