Magnitude Of Tensile Stress Research Articles

The effect of stiffness on the localization of tensile stress at the surface of bonded posterior restorations

Objectives. The aim of this study is to assess the distribution of tensile stress under the conditions representative of tooth–food contact in a restored posterior tooth. The ideal stiffness for a bonded restoration will be predicted. Methods. A two-dimensional plane–strain finite element representation of a restored first maxillary molar is created. The restoration with Class I/II bucco-lingual geometry is assigned a range of Young's moduli ( E=10–80 GPa), representative of the range of materials available. In addition, a hypothetical multi-phase model, in which stiffness is gradually increased from the intercuspal concavity to the adjacent enamel, is also created. A food particle is modeled and pushed close to the interface onto the site of (1) intact enamel and (2) restored surface. Results. For all single-phase models tested, the magnitude of tensile stress at the surface is far greater than that present elsewhere in the tooth. However, the exact position and magnitude of the maximum tensile stress varied according to the modulus assigned to the restoration and the position of load. The results of the model imply that interfacial problems are likely in low-modulus restorations ( E=10–20 GPa), whereas stress-related problems could occur in the intercuspal concavity of high-modulus restorations ( E=40–80 GPa). Of all the single-phase models, tensile stresses are lowest when the Young's modulus assigned to the restoration is 30 GPa. These tensile stresses are reduced in the multi-phase model. Significance. Given the limitations of the model, the results indicate that the most suitable modulus for a single-phase posterior bonded restoration is around 30 GPa. Such a modulus is approached in compact-filled composites. In addition, the highly desirable dissipation of load in the multi-phase model should warrant further investigation. It is suggested that the use of an incremental filling technique with region-specific proportions of hard-filler could be one way forward.

The fragmentation of wires carrying electric current

Exploding wires are widely used in pulsed power systems. However, many aspects of the wire explosion remain unclear. If the electric current density is not too high, the wires may disintegrate in the solid state without signs of significant melting. The experiments show that the wires break in tension due to some longitudinal force, the nature of this force being unknown. Simple estimates made in the past by other authors showed that neither the pinching effect nor thermal expansion was responsible for the disintegration of the wire, since the tension produced was too low to extrude the wires. A search for a longitudinal force had even led to a certain controversy in electrodynamics. In this investigation we employ the equations of magneto-thermo-elasticity to study stress waves induced in metal wires by a high pulsed current, when the current increases from zero to a constant value in a step-function manner. Two main aspects are studied, namely (i) the possible amplification of stress waves induced by the electromagnetic pinch force and (ii) the dynamic stress induced by the thermal expansion. It is shown that, for wires with free ends, the magnitude of tensile stress produced by the thermal expansion may well exceed the ultimate strength of the material.

Stress-induced anisotropic etching in polymer track detectors

CR-39 track detectors irradiated with alpha particles were chemically etched while under mechanical stress. Depending on tensile stress magnitude the response of track size and shape was observed to form two distinct domains: in the first domain sizes of track minor and major axis both increase slightly with increasing stress while in the second domain tracks show new shape due to a triangular extensions perpendicular to the stress direction. A simple model is proposed which explains the new etched track shape. With increasing compressive stress both minor and major axis sizes decrease but no change in chape occurs.

Testing mode and surface treatment effects on dentin bonding.

The goal of this project was to evaluate the effect of the following variables on shear dentin-bonding test results: mode of testing (cyclic fatigue versus static loading), surface treatments (32% phosphoric acid, 10% phosphoric acid, and no treatment [unetched]), and type of shear test (traditional planar versus push-out). All teeth were stored in distilled water and tested in a shear mode at a loading rate of 2 mm/ min. The specimens were loaded in static or cycled for 1000 cycles using a staircase approach or until fracture, whichever occurred first. On samples with etched dentin surfaces, the push-out test did not demonstrate a significant difference in measured bond strength when compared with results from the planar test, although sample preparation was more labor-intensive. The bond strength resulting from cyclic fatigue of the etched specimens was approximately 51% of the static loading value. Ten percent phosphoric acid was as effective as 32% phosphoric acid for dentin bonding. Finite-element analysis indicated that the traditional planar shear test produces flexure of the specimen and high tensile stress magnitudes within the resin bonding layer. The push-out test produces elevated compressive stresses localized in the composite along the circumference of the punch. Shear stresses in the resin bonding layer are similar for both testing methods at the same loading element contact force.

Testing mode and surface treatment effects on dentin bonding

The goal of this project was to evaluate the effect of the following variables on shear dentin-bonding test results: mode of testing (cyclic fatigue versus static loading), surface treatments (32% phosphoric acid, 10% phosphoric acid, and no treatment [unetched]), and type of shear test (traditional planar versus push-out). All teeth were stored in distilled water and tested in a shear mode at a loading rate of 2 mm/min. The specimens were loaded in static or cycled for 1000 cycles using a staircase approach or until fracture, whichever occurred first. On samples with etched dentin surfaces, the push-out test did not demonstrate a significant difference in measured bond strength when compared with results from the planar test, although sample preparation was more labor-intensive. The bond strength resulting from cyclic fatigue of the etched specimens was approximately 51% of the static loading value. Ten percent phosphoric acid was as effective as 32% phosphoric acid for dentin bonding. Finite-element analysis indicated that the traditional planar shear test produces flexure of the specimen and high tensile stress magnitudes within the resin bonding layer. The push-out test produces elevated compressive stresses localized in the composite along the circumference of the punch. Shear stresses in the resin bonding layer are similar for both testing methods at the same loading element contact force. © 1996 John Wiley & Sons, Inc.

Dynamic tensile‐failure‐induced velocity deficits in rock

Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock discs were impacted with aluminum and polymethyl methacralate (PMMA) flyer plates at velocities of 10 to 25 m/s. Tensile stress magnitudes and duration were chosen so as to induce a range of microcrack growth insufficient to cause complete spalling of the samples. Ultrasonic P‐ and S‐wave velocities of recovered targets were compared to the velocities prior to impact. Velocity reduction, and by inference microcrack production, occurred in samples subjected to stresses above 35 MPa in the 1.3 μs PMMA experiments and 60 MPa in the 0.5 μs aluminum experiments. Using a simple model for the time‐dependent stress‐intensity factor at the tips of existing flaws, apparent fracture toughnesses of 2.4 and 2.5 MPa m½ are computed for the 1.3 and 0.5 μs experiments. These are a factor of ∼ 2 to 3 greater than quasi‐static values. The greater dynamic fracture toughness observed may result from microcrack interaction during tensile failure. Data for water‐saturated and dry targets are indistinguishable.

Penetration of soil aggregates of finite size. III. wetting-drying and aggregate confinement effects on the penetrometer pressure

Aggregates in the size range 2–29 mm were prepared from a loam soil in the laboratory. Half of the air-dry aggregates were coated with a thin layer of plaster, to provide a partial confinement to aggregates. Five wetting and drying treatments were imposed upon the aggregates. All aggregates were wetted from air-dry to −2 kPa or to saturation and dried to −2 kPa (T 1 and T 2) or to −50 kPa (T 3, T 4 and T 5) with varying levels of intermediate drying. Maximum penetrometer pressure ( P max) of individual aggregates was measured in the laboratory with a 1-mm diameter steel probe of cone angle 60°, driven at 3mm min −1. P max increased with increased size of aggregate, aggregate confinement and decrease in the water potential to which aggregates were dried. Radial and tangential stresses adjacent to and away from the probe were estimated using a previously developed theory. Tensile stress associated with the probe penetration of aggregates decreased with increased size of aggregates. The magnitude of tensile stress was greater in dry aggregates. (T 3, T 4 and T 5) than in wet aggregates (T 1 and T 2).

Influence of residual welding stresses on fatigue crack growth under compressive loading

Fatigue cracks have been grown in large as-welded BS4360 50 D structural steel specimens cut from massive plates. Notches and fatigue cracks were made to gape open by tensile residual stresses which rearranged and remained high perpendicular to the discontinuity tips. When specimens were cycled under applied compression crack growth rates were observed which were up to 90% as fast as those produced in stress relieved control samples under pulsating tension. Crack deceleration and arrest was associated with the proximity of the neutral axis and a reduction in residual tensile stress magnitude ahead of the crack tip which led to general crack closure under applied load.