We devise a multiple crack weight (MCW) method for the accurate and effective solution of strongly interacting cracks by meshless numerical methods. The MCW method constructs weight functions around cracks so that they simultaneously characterize all the cracks present in the single nodal domain of influence. This approach reduces the number of nodes necessary to achieve sufficient accuracy and consequently it decreases the computational effort. Numerical examples demonstrate that the method allows an accurate solution of multiple cracks problems. Convergence of the method is analyzed and discussed.

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In the last decade several different approaches have been developed to study arbitrary static and dynamic cracks. Among these methods meshless techniques play an important role. These methods provide an accurate solution of a wide range of fracture mechanics problems while traditional methods such as finite element and boundary element have limitations. We wish to increase the accuracy of the meshless approximations without increasing the nodal density. This is done by an appropriate modification of the weight function near crack tips. Earlier attempts still had limitations that result in a lack of accuracy, especially in the case when a linear basis is used. In this work a new technique, the spiral weight, is introduced that minimizes the drawbacks of existing methods. Numerical examples show that the spiral weight method is more efficient than existing methods, when using a linear basis, for the solution of crack problems.

Acoustic Emission (AE) nondestructive tests have attracted great interest for their use in the determination of structural properties and behavior of reinforced concrete (RC) elements. One of the applications this method can contribute to is in high-strength concrete (HSC) columns. These elements have a great advantage in the lower stories of high-rise buildings. However, the premature failure of the concrete cover and the brittleness nature of the failure is of a concern for engineers. This paper presents a study on the AE monitoring of HSC columns subjected to compressive axial loading. The study consists of four large-scale reinforced HSC columns with different confinement reinforcement and height. It is shown that the AE distributions in the columns are categorized by three stages. Moreover, the levels of loads reached at the first AE macro event are similar to the lower range levels of the nominal axial compressive strengths of the tested specimens, while the majority of macro AE events are located at the concrete cover. Based on the results of this study, AE monitoring can provide indications for the damage and load levels attained by reinforced high-strength concrete columns subjected to compressive axial loading.

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Damage and failures in high-pressure equipment and in high-energy piping have increased significantly during the second part of the twentieth century, in spite of improved construction procedures and the high quality of materials used. The result has been a grave expansion in the number of fatal disasters and ecological catastrophes, and their harmful social and economic consequences. This trend is apparent from a brief analysis of extensively developing industrial activities in different countries of the world, such as chemical, refinery and gas-treatment enterprises, power and the nuclear power industry. The statistics of failures in these industries show that most of the damage was caused by systems of interacting flaws.To numerically tackle these problems a previously developed code by the authors, based on the Element Free Galerkin (EFG) solution of systems of strongly interacting static cracks, was modified and adapted for dynamic problems in fracture mechanics. Several numerical examples of single crack propagation under impulse loading are solved. Accuracy of the results is verified comparing several analytical and numerical methods. The developed method is then applied to the physical model of dynamic crack propagation in the field of interacting flaws.

Quantitative Acoustic Emission (QAE) technology, physical and mathematical models were created for the reliable and precise identification and evaluation of the danger level (the J-integral value) of a developing main crack in a system of interacting micro-cracks, and the reliable assessment of the remaining lifetime of low density polyethylene (LDPE) reactor tubes that contain cracks. These innovations made it possible to carry out pioneer investigations and established previously unknown dependences, phenomena and criteria, such as:• Interdependence between the J-integral value of the flaw and the remaining lifetime of tubes from steel in design condition that contained system of inclusions, micro-cracks or had undergone stress corrosion cracking (SCC) attack and/or hydrogen embrittlement. • Criteria for tube rejection.• The optimal interval between repeated inspections (monitoring) of the reactor together with the time of analysis and decision. • Criteria for acceptable flaw danger level that would allow continued use of tubes in operation for two years.It was established that a main crack in a system of micro-cracks under dynamic pulse loading could start to propagate earlier and faster, and reach greater lengths and take longer time to brake than an individual main crack. At the same time it was shown that the remaining lifetime could decrease significantly when a main crack interacts with a field of micro-cracks. The danger level (the J-integral value) of combined flaw increases significantly and may provoke dramatic failure within a few weeks. Therefore, only continued monitoring can eliminate the risk of tube fracture in this case.Tension tests, optical and electron fractography, micro-sclerometric and AE image recognition investigations, spectral and chemical analysis, TOFD, X-ray, all established a good correlation between the results obtained from steel specimens and full-size tube specimens tests, LDPE reactor tubes examinations and theoretical calculations. GENERAL INFORMATION AND MOTIVATIONSPreviously, we have created our QAE Image Recognition method for revealing, identifying and assessing flaws in LDPE reactors, operating at high pressure (3000 bar) 3-5 . This enabled increasing the operational safety of reactors and prevents unexpected and rarely predictable failures. Nevertheless there were no:• Reliable methods for assessing the remaining lifetime of LDPE reactor tubes with individual cracks or systems of cracks having known J-integral values. • Relevant design criteria for acceptable J-integral values of flaws for the specific tubes and criteria for rejecting tubes from operation. • Universal tools for analytical or numerical calculations of flaws in tubes that undergoing dynamic loading. These problems have motivated us to clarify complications, and create and develop technology for the reliable assessment of the remaining lifetime of LDPE reactor tubes that contain individual and multiple flaws.