Velocity Dependence Of Strength Research Articles

Velocity dependence of eccentric strength in young and old men: the need for speed!

Older adults better maintain eccentric strength relative to isometric strength, as indicated by a higher ratio of eccentric:isometric torque as compared with younger adults. The effect of increasing angular velocities (>200°/s) on the age-related maintenance of eccentric strength has not been tested and thus it is unknown whether the eccentric:isometric ratio is velocity dependent in old age. The purpose of this study was to investigate eccentric strength of the ankle dorsiflexors over a large range of lengthening angular velocities in young and older men. Isometric neuromuscular properties were assessed on a HUMAC NORM dynamometer. Nine young (∼24 years) and 9 older (∼76 years) healthy men performed maximal voluntary eccentric contractions at angular velocities of 15-360°/s. Despite near full voluntary activation (>95%), the older men were ∼30% weaker than the young men for isometric strength (P < 0.05). Across all lengthening velocities, older men had a greater eccentric:isometric ratio than young men (P < 0.05). Additionally, there was a velocity dependence of strength in both young and older men: eccentric strength increased as velocity increased up to 120°/s (P < 0.05) and plateaued thereafter. In young and older men, eccentric strength at 15°/s was ∼20% and ∼40% greater than isometric strength (P < 0.05), while at 360°/s eccentric strength was ∼50% and ∼90% greater, respectively (P < 0.05). These findings indicate that with increasing angular velocity, both young and older men have considerable increases in the eccentric:isometric ratio of torque production.

Velocity dependence of strength and healing behaviour in simulated phyllosilicate-bearing fault gouge

Despite the fact that phyllosilicates are widespread in fault zones, little is known about the strength of phyllosilicate-bearing fault rocks under brittle–ductile transitional conditions. In this study, we explored the steady state strength and healing behaviour of a simulated phyllosilicate-bearing fault rock, i.e. muscovite plus halite and brine, at room temperature, normal stresses of 1–9 MPa, atmospheric fluid pressure and sliding velocities of 0.001–13 μm/s, using a rotary shear apparatus. While 100% halite and 100% muscovite samples exhibit rate-independent frictional/brittle behaviour, the strength of mixtures containing 10–50% muscovite is both normal stress and sliding velocity dependent. At low velocities (< 1 μm/s), strength increases with increasing velocity and normal stress, and a mylonitic foliation develops. This behaviour results from pressure solution in the halite grains, which accommodates frictional sliding on the phyllosilicate foliation. The pervasive muscovite foliation, which coats all halite grains, prevents significant healing. At high velocities (> 1 μm/s), velocity-weakening frictional behaviour occurs, along with the development of a structureless, intermixed, cataclastic microstructure. The steady state porosity of samples deformed in this regime increases with increasing sliding velocity. We propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution. Towards higher sliding velocities, dilatation increasingly dominates over pressure solution compaction, so that porosity increases and frictional strength decreases. During periods of zero slip, pressure solution compaction occurs, causing a significant strength increase on reshearing. Our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions, but can simply result from oscillating fault motion rates. Our healing data suggest that foliated, aseismically creeping fault segments will remain weak and aseismic, whereas segments that have slipped seismically will rapidly re-strengthen and remain in the unstable, velocity-weakening regime.

Frictional behavior of synthetic gouge-bearing faults under the operation of pressure solution

Two recent experimental studies on the frictional behavior of synthetic gouge-bearing faults under the operation of pressure solution are compared. One is triaxial shear experiments on quartz gouge at high pressure-temperature hydrothermal conditions (Kanagawa et al., 2000), and the other is rotary shear experiments on halite gouge at atmospheric pressure and room temperature in the presence of methanol-water mixtures (Bos et al., 2000). In spite of quite different experimental settings and conditions, the results of these two series of experiments are strikingly similar; both cataclasis and pressure solution being active during the experiments, gouge strength rate-controlled by cataclasis, two different frictional behaviors of slip hardening and softening, slip hardening associated with gouge compaction, distributed deformation and wall-rock failure, slip softening associated with localized slip along the gouge-wall-rock interface, and the transition from slip-hardening to slip-softening behavior according to decreasing rate of pressure solution. Although there is a difference in velocity dependence of strength between quartz and halite gouges, these similarities clearly demonstrate the important effects of pressure solution on the frictional behavior of gouge-bearing faults.