Gradual Load Increase Research Articles

Evaluation of load testing of postendodontic restorations in vitro: Linear compressive loading, gradual cycling loading and chewing simulation

This study introduces a modified dynamic testing mode with gradual load increase and examines whether this could be an alternative to customary methods of static loading or chewing simulation. Seventy-two extracted human maxillary root-canal-treated central incisors were randomly divided into six groups with 12 teeth each. Three groups were restored with titanium posts cemented with chemically curing resin cement. The other three groups were restored with glass-fiber posts cemented with dual-curing composite cement. All teeth were capped with full ceramic crowns. Both kinds of restoration were tested by linear compressive (static) loading, a modified gradual (cycling) dynamic loading, and by chewing simulation followed by static loading until failure occurred. The maximum load capacity was recorded. Statistical comparison showed that maximum load capacities of the same post material obtained from gradual dynamic loading did not differ significantly from that of linear compressive loading or of chewing simulation. In contrast, comparisons of different post materials under static loading resulted in significantly different load capacities. Dynamic testing with gradual load increase can be considered an economic alternative for chewing simulation, because it provides equivalent results. Both procedures, however, imply different conclusions than static loading with respect to post materials.

Effect of storage in aqueous environments on polymer–metal interfacial fracture

The effect of environmental exposure on the fracture characteristics of two polymer–metal interfaces was measured using a four-point bend delamination test. Films of high-molecular-weight polymethylmethacrylate (PMMA) resin were spin-cast on metal-coated silicon wafer substrates, for which the metal was either titanium or aluminum. Sandwich beam specimens were fabricated and stored in one of the following conditions prior to the bending tests: vacuum desiccator at 25 °C, vacuum desiccator at 65 °C, distilled water at 25 °C, or distilled water at 65 °C. Load–displacement behavior measured during bending revealed significant differences between the delamination characteristics of the two PMMA–metal interfaces after vacuum exposure which were eliminated after the degrading effects of water exposure. Three distinctive load–displacement behaviors were observed: plateau, fracture at a single load; R-curve, fracture at increasing load with crack extension; and stick-slip, cycles of gradual load increase and sudden load drop with crack extension. PMMA–Al samples stored in vacuum desiccator at 25 °C exhibited R-curve fracture and the largest average crack driving force, GC, 12.2 (±0.5) J/m2, of all samples tested. After storage in 25 °C water, these PMMA–Al samples exhibited stick-slip fracture and GC decreased to 7.1 (±2.6) J/m2; storage in 65 °C water further decreased GC to 2.1 (±0.7) J/m2. PMMA–Ti samples exhibited stick-slip fracture after storage in vacuum desiccator at 25 °C, with an average GC of 8.0 (±2.6) J/m2; storage in 65 °C water resulted in a transition to plateau fracture and a decrease in GC to 1.6 (±0.3) J/m2. The initial difference and subsequent similarity are interpreted in terms of surface roughness and hydrolysis, respectively.

A biomechanical study on fixation stability with twin hook or lag screw in artificial cancellous bone

The twin hook has been developed as an alternative to the conventional lag screw to be combined with a barrelled side-plate in the treatment of trochanteric hip fractures. With two oppositely directed apical hooks introduced into the subchondral bone of the femoral head, the twin hook provides different stabilising properties to the lag screw. The femoral head purchase of the twin hook and the lag screw were compared in a biomechanical study using artificial cancellous bone, and responses to axial and torsional loading was determined. A distinct yield point in load and torque was noted for the lag screw, representing failure of the laminas supporting the threads. For the twin hook, gradual increase of load and torque occurred during impaction of the bone supporting the hooks. The peak loads and torques were higher for the lag screw, but were similar for both devices after 8 mm deformation. The stiffness was higher for the lag screw, but in counter-clockwise rotation the stiffness for the lag screw was negligible. The twin hook appeared to provide fixation stability comparable to that offered by the lag screw, but with conceivable advantages in terms of a deformation response involving bone impaction and gradually increasing stability.

マウスプロテクターが競技者の運動能力に及ぼす影響 第２報 呼吸機能への影響について

Custom-made mouth protectors were inserted into the mouth of seven baseball players, and the effects on respiratory function while playing baseball were studied. Actually, exhalation gas was gathered and analyzed using a gradual load increase method.The results were as follows:1. Heart rate was not influenced at all, even though mouth protectors were inserted.2. Ventilation with mouth protectors was less than without them, but with no significant differences except at 140 and 160 m/min treadmill speed.3. Vo2between 140 and 220 m/min of treadmill speed with mouth protectors and V02per weight were significantly larger than without them.4. Ventilation per Vo2between 140 and 220 m/min of treadmill speed with mouth protectors was significantly lower than without them.5. Oxygen pulses without mouth protectors increased remarkably between 200 and 220 m/min, while with mouth protectors they increased earlier, between 160 and 180 m/min.6. No large differences were found in Vco2, between with and without mouth protectors.7. Mouth protector can be used in the mouth without athletic impairment.

In-situ crack propagation in pressureless sintered fiber reinforced composites

Crack propagation in sintered SiC fiber reinforced alumina matrix composites was studied in situ inside a scanning electron microscope. Both crack front and crack wake toughening mechanisms were studied in isolation. In the crack front study, the crack propagation behavior varied with fiber coatings. For the uncoated fibers, a three-fold load increase was necessary to advance the crack over the fibers. A gradual load increase was necessary to propagate the crack over the fibers that were initially polymer coated. In samples where the fibers were either metal or polymer/metal coated, the crack becomes unstable before reaching the fiber positions and the fibers had no effect in resisting the crack propagation. However, the metal coated samples reached the highest toughness values during the crack wake study, with an increase in toughness of 75% over the monolithic samples. Crack profiles were measured and the crack opening displacements revealed direct evidence of the crack fiber interactions.

Dynamic activity in medium-scale ice indentation tests

Analysis of a series of medium-scale ice indentation experiments at Pond Inlet and Hobson's Choice Ice Island is presented. Load-time and pressure-time histories for the indentations were characterized by “sawtooth” dynamic response. Alternate ice crushing and extrusion of pulverized ice products from the impact zone is suggested as the primary mechanism for the observed dynamics. Both fast and slow extrusion processes likely occur. Fast extrusion corresponds to the rapid ejection of crushed ice at the peak failure load, while slow extrusion takes place during the gradual load increase prior to failure. This analysis concerns simulation of the slow extrusion component of the dynamic cycle using two models that treat crushed ice as a linear viscous material. Both models require, as input, estimates of crushed ice layer thickness, viscosity and contact area. Values of these parameters that resulted in the “best fit” to both the total load and center pressure were developed within ranges specified by previous research and observations of the impact zone following the tests. The results suggest that the models are a useful approximation to the slow extrusion component of the observed dynamics.