Cool-down Stage Research Articles

Effects of carbon-fibre Z-pins on the through-thickness tensile strength of curved composite laminates under four-point bending

This study investigates the influence of discrete through-thickness reinforcement, i.e. Z-pins, on the through-thickness tensile strength (TTS) of curved laminates through four-point bending experiments. Three types of samples are considered: unpinned, and Z-pinned with 0.27 % and 0.54 % areal densities. HexPly® IM7/8552 carbon/epoxy unidirectional prepreg with 0/±45 layup and 0.28 mm diameter T300/BMI pins were employed to manufacture the specimens. The Z-pinned laminates have comparable TTS with the unpinned samples in terms of the first observable load-drop. However, the TTS corresponding to the ultimate load-drop for low-density and high-density Z-pinned samples are 29 % and 38 % lower than for the unpinned ones, respectively. Z-pinned samples show less scatter than unpinned ones in terms of the ultimate TTS values. All specimens failed suddenly, i.e. with no evident damage progression. This implies that Z-pins were not able to form a progressive bridging zone to dissipate mechanical energy. Through a detailed meso-scale finite element analysis (FEA), it was found that the high through-thickness tensile residual stress in the Z-pin neighbourhood generated from the cool-down stage of cure is an important factor in causing the reduction in TTS of Z-pinned laminates. CT scan images of tested Z-pinned specimens show that the carbon-fibre pins experience fracture inside the laminates.

Experimental and numerical investigation of tool-part interaction on the process-induced distortions in composite structures

The tool-part interaction was experimentally carried out using FBG sensors adhesively bonded to the thin tool in a tool-part assembly, and corresponding interfacial properties parameters were theoretically quantified according to the acquired data and shear-lag theory. Subsequently, these derived parameters were adjusted and fed into FE model of C-shaped composite structure cured on the aluminium tool to numerically predict its spring-in, in company with experimental validation. Meanwhile, two comparative simulations neglecting tangential friction and tool expansion were provided to clarify the significance of tool-part interaction. The results from the assembly show that the sticking condition mainly exists until interfacial shear stress reaches a critical value at a certain time in the cool-down stage, after which the sliding friction dominates. The good agreement of spring-in between experiment and simulation provides a favourable support for the numerical simulation scheme, and comparative simulations reveal that tangential friction contributes to spring-in but the tool expansion alleviates it.

Characteristics of Physical Exercise Programs for Older Adults in Latin America: A Systematic Review of Randomized Controlled Trials.

Aim: To characterize physical exercise programs for older adults in Latin America. Methods: This review was conducted in accordance with the PRISMA statement. A search for randomized controlled trials (RCTs) published between the years 2015 and 2020 was performed in the Scopus, MedLine and SciELO databases. Results: A total of 101 RCTs were included. A large percentage of the studies had an unclear risk of bias in the items: selection, performance, detection and attribution. Furthermore, a heterogeneous level of compliance was observed in the CERT items. A total sample of 5013 older adults (79% women) was included. 97% of the studies included older adults between 60–70 years, presenting an adherence to the interventions of 86%. The studies were mainly carried out in older adults with cardiometabolic diseases. Only 44% of the studies detailed information regarding the place of intervention; of these studies, 61% developed their interventions in university facilities. The interventions were mainly based on therapeutic physical exercise (89% of the articles), with a duration of 2–6 months (95% of the articles) and a frequency of 2–3 times a week (95% of the articles) with sessions of 30–60 min (94% of the articles) led by sports science professionals (51% of the articles). The components of physical fitness that were exercised the most were muscular strength (77% of the articles) and cardiorespiratory fitness (47% of the articles). Furthermore, only 48% of the studies included a warm-up stage and 34% of the studies included a cool-down stage. Conclusions: This systematic review characterized the physical exercise programs in older adults in Latin America, as well the most frequently used outcome measures and instruments, by summarizing available evidence derived from RCTs. The results will be useful for prescribing future physical exercise programs in older adults.

Functional Fitness in Older Adults

Background: Using multiple interventions can increase muscle strength in older adults. A functional fitness test is a useful instrument to identify the effectiveness of physical functioning among older adults. However, the results in previous studies with regard to improvements in their physical function are inconsistent. Aim: To synthesize the intensity and interval of the effective interventions on physical functioning in community-dwelling older adults. Methods: A systematic review and meta-analysis on the related literature was conducted. The databases searched were PubMed, EBSCO, and Cochrane Trials. Papers published from January 1, 2013, to December 31, 2017, were subjected to a computerized search. The inclusion criteria were that the randomized controlled trials examined a healthy and community-dwelling older adult population, and the instrument used to evaluate functional fitness was based on “functional fitness” or “senior fitness.” Two reviewers independently assessed the methodological quality of each trial, including study participants, allocation, blinding, data analysis, duration of follow-up and loss to follow-up, intervention, inclusion criteria, outcome measurements, and other biases. Results: Among the 218 trials, only 5 randomized control trials were synthesized in this systematic review. These works examined the effects of the intensity, interval, and duration of intervention programs based on elastic band exercise, aerobic training and combined aerobic and resistance training, Tai Chi with TheraBand resistance exercise, and Tai Chi and Thai Yoga exercise. Conclusions: A 1-hour intervention program, including 10 to 15 minutes of warm-up, 30 to 40 minutes of main exercise, and 5 to 10 minutes of cool-down stages, for 2 to 3 sessions per week, and continuing for 12 to 16 weeks, was the effect intervention for community-dwelling healthy older adults to enhance their upper body strength, lower body flexibility, and agility balance.

Unsteady numerical simulation on helium cooldown process for the 650 MHz two-cell superconducting cavity

Superconducting cavity is usually needed to be gradually cooled from room temperature to the superconducting temperature zone (4.2 K and below) in the testing and sophisticated operation process of superconducting cavity. The purpose of this paper is to study the cooling law on the helium cooldown process for the 650 MHz two-cell superconducting cavity with the unsteady numerical simulation. A three-dimensional coupled heat-flow model of 650 MHz two-cell superconducting cavity was established. The unsteady numerical simulation of different inlet temperatures, flow rates and pressure conditions was carried out. The equivalent convective heat transfer coefficient and temperature distribution of 650 MHz two-cell superconducting cavity during cooldown process were obtained. The effects of cooling time and entrance parameters on the cooldown process were analyzed. The temperature distribution of the lower intersection lines has a large drop in the initial stage of cooldown process (120 s), while the temperature near the flanges at the both ends is still higher (remaining at the initial temperature of 300 K). With the passage of time, the temperature of the upper and lower intersection lines decreases. The maximum temperature difference on the lower intersections is within 2 K in the final stage of cooldown process (3600 s). The maximum temperature difference increases by 180%, and the difference between the maximum temperature and the minimum temperature (dT) at the end of a cooldown stage increases by 130% after 1 h, respectively, when the inlet temperature drops from 290 to 270 K (under the condition of the initial temperature of 300 K). The maximum temperature difference and the dT at the end of a cooldown stage increase with the decrease in the inlet temperature. The maximum temperature difference increases with the increase in the inlet flow rate, while the dT at the end of a cooldown stage decreases with the increase in the inlet flow rate. The effect of changing the inlet flow rate on the cooling rate is not as obvious as changing the inlet temperature. Once there is a certain flow rate, the advantage of further increasing the flow rate to reduce the temperature of the superconducting cavity is not so great.

Thermal-hydraulic analysis of VVER-1000 residual heat removal system using RELAP5 code, an evaluation at the boundary of reactor repair mode

Removing the residual heat from a nuclear reactor is an important safety aspect of thermal hydraulic analysis. In this study, a typical VVER-1000 reactor residual heat removal system has been evaluated using RELAP5 thermal hydraulic loop code during cool-down. Reactor cooling down starts from hot state temperature and then continues to the cool-down stages with 130°C and 70°C, respectively. The second stage of cooling down is the boundary of the reactor repair condition. Main cooling pump head, steam generator (SG) water level, system pressure, the level of coolant in the pressurizer (PRZ), and the temperature of fuel element are examined in a steady state condition. PRZ level, primary circuit and secondary circuit pressure/temperature, and SG water level are evaluated during 32,000s after cool-down scenario. By comparison, it concluded that the results of RELAP5 code are in agreement with plant experimental data and final safety analysis report (FSAR). Thus, it is proved that the studied reactor is capable to remove the residual heat generated during shutdown. Moreover, RELAP5 is properly recommended for analysis of the VVER-1000 pressurized water reactor during cool-down.

고체 추진제 장시간 물성거동 반응 연구

고체 추진기관이 노즐마개에 의해 외기와의 교환이 완벽히 차단된다면, 구조적 안정성은 제작초기 조건인 내부조성간의 잔류반응(Post Cure, Migration etc.), 그리고 자유공간 내의 산소(또는 산화방지제)와 습기(제습제)와의 반응에 종속된다. 이로부터 발생하는 혼합형 고체추진제의 기계적 특성은 매우 복잡하며, 추진기관은 발사직전까지 일교차/년교차의 끊임없는 열하중을 받게 된다. 본 연구에서는 고체 추진기관의 제조공정인 성형오븐에서 출고 후 저장안정화까지의 거동을 고체추진제의 열유변학적 단순특성을 적용하여 신속하게 산출할 수 있는 방법을 제시하였다. 이를 위해 온도제어 가능한 Endurance Test 장치를 고안 제작하였으며, 추가적으로 점진적 응력과 변형율 증가에 따른 비선형 특성도 검토한다. Structural integrity of solid rocket depends on the residual reactions between constituents of its composition(post cure, migration etc.), the oxygen(or anti-oxydent) in the free volume and humidity (desiccant) under the perfect sealed condition. Mechanical Properties of composite solid propellant arising from those factors are very complex. Moreover the propulsion are faced with thermal loads from diurnal & seasonal cycle till firing. In this study, the fast evaluation method of long term mechanical properties is suggested based on Thermo-Rheological Simplicity from curing oven to cool-down stage in view point of thermal stabilization. For this subject, endurance tester having temperature control capability are devised. From the results from incremental load and strain, non-linear characteristics are discussed.

An Investigation of the Effects of Aerobic Exercise on Serum Brain Natriuretic Peptide and C-Reactive Protein in women with Cardiovascular Diseases

A B S T R A C T Article type: Original Article Introduction: The aim of this study was to evaluate the effects of aerobic exercise on brain natriuretic peptide (BNP) and C-reactive protein (CRP) levels in women with cardiovascular diseases. Materials and Methods: In this semi experimental study, thirty female patients with at least one coronary artery stenosis (more than70%) were chosen and divided into two groups of aerobic training (n=15) and control (n=15). Blood samples were obtained at the beginning and end of the study to measure brain natriuretic peptide (BNP) and C-reactive protein (CRP) levels. The aerobic group cycled on a stationary ergometer for three sessions per week (period of eight weeks). The control group did not receive any exercise. Each exercise session included a10-minute warm-up, a 15-minute or more aerobic training program and a 5-minute cool-down. In the warm-up and cool-down stages, running, walking and stretching activities were used. During the first week of training, subjects exercised for 15 minutes at 55-60% of their target heart rates. Each week, exercise duration extended by five minutes, while the intensity was unchanged. Results: Results indicated that 8 weeks of aerobic training had a significant effect on decreasing the BNP and CRP levels in females with Cardiovascular Diseases (P=0.005 and P=0.017 , respectively). Moreover, a significant Difference was seen between the control and experimental groups in mean BNP and CRP values (P=0.0001 and P=0.001 respectively), while no significant difference were seen between the pre and post tests in the Control group for BNP (P=0.21) and CRP (P= 0.28). There were significant reduction in BNP (p<0. 05) and CRP levels (p<0.01) after 8 weeks aerobic exercise in experimental group, But no change was seen in the control group in both BNP and CRP levels. Conclusion: Aerobic exercise can attenuate BNP and CRP levels in females with cardiovascular diseases; hence it can be used as a part of treatment.

Moving-Boundary Model of Cryogenic Fuel Loading, II: Theory Versus Experiments

A moving-boundary model of two-phase cryogenic flow, which was developed in the companion paper (HafiychukV.FoygelM.Ponizovskaya-DevineE.SmelyanskiyV.WatsonM.BrownB.GoodrichC., “Moving-Boundary Model of Cryogenic Fuel Loading, I: Two-Phase Flow in a Long Pipe,” Journal of Thermophysics and Heat Transfer, Vol. 29, No. 3, 2015, pp. 524–532.), is applied toward simulation of liquid nitrogen flow data collected for the cooldown regime in two different experimental fuel transfer lines: 1) the 1966 National Bureau of Standards (now the National Institute of Standards and Technology) setup and 2) the novel NASA Kennedy Space Center cryogenic testbed. With relatively small computational effort compared to full-scale schemes, the model describes pressure and temperature histories, kinetics of the vapor void fraction, and interphase boundary motion in two different parts of the transfer lines. The aforementioned time-dependent characteristics are shown to be in a good agreement with the experimental data on the cooldown stage of liquid nitrogen loading obtained at the National Bureau of Standards setup (with no fitting parameters used) and in a fair agreement with the NASA Kennedy Space Center testbed setup (with a small fitting) for chilldown and fast fill operations. The fast and accurate modeling procedure accounts for cryogen fueling operations in both the nominal and major fault regimes.

Temperature–density hysteresis in X-ray CT during HIFU thermal ablation: Heating and cooling phantom study

Purpose: This paper investigated the effects of thermal ablation treatment on imaged X-ray computed tomography (CT) Hounsfield units (HU), for the purpose of monitoring tissue denaturation and coagulation.Materials and methods: Eight phantoms of water, oil, and chicken serum albumin as well as 15 ex vivo tissue samples were heated by applying high intensity focused ultrasound (HIFU) for 10 to 29 min to obtain denaturation temperatures, (i.e. >50 °C). X-ray CT scanning was performed simultaneously during heating and post-ablation cooling stages, and the HU at the focal zone were registered. The temperature profile versus time was also monitored under similar conditions using a thermocouple probe. The results were plotted and correlated as curves of HU versus temperature.Results: In all specimens studied, HU values depicted an exponential curve as a function of temperature during the heating stage. However, linear behaviour was observed during the cool-down stage for both chicken serum albumin and ex vivo bovine liver. Thus, a hysteresis phenomenon occurred only when the thermal conditions induced irreversible changes in the sample with quantification demonstrating high correlation with the maximal temperature reached during treatment (R2 > 0.9) for the chicken serum albumin.Conclusions: Our results demonstrate a HU–temperature hysteresis phenomenon for HIFU ablation, which is detectible by X-ray CT. This hysteresis is related to the amount of heat induced into the tissue and could potentially indicate irreversible tissue damage. Accordingly, this measurable phenomenon can be utilised as a quantitative method for non-invasive monitoring of thermal ablation.

Dependence of SLS Parameters on Thermal Properties of Composite Material of Cement with Polyamide 12

SLS employs powder-processing in the construction of parts and SLS parameters depending on the thermal properties of the powder. An average of 80% to 90% of the powder in the build chamber is not sintered during the SLS process and could be reused in relation to its properties. However, the properties of un-sintered powder deterioratedue to exposure to a various temperature for extended periods of time during the SLS system material build-processin three stages, starting from warm up stage, the build stage, where the powder is exposed to just below the melting point of the material, and the cool-down stage. An experimental study of the thermal properties of composite material of cement and PA12 to determine the optimum parameters of SLS process are being investigated. The investigationuses the thermal properties of different proportions of composite material of cement and PA12. In addition, an experimental study of the thermal properties and physical properties of used or un-sintering powder to expand a methodology of controlling the SLS parameters, in turn to obtain consistent, good quality fabricated SLS specimens. ``

Optimization of the cooling profile to achieve crack-free Yb:S-FAP crystals

Yb:S-FAP [Yb 3+:Sr 5(PO 4) 3F] crystals are an important gain medium for diode-pumped laser applications. Growth of 7.0 cm diameter Yb:S-FAP crystals utilizing the Czochralski (CZ) method from SrF 2-rich melts often encounters cracks during the post-growth cool-down stage. To suppress cracking during cool-down, a numerical simulation of the growth system was used to understand the correlation between the furnace power during cool-down and the radial temperature differences within the crystal. The critical radial temperature difference, above which the crystal cracks, has been determined by benchmarking the simulation results against experimental observations. Based on this comparison, an optimal three-stage ramp-down profile was implemented, which produced high-quality, crack-free Yb:S-FAP crystals.

The cool-down characteristics of a miniature Joule–Thomson refrigerator

Cool-down characteristics of a miniature J–T refrigerator with a pressurized vessel, which has different initial pressures of nitrogen gas, were investigated by experiment. The supply and exhaust pressures, the temperatures on the surface of the cryostat, and the mass flow rate were measured simultaneously to analyze the cool-down characteristics of the J–T refrigerator with a pressurized vessel or reservoir. The results show the effect of the supply pressure on the cold end temperature of the J–T refrigerator. The influence of the supply pressure and the temperature on the mass flow rate during the cool-down stage is discussed in details.

Growth of ZnS nanocombs with ZnO sheath by thermal evaporation

Self-assembled zinc sulfide (ZnS) of wurtzite structure nanocombs with a ZnO sheath have been grown using a simple catalyst-free thermal evaporation technique in the alumina furnace. The ZnS nanocombs are as long as to several tens of micrometers. The teeth with ZnO sheath are about 200 nm in length and 30 nm in width. The ZnO sheath is likely formed at cool-down stage due to the residual O 2.

Process-Induced Residual Stress Analysis by Resin Transfer Molding

The objective of this study is to investigate the residual stress development during the resin transfer molding process. The analysis was based on the strain measurement tests during the entire cure process of unidirectional carbon fiber–epoxy matrix composite laminate using aluminum mold. The investigation efforts were particularly focused on the verification of the strain development mechanism. Although complex interference between the mold structure and the composite laminate was observed during the process, it was found that the final residual strain of the laminate was induced only by thermal mismatch during the cooldown stage. Based on the observation, the residual stresses were calculated by a viscoelastic classical laminated plate theory. The viscoelastic modulus in the calculations was obtained by a series of stress relaxation tests of the material. The curvatures of [0°/90°] asymmetric laminate were calculated, and the results were compared with the experimental data. The results showed that the residual stress developed by the resin transfer molding could be calculated by considering only the cooldown stage of the process.

Simultaneous strain and temperature monitoring of the composite cure with a Brillouin-scattering-based distributed sensor

A Brillouin-scattering-based distributed sensing system has been used for in-situ strain measurement during the curing of AS4-3501 composite panels. It has a pulse length of 1.5 ns, which is equivalent to spatial resolution of 15 cm and read-out resolution of 5 cm. Distributed Brillouin scattering sensors have been used to monitor either tempera- ture or strain for concrete structures, dams, and fiber cables by NTT Transmission System Labs (Japan) and Ecole Polytechnique Federale de Lausanne (EPFL) groups with spatial resolutions .1 m. We report the first application for a Brillouin scattering sensor in a composite curing process. The challenges for this application are the requirements of cen- timeter spatial resolution due to the small size of the composite material; simultaneous temperature and strain measurement during the fast tem- perature changing process of the curing, and short signal processing times. We present experimental results on strain measurements from optical fiber embedded in the eight-ply panels during the heat-up, iso- therm, and cool-down stages of the cure process. The materials studied are 177°C cure thermo-set materials used extensively in the manufac- ture of composite parts for the aerospace industry. The distributed sen- sor can detect the reaction advancement by measuring the cure shrink- age at the gelation and vitrification stages. Shrinkage is then correlated with the degree of cure data from a differential scanning calorimeter (DSC). The thermal response of the solidified composite during cooling is profiled. Details of the data processing of the Brillouin-scattering- based distributed sensor to obtain the strain variations as a function of cure temperature, time, and location are explained. © 2002 Society of Photo-

Characterization of the Firing Schedule for Positive Temperature Coefficient of Resistance BaTiO3

The development of positive temperature coefficient of resistance (PTCR) behavior during the firing procedure of semiconducting BaTiO3 was characterized. The PTCR properties of BaTiO3 were shown to be sensitive to the material's microstructure, liquid‐phase distribution, and extent of grain‐boundary oxidation. The PTCR behavior first became pronounced as the material cooled from the sintering stage at 1350°C to the annealing stage at 1175°C. Within this region, rapid oxidation of the grain boundaries occurred, which resulted in significant formation of charge carrier traps and a potential barrier. The rapid oxidation of the grain boundaries corresponded with the redistribution and solidification of the liquid phases. Once the carrier traps were established, the magnitude and slope of the PTCR jump increased during the annealing and cool‐down stages of the firing procedure because of further oxidation of the grain boundaries.

Numerical study of Czochralski silicon full process thermokinetics

Presents a finite-element analysis of the whole Czochralski process thermal history for a semiconductor silicon single-crystal production run. Conduction-dominated quasi-steady and transient heat transfer models are solved to simulate step-wise crystal growth and final system cool-down stages, respectively. The implemented numerical solution procedure guarantees an effective handling of the non-linear, free-boundary features of the mathematical statements, working out temperature field, melt/crystal freezing interface shape and growth heating power level for a comprehensive geometrical description of an actual Czochralski silicon furnace. The reliability of the adopted modelling approach in predicting the problem unknowns is checked by comparison between a few numerical results and the corresponding measured values. The estimated mean disagreement between the theoretical and experimental data is less than 10%.

防衛大学校低温風洞の冷却過程の改善について

One of the most critical stages of operation for an externally insulated cryogenic wind tunnel is the cooldown stage. The NDA cryogenic wind tunnel is such a tunnel, and has suffered from the rather inefficient cryogenic operation due to its poor cooldown characteristics. To correct this defect, the original total pressure control system and the cooldown preocedure were modified. The test results with the new ones showed that the modified system and preocedure save substantial amount of liquid nitrogen and time for the cooldown.