Notch Bottom Research Articles

A Numerical Study of the Damage Mechanisms for CT Tensile Specimens of P265GH Steel Material

The aim of this paper is to determine the damage mechanisms of P265GH steel, commonly used for pressure equipment. First, an experimental study using tensile and Charpy tests allowed us to determine the mechanical properties (Young modulus E = 200 GPa, elongation ε = 35%, yield se = 320 MPa, ultimate stress su = 470 MPa, and KIC = 96 MP√m). Then, numerical finite element modeling on a CT specimen using the CASTEM calculation code allowed us to determine the damage of the material when the notch depth varies. The analysis of the results shows that the numerical values of the stress concentration coefficient Kt and the stress intensity factor KI are comparable with the analytically calculated values, thus validating our numerical study. The numerical results obtained revealed that the maximum stress σmax is located in the vicinity of the notch bottom and the high probability density corresponds to a high loading level.

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.

Assessing overload-induced delaminations in glass fiber reinforced polymers by its geometry and thermal resistance

The one-dimensional thermal quadrupole method is used to evaluate a pulsed thermography measurement at delaminations in a glass-fiber reinforced plastic plate quantitatively. The large-scale delaminations have been induced by tension overload and are air-filled and are usually located at the same depth as the notch bottom of a notch on the rear side. While classical evaluation methods like pulsed phase thermography and thermal signal reconstruction are focused on the delamination depth only, the thermal quadrupole method determines spatially resolved two parameters for delaminations, delamination depth and local thermal resistance. Interestingly, lateral heat flows do not disturb this kind of depth evaluation.

Fatigue strength prediction of TC4 titanium alloy following foreign object damage based on contour feature

Foreign object damage (FOD) is a common accident that happens in aeroengines, especially in fan and compressor blades. Accurate prediction of the fatigue strength of the fan/compressor blade materials following FOD is important for the repair and maintenance of aeroengines. This paper was based on the experiment part of Ref. [13], and fatigue strength prediction of TC4 alloy following simulated FOD was carried out by the theory of critical distances (TCD). The FOD notch bottom morphology was characterized by structured light scanning, and a superimposed notch model containing the characteristic of FOD contour was proposed for fatigue prediction. The results show that the prediction based on the FOD macro size overrates the fatigue strength. This overrated prediction is mainly caused by ignorance of the irregular local defects at the FOD notch bottom. By taking the local defects into consideration, the superimposed notch model can effectively improve the prediction accuracy of fatigue strength, and the relative error is within ±30 %.

Comparisons on impact fracture behavior between three-dimensional four directional and five directional braided composite materials

Fracture behaviors of three-dimensional braided composites could be designed from braided preform structures. Here, we compare the fracture behaviors of three-dimensional four direction (3D4d) and five directional (3D5d) braided composites under impact loading. A drop weight impact tester combined with a high-speed photography system was employed to characterize the progressive damages and fracture behaviors, which include crack growth and crack mouth opening displacement. The two-dimensional digital image correlation method was utilized for evaluating deformation contours and strain field. A finite element analyses model was established to reveal impact damage evolution, crack growth, stress distribution, and energy absorption at microstructure level. We found that axial yarns had great effect on the crack propagation paths. The stress at the notch bottom caused yarn breakage and then resulted in a sharply growing crack. The axial yarns in the 3D5d braided composites impede the progress of cracks under transverse impact. The 3D5d braided composites have smaller crack mouth opening displacement than that of the 3D4d composites. The braided preform is the main energy absorption component. With the same volume fraction, axial yarns exhibit the highest energy absorption. Due to the existence of axial yarns, the resistance to the fracture and crack growth of the 3D5d braided composites is better than the 3D4d braided composites.

Fracture Energy and Fracture Morphology after Three-Point Bending Test of Welded Joints Made of Cast Steel Designed for Use in Power Sector, with and without the Addition of Rare Earth Metals

The paper reports the three-point bending test results of welded joints. The joints were made of chromium-molybdenum cast steel designed for the use at elevated temperatures. TIG (tungsten inert gas) welding technique was used. The fracture energy for particular joint zones and the stretch zone width (SZW) under the notch bottom were determined in a qualitative fracture toughness assessment. Fracture surface morphology was analyzed. The stretch zone measurement indicated a qualitative relationship between its width and the values of fracture energy. The results confirmed the influence of the modification on the character of fracture and the portions of brittle and ductile fracture in particular areas of welded joints.

1217 鉛フリーはんだ材の疲労強度と切欠き形状に関する研究(OS8-7 材料力学と計算力学の新展開)

In this paper, low cycle fatigue characteristics of the lead free solder Sn-3.0Ag-0.5Cu material with a very sharp notch were discussed with tests at 100℃ and the numerical simulation. At the 1,000 cycle, the fatigue reduction factors β' were derived as 1.60. Inelastic numerical simulation depending on time was carried out under low cycle fatigue test conditions for lead free solder test piece with V notch with the tip radius of 30μm and 50μm at 100℃. It was shown that the depths from the bottom of the V notch with the radius of 30μm and 50μm, where calculated strain ranges agree with the low cycle fatigue life by test pieces with V notch, were almost same as the tip radius. For the life prediction, the equivalent depth from the notch bottom depends on the notch shape. However, the equivalent depth from the notch bottom was independent them the load level.

アルミニウム単結晶延性損傷評価のための放射光白色X線による検討

The ductile damage progress of the FCC single crystal was verified by the profile analysis using the white X-ray obtained in BL28B2 beam-line of SPring-8. In this study, an aluminum single crystal of the purity 6N was used as a specimen prepared in the I-type geometry for tensile test. The notch was introduced into one side of the center of a parallel part of the specimen by wire electric discharge machining. White X-ray, which has 200 microns in height and 100 microns in width, was incident into the specimen on the Bragg angle θ of 3 degrees in the horizontal plane using energy dispersive X-ray diffraction technique. The specimen was deformed by elongation along crystal orientation [001], and the diffraction profile of the white X-ray which penetrated it was analyzed. In profile analysis, the instrumental function was defined in consideration both of a divergence by a slit and a response function peculiar to an energy dispersive method. The Gauss component of integral breadth related to non-uniform strain and the Cauchy component of integral breadth related to crystallite size were determined by removing the broadening by the instrumental function from the diffraction profile of white X-ray. As a result, in 1.1% of applied strain, increase of the lattice strain was observed in the direction of about 45 degrees of slant from the notch bottom. However, the Gauss component and the Cauchy component of the integral breadth did not indicate the distinct tendency. In 2.2% of applied strain, the lattice strain reduced in the direction of about 45 degrees of slant from the notch bottom. Non-uniform strain and dislocation density decreased similarly. On the other hand, the development of crystallite was estimated in the area due to the increasing of the Cauchy component. The characteristic of the ductile damage progress near the notch of the aluminum single crystal which has crystal orientation [001] along the tensile direction was confirmed by this method.

Estimation of shear-banding resistance in metallic glass containing nano-crystalline particles

Abstract Bulk metallic glasses (BMGs) have a variety of excellent properties compared with the majority of conventional crystalline alloys. However, they exhibit limited global plasticity at room temperature because of shear banding. Several methods have been proposed to improve the limited ductility of BMG; one method is the homogeneous distribution of crystalline particles. However, our understanding of the interaction between the crystalline particles and shear bands (SB) is not sufficient. Here, we performed molecular dynamics (MD) simulations of mode II deformation of a notched BMG plate and BMG plates containing one nano-crystalline particle ahead of the notch bottom. To compare the effect of crystalline particle size on the resistance to SB propagation, we used the J-integral. By comparing J–R curves and the deformation behavior of the BMG plates with and without nano-crystalline particles, we found that the resistance to shear banding is efficiently improved by introducing crystalline particles with sufficient size, compared to the SB width.

Stress concentration factor analysis for notched welded tubular T-joints

In this study, the finite element method (FEM) is applied on a welded tubular T-joint, in order to analyse stress distribution in the vicinity of the weld fillet. The weld has been modelled all around the joint. A notch is to be simulated in the weld element, all around the joint as well. Using symmetry, the tubular T-joint is submitted to three loading cases: axial loading, in-plane bending (IPB), and out-of-plane bending (OPB). The stress distribution analysis, conducted using the (ARSEM, API, RP2A) code, has enabled locating the peak hot-spot stresses. The finite element method analysis shows that stresses are very high on the brace member, in the vicinity of the weld, and decrease gradually, with a quasi-stable difference, in the direction of the brace extremity. Both on the brace member and along the weld (from crown to saddle), the stresses are high at the crown toe, decrease in the middle and increase once again at the saddle point. Consequently, these zones are susceptible to fatigue damage and require reinforcement solutions in order to ensure sufficiently-long fatigue life for tubular T-joints. For the V-notch, the stress intensity factor at the notch is the parameter selected when processing crack-related problems. Stress distribution from the notch bottom towards the load-bearing segment indicates that the stress intensity factor rises as the radius increases at the notch bottom. The stress concentration factor is higher in the bottom of the notched weld at the load-bearing segment, which in most instances causes crack problems with an increase in stress intensity factor; this finding explains the harmful character of such defects and reveals the extent to which the presence of a notch in a weld fillet lowers the resistance of offshore structures to fatigue cracks.

Effect of Roller Working on Fatigue Strength of Structural Metals

Three kinds of materials, pure aluminum (Al1050), carbon steel (S25C) and eutectoid steel (NHH) with V-notch are used in this study to compare the effect of roller working on the fatigue properties among the test materials. As the result, all of the fatigue strengths are improved after roller working and the optimum deformation values exist for every test materials. Fiberized microstructure at the notch bottom, work hardening and compressive residual stress caused by roller working are considered as the main reasons for the fatigue strength improvement. The highest fatigue strength improvement of Al1050, S25C and NHH was 30%, 118% and 155%, respectively. The optimum fatigue improvement of NHH is the highest and it is much higher than that of Al1050, and the optimum deformation value of Al1050 is much lower than that of the other materials. Moreover, the roller working effect is much remarkable for the materials with better hardenability.

Effect of Plastic Deformation on Torsional Fatigue Properties of a Low Carbon Steel

This paper is aimed to investigate the effect of roller working on the torsional fatigue properties of a typical low carbon steel referring by the resutls of FEM analysis. Three types of specimens had been prepared with plastic deformation value of 0, 0.5 and 1.0mm, respectively. The main results obtained in this study are as follows: (1) The torsional fatigue strength of roller-worked specimen with 1.0mm plastic deformation is increased more than twice of that of non-rollerworked specimens. (2) Surface hardness of roller worked specimen is much higher than that of the non-roller worked one, and compressed and elongated structure is formed at the notch bottom of the specimen. (3) Crack length along the axial direction of roller worked specimens is longer than that of the non-roller worked specimen, and plural cracks initiated and propagated in the roller worked specimens. (4) The residual stress distribution at the specimen’s notch bottom was analysed using FEM analysis and it is verified the same tendency as the experimental results.

A Study of Fatigue Fracture Properties of High Strength Notched Steel in High Cycle Fatigue Region with Special Focus on the Statistical Variation of the Local Fatigue Strength at the Notch Bottom

This study was conducted to examine how and why a difference of fatigue strengths between unnotched and notched specimens, in terms of maximum stress amplitude at the notch bottom, depends on the change of hardness level in steel. As a result, the local fatigue strength of the notch bottom in the notched specimen became higher than that of the un-notched one with increase of notch sharpness and this was remarkable in high strength steel. This is because the transition of the fatigue fracture mode occurs from a slip induced crack initiation mode to a defect controled one with increasing hardness in steel. In addition, using a new experimental approach combined with a simulation technique based on the concept of risk competition of defects, a computer simulation was conducted on an effect of inclusion on the local fatigue strength at the notch bottom of high strength steel. It was clarified that the difference of fatigue strength between notched and un-notched specimens in high strength steel was caused by the statistical variation of the largest inclusion size contained in the critical volume at the notch bottom.

Influence of Solidification Microstructure and Distribution of Reinforcement on Fatigue Characteristics of Notched SiC Reinforced AC4B Alloy Composites

The influences of SiC particle distribution, surface notch size and solidification microstructure of the matrix on the fatigue characteristics of SiC reinforced JIS-AC4B alloy composites were investigated. Al–6.79 mass% Si–2.93 mass% Cu–0.17 mass% Mg–0.59 mass% Fe matrix composites with relatively homogeneously dispersed 11 mm SiC particles were fabricated through a combination of pressure infiltration and a melt stirring casting method. The matrix microstructure consisted of a dendritic alpha phase and eutectic Si with a few volume fractions of Fe intermetallic compound among the dendrites. All specimens contained some gas and shrinkage porosities, and all composite specimens contained SiC particle clusters. Vickers hardness of composites clearly increases due to the dispersion of SiC particles and age hardening. The hardening ability increases with an increasing volume fraction of SiC. Rotating bending fatigue tests were carried out on notch-free and notched specimens that had peak aging. In the notch-free matrix alloy specimen, cracks generated from porosities, whereas cracks generated from the SiC particles/the matrix interfaces in the composite specimens. Thus, the fatigue strength decreased with an increase in the SiC volume fraction. In the notch-introduced matrix alloy specimen, where the stress concentration factor is high, the notch becomes the crack generation site and dominated the fatigue strength. The cracks, however, generate near SiC particles instead of the notch bottom in the composite specimen. Moreover, it was found that the fatigue limit stress is unchanged in composite specimen even when the notch is introduced, although the critical stress for crack generation declines. Microstructural observation revealed that the cracks were spread and diverted in and around the cluster of SiC particles, suggesting that crack propagation resistance was improved in the composite specimen.

Study on Fatigue Strength of Aluminum-Silicon Alloy and Spheroidal Graphite Cast Iron under Compressive Load

Fatigue tests of aluminum-silicon alloy A4032FD and spheroidal graphite cast iron FCD600 were carried out under cyclic compressive loading using circumferential notched round bar specimens. Non-propagating cracks were observed at the notch bottom of the specimens after 1×107 load cycles. It is considered that residual tensile stress caused by plastic deformation at the notch bottom is the driving force of crack propagation. Therefore we calculated the range of stress intensity factor ΔK of residual stress at the notch bottom. The ΔK was also compared with effective threshold stress intensity U·ΔKthto predict the crack depth. It is considered that the crack propagation rate disappears when ΔK<U·ΔKth. The predicted value of crack propagation depth was in line with the experimental value.

730 高強度鋼の長寿命疲労特性に及ぼす表面処理の影響

In order to investigate the fatigue properties of high strength steels and surface hardened steels in the super-long life range (N&ge;1×(10)^7), cantilever type rotating bending fatigue tests were carried out for 2 kinds of SNCM 439 steels, which the cleanliness was different on these steels. Also, the surface of notch bottom was removed up to 30μm deep by electro-polishing, because the depth of compression residual stress in the notch bottom of grinding specimen has been 8μm. As a result of fatigue tests, in the case of SNCM 439 (D material) speciem and "high stress amplitude & short life area", it was found that the fatigue life of electro-polishing specimen were almost equal to that of grinding specimen, and in the case of SNCM 439 (S material) speciem and "low stress amplitude & long life area", it was found that the fatigue life of electro-polishing specimen were shorter than that of grinding specimen.

タルク強化共重合ポリプロピレンの衝撃吸収エネルギーに関する研究 第２報 タルク強化共重合ポリプロピレンの衝撃エネルギー吸収特性に及ぼすクリープ損傷の影響

Creep tests were conducted using talc-reinforced Polypropylene copolymers. Then, tensile tests were conducted using the creeped samples to reveal the effect of creep damage on the impact energy-absorption. The creep damage progression mechanism was investigated due to fracture surface observation, density measurement and local elastic property. The following conclusions have been drawn from the experimental results: 1) From the density variation of damaged samples, we clarified that creep damage occurs due to an accumulation of voids in the material. 2) The residual impact resistance does not decrease significantly until a lifetime ratio becomes 0.7, and apparently increases at certain notch radius values. At a lifetime ratio of 0.9, we identified a significant decrease in residual impact resistance due to void accumulation, as reflected in the measured density data. 3) The fractured surfaces of samples after high speed tensile tests were slightly whitened, but they were otherwise very similar to those of undamaged specimens. We found that this phenomenon coincides with the phenomenon in which the retained impact resistance did not significantly decrease. 4) The local elastic property measurement revealed that the vicinity of notch bottom was hardened due to strong molecular chain orientation, in turn contributing to the retention and improvement of the residual impact resistance. 5) The creep damage involves two causative factors: hardening from the strong molecular chain orientation, and softening of the material due to void accumulation. We also clarified that these factors affected the residual impact resistance.

Stresses in a weighable half-plane with a semicircular notch

The influence of the surface roughness on the stress state of a rock is studied. For an elastic half-plane in the gravity field that contains a notch shaped like a semicircle, the stress distribution is constructed. It is shown that depending on the Poisson ratio, the notch bottom can be in a state of tension or compression. The polynomial dependence of pressure on depth is given on the axis of symmetry.

635 高炭素クロム鋼の超長寿命疲労特性に及ぼす残留応力の影響

In order to investigate the fatigue properties of high strength steels and surface hardened steels in the super-long life range (N&ge1×(10)^7), cantilever type rotating bending fatigue tests were carried out for the SUJ2 high carbon chromium bearing steel which was finished by the electro-polishing. The surface of notch bottom was removed up to 30μm deep by electro-polishing because the depth of residual stress in the notch bottom of grinding specimen has been 20μm. As a result of fatigue tests, in the case of "surface crack origin type fracture", it was found that the fatigue life of eletctro-polishing specimen were shorter than that of grinding specimen. In the case of "interior crack origin type fracture", it was found that the fatigue life of electro-polishing specimen were almost equal to that of grinding specimen. The causes for this are discussed.

Study on Fatigue Strength of FCD 600 under Cyclic Compressive Load.

Fatigue tests of spheroidal graphite cast iron FCD600 were carried out under cyclic compressive load using round bars with circumferential notch.Cracks were observed propagating at the notch bottom of the specimens that were not fractured after 1×107 cycles under the repeated compressive load.The change of the stress intensity factor range =K accompanied by the crack propagation was studied based on fracture mechanics, considering the residual tensile stress distribution due to the plastic deformation.The applied and residual stresses were calculated by elastic-plastic finite element method.=K was compared with =Kth of short crack taking intrinsic crack length ao into consideration, and it was considered that crack propagation is stop at the depth of =K<=Kth.The estimated value of crack propagation depth is in good agreement with the experimental value.