Three-Dimensional Nonlinear Seismic Response of Shield Tunnel Spatial End Structure

The stiffness mutation of shield tunnel-shaft junction makes the tunnel structure affected by the differential displacement and forms a complex spatial effect. Taking the subsea shield tunnel crossing under the Shantou Gulf, China, as a case study, a three-dimensional finite element global model and a refined local spatial end submodel are established. The nonlinear dynamic behaviors of the seabed soil and concrete, the simulation of the bolt joints between ring segments by using cohesive models and the SMA shape memory alloy flexible joints, and the input ground motions produced by scaling from the high-level earthquake records are considered in detail. The results show that the shield tunnel spatial end structure increases nonlinearly in response to the increase of seismic motion intensity. The opening width and the deformation between ring segments at the vault and the outside spandrel are larger, and serious seismic damage and stress concentration exist at the conjugate 45° directions of shaft. The seismic responses of the tunnel-shaft junction subjected to the seismic motions with rich low-frequency components are much stronger than those of seismic motions with rich high-frequency components. Adding SMA flexible joints, the structural deformation caused by seismic motion propagation can be induced to the preset flexible joint, and the structural damage and stress concentration can be effectively reduced. The seismic response characteristics of shield tunnel spatial end structure calculated by the global model are consistent with those calculated by the submodel, while the seismic response of the submodel is greater than that of the global model.

Selected Aspects of Cohesive Zone Modeling in Fracture Mechanics

This review paper discusses the basic problems related to the use of cohesive models to simulate the initiation and development of failure in various types of engineering issues. The most commonly used cohesive zone models (CZMs) are described. Recent achievements in the field of cohesive modeling are characterized, with particular emphasis on the problem of mixed mode loading, the influence of the strain rate, the stress state triaxiality, and fatigue. A separate chapter of the work is devoted to the identification of cohesive parameters. Examples of the use of CZMs for the analysis of the fracture and failure process in various applications, both on the macro and microscopic scale, are given. The directions of CZMs development were indicated as well as the issues that are currently under particularly intensive development.

Rate-Dependent Cohesive Models for Dynamic Mode I Interfacial Propagation and Failure of Unidirectional Composite Laminates

With the increasing application of composite materials in anti-impact structure, the development of reliable rate-dependent interlaminar constitutive model becomes necessary. This study aims to assess and evaluate the applicability of three types of rate-dependent cohesive models (logarithmic, exponential and power) in numerical delamination simulation, through comparison with dynamic test results of double cantilever beam (DCB) specimens made from T700/MTM28-1 composite laminate. Crack propagation length history profiles are extracted to calibrate the numerical models. Crack propagation contours and fracture toughness data are predicted, extracted and compared to investigate the difference of the three different rate-dependent cohesive models. The variation of cohesive zone length and force profiles with the implemented models is also investigated. The results suggest that the crack propagation length can be better predicted by logarithmic and power models. Although crack propagation length profiles are well predicted, the numerical calculated dynamic fracture toughness tends to be higher than that of experimental measured results. The three models also show differences in the prediction of maximum loading forces. The results of this work provide useful guidance for the development of more efficient cohesive models and more reliable interface failure simulation of impact problems.

Wood-adhesive bond loaded in mode II: experimental and numerical analysis using elasto-plastic and fracture mechanics models

The goal of the study was to analyze fracture properties of adhesive bond using a three-point end-notched flexure test and the compliance-based beam method. Critical strain energy release rates (GIIc) and cohesive laws were obtained for adhesive bonds made of European beech (Fagus sylvatica L.) and adhesives such as EPI, MUF, PRF and PUR. The experiments were assisted with FE analyses employing three different material models of wood: elastic (Elas), symmetric elasto-plastic (EP) and elasto-plastic with different compressive and tensile yield stresses parallel to fiber (EP+). The highest mean GIIc was achieved for PUR (5.40 Nmm−1) and then decreased as follows: 2.33, 1.80, 1.59 Nmm−1 for MUF, EPI, and PRF, respectively. The failure of bondline was brittle and occurred at bondline for EPI, MUF and PRF, and ductile and commonly occurring in wood for PUR adhesive. The FE simulations employing cohesive models agreed well with the experimental findings for all adhesives. FE model with Elas material was found accurate enough for EPI, MUF and PRF adhesives. For PUR adhesive, the model EP+ was found to be the most accurate in prediction of maximal force. The impact of friction between lamellas may be up to 4.2% when varying friction coefficient from 0 to 1. The impact of the grain angle distortion (α) with respect to longitudinal specimen axis showed its high influence on resulting stiffness and maximal force. It was found that three-point end-notched test is suitable for EPI, MUF, and PRF, while it is less appropriate for a bond with PUR adhesive due to notable plastic behavior.

A comparative study of cohesive zone models for predicting delamination fracture behaviors of arterial wall

Arterial tissue delamination, manifested as the fracture failure between arterial layers, is an important process of the atherosclerotic plaque rupture, leading to potential life-threatening clinical consequences. Numerous models have been used to characterize the arterial tissue delamination fracture failure. However, only a few have investigated the effect of cohesive zone model (CZM) shapes on predicting the delamination behavior of the arterial wall. In this study, four types of CZMs (triangular, trapezoidal, linear–exponential, and exponential–linear) were investigated to compare their prediction of the arterial wall fracture failure. The Holzapfel–Gasser–Ogden (HGO) model was adopted for modeling the mechanical behavior of the aortic bulk material. The CZMs optimized during the comparison of the aortic media delamination simulations were also used to perform the comparative study of the mouse plaque delamination and human fibrous cap delamination. The results show that: (1) the numerical predicted the relationships of force–displacement in the delamination behaviors based on the triangular, trapezoidal, linear–exponential, and exponential–linear CZMs match well with the experimental measurements. (2) The traction–separation relationship results simulated by the four types of CZMs could react well as the corresponding CZM shapes. (3) The predicted load–load point displacement curves using the triangular and exponential–linear CZMs are in good agreement with the experimental data, relative to the other two shapes of CZMs. All these provide a new method combined with the factor of shape in the cohesive models to simulate the crack propagation behaviors and can capture the arterial tissue failure response well.