Cumulative Acoustic Emission Events Research Articles

Simulation of AE Generation Behavior in 2-Dimensional Polycrystals by Using the Body Force Method

AE (Acoustic Emission) generation behavior during microfracture process of 2-dimensional alumina polycrystals is simulated along with its characteristic parameters, by assuming that a microfracture corresponds to an AE event. The microfracture process is simulated by using the body force method. Various AE generation behaviors are predicted as a function of the grain boundary toughness (Kcb), the grain size and the distribution state of residual stresses due to crystalline anisotropy. The predicted AE events have a tendency to occur at higher stress with increasing Kcb and with decreasing grain size. By the fracture-mechanical calculation of the crack opening volume, the relative variation of AE amplitude is also simulated. The actual AE measurements are tarried out in vacuum for two kinds of alumina with different grain size, and compared with the simulated results. The cumulative AE event curve for 11.5 μm grain size shows good agreement with the simulation when the Kcb is 0.35 times the grain toughness (Kcg), and that for 28.2 μm shows good agreement with the simulation at Kcb = 0.45Kcg. The results of AE location for both the simulation and the measurement show the similar characteristics of the scattered distribution over the stressed fields.

Simulation of AE Generation Behavior in Microfracture Process of 2-Dimensional Polycrystals.

AE (Acoustic Emission) generation behavior during microfracture process of 2-dimensional alumina polycrystals is simulated by assuming that a microfracture corresponds to an AE event. Various AE generation behaviors are predicted as a function of the grain boundary toughness (Kcb), the grain size and the distribution state of residual stresses due to crystalline anisotropy. The predicted AE events have a tendency to occur at higher stress with increasing Kcb and with decreasing grain size. By the fracture-mechanical calculation of the crack opening volume, the relative variation of AE amplitude is also simulated. The actual AE measurements are carried out in vacuum for the two kinds of alumina with-different grain size, and compared with the simulated results. The cumulative AE event curve for 11.5 μm grain size shows good agreement with the simulation when the Kcb is 0.35 times the grain toughness (Kcg), and that for 28.2 μm shows good agreement with the simulation at Kcb=0.45Kcg. The results of AE location for both the simulation and the measurement show the similar characteristics of the scattered distribution over the stressed fields.

On the failure mode in dry and hygrothermally aged short fiber-reinforced injection-molded polyarylamide composites by acoustic emission

The failure mode in injection-molded short glass (GF) and carbon fiber (CF) reinforced polyarylamide (PAR) composites was studied on compact tension (CT) specimens in as-received (AR), hygrothermally aged (HA) and re-dried (RD) states, respectively, using acoustic emission (AE) and fractography. A significant difference was revealed in the failure manner characterized by the cumulative run, amplitude and energy distribution of the AE events as a function of the water content of the composites. Furthermore, a correlation was found between the cumulative AE events up to the maximum load and the fracture toughness of the composites. It was shown that the fracture response and thus the failure behavior of the water-saturated PAR composites can be restored by drying. This fact indicates that the water absorption and desorption are of a purely physical nature, i.e. they are reversible processes. It was established that chopped fiber-reinforced PAR composites fail by matrix deformation along with fiber/matrix debonding in the crack initiation, whereas fiber pull-out becomes dominant in the crack propagation range. Water uptake shifts both the AE amplitude and energy curves toward lower values, a phenomenon attributed to plastification of the PAR matrix by water.

Characterization of deformation and fracture behavior in amorphous and/or ceramic coatings and aluminum alloy substrates by small punch testing and acoustic emission techniques

Deformation and fracture behavior in Al alloy substrates coated with and without amorphous Ni-10wt.%P (10 μm thick) and overlaying Y 2O 3-stabilized ZrO 2 (0–1.0 μm thick) ceramics has been investigated by small punch (SP) testing and acoustic emission (AE) techniques. SP tests showed that an Ni P coating increased the yield strengthσ y by 45% and slightly decreased the fracture toughness J Ic compared with the result of an uncoated Al alloy. All overlaying ceramic coatings except a 1.0 μm thick coating did not strongly affectσ y. Ceramic coatings with thickness of 0.25 and 1.0 μm lead to 15%–20% decreases in J Ic while a 0.5 μm thick coating maintained almost the same level of J Ic as the uncoated alloy. It was found that cracks in the Ni P coating propagate more extensively along the radial and tangential directions under the biaxial stress condition than in the Ni P and ceramics coatings. The measurement of cumulative AE events was capable of identifying the severity of coating cracking in the alloys coated with and without overlaying ceramics. The coating effect on J Ic is discussed in terms of residual stress Induced during coating processing.

Fracture behavior in Amorphous and Ceramic Coatings on Aluminum Alloy Substrate

AbstractFracture behavior in Ni-10 wt.%P amorphous (10 µm thick) and overlay Y2O3 stabilized ZrO2 (0-1.0 µm thick) ceramic coatings on Al alloy substrates has been investigated using small punch (SP) testing and acoustic emission (AE) techniques. SP tests showed that coating cracks extensively propagate along both the radial and tangential directions under the biaxial stress condition. The formation of cracks was more profoundly found in Ni-P amorphous coatings than in amorphous/ceramic coatings. The cumulative AE events were well correlated with the severity of coating cracking independent of the type of coatings. Ceramic coatings with a thickness of 0.25 and 1.0 µm led to a decrease in the fracture toughness by 15–20%, compared with a uncoated alloy. The coating effect on the fracture toughness is discussed in light of residual stresses induced during coating processing and the presence of coating cracks.

Mechanical properties of osteopenic vertebral bodies monitored by acoustic emission

A microdamage event, either bone microfracture or microcrack propagation, releases energy. Some of this energy is in the form of acoustic waves. We measured acoustic emission (AE) in normal and osteopenic vertebral bodies during compression loading to confirm the microdamage accumulation. The 2nd to 4th lumbar vertebrae taken from the embalmed cadavers of a 37-year-old male and a 75-year-old female were used. Bone mineral densities (BMDs) were measured by dual energy X-ray absorptiometer (DEXA). The male vertebrae had normal BMDs, while the female vertebrae had lower BMDs than the normal range and were graded osteopenic. Mechanical parameters (maximum load, maximum stress, stiffness, strain at maximum load, and apparent elastic modulus) and AE event count rates in the load-deformation curve were measured during quasi-static compression loading (deformation rate 0.1 mm/min). For all mechanical parameters, the normal vertebrae had higher values than the osteopenic vertebrae. Cumulative AE event counts until maximum load of the osteopenic vertebrae were much greater than that of normal vertebrae. The vertebrae which were well compressed to the plastic range in the loaddeformation curve displayed dome-shaped fracture lines just above the end plates. These results are consistent with the hypotheses that microcracks of osteopenic vertebral bodies are generated and accumulate at lower strains than those of normal vertebrae at a specific site.

Acoustic emission and fracture of Carbon Fiber Reinforced Thermosoftening Plastic (CFRTP) materials under monotonous tensile loading

The quantitative relationships between the parameters of fracture mechanics and acoustic emission (AE) are investigated using two typical carbon fiber reinforced thermosoftening plastics (CFRTP) with evident differences in AE characteristics when undergoing monotonous tensile deformation. It is shown that the relation between cumulative AE events and the stress intensity factor does not hold well for the experimental results. The reason for this is that there exists a wide distribution of AE signal levels in composites. Therefore, a formula for the cumulative AE energy E and the stress intensity factor K is proposed as E = CK m . This gives a good result for the experiments in tensile deformation. Since the AE signal attenuation affects the cumulative AE energy, it is discussed for the proposed formula. It is found that the attenuation of AE signals has an influence on the coefficient C but not on the exponent m.

Fracture Mechanisms of Short-Fiber-Reinforced PETby Acoustic Emission Method. (Effects of Fiber Content and Loading Direction).

The fracture mechanisms of short-fiber-reinforced PET (FRPET) are studied by the acoustic emission (AE) method to examine the effects of fiber content and loading direction. The maximum loads of test specimens increase with increase in the fiber content, and the loads decrease with increase in the angle between the directions of injection and loading. The cumulative AE event count and the total AE energy are affected by the fiber content and the loading direction. The effects of the fiber content and the loading direction on the total AE energy can be explained by AE amplitude distribution. The loads of Pb and Pc determined by the stress intensity factor method are in agreement with the loads determined by the AE energy gradient method. Damage is generated by the debonding between fiber and resin at the load point of Pb and by the fiber breaking at the load point of Pc in materials with fiber content of 15 and 30% by weight.

ACOUSTIC EMISSION CHARACTERIZATION OF A GLASS-MATRIX COMPOSITE

Abstract Acoustic emission (AE) analyses have been performed on a high compliance, high failure strain randomly oriented carbon reinforced glass matrix composite. The composite exhibits a unique nonlinear stress-strain behavior with high failure strain during both uniaxial tensile and flexural testing. The extensive matrix cracking of this material during loading provides an excellent opportunity to study the fracture mechanism. In this study, two types of composites using similar constituents but different processing procedures resulting in different mechanical behavior, were studied. One composite showed a very pronounced non-linear stress-strain behavior during tensile testing. The curve consists of an initial linear portion, followed by a deviation from linearity and a large amount of longitudinal strain is produced with very little increase in the stress. The second composite did not show similar behavior with the slope of the stress-strain decreases continuously. The apparent different mechanical behavior of the two composites provided an excellent opportunity to use the AE characterization to distinguish two different fracture processes. This study concentrates on monitoring damage initiation and damage progression during tensile and flexural testing using acoustic emission sensors. Detailed examination of AE characterization, including cumulative AE events, event occurrence and its rate as a function of load, and amplitude distribution histograms of events at different load levels, reveals a different emission activities during tensile and flexural loading conditions, indicating different fracture patterns. The examination of amplitude distributions at various loads revealed that the method is sensitive enough to distinguish various fracture processes.

Acoustic emission responses from cyclic loading of a nuclear graphite

An analysis is presented of acoustic emission from samples of a nuclear graphite, IMI-24, subjected to three consecutive, load-unload cycles to 5, 10, and 15 MPa, in tension, compression, and flexure. On the first cycle, cumulative acoustic emission events in all loading modes increased progressively from zero stress. However, in subsequent cycles, acoustic emission recurred at stresses approaching, but less than, the previous peak stresses (i.e., a Felicity effect was observed rather than a Kaiser effect). A new parameter, the Recovery ratio, B, is proposed for characterizing the pattern of acoustic emission on cyclic loading of graphites.

A study of Fracture Mechanisms of Short Fiber Reinforced PET by Acoustic Emission Method. Effects of Notch Length and Fiber Content.

Effects of notch length and fiber content on the tensile fracture behavior of short fiber reinforced polyethylene terephthalate (FRPET) are studied using the acoustic emission (AE) method. The cumulative AE event count and the total AE energy increase with increases in the notch length. The total AE energy decreases with increases in the fiber content, but the cumulative AE event count has a maximum value at a fiber content by volume of 17.3%. The log-log plot of the relationship between the total AE energy and the stress intensity factor has two broken points at Kb and KC which correspond to the changes in failure mode, respectively. The ratios of Pb/Pmax are varied, and Pc/P<max> values are roughly constant at the various notch lengths and fiber contents, where the Pb and Pc are loads determined by Kb and Kc, and Pmax is the maximum load. The relationship between the damage state at the notch tip and the AE frequency spectrum determines the fracture mechanisms of FRPET at the applied loads of Pb/Pmax and Pc/Pmax.

Monitoring concrete reinforced with bonded surface plates by the acoustic emission method

Abstract Acoustic emission (AE) was studied in stressed structural beams composed of internally reinforced concrete with additional external reinforcement from thin bonded plates. A control beam without a bonded plate was compared to four beams having plates composed of carbon fibre composite, glass fibre composite, aramid fibre composite and steel. The dependence of acoustic emission on beam stiffness is described as well as a relation of the number of AE energy peaks to the number of visible cracks in the concrete. The Kaiser effect was observed in repeated loadings for all tested beams. Attenuation was measured to a distance of 2.75 m and was not found to be a limitation in acquiring useful data. The change in event rate and the energy above threshold were closely related to the number of cracks visible at the end of the test. The event rate consistently decreaed as the number of loading cycles increased. The results suggest that an examination of cumulative acoustic emission events and energy as a function of load can assist in analysing the cracking behaviour, loading history and stiffness of concrete beams externally reinforced with bonded surface plates.

Acoustic emission characterization of the fracture mechanism of a high compliant, glass-matrix composite

Reinforcing brittle glass-matrix with discontinuous carbon fibers provides a high compliant, high failure strain glass-matrix composite with a unique non-linear stress-strain behavior for both uniaxial tensile and flexural testings due to the extensive matrix cracking. The tensile strength of this material was less than that of the flexural test. The difference could be attributed to the shift of the neutral axis of the beam during flexural loading due to the matrix cracking. An analytical model was established to describe the mechanical response of the composite. Acoustic emission (AE) analyses were performed on this material during both tensile and flexural testings to study the damage initiation and progression. Data recorded included cumulative AE events, counts, and count rates as a function of load, and amplitude distribution histograms of events at different load levels. It was shown that the acoustic emission signal patterns were very different during tensile and flexural loading conditions, indicating different fracture patterns. The examination of amplitude distributions at various loads revealed that the method was sensitive enough to distinguish various fracture processes.