Papers by Keyword: Strength Degradation

Abstract: Bilinear hysteretic models are an attractive choice in modeling the elasto-plastic behavior of various materials to alternate loads, due to their simplicity and ease of interpreting the results. However, choosing the parameters of the model without taking into account some restrictions can lead to highly unrealistic results. These restrictions are analyzed, and the allowable ranges for the model parameters are determined. The conclusions can be used for a wide range of plastic and composite materials. The significant errors that can occur in the case of the inadequate selection of model parameters are illustrated by a practical application.

Abstract: How to establish the proper rheological model to describe and simulate the relationship between rock mechanic characteristics and time is one of the difficulties in the tunnel long-term stability analysis. In this paper, a numerical model to replicate the time-dependent deformation of rock mass was presented. In the model, the time-dependent deformation is described in terms of degradation of intrinsic physical and mechanical properties of rock and accumulation of mesoscopic damage inside the rock. Based on the model, uniaxial numerical experimentation and tunnel numerical model test are numerically constructed and investigated respectively. The model well reflects the initial creep, steady creep of rock and accelerated creep, which preliminary prove the validity of the proposed model. The results of the tunnel numerical model test show that the displacement curves from the numerical simulation were generally consistent with those from physical model tests. Furthermore, the macroscopic failure modes and local mesoscopic damage evolution of the tunnels were simulated.

Abstract: Stress-strength interference model has been widely used for reliability analysis of structures.Strength degradation of a structure is a common phenomenon in engineering. To evaluate the structure reliability in the case of strength degradation over time, the strength degradation is represented by a gamma process and the stress applied on a structure is represented by the Poisson process in this paper. Then a time-dependent stress-strength interference model is suggested by employing numerical integration method. With this model, the structure reliability can be evaluated more accurately.

Abstract: Reliable assessment procedures of existing RC buildings are currently available, and have been introduced in the Italian and European codes reporting new rules for seismic analysis. However, further studies are required in order to further improve such procedures and, specifically, obtain more accurate data on the behavior of beam-column joints, whose role on the global behavior of framed RC buildings can be crucial. Until now studies on this issue have been mainly devoted to joint specimens with stiff beams, however frame structures having wide (also called flexible or flat) beams are widely used in the European residential building stock. To this purpose, given the lack of knowledge, an experimental investigation on full scale beam-column joints with wide beam has been planned and is currently in progress. In the present paper the main results of three tests are reported and discussed. The role of different earthquake resistant design levels on joint performances is pointed out.

Abstract: Under cyclic loading, the material weakening processes in structural members inevitably involve multiple cracking originating from some of the spatially-distributed initial flaws and imperfections, and hence diverse cracking behaviors can be expected. It is known from previous studies on multiple cracks that, the cracking behavior in a structural member can abruptly change as a crack or a number of cracks reach a critical value of crack propagation, causing sudden strength degradation. In this study, by applying sequential loads at different locations of the same FE model of a notched beam, it is shown that this unique strength degradation mechanism can repeatedly occur as cracks propagate under sequential loads, leading to multistage strength degradation of the member. This result is in line with early experimental findings that the load-carrying capacity of a notched concrete beam under bending decreases in a similar fashion as the sizes of multiple initial notches are arbitrarily increased. This study has important implications for understanding the fundamental fatigue mechanisms of various engineering materials.

Abstract: Concrete filled steel tubular (CFST) column-beam joint with the column tube discontinuous in joint zone is a new type of joint. Firstly, the results of the cyclic loading experiments on this joint were introduced briefly. Then numerical simulation of the experiment process was carried out by OPENSEES, and the load-displacement curves, strength degradation, stiffness degradation of the specimens were studied. The analysis results exhibit good agreement with the experimental ones. OPENSEES can simulate two types of failure, including joint failure and beam failure. Based on specimen S3, parametric analysis were conducted to research the strength degradation and stiffness degradation in models with different reinforcement ratios and reinforcement diameters. The results indicate that models with higher reinforcement ratio of the beams will result in joint failure under cyclic reverse loading, and their load-displacement curves are pinched and ultimate bearing capacity would not increase while strength degradation and stiffness degradation would appear obviously.

Abstract: Casing damage is a very severe problem in steam-injection well due to thermal stress. In this paper, the computational model of casing-cement-formation coupling system is established. Considering strength degradation of casing, the temperature field and stress field are coupled to calculate casing stress. Related factors, such as concentric casing, cement thickness, casing specification and internal pressure, are considered to analyze casing safety. The results show that yield strength of N80 casing reduces as a linear function with temperature variation. There is little effect of cement thickness and casing specification on the thermal stress. Von Mises stress decreases with increase of steam injection pressure in wellbore as the casing doesn't yield. Based on the results, some measures against casing damage are presented.

Abstract: Failure rate functions were usually constructed in terms of life distribution. The present paper defines “discrete failure rate”, analyzes the dependence of failure rate curve shape on the probabilistic property of load-strength interaction, and develops load-strength-interaction based system failure rate model. For the majority of mechanical equipment, operation load is normally a random process, components are subjected to a large number of stochastic load actions during its service life. Meanwhile, material property degrades continuously owing to the failure mechanisms such as fatigue. It is the ever changing load-strength relationship that yields the variation in failure rate. Based on the criterion that failure occurs when a load higher than the relevant strength appears, failure rate model is developed based on the interaction mechanism between load and residual strength, and their statistical characteristics as well. Based on the interpretation to the variation of failure rate in terms of load uncertainty and strength uncertainty, the effect of statistical risk of load and the effect of strength degradation on failure rate curve shape are highlighted.

Abstract: Three specimens with ribbed stainless steel rebar and one specimen with ribbed ordinary steel rebar are tested concerning the strength degradation and energy dissipation of stainless steel reinforced concrete columns. The tests results indicate that the damage of the specimens exhibit ductile failure characteristics, and the reinforced concrete columns with stainless steel rebar damage to a lesser extent, appear good ductility and energy dissipation. The strength degradation of stainless steel reinforced column with high axial compression ratio is quite obvious, and with the increasing of the stirrup ratio of column with stainless steel rebar, the strength of column is enhanced.

Abstract: In nonlinear dynamic analyses of RC structures based on fiber-based discretization of member cross-sections, the constitutive model used to represent the cyclic behavior of reinforcing steel typically plays a significant role in controlling the structural response especially for nonductile systems. The accuracy of a fiber-section model is almost entirely dependent on the ability of both the concrete and reinforcing steel constitutive material models to represent the overall inelastic behavior of the member. This paper describes observations related to the fundamental properties of reinforcing steel such as buckling, hardening, diminishing yield plateau and growth of curvature, Bauschinger effect, and low-cycle fatigue and strength degradation that are relevant to the overall task of developing an accurate material model for use in seismic response analysis of reinforced concrete structures.