Minimum Stress Level Research Articles

Bioluminescence response of four species of dinoflagellates to fully developed pipe flow

Dinoflagellate bioluminescence provides a nearly instantaneous index of flow sensitivity. This study compared flow sensitivity in four species of morphologically diverse luminescent dinoflagellates (Ceratium fusus, Ceratocorys horrida, Lingulodinium polyedrum and Pyrocystis fusiformis) using fully developed laminar and turbulent pipe flow. Bioluminescence response thresholds always occurred in laminar flows with wall shear stress levels that, depending on species, ranged from 0.02 to 0.3 N m � 2 . With few exceptions, such as breaking waves and wave-forced bottom shears in shallow nearshore areas, these threshold shear stress levels are several orders of magnitude larger than typical oceanic ambient flows. For laminar flows above threshold, species also differed in the proportion of organisms responding and the minimum shear stress level where individual flashes reached their highest intensity. Following transition to turbulent flow, there was never a dramatic increase in bioluminescence, even when energetic turbulent length scales were similar to the cell size. On the basis of their bioluminescence response in laminar flow, these species were ranked in order of decreasing sensitivity as C. horrida > P. fusiformis > C. fusus > L. polyedrum. This ranking, though not conclusive, is consistent with increased flow sensitivity due to increasing size and the presence of spines. With the exception of a small fraction of the C. horrida population that is sensitive enough to flash within the feeding current of a predator, the present study suggests that flashes only occur with predator contact. Nevertheless, flow sensitivity may serve as an index of the response to mechanical agitation during predator contact/handling. Flow sensitivity may be constrained to maximize the response to predator contact/handling while minimizing stimulation by background oceanic flows to avoid depleting luminescent reserves.

Fatigue resistance of welded details enhanced by ultrasonic impact treatment (UIT)

Enhancement of the fatigue resistance of welded transverse stiffeners and cover plate details by ultrasonic impact treatment (UIT) was evaluated in 18 full-scale W27×129 rolled beam specimens. Fatigue tests were conducted under constant amplitude loading at various stress range levels and at two minimum stress levels simulating the effect of sustained load. The test specimens were investigated for fatigue crack initiation and propagation. Distributions of residual stresses adjacent to the weld toe were determined before and after the treatment. Test results indicated that UIT enhanced the fatigue performance of all treated details by improving the weld toe profile, changing microstructure and introducing beneficial compressive residual stresses at the treated weld toe. The treatment effectively elevated the fatigue crack growth threshold and the fatigue limit without changing the slope of the S– N curve.

Geometry effects of substrate and coating layers on the thermal stress response of FGM structure

Due to transient temperature change, the plane strain elastic-plastic problem for a functionally graded material (FGM) bonded to a homogeneous coating layer and a metal substrate is considered by the use of the finite element method (FEM). The substrate and the coating are assumed to be aluminum and partially stabilized zirconia, respectively. The FGM layer is a particulate composite of aluminum and partially stabilized zirconia with volume fractions continuously varying through the thickness. Generally in high temperature applications, the FGM system is sandwiched between a substrate layer and a coating layer. The coating layer increases the protection from heat but decreases the thermal shock resistance while the substrate layer increases the rigidity of the structure and decreases strength-related properties at high temperature. In order to compromise the thickness of both the coating and substrate layers, different values of the substrate and coating thickness are studied in order to evaluate their effects on the thermal stress response of the FGM structure. Since the main objective of the FGMs is using them in different applications with severe thermal loading conditions, the thermal stresses may be so high that some reinforcements may be fractured and/or debonded from the matrix giving a weakening effect instead of a reinforcing one. Hence, the behaviors of the reinforcements and the matrix are essential to be studied. In this regard, microscopic constitutive equations along with the temperature-dependent properties of the constituent materials are considered to enable us obtaining more realistic results of thermal stresses. Since the FGM structures are fabricated at high temperatures, thermal residual stresses are produced. In order to find out the importance of the consideration of the residual stresses arising from the fabrication process, the FGM structure with stress-free conditions is heated to the operating temperature, and its thermal stress response is compared with that one where the residual stresses are taken into account. Also, several functional forms of gradation of the constituents in the FGM layer are examined to reach the optimum profile giving the minimum stress level for the FGM structure under thermo-elasto-plastic behavior.

FLUID SHEAR‐STIMULATED DINOFLAGELLATE BIOLUMINESCENCE

Bioluminescence studies provide insight into the properties of water motion that are stimulatory to flow‐sensitive organisms such as dinoflagellates, the most common sources of near‐surface oceanic bioluminescence. Previous laboratory studies employing steady flows have characterized the luminescent response of dinoflagellates in terms of shear stress. In the present study, computational and experimental approaches were used to investigate the contributions of shear and acceleration to cells responding in a laminar converging flow field, where regions of high acceleration and shear are spatially separated. Flow‐stimulated flashes by the dinoflagellates Lingulodinium polyedrum and Ceratocorys horrida were used as a near‐instantaneous monitor of cell response. By combining video analysis of flash trajectories with computational methods, the location of each stimulated cell was determined and flow parameters at that location were calculated. Based on several criteria, shear stress was considered the flow parameter most stimulatory to cells. For both dinoflagellates species and for all flow rates, essentially all cells responded downstream near the wall where shear stress levels were maximal, and levels of acceleration and extensional stress were as much as two orders of magnitude less than locations away from the wall. Minimum shear stress levels at the cell positions were consistent with response thresholds based on previous studies. Bioluminescence is an excellent tool for examining how organisms respond to flow at the small temporal and spatial scales relevant to planktonic organisms.

Analysis and testing of a prototype pultruded composite causeway structure

The preliminary design of a floating causeway system made from pultruded composite materials is described. As part of the design effort a 24-ft long × 15-ft wide × 5-ft high (7.3 m × 4.6 m × 1.5 m) pultruded GFRP scaled prototype was fabricated. Details of, and selected results from, the experimental testing and finite element modeling of this prototype are given. Testing revealed that the prototype composite structure was capable of supporting the expected service sustained loads, as well as, the lifting loads. Strain and deflection data indicated that minimal stress levels existed in the structure during testing. For predictive and comparative purposes, two finite element models were developed that corresponded to the sustained load and lifting load tests. These were relatively simple two-dimensional models, however, they effectively captured global and localized structural behavior.

FLEXURAL FATIGUE OF CONCRETE FOR AIRPORT PAVEMENTS

Through a series of experiment in laboratory, this paper mainly clarifies the effects of the factors such as maximum stress level, minimum stress level, loading frequency and rest period on the fatigue life of concrete for airport pavement, and analyzes the failure probability due to flexural fatigue under different stress level. As a result, it is found that under high stress level, namely the maximum stress level is equal to or greater than 80%, the minimum stress level has no clear influence on the fatigue life of concrete, but when the maximum stress level is 70% its effect can not be ignored. Accordingly, the flexural fatigue equation of concrete for airport pavement, considering the minimum stress level, is derived.

A direct comparison of matched-die versus diaphragm forming

Continuous fibre reinforced thermoplastic laminates, in comparison with monolithic metallic sheets, are effectively inextensible in the direction of their reinforcements and therefore severely limited in the range of possible deformations they can undergo during forming. Inter-ply and intra-ply shearing plays a major role in the determination of the degree of formability of such laminates. The present study focuses on the shear behaviour of CFRT sheets in inter-ply and intra-ply modes aiming to assess their significance in matched-die and double diaphragm forming processes. In order to gain a better understanding of the minimum stress levels necessary to induce the two different shear modes, basic shearing experiments on single plies and unidirectional reinforced laminates were conducted prior to forming spherical shaped dome parts. By forming down-scaled yacht hulls it was intended to apply the findings obtained with the spherical dome parts to other 3-D shapes and to demonstrate the versatility and limitations of both processes.

Increased shear stress overcomes the antithrombotic platelet inhibitory effect of aspirin in stenosed dog coronary arteries.

Shear stress is one of the known platelet activating mechanisms that leads to thrombosis. Increased shear stress has also been postulated to reverse the antithrombotic effect of some drugs such as aspirin (ASA). Experiments were conducted in five dogs to determine the minimal shear stress levels that produce acute platelet thrombus formation in mechanically stenosed arteries and the increase in shear required to reverse the antithrombotic effect of ASA. After intimal and medial damage, stenosis was produced in the circumflex coronary artery. We used the finite-difference numerical solution of the Navier-Stokes equation to determine the wall shear stresses in the area of stenosis. At 70+/-6% coronary diameter reduction, cyclic flow reductions (CFRs) caused by acute platelet thrombus formation were observed in the stenosed lumen. At this level of stenosis, the shear stress was 144+/-15 Pa. ASA given at a dose of 5 mg/kg IV inhibited in vivo acute platelet-mediated thrombus formation and abolished CFRs in all dogs. However, increasing the stenosis level to 80+/-5% caused the CFRs to return. The shear stress increased with the increased level of stenosis to 226+/-22 Pa. Thus, an average 10% increase in diameter narrowing caused a 56+/-20% increase in shear stress (P<.005) and renewed platelet activation and thrombus formation despite ASA pretreatment. Individuals who take ASA daily to prevent coronary artery thrombus formation may not be well protected when a change in hemodynamics, such as an acute hypertensive episode, or an increase in stenosis severity due a ruptured atherosclerotic plaque causes an increase in shear stress.

Thermal residual stresses in a functionally graded material system

A one-dimensional calculation of thermal residual stresses, arising from the fabrication of a Functionally Graded Material (FGM) system, is presented. As a first step, calculations have been limited to the linear elastic case. The FGM system consists of ceramic (Al 2O 3) and metal (Ni) phases varying with distance in one direction. Several functional forms of gradation of constituents were examined to arrive at the optimum profile giving the minimum residual stress level. A linear variation in composition from fully ceramic to fully metal showed the least residual stress. Residual stresses were found to increase when fully ceramic and/or fully metal regions are included in the structure, adjoining the graded zone. The character (tension or compression) of stresses is dependent on the functional form of gradation. A low residual stresses state close to that of the continuously graded structure can be achieved also in a multilayer FGM system having several layers (typically > 11) of constant composition. The effects of temperature dependent elastic and thermal expansion characteristics of constituents on residual stress were found to be small.

Effect of mean strain on the cyclic deformation and stress relaxation in polypropylene

AbstractThe influence of mean strain Changes in the range from 0 to 4% on the cyclic deformation and stress‐relaxation properties is investigated using rod specimens of polypropylene. An extensometer measures and controls the axial strain in a closed‐loop, electrohydraulic, servocontrolled mechanical testing machine. The hysteresis loop at different numbers of cycles N of N = 35, 45, and 50 are irregular in shape, and the tensile portion decreases in size as the number of cycles was increased. The stress value of the tensile portion for three mean strains of ϵm = 1.0%, 2.0%, and 4.0% remains constant, but the value of the compressive portion decreases as the mean strain was increased. The stress level at a strain width of 3% changes little with mean strain. For the strain widths of 5% and 7%, however, the minimum stress levels at N = 35, 45, and 50 increase with increasing mean strain, in contrast to the behavior of maximum stress level. The stressrelaxation tests show that the drop of stress decreases with an increase in number of cycles. The discrepancy in the results of relaxation tests is due to the effect of the difference in strain rates.

Residual stress characteristics of a FGM joint: T. Itoh et al, (Toshiba Corp, Yokohama, Japan), J Japan Soc Powder and Powder Metallurgy, Vol 39, No 4, 1992, 287–290. (In Japanese)

A one-dimensional calculation of thermal residual stresses, arising from the fabrication of a Functionally Graded Material (FGM) system, is presented. As a first step, calculations have been limited to the linear elastic case. The FGM system consists of ceramic (Al2O3) and metal (Ni) phases varying with distance in one direction. Several functional forms of gradation of constituents were examined to arrive at the optimum profile giving the minimum residual stress level. A linear variation in composition from fully ceramic to fully metal showed the least residual stress. Residual stresses were found to increase when fully ceramic and/or fully metal regions are included in the structure, adjoining the graded zone. The character (tension or compression) of stresses is dependent on the functional form of gradation. A low residual stresses state close to that of the continuously graded structure can be achieved also in a multilayer FGM system having several layers (typically > 11) of constant composition. The effects of temperature dependent elastic and thermal expansion characteristics of constituents on residual stress were found to be small.

Reliability considerations in accelerated life testing of electrical insulation with generalized life distribution function

The authors introduced a new approach to estimate life distributions at nominal conditions from the results of accelerated life testing of electrical insulating materials. A very general family of probability distributions is introduced, and a best fit member of this family is used to represent life data at each stress level. Nonlinear optimization techniques are applied in conjunction with linear regression analysis. In any accelerated life testing study important questions pertain to the minimal and maximal stress levels to be applied. A method of determination of the minimal stress level as well as the suitable number of tests based on reliability considerations is presented. A numerical example based on test data and a user-friendly computer program are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Repeated impact on fatigue fracture of carbon spring En-42J steel: Dhar, S.Theor. Appl. Fract. Mech. Oct. 1988, 10, (2), 165–171

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 × 460 mm in size. The tests were conducted at room temperature with the loading frequencies in the range of 0.5–1.5 Hz. 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.

An assessment of carbon steel containers for radioactive waste disposal

The report describes a study of the corrosion of High Level Nuclear Waste containers in geological disposal. This has concentrated on the use of carbon steel in granitic environments, and has aimed at estimating the corrosion allowance required to achieve a 1000-year life. Experimental and mathematical modelling studies have indicated that 216 mm of steel should be more than sufficient to prevent penetration by general or localized corrosion. There are sound mechanistic reasons to regard this value as an overestimate. The possibility of stress corrosion cannot be dismissed, but, because the process requires a certain minimum stress level before it can occur, it should be possible to avoid the problem by giving the containers a stress relief heat treatment. A conservative assessment has shown that microbial corrosion may cause a maximum additional metal loss of 13 mm.

The development of molecular orientation and morphological texture in thermotropic copolyesters

AbstractLiquid crystalline polymers can be processed to form high strength/modulus materials. In processing these materials, it is apparent that molecular orientation is an important factor in determining the physical strength of the processed materials. In this study a systematic investigation was carried out to determine how a thermotropic copolyester of parahydroxybenzoic acid (PHB) and polyethylene terephthalate (PET) responds to two basic types of flows: shear and extensional flow. This was accomplished by preparing sheared and extended samples under controlled conditions of temperature and flow history. Sheared disks were prepared using a disk and plate geometry of a Rheometrics Mechanical Spectrometer (RMS model 605), while extended ribbons were prepared using a slit die attached to an Instron capillary rheometer. Two copolymerer compositions of 60 mole percent and 80 mol percent PHB were investigated. The sheared disks and extended ribbons were investigated for molecular orientation and morphological textures using wide angle x‐ray scattering (WAXS) and scanning electron microscopy (SEM) analysis, respectively. It was found that extensional flow has a greater capacity for orienting such materials than shear flow. Samples annealed at their softening points for 1 minute (240°C for the 60 mole percent PHB/PET copolymer and 300°C for the 80 mole percent PHB/PET copolymer) showed no significant loss of orientation, indicating that once orientation is produced it may remain in the melt for a long period of time. Sheared samples prepared by shearing the sample while cooling showed significantly higher degrees of orientation than those not cooled while being sheared. This may indicate that a minimum stress level exists for the production of orientation in shear flow.

変動繰返し圧縮荷重下におけるコンクリートの疲労寿命に関する研究

In order to investigate the fatigue life of concrete under varying repeated load, several stress fatigue tests were performed. In these tests, concrete cylinder specimens were subjected to the repeated load in which load levels were selected as to be between a constant minimum stress level and five different maximum stress level Sj (72∼82 percent of the ultimate static strength at each interval of 2.5%). The triangular-, normal-, uniform-, and exponential-distributions were used as the shape of the stress block for the varying repeated load. The order of the applied maximum stress level was selected at random.From the experimental results, it can be concluded as follows. (1). At each test condition, the sun of cycles ratio follows a logarithmic normal distribution. (2). Miner's rule may possibly be applied to predict the fatigue life under varying repeated load in which the magnitude of stress is changed quite very frequently.

A regression model of fatigue crack propagation under flight simulation loading

A multi-variable regression analysis is made of crack propagation data under flight simulation loading for the 2024-T3 and 7075-T6 alloys. The study is made using an effective stress range concept based on cracl closure considerations. A two-variable regression model of the equivalent stress range is suggested. The model is used to calculate fatigue crack propagation lives. Predictions using the proposed model are compared with empirical results. It is shown that the model accounts for the influence of maximum, minimum and mean stress levels of the load spectrum.

Inhomogeneous flows of non-newtonian fluids: generation of spatial concentration gradients

The conformation of polymer molecules or of micelles is known to be changed when a stress or deformation-rate field is applied to a fluid. Thermo dynamic considerations suggest that the resulting entropy changes must lead to diffusion of the species effected toward regions of minimum stress level in any system in which the stress varies with position. That is to say, spatial stress gradients inevitably appear to give rise to spatial concentration gradients. Such flows differ intrinsically, therefore, from homogeneous flows in which the stress field, hence the fluid concentration levels and properties, are uniform throughout the continuum. Preliminary measurements of these concentration fields are reported and consequences of this phenomenon in several flow processes are considered.

Constitutive equations for plastic deformation of porous materials

It is shown that a one-parameter model for the yield surface of porous materials is sufficient to analyse the plastic behaviour of those materials. Also, the concept of “limiting condition” is introduced to determine the minimum stress level for a plastic deformation. Application of the model gave satisfactory results.

Method of analysis and prediction for variable amplitude fatigue crack growth

Fatigue crack growth behavior of structure subjected to variable amplitude loading is very complex. Both the truncation and the load sequence have been shown to have a significant influence on the test results because of the load level interaction effects. To understand these interaction effects and the possible influence they can have on the results obtained a test program was performed. Fatigue crack growth tests were conducted on the program using 7075-T6 and 2024-T3 aluminum and titanium 6A1-4V mill anneal. Using the test data, an analysis method was developed. In this analysis method the crack growth rate is evaluated for each load cycle using a modification of the fracture mechanics correlation technique. The crack growth for each cycle was evaluated as a function of the stress intensity factor excursion with a correction factor for the maximum and minimum peak stress levels in the test spectrum. The fatigue crack growth correction for the peak stresses in the spectrum is given as a growth rate correction factor r. The relationship for r, is termed the ‘fatigue crack growth rate interaction model’. For verification, the interaction model was applied to test data from spectrum loading tests. The correlation obtained for the example, indicated that the model properly predicts the interaction effects and its use could significantly improve the accuracy of crack growth life calculations for programmed spectrum tests.