Effect Of Long-term Loading Research Articles

Mechanosensitivity of the rat skeleton decreases after a long period of loading, but is improved with time off

After the initial adaptation to large mechanical loads, it appears as though the skeleton's responsiveness to exercise begins to wane. To counteract the waning effects of long-term mechanical loading, “time off” may be needed to improve the responsiveness of bone cells to future mechanical signals and reinitiate bone formation. The aim of this study was to determine whether bone becomes less sensitive to long-term mechanical loading and whether time off is needed to improve mechanosensitivity. Fifty-seven female Sprague–Dawley rats (7–8 months of age) were randomized to one of following groups: Group 1 loading was applied for 5 weeks followed by 10 weeks of time off (1 × 5); Group 2 loading was applied for 5 weeks, followed by time off for 5 weeks and loading again for 5 weeks (2 × 5); Group 3 loading was applied continuously for 15 weeks (3 × 5); Group 4 age-matched control group; and Group 5 baseline control group. An axial load was applied to the right ulna for 360 cycles/day, at 2 Hz, 3 days/week at 15 N. At the end of the intervention, all three loaded groups showed similar increases in bone mass, cortical area, and I MIN in response to mechanical loading . Bone formation rate of the loaded ulna was increased in the first 5 weeks of loading for all three loaded groups; however, during the last 5 weeks, it was only significantly increased in the group that had time off (2 × 5) ( P < 0.05). The group that had time off (2 × 5) also showed greater improvements in work to failure compared to the group loaded for 5 weeks (1 × 5) and the entire 15 weeks (3 × 5). A second experiment showed that the waning effect of long-term loading on the skeleton is not a result of aging. In conclusion, mechanical loading of the rat ulna results in large improvements in bone formation during the first 5 weeks of loading, but continual loading decreases the osteogenic response. Having time off increases bone formation and improves the resistance to fracture.

Performance of Protective Cover Systems for Landfill Geomembrane Liners Under Long-Term MSW Loading

This paper presents the results of large-scale laboratory simulation tests conducted to evaluate the relative performance of different cover systems, consisting of a granular soil layer (i.e. a drainage layer) both with and without the presence of a needle-punched nonwoven geotextile, to protect a 1.5 mm thick smooth HDPE geomembrane liner under long-term municipal solid waste (MSW) loading conditions. Five different granular soils that range from a coarse gravel to a medium sand were used in the testing program. The protective cover system and the geomembrane liner were subjected to incremental loading to a maximum pressure of 1.4 MPa. The effect of long-term loading on the characteristics of the cover soils was assessed by performing particle size analyses, and the physical damage that occurred to the geomembrane liner was visually assessed in addition to performing multi-axial tension, wide strip tension, and water vapor transmission tests. The test results revealed that the degree of geomembrane liner protection decreases as the soil mean particle size increases and as the soil particle sphericity decreases. This study demonstrates that a 300 mm thick granular soil layer consisting of particles with a mean size less than 30 mm and a sphericity greater than 0.8, combined with a 270 g/m2 nonwoven geotextile provides adequate protection for a 1.5 mm thick HDPE geomembrane liner from MSW loading.

Relaxation-systolic pressure relation. A load-independent assessment of left ventricular contractility.

This contribution reviews the regulation of left ventricular pressure (LVP) fall by load and relates this regulation to left ventricular contractility. Load regulation of LVP fall has to be distinguished from neurohumoral regulation, from effects induced by arterial reflected waves and from long-term load effects on contractility. The response of LVP fall to a moderate elevation of systolic LVP is highly variable. It depends on the ratio between the actual systolic pressure and peak isovolumetric pressure, defined as "relative load". Up to a relative load of 81% to 84%, LVP fall accelerates. Above this relative load, LVP fall decelerates. Depending on the level of relative load there is a wide variety of effects ranging from moderate acceleration of LVP fall to marked deceleration of LVP fall. Acceleration of LVP fall in response to a load elevation is associated with normal cardiac function, while slowing of LVP fall is associated with impaired cardiac function. Similar but opposite effects are observed with reductions of systolic LVP. Effects of changes in systolic LVP on time constant tau reveal a fair correlation with systolic elastance (Ees), peak dP/dtmax and regional fractional shortening (or ejection fraction). There is an excellent correlation with measured isovolumetric LVP, indicating that contraction-relaxation coupling is close when contractility is expressed in terms of peak isovolumetric pressure. Assessment of contractility with systolic LVP-relaxation relation is precise and load independent and can be performed with the sole use of a high-fidelity pressure gauge positioned in the left ventricular cavity.

Long-term behaviour of plain and steel fibre reinforced concrete rings

The effects of long-term loading on plain (PLC) and steel fibre reinforced concrete (SFRC) rings under two-edge sustained load at five different levels were investigated. The results have led to the conclusion that a linear relationship between the initial and time-dependent deformations can be supposed. The intercept term of the regression straight line was found to be equal to the deformation caused by pure shrinkage. In spite of cracks appearing at higher loading levels the linearity was not distorted. The results indicate that an exponential function including one retardation time can be applied in practical computation of long-term deformations.

Convergence calculations for back-filled tunnels in rock salt

After the final disposal of radioactive waste in a mine, the drifts are to be closed by back filling with crushed salt. The time-dependent constitutive law for crushed salt obtained from compression tests need be employed for the description of the behavior of the back fill. The thermomechanical behavior of compacted rock salt, considering the long-term load effects, is a highly nonlinear function of time, stress, and temperature. The viscous behavior is described in general by a creep law like the Arrhenius formula. Finite-element calculations simulate open and flooded drifts at a depth of 800 m. The calculations were repeated with various pressures of the brine in the flooded drifts.

Effect of long-term loading at high temperatures on the structure and morphology of the carbide phase in a Nb?Zr?C alloy

Effect of long-term loading at high temperatures on the structure and morphology of the carbide phase in a Nb?Zr?C alloy