Notched Plate Specimens Research Articles

A triaxiality‐dependent fracture model for hot‐rolled sections made of S355 steel

AbstractFracture at net areas of steel sections often leads to premature failures of steel members, especially in the region of joints (bolted connections). This study develops a fracture model for hot rolled S355 steel for enabling a better understanding of the ultimate behaviour of steel sections through numerical simulations using the general‐purpose finite element software ABAQUS. Monotonic tensile tests are conducted on traditional dog‐bone plate specimens with a uniform cross‐section and notched plate specimens extracted from hot‐rolled I‐beams. Two material thicknesses and three notch radii per thickness are considered thus obtaining equivalent plastic strains at fracture over a wide range of stress triaxialities. The experimental fracture displacement is used as a threshold for the determination of the average stress triaxiality – equivalent strain history at the critical location (fracture initiation) of each plate model. Using regression analysis, an exponential approximation of the fracture locus is proposed to correlate the average stress triaxiality for a given equivalent plastic strain at fracture. The proposed model is then used to predict fracture initiation of unnotched tensile coupons and steel plates with a bolt hole in their centre extracted from the same steel sections. The test results are in good agreement with the numerical predictions thus demonstrating the efficiency of the proposed method.

Determination of fracture toughness through small punch test of notched-tube specimen

Fracture toughness is an important mechanical property for designing structures or for conducting safety assessments. In this study, determination methods of fracture toughness of P91 material were studied using finite element simulation based on the Gurson-Tvergaard-Needleman (GTN) model. Results indicated that the JIC of P91 could be exactly determined using the simulation of standard fracture toughness test based on the GTN model. When the small punch test (SPT) was used to determine JIC, a notched-plate specimen provided better results to determine the JIC of P91 compared to the other two methods. A notched-tube specimen of SPT was proposed to determine JIC of nuclear reactor fuel cladding tubes of P91. Simulation results indicated that the notched-plate specimen and the notched-tube specimen provided identical accuracy for determining JIC. Therefore, the determination method derived from the notched-plate specimen could be used to the notched-tube specimen.

Vibration fatigue life prediction method for needled C/SiC composite based on frequency response curve with low signal strength

The experimental results show that needled C/SiC composite exhibits nonlinear stiffness characteristics in the process of vibration. In order to consider the effect of the number of fiber layers on the fatigue properties, the SN curve related to the number of 0° layer fibers for needled C/SiC composite is proposed in this investigation. According to the relationship between the frequency response curve and the signal strength, an amplitude probability density function under narrow band random vibrations was proposed for predicting the fatigue life for the notched plate specimen. The results showed that the proposed method has a good prediction effect.

A physically-based representative stress methodology for predicting stress rupture life of Ni-based DS superalloy

Smooth and three types of U-shape single-edge notched plate specimens adopted to experimentally investigate stress rupture behavior of Ni-based Directionally Solidified (DS) superalloy at 850 °C exhibit notch weakening effect and multi-source cracking initiation near the notch root. However, stress rupture behavior of smooth and V-shape notched round bars at 1040 °C revealed by Li et al indicates notch strengthening effect and creep micro-holes originating mostly from the central portion. A combined creep-viscoplastic constitutive model is employed to analyze the distribution of stress, strain and stress Triaxial Factor (TF) near the notch root. The different stress distribution and creep restraint between asymmetric notched plate specimens and symmetric notched round bars are the main reasons for the corresponding failure mechanism. Meanwhile, a good qualitative relationship exists between TF value and stress rupture life of notched specimen. Especially, the area with maximum TF value (TFmax) is highly consistent with creep damage initiation region. Hence, based on the distribution characteristics of the initial tensile loading, a representative stress method independent of time -changing creep load at the location of TFmax is conducted for life prediction. The predicted results of both smooth and notched plate specimens and round bars agrees well with the experimental results.

An experimental investigation on the net cross-section failure of damaged plates containing holes

In Eurocode 3, a design rule is given for predicting the ultimate resistance of the net cross-section at bolt holes. This is based on the average stress evaluated in the net cross-section, and contains both a reduction factor of 0.9 and a partial factor. According to previous research, this reduction factor makes the design rule over-conservative for uncracked components. There are indications that this conservatism accounts for the possible presence of small cracks at the bolt holes. Very limited experimental research has been conducted on the influence of potential cracks at the bolt hole, particularly at the notch root. This paper presents a few experimental results on the ultimate load of pre-cracked notched plate specimens made of S275JR grade steel. Four pre-cracked specimens, with crack depths ranging from 1 to 7mm, have been tested and their ultimate load has been compared with the one of non-pre-cracked specimens, exhibiting lower ultimate load and ductility as compared to the undamaged specimens. However, despite the significant pre-cracking, the ultimate strength is slightly lower than calculated with the design rule of Eurocode 3, considering mean material properties and excluding the partial factor. Moreover, the pre-cracked specimens have been assessed using a constraint-corrected version of the failure assessment diagram validating its prediction with the experimental results.

Stress Rupture Life Prediction Method for Notched Specimens Based on Minimum Average Von Mises Equivalent Stress

Creep tests were carried out on notched plate specimens of nickel-based superalloy GH4169 with different stress concentration coefficients. It was found that the duration of the first stage of the creep curve increases with the increase of stress concentration coefficient, while the fracture ductility decreases with the increase of stress concentration coefficient. To predict the life of notched plate specimens, four constitutive models were used to analyze the stress and strain of the notches. It was found that the average Von Mises equivalent stress (AVES) on the minimum notch section first decreases and then increases with the creep time, resulting in a minimum value. The minimum average Von Mises equivalent stress (MAVES) is considered as the characteristic stress of notched specimens in this paper. The creep life equation is fitted according to the results of creep tests of smooth specimens, and then the predicted life of notched specimens is obtained by substituting the minimum average Von Mises equivalent stress of notched specimens into the creep equation. The prediction results of the four constitutive models are within 2 times the dispersion band, and the three-stage model is within the 1.5 times dispersion band.

Total fatigue life prediction of TC4 titanium alloy based on surface notch

A surface notch will be produced in crack grinding process of cracked blade, and fatigue life of surface notch is a critical measure to assess its remanufacturability. In this paper, a total fatigue life separation model based on surface notch is studied, which is defined as the sum of crack initiation life and crack propagation life. Tensile fatigue experiments were performed on surface notched plate specimens. At a given notch depth, there is a negative correlation between the secondary crack density and the notch width. Combined with Neuber's rule and elastic–plastic finite element results, a residual thickness correction coefficient Sε is proposed to reflect the effect of finite thickness on local strain. Depending on simulation results, it is found that Neuber's rule underestimates the notch-root strains of surface notch, which is contrary to the current conclusions. Crack initiation life is estimated by modifying Morrow method with the proposed Sε. The initial crack size a0 of surface notch is qualified by equivalent initial flaw size (EIFS) modified by fatigue strength calculated by Neuber approach. Crack initiation life is estimated by Labg equation with the improved a0. The proposed model is verified by fatigue experiment results with a prediction accuracy falling into the scatter band of factor 1.5. This study enriches the current notch fatigue analysis, which can provide reference suggestions by the model for crack grinding design in remanufacturing pre-treatment process.

A multiaxial fatigue life prediction method for metallic material under combined random vibration loading and mean stress loading in the frequency domain

A spectral method for the determination of the critical plane is proposed for metallic material under random vibration loadings based on the criterion of maximum shear stress amplitude, where the direction of critical plane is obtained by solving the maximum value of a function using mathematical optimization method and the fatigue critical location of the structure can be determined. A multiaxial fatigue damage parameter is proposed for combined random vibration loading and mean stress loading cases based on Susmel’s criterion, which is a Power Spectral Density (PSD) of the equivalent shear stress and can be adopted for fatigue life evaluation. The fatigue life estimation of structures under multiaxial random vibration loadings with mean stresses in the frequency domain could be carried out based on this damage parameter constructed at the fatigue critical location with the spectral method for random vibration fatigue life. The multiaxial random vibration fatigue experiment with mean stress loadings is conducted on the notched plate specimens made of 2297-T87 aviation grade aluminum alloy, and the initiation sites of the fatigue cracks as well as the experimental fatigue lives are compared with those calculated by the proposed models, where satisfactory prediction capabilities on both the fatigue crack initiation locations and fatigue lives of the models are demonstrated.

Critical distance model for the fatigue life analysis under low-velocity impacts of notched specimens

A fatigue life estimation method is proposed for the notched specimens under low-velocity repeated impact loadings. The stress concentration effect at the notch root is taken into account by the implementation of the Theory of Critical Distance, and the continuous variation of strain ranges at the critical point due to the plastic strain accumulation is considered by the mean strain modification. The impact fatigue experiment is conducted on the notched plate specimen of material S45C with the pendulum-type test machine, and the comparison with the numerical analysis results indicates that the proposed model gives a satisfactory prediction on the impact fatigue lives.

Study on local failure conditions of nuclear containment steels

Tensile tests were performed for notched round-bar and plate specimens to acquire the local failure characteristics of SGV480 and SPV490 steels for nuclear containment structures. The notched round-bar specimens were sampled from base metals, weld joints including heat affected zone (HAZ), and simulated-HAZ materials. The simulated-HAZ materials were made using multiple heating and cooling processes with induction heating and forced air cooling. The limit strain was calculated using the Bridgman’s solution with the notch cross-sectional dimensions of the specimens, which were continuously measured with a profile projector. All notched round-bar specimens caused the tensile type ductile fracture, and there was no significant difference in the limit strains among the base metal, HAZ, and simulated-HAZ material. The limit strains were higher than the ASME VIII-2 design curve, and showed conservative results. Tensile tests using the full or partial width notched plate specimens were performed for the base metals of the two steels. In the cases of small curvature notched specimens of both full and partial widths, cracks occurred on the surface of the notch bottom and propagated into the depth of the material, until the specimens finally fractured. The limit strain plots were concentrated in a low strain and stress triaxiality position. In the cases of large curvature notched specimens of both types, no cracks occurred, and the specimens would suddenly fracture in shear after a slight shrinking of thickness. The limit strains of the notched plate specimens were lower than those of the notched round-bar specimens.

Experimental investigation and simulation on stress rupture behavior of a Ni-based DS superalloy affected by initial elastic-plastic multi-axial stress state

In this study, the stress rupture behavior of a Ni-based directionally solidified (DS) superalloy affected by initial elastic-plastic multiaxial stress state at 850 °C has been investigated. Smooth and three types of U-shape notched plate specimens have been adopted. The notch effect on stress rupture lives is exhibited as notch softening, and there is a positive correlation between the degree of notch softening and theoretical stress concentration factor. Scanning electron microscopy (SEM) observation is conducted to reveal failure mechanism. For both smooth and notched specimens, thickness dependence on crack initiation due to the changed crack tip stress state is exhibited. The initiation of microcracks is mainly caused by interfacial decohesion at the carbide/matrix interface, non-uniform deformation field at eutectic/matrix interface and stress concentration around cavities. The plastic fiber zone of smooth plate is located in the intermediate region, which is occurred throughout a wide region near notch root of notched specimen. Finally, a finite element analysis (FEA) is performed based on the transverses elastic viscoplastic constitutive model with the Kachanov-Rabotnov damage rule. The simulation results show that the effect of notch softening is due to the fact that the redistributed tensile stress is higher than the value of net section stress in a very wide region.

Image-driven structural steel damage condition assessment method using deep learning algorithm

This paper presents a deep learning based structural steel damage condition assessment method that uses images for post-hazard inspection of ultra-low cycle fatigue induced damage in structural steel fuse members. The deep learning model – Convolutional Neural Network (CNN) model, can be trained to represent the high dimensional features in huge amount of raw data which traditional mathematical models are unable to describe. A saliency-based visualization method is employed to visualize the feature-related pattern recognized by the deep learning model. To quantify the damage condition of the inspected structure fuse members, a micromechanical fracture index is defined as the damage index and used for labeling the images in the training data set. To provide large training dataset, cumulative plastic strain contour plot images generated through finite element (FE) simulation are adopted for the training data. Parametric studies were performed to validate and optimize the deep learning based damage condition assessment method. The method and findings are further examined using real experimental images collected from cyclic testing of a steel notched plate specimen. The results suggest that the proposed method provides a promising automation tool for rapid inspection of structural steel fuse member damage condition.

Experimental investigation on low cycle fatigue and fracture behaviour of a notched Ni‐based superalloy at elevated temperature

ABSTRACTIn order to investigate the effects of stress concentration on low cycle fatigue properties and fracture behaviour of a nickel‐based powder metallurgy superalloy, FGH97, at elevated temperature, the low cycle fatigue tests have been conducted with semi‐circular and semi‐elliptical single‐edge notched plate specimens at 550 and 700 °C. The results show that the fatigue life of the notched specimen decreases with the increase of stress concentration factor and the fatigue crack initiation life evidently decreases because of the defect located in the stress concentration zone. Moreover, the plastic deformation induced by notch stress concentration affects the initial crack occurrence zone. The angle α of the crack occurrence zone is within ±10° of notch bisector for semi‐circular notched specimens and ±20° for semi‐elliptical notched specimens. The crack propagation rate decreases to a minimum at a certain length, D, and then increases with the growth of the crack. The crack propagation rate of the semi‐elliptical notched specimen decelerates at a faster rate than that of the semi‐circular notched specimen because of the increase of the notch plasticity gradient. The crack length, D, is affected by both the applied load and the notch plasticity gradient. In addition, the fracture mechanism is shown to transition from transgranular to intergranular as temperature increases from 550 to 700 °C, which would accelerate crack propagation and reduce the fatigue life.

Fatigue Evaluation of Notched Plate Specimens by the Battelle Structural Stress Method

Fatigue Evaluation of Notched Plate Specimens by the Battelle Structural Stress Method

Crack Initiation Process for Semicircular Notched Plate in Fatigue Test at Elevated Temperature

Crack initiation and propagation process of fatigue test in semicircular notched plates at elevated temperature were conducted under strain-controlled condition in order to study the crack initiation/propagation/coalescence process and load decrease in structure having stress concentration. Test specimens made of SUS304 stainless steel are tested at 550°C and the geometry of the semicircular notched plate specimens are changed by diameter size of the circular notch in both of single-notched specimens and double-notched specimens. Photographs in all cycles were recorded to investigate crack initiation/propagation/coalescence as a function of number of applied cycle. The typical crack process is predicted by using the simplified method, namely, stress redistribution locus method as well as Neuber’s formula.

S031011 ステンレス鋼SUS316Lの切欠き付き平板のねじり疲労き裂進展挙動([S03101]低サイクル疲労とその応用(1))

It is necessary to investigate the crack growth behavior of large deformation under the complex loading in order to maintain reliability of actual structures subjected to seismic forces. Therefore, a large deformation torsion fatigue testing machine is developed. Then, the crack growth behavior of notched plate specimens of stainless steel, SUS316L, used for piping is observed. The fatigue crack growth behavior of surface changed from the shear mode to the opening mode. As a result of fracture surface observation, it is found from crack front geometry that crack growth rate of surface is faster than that of inside. J-integral range of crack front for SUS316L is calculated by finite element method and crack growth characteristic is evaluated by J-integral range. These results corresponded to the mode I crack growth characteristics.

Stress State Dependent Cohesive Zone Model for Thin Walled Structures

A new stress-state dependent cohesive zone model for thin walled structures is proposed. The model incorporates the stress-state explicitly within the traction-separation law using basic elasticity-plasticity equations combined with a model parameter. The numerical implementation of the model is able to reproduce ductile fracture observed in a pre-cracked C(T) specimen as well as a notched plate specimen of the same material.

Elastoplastic characterization and damage predictions under evolving local triaxiality: axysimmetric and thick plate specimens

Ductile failure is a phenomenon involving large plastic strains, thus any method for predicting fracture of mechanical components needs to be supported by a reliable procedure for the material stress–strain characterization, whose accuracy is an essential prerequisite for allowing any damage model to deliver accurate predictions. Sometimes the true stress–true strain curves from smooth tensile specimens are used in the literature for identifying a metal despite their low accuracy at the post-necking deformation levels, but usually the experimental true stress is corrected by the Bridgman factor for the necking effect. Two mathematical models, of the stress–strain characterization in the elastoplastic post-necking phase and of the ductile failure prediction, are used in this paper for a FE 370 carbon steel. The stress–strain data accounting for the necking-induced triaxiality and the critical value of a damage indicator based on the cumulated stress triaxiality are obtained by applying the model called MLR and the Bao–Wierzbicki model, respectively, to experimental data from tensile tests on smooth and notched specimens. Then, the acquired data are used to simulate the material behavior with reference to two series of differently notched plate specimens. Simulated behavior is then compared with experiments for evaluating the achieved degree of accuracy.

Tear Toughness of Permanent Mold Cast and DC Cast A356 Aluminum Alloys

Cast products of A356 with different microstructural features were prepared; permanent-mold cast (PM) and direct-chill cast (DC) products. For each casting, a sharp notched plate specimen was subjected to static tensile loading until a crack initiated at the notch root and propagated across the width of the specimen. Both maximum load and energy to fracture (the integrated area under the load-displacement curve) increased with decreasing dendrite arm spacing (DAS). The curve of DC was smooth and the energy to fracture was quite large. The load-displacement curve was divided into two segments by a vertical line through the maximum load. The area under the first segment is a measure of the energy necessary to initiate the crack. The second segment represents the energy necessary for crack propagation. Unit energy was computed by dividing the measured energy by the net area of the specimen. Refinement of DAS and grain size increased unit energies for crack initiation (UEi) and propagation (UEp). Comparison among PM products revealed that DAS refinement was effective for increasing UEi. Among the present castings, the DC product with the finest DAS exhibited a significant increase in UEp. Observation of the crack propagation path revealed that the fracture surface was normal to the loading direction for PM. In contrast, for DC, a slanted crack path was dominant through the specimen ligament. The features of the crack propagation path are considered to have affected quantitative balance between UEi and UEp. The increased UEp in DC is considered to be due to the introduced slanted crack. Tear tests were confirmed to provide useful information concerning the effect of solidification structure on toughness, which can serve as a guide for further toughening of aluminum alloy castings.

502 熱応力を用いた純チタン薄板試験片の引張りとその場観察

In the present study, a new method of tensile test for small and/or thin specimens was proposed where tensile force was generated in the specimen by the thermal stress between the specimen and the loading plate. The basic mechanism of the method is as follows. Thin notched plate specimen was set on the loading plate and both ends of the specimen were adhered to the loading plate with quick-drying glue. As the loading plate has much higher linear expansion coefficient than the specimen, tensile stress was generated in the specimen with simultaneous heating of the specimen and the loading plate. A strain gauge was also glued on the loading plate in order to evaluate the load acting on the specimen. As all devices can be set on the stage of a microscope, in-situ observation of the plate specimen during tension was realized.