Higher Fracture Toughness Values Research Articles

Microstructural and fracture toughness characterisation of a high-strength FeCrMoVC alloy manufactured by rapid solidification

A newly-developed high-speed steel manufactured using a rapid solidification technique was investigated. Microstructural observations were conducted with the aim of comprehensively characterising the material. Furthermore, fracture toughness tests were carried out to determine the linear-elastic plane-strain fracture toughness KIc at both room temperature (20°C) and −40°C. A fatigue precracking method was developed to provide stable crack propagation from a manufactured chevron-notch at a ratio of minimum to maximum stress of R=−1. The results of the fracture toughness tests demonstrated that, in comparison to other tool steels, the material exhibits relatively high fracture toughness values. Furthermore, the lower shelf of KIc is already reached at room temperature. Fracture toughness is considered to be a matrix property due to the high matrix hardness. An analysis of the influence of notch tip radius ρ on notch fracture toughness KA revealed a linear correlation between the square root of the notch tip radius and KA. Upon examination of the influence of notch tip radius on fracture toughness measurements, the assumption was made that the critical notch tip radius is approximately 90μm.

OS1530 ISB法による部分安定化ジルコニアのR曲線挙動

R-curve and crack propagation behabior were studied for partially stabilized zirconia. R-curve obtained by the indentation strength in bending method, where beam strength measurements were made using the specimens with indented loads in the range of48-294 N, indicated that there was no increasing crack extension resistance, i.e. flat R-curve behavior. On the other hand, .KI-V curve obtained by constant stress rate tests using indented beam specimens indicated that there was a large effect on fatigue fracture of slow crack growth, i.e. the small crack propagation parameter of n=7.3. As a result, it is considered that the high fracture toughness value in partially stabilized zirconia is not caused by process zone wake, but is done by only frontal process zone produced in the crack tip.

Mechanical properties of AISI 1045 ceramic coated materials by nano indentation and crack opening displacement method

An effective approach was conducted for estimating fracture toughness using the crack opening displacement (COD) method for plasma enhanced chemical vapor deposition (PECVD) coating materials. For this evaluation, an elastoplastic analysis was used to estimate critical COD values for single edge notched bending (SENB) specimens. The relationship between fracture toughness (K IC) and critical COD for SENB specimens was obtained. Microstructure of the interface between Al2O3-TiO2 composite ceramic coatings and AISI 1045 steel substrates was studied by using scanning electron microscope (SEM). Chemical compositions were clarified by energy-dispersive X-ray spectroscopy (EDS). The results show that the interface between of Al2O3-TiO2 and substrate has mechanical combining. The nanohardness of the coatings can reach 1 200 GPa examined by nanoindentation. The K IC was calculated according to this relationship from critical COD. The bending process produces a significant relationship of COD independent of the axial force applied. Fractographic analysis was conducted to determine the crack length. From the physical analysis of nanoindentation curves, the elastic modulus of 1045/Al2O3-TiO2 is 180 GPa for the 50 μm film. The highest value of fracture toughness for 1045/Al2O3-TiO2-250 μm is 348 MPa·ml/2.

High strain rate behavior, transformation-induced plasticity and fracture toughness characterization of cast and additionally tempered Fe85Cr4Mo8V2C1 alloy manufactured using a rapid solidification technique

This study focuses on the characterization of the microstructures of an FeCrMoVC alloy in two states (an as-cast and a heat-treated state) as well as the compressive strain rate-dependent material and fracture toughness behavior. Both microstructures consist of martensite, retained austenite and complex carbides. Tempering results in a transformation of retained austenite into martensite, the precipitation of fine alloy carbides, and diffusion processes. High yield stresses, flow and ultimate compressive strength values at a relatively good deformability were measured. The yield and flow stresses at the onset of deformation are higher for the heat-treated state due to higher martensitic phase fractions and fine precipitations of alloy carbides respectively. Compressive deformation causes a strain-induced transformation of retained austenite to α′-martensite. Hence, both high-strength alloys are TRIP-assisted steels (TRansformation-Induced Plasticity). However, the martensitic transformation is more pronounced in the as-cast state due to higher phase fractions of retained austenite already in the initial state. Examinations of strained microstructures showed decreased crystallite sizes with increasing deformation. It is assumed that, during plastic deformation, the amount of low angle grain boundaries increases while the incremental formation of α′-martensite leads to decreased crystallite size. In general, lower microstrains were determined in the heat-treated state as a consequence of stress relaxation during tempering. In comparison to commercially available tool steels, the determined fracture toughness K Ic of both variants revealed relatively high fracture toughness values. It was found that the lower shelf of K Ic is already reached at room temperature. Higher loading rates \( \dot{K} \) resulted in lower dynamic fracture toughness K Id values. Notch fracture toughness K A measurements indicate that the critical notch tip radii of the examined materials are slightly smaller than 0.09 mm.

Effect of Interfacial Treatment Process on the Microstructure and Properties of C<sub>f</sub>/SiC Ceramic Matrix Composites

Abstract. This paper investigated the Cf/SiC ceramic matrix composites. By utilizing different interfacial treatment processes to prepare carbon-fiber preform, the preform was then densified by infiltration and pyrolysis(PIP) with polycarbosilan/xylene solution as precursor, and the Cf/SiC ceramic matrix composite specimens were fabricated. Mechanical tests such as bending test and fracture toughness were performed for Cf/SiC samples. The results show that the interfacial bonding strength in the sample with high-temperature treatment process was improved due to removing surface sizing. The samples which were treated up to 1400°C exhibited the highest three-point flexural strength, up to 595MPa; The samples which were treated up to 1400°C and deposited by pyrolytic carbon(PyC) coating shows the highest fracture toughness value which was 20.70MPa•m1/2.

Improvement of fracture toughness and tensile ductility of massive rings made of 18 % nickel maraging steel

Abstract Massive rings of 18% nickel maraging steel produced using the ring rolling process do not consistently attain high fracture toughness (KIc) and tensile ductility values. Due to large section sizes involved, it becomes difficult to suppress precipitation of carbonitride particles and prevent thermal embrittlement. The conventional heat treatment of solutioning at 820 °C followed by aging at 480 °C is not effective in undoing this embrittlement. Different multi-stage solution treatments were designed and tried out to improve the mechanical properties. A two-stage (950 °C + 820 °C)/three-stage (950 °C + 950 °C + 820 °C) solution treatment, depending on the section size, proved to be effective. This treatment leads to recrystallisation of the as-hot-worked microstructure and a fine grain size; it is believed that a delinking of grain boundaries and carbonitrides occurs simultaneously. The observed improvement of fracture toughness and tensile ductility can thus be explained.

Microstructure and mechanical behavior of alumina–zirconia–mullite refractory materials

The synthesis of Al 2O 3–ZrO 2–SiO 2 (AZS) refractory materials for use in furnace chambers for glass melting has been studied in this work. Several mixtures with different alumina–zirconia–silica ratios, corresponding to compositions selected within the ternary phase diagram Al 2O 3–ZrO 2–SiO 2, were processed by attrition milling followed by pressing and reactive sintering at different temperatures (1450, 1550 and 1650 °C). The density of the sintered samples varied significantly with respect to that of the green materials, with increases of up to 90% in this property after sintering at 1650 °C. The X-ray diffraction patterns revealed the formation of the predicted phases (ZrO 2, Al 6Si 2O 13 and Al 2O 3) according to the ternary phase diagram for all compositions studied, with varying amounts of each formed phase. SEM characterization showed that the sintering temperature employed made a significant difference regarding the final microstructure of the sintered materials. For the studied compositions, the maximum resistance to fracture (MRF) was 185 MPa, with the highest values of fracture toughness obtained at 1650 °C.

Determination of the Optimum Fitting Probabilistic Distribution for Fracture Toughness in Small Sample Size

Plane strain fracture toughness is an important mechanical performance index for damage tolerance design, so how to obtain this value through test deserves consideration. In this paper, the ductile fracture toughness test was performed. It was difficult for the specimen with big dimensions in the case of plane strain to be carried out through ordinary testing machine with lower capacity because of the higher fracture toughness value of the material, that it was attained indirectly for plane strain fracture toughness value by the way of ductile fracture toughness test and pertinent formula calculation if proper specimen selected. The fracture property tests according to the corresponding standard were conducted. The compact tension specimens were obtained from the main rotor butt of a helicopter in service. Based on the single specimen measuring ductile fracture toughness method, the experiments of measuring the ductile fracture toughness were carried out. A method was given to determine the optimum fitting probabilistic distribution function of fracture toughness in the small sample size. The statistic results show that the optimum probabilistic distribution function of ductile fracture toughness is the Extreme Maximum. Value distribution. The following factors were taken into account, the linear relative coefficient, total fit effect probability relative coefficient, consistency with the relevant fatigue physics and tail most importantly, safety of design evaluation. The shape parameter, scale parameter, and location parameter are -1.1231, 860.53, 6036.4, respectively. The statistical variation coefficient is 11.22%. The result shows that there is a large risk probability for a definite value to fracture toughness only with one or two pieces of samples according to the test.

Effect of CeO 2 addition on densification and microstructure of Al 2O 3–YSZ composites

Alumina (Al 2O 3) and alumina–yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO 2) were prepared by spark plasma sintering (SPS) at temperatures of 1350–1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al 2O 3 based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl 11O 18 grains was observed in the ceria containing composites sintered at 1400 °C. Al 2O 3–YSZ composites without CeO 2 had higher hardness than monolithic Al 2O 3 sintered body and the hardness of Al 2O 3–YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO 2 increased from 10 to 30 vol%. The fracture toughness of Al 2O 3 increased from 2.8 MPa m 1/2 to 5.6 MPa m 1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m 1/2, was achieved with the addition of 30 vol% YSZ.

Nanostructured TiC–TiB 2 composites obtained by adding carbon nanotubes into the self-propagating high-temperature synthesis process

The synthesis of nanostructured TiC–TiB 2 by self-propagating high-temperature synthesis (SHS) has been investigated by using carbon nanotubes as precursor materials in partial substitution of graphite according to the following reaction: 6Ti + B 4C + (3− x)C + x CNT → 4TiC + 2TiB 2. Different amounts of CNTs addition have been studied in order to achieve structural refinement of the SHS products. The CNT molar content was varied in order to define the optimal composition, which allows to obtain nanostructured TiC–TiB 2 powders morphologically homogenous. The optimized composition has been chosen for the further densification step. The Pressure Assisted Fast Electric Sintering (PAFES) technique gave bulk composites with ultrafine grained microstructure. The mechanical characterization showed very high hardness and good fracture toughness values if compared to literature data.

Improvement in fracture toughness of austempered ductile iron by two-step austempering process

Ductile cast iron samples were austenitised at 900°C and subjected to two types of austempering called as conventional austempering and two-step austempering. Five different temperatures, 280, 300, 320, 350, 380 and 400°C, with an austempering time of 2 h, were chosen for conventional austempering. For two-step austempering process, the first step temperatures were 280, 300 and 320°C. The samples were austempered at each of these temperatures for different times, i.e. 10, 20, 30, 45 and 60 min, and then upquenched to higher temperature of 400°C for 2 h. Fracture toughness and tensile studies were carried out under all these austempering conditions. During conventional austempering, the fracture toughness initially increased with increasing austempering temperature, reached a peak value of 63 MPa m1/2 and dropped with further increase in temperature. During the two-step austempering, fracture toughness was found to increase with increasing first step time. The curve shifted to higher values of fracture toughness as the first step temperature was decreased and the maximum value of 78 MPa m1/2 was obtained. The results of the fracture toughness study and the fractographic examination were correlated with microstructural features such as bainitic morphology, the volume fraction of retained austenite, and its carbon content. Ferrite lath size and stability of the retained austenite were found to influence the fracture toughness.

Microstructure and mechanical properties of silicon nitride–titanium nitride composites prepared by spark plasma sintering

▶ Si 3 N 4 –TiN composites were successfully prepared by using Spark Plasma Sintering . ▶ Homogeneous distribution of equiaxed TiN grains in Si 3 N 4 matrix for SPS1 (1550 °C). ▶ High hardness value (21.7 GPa) and bending strength (621 MPa) for SPS1 (1550 °C). ▶ Highest fracture toughness value (8.39 MPa m 1/2 ) SPS2 (1300 °C). Si 3 N 4 –TiN composites were prepared by spark plasma sintering (conventional sintering (SPS1) and in situ reaction sintering (SPS2)). Homogeneous distribution of equiaxed TiN grains in Si 3 N 4 matrix results in the highest microhardness (21.7 GPa) and bending strength (621 MPa) of sample SPS1 sintered at 1550 °C. Dispersion of elongated TiN grains in Si 3 N 4 matrix results in the highest fracture toughness (8.39 MPa m 1/2 ) of sample SPS2 sintered at 1300 °C.

Effect of Al2O3/ZrO2 reinforcement on the mechanical properties of PMMA denture base

A study on the properties of denture base materials made from PMMA filled with Al2O 3/ZrO2 was carried out. The amount of Al2O 3/ZrO2 filler was fixed at 5 wt%. However, the ratio of Al 2O3 to ZrO2 added was from 0 to 100. Samples were prepared for fracture toughness, flexural, and tensile tests. The fracture surface was analyzed using SEM and EDS. It was found that the Al2O 3/ZrO2 of 80 : 20 ratio shows the highest values of fracture toughness and flexural properties. SEM micrographs indicate that distribution of Al2O3 and ZrO2 in the PMMA matrix is fairly homogeneous. The interface between the reinforcement particles and matrix is good. Therefore, addition of Al2O3/ZrO2 in PMMA improves the mechanical properties of this denture base material.

Effect of intense plastic straining on microstructure and mechanical properties of an Al-Mg-Sc alloy

An Al-5%Mg–0.18%Mn–0.2%Sc-0.08%Zr-0.002%Be was subjected to equal-channel angular extrusion up to true strains of ~3 and ~8, that resulted in the formation of partially recrystallized and fully recrystallized structure, respectively. It was shown that the alloy with partially recrystallized structure exhibits highest strength and ductility. The material with fully recrystallized structure showed lowest fatigue crack growth rate and highest value of fracture toughness. Reasons of this unusual effect of microstructure on crack propagation resistance under fatigue are discussed.

Effect of salivary esterase on the integrity and fracture toughness of the dentin‐resin interface

Human Salivary Derived Esterases (HSDE) are part of the salivary group of enzymes which show strong degradative activity toward the breakdown of one of the most common monomers used in dental composites and adhesives, 2,2-[4(2-hydroxy 3-methacryloxypropoxy)-phenyl] propane (Bis-GMA), to form the degradation product 2,2-bis [4 (2,3-hydroxy-propoxy)phenyl] propane (Bis-HPPP). This study was aimed to evaluate the effects of HSDE on the biodegradation and fracture toughness of the adhesive resin-dentin interface. Adhesive resin (Scotchbond Multi Purposes), resin composite (Z250) and mini short-rod specimens, were either not incubated; or incubated in phosphate-buffered saline (PBS) or HSDE media for up to 180 days (37 degrees C, pH 7.0). The amount of Bis-HPPP was analyzed by high performance liquid chromatography and mini-SR specimens were tested for fracture toughness using universal testing machine following 30, 90, or 180-day incubation periods. Significantly higher amounts of Bis-HPPP were produced in HSDE than in PBS incubated specimens (p < 0.05). Non-incubated mini-SR specimens had the higher fracture-toughness values, while specimens incubated for 180-days in HSDE had the lowest fracture toughness (p < 0.05). This study suggests that biodegradation is an on-going clinically relevant process that progressively compromises the integrity of the critical resin restoration-adhesive interface, as well as the resin-composite component with time.

Improvements in microstructural, mechanical, and biocompatibility properties of nano-sized hydroxyapatites doped with yttrium and fluoride

Hydroxyapatite was doped with Y 3+ (2.5, 5 and 7.5 mol%) and F − (2.5 mol%) ions (2.5YFHA, 5YFHA, 7.5YFHA, respectively) to compare its structural and mechanical properties and cellular response with pure-hydroxyapatite. No second phases were observed by X-ray diffraction spectra of 2.5YFHA. Doped hydroxyapatites had F − bonds in addition to OH − bonds. Hydroxyapatites sintered at 900 and 1100 °C were in nano-size. 7.5YFHA sintered at 1300 °C had the highest microhardness value. 2.5YFHA sintered at 1100 °C had the highest fracture toughness value. MTT viability assays showed high cell attachments on 2.5YFHA. Cell proliferation on 2.5YFHA and 5YFHA sintered at 1100 and 1300 °C was comparable with the control after 5-day culture. The highest ALP production and calcium deposition were observed on all hydroxyapatites sintered at 1100 °C. 2.5YFHA sintered at 1100 °C can be an alternative for hydroxyapatite in orthopedic applications.

Alumina-Copper Composites with High Fracture Toughness and Low Electrical Resistance

Through an intense mixing process of Al2O3 powder with different copper contents, Al2O3-Cu composites were fabricated by sintered at 1300°C during 1h, where the likely liquid sintering mechanism, lead to obtain composites with relative densities greater than 95%. Scanning electron microscopy was used to observe the resulting microstructures, which indicated that these composites were mostly formed by a fine and homogeneous Al2O3-ceramic matrix with immerse nano-metallic copper particles. The behavior of both fracture toughness and electrical resistance of the composites is directly dependent with the copper content in the matrix. As the copper contents increased, the composites exhibited high values of fracture toughness, whereas, their electrical resistance is reduced considerably.

Evaluation of alternative polymer bracket materials

Polymer brackets still have some disadvantages because of decreased wear resistance and hardness. The aim of this study was to investigate the mechanical properties of alternative bracket polymers; urethane-dimethacrylate, high-density polyethylene, and an experimental bracket polymer (EBP) consisting of polyethylene and a copolymer were tested. Polycarbonate and polyoxymethylene brackets served as controls. The mechanical properties of urethane-dimethacrylate, high-density polyethylene, EBP, polycarbonate, and polyoxymethylene bracket materials were evaluated after thermocycling (6000 x 5 degrees C-55 degrees C) in a mastication device before testing. Three medium-wear, fracture toughness, and Vickers hardness tests were performed. High-density polyethylene had the highest values of wear and the lowest values of fracture toughness and Vickers hardness. The urethane-dimethacrylate bracket material and the EBP had better mechanical properties than polycarbonate. The polyoxymethylene bracket material had the highest values of fracture toughness and Vickers hardness, and the lowest values of wear compared with the other investigated polymers. High-density polyethylene seems to be unsuitable as bracket material because it demonstrated excessive wear and insufficient fracture toughness. Polyoxymethylene had the best performance during mechanical testing.

Wear and fracture toughness of partially stabilized zirconia ceramics under dry friction against steel

The wear of two ceramic materials containing partially stabilized zirconia is studied under unlubricated friction against steel. The first material, with ZrO2 and 3 mol % Y2O3, was obtained by cold isostatic pressing and sintering. The second material, comprising ZrO2 and 4 mol % Y2O3, was fabricated by additional hot isostatic pressing. The samples of both materials were fabricated with high and low values of fracture toughness. The samples with high fracture toughness are found to wear more intensively. This fact can be explained by surface micro-cracking during braking as a result of phase transformations.

In vitro wear and fracture toughness of an experimental light-cured glass–ionomer cement

Objective The objective of this study was to evaluate the in vitro wear and fracture toughness (FT) of an experimental resin-modified glass–ionomer cement (RMGIC) formulated with the newly synthesized 6-arm star-shape poly(acrylic acid) and Fuji II LC glass fillers and investigate the effects of several important formulation parameters on wear-resistance and FT of the cement. Materials and Methods The in vitro abrasive and attritional wear as well as FT of the newly developed RMGIC were evaluated. The resin composite P-60 and RMGIC Fuji II LC were used as controls. The effects of glycidyl methacrylate (GM)-grafting ratio, powder/liquid ( P/ L) ratio, polymer/water ( P/ W) ratio and aging in water were investigated. All the specimens were conditioned in distilled water at 37 °C for 1 day prior to testing, unless specified. Results The optimized experimental cement exhibited almost the same high initial wear-resistance to abrasion as P-60 and much higher than Fuji II LC. The experimental cement showed 1.4 times higher in resistance to attritional wear than Fuji II LC but 6 times lower than P-60. After 1-month aging, the cement can compete with P-60 in resistance to attritional wear by showing only 1.3 times more in wear depth. The experimental cement also showed a significantly higher FT value than Fuji II LC but a similar value to P-60. Conclusions It appears that this novel experimental cement may be potentially used for high wear and high stress-bearing site restorations such as Class I and II.