Ultrasonic inspection of studs (bolts) using dynamic predictive deconvolution and wave shaping

Bolt degradation has become a major issue in the nuclear industry since the 1980's. If small cracks in stud bolts are not detected early enough, they grow rapidly and cause catastrophic disasters. Their detection, despite its importance, is known to be a very difficult problem due to the complicated structures of the stud bolts. This paper presents a method of detecting and sizing a small crack in the root between two adjacent crests in threads. The key idea is from the fact that the mode-converted Rayleigh wave travels slowly down the face of the crack and turns from the intersection of the crack and the root of thread to the transducer. Thus, when a crack exists, a small delayed pulse due to the Rayleigh wave is detected between large regularly spaced pulses from the thread. The delay time is the same as the propagation delay time of the slow Rayleigh wave and is proportional to the site of the crack. To efficiently detect the slow Rayleigh wave, three methods based on digital signal processing are proposed: wave shaping, dynamic predictive deconvolution, and dynamic predictive deconvolution combined with wave shaping.     

Metallurgical failure analysis of fasteners in an impeller assembly

This paper examines a failure analysis of the bolts from a failed joint between an impeller blade and a rotating assembly unit. The bolts failed due to poor thread manufacture and installation. Sharpened thread roots led to high stress concentrations that favored crack initiation. An oddly shaped thread profile allowed friction between mismatched thread surfaces. Poor installation procedures allowed for the possibility of overtightening to nucleate cracks in the head-to-shank interface (which had a smaller radius and therefore a higher stress concentration) and possibly also in the thread roots. Each of these influences contributed to crack initiation in the bolts. After cracks had formed, bending fatigue then propagated the nucleated cracks to final fracture. The failure analysis also recommended using bolts with rolled threads, which allow a more complete fit between mating male and female threads, and assuring that an appropriate preload is placed on bolts during installation.     

Noncontact, Nondestructive Evaluation of Realistic Cracks with Surface Acoustic Waves by Scanning Excitation and Detection Lasers

Nondestructive evaluation of surface-breaking cracks with a scanning laser source, a scanning laser probe, or a scanning laser pump–probe setup is discussed. Multimode scattering of laser-excited surface acoustic wave pulses by artificial slots, realistic fatigue, or impulsive cracks is considered. This includes measuring the size of cracks in the micrometer-to-millimeter range by optical recording of the complete displacement or velocity field around the crack. Results obtained with a scanning pump–probe setup for a partially closed microcrack, generated by an elastic shock pulse in silica, are compared with those achieved with a scanning source or scanning probe for machined open notches. Crack size analysis based on the frequency spectrum of the reflected Rayleigh wave and the time lag of the transmitted Rayleigh wave is discussed. Signal enhancement effects observed in the displacement and velocity field near the crack are studied.     

Simulations of dynamic crack propagation in brittle materials using nodal cohesive forces and continuum damage mechanics in the distinct element code LDEC

Experimental data indicates that the limiting crack speed in brittle materials is less than the Rayleigh wave speed. One reason for this is that dynamic instabilities produce surface roughness and microcracks that branch from the main crack. These processes increase dissipation near the crack tip over a range of crack speeds. When the scale of observation (or mesh resolution) becomes much larger than the typical sizes of these features, effective-medium theories are required to predict the coarse-grained fracture dynamics. Two approaches to modeling these phenomena are described and used in numerical simulations. The first approach is based on cohesive elements that utilize a rate-dependent weakening law for the nodal cohesive forces. The second approach uses a continuum damage model which has a weakening effect that lowers the effective Rayleigh wave speed in the material surrounding the crack tip. Simulations in this paper show that while both models are capable of increasing the energy dissipated during fracture when the mesh size is larger than the process zone size, only the continuum damage model is able to limit the crack speed over a range of applied loads. Numerical simulations of straight-running cracks demonstrate good agreement between the theoretical predictions of the combined models and experimental data on dynamic crack propagation in brittle materials. Simulations that model crack branching are also presented.     

Depth measurement for cracks in corners using ultrasonic Rayleigh waves

The scattering of ultrasonic Rayleigh waves incident normally on corners containing cracks is considered by using elastodynamic ray theory. Detailed calculations are presented for vertical and horizontal cracks in right-angle corners in aluminium. It is shown that crack depth can be measured simply from the spacing of interference fringes in the high-frequency spectra of either the back- or forward-scattered Rayleigh waves, given only a knowledge of the Rayleigh wave speed. Use of the back-scattered wave is preferable because its fringes show stronger modulation, and because an experiment requires a single transducer and access to only one face of the specimen. The technique is applicable without modification to the more general case of a crack at any angle in a corner of any angle.     

The effect of manufacturing processes on the fatigue lifetime of aeronautical bolts

Many aeronautical fastners are exposed to cyclic stresses during service. Therefore, such parts are usually designed for limited fatigue lifetime. Various combinations of process type and sequence may be employed to produce threads, each resulting in different fatigue properties. Specifications of aircraft bolts often require production of threads by heat treatment followed by rolling, in order to improve the fatigue properties. Unfortunately, these specifications are not always followed to the letter. Therefore, for either quality assurance or failure analysis purposes, it is important to be able to determine unambigiously the process by which threads were produced. The objectives of this work were to study the effect of varied thread manufacturing process type and sequence on the mechanical properties of AISI 4340 stud bolts, and to develop a laboratory procedure for distinguishing between them. Threads were produced on heat-treated and non-heat-treated stud bolts either by machining or cold-rolling. The non-heat-treated bolts were subsequently heat-treated. All bolts were then subjected to mechanical testing (static tensile, dynamic fatigue, hardness and microhardness tests), metallographic and fractographic examinations. While the fatigue properties were significantly affected by the manufacturing process used, no effects on the tensile strength of the bolt were observed. Metallographic inspection and microhardness testing, but not fractographic inspection, were found to be effective for distinguishing between different manufacturing procedures.     

Rayleigh wave interaction with,and the extension of,microcracks

Intense Rayleigh waves produced by the impact of high-velocity liquid jets on brittle solids were arranged to interact with well-defined surface flaws of dimensions 50 to 200 μm. The extent of crack growth was monitored as a function of distance from the impact site. It was found that considerable crack growth as well as crack branching occurred for cracks parallel to the incident wavefront and little or no growth for orthogonal cracks. The form of the surface wave was monitored using piezoelectric crystals attached to the surface. The results are discussed in terms of recent fracture mechanics analysis of stress-wave interaction with cracks. The significance of this study to strength degradation of brittle bodies subjected to rain-drop impact is pointed out.     

Surface acoustic wave measurements of surface cracks in ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.     

Effect of loading and geometry on the subsonic/intersonic transition of a bimaterial interface crack

An experimental investigation was conducted to study the nature of intersonic crack propagation along a bimaterial interface. A single edge notch/crack oriented along a polymer/metal interface was loaded predominantly in shear by impacting the specimen with a high velocity projectile fired from a gas gun. The stress field information around the propagating crack tip was recorded in real time by two different optical techniques--photoelasticity and coherent gradient sensing, in conjunction with high speed photography. Intersonic cracks on polymer/metal interfaces were found to propagate at speeds between the shear wave speed (c_s) and of the polymer. The nature of the crack tip fields during subsonic/intersonic transition and the conditions governing this transition were examined. Experimental observations showed the formation of a crack face contact zone as the interfacial crack speed exceeds the Rayleigh wave speed of the polymer. Subsequently, the contact zone was observed to expand in size, shrink and eventually collapse onto the intersonic crack tip. The recorded isochromatic fringe patterns showed multiple Mach wave formation associated with such a scenario. It is found that the nature of contact zone formation as well as its size and evolution differ substantially depending on the sign of the opening component of loading.     

Process region influence on energy release rate and crack tip velocity during rapid crack propagation

A finite element model of a plate with an edge crack is investigated. A cell model of the material, with the cell size representing some characteristic intrinsic material length, is adopted. The size of the process region depends on the number of cells that have reached a state which is unstable at load control. The results show that the growth of the process region is a main factor responsible for the lack of a unique relation between the small scale yielding energy release rate and the crack tip velocity and also for the observed constant crack velocities that are significantly below the Rayleigh wave velocity. A rapidly propagating crack appears to meet an increase of the energy flow to the crack edge per unit of time by increasing the size of the process region rather than increasing its edge velocity.     

Analysis of Slide Gate Mounting Bolt Failure in Steel Ladle Due to Fatigue

The mounting bolt failed during slide gate operation of a steel ladle was investigated. Metallurgical analysis confirms the bolt as IS 1367 10.9 grade high-tensile 42CrMo₄ steel. On comparative study with a good un-fractured sample, a striation of fine banded ferrite in pearlite matrix was revealed in failed bolt under optical microscopy. The bolts failed due to fatigue and crack initiated from the surface of machined bolt threads. Comparatively lower hardness, low UTS associated with lower %Cr and %Mo content found to aggravate premature failure of bolts during ladle operation. Microstructure of un-fractured sample found with tempered bainite phase. The fatigue failure of bolts occurred due to repetitive nature of shear force development during steel pouring through slide gate system. Preventive measures to reduce fatigue failure of the mounting bolts are proposed.     

Subsurface crack determination by one-sided ultrasonic measurements

Corrosion of steel reinforcement in concrete is a common type of damage. The cracks propagate from the steel bar to the surface without giving any visual sign prior to surface crack formation. As long as the surface material is intact, the sensitivity of the longitudinal wave velocity to the subsurface cracks is doubtful. In this paper, cracks were created in steel fiber reinforced concrete specimens by four point bending. Wave measurements took place on the intact surfaces (compression side) using common acoustic emission transducers. Although there was no visual sign of the crack, Rayleigh as well as longitudinal wave velocities clearly decreased relative to those of the sound material. Other parameters like the amplitude and the experimental scatter of the waves were much more sensitive to damage. Numerical simulations were conducted in order to make a parametric study concerning the depth of the sound layer, the propagating wavelength and the measured wave parameters and propose a firm methodology. It is concluded that by scanning a surface with simple acoustic one-sided measurements, the identification of the location of the subsurface damage is possible, while the propagating wave gives information about the depth of the crack.     

Detection of the Subsurface Cracks in a Stainless Steel Plate Using Pulsed Eddy Current

The nondestructive method to detect subsurface defects is limited because conventional eddy current are concentrated near to the surfaces adjacent to the excitation coil. The PEC technique enables detection of cracks buried deeper under the surface with relatively small current density. In the present study, an attempt has been made to investigate detection of subsurface cracks using a specially designed double-D differential probe. The tested sample is a SS304 with a thickness of 5 mm; small EDM notches were machined in the test sample at different depths from the surface to simulate the sub surface cracks in a pipe. The designed PEC probe has two excitation coils and two detecting Hall-sensors. The difference between two sensors is the resultant PEC signal. The cracks under the surface were detected using peak amplitude of the detected pulse; in addition, for a clear understanding of the crack depth, the Fourier transform is applied. In time domain, the peak amplitude of the detected pulse is decreased, and in the frequency domain, the magnitude of the lower frequency component has been increased with an increase in the crack depth. The experimental results have indicated that the proposed differential probe has the potential to detect the sub surface cracks in a stainless steel structure.     

基于摄动法的多条裂纹欧拉梁特征模态分析

基于摄动理论推导了带多条开口裂纹的欧拉梁的特征模态参数的理论计算公式。采用最直接的方式将梁开口裂纹模拟成梁微段内的横截面折减并用δ函数表达了带开口裂纹的梁沿轴线的截面惯矩和线质量等物理参数。基于此,建立了裂纹梁动力微分方程,并采用一阶摄动理论推导得到了梁的模态频率和振型计算公式。简支梁及悬臂梁算例研究表明,该方法具有很好的精度,与有限元模拟结果及实验结果都能很好地吻合。并采用此方法分析了裂纹深度和位置对带多条开口裂纹梁的特征模态参数的影响。结果表明,裂纹对各阶模态频率虽然影响有限,但其引起的各阶频率变化有着明显的模式,可用于结构损伤定位;裂纹对模态振型影响不明显,但对模态曲率影响比较大,可用于结构损伤位置和程度的诊断。     

Quantitative acoustic emission source characterization of microcrackings in steel

Acoustic emission (AE) source wave analysis is a new NDE technique for the investigation of dynamic fracture process. We applied this technique to the quantitative characterization of crack sources in ductile fracture. Using two samples of ASTM A533B steel with different sulfur content, acoustic emissions during fracture toughness tests were detected, located, and analyzed. The detected AE signals were classified into two types according to the analyzed source waveforms. One was a signal due to microcracking at the MnS inclusion, and the other was a signal due to coalescence of the voids. The results of the source wave analysis showed that microcracking at the inclusions was due to Mode I type tension crack with sizes of 10–30 µm, and the coalescence of the voids was due to tension shear mixed cracks with sizes of 60–100 µm. It was confirmed that this technique is very effective for the quantitative evaluation of microcrackings and for the detection of the nucleation and growth of cracks.     

Microwave Detection of Stress-Induced Fatigue Cracks in Steel and Potential for Crack Opening Determination

Fatigue crack detection in metals is an important practical issue in many industries. In this paper the results of detecting fatigue cracks, using the dominant mode approach, employing flange-mounted, open-ended, rectangular waveguides at several microwave frequencies are presented. The goal of this investigation has been to demonstrate the capability of this approach for detecting stress-induced cracks under various static loads. In addition, a correlation between the features of the measured crack characteristic signals and crack opening has been sought. The results show that at all of the investigated frequencies, cracks from being nearly closed to having openings of up to 0.0508 mm are detected effectively. Furthermore, it is found that the interaction of the flange edge with a crack results in features that can be used to enhance crack detection robustness significantly (i.e., increased probability of detection). Several features associated with these measured crack characteristic signals are shown to correlate linearly with crack opening. Such simple correlations may then be used to estimate a crack opening closely after it has been detected using this approach. A complete discussion of the results is also provided in this paper.     

Relationships between crack opening displacement,alloy variables,and crack detection sensitivity

Alloy grain size is known to influence both the propagation path and the closure stress of surface fatigue cracks in many alloys. The general trend is for the path to be more tortuous and the closure stress to be larger, the larger the grain size. By use of Ti Al-4V and Al 7075-T6, the effects of grain size on the nondestructive detection of surface cracks which might arise from closure stress and path irregularity variables were evaluated. Titanium specimens were inspected using an acoustic harmonic generation technique, and it was discovered that the major source of harmonic signals was grain sized crystallographic cracks. Harmonic signals were larger during fatigue in an 8-µm compared to a 4-µm grain sized alloy, as there were more grain sized cracks in the large grain material. Crack closure was found to be extremely important in determining the reflected acoustic amplitude obtained in inspecting small (100–1000 µm) cracks in Al 7075-T6 using a critical angle technique. Average received amplitudes were an order of magnitude smaller for cracks at zero load than for those opened by a tensile stress. The scatter in the reflected amplitude was also large, apparently as the result of variations in the degree of the closure from crack to crack. For the 7075 material, the important effect of larger grain size was to increase the irregularity of the crack path, making the small cracks more visible acoustically at azimuthal angles not normal to the crack plane.     

Microwave Detection of Stress-Induced Fatigue Cracks in Steel and Potential for Crack Opening Determination

Abstract

Fatigue crack detection in metals is an important practical issue in many industries. In this paper the results of detecting fatigue cracks, using the dominant mode approach, employing flange-mounted, open-ended, rectangular waveguides at several microwave frequencies are presented. The goal of this investigation has been to demonstrate the capability of this approach for detecting stress-induced cracks under various static loads. In addition, a correlation between the features of the measured crack characteristic signals and crack opening has been sought. The results show that at all of the investigated frequencies, cracks from being nearly closed to having openings of up to 0.0508 mm are detected effectively. Furthermore, it is found that the interaction of the flange edge with a crack results in features that can be used to enhance crack detection robustness significantly (i.e., increased probability of detection). Several features associated with these measured crack characteristic signals are shown to correlate linearly with crack opening. Such simple correlations may then be used to estimate a crack opening closely after it has been detected using this approach. A complete discussion of the results is also provided in this paper.     

Cracking due to multiple indentations in silicon: effect of indentation spacing and relative orientations

The issue of multiple cracks in materials and their interaction is central in understanding the overall fracture behavior of materials. In the case of materials used in the microelectronics industry, indentation cracking has been extensively used for the measurement of fracture toughness due to its small sample size requirements as well as a relatively good correlation with values obtained from traditional fracture mechanics tests. The majority of these studies have focused on the fracture behavior of a single indent. The present study was aimed at understanding the effect of interaction between the cracks generated on Si from a pair of sequential indents as well as a set of four sequential indents placed at the corners of a square. The distance between the indents was varied from a level comparable to the crack size to a level where interaction could be ignored. This paper discusses the changes in the nature as well as the sizes of cracks due to interaction between the stress fields of the indents.     

Automatic Crack Detection and Characterization During Ultrasonic Inspection

The creation of a non-destructive technique that enables the automatic detection of defects is desirable, and TOFD (Time-Of-Flight Diffraction) technique is gaining rapid prominence due to its high accuracy in detecting, positioning and sizing flaws in steel structures. In this type of imaging, cracks are characterized using sets of hyperbolas, where summit positions correspond to crack tip positions. However, ultrasonic diffracted signals are often too low and difficult to distinguish from noise, and when large structures are inspected, the quantity of data can be extremely large, with the area of interest being very small in comparison to the image size. This paper describes a method that avoids the image formation, replacing it with a sparse matrix (as there is no reason to store and operate on an excessive number of zeros), and automates crack detection by analyzing the curve formed by the sparse matrix elements. The sparse matrix is formed using Split-Spectrum Processing, which enhances the signal-to-noise ratio. The Randomized Hough transform is then applied on the sparse matrix elements to detect the hyperbolas that characterize the crack defects.