Journal of Engineering Mechanics

The fracturing of materials is well known on the basis of the theory of fracture mechanics. An important concept in fracture mechanics theory is that crack propagation is governed by the fundamental material properties of energy dissipation and energy threshold. However, to apply the fracture mechanics approach and measure these properties, a notch needs to be introduced into a continuum body. This requires additional effort to make the notch, which is often sensitive to the formation of materials surrounding the area near the crack tip. These effects become more complex for a material that exhibits time, rate, and temperature dependency. To overcome these complexities and problems regarding fracture testing, this study used the analogy between the material’s behaviors with and without a notch. A transfer of the fracture mechanics principle to continuum viscoelastic media was the key to the model developed in this study. For this purpose, the energy release rate ($G$) which is theoretically derived from a viscoelastic cracked body, was transferred to a continuum body by employing the same energy principles but used in continuum damage mechanics. From the evaluation of the model for various mixtures at multiple temperatures, predictions made were in agreement with the expected results from the well-known fatigue model. Consequently, it is expected that the developed model will bridge the gap between viscoelastic fracture and continuum damage mechanics, and provide better cracking performance predictions of asphalt mixtures.

The modified cross-correlation (MCC) blind identification method is extended to handle the underdetermined case of structural system identification. The underdetermined case is one in which the number of sensors is less than the number of identifiable modes. The basic framework of the modified cross-correlation method is retained in cases in which multiple covariance matrices constructed from the correlation of the responses are diagonalized. The solution to the underdetermined blind identification consists of two stages: the generation of intrinsic mode functions (IMFs) from the measurements by using empirical mode decomposition (EMD) and the application of the modified cross-correlation method to the decomposed signals. The available measurements are first decomposed into IMFs by using the sifting process of EMD. Subsequently, the IMFs are used as initial estimates for the sources, and the MCC method is implemented in an iterative framework. Initial estimates for the mixing matrix necessary to start the iterative process are selected using assumed shape functions that satisfy the essential boundary conditions. The need for sensor measurements at all the relevant degrees of freedom (DOF) to identify the mode shapes is alleviated in this approach. This is the main advantage of the proposed method. Vibration responses collected from the apron control tower located at the Toronto Pearson International Airport are used for demonstration.

Understanding of cracking behavior of cover concrete is important for assessment of the remaining service life of corrosion-affected concrete structures. Predicting initiation of concrete cracking is commonly based on the evolution of stresses caused by the volume expansion of corrosion products on reinforcing bars. This paper presents an analytical method that is capable of providing the solution to two-dimensional (2D) elastic stress field caused by nonuniform volume expansion around the corroding reinforcing bars. The developed method features the formulation of a displacement model, which is applied as a boundary condition to simulate the effect of nonuniform corrosion on the deformation of the surrounding concrete. A closed-form solution to the stress field is obtained using the complex variable method of Muskhelishvili. Validation of the solution is supported by a comparison with finite-element-based simulation results. The solution reveals the evolution of stresses surrounding two typical locations of steel bars in reinforced-concrete components: the middle bar and the corner bar. Further, a comparison of stresses induced by nonuniform corrosion and uniform corrosion indicates that nonuniform corrosion pattern could lead to earlier initiation of crack given the same amount of corrosion weight loss.

The present work uses a finite-element approach to study vibration, buckling, and dynamic instability characteristics in damaged cross-ply and angle-ply curved panels. The panels are subjected to nonuniform, centrally, and edge-distributed follower loading. First order shear-deformation theory was used to model the doubly curved panels and was formulated in accordance with Sanders’ first approximation. An anisotropic damage formulation was used to model damage. An analysis was carried out on plate and shallow shells to obtain vibration, buckling, and static instability (i.e., divergence) and dynamic instability (i.e., flutter) behavior. The effects of load type, load width, damage, and damage location on natural frequency, buckling load, divergence load, flutter load and flutter frequency were studied. The effect of curvature to improve the stability characteristics of panels is discussed. The desirable position of damage on a panel is discussed on the basis of different stability behavior. Results indicate that narrow edge loading is undesirable in most cases.

The paper presents the results of a study that investigated the effects of wall porosity, inlet-to-outlet-ratio, opening location, and blockage on internal pressure in buildings with large façade openings located on opposite or adjacent walls considering data from simultaneous measurements on multiple internal pressure taps. The study found that the internal airflow field has a significant effect on mean and peak values of internal pressure, which is not uniform for configurations with wall porosity higher than 10% and openings located on adjacent walls. Peak internal pressure coefficients were found to exceed (1) the recommended ASCE 7-10 value for all configurations tested with inlets located above the midheight of the building and particularly those with inlet-to-outlet-ratios equal to 1 and (2) the recommended design value for building category 3 for configurations with openings located above the midheight of the building, leeward wall outlets, and inlet-to-outlet-ratios greater than 2.

Fatigue damage in plain portland cement concrete is a nonlinear process that exhibits two distinct crack stages: a deceleration stage (transient crack growth) and an acceleration stage (steady-state crack growth). In the past, either the transient crack growth stage was completely neglected or was described using a power law that used crack extension as its argument. This works well for constant amplitude loading and for one geometry. However, to extend the fatigue prediction to variable amplitude loading, and also account for size effect, a fatigue crack resistance curve should be obtained so that the bridging stresses behind the crack tip can be calculated. In this paper, a new method has been developed to determine the fatigue crack resistance curve from crack rate and stress intensity data obtained from three point bending single edge notched specimens. The results of this study suggest that the fatigue crack resistance curve has a similar shape to the quasi-static crack resistance curve. Primarily, the critical crack extensions are equivalent, and the maximum fatigue crack resistance value is approximately 40–45% of the quasi-static fracture toughness at the critical crack extension. In addition, the results suggest that the $K_{\mathrm{IC}}$ in fatigue is larger than the $K_{\mathrm{IC}}$ in quasi-static loading. The reason for this is that the quasi-static resistance continues to rise after peak load, which will tend to increase the fracture toughness in fatigue. Finally, it is shown that by using a fatigue crack resistance curve, a unique set of Paris parameters $C$ and $n$ can be defined for both the transient and steady-state crack stages, which can then be used to predict the fatigue life without having to separate the two crack stages.

This paper discusses the estimation of the failure probability of wind turbines required by codes of practice for designing them. The Standard Monte Carlo (SMC) simulations may be used for this reason conceptually as an alternative to the popular Peaks-Over-Threshold (POT) method. However, estimation of very low failure probabilities with SMC simulations leads to unacceptably high computational costs. In this study, an Enhanced Monte Carlo (EMC) method is proposed that overcomes this obstacle. The method has advantages over both POT and SMC in terms of its low computational cost and accuracy. The method is applied to a low-order numerical model of a 5 MW wind turbine with a pitch controller exposed to a turbulent inflow. Two cases of the wind turbine model are investigated. In the first case, the rotor is running with a constant rotational speed. In the second case, the variable rotational speed is controlled by the pitch controller. This provides a fair framework for comparison of the behavior and failure event of the wind turbine with emphasis on the effect of the pitch controller. The Enhanced Monte Carlo method is then applied to the model and the failure probabilities of the model are estimated to the values related to the required 50-year return period of the wind turbine.