Compressive Mean Stress Research Articles

Use of the AE Effect to Determine the Stresses State in AAC Masonry Walls under Compression.

Safety and reliability of constructions operated are predicted using the known mechanical properties of materials and geometry of cross-sections, and also the known internal forces. The extensometry technique (electro-resistant tensometers, wire gauges, sensor systems) is a common method applied under laboratory conditions to determine the deformation state of a material. The construction sector rarely uses ultrasonic extensometry with the acoustoelastic (AE) method which is based on the relation between the direction of ultrasonic waves and the direction of normal stresses. It is generally used to identify stress states of machine or vehicles parts, mainly made of steel, characterized by high homogeneity and a lack of inherent internal defects. The AE effect was detected in autoclaved aerated concrete (AAC), which is usually used in masonry units. The acoustoelastic effect was used in the tests described to identify the complex stress state in masonry walls (masonry units) made of AAC. At first, the relationships were determined for mean hydrostatic stresses P and mean compressive stresses σ3 with relation to velocities of the longitudinal ultrasonic wave cp. These stresses were used to determine stresses σ3. The discrete approach was used which consists in analyzing single masonry units. Changes in velocity of longitudinal waves were identified at a test stand to control the stress states of an element tested by the digital image correlation (DIC) technique. The analyses involved density and the impact of moisture content of AAC. Then, the method was verified on nine walls subjected to axial compression and the model was validated with the FEM micromodel. It was demonstrated that mean compressive stresses σ3 and hydrostatic stresses, which were determined for the masonry using the method considered, could be determined even up to ca. 75% of failure stresses at the acceptable error level of 15%. Stresses σ1 parallel to bed joints were calculated using the known mean hydrostatic stresses and mean compressive stresses σ3.

Correction to: Effects of residual stresses and compressive mean stresses on the fatigue strength of longitudinal fillet-welded gussets

Correction to: Effects of residual stresses and compressive mean stresses on the fatigue strength of longitudinal fillet-welded gussets

Analysis of the fully-reversed creep-fatigue behavior with tensile-dwell periods of superalloy DZ445 at 900 °C

High total strain-controlled creep-fatigue tests of 1.8% were performed for a directionally-solidified Ni-based superalloy, DZ445, at 900 °C in air. Dwell times of 0, 2, and 5 min. were introduced at the tensile peak strain for each cycle. Experimental results show a decrease in fatigue life with increasing dwell time. The significant cyclic softening is observed after the application of the dwell time. The mean compressive stress and large plastic-strain accumulation are produced due to the stress relaxation in the tensile-dwell period, resulting in the increase of the hysteresis loop area. The typical single-source crack-initiation and transgranular crack-propagation are transformed into multi-source crack-initiation and mixed transgranular and intergranular crack-propagation mode by these characteristics. Dislocation features are transformed from cutting of γ' precipitates by multiple stacking faults to the mixed mode of dislocation climbing and slipping. The reduction in the fatigue life with increasing the dwell time is well explained on the basis of the mechanical response and damage mechanism. The fatigue life is also predicted by the energy-based life-prediction model.

Mean stress effect in stress-life for hard steels

This paper examines the effect of mean stress on the fatigue behavior of very hard carburized steels. Current theories suggest that the maximum stress at the fatigue limit continuously decreases with decreasing minimum stress whereas the results of this study indicate that the maximum stress remains constant for minimum stresses less than the fully reversed constant amplitude fatigue limit minimum stress. The theory proposed by the authors corrects this error by maintaining the maximum stress at the fatigue limit constant with decreasing minimum stress in the compressive mean stress region.

Effect of Compressive Mean Stress on the Fatigue Strength and Crack Growth Characteristics of Aluminum Alloys

Effect of Compressive Mean Stress on the Fatigue Strength and Crack Growth Characteristics of Aluminum Alloys

Evaluation of residual stress of metal micro structure electroformed with megasonic agitation

Micro electroforming has become one of the key technologies for metal micro devices fabrication. However, residual stress in electroformed layer of micro devices will reduce their dimensional accuracy and service life. Affected by residual stresse, there is serious bending deformation occurred in cantilevered micro structure during electroforming process. In this paper, megasonic agitation method was employed to obtain low stress in the cantilevered micro structure firstly. Then, the cantilevered micro structures electroformed with and without megasonic were tested by nanoindentation method. The difference of Suresh model and Lee model on the stress calculation results was discussed. By contrast with X-ray diffraction (XRD) method, the calculation results obtained by Lee model in nanoindentation testing are closer to the actual residual stress of the nickel electroformed layer. The nanoindentation testing results obtained by Lee model show that the mean compressive stress of micro structures electroformed with megasonic has a decrease of 57.84 % compared with that without megasonic. The research results indicated that megasonic agitation method is a low residual stress fabrication technique in micro electroforming process. Besides, nanoindentation is a feasible method to measure the residual stress of metal micro structure, due to its excellent properties of high spatial resolution and wide application range.

Long-term in vivo integrity and safety of 3D-bioprinted cartilaginous constructs.

Long-term stability and biological safety are crucial for translation of 3D-bioprinting technology into clinical applications. Here, we addressed the long-term safety and stability issues associated with 3D-bioprinted constructs comprising a cellulose scaffold and human cells (chondrocytes and stem cells) over a period of 10 months in nude mice. Our findings showed that increasing unconfined compression strength over time significantly improved the mechanical stability of the cell-containing constructs relative to cell-free scaffolds. Additionally, the cell-free constructs exhibited a mean compressive stress and stiffness (compressive modulus) of 0.04 ± 0.05 MPa and 0.14 ± 0.18 MPa, respectively, whereas these values for the cell-containing constructs were 0.11 ± 0.08 MPa (p = .019) and 0.53 ± 0.59 MPa (p = .012), respectively. Moreover, histomorphologic analysis revealed that cartilage formed from the cell-containing constructs harbored an abundance of proliferating chondrocytes in clusters, and after 10 months, resembled native cartilage. Furthermore, extension of the experiment over the complete lifecycle of the animal model revealed no signs of ossification, fibrosis, necrosis, or implant-related tumor development in the 3D-bioprinted constructs. These findings confirm the in vivo biological safety and mechanical stability of 3D-bioprinted cartilaginous tissues and support their potential translation into clinical applications.

Identification of Stress States in Compressed Masonry Walls Using a Non-Destructive Technique (NDT).

The structure safety can be assessed, but only indirectly, by identifying material properties, geometry of structures, and values of loads. The complete and comprehensive assessment can be done only after determining internal forces acting inside structures. Ultrasonic extensometry using an acoustoelastic effect (AE) is among the most common non-destructive techniques (NDT) of determining true stresses in structures. Theoretical bases of the method were described in the mid 20th century. They were founded on the correlation between ultrasonic waves and the value and direction of stresses. This method is commonly used to determine stresses mainly in homogeneous materials without any inherent internal defects. This method is rarely applied to porous or composite materials, such as concrete or rock due to a high dispersion of results. Autoclaved aerated concrete (AAC), characterized by high homogeneity and porosity, is the popular material in the construction sector, used to produce masonry units. The discussed tests involved the acoustoelastic effect to determine stresses in the masonry wall made of AAC. This paper presents a widely theoretical background for the AE method, and then describes the author’s own research on AAC divided into two stages. At first, the empirical relationships between compressive stress and velocity of longitudinal ultrasonic wave, including humidity, were determined. In stage II, nine masonry walls were tested in axial compression. Mean compressive stresses in the masonry wall determined with the proposed method were found to produce a satisfactory confidence level up to ca. 50% of failure stresses. Results were significantly understated for stresses of the order of 75% of failure stresses.

In-situ stress evolution and its correlation with structural characteristics of GaN buffer grown on Si substrate using AlGaN/AlN/GaN stress mitigation layers for high electron mobility transistor applications

In-situ stress evolution as a function of thickness has been investigated and correlated with the structural properties and surface morphology of GaN buffer layer grown on AlGaN/AlN/GaN stress mitigating layers (SMLs). For comparison, GaN buffer was also grown on AlN/GaN SMLs. AlGaN/AlN/GaN SMLs exhibited efficient stress mitigation characteristics resulting in higher compressive mean stress during the growth and convex bow at the end of the growth. Horizontal screw-type misfit dislocations generated at the GaN/AlGaN and AlGaN/AlN interfaces were attributed to the stress mitigation property. The residual compressive stress in the GaN buffer was found to be lower with the AlGaN/AlN/GaN SMLs, which resulted in rough surface morphology. Increased V/III ratio used for GaN buffer growth was found to result in reduced stress relaxation at the interface leading to higher residual compressive stress and enhanced diffusion of ad-atoms. This consequently reduced the kinetic roughening and improved surface morphology. Thus, stress engineering by using AlGaN/AlN/GaN SMLs and by changing of the V/III ratio of GaN buffer, the mean stress of heterostructure was controlled and relatively smoother surface morphology was achieved, respectively. Reasonably good uniformity in electrical characteristics with a standard deviation of 7%, 1% and 8% for the sheet resistance, carrier concentration and mobility, respectively, were achieved for GaN high-electron-mobility transistor heterostructures across the 100 mm substrate.

Effect of constraint factor on the thermo-mechanical fatigue behavior of an Al-Si eutectic alloy

The thermo-mechanical fatigue (TMF) behaviors and corresponding damage mechanisms of Al-Si piston alloy were investigated in the temperature range of 120~425 °C under different constraint factors (η). The TMF hysteresis loops exhibit asymmetrical, because the yield strength and elastic modulus decrease significantly with the increasing temperature. Taking the phase angles into consideration, the mean tensile stress for out of phase TMF (OP-TMF, η<0) and mean compressive stress for in-phase TMF (IP-TMF, η>0) can be found. The fatigue life of IP-TMF is longer than that of OP-TMF except for higher constraint factor, and the life of TMF decreases with the increasing absolute value of η. There are two different crack initiation mechanisms under the constrain TMF: primary phases broken mainly for OP-TMF and matrix deboned mainly for IP-TMF. The oxidation may have only little influence on the crack behavior and TMF life, considering the obvious damage by deboned or broken primary phases. The tensile stress is key factor for fatigue crack initiation and growth in primary phases (Si and intermetallic) under cyclic deformation. The TMF life under different constrain factors can be predicted by the strain energy model with good precision.

Comparison of failure loads and compressive stress in Press on metal and Press on Y-TZP copings.

Objective:The aim of the study was to assess the failure loads and compressive stresses among bilayered press on Y-TZP (POZ) and press on metal (POM) crowns with different core-veneer thickness.Methods:Thirty metal and Y-TZP copings were fabricated using CAD-CAM technology with specified thickness. All copings were veneered with ceramic materials using hot pressing technique, with 2mm and 2.5mm thickness. The different coping veneer thickness of crowns resulted in six study groups, including, POM: Coping/ veneer thickness of 0.7/2mm (Gp1), 0.7/2.5mm (Gp 2) and 1mm/2mm (Gp 3)-POZ: 0.7/2mm (Gp A), 0.7/2.5mm (Gp B) and 1mm/2mm (Gp C). Crowns were cemented to a standard implant analog and failure loads (FL) and compressive stress (CS) was ascertained by controlled load application in a universal testing machine. Data was analysed using ANOVA and multiple comparisons test.Results:The maximum FL were observed in the POM specimens with a C/V ratio of 1/2 (Group 3-1880.67± 256.78 N), however the lowest FL were exhibited by POZ crowns with 1/2 C/V ratio (Group C-611.89± 72.79 N). Mean FL and CS were significantly higher in POM compared to POZ crowns in respective groups. Increasing the coping-veneer thickness increased FL and CS among POM crowns. Increasing veneer and decreasing coping thickness improved FL and CS among POZ crowns.Conclusions:Press on metal specimen showed higher resistance to fracture than Press on Y-TZP specimens. Improved failure loads were observed in thin coping and thick veneers among Press on Y-TZP crowns.

Evaluating the fatigue limit of metals having surface compressive residual stress and exhibiting shakedown

AbstractThe objective of the study described in this article is to evaluate the effect of shakedown of surface compressive residual stresses introduced by shot peening on fatigue limit of stainless steel. First, the tension‐compression fatigue tests were conducted on ASTM CA6NM specimens under controlled load and displacement conditions to acquire a fatigue limit diagram under various compressive mean stress. The results showed that shakedown of negative mean stress occurs under controlled displacement. We then carried out in‐plane–bending fatigue tests under controlled load conditions on welded ASTM 309 stainless steel specimens with surface compressive residual stress introduced by ultrasonic shot peening. The results provide a fatigue limit of 415 MPa, which agrees with the value of 404 MPa calculated based on a modified Goodman line considering shakedown. Therefore, it is suggested that the surface layer is restricted by the internal bulk that creates controlled displacement conditions and the shakedown of surface compressive residual stress occurs.

Analysis of strain and dislocation evolution during MOCVD growth of an AlGaN/GaN power high-electron-mobility transistor structure

We present the results of a comprehensive analysis of GaN-on-Si based HEMT epi-wafers grown by metal-organic chemical vapor deposition (MOCVD) in a production-scale reactor. An AlGaN/AlN superlattice was used as the buffer layer. Detailed characterization was combined with process modeling by STREEM-AlGaN software. Comparative analysis of modeling results, characterization data, and in situ curvature measurements allows the study of the evolution of structural properties of the epi-wafer during growth. The initial compressive mean stress in the superlattice gradually decreases during starting period of the growth and then becomes almost constant. The filtering of the dislocations is more effective in the bottom part of the SL, as both experiment and modeling demonstrate large inclination of dislocations in AlGaN layers of the superlattice, while the predicted dislocation density decreases due to annihilation. Proposed buffer layer and growth recipe resulted in final reduction of the dislocation density to ∼2 · 108 cm−2 with good structural uniformity over 6″ wafers and a residual bow below 50 μm.

Comparisons of low cycle fatigue behavior of CP-Ti under stress and strain-controlled modes in transverse direction

Low cycle fatigue (LCF) behavior of CP-Ti along transverse direction under symmetrical strain and stress-controlled modes was compared systematically. For the strain-controlled tests, tension-compression asymmetry (TCA) has little effect on LCF behavior due to the negligible compressive mean stress. For the stress-controlled tests, TCA is detrimental to LCF resistance. The microstructure evolution during cycling deformation was characterized by EBSD and TEM to identify different deformation mechanisms. It was found that strain cycling deformation was dominated by dislocation slip and exhibited the transition from planar slip to wavy slip with the increase of strain amplitude. For the stress-controlled tests, tensile twinning together with dislocation slip accommodated the increased ratcheting deformation. The strain-life curve under stress-controlled tests is lower than that at strain-controlled tests due to the heterogeneous plastic deformation caused by ratcheting deformation. In order to obtain a unified fatigue life prediction model for different control modes, a novel effective strain range related model was proposed by introducing the TCA factor into the original Manson–Coffin relation. Furthermore, the application of this effective strain range related model was extended to the condition of asymmetrical stress cycling by correcting fatigue ductility coefficient and exponent.

Effect of Precompression Deformation on the Strain-Controlled Low-Cycle Fatigue Behavior of Extruded AZ31 Magnesium Alloy

In this work, the effect of precompression along the extrusion direction on the strain-controlled low-cycle fatigue behavior of an extruded AZ31 magnesium alloy was investigated. After precompression deformation occurred, the tensile yield strength decreased, whereas the tensile yield strength and ultimate tensile strength increased gradually as precompression deformation increased. The shape of the stress–strain hysteresis of a specimen with 5% precompression deformation changed from asymmetrical to symmetrical compared with that of the as-extruded specimen. Hence, the whole process was likely dominated by twinning–detwinning behavior. Fractographic analysis revealed that the crack propagation region of the specimen with 5% precompression deformation was flatter than that of the as-extruded specimen because of the compressive mean stress and finer grains in the specimen with 5% precompression deformation. The fatigue lives of the as-extruded and precompression specimens were predicted by the Coffin–Manson model, the Ellyin energy model, the Smith–Watson–Topper model, the Fatemi–Socie model, and the Jiang model, respectively. The life predicted by the Ellyin energy model and the Jiang model was consistent with the experimental data.

Manufacture of polymeric foam and polyurethane composites with fiberglass boosters

The purpose of this research is to know the mechanical characteristics of polymeric foam and polyurethane with fiberglass reinforcement. The focus of the observations on the average maximum compressive force, mean maximum stress and elastic modulus where the observed characteristics are obtained from static press test. Furthermore, tensile testing is performed to observe the mechanical characteristic of mean maximum force, mean maximum stress and elastic modulus. The polymer as a matrix is use 157 BQTN-EX with thermoset resin with fiberglass reinforcement. Tensile test specimens were made according to ASTM D638 standard and ASTM 1621-00 standard for static press test. Three variation of composition is A, B and C where fiberglass used 10%, catalyst 5%, resin 55% and polyurethane 30%. The test result showed that the test specimen on composition C value of characteristic compressive force average 22.838 kN, the mean maximum compressive stress average 19.128 MPa and average modulus of elasticity is 161.447 MPa. While on tensile test the variation of material composition with the same fiber was obtained on compositional test specimen C value of the mean gravity characteristics of the average maximum of 1.214 kN, the mean maximum tensile stress is 20.243 MPa and average modulus of elasticity is 803.634 MPa.

Fatigue limit and life evaluation formulae for compressive mean stress states

ABSTRACTConsidering the difference of stress concentration behaviour near micro-defects in materials, a unified fatigue limit evaluation formula has been developed, which can be applied to compressive mean stress states without the necessity of introducing additional material’s properties except for the modification parameter of mean stress. A generalised life evaluation formula has also been proposed and the dependence of fatigue life on mean stress can be simply expressed by the effect of fatigue limit corresponding to the mean stress. By considering the effect of fatigue limit corresponding to the compressive mean stress, the fatigue life can be evaluated by the same generalised formula as that developed for tensile mean stress state, without the necessity of carrying out additional S–N curve fatigue tests.

Residual stress determination of silicon containing boron dopants in ceramic matrix composites

AbstractRaman spectroscopy was utilized to investigate residual stresses found within a SiC/SiC ceramic matrix composite containing Hi‐Nicalon™ fibers, a slurry melt‐infiltrated matrix of silicon carbide particles, and silicon matrix. Large gradients of electrically active boron are found throughout various regions within the crystalline lattice of the silicon matrix. The regions were identified by the varying degrees of asymmetry and peak width measured in the resonant Fano profile of the doped silicon. A methodology to determine the residual stress state of silicon exhibiting varying degrees of electrically active boron is presented by utilizing the changes in the Raman profile parameters. Previous works on similar SiC/SiC CMCs have attributed spatial gradients in the wavenumber to large fluctuations in stress. By applying the proposed methodology, we show that these observations are related to active boron that is segregated in various matrix areas. Utilizing this methodology, mean compressive stresses in various silicon regions were found to be approximately 300 MPa, with complementary tensile silicon carbide particle stresses of approximately 300 MPa.

Influence of mean stress and light water reactor environment on fatigue life and dislocation microstructures of 316L austenitic steel

Influence of mean stress on fatigue life of the austenitic stainless steel 316 L in air and light water environments (boiling water reactor/hydrogen water chemistry) at 288 °C was determined with a series of tests carried out in load-control mode. Fatigue life was found to increase with application of compressive and tensile mean stress in air and light water reactor environments. Secondary hardening was regarded as the main reason for this behavior. A modified Smith-Watson-Topper (SWT) model was considered to account for mean stress and was shown to predict fatigue life accurately in air and water environments. The reduction of fatigue life in water environment, determined with the SWT curves, was about 2.5. Observations of the end-of-life dislocation arrangements by transmission electron microscopy showed that the dislocation microstructure depends essentially on plastic strain amplitude, which in turn is strongly correlated to stress amplitude and mean stress. The microstructures were found consistent with those usually observed after strain-controlled experiments. At rather low plastic strain amplitudes, corduroy structure consisting of small dislocation loops was observed. Acting as significant obstacle to dislocation motion, corduroy structure affects overall dislocation mobility therefore contributing to notable secondary cyclic hardening.

A phase-field model integrating reaction-diffusion kinetics and elasto-plastic deformation with application to lithiated selenium-doped germanium electrodes

Recent experiments revealed micrometer (µm)-sized selenium (Se)-doped germanium (Ge) particles forming a network of inactive phase (Li–Ge–Se) bring superior performance in cycling stability and capacity over un-doped Ge particles. Therefore, based on two states of Li (one for diffusion and another for alloyed reaction), a phase-field model (PFM) is developed incorporating both chemical reaction and Li diffusion to investigate remaining elusive underpinning mechanism. The reaction-diffusion PFM enables us to directly determine the conditions under which the lithiation process is diffusion- and/or reaction-controlled. Moreover, coupling the elasto-plastic deformation, the model allows us to investigate the role of the inactive phase in morphology and stress variation of Se-doped Ge electrode upon lithiation. The numerical results reveal that the tensile hoop stress at the surface of the particles is significantly suppressed due to softness of the inactive Li–Ge–Se phase, in line with the experimental observation of surface fracture-free behavior. Further, we find that the soft Li–Ge–Se phase reduces a compressive mean stress at the reaction front, thus alleviating the stress retardation effect on the lithiation kinetics.