High Mean Stress Research Articles

Thermal cycling aging effects on the ratcheting behavior of anisotropic conductive film

Purpose – The purpose of this paper is to determine: how the thermal cycling aging affects the ratcheting behavior of anisotropic conductive adhesive film (ACF); how the loading conditions and loading history affect the ratcheting strain and strain rate of ACF with different thermal cycling aging histories. Design/methodology/approach – The ACF of CP6920F was cured at 190°C in an electro-thermal vacuum drying apparatus for 30 s. The cured specimens were put into the thermal cycling chamber (−40-150°C) for aging to 25, 50, 100, 200 and 500 cycles. A series of uniaxial ratcheting tests of aged ACF after different thermal cycles was carried out under stress control at 80°C. Findings – The ACF subjected to larger number of thermal aging cycles exhibits less ratcheting strain under the same loading conditions. The ACF with the same thermal cycling aging history shows more ratcheting strain and a higher ratcheting strain rate when loaded under a larger mean stress or stress amplitude or a lower loading rate. The ratcheting behavior of aged ACF is found to be more sensitive to the lower loading rate. The higher mean stress (or stress amplitude) enhances the deformation resistance and consequently restrains the ratcheting strain of subsequent cycling with a lower mean stress (or stress amplitude). The prior lower loading rate accelerates the plastic deformation more significantly than the higher one. Originality/value – The influencing trends of thermal cycling aging, loading condition and loading history on ratcheting behavior of ACF are obtained, which is important for the design and safety assessment of ACF joints.

Recent advances in mechanical properties of anisotropic conductive adhesive film for microelectronic packaging

Purpose– This review paper aims to provide a better understanding of formulation and processing of anisotropic conductive adhesive film (ACF) material and to summarize the significant research and development work for the mechanical properties of ACF material and joints, which helps to the development and application of ACF joints with better reliability in microelectronic packaging systems.Design/methodology/approach– The ACF material was cured at high temperature of 190°C, and the cured ACF was tested by conducting the tensile experiments with uniaxial and cyclic loads. The ACF joint was obtained with process of pre-bonding and final bonding. The impact tests and shear tests of ACF joints were completed with different aging conditions such as high temperature, thermal cycling and hygrothermal aging.Findings– The cured ACF exhibited unique time-, temperature- and loading rate-dependent behaviors and a strong memory of loading history. Prior stress cycling with higher mean stress or stress amplitude restrained the ratcheting strain in subsequent cycling with lower mean stress or stress amplitude. The impact strength and adhesive strength of ACF joints increased with increase of bonding temperature, but they decreased with increase of environment temperature. The adhesive strength and life of ACF joints decreased with hygrothermal aging, whereas increased firstly and then decreased with thermal cycling.Originality/value– This study is to review the recent investigations on the mechanical properties of ACF material and joints in microelectronic packaging applications.

VHCF Behavior and Word Hardening of a Ferritic-Martensitic Steel at High Mean Stresses

Low-pressure steam turbine blades undergo VHCF-loadings induced by inhomogenous flow behind the vanes resulting in excitation frequencies of ≈ 2 kHz for rotational speeds of 50 Hz and a typical number of stator vanes of ≈ 60. The VHCF loading is superimposed by considerable mean stresses caused by centrifugal forces. In the present study, the VHCF-behavior of the ferritic-martensitic turbine blade steel X10CrNiMoV12-2-2 is investigated using an ultrasonic fatigue testing system up to cycle numbers of 5∙109 at stress ratios from R = -1 up to 0.7, i.e. up to very high mean stresses. Generally, crack initiation changes from the surface to internal inclusions at fatigue lives around 4∙107. The transition between fatigue failure and run-outs is shifted to higher lifetime with increasing R, and fine grained areas (FGAs) at the crack initiation sites only occur at R < -0.1. However, the fracture mechanics approach proposed by Murakami consistently describes the lifetime behavior for all load ratios over 4 decades of lifetime. At R up from 0.5 considerable cyclic creep occurs, even for lifetimes above 108 cycles, resulting in cyclic hardening which was proved by microhardness measurements at longitudinal sections. This effect at least partially explains the high maximum stresses close to the tensile strength of the material occurring in the VHCF regime at load ratios ≥ 0.5.

Fatigue testing and analysis of notched specimens with typical subsea design features

Specimens with typical subsea design features have been fatigue tested in order to quantify the fatigue life prediction accuracy of the local stress approach as prescribed by the recommended practice DNVGL-RP-0005 (former DNV-RP-C203). A second aim has been to explore whether the use of the notch support factor method or the weakest-link method would improve the predication accuracy compared to the use of the local stress approach. Nine different notched specimen types, and a total of 139 notched specimens have been fatigue tested in air. All specimens have been subjected to an uniaxial cyclic load. Specimens have been tested at stress ratios R in the range from −1 (zero mean stress) and 0.5 (high mean stress). The specimens have stress concentration factors in the range from 1.85 and 5.45 and fatigue notch factors ranged from 1.16 to 2.03.A statistical based procedure has been used for establishing an indicator of the degree of the prediction accuracy (test results versus estimated fatigue life). This predictability indicator showed that the local stress-life predictions according to DNVGL-RP-0005 resulted in conservative fatigue life predictions for all the tested notched specimens. The degree of conservatism was found to increase with increasing stress concentration factors. DNVGL-RP-0005 yielded fatigue life predictions closer to the observed data for the mildly notched specimen types tested with a high net-section mean stress (R=0.5). The use of the notch support factor method and the weakest-link method improved the fatigue life predictions significantly compared to the use of the local stress approach for all specimen types. The highest predictability was obtained with the use of the notch support factor method. Also the weakest-link method yielded good fatigue life prediction accuracy for all specimen types.

Stress Estimation in Different Bone Layers Subject to Therapeutic Ultrasound in an Intelligent Bone System

Physiological effects caused by power ultrasound radiation are of therapeutic benefits for fracture healing. However, these effects are hard to detect with current instrumentations. The aim of this paper is to analyze the behavior of bone subject to therapeutic ultrasound and provide data reference for an intelligent bone ultrasonic system. In this paper, we adopted a 3-D finite element method as a virtual measurement tool to study the acoustic-radiation-induced stress fields inside and on the surface of bone. The equivalent long bone model was built and the soft tissue was involved by establishing coupling connections with bone surface points. The ultrasound radiation was generated by a 2-MHz excitation and was applied on the surface of soft tissue. In this paper, we first defined six paths in different bone layers to quantitatively study the longitudinal stress distribution and examined the concentration center positions and width with sliding windows. Then, the circumferential stress evolution from relaxation fields to concentration fields was investigated by computing the stress fields on cross sections. Analytical dispersion curves were measured to characterize the guided wave modes. The results show that the middle bone tissue has a higher mean stress (2027.7 Pa) than the surface (763.3 Pa) and the outer bone layer (1898.1 Pa), and the stress distribution of the middle layer is less disturbed (coefficient of variation $= 39.8$ %). Also, on cross section of the concentration zones, periodical fields with a distance of half-wavelength are obtained. From 0.2 to 2 MHz, the acoustic intensity grows proportionally with excitation amplitude.

Fatigue assessment of high‐frequency mechanical impact (HFMI)‐treated welded joints subjected to high mean stresses and spectrum loading

AbstractThe fatigue strength of welded joints can be improved with various post‐weld treatment methods. High‐frequency mechanical impact treatment is a residual stress modification technique that creates compressive residual stresses at the weld toe. However, these beneficial residual stresses may relax under certain loading conditions. In this paper, previously published fatigue data for butt and fillet welded joints subjected to high stress ratios and variable amplitude cyclic stresses were evaluated in relation to the current International Institute of Welding (IIW) recommendations on fatigue strength improvement and a proposed IIW design guideline for high‐frequency mechanical impact‐treated welded joints. The evaluation showed that the current IIW recommendations resulted in both non‐conservative and overly conservative fatigue strength estimations depending on the applied stress level, whereas the proposed fatigue assessment guideline fitted the current data well.

Failure analysis of thermowell weldment cracking

This study presents the root cause analysis of a weldment failure in a thermowell assembly operating in a natural gas processing plant. Laboratory investigations indicated inferior quality for the fillet weld joining the thermowell flange to the pipe supporting the thermowell to the main pipe. The fillet weld exhibited excessive concavity, lack of penetration and lack of fusion. This led to minute weldment cracking, which was exacerbated by flow-induced vibrations exceeding safe operational limits. This promoted small magnitude, high frequency stress cycling coupled with high mean stress at the poorly welded joint, which led to accumulation of high cycle fatigue damage and final fracture causing gas leakage. Mitigation plans included use of a shorter support pipe and revised safe operational envelope of the pipeline under consideration.

Investigations into crazing in glassy amorphous polymers through molecular dynamics simulations

In many glassy amorphous polymers, localisation of deformation during loading leads to crazes. Crazes are crack like features whose faces are bridged either by fibrils or a cellular network of voids and fibrils. While formation of crazes is aided by the presence of surface imperfections and embedded dust particles, in this work, we focus on intrinsic crazes that form spontaneously in the volume of the material. We perform carefully designed molecular dynamics simulations on well equilibrated samples of a model polymer with a view to gaining insights into certain incompletely understood aspects of the crazing process. These include genesis of the early nanovoids leading to craze nucleation, mechanisms of stabilising the cellular or fibrillar structure and the competition between chain scission and chain disentanglement in causing the final breakdown of the craze. Additionally, we identify and enumerate clusters of entanglement points with high functionality as effective topological constraints on macromolecular chains. We show that regions with low density of entanglement clusters serve as sites for nanovoid nucleation under high mean stress. Growth occurs by the repeated triggering of cavitation instabilities above a growing void. The growth of the void is aided by disentanglement in and flow of entanglements away from the cavitating region. Finally, for the chain lengths chosen, scission serves to supply short chains to the growing craze but breakdown occurs by complete disentanglement of the chains. In fact, most of the energy supplied to the material seems to be used in causing disentanglements and very little energy is required to create a stable fibril.

An unusual failure of a nickel-aluminium bronze (NAB) hydraulic valve

Investigation of a hydraulic leak from a nickel-aluminium bronze (NAB) valve in a naval vessel revealed that two corners of a square flange had fractured in a macroscopically brittle fashion such that the valve was in three pieces – an unusual occurrence for a ductile material. Moreover, fracture surfaces were almost entirely intergranular, and there were no signs of dimpled fracture surfaces or shear lips typical of fast fracture in NAB. A few macroscopic progression markings were evident and, at high magnifications, intergranular facets exhibited slip lines and (probably) fatigue striations. Fracture surfaces produced by fatigue of specimens cut from the valve were similar to the in-service fractures for fatigue at very low (near-threshold) crack-growth rates, but not for fatigue at higher crack-growth rates. Data in the literature for NAB, especially in regard to previous failure analyses of NAB hydraulic couplings, indicated that environmentally assisted cracking modes such as stress-corrosion cracking were not involved. All the above (and other) observations indicated that failure of the valve occurred entirely by high-cycle fatigue, probably in the gigacycle regime, at very low stress amplitudes. The cyclic stresses are most probably associated with high-frequency vibration/resonance, but the source of the excitation causing this problem was not identified. High mean stresses, due to torquing bolts to specified levels on the square flange (which is free to deflect) probably facilitated fatigue crack initiation from machining grooves. Various recommendations were made in regard to preventing further failures.

Improving fracture initiation and potential impact on fracture coverage by implementing optimal well-planning and drilling methods for typical stress conditions in the Cooper Basin, central Australia

Drilling conditions involving high mean and deviatory stresses, and natural fractures in the Cooper Basin present challenges in drilling and introduce wellbore rugosities, leaving a damaged wellbore subject to a stress cage effect. Fracture initiations have been problematic in vertical Cooper Basin wells, exhibiting high initiation and treating pressure frac treatments, and with high stress conditions posing greater risks in non-vertical completions. While far-field fracture complexity should be simplified, the near wellbore complexity results in reduced fracture conductivity. The authors believe that present drilling practices and wellbore azimuths may be contributing to sub-optimal hydraulic fracture initiations and complexities. Present analytical modelling methodologies can derive initiation pressures for circular wellbores, but require more complex numerical models to include flaws and ellipticity to represent natural fractures and wellbore rugosities. This study compares initiation pressures and presents graphical results, comparing circular and elliptical wellbore cases with flaws. This extended abstract outlines the criteria used in these models and remarks on areas needing further research and model development. The authors also propose improved drilling techniques to achieve more stable, smoother wellbores, potentially reducing some rugosity and drilling induced fractures. By using data from recent research and other cases with complex stress environments, it is proposed that initiation pressures might be reduced by inclining wells for hydraulic fracturing treatments in a favourable alignment in the maximum horizontal stress direction (σH-Max), and implementing completion techniques that aid better fracture initiation.

Fatigue of Clip Connectors for Offshore Drilling Risers under the Combined Influence of High Mean Stress and Biaxial Tension

Push-pull fatigue tests at various R ratios (σmin/σmax), as well as combined cyclic tension and internal pressure tests with various proportions of each loading were run on a 2.5%Cr-1%Mo steel to investigate separately the effects of a mean stress and of positive stress biaxiality. Fatigue lives were found to decrease with increasing mean stress at fixed biaxiality and a non-linear effect of stress biaxiality on fatigue lives was observed at a fixed R ratio. The damage mechanisms were found to change with both parameters and these modifications were analyzed. Many popular multiaxial fatigue criteria were unable to describe all the data. Thus, a new fatigue criterion based on Gerber's parabola, has been proposed that captures the evolution of the endurance limit under the combined effect of mean stress and biaxial tension.

Strain localisation and grain breakage in sand under shearing at high mean stress: insights from in situ X-ray tomography

This work presents results from a series of triaxial compression tests on two quartz sands (differing principally in grain shape), at confining pressures high enough to cause grain breakage during shearing. Tests are performed inside an X-ray scanner, which allows specimens to be imaged non-destructively as they deform. Observation of the acquired images clearly shows different mechanisms of deformation, including shearing, dilation, compaction and grain breakage. These mechanisms are investigated quantitatively through 3D measurements of local porosity, as well as strain (obtained by 3D Digital image correlation), which is analysed in terms of volumetric and shear components. These tools allow the transition between macroscopically dilative (typically of a dense sand at low mean stress) and compactive behaviour to be investigated. The analysis reveals that at the high end of the confining pressure range studied (100–7,000 kPa), the more rounded sand deforms with highly localised shear and volumetric strain—the porosity fields show a dilative band within which a compactive region (due to grain crushing) grows. The more angular material shows shear strain localisation; however, its faster transition to compactive behaviour (due to a higher propensity for individual grains to crush) translates to much more distributed compactive volumetric strain.

Effects of loading conditions on the fatigue failure characteristics in a polycarbonate

In this study, fatigue properties and crack growth characteristics of a polycarbonate (PC) were examined during cyclic loading at various mean stress (amp) and stress amplitude (mean) conditions. Different S vs. N and da/dN vs. K relations were obtained depending on the loading condition. The higher fatigue strength and the higher resistance of crack growth are seen for the PC samples cyclically loaded at the higher mean stress and lower stress amplitude due to the low crack driving force. Non-linear S – N relationship was detected in the examination of the fatigue properties with changing the mean stress. This is attributed to the different crack growth rate (longer fatigue life): the sample loaded at the high mean stress with lower stress amplitude. Even if the higher stress amplitude, the low fatigue properties are obtained for the sample loaded at the higher mean stress. This was due to the accumulated strain energy to the sample, where severe plastic deformation occurs instead of crack growth (plasticity-induced crack closure). Shear bands and discontinuous crack growth band (DGB) are observed clearly on the fracture surfaces of the sample cyclically loaded at the high stress amplitude, where the lower the mean, the narrower the shear band and DGB. On the other hand, final fracture occurred instantly immediately after the short crack growth occurs in the PC sample loaded at the high mean with the low amp, i.e., tear fracture, in which the shear bands and DGB are not seen clearly.

Naval Centrifugal Compressor Design Using CAD Solutions

Centrifugal compressors of turbochargersoperate in a wide range of rotational speeds, which depends on the load of the supercharged engine. Current designs of turbocharger compressors exhibit high efficiencies accompanied by high flow capacities [1]. Consequences of aerodynamic optimization are high mean stress values in the blades due to centrifugal loading as well as dynamic stresses due to blade vibrations. Blade vibrations in a turbocharger compressor are assumed to be predominantly excited by unsteady aerodynamic forces [2]. These forces are caused by a variety of sources influencing the flow. Examples include the geometry of the flow channel, elbows, the diffuser vanes or struts. Therefore, an understanding of FSI is essential for further design optimizations.

Fretting fatigue behavior of SUS304 stainless steel under pressurized hot water

Fretting fatigue behavior of the sensitized SUS304 stainless steel under a pressurized hot water at 7.3MPa and 288°C was investigated. The tests were carried out under a contact pressure of 100MPa and a frequency of 20Hz. From the experimental result, combined effect of pressurized hot water and localized high tangential stress due to fretting resulted in nucleation of intergranular crack along the outer edge of contact region at lower stress amplitudes, while a fretting fatigue crack was nucleated at the highest tangential force point independently from these intergranular cracks at higher stress amplitudes. No intergranular crack nucleation was observed for fretting fatigue at the same temperature in air. The higher stress ratio reduced the fatigue strength, where the crack tip was exposed more in corrosive environment due to the high mean stress compared to the lower stress ratio.

かえりなしせん断加工の成功条件および切り口面品質に及ぼす材料延性の影響

In push-back blanking, burr-free products are produced by compressing a half-blanked sheet between platens, where success conditions are affected by the ductility of materials. In this study, the mechanism of success and the cut edge quality in the push-back blanking process using a ductile and a brittle steel sheet were discussed. Experiments were carried out, varying the punch penetration and clearance in the half-blanking step. Brittle steel showed a wider successful range and less shear droop on the cut edge than ductile steel. Corresponding simulations were conducted and the damage value based on a ductile fracture criterion was calculated around the shear zone. Negative clearance was effective in keeping the damage value low and allowed deep punch penetration without fracturing in the half-blanking step. In the following push-back step, a rapid increase in the damage was observed when the high mean stress zone prevailed over the half-blanked portion. The timing of increase in damage was earlier for brittle steel owing to the localization of deformation, resulting in a wider successful range than that of ductile steel.

Fatigue of shaft flange bolted joints under preload force and dynamic response

The bolted joints are used for flange connections which are subjected to preload forces and dynamic loads. This combination of static and dynamic loads in threaded joints result with complex interaction of high mean stresses, high notch effect, thread flanks contact forces and moments, and contact surfaces slippage which leads to fatigue damage. The Multi-body system dynamic model of shaft was used for assessment of dynamic behavior. Finite element model of shaft flange connection, with detailed thread joints made of heat treatable steel 30CrNiMo8 under preload condition and nonlinear thread flanks contact, was created. The Rainflow cycle counting was used as a cycle count method for describing the load cycle with local stress-strain hysteresis loop. Influences of mean stresses, bolt diameter, and stress gradients were taken into account according to FKM guidelines. The most critical fatigue locations were obtained at thread roots.

A fatigue assessment technique for modular and pre-stressed orthopaedic implants

Orthopaedic implants experience large cyclic loads, and pre-clinical analysis is conducted to ensure they can withstand millions of loading cycles. Acetabular cup developments aim to reduce wall thickness to conserve bone, and this produces high pre-stress in modular implants. As part of an implant development process, we propose a technique for preclinical fatigue strength assessment of modular implants which accounts for this mean stress, stress concentrating features and material processing.A modular cup's stress distributions were predicted computationally, under assembly and in vivo loads, and its cyclic residual stress and stress amplitude were calculated. For verification against damage initiation in low-cycle-fatigue (LCF), the peak stress was compared to the material's yield strength. For verification against failure in high-cycle-fatigue (HCF) each element's reserve factor was calculated using the conservative Soderberg infinite life criterion.Results demonstrated the importance of accounting for mean stress. The cup was predicted to experience high cyclic mean stress with low magnitude stress amplitude: a low cyclic load ratio (Rl=0.1) produced a high cyclic stress ratio (Rs=0.80). Furthermore the locations of highest cyclic mean stress and stress amplitude did not coincide. The minimum predicted reserve factor Nf was 1.96 (HCF) and 2.08 (LCF). If mean stress were neglected or if the stress ratio were assumed to equal the load ratio, the reserve factor would be considerably lower, potentially leading to over-engineering, reducing bone conservation.Fatigue strength evaluation is only one step in a broader development process, which should involve a series of verifications with the full range of normal and traumatic physiological loading scenarios, with representative boundary conditions and a representative environment. This study presents and justifies a fatigue analysis methodology which could be applied in early stage development to a variety of modular and pre-stressed prosthesis concepts, and is particularly relevant as implant development aims to maximise modularity and bone conservation.

Use of acoustic emissions to investigate localization in high-porosity sandstone subjected to true triaxial stresses

Acoustic emissions were used to investigate the evolution of damage and strain localization in Castlegate sandstone specimens subjected to true triaxial states of stress, where the intermediate principal stress ranged from equal to minimum compression to equal to maximum compression. Three failure modes were observed: shear band formation at low mean stresses, compaction localization at intermediate stresses, and no localization at high mean stresses. For shear bands, the onset of localization consistently occurred prior to peak stress, while compaction localization initiated at the beginning of a stress plateau. The band angle (defined as the angle between the band normal and the maximum compression direction) determined by fitting a plane through the localized acoustic emission events corresponded well with the shear band angle expressed on the specimen jacket. As expected, the band angle decreased with increasing mean stress. The theoretically predicted dependence of band angle on deviatoric stress state was not confirmed; however, data scatter due to natural variations in material could obscure such a trend. Each failure mode displayed a unique acoustic emission rate response; therefore, this rate response alone can be used to determine the failure mode and the onset of localization in Castlegate sandstone.

Effect of cyclic loading on the nanoscale deformation of hydroxyapatite and collagen fibrils in bovine bone

Cyclic compressive loading tests were carried out on bovine femoral bones at body temperature (37 °C), with varying mean stresses (-55 to -80 MPa) and loading frequencies (0.5-5 Hz). At various times, the cyclic loading was interrupted to carry out high-energy X-ray scattering measurements of the internal strains developing in the hydroxyapatite (HAP) platelets and the collagen fibrils. The residual strains upon unloading were always tensile in the HAP and compressive in the fibrils, and each increases in magnitude with loading cycles, which can be explained from damage at the HAP–collagen interface and accumulation of plastic deformation within the collagen phase. The samples tested at a higher mean stress and stress amplitude, and at lower loading frequencies exhibit greater plastic deformation and damage accumulation, which is attributed to greater contribution of creep. Synchrotron microcomputed tomography of some of the specimens showed that cracks are produced during cyclic loading and that they mostly occur concentric with Haversian canals.