Hydrostatic stress

About: Hydrostatic stress is a research topic. Over the lifetime, 1568 publications have been published within this topic receiving 37773 citations.

Fatigue crack propagation in biaxial stress fields

Abstract: Biaxial tension-compression fatigue tests were conducted with cruciform shaped specimens in a closed-loop servo hydraulic testing machine. The effects of static and cyclic non-singular stresses acting parallel to the crack plane on the crack growth rate are discussed based on the experimental observations of crack opening behaviour and fractography. Those non-singular stresses did affect the growth rate significantly under certain conditions. The range of crack-tip opening displacement was found to be a better parameter in correlating the growth rate than the stress intensity range or its effective range. The rate tended to increase with increasing non-singular stress which is correlated to the opening displacement range. This tendency was explained by the shift of fracture mechanisms to a more brittle type due to a higher elevation of hydrostatic stress near the crack tip for the case of a larger non-singular stress term.

Development of a model for the simulation of orthodontic load on lower first premolars using the finite element method.

Abstract: This study was undertaken to calculate the stress in the tooth, surrounding periodontal ligament, and in the alveolar bone when a lower first premolar is subjected to intrusion or torque movement using a constant moment. Root resorptions occur even when very low forces and moments are used in orthodontic therapy. It is therefore of great interest to determine and measure the stress that occurs under particular treatment conditions in the periodontal ligament. In this study, three finite element calculations were carried out with a realistic 3D model developed by CT data that consisted of a lower premolar, the surrounding periodontal ligament and alveolar bone. In close reference to the in-vivo experiments carried out by Faltin et al. [3, 5, 6] in Sao Paulo, Brazil, our model was subjected to an intrusive force on the premolar of 0.5 N and a lingual root torque of 3 Nmm. The three main stress directions and hydrostatic stress were quantified in all the surrounding tissues, revealing that the hydrostatic stress profile in the periodontal ligament correlated closely with resorption findings in Faltin et al.’s patients. Resorption occurred in the experimental study in Brazil when the hydrostatic stress exceeded capillary blood pressure in the periodontal ligament. We maintain that hydrostatic stress represents a suitable indicator for potential root resorptions caused by higher forces and moments, making it a helpful tool in the development of new orthodontic appliances. We must of course mention that there are many factors other than forces that are responsible for resorptions. But at the moment, only the force can be influenced by the orthodontist.

Brittle versus ductile deformation as the main control on the transport properties of low-porosity anhydrite rocks

Abstract: [1] An experimental approach has been taken to study the prefailure fluid flow properties of borehole samples of low-porosity anhydrites and the evolution of permeability (k) during deformation leading to brittle and ductile failure. The permeability measured under hydrostatic stress conditions prior to loading at room temperature is generally low (k = 10−21–10−19 m2) and displays an anisotropy and pressure sensitivity controlled by grain size and fabric orientation (foliation). Triaxial loading test results show that the brittle-ductile transition occurs for effective pressure Pe < 20 MPa and is almost independent of fabric orientation and grain size. All samples, whether deforming in a brittle (localized deformation) or ductile (distributed deformation) mode, show dilatancy after an initial phase of compaction. During loading, the k starts to increase prior to the phase of sample dilation and before the yield stress is attained. The k rise is characterized by an upward concave trend, prior to localized deformation (brittle failure), and by a downward concave trend, during distributed deformation (ductile failure). The k increase prior to brittle failure is about 1 order of magnitude higher than during ductile failure. We interpret the different shape of the k curve as due to the observed different degrees of fracture connectivity (widespread development of intragranular and intergranular fractures) reached during brittle (low) and ductile (high) deformation, respectively. Our experimental results imply that for low-porosity rocks the mode of failure, controlled by Pe, has an overwhelming effect on the evolution of permeability, compared to other factors such as grain size and fabric orientation.

Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

Abstract: The true stress-true strain behavior of polyoxymethylene, n(-CH2O), as an example of a bulk semi-crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure. In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure-induced increase in the β-transition temperature which points to the potential usefulness of the concept of pressure-temperature super-position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb-Navier criterion, are not valid for general deformation (non-plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi-crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.