Minimum Stress Level Research Articles

Two-way shape memory effect under multi-cycles in [001]-oriented Ni49Fe18Ga27Co6 single crystal

The effects of prior thermo-mechanical treatment on two-way shape memory effect (TWSME) and its stability under thermal cycles in [001]-oriented Ni49Fe18Ga27Co6 single crystals were investigated. In this study, TWSME in quenched, stress-free and stress-assisted aged crystals was induced by 100 isothermal loading/unloading cycles (ε = 6%, σmax = 140MPa). Maximum reversible TWSME strain up to 5.5% was observed in stress-assisted aged single crystals in which a full one-way transformation strain εtr is achieved under minimum stress levels and the favourably oriented internal stress fields are formed by the selection of certain precipitate variants during stress-assisted ageing. In quenched and stress-free aged crystals TWSME strains are less than 2.2%. The mechanisms of cycling degradation of TWSME (decrease of TWSME strain and change in martensitic start temperature Ms) have also been discussed. In quenched single crystals, 100 thermal cycles result in irreparable degradation of TWSME. In contrast, age hardening in stress-free and stress-assisted aged crystals results in unchanged Ms temperature during thermal cycles and the restoration of the TWSME strain due to additional training (10 isothermal loading/unloading cycles).

High-Speed Solid Rotor Permanent Magnet Machines: Concept and Design

This paper proposes a novel solid rotor topology for an interior permanent magnet (IPM) machine, adopted in this case for an aircraft starter-generator design. The key challenge in the design is to satisfy two operating conditions that are a high torque at start and a high speed at cruise. Conventional IPM topologies that are highly capable of extended field weakening are found to be limited at high speed due to structural constraints associated with the rotor material. To adopt the IPM concept for high-speed operation, it is proposed to adopt a rotor constructed from semimagnetic stainless steel, which has a higher yield strength than laminated silicon steel. To maintain minimal stress levels and also minimize the resultant eddy current losses due to the lack of laminations, different approaches are considered and studied. Finally, to achieve a better tradeoff between the structural and electromagnetic constraints, a novel slitted approach is implemented on the rotor. The proposed rotor topology is verified using electromagnetic, static structural, and dynamic structural finite-element analyses. An experiment is performed to confirm the feasibility of the proposed rotor. It is shown that the proposed solid rotor concept for an IPM fulfils the design requirements while satisfying the structural, thermal, and magnetic limitations.

Compressive fatigue performance of fiber-reinforced lightweight concrete with high-volume supplementary cementitious materials

As a part of the fundamental investigation to evaluate the compressive fatigue damage of fiber-reinforced lightweight concrete using high-volume supplementary cementitious materials (SCMs), the stress–strain response of the concrete was examined under cyclic loading within maximum stress levels of 0.9, 0.8, and 0.75 fc’ and a minimum stress level fixed at 0.1 fc’, where fc’ is the concrete compressive strength. To produce a high-volume SCM binder, 20% fly ash and 50% ground granulated blast-furnace slag were used as a partial replacement for cement. Amorphous steel (AS) and polyvinyl acetate (PVA) fibers were added individually or in combination considering their conventionally recommended volume fractions. The fatigue tests were carried out by controlling the load between two limits with a sinusoidal variation at a frequency of 1 Hz. Test results showed that PVA fiber was preferable to AS fiber in enhancing the fatigue life of HVS-LWC, whereas the fatigue damage of the PVA fiber concrete was lower than that of the AS fiber concrete. This trend was more notable under a higher maximum stress level. To improve the fatigue life and fatigue damage of concrete, hybridization of both fibers rather than monolithic use is recommended.

Tests on the Compressive Fatigue Performance of Various Concretes

AbstractIn the present study, six concrete mixtures were prepared to examine the effect of the types of binder and concrete on the fatigue performance of concrete under compression. The selected binder types include ordinary portland cement (OPC); high-volume supplementary cementitious material (SCM) composed of 30% OPC, 20% fly ash (FA), and 50% ground granulated blast-furnace slag (GGBS); and alkali-activated (AA) binder composed of 50% FA and 50% GGBS activated by the combination of Na2SiO3 and Ca(OH)2. Using the prepared binders, normal-weight concrete (NWC) and lightweight concrete (LWC) with a unit weight of 1,700 kg/m3 were produced. For cyclic loading of concrete samples, the constant maximum stress level varied among 75, 80, and 90% of the static uniaxial compressive strength of concrete, whereas the constant minimum stress level was fixed at 10% of the static strength. On the basis of a regression analysis conducted using test results, the fatigue life and fatigue stress-strain curve for concre...

Soda Lime Silicate Glass-Copper Metal Joining by Controlled Heat Treatments in Air

Metal–glass joints are needed to improve efficiencies of heat collecting units used in linear parabolic sun collector systems. Soda lime silicate glass–copper metal joints were prepared at 800 ◦C for 5, 10 and 20 min by furnace treatment in air. Cu is rapidly oxidized with heat treatment in air, reacted with glass and formed some bubbles at reaction interface. Scanning electron microscope investigation suggested copper oxide formation at copper interface, reaction between copper oxide and glass and formation of bubbles at joining interface and some microcracking for the studied glass disk on Cu disk sample profiles. ANSYS14 software simulations suggested joined glass experienced complicated and high residual stress levels due to big thermal expansion coefficient difference and bubble formation at the joining interface. The residual minimal principal stress levels were at the order of −400 MPa for joined glass at interface produced microcracking in glass starting from joined interface growing into glass layer.

Anesthetic and anthelminthic effects of clove basil (Ocimum gratissimum) essential oil for tambaqui (Colossoma macropomum)

Expensive imported drugs that are sometimes not available in the market are used to anesthetize and avoid handling stress in farmed tambaquis and to control and prevent parasites, especially monogeneans. Given that the use of these chemicals poses intoxication risks to fish, farm workers, consumers and the environment; the present study evaluated the anthelmintic and anesthetic properties of a natural product, clove basil (Ocimium gratissimum) essential oil (OgEO), in tambaqui farming. The first experiment assessed the anthelmintic effects of tambaqui exposure to OgEO at 0, 5, 10 and 15mgL−1 in 15-min immersion baths; the second evaluated fish blood parameters after 15-min baths in 15 and 60mgL−1 OgEO; and the final experiment assessed anesthesia induction time using OgEO at 20, 50, 100, 200 and 300mgL−1. OgEO showed anthelmintic and anesthetic effects at concentrations of at least 15mgL−1. Fish serum ammonia increased after the baths because of both handling and OgEO exposure, but after 24h it returned to normal levels, which were exhibited by fish that were not subjected to stress conditions. The findings indicate that the essential oil of clove basil is a safe and efficient ingredient to be used in natural anthelmintic and anesthetic products for tropical fish farming. Statement of relevanceEssential oils can anesthetize and treat ectoparasites of farmed fish at minimum stress level.

対称アングルプライ CFRP 積層板の高温サイクル変形挙動

Strain-controlled cyclic behavior of angle-ply CFRP laminates [±θ] at high temperature has been studied. Strain-controlled symmetric tension-compression cyclic tests for different strain rate were carried out on coupon specimens with fiber orientation θ = 45°. On the basis of the cyclic tests, the changes of maximum and minimum stress levels with cyclic straining are examined. The experimental results showed that hysteretic stress-strain response to cyclic straining appears, regardless of the strain rate. These observations reveal that the cyclic softening occurs in the angle-ply CFRP laminates tested in this study. The cyclic softening terminates rapidly and a cyclic saturation stage is reached in all the strain-controlled cyclic tests.

OS1301-376 対称アングルプライCFRP積層板の高温におけるサイクル軟化挙動

Strain-controlled cyclic behavior of angle-ply CFRP laminates [±θ] at high temperature has been studied. Strain-controlled symmetric tension-compression cyclic tests for different strain amplitudes were carried out on coupon specimens with different fiber orientations (θ = 30°, 35° and 45°), respectively. On the basis of the cyclic tests, the changes of stress amplitude, mean stress, non-elastic strain amplitude, maximum and minimum stress levels and loading and unloading tangent moduli with cyclic straining are examined. The experimental results showed that hysteretic stress-strain response to cyclic straining appears, regardless of the fiber orientation. The stress amplitude decreased with increasing number of cycles, and accordingly the non-elastic strain amplitude increased with cyclic straining. These observations reveal that the cyclic softening occurs in the angle-ply CFRP laminates tested in this study. The cyclic softening terminates rapidly in a small number of cycles, and a cyclic saturation stage is reached in all the strain-controlled cyclic tests, except some cases of large strain amplitude or small fiber orientation angle.

혼화재를 다량 치환한 경량 및 보통중량 콘크리트의 압축피로 특성 평가

(Received December 20, 2014 / Revised December 24, 2014 / Accepted December 25, 2014)The objective of this study is to examine the fatigue behavior in compression of normal-weight and lightweight concrete mixture s withhigh volume supplementary cementitious material(SCM). The selected binder composition was 30% ordinary portland cement, 20% fly-ash, and 50% ground granulated blast-furnace slag. The targeted compressive strength of concrete was 40 MPa. For the cyclic loading, the constant maximum stress level varied to be 75%, 80%, and 90% of the static uniaxial compressive strength, whereas the constant minimum stress level was fixed at 10% of the static strength. The test results showed that fatigue life of high volume SCM lightweight concrete was lower than the companion normalweight concrete. The value of the fatigue strain at the maximum stress level intersected the descending branch of the monotonic stress-strain curve after approximately 90% of the fatigue life.키워드 : 다량치환 혼화재, 경량콘크리트, 압축 피로, 응력-변형률 관계Keywords : High volume SCM, Lightweight concrete, Fatigue in compression, stress-strain relationship* Corresponding author E-mail: whenwhere@kgu.ac.kr

An Educational Platform in Structural Mechanics

Thermal residual stresses often arise due to a manufacturing process, involving localised thermal induction, or to the existence of structural components with different thermal expansion coefficients. The existence of thermal residual stresses within a structural member is usually undesired, as it decreases the mechanical resistance of structures. Hence, it is desirable to obtain both a minimum level of residual stresses and smoother stresses transitions in the materials interfaces. Regarding the mitigation of thermal residual stress concentration, the use of materials which properties can vary along the component directions has great interest. This work addresses the use of dual-phase functionally graded materials, which microstructure varies gradually from a material to another according to a given gradation function. On the order hand, it is also addressed the use of a population based optimization algorithm in order to attain the referred minimum stress level. Summarizing, the current work presents an educational platform directed to structural mechanics students, which aims to give the tools to understand both the influence of design parameters in the thermal residual stress level and distribution along the material and the advantages of using a structural optimization technique in order to minimize the drawback thermal residual stresses effects.

Mechanism for fatigue striations as formed under variable negative R-ratio in Al-based structural alloys

Effect of varying negative R-ratio on the fatigue striation patterns was examined for Al-based V95-T4 (analogue of 7057T6) and D16T (analogue of 2024T3) alloys. Three different sequences of cyclic loads and of the varying maximum and minimum stress levels were applied in air atmosphere to the test alloy panels 10×160×460mm in size. The tests were conducted at room temperature with the loading frequencies in the range of 0.5–1.5Hz. Fractographically, the magnitudes of striation spacing were found increasing for increasingly negative R-ratios. To estimate the effects of negative R-ratio on the crack growth rate correction-function surfaces were constructed for the stress-intensity factor KI. The fractographic analysis suggests that the unloading portions of cycling trajectory are only responsible for the fatigue striation effects. Such a mechanism appears dominating in the positive as in the negative ranges of R-ratio.

NONLINEAR ASSOCIATION OF DIURNAL TEMPERATURE RANGE WITH OXIDATIVE AND PHYSIOLOGIC STRESS MARKERS IN THE KOREAN ELDERLY ENVIRONMENTAL PANEL STUDY (KEEPS)

Background and Aims: While diurnal temperature range (DTR) has been found to be a risk factor for mortality in a previous study with an assumption of linearity, some disease mortality, such as that associated with rheumatic heart disease (RHD), showed a threshold effect for DTR. To define the nonlinear relationship pathway between DTR and health effects, we investigated how physiologic and oxidative stress markers responded to DTR. Methods: This study was part of the Korean Elderly Environmental Panel Study (KEEPS). Data was obtained from 569 participants who regularly attended a community elderly care center located in Seoul, Korea. The study was conducted a total of five times over a three years period beginning in August, 2008. We examined physiologic stress markers including heart rate variability (HRV) and indices such as mean heart rate, standard deviation of normal-to-normal (SDNN), normalized low frequency (LF) and high frequency (HF), and low/high frequency ratio (LF:HF). Urinary malondialdehyde (MDA) was evaluated as an oxidative stress marker. Effects of DTR below or above the median level were estimated using a generalized linear mixed model. Results: Significant differences in effects of DTR above and below the median level indicated the presence of a threshold effect or a nonlinear association between DTR and HRV markers. Estimated effects for log-transformed SDNN were 0.211 (standard error [SE] 0.069, P =0.0021) below and -0.093 (SE 0.038, P =0.0129) above the median DTR. While the mean heart rate slope above median DTR was significant, slopes of normalized HF and LF:HF ratio below median DTR were statistically significant. In addition, log-transformed MDA showed a pattern similar to HRV indices. Conclusions: The study demonstrated that physiologic and oxidative stress increased at low and high DTR with minimal stress levels around the median level of DTR indicating a physiologic comfort point.

Accumulation of time-dependent strain during dwell-fatigue experiments of iBN-Sylramic melt infiltrated SiC/SiC composites with and without holes

A 2 h tensile load cycle is repeated for specimens without holes and specimens with a central hole under the following conditions: (i) a stress ratio of 0.05, (ii) various stress levels, (iii) 815 °C and 1204 °C, (iv) different times up to 600 h and (v) different hole diameters. An increase in the accumulation of time-dependent strain with the increase in time and stress level is observed for specimens with and without holes. All specimens recovered part of their time-dependent strain at the minimum stress level. Residual strain to failure for specimens without holes showed a reduction with the increase in the amount of strain accumulated during the experiment. A curve fitting procedure of this experimental data, as well as data from archived literature, pointed at the possibility of the existence of a similar finite amount of strain to failure of all specimens tested at both temperatures.

Numerical modelling of twin helical spring under tensile loading

This paper presents a 3D geometric modelling of a twin helical spring and its finite element analysis to study the spring mechanical behaviour under tensile axial loading. The spiralled shape graphic design is achieved through the use of Computer Aided Design (CAD) tools, of which a finite element model is generated. Thus, a 3D 18-dof pentaedric elements are employed to discretise the complex “wired-shape” of the spring, allowing the analysis of the mechanical response of the twin spiralled helical spring under an axial load. The study provides a clear match between the evolution of the theoretical and the numerical tensile and compression normal stresses, being of sinusoidal behaviour. The overall equivalent stress isovalues increases radially from 0° to 180°, being maximal on the internal radial zone at the section 180°. On the other hand, the minimum stress level is located in the centre of the filament cross section.

Notch FCI life under constant amplitude uniaxial loads

PurposeThe purpose of this paper is to estimate the number of cycles for fatigue crack initiation in notched plates.Design/methodology/approachPreviously published experimental results for tests with initially uncracked U‐shaped notches of different sizes and geometries are utilized. The present work defines the fatigue initiation period, Ni, as number of loading cycles required for the formation of a through‐thickness Mode I crack at the notch root macroscopically propagating along the plate width. Thus, backward extrapolation of the observed early decreasing fatigue growth rates of the initiated crack at the notch root on the specimen surface to zero surface crack size enables the estimation of Ni.FindingsA parameter based on the normal strain range and the maximum normal stress acting at the notch root best fitted Ni.Research limitations/implicationsThis paper provides an estimation of Ni in 3 mm thick mild steel plates subjected to constant amplitude uniaxial cyclic stresses. Initiated‐but‐non‐propagating cracks possibly found at the notch surface and at the notch root along the plate width are distinguished.Originality/valueBased on the present analysis, a threshold stress is devised to represent the minimum stress level required for the appearance of a through‐thickness crack at the notch root running along the specimen width. Notched plates simulate many engineering applications particularly in aerospace industry.

A probabilistic fatigue strength model for brick masonry under compression

The paper presents the development of a model for the fatigue resistance of brick masonry under compression. The model takes into account the random nature of the fatigue strength phenomena and, as a consequence, proposes different model parameters as a function of the desired probability level. The model, based on a Weibull distribution, has been calibrated with the still limited experimental data on brick masonry under high cycle compression loading. Very good correlation is obtained. The results show that there is a clear trend of the experimental data to follow the Weibull distribution. It is also shown how the fatigue resistance of brick masonry is not only dependent on the magnitude of the stress cycles, but also on the magnitude of the minimum stress level. The paper also shows how the proposed model can be used in the reliability-based assessment of the fatigue performance of existing masonry arch bridges. Either the nominal probability of failure (reliability index) to fatigue or the remaining service life with a predefined probability level for an existing bridge can be predicted with the proposed method.

Effect of Mixed Mode Loading Induced by Asymmetrical Stress upon Crack Initiation from Corrosion Pits

This work focuses on determining the influence of mixed stress conditions on the initiation of fatigue cracks from corrosion pits in a UNS S31603 stainless steel. Fatigue limit was established as a threshold stress criteria for fatigue crack initiation from semi-elliptic surface pits. The strain energy release rate range (G) around semi-elliptic surface pits was determined as a function of the pit aspect ratio (depth/major width axes: a/c), pit contour and the orientation of the nominal stress from a linear elastic fracture mechanics approach. Results indicated that stress conditions for crack initiation in mixed mode overcome the minimum stress level or single threshold stress established from the fatigue limit concept. Under mixed mode condition a crack will grow at the surface zone (φ = 0˚) when a/c ≥ 0.60 and for orientation angles from 5˚ to 75.

Performance of CFRP tendon–anchor assembly under fatigue loading

Abstract This paper presents the results of an experimental study to examine the fatigue performance of a novel wedge-type anchor for carbon fiber reinforced polymer (CFRP) post-tensioning tendons. The CFRP tendon–anchor systems were tested under different cyclic loading conditions. Two minimum stress levels, representing 40% and 47% of the ultimate strength of the CFRP tendon, were considered. Four stress ranges varying between 7% and 17% of the ultimate strength of the tendon were applied. The anchor system successfully met the Post-Tensioning Institute provisions for cyclic loading. The fatigue life of the system was primarily affected by the applied stress range with little effect by the minimum stress. The fatigue limit of the CFRP tendon–anchor system could be taken at a stress range of 10% for a maximum stress of 50% of its ultimate capacity. Below this stress level, the assembly is expected to withstand an infinite fatigue life.

Broodstock management and induced spawning of flounder Paralichthys orbignyanus (Valenciennes, 1839) under a closed recirculated system

The present study was conducted to determine the parameters regulating spawning performance of P. orbignyanus wild broodstock. The adaptation to captivity was successfully achieved with minimal stress levels. In three consecutive spawning seasons, the annual photothermal manipulation was not effective for the maturation and natural spawning of females although males were running. Females were HCG injected, releasing non fertilized eggs. Artificial fertilization was carried out after stripping the fish with a total production of 18,000 fertilized eggs in 2001 (egg diameter= 800±10 µm), 55,300 in 2002 (egg diameter= 850±12 µm) and 1,635,000 fertilized eggs in 2003 (egg diameter 818.4 ±30.08 µm, oil drop 98.1 ±2.96 µm). For the study of the ovulation time, females were hand-stripped every 48 h (female 1), 96 h (female 2) and 144 h (female 3), after being induced by hormones. No statistical differences were found for fertilization and hatching rates and SAI. This could be attributed to the small sample sizes, individual variation and/or stripping frequencies. Nevertheless, qualitative differences were evident. Although long-term studies are needed to confirm these observations, 96 h stripping frequency could be considered a good technique to obtain eggs and larvae experimentally. The estimated fecundity by this technique varied from 185,951 to 399,118 total eggs per kg of female.

The effect of a cognitive and a physical stress-reducing programme on psychological complaints

To investigate the short-term and long-term effectiveness of two, brief, preventive, work stress management programmes. One programme was a cognition-focused programme, the other was a newly developed intervention in which physical exercise and relaxation were combined. It was hypothesised that the newly developed intervention would be more effective in reducing psychological complaints than the cognitive intervention. Both programmes consisted of four sessions in a period of 10 weeks. From a working population engaged in a periodic health check-up, employees above a minimum stress level (n=396) were invited to participate in a randomised comparative outcome study with pre-trial, post-trial and 6-month follow-up measures. After giving informed consent 130 participants entered the study (response rate 33%). Outcome measures consisted of three self-reported questionnaires on psychological complaints. It was found that both interventions revealed a positive impact on psychological complaints, burnout and fatigue, both at short-term and at 6-month follow-up. No statistical interaction effects between the two interventions were found. Calculation of the clinical significance of the effects indicated that 50% of the employees with psychological complaints who participated in the physical intervention and 60% of the employees who participated in the cognitive intervention improved and returned to functioning within normal range both in the short term and in the long term at 6 months. The data indicate that interventions were equally effective on psychological complaints, burnout and fatigue.