Frontal Process Zone Research Articles

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.

Toughening mechanism and frontal process zone size of ceramics

In order to improve the fracture toughness of intrinsically brittle ceramics, it is essential to expand the frontal process zone (FPZ) ahead of a crack tip. Nanocomposites proposed by Niihara utilize dislocations generated around the dispersed nano-sized particles in matrix and expand the FPZ. In this paper, we discussed the toughening mechanism of ceramics using our experimental data on alumina-based nanocomposites, focusing on fundamental theories of fracture mechanics, such as Griffith-Irwin energy equilibrium and the local fracture criterion. We estimated the FPZ size using an indirect technique proposed recently, and clarified that the FPZ expansion toughening mechanism is achieved in nanocomposites by means of dislocation activities. The results revealed that ceramics with a larger FPZ size had higher fracture toughness and properly annealed nanocomposites had the largest FPZ size. The FPZ expansion mechanism in nanocomposites was considered that sessile dislocations generated around a crack tip in matrix, served as nano-crack nuclei, and hence expanded the FPZ size. It is clarified that the critical local stress is the strength of an infinite plate with no crack, and must be the true strength with no size effect. The fracture toughness, the critical local stress, and the FPZ size are the mutually dependent material properties, and the relation among them is clarified.

Evaluation of Nano-Sized Nickel Dispersed Alumina Materials

In order to develop new ceramics for a power electric device, alumina/nickel (Ni) nanocomposites were fabricated using the soaking method and the pulse electric current sintering technique. The volume fractions of Ni were 0.5, 1, and 5% in γ–alumina powder. 10mass% α–alumina powder was added as a sintering seed. SEM and TEM observations revealed that the nanocomposites exhibited an intra-granular fracture mode, and agglomerated Ni particles existed mostly along alumina grain boundaries especially for 5vol% Ni. The experimental results showed that the strength and fracture toughness values of the nanocomposites were higher than those of monolithic alumina, and the thermal conductivity decreased slightly with increase in Ni content. Moderate improvements in the strength and fracture toughness seemed to result from the fact that the nano-sized Ni particles dispersed within alumina grains creating dislocations around the Ni particles, which reduced sintering residual stresses in alumina grains and expanded the frontal process zone size. It was concluded that nanocomposites with a lower Ni content had better mechanical, electrical, and thermo-mechanical properties.

Fabrication and Characterization of Alumina/Silver Nanocomposites

Alumina/silver nanocomposites were fabricated using a soaking method through a sol-gel route to construct an intra-type nanostructure. The pulse electric-current sintering (PECS) technique was used to sinter the nanocomposites. Several specimens were annealed after sintering. The microstructure, mechanical properties, critical frontal process zone (FPZ) size, and thermo-mechanical properties of the nanocomposites were estimated. The relative densities of the specimens sintered at 1350 and 1450℃ were 95% and 99%, respectively. The maximum value of the three-point bending strength was found to be 780 ㎫ for the 2×2×10 ㎜ specimen sintered at 1350℃. The fracture toughness of the specimen sintered at 1350℃ was measured to be 3.60 ㎫·m 1/2 using the single-edge V-notched beam (SEVNB) technique. The fracture mode of the nanocomposites was transgranular, in contrast to the intergranular mode of monolithic alumina. The fracture morphology suggested that dislocations were generated around the silver nanoparticles dispersed within the alumina matrix. The specimens sintered at 1350℃ were annealed at 800℃ for 5 min, following which the maximum fracture strength became 810 ㎫ and the fracture toughness improved to 4. 21 ㎫m 1/2 . The critical FPZ size was the largest for the specimen annealed at 800℃ for 5 min. Thermal conductivity of the alumina/silver nanocomposites sintered at 1350℃ was 38 W/mK at room temperature, which was higher than the value obtained with the law of mixture.

Nanocomposites – Toughened Ceramics

In order to improve fracture toughness of ceramics, an intrinsically small frontal process zone (FPZ) size must be expanded. An intra-type nano-structure, where nano-particles are embedded within matrix grains, yields dislocations around the dispersed particles due to residual stresses. These dislocations become sessile dislocations at room temperature, operate as origins of small stress concentration in the matrix, and create nano-cracks in the FPZ. To produce the intra-type nano-structure, we developed a soaking method. This method makes it possible to produce nano-sized metallic particles dispersed within ceramic powders. In this research, alumina-nickel nanocomposite powder was obtained using the soaking method. The powder mixed with α-alumina as a seed was sintered using a pulse electric current sintering technique. The sintered nanocomposites are then annealed to disperse dislocations around the nanoparticles into alumina grains. Results showed that the maximum fracture toughness was 7.6 MPam1/2, which was two times higher than that of alumina.

Novel Estimation of Critical Frontal Process Zone of Ceramics by a Single-Edge V-Notched-Beam Technique

A novel estimation for the critical size of the frontal process zone of ceramics is proposed using a single-edge V-notched beam (SEVNB) technique. A three-point flexure test is carried out on aluminum titanate ceramics containing a sharp V-shaped notch with different depth. An exact solution of the critical local stress is analyzed at a critical distance from the notch tip. The critical frontal process zone size is determined as the distance between the notch tip and the point where the critical local stress equals the flexural strength of specimens without notches, based on the local fracture criterion and the Griffith-Irwin criterion. The critical size of the frontal process zone, the fracture toughness and the flexural strength were also estimated for several materials, such as, alumina, porous alumina, and alumina-based nanocomposites. The relationship between these mechanical properties indicated that there was an almost linear relationship between the fracture toughness and the resultant of strength and square root of the critical frontal process zone size, and that both of them must be increased to improve the fracture toughness of ceramics.

Critical Frontal Process Zone Size and R-Curve Behavior of Porous Ceramics

Although porous ceramics are materials with high potential for helping conserve the environment, the characteristics of pore-related mechanical properties have not yet been examined sufficiently. The R-curve behavior of porous ceramics was estimated using the crack stabilizer technique developed by Nojima et al. Also, the critical frontal process zone (CFPZ) size for porous ceramics was estimated from the strength and fracture toughness of the materials used. The results revealed that the R-curve behavior was almost flat in porous ceramics, in contrast with a steeply rising R-curve behavior for porous silicon carbide observed previously, and that the CFPZ size of porous ceramics was larger than that of dense ceramics. A schematic explanation for the crack extension in porous materials was presented to discuss the R-curve behavior of porous ceramics.

Temperature dependence of mechanical properties of aluminum titanate ceramics

Aluminum titanate (Al 2TiO 5: AT) is a synthetic ceramic material of potential interest for many structural applications. A critical feature, which greatly limits the mechanical properties of polycrystalline AT, is the considerable intergranular microcracking, which occurs due to the large thermal anisotropy of individual grains during cooling after sintering. This study discusses the temperature dependence of mechanical properties, and presents observations on the microstructure morphology. Both the fracture strength and fracture toughness increased considerably with increasing temperature. The critical frontal process zone (FPZ) size was estimated from the mechanical properties. A decrease in FPZ size was observed with increasing temperature in the first thermal treatment. These phenomena were explained on the basis of the stress redistribution and unique microscopic feature on the fracture surface of AT ceramics. The experimental results revealed that further thermal treatment increased the fracture strength and reduced the fracture toughness, while it had almost no effect on the FPZ size.

Relation between Strength, Fracture Toughness, and Critical Frontal Process Zone Size in Ceramics

In order to improve the fracture toughness of inherently brittle ceramics, a new material design concept must be developed. One suitable concept involves the use of dislocation activities. Intra-type ceramic-based nanocomposites use dislocation activities to enhance strength and fracture toughness. The dislocations are generated by residual stresses induced during sintering process around the second-phase nanoparticles dispersed within the matrix grains. In this paper, first, we proposed an indirect technique for estimating a critical frontal process zone (CFPZ) size for ceramics and clarified the relation between the strength, fracture toughness, and CFPZ size. The fracture toughness of ceramics is closely related to the CFPZ size because ceramics with larger CFPZ size consume higher fracture energy during crack extension and consequently have higher fracture toughness. Second, we fabricated toughened alumina-nickel nanocomposites using a soaking method to construct an intra-type nanostructure, and found that the appropriate annealing conditions after sintering could achieve toughened nanocomposites. Finally, we discussed the relation between the experimentally obtained fracture toughness and the CFPZ size of monolithic alumina, as-sintered nanocomposites, and annealed nanocomposites. The results revealed that nanocomposites showed the largest CFPZ size and the highest fracture toughness after appropriate annealing: the sessile dislocations ahead of a crack tip in a nanocomposite are thought to serve as stress concentration, create many nanocracks in the CFPZ, and expand the CFPZ size.

セラミックスの強度，破壊靱性と臨界損傷域寸法の関係

In order to improve the fracture toughness of ceramics, we need to develop a new material design concept. One of the suitable concepts is to utilize dislocation activities even in brittle ceramics. Intra-type nanocomposites use dislocation activities to enhance the strength and fracture toughness. The dislocations are caused by sintering residual stresses around the second-phase nano-particles dispersed within the matrix grains. In this research, first, we clarified the relation between the strength, fracture toughness, and critical frontal process zone (CFPZ) size using alumina ceramics. The fracture toughness of ceramics is related closely to the CFPZ size, because ceramics with larger CFPZ size consume higher fracture energy during crack extension and indicate larger fracture toughness. Second, we fabricated toughened alumina-nickel nanocomposites using a soaking method which we developed recently to create an intra-type nano-structure, and found that the suitable annealing after sintering could achieve toughened nanocomposites. Finally, we discussed the relation between the fracture toughness and CFPZ sizes of the monolithic alumina, as-sintered nanocomposites, and annealed nanocomposites. The results revealed that the annealed nanocomposites had the highest CFPZ size and fracture toughness, because it was conceivable that the sessile dislocations in the CFPZ became nuclei of nano-cracks, created many nano-cracks, and expanded the CFPZ size.

Evaluation of Fracture Energy for Chamotte Refractory

Fracture energy is one of important parameters in estimating the resistance of crack propagation for refractories. However, the effect of the dimension of a test specimen on the fracture energy has not been sufficiently studied. In the present study, effective fracture energy γeff of chamotte refractory was evaluated using WOF (work of fracture) technique, in order to investigate the relationship between the fracture energy and specimen dimension. The γeff was significantly dependent on specimen dimension and notch length. Larger specimen dimension and shorter relative notch length made the γeff increased. For instance, the γeff of the large specimen was approximately three times as large as that of the small specimen. To evaluate accurate γeff for the refractory, a specimen with ligament longer than a given length was required. The ligament length was discussed in relation to the size of a frontal process zone.

Nanocomposites—a new material design concept

Ceramic-based nanocomposites were reviewed, emphasizing the newly developed concept of material design for ceramics. First, characteristics of the nanocomposites observed by previous researchers were summarized as, significant or moderate improvement in strength, drastic change of the fracture mode from intergranular fracture of monolithic ceramics to transgranular fracture of nanocomposites, moderate enhancement of fracture toughness, improvement of other mechanical properties, and observations of dislocations. Second, several mechanisms proposed previously to explain these characteristics were reviewed. Third, our strengthening and toughening mechanisms of nanocomposites on the basis of dislocation activities were explained. In nanocomposites, the highly localized residual stresses in the matrix grains are generated by the mismatch of thermal expansion coefficients between the matrix and the dispersed particles, and the dislocations are yielded during the cooling process after sintering. These dislocations then release the tensile residual stresses intrinsically existing in the matrix grains of sintered ceramics and improve the strength of the materials. In addition, as these dislocations cannot move at room temperature the sessile dislocations in the matrix operate as nano-crack nuclei in a frontal process zone (FPZ) ahead of the crack tip when the tip of a propagating crack approaches this area. Therefore, the size of the FPZ is expanded and as a result the fracture toughness is improved. Finally, estimation of the critical FPZ size was explained in order to clarify its toughening mechanism in nanocomposites.

Critical Frontal Process Zone Size and Fracture Toughness

Critical Frontal Process Zone Size and Fracture Toughness

セラミックスの破壊じん性とき裂前方損傷域寸法

Our previous technique for estimating critical sizes of the frontal process zone (FPZ) in ceramics was improved using exact solutions of stress distributions around crack and elliptical hole tips in an infinite plate for calculating local fracture stresses. The local fracture stress was calculated at the characteristic distance from the notch tip on the basis of a local fracture criterion. A three-point flexure test for alumina was carried out using V-notched specimens, and a formula for the relation between the strength and the notch depth including the short notch depth region was successfully established. The critical FPZ size of alumina was estimated from the fracture toughness and strength of the material. The critical FPZ sizes for alumina-based nanocomposites were also estimated. A concept of material design for toughened ceramics was discussed on the basis of these data. It was proved that there was a linear relationship between the fracture toughness and the product of strength and square-root of the FPZ size for these materials. This relation suggests that both the strength and the critical FPZ size must be increased to enhance the fracture toughness of ceramics.

Mechanisms of crack propagation in dry plaster

Controlled crack propagation tests were performed on single edge notched bend samples to investigate the crack growth behaviour of dry plaster. The influence of the relative notch depth on the crack resistance curve has been studied and appears to be very important. The results are discussed considering a qualitative model based on the specific microstructure of plaster and in situ observations of the crack propagation. Two mechanisms acting at different scales and undergoing complex interactions are involved: crack bridging by small gypsum crystals acting locally behind the crack tip and secondary cracking in a macroscopic frontal process zone. Interaction of the main crack with secondary ones undergoes substantial branching and crack accelerations leading to bridging destruction due to sudden crack opening.

アルミナマトリックスナノ複合体の高強度化機構

In a previous report, we proposed strengthening and toughening mechanisms in nanocomposites based on thermally induced residual stresses around nano-particles within matrix grains. These residual stresses generate dislocations at elevated temperatures and the dislocations release tensile residual stresses along grain boundaries in the matrix. Conversely, the dislocations in alumina are immovable at room temperature, act as nano-crack nuclei when a main crack tip approaches and expand the size of the frontal process zone. In this paper, alumina matrix/silicon carbide dispersed nanocomposites were fabricated to clarify the strengthening mechanism in nanocomposites by means of hot-press processing. To compare the effects of the annealing process on the mechanical properties of sintered specimens, heat treatment was carried out at 1300°C (1573K) in Ar gas atmosphere after sintering at 1600°C (1873K) for 1h. Bulk densities of the specimens were more than 98% of relative density, but some specimens had relative densities lower than 95%. The mean values of fracture strength and fracture toughness were 862MPa for the 5vol% SiC and 3.90MPa·m1/2 for the 3vol%SiC, respectively. Even though the fracture strength of the nanocomposites was 2 to 3 times higher than that of the monolithic alumina, the fracture toughness was almost same as the monolithic alumina.

作動温度下での多孔質ランタンストロンチウムマンガナイトの機械的および電気的特性

Porous lanthanum strontium manganite (LSM) for the cathode in solid oxide fuel cells (SOFCs), was prepared with the fine perovskite oxide powder and organic PMMA particles as the pore-former. After calcination of organic particles, porous LSM with different porosity was synthesized at 1673K for 1hr by pressureless sintering. Porosity and microstructure of porous LSM were characterized. At room temperature or at 1273K of operating temperature, fracture strength was estimated by three-point bending test and Young's modulus was by the strain under bending test. The fracture toughness was also measured by SEVNB method. Electrical conductivity at operating temperature was confirmed by the four-terminal method, using the specimen similar to that measured mechanical properties. With addition of PMMA particles, the uniformly spherical and isolated pores with approximetely 10μm in diameter were observed below 0.25 porosity. Above 0.25, continuous pores were mainly observed. The fracture strength decreased with the increase of the porosity independent of testing temperature. Isolated or continued pores in LSM matrix changed the decreasing rate of strength. Normalized strength, strength for porous LSM divided by that for dense LSM, was agreed at each porosity, though strength at operating temperature was higher than that at room temperature. Apparent Young's modulus for porous LSM also showed similar tendency. Slightly decrease of fracture toughness for porous LSM at operating temperature was observed due to the healed wake zone or enlarged the frontal process zone. Electrical conductivity at operating temperature and thermal conductivity at room temperature decreased similarly with porosity. These properties of porous LSM were discussed using the effective volume of solid phase.

Critical Frontal Process Zone Size and Fracture Toughness

Critical Frontal Process Zone Size and Fracture Toughness

Experimental validation of a microcracking-based toughening mechanism for cortical bone

It has been proposed that cortical bone derives its toughness by forming microcracks during the process of crack propagation (J. Biomech. 30 (1997) 763; J. Biomech. 33 (2000) 1169). The purpose of this study was to experimentally validate the previously proposed microcrack-based toughening mechanism in cortical bone. Crack initiation and propagation tests were conducted on cortical bone compact tension specimens obtained from the antlers of red deer. For these tests, the main fracture crack was either propagated to a predetermined crack length or was stopped immediately after initiating from the notch. The microcracks produced in both groups of specimens were counted in the same surface area of interest around and below the notch, and crack growth resistance and crack propagation velocity were analyzed. There were more microcracks in the surface area of interest in the propagation than in initiation specimens showing that the formation of microcracks continued after the initiation of a fracture crack. Crack growth resistance increased with crack extension, and crack propagation velocity vs. crack extension curves demonstrated the characteristic jump increase and decrease pattern associated with the formation of microcracks. The scanning electron micrographs of crack initiation and propagation displayed the formation of a frontal process zone and a wake, respectively. These results support the microcrack-based toughening mechanism in cortical bone. Bone toughness is, therefore, determined by its ability to form microcracks during fracture.

Fracture Toughness of Microfiber Reinforced Cement Composites

An investigation into the reinforcing behavior of microdiameter fibers in cement composites, with an emphasis on their behavior in the frontal process zone (FPZ) ahead of the crack tip, is presented. Fracture toughness tests were conducted on thin sheet cement composites reinforced with polypropylene (PP), polyvinyl alcohol (PVA), and refined cellulose (RC) fibers under air-dry conditions. Fracture toughness in the absence of microscopic damage was quantified for these composites, as was fracture toughness that included the energy absorbed in the microcracked region prior to failure crack localization. These tests revealed that the PVA and RC fibers were able to effectively postpone microcrack formation, thereby delaying the localization of the failure crack. The PP fibers were not able to provide the same level of reinforcement. An optical microscope was used to monitor the progression of damage during testing. In addition, the acoustic emission technique was used to confirm the accuracy of the fracture toughness values.