Translaminar fracture toughness: The critical notch tip radius of 0° plies in CFRP

The relationship between translaminar fracture toughness measured at initiation and specimen initial notch root radius is investigated for the translaminar failure mode of cross-ply IM7/8552 carbon/epoxy laminates. Compact tension specimens with four sizes of notch root radii were tested; the true initiation toughness of the laminate was measured from specimens with notch tip radii of ρ ? 250 μm. Testing of specimens with larger notch root radii, ρ = 750 μm, yielded an apparent toughness that was found to be 30% higher than the true toughness of the laminate. The propagation toughness corresponding to the R-curve plateau was found not to be affected by the initial notch tip radius. Investigation of the fracture surfaces of failed specimens revealed that there is no interaction between the 0° and 90° ply failure mechanisms, and that the critical notch radius is a property intrinsic to the 0° plies of the laminate.     

An investigation of residual stresses in brazed cubic boron nitride abrasive grains by finite element modelling and raman spectroscopy

Joining cubic boron nitride (CBN) abrasive grains and tool body made of steel using brazing always creates residual stress due to thermal mismatch of the components when cooling down from the brazing temperature. A large tensile stress perhaps causes grain fracture during the grinding process with single-layer brazed CBN abrasive tools. To evaluate the residual stresses occurring in brazed CBN grains, values and distribution of residual stresses are calculated using the finite element method. Effects of bonding materials, embedding depth, gap thickness and grain size on brazing-induced residual stresses are discussed. Results show that the Cu–Sn–Ti bonding alloy always results in a larger tensile stress in the CBN grains, when compared to Ag–Cu–Ti alloy during the cooling phase of the brazing process. The maximum tensile stress is obtained at the grain–bond junction region irrespective of the choice of bonding material and embedding depth. When the grain side length is 100 μm, gap thickness is 10 μm and grain embedding depth is 30%, the maximum magnitude of the tensile stresses is obtained. The maximum stress is 401 MPa with Ag–Cu–Ti alloy and 421 MPa with Cu–Sn–Ti alloy. The brazing-induced residual stresses have been finally measured experimentally by means of the Raman spectroscopy. The current simulated results are accordingly verified valid.     

The effects of texture and grain morphology on the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

The fracture toughness and fatigue crack growth resistance of nanocrystalline materials are significantly affected by the thickness of the specimen. In this work we relate the mechanical properties of nanocrystalline platinum films to their texture and grain morphology. Tensile, creep and fatigue testing of annealed, ∼1 μm films resulted in mechanical properties similar to the as-received films (yield strength of ∼1.2 GPa, fracture toughness ∼17.8 MPa √m, and a fatigue crack growth power law exponent of ∼4.2). However, the breakdown of the initially columnar grain morphology had a marked effect on the transition point from an intergranular to transgranular fatigue cracking mode. Finite element modeling suggests that cyclic (fatigue) grain coarsening and the transition from inter- to transgranular cracking modes are a result of the relative importance of dislocation slip accommodation on in-plane and through-thickness oriented slip directions.     

Effect of grain size on mechanical,surface and biological properties of microwave sintered hydroxyapatite

Hydroxyapatite (HA) compacts having average grain sizes of 168 ± 0.086 nm, 1.48 ± 0.627 μm and 5.01 ± 1.02 μm are processed from synthesized HA powder by microwave sintering at varying sintering temperature for different times. Superior mechanical and biological properties are shown by nano-grain HA compacts as compared to their micron grained counterparts. Compressive strength, indentation hardness, and indentation fracture toughness are increased with the decrease in HA grain size. The highest surface energy and maximum wettability are exhibited by nano-grain HA. HA compacts are assessed for cell–material interaction by SEM, MTT and immunochemistry assays using human osteoblast cell line for 1, 5 and 11 days. MTT assays showed higher number of living cells and faster proliferation on nano-grain HA surface. Osteoblast cells on nano-grain HA surface expressed significantly higher amount of vinculin and alkaline phosphatase (ALP) protein markers for cell adhesion and differentiation respectively. This study shows the effect of grain size on physical, mechanical and in vitro biological properties of microwave sintered HA compacts.     

Fracture properties of bamboo

Bamboo is a typical natural composite material, which is longitudinally reinforced by strong fibers. The fibers are distributed densely in the outer surface region, and sparsely in the inner surface region, and their volume fraction changes with respect to radius. The structure of bamboo has been characterized by tensile tests and its mechanical properties have been related to its structure.This paper presents the fracture toughness of bamboo culms and nodes. A notch is inserted into the culm and node specimens using a razor blade with a thickness of 0.4 mm. Tensile tests are carried out to evaluate fracture toughness. The average value obtained was 56.8 MPa m^1/2, which is higher than that of Al-alloy. It was concluded that the fracture toughness of the bamboo culm depends on the volume fraction of fibers.     

Structure and strength of aluminum with sub-micrometer/micrometer grain size prepared by spark plasma sintering

A spark plasma sintering (SPS) technique has been applied to prepare fully dense Al samples from Al powder. By applying a sintering temperature of 600 °C and a loading pressure of 50 MPa, fully recrystallized samples of nearly 100% density with average grain sizes of 5.2 μm, 1.3 μm and 0.8 μm have been successfully prepared using a sintering time of less than 30 min and without the need for a nitrogen atmosphere. A similarity between the grain size and powder particle size is found, which suggests a potential application of the SPS technique to prepare samples with a variety of grain sizes by tailoring the initial powder particle size. The SPS samples show higher strength than Al samples with an identical grain size prepared using thermo-mechanical processing, and a better strength–ductility combination, with the 1.3 μm grain size sample showing a yield strength (σ_0.2%) of 140 MPa and a uniform elongation of more than 10%. This higher strength is related to the presence of oxide particles in the grain boundaries of the samples. It is concluded that SPS is an excellent technique for the production of very fine grained Al materials with high strength, by combining both grain boundary and oxide dispersion strengthening.     

Effect of oxygen content on impact toughness of a fine-grained magnesium alloy

Fine-grained Mg-3Al-Zn alloys with various oxygen contents were prepared by a powder metallurgy process, and the effect of oxygen content on the impact toughness of the fine-grained magnesium alloys was quantitatively investigated. It is found that the impact toughness of magnesium alloys with mean grain size smaller than 3 μm is extremely high when the oxygen content is less than 400 pp, but it becomes very low as the oxygen content is higher than 1000 ppm. Both magnesium oxides and hydroxides are detected in the magnesium alloys. The presence of excessive magnesium oxides and hydroxides deteriorates the dynamic plastic deformation ability of the fine-grained magnesium alloys.     

Effects of initial grain size on hot deformation behavior of commercial pure aluminum

The effects of initial grain size of commercial pure aluminum on hot deformation behavior were investigated using hot compression tests. The hot compression tests were carried out on the pure aluminum samples with the initial grain sizes of 50, 150 and 450 μm using various strains, strain rates and different deformation temperatures. It was found that the hot deformation behavior of used material was sensitive to deformation conditions and initial microstructure. Results indicate that the initial grain size has significant effect on the flow stress. Flow stress decreases when the grain size decreases from 450 to 50 μm and when strain rate is lower than 0.05 s⁻¹. This procedure is reversed at strain rate of 0.5 s⁻¹. Furthermore, effects of other parameters like the strain rates and deformation temperatures on the flow stresses and hardening rates were investigated. It was also found that the inhomogeneity of microstructure distribution at different positions of the deformed specimens depended on the amount of deformation concentration at particular points and other processing parameters such as initial grain sizes, strain rates and deformation temperatures. In addition the geometric dynamic recrystallization (GDRX) was observed in the specimens highly strained (0.7) at elevated temperature (500 °C) using polarized light microscope and sensitive tint (PLM + ST).     

Effect of granulated sugar as pore former on the microstructure and mechanical properties of the vitrified bond cubic boron nitride grinding wheels

For vitrified bond cubic boron nitride (CBN) grinding wheel, introduced pores play a very important role for its mechanical properties and performance. In this paper, granulated sugar was used as pore former of the vitrified bond in CBN grinding wheel. The effects of content and particle size of the granulated sugar on the porosity and the flexural strength of the sintered vitrified bond CBN wheel samples have been investigated. It was found that the porosity of the vitrified bond CBN wheel is positively correlated with the content of the granulated sugar. The smaller and more irregular shaped pores are uniformly distributed in the bond when the content of granulated sugar is between 1 and 3 wt.%. Larger and more non-uniform pores and pore channels appear as the content of granulated sugar is increased from 5 to 7 wt.%. The flexural strength of the vitrified bond CBN wheel specimens decreases with an increase in pore former’s content and the porosity. With the increase of pore former’s particle size at the content of 3 wt.%, the flexural strength reaches to a peak value of 49 MPa with average particle size of granulated sugar is 250 μm. When the average size of granulated sugar is from 100 to 125 μm, the pores’ size is similar with the size of pore former and distributed homogeneously. The larger granulated sugar with the size from 160 to 500 μm can introduce different size of pores which could be smaller or larger than the size of pore former.     

Constitutive modeling of alumina sintering: grain-size effect on dominant densification mechanism

In the present work, alumina powders with the initial grain sizes of 0.9 and 7.0 μm, respectively, were sintered at different temperatures. Constitutive laws for densification were employed to model the sintering process of alumina ceramics. Based on the constitutive laws employed and the experimental results obtained, the dominant densification mechanism was identified and the effect of grain size on dominant densification mechanism was discussed. The activation energy for densification was also evaluated. In the investigated sintering temperature range, interface reaction was identified as the controlling process in sintering of alumina powders with the initial grain size of 0.9 μm, while grain-boundary diffusion was identified as the dominant process in sintering of alumina powders with the initial grain size of 7.0 μm. The activation energies for densification of the finer and coarser grain size alumina ceramics were determined as 342 and 384 kJ mol⁻¹, respectively, which provided a strong support on the densification mechanism investigation.     

Mechanically induced self-propagating reaction and consequent consolidation for the production of fully dense nanocrystalline Ti55C45 bulk material

We employed a high-energy ball mill for the synthesis of nanograined Ti₅₅C₄₅ powders starting from elemental Ti and C powders. The mechanically induced self-propagating reaction that occurred between the reactant materials was monitored via a gas atmosphere gas-temperature-monitoring system. A single phase of NaCl-type TiC was obtained after 5 h of ball milling. To decrease the powder and grain sizes, the material was subjected to further ball milling time. The powders obtained after 200 h of milling possessed spherical-like morphology with average particle and grain sizes of 45 μm and 4.2 nm, respectively. The end-products obtained after 200 h of ball milling time, were then consolidated into full dense compacts, using hot pressing and spark plasma sintering at 1500 and 34.5 MPa, with heating rates of 20 °C/min and 500 °C/min, respectively. Whereas hot pressing of the powders led to severe grain growth (~ 436 nm in diameter), the as-spark plasma sintered powders maintained their nanograined characteristics (~ 28 nm in diameter). The as-synthesized and as-consolidated powders were characterized, using X-ray diffraction, high-resolution electron microscopy, and scanning electron microscopy. The mechanical properties of the consolidated samples obtained via the hot pressing and spark plasma sintering techniques were characterized, using Vickers microhardness and non-destructive testing techniques. The Vickers hardness, Young's modulus, shear modulus and fracture toughness of as-spark plasma sintered samples were 32 GPa, 358 GPa, 151 GPa and 6.4 MPa·m^1/2, respectively. The effects of the consolidation approach on the grain size and mechanical properties were investigated and are discussed.     

The Cu matrix cermets remarkably strengthened by TiB2 “in situ” synthesized via self-propagating high temperature synthesis

The TiB₂–Cu cermets with predominant concentration of superhard TiB₂ (from 45 to 90 vol.%) were fabricated using elemental powders by means of SHS (self-propagating high-temperature synthesis) process and simultaneously densified by p-HIP (pseudo-isostatic pressing technique). The heat released during highly exothermic SHS reaction was “in situ” utilized for sintering. The combustion occurred even for 50 vol.% Cu dilution. According to XRD metallic copper binder was formed in those cermets in whole range of investigated compositions. The TiB₂ volume fraction significantly influenced the properties of fabricated materials, especially grain size and hardness. Both the average grain size and hardness significantly increased with TiB₂ content, so the maximum value of 18 GPa was measured for TiB₂–5 vol.%Cu composite. Coarse grains of 6.4 μm in size were observed for this composite while TiB₂-based submicro-composites were formed for 40–50% of Cu where the average grain size did not exceed 0.6 μm. The Vickers hardness of 16–18 GPa obtained for cermets containing from 85 to 90 vol.% of TiB₂ and no radial cracks in Vickers hardness test proved that in term of hardness and fracture toughness the composites might be competitive to WC–Co cermets.     

Impact behavior of entangled steel wire material

Entangled steel wire (Q195F) with total porosity of 36.3 ± 1.3 to 61.8 ± 2.4% and pore sizes of 15–825 µm have been investigated in terms of the porous morphologies, impact deformation and failure behavior. The results reveal that the impact toughness increases with the decrease of the porosity. The sintered entangled steel wire materials with 61.8 ± 2.4% porosity exhibit an average of 11.8 J/cm² impact toughness. With 36.3 ± 1.3% porosity, the sintered materials have an average of 45.5 J/cm² impact toughness. Impact absorbing energy and impact toughness have been obtained by Charpy impact testing. Essential impact deformation and failure mechanisms such as pore edges (i.e. fibers) bending, bulking, rotating, yielding, densification and fracture, as well as break (or avulsion) of sintering points in the steel wire framework contribute to the excellent energy-absorbing characteristics under impact loading condition.     

Quantitative measurements of small scaled grain sliding in ultra-fine grained Al–Zn alloys produced by friction stir processing

An Al-15 wt.% Zn alloy was processed by friction stir processing to produce grain sizes of ~ 0.5 μm, ~ 1 μm, and ~ 2 μm. A simple and effective method was developed to determine the true strain by scribing marker lines with scaled division using focused ion beam micromachining prior to deformation. The “microscopic” grain boundary sliding, with displacements of adjacent grains of the order of a nanometer, can easily be detected by the proposed technique, providing a surface analysis with high accuracy that could be used to observe the changes in relief with increasing strains. Moreover, the occurrence of grain boundary sliding at room temperature was considered a major cause for higher strain rate sensitivity in fine-grained Al–Zn alloys.     

Preparation of YAG gel coated ZrB2–SiC composite prepared by gelcasting and pressureless sintering

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB₂–SiC (YZS) composite were conducted. YAG gel coated ZrB₂–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6 wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB₂ and SiC in the YZS composite were 3.8 and 1.3 μm, respectively. The flexural strength, fracture toughness and microhardness were 375 ± 37 MPa, 4.13 ± 0.45 MPa m^1/2 and 14.1 ± 0.5 GPa, respectively.     

Recent developments in local concepts of fatigue assessment of welded joints

Several lately proposed modifications or variants of the structural stress or strain concepts, of the notch stress or strain concepts (also termed ‘local stress or strain concepts’) and of the fracture mechanics concepts of fatigue assessment of welded joints are reviewed, whereas the wider context is presented in a recently republished and actualised standard work. The structural stress concepts described first are based on a linearisation of the stress distribution across the plate thickness or along the anticipated crack path and, alternatively, on the structural stress 1 mm in depth below the weld toe. The structural stress is defined and set against design S–N curves. A further structural stress concept is presented for welded joints in thin-sheet steels and aluminium alloys. Among the elastic notch stress concepts, the variant with the reference notch radius, ρ_r = 1 mm, recently verified also for welded joints in aluminium alloys with plate thicknesses t ? 5 mm and the variant with a small-size reference notch radius, ρ_r = 0.05 mm, applicable to welded joints in thin-sheet materials, are outlined. The elastic–plastic notch strain concept is applied to a spot-welded tensile-shear specimen starting from a small-size keyhole notch at the nugget edge. The novel notch stress intensity factor (NSIF) approach relating to crack initiation and extrapolated to final fracture of seam-welded joints in steels and in aluminium alloys is reviewed. A more recently developed crack propagation approach for spot welds is finally described.     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

Fatigue strength of notched specimens made of 40CrMoV13.9 under multiaxial loading

The work deals with multiaxial fatigue strength of notched round bars made of 40CrMoV13.9 steel and tested under combined tension and torsion loading, both in-phase and out-of-phase. The axis-symmetric V-notches present a constant notch root radius, 1 mm, and a notch opening angle of 90°; the notch root radius is equal to 4 mm in the semi-circular notches where the strength in the high cycle fatigue regime is usually controlled by the theoretical stress concentration factor, being the notch root radius large enough to result in a notch sensitivity index equals to unity. In both geometries the diameter of the net transverse area is 12 mm.The results from multi-axial tests are discussed together with those obtained under pure tension and pure torsion loading from notched specimens with the same geometry. Altogether more than 120 new fatigue data are summarised in the present work, corresponding to a one-year of testing programme.All fatigue data are presented first in terms of nominal stress amplitudes referred to the net area and then re-analysed in terms of the mean value of the strain energy density evaluated over a given, crescent shape volume embracing the stress concentration region. For the specific steel, the radius of the control volume is found to be independent of the loading mode.     

The effect of grain size and martensitic transformation on the wear behavior of AISI 304L stainless steel

In the present study, a combination of cold rolling and subsequent annealing was used to produce an AISI 304L stainless steel with different grain sizes (650 nm, 3 μm and 12 μm). Wear behavior of the steel was subsequently examined using dry sliding wear test under different loads of 10 N, 20 N and 30 N. Different microstructural characterizations were conducted on the samples. The results demonstrated that the ultra-fine grained steel (650 nm grain size) had better wear resistance under normal loads of 10 N and 20 N, whereas under the normal load of 30 N, it showed weak wear resistance as compared to the steel with larger grain size (3 μm and 12 μm). This behavior can be attributed to the amount of induced martensitic transformation formed during the wear test. This transformation was evaluated using XRD analysis and quantified by Ferritescope measurements. Wear mechanism was recognized as delamination in the early stages of the wear test and the mixture of delamination and abrasion for higher distances.     

Analysis of ceramics toughened by non-conventional fiber reinforcement

Many refractory materials that are not readily available in fiber form, including ultra-high temperature ceramics (UHTC), are attractive candidates for use in high temperature structural components. This work explores the possibility of using non-conventional fiber forms that can be fabricated by “fibrous monolith” techniques to design composites with high strength and toughness along with 2D isotropy. The use of low-aspect ratio bone-shaped short fibers (BSSF) to improve fracture toughness and the use of composition tailoring to increase fiber strength were analyzed and it was found that both concepts need to be used in combination to achieve significant toughening. Computational models using UHTC as model materials indicate, for example, that significant improvements in fracture toughness can be realized with an aspect ratio of just 15, but only if the fiber strengths can be raised to 1.5 GPa. The use of a single outer layer of lower thermal expansivity composition is predicted to increase low temperature strength by a factor of 2, while multilayers of reasonable thickness (10 μm) result in strengthening by a factor of 3. For UHTCs, processing improvements that reduce flaw sizes will be necessary to take advantage of these results, but considerable improvement in properties can result from such progress.