Specimen Gauge Length Research Articles

Current-rate flash sintering of gadolinium doped ceria: Microstructure and Defect generation

In current-rate flash-sintering experiments the current is injected into the specimen from the very start and then increased at a constant rate, while the furnace is held at a constant temperature. The power supply remains under current control. The flash is induced at low current densities which reduces local heating at the electrodes. It leads to a uniform grain size across the entire gage length of the dog-bone specimen. This work pertains to 10 mol.% gadolinium-doped ceria flash sintered at current-rates ranging from 50 mA min−1 to 1000 mA min−1 at a furnace temperature of 680 °C. Full densities are obtained at a current density limit of 200 mA mm–2. Densification is shown to depend only on the instantaneous value of the current density, and not on the current-rate. The grain size, however, is shown to become finer at higher current-rates. A preliminary analysis of the “energy deficit”, that is, the estimated power input corresponding to the temperature as measured with a pyrometer, and the actual power consumption, estimates that huge concentrations of Frenkel defects may be introduced in the flash process.

Current-Rate Flash Sintering of Doped Ceria: Microstructure and Defect Generation

In current-rate flash sintering the current is injected into the specimen and then increased at constant rate, while the furnace is held at a constant temperature. The flash is induced at low current densities which to some extent obviates the reduction of the ceramic at the cathode. It leads to uniform density and microstructure across the entire gage length of the dog-bone specimen. This work pertains to 10 mol.% gadolinium-doped ceria flash sintered at current-rates ranging from 50 mA min-1 to 1000 mA min-1 at a furnace temperature of 680 °C. Densification is shown to depend on the instantaneous value of the current density, regardless of the current-rate. The grain size, however, is shown to become finer at higher current-rates. A preliminary analysis of the energy deficit, that is, the difference between the temperature predicted by the black body radiation model and the temperature measured with the pyrometer, estimates that huge concentrations of defects may be introduced in the flash process.

Improvement of Low-Alloy Pipe Steel Sulfide Stress Cracking Resistance

The influence of sheet microstructure formed by various thermomechanical processing schedules on sulfide stress cracking (SSC) resistance under uniaxial tension for low-alloy pipe steels grades X42–X65 is studied. A favorable role is established for substitution of a ferritic-pearlitic by a ferritic-bainitic microstructure and improvement of structural component dispersion is established. The favorable effect is facilitated by using high intensity post-deformation cooling from the austenite region. It is shown that segregation bands and non-metallic inclusions in a sheet axial zone do not affect the duration of specimens withstanding SSC, but they can lead to formation of hydrogen cracks (HIC) on a specimen surface that is a defective feature. In order to prevent crack formation in a specimen surface and premature failure it is necessary to lay down high specifications for specimen gauge length and fillet surface quality.

Constitutive parameters identification based on DIC assisted thermo-mechanical tensile test for hot stamping of boron steel

Hot stamping of Boron steel is a thermal-mechanical and phase transformation coupled process. The reliable testing and parameter identification method of the constitutive behavior is crucial for accurate simulation on hot stamping. The imprecise strain and strain rate calibration due to the inevitable temperature gradient along the gauge length in the Gleeble tensile test has been a great challenge to the accurate description on constitutive behavior at elevated temperatures. In the present work, a novel design of grips is developed to flexibly regulate the length of Uniform Temperature Zone (UTZ) along the specimen gauge length. A system with new speckle preparation method and great optical anti-interference ability is developed for the accurate strain calibration at elevated temperature and high-speed deformation based on digital image correlation (DIC). The influence of the length of UTZ and the strain measurement methods on the accuracy of parameters identification is further investigated. The DIC based calibration method on full-field strain and strain rate makes the parameter identification more robust on the length variation of UTZ, that is, the identified parameters are nearly the same using different UTZ length of specimen. The appropriate UTZ length should be satisfied to prevent the ends of gauge length from blue brittleness and the undesirable end failure. The identified constitutive parameters are verified by comparison between simulation and experiments of the hot stamping hat-type parts. It shows that the newly developed DIC-based method is the most precise within the context of the tested approaches to determine constitutive parameters of hot stamping process.

Experimental study of mechanical properties of steel 40Cr in the necking area of specimen during the postcritical deformation

The work is devoted to the experimental study of the evolution of mechanical properties of the structural steel 40Cr during postcritical deformation of solid cylindrical specimens in conditions of strain localization formation in the form of neck at tension. The methodical issues of providing tests by ‘specimen from specimen’ scheme are observed. Different levels of preliminary postcritical deformation of initial samples are realized, and the results of testing specimens cut from the initial samples with a neck are obtained. To measure the displacement and strain fields in the gauge length of specimens with the neck the noncontact 3D video system Vic 3D, based on the digital images correlation technique, was used. On the base of test results, the stiffness and strength of steel 40Cr were evaluated in a necked specimen at various stages of postcritical deformation. The hardness distribution on the specimen after necking was analyzed too. It is shown that the material in the peripheral areas of the gauge length of the sample is in a strengthened state, which does not depend on the degree of previously achieved postcritical deformation, and the strength of the material in the neck-forming zone on the initial specimens is thereby increased.

Stable crack growth in Al-Cu-Mg alloy under various stiffness of loading system in bodies with concentrators

In the present work, the influence of the stiffness of the loading system on regularities of inelastic behavior and on processes of the equilibrium crack growth during uniaxial tension in bodies with stress concentrators was studied. Flat specimens (Al-Cu-Mg alloy) with a center notch and initial fatigue crack were extended with a biaxial servo-hydraulic testing system Instron 8850 at room temperature. The mechanical tests were complemented by a digital image correlation (DIC) and an infrared thermography. By using the 3D DIC-measurement system Vic-3D and the software module ‘virtual extensometer’, it was possible to evaluate the crack opening value, to analyze the evolution of heterogeneous strain fields during the initiation and propagation of cracks. It was revealed that with increasing stiffness of the loading system (by decreasing the gauge length of specimens), the realization of the stable postcritical deformation stage is increasing as well as the inhomogeneity of plastic flow and temperature in the crack tip.

Measurement of Plastic Strain Ratio Using Digital Image Correlation

Abstract Plastic strain ratio, or r-value is used extensively as an indicator of anisotropy of a material. The r-value is usually measured using the “line method” as defined in ASTM E517-00 using an automatic or manual line method. In this contribution, digital image correlation is introduced as an alternative measuring technique to determine r-value using the “line method” or “point method” that utilize a small gauge length of 0.5 mm. These measuring methods have been used to measure the r-values of six different sheet alloys (i.e., AA5754-O, AA6063-T6, AA7055-T751, IF-steel, AZ31, and ZEK100) with different starting textures. The results show that the r-values calculated using the “line method” are very sensitive to specimen geometry and gauge length, and the measurements are also limited to maximum uniform strain corresponding to the maximum load. However, the newly proposed “point method” using a local gauge length of 0.5 mm produces consistent r-value results up to large true axial strains and is also not sensitive to the specimen geometry being used. The measured r-values from the ASTM specimen and a new test specimen with a 25-mm gauge length using both methods are consistent up to maximum uniform strain. This indicates that the new test specimen with a 25-mm (1-in.) gauge length is sufficient to measure r-value accurately using digital image correlation.

An Alternative Method for Strain-Controlled Fatigue Test at Elevated Temperature

For strain-controlled fatigue tests, especially at elevated temperature, implementation of knife-edges and punching dimples within gauge length of specimen may cause premature failure on the test section. In this paper, an alternative approach is introduced to control and measure the strains in fatigue tests. Two dimples outside the gauge length section are punched and used to mount the extensometer. The correlation between the controlled (measured) strain and the strain within the gauge length section is established. By comparing the hysteresis loops obtained through the correlation and the one directly measured within the gauge length, it is found that the established correlations are in good agreement with the latter result.

Tensile Split Hopkinson Bar Technique: Numerical Analysis of the Problem of Wave Disturbance and Specimen Geometry Selection

Abstract A method of tensile testing of materials in dynamic conditions based on a slightly modified compressive split Hopkinson bar system using a shoulder is described in this paper. The main goal was to solve, with the use of numerical modelling, the problem of wave disturbance resulting from application of a shoulder, as well as the problem of selecting a specimen geometry that enables to study the phenomenon of high strain-rate failure in tension. It is shown that, in order to prevent any interference of disturbance with the required strain signals at a given recording moment, the positions of the strain gages on the bars have to be correctly chosen for a given experimental setup. Besides, it is demonstrated that - on the basis of simplified numerical analysis - an appropriate gage length and diameter of a material specimen for failure testing in tension can be estimated.

Study of Damage in the Medium Density Polyethylene Using Digital Image Correlation

The non-linear response of solids is an expression of irreversible processes that originate in microdefects, which are understood as initial material damage. This work aims to analyze the degradation of the mechanical properties of medium density polyethylene through continuum damage mechanics using Digital Image Correlation (DIC). With this technique, displacement and strain fields are obtained throughout the specimen gauge length and therefore optical gauges in any region of interest can measure the strain. Mechanical testing with successive loading and unloading were performed in order to obtain the actual magnitude of the material stiffness in certain strains. Finally, through the damage versus strain diagrams, the results showed that the damage in medium density polyethylene increases from a certain deformation.

Effects of Preform Shear on Tensile Properties of a Woven C/SiC Composite

This article addresses effects of weave defects in an angle‐interlock C‐fiber preform on the tensile properties of the resulting fully processed C‐fiber/SiC‐matrix composite. For this purpose, a preform was intentionally sheared in a controlled manner after weaving. The resulting distortions were quantified by analyzing high‐resolution images of the preform surface after the first step of matrix processing, while the tows were still clearly visible. Comparisons are made of tensile test results on specimens cut from this composite panel and from a pristine panel in select loading orientations. Strain maps obtained by digital image correlation are used to identify local strain variations that are attributable to weave defects. The results are discussed in terms of: (i) the shear‐normal coupling that arises in loading orientations of present interest, and (ii) the geometric effects of tow misalignment on tow continuity along the specimen gauge length. The composite is found to perform in a robust manner, in the sense that the tensile properties are not sensitive to the presence of the defects.

Effect of Applied of Pre-Stresses on Corrosion Behavior of 304 Stainless Steel in 1N H2SO4

This research is devoted to study the effect of applying different pre-tensile stresses (255,305,355,405,455,505 555 and 605) MPa on the microstructure, hardness and corrosion behavior of 304 stainless steel in 1N H2SO4. The stress-strain curve was drawn for standard (304) stainless steel in laboratory environment. The curve was divided into three zones. At zone one the values of elastic pre-stresses vary between (σ0 - σpro.) MPa. The results showed; that the corrosion rate was very little increased compared with that of as received (304) stainless steel. The microstructure presented undeformed austenitic grains and the hardness value was (157.433) Hv. At zone two the value of plastic pre-stress varies between (σpro.- σ U.T.S) MPa. The corrosion rate increases after applying pre-stress between (σ255 - σ 455) MPa. The microstructure showed that the austenitic grains begin to deform in the direction of applied pre-stresses. The maximum hardness at this region was (229.2) Hv, but at higher pre-stress (σ455 – σ 605) MPa, the corrosion rate decreases. The microstructure inspection shows the deformed austenitic grain and ά-martensitic phase needle are appeared inside austenitic grains and the hardness reached the maximum value (332.433) Hv. At zone three the values of pre-stresses are between (σ605 - σf) MPa. The results showed that the corrosion rate increases. The investigation of microstructure showed that there are distortion in austenitic grains and ά- martensite phase observed inside austenitic grains. The hardness reached the maximum value at (354.3) Hv. The necking of gauge length of specimens occurs in specimens and this leads to deterioration in original properties

Special Testing Equipment and Validation of Measurement Methodologies for High Temperature Low Cycle Fatigue Testing of Miniature Metallic Specimens

A technique for high temperature low cycle fatigue testing of metallic materials has been developed, to determine fatigue behaviour through the testing of miniature specimens. The miniature specimen geometry was specifically designed, such that it could be manufactured from a small volume of material removed by chain-drilling extraction. An extensometry method to measure and control strain at the specimen shoulders during testing was adopted. This was undertaken to minimise the deleterious contact effects that can occur via extensometry attached at the gauge length of specimens, hence leading to premature failure and inaccurate fatigue data. By the application of this technique, the high temperature low cycle fatigue behaviour of 2.25Cr-1Mo steel was successfully characterised at 540 °C, under a fully reversed strain-controlled regime. The fatigue properties of the steel obtained from testing miniature specimens were shown to correlate well with existing literature for the material under comparable conditions, as determined by the testing of conventional standard-sized specimens.

Influence of bias angle of stitching on tensile characteristics of lapped seam parachute canopy fabric – Part I: Mathematical modelling for determining test specimen size

This study embodies evaluation of ideal test specimen dimension for the study of tensile characteristics of lapped seam parachute canopy fabric. Due to anisotropic nature of textile fabrics, the mechanical property of unseamed and seamed fabric changes with the change in specimen test direction. Therefore, measurement of tensile property at different bias angle (an acute angle between warp and specimen length/test direction) becomes more important in parachute canopy where joining of its various parts is not only in principle direction but also at different bias angle. The present paper deals with the influence of bias angle of stitching on breaking strength and elongation of unseamed and lapped seam parachute canopy fabric using test specimen dimension based on previous practice. Present study reveals that there is need of re-specification of specimen dimension for evaluation of tensile properties of parachute fabric. With the changes in angle of bias, width and gauge length of test specimen, the number of different category of warp and weft yarns available between the specimen grip lines changes and can be calculated mathematically. This has important role in affecting the ultimate properties of parachute fabric. Based on the study, an optimized dimension of test specimen has been postulated for evaluation of tensile characteristics of parallel/opposite stitched specimen through which reliable test result can be obtained. The proposed dimension can be used for comparative analysis of breaking strength and elongation of unseamed and seamed fabric at different bias angle.

Tensile creep measurements of ordinary ceramic refractories at service related loads including setup, creep law, testing and evaluation procedures

Abstract 1 The performance of refractories under operating conditions is often influenced by tensile creep. This paper proposes a high-temperature tensile testing device which allows for long term creep measurements at operating loads. The chosen specimen geometry is more adequate for the testing of ordinary ceramic refractories which may be heterogeneous with respect to grain size (maximum grain size e.g. 5 mm) and chemical composition. For this application the innovative design exhibits not only improved specimen and loading alignments, but also reliable specimen holding and cooling systems. Specific procedures were followed to avoid uneven stress distribution along the specimen gauge length. The testing procedure was optimized by simulating experimental creep conditions with a finite element (FE) model built with the software Abaqus. Measurements were performed on magnesia–chromite material at different temperatures and applied stresses. Under these testing conditions, three creep stages emerged. The Norton-Bailey creep rate equation was employed to describe the creep behavior for the three stages. An evaluation using the Generalized Reduced Gradient (GRG) algorithm was then performed in order to identify the three creep stages and inversely estimate the Norton-Bailey creep parameters n , a and K for each stage.

Alternative Approach for Strain Measurement and Control in Fatigue Tests at Elevated Temperature

Abstract For strain-controlled fatigue tests, especially at elevated temperature, implementation of knife-edges and punching dimples within the gauge length of a specimen is a common practice to mount the extensometer. Premature failure on the test section of the specimens might be introduced by implementing these techniques. In this paper, an alternative approach is introduced to control and measure the strains in fatigue tests. Two dimples outside the gauge-length section are punched and used to mount the extensometer. The correlation between the controlled (measured) strain and the strain within the gauge-length section is established. For the isothermal fatigue tests (strain cycling at constant elevated temperature), the correlation can be established either through analytical analysis or finite-element analysis (FEA). By comparing the hysteresis loops obtained through the correlation and the one directly measured within the gauge length, it is found that the established correlation obtained from FEA has better agreement with the latter result. In addition, this technique can also be used for thermal-mechanical fatigue (TMF) tests (both temperature and strain cycling). However, it is found that the correlation developed through FEA does not give accurate result. Therefore, the interconnection should be established through a cyclic test, in which two extensometers are mounted on the specimen, one measures the strain within the gauge length and the other measures the strain outside the gauge length.

Influence of the Casting Process in High Temperature Fatigue of A319 Aluminium Alloy Investigated By In-Situ X- Ray Tomography and Digital Volume Correlation

In order to satisfy the economic constraints together with environmental requirements, the automotive industry has been forced to adopt a strategy of down-sizing, which has led into process modification of some engine parts like cylinder heads. Nowadays, the Lost Foam Casting process (LFC) replaces the conventional Die Casting (DC) process due to cost reduction and geometry optimization goals. However, aluminum alloy automotive parts produced by the LFC process have a coarser microstructure and more porosity defects than parts produced with conventional casting processes at faster cooling rates. In the cylinder heads produced by the LFC process, the microstructure consists of hard intermetallic phases and large gas and microshrinkage pores. In order to study the influence of this complex 3D microstructure on fatigue crack initiation and propagation, an experimental protocol using laboratory, synchrotron tomography, SEM images and 3D Digital Volume Correlation (DVC) has been used. Fatigue tests performed at temperatures characteristics of in-service conditions (250°C) revealed the initiation of 3D cracks at large pores and a propagation along the hard inclusions around the main pore that drove to failure but also in other areas of the specimen gauge length.

EVALUATION ON ELONGATION OF REBAR BY USING IMAGE PROCESSING METHOD

In this study, longitudinal strain distributions of rebars in the uniaxial tension test are evaluated with an image processing method (IPM). The measurements of strain are conducted till rebar is ruptured. As a result of the test, the detailed longitudinal strain distributions of rebar in the tension test are clarified and the results show that the strain at necking region are almost 2-3 times of average strain in the gauge length of specimen. After the stress of rebar is reached at ultimate strength, the most of elongations are concentrated at necking region. In the other region except necking region, therefore, the strains are merely increased till the end of test. From these results, it is estimated that the necking region can make the over estimation of elongation of rebar, therefore, the estimation except the necking region can be suggested to evaluate the true elongation of rebar. Also the measured results with IPM are compared with the measured results by using conventional strain gauge. It is estimated that the IPM shows some noise in data, but the IPM has advantages to measure the strain distribution to the range of rupture. Therefore if the image acquisition of the better resolution and better quality are possible, it can be a good alternative measuring method for the strain measurement.

Small crack propagation behaviour during thermo-mechanical creep-fatigue loading of Ni based superalloy specimen under non-uniformly distributed temperature condition

A new testing system has been developed to get a basic understanding on the thermo-mechanical creep-fatigue (TMCF) small crack propagation behaviour around the cooling holes in high temperature Ni base superalloy used for advanced gas turbine components. By means of the test system the TMCF tests under creep-fatigue condition were carried out for a directionally solidified Ni base superalloy hollow specimen with nine cooling holes which were machined in a 3×3 square array within the specimen gauge length, simulating the cooling system in air cooled gas turbine blades. Special attention was paid to the early growth of creep-fatigue crack(s) around the holes. Some fracture mechanics discussions are made to interpret the experimental results and the effect of cooling system on the TMCF failure, taking account of the roles of stress concentration and the local thermal stress induced under a significant temperature gradient.

Design and fabrication of compression test rig for fibre-reinforced polymer composites

Compressive strength is one of the most important considerations when designing fibre-reinforced polymer (FRP) composite structures. This property is obtained from the stress-strain response of FRP specimen loaded in compression. Compression tests using shear-loading or end-loading methods usually cause premature failure of the specimens or failure near to/in tabs. Therefore, the purpose of this study is to design a test rig for compression testing that combines both shear- and end-loading methods. Computer-Aided Three-Dimensional Interactive Application software was used to design parts and the whole system of the test rig, namely UiTM dual guide compression test fixture. The performance of the test rig was evaluated by conducting compression tests on woven carbon, glass and Kevlar fibre-reinforced polymer composites samples. It was observed that all specimens failed within the specimen gauge length, implying that the tests were successful.