Efficient numerical integration of an elastic–plastic damage law within a mixed velocity–pressure formulation

This study focuses on numerical integration of constitutive laws in numerical modeling of cold materials processing that involves large plastic strain together with ductile damage. A mixed velocity–pressure formulation is used to handle the incompressibility of plastic deformation. A Lemaitre damage model where dissipative phenomena are coupled is considered. Numerical aspects of the constitutive equations are addressed in detail. Three integration algorithms with different levels of coupling of damage with elastic–plastic behavior are presented and discussed in terms of accuracy and computational cost. The implicit gradient formulation with a non-local damage variable is used to regularize the localization phenomenon and thus to ensure the objectivity of numerical results for damage prediction problems. A tensile test on a plane plate specimen, where damage and plastic strain tend to localize in well-known shear bands, successfully shows both the objectivity and effectiveness of the developed approach.     

工业广场保护煤柱开采可行性研究

林南仓矿业公司西一采区8-1煤层局部进入工业广场煤柱。文章对西一采区8-1煤层局部进入工业广场保护煤柱开采可行性及开采对工业广场和相邻村庄的影响程度进行分析论证,并对公司后续开采提出规划与建议。     

Modeling of concrete behavior under high strain rates with inertially retarded damage

The paper proposes a novel approach to model the influence of high strain rates on the behavior of quasi-brittle materials like concrete. It is based on gradient continuum damage, where the gradient part is extended with an inertia of damage. This causes a retardation of damage due to the fact that micro-cracks cannot spread out arbitrarily fast. The application is demonstrated with uniaxial tensile wave propagation and for a plane stress case. Increasing strain rates lead to an expansion of the linear stress–strain behavior with stresses exceeding the quasistatic material strength.     

玄武岩纤维加固震损混凝土框架节点承载力计算分析

对5个不同损伤程度的框架节点采用玄武岩纤维加固后进行低周反复荷载试验。考虑混凝土框架节点受损修复后初始损伤的影响,提出构件修复后混凝土承载力折减系数;基于现行设计规范,建立玄武岩纤维加固混凝土框架节点考虑构件初始损伤的承载力计算公式,为震损混凝土构件的加固设计提供参考。     

煤矿违序开采带来的危害

违序开采带来安全隐患和经济效益的负面影响,对煤炭资源造成浪费,增加成本投入,对煤矿发展不利。文章针对违序开采带来的安全隐患,提出了具体的预防措施。     

Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 2. Fatigue test results and damage mechanisms

The analysis of stiffness degradation and the identification of damage mechanisms during and after fatigue tests of sandwich panels with PVC foam cores have been performed. The sandwich panels with cross-ply laminates skins made of glass fiber and epoxy resin were manufactured by vacuum moulding and subjected to three-point bending tests. Two PVC cores of similar type but with differing densities were investigated. The effect of core density and thickness on the damage behavior was highlighted. Using the cyclic life criterion, fatigue curves were plotted according to two models and compared with those of the literature. It has been demonstrated that the sandwich SD 2, with the higher core density, withstands a higher load and possesses greater rigidity in static tests, combined with an enhanced fatigue resistance, when compared to sandwich SD 1 which has a lower core density. Translated from Problemy Prochnosti, No. 3, pp. 32–44, May–June, 2009.     

Multi-hazard loss analysis of tall buildings under wind and seismic loads

This paper presents a framework for life-cycle loss estimation for non-structural damage in tall buildings under wind and seismic loads. Life-cycle cost analysis is a useful design tool for decision- makers, aimed at predicting monetary losses over the lifetime of a structure, accounting for uncertainties involved in the problem definition. For tall buildings, sensitive to dynamic excitations like earthquake and wind, it can be particularly suitable to base design decisions not only on initial cost and performance but also on future repair expenses. The proposed approach harmonises the procedures for intervention costs evaluation of structures subjected to multiple-hazards, taking into account the peculiar differences of wind and earthquake, in terms of load characterisation, type and evolution of damage. Relative effect of the two hazards on damage to drift- and acceleration-sensitive non-structural elements are examined. The influence of uncertainty in structural damping is also taken into account. It is shown that, although it is commonly believed that the design of a given structure is usually dominated by either winds or earthquakes, when LCC-based design is performed, both winds and earthquakes may be important.     

An improved damage diagnostic technique based on Singular Spectrum Analysis and time series models

An approach to enhance the sensitivity of the damage index using the prediction errors of autoregressive – autoregressive exogenous models by augmenting Singular Spectrum Analysis (SSA) to detect and locate minor damage is presented. Numerical simulation studies are carried out by considering a simply supported beam with single and multiple cracks. An experimental study on a reinforced cement concrete beam has been carried out to validate the technique using different levels of damage. The studies emphasise that the SSA improves the sensitivity of the damage index for detection and also localisation, handling environmental/operational variability and measurement noise.     

Oxide scale damage and spallation in P92 martensitic steel

Abstract

9 Cr martensitic steels are widely used in high temperature steam environments for their combination of creep strength and oxidation resistance. These materials are pushed to their limit in the quest for higher efficiencies in boilers and steam turbines and loss of the protective oxide scale may result in premature failure of critical components.

The major cause of oxide scale spallation is a temperature drop. The mechanism behind the spallation process is the generation of internal stresses in the oxide scale due to the mismatch of thermal expansion coefficients between the substrate and the haematite, magnetite and spinel layers in the oxide scale. These scales can be partially relaxed due to creep processes if the cooling rate is sufficiently slow.

Specimens of P92 have been oxidised in flowing steam for periods up to 2000 h and cooled at a linear rate of 100°C h–1. Acoustic emission (AE) of the samples was monitored during the oxidation and cooling periods. Specimens were sectioned and prepared metallographically and the damage in each layer of the oxide scale was examined. The spall from the specimens was also collected and characterised.

The damage observed in the oxide scale and the AE signals have been explained by comparison with a simple stress analysis arising from thermal expansion mismatch. Future work will extend the modelling approach to incorporate creep relaxation effects.