Notch Effect Research Articles

Stanovenie únavovej pevnosti zvarových spojov ocele S960MC zváraných laserovým lúčom

The work deals with determining the fatigue strength of butt-welded joints of S960MC steel. The fatigue strength of two versions of laser beam welded joints as well as the base material was assessed. To reduce the notch effect of the weld beam, the assessed weld joints were produced in a variety of methods utilizing filler material and additional dressing of the weld beam. The measurements determined variations in each type. The value of the fatigue strength decreased by 50 % to 67 % with respect to the base material, indicating that the notch effect of the weld joint had an impact on the value of the fatigue limit. Despite having relatively low fatigue strengths, both values meet normative standards and can be used in practical circumstances.

Notch Effects on the Stress Intensity Factor and on the Fatigue Crack Path for Eccentric Circular Internal Cracks in Elliptically Notched Round Bars under Tensile Loading.

In this paper, stress intensity factor (SIF) solutions are numerically obtained for notched bars subjected to tensile loading containing an eccentric circular inner crack located in the cross-section corresponding to the notch root. The finite element method and the J-integral have been used to obtain the SIF and to analyze the effect on it of three elliptical notch geometries (of equal radial depth). The results show how the SIF is greater in the notched bars than in the smooth bar and within the former when the axial semi-axis of the notch rises, its effect being greater as the diameter and eccentricity of the inner crack increase. In addition, the fatigue growth of an eccentric crack induces an increase in such eccentricity, greater as the notch axial semi-axis increases. The cause of these phenomena can be attributed to the constraint loss caused by the notch, which also facilitates bending of the specimen due to the asymmetry generated by the crack eccentricity.

Fatigue life prediction for notched specimen considering modified critical plane method

AbstractTo meet the different functional requirements of the components, irregularities and geometric discontinuities occur in the structural design of the components. These geometrical discontinuities lead to local stress concentrations, which inevitably introduce notch effects. The unyielding internal material still supports the high‐stress area on the plane due to the supporting effect of the notch. This effect can lead to inaccurate prediction of fatigue life. So, the notch support factor and strain were used as damage parameters to modify the life prediction model of the notched specimens in this research. And an improved critical plane method is applied in selecting damaged planes. It considers the damage at the notch before loading and uses dynamic weights to measure the effect of the maximum stress plane on the damaged plane. The model was validated using TC4 and GH4169 notch data and compared with other models, and the revised model has higher accuracy. The damaged plane and damage parameters to modify the fatigue life of the notched specimen make more physical sense and are more consistent with the fatigue failure mechanism.

The effects of building position on surface and fatigue of DED-arc steel components

Recently, additive manufacturing (AM) of structural metallic components is analyzed regarding its potential use by industry and research. Next to the development of manufacturing processes, the mechanical properties are under investigation today. One of the quality measures of metallic components is the surface topography. DED-arc processes (direct energy deposition) result in relatively coarse surfaces, characterized by a distinct waviness with wave amplitudes in the mm-range. This is enhanced when applying horizontal building position in comparison to vertical position. Next to increased waviness, the load-bearing net cross sections are reduced as well. The surface topography determines the fatigue life properties of metallic components. While stress raising surface effects are generally well understood and fatigue (Structures 31: 576–589, 2021) of welded metals is established well, the fatigue behaviour of additively manufactured components is less investigated yet. In order to define surface quality levels for DED-arc components, the effects of surface topography on mechanical performance need to be understood. This article presents the manufacturing of high strength steel test coupons by the DED-arc process. The process parameters were varied with regard to the building position and different levels of surface quality were generated. The surfaces of different specimens were characterized and fatigue tests were conducted. The results were used to derive the surface influence on both, the effective load-bearing wall thickness and notch effects induced by the layer-by-layer building approach. A correlation between building position, surface waviness and fatigue strength was proven. In general, higher waviness resulted in reduced effective wall thickness and lowered fatigue strength. A difference in fatigue strength at 2 million load cycles of 20 to 30% was proven when printing in different building positions. The surface effect can be captured in the design concept when applying the effective notch stress approach with an averaging length of of ρ* = 0.4 mm. The fatigue strength is describable by a design S–N curve FAT160 and a k-value of 4.

Manufacturing a prototype with laser direct metal deposition and laser welding made from martensitic steel 1.4313

Burckhardt Compression Holding AG, based in Winterthur, is an internationally active manufacturer of reciprocating compressors who uses three-piece pistons in its Laby® reciprocating compressors. Due to their design for casting, the pistons have a high weight, which limits the size of the piston, particularly for the large diameters. For this reason, solutions are being looked for to produce pistons in lightweight design using metal additive manufacturing processes to counteract these challenges. One of the innovative techniques for weight reduction that has been applied in various fields of science and industry is laser direct metal deposition (DMD). Therefore, a project was started with Burckhardt Compression to reduce the mass enabling higher operating speeds. This study presents a workflow to manufacture a lightweight piston from martensitic steel 1.4313 by direct metal deposition (DMD) with a diameter of approximately 342 mm and a height of 140 mm. The piston is characterized by different segments, which are conventionally and additively manufactured to overcome machine limitations. The piston crown was joined to the additive manufactured part and sealed by CO2 laser welding. Reducing the laser power in DMD reduced the temperature, and hence, oxidation of manganese and silicium and reducing the carrier gas flow improved the buildup rate and reduced the turbulence induced oxidation. Alternating the feed direction per layer improved the geometrical accuracy and avoided material accumulation at sharp corners. A method was found to indicate quantitatively the geometrical accuracy of a radius in buildup direction. The welding types and seams for laser welding were selected to enable a good force flow; however, a clamping device was necessary. A double weld strategy was considered in order to reduce a notch effect at the hidden T-joints. The design enabled a 40% weight reduction resulting in a weight of 24 kg compared to the cast piston with a weight of 40 kg. Metallographic analysis and 3D scans were performed in order to evaluate the material quality and geometrical accuracy. The study shows the limitations and challenges of DMD and how to overcome machine limitations by part segmentation.

Thermodynamics-based method considering orientation and notch effect to predict the high cycle fatigue life of a nickel-based single crystal superalloy

The high cycle fatigue (HCF) life of DD6 at 760 ℃ was studied by performing a large number of experiments. A new thermodynamics-based life prediction method was proposed by considering the thermodynamic entropy change during the fatigue process. The model is not only suitable for smooth specimens but also can be applied to the life evaluation of notched specimens and those with different orientations. Finally, the life of an aeroengine blade with different orientations was qualitatively evaluated by the proposed model, which demonstrated that the model can be used to analyze the influence of crystal-axis deflection angle on blade life.

Heat treatment improved the corrosion resistance of laser- welded joints of SK5 steel with the notch effect

Heat treatment improved the corrosion resistance of laser- welded joints of SK5 steel with the notch effect

Plastic stress concentration effects in fatigue strength

Elastoplastic (EP) stress gradient factors (SGF) ahead of notch tips are used to evaluate the actual notch effects in fatigue strength due to their tolerance to non-propagating short cracks, which depends on the notch stress concentration factor, on the stress gradient ahead it, and on the material. Considering the significant role of local plasticity in the growth behavior of short cracks within the notch plastic zone, a sound mechanical methodology is proposed to account for the effect of EP stress and strain fields in the actual fatigue stress concentration factor value, which includes its tolerance to short cracks. To validate the proposed methodology, life predictions based on numerical EP SGF are compared to experimental SN data of notched specimens made of different materials tested under uniaxial loads collected from the literature.

Probabilistic notch fatigue assessment under size effect using weakest link theory

Notch and size effects exerts significant influence on the fatigue performances of engineering components. In the present study, a probabilistic framework for fatigue life prediction of the notched components is proposed based on the calibrated weakest link theory considering size effect. Specifically, a probabilistic method considering the notch effect is elaborated utilizing FE analysis, which inherently considers the size effect and can be used for fatigue life prediction of components with arbitrary shapes. Model validation and comparison are carried out using experimental data of GH4169, TC4 and TC11 alloys with various specimen geometries as well as a full-scale compressor disc. Results indicate that the proposed model presents better prediction accuracy than the other three traditional methods.

Evaluating the fatigue property of S355J2W steel butt-welded joint: Multiple notch effects

The fatigue fracture mechanisms and the fatigue property evaluations of S355J2W steel butt-welded joints relieved welding residual stresses with and without welding reinforcement were systematically investigated by microstructure characterization and fatigue tests. Experimental results show that the complex evolution of microstructure at welded joint results in the notable degradation of mechanical properties. The fatigue performance of the as-welded specimen with welding reinforcement is significantly poorer than that of the smooth-welded specimen without welding reinforcement. Stress concentration under the fish-scale pattern at the weld toe caused by geometric dimension change is the major reason for the fatigue fracture of the as-welded specimen. By comparison, the cementite segregation at grain boundary in the partial recrystallization zone of the heat affected zone caused by the sharp change of microstructure should be responsible for the fatigue fracture of the smooth-welded specimen. The fatigue notch coefficient (Kf=1.89) calculated by the effective notch stress method is consistent with that (Kf=1.9) measured by the fatigue tests, which shows that the effective notch stress method can well evaluate the multiple notch effect under the coupling of the weld toe and the fish-scale pattern. Furthermore, the fatigue risk factor (Rf) model was employed to evaluate the fatigue performance of welded joint, and the intuitive Rf curves show that the fatigue fracture occurs at the fatigue life exceeding 2 × 106 cycles, i.e., Rfmax exceeds 0.7.

Notch effect in 20% short carbon fibre-PA reinforced composites under quasi-static tensile loads

In this work the static behaviour of notched specimens made of a polyamide-6 thermoplastic material reinforced with 20% of short carbon fibres was investigated. In more details, U and V-shaped notched specimens were manufactured by a wire cutting process from injected plates and tested under tensile quasi static loadings. All in all, 42 new experimental data have been provided, related to different notch root radii and notch opening angles, investigating the effect of the local geometry on the notch strength. Taking advantage of the scanning electron microscopy, a detailed damage analysis was also carried out on the fractured specimens, identifying the main damaging mechanisms occurring in the highly stressed regions close to the notch tip. Eventually, all the experimental data from the notched specimens have been reconverted in terms Generalised Notch Stress Intensity Factors, GSIFs, and summarised in a single scatter band.

Delineating cooperative effects of Notch and biomechanical signals on patterned liver differentiation

Controlled in vitro multicellular culture systems with defined biophysical microenvironment have been used to elucidate the role of Notch signaling in the spatiotemporal regulation of stem and progenitor cell differentiation. In addition, computational models incorporating features of Notch ligand-receptor interactions have provided important insights into Notch pathway signaling dynamics. However, the mechanistic relationship between Notch-mediated intercellular signaling and cooperative microenvironmental cues is less clear. Here, liver progenitor cell differentiation patterning was used as a model to systematically evaluate the complex interplay of cellular mechanics and Notch signaling along with identifying combinatorial mechanisms guiding progenitor fate. We present an integrated approach that pairs a computational intercellular signaling model with defined microscale culture configurations provided within a cell microarray platform. Specifically, the cell microarray-based experiments were used to validate and optimize parameters of the intercellular Notch signaling model. This model incorporated the experimentally established multicellular dimensions of the cellular microarray domains, mechanical stress-related activation parameters, and distinct Notch receptor-ligand interactions based on the roles of the Notch ligands Jagged-1 and Delta-like-1. Overall, these studies demonstrate the spatial control of mechanotransduction-associated components, key growth factor and Notch signaling interactions, and point towards a possible role of E-Cadherin in translating intercellular mechanical gradients to downstream Notch signaling.

Microstructure-Sensitive modeling of surface roughness and notch effects on extreme value fatigue response

Fatigue life knockdown due to surface roughness induces local stress concentrations. Physical fatigue experiments are crucial to qualify materials for use but are limited in their capability to examine the mechanisms underlying the fatigue crack formation driving force due to these stress concentrations. Microstructure-sensitive computational models can aid in this regard and are employed in this work to examine the effects of surface roughness in Al 7075-T6. Digital microstructure models are subjected to crystal plasticity finite element method (CPFEM) simulations to examine the combined effects of microstructure and grain scale asperities on the driving force for fatigue crack formation. Following elastic–plastic shakedown, mesoscale volume-averaged fatigue indicator parameters (FIPs) are computed within fatigue damage process zones of grains. An increase in the intensity of realistic surface roughness profiles corresponds accordingly to larger FIPs but it is difficult to precisely interpret the mechanisms and effects of this intensification. We thus parametrically investigate surface asperities that couple with microstructure and lack of constraint on slip at the free surface to describe microstructure-sensitive surface roughness knockdown effects on fatigue resistance. The depth of a single valley (i.e., notch) due to surface roughness is found to be more detrimental to fatigue resistance than the radius (i.e., notch width, not to be confused with notch acuity). We also find that different crystallographic textures, grain morphologies, and number of sampled grains have a limited effect on the depth of influence of these notches.

Substrate geometry effect on the strength of repaired plates: Combined XFEM and CZM approach

Aluminum alloys are commonly used in aeronautical applications because of their specific strength and improved corrosion resistance. These structures, during their service, are exposed to various loading cycles, eventually leading to failure at the loci of geometric discontinuities. Repairing by metal or composite patch bonding is widespread to extend the structures’ life by limiting stress concentrations and delaying crack initiation. This work consists of a numerical study, validated by experimental test data, to assess the effect of a central circular notch in an aluminum plate, either reinforced or not by an adhesively bonded composite patch, on the global tensile response of the structure. The constitutive law of the aluminum and adhesive is assumed nonlinear and follows Von Mises equivalent stress flow theory with a hardening variable in incremental form. Damage initiation in the aluminum alloy is modeled by the XFEM (eXtended Finite Element Method), using the maximum principal stress criterion (MAXPS) for damage initiation prediction. Damage evolution is based on the energy approach. The adhesive layer was modeled by CZM (Cohesive Zone Model). A good agreement was found between the experimental results of the tensile curves of the repaired and unrepaired plates with those resulting from numerical modeling. Once the numerical model was validated, several parameters werenumerically studied, namely the shape of the composite patch, the size of the notch, the nature of the adhesive and repair mode by single and double patch, to reduce maximum stress of the damaged area and provide maximum repair efficiency.

Comparative study on numerical simulation methods of stress concentration calculation and fatigue life prediction for welds

The effective notch stress considers the influence of notch effect and solves the stress singularity in FEA by setting virtual notch radius. However, this method also has limitations such as inaccurate calculation results affected by virtual radius and modeling difficulty. The structural stress method based on fracture mechanics recommended in this paper has no such limitation, but also takes notch effect and stress singularity into account. To prove this, the engineering application ability of the two methods is compared through two cases. The results show that the structural stress method has better engineering applicability and is worth popularizing.

Experimental study on mechanical properties and low-cycle fatigue behaviour of stainless steels subjected to salt spray and dry/wet cycle

This paper presents experimental studies on the mechanical properties and low-cycle fatigue behaviour of corroded austenitic stainless steel (304) and ferritic stainless steel (430). Tensile and low-cycle fatigue tests are conducted on 28 specimens. The corrosion process of the specimens is finished by using salt spray and dry/wet cycle tests. In the tensile tests, the key parameters of the constitutive model proposed by both Rasmussen and Gardner were obtained for austenitic stainless steel (304) and ferritic stainless steel (430). In the low-cycle fatigue test, the relationships of the stress amplitude and fatigue life of the specimens were obtained. The relationships of the mass loss, corrosion rate and cross-sectional area loss with the corrosion time are established. The influences of corrosion pits and fatigue cracks on the low-cycle fatigue behaviour of these two materials are studied through both macro and micro fracture morphology analysis. It is found that compared with the salt spray, the dry/wet cycle condition causes more degradation on the strength and fatigue life of the ferritic stainless steel (430). The stress concentration and notch effect of corrosion pits are the main reasons for the degradation in fatigue life of ferritic stainless steel (430). Moreover, current codes Eurocode 3 (EC3), Australian Standard (AS), BSI Standard (BSI) and DNV Standard (DNV) could not provide accurate fatigue life assessment for these two materials after salt spray and dry/wet cycle corrosions.

An Experimental and Numerical Investigation of the Effect of Notches on the Root of Gas Turbine Compressor Blades on Critical Speeds

An Experimental and Numerical Investigation of the Effect of Notches on the Root of Gas Turbine Compressor Blades on Critical Speeds

Comparison of machine learning and stress concentration factors‐based fatigue failure prediction in small‐scale butt‐welded joints

AbstractFatigue behavior of welded joints is significantly influenced by numerous factors, for example, local weld geometry. A representative quantity for the influence of the notch effect created by the local weld geometry is the stress concentration factor (SCF). Thus, SCFs are often used to estimate fatigue failure locations and fatigue strength; however, this simplifies the mutual effect of other influencing factors. Consequently, fatigue strength estimates for welded joints may deviate from experimental results. Machine learning techniques offer an alternative to traditional fatigue assessment approaches based on SCFs. This study presents a comparison of failure location predictions and number of cycles to failure for 621 fatigue tests of small‐scale butt‐welded joints. In addition, an understanding of importance and mutual influence of the factors is desired. We used gradient boosted trees in combination with the SHapley Additive exPlanation framework to identify influential features and their interactions.

Fatigue life prediction of notched components under size effect using critical distance theory

Notch and size effects have shown significant influences on fatigue performance of engineering components, which deserve special attentions when performing structural health assessment. Thus, a possible size effect on critical distance is identified, when evaluating fatigue lives of notched specimens using the theory of critical distance.. In this work, by introducing the relative stress gradient concept, a novel reformulation of critical distance theory is proposed to consider the size effect in notch fatigue analysis. Experimental data of GH4169 Ni-based alloy, Al 2024-T351 alloy, and En3B low carbon steel notched specimens are applied for model validation and comparison. Results indicate that all predicted lives based on the proposed method agree well with the experimental values.

Influence of the Laser Welding Direction of Thin DP600 and the Residual Stresses Relaxation Obtained by Simulation and Neutron Diffraction

Laser-welded structures are often subjected to dynamic service loads ranging from cyclic fluctuations to completely random ones. The laser-welded lap joints suffer from defects resulting in notch effects, surface cracks, residual strains and stresses. The fatigue strength of the laser-welded lap joints is reduced significantly because of the presence of these defects, and the size of the welded joints is small. Therefore, the mechanical strength of laser-welded structures must be defined in terms of the fatigue strength and residual stress of the obtained joints or assemblies. To analyze the above-discussed effects, this paper proposed two approaches: numerical and experimental methods. The originality of the work is to weld the rolled sheet in three directions (0°, 45°, 90°). The residual stresses before and after low cyclic tensile tests of assemblies obtained from overlapped thin DP600 steel sheets were calculated by ABAQUS. The obtained results were compared to the experimental data by neutron diffraction. The presented results in terms of residual stresses curves, spatial distributions of residual stresses obtained at the end of laser welding and low-cycle fatigue. It showed the relaxation effect of residual stresses and the direction effect of welding. These results have been explained by several factors.