Improved Failure Model Research Articles

An Improved Failure Model Considering the Arching Effect for Tunnel Face Stability in a Weak and Fractured Rock Mass

An Improved Failure Model Considering the Arching Effect for Tunnel Face Stability in a Weak and Fractured Rock Mass

Numerical modelling of brittle fracture using lattice particle method with applications to fluid structure interaction problems via SPH coupling

This paper presents an improved failure model for simulating brittle fracture using the mesh-less Lattice Particle Method (LPM). By modelling the initial crack line using the Remove Bond (RB) approach as outlined in this paper, a new formulation is then developed for predicting the mode-I Stress Intensity Factor (SIF) near the crack tip. Compared to the conventional Remove Particle (RP) approach, it is found that the accuracy of the present SIF formulation based on the RB method is superior. A series of benchmark test cases are simulated to test the numerical accuracy and numerical convergence of the method. Finally, the LPM method is coupled with the Smoothed Particle Hydrodynamics (SPH) method for studying Fluid Structure Interaction (FSI) problems involving solid fracture and free surface. The coupled SPH-LPM method is implemented in the open-source code, DualSPHysics, which has been optimized for both CPU and GPU performances. Upon integrating LPM with SPH, the proposed FSI method is suitable for modelling fracture phenomena caused by natural hazards such as tsunami and flood.

Stability of Anchored Retaining Walls Under Seismic Loading Conditions to Obtain Minimal Anchor Lengths Using The Improved Failure Model

Anchored retaining walls are structures designed to support different loading applied in static and dynamic cases. The purpose of this work is to design and study the stability of an anchored retaining wall loaded with different seismic actions to obtain minimal anchor lengths. Mononobe-Okabe theory has been applied for the evaluation of seismic earth pressures developed behind the anchored wall. Checking the dynamic stability of anchored retaining walls is usually done using the classic Kranz model. To take into consideration the effects of the internal forces developed during failure, we have proposed a new model, based on the Kranz model, which will be used as the Kranz model to find the critical angle failure performed iteratively until the required horizontal anchor length is reached for a minimum safety factor. The results of this study confirm that the effect of the seismic load on the design of an anchored retaining wall, and its stability, has a considerable influence on the estimation of anchor lengths. To validate the modifications made to the new model, a numerical analysis was carried out using the Plaxis 2D software. The interpretation of the obtained results may provide more detailed explanation on the effect of seismic intensities for the design of anchored retaining walls.

Long-term evolution of spherical shell with boron carbide layer after explosive compression

Predictive simulation of the long-term response of multilayer targets with ceramics layers to shock compression demands appropriate material models. Because ceramics are complex brittle materials, which tend to lose their strength under heavy loads, such simulation requires the failure models well-proven for a wide range of strains and strain rates. Standard plate impact experiments provide the main data utilized for developing and validating the mechanical models of material response to shock compression. However, apart from the fact that such experimental data are inherently one-dimensional, they can be insufficient to verify the failure model at relatively low strain rates typical for long unloading waves. Here, we present the experimental results for explosive compression of a spherical multilayer shell initiated by a single detonator. The explosive-coated shell consists of the nested spherical layers: the outer made of boron carbide and the inner of lead. X-ray images showing the evolution of those layers after detonation are then compared with simulation results. Propagation of the compression wave through the layers resulting in ceramics damage is analyzed in detail. We demonstrate that the failure model of boron carbide should be adjusted for compressions below 10 GPa to achieve a good agreement with our experimental images. Such an improved failure model provides the predictive simulation of long-term dynamics of targets after unloading, and it has almost no effect on wave profiles after plate impact.

Explicit failure model for boron carbide ceramics under shock loading

Ceramic materials have a long-term industrial demand due to their high mechanical hardness and chemical and temperature resistance. They are brittle and tend to lose strength under heavy loads which complicates the development of a comprehensive material model for simulation of engineering prototypes containing ceramic parts. We developed an improved failure model of ceramics based on the well-known Johnson–Holmquist approach. This model redefines the damage rate equation using a consistent definition of the total plastic strain in the failed material. It reduces the number of free model parameters and enables the plastic strain to be explicitly accumulated during the failure process. The corresponding non-iterative algorithm utilizing this explicit failure model is developed. It is successfully validated by simulation of the wave profiles obtained in plate-impact experiments with boron carbide using the contact smoothed particle hydrodynamic method.

Constitutive equations of a ballistic steel alloy as a function of temperature

In the present work, dynamic tests have been performed on a new ballistic steel alloy by means of split Hopkinson pressure bars (SHPB). The impact behavior was investigated for strain rates ranging from 1000 to 2500 s −1 , and temperatures in the range from −196 to 300 ◦ C. A robotized sample device was developed for transferring the sample from the heating or cooling device to the position between the bars. Simulations of the temperature evolution and its distribution in the specimen were performed using the finite element method. Measurements with thermocouples added inside the sample were carried out in order to validate the FEM simulations. The results show that a thermal gradient is present inside the sample; the average temperature loss during the manipulation of the sample is evaluated. In a last stage, optimal material constants for different constitutive models (Johnson-Cook, Zerilli-Amstrong, Cowper-Symonds) has been computed by fitting, in a least square sense, the numerical and experimental stress-strain curves. They have been implemented in a hydrocode for validation using a simple impact problem: an adapted projectile geometry with a truncated nose (.50 calibre fragment simulating projectiles) was fired directly against an armor plate. The parameters of the selected strength and failure models were determined. There is a good correspondence between the experimental and computed results. Nevertheless, an improved failure model is necessary to get satisfactory computed residual projectile velocities.

Sensitivity Analysis of a Thermo-Structural Model for Materials in Fire

A new thermo-structural model was developed and validated to predict the failure of compressively loaded fiber-reinforced polymer (FRP) laminates during one-sided heating from a fire. The model consists of the best thermal and structural models in the literature integrated into a single predictive model. This includes a one-dimensional pyrolysis model to predict the thermal response of a decomposing material. Using the thermal response to calculate the mechanical properties, an integral structural model was developed considering thermally induced bending caused by one-sided heating. The thermo-structural model predicts out-of-plane deflections and compressive failure of laminates in fire conditions. This paper also provides an improved failure model for FRP laminates exposed to fire, a first validation study on the modeling approach using intermediate-scale compression load failure tests with a one-side heat flux exposure, and a first sensitivity study of the input parameter effects on the structural response of FRP laminates. Through the sensitivity study, the out-of-plane deflection predictions exhibited little sensitivity to the thermal inputs. However, the time-to-failure predictions were significantly affected by the virgin conductivity and specific heat capacity. The structural inputs exhibited a significant impact on the out-of-plane deflection predictions. The in-plane thermal expansion, residual elastic modulus above the glass transition temperature, and vertical temperature profile significantly affected the magnitude of the out-of-plane deflection; however, only the in-plane thermal expansion and residual elastic modulus affected the failure direction. The time-to-failure prediction was only significantly affected by the residual elastic modulus. A better agreement between the predicted and observed times-to-failure was achieved by reducing the residual elastic modulus.

Experimental and Numerical Investigations on the Origins of the Bodywork Effect (K-Effect)

The integration of a high-hardness steel armor plate inside the bodywork of a vehicle may result in a decrease in the overall ballistic resistance. This phenomenon is referred to as the bodywork effect. The effect was examined for a 5.56×45 mm North Atlantic Treaty Organization (NATO) Ball projectile. Previously reported experimental work has confirmed the numerically based assumption that the bodywork effect was due to the flattening of the tip of the projectile upon perforation of the frontal bodywork plate prior to hitting the integrated armor. The amount of qualitative and quantitative experimental data has now been extended. In order to eliminate the data dispersion observed after perforating the bodywork, an adapted projectile geometry with a truncated nose was fired directly against the armor plate. Ballistic testing also involved firing a soft-core 5.56×45 mm projectile for which a similar mechanism was observed. A finite element code was used to simulate the impact process for the different types of projectiles. The parameters of the selected strength and failure models were experimentally determined for the high-hardness armor plate. As to the ballistic limit velocity and plugging morphology there is a good correspondence between the experimental and computed results. Nevertheless, an improved failure model is necessary to get satisfactory computed residual projectile velocities.