Increasing Specimen Thickness Research Articles

Dynamic crack propagation in large steel specimens

Dynamic fracture experiments on crack initiation and crack growth in single edge bend specimens are performed. The impact velocity is in the range of 14 to 50 m/s and the specimen size is 320×75 mm with a thickness varying from 18 to 40 mm. The experiments are recorded by high speed photography. Two different steel qualities are investigated and their constitutive characterisation are obtained from uni-axial tension tests and shear tests with strain rates in the range 10 −4 to 10 3 s −1 and tension tests at temperatures between −196 and 600°C. One of the materials exhibits a transition from a ductile dimple fracture to a brittle cleavage fracture as the loading velocity increases and as the specimen thickness increases. Scanning electron microscope fractographs show that the density of plastic bridges within cleavage ligaments decreases with increasing impact velocity and with increasing specimen thickness. It is also noted that the local crack propagation direction deflects from the global one in cleavage fracture areas with a high density of plastic bridges. The other material fails in a ductile mode in all the investigated cases.

プラスチックのレーザフォーミングに関する研究 試料厚さが変形特性に及ぼす影響

In this study, the effect of specimen thickness on machining characteristics and deformation mechanism of laser forming for plastic material with YAG laser were made clear by the experimental analysis. Each specimen with different thickness can be bent to 90 degrees under proper machining conditions. The optimum laser power, in which bending angle takes a maximum under the same feed condition, increases with an increase of specimen thickness. In the case of thick specimen, the deformation occurs without a delay between the beginning point and the end one of irradiation just after finishing of laser scan. On the other hand, thin specimen is deformed from the beginning point of irradiation during laser scan at a small number of laser scan, since the rigidity of thin specimen is lower compared to that of thick specimen. Consequently, the bending angle per one laser scan is small in the case of thin specimen. However, the bending angle per one laser scan becomes larger at a large number of laser scan. Because it is considered that the increase of rigidity of specimen leads to the stable deformation, since the hardness of irradiated area increases with an increase of laser irradiation.

Three-dimensional effects near the interface in a functionally graded Ni–Al2O3 plate specimen

Abstract Functionally graded materials composed of nickel–Al 2 O 3 composites with a variety of compositions and microstructures have been fabricated at the Idaho National Engineering and Environmental Laboratories. Finite element analysis has been utilized to numerically model the residual stress distribution in a graded nickel–Al 2 O 3 joint, primarily for the purpose of optimizing the architecture of the interface to resist failure. In this paper, the applicability of using computationally efficient two-dimensional (2-D) analyses for modeling complicated three-dimensional (3-D) plate geometries are quantified for a specimen consisting of nickel and alumina plates joined by a 60 vol% Ni particulate reinforced composite interlayer to form a functionally graded joint. A 2-D generalized plane strain model is also introduced for comparison. Overall, the results indicate that the generalized plane strain model with or without rotation of the rigid bounding planes provides the most accurate description of the 3-D stress state, especially at the corner interface within the elastic region. However, the plane stress model predictions are slightly better in the ductile regions while the plane strain model predictions are better along the edge of the interface in the elastic region. Furthermore, 3-D effects are prevalent at distances within approximately 1/2 of the specimen thickness from the edge singularity in the elastic region of the specimen on the free surface and within 1/8 of the specimen thickness below the free surface. In addition, the intensity of the stresses near the corner of the 3-D model appears to increase as the specimen thickness increases, and is greater than the intensity of the stresses predicted at corresponding locations in the 2-D models.

Linear shrinkage of photo‐activated composite resins during setting

The linear shrinkages during setting of five commercially available photo-activated composite resins were measured using a developed weight-loaded plate plastometer. Six specimens of thickness ranging from 0.25-2.00 mm were used and the linear shrinkage was measured in three stages: under no-load or load of 200 or 400 g. Two light-irradiation times of 20 and 60 s were chosen for each specimen. All measurements were carried out in an air cabinet maintained at either 23 degrees C or 37 degrees C. The measuring time of linear shrinkage was 11 min from the start of light-irradiation. The ratio of apparent linear shrinkage decreased according to the increase in specimen thickness and decreased when load was added during setting. Furthermore, the ratio of linear shrinkage of the specimens increased according to the increase in light-irradiation time and the ambient temperature of the specimens. When the adsorptivity or stickiness of material to a tooth was high, detectable initial time (Ti) of the linear shrinkage became longer and the magnitude of linear shrinkage to the adherent direction decreased.

Chapter 14 Application of electron tomography to mitochondrial research

Publisher Summary This chapter discusses the advantages and limitations of the electron tomography technique as applied to mitochondrial research and also outlines the important steps in a typical tomographic investigation, with special attention to optimization of specimen preparation. Some of the biological applications of transmission electron microscopy (TEM) are investigations into the internal organization of the mitochondrion. The two dimensionality of conventional TEM imaging is imposed by the physics of the interaction of electrons with the specimen. Most structural information in electron micrographs is provided by electrons scattered elastically (without energy loss to the specimen) only once. However, for incident electrons of a given energy, as specimen thickness increases, the fraction of singly elastically scattered electrons drops and the fractions of singly inelastically and multiply elastically and inelastically scattered electrons increase, reducing image contrast greatly. Moreover, tomography of thick, plastic section specimens has the potential to become a routine tool for the three-dimensional (3D) analysis of biological structure. Furthermore, advances in methodology are leading to the extension of this 3D imaging technique to frozen-hydrated specimens.

Effect of specimen thickness on fatigue crack growth rate

Fatigue tests were performed on the compact tension (CT) specimens of Type 304 stainless steel and Inconel 718. To investigate the effects of specimen thickness on crack tip deformation and fatigue crack growth rate (FCGR), specimens of different thickness were used. In the analysis, the elastic plastic fracture mechanics (EPFM) parameter known as the cyclic J-integral, Δ J was adopted to observe the local plasticity at the crack tip and compared with the linear elastic fracture mechanics (LEFM) parameter known as the stress intensity factor range, Δ K. The results show that FCGR is a function of specimen thickness, the effect of which is accelerated as specimen thickness increases. Therefore, it is thought that not only applied stress level but also specimen thickness should be taken into account in the measurement of FCGR, which is not considered in ASTM E 647 (ASTM E 647, 1995. Standard test method for measurement of fatigue crack growth rates.).

Effect of materials variables on the tear behaviour of a non-crystallising elastomer

Crack growth rates (r) were measured in pure shear test specimens as a function of strain energy release rate (G) for a non-crystallising SBR elastomer. Measurements were made as a function of: extent of swelling with Dibutyl Adipate; carbon black content; and crosslink density. In some cases experiments were carried out over a range of temperatures. In most cases the resulting G versus r plots showed a clear transition from rough to smooth crack surface behaviour with increasing crack growth rate, with an intervening slip/stick region. In the high speed/steady tear/smooth region the value of G necessary to drive a crack at a given rate was determined largely by the magnitude of the visco-elastic losses in the crack tip region, increasing with: decreasing temperature; increasing molar mass between crosslinks; decreasing extent of swelling; and increasing carbon black content. However G was independent of specimen thickness in this region suggesting that crack tip effects were minimal. In the low speed/rough region changes in the magnitude of G with materials and temperature/rate variables could not be explained by changes in visco-elastic loss alone. Furthermore the magnitude of G increased significantly with increasing specimen thickness. This suggested that in this region cavitation ahead of the growing crack tip resulting from dilatational stresses determined the crack tip diameter, and hence the magnitude of G.

ポリカーボネート板の疲労き裂進展の板厚依存性

The effect of the specimen thickness on the fatigue crack propagation in polycarbonate plates was investigated. Fatigue cracks were propagated under axial load conditions with constant amplitude loading, a single peak overload and two-step loading. The crack propagation rates both on the surface and in the interior of the specimen were measured. The crack closure behavior and the appearance of fracture surface were also examined. The fatigue crack propagation rate, da/dN, under constant amplitude loading increased with increasing specimen thickness at low ΔK levels. The crack closure behavior was not appreciably affected by the specimen thickness. The measurements of shear lip width on the fracture surface indicate that the thickness dependence of da/dN is closely related with the formation of shear lips. The crack extension during a single peak overload was larger in the thick specimen. The fatigue crack retardation after the peak overload was more marked in the thin specimen, which was similar to the case of stress decrease under two-step loading. These results can also be understood in terms of the contribution of shear lips to fatigue crack propagation.

EFFECT OF SPECIMEN THICKNESS ON FATIGUE CRACK GROWTH BEHAVIOUR UNDER CONSTANT AND VARIABLE AMPLITUDE LOADING

Mode I fatigue crack growth behaviour was experimentally investigated under constant and following the application of a single spike overload. Different specimen thicknesses were investigated at different overload ratios. A previously developed crack tip deformation model to correlate fatigue crack growth behaviour following k a single overload cycle was used to correlate that behaviour for different specimen thicknesses. It was found that the effect of specimen thickness on the fatigue crack growth behaviour under constant amplitude of loading is marginal especially at growth rates greater than le mm/cycle. The retardation due to the application of a tensile single overload decreases with the increase in specimen thickness. Good agreement between relevant experimental results and those predicted using three dimensional crack tip deformation parameter was found.

炭素繊維織物を用いたＣＦＲＰの衝撃損傷

CFRPs, reinforced with fibers having different modules/strain-to-breaks but a similar strength, were impacted by a 4 mm diameter steel sphere at a velocity of 900 m/s to 1, 800 m/s. Spall area and kinetic energy absorption of the sphere by CFRPs were investigated. In this velocity range, energy absorption ratio was independent with impact velocity in both CFRPs. A fiber cloth and a CFRP with low modules, i.e., high strain-to-breaks showed a high-energy absorption. Spall area increased with increasing specimen thickness. Tensile-stress applied areas simulated by a computer code also increased; The tendency of the experiments and simulation was quite similar. Most kinetic energy of spheres was absorbed at early stage of the impact.

Characterization of multilayers by means of EDXS in the analytical TEM

Referring to the characterization of nanoscale multilayers in cross section, the resolution limits of the EDXS method have been investigated. For that purpose EDXS line scan profiles of nanoscale Fe-Cr multilayers have been calculated assuming an increased specimen thickness and different tilts between electron beam and layer interface. The resolution limit seems to be greater than 2 nm layer thickness for regular multilayers. Experimentally a limit of 5 nm was reached.

Effect of temperature on measuring the thermal diffusivity of ceramic of different specimen thicknesses by using a laser-pulse method

Effect of temperature on the thermal diffusivity of (Sr, K)Zr 4(PO 4) 6 ceramic (abbreviated as SrKZP) of different specimen thicknesses ranging from 0.68 to 3.0 mm has been investigated using a laser-pulse method with different pulse energies. The thermal diffusivity of the ceramic shows a strong dependence on the specimen thickness as well as pulse energy at room temperature, which is attributed to excessive rise in the specimen temperature (Δ T) associated with a non-linear detector response. However, such a dependence becomes weak at elevated temperatures. The thermal diffusivity ( α) of the ceramic illustrates a strong thickness-dependence with Δ T ⇒ 0 α 0), i.e. α 0 increases with increasing specimen thickness for increasing specimen thickness for various temperatures. The thickness-dependent diffusivity of the SrKZP ceramic is modified by considering radial heat conduction effect, resulting in values of thermal diffusivity which are independent of specimen thickness at various temperatures.

Reduction in compressive strain to failure with increasing specimen size in pin-ended buckling tests

Flexural tests were performed on unidirectional T800/924 carbon-fibre/epoxy composites in a pin-ended buckling rig producing consistent compressive failures. Three sets of tests were carried out with all dimensions geometrically scaled. The results showed a strong decrease in strain at failure with increasing specimen size. Three further sets of tests were conducted where the thickness was kept constant and the length and width of the specimens varied. These showed similar strains at failure, suggesting that the size effect observed with the scaled specimens is predominantly due to the reduction in strain gradient with increasing specimen thickness.

Plastic zone size in fatigue cracking

Fatigue tests were performed on the specimens of Type 304 stainless steel and Inconel 718. To investigate the effects of specimen thickness on crack tip deformation and fatigue crack growth rate (FCGR), specimens of different thickness were used. To validate fatigue crack propagation in terms of plastic zone size, elastic plastic fracture mechanics (EPFM) are studied in this investigation. Results show that FCGR is a function of specimen thickness, which worsens as the specimen thickness increases. It is considered that plastic zone size is an important fatigue crack propagation parameter in conjunction with applied stress level, specimen thickness and crack closure.

Elastic-plastic fracture toughness of PC/ABS blend based on CTOD and J-integral methods

The elastic-plastic fracture toughness of a PC/ABS blend has been investigated simultaneously by the conventional J-integral and the crack tip opening displacement (CTOD) methods for specimen thickness varying from 4 to 15 mm. The critical J (Jc) and the critical CTOD (δc) are based on crack initiation. Another new method to determine Jc based on the close relationship between the J-integral and CTOD has also been developed. Those three critical Jc values, Jc−81, Jc-NEW, and Jc-BS obtained from ASTM E813-81, the new J method, and the equation Jc = mσyδc (1−ν2) of the CTOD method, are comparable and are independent of the specimen thickness. The constraint factor m evaluated from the linear elastic fracture mechanics (LEFM) approach using the Dugdale model is about 1.5. A rotation factor rg of approximately 2 was obtained which is also independent of the specimen thickness. The crack propagation resistance, J-curve (dJ/da) or δ-curve (dδ/da), decreases with increasing specimen thickness. A close relationship between CTOD and J-integral has been demonstrated in this study.

Effect of specimen thickness on the thermal diffusivity of (Sr, K)Zr4(PO4)6 ceramic via a laser-flash technique

It has recently been found that the thermal diffusivity of materials determined by means of a pulse method is strongly dependent on the specimen size, due to factors such as a strongly negative temperature-dependent diffusivity and the non-linearity of the detector response, etc. In polycrystalline (Sr, K)Zr4(PO4)6(SrKZP) ceramic, which has a low and weak temperature-dependent thermal conductivity, it has been found the thermal diffusivity increases with increasing specimen thickness and decreases with increasing specimen temperature rise, ΔT. In the limiting of ΔT→0, the SrKZP ceramic shows a thickness-dependent thermal diffusivity. The limiting diffusivity of the ceramic for different thicknesses can be corrected to a value of 7.84 × 10−3cm2s−1 by taking the radial heat conduction effect into account. A reliable measurement of the thermal diffusivity of the SrKZP ceramic can be obtained by using a specimen with thickness not exceeding ∼1 mm and keeping the specimen temperature rise to less than ∼3 °C.

加熱加圧成型により作製するオールセラミッククラウン―ＩＰＳ Ｅｍｐｒｅｓｓ―の色調に関する研究

The aim of this study was to investigate the influences of the color of abutments on IPS Empress' crowns.IPS Empress® ceramic disc specimens, metal disc abutment specimens (type IV gold and gold- silver - palladium alloy) and porcelain disc abutment specimens (A 1, A 2, A 3, A 3.5 and A 4) were used to investigate the influence of the color of abutment specimens on ceramic specimens.The color of ceramic specimens was measured with a small area colorimeter and evaluated by the CIE L*a*b* color system.The results were as follows:1. Both metal and porcelain abutment specimens influenced the color of ceramic specimens with a uniform thickness (1.0 mm) and different colors.2. The colorimetric influences of abutment specimens were decreased with the increase in thickness of ceramic specimens.3. Metal abutment specimens were brighter and more yellow than porcelain abutments.

Observations of the micro-mechanisms of fatigue-crack initiation in polycarbonate

The mechanisms of crack initiation in tensile fatigue of single-edge notched specimens of polycarbonate of varying thickness have been elucidated. At low stresses and long times microcracking and localized yielding occurred to form regular diamond-shaped cells on a scale of 2–4 Μm. On increasing the stress level with thin specimens (<1 mm), the microshear bands coalesced to form macroscopic damage zones of yielded material around the notch, followed by crack tearing from the notch surface. With increasing specimen thickness, restriction of shear banding ensued and a stable, semi-elliptical cavitation, or pop-in, formed about 10–100 Μm ahead of the notch, dependent on specimen geometry. As a result, the ligament formed between the notch and pop-in consists of yielded material. Brittle behaviour resulted with further increases in specimen thickness on loading, i.e. when the ligament could not be stabilized.

Variation of Young's modulus with the test specimen's aspect ratio

A study of Young's modulus in flexure of a number of methacrylate polymers indicated that the results were dependent on the test specimen thickness. The Young's modulus calculated from simple bending theory decreased with increased specimen thickness. This effect could not be accounted for either by the usual corrections for shear or corrections for the effect of the width/ length ratio. It transpires that the effects noted experimentally are due to the combined effects of shear and the width/length ratio.

Damage Development in a Short Fiber Reinforced Composite

The important issue of finding a relationship between a macroscopic damage variable and the process of damage accumulation within a material is addressed in this article. Monotonic and cyclic deformation behavior has been studied in a randomly distributed glass reinforced polyester matrix composite. The composite material is called sheet molding compound, abbreviated as SMC, with a short glass fiber weight of 30%. The damage evolution in this planar isotropic material is studied by continuously monitor ing axial, lateral, and specimen thickness strain components on the specimen. Based on these measurements, the damage volumetric strain was calculated that increased continu ously with cycles in load-controlled experiments. However, in the strain-controlled experi ments, where tensile stresses decayed rapidly with cycles, the corresponding damage volumetric strain reached a saturation level. Microscopic observations revealed that cracks grew along the matrix/fiber interfaces as well as the matrix/filler interfaces. Many of these cracks are normal to the specimen thickness direction. This observation is consistent with the increasing specimen thickness strain measured in the experiments.