Journal of Engineering Mechanics

A smeared dynamic constitutive model is proposed for reinforced concrete based on microplane Model M4, in which the modified Menegotto-Pinto model for steel was adopted and the strain rate effect was taken into account by introducing parallel moving of envelope line. The model was established based on the hypothesis that the strains of concrete and steel bars have parallel coupling. Then the contribution of steel to total stress tensor was derived by projecting the section area of steel bars to three orthogonal directions. This model was calibrated by fitting with the test data, and its validity was verified by simulating a bridge-ship collision application using LS-DYNA embedded with a user-defined material subroutine.

In this paper, we examine the use of boundary springs as approximate external boundary conditions for the typical soil islands encountered in finite-element (FE) models of soil–foundation interaction problems. We present a set of simple uniformly distributed boundary springs and a set of variable boundary springs that account for the lateral distribution of contact forces between the foundation and the soil. We evaluate the accuracy of the approach for shallow foundations resting on soil islands of different dimensions. It is shown that significantly smaller errors can be achieved with these boundary springs than those obtained when fixed-fixed or fixed-free external boundary conditions are used. We present an iterative approach to determine the boundary springs for cases in which extremely small soil islands are required. Although the approach suggested to determine the boundary springs is best suited to shallow foundations, good results are also obtained for pile foundations. The approach presented applies to a uniform elastic half-space, but extensions to layered media are possible.

Conoidal shells are very popular roofing units owing to their aesthetic elegance and stiffness. Many parts of the globe, which were earlier assumed to be seismologically stable, are now being considered as earthquake prone. Hence the necessity to build light structures using composites has become very important. In this paper an eight-noded isoparametric shell element is applied for analyzing the bending behavior of delaminated composite conoidal shells under a uniformly distributed load with different practical boundary conditions. To ensure compatibility of deformation and equilibrium of forces and moments at the delamination crack front, a multipoint constraint algorithm is incorporated, which leads to an unsymmetrical stiffness matrix. This formulation is validated through the solution of benchmark problems. Lamination, curvature, and extent of delamination area are varied to compare the performances of delaminated conoidal shells against those with no damage. The results are carefully observed, and a set of conclusions is presented at the end of the paper.

Dynamic tensile tests are conducted on aluminum alloy (AA) 6061-T6 using a high-speed servohydraulic machine at intermediate strain rates to validate the testing technique and to investigate the strain-rate effect on the material’s stress-strain behavior and failure mode. We present the experimental procedures and results discussing the constitutive response of the alloy at strain rates up to approximately $200{\mathrm{s}}^{-1}$. The predominant frequencies of the high-speed testing machine were characterized by modal analysis, and we analyzed the effect from vibration of the system and loading rate on flow stress by using a single degree-of-freedom (SDOF) spring-mass-damper model. We tested two different specimen sizes at a wide range of actuator velocities to achieve the desired strain rates. Results show that the yield strength, ultimate strength, and failure strain were dependent on strain rate. We fitted the data to the Johnson-Cook (JC) constitutive model, and the resulting parameters are comparable to published results in the literature. The fracture surface of specimens tested at different strain rates obtained by scanning electron microscopy (SEM) showed that the ductile failure mode was dominant for the alloy, and strain rates within the range examined affected the fracture morphology.

Bridge-level failure event definitions per limit state have evolved from failure of one key bridge component as representative of the whole bridge system to failure of at least one of multiple components. However, an entire set of bridge failure event possibilities exists between these two extremes in the same limit state, such as failure of any two, any three, or any desired subset of bridge components. This paper proposes a closed-form combinatorial method to evaluate all possible ways in which bridge components can fail within and across limit states. It also highlights bridge component importance measures as key by-products of the closed-form solution. Calculations are illustrated with a particular yet illustrative system failure event, called the augmented event, which incorporates failures of at least one component in a given limit state and joint failures of multiple important components in a previous limit state. Bridges in as-built and retrofitted conditions are used to illustrate the augmentation calculation under seismic loads and the application of the proposed system reliability method. The results reveal an increase in median system fragility at the moderate limit states in the range of 4–20% relative to traditional approaches that neglect augmentation. This methodology to connect bridge components to bridge system reliability can readily support infrastructure stakeholder decision making and risk management through an efficient approach that can adapt to evolving system failure event definitions.

This work examines the influence of microcracking on a material’s tendency to shear localize under compressive loading. A two-dimensional (2D) finite-element framework with explicit crack representation using cohesive-element methodologies is employed. The influence of microcracking is examined by taking the fracture toughness of the cohesive elements as a free parameter. The simulations suggest that an optimum fracture toughness exists for promoting shear localization. This value corresponds to the limiting mode  I fracture toughness, below which microscopic material defects lead to brittle compressive failure, as opposed to shear localization. While in the presence of confinement, this value is shown to be close to zero; in the absence of confinement, it is computed to be 28% of the shear band toughness for the specific case of ultrafine-grained tungsten. More generally, it is found that the ratio of mode I fracture toughness to shear band toughness provides a crude indicator for predicting whether material defects are likely to lead to brittle failure or enhanced shear localization.

The validity of objectivity or frame-indifference in turbulence is examined simultaneously from the dual perspective of change of reference frame and superposed rigid-body motion. Similarities and differences arising from the two approaches and their effect on modeling in turbulence are discussed in detail. The use of the absolute spin for noninertial frames and of the net spin for superposed rigid-body motions in closure representations for the pressure-strain-rate correlations tensor are critically discussed. The enigmatic statement made by Speziale that the frame-indifference or objectivity of Reynolds stress does not establish the validity of material frame-indifference in turbulence and the subsequent case-study test of objectivity under superposed rigid-body mean motion within a rotating frame presented to prove it are investigated and fully explained. It is shown analytically that such apparently contrasting conclusions on the validity of objectivity result from the application of the objectivity test to the mean rather than the instantaneous motion, a case not encountered in classical continuum mechanics, which considers only one motion, the instantaneous.

A novel formulation is used to study the thermal postbuckling behavior of circular plates, with the edges supported to not have lateral deflection and elastically restrained against rotation. The elastic restraint is mathematically represented by an elastic rotational spring. The circular plate is subjected to a uniform edge compressive radial load, developed because of a uniform temperature rise. The formulation is on the basis of on the radial tensile load developed in the plate because of the large deflections of the plate with edges immovable in the plane normal to the edge and the linear buckling load corresponding to the uniform edge radial compressive load. The developed radial tensile load is obtained by using Berger’s approximation. The numerical results obtained from the present investigation in terms of the ratios of the postbuckling to the buckling loads for several rotational spring stiffness values compare well with those obtained by using the versatile finite-element analysis.