Source Of Stress Concentration Research Articles

Approximation method for arbitrary-shaped stress concentration source; application of physical and mathematical approximations of arbitrary parent–child notch

The degree of stress concentration in structures is of critical importance in safety designs. Many stress concentration factors for various shapes of notches with defined their geometry have been published, and if not published, the stress concentration factors can be obtained by FEM. However, when a complex, non-reproducible, and diverse notch exists on the surface of an actual machine that has undergone processing or is in a corrosive environment, it is usually more rational to use an immediate approximate solution rather than a rigorous but time-consuming analytical solution. There are two methods proposed for obtaining such approximate solutions. Although physical and mathematical approximate methods are separately used to estimate stress concentration factors in a specific-shaped notch, the application limits to arbitrarily sized and shaped notches are not yet clarified. In this study, we first extended the double-notch concept to the “parent–child notch” concept in the mathematical method, and then conducted finite element modelling for typical and complex notches. Further, we introduced the two representative lengths in the stress distribution induced by the parent notch from the mechanical consideration, and we deductively and inductively elucidated the application limits of the physical and mathematical approximations for an arbitrary parent–child notch. It was finally found that the stress concentrations of an arbitrary parent–child notch could be estimated only with the physical or mathematical approximation. Furthermore, a method for predicting fatigue limits of real structures subjected to cyclic loading was also proposed based on the present results.

A Comparative Investigation of Random Forest Regression and Artificial Neural Networks for Predicting Crack Growth Life of a Fighter Aircraft Wing Joint Under Spectrum Loading

Estimating fatigue life is challenging due to the input parameters’ statistical natures, such as the manufacturing process, scatter of service loads, microstructure, etc. Regarding fatigue life calculation in the aerospace industry, the importance of an accurate estimation becomes more critical due to strict safety, certification, service costs, and competitiveness regulations. The ability of soft computing methods to reveal complex relationships between multiple parameters and their computational speed could help predict fatigue life, especially in the service. This study compares random forest regression and artificial neural network methods to estimate the crack growth life of a fighter jet aircraft wing joint in terms of their computational time and accuracy. In addition, permutation feature importance and hyper parameter optimisation studies are conducted to extract essential features, investigate their effects on estimation performance, and fine-tune model parameters. The analysed joint is made of 7050-T7451 aluminium, widely used as a structural element in the aerospace industry. Since a hole is one of the major sources of stress concentration, and there may be many holes involved in any engineering structure, it is reasonable to assume that fatigue cracks may initiate at some of these holes during the service life of engineering structures. The crack type considered is a thru-crack around a hole, which is more severe than a corner crack. Load spectra are derived using the Fighter Aircraft Loading Standard for Fatigue (FALSTAFF) to calculate crack growth life. Considering particular service load conditions, ninety different spectra are developed, and the crack growth life of the joint is calculated based on linear elastic fracture mechanics correspondingly. Also, to simulate the worst-case scenario, friction between members and the retardation effect of load spectra are not considered when calculating crack growth life. Python’s Tensor Flow and Scikit-learn libraries are utilised to build machine learning models......

INVESTIGATION OF STRUCTURAL CHARACTERISTICS ON FERRITIC NODULAR CAST IRON

The linear form of graphite in gray cast iron is not favorable due to the weakening of the metal base and strong sources of stress concentration at the ends of the lamellae. Converting the lamellar form of graphite into nodular form is done by adding modifiers and pre-secreting smaller amounts of magnesium or cerium. In this way, the stress concentration due to graphite inclusions in nodular cast iron is significantly lower than in gray cast iron. “Nodular cast iron is a foundry pseudobinary alloy of iron and carbon, which was mostly excreted in the form of spherical graphite”. Depending on the structure of the metal base, the properties of strength and plasticity also depend, but certainly the shape, size and arrangement of graphite nodules has a significant influence. For these reasons, the size of graphite nodules is also determined, the highest values of the average nodule being from 50 to 10 μm. In this paper, a researched nodular cast iron with a ferrite structural base is given. The mentioned nodular casting was isothermally improved and tribologically tested, on a “pin and disc” tribometer. The obtained results are presented tabularly and diagrammatically.

Square Openings as Sources of Stress Concentration in Parts of Machines and Devices

Square Openings as Sources of Stress Concentration in Parts of Machines and Devices

Developments in Laminate Modification of Adhesively Bonded Composite Joints.

The use of carbon fibre reinforced polymer (CFRP) materials is increasing in many different industries, such as those operating in the aviation, marine, and automotive sectors. In these applications, composite parts are often joined with other composite or metallic parts, where adhesive bonding plays a key role. Unlike conventional joining methods, adhesive bonding does not add weight or require the drilling of holes, both of which are major sources of stress concentration. The performance of a composite joint is dependent on multiple factors and can be improved by modifying the adhesive layer or the composite layup of the adherend. Moreover, joint geometry, surface preparation, and the manufacturing methods used for production are also important factors. The present work reviews recent developments on the design and manufacture of adhesively bonded joints with composite substrates, with particular interest in adherend modification techniques. The effects of stacking sequence, use of thin-plies, composite metal laminates and its specific surface preparations, and the use of toughened surface layers in the composite adherends are described for adhesively bonded CFRP structures.

Peridynamic stress is the static first Piola–Kirchhoff Virial stress

The peridynamic stress formula proposed by Lehoucq and Silling (2008) is cumbersome to be implemented in numerical computations. In this paper, we show that the peridynamic stress tensor has the exact mathematical expression as that of the first Piola–Kirchhoff static Virial stress originated from the Irving–Kirkwood–Noll formalism (Irving and Kirkwood, 1905; Noll, 1955) through the Hardy–Murdoch procedure (Hardy, 1982; Murdoch, 1983), which offers a simple and clear expression for numerical calculations of peridynamic stress.Several numerical verifications have been carried out to validate the accuracy of the proposed peridynamic stress formula in predicting the stress states in the vicinity of the crack tip and other sources of stress concentration. By evaluating the peridynamic stress in the prototype microelastic brittle (PMB) material model of bond-based peridynamics, it is found that the PMB material model may exhibit nonlinear constitutive behaviors at large deformations. The stress fields calculated through the proposed peridynamic stress formula show good agreements with these calculated by using finite element methods, analytical solutions, as well as experimental data, demonstrating the merit of the peridynamic stress formula in predicting stress states for material failure problems.

Theoretical examination of the fracture behavior of BC3 polycrystalline nanosheets: Effect of crack size and temperature

2D carbon graphene nanostructures are elements of advanced materials and systems. This theoretical survey provides explanation to the mechanical and fracture behavior of mono- and polycrystalline BC3 nanosheets (denoted as MC- and PCBC3NS, respectively) as a function of temperature and the type of crack defects. The mechanical performance of PCBC3NS at elevated temperatures was monitored varying the number of grain boundaries (the main source of stress concentration) by considering structural defects forming during the crystal growth. Molecular dynamics (MD) simulation was applied as a cost-effective technique to model and test MC- and PCBC3NS by selecting the proper potential function and boundary conditions. The results demonstrated that the mechanical properties of the perfect crystalline PCBC3NS was decreased by increase of the number of grains, particularly when the grain numbers were equal to or more than 36. For defective PCBC3NS, the mechanical properties were decreased by the crack length and the temperature. The lowest values of the Young's modulus, failure stress, and failure strain were assigned to the PCBC3NS having the crack length of L/2 at 1000 K, respectively by 23%, 46%, and 33% lower than the corresponding defect-free PCBC3NS. The crack tip played a key role in failure behavior, even more that the number of grain boundaries. Eventually, the critical stress intensity was decreased gradually by increasing the temperature. The results of this work can be generalized to more complicated cases to deepen understanding and predict fracture fingerprint of the next generations of 2D nanostructures.

Application of Failure Assessment Diagram (FAD) for Steel Welded Connection Based on BS7910 and DNV-RP-108

The failure assessment diagram (FAD) method has been widely accepted to evaluate the extent to which cracks may affect structural safety. The usage of this FAD method has been validated and included in [1]-[3]. The structure under investigation, described in four fully welded T-joint (BCC5) specimens, where these welded joints are a source of stress concentration and defects from which fatigue cracks can grow. The four specimens were modeled under different displacement loading using a finite element analysis program Ansys and SolidWorks software. In this work, the application of a FAD (Lr, Kr) using maximum stress, cumulative stress ranges, and the last half-cycle stress range was investigated. The results are showing that all the points were lying outside the FAD curve except for the BCC5D specimen point was inside FAD when using maximum stress.
 Conclusions made that the cumulative stress gives Lr and Kr are extremely large and hence predict failure too early. With the Crack Tip Opening Displacement (CTOD) of the test specimen assumed to be about 1mm rather than 0.1mm it was found that, if a FAD is to be used to indicate failure, then both Lr and Kr should be based on the maximum stress. It appears that the FAD methodology does help to predict the final failure (which is the usual application in such cases). This represents more effectively the structural behavior and would be more easily used by designers.

Application of Failure Assessment Diagram (FAD) for Steel Welded Connection Based on BS7910 and DNV-RP-108

The failure assessment diagram (FAD) method has been widely accepted to evaluate the extent to which cracks may affect structural safety. The usage of this FAD method has been validated and included in [1]-[3]. The structure under investigation, described in four fully welded T-joint (BCC5) specimens, where these welded joints are a source of stress concentration and defects from which fatigue cracks can grow. The four specimens were modeled under different displacement loading using a finite element analysis program Ansys and SolidWorks software. In this work, the application of a FAD (Lr, Kr) using maximum stress, cumulative stress ranges, and the last half-cycle stress range was investigated. The results are showing that all the points were lying outside the FAD curve except for the BCC5D specimen point was inside FAD when using maximum stress.
 Conclusions made that the cumulative stress gives Lr and Kr are extremely large and hence predict failure too early. With the Crack Tip Opening Displacement (CTOD) of the test specimen assumed to be about 1mm rather than 0.1mm it was found that, if a FAD is to be used to indicate failure, then both Lr and Kr should be based on the maximum stress. It appears that the FAD methodology does help to predict the final failure (which is the usual application in such cases). This represents more effectively the structural behavior and would be more easily used by designers.

Failure analysis on abnormal blockage of electro-hydraulic servo valve in digital electric hydraulic control system of 125 MW thermal power plant

Electro-hydraulic servo-valves, which are considered as the crucial core element of digital electric hydraulic control system, are widely applied in thermal power plants. In this paper, motivated by the long stable operation of thermal power plant, an abnormal blockage failure of the electro-hydraulic servo valve in digital electric hydraulic control system of 125 MW thermal power plant was investigated systematically. Three body abrasive wear was identified as the main failure mechanism. Comprehensive analysis results finally revealed that the unqualified material of the valve spool with abnormal coarse carbides caused by improper preparation process was ascribed to be the root cause of the failure. During the operation, the abnormal coarse carbides acted as stress concentration source and were peeled off from the valve spool under the effect of the friction between the feedback wire and the valve spool. After that, the exfoliated carbides served as third bodies, resulting in three body abrasive wear. In this process, some of the carbides with size exceeding 5 μm driven by fire-resistant oil entered the clearance between the valve spool and the valve sleeve, leading to the abnormal blockage failure since the maximum allowable clearance was 5 μm. Countermeasures were then proposed to address corresponding problems. Considering that practical failure of the electro-hydraulic servo-valves is seldom reported before, the finding of the paper could be helpful for better understanding the failure mechanisms of the electro-hydraulic servo-valves under actual service conditions.

Failure of induced draft-ID fan blade in coal fired boiler

In the present work, failure analysis has been carried out for induced draft ID fan blade used in coal fired boiler of thermal power plant. The vibrations were recorded in bearing housing prior to breakdown. Severe looseness in bearing causes an excessive vibration which in turn induced alternating stress in fan blade. The base material of failed blade was carbon steel coated with Ni-Cr-Fe alloy for erosion resistance. Although fatigue tests demonstrated that failed and virgin blade have no significant variation in fatigue strength but blade exhibited higher fatigue strength in longitudinal axis compare to its transverse axis. Higher fatigue strength in longitudinal axis was attributed to pearlite bands. Visual inspection evidenced beach marks and also depletion of coating on leading edge. The mechanical collision of silica, fly ash & un-burnt coal particles have eroded coating and presence of chloride and sulfur in flue gas caused pitting corrosion. Fractographic evidences suggested that fatigue was initiated from pits. Under actual operating conditions of blade, the pits acted as a source of stress concentration to initiate fatigue crack and ultimately leads to fatigue fracture. Recommendations were suggested to prevent the failure of investigated blade for similar applications.

Study on microstructure and fracture characteristics of 5083 aluminum alloy thick plate

The microstructure and fracture characteristics of 105 mm thick 5083 aluminum alloy hot rolled plate are investigated by X-ray diffractometer, metallurgical microscope, scanning electron microscope, transmission electron microscope, energy dispersive spectrometer and tensile testing machine in this paper. The results show that the plate is mainly composed of α-Al, Al21(Fe, Mn)4Si, Al6(Fe, Mn) and Mg2Si. The second phase distribution is inhomogeneous along the thickness direction, resulting in an U shape change of elongation. The minimum elongation in Layer 5 is related to the dependence of Mg2Si on Fe-containing intermetallic compounds, state of dislocations and grooves. Al6(Fe, Mn) presents acicular in the surface layer and fragmented in the center, and its fracture mode can be used to roughly judge the stress. Under SEM, the common black phases in high Mg-containing aluminum alloys may be not Mg2Si, but holes formed by their shedding. Mg2Si tends to adhere to the surface of Fe-containing intermetallic compounds and it is more easily separated from Al matrix. This indicates that Mg2Si may be the source of stress concentration initially causing materials fracture. A comprehensive summary has been made to clarify the influence of structures on fracture characteristics of thick plate.

Fatigue performance of hydraulic unit of high-pressure plunger pump

The present paper considers a hydraulic unit of high-pressure plunger pump that relates to the main types of equipment for oil-and-gas well drilling, completing or repairing. The problem of fatigue breakdowns of hydraulic units of high-pressure plunger pumps is posed in the paper. According to the leading pump manufacturers, the main source of stress concentration in a valve casing of a pump is the edgings of crossing of cylindrical surfaces of seating fits, valves or plunger bores. The value of stresses in the concentrator zone was assessed, and values of safety coefficients of fatigue strength depending on geometric parameters of stress concentrators were compared. To solve the problem set by the leading manufacturers of pumps, the full-scale model of hydraulic unit was elaborated, stress condition and its effect on fatigue parameters of valve casing of hydraulic unit of oilfield high-pressure plunger pump were analyzed. The obtained calculation results quantitatively clarify the pattern of stress distribution in zones of stress concentration in a valve casing of hydraulic unit of a high-pressure plunger pump. Certain conclusions were made on how the parameters under consideration affect the values of fatigue stresses and on priority research directions for the problem under study.

Comparative study for material effect on stress behaviourial characteristics of rectangular plate

The circular cutout is considered as a typical source of stress concentrations in rectangular plate type structures. The proper position of a cutout in a rectangular plate can be significantly affected by the choice of material, cutout size, and various end conditions which are relative to the externally applied force. The research work entitled in this paper deals with the finite element formulation applied to a plate made with isotropic and laminated plates subjected to uniform pressure with various end conditions. The vicinity of stress distribution for the rectangular plate made of these two materials with and without the effect of circular cutouts is discussed. The accuracy of the solution verified with the available literature, and from the comparison, a fairly good agreement seen between the Finite element formulations in this paper with previously available results.

Summary of research on fatigue life assessment of welded joints

In order to deepen the understanding of the fatigue problem of welded joints, this paper compares the research principles and limitations of four commonly used welding fatigue life prediction methods, including nominal stress method, hot spot stress method, notched stress method and fracture mechanics method. The nominal stress method is the most widely used in the four fatigue prediction methods, but it is too conservative. The hot spot stress method has high calculation accuracy, but it cannot fully reflect the fatigue behavior of the joint details. The notch stress method includes all sources of stress concentration, but the S-N curve recommended by IIW does not apply to the fatigue evaluation of butt joints. The fracture mechanics method is applicable to the crack propagation stage of joints. Through the analysis and research on the fatigue life assessment methods of four welded joints, it lays a theoretical foundation for the fatigue life prediction of welded joints.

Effect of Hardness HV on Plastic Strain Gradient Around Sharp Stress Concentration Source of S45C Steel with a Mechanical Long Crack

Effect of Hardness HV on Plastic Strain Gradient Around Sharp Stress Concentration Source of S45C Steel with a Mechanical Long Crack

Micro failure analysis of adhesively bonded joints enhanced with natural cork particles: Impact of overlap length and particles volume fraction

In this work, the effects of natural particles on the micro failure mechanisms and static strength of an epoxy are studied using scanning electron microscope (SEM) technique. To this end, bulk specimens and adhesively bonded single lap joints (SLJs) containing different amount of micro cork particles were fabricated and tested. To investigate the toughening mechanisms, the fracture surfaces of the specimens were analyzed using SEM and a magnifying glass. It was shown that the deviation of the crack path and formation of micro-cracks generate multi fracture surfaces which is one of the main sources of energy absorption and resulted in higher fracture energy of the enhanced adhesives. Acting as a crack stopper and consequently stabilizing the crack propagation, prevents developing large and unstable cracks which is another source of energy consumption. However, the micro analysis showed that higher contents of cork lead to agglomeration of the particles which acts as a source of stress concentration, with a higher possibility of delamination between the matrix and the micro corks. Accordingly, there is an optimum amount of cork where the best adhesive fracture toughness is reached. The results showed that the optimum value of micro particles depends on the loading conditions.

Bond-based peridynamic modelling of singular and nonsingular crack-tip fields

A static meshfree implementation of the bond-based peridynamics formulation for linearly elastic solids is applied to the study of the transition from local to nonlocal behavior of the stress and displacement fields in the vicinity of a crack front and other sources of stress concentration. The long-range nature of the interactions between material points that is intrinsic to and can be modulated within peridynamics enables the smooth transition from the square-root singular stress fields predicted by the classical (local) linear theory of elasticity, to the nonsingular fields associated with nonlocal theories. The accuracy of the peridynamics scheme and the transition from local to nonlocal behavior, which are dictated by the lattice spacing and micromodulus function, are assessed by performing an analysis of the boundary layer that surrounds the front of a two dimensional crack subjected to mode-I loading and of a cracked plate subjected to far-field tension.

Ductile-to-brittle transition in tensile failure due to shear-affected zone with a stress-concentration source: a comparative study on punched-plate tensile-failure characteristics of precipitation-hardened and dual-phase steels

The effect of shear-affected zone (SAZ), with a stress-concentration source induced by the punching process, on tensile properties was investigated. Tests using honed specimens (which have the same shapes and stress-concentration without any SAZ) and smooth specimens were conducted to compare the effect with that of the punched specimens. Dual-phase steel, which has a high work-hardening ability and low yield strength, and precipitation-hardened steel, which has a low work-hardening ability and high yield strength, were used in the tests. Materials with two tensile strength grades were prepared from both types of steel. Only the precipitation-hardened steel with higher strength grade punched specimen showed a brittle fracture with extremely short fracture-elongation, whereas the other specimens showed a ductile fracture. The fracture surface analysis revealed that cracks initiated in the maximum shear stress plane of the SAZ under tensile loading at first. We call the crack “shear crack.” The steel which showed brittle fracture used in this study easily exhibited plastic-strain localization compared with the other steels. If the shear crack is sharp, then the transition from ductile to brittle failure tends to occur. Furthermore, the strength characteristics of the punched specimen depend on the crack length dependency of the strength resistance and the failure phenomenon of the original material.

Fatigue resistance of an aluminium one-component polyurethane adhesive joint for the automotive industry: Effect of surface roughness and adhesive thickness

Fatigue is a crucial type of loading for many structural components that contain adhesive bonding systems such as the automotive industry. One of the main advantages of adhesives is their weight-reduction aptitudes. Furthermore, adhesively bonded joints allow a good damping to the fatigue's solicitation with fewer sources of stress concentration. In addition, a suitable surface treatment allows a great joint interface adhesion under cyclic loading. Hence, the mechanical preparation of the bonded surface influences directly the bonded joint performance. Therefore, a careful consideration should be given to the choice of the bonded adhesive thickness. The aim of this paper is to investigate the influence of surface roughness of an aluminium alloy and the adhesive thickness of a one-component polyurethane adhesive, to be used in several parts for buses structure, on the fatigue behaviour of single lap joints. To achieve this purpose, four different surface roughnesses were prepared on aluminium specimens, with an arithmetic average height from Ra ≈ 0.6 µm to Ra ≈ 1.5 µm. Bonded specimens with four adhesive thicknesses from 0.3 mm to 2 mm were manufactured and tested in dynamic tests. The results confirm that there is an optimum combination of surface roughness and adhesive thickness providing the maximum fatigue life. Moreover, good agreement was found between this investigation and a previous work using the same parameters under static loading.