Circumferential Notch Research Articles

Evaluating stress corrosion cracking behaviour of high strength AA7075-T651 aluminium alloy

AbstractThe objective of the present study is to determine the threshold stress level of stress corrosion cracking (SCC) in AA7075-T651 aluminium alloy by suitable experimentation. The test was carried out using a circumferential notch specimen in a horizontal-type constant load SCC setup in a 3.5 wt.% NaCl solution. The time to failure by SCC was determined at various loading conditions. The threshold stress of AA7075-T651 alloy was found to be 242 MPa in a 3.5 wt.% NaCl solution. The various regions of the fractured surface specimen such as machined notch, SCC region and final overload fracture area were examined using scanning electron microscopy (SEM) in order to identify the SCC mechanism.

Critical Void Volume Fraction fc at Void Coalescence for S235JR Steel at Low Initial Stress Triaxiality

The paper is concerned with the nucleation, growth and coalescence of microdefects in the form of voids in S235JR steel. The material is known to be one of the basic steel grades commonly used in the construction industry. The theory and methods of damage mechanics were applied to determine and describe the failure mechanisms that occur when the material undergoes deformation. Until now, engineers have generally employed the Gurson-Tvergaard- Needleman model. This material model based on damage mechanics is well suited to define and analyze failure processes taking place in the microstructure of S235JR steel. It is particularly important to determine the critical void volume fraction fc, which is one of the basic parameters of the Gurson-Tvergaard-Needleman material model. As the critical void volume fraction fc refers to the failure stage, it is determined from the data collected for the void coalescence phase. A case of multi-axial stresses is considered taking into account the effects of spatial stress state. In this study, the parameter of stress triaxiality η was used to describe the failure phenomena. Cylindrical tensile specimens with a circumferential notch were analysed to obtain low values of initial stress triaxiality (η = 0.556 of the range) in order to determine the critical void volume fraction fc. It is essential to emphasize how unique the method applied is and how different it is from the other more common methods involving parameter calibration, i.e. curve-fitting methods. The critical void volume fraction fc at void coalescence was established through digital image analysis of surfaces of S235JR steel, which involved studying real, physical results obtained directly from the material tested.

Shear Mode Stress Intensity Factors for Serrated Crack Fronts

The paper is related to experiments on near-threshold fatigue cracks under shear modes II, III and II+III in bcc metals. Cylindrical bars with circumferential cracked notch were loaded by shear force. In-plane precracks with microtortuous geometry were created by compressive cyclic loading in mode I to measure the effective values of the remote crack driving force. Fatigue cracks in bcc metals loaded under remote shear modes II, III and II+III always grew by creation of local tongues loaded in mode II and their coalescence. Therefore, serrated precrack fronts of a linear roughness identical to those of the real fronts were modeled and the related local stress intensity factors k2 were calculated. Since such FEM calculation for various values of roughness were time consuming, a further task was to identify a lowest number of isolated teeth that produces k2 components identical with those created by the continuously serrated crack front. The results reported in this article reveal that this condition is fulfilled by only two isolated teeth.

Static Multiaxial Fracture Behavior of Graphite Components: A Review of Recent Results

The problem of mixed mode (I+III) brittle fracture of polycrystalline graphite is investigated systematically here for the first time. The present study considers cylindrical specimens weakened by circumferential notches characterized by different acuities. A new complete set of experimental data is provided considering different geometrical configurations by varying the notch opening angle and the notch tip radius. The multiaxial static tests have been performed considering different values of the mode mixity ratio. A criterion based on the local Strain Energy Density previously applied by the same authors only to pure modes of loading is extended here to the case of tension and torsion loadings applied in combination.

Effect of fly ash addition on the fracture toughness of plain concrete at third model of fracture

A new testing method and estimation of the fracture toughness in Mode III (antiplane shear) of concretes contain­ing: 0, 20 and 30% volume content of the class F fly ash (FA) was proposed. Fracture toughness tests were performed on axial torsional machine MTS 809 Axial/Torsional Test System using special device. The Device for the fracture toughness test at the Mode III fracture consisted of the cylindrical specimen with the formed initial crack, steel plates, and screws with washers securing the specimen in the press holders. The specimens applied for experiments were cylinders with dimensions of 150/300 mm, having an initial circumferential notch made in the half-height of cylinders. The specimens were subjected to pure torsion. The studies examined effect of FA additive on the parameter KIIIc. 20% FA additive causes a small increase of the KIIIc in comparison to the concrete without the FA, while 30% FA additive causes a significant decrease in fracture tough­ness. The fracture surfaces of the specimens analysed macroscopically were planar. Scanning microscopic observations (SEM) indicate that in the fracture surfaces multilevel cracks occur in the shape of semicircles.

Investigation of role of alloy microstructure in hydrogen-assisted fracture of AISI 4340 steel using circumferentially notched cylindrical specimens

The present study investigates the role of three different microstructures of a high strength steel AISI 4340 in relation to its susceptibility to hydrogen-assisted cracking (HAC), and demonstrates the ability of circumferential notch tensile (CNT) testing in distinguishing the role of the three microstructures. Critical stress intensity factors (SIFs) of CNT specimens of the steel with different microstructures (viz., the spheroidite (the as-received steel), the untempered martensite (the oil-quenched steel) and the tempered martensite (the oil-quenched + tempered steel)) were determined under two conditions at the room temperature, i.e., in air and under in-situ cathodic charging in 3.5% NaCl solution. The CNT tests under the two conditions show the as-received steel (with the spheroidite microstructure) to have a considerably high critical SIF (~58MPam0.5) in air, and an insignificant decrease in critical SIF data when the test condition was changed to the cathodic charging, suggesting an insignificant susceptibility of the steel with the spheroidite microstructure to HAC. In contrast, the oil-quenching of the steel dramatically decreased the critical SIF to ~17MPam0.5 in air, suggesting the highly strained martensitic microstructure to be more susceptible to cracking in air. The low critical SIF of the oil-quenched steel became even lower (~10MPam0.5) under the cathodic charging condition, suggesting increased susceptibility of the martensitic microstructure to HAC. However, tempering of the martensitic microstructure at 300°C for 2h raised the critical SIF in air (by ~3 times), i.e., fracture resistance increased dramatically, but no significant improvement in the resistance to HAC was noticed. The hardness data of steels with the different microstructures are consistent with the trend in the corresponding SIF data. The study also provides two practical information: (a) an SIF below 10MPam0.5 under operating load would avoid HAC in AISI 4340 steel in applications, and (b) the relatively new CNT technique is a viable low-cost testing option in assessing the role of microstructural variations in the context of HAC of high strength structural steels.

Determination of fracture toughness in concretes containing siliceous fly ash during mode III loading

This paper describes laboratory tests carried out to evaluate the influence of class F fly ash (FA) on fracture toughness of plain concretes, specified at the third model fracture. Composites with the additives of: 0%, 20% and 30% siliceous FA were analysed. Fracture toughness tests were performed on axial torsional machine MTS 809 Axial/Torsional Test System, using the cylindrical specimens with dimensions of 150/300 mm, having an initial circumferential notch made in the half-height of cylinders. The studies examined effect of FA additive on the critical stress intensity factor KIIIc. In order to determine the fracture toughness KIIIc a special device was manufactured.The analysis of the results revealed that a 20% FA additive causes increase in KIIIc, while a 30% FA additive causes decrease in fracture toughness. Furthermore, it was observed that the results obtained during fracture toughness tests are convergent with the values of the compression strength tests.

The fracture toughness the KIIIc of concretes with F fly ash (FA) additive

The aim of this study is elaboration of a new testing method and estimation of the fracture toughness in Mode III of concretes with F fly-ash (FA) additive. The applied specimens were cylinders with a circumferential notch at their halfheight, which are made of plain concretes with different volume content of the FA additive. In fact, the influence of the FA additive evidently changes the fracture toughness KIIIc, like in case of pure shear KIIc (Golewski and Sadowski 2014). Namely, 20% FA additive causes a small increase of the KIIIc in comparison to the concrete without the FA, while 30% FA additive significantly decrease in the fracture toughness.The experimental program was completed by analysis of fracture surfaces of broken specimens. SEM observations indicate that the longest cracks in ITZ between paste and coarse aggregates took place in the concretes containing 30% FA. For 20% FA additive we observed significantly smaller crack lengths.The obtained results are significant in designing of any civil engineering structural elements subjected to the influence of torsion loading (where unstable cracking process leading to failure begins at KIII⩾KIIIc) or more complex stress states due to bending and torsion.

円周切欠きを有する丸棒試験片におけるひずみ速度集中係数の提案と応力集中係数の関係についての考察

円周切欠きを有する丸棒試験片におけるひずみ速度集中係数の提案と応力集中係数の関係についての考察

Multiaxial fracture of graphite components: a review of recent results

Abstract: While a large bulk of experimental results from cracked specimens of polycrystalline graphite under pure modes of loading, in particular under mode I loading, can be found in the literature, only a very limited number of tests have been carried out on notches. At the best of authors' knowledge dealing with the specific case of V-notches under mixed mode loading (tension + torsion) no results can be found in the literature. With the aim to fill this lack, the problem of mixed mode (I+III) brittle fracture of polycrystalline graphite is investigated systematically here for the first time. The present study considers cylindrical specimens weakened by circumferential notches characterized by different acuities. A new complete set of experimental data is provided considering different geometrical configurations by varying the notch opening angle and the notch tip radius. The multiaxial static tests have been performed considering different values of the mode mixity ratio (i.e. the ratio between the nominal stress due to tension and that due to torsion loading). A criterion based on the local Strain Energy Density previously applied by the same authors only to pure modes of loading is extended here to the case of tension and torsion loadings applied in combination. The proposed criterion allows a sound assessment of the fracture loads.

Detection of axial and circumferential notches using transceiver rotating field probe for tube inspection

Detection of axial and circumferential notches using transceiver rotating field probe for tube inspection

The influence of various types of small defects on the fatigue limit of precipitation-hardened 17-4PH stainless steel

Fatigue tests were performed on 17-4PH stainless steel specimens containing small artificial defects, with area ranging from 30 to 900μm at stress ratios, R, of −1, 0.05 and 0.4. An investigation was thus conducted to examine the influence of various types of small artificial defects on fatigue strength, including circumferential notches, corrosion pits, drilled holes and pre-cracked holes. The fatigue limit was determined by the threshold condition for the propagation of a crack emanating from the defects. The threshold stress intensity factor range, ΔKth, exhibited a defect size dependency for area≤80µm , and it became a constant value for area>80µm independent of R. Based on the area parameter model and a material constant of ΔKth for long cracks, the fatigue limit could be predicted as a function of R, with the exception of drilled holes with relatively large diameters of 100 and 300μm, for which the fatigue limit was determined by the critical condition for crack initiation. When artificial defects were absent or non-detrimental, intrinsic defects, such as non-metallic inclusions, were found to control the fatigue strength and, in addition, were responsible for the scatter in the fatigue limit. The proposed method enables the quantitative evaluation of the lower bound of the scatter as a function of the number of test specimens, or the overall control volume of fatigue-loaded components.

A Prediction Model for Mode-III Fatigue Crack Growth

In the fracture mechanics, the failure of mode-III crack belongs to the anti-plane shear (also called out-plane shear or longitudinal shear). Fatigue crack growth (FCG) behavior under mode-III reversed load (load ratio R=-1) was studied by using 30Cr2Ni4MoV rotor steel with types of circumferential notches. Under the remote cyclic torsional load, the precrack process was applied to specimens with notch in the round bar, and the ideal sharp-crack was obtained. Then, mode-III FCG test under the remote cyclic torsional load and the axial constant small load was carried out until to fatigue failure. The axial constant small load is aimed at reducing the cracks interaction in the mode-III FCG experiment. Due to experimental data of 30Cr2Ni4MoV rotor steel, the mode-III FCG rate in the range of KIII from 12MPa·m1/2 to 40 MPa·m1/2 and the threshold value were got. Furthermore, a prediction model for mode-III FCG is proposed with considering the small scale yield of plane stress crack tip zone and the plastic strain energy density failure criteria. The predicted mode-III FCG rate is found to agree well with the experimental curves of 30Cr2Ni4MoV rotor steel.

Ductile fracture of EN-AW 2024 aluminum alloy specimens with notches under biaxial loading. Part 1 – Experimental research

This paper presents the results of the experimental studies on ductile fractures of axisymmetric specimens with circumferential notches. A range of notch root radii is considered. The specimens underwent proportional biaxial loading (tensile–torsion) until failure. The load was forced onto the specimens by elongation as well as angular movement and was controlled by a device for the non-contact measurement. The EN-AW 2024 T351 aluminum alloy was used in this research. Special attention was paid to the change in shape of the fracture surface, changing the force-elongation curve and torsional moment–torsional angle curve, which are dependent on loading.

Ductile fracture of EN-AW 2024 aluminum alloy specimens with notches under biaxial loading. Part 2 – Numerical research and ductile fracture criterion

This paper presents the results of the numerical modelling of axisymmetric specimens with circumferential notches. A range of notch rood radii is considered. The specimens underwent proportional biaxial loading. Stress and strain fields in whole specimens with notches were calculated using finite element analysis (FEM). Special attention was paid to changing distributions of stresses and plastic strains induced biaxial loading and notch radius. By taking advantage of numerical calculations, new ductile fracture criterion for notched specimens under biaxial loading was proposed.

Novel Rotating Current Probe With GMR Array Sensors for Steam Generate Tube Inspection

This paper presents a novel probe design that uses a rotating current excitation scheme and giant magnetoresi-stance (GMR) sensors for steam generator tube inspection. Rotating eddy currents in the tube wall under test is induced using a pair of axial and circumferential excitation coils. A circumferential array of GMR sensors measures the radial component of the magnetic field. The perturbation in eddy current flow caused by a defect produces a radial component field that is measured by the GMR sensors. A C-scan image of the induced magnetic field obtained with the axial motion of probe shows defect location and orientation. The probe is sensitive to both the axial and circumferential defects and offers the advantages of high sensitivity over a wide frequency range. A finite-element model was developed to mimic the underlying physical process and predict the output images of defects with different orientations and sizes. A prototype unit consisting of excitation coils and GMR sensor array along with a data acquisition system was developed and tested. A steam generator tube sample with machined axial and circumferential notches was inspected using the prototype probe. Experimental results demonstrating the feasibility of the concept are presented.

A Longitudinal Mode Electromagnetic Acoustic Transducer (EMAT) Based on a Permanent Magnet Chain for Pipe Inspection.

A new electromagnetic acoustic transducer (EMAT) design, employing a special structure of the permanent magnet chain, is proposed to generate and receive longitudinal guided waves for pipe inspection based on the magnetostriction mechanism. Firstly, a quantitative analysis of the excitation forces shows the influence of the radial component can be ignored. Furthermore, as the axial component of the static magnetic field is dominant, a method of solenoid testing coils connected in series is adopted to increase the signal amplitude. Then, two EMAT configurations are developed to generate and receive the L(0,2) guided wave mode. The experimental results show the circumferential notch can be identified and located successfully. Finally, a detailed investigation of the performance of the proposed EMATs is given. Compared to the conventional EMAT configuration, the proposed configurations have the advantages of small volume, light weight, easy installation and portability, which is helpful to improve inspection efficiency.

Assessment of Rupture Behaviour of 9Cr-Steels Under Multiaxial State of Stress

In this investigation, detailed finite element analysis has been carried out to assess the creep behaviour of 9Cr-1Mo and modified 9Cr-1Mo steels under multiaxial state of stress. The multiaxial state of stress was introduced by incorporating the circumferential notches of different notch radii in smooth cylindrical specimens. The creep experiments were carried out on smooth and notched specimens of the steels at 873 K and in the stress range of 110–230 MPa. The modified 9Cr-1Mo steel possessed significantly higher creep strength than the 9Cr-1Mo steel. The rupture life was found to increase in presence of notch for both the steels. However, the extent of strengthening was found to be significantly higher for modified 9Cr-1Mo steel compared to 9Cr-1Mo steel. The fracture appearance was found to be dependent on the applied stress and the notch geometry. Typical transgranular ductile fracture was observed for shallow notches and mixed mode fracture consisting of ductile and creep cavitation for relatively sharper notches, which was more prevalent at lower applied stress. Finite element analysis was carried out to understand the distribution of stresses across the notch throat plane. The stresses were found to relax in the notch throat plane. The stress relaxation at the notch root was found to be more for modified 9Cr-1Mo steel than that of 9Cr-1Mo steel, resulting in higher extent of strengthening in modified 9Cr-1Mo steel.

Influence of intrinsic and artificial defects on the VHCF properties of 17-4PH stainless steel

In the present work, fatigue test results in the very high cycle fatigue (VHCF) regime are discussed. Experiments were performed with smooth specimens in which crack initiation mostly originated at non-metallic inclusions. Additionally, the influence of small artificial defect, i.e. circumferential notches, drilled holes and corrosion pits, was investigated. Fatigue tests in the VHCF regime were conducted using ultrasonic fatigue testing technique. Further experiments in the high cycle fatigue regime were performed using rotating bending and servo-hydraulic testing machines.For smooth specimens, fatigue fracture was observed even at more than 1010 number of cycles. In contrast, no fatigue failure occurred above approximately 107 cycles in the presence of artificial defects, i.e., a pronounced fatigue limit was determined. Furthermore, it was found that the fatigue limit is not only depending on the defect size but also on the notch root radius of a defect. Consequently, different approaches were used to predict the fatigue limit in the presence of different defects.

Fracture assessment of graphite components weakened by rounded V-notches and subjected to static multiaxial loading

While a large bulk of experimental results from cracked specimens of polycrystalline graphite under pure modes of loading, in particular under mode I loading, can be found in the literature, only a very limited number of tests have been carried out on notches. At the best of authors’ knowledge dealing with the specific case of V-notches under mixed mode loading (tension + torsion) no results can be found in the literature. With the aim to fill this lack, the problem of mixed mode (I+III) brittle fracture of polycrystalline graphite is investigated systematically here for the first time.The present study considers cylindrical specimens weakened by circumferential notches characterized by different acuities. A new complete set of experimental data is provided considering different geometrical configurations by varying the notch opening angle and the notch tip radius. The multiaxial static tests have been performed considering different values of the mode mixity ratio (i.e. the ratio between the nominal stress due to tension and that due to torsion loading).A criterion based on the local Strain Energy Density previously applied by the same authors only to pure modes of loading is extended here to the case of tension and torsion loadings applied in combination. The proposed criterion allows a sound assessment of the fracture loads.