High Stress Regime Research Articles

Development of crystalline order during hot-drawing of poly(ethylene terephthalate) film: influence of strain rate

We have studied the evolution of crystalline order during drawing of poly(ethylene terephthalate) at 90°C and at strain rates in the range 0.01–2.1 s −1. The onset of crystallization always occurs at the inflection point in the stress-strain relationship, which shifts to higher strain levels and lower stress levels as strain rate decreases. Crystallinity develops in two regimes: in the low stress regime (regime 1) crystallinity increases more rapidly than in the high stress regime (regime 2). The onset of regime 2 occurs in the region of the sharp upturn in the stress-strain relationship, which shifts to higher strain levels as strain rate decreases, and at a characteristic level of crystallinity, which is independent of strain rate. We suggest that regime 1 involves the formation of a crystallite network which, at the characteristic level of crystallinity, has sufficiently crosslinked and reinforced the polymer that further deformation generates large stresses. WAXS determinations of lateral crystallite dimensions, normal to the 010 and 100 planes, reveal that crystallization in regime 2 involves modest growth of small crystallites (∼2.5−3.5 nm), with preferential growth occurring normal to 010. A preliminary investigation of the influence of molecular weight indicates that a lower rate of molecular relaxation at higher molecular weight reduces the strain-rate dependent shift in the onset of crystallization.

Microstructural characterization of a creep-deformed SiC whisker-reinforced Si 3N 4

Constant compressive stress-creep experiments have been conducted in the temperature and stress ranges of 1470–1720 K and 50–350 MPa, respectively, on a 30 vol% SiC whisker-reinforced Si 3N 4 composite containing sintering aids of 1.5 wt% Al 2O 3 and 6.0 wt% Y 2O 3. This work focuses on two independent creep experiments conducted at 1470 and 1620 K, both in the stress range 50–350 MPa. At 1620 K, the stress exponent, n, exhibited a bilinear behavior with n<1 in the low-stress regime and n>1 in the high-stress regime. This behavior was not observed at 1470 K. Various electron microscopy and X-ray diffraction techniques have been used to completely characterize the microstructure of the as-received, annealed, and crept materials. Transmission electron microscopy observations on samples taken before and after the change in n at 1620 K were compared to observations on material crept at 1470 K to clarify the microstructural phenomena controlling the break in the creep curve. Interfaces between Si 3N 4 grains and the whisker reinforcement were examined using high-resolution electron microscopy. The relationships between observed microstructural changes and high-temperature mechanical properties are presented.

The influence of acclimatization treatment and plantlet morphology on early greenhouse-performance of tissue-cultured Douglas fir [Pseudotsuga menziesii (Mirb.) Franco

The objective of this investigation was to test for the possible effects of plantlet morphology and environmental stress on survival and growth during the acclimatization of tissue-cultured Douglas fir [Pseudotsuga menziesii (Mirb.) Franco]. Under a high-stress environmental regime of 40–70% relative humidity and 22–28°C, survival was 33%, compared to 89% under a low-stress regime of 80–90% RH and 15–20°C. Shoot elongation under low stress was twice as great as that under high stress. Certain morphological features were associated with improved survival under high stress, and they included a large (10+) root number, tall (40 + mm) shoots, and upright needles. Plantlets with 10 + roots had 52% survival, compared to 25% or 31% for those with 1–3 or 4–10 roots, respectively. Also, plantlets with an initial shoot height of 41–60 mm had a 53% survival rate compared to 20% or 37% for those with 21–30 mm or 31–40 mm shoots. Tall plantlets and those with 10+ roots also underwent the greatest shoot elongation during the 7 week observation period. However, plagiotropism was frequent on tall shoots.

Mechanical behaviour and physical ageing of semi-crystalline polymers: 4

It is shown that for some semi-crystalline polymers (PP, HDPE at 20°C and higher) the effect of previous physical ageing on creep decreases with increasing level of stress, as found previously for amorphous polymers. This finding suggests that for semi-crystalline materials, high stresses can erase previous ageing and can rejuvenate the material. As a consequence, the methods developed for the prediction of long term creep could be generalized to the high stress regime. It is further shown that for compression moulded, annealed, PP, the upper T g, T U g, lies around + 100°C. Moreover the effect of stress level on ageing disappears for temperatures above 100°C. Finally, it is shown that a change from compression to injection moulding strongly influences the creep properties. The material becomes softer (factor of 2–3) and T U g drops by at least 25°C. This overall effect on T U g seems more important (for the creep properties) than the frozen-in stresses and orientations.

Further Development of a Model for Predicting Corrosion Fatigue Crack Growth in Reactor Pressure Vessel Steels

Analysis of fatigue crack growth data for low-alloy steel shows that the influence of cyclic frequency in simulated LWR environments can be interpreted as the superposition of a time-dependent, corrosion-assisted crack growth rate upon an increment predicted by a Paris law. The time-dependent component increases monotonically to a maximum of about 6×10−5 mm/s as stress cycling becomes more aggressive. A useful measure of aggressiveness is the average time rate of crack advance due to the Paris law component alone; i.e., AΔKn × frequency. The result suggests that current ASME Code methods for flaw assessment are highly conservative in some regimes of stress and frequency, but there is a possibility of growth rates well above the ASME XI, Appendix A curves in a very low-frequency, high-stress regime. An upper bound to the time rate of corrosion-assisted crack growth in low-alloy steel is well supported by the data. The threshold conditions for the onset of this high rate are less well defined and require further investigation.

A technique for determination of stress fields in the contact loading of highly deformable polymeric materials

AbstractSoft, deformable padding materials are widely used for the fabrication of cushions and pads used to reduce contact loading stresses in several industrial and biomedical applications. This paper, presents an indentation technique that can be used to evaluate cushioning materials. The technique uses a soft‐headed indenter made of poly(vinyl chloride) gel that is impressed upon the polymer cushion to be tested. The strain fields and the high stress regimes that develop in the head are then mapped using a grid technique and finite strain theory. Results of indentation tests carried out on a number of cushions of varying stiffness are presented and analyzed.

The endurance limit of JPCA alloy at 703 K in vacuum

The high-cycle fatigue behavior of the Japanese Primary Candidate Alloy (JPCA) was determined at 703 K in vacuum. Four different thermomechanical treatments produced the changes in dislocation structure and dispersion of Ti-rich MC type carbides. Applying load controlled sine waves of 30 Hz in tension-compression mode, ductile fractures with striations and dimples were observed for both solution annealed and subsequently aged alloys at 1073 K, while brittle facets and wavy patterns with tearing by shear were observed for 15% cold worked alloy. The fatigue life was shown to be improved by the cold work in the relatively high-stress and short-life regime. But the fatigue endurance limit at 10 7 cycles for cold worked JPCA was 550 MN/m 2, slightly lower than that for solution annealed JPCA, and was furthermore decreased by aging for the precipitation of MC type carbides.

Flow of metal into the flash gap in the last stages of a plane-strain closed-die forging operation

In closed-die forging the flash geometry should be such as to ensure that the cavity is completely filled just as the two dies come into contact at the parting plane. If metal is caused to extrude through the flash gap as the dies approach the point of contact — a practice generally resorted to as a means of ensuring complete filling — dies are unnecessarily stressed in a high-stress regime (as the flash is quite thin and possibly cooled by then), which reduces the die life and unnecessarily increases the energy requirement of the operation. It is therefore necessary to carefully determine the dimensions of the flash land and flash thickness — the two parameters, apart from friction at the land, which control the lateral flow. The dimensions should be such that the flow into the longitudinal cavity is controlled throughout the operation, ensuring complete filling just as the dies touch at the parting plane.The design of the flash must be related to the shape and size of the forging cavity as the control of flow has to be exercised throughout the operation: it is possible to do this if the mechanics of how the lateral extrusion into the flash takes place is understood for specific cavity shapes and sizes.The work reported here is part of an ongoing programme investigating flow in closed-die forging. A simple closed shape (no longitudinal flow) which may correspond to the last stages of a real forging operation is analysed using the stress equilibrium approach. Metal from the cavity (flange) flows into the flash by shearing in the cavity in one of the three modes considered here: for a given cavity the mode with the least energy requirement is assumed to be the most realistic. On this basis a map has been developed which, given the depth and width of the cavity as well as the flash thickness, will tell the designer of the most likely mode (of the three modes considered) in which metal in the cavity will shear and then flow into the flash gap. The results of limited set of experiments, reported herein, validate this method of selecting the optimum model of flow into the flash gap.

An investigation on the creep and fracture behavior of cast nickel-base superalloy IN738LC

The creep-rupture properties of cast nickel-base superalloy IN738LC were studied over the temperature range 750 to 950 °C. Our results show that primary and steady-state creep should not be regarded as distinct stages and that they have basically the same deformation mechanism. The dependence of the steady-state creep rate,es, on stress,δ, and on temperature,T, for this superalloy can be described ases = Aδ nexp(−Qc/RT).n = 8.3 - 9.8 andQc = 570 - 730 kJ mol−1 at high stress levels, whereasn = 4.1 - 4.9 andQc = 370 - 420 kJ mol−1 at low stress levels. The observations of dislocation structures during steady-state creep confirm that the creep mechanism is different in the high and low stress regimes. The observations of the microstructure show that the initial acceleration in creep rate during the tertiary stage is connected with changes in the size and distribution ofγ′ particles during creep. Rupture occurs by the propagation of oxidized intergranular cracks which initiate at the specimen surface, and the rate of crack propagation is controlled by the deformation behavior of the superalloy.

Cataclastic rocks of the San Gabriel fault—an expression of deformation at deeper crustal levels in the San Andreas fault zone

The San Gabriel fault, a deeply eroded late Oligocene to middle Pliocene precursor to the San Andreas, was chosen for petrologic study to provide information regarding intrafault material representative of deeper crustal levels. Cataclastic rocks exposed along the present trace of the San Andreas in this area are exclusively a variety of fault gouge that is essentially a rock flour with a quartz, feldspar, biotite, chlorite, amphibole, epidote, and Fe-Ti oxide mineralogy representing the milled-down equivalent of the original rock (Anderson and Osborne, 1979; Anderson et al., 1980). Likewise, fault gouge and associated breccia are common along the San Gabriel fault, but only where the zone of cataclasis is several tens of meters wide. At several localities, the zone is extremely narrow (several centimeters), and the cataclastic rock type is cataclasite, a dark, aphanitic, and highly comminuted and indurated rock. The cataclastic rocks along the San Gabriel fault exhibit more comminution than that observed for gouge along the San Andreas. The average grain diameter for the San Andreas gouge ranges from 0.01 to 0.06 mm. For the San Gabriel cataclastic rocks, it ranges from 0.0001 to 0.007 mm. Whereas the San Andreas gouge remains particulate to the smallest grain-size, the ultra-fine grain matrix of the San Gabriel cataclasite is composed of a mosaic of equidimensional, interlocking grains. The cataclastic rocks along the San Gabriel fault also show more mineralogiec changes compared to gouge from the San Andreas fault. At the expense of biotite, amphibole, and feldspar, there is some growth of new albite, chlorite, sericite, laumontite, analcime, mordenite (?), and calcite. The highest grade of metamorphism is laumontite-chlorite zone (zeolite facies). Mineral assemblages and constrained uplift rates allow temperature and depth estimates of 200 ± 30° C and 2–5 km, thus suggesting an approximate geothermal gradient of ~50°C/km. Such elevated temperatures imply a moderate to high stress regime for the San Andreas, which is consistent with experimental rock failure studies. Moreover, these results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress. Much of the mineralogy of the cataclastic rocks is still relict from the earlier igneous or metamorphic history of the protolith; porphyroclasts, even in the most deformed rocks, consist of relict plagioclase (oligoclase to andesine), alkali feldspar, quartz, biotite, amphibole, epidote, allanite, and Fe-Ti oxides (ilmenite and magnetite). We have found no significant development of any clay minerals (illite, kaolinite, or montmorillonite). For many sites, the compositions of these minerals directly correspond to the mineral compositions in rock types on one or both sides of the fault. Whole rock major and trace element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform, and that originally micaceous foliated, or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material. As previously found for the gouge along the San Andreas, chemical mobility is not a major factor in the formation of cataclastic rocks of the San Gabriel fault. We see only minor changes for Si and alkalies; however, there is a marked mobility of Li, which is a probable result of the alteration and formation of new mica minerals. The gouge of the San Andreas and San Gabriel faults probably formed by cataclastic flow. There is some indication, presently not well constrained, that the fine-grained matrix of the cataclasite of from the San Gabriel fault formed in response to superplastic flow.

Model experiments to study the stress distributions in a seated buttock

Mechanical stress states that develop in the buttock during sitting may exceed tissue tolerance and lead to decubitus ulcer formation in susceptible patients, such as those with spinal cord injury. The danger of this complication can be reduced by using suitable cushions to minimize stress magnitudes and gradients within soft tissues. In this investigation, a two-dimensional physical model of the buttock-cushion system was developed to aid in cushion design. The model consists of PVC gel simulating flesh, cast around a wooden core simulating the ischium bone. A grid etched on the gel permits measurement of strains via photographs of the underformed and deformed model buttock supported by various cushion materials. The displacement field is analyzed, using a finite strain theory and a strain energy function, to obtain the ‘tissue stresses’. In this manner, the performances of five clinically used cushion materials were compared with respect to the high stress regimes developed in the model buttock.

Microstructural stability during creep of a niobium-stabilized austenitic steel

Creep tests combined with transmission electron microscopy have been carried out on a niobium stabilized austenitic stainless steel which has been subject to a multi-mechanical thermal treatment (MMTT). It is found that the substructure introduced by the MMTT creates two distinct creep regimes, separated by a narrow transition temperature or stress range. In the low stress or low temperature regime creep is characterised by a low stress exponent (about 5) and a low activation energy (~300kJ/mole). The diffusion-controlled motion of jogged screw dislocations is suggested to be the most likely rate-limiting process. In the high stress or high temperature regime creep is characterised by a high stress exponent (about 10) and a high activation energy (~ 600 kJ/mole). Subgrains stabilized by the NbC precipates are observed to elongate during creep and the rate-limiting process is the breakaway of subgrain boundaries from small particles.

Effect of stress and temperature on the creep and rupture behavior of a 1.25 Pct chromium—0.5 Pct molybdenum steel

The creep and stress rupture behavior of a normalized 1.25 pct chromium-0.5 pct molybdenum steel has been investigated over a temperature (T) range of 510 to 620°C and a stress(σ) range of 65 to 425 MN/m2. The creep rate ( $$\dot \in $$ ) and time to rupture (t r ) data have been analyzed in terms of the general expression $$\dot \in $$ ort r -A σn exp (Q/RT), whereA is a constant,n is the power exponent of stress,Q is an empirical activation energy for the rate controlling process andR is the universal gas constant. At each temperature, the logarithmic plots of creep rate and time to rupture as functions of stress consist of two linear segments, separating the data into low stress and high stress regimes. The stress exponent has approximate values of 4 and 10 in the low stress and high stress regimes respectively in the appropriate expressions for both creep rate and for time to rupture. The activation energy has values of 367 and 420 kJ/mole in the low stress regime for time to rupture and creep rate respectively. In the high stress regime, the respective values of activation energy are 581 and 670 kJ/mole. Fractographic observations show that the changes from low stress to high stress behavior in creep rate and time to rupture approximately coincide with the transition in fracture mode from intergranular to transgranular cracking as well as with the transition in the rupture ductility from a region of linear variation with stress to one of constant ductility. These observations suggest that the transition from low stress to high stress behavior may be associated with a change in deformation mode from predominantly grain boundary sliding at low stress to transgranular matrix deformation at high stress. Analysis of the creep rate data based on this premise enables calculation of the ratio of the contributions of the grain boundary sliding mode to the total deformation (e gb /e T ) at various values of stress and temperature. Results of this analysis are consistent with numerous experimental observations reported in the literature.

The Effect of Particle Size and Spacing on Creep of Two-Phase Copper–Cobalt Alloys

The effects of variations in particle diameter and spacing on the steady creep rate (έ s ) of three copper–cobalt alloys containing 0·88, 2·48, and 4·04%Co have been studied over a range of stresses at 712 K. At constant particle size and spacing, the stress (σ) and temperature-(T) dependence of έ s for each alloy can be described as <disp-formula> <mml:math> <mml:mrow> <mml:msub> <mml:mrow> <mml:mover> <mml:mi>ɛ</mml:mi> <mml:mi>˙</mml:mi> </mml:mover> </mml:mrow> <mml:mi>S</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mi>A</mml:mi> <mml:msup> <mml:mi>σ</mml:mi> <mml:mi>n</mml:mi> </mml:msup> <mml:mtext> exp</mml:mtext> <mml:mo>−</mml:mo> <mml:msub> <mml:mi>Q</mml:mi> <mml:mi>c</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:mi>R</mml:mi> <mml:mi>T</mml:mi> </mml:mrow> </mml:math> </disp-formula>A change in stress exponent, n, and activation energy for creep, Q c , occurs at a stress comparable with the macroscopic yield stress. It is proposed that with stresses < YS, when n ≃ 4·5 and Q c ≃ 140 kJ/mol, only limited deformation of the grains takes place and creep is attributable primarily to grain-boundary sliding and accommodating deformation. In the high-stress regime, when n ≃ 12 and Qc ≃ 210 kJ/mol, deformation occurs both in the grains and in the grain-boundary regions. For each alloy tested at σ > YS, έ s decreases with increasing particle diameter until a minimum value is reached and then increases rapidly with further increase in particle size. The decrease in έ s appears to be a result of the increase in work done by the dislocations in cutting particles as their size increases, until at maximum creep-resistance the dislocations are forced to bow between the particles. The creep rate increases with further increase in particle size and spacing as dislocation bowing becomes progressively easier. Under all conditions, the effect of particles on the creep rate is shown to be largely a consequence of their effect on the rate of recovery.

Internal friction: Some considerations of the frequency response of rocks and other metallic and non-metallic materials

Although several workers, including the authors, have shown that the attenuation exponent in rocks tends to increase linearly with frequency, such behaviour is not general for all materials: metals, in particular, project several internal friction peaks over a spectrum of frequencies, each peak being ascribed to a certain physical process operating in the material. This article reviews in a simple manner some basic concepts behind the frequency response of various materials since a general knowledge of the behaviour of solids under constraint is an essential prerequisite to a more fundamental investigation of a particular material. All strains are considered recoverable, the vibrating stress levels being low and the behaviour of the material being described by its anelastic properties. The review takes no consideration of the high stress regime within which the logarithmic decrement has been found to increase strongly due usually to plastic losses.

Engineering progress in 1932. Structural advance

China's southern Shanxi Province contains thick, gassy coals of anthracite rank. These high-methane-content (higher than 500 ft3/ton) coals necessitate pre-mining methane drainage to enhance the health and safety of underground coal workers. Both traditional vertical wells and multilateral horizontal wells, which were shown to perform better, have been drilled in the area in order to effectively reduce the gas content of coals. However, optimization of well drilling patterns is crucial to improve the gas drainage by boosting well performance and resulting in significant additional recovered gas in a cost-effective manner. In this study, exploration borehole data from an undeveloped coalbed methane field in China's Qinshui Basin was obtained where permeability is expected to range from 0.1 to 10 md.Reservoir modeling conducted in this study concludes that in the presence of low permeability coals (less than 1.0 md) well patterns can be optimized to maintain high-methane production rates. In this work, relationships to determine combinations of horizontal lateral spacing and permeability that yield similar production curves were developed in order to maximize production. Pinnate wells with laterals spaced at 600-ft intervals in reservoirs with 2.0 md permeability performed similar to those with 200-ft lateral intervals in 0.5 md reservoirs. Additional similar relationships were drawn for reservoirs ranging from 0.1 to 8.0 md. Furthermore, because of the Qinshui Basin's potential high stress regime, Pitchfork and Pinnate horizontal well patterns were analyzed based on their potential for catastrophic wellbore failure at well junctions and producing well branches. Analyses concluded that Pitchfork wells had a significantly lower risk for significant wellbore loss due to borehole collapse, while providing the opportunity for higher gas recovery in reservoirs exhibiting directional permeability.