Ultrasonic inspection of long steel pipes using Lamb waves

An ultrasonic inspection technique using Lamb waves was evaluated to detect and determine the exact location of flaws present in long steel pipes. Since multiple modes of Lamb waves are generated due to their dispersive characteristics in the inspected pipes, the selection of a specific Lamb wave mode is very important for inspection of flaws. Experimental studies of flaw detectability with the use of each Lamb wave mode, namely, the A₀ S₀, A₁, and S₁ modes and their ultrasonic attenuation characteristics were conducted. The experimental results showed that the A₀ mode is the most effective for detection and exact determination of the location flaws. A lucite wedge containing a water column that generates the A₀ Lamb wave mode was developed and used in the present inspection study. It was found that the ultrasonic beam divergence after wrapping around the inspected pipe once interferes with exact determination of the location of flaws and that the maximum reflection signals are obtained when the transducer is offset axially from the straight line with the position of the flaw. The present study showed feasibilities of ultrasonic inspection with the use of Lamb waves for detection of flaws in insulated or inaccessible steel pipes.     

Non-contact estimation of thickness and elastic properties of metallic foils by laser-generated Lamb waves

Non-contact estimation of the thickness and elastic properties of metallic foils was attempted by quantitative analysis of velocity dispersion of laser-generated Lamb waves. Lamb waves were generated in stainless steel (AISI304) foils with a thickness of less than 40 μm by a Q-switched Nd:YAG laser. Both the zeroth order symmetric S₀ and anti-symmetric A₀ waves were monitored using a heterodyne-type laser interferometer. Dispersion of group velocity of the A₀ mode was obtained by the wavelet transformation, and was found to agree well with the numerical solution of the Rayleigh–Lamb equation. A modified method to estimate both the thickness and acoustic (or elastic) properties from the sheet wave velocity and the group velocity dispersion of the A₀ mode was proposed. The modified method was found to provide a correct estimate for stainless steel foils thinner than 30 μm.     

Interface fracture toughness and fracture mechanisms of thermal barrier coatings investigated by indentation test and acoustic emission technique

Interface fracture toughness and fracture mechanisms of plasma-/sprayed thermal barrier coatings (TBCs) were investigated by interfacial indentation test (IIT) in combination with acoustic emission (AE) measurement. Critical load and AE energy were employed to calculate interface fracture toughness. The critical point at which crack appears at the interface was determined by the IIT. AE signals produced during total indentation test not only are used to investigate the interface cracking behavior by Fast Fourier Transform (FFT) and wavelet transforms but also supply the mechanical information. The result shows that the AE signals associated with coating plastic deformation during indentation are of a more continuous type with a lower characteristic frequency content (30-60 kHz), whereas the instantaneous relaxation associated with interface crack initiation produces burst type AE signals with a characteristic frequency in the range 70-200 kHz. The AE signals energy is concentrated on different scales for the coating plastic deformation, interface crack initiation and interface crack propagation. Interface fracture toughness calculated by AE energy was 1.19 MPam_1/2 close to 1.58 MPam_1/2 calculated by critical load. It indicates that the acoustic emission energy is suitable to reflect the interface fracture toughness.     

Acoustic Emission Methodology to Evaluate the Fracture Toughness in Heat Treated AISI D2 Tool Steel

In this article, fracture toughness behavior of tool steel was investigated using Acoustic Emission (AE) monitoring. Fracture toughness (K _IC) values of a specific tool steel was determined by applying various approaches based on conventional AE parameters, such as Acoustic Emission Cumulative Count (AECC), Acoustic Emission Energy Rate (AEER), and the combination of mechanical characteristics and AE information called sentry function. The critical fracture toughness values during crack propagation were achieved by means of relationship between the integral of the sentry function and cumulative fracture toughness (KICUM). Specimens were selected from AISI D2 cold-work tool steel and were heat treated at four different tempering conditions (300, 450, 525, and 575?°C). The results achieved through AE approaches were then compared with a methodology proposed by compact specimen testing according to ASTM standard E399. It was concluded that AE information was an efficient method to investigate fracture characteristics.     

Wave propagation in thin Plexiglas plates: implications for Rayleigh waves

Two-dimensional (2D) experimental models are often used to study wave propagation problems. The advantages of using 2D experimental models, as opposed to 3D models, is the reduction of both extraneous reflections and mathematical complexity. Further, many structural elements conform to this geometry. The following study examines Rayleigh wave motion in thin Plexiglas sheets. Source–receiver time domain measurements were made at different locations on the Plexiglas sheet. The time–distance space was 2D-Fourier transformed into the frequency–wavenumber space to facilitate the analysis of wave modes propagating in the Plexiglas sheet. Experimental results showed that fundamental symmetric (S₀) and antisymmetric (A₀) Lamb waves propagated through the plate. Along the thickness of the plate, a non-dispersive Rayleigh wave was generated. Lamb waves were found to interfere with the Rayleigh wave. The assumption of generalized plane stress is preserved if higher mode Lamb waves have low energy content.     

Monitoring of the moisture content in carbon-epoxy plates using Lamb waves

Non-destructive material health monitoring is of great importance for aerospace industries, in particular for tanks made of composite materials, the mechanical properties of which can be altered by hydrothermal environment. In this context, the sensitivity of Lamb waves to various levels of moisture in a carbon-epoxy plate has been investigated. The plate has been manufactured and submitted to different cycles of hydrothermal aging and drying. For various steps of these cycles, both changes in weight and ultrasonic data are measured. Low-order A₀, S₀, S₁ Lamb modes are generated and detected using air-coupled transducers, and signal processing allows both real and imaginary parts of the wave-numbers to be measured. The real parts of the wave-numbers of all the generated modes are shown to be not sensitive to the moisture level of the material. However the imaginary part of the wave-number, i.e. the attenuation, of the A₀ mode is very sensitive to the moisture content, and its changes follow those of the plate weight, thus making it a promising mode for NDT of composite materials used in humid environments.     

Quantitative assessment of through-thickness crack size based on Lamb wave scattering in aluminium plates

The interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analysed in terms of finite element method and experimental study. For oblique-wave incidence, both numerical and experimental results showed that the wave scattering from a crack leads to complicated transmission, reflection and diffraction accompanied by possible wave-mode conversion. A dual-PZT actuation scheme was therefore applied to generate the fundamental symmetrical mode (S₀) with enhanced energy to facilitate the identification of crack-scattered wave components. The relationship between crack length and the reflection/transmission coefficient obtained with the aid of the Hilbert transform was established, through which the crack length was quantitatively evaluated. The effects of wavelength of Lamb waves and wave diffraction on the properties of the reflection and transmission coefficients were analysed.     

Air coupled Lamb waves evaluation of the long-term thermo-oxidative ageing of carbon-epoxy plates

Thermo-oxidative ageing of carbon-epoxy plates has been done for up to 3500 h at two temperatures. Non-destructive evaluation by means of non-contact ultrasonic methods has been tested to characterize the aged plates. Experimental results obtained using Lamb waves propagation in the plates are presented here. Experimental and theoretical dispersion curves are compared. It is shown that the phase velocity of the A₀ and S₀ Lamb modes is decreasing when ageing duration increases. Assuming the plate to be homogeneous and orthotropic, we demonstrate that this phenomenon is related to an important decrease of the C₅₅ elastic parameter of the plate.     

Application of signal processing techniques to ultrasonic testing of plates by S0 Lamb wave mode

In recent years, much attention has been paid to the use of Lamb waves for structural health monitoring. This choice is prompted by the high speed of Lamb wave inspection, although their dispersive nature can complicate the interpretation of results, especially when dealing with closely-spaced reflectors. In this paper, the objective is to improve the time resolution and signal-to-noise ratio of signals obtained from inspection of plates by the S₀ Lamb mode. The signal processing scheme used is based on deconvolution of the measured signal by Wiener filtering, followed by autoregressive spectral extrapolation. The deconvolution technique is applied to signals obtained from finite element models and also to experimentally measured signals; both sets of data are based on plates with various types of notch discontinuities. Using this technique, the separation distance between adjacent notches was estimated with high accuracy in both simulated and experimental ultrasonic signals.     

FE modeling of Lamb mode diffraction by defects in anisotropic viscoelastic plates

A Finite Element code is used in a stationary mode to compute, at a single frequency, the stress and displacement fields in plates made of anisotropic, viscoelastic materials. An appropriated spatial load at one boundary of the plate is applied to generate a guided mode. This step is included in a frequency loop for which the number of iterations (≈ usually less than 50) is defined by the frequency spectrum of a temporal excitation. Then the temporal response at any location in the plate can be reconstructed with inverse Fourier transform. The phase velocity, attenuation and nature of one or several propagating modes can be identified in the classical frequency/wave number representation. The first example concerns the interaction of the symmetric Lamb mode S₀ with an opening notch in a Perspex plate. The proportion of S₀ and converted antisymmetric A₀ mode, transmitted past the notch is evaluated by the FE method and validated by a comparison with measurements made on a real system using an air coupled transducer. The second example shows the effect of the viscoelasticity on the propagation of the symmetric Lamb mode S₀ in a cross-ply, carbon-epoxy material plate. Focus is made on some difficulties that the attenuation causes on the detection of a delamination between plies.     

Einfluß von Kohlenstoff,Stickstoff und Phosphor auf die interkristalline Korrosion und Spannungsrißkorrosion des Eisens

Effects of carbon, nitrogen and phosphorous on the intergranular corrosion and the stress-corrosion cracking of iron Experiments without stress, with constant load, or with constant strain rate were performed in boiling 55% Ca(NO₃)₂ solution (115°C) to elucidate the effects of carbon, nitrogen and phosphorous on the intergranular corrosion (IC) and stress-corrosion cracking (SCC) of iron. The original material contained 20 to 40 ppm C, 17 ppm N, and 20 ppm P. One batch of this material was normalized only, a second batch was also decarburized up to under 10 ppm C prior to normalizing, and a third batch was decarburized, and then nitrided up to 140 to 220 ppm N prior to normalizing. Some of the specimens were tempered at 550°C for one month. All normalized specimens showed no susceptibility to IC at applied potentials from 800 to 1000 mV (SHE). In contrast, specimens tempered at 550°C were very susceptible to IC, which was observed even at 200 mV after decarburization. Auger-spectra of intergranular fracture surfaces of a tempered specimen produced at liquid-nitrogen temperature revealed grain boundary segregation of P (about 10 atomic %). The as-normalized specimens fractured intergranularly within 1 to 2 h, when held at constant load of 0.8 and 0.9 of ultimate tensile strength (UTS), and 0 mV. By contrast, under identical conditions, the decarburized specimens and the nitrided specimens did not fracture in 30 d. The fracture energy tested with the constant strain-rate method at 0 mV, as compared with experiments in oil, was strongly lowered in the as-normalized specimens and significantly lowered in the nitrided specimens. The values of fracture energy obtained for the decarburized specimens were mostly similar to those in oil at 115°C. Only some of the decarburized specimens showed substantial decreases in fracture energy. However, the decarburized and tempered specimen showed an even larger decrease in fracture energy, with intergranular fracture mode.     

P92钢高温断裂韧性的试验

文中以P92耐热钢为研究对象，研究了不同尺寸的开侧槽和不开侧槽的紧凑拉伸试样在630 °C下的断裂韧性，得到了相应的的阻力曲线及断裂韧度J_Q. P92钢在高温下为典型的韧性断裂机制. 基于三维有限元计算对侧槽的拘束效应进行表征，结果表明，侧槽可明显提高试样的拘束水平，试样尺寸越小，J阻力曲线差异越明显. 随载荷增大，非侧槽试样的拘束变化更明显，开侧槽将导致试样阻力曲线不同. 试样尺寸及结构的改变，对韧性材料的阻力曲线影响较大，而对断裂韧度值影响较小. 试样开侧槽之后裂纹扩展更平齐，可优化断裂韧性试验过程.     

Impact properties of spark sintered titanium aluminides at elevated temperatures

The two kinds of spark sintered titanium aluminides, series 1 (70% TiAl, 30% TiAl/Ti₃Al lamella) and series 2 (10% TiAl, 90% TiAl/Ti₃Al lamella) specimens, having different volume fraction of the TiAl and TiAl/Ti₃Al lamellar grains at or near the notch tip, are prepared for Charpy impact testing. These tests are conducted in air at various temperatures between 293 and 1373 K. Charpy impact value increases monotonously as temperature rises and its value is 2.0 to 8.1 kJ/m², regardless of microstructural differences in the vicinity of the notch tip. However, the value (0.25×10⁻³m) of the deflection to failure is very low, even in the result obtained from the highest temperature (1373 K). On the other hand, the fracture load or maximum fracture load tends to increase as the test temperature is raised to 793 and 873 K in the series 2 and 1 specimens, respectively, and thereafter it decreases significantly. The maximum fracture load of the series 1 specimen is higher than that of the series 2 specimen throughout the test temperatures. The predominant mode of failure changes from transgranular fracture to intergranular fracture as the temperature is increased, which corresponds with the behaviour of the maximum fracture load.     

Study of Cr2O3 coatings Part II: Adhesion to a cast-iron substrate

The interfacial indentation technique for determining toughness is applied to plasma-sprayed Cr₂O₃ coatings. In this investigation, another adhesion test, i.e. the four-point bend test coupled with acoustic emission (AE), is performed. AE is monitored during the test from the initiation of load application until fracture in order to detect the damaging of the coating and to identify the different crack growth proc-esses. The residual stresses of coatings are determined by a step-by-step hole drilling method. Correlation of the residual stresses and the two determined parameters to failure (crack length in the case of indenta-tion test and failure displacement in the case of bending test) is discussed.     

Hydrogen damage and delayed fracture in bulk metallic glass

Initiation, propagation, arresting and breaking of hydrogen blistering and hydrogen-induced delayed fracture (HIDF) under sustained load in a bulk metallic glass of Zr_41.2Ti_13.8Ni₁₀Cu_12.5Be_22.5 have been investigated. Results show that when the current density, i, is smaller than 20 mA/cm² corresponding to total hydrogen concentration of 3310 wppm, there are no hydrogen blistering and microcrack on the surface of the specimen without loading, but HIDF under sustained load can occur, and the threshold stress intensity factor for a single edge notched sample, K_IH, is 0.63 times of the notched toughness, K_Q, which is 62.2 MPa m^1/2. When i is equal to or larger than 20 mA/cm², hydrogen blistering or blistering plus microcrack appears on the surface of specimen without loading, as well as K_IH is 0.26 times of K_Q and independent upon i. The relative losses of the notched toughness induced by both atomic hydrogen and the blistering are all 37%. The critical pressure necessary for a stable blister formation, P_i, is 3.6 GPa, and that for cracking of the blister, P_C, is 3.9 GPa. The blister cracking will arrest after propagating for 20-30 μm, and the arrested crack will propagate again because of entering of hydrogen atoms. At last, the blister with cracking will break and leave local cleavage fracture surface with arrested lines on the sample surface without loading.     

Study of effect of 90° domain switching on ferroelectric ceramics fracture using the moiré interferometry

Fracture behavior of ferroelectric ceramics during in-plane and out-of-plane 90° domain switching was studied using the moiré interferometry technique. The specimens used in the experiment were three-point-bending beams, each with a single through-notch, which were subjected to a mechanical load, an electrical load and a combined electrical and mechanical load, respectively. The main subject of interest is to investigate the influence of 90° domain switching induced by the electrical load on fracture toughness and material brittleness. In the experiment, compared with out-of-plane 90° domain switching, in-plane 90° domain switching occurs in the region of approximately a 45° band. It causes larger in-plane tensile strain ε_xx in almost all regions of a specimen, especially in the 45° band. In-plane 90° domain switching greatly decreases fracture toughness and weakens the material brittleness of ferroelectric ceramics. The out-of-plane 90° domain switching does not exhibit such a great influence on fracture toughness and material brittleness as the in-plane 90° domain switching does.     

Residual Strain and Fracture Response of Al2O3 Coatings Deposited via APS and HVOF Techniques

The aim of this investigation was to nondestructively evaluate the residual stress profile in two commercially available alumina/substrate coating systems and relate residual stress changes with the fracture response. Neutron diffraction, due to its high penetration depth, was used to measure residual strain in conventional air plasma-sprayed (APS) and finer powder high velocity oxy-fuel (HVOF (θ-gun))-sprayed Al₂O₃ coating/substrate systems. The purpose of this comparison was to ascertain if finer powder Al₂O₃ coatings deposited via θ-gun can provide improved residual stress and fracture response in comparison to conventional APS coatings. To obtain a through thickness residual strain profile with high resolution, a partially submerged beam was used for measurements near the coating surface, and a beam submerged in the coating and substrate materials near the coating-substrate interface. By using the fast vertical scanning method, with careful leveling of the specimen using theodolites, the coating surface and the coating/substrate interface were located with an accuracy of about 50 μm. The results show that the through thickness residual strain in the APS coating was mainly tensile, whereas the HVOF coating had both compressive and tensile residual strains. Further analysis interlinking Vickers indentation fracture behavior using acoustic emission (AE) was conducted. The microstructural differences along with the nature and magnitude of the residual strain fields had a direct effect on the fracture response of the two coatings during the indentation process.     

兰姆波在薄钢板无损检测中的应用研究

通过在钢板上钻制直径不等的孔来模拟钢板中的孔洞类缺陷，采用线切割加工出不同深度的狭缝来模拟钢板中的分层缺陷。采用一发一收法，分别在两种厚度的钢板上激励出兰姆波进行检测，通过对比分析检测到的缺陷前后板中兰姆波信号，采用时频分析方法识别出板中激发的兰姆波模式以及模态转换现象，初步确定特定缺陷形态对兰姆波在板中传播的影响。     

Fatigue behaviour of WC-Co hard metal under stress ratio and effectively loaded volume relevant to metalworking tool failure

WC-Co hard metals have proven to be excellent tool materials capable of coping with the elevated cyclic load levels that arise in e.g. metalworking tool application. The design of cyclically loaded tools requires reliable information on the fatigue behaviour of hard metals, also as a function of the acting mean stress. Since hard metals show defect controlled fracture behaviour, the nature of defects and their size distribution in the volume of given loaded structures is relevant to their fatigue behaviour. The concept of effectively loaded volume V_eff allows for the comparison of the fracture probability of loaded structures with different kinds of stress distributions. To date, V_eff of laboratory test setups and metalworking tools and its influence on the fatigue behaviour was not considered in the respective design processes. Instrumented milling experiments in combination with finite element based load analysis were used in the current work to provide information on typical values of stress ratio and V_eff for end milling tools. A newly developed non-conventional type of a six point bending test setup was designed to induce values of V_eff and mean stress which were found to be representative for the metalworking tool. The shape of the used central specimen support structures was designed to avoid contact damage to the specimen at the sites of load transfer. The knowledge-based design of the six point bending test setup assures that a region of the hard metal's bimodal defect size distribution is probed that leads to fatigue limits relevant to the failure behaviour of end milling tools.     

超细晶纯钛的微动疲劳特性

利用自行设计的微动疲劳实验夹具装置研究超细晶纯钛在柱面-平面接触下的微动疲劳特性,分析循环应力对其微动疲劳寿命的影响,通过观察接触区磨损和断口形貌,分析其微动损伤机制。结果表明,当法向载荷不变时,超细晶纯钛的微动疲劳寿命随着循环应力的增加而减小,比常规疲劳寿命更小。微动疲劳裂纹于接触区边缘萌生,磨损区破裂严重且附着有磨粒,在磨粒磨损作用下加速了试样的疲劳失效。断口同时呈现出疲劳形貌和微动形貌,形貌从平滑转向粗糙直至断裂,裂纹由小变大,裂纹扩展速率也逐渐增加,且在裂纹扩展区存在二次裂纹;由于受力不均在裂纹扩展区与断裂区之间存在山脊状形貌。