Increase In KIC Research Articles

Effect of aging temperatures on the microstructure and mechanical properties of TC21 forgings with basket–weave microstructure

This study aims to explore the influence of various aging temperatures on the microstructure and mechanical properties of TC21 forgings. Basket-weave microstructures were prepared via quasi-β forging followed by annealed at 900 °C for 1.5 h and aged at 530 °C, 560 °C, and 590 °C for 4 h, and then their tensile performance, impact toughness, and fracture toughness were examined. Results indicate that higher aging temperatures not only facilitate the segregation of Al element, resulting in a reduction in the effective size (dαlaths, the width/length ratio) of primary α laths (αlaths) and their amount, but also promote phase transition, inducing an increase in the effective size (dαfine) of secondary fine α (αfine) lamellae. While both dαlaths−1/2 and dαfine−1/2 have a linear relationship with the strength, which was analyzed using the Hall–Petch relationship, the more pronounced interface strengthening effect and hindering crack propagation mainly results from the αlaths. Moreover, the plasticity declines significantly as the proportion of the αlaths phases decreases. Thus, for strength and plasticity, the αlaths are a much more important factor than the αfine lamellae. However, although the synergistic deformation and fracture of both the αlaths and αfine phases can consume a larger impact initiation energy, the enhancement of the impact energy of the alloy is primarily related to a high precipitation amount of the αfine lamellae. Furthermore, the decrease in dαlaths promotes the misorientation and accumulation of dislocations between αlaths interfaces, hindering the enhancement of KIC (fracture toughness) by forming transgranular fractures. Conversely, the increase in KIC attributed to the energy consumed in the plastic deformation region at the crack tip, due to the αfine lamellae compensates for the energy, resulting in a reduction in the crack propagation path. Therefore, the αfine lamellae may play a more crucial role in increasing the toughness of alloys, in particular the impact toughness of the TC21 alloy.

Predicting the Effect of the Loading Rate on the Fracture Toughness of Hydraulic Asphalt Concrete Based on the Weibull Distribution.

The cracking problem of asphalt concrete panels is a crucial consideration in the design of hydraulic asphalt concrete seepage control bodies. Panels experiencing uneven rises or falls of water levels during impoundment may exhibit loading rate effects. Investigating the fracture toughness value of asphalt concrete under varying loading rates is essential. This study employs a statistical method to calculate the fracture index KIC, using the semi-circular bending test (SCB) to examine the effect of loading rates on the Type I fracture mode of hydraulic asphalt concrete. The data are analyzed using the two-parameter Weibull distribution curve, offering insights into the minimum number of KIC test specimens. The results indicate an increase in KIC with loading rate, with greater data dispersion at faster rates. The Weibull distribution curve successfully fits the fracture behavior under different loading rates, providing valuable predictions. This study estimates the minimum number of SCB test specimens to be nine, based on a confidence level of 0.95 and a relative deviation not exceeding 5%.

Experimental characterization of Mode I fracture toughness of the undisturbed Guadalquivir Blue Marl: Effect of suction

Fracture Mechanics is an extensively developed discipline for a wide range of materials used in scientific and engineering applications. However, the thorough understanding and characterization of fracture phenomena in many soil and rock-type materials are still under development. This work aims to characterize the suction-dependent fracture response in undisturbed clay soils, which will lead to identifying a ductile–brittle behavior threshold. The undisturbed condition will provide information on the behavior of the soil in its natural state. For this purpose, a new experimental procedure is presented in order to obtain a closed-form relationship between mode I fracture toughness (KIc) and a suction range (Ψ) for the Guadalquivir blue marl. In order to achieve this objective, an experimental campaign was carried out using a series of semi-circular specimens subjected to the three-point bending test with different suctions. The corresponding loading–displacement curves from the tests are qualitatively analyzed to identify the fracture response of this material, and the linear regression technique is used to finally obtain the desired relationship. Current results show that an increasing value of suction increases both the maximum load and the stiffness, yielding a more brittle behavior associated with an increase in KIc. Finally, an alternative expression for the determination KIc with respect to that proposed by Kuruppu and coauthors (Kuruppu et al., 2013) is presented, which involves the explicit inclusion of the suction variable for the more brittle behavior, limited by the threshold found. This provides a new perspective on how moisture content affects the fracture mechanics of clays.

Anisotropic fracture in nacre-like alumina

Fracture in nacre-like alumina is studied experimentally and numerically with alumina platelets main alignment direction perpendicular (along-configuration) to the one commonly tested (across-configuration) in the literature. In along-configuration, brittle failure is noted after crack initiation with no macroscopic crack deviation, unlike what is observed in across-configuration. However, NLA exhibits in along configuration initiation critical stress intensity factor KIc = 5.3 ± 0.5 MPa m1/2, in the same order of magnitude as in across-configuration. The increase in KIc compared to monolithic alumina can be explained by a phenomenon of crack initiation containment at the interface between platelets. Unstable crack propagation occurs just after initiation, mainly following the platelet interfaces which results in crenelated fracture surfaces with crack decentering with respect to the initial notch tip.

Conformational Flexibility-Controlled Piezochromic Behavior of Organic Fluorophores

The pressure-responsive behaviors of organic fluorophores are often irregular and highly system-dependent. Unraveling the pressure effects on them is the key to the precise design of piezochromic materials. Here, we demonstrate that the different conformational flexibilities of organic fluorophores can lead to their distinct piezochromic behaviors. By combining quantum mechanics/molecular mechanics models and thermal vibration correlation function formalism, we show that two organic fluorophores, dibenzo[b,d]thiophene 5,5-dioxide (DBTS) and carbazole (Cz), with similar chemical structures, display different conformations and conformational flexibilities, which determine the responsiveness of their piezochromic behaviors. It is found that pressurization continuously improves the planarity between the sulfonyl group and the aromatic skeleton in cross-shaped DBTS, leading to red-shifted emission. In addition, the fluorescence quantum efficiency (FQE) of DBTS increases monotonically in a wide range of 0–7 GPa and then shows a dramatic reduction upon further compression to 12 GPa. The non-monotonical FQE of DBTS is attributed to the continuously decreased radiative decay rate constant (kr) and non-monotonically changed non-radiative decay rate constant (kic). The initial decrease in kic is due to the restriction of aromatic skeleton vibrations in the high-frequency region, and the subsequent increase in kic is ascribed to the acceleration of the bending and rotational vibrations of the sulfonyl group in the low-frequency region. In contrast, pressurization hardly influences the conformation of planar Cz, resulting in a slight change in emission and FQE, which is well consistent with experimental results. This study opens an efficient shortcut to the rational design of advanced piezochromic luminescent materials.

Evaluation of fracture toughness in graphene-based cementitious nanocomposites via electrical impedance

The present work investigates experimentally the improvement of fracture toughness and electrical conductivity in the hardened cementitious matrix after the addition of graphene nanoplatelets (GnPs), at four different concentrations varying between 0.05 wt% and 0.4 wt% per cement. The experimental investigation is conducted through flexural and electrical impedance spectroscopy (EIS) tests on prismatic specimens. The study is oriented towards establishing a correlation between the electrical response of the nanocomposites under alternate current and the exhibited fracture toughness values (KIc). The flexural test results show that the incorporation of the GnPs in the matrix can increase the average KIc values and can decrease the average resistivity (ρ) when compared with the reference matrix. The maximum increase in KIc (+ 29 %), and the decrease in ρ (- 68 %) were found in the mixture with the lowest amount (0.05 wt%) of GnPs. Moreover, the comparison of the ρ values and the KIc values, as a function of the GnPs concentration, reveals a reverse relation between the two parameters (KIc and ρ). The functional correlation between these parameters was also confirmed by linear regression analysis, resulting from the experimental data fitting. The analysis provides evidence that EIS can be used as a non-destructive tool to assess the fracture toughness of cementitious nanocomposites.

Four-Point Bending Fatigue Behavior of Al2O3-ZrO2 Ceramic Biocomposites Using CeO2 as Dopant

This work investigated the effect of adding ceria-stabilized tetragonal zirconia (Ce-TZP) on the fatigue behavior of alumina-based ceramic composites. Alumina powder (control group) and mixtures containing 5 wt.% (group A) and 20 wt.% (group B) of a commercial m-ZrO2/Al2O3/CeO2 powder mixture were milled/homogenized, compacted, sintered at 1600°C-2h, and submitted to hydrothermal degradation. The samples were characterized by relative density, microstructure, crystalline phases, and static mechanical properties. The cyclic fatigue strength was determined using the modified staircase method in 4-point bending tests. The results indicate that adding the m-ZrO2/Al2O3/CeO2 powder mixture to the Al2O3-matrix increases the tetragonal-ZrO2 grains (Ce-TZP) content, presenting 2.9 wt.% of Ce-TZP and 11.9 wt.% of Ce-TZP for group A and group B, respectively. Furthermore, the addition of Ce-TZP improves densification (98.5% → 99.1%) with a slight reduction in hardness and modulus of elasticity and a significant KIC increase of the composite (KIC = 6.7 MPa.m1/2, group B) when compared to monolithic alumina (KIC=2.4 MPa.m1/2). The fatigue strength limit of the control group was around 100 MPa, while the composites (groups A and B) presented the values ​​of 279 MPa and 239 MPa, respectively. The results indicated that the incorporation of Ce-TZP significantly improves the fracture toughness of alumina-based ceramics. On the other hand, regarding the fatigue behavior, there was an increase in fatigue resistance in group A, resulting from the benefits of the t→m Ce-TZP grains transformation, which occurs during cyclic loading, producing a zone shielding that involves the tip of the crack, slowing its growth. The increase in the amount of Ce-TZP (group B) leads to an increase in the internal residual stresses between the phases due to anisotropy and difference in the thermal expansion coefficients, which accelerates the phase transformation and formation of microcracks at grain boundaries, reducing the fatigue strength of composites of group B.

Grain size dependence of cracking performance in polycrystalline NiTi alloys

Nickle-titanium (NiTi) alloys are widely considered as one of the most intelligent materials with great commercial values. This study focuses on investigating the grain size (GS) effect on crack propagation of polycrystalline NiTi alloys through molecular dynamics (MD) simulations combined with cohesive zone model (CZM). The GS distribution is statistically achieved by electron backscatter diffraction experiment, while the Voronoi tessellation is applied to characterize the microstructures. MD modeling is conducted to determine the traction-separation (T-S) law of CZM. The plastic behavior and phase transition are examined as well. Subsequently, the characteristic parameters extracted from the T-S law are embedded into the cohesive elements along grain boundaries, and then the intergranular fracture of NiTi alloys with various grain sizes are reproduced by conducting finite element simulations. Interestingly, an unexpected subcritical crack growth (SCG) is found during the simulations, and its mechanism is explored at the atomic scale. Moreover, the critical stress intensity factor (KIC) of compact tension specimens with average grain size from 17 to 45 µm is predicted. It is discoverable that the SCG duration and KIC increase as the grains become coarsened. Besides, the internal toughening mechanism of GS is probed from the aspects of stress-induced martensitic transformation and crack-path configuration. A comparison between numerical results and published experimental data demonstrates that this hybrid method could be used to successfully simulate the crack growth behavior.

Environment dependence of KIc of glass

The role of temperature (T) and moisture on fracture toughness (KIc) of glasses, as measured by the single-edge precracked beam method (SEPB), is investigated. The brittle to ductile temperature, shown by an increase of KIc, is ~Tg. At T below 0.9Tg, the decrease of the bond strength with T results in the decrease of both the fracture surface energy and Young's modulus, so that KIc decreases with T. From above Tg to below 1.1Tg, the work of fracture increases because of dissipative processes, including confined viscoplasticity and blunting at the crack front. From 1.1Tg, viscous flow extends to the whole specimen and becomes more uniform, resulting in the macroscopic deformation of the specimen, eventually preventing against fracture. The results on humidity sensitive glasses show that both precracking and final fracture steps need to be performed in inert environment to meet the intrinsic KIc value, unless the test is fast enough.

Research on size effect of fracture toughness of sandstone using the center-cracked circular disc samples

In order to investigate the size effect and reason of rock fracture toughness, the split loading test was carried out in the GCTS rock mechanics testing machine using the center-cracked circular disc (CCCD) samples with diameters of 50 mm, 100 mm, 150 mm and 205 mm, respectively. The lengths of fracture process zone (lFPZ) were determined by the maximum normal stress criterion. The size effect of the lFPZ , fracture toughness test value (KIc), and fracture energy (Gf) were analyzed. The influence of the lFPZ on the size effect of the KIc was revealed. The crack growth paths and failure characteristics of the samples were discussed. The results show that the lFPZ and the KIc increase linearly with the sample size, and there is obvious size effect within the range of the test. The KIc and the lFPZ do not always increase linearly with the size in a larger size range, but gradually stabilize by extrapolation analysis. The lFPZ increases with the size, which means the release of stored energy increases with the size corresponding to the size effect of the Gf, and in turn leads to the size effect of the KIc. It is helpful to deeply understand the cause of the size effect reason of rock fracture toughness.

Microstructure and mechanical properties of WC-(Ti,W)C-Co hardmetals fabricated by in-situ carbothermal reduction of oxides and subsequent liquid phase sintering

WC-(Ti,W)C-Co hardmetal was fabricated using carbon black, WC, Co, WO3, TiO2 as raw materials, through an energy-efficient and cost-efficient method that the process of carbothermal reduction of oxides and the liquid phase sintering of hardmetal were combined in one heating cycle. Phase transformation and microstructure characteristics of the hardmetals were examined by XRD, SEM, EDX, and TEM. Moreover, mechanical properties were tested with related mechanical property testing instruments. Results demonstrated that the in-situ carbothermal reactions and outgassing processes had finished below 1200 ℃. Microstructures of hardmetal consisted of WC grains, (Ti,W)C solid solution, and Co binder phase. Moreover, the mean size of WC and (Ti,W)C grains were both smaller than those of hardmetal prepared with TiC powders. A coherent relationship between Co and (Ti,W)C grain was noticed. Moreover, a coherent relationship between two (Ti,W)C grains was observed. Compared with hardmetal prepared with TiC powders, TRS and KIC were improved. The increase of TRS was chiefly due to the smaller grain size and the coherent interface structure. Moreover, the increase of KIC was ascribed to the (Ti,W)C solid solution and the coherent interface structure. Hardmetals prepared through this method showed good mechanical properties with a hardness of 90.7 ± 0.2 HRA, a TRS of 3007 ± 87 MPa, a KIC of 13.1 ± 0.2 MPa·m1/2.

Synthesis, Structure, and Properties of Zirconia-Modified Aluminosilicate Glass-Ceramics

Strontium aluminosilicate glass-ceramics modified with zirconia additions in the presence of yttria as a stabilizing oxide and without it have been prepared by a sol–gel process. Increasing the zirconia content from 5 to 15 wt % has been shown to reduce the gelation time of the starting solutions, lower the gel crystallization onset temperature, activate the glass-ceramic sintering process, and increase the critical stress intensity factor (KIc) of the glass-ceramics by more than a factor of 2. The present results confirm that the increase in KIc on the addition of ZrO2 is due to transformation toughening. At the same time, the addition of yttria as a stabilizing oxide has been shown to hinder the martensitic transformation of tetragonal ZrO2 into the monoclinic phase.

Synergistic effects of double-walled carbon nanotubes and nanoclays on mechanical, electrical and piezoresistive properties of epoxy based nanocomposites

Many studies performed on multifunctional properties of epoxy based nanocomposites reinforced with carbon nanotubes (CNTs) and nanoclay (NC) whereas their synergetic effects on piezoresistive behaviour of ternary state nanocomposites still remains unaddressed. Therefore, the hybrid effects of double-walled CNTs (DWCNTs) and NC on the mechanical, electrical and piezoresistive performances of the epoxy were addressed in this study. Nanocomposites were prepared in two different states, i.e. the binary state (DWCNTs/epoxy) and the ternary states (DWCNTs-NC/epoxy). SEM, FESEM, and XRD were used for the microstructural analysis of the materials while tensile and mode I fracture tests were performed for mechanical and piezoresistive characterizations. The addition of NC to CNTs doped epoxy resulted in a better CNT dispersion, hindering CNT re-agglomeration. A significant increase in KIC (94%) and GIC (254%) compared to the neat epoxy was obtained for the hybrid nanocomposites loaded at 1 wt% NC due to crack bridging and crack deflection. The electrical conductivity of the ternary state materials increased by 700% and 400% with respect to the binary nanocomposite, for 0.5 wt% and 1 wt% NC loadings, respectively. The hybrid nanocomposites also manifested higher piezoresistivity and a more robust signal in tensile and fracture tests, respectively.

Quantitative multi-scale characterization of single basalt fibres: Insights into strength loss mechanisms after thermal conditioning

This article presents an experimental investigation to quantify the effects of high temperature exposure (400–600 °C) on the mechanical properties of single basalt fibres. To this purpose, a combination of single edge notch tension and nanoindentation micro-pillar splitting methods was used to provide an assessment of the fracture toughness of as-received and thermally treated basalt fibres. Similar values were obtained by the two different methods, and interestingly both highlighted an increase in KIc after heat treatment, up to 22% after exposure at 600 °C for 1h (1.59±0.06MPam). The increase in KIc suggests that microstructural changes occur in the fibres, as confirmed by high-speed nanoindentation mapping. Local radial heterogeneity in the fibre structure and elastic modulus and, possibly, the loss of defect orientation originally induced during the fibre drawing process are envisaged to control the decay of basalt fibres tensile strength during high temperature exposure, mimicking a thermal recycling process for composites.

Fluid‐Driven Tensile Fracture and Fracture Toughness in Nash Point Shale at Elevated Pressure

AbstractA number of key processes, both natural and anthropogenic, involve the fracture of rocks subjected to tensile stress, including vein growth and mineralization, and the extraction of hydrocarbons through hydraulic fracturing. In each case, the fundamental material property of mode‐I fracture toughness must be overcome in order for a tensile fracture to propagate. While measuring this parameter is relatively straightforward at ambient pressure, estimating fracture toughness of rocks at depth, where they experience confining pressure, is technically challenging. Here we report a new analysis that combines results from thick‐walled cylinder burst tests with quantitative acoustic emission to estimate the mode‐I fracture toughness (KIc) of Nash Point Shale at confining pressure simulating in situ conditions to approximately 1‐km depth. In the most favorable orientation, the pressure required to fracture the rock shell (injection pressure, Pinj) increases from 6.1 MPa at 2.2‐MPa confining pressure (Pc), to 34 MPa at 20‐MPa confining pressure. When fractures are forced to cross the shale bedding, the required injection pressures are 30.3 MPa (at Pc = 4.5 MPa) and 58 MPa (Pc = 20 MPa), respectively. Applying the model of Abou‐Sayed et al. (1978, https://doi.org/10.1029/JB083iB06p02851) to estimate the initial flaw size, we calculate that this pressure increase equates to an increase in KIc from 0.36 to 4.05 MPa·m1/2 as differential fluid pressure (Pinj − Pc) increases from 3.2 to 22.0 MPa. We conclude that the increasing pressure due to depth in the Earth will have a significant influence on fracture toughness, which is also a function of the inherent anisotropy.

Toughening of Bismaleimide Resin Based on the Self-Assembly of Flexible Aliphatic Side Chains

The poor toughness of bismaleimide (BMI) resin hampers its application in the aeronautics and space field. Herein, a flexible monohydric aliphatic amine (MAA) was introduced to in-situ toughen BMI resins, in which a novel self-assembled second-phase structure was formed. The micro-nanostructure can be adjusted by changing MAA to perfectly improve the comprehensive performance of BMI. In addition, only a small amount of MAA can achieve substantial increase of toughness of BMI. The morphology of MAA-modified BMI was observed via scanning electron microscopy and atomic force microscopy. Consequently, after adding 1% MAA, the elongation at break of MAA-modified BMI increased by 32.3%. Compared to the neat BMI, the flexural strength of MAA-modified BMI exhibited enhancement by 30.1%. Significantly, the presence of MAA in the BMI resin resulted in a remarkable increase in KIC of 24.3% and GIC of 54.2% without sacrificing the processability and heat resistance.

Elastic properties and fracture toughness of SiOC‐based glass‐ceramic nanocomposites

AbstractFour different SiOC glass ceramics were synthesized and their fracture toughness (KIc) and fracture surface energy (γ) were assessed by means of the single‐edge precracked beam (SEPB) method. In addition, the elastic moduli were measured and the Vickers indentation behavior (hardness and microcracking) was characterized. In particular, the dependence of KIc on the free carbon content and on the fraction of crystallized nanoparticles (SiC, ZrO2, HfO2) was investigated. An increase in KIc, from about 0.73 to 0.99 MPa √m is observed as the free carbon content is increased from less than 1 to 12 vol%. The addition of Hf and Zr (resulting in 4.5 to 7.8 vol% HfO2 and ZrO2 nanoparticles) was found to increase KIc to an extent similar to the free carbon content. Moreover, predicted KIc values, assuming that the crack travels through all phases accounting for their respective volume fractions, disrupting the weakest links within the structural units, are in agreement with the experimental values.

Static and dynamic mode I fracture toughness of rigid PUR foams under room and cryogenic temperatures

The research presented in this paper is an effort to better understand the fracture toughness of closed-cell rigid polyurethane (PUR) foams under different loading and temperature conditions. The effect of density (100, 145 and 300 kg/m3) and anisotropy (in-plane and out-of-plane loading directions) on both quasi-static and dynamic fracture behavior was also experimentally investigated. The three-point bending (3PB) tests were performed on Single Edge Notched Bend (SENB) samples, at room (25 °C) and cryogenic (−196 °C) temperatures, and the mode I fracture toughness (KIC) was calculated from their load-displacement curves. It was observed that all PUR foam samples, regardless of foam density and loading direction, showed a significant increase in KIC at the cryogenic temperature. The out-of-plane obtained samples showed a slight improvement in fracture toughness (highlighting an anisotropic behavior), both under quasi-static and dynamic 3PB loads. The dynamic KIC values were found higher than quasi-static ones, and irrespective of foam density and test condition, a brittle deformation mechanism without plastic deformation was observed for all samples. Finally, empirical formulations for cryogenic and dynamic KIC based on room temperature mode I fracture toughness were proposed.

Effect of Hybridization of Carboxyl‐Terminated Acrylonitrile Butadiene Liquid Rubber and Alumina Nanoparticles on the Fracture Toughness of Epoxy Nanocomposites

The aim of this work was to study the effect of carboxyl‐terminated acrylonitrile butadiene (CTBN) liquid rubber and alumina nanoparticles (Al2O3) on the fracture toughness of an epoxy resin. Hybrid nanocomposite samples were prepared by the inclusion of the optimum concentration of CTBN (15 phr) and 0–10 phr Al2O3 into the epoxy matrix. Samples were examined using Fourier transform infrared spectroscopy, scanning electron microscopy, and fracture toughness test. Fractal dimension values were also estimated from the edge of the fracture surfaces for the hybrid samples. The maximum value of critical stress intensity factor, K IC, was found for the non‐hybrid sample containing 15 phr CTBN and the G IC values showed an increasing trend as it was expected from the increase in K IC values and decrease in Young's modulus. The maximum K IC for the hybrid samples was obtained for the one containing 7 phr Al2O3 and an almost similar trend was observed for G IC, accompanied with the significant changes in the fracture surface morphology. These were consistent with the fractal dimension results, where the sample containing 7 phr Al2O3 exhibited the maximum roughness. Therefore, the hybrid system of epoxy/CTBN (15 phr)/Al2O3 (7 phr) was found to be the optimum formulation. POLYM. COMPOS., 40:2700–2711, 2019. © 2018 Society of Plastics Engineers

Fracture toughness and structural evolution in the TiAlN system upon annealing

Hard coatings used to protect engineering components from external loads and harsh environments should ideally be strong and tough. Here we study the fracture toughness, KIC, of Ti1−xAlxN upon annealing by employing micro-fracture experiments on freestanding films. We found that KIC increases by about 11% when annealing the samples at 900 °C, because the decomposition of the supersaturated matrix leads to the formation of nanometer-sized domains, precipitation of hexagonal-structured B4 AlN (with their significantly larger specific volume), formation of stacking faults, and nano-twins. In contrast, for TiN, where no decomposition processes and formation of nanometer-sized domains can be initiated by an annealing treatment, the fracture toughness KIC remains roughly constant when annealed above the film deposition temperature. As the increase in KIC found for Ti1−xAlxN upon annealing is within statistical errors, we carried out complementary cube corner nanoindentation experiments, which clearly show reduced (or even impeded) crack formation for annealed Ti1−xAlxN as compared with their as-deposited counterpart. The ability of Ti1−xAlxN to maintain and even increase the fracture toughness up to high temperatures in combination with the concomitant age hardening effects and excellent oxidation resistance contributes to the success of this type of coatings.