查看更多>>摘要：The flame straightening of steel components is based on heating a local region of the part by means of a torch in order to induce a permanent deformation through a field of residual stresses. Although this is a very common practice, it is not devoid of serious drawbacks. In this paper, the influence of the flame bending procedure on the microstructure of three very different structural steels (S235 JR, S460 ML and S690 QL, respectively), widely used for the construction of metallic structures, is analysed. The consequences of the heat treatment on the mechanical and fracture properties were characterised through micro-hardness Vickers and Charpy impact tests; in addition, some elastic-plastic fracture tests were performed on precracked Charpy specimens manufactured with the S235 JR steel. The relationship between the microstructural features and the material mechanical and fracture behaviour was studied in depth in all cases, correlating the changes induced by the flame heat treatment on the microstructure with the macroscopic mechanical and fracture response. For a proper understanding of the microstructural consequences of this straightening heat treatment, it was necessary to develop a Finite Element numerical model. Based on the experimental results, this study has revealed that the consequences of the flame straightening on the microstructure, mechanical or fracture behaviour strongly depend on the nature of the material; for this reason, it is not possible to establish general recommendations. Nevertheless, the paper proposes a series of guidelines for good practice for steels similar to those characterised here.
V. A. Acosta SantamariaJ. M. Garcia AznarI. OchoaM. Doblare...
7页
查看更多>>摘要：Mechanical characterization of articular cartilage has been widely analyzed in the literature, both experimentally and numerically, since load support (strength) and deformability (compliance) are among the most important physiological functions of this living tissue. They are also important indicators of cartilage degradation and regeneration. Cartilage elastic properties (Young's Modulus, Aggregate Modulus and Poisson ratio) are usually determined experimentally by uniaxial confined and unconfined compression tests. Relevant differences can be found in the literature for these variables even when similar experimental protocols are used. Before starting the actual compression test, a pre-contact protocol has to be carried out in order to obtain optimum contact between the test tool and the cartilage sample. The aim of this work is to analyze the effect of sample pre-contact on the test results comparing three different protocols available in the literature (predeformation of 4 and 10% of the total thickness of the sample and a preload equivalent to 12.5 KPa). The implementation of these protocols achieved contact, but only the predeformation allowed normalized experiments and ensured repeatability of the tests. Additionally, under the predeformation protocols, the results and trends for uniaxial compression tests were consistent with the cartilage physiological function. However, for 10% predeformation, the experimental data showed maximum dispersion at the final stress levels. The statistical treatment determined significant differences between the 10% predeformation and the preload results (Aggregate Modulus and Poisson ratio), defining them as non comparable protocols. Finally, a new pre-contact protocol is proposed introducing an initial approximation point between the test tool and the cartilage sample, and followed by a predeformation of 4% for confined and unconfined tests.
查看更多>>摘要：The feasibility of determining the creep compliance of asphalt concrete from a notched Semi-Circular Bend (SCB) test specimen was investigated. The objective of this study was to propose a combined test methodology that can provide both viscoelastic and fracture properties of asphalt concrete mixtures tested at low temperatures. Finite element (FE) analyses were performed to understand the stress state in the SCB, and an optimal load range producing appreciable displacement measurements while preserving the linear viscoelastic conditions was identified. Expressions that relate displacement measurements, from particular regions of the SCB specimen, to creep function were derived. The validity of the proposed SCB creep method was tested both by numerical simulations and experimental testing. Good agreement was found between the creep function obtained from SCB and those obtained from the Three-Point Bending Beam (3PBB) and the Indirect Tensile (IDT) creep test.
查看更多>>摘要：Instrumented indentation is a technique that can be used to measure the elastic properties of soft thin films supported on stiffer substrates, including polymer films, cellulosic sheets, and thin layers of biological materials. When measuring thin film properties using indentation, the effect of the substrate must be considered. Most existing models for determining the properties of thin films from indentation measurements were developed for metal and dielectric films bonded to semiconductor substrates and have been applied to systems with film-substrate modulus ratios between 0.1 and 10. In the present work, flat punch indentation of a thin film either bonded to or in contact with a substrate is examined using finite element modeling. A broad range of film-substrate modulus ratios from 0.0001 to 1 are investigated. As the substrate is effectively rigid compared to the film when the film-substrate modulus ratio is less than 0.0001, the results are also useful for understanding systems with lower film-substrate modulus ratios. The effects of the contact radius, film thickness, elastic properties, and friction between the film and the substrate on the measured stiffness were quantified using finite element modeling in order to understand how the elastic properties of the film can be extracted from indentation measurements. A semi-analytical model was developed to describe the finite element modeling results and facilitate the use of the results to analyze experimental measurements. The model was validated through analysis of indentation measurements of thin polyethylene sheets that were supported on substrates of various stiffness.
查看更多>>摘要：Wheel-rail rolling contact at railhead edge, such as a gap in an insulated rail joint, is a complex problem; there are only limited analytical, numerical and experimental studies available on this problem in the academic literature. This paper describes experimental and numerical investigations of railhead strains in the vicinity of the edge under the contact of a loaded wheel. A full-scale test rig was developed to cyclically apply wheel/rail rolling contact load to the edge zone of the railhead. An image analysis technique was employed to determine the railhead vertical, lateral and shear strain components. The vertical strains determined using the image analysis method have been validated with the strain gauge measurements and used for the calibration of a 3D nonlinear Finite Element Model (FEM) that simulates the wheel/rail contact at the railhead edge and use suitable boundary conditions commensurate to the experimental setup. The FEM was then used to determine other states of strains.
查看更多>>摘要：Coherent gradient sensing (CGS) method, a real time, full-field optical technique, is insensitive to vibrations and able to provide slope and curvature maps and surface topographies, to investigate non-uniform deformations. In this paper, we analyze the thermal effects on the optical path in CGS due to air convection, and the influence of grating thickness and refractive index on the measurement accuracy. A modified governing equation is derived considering the grating thickness, which is demonstrated by testing a standard sample. Finally, we apply CGS method to measure the full-field deformation of a specimen at high temperature.
查看更多>>摘要：The residual stress state in thick film systems, as for example thermal spray coatings, is crucial for many of the component's properties and for the evaluation of the integrity of the coating under thermal and/or mechanical loading. Therefore it is necessary to be able to determine the local residual stress distribution in the coating, at the interface and in the substrate. The incremental hole-drilling method is a widely used method for measuring residual stress depth profiles, which was already applied for thermally sprayed coatings. But so far no reliable hole-drilling evaluation method exists for layered materials having a stress gradient in depth. The objective was to investigate, how far existing evaluation methods of the incremental hole-drilling method that are only valid for residual stress analysis of homogenous material states can be applied to thick film systems with coating thicknesses between 50 μm and 1000 μm and to point out the application limits for these already existing methods. A systematic Finite Element (FE) study was carried out for coating systems with an axisymmetric residual stress state σ_1 = σ_2. It is shown that conventional evaluation methods developed for homogeneous, non-layered material states can be successfully applied for a stress evaluation in the substrate and the coating for small and for sufficiently large coating thicknesses, respectively, regardless of the type of evaluation algorithm used i.e. the differential or the integral method. The same accounts for material combinations that have a Young's modulus ratio close to one, between 0.8 and 1.2. The studies indicated that outside the given ranges case specific calibration must be applied to calculate reliable results. Further, calibration data were determined case specifically for a selected model coating system. The accuracy of a residual stress determination using these case specific calibration data was examined and the sensitivity of the evaluation with respect to an accurate knowledge of the boundary conditions of the coating system i.e. the coating thickness and the Young modulus was studied systematically. Finally, the calibration data were applied on a thermally sprayed aluminium coating on a steel substrate analysis and the results for using the incremental hole drilling method were compared to results from X-ray stress analysis.
C. A. SciammarellaA. BoccaccioL. LambertiC. Pappalettere...
11页
查看更多>>摘要：Thin film technology is an area of great importance in current applications of opto-electronics, electronics, MEMS and computer technology. A critical issue in thin film technology is residual stresses that arise when the coating is deposited onto a substrate. Residual stresses can be very large in magnitude and have detrimental effects on the role that the thin film must play. To save development time on coating deposition processes it is important to perform accurate residual stresses measurements in situ in real time where the deposition is made. A novel optical set up is developed in this study to measure deflections and residual stresses generated in coated specimens that can be applied directly in the reactor utilized in the deposition process. Experimental results are in good agreement with other measurements carried out independently and other data reported in literature for thin films like those tested in the experiments.
查看更多>>摘要：An extensive suite of experiments was conducted to characterize the mechanical response of an S-2 glass composite. The primary interest was the response of a 3-D composite, consisting of unidirectional (non-woven) layers of glass fibers interlaced by through-thickness Z-yarns. A plain-weave material was also characterized for comparison purposes. Additionally, epoxy-only specimens were fabricated to assist in understanding the contribution of the SC-15 epoxy resin in the response of the composite system. Two new specimen geometries (torsion and hourglass) were developed specifically for this characterization effort. The response of these specimens provides considerable insight into the failure mechanics of the plain weave and 3-D weave composites. It was shown that the matrix material has an elastic-plastic response, but with different strengths in tension and torsion. The response of the composite in tension is controlled by the epoxy until failure at the glass-resin interface. The strength falls to zero for the plainweave composite, but the Z-yarns can support tensile stress until the yams begin to fail. The fibers contribute to the elastic stiffness in shear for the plain-weave material, but the failure strength in shear is the same as the matrix. The 3-D weave composite also fails at the failure strength of the matrix, but retains some shear strength because of the Z-yarns.
查看更多>>摘要：This paper presents an effective methodology to characterize all the constitutive (elastic) parameters of an orthotropic polymeric foam material (Divinycell H100) in one single test using Digital Image Correlation (DIC) in combination with the Virtual Fields Method (VFM). A modified Arcan fixture is used to induce various loading conditions ranging from pure shear or axial loading in tension or compression to bidirectional loading. A numerical optimization study was performed with different loading angles of the Arcan test fixture and off-axis angles of the principal material axes. The objective is to identify the configuration that gives the minimum sensitivity to noise and missing data on the specimen edges, which are the two major issues when identifying the stiffness components from actual DIC measurements. Two optimized Arcan test configurations were chosen. The experimental results obtained for these two optimized test configurations show a significant improvement of the measurement accuracy compared with a pure shear load configuration. The larger sensitivity of the pure shear test to missing data as opposed to the tensile test is also evident from the experimental data and confirms the analysis from the optimization study. The recovery of missing data along the specimen edges is a promising way to further improve the identification results.
NSTL
Springer Nature