Higher Fracture Toughness Values Research Articles

Development and characterization of zirconia–alumina composites for orthopedic implants

In this study, three different grades of alumina toughened zirconia composites (ATZ) (from 80 to 90wt% of zirconia), and three grades of zirconia toughened alumina (ZTA) from 80 to 90wt% of alumina were developed. Two types of stabilized zirconia were used: 3mol% yttria stabilized zirconia used on the ATZ, and 2mol% yttria stabilized zirconia for the ZTA. The composite powders were obtained by spray-drying with a controlled granulometric distribution. Uniaxial and cold isostatic pressing were performed. High density ceramics with densification above 97% were obtained after a low temperature sintering (1400°C) leading to enhanced mechanical properties. Higher values of fracture toughness and flexural strength were achieved for the ATZ samples (up to 5MPam1/2 and 1394MPa respectively), while ZTA samples presented higher values of hardness (up to 1846HV). Aging tests showed that ZTA samples did not present monoclinic zirconia after 96h. MTT and alkaline phosphatase activity assays were performed on the sintered samples. This study shows that ATZ and ZTA composites with good mechanical properties, aging resistance and osteoblastic cytocompatibility can be attained.

The Effect of High Concentration and Small Size of Nanodiamonds on the Strength of Interface and Fracture Properties in Epoxy Nanocomposite.

The concentration and small size of nanodiamonds (NDs) plays a crucial role in the mechanical performance of epoxy-based nanocomposites by modifying the interface strength. Herein, we systemically analyzed the relation between the high concentration and small size of ND and the fracture properties of its epoxy-based nanocomposites. It was observed that there is a two-fold increase in fracture toughness and a three-fold increase in fracture energy. Rationally, functionalized-NDs (F-NDs) showed a much better performance for the nanocomposite than pristine NDs (P-NDs) because of additional functional groups on its surface. The F-ND/epoxy nanocomposites exhibited rougher surface in contrast with the P-ND/epoxy, indicating the presence of a strong interface. We found that the interfaces in F-ND/epoxy nanocomposites at high concentrations of NDs overlap by making a web, which can efficiently hinder further crack propagation. In addition, the de-bonding in P-ND/epoxy nanocomposites occurred at the interface with the appearance of plastic voids or semi-naked particles, whereas the de-bonding for F-ND/epoxy nanocomposites happened within the epoxy molecular network instead of the interface. Because of the strong interface in F-ND/epoxy nanocomposites, at high concentrations the de-bonding within the epoxy molecular network may lead to subsequent cracks, parallel to the parent crack, via crack splitting which results in a fiber-like structure on the fracture surface. The plastic void growth, crack deflection and subsequent crack growth were correlated to higher values of fracture toughness and fracture energy in F-ND/epoxy nanocomposites.

Ультрамелкозернистая структура, текстура и механические свойства низкоуглеродистой стали, полученной различными методами пластической деформации

The paper reports on the investigation of the effect of various methods of plastic deformation on the structure, texture and mechanical properties of low carbon 12GBA steel. It is found that multiple isothermal forging results in formation of equiaxed ultrafine-grained (UFG) structure with a uniform distribution of the second phase, the size of which ranges from 50 to 500 nm. The warm rolling leads to formation of UFG fibrous structure type with presence of carbides sized less than 100 nm. X-ray analysis revealed that both deformation schemes result in formation of two-component (110) [112] and (111) [123] texture. Low carbon steel 12GBA with fibrous UFG structure is characterized by high strength, with satisfactory ductility and toughness values similar to ones in case of the coarse-grained (CG) structure. Equiaxed UFG structure steels possess 2 times higher values of fracture toughness and 1.5 times increased tensile strength by comparison with the CG structured steel. Internal strains relaxation annealing of UFG fibrous structure leads to second phase coagulation retaining the two component texture and insignificant change of mechanical properties. It was shown that UFG fibrous steels retains its high level due to 1.5-2 times increase of crack initiation work. Considerable growth of fracture toughness in equaxed UFG steel takes place due to 7 times increase of crack initiation and propagation work.

TiC-SiC硬質セラミックスの合成と機械的性質

TiC-SiC ceramics were sintered at 1600–1800 °C using a resistance-heated hot pressing machine. No phase except TiC and α-SiC was detected after sintering. Dense ceramics were obtained in the range of 0–70 mol% SiC at a sintering temperature of 1800 °C. The Young’s moduli for dense TiC-SiC ceramics ranged from 426–457 GPa, regardless of the SiC additive amount. The addition of SiC above 10 mol% increased Vickers hardness by 2–3 GPa. The addition of SiC above 10 mol% inhibited the grain growth of TiC, and brought fine equiaxed granular structure to sintered bodies. The addition of SiC increased fracture toughness in the conditions 0–50 mol% SiC at 1800 °C, 0–20 mol% SiC at 1700 °C, and 0–10 mol% SiC at 1600 °C. The highest fracture toughness value 6.5 MPa m1/2 was obtained for the TiC-50 mol% SiC ceramic sintered at 1800 °C. The addition of SiC greatly increased thermal conductivity from 27 W m−1 K−1 for pure TiC to 98 W m−1 K−1 at 70 mol% SiC.

Mechanical properties of soda–lime–silica glasses with varying alkaline earth contents

The effects of varying the alkaline earth oxide content of three series of soda–lime–silica glasses with the general formulae 13.5Na2O·xMgO·10CaO·1.5Al2O3·(75−x)SiO2 (mol.%) where x=0, 1, 2, 3, 4, 5, 6, and 7; 13.5Na2O·3MgO·(7+y)CaO·1.5Al2O3·(75−y)SiO2 (mol.%) where y=0, 1, 2, 3, 4, 5, 6, and 7 and 13.5Na2O·zMgO·(13−z)CaO·1.5Al2O3·72SiO2 (mol.%) where z=1, 3, 5, 7, 9, and 11 have been examined. Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy have been used to assess network connectivity. In the first two glass series network connectivity decreases with increasing alkaline earth addition whereas in the third series connectivity tends to be greater for the more magnesia rich glasses suggesting that magnesia does have a different effect to lime on network connectivity, but only when magnesia is the dominant alkaline earth species. Fracture toughness has been measured using bend testing, which avoids many of the questions raised by the widely used indentation technique. Moduli have been assessed using acoustic means. It was found that the mechanical properties tend to decrease with increasing network connectivity for all three glass series. For the MgO–SiO2 series and CaO–SiO2 glasses increasing the alkaline earth content at the expense of the silica content resulted in increased network depolymerization, whereas for the MgO–CaO series when MgO became the dominant alkaline earth species, network depolymerization was reduced. Thus whilst MgO and CaO both act as network modifiers when more CaO than MgO is present in soda–lime–silica glasses, a difference in behaviour is seen with magnesia rich soda–magnesia–silica glasses. In contradiction to previous data no significant advantage of replacing CaO by MgO is observed. In addition it appears that the glasses with the lowest fracture toughness values may be more resilient to contact damage than those with the higher fracture toughness values.

Skin‐core debonding in resin‐infused sandwich structures

Techniques for enhancing the interfacial fracture properties of foam‐based sandwich structures are investigated. Prior to manufacture, holes were drilled into a PET foam core in order to facilitate resin flow during the subsequent resin‐infusion process. Glass fibers were then inserted into the perforations in an attempt to increase the interfacial fracture toughness of the sandwich structure. The results from these tests are compared to data generated from similar tests on a plain PET core, as well as on samples in which no fiber reinforcement was incorporated into the vertical holes. The inclusion of fibers in the through‐thickness holes served to increase the skin‐core interfacial fracture toughness of the sandwich structures. Here, it was noted the highest values of fracture toughness were more than three times the value measured on the plain foam system. POLYM. COMPOS., 37:2974–2981, 2016. © 2015 Society of Plastics Engineers

Effect of particle size, heating rate and CNT addition on densification, microstructure and mechanical properties of B4C ceramics

Monolithic B4C ceramics and B4C–CNT composites were prepared by spark plasma sintering (SPS). The influence of particle size, heating rate, and CNT addition on sintering behavior, microstructure and mechanical properties were studied. Two different B4C powders were used to examine the effect of particle size. The effect of heating rate on monolithic B4C was investigated by applying three different heating rates (75, 150 and 225°C/min). Moreover, in order to evaluate the effect of CNT addition, B4C–CNT (0.5–3 mass%) composites were also produced. Fully dense monolithic B4C ceramics were obtained by using heating rate of 75°C/min. Vickers hardness value increased with increasing CNT content, and B4C–CNT composite with 3 mass% CNTs had the highest hardness value of 32.8GPa. Addition of CNTs and increase in heating rate had a positive effect on the fracture toughness and the highest fracture toughness value, 5.9MPam1/2, was achieved in composite with 3 mass% CNTs.

Evaluation of Addition ABS and EPDM Effect on the Mechanical Properties of Ternary Polymer Blends

Polymer blends are capable of providing materials with extended useful properties beyond the range that can be obtained from single polymer. Blends of polystyrene (PS) and polypropylene (PP) with adding acrylonitrile-butadiene-styrene (ABS) or ethylene propylene diene monomer (EPDM) are prepared in different ratios by melt blending technique which was carried out using a double screw extruder. The tensile, flexural, impact and hardness tests of all the prepared samples were study. The results from this work show that the elongation property and impact properties for blend system increase with increase PP content in polymer blend of (PS:PP), also increase value of these properties with increase the ratio of adding (ABS or EPDM) for polymer blend (PS:5%PP).The highest values of impact strength was (10.68Kj/m2) for ternary polymers blend (90%PS:5%PP:5%EPDM) and the high values of fracture toughness (6.125MPa√m ) for ternaty polymer blend (90%PS:5%PP:5%ABS).

The interrelationship of fracture toughness and microstructure in a new near β titanium alloy Ti–7Mo–3Nb–3Cr–3Al

A new near β titanium alloy Ti–7Mo–3Nb–3Cr–3Al (Ti-7333) with ultrahigh tensile strength and reasonable ductility after subtransus solution followed by aging treatment has been developed in China. The fracture toughness tests were carried out on the material at room temperature in order to understand the fracture mechanism. The results show that Ti-7333 alloy has a high fracture toughness value (>140MPa·m1/2) under the β solution followed by slow cooling, and then aging heat treatment process. Meanwhile, the elongation and reduction of area remain at about 19% and 45% respectively, and its corresponding tensile strength maintains at about 1000MPa. The microstructure observation and analyses show that the intragranular α phase laths and grain boundary α become coarser roughly with the increasing of aging temperature, whereas, the continuity of grain boundary α is subjected to change and turned to zigzag. The coarser and longer intragranular α and more multidirectional arrangement of the colony of α lamellae, as well as discontinuous α precipitates on the grain boundary lead to a more tortuous crack propagation path and higher fracture toughness. In addition, the bigger the size of the plastic zone of the crack tip is, the higher fracture toughness it has. Compare with other similar alloys (Ti-5553, Ti-1023, VT22, etc.), Ti-7333 exhibits higher fracture toughness and close tensile strength. That indicates that Ti-7333 alloy has excellent combinations of mechanical properties and giant competitiveness.

Fracture and fatigue of rock bit cemented carbides: Mechanics and mechanisms of crack growth resistance under monotonic and cyclic loading

In an attempt to improve the material selection, design and reliability of rock bit WC–Co cemented carbides (hardmetals), an extensive and detailed study is conducted with the main goal of characterizing the fracture and fatigue crack growth (FCG) behavior of four hardmetal grades. Work includes basic microstructural and mechanical characterization of the materials, assessment of fracture toughness and FCG kinetics. It is found that rock bit cemented carbides exhibit relatively high fracture toughness values (between 17 and 20MPa√m) in direct association with their specific microstructural characteristics, i.e. medium/coarse carbide grain size and medium cobalt content. The influence of microstructure on the measured crack growth mechanics under monotonic loading may be accounted by considering the effective operation of ductile ligament bridging and crack deflection as the prominent toughening mechanisms. Regarding FCG behavior, it is observed to exhibit a significant Kmax influence. Furthermore, relative increments in toughness are maintained, in terms of crack growth threshold, under cyclic loading. As a consequence, fatigue sensitivity for rock bit cemented carbides is found to be lower than that extrapolated from data reported for fine-grained grades. Crack growth resistance under cyclic loading for the hardmetals studied may be understood on the basis that prevalent toughening mechanisms (ductile ligament bridging and crack deflection) show distinct susceptibility to fatigue degradation and are thus critical in determining fatigue sensitivity.

Low temperature spark plasma sintering of Al2O3–TiC composites

Al2O3–TiC composites were fabricated using spark plasma sintering (SPS) technology between 1300°C and 1500°C. The composite densification process starts at 950°C, with a maximum densification rate at 1170°C. After thirty minutes soaking at 1400°C, a relative density of 99.5% was achieved. Microstructure analysis reveals a submicron Al2O3 grain size and homogenous distribution of titanium carbide in the matrix. The fully dense Al2O3–20wt% TiC composite exhibits high values of fracture toughness (8.3MPam1/2), Young׳s modulus (406GPa), flexural strength (850MPa) and hardness (21GPa).

Spark plasma sintered Al2O3–YSZ–TiO2 composites: Processing, characterization and in vivo evaluation

Al2O3 and Al2O3–YSZ composites containing 3 and 5wt.% TiO2 were prepared by spark plasma sintering at temperatures of 1350–1400°C for 300s under a pressure of 40MPa. The grain growth of alumina was suppressed by the addition of YSZ. Al2O3–YSZ composites showed higher hardness than monolithic Al2O3. There was not a considerable difference in hardness values for Al2O3–YSZ composites containing 10 and 20vol.% YSZ and the addition of TiO2 decreased the hardness of the composites. The fracture toughness of Al2O3 increased from 2.8MPa·m1/2 to 4.3MPa·m1/2 with the addition of 10vol.% YSZ, further addition resulted in higher fracture toughness values. The fracture toughness values were increased with TiO2 addition and the highest value of fracture toughness, 5.3MPa·m1/2, was achieved with the addition of 20vol.% YSZ and 5wt.% TiO2. Preliminary in vivo tests demonstrated the biocompatibility and osseointegration of the composites after 6week post-implantation in femur of Wistar rats.

Improvements of strength and fracture resistance by spatial material property variations

A material with spatial variation in the elastic modulus E can have a much higher apparent fracture resistance and fracture stress than a comparable homogeneous material. The effect occurs due to the strong decrease of the crack driving force, which leads to crack arrest when the crack tip is located in the region with low elastic modulus. From the results of exemplary numerical studies and simple fracture mechanical considerations, models are derived in order to predict the fracture stress and fracture toughness of the inhomogeneous materials. It is shown that high values of fracture stress and fracture toughness can be reached if the amplitude of the E variation is high enough to provide crack arrest and the wavelength of the E variation is small. The beneficial effect of material property variations also occurs if the width of the compliant region is very thin and the loss in stiffness of the structure is almost negligible. The concept is applicable for various types of composite materials; examples are presented.

Study of the effect of moisture content and bending rate on the fracture toughness of rocks

Generally, rock material failure is controlled by cracks under specific conditions. The study of rock fracture toughness belongs to the current frequent directions of research in the area of rock failure. The present paper describes the effects of parameters influencing the resultant properties of rock materials (bending rate, rock moisture) during fracture toughness measurement of different kinds of rocks (sandstone, marble, granite). The highest fracture toughness values were found in the marble samples. This is probably due to the inner structure of analysed marble, which is composed of only one mineral (calcite) and also has a lower porosity than the used granite. The lowest fracture toughness values were found in the sandstone sample, and reached c. 17–30 % of the measured fracture toughness values of the analysed granite and marble samples. As in the case of the other mechanical properties of rocks (e.g. uniaxial compressive strength) also in the case of higher sandstone (carboniferous) moisture the fracture toughness values decrease and its deformation ability increases. Preparation of samples for fracture toughness tests and performance of these tests are more complicated than in the case of tensile tests (e.g. the Brazilian test) and therefore this contribution presents a comparison between fracture toughness of analysed rocks and tensile strength values. The measured data in this study considering the fracture toughness tests and Brazilian tests were compared with results published by Zhang (2002).

On the effect of the different notching techniques on the fracture toughness of PETG

The fracture toughness of poly (ethylene terephthalate) modified glycol (PETG) has been evaluated using notch sharpening techniques which could be grouped into contact and non-contact procedures. Razor blade tapping, razor blade sliding, razor pressing and broaching are part of the first group, while the femtolaser technique belongs to the second one. Not all the contact procedures generated valid sharp cracks for fracture parameter assessment; indeed none of the samples sharpened via razor sliding generated acceptable sharp cracks. The results revealed that the non-contact femtolaser technique produced the sharpest cracks in this polymer, with crack tip radii of only 0.5 μm, leading to the lowest fracture toughness values. On the contrary, the traditional notch sharpening technique based on razor tapping, recommended in ISO, ESIS and ASTM protocols and standards, generated larger crack tip radii than those introduced via femtolaser and, consequently, resulted in higher fracture toughness values. Both broaching and pressing methods created specimens with smaller crack tip radii compared to those obtained by razor tapping, and hence resulting in intermediate fracture toughness values.

Dynamic fracture toughness of aluminium 6063 with multilayer composite patching at lower temperatures

Aluminium alloys provide many benefits and challenges for use in marine environment. Attention on lightweight hulls leads to power saving, better handling and easy transportation. The stiffness and mass of the structure or member is responsible for its natural vibration. The hull panels and stiffeners vibrate at different frequencies when their stiffness is reduced by formation of a crack or any structural problems. If there is a monitoring system on board and by using a sensor detecting the modified vibrations, the maintenance crew senses the spot where urgent repair is needed. Application of composite laminates is convenient in view of portability and simplicity in procedure such as hand lay-up. In this study, the edge-cracked aluminium plates were repaired with one-sided composite patches and their temperature were varied to both low and high temperature levels. Dynamic fracture toughness was determined using Charpy impact test. Three different types of materials were used for composite patching: GFRP (glass-fibre-reinforced plastic), CFRP (carbon-fibre-reinforced plastic) and Kevlar. For each temperature level, two lay-ups were used, viz. three layered and five layered. Aluminium specimens with edge cracks of two different lengths, keeping the width of the crack constant, were prepared. Impact tests were performed for the prepared specimens at different temperatures. The fracture toughness of aluminium strips reinforced with patches was determined and compared with the energy absorbed during the impact of the corresponding specimen without the patch. It was observed that the specimen with patches gave a higher fracture toughness value when compared with that without the patch; corresponding observation on aluminium has been made along with patch at these temperatures. The patch using Kevlar was found to be the best among the three composites. Followed by these experiments, simulations using FRANC 2D software were carried out for finding stress intensity factor at different crack lengths with and without patches. The research has a spin-off application in the selection of materials used for patching and hence in the maintenance of LPG (liquefied petroleum gas) and LNG (liquefied natural gas) storage tanks under cryogenic conditions.

FRACTURE MECHANISM IN Si3N4 – GRAPHENE PLATELETS COMPOSITES

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPLs) as a filler. Two different sintering routes were applied which resulted in different microstructure: hot isostatic pressing (1700°C/3h/20 MPa) and gas pressure sintering (1700°C/0h/2 MPa). The influence of the GPLs addition and of processing routes on the fracture toughness and fracture mechanism of Si 3 N 4 +GPLs was investigated. The main toughening mechanisms, which originated from the presence of the graphene platelets are crack deflection, crack branching and crack bridging . These mechanisms are responsible for the increase of fracture toughness which is higher than that of monolithic Si 3 N 4 . The highest value of fracture toughness was obtained in the case of the composite processed by hot isostatic pressing using the GPLs with lowest dimension.

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.