Various States Of Stress Research Articles

INTERFACIAL BOND STRENGTH BETWEEN MICRO-SYNTHETIC FIBER-REINFORCED PATCH REPAIR MORTAR AND CONCRETE SUBSTRATE BASED ON PUSH-OFF TEST

A micro synthetic fiber-reinforced mortar has been developed as a patch repair material. The bond strength of this material is a crucial parameter for successful repair applications. The state of stress occurring at the interfacial plane between the repair material and the concrete substrate influences the bond strength requirements of patch repair material. Therefore, bond strength envelope criteria should be established using, among others, the interfacial adhesion and friction coefficient parameters. This research aims to determine the composite’s interfacial bond strength parameters (adhesion and friction coefficient) between micro-synthetic fiber-reinforced patch repair mortar (MSFRPRM) and concrete substrate by push-off tests. At this stage, the smooth joint texture limits the investigation. The fiber volume fraction and age of repair are considered. The tests were conducted at the age of 1, 3, 7, and 28 days. In addition to the laboratory experiment, this research used ATENA Science GiD to numerically simulate the push-off test and study the influence of the fiber volume on the adhesion and friction coefficient. The results show that at a fiber volume of 0.06%, the adhesion and friction coefficients increase from 1.15 to 2.33 MPa and 0.74 to 0.85, respectively, corresponding to age. Numerical simulation indicates that the maximum adhesion and friction coefficient occurs at the fiber volume fraction of 0.06%. Thus, the overall simulation results can be used in the future as a parameter to establish MSFRPRM and concrete substrate bond strength envelopes that will be useful for evaluating the suitability of MSFRPRM as repair materials under various states of stress. Keywords: Bond Strength, Concrete Patch Repair, Concrete Substrate, Micro-Synthetic Fiber-Reinforced Repair Mortar, Push-Off Test DOI： https://doi.org/10.35741/issn.0258-2724.58.1.20

On the photoelastic constants for anisotropic stressed crystals

Abstract The need for high-quality and high-performance crystals for high-energy physics (e.g. scintillators) and biomedical applications require a good knowledge of their mechanical and optical properties, amongst the others. This is a mandatory step in many quality control process aimed at the improvement of the technological processes for crystals growth. Following the first analysis already done for Tetragonal crystals, in this paper we study the elasto-optic behavior of optically biaxial (Monoclinic and Orthorhombic groups) and uniaxial crystals (Trigonal and Hexagonal groups). We evaluate the photoelastic constants, the optic angle and the optic plane associated to various states of stress, in terms of the components of the Piezo-Optic tensor.

Strength Testing of Sandstone Under Multi-Axial Stress States

A true triaxial apparatus was designed and fabricated to achieve multi-axial stress states with prismatic specimens. The device, which is a modification of the University of Minnesota plane-strain apparatus, is equipped with piston assemblies that generate the intermediate principal stress. A closed-loop, servo-hydraulic load frame is used to control the major principal stress, while the minor principal stress is developed by fluid pressure. A stress path was developed to perform experiments under various states of stress at constant mean stress. Results from strength testing on Dunnville sandstone under conventional compression and extension are compared with the results performed under multi-axial stress states. Failure data were plotted in principal stress space to investigate the effect of intermediate stress on strength. A plane fitting method was used to obtain the material parameters of two, six-sided Paul–Mohr–Coulomb failure surfaces, which capture the strength characteristics of the sandstone over a range of mean stress.

CO2 and CH4 sorption properties of granular coal briquettes under in situ states

The aim of this work was an experimental research into the sorption properties of coal briquettes under the various states of stress. The present state of knowledge when it comes to the impact of stress on the CO2 and CH4 sorption properties of coal is limited. Hence, in situ sorption capacity or sorption-induced deformation may vary considerably from these specified in the unloaded conditions. As part of this paper, a research device that enables the sorption measurements for coal samples submitted to loads of up to 40 MPa is presented. Sorption experiments with pure CO2 and pure CH4 were carried out by means of the volumetric method, under isobaric-isothermal conditions. Dry coal material in the form of granular coal briquettes was used. Presented results aimed to prove the existence of a relation between the stress state, sorption capacity and volumetric deformation of coal. Gas sorption and desorption processes on coal may be triggered by the action of mechanical factor. The mechanical load applied to the coal sorbent reduces its sorption capacity but has no direct effect on swelling. The sorption or desorption induced volumetric changes of coal occur regardless of the stress state and regardless of the mechanism of the sorption process itself. Deeper coal seams with reduced CO2 and CH4 sorption capacity may not be as useful for ECBM techniques as the shallower coal seams. Reduction of the sorption capacity with depth may also explain the growing threat of coal and methane outbursts in the USCB mines observed in recent years.

On the photoelastic constants and the Brewster law for stressed tetragonal crystals

An exact knowledge of the mechanical and optical properties of crystals allows not only for theoretical advances, but it is also a useful tool to asses crystal quality in the technological processes of growth and production of advanced crystals like, eg, scintillators. In this paper, we study the elasto‐optic behavior of tetragonal crystals to evaluate the photoelastic constants, associated to various states of stress, in terms of the components of the piezo‐optic tensor. Moreover, we arrive at a generalization, for tetragonal crystals, of the Brewster law for optically isotropic materials.

Discrete Model for the Structure and Strength of Cementitious Materials

Cementitious materials are characterized by brittle behavior in direct tension and by transverse dilatation (due to microcracking) under compression. Microcracking causes increasingly larger transverse strains and a phenomenological Poisson’s ratio that gradually increases to about $$\nu =0.5$$ and beyond, at the limit point in compression. This behavior is due to the underlying structure of cementitious pastes which is simulated here with a discrete physical model. The computational model is generic, assembled from a statistically generated, continuous network of flaky dendrites consisting of cement hydrates that emanate from partially hydrated cement grains. In the actual amorphous material, the dendrites constitute the solid phase of the cement gel and interconnect to provide the strength and stiffness against load. The idealized dendrite solid is loaded in compression and tension to compute values for strength and Poisson’s effects. Parametric studies are conducted, to calibrate the statistical parameters of the discrete model with the physical and mechanical characteristics of the material, so that the familiar experimental trends may be reproduced. The model provides a framework for the study of the mechanical behavior of the material under various states of stress and strain and can be used to model the effects of additives (e.g., fibers) that may be explicitly simulated in the discrete structure.

Micromechanics-based modeling of stress–strain and fracture behavior of heat-treated boron steels for hot stamping process

In the automotive industry, hot stamped parts with tailored properties have shown advantageous safety performance. Such components are produced by applying different heat treatment conditions after forming for different zones in order to obtain various combinations of hard and soft microstructures. In this work, pure martensitic, pure bainitic, and three martensitic/bainitic phase microstructures were initially generated from the boron steel grade 22MnB5 by a two-step quenching procedure in which different holding times in the bainitic temperature range were varied. Increased phase fraction of bainite due to longer holding time led to decreased yield and tensile strength; however, elongation and resulting energy absorbability became significantly higher. To describe mechanical properties and failure behavior of hot stamped parts containing multiphase microstructures, influences of microstructure characteristics should be considered on the micro-scale. Using modeling, 2-D representative volume elements (RVE) were generated from observed real microstructures and flow curves of the individual single phases were defined, taking into account a dislocation theory based model and local chemical compositions. Then, effective stress–strain curves of the heat-treated boron steels were calculated by using the isostrain and non-isostrain methods and compared with tensile test results. Regarding fracture behavior, damage curves of fully martensitic and bainitic structures were determined by means of tensile tests of different notched samples and a hybrid digital image correlation (DIC)–finite element (FE) approach. 2-D RVE simulations of a martensite/bainite mixture were carried out under various states of stress, in which the obtained damage curves were individually applied for each phase. The predicted damage curve from RVE simulations for two-phase boron steel fairly agreed with experimental fracture strains. Moreover, correspondingly normalized Lode angle could be calculated, and the 3-D damage locus was also constructed.

The Second Blind Sandia Fracture Challenge: improved MBW model predictions for different strain rates

Sandia National Laboratories have carried out the Sandia Fracture Challenge in order to evaluate ductile damage mechanics models under conditions which are similar to those in the industrial practice. In this challenge, the prediction of load-deformation behavior and crack path of a sample that is designed for the competition under two loading rates is required with given data: the material Ti–6Al–4V, and raw data of tensile tests and V-notch tests under two loading rates. Within the stipulated time frame 14 teams from USA and Europe gave their predictions to the organizer. In this work, the approach applied by Team Aachen is presented in detail. The modified Bai–Wierzbicki (MBW) model is used in the framework of the Second Blind Sandia Fracture Challenge (SFC2). The model is made up by a stress-state dependent plasticity core that is extended to cope with strain rate and temperature effects under adiabatic conditions. It belongs to the group of coupled phenomenological ductile damage mechanics models, but it assumes a strain threshold value for the instant of ductile damage initiation. The initial guess of material parameters for the selected material Ti–6Al–4V was taken from an in-house database available at the authors’ institutes, but parameters are optimized in order to meet the validation data provided. This paper reveals that the model predictions can be improved significantly compared to the original submission of results at the end of SFC2 by two simple measures. On the one hand, the function to express the critical damage as well as the amount of energy dissipation between ductile damage initiation and complete ductile fracture were derived more carefully from the data provided by the challenge’s organizer. On the other hand, the experimental set-up of the challenge experiment was better described in the geometrical representation used for the numerical simulations. These two simple modifications allowed for a precise prediction of crack path and estimation of force–displacement behavior. The improved results show the general ability of the MBW model to predict the strain rate sensitivity of ductile fracture at various states of stress.

Determination of 3D Ductile Failure Criteria for Advanced High Strength Steel Sheet

In this work, 3D ductile fracture locus was determined for the advanced high strength (AHS) steel sheet grade DP780 using a hybrid approach between experiment and FE simulation. Tensile tests of different sample geometries were performed for the investigated dual phase steel, by which varying stress triaxiality (η) and lode angle (θ) values developed in the material during loading were introduced. During the tests, the direct current potential drop (DCPD) method and digital image correlation (DIC) technique were applied for identifying crack initiation on the micro-scale and fracture of the specimens due to local plastic deformation. Obtained force and displacement curves were correlated with the electric potential curves. Then, the moments of crack onset were determined for various states of stress. In parallel, the most critical areas of deformed samples before fracture were observed by the DIC method. Subsequently, FE simulations of the tensile tests were carried out and calculated local stresses and strains were gathered. The stress triaxialities, equivalent plastic strains and lode angles were evaluated for the corresponding detected areas. These threshold variables obtained from different specimens were plotted as the 3D failure locus for defining crack initiation and fracture occurrence in the DP steel.

New calibration method for high and low triaxiality and validation on SENT specimens of API X70

The determination of the exact mechanical properties of material is essential for an optimal and safe design of linepipes. It is especially important for the prevention of over-engineering and the reliable assessment of complex accidental loading, such as extreme bending due to loss of buoyancy of support, or abrupt ground movement. Currently, the focus of research in offshore deepwater installations and linepipes is towards pre-cracked structures with high triaxiality stress states and complex loading histories. At the same time, low triaxiality stress states must be correctly studied in order to represent shear dominated failure in pipes. A comprehensive experimental and numerical program was undertaken to determine the mechanical properties of the traditional API X70 grade of steel. The material was characterized for anisotropic plasticity, fracture initiation and uncracked ductility for various states of stress. The same material was also used for pre-cracked fracture toughness assessment. The experimental program included flat and round specimens. The first type of tests on flat butterfly-shaped, central hole, notched and circular disk specimens; were selected to address the low stress triaxiality range. Tests on round notched bar specimens and SENT fracture mechanics tests extended the characterization and verification process to higher stress triaxiality values. This program covered a wide range of stress conditions and demonstrated their effect on the material resistance to crack extension. Each test conducted was numerically simulated using solid finite element models, matching the exact geometric and loading history features. The numerical simulation provided information on the local stress and strain fields around the location of the potential or existing cracks. Based on the above hybrid experimental/numerical technique tailored for pipe applications, the MMC fracture model was calibrated. The model relates the material ductility not only to stress triaxiality but also to the Lode parameter. The predictive capabilities of the MMC were then evaluated in the case of SENT testing, used extensively in the pipeline industry. It was shown that the present fracture model calibration can describe fracture behavior of SENT experiments.

Fatigue life prediction of nanoparticle/fibrous polymeric composites based on the micromechanical and normalized stiffness degradation approaches

In this paper, a new model based on the micromechanical and normalized stiffness degradation approaches is established. It has been assumed that during the fatigue condition, only material properties of composites (fiber and matrix) were degraded and nanofillers remain intact under different states of stress. A normalized stiffness degradation model was proposed for laminated fibrous composites reinforced with nanoparticles to derive a novel model to predict the stiffness reduction. The developed model is capable of predicting the fatigue life of nanoparticle-filled fibrous composites based on the experimental data of fibrous composites without nanofillers. The new fatigue model is verified by applying it to different experimental data provided by different researchers. The obtained results by the new fatigue model are in very good agreement with the experimental data of nano-silica glass/epoxy composites under constant cyclic stress amplitude fatigue and also for silica/epoxy nanocomposites in various states of stress with negligible error.

Final state of soils under vacuum preloading

Vacuum preloading of clay deposits is becoming an increasingly popular ground improvement technique. Although many studies have been reported in literature, the mechanism of vacuum preloading is still not properly understood. Soil under vacuum preloading is expected to undergo an inward lateral deformation, therefore the influence of lateral deformation on volumetric strain requires further analysis. This paper addresses this aspect through laboratory studies on reconstituted samples of kaolinite. The Rowe cell apparatus was modified to measure lateral deformation under various states of stress because lateral and vertical deformation of soil depends on its state of stress and associated lateral pressure. A method for predicting the volumetric and lateral strains under vacuum consolidation is proposed and then applied to two case studies in China.

Analysis and design procedures for the flexural behavior of glass fiber-reinforced polymer composite poles

Earlier research progress on using fiber-reinforced polymer (FRP) composite poles for transmission lines revealed that the use of these poles is economically viable and industrially reliable. While this promising alternative technique is coming to be widely accepted in practice and evolving to become the future of the construction in transmission lines, the development of appropriate design guidelines that deal with detailed analysis of such poles has lagged behind. Design approaches for conventional steel/wood poles were influential in providing safety and robustness to these poles. However, little advanced approaches focused on the design of fiber-reinforced polymer poles. This is the focus of the present paper. The available design procedures of fiber-reinforced polymer poles are based on the allowable stress design theory of composite material under various states of stress. However, various experimental results show that cracking and early failure of fiber-reinforced polymer poles are normally controlled by the relative location of the opening from the base of the pole. Most of the design guidelines ignore such effect and accounts only for the effect of these holes by considering their influence on the abrupt reduction in the cross-sectional area of the poles. These guidelines do not give a specific attention to the impact of the opening on the generated stress concentrations in their vicinity. Furthermore, local buckling at a nearby area of the opening generally dominates the mode of failure of such poles that requires more attention to the relative locations of the openings. A new design approach is introduced to accurately evaluate the design equations for the flexural behavior of fiber-reinforced polymer -composite poles. The accuracy of both the developed design procedures and that of existing design approaches are verified by comparison with documented test results of an early experimental program of the authors.

THE INFLUENCE OF SHEAR SPAN RATIO ON LOAD CAPACITY OF FIBRE REINFORCED CONCRETE ELEMENTS WITH VARIOUS STEEL FIBRE VOLUMES

This paper analyses the influence of steel fibre volume and shear span ratio on the strength of fibre reinforced concrete elements in various states of stress. 36 beams with three different shear spans (a/h = 1, 1,5, and 2 %) and three different fibre volumes (1, 1,5, and 2 %) were tested to examine how these factors influence the behaviour of such elements. Test results suggest that steel fibre volume and shear span can increase load capacity, plasticity and cracking. Experimental research showed that steel fibre volume has different influence at different shear span ratios. Regression analysis of experimental data was carried out and empirical approach showing different effect of these factors was proposed. Furthermore, test results were compared with different theoretical and empirical approaches of other authors.

The influence of shear span ratio on load capacity of fibre reinforced concrete elements with various steel fibre volumes

Abstract This paper analyses the influence of steel fibre volume and shear span ratio on the strength of fibre reinforced concrete elements in various states of stress. 36 beams with three different shear spans (a/h = 1, 1,5, and 2 %) and three different fibre volumes (1, 1,5, and 2 %) were tested to examine how these factors influence the behaviour of such elements. Test results suggest that steel fibre volume and shear span can increase load capacity, plasticity and cracking. Experimental research showed that steel fibre volume has different influence at different shear span ratios. Regression analysis of experimental data was carried out and empirical approach showing different effect of these factors was proposed. Furthermore, test results were compared with different theoretical and empirical approaches of other authors.

Coupled damage and plasticity modelling in transient dynamic analysis of concrete

AbstractIn a concrete structure subjected to an explosion, for example a concrete slab, the material is subjected to various states of stress which lead to many modes of rupture. Closer to the explosive, a state of strong hydrostatic compression is observed. This state of stress produces an irreversible compaction of the material. Away from the zone of explosion, confinement decreases and the material undergoes compression with a state of stress, which is slightly triaxial. Finally, the compression wave can be reflected on a free surface and becomes a tensile wave, which by interaction with the compression wave, produces scabbing. We present, in this paper, a model aimed at describing these three failure modes. It is based on visco‐plasticity and rate dependent damage in which a homogenization method is used in order to include the variation of the material porosity due to compaction. The model predictions are compared with several experiments performed on the same concrete. Computations of split Hopkinson tests on confined concrete, a tensile test with scabbing, and an explosion on a concrete slab are presented. Copyright © 2001 John Wiley & Sons, Ltd.

Constitutive properties of hard-alpha titanium

The flow and fracture behavior of hard-alpha Ti was studied as a function of nitrogen content, stress state, strain rate, and temperature. Hard-alpha Ti specimens with nitrogen contents ranging from 2 to 11.6 wt pct were fabricated by powder-metallurgy techniques. Stress-strain curves were obtained under various states of stress by performing uniaxial compression, indirection tension, indentation, and plane-strain compression tests at two strain rates. By varying the test technique and the specimen geometry, deformation and fracture in hard-alpha Ti was characterized for mean pressures as high as 6 times the flow stress. Most of these tests were conducted at 954 °C, but some were performed at 25 °C, 927 °C, and 982 °C. The experimental results indicated that, during compression testing at 927 °C to 982 °C, hard-alpha Ti exhibited substantial plastic deformation for nitrogen contents less than 4 wt pct, but showed brittle fracture with little plastic flow for nitrogen contents of 5.5 to 11.6 wt pct. Both the yield and fracture strengths increase with increasing nitrogen content and pressure, but decrease with increasing temperature. The yield strength increases with strain rate, while the fracture strength shows little or no rate sensitivity. The fracture strength in tension is substantially lower than that in compression. These observed deformation and fracture characteristics are explained on the basis of microcrack formation during inelastic deformation.

Tensile strain capacity of concrete under various states of stress

The direct-tension-test method and the mechanical properties of concrete in compression, flexure and tension have been investigated. The mould used for preparing the specimens in the direct tension test has been modified to overcome the difficulties in centralizing and aligning the two embedded bars in the specimens. The method adopted improves the weak bond strength between the embedded bar and concrete and reduces the stress concentration at the end of the embedded bar. Concrete is most brittle under tension. The state of stress affects the failure characteristics of concrete. The tensile strain capacity, which is used to evaluate cracking in concrete, should be chosen judiciously depending on the state of stress in the concrete. The tensile strain capacity obtained from the flexure test has a higher standard deviation, indicating wider variability of the results. Relationships between tensile strength and flexural strength and between compressive strength and tensile or flexural strength are also presented in this paper. The modulus of elasticity from the tension test is similar to that obtained from the compression test for low-grade concrete, but the values obtained from the tension test are lower than those from the compression test for concrete of high grade. The values of Poisson's ratio in tension and flexure decrease when the tensile or flexural stress in the concrete is raised. The values of Poisson's ratio in compression vary insignificantly before reaching the discontinuity point, and increase sharply afterwards.

A stress integration algorithm for plane stress elastoplasticity and its applications to explicit finite element analysis of sheet metal forming processes

A stable stress integration algorithm of the elastoplastic constitutive equations for materials under a plane stress condition has been developed. In the algorithm, the trial stress obtained by elastic prediction is geometrically relaxed to the closest point on the yield surface in the principal stress plane. The direction of return-mapping is first obtained through the minimization problem using the Newton-Raphson method, and thereafter the plastic parameter is determined. As a model of yield criterion, Hill's quadratic normal anisotropic yield function has been adopted. The accuracy of the algorithm has been shown by iso-error maps for several materials. It has been revealed that errors are within 5% when the explicit time-integration method is adopted. In order to show the applicability of the algorithm, two kinds of sheet metal forming processes being able to represent various states of stress on the yield surface have been analyzed by FORMSYS-DE, the in-house explicit FEM code. The results of the simulations show good general coincidence with those of the experiments reported in the literature.

Calculations of the elastic compliance and piezoelectric constants of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) crystals

We present calculated compliance and piezoelectric constants for polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene P(VDF/TrFE). The constants were derived by minimizing the energy of the unit cell under various states of stress, using a modified molecular mechanics force field. The model was first validated against experimental data and theoretical calculations for the properties of PVDF. The behavior of the compliance and piezoelectric constants of P(VDF/TrFE) are examined as a function of the molecular percentage of VDF. It was found that an optimum value for the dielectric constant d 33 of P(VDF-TrFE) exists between 55 and 65 mol % VDF.