Stress Concentration Features Research Articles

The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature

AbstractThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

Microscale deformation behavior of sandstone mineral particles based on XCT scanning

This work aimed to quantify the physical and mechanical behavior of three-dimensional microstructures in rocks under uniaxial compression. A high-precision in situ XCT (X-ray transmission computed tomography) technology was applied to investigating the behavior of mineral grains in sandstone: the movement, the rotation deformation, and the principal strains between fault zone and non-fault zone. The results indicate that after unloading, the shear strain of mineral grains is periodic in the radial direction, the strain of mineral grains in the fracture zone is about 30 times of the macro strain of the specimen, which is about 5 times in the non-fracture zone, and the shear strain near the fault zone is larger than the compressive strain, and there is the shear stress concentration feature.

Using the theory of critical distances to predict notch effects in fibre composites

The Theory of Critical Distances (TCD) is a well-established method for predicting the effect of notches and other stress concentration features on the fracture and fatigue strength of materials and structures. It has been applied to fibre composite laminate materials but only in a limited manner: previous work was largely limited to two particular cases: circular holes and sharp cracks, both being loaded in axial tension. Long-fibre composites are increasingly replacing metals in many applications, but they are highly sensitive to stress concentration and damage. The present work aimed to develop a more complete picture of notch effects and to investigate the accuracy of the TCD as a predictive tool for this class of materials. Using a quasi-isotropic carbon-fibre/epoxy laminate, we carried out experiments on samples containing two types of stress concentration (notches and corners), varying notch length from 1 to 10 mm and root radius from 0.5 to 20 mm giving stress concentration (Kt) factors from 1.16 to 3.92. Samples were tested in axial tension, in axial torsion and also in mixed tension/torsion loadings. Finite element analysis (FEA) was used to determine stresses in the vicinity of the notches. Predictions using the TCD with a constant value of the critical distance L, were found to be very accurate for the specimens loaded in tension. Specimens loaded in torsion tended to fail at applied torques which were higher than predicted by the TCD, by factors of the order of two. This difference may be explained by the three-dimensional nature of these stress concentrations. The torsion failures could be predicted accurately by using different constants from those used for tension, but this solution is not satisfactory in the general case and should be investigated further.

Elasto-plastic behaviour of cantilever beams containing varied stress concentration cut out features

In machine component design, it is necessary to introduce openings or cut outs to carry out specific service functions such as electrical wiring, piping, and inspection, among others. The introduction of these cut outs results in a high concentration of stress leading to degradation of the component strength, especially under cyclic loading. The geometrical features and orientation of these cut outs relative to the support and the applied loads will also influence the severity of the resulting stress concentration. Of particular interest to this study is the elastic and elasto-plastic behaviour of a cantilever beam containing rectangular, circular or diamond cut outs under plane strain conditions. Four aspects of the work were accordingly examined. The first is concerned with the elasto-plastic load–deflection diagram of the beam containing the cut out features. The second with the finite element predictions of the elastic behaviour and the associated stress concentration factor of the beam in the presence of the cut out features. The third with the elasto-plastic behaviour and etching results showing the remarkable agreement between the two. Finally, in the fourth, we focus our attention to unloading residual stresses. The results offer a complete coverage of the problem and should help in the elasto-plastic design of beams containing stress concentration features.

Theory of critical distances: A discussion on concepts and applications

Theory of Critical Distances (TCD) collects several methods adopted in failure prediction of components provided with stress concentration features. The idea of evaluating stress effect in a zone rather than in a single point was proposed decades ago but, only thanks to relatively recent works, TCD concepts showed to be a successful extension of Linear Elastic Fracture Mechanics (LEFM), able to assess strength and fatigue life. The increasing computational power has made Finite Element Method (FEM) widespread, hence stress fields can be easily extracted and used as input data for fatigue post-processing and durability analyses. In this scenario, TCD reveals as a powerful tool which, thanks to the introduction of a single material parameter (critical distance, [Formula: see text]), integrates classical fracture models by considering the presence of microscale phenomena acting in fracture process. In this sense, TCD behaves as a link between continuum mechanics and LEFM. Modalities and reasons for this connection to occur are interesting points of further investigations. Literature on TCD and its theoretical-experimental background is quite extended, nevertheless few industrial applications are available in literature to the best of authors’ knowledge. In this paper, an overview of concepts and applications related to TCD are reported highlighting the relevance of theoretical arguments in actual applications.

The introduction of critical distance in the description of dynamic experiments based on simulation mesodefect evolution

The Theory of Critical Distances is a group of phenomenological methods for fracture prediction of materials subjected different loading conditions with accounting effect of stress concentration features. When estimating strength of components under dynamic loading by directly postprocessing the elastoplastic stress fields acting on the material in the notch tip the critical distance is a material parameter. Based on research of physical processes taking place at a micro/mesoscopic level near the notch tip the physical meaning of the parameters of theory of critical distances was investigated. The modeling of solids with defects behavior based on self-similar solutions for stress and defect density fields in one- and three-dimensional cases was carried out. Established link between critical distance and fundamental length of dissipative structure growing in a blow-up regime.

Effect of secondary crystal orientations on the deformation anisotropy for nickel-based single-crystal plate with notch feature

The effects of the secondary crystal orientations on the nickel-based single-crystal superalloy turbine blades were investigated. The stress concentration features were used for investigation of the optimal secondary crystal orientation leading to the higher strength of the single-crystal turbine blades. The crystal plastic finite element method coupled with micromechanics constitutive model is applied to study the effect of secondary crystal orientation on plastic deformation and mechanical behavior around the cooling holes and notches with the primary (load) orientation fixed at [001] direction. For nickel-based superalloy plates with holes or notches, the secondary crystal orientation effect on the strength needs to be clarified at various load levels. The maximum von Mises stress in the single-crystal alloy varies significantly with variation in the secondary crystal orientations. It was found that only two slip systems dominate the deformation process of the material owing to their favorable orientation with loading. The secondary orientation of 45° was identified with lowest resolved shear stress in the dominating slip systems and potential of producing higher strength for single-crystal turbine blades.

Effects of rolled edges on the fatigue life of shot peened Inconel 718

Shot peening on part edges is a critical operation in aerospace manufacturing. Edges are plastically deformed by the shot peening media, which can create stress concentration features known as “rolled edges”. The detrimental impact of rolled edges on fatigue life has already been observed in the literature. However, the effects of the geometries of the unpeened edges on rolled edge formation and on the resulting fatigue life have never been published in the open literature. This study presents the effects of different shot peened edge geometries on the fatigue life of Inconel 718, in high and low cycle fatigue. Chamfered, sharp and rounded edges were prepared, shot peened, and fatigue tested. An analysis of crack initiation mechanisms and fatigue lives showed that shot peened sharp edges represent the worst configuration tested in terms of fatigue life and that shot peened rounded edges do not improve the fatigue life when compared to chamfered shot peened edges.

Effect of microstructure on fatigue behaviour of advanced high strength ductile cast iron produced by quenching and partitioning process

The fatigue behaviour of high strength ductile cast iron produced by a quenching and partitioning process (Q&P) is evaluated. The Q&P process is receiving increased attention as a new way to produce ultra-high strength steels with multiphase microstructures composed of a martensite matrix and substantial amounts of carbon-stabilized retained austenite. Currently, the mechanical properties arising from applying the Q&P process in ductile cast irons have not been investigated. Thermal treatments consisted of heating the material to 880°C for a 2h soaking time followed by quenching in oil at 140°C and 170°C, intermediate temperatures between Ms and Mf allowing the formation of a controlled amount of athermal martensite. The material was reheated to 300°C and 375°C (the partition treatment) for different times between 15 and 120min and subsequently air-cooled to room temperature. Microstructural evolution and carbon partitioning was investigated by in situ synchrotron X-ray diffraction. With a microstructure composed of tempered martensite, bainitic ferrite and carbon-enriched stabilized austenite, a new class of properties was obtained, with an enhanced strength when compared to ADI (austempered ductile iron) while still maintaining reasonable elongation. Fatigue testing was undertaken using polished plain bend bars (no stress concentration features) assessed under four-point bending. Uninterrupted tests at varying loads show that the higher partitioning temperature is beneficial for fatigue life. Fracture initiation sites are primarily from pores and a number of decohered graphite nodules. A strong influence of the microstructure on subsequent fatigue crack growth is observed from interrupted testing with replica records and SEM examination of tested samples, with cracks exhibiting significant tortuosity, at times even appearing to grow approximately parallel to the tensile axis. Using these results, the effects of the Q&P treatment, particularly the role of the partitioning time and temperature, on the fatigue properties of ductile cast iron are assessed.

Implementation of the Theory of Critical Distances using mesh control

The Theory of Critical Distances (TCD) is a method for predicting fatigue and fracture at stress concentration features. It is extensively used in the design of engineering components, being a relatively simple approach based on post processing of finite element analysis (FEA). It requires only one additional material parameter, the so-called critical distance, L. However its implementation for engineering components of complex shape can be difficult because of the need to use a fine mesh and the requirement to obtain stress/distance path information for the feature being investigated. In this paper we propose a new approach by which predictions can be made more easily through control of the size and type of mesh used. We found that by setting the element size to be a simple multiple of L and by using certain other mesh controls which are generally available in commercial software such as ANSYS, fatigue behaviour could be predicted by simply using the “hot spot stress”, i.e. the maximum stress at the notch root. With this approach the user avoids excessive mesh refinement and does not need to obtain the stress at the critical distance in order to make the prediction. We tested eight different stress concentration features including cracks, sharp notches, blunt notches and V-shaped notches, along with an example component (a welded joint). We found that the error in using this approach, compared with that of the usual implementation of the TCD (Point Method) was acceptably small (less than 20%) in the great majority of cases. A few cases in which larger errors arose were found to be due to particular combinations of mesh element size, feature size and component size which caused difficulties for the automatic mesh generation software. This new approach will allow the TCD to be used more easily, for a wider range of industrial components, structures and materials.

The Theory of Critical Distances: A link to micromechanisms

The Theory of Critical Distances (TCD) is a methodology for prediction of the effect of stress concentration features on material failure, especially cracking-related mechanisms such as fatigue and brittle fracture. It has a long history but has become much more relevant to industrial problems in recent years thanks to the widespread availability of finite element analysis and other numerical simulation tools. An ongoing fundamental challenge is to understand the physical significance of the TCD parameters, in particular the value of the critical length L, in relation to the underlying micromechanisms of crack extension and toughening. A novel approach is presented here, in which the TCD is used to make predictions of the notch fracture behaviour of three different model materials having different crack extension mechanisms. Three different notch types (circular holes, long slots and short cracks) were investigated. In all cases the value of L was shown to be simply related to a microstructural length parameter but this relationship depended on the mechanism involved. These results provide some insights into the distinctive behaviour of different materials, especially polymers such as PMMA which display crazing behaviour, and fibre composites such as carbon/epoxy laminates.

The effect of stress concentration on the bending behavior of a ZrCuNiAl bulk metallic glass

The bending behaviors of a Zr55Cu30Ni5Al10 bulk metallic glass (BMG) samples with different notch radii are studied. The artificial notch will introduce stress concentration within the BMG sample during bending. That means, smaller notch radius leads to greater stress concentration around the notch area, and thus results in earlier failure and smoother fracture surface. Then, the sample with smaller notch radius possesses smaller fracture toughness. The relationship between the stress concentration and deformation feature has been discussed in terms of grease model.

Application of the Microstructural Finite Element Alternating Method to assess the impact of specimen size and distributions of contact/residual stress fields on fatigue strength

This paper shows an implementation of the Microstructural Finite Element Alternating Method (MFEAM) to calculate fatigue strength of engineering components subjected to complex loading conditions. These conditions include combination of primary cyclic loading and residual stress fields or localised stress distributions due to the contact of two solids. The alternating method uses the finite element method (FEM) to solve the boundary value problem of the un-cracked body, allowing treatment of arbitrary shaped components, in conjunction with a short crack model to account for the interaction of the crack with the microstructural barriers, implemented within the distributed dislocation technique (DDT) to assess the crack problem in an infinite medium. An iterative scheme between the FEM and the DDT solutions is proposed in order to predict the required applied load to propagate the crack in the finite size body. Comparisons of results with reported data in the literature show that the MFEAM can be used effectively to analyse finite size components and that it is capable of capturing the effect of localised stress concentration features on cyclic behaviour.

Correlation of circumferentially notched and smooth specimen fatigue test results in two nickel base superalloys

Prediction of the fatigue lives of components in engineered products is an important area of study; fatigue failure of a component may have product safety implications. Nearly all components contain stress concentration features and fatigue cracks will typically develop from these stress concentrations. Therefore, it is important that fatigue life prediction methods take into account the presence of stress concentrations. This article compares two methods of correlating fatigue test results between smooth and notched specimens in order to be better able to predict the fatigue lives of full-scale components. The first method uses the Neuber analysis approach to predict the strain range and mean strain at the root of the notch; the second uses elastic-plastic finite element analysis of the notched specimens. Two circumferentially notched specimen geometries are considered, with Kt values of 2.19 and 3.43. The analyses show that the Neuber method under-predicts the fatigue lives of the notched specimens, whereas a closer correlation is obtained using the finite element models. This is explained in terms of the different levels of constraint on yielding in a circumferentially notched specimen relative to the pure shear loading that is the basis of the Neuber analysis. Four methods of enhancing the finite element correlations are described. The first uses a method developed by Peterson to define a fatigue stress concentration factor. The second uses a mean stress and stress range approach developed by Smith, Watson and Topper. The third uses a critical distance approach developed by Pluvinage. The final method takes into account the volume of highly stressed material at the root of the notch using an approach first described by Waters and Norris to define a fatigue stress concentration factor. This approach has many parallels with the strain energy density approach.

Biocompatible Pressure Sensing Skins for Minimally Invasive Surgical Instruments

This paper presents 800-μm thick, biocompatible sensing skins composed of arrays of pressure sensors. The arrays can be configured to conform to the surface of medical instruments so as to act as disposable sensing skins. In particular, the fabrication of cylindrical geometries is considered here for use on endoscopes. The sensing technology is based on polydimethylsiloxane synthetic silicone encapsulated microchannels filled with a biocompatible salt-saturated glycerol solution, functioning as the conductive medium. A multi-layer manufacturing approach is introduced that enables stacking sensing microchannels, mechanical stress concentration features, and electrical routing via flexcircuits in a thickness of less than 1 mm. The proposed approach is inexpensive and does not require clean room tools or techniques. The mechanical stress concentration features are implemented using a patterned copper layer that serves to improve sensing range and sensitivity. Sensor performance is demonstrated experimentally using a sensing skin mounted on a neuroendoscope insertion cannula and is shown to outperform previously developed non-biocompatible sensors.

An experimentally validated contour method/eigenstrains hybrid model to incorporate residual stresses in glass structural designs

Contour method–based finite element models together with knowledge of the surface deformation resulted from the stress relaxation along a newly cut-plane were used to construct the residual stresses in commercially available float glass. The results show that the residual stress depth profile of float glass is parabolic. The constructed residual stress profiles, validated to some extent against results of scattered light polariscope experiments, were then used to establish the misfit strains (i.e. eigenstrains) existed in the original glass specimens. It is shown that despite the modelling uncertainty of the contour method and the limitations associated with the scattered light polariscope measurements, the eigenstrain depth profile in a given float glass specimen can be determined to an acceptable accuracy. The article shows that once the underlying eigenstrain distribution in a given thickness of glass has been determined, the complete residual stress distribution can simply be determined by incorporating the eigenstrain profile as a misfit strain in an appropriate finite element model. It is also shown that the hybrid contour method/eigenstrain model enables modelling the residual stress around stress concentration features such as holes and/or stress evolution during subsequent applied loadings, by simply using the knowledge of eigenstrains.

Fatigue in bamboo

This paper describes the results of an experimental programme to determine the fatigue behaviour of bamboo. Bamboo is subjected to cyclic loading, both in the plant itself and subsequently when the material is used in load-bearing applications in the construction industry. However, there is currently no data in the literature describing fatigue in this material. We found that sections of bamboo culm loaded parallel to the culm axis did not undergo fatigue failure: samples either failed on the first loading cycle, or not at all. By contrast, fatigue was readily apparent in samples loaded in compression across the diameter of the culm. The number of cycles to failure increased as the cyclic load range decreased in a manner similar to that found in many engineering materials: fatigue occurred at applied loads as small as 40% of the ultimate strength. Two different species of bamboo were tested and found to have different ultimate strengths but similar high-cycle fatigue strengths. Finite element analysis was used to help understand the progression of fatigue damage and the effect of stress concentration features. Some tentative design rules are proposed to define stress levels for the safe use of bamboo, taking fatigue into account.

Field Load Acquisition and variable amplitude fatigue testing on maxi-scooter motorcycles

Aim of the present work was the instrumentation of a maxi scooter for the field collection of service loads acting on the scooter main components such as frame, fork, handlebar, rear frame and suspension. Service loads were collected on an instrumented Yamaha Tmax scooter equipped with 22 channels during a set of field tests that were representing a predefined road mix, covering a mileage of 270 km. Field load histories were used to develop an accelerated test procedure for the accelerated bench fatigue testing of a new model prototype whose mission was set to 50000 km. The acceleration procedure allowed a time reduction from 1600 hrs to 122 hrs bench equivalent testing. Both the benchmark scooter Tmax and a maxi-scooter prototype under development underwent the bench variable amplitude fatigue testing. The results of the fatigue tests on the prototype allowed to identify some critical bolted connections and to reduce some stress concentration features causing the appearance of small cracks that were found also after during 50000 km of driving tests.

Static assessment of brittle/ductile notched materials: an engineering approach based on the Theory of Critical Distances

Engineering components often contain notches, keyways or other stress concentration features. These features raise the stress state in the vicinity of their apex which can lead to unexpected failure of the component. The Theory of Critical Distances has been proven to predict accurate results, but, conventionally, requires two key ingredients to be implemented: the first is a stress-distance curve which can be obtained relatively easily by means of any finite element software, the second is two additional material parameters which are determined by running appropriate experiments. In this novel reformulation, one of these additional parameters, namely the critical distance, can be determined a priori, allowing design engineers to assess components whilst reducing the time and cost of the design process. This paper investigates reformulating the Theory of Critical Distances to be based on two readily available material parameters, i.e., the Ultimate Tensile Strength and the Fracture Toughness. An experimental data base was compiled from the technical literature. The investigated samples had a range of stress concentration features including sharp V-notches to blunt U-notches, and a range of materials that exhibit brittle, quasi-brittle and ductile mechanical behaviour. Each data set was assessed and the prediction error was calculated. The failure predictions were on average 30% conservative, whilst the non-conservative predictions account for less than 10% of the tested data and less than 2% of the non-conservative error results exceed -20%. It is therefore recommended that a safety factor of at least 1.2 is used in the implementation of this version of the Theory of Critical Distances.

A Unified Viscoplastic Model for High Temperature Low Cycle Fatigue of Service-Aged P91 Steel

The finite element (FE) implementation of a hyperbolic sine unified cyclic viscoplasticity model is presented. The hyperbolic sine flow rule facilitates the identification of strain-rate independent material parameters for high temperature applications. This is important for the thermo-mechanical fatigue of power plants where a significant stress range is experienced during operational cycles and at stress concentration features, such as welds and branched connections. The material model is successfully applied to the characterisation of the high temperature low cycle fatigue behavior of a service-aged P91 material, including isotropic (cyclic) softening and nonlinear kinematic hardening effects, across a range of temperatures and strain-rates.