New tension-compression damage model for complex analysis of concrete structures

A new damage model, based on continuum damage mechanics and simulating the opening, closing, and reopening of cracks in concrete using only one surface of discontinuity, is proposed in this article. The model complies with the thermodynamics principles of nonreversible, isothermal, and adiabatic processes. Two scalar internal variables have been defined: a tensile damage variable d+d+ and a compressive damage variable d-d-; the threshold of damage is controlled by only one surface of discontinuity and a new parameter controlling the damage variable that should be activated. This new parameter represents the ratio of tensile stress to compressive stress in the damaged material. The continuity of response under complex loads, which is one of the aims of this work, is ensured. An adequate response under different types of loads leads to the conclusion that the proposed model provides a powerful tool to numerically analyze reinforced concrete structures. Validation and illustrative examples are included in the article.Peer ReviewedPostprint (author's final draft

Calculation of the incremental stress-strain relation of a polygonal packing

The constitutive relation of the quasi-static deformation on two dimensional packed samples of polygons is calculated using molecular dynamic simulations. The stress values at which the system remains stable are bounded by a failure surface, that shows a power law dependence on the pressure. Below the failure surface, non linear elasticity and plastic deformation are obtained, which are evaluated in the framework of the incremental linear theory. The results shows that the stiffness tensor can be directly related to the micro-contact rearrangements. The plasticity obeys a non-associated flow rule, with a plastic limit surface that does not agree with the failure surface.Comment: 11 pages, 20 figur

The application of ultrasonic NDT techniques in tribology

The use of ultrasonic reflection is emerging as a technique for studying tribological contacts. Ultrasonic waves can be transmitted non-destructively through machine components and their behaviour at an interface describes the characteristics of that contact. This paper is a review of the current state of understanding of the mechanisms of ultrasonic reflection at interfaces, and how this has been used to investigate the processes of dry rough surface contact and lubricated contact. The review extends to cover how ultrasound has been used to study the tribological function of certain engineering machine elements

Evolution, Monitoring and Predicting Models of Rockburst: Precursor Information for Rock Failure

Load/unload response ratio predicting of rockburst; Three-dimensional reconstruction of fissured rock; Nonlinear dynamics evolution pattern of rock cracks; Bayesian model for predicting rockburs

Positive Feedback, Memory and the Predictability of Earthquakes

We review the "critical point" concept for large earthquakes and enlarge it in the framework of so-called "finite-time singularities". The singular behavior associated with accelerated seismic release is shown to result from a positive feedback of the seismic activity on its release rate. The most important mechanisms for such positive feedback are presented. We introduce and solve analytically a novel simple model of geometrical positive feedback in which the stress shadow cast by the last large earthquake is progressively fragmented by the increasing tectonic stress. Finally, we present a somewhat speculative figure that tends to support a mechanism based on the decay of stress shadows. This figure suggests that a large earthquake in Southern California of size similar to the 1812 great event is maturing.Comment: PostScript document of 18 pages + 2 eps figure

A comparison of single-cycle versus multiple-cycle proof testing strategies

An evaluation of single-cycle and multiple-cycle proof testing (MCPT) strategies for SSME components is described. Data for initial sizes and shapes of actual SSME hardware defects are analyzed statistically. Closed-form estimates of the J-integral for surface flaws are derived with a modified reference stress method. The results of load- and displacement-controlled stable crack growth tests on thin IN-718 plates with deep surface flaws are summarized. A J-resistance curve for the surface-cracked configuration is developed and compared with data from thick compact tension specimens. The potential for further crack growth during large unload/reload cycles is discussed, highlighting conflicting data in the literature. A simple model for ductile crack growth during MCPT based on the J-resistance curve is used to study the potential effects of key variables. The projected changes in the crack size distribution during MCPT depend on the interactions between several key parameters, including the number of proof cycles, the nature of the resistance curve, the initial crack size distribution, the component boundary conditions (load vs. displacement control), and the magnitude of the applied load or displacement. The relative advantages of single-cycle and multiple-cycle proof testing appear to be specific, therefore, to individual component geometry, material, and loading

A Structural Engineer’s Approach to Efficient SFSI: Towards Performance Based Design

Performance-based design (PBD) involves designing structures to achieve specified performance targets under specified levels of seismic hazard. This involves analyzing the entire soil-structure system and requires structural and geotechnical expertise. This paper is focused on soil-foundation-structure interaction (SFSI) in relation to PBD. A Beam-on-Nonlinear-Winkler- Foundation (BNWF) model is developed to incorporate important SFSI aspects into structural analysis software. The model accounts for: nonlinearity due to soil yield and/or footing uplift; cyclic degradation of stiffness and strength due to variable-amplitude loading; distribution of soil resistance underneath the footing for different loading conditions; reduction in radiation damping with increased nonlinearity; and coupling effects between different responses of the foundation. The coupling between different responses is achieved by appropriate mathematically derived bounding surfaces. The model utilizes a rotation hinge governed by a bounding surface to model coupling between rocking (in two directions) and vertical responses, and a shear hinge governed by another bounding surface to couple the horizontal responses. These models are implemented in readily available structural packages, and hence allow structural engineers to properly account for SSI effects when performing PBD. The application of the developed models to analysis of experiments on model foundations showed good agreement between the calculated and observed behavior

Use of cohesive elements in fatigue analysis

Cohesive laws describe the resistance to incipient separation of material surfaces. A cohesive finite element is formulated on the basis of a particular cohesive law. Cohesive elements are placed at the boundary between adjacent standard volume finite elements to model fatigue damage that leads to fracture at the separation of the element boundaries per the cohesive law. In this work, a cohesive model for fatigue crack initiation is taken to be the irreversible loadingunloading hysteresis that represents fatigue damage occuring due to cyclic loads leading to the initiation of small cracks. Various cohesive laws are reviewed and one is selected that incorporates a hysteretic cyclic loading that accounts for energetic dissipative mechanisms. A mathematical representation is developed based on an exponential effective load-separation cohesive relationship. A three-dimensional cohesive element is defined using this compliance relationship integrated at four points on the mid-surface of the area element. Implementation into finite element software is discussed and particular attention is applied to numerical convergence issues as the inflection point between loading and 'unloading in the cohesive law is encountered. A simple example of a displacementcontrolled fatigue test is presented in a finite element simulation. Comments are made on applications of the method to prediction of fatigue life for engineering structures such as pressure vessels and piping

Creep and Anelastic Deformation in Austenitic Steels

This study examines the creep behaviour of austenitic steels under service temperatures, to determine the effect of creep on material performance. Nuclear power plant components are in regular use at temperatures greater than 450°C, where creep deformation plays a dominant role in limiting the lifetime of the material. The prime aim of this study was to characterise the effect of load-reductions on the creep behaviour of austenitic steels (AISI type 316H). In-service materials seldom operate at a constant load and/or temperature. The supply demand, maintenance operations, refuelling, etc. will result in large variation of load and temperature acting on the material. Experiments where load/temperature removals during a creep test were therefore conducted. These unloading procedures result in material recovery of the accumulated creep strain (anelasticity). This phenomenon will influence the material properties such as creep life and ductilities. Creep life was found to increase by 2-3 times whereas creep ductilities decreased by 50% when compared to steady-load creep data under identical conditions. The occurrence of anelasticity suggested the presence of a material backstress. The origin and evolution of this internal stress was investigated using neutron diffraction and TEM microscopy. Lattice strain measurements were conducted in-situ using neutron diffraction during a creep test which consisted of load/unload cycles. Experimental results suggest that creep strain is equivalent to plastic strain at a granular level. The data also shows intergranular micro-stresses are introduced into the material by primary creep. Anisotropic behaviour of the individual crystal planes results in formation of tensile and compressive intergranular stresses in individual grain families. Residual compressive stresses drive this anelastic deformation. TEM examinations of samples stopped during the unload show changes in dislocation and precipitate morphologies during the plastic strain recovery phase. Evidence of a changing dislocation substructure during the load-reduction period was found. Examinations have also shown carbide densities change during the unload. Pipe diffusion is a possible mechanism which can be used to explain this occurrence. The changing precipitate and dislocation state will influence the strengthening mechanisms, which in-turn will affect the deformation characteristics. These microstructural observations were introduced into a damage mechanics model. Predictions of material behaviour using this model have shown good agreement with experimental data. Outcomes of this project, have established that changes in creep deformation mechanisms will greatly influence material properties. Deformation history of the material will affect the intergranular stress state which in turn will affect the elastic and plastic response of the material. The effect of plastic strain history must be considered and incorporated accounted in any design and assessment procedure