Artificial Surface Defects Research Articles

The effect of quenching and defects size on the HCF behaviour of Boron steel

This work investigates the effect of natural and artificial surface defects and quenching on the fatigue strength of a Boron steel (22MnB5). A vast experimental campaign has been undertaken to study the high cycle fatigue behaviour and more specifically the fatigue damage mechanisms observed in quenched and untreated materials, under different loading conditions and with differents artificial defects sizes (from 25μm to 370μm radius). In order to test the sheet metal in shear an original test apparatus is used. The critical defect size is determined to be 100±50μm. This critical size does not appear to depend on the loading type or the microstructure of the material (i.e. ferritic–perlitic or martensitic). However, for large defects, the quenched material is more sensitive to the defect size than the untreated material. For a defect size range of 100–300μm the slope of the Kitagawa–Takahashi diagram is approximately −1/3 and −1/6 for the quenched and untreated materials respectively. A probabilistic approach that leads naturally to a probabilistic Kitagawa type diagram is developed. This methodology can be used to explain the relationship between the influence of the heat treatment and the defect size on the fatigue behaviour of this steel.

Formation Mechanism of Specific Fracture Surface Region in the Sub-Surface Fracture of Titanium Alloy

In Ti–6Al–4V alloy, fatigue properties have been widely investigated, and the origin of fatigue fracture is usually at the surface in the high stress and lower fatigue life region, whereas in low stress and longer fatigue lifetimes origins are generally sub-surface in nature. Very high cycle fatigue tests were conducted, and observation of fracture surfaces revealed that a unique fine concave and convex agglutinate (hereinafter called Granular Region) formed on the fracture surface of sub-surface fractures. The granular region was not observed on the fracture surface of surface fractures. To clarify the formation mechanism and process of forming the granular region, which is a unique phenomenon in the very high cycle fatigue, fatigue tests using specimens with an artificial surface defect were conducted in air and vacuum. The fatigue tests were based on the idea that the environment around a sub-surface fatigue crack is a vacuum-like environment. During the tests, fracture surfaces were intentionally contacted in air and vacuum under different loading conditions. Fracture surface observations revealed that repeated contact of the fracture surfaces and a vacuum environment are necessary for the formation of the granular region. A mechanism for the formation of the granular region will be proposed.

The Effect of the Microstructure and Defects on Crack Initiation in 316L Stainless Steel under Multiaxial High Cycle Fatigue

The aim of this study is to analyse the influence of both the microstructure and defects on the high cycle fatigue behaviour of the 316L austenitic stainless steel, using finite element simulations of polycrystalline aggregates. High cycle fatigue tests have been conducted on this steel under uniaxial (push-pull) and multiaxial (combined in-phase tension and torsion) loading conditions, with both smooth specimens and specimens containing artificial semi-spherical surface defects. 2D numerical models, using a cubic elastic constitutive model, are created to determine the degree of heterogeneity of the local stress parameters as a function of the defect size. This has been done for one microstructure using several orientation sets generated from the initial texture of the material. The grains are explicitly modelled and the anisotropic behaviour of each FCC crystal is described by the generalized Hookes law with a cubic elasticity tensor. From the simulations carried out with different defect sizes and orientation sets that are representative of the real texture of the tested material, statistical information regarding mesoscopic mechanical fields provides useful insight into the microstructural dependence of the driving forces for fatigue crack nucleation at the mesoscopic scale (or the scale of individual grains). The results in terms of the stress fields and fatigue crack initiation conditions are determined at both the mesoscopic and macroscopic scales. The results from these FE models are used along with an original probabilistic mesomechanics approach to quantify the defect size effect. The resulting predictions, which are sensitive to the microstructure, include the probability distribution of the high cycle fatigue strength.

Fatigue behavior of nitrided and shot peened steel with artificial small surface defects

In this paper the effect of small defects on fatigue threshold of different series of nitrided and nitride-shot peened low alloy steel specimens is investigated by means of experimental rotating bending tests on sandglass specimens. Micro-holes acting as pre-cracks are introduced by means of different methods (electro-erosion and indentation) in order to evaluate the effect of defect size as well as the influence of the process to generate them on the fatigue threshold. The results, interpreted in terms of ΔKth, show good agreement with estimated ones, calculated by a formula that considers the experimentally measured residual stresses, FWHM parameter, micro-hardness values and SEM observation of broken and run-out specimens. A critical discussion of the obtained results highlights the influence of the defect size and of the method used to generate the micro-holes on the fatigue threshold.

Effects of shot peening on the torsional fatigue limit of high‐strength steel containing an artificial surface defect

PurposeThe purpose of this paper is to clarify the effects of shot peening (SP) on the torsional fatigue limit of high‐strength steel specimens containing an artificial small defect.Design/methodology/approachSpecimens containing a drilled hole 0.1‐0.4 mm deep or a semi‐circular slit 0.15 or 0.3 mm deep were subjected to SP. Torsional fatigue tests were then carried out.FindingsThe torsional fatigue limits of specimens containing a drilled hole and those with a semi‐circular slit were increased 25‐64 per cent and 156‐186 per cent by SP, respectively. The torsional fatigue limits of the specimens subjected to SP and containing a drilled hole less than 0.1 mm in depth or a semi‐circular slit less than 0.15 mm in depth were almost equal to those of SP specimens without a defect. Based on these results, it can be concluded that a drilled hole less than 0.1 mm in depth and a semi‐circular slit less than 0.15 mm in depth could be rendered harmless by SP.Practical implicationsThe proposed method can be applied to mechanical parts subjected to cyclic torsion, such as coil springs, crank shafts and drive shafts.Originality/valueThis is the first paper to investigate the torsional fatigue limits after SP in materials containing a surface defect. In this paper, the effect of SP on the torsional fatigue limit having a surface defect is investigated.

Reduction of Lift-Off Effects in Pulsed Eddy Current for Defect Classification

Pulsed eddy-current (PEC) testing is an electromagnetic nondestructive testing & evaluation (NDT&E) technique and defect classification is one of the most important steps in PEC defect characterization. With pulse excitation, the PEC response signals contain more features in time domain and rich information in frequency domain. This paper investigates feature extraction techniques for PEC defect classification including rising time, differential time to peak and differential time to zero, spectrum amplitude, and differential spectrum amplitude. Experimental study has been undertaken on Al-Mn 3003 alloy samples with artificial surface defects, sub-surface defects, and defects in two-layer structures under different lift-off. Experimental results show that methods are effective to classify the defects both in single-layer structures and two-layer structures. Comparing the results of different methods, it is found that differential process can eliminate the lift-off in defect classification in both time domain and frequency domain. The study can be extended to defect classification in complex structures, where lift-off effects are significant.

Optimal features combination for pulsed eddy current NDT

This paper investigates current feature extraction techniques for pulsed eddy current NDT including peak value and peak time, spectral characteristics analysis, principal component analysis (PCA) projection coefficient and response shape curvature. Experimental study has been undertaken on samples with artificial surface and subsurface defects and metal thickness changes. Based on a comparative study of the feature extraction techniques, the most suitable features and their combination are discussed. The present study will provide a framework for future applications comprising the characterisation of real defects and stress profiles in engineering components.

Surface Flaws Detection Using AC Magnetic Field Sensing by a Thin Film Inductive Microsensor

Alternating magnetic field was used for detection of surface flaws on conductive metallic specimens. The nondestructive sensor probe was composed of the planar coil with inductive magnetic thin film yoke as a sensing component and a single straight typed exciting coil. The planar inductive coil sensor with magnetic yoke was fabricated by sputtering, electroplating, dry etching, and photolithography process. The exciting coil was generated by the alternative current with the range of 0.1-1.2 A with frequency rage of 0.6-1.8 MHz, respectively, which alternating current (ac) magnetic fields by exciting coil were applied to the specimen with artificial flaws. The specimens were prepared with the slit shaped artificial surface flaws (minimum depth and width; 0.5 mm) on metallic plate (Al; nonmagnetic metal and FeC; magnetic metal). The detected signal for the positions and shapes of surface flaws on specimens were obtained with high sensitivity and high signal to ratio by the planar inductive microsensor probe. The measured output signals by the noncontacted scanning on surface of coins (one cent and two euros) were converted to the images of the flaws. These results were compared with the optical images of real coins, respectively.

Improvement of Bending Fatigue Limit by Cavitation Peening for Specimens Containing an Artificial Surface Defect

Improvement of Bending Fatigue Limit by Cavitation Peening for Specimens Containing an Artificial Surface Defect

1040 ショットピーニングによる人工表面欠陥の無害化(J12-2 金属材料の疲労特性と破壊機構(2) 表面改質処理,ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

1040 ショットピーニングによる人工表面欠陥の無害化(J12-2 金属材料の疲労特性と破壊機構(2) 表面改質処理,ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Nucleation and Growth of Gold on MgO Thin Films: A Combined STM and Luminescence Study

Electroluminescence measurements have been performed with a scanning tunneling microscope (STM) to study the growth of Au particles on thin MgO films prepared on Mo(001). With increasing Au coverage, rapid quenching of the characteristic, defect-related MgO emission is observed, while a new peak emerges in the optical spectra that is attributed to Mie plasmon resonances of the Au particles. The fast decline of the MgO signal with metal exposure indicates that Au nucleation preferentially takes place at the optically active centers in the MgO surface. The nucleation sites have been identified in STM measurements as being located along the step edges of the MgO surface. A sputter-deposition of Au results in a significantly slower decline of the MgO optical signal, manifesting the delayed blocking of optically active MgO centers due to Au nucleation on artificial surface defects.

Improvement of fatigue strength by shot peening for spring steel specimens containing an artificial surface defect

Effects of shot peening on the bending fatigue strength of spring steel specimens (SUP9A) containing an artificial small hole were investigated. Shot peening (SP) and stress shot peening (SSP) were carried out with the specimens containing an artificial drilled hole 0.2, 0.4 and 0.8 mm diameter. Then, bending fatigue tests were carried out with the specimens. The fatigue strengths of specimens containing an artificial small hole were increased by shot peening. Stress shot peening (SSP) was effective in improving fatigue strength. The specimens containing an artificial hole 0.2 mm diameter which received shot peening fractured elsewhere the hole, and they had considerably high fatigue strengths almost equal to those of the shot peened smooth specimens. It was found that the fatigue limit of specimens having an artificial hole of under 0.2mm diameter determined by the threshold condition for non-propagation of fatigue cracks emanated outside the drilled hole. From these results, it can be concluded that an artificial drilled hole under 0.2 mm diameter could be made acceptable by shot peening.

608 人工表面欠陥を有する高強度鋼のショットピーニングによる疲労強度向上(GS.F-2 疲労)

608 人工表面欠陥を有する高強度鋼のショットピーニングによる疲労強度向上(GS.F-2 疲労)

The effect of small flaws on the fatigue strength of HAZ at the weld toe

The effects of small artificial surface flaws on the fatigue strength of HAZ material are studied. The samples of coarse grain HAZ material were prepared using a welding thermal-cycle simulator. The artificial flaws were produced by indenting with a Vickers pyramid at different loads as either single indentations or as a series. The size of the flaws did not exceed the primary austenitic grain size of the material, which is the most relevant microstructural unit of carbon steels. The dependence of the experimentally determined bending fatigue strength of treated coarse grain HAZ materials on the properly evaluated size of the artificial flaws was compared with the law of long-crack propagation. The biggest flaws evaluated with the flaw-size parameter √area ≅ 100 m are still small for the studied coarse grain HAZ. In further experimental work, the effects of flaw size and the effects of residual stresses should be analysed separately.

Inspection of Thin Copper Heat Exchanger Tubes Using SQUID-NDE System

This study is aimed at developing an eddy-current-based SQUID-NDE system for in-situ inspection of micro flaws on thin copper heat exchanger tubes. The system employs a high temperature superconductor (HTS-) SQUID gradiometer and a Helmholtz-coil-type inducer. As specimens, copper tubes 6.35 mm in outer diameter and 0.8 mm in thickness with artificial surface flaws of several tens μm in depth were inspected using the system. Magnetic response due to a flaw of 10 μm in depth on the tube moving at 32 m/min was successfully detected while applying an excitation field of 10 μT at 3 kHz to the tube. Numerical simulations were also conducted to evaluate how many sensors would be required for inspection around the circumference of an entire tube.

Exploration of the ultimate patterning potential achievable with high resolution focused ion beams

Controlled and reproducible fabrication of nano-structured materials will be one of the main industrial challenges in the next few years. We have recently proposed exploitation of the nano-structuring potential of a high resolution Focused Ion Beam Tool, to overcome basic limitations of current nano-fabrication techniques. The aim of this article is to present some new routes for material patterning, which benefit from ion-induced local property modifications or damage. In the experiments we describe hereafter an ultra-sharp pencil of 30 keV gallium ions is used to tailor the characteristics of several materials at a scale of a few nanometres. The experimental results are then compared to simulations. First, we simulate the control of collisional defects generated in a thin magnetic layer under FIB irradiation. The results explain the stable magnetic structures we have obtained experimentally. This was achieved with a low surface ion dose (1012 to 1014 ions/cm2). In addition we have explored the promising direction of “Bottom-up” or “self-organization” processes using a FIB instrument. We have defined artificial surface defects. These defects created by the impact of an 8-nm FWHM probe were used to pin the diffusion and to organize nanometre-sized gold clusters on a graphite surface.

Effect of surface defects and cross-sectional configuration on the fatigue fracture of NiTi rotary files under cyclic loading

The purpose of this in vitro study was to evaluate the effect of surface defects and cross-sectional configuration of NiTi rotary files on the fatigue life under cyclic loading. Three NiTi rotary files () with ＃30/.04 taper were evaluated. Each rotary file was divided into 2 subgroups : control (no surface defects) and experimental group (artificial surface defects), A total of six groups of each 10 were tested. The NiTi rotary files were rotated at 300rpm using the apparatus which simulated curved canal (40 degree of curvature) until they fracture. The number of cycles to fracture was calculated and the fractured surfaces were observed with a scanning electron microscope. The data were analyzed statistically. The results showed that experimental groups with surface defects had lower number of cycles to fracture than control group but there was only a statistical significance between control and experimental group in the (p

Fracture toughness of PAN-based carbon fibers estimated from strength–mirror size relation

Fracture toughness ( K IC) of representative high-strength type PAN (polyacrylonitrile)-based carbon fibers, Torayca™ T300 and T800H, with or without artificial surface defects, were estimated to be ca. 1 MPam 1/2 from the tensile strength vs. fracture mirror size relation, assuming a constant crack-to-mirror size ratio. The corresponding critical energy release rate ( Γ) was ca. 7.4 J m −2, which was close to the value derived from the reported surface energies for a graphite crystal. Similar K IC values were obtained for the old-type PAN-based carbon fibers from the reported data by the use of the present estimation procedure.

GSW0461 Application of J Integral to Fatigue Crack Propagation in Vulcanized Natural Rubber

Fatigue tests of vulcanized natural rubber were conducted under displacement-controlled conditions. The specimens were of three types: smooth specimen, specimen with an artificial surface defect of 0.2mm in diameter, and specimen with an artificial through-thickness defect of 2mm in length. In smooth specimens, a fatigue crack started from natural defects of about 0.15mm in size, and grew as an internal crack, and then became a through-thickness crack. The spring constant of the specimen decreased slowly in the range of about 80% of the total life (Stage A), followed by a rapid decrease (Stage B). The transition from Stage A to B took place when a crack penetrated through the thickness. The rate of fatigue crack propagation is expressed as a second-power function of the J-integral range both for surface cracks and through-thickness cracks. The J-integral was evaluated from the crack-center opening displacement. Fracture surfaces for three types of specimens were observed by scanning electron microscope (SEM). In smooth specimens and specimens with an artificial surface defect, fatigue cracks propagated in the shape of a concentric circle centering on natural or artificial defects. Fracture surfaces neighboring natural or artificial defects were very flat. After a crack penetrated through the thickness, the fracture surface became rough as the crack propagated. In specimens with an artificial through-thickness, the same tendency was also obtained.

J-Integral Approach to Fatigue Crack Propagation in Vulcanized Natural Rubber

Fatigue tests of vulcanized natural rubber were conducted under displacement-controlled conditions. The specimens were of three types: smooth specimen, specimen with an artificial surface defect of 0.2 mm in diameter, and specimen with an artificial through-thickness defect of 2 mm in length. In smooth specimens, a fatigue crack started from natural defects of about 0.15 mm in size, and grew as an internal crack, and then became a through-thickness crack. The spring constant of the specimen decreased slowly in the range of about 80% of the total life (Stage A), followed by a rapid decrease (Stage B). The transition from Stage A to B took place when a crack penetrated through the thickness. The rate of fatigue crack propagation is expressed as a second-power function of the J-integral range both for surface cracks and through-thickness cracks. The J integral was evaluated from the crack-center opening displacement.