Higher Fracture Toughness Values Research Articles

Enhanced thermo-mechanical damage tolerance of functionally graded ZrB2-20SiC ceramic reinforced with carbon nanotubes

The demand of a new generation spacecraft component has led to the development of a novel structure, which combines the traits of both functionally graded material (FGM) as well as an ultra-high temperature ceramics (UHTC). Herein, FGM based on the well-established ZrB2-20SiC (vol%) UHTC with the reinforcement of carbon nanotubes (CNT) is processed via powder metallurgy and spark plasma sintering (SPS). A densely graded microstructure with a crack-free seamless interface is witnessed along the cross-section of the FGM. The CNT in the optimized FGM is found to be efficaciously embedded in the matrix retaining its structure during sintering (at 1850 °C), shown microstructurally and supplemented by Raman spectroscopy. Enhanced mechanical and wear resistance of the FGM when compared to that of the conventional ZrB2 UHTC is attributed to the dense, crack-free microstructure and CNT pull-out mechanism. Oxidation test revealed an enhancement in the thermal stability as the onset of oxide formation starts above 886 °C (i.e. > 30% when compared to that of monolithic ZrB2). The thermo-structural performance of the processed FGM simulated using finite element method (FEM) indicated a reduction in the thermal residual tensile stresses (~ 8.1 times) which mitigates the crack propagation. The FEM analysis complements the high indentation fracture toughness values of the FGM even at the interfaces (similar to that found in each layer), establishing FGM approach superior to that of the conventional UHTC composites. Thus, addition of CNT in ZrB2-20SiC based FGM are conducive enough to be engineered as a potential material, poised to shape the future of re-entry vehicles.

Strain-rate dependent occurrence of cleavage fracture in Fe−Si−Al alloys

Iron−silicon steels have excellent magnetic and electrical properties, especially, with increased silicon and aluminum contents. However, increasing alloying with Si and Al reduces the workability of the material, and spontaneous cleavage fracture may occur during production. The main influence factors on the occurrence of cleavage fracture of Fe−Si−Al alloys are investigated in this paper. Fracture mechanics tests are performed at various temperatures and loading rates. The effects of specimen orientation and strain aging are studied. The tests result surprisingly high fracture toughness values. However, cleavage fracture regions are found in the pre-fatigue regions of the specimens, i.e. at much lower loads. In order to resolve this effect, the strain rates during the various fatigue- and fracture experiments are estimated by an analytical model. Additional fatigue experiments are performed at various frequencies where the stress intensity range is increased until cleavage fracture is detected. Similar experiments are conducted at increasing temperatures. The analyses reveal a significant transition behavior of the stress intensity at the onset of cleavage fracture, where the effects of decreasing temperatures and increasing strain rates are equivalent. Measures for avoiding cleavage fracture can be deduced from the results of this investigation.

Exceptionally high fracture toughness of carbon nanotube reinforced plasma sprayed lanthanum zirconate coatings

Lanthanum zirconate (La2Zr2O7), an emerging candidate for TBC system, faces stringent concern owing to poor fracture toughness. We report an exceptionally high fracture toughness (5.3 ± 0.4 MPa m0.5) value for a La2Zr2O7 coating prepared by atmospheric plasma spray technique reinforced by carbon nanotubes (2 wt %); the enhancement in fracture toughness was noted to be more than 300% compared to all previous works. Several factors, viz. increased density, uniform distribution of CNTs in La2Zr2O7 matrix, stabilization of pyrochlore La2Zr2O7 phase and various associated toughening mechanism offered by CNTs (e.g., CNT pull out, CNT bridging and crack arresting etc.) was attributed for this surprising enhancement. In addition, splat sandwiching and nanomechanical interlocking between CNT and La2Zr2O7 was found to be unique toughening mechanism. Apart from it, thermal shock resistance of LZC-SD coating performed at 1300 °C, showed the 145 numbers of cycle before the partial delamination of the coating. While, it was only 87 numbers of cycle for LZ-SD coating. The density, hardness, elastic modulus values also depicted a suitable increase. The introduction of CNTs in the La2Zr2O7 matrix did not alter/affect any of the physical properties (phase purity, compositional integrity etc.) of the pristine La2Zr2O7 system.

On sample size effects in fracture toughness determination of Bulk Metallic Glasses

Bulk Metallic Glasses (BMG) show extraordinarily high values of strength and elastic strain. Despite the, in most cases, macroscopically brittle behaviour (especially in tension and bending), extremely high fracture toughness values are reported. Even though it is sometimes stated that huge plastic zones and strain softening make K-based approaches (ASTM E399) or J-integral based approaches (ASTM E1820) difficult for BMG, changes in material behaviour or intrinsic metallic glass materials properties are still inferred by many authors in order to explain the results. In addition, the limited dimensions of fully glassy bulk ingots mean that the available specimen sizes differ from those required by the common standards of fracture toughness testing. In this context, reported results from fracture toughness of prominent Pd-, Pt- and Zr-based BMG are analysed. An in-depth discussion of the size dependent effects on the mechanical loading conditions during fracture toughness testing is provided. As a result, it is shown that the differences and peculiarities of characterized BMG toughness mainly relate to geometrical/mechanical effects of small scale sample testing. They are not due to an intrinsic material property or to a size dependent ramification of intrinsic material behaviour.

Microstructure, Mechanical Properties and in vitro Biological Behavior of Silicon Nitride Ceramics

The bioinert surface of silicon nitride ceramics led us to investigate the additions of SiO2, CaO and Al2O3 in order to aid the liquid phase sintering and improve the mechanical properties and biological behavior of the final materials. The sintered materials reached ca. 97% of theoretical density and total α→β-Si3N4 transformation. The samples had relatively high values of fracture toughness while their elastic modulus values were lower than those of conventional silicon nitride ceramics. Apatite deposits were observed on the surfaces analyzed after SBF (simulated body fluid) immersions, suggesting their bioactivity. Osteoblasts proliferation and calcified matrix were also detected as response to cells/materials contact. This combination of properties suggests that all studied compositions are promising for applications in biomedical devices. Moreover, compositions with alumina additions and higher silica content had better in vitro biological behavior, densification and mechanical properties, suggesting greater potential to be used in bone substitute devices.

The Fracture Resistance of Fiber Reinforced Composite Restorative Material has Higher Yield than Nanohybrid Resin Composite

Restorative materials used in dentistry should have aesthetic appeal and good strength for use in restorations on anterior and posterior teeth. Nanohybrid resin composite is a composite resin with nano-filler particles having both aesthetic appeal and good durability against fracture. The newest composite resin, fiber-reinforced composite, comprises resin matrix, E-glass fiber and organic particulate filler. Composite resins with E-glass fibers are expected to have higher fracture toughness values than composite resins without E-glass fibers. This research aimed to study the difference in the fracture toughness between nanohybrid resin composite and fiber-reinforced composite. Twenty samples were divided into 2 groups: Group I-nanohybrid resin composite and group II-fiber-reinforced composite. Each sample was applied incrementally on a glass mould and individually exposed to light for cure stiffening for 20 s. Before polymerisation of the second layer, the central part of the sample was given a sharp crack. The sample was then maintained at 37°C for 48 h, followed by testing for fracture toughness by the universal testing machine specification. Independent t-test revealed significant differences in fracture toughness values between the two groups (p = 0,000).

Quasi-static and dynamic mechanical behavior of transparent graft-interpenetrating polymer networks (graft-IPNs)

Mechanical characterization of transparent graft Interpenetrating Polymer Networks or graft-IPNs based on polyurethane and acrylic copolymer are reported under both quasi-static and dynamic loading conditions. This study builds on an earlier work detailing the synthesis of the graft-IPNs [1]. Optically transparent graft-IPNs were synthesized using elastomeric polyurethane (PU) phase and stiff acrylate-base copolymer (CoP) phase. The ratios of CoP:PU were varied from 90:10 up to 60:40 by weight. Two grades of graft-IPNs were synthesized by using 650 g mol−1 and 1400 g mol−1 poly (tetramethylene ether) glycol (PTMG) during synthesis to study the molecular weight effect. Quasi-static tensile and fracture tests as well as dynamic fracture tests were performed. The dynamic fracture tests were carried out at high strain rate using a modified-Hopkinson pressure bar in conjunction with an optical technique called Digital Gradient Sensing (DGS) and ultrahigh-speed photography. Quasi-static tests indicate significant enhancements in crack initiation toughness and change in failure mode (brittle to ductile) when the constituents were varied. Enhancements in crack inititation toughness were also observed under dynamic loading. These results were further compared with those for commercial poly (methyl methacrylate) (PMMA) and polycarbonate (PC) sheet stock. Further improvement in fracture properties were observed under quasi-static loading conditions with increase in molecular weight of PTMG. The relatively high values of fracture toughness obtained for graft-IPNs is attributed to the crosslinks generated between the CoP and PU networks.

Spherical engineering Ca-TZP ceramics made from baddeleyite: fabrication, structure and mechanical properties

The spherification method was adapted to baddeleyite-based ceramics fabrication. Novel calcia stabilized tetragonal zirconia (Ca-TZP) ceramics with sufficiently high values of hardness (11.73 ± 0.12 GPa), fracture toughness (9.15 ± 0.24 MPa m0.5) and Young's modulus (217 ± 14 GPa) was produced. Dependences of the material microstructure, its phase composition and mechanical properties on sintering temperature and the doping agent concentration were investigated. Fabricated spherical Ca-TZP ceramics made from baddeleyite may be considered as perspective engineering ceramics, for example, for milling processes as an alternative to grinding media made of chemically precipitated zirconia.

Effect of Ni content and Al2O3 particle size on the thermal and mechanical properties of Al2O3/Ni composites prepared by spark plasma sintering

Alumina-nickel composites were prepared by carrying out spark plasma sintering (SPS) of nano-sized and micro-sized Al2O3 particles with 15–45 wt% Ni powders. The powder materials were sintered at a temperature of 1400 °C under a constant uniaxial pressure of 50 MPa. FESEM micrographs of the products showed uniformly dispersed nickel inclusions in both matrices at intergranular positions. Presence of Al2O3 as the major phase along with Ni as the minor phase was confirmed using XRD analysis. Thermal and mechanical properties of the nano- and micro-sized Al2O3/Ni composites were investigated. The thermal conductivity of nano-sized alumina composites was seen to increase with the increase in nickel content, however, an opposite trend was observed for micro-sized alumina-based composites. Moreover, thermal conductivities of all the composites decreased with increase in temperature. The composites also showed high hardness and fracture toughness values of up to 19.6 GPa and 4.71 MPa ∗ m1/2, respectively, and relative density values, between 79 and 99%, that decreased with increasing Ni content. Furthermore, the nano-sized Al2O3/Ni composites showed thermal and mechanical properties superior to those of the micro-sized Al2O3/Ni composites.

Ferrimagnetic wollastonite ceramics based on waste valorization

AbstractIndustrial residues may be considered as an economic and environmentally friendly option for the manufacture of ceramic materials for civil and energy engineering. In this work, granite sludge, mussel shells and glass cullet were used to produce a ceramic material that presents high hardness (HV = 5.57 GPa) and fracture toughness values (KIC = 1.78 MPa·m1/2) as well as soft ferrimagnetic behavior. The presence of wollastonite confers characteristics typical of porcelain stoneware (σ = 64 N/mm2, WA = 0.361%) to the final products.

Fracture toughness of a lamellar orientation-controlled TiAl-based alloy processed by either one-step or two-step compression at high temperature

In this work, the effects of lamellae orientation and the distribution of fine grains and fine lamellar colonies on the fracture toughness of orientation-controlled Ti-43 mol%Al alloy were examined. Fracture toughness was determined by three-point bending tests following the introduction of a chevron notch, and thermomechanical processing to control lamellar orientation consisted of high temperature compression in the α single-phase and (α + γ) two-phase regions. Materials processed via one-step compression in the α single-phase region exhibited tilting of the lamellae interface by approximately 20–30° relative to the compression plane. Further compression in the (α + γ) two-phase region (termed two-step compression) arranged the lamellae interface nearly parallel to the compression plane. Fine grains were also observed around the orientation-controlled lamellar colonies, and were converted into fine lamellar colonies during heat treatment following processing in the (α + γ) two-phase region. The proportion of fine grains decreased with an increase in the true strain rate during compression in the (α + γ) two-phase region. Specimens processed by one-step compression had fracture toughness values lower than those of two-step compression specimens. In addition, specimens processed using two-step compression and exhibiting a crack arrester orientation had higher fracture toughness values than those with a crack divider orientation. The fine grains and colonies formed at the lamellar colony boundaries were found not to affect the fracture toughness of specimens with either of these crack orientations. However, decreased quantities of fine grains and colonies may increase the toughness of specimens for which crack propagation occurs in conjunction with crack arrester orientation.

From Fibrils to Toughness: Multi-Scale Mechanics of Fibrillating Interfaces in Stretchable Electronics.

Metal-elastomer interfacial systems, often encountered in stretchable electronics, demonstrate remarkably high interface fracture toughness values. Evidently, a large gap exists between the rather small adhesion energy levels at the microscopic scale (‘intrinsic adhesion’) and the large measured macroscopic work-of-separation. This energy gap is closed here by unravelling the underlying dissipative mechanisms through a systematic numerical/experimental multi-scale approach. This self-containing contribution collects and reviews previously published results and addresses the remaining open questions by providing new and independent results obtained from an alternative experimental set-up. In particular, the experimental studies on Cu-PDMS (Poly(dimethylsiloxane)) samples conclusively reveal the essential role of fibrillation mechanisms at the micro-meter scale during the metal-elastomer delamination process. The micro-scale numerical analyses on single and multiple fibrils show that the dynamic release of the stored elastic energy by multiple fibril fracture, including the interaction with the adjacent deforming bulk PDMS and its highly nonlinear behaviour, provide a mechanistic understanding of the high work-of-separation. An experimentally validated quantitative relation between the macroscopic work-of-separation and peel front height is established from the simulation results. Finally, it is shown that a micro-mechanically motivated shape of the traction-separation law in cohesive zone models is essential to describe the delamination process in fibrillating metal-elastomer systems in a physically meaningful way.

Interfacial fracture toughness of universal adhesive systems treated with an antioxidant.

BackgroundSecondary caries, degradation of the dentin-resin interface and fracture of the restoration are the most common forms of failure. The aim of this study was to assess the interfacial fracture toughness of three different adhesive approaches and to compare their fracture toughness after surface treatment with antioxidant.Material and MethodsSeventy two dentin blocks with 3×4mm dimension and 3mm thickness were prepared and attached to precured 3×4×12mm composite blocks from both sides. Six adhesive groups (N=12), All-bond universal, Scotchbond universal and Clearfil SE bond with or without antioxidant treatment (Sodium Ascorbate 10% )were applied to dentin surfaces, a 160µ polyethylene paper formed the chevron in adhesive-dentin interfaces. Chevron-notched beam fracture toughness was measured following a modified ISO 24370 standard. The uniform composite-dentin-composite block was subjected to a 4-point test in universal machine. Data were analyzed by Kruskal-Wallis, Wilcoxon signed-rank and Mann-Whitney tests (α =0.05).ResultsDifferent adhesive approaches yielded different significant in fracture toughness rates. A significant increase of fracture toughness was observed between adhesive groups after antioxidant surface treatment. The difference in fracture toughness between Scotchbond universal and All-bond universal were significant.ConclusionsThe highest value of fracture toughness was reported for Clearfil SE bond and the lowest value was found for All-bond universal. Sodium ascorbate as antioxidant surface treatment had a significant effect in increasing the fracture toughness. Key words:Chevron-notched beam fracture toughness, fracture toughness, dentin, adhesive, antioxidant.

Improved toughness of layered architecture TiAlN/CrN coatings for titanium high speed cutting

Fracture toughness is an important characteristic for coatings for high speed cutting of titanium. This study focuses on improving the fracture toughness of the coatings using a layered design method. TiAlN/CrN multilayer coatings with bilayer periods between 12nm and 270nm were prepared by reactive magnetron sputtering. XRD results show the multilayer coatings have face-centred cubic structures. When the bilayer period of multilayer coating decreases from 270nm to 25nm, the coating hardness decreases from 24.5GPa to 18.1GPa. Increased fracture toughness and adhesion strength of multilayer coatings are observed with decreasing bilayer periods. A maximum fracture toughness (KIC) of 3.6MPam1/2 and adhesion strength of 107 N were obtained with a 25nm bilayer period. The inhibition of crack propagation is the main reason for the high fracture toughness values in multilayer coatings. However, when the bilayer period is decreased further (12nm), the coating hardness begins to increase rapidly to 23.4GPa, and the fracture toughness and adhesion strength begin to decrease. Titanium cutting results show that an improvement in cutting performance is obtained with a TiAlN/CrN_25 multilayer coating.

Effect of build orientation on the surface quality, microstructure and mechanical properties of selective laser melting 316L stainless steel

PurposeThe purpose of this paper is to investigate the density, surface quality, microstructure and mechanical properties of the components of the selective laser melting (SLM) parts made at different building orientations. SLM is an additive manufacturing technique for three-dimensional parts. The process parameters are known to affect the properties of the eventual part. In this study, process parameters were investigated in the building of 316L structures at a variety of building orientations and for which the fracture toughness was measured.Design/methodology/approachHardness and tensile tests were carried out to evaluate the effect of consolidation on the mechanical performance of specimens. Optical and electron microscopy were used to characterise the microstructure of the SLM specimens and their effects on properties relating to fracture and the mechanics. It was found that the density of built samples is 96 per cent, and the hardness is similar in comparison to conventional material.FindingsThe highest fracture toughness value was found to be 176 MPa m^(1/2) in the oz. building direction, and the lowest value was 145 MPa m^(1/2) in the z building direction. This was due to pores and some cracks at the edge, which are slightly lower in comparison to a conventional product. The build direction does have an effect on the microstructure of parts, which subsequently has an effect upon their mechanical properties and surface quality. Dendritic grain structures were found in oz. samples due to the high temperature gradient, fast cooling rate and reduced porosity. The tensile properties of such parts were found to be better than those made from conventional material.Originality/valueThe relationship between the process parameters, microstructure, surface quality and toughness has not previously been reported.

Titanium diboride–silicon carbide–boron carbide ceramics with super‐high hardness and strength

AbstractTriplex particulate composites composed of boride and carbide ceramics were found to have high strength, hardness, and fracture toughness values. Two compositions consisting of 70:15:15 and 1:1:1 volume ratios of TiB2, SiC, and B4C were produced from commercially available powders by hot‐pressing. The 70:15:15 ceramic exhibited a strength of ~1.3 GPa, while the 1:1:1 ceramic had a strength of ~0.9 GPa. These strengths are comparable to super‐strong Y2O3‐PSZ and β‐SiAlON based composites. The Vickers’ hardness values of these ceramics were ~32 GPa for indent loads of 9.8 N. Hardness increased as indentation load decreased. The 1:1:1 ceramic had a hardness of ~53 GPa at an indentation load of 0.49 N, higher than values reported for so‐called “super‐hard” ceramics, and comparable to c‐BN.

Microstructures and properties of silicon carbide- and graphene nanoplatelet-reinforced titanium diboride composites

Monolithic TiB2, TiB2-SiC, and graphene nanoplatelet (GNP)-containing TiB2SiC-GNP composites were prepared by spark plasma sintering (SPS). The Vickers hardness and thermal conductivity values of the TiB2-SiC composites decreased with increasing SiC addition. However, SiC addition increased the fracture toughness and oxidation resistance relative to TiB2. The GNP-containing composites exhibited different behaviours depending on the GNP content and strength of the interface between the GNPs and surrounding matrix. A strong interface improved the oxidation resistance and thermal transfer properties. The highest fracture toughness values were achieved for the composites with 5 vol% GNPs, but the higher GNP loadings resulted in poorer mechanical properties and thermal conductivities. The formation of Si-depleted layers was also observed after oxidation at 1200–1400 °C for all composites.

Effect of mechanical mismatch on fracture mechanical behavior of SA 508 – Alloy 52 narrow gap dissimilar metal weld

In modern pressurized water reactor (PWR) designs, dissimilar metal welds (DMW) between the reactor pressure vessel (RPV) nozzles and safe-ends connected to the main coolant pipes are manufactured using a narrow-gap welding (NGW) technique. In addition to the new weld design, the filler metals have been changed from conventional Alloys 82 and 182 to higher Cr containing Alloys 52 and 152 to ensure the structural integrity of the welds during long term operation. The changes in weld design and filler metal have an effect on the mechanical mismatch state of the DMW which has an effect on the fracture behavior of the weld. For the experimental determination of the local strength variations and the fracture mechanical behavior of a modern NG-DMW, a weld mock-up consisting of SA 508 pressure vessel steel with AISI 309L/308L cladding, AISI 304 piping steel, and Alloy 52 weld metal was manufactured. The NG-DMW was characterized in both as-welded and post-weld heat treated (PWHT) conditions. The results showed that the highest strength mismatch existed between the carbon-depleted zone (CDZ) in the SA 508 heat-affected zone (HAZ) and the Alloy 52 weld metal. High fracture toughness values were observed for the different zones of the weld and the crack propagation occurred towards the softer SA 508 HAZ CDZ.

Fracture toughness of injection molded, man-made cellulose fiber reinforced polypropylene

This investigation focuses on the fracture toughness of injection molded compact tension (CT) specimen of man-made cellulose fibers reinforced composites with PP as their matrix and different varying fiber contents. The influence of the fiber orientation and the addition of a coupling agent on the fracture toughness was determined using an optical strain measurement and a micro computer tomography. It was verified that a reinforcement with man-made cellulose fibers leads to significantly higher values of fracture toughness and J-Integral in comparison to glass fiber reinforcement. Furthermore, it was demonstrated that the majority of fibers in the CT specimen show an orientation perpendicular to the flow direction of the injection molding process. Thus, a notch direction parallel to the flow direction leads to significantly higher values. This is a result of less local strains around the crack path, as well as of a higher amount of fiber pull-outs in the fractured surface. The coupling agent MAPP creates stronger fiber-matrix adhesion, which results in increasing values of the fracture toughness but a decreasing of the J-Integral values due to less fiber pull-outs.

A high strength alumina-silicon carbide-boron carbide triplex ceramic

A ceramic particulate composite composed of oxide, and carbide ceramics was found to have high strength, hardness, and fracture toughness values. A composition consisting of Al2O3 with 15vol% SiC and 15vol% B4C additions was produced by hot-pressing at 1650°C for 30min, with full density reached after ~5min at temperature. Both WB and WB2 were observed, with the W source presumably an impurity from WC milling media, and Al18B4O33 was also detected following densification. Strength was ~880MPa, which is greater than values reported for comparable composites of Al2O3 containing 30vol% SiC or B4C. Vickers hardness was ~21GPa, and fracture toughness was ~4.5MPam½, comparable to values reported for the binary mixtures. The calculated critical flaw size of the material was similar to the size of the SiC/B4C clusters and microcracking at grain boundaries. The latter resulting from thermal expansion mismatch between the Al2O3 matrix and SiC/B4C reinforcing phases.