TO THE QUESTION OF STRUCTURAL FEATURES OF IRON COATINGS OF HIGH THICKNESS OBTAINED BY THE ELECTROGALVANIC METHOD WITH MECHANICAL LAYER-LAYER HARDENING

The process of electrogalvanic production of coatings of increased thickness with layer-by-layer mechanical hardening is considered. The mode of deposition and the scheme of coating deposition, the research technique and the results of the X-ray analysis of internal stresses are presented.

Reliability as a Key Driver for a Sustainable Design of Adaptive Load-Bearing Structures

The consumption of construction materials and the pollution caused by their production can be reduced by the use of reliable adaptive load-bearing structures. Adaptive load-bearing structures are able to adapt to different load cases by specifically manipulating internal stresses using actuators installed in the structure. One main aspect of quality is reliability. A verification of reliability, and thus the safety of conventional structures, was a design issue. When it comes to adaptive load-bearing structures, the material savings reduce the stiffness of the structure, whereby integrated actuators with sensors and a control take over the stiffening. This article explains why the conventional design process is not sufficient for adaptive load-bearing structures and proposes a method for demonstrating improved reliability and environmental sustainability. For this purpose, an exemplary adaptive load-bearing structure is introduced. A linear elastic model, simulating tension in the elements of the adaptive load-bearing structure, supports the analysis. By means of a representative local load-spectrum, the operating life is estimated based on Woehler curves given by the Eurocode for the critical notches. Environmental sustainability is increased by including reliability and sustainability in design. For an exemplary high-rise adaptive load-bearing structure, this increase is more than 50%.

A Comparison of the Effects of Ultrasonic Cavitation on the Surfaces of 45 and 40Kh Steels

The ultrasonic treatment of metal products in liquid is used mainly to remove various kinds of contaminants from surfaces. The effects of ultrasound not only separate and remove contaminants, they also significantly impact the physical–mechanical and geometric properties of the surfaces of products if there is enough time for treatment. The aim of this study was to compare the dynamics of ultrasonic cavitation effects on the surface properties of 45 (ASTM M1044; DIN C45; GB 45) and 40Kh (AISI 5140; DIN 41Cr4; GB 40Cr) structural steels. During the study, changes in the structure, roughness, sub-roughness, and microhardness values of these materials were observed. The results showed significant changes in the considered characteristics. It was found that the process of cavitation erosion involves at least 3 stages. In the first stage, the geometric properties of the surface slightly change with the accumulation of internal stresses and an increase in microhardness. The second stage is characterized by structure refinement, increased roughness and sub-microroughness, and the development of surface erosion. In the third stage, when a certain limiting state is reached, there are no noticeable changes in the surface properties. The lengths of these stages and the quantitative characteristics of erosion for the considered materials differ significantly. It was found that the time required to reach the limiting state was longer for carbon steel than for alloy steel. The results can be used to improve the cleaning process, as well as to form the required surface properties of structural steels.

Unraveling the origin of extra strengthening in gradient nanotwinned metals

Materials containing heterogeneous nanostructures hold great promise for achieving superior mechanical properties. However, the strengthening effect due to plastically inhomogeneous deformation in heterogeneous nanostructures has not been clearly understood. Here, we investigate a prototypical heterogeneous nanostructured material of gradient nanotwinned (GNT) Cu to unravel the origin of its extra strength arising from gradient nanotwin structures relative to uniform nanotwin counterparts. We measure the back and effective stresses of GNT Cu with different nanotwin thickness gradients and compare them with those of homogeneous nanotwinned Cu with different uniform nanotwin thicknesses. We find that the extra strength of GNT Cu is caused predominantly by the extra back stress resulting from nanotwin thickness gradient, while the effective stress is almost independent of the gradient structures. The combined experiment and strain gradient plasticity modeling show that an increasing structural gradient in GNT Cu produces an increasing plastic strain gradient, thereby raising the extra back stress. The plastic strain gradient is accommodated by the accumulation of geometrically necessary dislocations inside an unusual type of heterogeneous dislocation structure in the form of bundles of concentrated dislocations. Such a heterogeneous dislocation structure produces microscale internal stresses leading to the extra back stress in GNT Cu. Altogether, this work establishes a fundamental connection between the gradient structure and extra strength in GNT Cu through the mechanistic linkages of plastic strain gradient, heterogeneous dislocation structure, microscale internal stress, and extra back stress. Broadly, this work exemplifies a general approach to unraveling the strengthening mechanisms in heterogeneous nanostructured materials.

Study of anisotropy through microscopy, internal friction and electrical resistivity measurements of Ti-6Al-4V samples fabricated by selective laser melting

Purpose This paper aims to investigate the microstructural anisotropy of Ti-6Al-4V samples fabricated by selective laser melting. Design/methodology/approach Specimens are fabricated through a Renishaw AM400 selective laser melting machine. Three microstructures (as-built, 850°C annealed and 1,050°C annealed) and two building orientations, parallel (PA) and perpendicular (PE) to the building platform, are considered. Starting from in-depth microscopic observations and comprehensive electron backscattered diffraction imaging, the study addresses non-conventional techniques such as internal friction and electrical resistivity measurements to assess the anisotropy of the fabricated parts. Findings Microscope observations highlight a fine texture with columnar grains parallel to the building direction in the as-built and 850°C annealed samples. Besides, coarse grains characterized the 1,050°C annealed specimens. Internal friction measurements pointed out the presence of internal stress while storage modulus analyses appear sensitive to texture. Electrical resistivity is resulted to be dependent on grain orientation. Originality/value The work uses some novel characterization techniques to study the anisotropy and internal stresses of Ti-6Al-4V samples processed by selective laser melting. Mechanical spectroscopy results suitable in this kind of study, as it mimics the operating conditions of the material.

Assessment of a Heterogeneous Environment

An approach to the assessment of a heterogeneous environment in accordance with the proposed concept of a heterogeneous environment is described. The classification of industrial heterogeneous media by technological feature is proposed. The processes described by the proposed approach include the process of metal cutting, surface plastic deformation, chemical-thermal treatment, formation of coatings, welding, most of the processes that change the level and gradient of hardness and internal stresses in the surface layer of the material. Application of the proposed principles makes it possible to find new directions in the study of such processes.

MODEL OF HEAT GENERATION DURING DEFORMATION OF MEDIA WITH INTERNAL STRESSES

Observations of a solid body with internal stresses have been carried out, this model allows us to describe the energy dissipation during the change of the elastic stage of deformation to the plastic one. The dependence of heat release on the heat physical properties of the contacting structures is noted.

Design of tension structures and shells using the Airy stress function

Discontinuities in the Airy stress function for in-plane stress analysis represent forces and moments in connected one-dimensional elements. We expand this representation to curved membrane-action structures, such as shells and cable nets, and graphically visualise the internal stresses and section forces at the boundary necessary for equilibrium. The approach enhances understanding of the interplay between form and forces and can support design decisions related to form-finding and force efficiency. As illustrative examples, the prestressing needed for three existing cable nets is determined, and its influence on the edge-beam bending moment is explored.

Thermal residual stresses in thermoplastic CFRP-steel laminates: Modification and influence on fatigue life

Thermal residual stresses (TRS) in hybrid materials and structures occur by the mismatch of thermal expansion of different materials. Especially when combining metals with carbon fiber reinforced plastics (CFRP), a significant level of internal stresses can be reached. High processing temperatures and high stiffness of the constituents are also responsible for high stress levels. Laminates of thermoplastic CFRP (unidirectional carbon fiber reinforced polyamide 6) and stainless steel foils are a suitable material system to examine the TRS in detail. Since TRSs in the steel fraction are of tensile nature, these superpose to externally applied loads, resulting in higher efforts for the material and thus reduced lifetimes under cyclic fatigue loading. Therefore, a reduction of TRS is desired. Two methods for TRS reduction were applied, and its influence on fatigue lifetime was investigated. Firstly, specimens were stretched by a preloading to reduce TRS by yielding of the metal. Secondly, non-symmetric laminates were gradually cooled down after consolidation to compensate TRS formation by non-symmetric shrinkage. While preloading of materials and structures is known for TRS modification, the gradually cooling is not established, yet. Both modification principles were numerically investigated before experimental validation. A significant increase of lifetime was reached by TRS reduction.