Key Engineering Materials Vols. 577-578

Abstract: Austenitic chromium-nickel stainless steel CrNiNb 18-10 was studied using TEM technique. Characterizations of thin films prepared from bulk cylindrical samples after low-cycle fatigue (LCF) tests were conducted. Focus was made on the dislocation clusters, slip bands, defects and microstructure changes taking place in the steel during LCF. It is shown that microcracks occur in slip bands. Stereographic and trace analyses revealed the microcrack propagation directions. Two types of microcracks were observed: wedge-shaped and with parallel sides. The obtained results on possible reasons and mechanisms of microcrack formation in the above places are discussed in line with the theoretical assumptions and the existing literature.

Abstract: Damage detection is a wide field of research and different approaches can be used to monitor structure integrity. Continuous monitoring of critical components is of vital importance to guarantee the safety of a structure. Variations in the dynamic response, in particular the curvature of mode shapes, are considered good indicators of the presence of possible defects. However noise, which often affects data, can lead to an erroneous calculation of the curvature, preventing the location of possible damage. The aim of this paper is to investigate the capability of the proper orthogonal decomposition (POD) to overcome noise, computing the curvature of proper orthogonal modes with a modified Laplacian operator. A numerical investigation on a cracked beam is compared to an analytical case present in literature. An extension of the mono-dimensional modified Laplacian scheme is introduced to study also plate-like structures. An experimental application on a vibrating composite plate will be presented in order to validate the numerical model.

Abstract: The paper presents the results of fatigue crack growth rate test of iron sinters. The samples were produced by means of the impact sintering. Applied production method allowed to obtain dense sinters with fine grain size resulted from large shear strains. Due to the limited size of the final products the mechanical tests were carried out in mini-samples. Optical, non-contact method of displacement measurement, namely Digital Image Correlation (DIC), was applied for determination of the displacement fields near the crack tip at the maximal force of selected loading cycles. The results of DIC measurement were utilized in the calculations of stress intensity factors and crack tip coordinates by means of the iterative procedure based on inverse method. These parameters were used for measuring crack development rate. There were investigated two types of materials produced by the consolidation of two different kinds of Fe powders and sintered in different temperatures. The results of crack growth rate tests were correlated with the microstructure changes, as well as yield and ultimate strength of the materials.

Abstract: An up to date approach to material cold formability, based on a conventional J₂ theory for plasticity together an uncoupled linear plastic damage evolution theory, is applied to the study of steel bolt head cold forming. Specific laboratory tests to characterize the material formability have been used. Finite element simulations have been performed to characterize the strain and stress state both in the forming process and in experimental tests to determine the material ductile fracture locus. The influence of chemical composition and initial microstructure is discussed.

Abstract: This paper presents finite element modeling effort to predict possible microcracking of the matrix in 3D woven composites during curing. Three different reinforcement architectures are considered: a ply-to-ply weave, a one-by-one and a two-by-two orthogonal through-thickness reinforcement. To realistically reproduce the as-woven geometry of the fabric, the data from the Digital Fabric Mechanics Analyzer software is used as input for finite element modeling. The curing processed is modeled in a simplified way as a uniform drop in temperature from the resin curing to room temperature. The simulations show that the amount of residual stress is strongly influenced by the presence of through-thickness reinforcement.

Abstract: Microcracking is often observed in the resin pockets of as-cured 3D woven composites with significant through-the-thickness fiber constraint. The resin is subjected to a triaxial tensile stress during cooling due to thermal expansion mismatch and shrinkage during curing. However, the temperature dependence of the failure surface for resins subjected to triaxial tensile stress is not known and there is no standard measurement method for applying a triaxial tensile stress. We have developed a novel method for measuring the triaxial tensile failure stress surface by confining the shrinkage of the resin to tubes of different thicknesses and made of materials with different thermal expansion coefficient. The difference in thermal contraction and shrinkage between the resin and the confining tube subjects the resin to a triaxial tensile stress during cooling and curing. We vary the stress-temperature state by selecting tubes with different coefficient of thermal expansion. We infer the stress in the resin from the deflections of the tube measured by a high resolution dilatometer assuming that the tube is a linear elastic, thick-walled pressure vessel.

Abstract: Corrosion fatigue performance of two copper alloys (admiralty brass and cupronickel 90/10) is investigated by conducting fatigue tests in artificial seawater. Two different experimental setups are developed and used: immersed rotating beam bending of round wires and immersed flexural cycling of rectangular plates. For the second setup, two sets of specimens are used: as-manufactured and after 1-year exposure to natural seawater in North Atlantic. In addition, the fatigue performance is compared between the dry and immersed tests. It is observed that the fatigue life of copper alloys in seawater environment depends on their composition and manufacturing parameters. Immersion in seawater does not affect low-cycle fatigue, however, high-cycle fatigue behavior shows significant differences. It is also observed that one-year preliminary exposure to natural seawater (stress-free corrosion) results in up to three times reduction of fatigue life at stress amplitudes corresponding to high-cycle fatigue.

Abstract: The paper is focusing on the investigation of the effective crack length obtained from bending test on concrete notched beams with the complementary measurements of electrical resistivity of the tested concrete. The electrical resistivity measurements are performed on several stages of the fracture process along the specimen ligament for each tested notched beam. Gained results of the concrete resistivity during the fracture process, i.e. its dependence on the crack length or opening, provide information which should be taken into account in structural durability analyses. The fracture tests are conducted for a set of specimens differing in the notch length. Changes of the concrete resistivity with increasing effective crack length are observed and discussed.

Abstract: Analysis on the aspects of the energy dissipation in the case of quasi-brittle fracture is presented. Dissipation both via cohesive forces at the crack faces and the one taking place within the volume of the fracture process zone is considered. Tools from the field of soft computing techniques are employed. The analysis is conducted on results from extensive experimental campaign.

Abstract: By applying original experimental methodology and software system for non-contact research of deformation field investigated conditions of DNP in the test setup of given stiffness. Established that at the speed of additional impulse loading within 300-500 MN / s, specimens of aluminum alloy 2024 - T3 and D16 exposed low cycle, high-frequency oscillations with a frequency of 1.5 ... 2 KHz. This process of load with deformations speeds of the material 2 ... 60s^-1 promotes plasticizing of aluminum alloys and delays in "neck forming.".