Effect of surface treatments on strength of PZT polycrystalline ceramics

In bending strength determinations of ceramics, the fracture usually initiates from the tensile surface or the edges. The edge effect could be minimized by carefully rounding off the edges of specimens subjected to four-point bending. The effect of tensile surface polishing, polishing and rounding off the edges, and the acid treatment of the polished surface on the strength of PZT polycrystalline ceramics was determined along with the failure mechanism.Deceased.     

Investigation of explosive welding parameters and their effects on microhardness and shear strength

The aim of this study was to investigate the strength of explosive welded metals with the same chemical compositions. Different welding interfaces (straight, wavy and continuous solidified-melted) were used with changing explosive welding parameters [stand-off distance (s), explosive loading (R) and anvils]. Joined metals were investigated under heat-treated and untreated conditions. Results on the microstructure, microhardness, tensile shear strength and bending tests are reported. According to the experimental results, the effect of the anvil on the explosive welding process was only the joining or not-joining performance. It was shown that the bonding interface changed from a straight to a wavy structure when the explosive loading and stand-off distance were increased. For wavy interfaces, when the explosive loading was increased the wavy length and amplitude increased. Results of tensile shear and bending tests showed that heat-treated specimens have more strength than untreated samples. According to tensile shear test results, straight and wavy interfaces had similar strength. In addition, in bending tests of untreated specimens it was shown that the bending zone had some cracks.     

Impact of fibre orientation on tensile,bending and shear behaviors of a steel fibre reinforced concrete

Fibre orientation and density are known to have a significant influence on steel fibre reinforced concretes (SFRC) mechanical properties. In practice, parameters such as fresh state properties, restriction to concrete flowability and placing methods are likely to induce different fibre orientations in characterisation specimens and structural components. This difference in fibre orientation can impact the mechanical behavior of the structural component and therefore provide an unsafe design if not considered. This project consisted to produce a large SFRC slab, extract specimens with different fibre orientations, and submit specimens to tensile, bending and shear tests to evaluate the impact of fibre orientation and density on mechanical and post-peak strengths. Test results have shown that tensile and bending behaviors are mainly influenced by the fibre orientation, while the shear behavior is mainly impacted by fibre density. Test results were processed to allow comparison between tensile and bending tests. Linear correlations between tensile residual stresses and fibre orientation where found, linear or power type correlations according to bending residual stresses, as well as linear correlations between shear stresses and fibre density.     

Bending strength of silica glass

Tensile and bending tests are useful to characterize the mechanical behavior of ceramics. Theoretical comparisons between results of both tests are usually done based on Weibull statistics. In previous experiments on borosilicate glass, no agreement was found between experimental and theoretical values of the ratio of the maximum bending and tensile stresses at 50 percent probability of fracture. In this investigation, additional experiments in bending have been performed, to measure the distribution of fracture initiation points. Good agreement with theory is found. The previous disagreement could be attributed to fatigue effects.     

Fracture behaviour of a TiAl alloy under various loading modes

Tensile tests, compression tests, in situ tensile tests, bending tests, tensile fatigue tests and bending fatigue tests were carried out for a TiAl alloy. Based on the global experimental results and microscopic observations of the fracture surfaces and cracking behaviour on the side surfaces of tested specimens, the fracture mechanisms of fully lamellar (FL) TiAl alloys under various loading modes are summarized as following: (1) Cracks initiate at grain boundaries and/or interfaces between lamellae. (2) When a crack extends to a critical length, which matches the fracture loading stress the crack propagates catastrophically through entire specimen. (3) The crack with the critical length can be produced promptly by the applied load in the tensile and bending test or be produced step-by-step by a much lower load in the fatigue tensile test. (4) For fatigue bending tests, the fatigue crack initiates and extends directly from the notch root, then extends step-by-step with increasing the fatigue bending loads. The fatigue crack maybe extends through entire specimen at a lower fatigue load or triggers the cleavage through the whole specimen at a higher load. (5) In compressive tests, cracks initiate and propagate in directions parallel or inclined to the compressive load after producing appreciable plastic strains. The specimen can be fractured by the propagation of cracks in both directions.     

Experimental and numerical assessment of mechanical properties of welded tubes for hydroforming

The identification of welded tubes properties considering the weld bead and Heat Affected Zone (HAZ) is important for reliable and accurate finite element simulation of tubular plastic forming processes such as tube hydroforming and rotary draw bending processes. Therefore, a simplified method is proposed to extract the weld bead and HAZ properties. Full size standard tensile specimens cut from the welded tube and comprising the weld parallel to the load direction are extended to failure. Mechanical properties obtained from uniaxial tensile test are correlated with the microhardness data measured across the welded specimen and by using the rule of mixtures; the constitutive model parameters of weld bead and HAZ regions are identified. Accuracy of the proposed method is assessed by comparing finite element simulation predictions to experimental measurements obtained from two mechanical tests: the first one is the uniaxial tensile test performed on specimens comprising the weld line perpendicular to the loading direction and the second test is the free bulge hydroforming test achieved on seamed tubular samples. This investigation has shown that the presented method is practical in use and sufficiently accurate to extract the weld metal properties of seamed tubes.     

An experimental study on the tensile strength of steel fiber reinforced concrete

This paper deals with steel fiber reinforced concrete mechanical static behaviour and with its classification with respect to fibers content and mix-design variations. A number of experimental tests were conducted to investigate uniaxial compressive strength and tensile strength. Different mixtures were prepared varying both mix-design and fiber length. Fibers content in volume was of 1% and 2%. Mechanical characterization was performed by means of uniaxial compression tests with the aim of deriving the ultimate compressive strength of fiber concrete. Four-point bending tests on notched specimens were carried out to derive the first crack strength and the ductility indexes. The tensile strength of steel fiber reinforced concrete (SFRC) was obtained both from an experimental procedure and by using an analytical modelling. The experimental tests showed the different behaviour of SFRC with respect of the different fiber content and length. Based on the experimental results, an analytical model, reported in literature and used for the theoretical determination of direct tensile strength, was applied with the aim of making a comparison with experimental results. The comparison showed good overall agreement.     

Cryogenic fracture of cracked piezoelectric ceramics in three-point bending under electric fields

This paper investigates theoretically and experimentally the cryogenic fracture behavior of cracked piezoelectric ceramics under electric fields. Fracture tests were performed in three-point bending with the single-edge precracked-beam specimens at room temperature and liquid nitrogen temperature (77 K), and the fracture loads under electric fields were obtained. Plane strain finite element analysis was also carried out using temperature-dependent material properties of the piezoelectric ceramics, and the dependence of the energy release rate on the electric field and temperature was discussed. In addition, possible mechanisms for cryogenic fracture were examined by scanning electron microscopy.     

Reduction in tensile and flexural strength of unidirectional glass fibre-epoxy with increasing specimen size

Size effects in tensile failure were investigated by means of tensile and four-point bending tests. Tapered tensile specimens with plies dropped off internally showed a reduction in strain at failure with increasing gauge length. Scaled bending tests also showed a reduction in strain with increasing specimen size. These two effects and the relationship between the tensile and flexural results could all be fitted satisfactorily with a Weibull strength model.     

Bending characterisation of a molten unidirectional carbon fibre reinforced thermoplastic composite using a Dynamic Mechanical Analysis system

The quality of forming simulations based on Finite Element methods is mainly determined by the accuracy of the material properties. Out-of-plane bending is one of the deformation mechanisms that govern the appearance of wrinkles while forming composite reinforcements. This paper proposes a new test method using a Dynamic Mechanical Analysis (DMA) system for the characterisation of longitudinal out-of-plane bending properties of molten unidirectional thermoplastics. Investigations are presented for a unidirectional carbon fibre reinforced polyamide 6 composite. Several standard bending test fixtures are assessed quasistatically at three temperatures and three test speeds with specimens of different geometries. Additional tests are conducted at forming temperature with the selected test arrangement. The evaluation of different approaches for the calculation of the bending modulus shows the interlaminar shear to be negligible. Results highlight an important material strain rate dependency. The evolution of the bending modulus satisfies a linear fitting within the range of data.     

Mechanical testing of directionally solidified eutectic ceramics (DSECs): specific problems and limitations

The specific problems of DSECs mechanical testing result from the particularities of these 3-D interconnected eutectic ceramics. First of all, 4-point bending tests ensure pure bending loading, whereas 3 PB tests only lead to a tensile and shear stress combination. Consequently, due to the 3-D microstructure of DSECs, interfaces between the various phases are subjected to a mixed (tensile and shear) loading which makes the interpretation of the results (strength) and of the fracture surfaces, rather difficult. For usual ceramics, biaxial flexure testing offers many advantages over 3- or 4-point beam-bending testing. The coaxial-ring test is free of edge influences (flaws): cracks initiate in the central area and propagate outwardly. However, in the case of DSECs, due to the presence of high internal thermal stresses (especially for ternary eutectics), interfaces can be subjected to a strong radial tensile and shear (near the free surface) stress combination. In the presence of the radial tensile stress resulting from biaxial loading, this internal thermal stress combination can lead to premature crack initiation leading to failure. Specimen machining through grinding leads to the formation of a strongly damaged layer. Annealing of this layer leads to the formation of a rough surface: slightly protruding phases and stress concentrations at the interfaces. The measured strength is ≈20% lower after annealing than that directly after grinding. Concerning the effect of the microstructure size, four representative sizes have been selected in the ≈10 µm to submicrometre range. A classical crack propagation criterion has allowed explaining the corresponding strength values.     

Comparison of mechanical properties of martensite/ferrite and bainite/ferrite dual phase 4340 steels

Different microstructures were produced by heat treatment of 4340 steel. These microstructures are bainite, martensite, ferrite–martensite and ferrite–bainite. Mechanical tests were carried out at room temperature. The results showed that steel with bainite–ferrite microstructure has better ductility and charpy impact energy than steels with martensite–ferrite and full bainite microstructures. But yield and tensile strengths of this steel are less than the yield and tensile strengths of the other two steels. Hardness measurements showed that their hardness is the same. Fracture surface observations of tensile specimens showed increase in toughness of bainite–ferrite in comparison to martensite–ferrite and full bainite microstructures.     

Influences of tensile and bending strains on the critical current in multifilamentary Nb3Sn composites processed by Nb tube method

The critical current degradation of multifilamentary Nb₃Sn superconducting composites strained mechanically at room temperature has been investigated. The experimental results show that the effects of the Nb₃Sn layer thickness and the specimen wire structure, such as monolithic and stranded cable on critical current degradation, are appreciable for the specimens strained by bending stress, but are not for specimens strained by tensile stress. The results of the critical current degradation by tensile strain were discussed, based on the stress-strain characteristics of the composites.It was clarified that the critical strain in the case of applying tensile and bending stresses simultaneously at room temperature lay around the line which was drawn from the point of the critical tensile strain to that of the critical bending strain, when the ordinate was tensile strain and the abscissa was bending strain.     

Tensile behaviour in natural building stone: Serena sandstone

The tensile behavior of Serena sandstone is investigated by means of uni-axial tensile and bending tests. In order to obtain an overall characterisation of a quasi-brittle material, such as Serena sandstone, it is necessary to investigate the post-peak softening branch by adopting a stable control system. Test results on different size specimens are presented. Also analysed in this paper is the effect of the boundary conditions in a uni-axial tensile test by comparing free and fixed platens tests. A laser interferometry technique (ESPI) was adopted to investigate the strain field and crack propagation in the material.A notable outcome of the research is the location of crack penetration before the peak load, that in the case of the bending test is almost constant in specimens of different size.     

Ca3Co4O9 ceramics consolidated by SPS process: Optimisation of mechanical and thermoelectric properties

Ca₃Co₄O₉ (349) thermoelectric (TE) oxide ceramics were successfully prepared by Spark Plasma Sintering process. The effects of the uniaxial pressure (30-100 MPa), the dwell temperature (700-900 °C) and the cooling rate were investigated. Microstructure analyses have revealed strong enhancements of the bulk density as the pressure level and the applied temperature during the SPS process are increased. Mechanical properties were investigated by using instrumented nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness were shown to improve drastically and depend on the processing parameters. Thermal expansion measurements reveal a noticeable anisotropy induced by unidirectional hot pressing. The mechanical, thermal and thermoelectric properties were correlated to the microstructure and crystallographic texture of the resulting ceramics.     

Characterisation of mechanical behaviour and damage analysis of 2D woven composites under bending

In this paper, flexural loading of woven carbon fabric-reinforced polymer laminates is studied using a combination of experimental material characterisation, microscopic damage analysis and numerical simulations. Mechanical behaviour of these materials was quantified by carrying out tensile and large-deflection bending tests. A substantial difference was found between the materials' tensile and flexural properties due to a size effect and stress stiffening of thin laminates. A digital image-correlation technique capable of full-field strain-measurement was used to determine in-plane shear properties of the studied materials. Optical microscopy and micro-computed tomography were employed to investigate deformation and damage mechanisms in the specimens fractured in bending. Various damage modes such as matrix cracking, delaminations, tow debonding and fibre fracture were observed in these microstructural studies. A two-dimensional finite-element (FE) model was developed to analyse the onset and propagation of inter-ply delamination and intra-ply fabric fracture as well as their coupling in the fractured specimen. The developed FE model provided a correct prediction of the material's flexural response and successfully simulated the sequence and interaction of damage modes observed experimentally.     

Fracture mechanism of AZ31 magnesium alloy processed by equal channel angular pressing comparing three point bending test and tensile test

A commercial magnesium alloy, AZ31 in hot-rolled condition, has been processed by equal channel angular pressing (ECAP) to get microstructure modified. Uniaxial tensile tests were conducted along the rolling/extrusion direction for as-received AZ31 alloy and ECAPed AZ31 alloy. Then, three point bending fracture tests were conducted for specimens with a pre-crack perpendicular to the extruded direction. Digital image correlation (DIC) technique was adopted to determine the deformation field around the crack tip. The fracture surfaces of the failed specimens after tensile tests and fracture tests were observed by Scanning Electron Microscope (SEM). To explore the deformation mechanism, the microstructure and texture of different regions on the deformed specimens were examined through electron backscatter diffraction (EBSD). The results show ECAP process improves both the tensile elongation and fracture toughness of AZ31 alloy. Different from the slip dominated deformation mechanism in the tensile test, deformation twinning presents in the deformation zone adjacent to the crack tip in the three point bending fracture tests. The fracture surface is characterized by co-occurrence of dimple and cleavage features.     

Cracking mechanisms in durable sisal fiber reinforced cement composites

Fiber reinforced cement composite laminates with long sisal fibers were manufactured using a cast hand lay up technique. A matrix with partial cement replacement by metakaolin and calcined waste crushed clay brick was used in order to improve the durability aspects. Mechanical response was measured under tension and bending tests while crack formation was investigated using a high resolution image capturing procedure. Crack spacing was measured using image analysis and correlated with the applied strain under both the tensile and bending response. Various stages of loading corresponding to initiation, propagation, distribution, opening, and localization of a crack system in the specimen are discussed. The effect of flexural cracking on the location of neutral axis during the bending tests was measured using strain-gages.     

Experimental equipment for the evaluation of the strength characteristics of carbon-carbon composite mateials within the temperature range 20–2200°C

We suggest procedures for testing and describe the design of a UKM-2200 specialized experimental installation for tensile and bending testing of specimens made of carbon-carbon composite materials in a vaccum or inert media within the temperature range 20–2200°C. We discuss the results of strength tests for specimens of carbon-carbon composite materials with multidirected spatial reinforcement of the structure and with felt-like structure in a vacuum for the same temperature range. Translated from Problemy Prochnosti, No. 3, pp. 130–138, May–June, 1999.     

Mechanical and Structural Properties of Similar and Dissimilar Steel Joints

The mechanical properties of specimens from similar and dissimilar weld joints were examined. A ferritic steel (St37-2) and an austenitic stainless steel (AISI 304) were joined by the gas tungsten arc weld (GTAW) process using an austenitic filler metal. Mechanical and metallographic properties of the specimens were obtained by means of microhardness testing, tensile testing, bending fatigue testing, and light optical and scanning electron microscopy. The highest microhardness values were recorded on the ferritic–austenitic dissimilar weld joint, whereas the highest tensile strength and bending fatigue life were obtained with the austenitic–austenitic joints. Ferritic and pearlitic structures were observed in the microstructure of the ferritic–ferritic joint. The microstructures of austenitic–austenitic and austenitic–ferritic joints showed small recrystallization grains in addition to the typical austenitic and ferritic structures. Scanning electron microscopy was used to observe the fracture surfaces of the specimens and the origins of the fatigue cracks.