Maximum Stress Intensity Research Articles

Corrosion Fatigue Crack Growth Rates of Sensitized AA5456-H116: Contribution of Stress Corrosion Cracking

The interplay between stress corrosion cracking (SCC) and corrosion fatigue was investigated for AA5456-H116 to determine the source of the inverse relationship between fatigue loading (f) and fatigue crack growth rates (da/dN). Sensitization in 5xxx series aluminum alloys refers to the precipitation of β on α-aluminum grain boundaries, which can occur in high-Mg 5xxx alloys after sufficient exposure to temperatures as low as 40°C. Recent research has established that da/dN in sensitized 5xxx series aluminum alloys can be inversely dependent on f. The severity of the inverse relationship between da/dN and f has been attributed to sensitization level, alloy composition, and temper. In this work, the effect that Kmax has on corrosion fatigue da/dN was quantified using fracture mechanics-based experiments conducted at different maximum stress intensity factors (Kmax) but the same ΔK in 3.5 wt% NaCl. Results suggest that the severity of the inverse relationship between da/dN and f (i.e., the slope of the da/dN vs. f trend) is governed by the magnitude of Kmax in relation to the threshold stress intensity factor for SCC (KISCC). In addition, results show that an inverse f-dependence, albeit with a lower slope, can exist even when Kmax is less than KISCC; in other words, absent SCC. This low-slope corrosion fatigue behavior may have been driven by an enhanced rate of hydrogen embrittlement facilitated by the more aggressive local crack tip environment present at progressively lower loading frequencies during corrosion of highly sensitized microstructures.

Fracture resistance of asphalt mixtures under mixed-mode I/II loading at low-temperature: Without and with nano SiO2

Increasing traffic congestion especially in cold climates has increased the damage caused by low-temperature cracks (LTCs) in the asphalt mix where the constituents are not able to withstand the loads required in these climates lonely and using the modifier is undeniable. In recent years, some research fields have been conducted on the use of nanomaterials as a modifier in asphalt mixtures to improve their properties. In accordance with previous researches, one of the nanomaterials that have appropriate effects on the performance of asphalt mixtures is Nano-SiO2. In this study, the effect of Nano-SiO2 on the incident of low-temperature cracks in asphalt mixtures is experimentally investigated. For this purpose, the semicircular bending test (SCB) under mixed-mode I/II loading is used for investigating the effect of Nano-SiO2 on forming cracks in asphalt (i.e., vertical and angular crack) at different temperatures of −5℃, −15℃, and −25℃. Results show that the maximum stress intensity factor (SIF) of the modified asphalt mixtures is related to specimens which have angular crack under pure opening mode, while the maximum critical SIF is improved when Nano-SiO2 is added to specimens which have vertical cracks under mixed-mode I/II with Me = 0.6 at −25℃. Furthermore, the critical SIF of all specimens having both vertical and angular cracks is significantly improved by adding 1.2% of Nano-SiO2 at all temperatures.

Internal Versus External Cracking—Their Impact on the Fatigue Life of Modern Smoothbore Autofrettaged Tank Gun Barrels

Abstract An extensive analysis of the fatigue life of a typical modern autofrettaged smoothbore tank barrel, cracked either internally or externally, in terms of the initial crack depth and shape, type and level of autofrettage, was conducted. Five overstraining cases were considered: no-autofrettage, 70% and 100% hydraulic autofrettage, and 70% and 100% swage autofrettage. KINmax, the maximum combined stress intensity factor (SIF) KINmax = (KIP + KIA) max, due to both internal pressure and autofrettage, as a function of crack depth for a large number of internal and external crack configurations was determined by the finite element method (FEM). A novel realistic experimentally based autofrettage model, incorporating the Bauschinger effect, was integrated into the finite element model, replicating both the hydraulic and swage autofrettage residual stress fields (RSFs) accurately. Fatigue lives were evaluated by integrating Paris' Law using the above values of KINmax. The following conclusions can be drawn from the results: hydraulic and swage autofrettage have a dramatic beneficial effect in extending the fatigue life of an overstrained barrel 4–11 times as compared to an identical nonautofrettaged tube. The fatigue life of overstrained barrels is controlled by internal cracking, for barrels overstrained by up to ε = 100% hydraulic autofrettage, by up to ε = 70% in the case of swage autofrettage, and by external cracking for ε = 100% swage autofrettaged. Eliminating or carefully designing stress concentrators on the tube's external face and keeping away from corrosive agents thus, extending the fatigue-crack initiation life of an external crack, enables the increase of the level of swage autofrettage to up to ε = 100%. Swage autofrettage is much more superior to hydraulic autofrettage. The fatigue life of a 70% swaged autofrettaged barrel is 1.5 times higher than that of a 100% hydraulically autofrettaged tube. If full swage autofrettage is permissible, the fatigue life of such a barrel is twofold that of a fully hydraulically autofrettaged tube. Unlike the commonly accepted concept, the level of hydraulic autofrettage should not be limited to 70%, and full hydraulic autofrettage should be used. Similarly, in the case of swage autofrettage, if the detrimental effect of external cracking is removed by proper design and maintenance of the tube's outer surface, the level of autofrettage can be increased to up to ε = 100%, thus, gaining an increase of 33% in the fatigue life as compared to overstraining the barrel to only ε = 70%. Initial crack depth and shape are major factors affecting the fatigue life of the barrel. The deeper the initial crack depth, a0, and the slenderer its shape, a/c→ 0, the shorter the fatigue life of the barrel.

Mechanical Integrity Assessment of Two-Side Etched Type Printed Circuit Heat Exchanger with Additional Elliptical Channel

Printed circuit heat exchangers (PCHEs) are often subject to high pressure and temperature difference between the hot and cold channels which may cause a mechanical integrity problem. A conventional plate heat exchanger where the channel geometries are semi-circular and etched at one side of the stacked plate is a common design in the market. However, the sharp edge tip channel may cause high stress intensity. Double-faced type PCHE appears with the promising ability to reduce the stress intensity and stress concentration factor. Finite element analysis simulation has been conducted to observe the mechanical integrity of double-etched printed circuit heat exchanger design. The application of an additional ellipse upper channel helps the stress intensity decrease in the proposed PCHE channel. Five different cases were simulated in this study. The simulation shows that the stress intensity was reduced up to 24% with the increase in additional elliptical channel radius. Besides that, the horizontal offset channels configuration was also investigated in this study. Simulation results show that the maximum stress intensity of 2.5 mm offset configuration is 9% lower compared to the maximum stress intensity of 0 mm offset. This work proposed an additional elliptical upper channel with a 2.5 mm offset configuration as an optimum design.

Failure load and stress intensity factor of single-lap steel joints bonded with nano-Al2O3 reinforced epoxy adhesive

The failure load of single-lap steel joints (subjected to tensile force) with epoxy/nano-Al2O3 adhesives was analysed experimentally. Spherical-shaped nano-Al2O3 of diameter 23–47 nm and rod-shaped nano-Al2O3 of diameter < 10 nm and length < 50 nm were included in the epoxy adhesive by wt% (0.5, 1.0, 1.5, and 2.0 wt%). Stress intensity factor of the adhesive layer at the corner of single-lap joints was calculated from the failure load. A substantial improvement in the failure load and stress intensity factor of epoxy/nano-Al2O3 adhesives was obtained compared to the base adhesive. The maximum failure load and stress intensity factor of epoxy/nano-Al2O3 adhesives were found at 1.0 wt% of Al2O3 nanorods and 1.5 wt% of Al2O3 nanospheres. The failure load and stress intensity factor of epoxy/nano-Al2O3 adhesives reduced on further addition of nano-Al2O3 (i.e. 1.5 and 2.0 wt% of nanorods and 2.0 wt% of nanospheres). Still, these were more than the base adhesive.

Evaluation of Deformation and Failure Behaviors of Nuclear Piping Components Under Beyond Design Basis Seismic Loads Using a Simulated Specimen

Abstract This study designed a specimen that simulates the deformation and failure behaviors of the piping components in nuclear power plants (NPPs) under excessive seismic loads beyond the design basis, and conducted ultimate-strength tests using this specimen at room temperature (RT) and 316 °C. SA312 TP316 stainless steel (SS) and SA508 Gr.3 Cl.1 low-alloy steel (LAS) were used in the experiments. Displacement-controlled cyclic loads with constant and random amplitudes of load-line displacement (LLD) were applied as input loads. A set of input cyclic loads consisted of 20 cycles, and the LLD amplitudes of the cyclic load were determined to induce the maximum membrane plus bending stress intensity of 6–42Sm on the specimen, where Sm is the allowable design stress intensity. Multiple sets of input cyclic loads, with increasing amplitude of LLD, were applied to the specimen until cracking initiated. The results demonstrate that the simulated specimen adequately showed the ratcheting deformation and fatigue-induced cracking of piping components under displacement-controlled excessive seismic loads. In addition, samples of both materials failed under displacement-controlled cyclic load levels that were several times higher than those of the design basis earthquake (DBE). The SA316 TP316 SS had greater resistance to failure under large-amplitude cyclic loads than did SA508 Gr.3 Cl.1 LAS. For both materials, resistance to failure was lower at 316 °C than at RT. This study confirmed that the evaluation procedure of the ASME design code predicted the fatigue failure of specimens very conservatively under large-amplitude cyclic loads simulating displacement-controlled excessive seismic loads.

Failure Analysis of Spur Gears Used in Transmission System Applied on a Hand Tractor

Damage or failure frequently occurs on gearbox gears. If this occurs in the gearbox of tractors, this can be severe. This study aimed to determine the cause of the spur gear fracture through empirical and simulation studies. The hardness test was undertaken employing the Rockwell method where the fracture surface was observed using a scanning electron microscope (SEM) in order to identify crack initiation and the type of fracture. The stress intensity factor was next analysed using the finite element method (FEM). The results of the chemical composition testing indicated that the material used was according to the AISI 8620 standard containing an element of Carbon (C) of about 0.142 %.The hardness value of the gear was 109 HRB. The observation of the fracture surface showed a brittle fracture surface, suggesting that an impact load had occurred. The simulation results using the FEM also showed that the maximum stress intensity factor and KI value occurred at the centre of the tooth. The value of KI was shown to be larger than the fracture toughness and (KIC). Therefore, this result indicates that a crack will continue to propagate until final failure.

Crack kinking in isotropic and orthotropic micropolar peridynamic solids

A micropolar peridynamic model is presented for characterizing crack propagation in isotropic and orthotropic brittle materials. The analytical formulation of the two-dimensional model is based on the definition of a micropotential energy function that accounts for the four independent elastic constants that define orthotropy and that in the limit can be reduced to isotropy. A distinctive feature of the model is that the bonds’ elastic parameters are continuous functions of orientation with respect to principal material axes. By defining three deformation parameters that quantify bond stretch, bond shear deformation and particles relative rotation, the first continuum bond-based peridynamic model is obtained for two-dimensional Cauchy orthotropic materials characterized by four independent material moduli that is suitable for describing fracture as well as homogeneous and non-homogeneous deformations.The accuracy of the computational model as applied to crack-tip analyses is assessed by comparing the displacement and stress fields within the boundary layer that develops in the immediate vicinity of a crack with the analytical asymptotic results for an orthotropic continuum. The extension of such cracks when they are subjected to mixed-mode loading is simulated under the assumption of illustrative crack extension criteria, and the predictions are compared to those of the maximum hoop stress intensity factor criterion (HSIF-criterion) and the maximum energy release rate criterion (G-criterion).

The Strain-Stress State of K-1000-60/3000 Turbine Rotor for Typical Operating Modes

The extension of the exploitation-time of nuclear power plants over the park service life-time requires the verification of the main elements of power equipment that determine the resource characteristics. According to the regulatory documents, the extension of the service life-time of steam turbine equipment requires the calculation of the thermal and stress-strain state of its main elements, namely the high-pressure rotor. The numerical mathematical study of high-pressure rotor of the steam turbine K-1000-60/3000 is investigated in the paper. The boundary task of unsteady heat conduction for typical operating conditions of a steam turbine installation is considered. To solve differential heat conduction equations, a finite-element method of discretization of the computational domain was used. The strain-stress state of the high-pressure rotor is calculated with taking into account the combined effect of temperature stresses, irregularity of the temperature field, stresses from pressure and centrifugal forces. The calculation of the nominal mode of exploitation regime is performed in a quasi-stationary setting. The calculation of the variable modes of operation is performed in a non-stationary setting. It has been established that when operating at exploitation conditions that are close to the nominal regime, the zones of maximum stress intensity concentration are axial bore in the area of the fourth and fifth pressure stages. At exploitation in variable operating modes, frequent stress concentrators are discharge openings and fillet transitions of all stages. It has been established by calculation that for a high-pressure rotor of a K-1000-60/3000 steam turbine, the maximum of stress intensity is 158 MPa at nominal operating mode. At variable operating modes, the value of the intensity of the conditional elastic stresses does not exceed 226-263 MPa for all types of turbine launches

Development of a Novel Test Method to Characterize Material Properties in Corrosive Environments for Subsea HPHT Design

Abstract High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. Fatigue endurance (i.e., S–N) and fracture mechanics design are both permitted, depending on the criticality of the component. Both design approaches require material properties generated in corrosive environments, such as seawater with cathodic protection and/or sour production fluids. In particular, it is necessary to understand sensitivity to cyclic loading frequency (for both design approaches), crack growth rates (CGR) (for fracture mechanics approach) as well as fracture toughness performance. For many subsea components, the primary source of fatigue loading is associated with the start-up and subsequent shutdown operation of the well, with long hold periods in-between, during which static crack growth (CG) could occur. These are the two damage modes of most interest when performing a fracture mechanics based analysis. This paper presents the preliminary results of a novel single specimen test method that was developed to provide fatigue crack growth rate (FCGR) and fracture toughness data in corrosive environments, in a timeframe that is compatible with subsea HPHT development projects. Test data generated on alloy 625+ in seawater with cathodic protection are presented along with a description of how the test method was developed. A crack tip strain rate based formulation was applied to the data to rationalize the effect of frequency, stress intensity factor range (ΔK), and maximum stress intensity factor (Kmax).

Investigation of subsurface fatigue crack growth behavior of D2 tool steel (JIS SKD11) based on a novel measurement method

In our preliminary study of the fatigue properties of D2 tool steel (JIS SKD11), it was found that owing to grinding in the specimen preparation process, high compressive residual stresses were introduced in the surface layers of the fatigue specimens, and as a result, the fatigue crack growth of the specimen was entirely subsurface. As relatively little is known about the subsurface growth of fatigue cracks, this finding gave us the opportunity to learn more about the subsurface fatigue crack growth process. An S–N curve covering fatigue lifetimes from 103 to 108 cycles was established for R = −1 loading condition. The velocity of fatigue crack growth as a function of the maximum stress intensity factor Kmax was determined to be in the range of 10−11–10−7 m/cycle. The Murakami √area method and a new experimental procedure were used to estimate the stress intensity factor and crack growth velocity under the specimen surface, respectively. An analytical equation for the velocity of crack growth as a function of the maximum stress intensity factor Kmax was developed, which agrees with the experimental results. The integration of this equation led to estimates of the fatigue lifetime for given stress ranges. These estimates were also in agreement with the experimental results.

One-way coupled three-dimensional fluid-structure interaction analysis of zigzag-channel supercritical CO2 printed circuit heat exchangers

Printed circuit heat exchangers (PCHEs), with supercritical carbon dioxide (sCO2) as the working fluid, are being considered for use as recuperators and condensers in Brayton cycles for Next Generation Nuclear Plant (NGNP) projects as well as other power generation and heat transfer applications. A few experimental and numerical structural assessments of these PCHEs have been conducted, but all have been somewhat limited due to the difficulty measuring actual stresses in an operating PCHE and the computer resources needed to accurately conduct a fluid–structure interaction (FSI) examination using finite element analysis (FEA). This paper examines a previous pseudo two-dimensional (2D) study of a sodium-sCO2 PCHE, linear elastic model and multilinear elastic hardening model results are included. Next, previously unperformed, three-dimensional (3D) one-way coupled FSI studies of two notional zigzag-channel, sCO2 PCHEs are conducted. All results are evaluated against the stress intensity limits set forth by the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Sections III and VIII. Most of the examined PCHEs meet the requirements for general use but exceed the maximum allowable stress intensities for application as nuclear components.

Fracture and flexural analysis of sandwich panel with polypropylene honeycomb as core and jute fabric reinforced epoxy matrix composite as skin layer

In this modern world, most of the structural components are being replaced by composite materials. Sandwich panels have major advantages and applications in light weight systems like aircraft wings and helicopter components because of their low weight and high strength. The sandwich panel is fabricated by joining two face sheets to a core. Here, the face sheets are made of jute reinforced polymer matrix composite and the core is poly-propylene (PP) honeycomb structure. In this work, the flexural and fracture strengths of sandwich panels with different face sheet thickness, core thickness and orientations are compared numerically and experimentally. The ANOVA (Analysis of Variation) evaluation is used to find out the influence of the factors on flexural strength and stress intensity factor of the sandwich panel. The MINITAB software is used to carry out ANOVA analysis. The regression equations are obtained for maximum flexural stress and stress intensity factor. From the regression equations, the jute fiber layer is most significant factor than other two factors i.e. core thickness and jute fabric orientation.

Numerical Analysis of Stress Causing Fracture Failure in the Gear Transmission System Applied on an Agricultural Machinery Equipment

In general, failures often occur in gear transmission systems, even this case also occurs in an agricultural machinery equipment for example in a hand tractor. This study aimed to determine the cause of the spur gear fracture through numerical studies. The fracture surface was observed using a scanning electron microscope (SEM) in order to identify crack initiation. The results of the chemical composition test indicated that the material used was according to the AISI 8620 standard. It is necessary to create a finite element analysis (FEA) modeling. The stress, strain and stress intensity factor was next analysed near the crack tip using the FEA. The results of the FEA pointed out that the value of the maximum stress intensity factor KI occurs in the center of the tooth which experiences an initial crack due to friction. The KI value obtained from the FEA indicates that the value is higher than the fracture toughness KIC. Finally, crack propagation occurs in the teeth of the spur gear that causes the tractor to stop working.

Fracture and fatigue of the surface layer of layered media due to adhesive sliding contact

Layer cracking in a layered medium consisting of an elastic-perfectly plastic half-space (substrate) and a relatively stiff, elastic surface layer in adhesive sliding contact with a rigid asperity was analyzed with the finite element method using linear elastic fracture mechanics. The directions of tensile and shear mode fracture were determined from the maximum tensile and shear stress intensity factors (SIFs), respectively, whereas the directions of tensile and shear mode fatigue crack growth were determined from the maximum range of tensile and shear SIFs, respectively. The effects of asperity position, interfacial adhesion (Maugis parameter), crack depth, asperity interaction distance, layer thickness, and material properties of the elastic layer and the elastic-plastic substrate on fracture and fatigue crack growth in the layer are interpreted in the context of simulation results. It is shown that the propensity for fracture and fatigue in the layer is enhanced with increasing asperity interaction distance and Maugis parameter and decreasing crack depth and layer thickness. The numerical results of this study reveal that, for similar tensile and shear fracture toughness, layer fracture in adhesive sliding contact predominantly occurs by the out-of-plane tensile mode of fracture, whereas fatigue crack growth in the layer is a manifestation of the in-plane shear and out-of-plane tensile modes of fatigue crack growth.

Observation of Crack Growth Behavior of Various Cracks in 4.762mm Diameter Silicon Nitride Balls under Cyclic Compressive Load

In order to investigate the effect of pre-crack lengths on strength of silicon nitride balls under cyclic pressure loads, growth behavior of 600~700μm pre-cracks were compared to those of 200μm~300μm and 400~500μm pre-cracks. Furthermore, the change in initial threshold limit of the maximum stress intensity factor was discussed. It was found that the increasing ratio of stress intensity factor during N=0 and N=1000 distinguished the failure and non-failure, and pre-crack length had strong effect on the threshold limits of the increasing ratio.

How to get a correct estimate of the plastic zone size for shear-mode fatigue cracks?

It was found that standardly calculated sizes of plastic zones ahead of mode II and mode III cracks during fatigue experiments close to threshold loading were very large and the stresses in the specimen net cross section without stress concentration were close to the yield stress. This represents a theoretical contradiction, since at threshold loading the plastic zones should be very small. In order to clarify the situation, results from three approaches were compared: (i) linear-elastic fracture mechanics, (ii) nonlinear Hutchinson-Rice-Rosengren (HRR) stress field and (iii) elastic-plastic finite element analysis considering nonlinear material behaviour according to the cyclic stress-strain curve. It was explained that, unlike under mode I loading, the plastic zone size should not be calculated using the applied maximum stress intensity factor (SIF). Instead, the effective SIF range needs to be used for calculation of stress fields ahead of the crack tip and, consequently the plastic zone sizes. Using these values realistic plastic zone sizes were obtained (less than 50μm). It also implies that stress in the specimen net cross section should not be calculated directly. A large part of loading applied to the specimen in terms of force or torque is transferred by fracture surfaces.

Observation of Crack Growth Behavior of 400μm Cracks in 4.762mm Silicon Nitride Balls under Cyclic Compressive Load

In order to investigate the effect of pre-crack lengths on silicon nitride balls under cyclic pressure loads, the pre-crack lengths ranging from 400μm to 500μm were observed. Their growth behavior was compared to that of 200μm to 300μm pre-cracks. Furthermore, the initial threshold limits of their maximum stress intensity factors were measured.

(Invited) Model and Knowledge Based Design of Lib Electrode Production Processes

Besides the active and passive materials and their compositions, the processing of these materials to form the electrodes of lithium-ion batteries is very crucial for the resulting cell performance. Due to the long process chain and high number of process parameters, a try and error procedure should be absolutely avoided. On one side models and subsequent simulation methods which are usually well known from other process technology fields like for pharmaceutics, ceramics and minerals. On the other side from systematic investigations of the different process steps for electrode production, so-called process-structure-property-relations are already derived showing the influence of different process parameters on the particle and electrode structure and the effect of these structures on the resulting cell performance. In the presentation, along the process chain from dry mixing to calendering important interrelations and models for knowledge based process design will be explained. Among others in dry and wet mixing processes stress intensity and stress frequency determine mainly the carbon black structure described for example by the inner porosity which can be well correlated with the cell performance especially at higher discharge/charge rates. For example, the viscosity and shear rates in a wet dispersing process determine the shear stress distribution and especially the maximum stress intensity, which is decisive for the degree of carbon black dispersion even for very long mixing times. The shear rate and shear stress distributions in planetary mixers and extruders can be well determined by CFD simulations. The results will show that extrusion is a very effective process to produce electrode suspensions and has a high potential for directly forming electrodes from very high concentrated electrode pastes. During drying the wet electrode coatings should be dried as fast as possible with a homogeneous structure. Especially the migration of binder and carbon black to the electrode surface has to be avoided which depends strongly on mass loading and drying temperature. Moreover, for thick electrodes the structure has to be designed in a way that fast ion diffusion is possible. An often-underestimated process is the calendering of the electrodes to the final density and porosity. The electrodes density and porosity change with increasing line load according to an exponential function and reach almost constant values at very high line loads. The parameters of the exponential function can be predicted as functions of the mass loading, the temperature and the material composition. Beside the simple models for the integrated properties density and porosity the change of the micro structure and with that of the pore size distribution during calendering can be predicted by so-called discrete element simulations. This simulation method is also able to predict changes in the mechanical behavior of the electrodes due to change of particle sizes, mass loading and binder content. Overall, it can be concluded that a lot of systematic process studies and several models and simulation tools are available today which enable a knowledge and model based design of the process chain based on a low number of experiments. Moreover, also models and simulation methods exist to evaluate the cell performance based on predicted electrode structure parameters.

Reliability Analysis of Symmetrical Columns with Eccentric Loading from Lindley Distribution

This paper shows the reliability of the symmetrical columns with eccentric loading about one and two axes due to the maximum intensity stress and minimum intensity stress. In this paper, a new lifetime distribution is introduced which is obtained by compounding exponential and gamma distributions (named as Lindley distribution). Hazard rates, mean time to failure and estimation of single parameter Lindley distribution by maximum likelihood estimator have been discussed. It is observed that when the load and the area of the cross section increase, failure of the column also increases at two intensity stresses. It is observed from the results that reliability decreases when scale parameter increases.