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Fatigue models that accurately resolve the complex three-dimensional failure mechanisms of concrete are numerically expensive. Especially the calibration of fatigue parameters to existing Wöhler lines requires solving for thousands or millions of cycles and a naive cycle-by-cycle integration is not feasible.
The proposed adaptive cycle jump methods provide a remedy to this challenge.
They greatly reduce the numerical effort of fatigue simulations and provide the basis for a development of those models.
Engineering Modelling
(2019)
This chapter presents the most commonly used approach to analyse the
thermo-mechanical behaviour of concrete structures subjected to high temperatures as in the case of fire loading. Prescriptions of the Eurocode are detailed for the thermal as well as the mechanical analysis. Finally, recommendations from two national (Austrian and German) guidelines give some improvements for Underground infrastructure.
Advanced Modelling
(2019)
The risk analysis of road tunnels faces a growing complexity in fire scenarios, e.g. caused by new energy carriers. Essentially, such complex scenarios involve many interactions between the tunnel users, the fire source and the safety measures. One example is the alarm of tunnel users either initiated by the perception of smoke or by the fire alarm system. To consider these interactions for the quantification of consequences, e.g. fatalities, risk analysis requires a complex model. However, the complex model can compute in practice only few discrete scenarios due to its high computational cost, whereas risk analysis generally needs the consequences of a high number of random scenarios. Metamodels can solve this contradiction. They are able to approximate the consequences of many random scenarios with low computational cost based on the consequences of few discrete scenarios computed with the complex model. The efficiency of metamodels depends on the required number of these discrete scenarios. In this sense, this dissertation proposes an efficient metamodel within an innovative methodology for risk analysis of road tunnels to allow to consider an increased complexity of scenarios. This metamodel applies the following methods or models: the projection array-based design method specifies the experimental design for the discrete scenarios; the combination of the fire model FDS and the microscopic evacuation model FDS+Evac constitutes the complex model; and moving least squares produces the response surface model. The response surface model approximates the consequences of the random scenarios and therewith introduces an uncertainty, called metamodel uncertainty, which is quantified with the prediction interval method. Additionally, stochastic individual characteristics of tunnel users in discrete scenarios computed with FDS+Evac attribute evacuation uncertainties to the consequences. An original development in this dissertation, the ’direct approach’, directly transfers the evacuation uncertainties of the discrete scenarios to any random scenario. The evaluation of the metamodel in this dissertation shows following results. Firstly, the response surface model sufficiently represents the consequences of the complex model. Secondly, the metamodel uncertainty is also essential for this representation, but the prediction interval method reveals a drawback in the risk analysis. Potential approaches to deal with this drawback are discussed. Finally, the direct approach reproduces the evacuation uncertainty of the complex model which then clearly affects the consequences of random scenarios. Therefore, the consideration of the evacuation uncertainty plays an important role for the risk analysis. Furthermore, the projection array-based design method was adapted in this dissertation with two approaches, namely the combination of the experimental designs for FDS and FDS+Evac as well as their sequential refinement. Both approaches contribute to the efficiency of the metamodel. These results lead to following conclusions. Firstly, the metamodel efficiently integrates the consequences of discrete scenarios into risk analysis and thus allows to consider an increased complexity. Secondly, the metamodel is an advancement for risk analysis not only for road tunnelsbutalsomoregeneralinfiresafetyengineering. Forthesetworeasons,themetamodel might be interesting for other methodologies for risk analysis. In addition, the metamodel is generic and is therefore widely applicable on other issues beside from risk analysis, e.g. to assess the safety of structures related to time-consuming experiments depending on multiple variables.
The simulation of the structural response for impact scenarios strongly requires an accurate simulation of both the impact event as well as the subsequent wave propagation. The numerical modeling of the impact event is intrinsically ill-posed due to the instantaneous changes of velocities in the contact area, leading to unbounded accelerations for decreasing time steps which causes oscillations in the contact stresses. These oscillations then propagate into the bulk material. Using a rate dependent material model, like concrete, they might lead to significant errors and a wrong prediction of the structural response. A regularization is thus required to avoid oscillations in the contact stresses. Another issue is related to the numerical computation of the contact conditions. In impact simulations, the nonlinear contact computation needs to be evaluated in every time step. A segmentation technique of the contact area is accurate but time consuming and may result in a bottleneck for the simulation and implementation, especially for 3D problems. The modeling of the subsequent wave propagation requires small time steps, which is primarily due to accuracy reasons. Implicit schemes are thus not affordable. Explicit time integration schemes are efficient only for diagonal mass matrices, as in this case no solution of a linear system is required. In this work, a coupled finite element - Non-Uniform Rational B-Spline (FE-NURBS) approach is applied to impact problems. The coupled approach uses an intermediate NURBS layer to compute the contact forces between the contacting bodies discretized by FEs. The advantages of a smooth isogeometric contact formulation are used to compute the contact forces. A segmentation of the contact area is avoided and an efficient element-based integration is used. The impact event is regularized using a mesh dependent nonlinear penalty approach. The penalty function is a polynomial which ensures a smooth transition between the noncontact and the contact state during the impact. For finer meshes, the penalty regularization becomes stiffer while still avoiding artificial oscillations in the contact stresses. Efficient higher order space and time discretizations are used to model the wave propagation. Explicit time integration is combined with higher order spectral element spatial discretization.
The full-notch creep test (FNCT) is a common method to evaluate the environmental stress cracking (ESC) behavior of high-density polyethylene (PE-HD) container materials . The test procedure as specified in ISO 16770 provides a comparative measure of the resistance against ESC using the time to failure of specimens mechanically loaded in a well-defined liquid environment. Since the craze-crack damage mechanism underlying the ESC process is associated with brittle failure, the occurrence of globally brittle fracture surfaces is a prerequisite to consider an FNCT measurement as representative for ESC . Therefore, an optical evaluation of FNCT fracture surfaces concerning their brittleness is essential. Due to the experimental setup, an inevitable increase of the true mechanical stress and the associated appearance of small ductile parts on fracture surfaces is induced in any case. Hence, an FNCT experiment is considered as 'valid', if the corresponding fracture surface is predominantly brittle . Based on laser scanning microscopy (LSM) height data of FNCT fracture surfaces , a universal and easy-to-use phenomenological criterion was developed to assess the validity of distinct FNCT experiments. This criterion is supposed to facilitate a quick evaluation of FNCT results in practical routine testing.
During their lifetime, polymer components subjected to mechanical loads and environmental influences show a loss of their mechanical properties required for their specific applications. In this respect, the craze-crack damage mechanism slow crack growth (SCG) is relevant for PE-HD components used in high-performance applications such as pipes and containers for the storage and transport of dangerous goods. SCG is considered to be the major failure mechanism in polyolefins and it typically occurs suddenly and unexpectedly. Due to the fields of application, SCG is a safety relevant issue. To test for the resistance of PE-HD pipe and container materials against SCG, the full-notch creep test (FNCT) is widely applied in Europe. In this study, SCG phenomena in PE-HD are investigated in detail based on an improved FNCT, especially including the consideration of the influence of environmental liquids effecting the damage mechanism. Using an enhanced fracture surface and a crack propagation analysis with imaging techniques such as light microscopy (LM), laser scanning microscopy (LSM), X-ray computed tomography (CT-scan) and scanning electron microscopy (SEM), detailed data concerning SCG are obtained.
The combined application of FNCT and such imaging techniques is explicitly advantageous and recommended to gain important information on damage occurring to PE-HD induced by mechanical stress and the influence of environmental liquids, which is essential within the Fourth Industry Revolution.
Fatigue safety verification of existing bridges that uses ‘‘re-calculation’’ based on codes, usually results in insufficient fatigue safety, triggering invasive interventions. Instead of “re-calculation”, Structural Health Monitoring (SHM) should be used for the assessment of the existing bridges. Monitoring systems provide data that can reduce uncertainties associated with the fatigue loading process and the structural resistance. The objective of this paper is to quantify the value of the SHM system implemented in a 60-years-old road viaduct to investigate its fatigue safety, through modeling of the fundamental decisions of performing monitoring in conjunction with its expected utility. The quantification of the conditional value of information is based on the decision tree analysis that considers the structural reliability, various decision scenarios as well as the cost-benefit assessments. This leads to a quantitative decision basis for the owner about how much time and money can be saved while the viaduct fulfills its function reliably and respects the safety requirements. The originality of this paper stands in the application of the value of information theory to an existing viaduct considering the fatigue failure of the system based on the monitoring data and the cost-benefit of monitoring method.
In this paper, the impact problem and the subsequent wave Propagation are considered. For the contact discretization an intermediate non-uniform rational B-spline (NURBS) layer is added between the contacting finite element bodies, which allows a smooth contact formulation and efficient element-based integration.
The impact event is ill-posed and requires a regularization to avoid propagating stress oscillations. A nonlinear mesh-dependent penalty regularization is used, where the stiffness of the penalty regularization increases upon mesh refinement. Explicit time integration methods are well suited for wave propagation problems, but are efficient only for diagonal mass matrices. Using a spectral element discretization in combination with a NURBS contact layer the bulk part of the mass matrix is diagonal.
Concrete is a complex material and can be modeled on various spatial and temporal scales. While simulations on coarse scales are practical for engineering applications, a deeper understanding of the material is gained on finer scales. This is at the cost of an increased numerical effort that can be reduced by the three methods developed and used in this work, each corresponding to one publication.
The coarse spatial scale is related to fully homogenized models. The material is described in a phenomenological approach and the numerous parameters sometimes lack a physical meaning. Resolving the three-phase mesoscopic structure consisting of aggregates, the mortar matrix and the interfaces between them allow to describe similar effects with simpler models.