Specimen Tip Research Articles

Application of atom probe tomography to fundamental issues of steel materials

Although steel research has a very long history, there are still various questions and uncertainties on the fundamental issues of steel materials. Our aim is to clarify the issues by atomic‐scale characterization using atom probe tomography (APT). This paper introduced our developed techniques in application of APT into steel materials. The first one is the specimen tip preparation technique from the site‐specific regions of interest in steel, which significantly expanded the application field of steel analysis. The second one is the observation technique of trapped hydrogen in steel using our developed “deuterium charge cell,” which accomplished direct observation of the hydrogen trapping site in alloy carbide precipitation strengthened steels. Such atomic‐scale analyses bring many new findings in steel materials science.

Filaman Sargılı Cam Elyaf Takviyeli Yarıklı Kompozit Tüplerin Burulma Yükleme Durumu Davranışları

The stress and strain behaviors of filament wound glass-fiber epoxy based cylindrical tube structures made of two different stacking sequences as [(±45°)5] and [(0/ 90)5] were numerically investigated under a constant torque value. The exterior surfaces of tubes were deliberately defected by longitudinally extending, rounded end, 0.6 mm deep, 2 mm wide but varying-length slots. Slot-less and full-length-slot structures were also included in the study. During torsional loading, the variations in stresses, strains and twisting angles of the specified thin-walled composite tube models were investigated, comparatively. Additionally, the effects of fiber winding angles and slot lengths on the specified quantities were parametrically examined. A considerable amount of stress accumulation around the slot tip of both type of tube model is measured which constitute a risk of damage progression. At the innermost laminas, slight fluctuations in the maximum stresses are observed in the slot lengths shorter than 140mm, whereas rapid increases are remarkable for exceeded lengths. On the other hand, the maximum stress changes in the outermost layer are quite uneven. From slot-less to full-length-slot, the change in slot dimensions provokes 8.32 and 9.11 % increments in twisting angles in the 45°/-45° angle-ply and 0°/90° cross-ply structures, respectively. Under the same loading condition, [(±45°)5] stacking sequence gives the structure averagely 0.66° lower total rotation at the specimen tip in comparison to [(0/ 90)5] fiber arrangement.

An innovative system for uncoupled bending/torsion tests by tri-axis shaker: numerical simulations and experimental results

This work describes a new testing system for applying a coupled/uncoupled bendingtorsion loading in vibratory tests by a tri-axis shaker. The system is composed of a cylindrical specimen with eccentric tip masses, excited by horizontal and/or vertical base accelerations. The specimen tip is constrained by a lateral thin and flexible plate which impedes any bending when the specimen is excited horizontally, but which permits the specimen torsional rotation. This layout then allows torsional and bending deformations to be produced and controlled independently, when vertical and horizontal base accelerations are applied simultaneously. A finite element model is first used to estimate the system dynamic response and the stresses in the notched specimen section. The model is then validated through experimental tests under harmonic base accelerations. The strains at clamping system are also monitored to indirectly estimate the bending and torsion moment in the specimen. Comparison of numerical and experimental results showed a close correlation and proved that bending-torsion loading are truly uncoupled. Preliminary fatigue tests with harmonic bending loading (vertical base excitation) are finally compared to the constant amplitude S-N curve, showing a quite satisfactory agreement.

Electron beam welding study of grade 1 CP Titanium

Mechanical properties at room temperature as well as at cryogenic temperatures (77K, 20K) were evaluated on parent metal as well as electron beam weldments of α–Ti (commercially pure grade I) in this work. The strength of the weldment as well as parent metal increased with decrease in temperature. Weld efficiency >95% at all the temperatures (ambient, 77K, 20K) was observed. Microscopy revealed single phase α in parent metal, which transformed to widmanstatten martensitic structure with feathery morphology in the weld.Increase in dislocation density near the failure tip of tensile test specimen and presence of dimples were observed indicating ductile failure/ sufficient amount of ductility even at 20K.

Determination of the Parameters of the Two-Parametric Fracture Mechanics along the Crack Front Based on the Digital Image Correlation Data

A mathematical processing technique and an algorithm for the numerical correction of experimental displacement fields on the specimen surface in the vicinity of a crack tip with consideration of the displacement of a body as a rigid whole under its loading and the actual position of a crack tip were proposed. The initial experimental displacement fields in the vicinity of a fatigue crack tip of a compact specimen were determined by digital image correlation. This approach was successfully used for a compact specimen and the possibility of its expansion to obtain the distributions of the stress intensity factors and the nonsingular T stresses along the spatial crack front was shown.

Impact of specific fracture energy investigated in front of the crack tip of three-point bending specimen

Presented study is focused on the analysis of the dependence of the specific fracture energy value on the assumed work of fracture in threepoint bending tests. Specimens of different sizes and relative notch lengths are assumed in this study, in order to take into account the size effect. The three-point bending test of cracked specimens is simulated numerically by means of commercial software based on the finite element method with implemented cohesive crack model. Three levels of the specific fracture energy are considered.

Controlling residual hydrogen gas in mass spectra during pulsed laser atom probe tomography

Residual hydrogen (H2) gas in the analysis chamber of an atom probe instrument limits the ability to measure H concentration in metals and alloys. Measuring H concentration would permit quantification of important physical phenomena, such as hydrogen embrittlement, corrosion, hydrogen trapping, and grain boundary segregation. Increased insight into the behavior of residual H2 gas on the specimen tip surface in atom probe instruments could help reduce these limitations. The influence of user-selected experimental parameters on the field adsorption and desorption of residual H2 gas on nominally pure copper (Cu) was studied during ultraviolet pulsed laser atom probe tomography. The results indicate that the total residual hydrogen concentration, HTOT, in the mass spectra exhibits a generally decreasing trend with increasing laser pulse energy and increasing laser pulse frequency. Second-order interaction effects are also important. The pulse energy has the greatest influence on the quantity HTOT, which is consistently less than 0.1 at.% at a value of 80 pJ.

Toward the Atomic-Level Mass Analysis of Biomolecules by the Scanning Atom Probe.

In 1994, a new type of atom probe instrument, named the scanning atom probe (SAP), was proposed. The unique feature of the SAP is the introduction of a small extraction electrode, which scans over a specimen surface and confines the high field, required for field evaporation of surface atoms in a small space, between the specimen and the electrode. Thus, the SAP does not require a sharp specimen tip. This indicates that the SAP can mass analyze the specimens which are difficult to form in a sharp tip, such as organic materials and biomolecules. Clean single wall carbon nanotubes (CNT), made by high-pressure carbon monoxide process are found to be the best substrates for biomolecules. Various amino acids and dipeptide biomolecules were successfully mass analyzed, revealing characteristic clusters formed by strongly bound atoms in the specimens. The mass analysis indicates that SAP analysis of biomolecules is not only qualitative, but also quantitative.

COMPARATIVE STUDY OF CRACK GROWTH DUE TO MECHANICAL AND THERMAL LOADS ON A CURVED SURFACE

In the high temperature regime, components fail by thermal damage at the defects, also failure can happen under mechanical loading of the parts, as it induces substantial amount of stresses around the defects. From the literature it is learnt that, much of the work has been done on study of combined mechanical and thermal loading effect on the life of the component with defects, hence mechanical and thermal loading effects on Stress Intensity Factor were studied independently. Finite Element Method is used to determine Stress Intensity Factor at the crack tip of an AT specimen with initial notch under mechanical and thermal loads separately. In case of thermal loading, both the inner and outer surface of the AT specimen was subjected to different temperature. Results so obtained were expressed in terms of the crack tip Stress Intensity factor, as defined by the theory of linear elasticity. Comparison of the results of mechanical and thermal loading are made and found in good agreement with the similar work done by the other authors.

Evaluation of the mechanical performance of silicon carbide in TRISO fuel at high temperatures

The HTR design envisions fuel operating temperatures of up to 1000°C and in case of an accident even 1600°C are conceivable. To ensure safety in all conditions a thorough understanding of the impact of an extreme temperature environment is necessary. This work assesses the high temperature mechanical performance of the silicon carbide (SiC) layer within the tristructural-isotropic (TRISO) fuel particle as it poses the main barrier against fission product release into the primary circuit. Therefore, simulated fuel was fabricated by fluidized bed chemical vapour deposition; varying the deposition conditions resulted in strongly differing SiC microstructures for the various samples. Subsequently the TRISO particles were annealed in inert atmosphere at temperatures ranging from 1600°C up to 2200°C. Scanning electron microscopy and Raman spectroscopy showed that strong disintegration of the SiC layer occurred from 2100°C onwards, but initial signs of porosity formation were visible already at 1800°C. Still, the elastic modulus and hardness as measured by nanoindentation were hardly impaired. After annealing stoichiometric SiC coatings showed a reduction in fracture strength as determined by a modified crush test, however the actual annealing temperature from 1600°C to 2000°C had no measureable effect.Furthermore, a technique was developed to measure the elastic modulus and hardness in situ up to 500°C using a high temperature nanoindentation facility. This approach allows conducting tests while the specimen and indenter tip are heated to a specific measurement temperature, thus obtaining reliable values for the temperature dependent mechanical properties of the material. For the SiC layer in TRISO particles it was found that the elastic modulus decreased slightly from room temperature up to 500°C, whereas the hardness was reduced more severely to approximately half of its ambient temperature value.

ステンレス鋼のスパッタエッチングにより形成した多機能微細突起物の圧縮強度におよぼすプラズマ窒化の影響

Argon ion sputter-etching of SUS420J2 martensitic stainless steel and SUS316 austenitic stainless steel were carried out to form fine protrusions on the specimen surface by using a radio-frequency magnetron sputter-apparatus. Plasma-nitriding was applied to the protrusions by using nitrogen gas of 0.53 Pa mixed with argon gas of 0.67 Pa at a power of 50 W for 1.8 ks. Micro-Vickers hardness tests on cone-shaped protrusions with diameter of about 20 μm formed on the SUS420J2 steel showed that the compressive strength of protrusion tip of the plasma-nitrided specimen is almost four times larger than that of the as-sputter-etched specimen. Nanoindentation tests on quasi-columnar protrusions with sizes smaller than 500 nm formed on the SUS316 steel showed not only much scatter of indentation force vs depth curves but also irregular changes of gradient of the curves. This means that an inhomogeneous contact of the indenter on the protrusions with different heights, sizes and spaces as well as a discontinuous deformation of protrusions occur with progress of indentation. Although the true compressive strength of one protrusion is not obtained directly by usual hardness tests, a measure of the deformation resistance of a group of protrusions can be obtained from the hardness that is extrapolated to zero indentation area on the indentation area vs hardness curve. The compressive resistance of the plasma-nitrided protrusions is almost six times as large as that of the as-sputter-etched ones.

New Development to Measure Mode I. Fracture Toughness in Rock

Tensile fracture toughness is one of the dominant characteristics of rocks that play an important role in fracture mechanics of rock structures. In spite of the substantial amount of work that has been conducted as suggested methods to determine the fracture toughness of rock in Mode I., research on new methods are still demanded. The compact tension (CT) specimen is widely used to determine the fracture toughness of metals and is a standard method in accordance with ASTM standard. To conduct similar direct tests in rock, preparation of rock specimen a difficult task and needs some special tools to create notched sample. To address these issues, experimental techniques based direct tensile apparatus have been developed. Developed experimental procedure involves the use of the specimens having a central hole and two notches or cracks at both inner side of proposed specimen. The stress intensity factor formula at the crack tip of a CT specimen can be availed through the literature. However a new expression of stress intensity factor is needed for the developed test in order to account geometry and loading configuration. To develop a new stress intensity factor expression, the numerical models of suggested method were worked out. The focus of the current work is on development of the experimental technique, which involves determining the optimum sample size and shape, loading procedure, fracture toughness for different rock types, and verification of suggested method based on CT test which has been carefully considered in this investigations.

Effect of Microscopic Structure on High-cycle Fatigue Damage in Polycrystalline Nano-copper

A high-cycle loading method under full load reversal using resonant vibration was developed to investigate the high-cycle fatigue damage of nano-polycrystalline copper in a multi-layered material. A nano-component specimen in which a thin layer of nano-polycrystalline copper was sandwiched between a Si substrate and SiN layer was prepared by the focused ion beam technique, and a cyclic load was applied to the specimen. In order to reduce the resonance frequency of the specimen was reduced to less than several hundred kilohertz to control the loading cycle, a gold weight was attached to the specimen tip by tungsten deposition. The high-cycle fatigue loading induced slip bands associated with extrusion/intrusion of approximately several tens of nanometers in width on the upper surface of the Cu layer. The slip band was formed by the microscopic stress field in the grain, which was generated by the deformation constraint between grains. Although the morphology of the extrusion/intrusion was very similar to that observed in fatigue of the bulk material, the size was significantly different. A crack along the grain boundary was observed and it seemed to be initiated by the collision of slip bands with the grain boundary.

Room-temperature creep behavior on crack tip of commercially pure titanium

Abstract The room-temperature creep behavior on crack tip of compact tensile (CT) specimen for commercially pure titanium (CP-Ti) was studied by experiment and finite element (FE) simulation in this paper. The experimental results indicated that the time-dependent deformation was observed on the crack tip of CP-Ti CT specimen at room temperature, which agreed with the primary creep, and crack propagation was not observed. In order to consider the creep behavior on crack tip, time-dependent J-integral was used to characterize the stress fields near crack tip. The room-temperature creep behavior on crack tip was analyzed by FE simulation, which was verified by experimental results. Then, the strain fields under different stress states were analyzed by FE simulation. The influences of the locations to crack tip and load on the room-temperature creep were analyzed, which shows that the creep significance on crack tip is enhanced with increasing of load and decreasing of distance to crack tip. The estimation formula of creep strain value along ligament direction of CP-Ti CT specimen was established and verified by FE simulation results.

The influence of experimental parameters and specimen geometry on the mass spectra of copper during pulsed-laser atom-probe tomography.

We have studied the influence of experimental factors and specimen geometry on the quality of the mass spectra in copper (Cu) during pulsed-laser atom-probe tomography. We have evaluated the effects of laser pulse energy, laser pulse frequency, specimen base temperature, specimen tip radius, and specimen tip shank half-angle on the effects of mass resolving power, (m/Δm), at full-width at half-maximum and at full-width at tenth-maximum, the tail size after the major mass-to-charge state (m/n) ratio peaks, and the mass spectra. Our results indicate that mass resolving power improves with decreasing pulse energy between 40 and 80 pJ and decreasing base temperature between 20 and 80 K. The mass resolving power also improves with increasing tip radius and shank half-angle. A pulse frequency of 250 kHz slightly improves the mass resolving power relative to 100 or 500 kHz. The tail size decreases with increasing pulse energy. The mass resolving power improves when the cooling time is reduced, which is influenced by the thermal diffusivity of Cu and the specimen base temperature.

Long-term degradation of resin-based cements in substances present in the oral environment: influence of activation mode

Indirect restorations in contact with free gingival margins or principally within the gingival sulcus, where the presence of organic acids produced by oral biofilm is higher, may present faster degradation of the resin-based cement pellicle.Objectives:To investigate the degradation of four resin-based cements: Rely X ARC (R), Variolink II (V), enforce (E) and All Cem (A), after immersion in distilled water (DW), lactic acid (LA) and artificial saliva (AS) and to analyze the influence of the activation mode on this response.Material and Methods:Two activation modes were evaluated: chemical (Ch) and dual (D). In the dual activation, a two-millimeter thick ceramic disk (IPS empress System) was interposed between the specimen and light-curing unit tip. Specimens were desiccated, immersed in distilled water, artificial saliva and lactic acid 0.1 M at 37ºC for 180 days, weighed daily for the first 7 days, and after 14, 21, 28, 90 and 180 days and were desiccated again. Sorption and solubility (µg/mm3) were calculated based on ISO 4049. The data were submitted to multifactor analysis of variance (MANOVA) and Tukey's HSD test for media comparisons (α=0.05).Results:Sorption was higher after immersion in LA (p<0.05) and increased significantly with time (p<0.05). Sorption was influenced by the activation mode: Ch>D (p<0.05). The lowest solubility was presented by R (p<0.05). Conclusions:Lactic acid increased the degradation of resin-based cements. Moreover, the physical component of activation, i.e., light-activation, contributed to a low degradation of resin-based cements.

Understanding the detection of carbon in austenitic high-Mn steel using atom probe tomography

A high-Mn TWIP steel having composition Fe-22Mn-0.6C (wt%) is considered in this study, where the need for accurate and quantitative analysis of clustering and short-range ordering by atom probe analysis requires a better understanding of the detection of carbon in this system. Experimental measurements reveal that a high percentage of carbon atoms are detected as molecular ion species and on multiple hit events, which is discussed with respect to issues such as optimal experimental parameters, correlated field evaporation and directional walk/migration of carbon atoms at the surface of the specimen tip during analysis. These phenomena impact the compositional and spatial accuracy of the atom probe measurement and thus require careful consideration for further cluster-finding analysis.

Severe Plastic Deformation of Steel Induced by Ultrasonic Vibrations

High-intensity ultrasonic vibration was focused on the tip of conical steel specimens to induce severe plastic deformation at room temperature. We found, for the first time, that grain size smaller than 200 nm was obtained. Furthermore, the sharp tip of the conical specimen became umbrella-shaped or disk-shaped. The tip size changed from 0.5 mm diameter to a disk about 5 mm diameter, representing a large amount of plastic deformation in the metal at the tip of the conical specimen.

An Experimental and Numerical Hybrid Technique for Analysis the Fracture Behaviors in Natural Fiber Reinforced Composites

Natural fiber reinforced composites have become excellent candidates for automotive applications and civil engineering facilities. The design procedures will require precisely evaluating the toughness of the materials, which demand accurate measurements of strain or displacement that are difficult to obtain for ductile materials. The primary objectives of this paper were to develop reliable experimental approaches which can effectively evaluate the fracture behaviors of random short natural fiber reinforced polypropylene (PP) composites. The digital image correlation method (DICM) was applied to compute the deformation fields around the crack tip of a compact-tension specimen in a process of loading. An experimental and numerical hybrid method was used to describe the fracture behaviors of the composites. The relationship between the micro-structure of random fibers and the macroscopic mechanical properties of the composites can also be understood.

Histopathology attributes of Fox–Fordyce disease

Fox-Fordyce disease is a rare chronic papular condition with a very characteristic clinical presentation but a nonspecific histopathology. Its traditionally described histopathologic features have been criticized as variable and indistinct. Recently, a perifollicular infiltrate of histiocytes with foamy cytoplasm has been described as a consistent and reliable diagnostic finding. To evaluate the traditional and most recently described histopathologic and immunohistochemical attributes of Fox-Fordyce disease, we performed a computerized search of specimens in two dermatopathologic databases in Puerto Rico from the years 2000-2010. An additional specimen was donated by a dermatopathologist from an outside institution. Three cases were evaluated using hematoxylin-eosin-stained sections. The tip of an axillary lipoma excision specimen was used as the control tissue. Periodic acid-Schiff, colloidal iron, and immunoperoxidase staining for CD68 and c-kit (CD117) were performed in all specimens. We were able to verify traditionally described histological features such as infundibulum dilation, hyperkeratosis, plugging, acanthosis, and lymphohistiocytic infiltrate. Infundibular spongiosis was also common. A perifollicular foam cell infiltrate was the most distinct pathologic feature among our cases. The periodic acid-Schiff staining patterns suggested that the foam cell cytoplasm material might be similar in nature to the apocrine gland secretion content. Our results confirmed that a perifollicular foam cell infiltrate is the most distinct histopathologic feature. In addition, findings suggest that the intracytoplasmic foam cell material may be similar in nature to the apocrine gland secretion.