Artificial Surface Defects Research Articles

Influence of surface condition, cycling frequency and ferritic zones on the high and very high cycle fatigue properties of a pearlitic steel

In the present work, the high and very high cycle fatigue (HCF/VHCF) properties of pearlitic steel R350HT are investigated. Servo-hydraulic (cycling frequency between 15 Hz and 60 Hz) and ultrasonic (19 kHz) fatigue tests were performed with different surface conditions (fine turned, mechanically polished and electropolished). At a load ratio of R = −1, solely failures in the HCF regime were found for all surface conditions, although runout specimens were loaded with more than 1010 cycles, and crack initiation was exclusively at the surface. At R = 0.1, in contrast, mechanically polished specimens showed internal failures for lifetimes between 106 and 109 cycles. The origin of fracture in the interior was identified as distributed ferritic phases embedded in the harder pearlitic microstructure. Crack initiation from ferritic zones could be explained by their larger size relative to inherent defects such as nonmetallic inclusions and precipitates. The absence of turning grooves in combination with compressive residual stresses near the surface of mechanically polished specimens led to an increase in HCF strength compared with fine-turned and electropolished specimens and facilitated VHCF failure from the interior. No frequency effect was observed when crack initiation originated from turning grooves or artificial surface defects, but slightly higher fatigue strengths were determined for electropolished specimens at 19 kHz than at conventional cycling frequency. This can be explained by the strain-rate sensitivity of ferrite where fatigue cracks preferentially initiate. Additional tests with specimens containing artificially introduced micro-holes were performed to systematically study material's defect tolerance and to quantify the mean-stress sensitivity.

Low-cycle fatigue performance and life prediction of a P/M nickel-based superalloy with artificial surface defect at elevated temperature

P/M nickel-based superalloys are widely used in turbine disks, defects such as inclusions and surface scratches inevitably arisen from manufacturing process will exert negative influences on the low-cycle fatigue performance of superalloys and make traditional life prediction method no longer reliable. The residual fatigue life in presence of defect is an important performance index for disc alloy in damage tolerance design criteria. In this work, LCF tests of FGH96 specimens with artificial surface defect were conducted at 600℃, 650℃, 700℃ and room temperature, the size of the defect is determined in accordance with the minimum flaw size which can be detected by NDT technique. The results show that the LCF life of the specimens with artificial surface defect decreases significantly with the increase of temperature and size of defect. Microscopic characterization technique SEM and EDS were executed to investigate the failure mechanism of the specimens, different mechanisms of crack initiation from artificial surface defect at room and elevated temperature were discussed. And then life prediction based on fracture mechanics was carried out by ABAQUS-FRANC3D co-simulation. This method allows that SIF can be conveniently calculated by finite element method and it is applicable to analysis of arbitrary engineering structure. Good life prediction results which fall within double dispersion band of experimental lives were obtained at elevated temperature, but the predicted life is excessively conservative at room temperature because the crack initiation process accounts for a large proportion of fatigue life. It seems that life prediction method based on fracture mechanics is more applicable to specimens with surface defect at elevated temperature due to the shorter crack initiation process. This work has reference significance to the damage tolerance analysis of turbine disks.

Frequency Characteristics Analysis of Remote Field Eddy Current Testing on Ferromagnetic Pipes

Pipes made of ferromagnetic alloys are widely installed in petrochemical plants and the steel plants. To implement non-destructive corrosion inspections on these pipes, remote field eddy current testing (ECT) which installs a search coil inside the pipe is a practical inspection method. Remote field ECT has the advantage of being able to detect external surface defects while the distance between the detection coil and the excitation coil is twice of the outer diameter of the testing pipe. Nevertheless, the inspection principle such as the frequency characteristics etc. of ECT signals have not been elucidated, inspection devices based on remote field ECT had not been employed widely. In this paper, we measured the frequency characteristics of ferromagnetic pipes with artificial outer surface defects by remote field ECT, and then detailed finite element analysis are carried out by using the nonlinear eddy current analysis to clarify the inspection principle of the remote field ECT. In both measurement and analysis, peak signal occurs directly under the defect and the optimal frequency range are the same. The phase angle of flux density due to the excitation current and flux density due to eddy current are found to greatly affect the frequency characteristics.

Technique for Introducing Internal Defects with Arbitrary Sizes and Locations in Metals via Additive Manufacturing and Evaluation of Fatigue Properties

Mechanical component failure is usually caused by metal fatigue originating from small defects in metallic materials. Thus, it is important to precisely capture the fatigue properties of materials containing small defects. Fatigue tests of materials with artificial surface defects introduced by drilling have been conducted. Using the resulting data, an equation for predicting the material fatigue limit has been proposed on the basis of the √area parameter model, and its effectiveness has been confirmed for various materials. However, for additive manufactured (AM) materials that contain internal defects resulting in failure, controlling the size of the defect where the fracture originates is extremely difficult. Therefore, verification of the predictive ability of the √area parameter model for AM materials is impossible, in contrast with other materials that fail because of surface defects. In this context, developing a technique to intentionally introduce internal defects with arbitrary sizes at arbitrary locations can provide insights that help predict the fatigue limit of AM materials. This study aimed to establish a technology for quantitatively evaluating the effect of internal defects on the fatigue properties of AM materials by introducing internal defects with arbitrary sizes at arbitrary locations via AM. Specimens with different defect sizes and locations were prepared. Prior to the fatigue tests, the defect sizes and locations were measured non-destructively via X-ray computed tomography (CT). The fatigue tests were conducted in air at room temperature. All the specimens failed because of the intentionally introduced internal defects, and the fatigue lives became shorter with increasing defect sizes, except for the specimens with defects adjacent to the surface. In those cases, fatigue cracks easily reached the surface; therefore, the fatigue lives were speculated to be shorter than those of the specimens with the same defect sizes. Moreover, the defect sizes determined from the fracture surfaces by scanning electron microscopy were nearly consistent with those determined by X-ray CT.

Very High Cycle Fatigue Data Acquisition Using High-Accuracy Ultrasonic Fatigue Testing Equipment

Abstract Fatigue testing at ultrasonic frequency is a powerful method for rapid generation of high and very high cycle fatigue data. High accuracy of cyclic loading is the basis for reproducible test results, which can be obtained with closed-loop control of vibration amplitude and resonance frequency. Self-heating of test specimens can be suppressed by intermittent (pulse-pause) loading. In the present work, the high-accuracy ultrasonic fatigue testing equipment developed at the University of Natural Resources and Life Science, Vienna (BOKU) is described, and systematic investigations are presented that demonstrate the effects of cycling frequency, specimen size, and intermittent loading. Data obtained with additively manufactured (AMed) austenitic stainless steel 316L at 19 kHz are compared with electromagnetic resonation test results measured at 90 Hz. No influence of cycling frequency on fatigue lifetimes and fatigue limit was found when specimens of comparable size were used for tests at 19 kHz and 90 Hz. In contrast, the use of as-built specimens with smaller testing volume resulted in an increase in cyclic strength, demonstrating a size effect on fatigue properties. Polishing the surfaces of AMed specimens increased the fatigue limit by a factor of two. Furthermore, ultrasonic fatigue tests with intermittent loading and four different pulse lengths as well as continuous cycling were performed. The tests were conducted with wrought aluminum alloy 7075-T651 and with specimens containing small artificial surface defects. Similar lifetimes were measured in all testing series. Moreover, the same fatigue limit is found for all pulse lengths and for continuous cycling. This demonstrates that pulsed loading is a most suitable method to avoid specimen heating at ultrasonic frequency and that there is no influence on measured fatigue properties if ultrasonic loading is appropriately controlled. Measurement artifacts could be identified and avoided by measuring and recording all load amplitudes during ultrasonic fatigue tests.

Ultrasonic evaluation for the detection of contact defects of the timber and fiber-reinforced polymer

Fiber-reinforced polymer (FRP) composites have been used tremendously to repair and rehabilitate timber structures due to their formability, ease of use, and high specific strength. The bond quality between FRP and timber substrate is critical for complete composite action. In this paper, a comprehensive set of linear and nonlinear ultrasonic methods was performed to investigate the bond between carbon-FRP (CFRP) and timber. For this purpose, 126 specimens of reinforced timber were prepared. Two techniques were considered to bond CFRP and timber: (1) externally bonded reinforcement (EBR), and (2) externally bonded reinforcement on the groove (EBROG). The effect of the number of CFRP layers, adhesive types, the extent of artificial surface defects, and the depth of the groove were contemplated. By analyzing linear methods, differences in stiffness and modulus of elasticity of CFRP and timber led to velocity variation and attenuation phenomena. Higher-order harmonics were also used to investigate the nonlinear behavior of ultrasonic waves at the bond. The second harmonic generation detected surface damage at CFRP and timber bond. Single-lap shear tests were employed to determine the bond strength of CFRP and timber and evaluate the accuracy of the linear and nonlinear methods. Finally, using the linear and nonlinear features extracted from the ultrasonic response signals and available physical parameters, three artificial neural networks (ANNs) were developed to predict three important CFRP-to-timber bond characteristics quantitively: (1) to predict the groove depth, (2) to predict the volume of voids, and (3) to predict the CFRP-to-timber bond strength. The three proposed models achieved an average R 2 of 0.95, 0.96, and 0.92 in predicting the groove depth, voids volume, and bond strength, respectively.

A Machine Vision-Based Method for Detecting Surface Hollow Defect of Hot-State Shaft in Cross Wedge Rolling

In order to solve the problems of low detection efficiency and safety of artificial surface defects in hot-state cross wedge rolling shaft production line, a machine vision-based method for detecting surface hollow defect of hot-state shafts is proposed. Firstly, by analyzing the high reflective properties of the metal shaft surface, the best lighting method was obtained. And by analyzing the image contrast between image foreground and image background, the most suitable optical filter type in image acquisition was determined. Then, Fourier Gaussian low-pass filtering method is used to remove the interference noise of rolled shafts surface in frequency domain, such as high-light, oxide skin and surface texture. Finally, by analyzing the characteristics of the surface hollow defect area, a defect identification method combining the Otsu threshold method and the adaptive threshold method is proposed to realize the effective extraction of surface hollow defect of rolled shafts. The test results show that the average recognition rate of the method based on machine vision is 95.7%. The results of this paper provide technical support to meet the production requirements of high quality and high performance of cross wedge rolling.

Mechanism of artificial surface defect induced cracking for very high cycle fatigue of Ti alloys

In-situ observation of crack growth and electron backscatter diffraction observation of microstructure around artificial surface defect and at mated crack surfaces are conducted for very high cycle fatigue (VHCF) of TC17 alloy. The artificial surface defect induced VHCF failure presents no slow process of crack initiation and early growth accompanied with nanograin formation commonly observed for internal cracking induced VHCF. It is attributed to the synergistic effect of cyclic stress with time dependent process (e.g. the influence of moisture, action of hydrogen, etc.). The nanograins and deformation twins are also observed in some α grains at or near crack surfaces.

Effects of micro-shot peening on the defect tolerance of Al-Si-0.6Mg alloy containing artificial defect

The effects of micro-shot peening (MSP) and artificial surface defects were investigated in relation to the defect tolerance of Al-Si-0.6Mg alloy in the framework of fracture mechanics. The results indicated that coral-shaped casting defects inside the alloy could be effectively quantified using the X-ray synchrotron radiation technique. An affected layer with a surface roughness (Ra) of 1.25 and residual stress of − 125 MPa was introduced on the MSP specimen compared to the un-peened (UP) specimen. The fatigue strengths of the UP and MSP specimens containing the artificial defects decreased as the defect size increased under rotating bending fatigue tests, except in the case of defects with the equivalent size of 395 and 490 µm, respectively. MSP could shift the crack initiation site toward the subsurface, while all the cracks of the specimens with the artificial defects originated from the surface and maintained an aspect ratio of 0.66 during the crack growth process. The relationship between the defect sizes and the fatigue strengths could be reasonably assessed by the modified Kitagawa-Takahashi (K-T) diagrams. The critical defect size of the MSP specimens was increased from 450 to 542 µm compared with that of the UP specimens, owing to the beneficial effect of the compressive residual stress. The defect tolerance of the Al-Si-0.6Mg alloy was improved by the MSP process.

Defect induced cracking and modeling of fatigue strength for an additively manufactured Ti-6Al-4V alloy in very high cycle fatigue regime

Additively manufactured (AM) alloy usually inevitably contains defects during the manufacturing process or in service. Defect, as a harmful factor, could significantly reduce the fatigue performance of materials. This paper shows that the location and introduced form of defects play an important role in high cycle fatigue and very high cycle fatigue (VHCF) behavior of selective laser melting Ti-6Al-4V alloys. The S-N curve descends approximately linearly for internal defect induced failure. While for artificial surface defect induced failure, the S-N curve descends at first and then exhibits a plateau region feature. The competition of interior crack initiation with fine granular area feature is also observed in VHCF regime. The paper indicates that only the size or the stress intensity factor range of the defect is not an appropriate parameter for describing the effect of defect on fatigue crack initiation. Finally, the effect of artificial surface defect on high cycle fatigue and VHCF strength is modeled, i.e., the fatigue strength σ, fatigue life N and defect size area (square root of the projection area of defect perpendicular to principal stress direction) is expressed as σ=CNaarean,N<N0CN0aarean,N≥N0, where C, a and n are constants, and N0 is the number of cycles at the knee point of the curve.

Integrity increasing of damaged transporting pipelines using fiber reinforced polymer composite wrap systems

The main objective of the ongoing research is data collection for integrity management tasks, helping the transporting pipeline operators in different decision situations. On the one hand, can be used the composite repairing and reinforcing systems for a long-term; or there are only appropriate for a short-term maintenance period. On the other hand, can be identified the reserves of the girth welds after a long-term operation; or the girth welds with different defects should be repaired and/or replaced. The paper introduces a full-scale pipeline investigation program. Fatigue and burst tests were executed on repaired pipeline sections containing artificial surface and through-wall defects, modelling gouges and leakages, respectively. Both separate and interacting flaws were investigated. Different composite reinforcing methods, in other words wrap systems, were applied, investigated and compared. Furthermore, long-term operated and cut gas transporting pipeline sections containing girth welds were investigated, too. The testing results and the damage histories were compared with each other; safety factor was defined and calculated for the assessment of the effectiveness of the reinforcing systems and the reliability of the girth welds.

Влияние различных факторов на чувствительность магнитопорошкового контроля

The magnetic powder method occupies one of the leading positions in the system of non-destructive quality control of materials. Approximately 80% of the controlled ferromagnetic products are controlled by the magnetic powder control method. The high sensitivity of the control method, its versatility, relatively low labor intensity of control, and the visibility of the results ensured that the method was widely used in industry. The main factors, characteristics and parameters of magnetic powder suspensions affecting the sensitivity of magnetic powder control are considered. A comparative analysis of the sensitivity of experimental suspensions in terms of the effectiveness of detecting artificial surface and subsurface defects of the MO-1 control sample was carried out. The qualitative efficacy of magnetic powders of the "Diagma" and "Crocus" type as a basis for the manufacture of the suspensions is clearly demonstrated, and the effect of autocoagulation on the sensitivity and reliability of the results of magnetic powder control is shown on the example of "Diagma-1100" and "Diagma-1200" powders. The verification of the effectiveness of the suspensions based on ferromagnetic particles γ-Fe₂O₃ and Fe₃O₄ in suspended state did not show their qualitative advantages in comparison with the other experimental suspensions. The further direction of the development of this work is to check the operability of experimental magnetic suspensions γ-Fe₂O₃ and Fe₃O₄ that are not in a suspended state. It is suggested that this state of magnetic suspensions γ-Fe₂O₃ and Fe₃O₄ negatively affects the attraction of magnetic particles by inhomogeneous magnetic fields, and also does not allow a clear indication pattern to form over the defects. In addition, the task of selecting the optimal concentration of magnetic particles in the finished magnetic suspension, in which a high degree of sensitivity to defects will be achieved with the minimum possible consumption of materials included in the magnetic suspension, can be considered as the next stage of the research. English version of the article is available at URL: https://panor.ru/articles/influence-of-various-factors-on-the-sensitivity-of-magnetic-particle-inspection/77764.html

The effect of artificial defects on the fatigue characteristics of AISI 4340 steel

This study examined the fatigue life (Nf) and fatigue limit (FL) of AISI 4340 steel in relation to surface roughness, artificial defects and shot peening. Artificial surface defects on fatigue specimens were obtained by electrical discharge machining (EDM) or by pre-corrosion in 3.5% NaCl and perchloric acid solutions, followed by a fatigue test. The presence of artificial defects of various sizes, taken into account by the area parameter, led to a significant decrease in fatigue characteristics, while shot peening contributed to their noticeable improvement. Defects resulting from electrochemical corrosion had a much greater negative effect on fatigue life than defects introduced by EDM.

Pixel frequency based railroad surface flaw detection using active infrared thermography for Structural Health Monitoring

With rapid increase in operation and development of high-speed trains, inspection of railroad surface flaws has become an important aspect for safe and reliable operation of rail network. Non-destructive testing using active infrared thermography has been useful in determining the structural health of different structures with additional benefit of robustness in overall inspection system. This study is based on detection of artificial surface flaws on an in-service railroad. Transverse and longitudinal flaws of various dimensions were machined on rough and smooth rail surface. The railroad surface was thermally stimulated to a temperature equivalent to practical conditions. Emitted radiations from rail surface were captured by an infrared camera to detect cracks. Results show a comparison between the surface flaws on rough and smooth rail surface. Subsequently, raw infrared images were post-processed by statistical image improvement to quantitatively analyse the results. Significant change in the frequency distribution of pixel intensity is observed as the flaw size and depth changes giving a clear quantification of crack topology. A comprehensive and inexpensive solution for damage diagnosis will be offered to railway authorities for Structural Health Monitoring (SHM) and NDT by the proposed framework.

High-Frequency Guided Wave Propagation and Scattering in Silicon Wafers

Abstract Thin monocrystalline silicon wafers are employed for the manufacturing of solar cells with high conversion efficiency. Micro-cracks can be induced by the wafer cutting process, leading to breakage of the fragile wafers. High-frequency guided waves allow for the monitoring of wafers and detection and characterization of surface defects. The material anisotropy of the monocrystalline silicon leads to variations of the guided wave characteristics, depending on the guided wave mode and propagation direction relative to the crystal orientation. Selective excitation of the first antisymmetric A0 wave mode at 5 MHz center frequency was achieved experimentally using a custom-made wedge transducer. Strong wave pulses with limited beam skewing and widening were measured using noncontact laser interferometer measurements. This allowed the accurate characterization of the Lamb wave propagation and scattering at small artificial surface defects with a size of less than 100 µm. The surface extent of the defects of varying size was characterized using an optical microscope. The scattered guided wave field was evaluated, and characteristic parameters were extracted and correlated with the defect size, allowing in principle detection of small defects. Further investigations are required to explain the systematic asymmetry of the guided wave field in the vicinity of the indents.

Influence of load ratio on torsion very high cycle fatigue of high‐strength spring steel in the presence of detrimental defects

AbstractThe influence of mean shear stress on torsion very high cycle fatigue (VHCF) for internal inclusion‐initiated fracture is investigated. Ultrasonic torsion fatigue tests are performed at load ratios R = −1, 0.1, and 0.35 with high‐strength spring steel SWOSC‐V featuring an increased number of nonmetallic inclusions. Based on Murakami and Endo's parameter model, an equation predicting the torsion fatigue limit in the presence of detrimental defects is rendered. Stress biaxiality is considered using a ratio of torsion to axial fatigue strength of τa/σa = 0.86. Considering the load ratio sensitivity according to ([1 − R]/2)α, the value for α is experimentally determined to be 0.550. A fine granular area (FGA) is generated at an artificial surface defect in a fully reversed cyclic torsion test in vacuum. Measured growth rates in the FGA were 10−14–10−12 m/cycle, and the formation of the FGA consumed 98% of the VHCF lifetime.

Effects of shot peening and artificial surface defects on fatigue properties of 50CrV4 steel

Shot peening processes are commonly used for improving the fatigue properties of steels. Shot peening introduces a compressive residual stress field in the near surface of steel, which can reduce or stop the growth of fatigue cracks and improve fatigue properties. This study experimentally investigated the effect of shot peening on the fatigue properties of 50CrV4 steel alloys with different artificial surface defects. Drilling tools were used to introduce different artificial defects with root radii of 0.585 mm and 0.895 mm on the surface of unpeened samples. The shot peening was applied to the drilled and undrilled samples. Scanning electron microscopy (SEM) observations, micro-hardness and X-ray diffraction residual stress measurements were conducted to analyse the characteristics of the shot-peened and unpeened samples. The results show that the shot peening leads to the transformation of the retained austenite to martensite in the near-surface microstructure. The hardness rates of the surface and near surface both increase by 8% after the shot peening. The peened samples exhibit compressive residual stresses with a high degree of isotropy in the near surface. The fatigue properties of samples were experimentally evaluated by conducting 3-point bending tests. The results indicate that the shot peening improves the fatigue life of drilled and undrilled samples. For the defects with the root radius of 0.895 mm, the shot peening leads to a 500% improvement in the fatigue life compared to unpeened samples regardless of defect depth. For the defects with the root radius of 0.585 mm, the improvement in fatigue life is 40% for the defect depth of 0.2 mm compared to unpeened samples. The improvement increases to 60% and 200% by increasing the defect depths to 0.4 mm and 0.6 mm. The fatigue properties are linked to the changes in the features of defects mainly caused by the deformation hardening and compressive residual stress after shot peening.

Sensitivity of Electromagnetic-Acoustic Multiple Shadow Method Using Rayleigh Waves in Inspection of Oil Country Tubular Goods

We propose algorithms for processing signals obtained by the multiple shadow method using Rayleigh waves propagating in the direction of the pipe envelope and electromagnetic-acoustic emission-reception technique. The results of studying the method susceptibility to artificial and natural surface and near-surface defects of workpieces intended for the production of drill pipes in two-frequency ranges are presented. The multiple detection coefficient and the probabilistic characteristics of variance, asymmetry, and kurtosis are proposed to be used as informative parameters in analyzing the series of multiple signal passes. The rejection criteria are substantiated. The research results can be used in the development of automated units and methods for monitoring small-diameter pipes.

Чувствительность электромагнитно-акустического метода многократной тени с использованием рэлеевских волн при контроле труб нефтяного сортамента

The paper proposes algorithms for signals processing obtained by the multiple shadow method using Rayleigh waves propagating in the direction of the envelope pipe, and an electromagnetic-acoustic method of emission-reception. The results of a study of the sensitivity of the method to artificial and natural surface and near-surface defects of workpieces of drill pipes in two frequency ranges are presented. As informative parameters, it is proposed to use the multiple coefficient of detectability, as well as the probabilistic characteristics of dispersion, asymmetry and kurtosis in the analysis of a series of multiple passes, and the criteria for rejection are substantiated. The research results can be used in the development of automated installations and methods for monitoring pipes of small diameter.

Effect of Shot Peening on Residual Stress Distribution and Microstructure Evolution of Artificially Defected 50CrV4 Steel

The aim of this paper is to clarify the effects of shot peening on fatigue life and strain hardening behavior of spring steel samples containing artificial surface defect. Artificial defect acting as pre-cracks is introduced and the effects of this defect on the material surface and the distribution of residual stress away from the defect are investigated before and after shot peening. Shot peening has provided the following modifications on the surface: (1) homogenous compressive residual stress, (2) high dislocation density, (3) superficial strain hardening, (4) retained austenite reduction. In this paper, shot peening is applied to 50CrV4 steel and its effect on surface roughness, microhardness, surface residual stress, crystallite size and dislocation density calculation is determined and SEM observations are used to reveal the properties severely strain-hardened layer. It can be shown that the shot peening can modify the crystallite size, however, the main effect is achieved by increasing the dislocation density and inducing the highly compressive residual stress. The effect of the surface modification to the 3-point bending fatigue life are analyzed.