Vicinity Of Notch Root Research Articles

Unforeseen, negative interaction-effect of adjacent circumferential notches on the fatigue limit of Ni-based superalloy 718

Investigations were conducted on the interaction-effects of adjacent, circumferential V-notches on the fatigue limit of Alloy 718, with the aid of fully-reversed, tension–compression fatigue tests. Contrary to expectation, specimen with seven adjacent circumferential notches had lower fatigue limit than that with a single circumferential notch. This phenomenon was attributed to the increased stress intensity factor (SIF) range of shear-mode fatigue cracks at notch-roots, resulting from competition between two factors: (i) stress mitigation through the mechanistic interaction of notches (decreased SIF) and (ii) the orderly, spatial alignment of crack-planes, due to the asymmetric stress-fields in the vicinity of notch-roots (increased SIF).

Ratcheting Examination of 1045 Notched Steel Plates under Low-High and High-Low Sequences

Abstract The present study examines the ratcheting response of 1045 notched steel plates undergoing uniaxial step-loading cycles. Loading steps were applied with different Low-High and High-Low sequences. Ratcheting tests were conducted on notched samples, and average local strains were measured with the use of strain gauges mounted in the vicinity of notch roots. Measured ratcheting strains near notch roots were found as high as 10 times those of tests conducted on unnotched samples. The magnitude of applied cyclic load as well as notch diameter influenced local ratcheting strain at the vicinity of the notch root. The lower nominal load level over the first step in the Low-High sequence substantially promoted ratcheting magnitude over subsequent loading steps with higher load magnitudes. In the High-Low loading sequence, the previous load step with higher magnitude retarded the local accumulated plastic strain of the subsequent step with a lower load level. The plastic zone at the notch root was affected over the loading steps. Microhardness tests were conducted at several positions within a small area of 1.5 by 4 mm near the notch root to map the plastic zone and its progress after each loading step. Microhardness data collected at the notch region verified an enlarged plastic zone over the Low-High loading sequence, while the plastic zone for the High-Low loading sequence was estimated about 50 % smaller in size. Cracking at the notch root and fracture surface taken within the plastic zone were examined through optical and scanning electron microscopies.

Ratcheting Response of SS316 Steel Samples with Different Notch Shapes under Various Loading Spectra

Ratcheting response of notched 316 stainless steel samples was examined under various low-high-high (LHH) and high-low-low (HLL) loading steps and sequences. Plate samples were tested under uniaxial asymmetric stress cycles contained circular or elliptical notches in center. Local ratcheting strain increased over loading steps of steel samples tested with LHH loading sequence, while it dropped in magnitude for samples tested with HLL loading histories. Local ratcheting strains were further evaluated through use of the Ahmadzadeh–Varvani (A–V) and Chaboche (CH) hardening models coupled with Neuber’s rule to predict ratcheting at the vicinity of notch root in steel samples. Finite element (FE) method was employed to numerically assess local ratcheting around the notch root through the use of ABAQUS software. Horizontal X-ellipse, circular and vertical Y-ellipse notch shapes lead to the highest, intermediate and lowest local ratcheting results for steel samples as predicted by means of the A–V and CH frameworks as well as those simulated results through FE method. Simulated ratcheting results through FE analysis positioned on or above the measured values for various loading steps and sequences. Predicted ratcheting results through the CH model positioned slightly above measured data over the first loading step and were turned to higher magnitudes over steps 2 and 3 in LHH samples with different notch shapes. This model underestimated ratcheting over loading steps 2-3 for HLL loading sequences. The predicted results by the A–V model consistently agreed with experimental data over the first step in notched samples tested under LHH and HLL sequences. Last two steps however resulted in lower ratcheting results through this model.

Ratcheting behavior of notched stainless steel samples subjected to asymmetric loading cycles

The ratcheting response of 316 stainless steel samples at the vicinity of notch roots under single- and multi-step loading conditions is evaluated. Multi-step tests were conducted to examine local ratcheting at different low–high–high and high–low–low loading sequences. The stress levels over loading steps and their sequences highly influenced ratcheting magnitude and rate. The change of stress level from low to high promoted ratcheting over proceeding cycles while ratcheting strains dropped in magnitude for opposing sequence where stress level dropped from high to low. Local ratcheting strain values at the vicinity of notch root were found noticeably larger than nominal ratcheting values measured at farer distances from notch edge through use of strain gauges. Ratcheting values in both mediums of local and nominal were promoted as notch diameter increased. To assess progressive ratcheting response and stress relaxation concurrently, the Ahmadzadeh-Varvani (A-V) kinematic hardening rule was coupled with Neuber’s rule enabling to calculate local stress at notch root of steel samples. Local stress/strain values were progressed at notch root over applied asymmetric stress cycles resulting in ratcheting buildup through A-V model. The relaxation of stress values at a given peak-valley strain event was governed through the Neuber’s rule. Experimental ratcheting data were found agreeable with those predicted through the coupled framework.

Local ratcheting assessment of steel samples undergoing various step and block loading conditions

Ratcheting response of notched SS316 samples was examined under various loading steps and blocks. Asymmetric stress cycles following various loading blocks of Low-High-High (LHH) and High-Low-Low (HLL) sequences were applied on steel plates with notch sizes of 5, 9 and 12 mm. Local strains were measured through strain gauges mounted at the vicinity of circular notch roots. Nominal ratcheting was found to gradually progress up to a saturation limit at which ratcheting magnitude and rate stayed unchanged. Over applied cyclic loads from the first block to the 100th, local ratcheting data have shown highest rate of progress between blocks 1 and 5. Local ratcheting data at the last step of blocks 1–5 have shown an overlap reflecting a constraint induced due to evolution of plastic zone at the vicinity of notch root. The Ahmadzadeh-Varvani (A-V) kinematic hardening rule was coupled with the Neuber’s rule to assess ratcheting at the notch root of steel samples. This framework enabled to predict local ratcheting over loading steps and blocks as asymmetric stress cycles were nominally applied. Local ratcheting curves predicted by the coupled framework were compared with those simulated through finite element method (FEM) through use of ABAQUS software and on the basis of Chaboche kinematic hardening model. The coupled framework showed a closer agreement with experimental data over loading steps and blocks than those simulated through FEM. Computed ratcheting curves were yet to address imbricating ratcheting curves over last steps of loading blocks in a manner similar to those of measured data.

Concurrent ratcheting and stress relaxation at the notch root of steel samples undergoing asymmetric tensile loading cycles

AbstractThe current study intends to develop a framework model to assess ratcheting and stress relaxation at the notch root of 1045 steel samples over asymmetric loading cycles. The framework involves the Ahmadzadeh‐Varvani (A‐V) kinematic hardening rule to control ratcheting progress and Neuber rule to accommodate for local stress and strain components at the vicinity of notch root. Plastic strain at notch root was first coupled with its counterpart in the A‐V model to establish a relation between local stress and backstress components. Calculated local stress and strain values at turning points enabled the A‐V model to assess ratcheting strain over each loading cycle. The stepwise drop in stresses at peak‐valley tips of hysteresis loops at the notch root was associated to coupled framework of the A‐V model and Neuber rule through constancy in local strain while ratcheting progressed over each cycle. This relaxed out the local stresses at tips of hysteresis loops to position on Neuber hyperbolic curve. Predicted ratcheting values at notch root of various diameters closely agreed with those of measured in steel samples over stress cycles.

Ratcheting progress at notch root of 1045 steel samples over asymmetric loading cycles: Experiments and analyses

AbstractThe present study examines ratcheting response of steel samples with various notch diameters through conducting several cyclic tests. Ratcheting strain values were measured through strain gauges mounted at different distances from the notch root. Local ratcheting at the notch region was highly influenced by notch diameter, notch shape, distance from the notch root, and magnitude of the nominal mean/amplitude of loading cycles. Nominal force‐controlled cycles were kept below the yield point and the Neuber's rule accommodated for the maximum/minimum local stress components along those local strains measured through the strain gauges at the notch region. Plastic strains at the vicinity of notch root over loading cycles were further accumulated by means of the Chaboche hardening model. The local ratcheting strain while progressed at the notch root plastic zone over loading cycles resulted in mean stress relaxation controlled by the model.

701 デジタル画像相関法の高精度化と切欠き底ひずみ計測への応用(GS-3 材料計測,研究発表講演)

An improved Digital Correlation Method (DCM) was developed to measure a small displacement and strain with a high accuracy. One additional technique was incorporated into the conventional DCM system. The Lagrange interpolation technique was newly developed in order to determine the value of the intensity at an arbitrary position. This technique was developed to raise a precision in determination of an identical position between two image planes; the sample surface images before/after deformation. The results obtained using various methods were compared. The modified DCM method with both the Lagrange interpolation and nonlinear strain distribution in subset was found to be good technique for detecting the two dimensional deformation. The DCM system was found to be effective method to strain measurement in the vicinity of notch root.

1463 高次デジタル画像相関法による応力急変部のひずみ分布計測(S20-5 応力・ひずみ分布計測法(1),S20 実験力学的手法による材料・製品の評価)

A high order Digital Image Correlation Method (DCM) was developed to measure a strain distribution at notch root with a high accuracy. Only a uniform deformation in a subset was considered in the conventional DCM system. However, the strain distribution near the stress concentrated area, like a notch, was not complex. For the application was incorporated into the conventional DCM system. From the measurement result, the developed DCM system was found to be effective to measure the strain in the vicinity of notch root.

応力集中部における微小表面き裂の応力拡大係数

A method of the estimation of approximate stress intensity factor for a semi-elliptical crack at notch root was proposed. The stress distribution in the vicinity of notch root was divided into two components: uniform and linear components. The linear component was correlated with the notch root radius ρ. The stress intensity factors for these two stress components were calculated using the equations which were made on the basis of numerical results for a semi-elliptical crack at the surface of a semi-infinite body.First, the accuracy of the proposed method was checked by comparing the present values with the accurate numerical results for a semi-elliptical crack at the inner wall of a pressurized hollow cylinder with an infinite outer radius. And then, the method was applied to a semi-elliptical crack in a pressurized hollow cylinder with a finite outer radius and to a semi-elliptical crack at notch root of CT specimen.The following closed form formula is proposed for the estimation of stress intensity factors for a semi-elliptical crack at notch root in other problems.KI, D=KIT+KIBKIT=σ1√πbFIT(λ), λ=b/aFIT(λ)=1.122-0.230λ-0.901λ2+0.949λ3-0.280λ4KIB=(σ2-σ1)√πbFIB(λ)FIB(λ)=0.443-0.310λ-0.104λ2+0.206λ3-0.061λ4where σ2 is the maximum stress at notch root and σ1 is that at the point corresponding to the deepest point of the elliptical crack front.

Fatigue Crack Propagation and Substructure of Tufftrided Steel

It has been pointed out by many researchers that substructure affects crack propagation in case of fatigue of metallic materials. However, the materials used in the past investigations were mostly homogeneous, and the microscopic behaviour of surface hardened materials such as tufftrided steel is only little known.In the present study, some investigations were made to obtain the relation between the crack propagation and substructure of tufftrided steel specimens with notch root through the observations of the crack propagation and the change in substructure in the vicinity of notch root and crack during the fatigue process by using the back reflection X-ray microbeam diffraction technique.The results obtained are summarized as follows;(1) The life to fracture or the period preceeding crack initiation of a tufftrided steel specimen increases remarkably as compared with that of a non-tufftrided steel specimen under the same stress amplitude.(2) The crack initiates at a comparatively early stage in the life to fracture and grows rapidly. However, when the crack tip passes through the position 1∼2mm away from the notch root, the crack propagation rate gradually decreases.(3) The substructure had already formed at the places with high content of soluted nitrogen after tufftriding, and the same substructure can be observed in the vicinity of crack tips irrespective of crack length.(4) It is evident from the observations by microbeam X-rays and by a scanning electron microscope that the plastically deformed area in the vicinity of crack in the tufftrided steel is very small and the magnitude of deformation of each crystal is also small as compared with that in the non-tufftrided steel.