Fatigue Crack Growth Characterisation of RAFM Steel using Acoustic Emission Technique (original) (raw)
Related papers
Fatigue and fracture assessment of cracks in steel elements using acoustic emission
2011
Single edge notches provide a very well defined load and fatigue crack size and shape environment for estimation of the stress intensity factor K, which is not found in welded elements. ASTM SE(T) specimens do not appear to provide ideal boundary conditions for proper recording of acoustic wave propagation and crack growth behavior observed in steel bridges, but do provide standard fatigue crack growth rate data. A modified versions of the SE(T) specimen has been examined to provide small scale specimens with improved acoustic emission(AE) characteristics while still maintaining accuracy of fatigue crack growth rate (da/dN) versus stress intensity factor (∆K). The specimens intend to represent a steel beam flange subjected to pure tension, with a surface crack growing transverse to a uniform stress field. Fatigue test is conducted at low R ratio. Analytical and numerical studies of stress intensity factor are developed for single edge notch test specimens consistent with the experimental program. ABAQUS finite element software is utilized for stress analysis of crack tips. Analytical experimental and numerical analysis were compared to assess the abilities of AE to capture a growing crack.
ACOUSTIC EMISSION MODEL OF FATIGUE CRACK IN LOW-CARBON STEEL
Transstellar Journal , 2019
One of the problems of the acoustic emission (AE) testing method is the parametric uncertainty of defect models. In this work, an empirical model of a fatigue crack is considered, which allows establishing quantitative relations between the parameters of a fatigue crack and AE data. The ambiguity in the interpretation of AE data is explained by the inhomogeneous stress-strain state of the material.
Using acoustic emission to understand fatigue crack growth within a single load cycle
Engineering Fracture Mechanics, 2018
Current methods for prediction of fatigue crack growth are based on empirical correlations which do not take the crack growth behaviour within a single cycle into account. To improve these prediction methods, more understanding of the physical mechanisms of crack growth is required. In this research the acoustic emission technique was used to investigate the crack growth behaviour during a single fatigue cycle. It was found that crack growth can potentially occur both during loading and unloading, but only while the strain energy release rate is above a crack growth (CG) threshold value. The results suggest this CG threshold value is the same in both quasi-static and fatigue loading. further work is necessary to fully understand the link between the received acoustic emission signals and the actual crack growth processes. Nevertheless, the paper shows the potential of acoustic emission to provide more insight into the physics of crack growth.
Ndt & E International, 1992
Acoustic emission (AE) is potentially an ideal technique for health monitoring of large structures due to the small number of sensors required and its high sensitivity. There has been much research conducted to characterize and provide qualitative understanding of the AE process in small specimens. Unfortunately, it is difficult to extend these results to real structures as the experimental data is dominated by geometric effects due to the small size of the specimens. The aim of this work is to provide a characterization of elastic waves emanating from fatigue cracks in plate-like structures. Fatigue crack growth was initiated in large 6082 T6 aluminium alloy plate specimens subjected to cyclic loading in the laboratory. A large specimen was used to eliminate signal reflections from the specimen edges and to enable signals from different wave modes to be separated in time. The signals were recorded using both resonant and non-resonant transducers attached to the surface of the specimens. Large numbers of AE signals were detected due to active fatigue crack propagation during the experiment. Analysis of experimental results from multiple crack growth events was used to characterize the modal and angular distributions of the radiated elastic waves. Experimental results are compared with finite element predictions to examine the mechanism of AE generation at the crack tip.
IRJET-Fatigue Crack Analysis Using Acoustic Emission
- The main objective of the paper is to discuss the use of Acoustic Emission technique to identify fatigues and cracks in different metals. The paper talks about acoustic emission (AE) and principles of AE testing. It gives information about ‘STRESS’ as a primary source of AE. Behavior of AE signals during Fatigue testing and micro-crack growth formation is given in the paper. Different types of AE testing methods for different materials are discussed for example, AE testing for detecting crack initiation for alluminium alloy, fatigue analysis for C40 and AISI 304 etc.
Fatigue & Fracture of Engineering Materials & Structures, 2007
An ultrasonic surface acoustic wave technique for studying the growth behaviour of small fatigue cracks is described. The technique allows crack depth and opening stress to be monitored continuously during the course of a fatigue test. Results are given for a 1740 MPa yield strength, silicon-modified, AISI 4340 steel tested under zero-to-tension cyclic loading. Good agreement is shown between acoustically determined crack depth and that measured by post-fracture optical microscopy. The monitoring of changing crack depth-to-surface length ratios during tests is also demonstrated. Acoustically determined crack opening stresses were found to be about 10% higher than values determined by measurements of crack tip opening displacements by scanning electron microscopy. Effects on crack growth of two different specimen surface preparations, electropolishing and diamond paste polishing, are also reported. Growth rates in electropolished specimens were as much as an order of magnitude higher than in diamond paste polished specimens which had a shallow but significant layer of compressive residual stress. NOMENCLATURE u/c = ratio of crack depth to half surface crack length C = crack front E = modulus of elasticity AK, A& = range of K , and of effective K , CTOD = crack tip opening displacement K , = mode I stress intensity factor N = number of cycles P = power input to acoustic transducer rb = reversed crack tip plastic zone size R = stress ratio = urn in/^,,, wb = beam width at transmitting transducer = crack reflection coefficient K = wave number (2n/I) J. = acoustic wavelength v = Poisson's ratio p = distance between origin of surface crack and tangent line to crack front u* = acoustic stress uOp = opening stress usat = saturation stress urnin, u,,, = minimum and maximum applied stress uCTOD = opening stress based on CTOD measurements Aueff = range of effective stress u; = cyclic yield strength @ = elliptic integral of second kind o : angular frequency of surface acoustic wave
The signatures of acoustic emission waveforms from fatigue crack advancing in thin metallic plates
Smart Materials and Structures
The acoustic emission (AE) waveforms from a fatigue crack advancing in a thin metallic plate possess diverse and complex spectral signatures. In this article, we analyze these waveform signatures in coordination with the load level during cyclic fatigue. The advancing fatigue crack may generate numerous AE hits while it grows under fatigue loading. We found that these AE hits can be sorted into various groups based on their AE waveform signatures. Each waveform group has a particular time-domain signal pattern and a specific frequency spectrum. This indicates that each group represents a certain AE event related to the fatigue crack growth behavior. In situ AE-fatigue experiments were conducted to monitor the fatigue crack growth with simultaneous measurement of AE signals, fatigue loading, and optical crack growth measurement. An in situ microscope was installed in the load-frame of the mechanical testing system (MTS) to optically monitor the fatigue crack growth and relate the AE signals with the crack growth measurement. We found the AE signal groups at higher load levels (75%-85% of maximum load) were different from the AE signal groups that happened at lower load levels (below 60% of load level). These AE waveform groups are highly related to the fatigue crackrelated AE events. These AE signals mostly contain the higher frequency peaks (100 kHz, 230 kHz, 450 kHz, 550 kHz). Some AE signal groups happened as a clustered form that relates a sequence of small AE events within the fatigue crack. They happened at relatively lower load level (50%-60% of the maximum load). These AE signal groups may be related to crack friction and micro-fracture during the friction process. These AE signals mostly contain the lower frequency peaks (60 kHz, 100 kHz, 200 kHz). The AE waveform based analysis may give us comprehensive information of the metal fatigue.
2017
The study of fatigue crack behavior in welded structures using AE technique during crack initiation and propagation is discussed in this paper. This was achieved both theoretically and experimentally using four point bending testing. A new averaging index known as AE intensity was used to analyze the fatigue crack initiation conditions in the specimen. A theoretical model based on acoustic emission energy rate was developed to determine the critical fatigue level and crack extension. The results were compared with fatigue crack growth using a non-contact non-destructive testing method known as the digital imaging correlation. It was realized that, AE technique was able to detect the unset of a crack at a length of 8 μm in welded connection and 10 μm in the base metal, this proved that, crack initiates faster in welded connection than base metal. Key words: Acoustic emission, fatigue, initiation, propagation, digital imaging correlation.