Key Engineering Materials Vols. 348-349

Abstract: Multiple Site Damage (MSD) is a typical problem of aging aircraft structures. On the other hand a similar situation may occur, if damages in the vicinity of the crack tip of a large crack is investigated. In some preceding papers the author has shown the possibility to assess the criticality of a MSD-scenario by comparing certain data compression measures of the crack scenario in an early stage. All of these scenarios were linked to cracks in one row of fasteners of equal distance, and the method was used in conjunction with the Monte-Carlo Simulation. The idea of using such ways to assess multiple crack scenarios is extended now in the paper presented here. The investigations are extended now in the sense that, the scenarios are more complex as they include large cracks and small cracks in the vicinity of the crack, where no fixed distance of the cracks is foreseen. The attempt is made to relate such results to experimental data on residual strength and therefore, to relate them to sophisticated models like the Gurson model.

Abstract: Using single crack solution and regular plane harmonic function, the Saint-Venant bending problem of a cracked cylinder with general cross section is formulated in terms of two sets of boundary-singular integral equations, which can be solved by using the methods for combination of boundary element and singular integral equation methods. The concept of bending center used in strength of materials is extended to this bending problem. Theoretical formulae to calculate the bending center and stress intensity factors in cracked cylinder are derived and expressed by the solutions of the integral equations. Based on these results, some numerical examples are given for different configurations of the cylinder cross section as well as the crack parameters.

Abstract: Based on the S-N relationship and statistical property of concrete static strength, a function of fatigue life of concrete,1 (a − blog N) , is found to follow the normal distribution. Thus a new probabilistic model of fatigue life distribution of concrete is presented in this paper. The model connects statistical properties of static strength and fatigue life of concrete together in theory, so it is of clear physical meaning. An experiment was conducted. The experiment was a part of the project of The State Natural Science Foundation—Failure Criterion of Plain Concrete Under Multiaxial Fatigue Loading. 2 χ -test and Kolmogorov-Smirnov test are employed to test the proposed model. Fuzzy optimization is used to compare the model with lognormal distribution. 2 χ -test, Kolmogorov-Smirnov test and fuzzy optimization are also conducted for test data from references. The results show that the new model is more flexible to fit test data.

Abstract: Many components from industry are subjected to repeated impacts, or in some cases these impacts can appear as additional loads. Repeated impacts define a fatigue phenomenon known under the name of Impact Fatigue. Because the strain rate changes the material characteristics it is to expect that the material properties at impact fatigue to be different in regard to those obtained at non-impact fatigue. First studies at repeated impacts were made at the middle of 19th Century, but the progress in this field is not as fast as non-impact fatigue, due to experimental difficulties and the lake of standards for impact fatigue tests. This paper presents a classification of repeated impact tests, and starting from this a series of parameters used for durability estimation will be analyzed. The high number of parameters used by different authors creates difficulties in comparison the different laboratories results. The importance of the shape and dimensions of specimens, and the stiffness of bearing are highlighted. In order to avoid these influences the authors proposed an experimental technique, based on testing of Charpy specimens, in similar conditions as single impact test. The paper presents a series of results obtained for additional impacts overlapped to a cyclic load.

Abstract: A study on the fatigue behaviour of friction stir butt welds of 3mm thick 6082-T6 aluminium alloy was carried out. Monotonic tensile and cyclic tests of welded joints and base material were performed to understand the influence of the welding process on the static and fatigue properties. Microhardness profiles were measured and fatigue crack growth curves were determined for cracks growing in different locations of the weldments. Friction stir material exhibited lower strength and ductility properties than the base material. However, an enhanced crack propagation resistance is observed.

Abstract: A numerical method from the mesoscopic point of view is proposed to describe the fracture process of concrete. At mesoscopic level, concrete is considered as a three-phase composite consisting of mortar matrix, coarse aggregate and interfacial transition zone (ITZ) between them. According to the grading of coarse aggregate obtained from sieve analysis, the random aggregate models with polygonal aggregates were generated by Monte Carlo random sampling principle. In this work, the tensile cracking is assumed to the only failure criterion at the mesoscopic scale; and the stress-separation law based on the fictitious crack model is adopted to allay the sensitivity on mesh size in the softening regime. The nonlinear finite element method is used in the simulation of concrete under bend loading. The influence of the shape of aggregate on the macroscopic response of concrete is also investigated. Numerical results show that the strength of the specimen with circular aggregate is higher than the specimen with arbitrary polygonal aggregate. The predicted bending strength agrees well with experimental data.

Abstract: Due to the coexistence of different micro structural components and their interactions, multiphase steels offer an excellent combination between high formability and strength. On the micro-scale, the fracture examination shows large influence of different phases and their distributions on the mechanical properties and failure mechanisms. Considering the influence of multiphase microstructure, an approach is presented using representative volume elements (RVE) in combination with continuum damage mechanics (CDM). Herein, the influence of the material properties of individual phases and the local states of stress on the material formability as well as the failure behavior can be examined. By means of the RVE-CDM approach, a precise criterion for the deformability characterization in sheet metal forming of multi phase steels is presented.

Abstract: TiCp/ZA-12 composites have been fabricated by XDTM method and stirring-casting techniques. Microstructure of the composites has been studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that TiC particles distribute uniformly in ZA-12 matrix alloy. The interface between reinforcements and matrix alloy is very clean, and there is not interface reaction between TiC particles and ZA-12 matrix alloy. The tests for mechanical properties reveal that the tensile strength, yield strength, elastic modulus and hardness of the composites are improved obviously due to the incorporation of TiC particles. The strengthening mechanisms are attributed to the following factors: dispersion strengthening of TiC particles, grain refinement of ZA-12 matrix alloy and high-density dislocations existing in ZA-12 alloy.

Abstract: A new approach for progressive failure and reliability analysis of carbon fiber reinforced polymeric (CFRP) composite pressure vessel with many base random variables is developed in the paper. The elastic constants of CFRP lamina and geometric parameters of the vessel are selected as the base design variables. CFRP lamina specimen and pressure vessel were manufactured and tested in order to obtain statistics of design variables. The limit state function for progressive failure analysis was set up. Then the progressive failure and reliability analysis of the vessel were performed according to the stiffness degradation model based on Monte Carlo simulation procedure using MATLAB. The distributions of failure loads and the probability of failure of the vessel were obtained. The feasibility and accuracy of the proposed method is validated by good agreement between the simulation and experimental results. Further analysis indicates that the lamina tensile strength in the fiber direction and hoop layer thickness of the vessel have significant influence on the probability of failure of composite pressure vessel.

Abstract: The topic of this paper lies in the field of non-destructive parametric identification. Its objective is to evaluate the mechanical characteristics of constituent bars in existing truss structures. In particular, it locates bars with reduced mechanical properties and quantifies the loss of stiffness. The suggested algorithm takes into account plane and three-dimensional structures, both statically determinate and indeterminate. In the case where it is possible to measure strains only on some bars, the procedure uses Genetic Algorithms to overcome the lack of information.