Notch Specimens Research Articles

МОДЕЛИРОВАНИЕ РОСТА КРАЕВОЙ УСТАЛОСТНОЙ ТРЕЩИНЫ ПРИ ВЫСОКОЧАСТОТНОМ НАГРУЖЕНИИ

Multiple fracture surface investigations of real aviation industry products have proved that low-amplitude high-frequency oscillations can lead to an unexpected failure of aviation elements and other structures. High-frequency loading is the reason for this, as it produces a large number of loading cycles that often exceed an investigated area of a material fatigue behavior. This new fatigue failure mode (very-high-cycle fatigue) requires a special experimental study with the use of both experimental and mathematical modeling. This paper is focused on a numerical determination of stress intensity factors (SIF) in the specimen with the edge notch specimen loaded by harmonic high frequency displacements of small amplitudes. The numerical calculations were performed for the case of loading with frequency close to its natural frequency. A dimensionless adjusting function was determined for SIF that takes into account a change in modal characteristics of the resonance system (the specimen with rectilinear edge notch) due to crack propagation. The obtained equation was used to model the position of the curvilinear fatigue crack front. A general scheme of a piezoelectric fatigue testing machine is introduced with a technique of tensile-compression fatigue tests on the titanium crack growth specimen with an edge notch in the range of very high cycle fatigue. The analysis of the fracture surfaces with the identification of the front stop lines and mathematical modeling of the crack front evolution under high-frequency loading are carried out. The results of mathematical modeling are comparing with experimental data obtained during high-frequency fatigue tests on a piezoelectric fatigue testing machine. The numerical calculations have shown that this approach allows us to qualitatively and quantitatively simulate the evolution of the edge fatigue crack with a curvilinear front under very-high-cycle fatigue (high-frequency) loading mode.

Estimation of Impact Toughness Transition Temperatures of As-Quenched Steels

High strength and sufficient toughness are key requirements for modern high-performance structural steels. In an attempt to develop a suitable estimation of impact toughness transition temperatures for as-quenched steels, we investigated the determiners of low-temperature toughness with a group of thermomechanically rolled direct-quenched steels with varying martensite contents. These were produced by altering chemical composition, finish rolling temperature and reduction below the non-recrystallization temperature, i.e. austenite pancaking, and characterised in terms of microstructural constituents, grain size distributions, texture and fractography. Provided the finish rolling temperature is high enough to avoid the formation of granular bainite on subsequent cooling, high levels of austenite pancaking yield the best combinations of low-temperature toughness and strength by effectively refining the size of the coarsest grains and randomizing the texture. While absolutely no direct correlation is found within as-quenched steels between the impact toughness transition temperatures and yield strength alone, T28J and T50 do closely follow a dynamic reference toughness, i.e. the opening stress intensity factor defined by yield strength and the size of the coarsest grains in the effective grain size distribution. This parameter reflects the transition temperatures – the lower the temperature, the lower the reference toughness needed to cause a local brittle fracture. Finally, we show that the impact toughness transition temperatures T28J and T50 of as-quenched steels can be accurately estimated, irrespective of the test specimen orientation, by utilizing just the dynamic reference toughness and the fraction of {100} cleavage planes within ± 15° of the specimen notch plane.

A notch critical plane approach of multiaxial fatigue life prediction for metallic notched specimens

AbstractAn approach based on the local stress response is proposed to locate the fatigue critical point for metallic blunt notched specimens under multiaxial fatigue loading. According to the stress analysis, both stress gradient and gradient of loading nonproportionality exist at notch root. The plane in the vicinity of the notch that passes through the fatigue critical point and experiences the maximum shear stress amplitude is defined as the critical plane for notch specimens (CPN). Furthermore, the Susmel's fatigue damage parameter is modified to assess fatigue life of notched components by combining CPN and the theory of critical distance (TCD). The multiaxial fatigue test of the thin‐walled round tube specimens made of Ni‐base alloy GH4169 is carried out to verify the above approaches. In addition, test data of two kinds of materials are collected. The results show that the maximum absolute error of the fatigue critical point is 9.6° and the majority of the predicted life falls within the three‐time scatter band.

Analysis of dissipated energy and temperature fields at severe notches of AISI 304L stainless steel specimens

In the last years, a large amount of fatigue test results from plain and bluntly notched specimens made of AISI 304L stainless steel were synthetized in a single scatter band by adopting the specific heat loss per cycle (Q) as a damage parameter. During a fatigue test, the Q parameter can be evaluated measuring the cooling gradient at a point of the specimens after having suddenly stopped the fatigue test. This measurement can be done by using thermocouples; however, due to the high stress concentration at the tip of severely notched components analysed in the present paper, an infrared camera achieving a much improved spatial resolution was adopted. A data processing technique is presented to investigate the heat energy distribution close to the notch tip of hot-rolled AISI 304L stainless steel specimens, having notch tip radii equal to 3, 1 and 0.5 mm and subjected to constant amplitude cyclic loads. A thermal finite element analysis was also performed by assigning heat generation in the appropriate region close to the notch tip. Then the numerical temperature values were compared with the experimental measurement.

A Method for Predicting the Effects of Specimen Geometry and Loading Condition on Fatigue Strength

Specimen geometry and loading condition usually have a great influence on the fatigue strength of metallic materials, which is an important issue in evaluating the reliability of component parts. In this paper, a rotating bending fatigue test is performed at first on an hourglass specimen and a notch specimen of a high strength titanium alloy. Experimental results indicate that, in terms of local stress, the notch specimen endures higher fatigue strength in comparison with the hourglass specimen due to its relatively smaller control volume. Then, a probabilistic control volume method is proposed for correlating the effects of specimen geometry and loading condition on the fatigue strength based on Weibull distribution and the concept of control volume. A simple formula is obtained for the fatigue strength in relation to control volumes, in which the parameter is the shape parameter of Weibull distribution of fatigue strength. The predicted results are in good agreement with the present experimental data for high strength titanium alloy and the data for the high strength steel and the full scale EA4T axle in the literature.

Development of Solute-solvent Separation Soft Lithography Grating for Measuring High-temperature In situ Deformation Using Scanning Electron Microscope Moiré Method

In this study, a technique is developed for fabricating high-temperature solute-solvent separation soft lithography (HSS-SL) grating for metallic materials. Using this technique, a 150 lines/mm crossing-type grating is directly fabricated on the surface of a miniaturized single edge notch tension (SENT) specimen. Further, the microstructure of the grating is analyzed using scanning electron microscope (SEM) and atom force microscope. The grating is found to be highly suited for forming high-contrast SEM moire fringes. In addition, the chemical composition of the grating is characterized using energy dispersive X-ray spectroscope (EDS), whose results indicate that the grating exhibits good resistance to high-temperature oxidation owing to the high heat endurance of its constituent materials, SiO and SiO2. With respect to high-temperature applications, the HSS-SL grating is employed successfully for measuring the linear thermal expansion coefficient of GH2036 alloy at temperatures of 25-600 °C. Moreover, the high-temperature displacement and strain fields around the semicircular notch of the SENT specimen are determined based on the SEM moire. These results confirm that thus-fabricated HSS-SL gratings show high potential for use in high-temperature in situ deformation measurements using the SEM moire method.

Residual Stress Field of a Single Edge Notch Specimen after Laser Shock Peening and Shot Peening Treatment

Abstract Laser shock peening (LSP) is a reliable, repeatable, and successful surface technique for introducing high magnitude, deep compressive residual stresses that can significantly increase the fatigue life of metallic components. However, depending upon how the LSP treatment is applied, the induced residual stresses can result in the undesirable deformation of the part. In this work, traditional shot peening has been applied over LSP as a means to optimize the stress distribution at the surface of a part while constraining deformation. A single edge notch test specimen of AA7075 was laser peened local to the notch region and then shot peened over the entire central region. The resulting residual stress distribution has been characterized using neutron diffraction to measure the stress distribution in the bulk, and it was compared with (1) incremental center hole drilling to measure the stress distribution up to depths of ∼1 mm and (2) near-surface stresses obtained in a previous X-ray diffraction (XRD) study on nominally identical specimens subjected to the same surface treatments. For regions where the two techniques overlap, the residual stresses are in good agreement (within uncertainty). Comparing the bulk stresses obtained from neutron diffraction in this study and XRD data published elsewhere, it can be shown that shot peening applied after LSP has a profound effect on near-surface stresses; however, these effects disappear at depths of ∼0.7 mm or more.

Experimental study and multiscale modelling of the high temperature deformation of tempered martensite under multiaxial loading

The microstructural deformation of ex-service 9Cr-1Mo steel, with a tempered martensitic microstructure, has been examined in this study, through the combined use of electron backscatter diffraction (EBSD) and multiscale modelling techniques. Both the experimental and predicted deformation of the material at a notch root on a range of scales from the specimen level down to the microstructural block level are compared. A tension loaded notch specimen of the material which was extracted from an ex-service power plant pipe was used for this analysis. The deformation at the specimen level was quantified by analysis of the load displacement curves and notch opening displacement, which showed excellent agreement with the predicted results from the experimentally calibrated elastic-plastic finite-element model of the specimen geometry. The microstructural deformation was experimentally measured through the use of EBSD carried out at the notch root before and after high temperature mechanical testing. The initial orientation of the microstructure as well as the displacement around the boundary of the area of interest in the macroscale model were applied to a representative volume element (RVE) and a slip based crystal plasticity modelling framework was implemented to model the in-elastic deformation of the material under high temperature loading.

Evaluation of ductile-brittle transition temperature of anisotropy material by small punch test with U-shaped notch

Miniature and standard specimens, were cut from the anisotropy materials with axial, central and radial directions to study the mechanical property. In the paper, main research focused on the small punch test (SPT) with un-notched and U-shaped notched specimen in low temperature. Through the small punch energy variation with temperature, the ductile-brittle transition temperature by the small punch test (T SP ) can be determined. The results indicated that there was no obvious difference among three different directions in transformation temperature of SPT with un-notch specimens, and it cannot represent upper plateau impact energy of three different directions. And the SPT with U-shaped notched specimens can determine the differences of upper plateau fracture energy of three different directions. Therefore, SPT with U-shaped notch specimens was more useful to evaluate the material anisotropy.

High cycle fatigue analysis in the presence of autofrettage compressive residual stress

AbstractAn experimental and numerical investigation of the effect of residual compressive stress on the high cycle fatigue life of notched low carbon steel test specimens is presented. Experimentally determined cyclic stress strain curves for S355 low carbon steel are utilized in a finite element analysis plasticity modelling framework incorporating a new cyclic plasticity material model representative of cyclic hardening and softening, cyclic mean stress relaxation, and ratcheting behaviors. Fatigue test results are presented for standard tensile fatigue test specimens and novel double notch specimens. Double notch specimens are tested with and without compressive residual stress prior‐induced through tensile overload. It is shown that cyclic plasticity phenomena have a significant influence on the induced residual stress distribution and also on material behavior when fatigue tested in the high cycle regime. It is observed that higher initial compressive residual stresses magnitude does not necessarily lead to a longer fatigue life. Finite element analysis using the new cyclic plasticity material model shows this behavior is due to combined residual stress redistribution under fatigue test cyclic loading and cyclic hardening effects. A fatigue life methodology based on the stress‐life approach augmented by a critical distance method is proposed and shown to give good agreement with experimental results for test specimens with no induced residual stress. The results obtained for specimens with induced residual stress are more conservative, but the degree of conservatism is significantly lower than that in the conventional stress life approach. The proposed methodology is therefore suitable for analysis and design assessment of components with pre‐service induced compressive residual stress, such as autofrettaged pressure components.

The Effect of Notch Depth on CTOD Values in Fracture Tests of Structural Steel Elements

Abstract In elements of steel structures working at low temperatures, there is a risk of appearance of brittle fracture. This risk is reduced through the use of certified materials having guaranteed strength at a given temperature. A method which is most frequently used to determine brittle fracture toughness is the Charpy impact test, preformed for a given temperature. For offshore structures intended to work in the arctic climate, the certifying institutions more and more often require Crack Tip Opening Displacement (CTOD) tests instead of conventional impact tests, especially for steel and welded joints of more than 40 mm in thickness in the case of high-strength steel, and more than 50 mm for the remaining steels. The geometry of specimens and the test procedure are standardised; however, these standards provide some margin for specimen notch depth. The paper analyses the effect of notch depth difference, within the range permitted by the standards, on the recorded CTOD values of a given material. The analysis was performed via numerical modelling of destruction of specimens with different notch geometries and further verification of the obtained numerical results in laboratory tests. The calculations were carried out at the Academic Computer Centre in Gdansk.

The Static and Fatigue Behavior of AlSiMg Alloy Plain, Notched, and Diamond Lattice Specimens Fabricated by Laser Powder Bed Fusion

The fabrication of engineered lattice structures has recently gained momentum due to the development of novel additive manufacturing techniques. Interest in lattice structures resides not only in the possibility of obtaining efficient lightweight materials, but also in the functionality of pre-designed architectured structures for specific applications, such as biomimetic implants, chemical catalyzers, and heat transfer devices. The mechanical behaviour of lattice structures depends not only the composition of the base material, but also on the type and size of the unit cells, as well as on the material microstructure resulting from a specific fabrication procedure. The present work focuses on the static and fatigue behavior of diamond cell lattice structures fabricated from an AlSiMg alloy by laser powder bed fusion technology. In particular, the specimens were fabricated with three different orientations of lattice cells—[001], [011], [111]—and subjected to static tensile testing and force-controlled pull–pull fatigue testing up to 1 × 107 cycles. In parallel, the mechanical behavior of dense tensile plain and notched specimens was also studied and compared to that of their lattice counterparts. Results showed a significant effect of the cell orientation on the fatigue lives: specimens oriented at [001] were ~30% more fatigue-resistant than specimens oriented at [011] and [111].

Experimental and Numerical Study of Notch Geometry Effects on Fatigue Life Under Mixed Mode I and III Loading

Fatigue is one of the main reasons of fracture in mechanical components. Generally performing experiments is a useful and effective method for determination of fatigue life. In this paper, results of different notch geometry effects on the fatigue life under mixed mode I and III were studied experimentally and numerically. For this purpose, fatigue specimens were manufactured according to the ASTM-A370 standard and all the specimens were quenched and tempered. Two types of notch geometries, V-shape and U-shape were considered in this study. The fatigue life was determined experimentally for each notch geometry and results were verified by finite element method simulation. Fractographic investigation was done broadly to determine cause of failure in the specimens. Our results showed that the notch geometry had significant effect on the fatigue life. Fatigue crack growth in the V-shape notch specimens happened quickly and the maximum fatigue life reduction occurred for that.

Method for Predicting Crack Initiation Life of Notched Specimen Based on Damage Mechanics

Based on the theory of damage mechanics, a method for fatigue crack initiation life prediction of notched components is proposed in this paper. The damage evolution equation of notched specimen under tension-compression loading is obtained in term of closed-form solution. The crack initiation life of notched specimen is estimated by the proposed method even when material and stress concentration factor are different. It has been verified that the result calculated by the proposed method agrees with the experimental result. The proposed method is concise, effective and feasible to practical application.

Impact Fatigue Life Prediction for Notched Specimen of Steel AerMet100 Subjected to High Strain Rate Loading

In this study, the ultra-low cycle fatigue (ULCF) behavior of a high-strength-ductile steel AerMet100 exposed to repeated high strain rate impact loadings is investigated. Three types of coupon-level experiments were performed, in which a three-point bending (TPB) specimen with a through-thickness notch was employed for multi-impact test. A special ratchet effect associated with complex stress state, dynamic impact loading and other physical mechanisms was observed through measured principal strain variations with a specific decay rate at the notch root surface. An improved ULCF predictive model based on the continuum damage mechanics was developed to quantify the relationship between the fatigue damage and the input of localized impact energy to the notch root. The model expressed in terms of damage growth rate introduces a new exponential term for better predictive accuracy and reduced number of nonlinear dynamic response analysis. As a computational efficient tool, the proposed model can predict impact fatigue life in acceptable timeframe for multiple critical locations in a complex engineering component.

Determination of Fracture Toughness of PMMA under Impact Loading Using Izod Impact Tester

The aim of this study is to develop a new test method for determining dynamic fracture toughness of brittle material using the izod impact tester. Poly (methyl methacrylate) (PMMA) is used in this study. Test setup and load calibration methods are proposed. The proposed method can be successfully used to determine near-mode I fracture toughness under dynamic load. The effects of notch tip preparation and specimen thickness are also studied and discussed. Stress intensity factor at maximum load (Kmax) is determined based on a numerical simulation. It is found that the notch preparation method has a minimal effect on total fracture energy, except in the case of saw notch. Moreover, fracture toughness increases with increasing of specimen thickness.

Fracture Assessment of Inclined Double Keyhole Notches in Isostatic Graphite

In the present contribution, the static strength of isostatic graphite using keyhole notch specimens under mixed mode loading is investigated. An experimental program was performed and in total, 18 new experimental data are provided. In addition, different loading mode ratios are considered by varying the inclination angle of the notch with respect to the direction of the applied load. The criterion based on the averaged value of the strain energy density over a control volume at the notch edge is applied to assess the static strength of specimens. A sound agreement is found between experimental data and the results obtained from strain energy density criterion.

A frequency dependent embrittling effect of high pressure hydrogen in a 17-4 PH martensitic stainless steel

The effects of hydrogen on tensile and fatigue-life properties of 17-4PH H1150 steel have been investigated by using a smooth, round-bar specimen for tensile tests and circumferentially-notched specimen for fatigue-life tests. The specimens were precharged by an exposure to 35-100 MPa hydrogen gas at 270°C for 200 h. For the 100 MPa hydrogen exposure, the steel showed a significant degradation in ductility loss, translated by a relative reduction in area, RRA, of 0.31. The fatigue-life test of the present notch specimen (stress concentration factor of 6.6) reflects the fatigue crack growth (FCG) for long cracks. The fatigue limit of the non-charged and H-charged notched specimens, defined by the threshold of non-propagation for long cracks, was not affected by hydrogen. At a higher stress amplitude, the H-charged specimen showed a significant FCG acceleration ratio compared to the non-charged specimen. Although, an upper bound of the FCG acceleration seemed to exist, this ratio was approximately 100. The fracture surface of the H-charged specimen was covered with quasi-cleavage (QC) at a lower stress amplitude and with a mixture of QC and intergranular (IG) facets at higher stress amplitudes. It has been suggested that a cycle-dependent crack growth accompanied by QC occurs at a lower stress amplitude, whereas a mixture of cycle-dependent crack growth (accompanied by QC) and time-dependent crack growth (accompanied by IG) occurs otherwise. This mixture justifies the 100 times FCG acceleration ratio in spite of the existence of the upper bound.

Measurement of opening displacement and stress intensity factor of two branches of bifurcated notch by Moiré interferometry

Static experiments on dynamic crack branching are carried out, in which Y-shaped notches in plate specimens are regarded as rapidly bifurcating cracks. Moiré interfrometry is applied to measure crack opening displacement, COD, of two branch notches as well as the mother notch of the Y-shaped notch. Stress intensity factor of the branch notches are calculated through the formula for COD of linear elastic fracture mechanics. From the measurement results, it is clarified that how far the singular stress field is developed around the branch crack tip.

Creep Damage Assessment of Various Shape of Round Bar Notch Specimens Mod.9Cr-1Mo Steel

Creep Damage Assessment of Various Shape of Round Bar Notch Specimens Mod.9Cr-1Mo Steel