Flat Specimens Research Articles

Dislocation detection of gas turbine materials using a nonlinear ultrasound modulation technique

Industrial gas turbines are used for generating electricity or driving other turbomachinery with the continuous development goal of further increasing machine efficiency. This is primarily achieved by raising the pressure ratio generated in the compressor and by increasing the turbine inlet temperature. Consequently, the hot gas components in gas turbines are subjected to extreme loads and the need for non-destructive testing and structural health monitoring techniques is becoming increasingly important to maintain these components. An important indicator for assessing the structural integrity is the determination of the initial plastic deformation.In this paper, a new method for the detection of plasticity was developed, which is based on a nonlinear ultrasonic two-frequency excitation. The one-dimensional wave equation was solved with a two-frequency excitation and combined with the expanded dislocation theory. As a result, various nonlinearity parameters were defined, showing a clear increasing or a decreasing behaviour with increasing plastic strain. This was experimentally proven with flat tensile specimen made of stainless steel and Inconel 718 (metal plates and additively manufactured). The new indicators allow the possibility to efficiently detect the initial plastic deformation in gas turbine components.

Progressive collapse mode induced by slab-column joint failure in RC flat plate substructures: A new perspective

Majority of progressive collapse incidents of concrete flat plate structures were caused by slab overloading in the form of punching shear failure at a slab-column joint, which exhibit distinctive resistance mechanism from those triggered by column losses. Relevant studies on the slab overloading scenario are scarce. To fill this gap in research and to broaden the perspective on different collapse modes, an experimental test was conducted on a 1/3-scale 2 × 2-bay flat plate substructure specimen by applying an increased downward uniform load on the slab, until the structure was completely damaged following the initial punching shear failure at the interior slab-column joint and was unable to take any further load. In addition, the flexural capacity of the specimen was evaluated using the yield line theory, and an analytical model was also proposed to estimate the post-punching peak load of the specimen. Finally, the experimental results obtained from this study were compared to those of a similar test under an interior column removal scenario. Comparisons indicate that the specimen under the slab overloading scenario displays better collapse resistance than that under the column removal scenario. Nevertheless, the residual resistant capacity of the overloaded specimen may not be adequate enough to prevent the collapse of the entire structure after the initial punching failure.

TO THE CALCULATION OF MACHINE PARTS FOR FATIGUE RESISTANCE

Purpose. The aim of the work was to summarize the reference and experimental data and to provide recommendations for the design of machine parts for fatigue resistance under conditions of cycle asymmetry.
 Research methods. To study the problem of resistance of structural materials to fatigue under conditions of cycle asymmetry, an installation for testing samples at any coefficients of cycle asymmetry, both at tensile and compressive average stresses, created at the National University “Zaporizhzhya Polytechnic”. On this installation, tests were carried out on flat smooth specimens with a cross section of 5×20 mm and specimens with a cross section of 5×24 mm, with a form-hole concentrator with a diameter of 5 mm, made of 09Г2С steel in accordance with DSTU 8541. The smooth specimens were tested at average cycle stresses of 0, 100, 200, 300, -100, -150, -200 MPa up to a service life of 5×106 cycles. The reference data of steel with a tensile strength of σв=520 MPa for a service life of 2×106 cycles and the effective stress concentration factor were processed after determining the ultimate amplitudes and the corresponding average cycle stresses at different cycle asymmetry factors.
 Results. The obtained results of studying the work of structural materials under cyclic loads with different cycle asymmetries allow us to better understand the physical processes of fatigue materials of machine parts, as well as to reveal the dependence of the effective stress concentration factor not only on the amplitude, but also on the value of the average cycle stress. The work shows that the effect of the stress concentrator depends on the value of the average cycle stress.
 Scientific novelty. A formula for obtaining a complete diagram of ultimate amplitudes that takes into account durability is proposed.
 Practical value. The work can be useful for designers to more effectively use the service properties of structural materials, as well as for researchers to save reimbursements in fatigue resistance tests.

Temperature-dependent deformation and fracture properties of low-carbon martensitic steel in different stress states

The temperature-dependent ductile-brittle transition (DBT) in low-carbon martensitic steel is investigated in different stress states. To capture the effects of the stress state, tensile tests have been performed with flat specimens and three-point bending tests have been conducted with Charpy specimens at dynamic (impact) and quasi-static loading rates, covering a broad temperature range of 77 K–423 K. The results reveal that ductile-brittle transition temperature (DBTT) is significantly affected by stress triaxiality, wherein the transition temperature in uniaxial tension is ∼142 K lower than that in plane strain condition of Charpy specimens. The energy absorption capacity in the ductile resistance domain during three-point bending tests is sensitive to the loading rate. Moreover, dynamic strain aging poses a decrease in ductile resistance upon quasi-static loading when the temperature exceeds 323 K and thus without a specific upper shelf plateau. Tensile strength and ductility tend to increase with decreasing temperature, and the associated failure mode changes from shear-dominated to nearly pure tension due to the specific discrepancy of temperature influence on the critical normal and shear strengths, wherein the formation of shear bands that dominate the tensile deformation could be strictly inhibited by the formation of nanoscale dislocation cells based on dislocation analysis. The failure mechanisms in tensile tests correlate with the tensile specimens' macroscopic fracture angle. In the end, a new local parameter is proposed for temperature-dependent toughness estimation in tensile tests.

Low-velocity impact behavior of elliptic curved composite structures

Although many composite structures are inconsistently curved, such as the leading edges of aircraft wings, the variety of research in impact engineering is almost limited to the impact performance of plates or cylindrically curved specimens. It is not known whether the findings obtained from standardized tests can be transferred to curved structures or which adaptions are required. Therefore, a deeper understanding of the deformation and damage behavior of inconsistently curved structures is essential to transfer the observed impact behavior of flat specimens to general curved structures and therefore to utilize the full lightweight potential of a load-specific design. An accurate description of the procedure as well as the results of the experimental and numerical study of the low-velocity impact behavior of differently single-curved elliptic specimens is presented. To close the research gap of the impact behavior of geometries with curvatures between the plates and simplified leading edges, novel specimens geometries have been derived from established impact test standards. Glassfiber-reinforced specimens are subjected to an instrumented impact test at constant impact energy. This is numerically investigated by a stacked-layer model, which used cohesive zone modeling to enable the simulation of matrix cracking, fiber fracture and delamination. The resulting projected damage areas, as well as the force and deflection histories, were evaluated and section cuts were examined to discuss the damage morphology, formation and propagation process. Significant effects on maximum deflection, compliance and dynamic behavior on the size and morphology of damage were found.

BIO20: In Vitro Thrombogenicity Testing of Various Silicone Dispersions for VAD Inflow Cannula

Study: Our group is developing next-generation ventricular cannula for use with the PediaFlow™ miniature totally magnetically levitated ventricular assist device. Various formulations of silicone elastomers are being investigated based on criteria of durometer and tear strength. In addition to mechanical properties, the dispersions employ various curing systems (e.g., platinum cure, acetoxy…) and solvent carriers (e.g., heptane, xylene…) which may affect their thrombogenicity. This study aimed to assess thrombus deposition on candidate materials with recalcified blood in vitro. Methods: Preliminary studies assessed a flat specimen of one-part acetoxy cure room-temperature-vulcanizing silicone compared to latex (positive control) and PTFE (negative control). Material samples 4.0x4.5 cm in size were cut lengthwise and both halves were placed in 10 mL no-additive BD Vacutainer blood collection tube as shown in Figure 1a. Tubes were filled with sheep whole blood in CPD (Lampire Biological Laboratories). The blood was heparinized and recalcified to achieve a target ACT value between 250-300 s. The tubes were rotated on a modified hot dog roller, shown on Figure 1b, at 2.5 RPM for 30 minutes. Afterwards samples were rinsed and assessed for gross thrombus deposition. Based on these preliminary studies, the protocol was optimized for further comparative testing of the remaining silicone candidates – which will be presented at the conference. Results: Figure 1d shows specimens after 30 minutes. While there was an appreciable difference between the positive and negative control materials, an artifact was suspected due to blood trapped between the sample and the wall of the tube. To circumvent this issue, an improved protocol was developed using short segments of PVC (Tygon®) tubing coated with the materials of interest. Latex tubing and uncoated PVC tubing will be used as the positive and negative controls, respectively.Figure 1. (a) Material sample placement in a 10 ml Vacutainer blood collection tube. (b) A hot dog roller with the heating element disabled was used to slowly spin the tubes. (c) The latex (positive control), PTFE (negative control), and Silicone samples before the test. (d) The three material samples after 30 minutes in whole blood.

On the Characteristics of Fatigue Fracture with Rapid Frequency Change.

The effect of sudden variations in working modes and fatigue behavior of CS 1018 is studied. A general model based on the framework of the fracture fatigue entropy (FFE) concept is developed to capture such changes. Fully reversed bending tests are performed on flat dog bone specimens with a series of variable frequency tests without turning the machine off to simulate fluctuating working conditions. The results are then post-processed and analyzed to assess how fatigue life changes when a component is subjected to sudden changes in multiple frequencies. It is demonstrated that regardless of the frequency changes, FFE remains constant and stays within a narrow band range, similar to that of a constant frequency.

Long-term viscoelastic properties of carbon fiber/epoxy composites using tow prepreg strand specimens

Carbon fiber/epoxy tow prepreg is an intermediate material for filament winding, consisting of fibers pre-impregnated with a matrix resin in a specific ratio. This study predicted the long-term viscoelastic properties of tow prepreg using DMA and TTSP. Typically, DMA involves applying a wound or laminated flat specimen, but a strand specimen was attempted to predict the characteristics of tow prepreg. The long-term viscoelastic properties were storage modulus, creep compliance, and relaxation modulus. Three test modes were performed in DMA to obtain each result. Smoothly overlapped master curves were obtained in all results. From the master curves, the viscoelastic properties of the material were predicted for 1 year, 10 years, and more extended periods. For 10 years, it was estimated that the storage modulus, creep compliance, and relaxation modulus changed by 7%, 132%, and 48%, respectively. When tests are conducted, considering future design variables, such as temperature, humidity, and out time, the results can be utilized to predict the long-term performance of tow prepreg and optimize composite manufacturing processes.

Using the Corner Reflection for Depth Evaluation of Surface Breaking Cracks in Concrete by Ultrasound

The corner echo is a well-known effect in ultrasonic testing, which allows detection of surface breaking cracks with predominantly perpendicular orientation to the surface as, for example, corrosion cracks in metal pipes or shafts. This echo is formed by two planes, the surface of the crack and the surface which the crack breaks. It can also be classified as a half-skip method, since a reflection of the pulse occurs on the backwall before the reflection at the defect takes place. In combination with the diffraction from the crack tip, the corner echo also allows crack sizing. As shown in this paper, the corner reflection can be used in civil engineering for nondestructive inspection of concrete. Commercially available low frequency ultrasonic arrays with dry point contact sources generate SH transversal waves with sufficient divergence of the sound field in order to detect corner reflections. Ultrasonic line-scans and area-scans were acquired with a linear array on flat concrete specimens, and the data were reconstructed by the Synthetic aperture focusing technique. If the angles and the area of reconstruction are chosen accordingly, the corner echo reflection can be distinguished from other ultrasonic information. The corner echo can thus be used as a method for deciding whether a crack is a partial-depth crack or a full-depth crack and thus for obtaining a statement about crack depth. This paper presents corresponding experimental results obtained on concrete specimens with artificial test defects and cracks induced under controlled conditions.

Structural Behaviour of Voided Flat Slab With Various Void Ratios Under Vertical Loading

Abstract In recent modern buildings, voided slabs have been implemented because of the high reduction of their weight up to 35 %. Also, CO2 emission takes place which was developed from mass concrete. Due to percentage reduction of concrete, it leads to help the pollution free environment, eco-friendly and also more sustainable. On voided flat slabs, a minimal study has been recorded. In this current study, an approach has been made to study the effect of the position of voids in the voided flat slabs. Two groups of seven specimens each, one slab in each group was a controlled flat slab specimen with zero number of voids kept as a reference specimen, while the others were voided slabs. In each group, the number of voids were used based on the positions. The designation of the specimens is made depending on the number of voids placed and the diameter of the rebar used in the specimens. Group-A was reinforced with 8 mm diameter rebars, and Group-B was reinforced with 6 mm diameter rebars. The parameters of the study are the void positions and the rebar diameter. The specimens were subjected to a gradually concentrated load through the short column up to failure. The load carrying capacity of both voided and controlled flat slabs tested have almost same. It indicates that, the inactive concrete below the neutral axis where high density polyethylene (HDPE) recycled hollow spheres have been provided in the flat slab has shown no impact on load carrying capacity of the test specimens. However, the presence of the punching shear can be seen in the specimens where the voids are positioned near the columns and supports. Besides, ABAQUS program (A finite element based software) was used to perform the finite element analysis for profoundly illustrating the experimental findings and performing a parametric study. It is concluded that the concrete has no role at below the neutral axis of structural voided flat slab cross-section for all specimens including controlled specimens (0VS6, 0VS8). Also, it is noticed that the finite element analysis obtained results have shown more or less same as experimental results. From the present study, it is evident that behaviour of voided flat slabs are different from simply supported voided flat slabs.

Effect of surface texture on the tribological behavior of sliding electrical contact interface

Two kinds of textured surfaces, i.e. square pit-textured surface (SPTS) and round pit-textured surface (RPTS), are manufactured on the flat specimen surface, respectively. A series of current-carrying tests are carried out to detect the ability of surface texture in affecting the tribological behaviors. Results show that the friction coefficient and friction force of RPTS is the lowest among all the surfaces. Moreover, the RPTS shows the best potential in suppressing the generation of friction-induced vibration (FIV). Although the contact resistance of RPTS is slightly larger than that of other surfaces, the continuity of electrical contact is acceptable. Worn surface analysis is performed to illustrate the different tribological performances of these surfaces. Finite element analysis is also performed to provide reasonable explanation for the test phenomenon.

Effect of tool offset distance variation and mechanical property evaluation for effective cladding of copper to steel by friction stir cladding

Deposition of copper on top of a steel substrate combines the advantages of the corrosion resistance of copper with the high tensile strength of steel. Due to their diverse properties, it is challenging to deposit copper on steel. In this work, the deposition of a 3 mm-thick copper layer has been carried out on top of a 6 mm thick steel substrate, utilizing the friction stir welding (FSW) process. During the process, several repetitive passes are required with a suitable tool offset to cover the entire plate effectively. A smaller tool pin offset results in the requirement of more passes, while too large a pin offset leaves the area between two passes unbonded. To optimize the tool offset distance, a series of trial experiments with offset values of 6, 8, 10, and 12 mm were carried out. Metallography examination along with X-ray radiography was carried out on the prepared clad samples. The average grain size of the steel substrate below the clad layer showed refined grains with a size of 5 μm, indicating an improvement of more than 60 % over the base steel. SEM and EDS map analysis revealed proper bonding of the cladding material with the substrate. Uniaxial tensile tests on flat specimens were carried out for base copper, base steel, and copper clad steel samples. The copper clad steel has a YS of 261 MPa and tensile strength of 359 MPa in comparison to the base copper, which has a YS of 131 MPa and a UTS of 227 MPa. Fractography analysis revealed stretching marks between copper and steel due to the difference in elongation rates of both materials. A guided bend test for face bend, root bend, and side bend revealed no delamination or crack initiation along the convex surface. Microhardness analysis was carried out across the interface from copper to steel, where a maximum hardness value of 226 HV was recorded near the interface close to the steel region.

Additive manufacturing of hexagonal lattice structures: tensile tests and validation

Abstract In this paper, different hexagonal lattice structures of the tensile test specimens were designed, manufactured by additive manufacturing, and tested in three different lengths. Moreover, flat shape tensile test specimens were designed without lattice structure. Mechanical behaviors were obtained using a tensile testing machine and analyzed at a finite element program. Effect of lattice shape and infill density on mechanical behavior was investigated. Results were compared with finite element analysis to validate the simulation. Main objective of this paper is to investigate the tensile test properties of polylactic acid (PLA) test specimens with different hexagonal-shaped lattice structures and verify their finite element model. It is concluded that increasing hexagonal lattice size increases ductility up to certain degree. Elastic modulus of 15% and 100% infill density specimens was nearly same at lattice region.

Discontinuous Yielding of Fe E420 under High Strain Rate Loading

The discontinuous yielding behaviour of Fe E420 steel was experimentally investigated through tensile tests conducted on laminated sheets. In order to assess the response under both quasi-static and dynamic loading conditions, uniaxial tensile tests were performed according to ASTM E8 standards, and dynamic tests were conducted on the Hopkinson bar using a specially designed flat specimen. Three nominal strain rates (2×10−4, 500, and 1000) s−1 were considered. The effect of heat treatment on the material was also considered. For this purpose, the proposed experimental campaign was carried out on both the as-is material and on one treated by water quenching. All dynamic tests were performed with the direct tensile split Hopkinson pressure bar and were recorded via a high frame rate camera. The post failure fracture surfaces were investigated using scanning electron microscopy technique.

Role of surface wetting on tribological behavior for laser nanotextured steel using ionic liquid lubricants

This research evaluates the effect of surface wettability on the tribological performance through ball-on-flat tribology testing. The substrate material, M2 tool steel, is laser processed and then functionalized with fluorocarbon and nitrile chemistry to achieve distinct oleophobicity and oleophilicity, respectively, but with a similar nanoscale surface texture. The baseline lubricant is poly-alpha-olefin (PAO) oil, and ionic liquids (ILs) are used as additives for this study. The interaction between the nanoscale textured steel surface and ionic liquid-based oils is investigated. A set of reciprocating wear tests are performed to investigate the tribological behavior of the tribo-system consisting of the surface-engineered, flat M2 tool steel specimen and a standard, surface-polished steel ball. Results show that the oleophobic flat surface results in a lower friction, while the oleophilic surface modification leads to a better wear protection to the flat surface. Ammonium-based IL provides the highest friction reduction, while the phosphonium-based ILs provide an improved wear protection.

Surface Optimization of Components Obtained by Fused Deposition Modeling for Air-Plasma-Sprayed Ceramic Coatings

Additive manufacturing is an emerging disruptive 3D printing technology that is stimulating innovations in design and engineering, materials, and manufacturing thanks to the prospects of reducing cost and waste and increasing efficiency; in doing so, it presents the potential to have marked industrial, economic, and societal impacts. Thermoplastic polymers show some ideal characteristics for the most common additive manufacturing methods, and this aids in the improvement of the design accuracy and reliability and makes inroads for the customized manufacturing of high-design-flexibility polymer parts. Despite this, this material family is strongly sensitive to temperature, and one of the viable ways of limiting this weak point is surface coating with thermal barriers. The focus of this work was the optimization of an additive manufacturing process for producing thermoplastic components and to improve the adhesion of a thermal barrier coating on their surface. In detail, flat plate specimens of ULTEM 1010 were obtained by the fused deposition modeling technique by varying two significant surface parameters, the enhanced visible rasters and the visible raster air gap; then, their surfaces were covered by a thin ceramic coating by an air plasma spray system. A micro-geometric analysis that was conducted using a confocal microscope and the coating thickness measurements highlighted that a global larger roughness value, the presence of more flat peaks with a large area, and the complexity of the texture can be considered as supporting factors for improving the mechanical gripping and allowing a uniform adhesion of the coating powders on the thermoplastic substrate.

The Analysis of Mechanical Properties and Geometric Accuracy in Specimens Printed in Material Jetting Technology.

The purpose of this research was to analyze polymer materials based on mechanical properties and geometrical parameters, such as the smallest material deviations and the best printing texture after three-dimensional (3D) printing in two methods of Material Jetting technology: PolyJet and MultiJet. This study covers checks for Vero Plus, Rigur, Durus, ABS, and VisiJet M2R-WT materials. Thirty flat specimens were printed both for 0 and 90 raster orientations. Specimen scans were superimposed on the 3D model from CAD software. Each of them was tested, paying attention to the accuracy and the layer thickness effect of printed components. Then, all specimens were subjected to tensile tests. The obtained data-Young's modulus and Poisson's ratio-were compared using statistical methods, focusing on the two most important parameters: the isotropy of the printed material in two directions and the characteristics close to linear. It was found that unitary surface deviation with general dimensional accuracy equal to ±0.1 mm was the common feature of printed models. Some small areas had lower accuracy depending on the material and printer device. Rigur material obtained the highest mechanical properties. Dimensional accuracy in Material Jetting technology as a function of layer parameters such as layer thickness and raster orientation was checked. The materials were checked in terms of relative isotropy and linearity. Additionally, similarities and differences between PolyJet and MultiJet methods were covered.

Size-dependence of AM Ti–6Al–4V: Experimental characterization and applications in thin-walled structures simulations

Previous studies show that the properties of parts manufactured via additive manufacturing, such as selective laser melting, depend on local feature sizes like lattice wall thickness and strut diameter. Although size dependence has been studied extensively, it was not included in constitutive models for numerical simulations. In this work, flat dog-bone tensile specimens of different thicknesses were manufactured and tested under quasi-static conditions to characterize the size-dependent properties experimentally. It was observed that key mechanical properties decrease with specimen thickness. Through curve-fitting to experimental data, this work provides approximate analytical expressions for the material properties values as a function of specimen thickness, furnishing a phenomenological size-dependent constitutive model. The interpolating capability of the model is cross-validated with existing experimental data. Two numerical examples demonstrate the application of the size-dependent material model. The axial crushing of thin-walled lattices at varying wall thicknesses was simulated by the size-dependent material model and one that ignores size effects. Results show that ignoring size effects leads to overestimated peak crushing force and specific energy absorption. The two material models were also compared in the topology optimization of thin-walled structures. Results show that the size-dependent model leads to a more robust optimized design: having higher energy absorption and sustaining less material fracture.

Impact of Irradiation on the Adhesive Performance of Resin-Based Dental Biomaterials: A Systematic Review of Laboratory Studies

Head and neck cancers are a significant global health burden, with radiation therapy being a frequently utilized treatment. The aim of this systematic review was to provide a critical appraisal of laboratory studies that assessed the effect of irradiation on the adhesive performance of resin-based biomaterials. The analysis included 23 laboratory studies obtained from five databases, with most studies using human enamel, dentin, or both, and bonding procedures involving the fabrication of direct restorations, standardized specimens, bonding of orthodontic brackets, and luting of endodontic fiber posts. The protocols used for irradiation varied, with most studies exposing specimens made from extracted teeth to irradiation using cabinet irradiators to simulate treatment of head and neck cancer. The findings indicate that irradiation reduces the bond strength of dental adhesives and resin-based composites on flat, ground enamel and dentin specimens, with different adhesives and timing of irradiation having a significant impact on adhesive performance. Irradiation also increased microleakage in most studies. The effect of irradiation on marginal adaptation of direct resin-based composite restorations was inconclusive. This systematic review indicates that irradiation has detrimental effects on the adhesive performance of resin-based biomaterials and highlights the need for further clinical and laboratory studies evaluating the performance of adhesive materials and approaches to improve it.

The fracture behaviour of notched PMMA specimens under simple loading conditions – Tension and torsion experimental tests

This paper presents the results of experimental testing of flat PMMA specimens during uniaxial loading conditions. Two separate tests were conducted: tensile and torsion. The specimens were weakened with V-type edge notches with different root radii: 0.5; 2 and 10 mm. The specimens were made in two thickness variants: 5 and 15 mm. Monotonic tensile and torsion tests were carried out while keeping the averaged strain rate constant, due to the strong influence of this parameter on the PMMA behaviour. The procedure for non-contact measurement of the torsion angle using the ARAMIS 3D 4 M vision system is described. A set of PHANTOM cameras was used in the study, thanks to which all fracture processes were recorded. The processing of the obtained recordings made it possible to precisely indicate the moment (critical value of tensile force, torsional moment) and the location of crack initiation, as well as determine the nature of their evolution. Microscopic observations showed the structure of the failure surface and clearly indicated the different initiation sites observed for the two simple loading states. Different crack initiation angles were indicated, depending on the type of test conducted, as well as the notch root radius. The material in the article provides a starting point for numerical modelling of the fracture processes of notched elements and verifying the fracture criteria.