End Of Specimen Research Articles

Study on dynamic relaxation time and modulus of cement mortar by ultrasonic technique

Ultrasonic wave technique is employed to determine relaxation time and modulus of cement mortar. First, the property of attenuation of amplitude at different cross sections of the specimen may be obtained by detecting the ultrasonic signal at the ends of specimens with different lengths. Secondly, the coefficient of attenuation of the ultrasonic wave is obtained by simulating experimental results. Thirdly, one-dimensional propagation of the ultrasonic wave in cement mortar is analysed by considering the transversal inertial effect. Two important parameters of viscoelastic constitutive model of cement mortar, namely, modulus and relaxation time, are obtained. Finally, the transversal inertial effect on the relaxation time and modulus of material is examined numerically. The results obtained show that the transversal inertial effect may lead to increase of relaxation time and modulus. The method proposed in this paper is effective to measure parameters of the constitutive relation of cement mortar by ultrasonic wave technique.

Damage Evaluation with P-Wave Velocity Measurements during Uniaxial Compression Tests on Argillaceous Rocks

In the study of time dependent behavior of rock, the main difficulty is to predict delayed failure, which is of the utmost importance in assessing the safety of underground structures, such as deep underground facilities designed for high-level radioactive waste disposal. In this context, the viscoplastic behavior associated with the rock damage must be taken into account. As the longitudinal and transversal wave velocities are related to the physical and mechanical characteristics of materials, ultrasonic measurements can give valuable information about the development of damage. In this study, P-wave velocity measurements were used to monitor damage evolution during uniaxial strain in controlled compression tests and long-term creep tests. These measurements were performed using sensors in a piezoelectric copolymer of polyvinyl-difluoride, which were placed on both ends of cylindrical rock specimens. Throughout the experiments, the dilating behavior of an argillite could be correlated with a decrease of...

박판재의 스프링백 해석(I)-잔류 변형율에 근거한 해석모델

As the springback of sheet metal during unloading may cause deviation from a desired shape, accurate prediction of springback is essential for the design of sheet stamping operations. When considering the case of a sheet metal being bent to radius <TEX>$\rho$</TEX> that is such that the maximum stress induced exceed the elastic limit of the material, plastic strain in the outer surface will occur and the material will take a permanent set: but since, on removing the bending moment, the recovery of the material is not uniform across the thickness, springback will occur and the radius <TEX>$\rho$</TEX> will not be maintained. Furthermore, when a tensile load being applied to each end of specimen, the tensile stress due to bending is increased and the compressive stress is decreased or cancelled and eventually the whole specimen may be in varying degree of tension. On the removal of the applied load the specimen loses its elastic strain by contracting around the contour of the block, the radius <TEX>$\rho$</TEX> will be determined by the residual differential strain. Therefore in this study the springback is analytically estimated by the residual differential strains between upper and lower surfaces of greatest radius after elastic recovery, and a springback model based on the bending moment is also analytically derived for comparison purpose.

Fatigue Fracture Behavior of Mg-Zn-Y Alloys

Recently, Mg-Zn-Y alloys with superior performance, which have a long period stacking order (LPSO) phase, have been developed. Therefore, it is important to understand fundamental fatigue properties in such materials. In this study, the fatigue fracture behavior of the Mg96Zn2Y2 alloy has been investigated with a plain bending testing machine, which was originally developed for thin sheet specimen at room temperature and 523K. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. To estimate stress amplitude of bending, deflections at the free end of the specimen oscillating at a frequency of about 200~500Hz was measured by a laser displacement gauge. For comparison, AZ31B alloy also has been investigated. S-N curve for the Mg96Zn2Y2 alloy was obtained using a stress ratio of R=-1, and the fatigue strengths were estimated as 200MPa at room temperature and 120MPa at 523K at 106~107 cycles. These values correspond to about 50% of 0.2% proof strengths of the Mg96Zn2Y2 alloy. Two types of fatigue surface were observed in the Mg96Zn2Y2 alloy. One was striation-like-pattern and the other was relatively flat surface. Striation-like-pattern was similar to fatigue surface of AZ31B. Therefore, these two types of fatigue surface correspond to crack passing through α-Mg phase and LPSO phase, respectively. The feature of fracture surface at 523K was almost the same as that at room temperature.

Numerical Studies of End Effect on Crack Propagation Behavior of Brittle Specimen Containing Pre-Existing Crack under Uniaxial Compression

In this paper, a Material Failure Process Analysis code (MFPA2D) was employed to investigate the interaction of end effect zone of specimen with the wing crack propagation inside the brittle specimen containing pre-existing flaws under uniaxial compression comparing with the experimental results. The numerical results show that the shorter the distance between the pre-existing flaw and the specimen's end , the slower the crack propagation process and the shorter wing propagation length is , and vice versa. In addition, the end effect zone was also influenced by the wing crack propagation.

Crack Orientation Dependence for Fatigue Behavior of Titanium Single Crystals

In this present study, the fatigue behavior of titanium single crystals containing cracks growing in different crystallographic orientations has been investigated. To investigate fatigue fracture behavior of small single crystals, plain bending fatigue test method for thin sheet specimen was developed. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. In A-specimen, the notch plane and the direction are (11-20) and [1-100], respectively, the crack propagated parallel to {1-100} plane and striation-like markings were observed on the fatigue surfaces. In B-specimen with a notch of (1-100) and [11-20], the crack also propagated parallel to {1-100} plane as similar to A-specimen. These cracks are deduced to extend by alternating shear on two intersecting prismatic slip systems at the crack tip. In F-specimen with a notch of (0001) and [11-20], the crack propagated parallel to (0001) and twins were observed near the crack. As a result, S-N curves of each specimen showed strong orientation dependence and the fatigue strength of F-specimen was the highest in these specimens.

Combustion of pressed specimens of heterogeneous systems subjected to a constant axial mechanical load

The growth dynamics of the height of burning pressed specimens with the specimen ends subjected to a compressive force of constant magnitude is studied using the heterogeneous Ti+C+20% TiC system as an example. It is found that in the examined range of compressive forces 0.1–2 kg/cm2, the growth of the height of the burning specimens with time obeys a linear law; under a compressive force of ≈0.1 kg/cm2, the specimen height increases by ≈100%, and under a force of ≈2 kg/cm2, it increases by about 25%. The method used to measure the growth dynamics of the specimen height during combustion proved a useful tool for the experimental determination of both the burning time of the specimens and the delay in the transfer of combustion through an obstacle.

Thermal Fatigue Characteristics of Heat-Resisting Stainless Steel for Automotive Exhaust

Thermal fatigue is a complex phenomenon encountered in materials exposed to cyclically varying temperatures in the presence or absence of external load. Continually increasing working temperature and growing need for greater efficiency and reliability of automotive exhaust require immediate investigation into the thermal fatigue properties especially of high temperature stainless steels. In this study, thermal fatigue properties of 304 and 429EM stainless steels have been evaluated in the temperature ranges of 200-800oC and 200-900oC. Systematic methods for control of temperatures within the predetermined range and measurement of load applied to specimens as a function of temperature during thermal cycles have been established. Thermal fatigue tests were conducted under fully constrained condition, where both ends of specimens were completely fixed. Thermal fatigue property of STS 304 was superior to that of STS 429EM. Load relaxation behavior at the temperatures of thermal cycle was closely related with the thermal fatigue property.

Specimen size effect in off-axis compression tests of fiber composites

Specimen size effects in off-axis compression tests were studied by conducting experiments with small block off-axis specimens of low modulus S2 glass fiber reinforced composites and high modulus AS4 carbon fiber reinforced composites. It was found that the off-axis compressive strength of the glass/epoxy composite decreased by a small amount (<5%) when increasing either specimen width or thickness. However, an appreciable reduction in off-axis compressive strength of the high modulus carbon/epoxy composite was observed as specimen width or thickness increased with lapped specimens. However, when a thin layer of titanium was applied on both contact ends of the specimen, the contact friction was significantly reduced, leading to much smaller reductions in off-axis compressive strength as the specimen width or thickness increases. It can be concluded that specimen end surface friction can produce specimen size effect in off-axis compression tests, and the specimen size effect is negligible when the friction between the specimen end and loading element is minimized.

Structural responses of restrained steel columns at elevated temperatures. Part 1: Experiments

This paper outlines a research program on an experimental investigation to determine the failure time of unprotected steel columns subjected to various axial restraint ratios. Axial restraints were applied to simulate the thermal restraint effects due to adjacent cooler parts of a steel framed structure in fire. All columns had an effective length of 1.74 m, and were divided into 4 groups according to their minor-axis slenderness ratios of 45, 55, 81 and 97, respectively. The columns were axially loaded and exposed to a monotonically increasing heating condition. Initial imperfections such as column crookedness and load eccentricity were measured by a specially designed facility. The test results show that axial restraints, as well as initial imperfections, significantly reduce the failure times of axially-loaded steel columns. By contrast, bearing friction substantially retards column failure times. These effects are represented by flexural springs at both ends of a column specimen. Besides, a simple but reliable Rankine approach is used to compute the column failure times. The Rankine predictions agree well with experimental results.

拡散法で作製したBi2212酸化物超伝導体電流リードの組織と通電特性の評価

Bi2212 HTS conical tubular conductors have been prepared by diffusion process for current lead. The Bi2212 HTS layers are synthesized through the diffusion reaction between a Sr-Ca-Cu oxide substrate and a Bi-Cu oxide coating with Ag addition. The HTS diffusion layers about 150 μm in thickness are formed around both outside and inside of the conical tubes 34/29 mm in outside/inside diameter at the larger end, 24/19 mm in outside/inside diameter at the smaller end, and 100 mm in length. The Ag added to the coating enhances the diffusion reaction, and precipitates on the surface of the specimen decreasing its contact resistance. The transport current properties were evaluated by measuring system using two cryocoolers at Railway Technical Research Institute. The critical temperature of transport current of 1000 A at 0.5 T for the specimen is 57.4 K, which corresponds to the current density of 33 A/mm2 for the Bi2212 layer. The transport current decreases with increasing temperature at the warm end of the conical specimen, and is about 800 A at 60.2 K and 600 A at 62.5 K. The joint voltage of 20 μV at cold end was generated at 50 K and 0.5 T with transport current of 1000 A, which corresponds to the small heat load of 20 mW resulted from the Joule heating. Present Bi2212 conical tubes seem to be promising as current leads with small heat loads for superconducting magnets.

Densitometric, morphometric and mechanical distributions in the human proximal femur

With the prevalent use of DXA-measured BMD to assess pathologic hip fractures and its recently reported lack of reliability to predict fracture or account for efficacy of anti-resorptive therapy, it is reasonable to assess whether variations in the primary and secondary tensile and compressive trabecular microstructure can account for variations in proximal femur strength in comparison to DXA-measured BMD. To that end, microstructural and densitometric measures of trabecular bone specimens, from discrete sites within the proximal femur, were correlated with their mechanical properties. We hypothesize that accounting for regional variations in trabecular microstructure will improve predictions of proximal femur strength and stiffness compared to bone density measured by DXA. Forty-seven samples (seven donors) from seven distinct sites of human proximal femur underwent DXA and μCT imaging and mechanical testing. The results revealed significant variations in BMC, morphometric indices and mechanical properties within the proximal femur. This work has demonstrated that the mechanical performance of each sub-region is highly dependent on the corresponding trabecular microstructure. BMD measured by DXA at standard regions of interest cannot resolve the variations in trabecular density and microstructure that govern the mechanical behavior of the proximal femur. This work suggests that a quantitative Singh index that uses high resolution QCT to monitor the trabecular microstructure at specific sub-regions of the proximal femur may allow better predictions of hip fracture risk in individual patients and an improved assessment of changing bone structure in response to pharmacological interventions.

Elimination of the effect of strain gradient from concrete compressive strength test results

Poor strength test results are sometimes not an indication of low concrete quality, but rather inferior testing quality. In a compression test, the strain distribution over the ends of the specimen is a critical factor for the test results. Non-uniform straining of a concrete specimen leads to locally different compressive stresses on the cross-section, and eventual premature breaking of the specimen. Its effect on a specimen can be quantified by comparing the compressive strength results of two specimens, one subjected to uniform strain and another to a specified strain gradient. This can be done with the help of a function that relates two parameters, the strain ratio and the test efficiency. Such a function depends on the concrete strength and cross-sectional shape of the specimen. In this study, theoretical relationships between the strain ratio and test efficiency are developed using a concrete stress-strain model. The results show that for the same strain ratio, the test efficiency is larger for normal strength concrete than for high strength concrete. Further, the effect of the strain gradient on the test result depends on the cross-sectional shape of the specimen. Implementation of the results is demonstrated with the aid of two examples.

Fatigue Fracture Behavior of Mg-Zn-Y Alloy

Recently, Mg-Zn-Y alloys with superior performance, which have a long period order (LPO) phase, have been developed. Therefore, it is important to understand fundamental fatigue properties in such materials. In this study, the fatigue fracture behavior of Mg-Zn2-Y2-Zr0.2 alloy has been investigated using a plain fatigue bending testing machine, which was originally developed for thin sheet specimen. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. To estimate stress amplitude of bending, deflections at the free end of the specimen oscillating at a frequency of about 250~700Hz was measured by a laser displacement gauge. For comparison, AZ31B alloy also has been investigated. S-N curve for the Mg-Zn2-Y2-Zr0.2 alloy was obtained using a stress ratio of R=-1, and the fatigue strength was estimated as about 200MPa at 106~108 cycles. The value corresponds to about 50% of 0.2% proof strength of the alloy. Two types of fatigue surface were observed in the alloy. One is striation like pattern and the other is relatively flat surface. The former is similar to fatigue surface of AZ31B. Therefore, these two types of fatigue surface correspond to crack passing through α-Mg phase and LPO phase, respectively.

Crack Orientation Dependence of Fatigue Behavior in Titanium Single Crystals by Thin Sheet Plain Bending

Fatigue test of titanium single crystals with different crystallographic orientations has been carried. To investigate fatigue fracture behavior of small single crystals, plain bending fatigue test method for thin sheet specimen was developed. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. In A-specimen, a notch plane and the direction are (11 20) and [1100], respectively, the crack propagates parallel to {1100} plane and striation-like markings are observed on the fatigue surfaces. In B-specimen with a notch of (1100) [1120], the crack also propagates parallel to {1100} plane as similar to A-specimen. These cracks are deduced to extend by alternating shear on two intersecting prismatic slip systems at the crack tip. In F-specimen with a notch of (0001) [11 20], the crack propagates parallel to (0001) and some trace of pyramidal slip was observed on the fatigue surface. As a result, S-N curves of each specimen showed strong orientation dependence. Fatigue strength of F-specimen is higher than that of A- and B-specimen.

Effect of shrinkage constraint on damage of C/SiC composites during thermal cycles in oxidising atmosphere

Thermal cycling response of a three-dimensional C/SiC composite subjected to a unidirectional shrinkage constraint in wet oxygen was investigated. Constraint stress generated on both ends of the composite specimens during thermal cycles is shown to decrease upon heating and increase with cooling. Nevertheless, the peaks of the constraint stress gradually reduced from the initial 41 MPa to the final constant value of 9·8 MPa after 20 cycles. Residual mechanical properties and microstructure characterisation suggest that the cycled composites suffer a little mechanical degradation and the shrinkage constraint is of advantage to oxidation resistance of the materials during thermal cycles.

Experimental study on complete stress-deformation curves of larger-size concrete specimens subjected to uniaxial tension

In order to provide parameters for numerical analyses of the huge Three-Gorge concrete dam (2309 m long by 175 m height), complete tensile stress-deformation curves for large-size plain concrete specimens were measured and studied by performing uniaxial tensile tests on large-size unnotched specimens (250 mm × 250 mm × 1400 mm). The specimens were prepared with the three-graded-aggregate materials provided by the client of the Three-Gorge project. To prevent a failure occurring near the ends of the unnotched specimens, both the ends of each specimen (450 mm in length) were cast using a higher-strength concrete than the middle part (i.e., active part). Tensile tests were completed on a specially-designed tensile testing machine, which can be easily re-assembled to accommodate different-size specimens. To make the specimens fail stably, a cyclic loading scheme was adopted after the peak strength was reached. Four of five tests in this study were successful, and four complete tensile stress-deformation curves were obtained. It was found that the post-peak curve of the large-size specimens used in this study is more gradual than those for the small-size specimens reported in the literature.

Factors Affecting Magnetic Flux Leakage Inspection of Tailor-Welded Blanks

The development of a laboratory-based tailor-welded blank (TWB) inspection system using the principles of magnetic flux leakage (MFL) is presented. The effects of variations in inspection system operating parameters are quantified to allow for optimized system performance. The parameters examined include the applied magnetic field strength, inspection scanning velocity, spatial resolution of acquired signals, specimen end effects, and pole piece liftoff. The results indicate that this inspection method is relatively insensitive to operating parameters and is therefore robust.

Strength and Energy Absorption of Aluminium Foam under Quasi-Static Shear Loading

Experiments were carried out to examine the behaviour of aluminium foams under quasistatic shear loading. Special fixtures were designed to clamp the both ends of a beam-like specimen while the load was applied via a punch, which led to failure by shearing along the clearance near the fixed end. Specimens were made of CYMAT foams with two nominal relative densities (12% and 17%) and several values of width. This study focuses on the maximum strength under shear and the essential energy in fracture. It has been found that the ultimate shear force increases linearly with the width of the beam, so does the total energy absorbed. Two empirical formulae have been obtained which relates to the relative density, respectively, the ultimate shear strength and energy absorbed in shearing.

Bi2212 HTS Conical Tubes Prepared by the Diffusion Process for Current Lead Application

Bi2212 HTS conical tubular conductors have been prepared by the diffusion process for current lead application. The Bi2212 HTS layers are synthesized through the diffusion reaction between a Sr-Ca-Cu oxide substrate and a Bi-Cu oxide coating with Ag addition. The HTS diffusion layers about 150 micrometer in thickness are formed around both outside and inside of the conical tubes 37/29 mm in outside/inside diameter at the larger end, 27/19 mm in outside/inside diameter at the smaller end, and 200 mm in length. The Ag added to the coating enhances the diffusion reaction, and precipitates on the surface of the specimen decreasing its contact resistance. The transport current of the specimen exceeds 7,000 A at 4.2 K and self-field, which corresponds to the current density of 32,000 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the Bi2212 layer. The transport current decreases with increasing temperature at the warm end of the conical specimen, and is about 4,600 A at 30 K, 3,200 A at 40 K or 2,000 A at 50 K. The total heat loads composed of heat leakage conducted through the conical tube and Joule heating at the joint are estimated to be about 320 mW at 3,000 A between the warm end of 42 K and the cold end of 4.2 K, and is 400 mW at 2,000 A between 50 K and 4.2 K, respectively. Present Pb-free Bi2212 conical tubes seem to be promising as large transport current leads with small heat loads for superconducting magnets