The fracture toughness of 617 Ni-based weld metal (WM) under different elevated temperatures was tested with a novel method and its fracture mechanism was investigated in this paper. It was found that the fracture toughness of WM was lower than that of base metal (BM) at the same temperature, which was mainly due to the coarse columnar structure, differences in misorientation, and precipitated phases. For both BM and WM, the fracture toughness was lower at elevated temperature due to decreased strength. Much more micro-voids caused by Ti(C, N) and M23C6 inside grains of BM could be observed adjacent to the crack path, which accounted for the dramatically decreased fracture toughness of BM at elevated temperature. In comparison, fewer micro-voids could be observed in WM due to the lack of those second particles. As a result, the J0.2 value and propagation path morphology both showed that the WM had more stable microstructure even though possessing lower toughness.

Fracture of materials at the microscopic level involves a characteristic length related to microstructure. However, a clear structure-property relationship is still absent in metallic glasses. Therefore, a physics-based expression is derived for the characteristic length (relevant to brittle fracture) in metallic glasses (MGs) in order to link the microscopic material features controlling the fracture process to the macroscopic material parameters. The derived characteristic length is associated to micro/nano structural fracture patterns, critical crack tip opening displacement as well as fracture toughness. Characteristic lengths of various metallic glasses are determined using the proposed expression and compared to the experimental results. Theoretical results are in very good agreement with the experimental results of various metallic glasses. Furthermore, the contribution of characteristic length as well as macroscopic material parameters such as Poisson’s ratio, yield strength, and Young’s modulus on fracture toughness (and fracture energy) is investigated and compared to the experimental results.

Carbon fibres with high tensile strength are being increasingly utilised in the manufacture of advanced composite aerostructures. A Modified Compact Tension (MCT) specimen is often deployed to measure the longitudinal intralaminar fracture toughness but a high tensile strength often leads to premature damage away from the crack tip. We present an approach whereby the MCT specimen is supported by external fixtures to prevent premature damage. In addition, we have developed a novel measurement technique, based on the fibre failure strain and C-scanning, to determine the crack length in the presence of surface sublaminate delamination which masks the crack tip location. A set of cross-ply specimens, with a ((90/0)s)4 layup, were manufactured from an IMS60/epoxy composite system Two different data reduction schemes, compliance calibration and the area method, are used to determine the fibre-dominated initiation and propagation intralaminar fracture toughness values. Propagation values of fracture toughness were measured at 774.9 ± 5.2% kJ/m2 and 768.5 ± 4.1% kJ/m2, when using the compliance calibration method and the area method, respectively. Scanning Electron Microscopy (SEM) is carried out on the fracture surface to obtain insight into the damage mechanism of high-tensile-strength fibre-reinforced unidirectional composites. The measured tensile fracture toughness value is used in a fully validated computational model to simulate the physical test.

As high-entropy alloys (HEAs) are being actively explored for next-generation structural materials, gaining a comprehensive understanding of their creep, fatigue, and fracture behaviors is indispensable. These three aspects of mechanical properties are particularly important because (i) creep resistance dictates an alloy’s high-temperature applications; (ii) fatigue failure is the most frequently encountered failure mode in the service life of a material; (iii) fracture is the very last step that a material loses its load-carrying capability. In consideration of their importance in designing HEAs toward applicable structural materials, this article offers a comprehensive review on what has been accomplished so far in these three topics. The sub-topics covered include a comparison of different creep testing methods, creep-parameter extraction, creep mechanism, high-cycle fatigue S–N relation, fatigue-crack-growth behavior, fracture toughness, fracture under different loading conditions, and fractography. Directions for future efforts are suggested in the end.

Effects of Mo addition on the microstructure, mechanical properties, and abrasive wear properties of an oriented bulk Fe2B crystal have been investigated systematically in the present paper. Five groups of pure Fe2B samples with different Mo contents have been examined by optical microscope, X-ray diffraction, scanning electron microscope integrated with energy disperse spectroscopy, microhardness tester, and three-point bending testing of fracture toughness. The results indicate that Mo tends to segregate on the grain boundaries after doping; with increasing Mo addition, interplanar spacing of the (002) crystal plane of Fe2B decreases firstly and then increases slightly while that of (200) increases gradually; microhardness on the transversal section changes little while that on the longitudinal section increases firstly and then decreases [possessing the opposite trend to interplanar spacing of (002)]; fracture toughness and wear resistance of both transversal and longitudinal samples can be improved to some extent with Mo addition less than 2.0 wt%. In conclusion, appropriate Mo addition plays a positive role in the improvement of mechanical properties of oriented bulk Fe2B.

Hydrogenated diamond like Carbon (H-DLC) is a promising lubricious coating that attracted a great deal of interest in recent years mainly because of its outstanding tribological properties. In this study, the nano-mechanical and -tribological properties of a range of H-DLC films were investigated. Specifically, four kinds of H-DLC coatings were produced on Si substrates in pure acetylene, pure methane, 25% methane + 75% hydrogen, 50% methane + 50% hydrogen discharge plasmas using a plasma enhanced chemical vapour deposition (PECVD) system. Nano indentation was performed to measure the mechanical properties such as hardness and young's modulus and nanoscartching was performed to investigate the frictional behavior and wear mechanism of the H-DLC samples in open air. Moreover, Vickers indentation method was utilized to assess the fracture toughness of the samples. The results revealed that there is a strong correlation between the mechanical properties (hardness, young's modulus, fracture toughness) and the friction coefficient of DLC coatings and the source gas chemistry. Lower hydrogen to carbon ratio in source gas leads to higher hardness, young's modulus, fracture toughness and lower friction coefficient. Furthermore, lower wear volume of the coated materials was observed when the friction coefficient was lower. It was also confirmed that lower hydrogen content of the DLC coating leads to higher wear resistance under nanoscratch conditions.

Tantalum carbide (TaC) is an ultrahigh temperature ceramic, where low damage tolerance limits its potential application in propulsion sector. In this respect, current work focuses on enhancing the toughness of TaC based composites via synergistic reinforcement of SiC and carbon nanotubes (CNTs). Spark plasma sintering of TaC, reinforced with 15 vol% SiC and 15 vol% CNT (processed at 1850 °C, 40 MPa, 5 min), has shown enhanced densification from ∼93% (for TaC) to ∼98%. Potential damage of the tubular CNTs to flaky graphite was revealed using transmission electron microscopy, and was supplemented via Raman spectroscopy. SiC addition has enhanced the hardness to ∼19.5 GPa while a decreases to 12.6 GPa was observed with CNT addition when compared to the hardness of TaC (∼15.5 GPa). The increase in the indentation fracture toughness (from 3.1 MPa m1/2 for TaC to 11.4 MPa m1/2) and fracture strength (from ∼23 MPa for TaC to ∼183 MPa) via synergetic reinforcement of SiC and CNT is mainly attributed to energy dissipating mechanisms such as crack branching, CNT bridging, and crack-deflection. In addition, the reduction of interfacial residual tensile-stresses with SiC- and CNT-reinforcement, resulting an overall increase in the fracture energy and toughening, is also established.

The influence of Pt layer thickness on the fracture behavior of PtNiAl bond coats was studied in situ using clamped micro-beam bend tests inside a scanning electron microscope (SEM). Clamped beam bending is a fairly well established micro-scale fracture test geometry that has been previously used in determination of fracture toughness of Si and PtNiAl bond coats. The increasing amount of Pt in the bond coat matrix was accompanied by several other microstructural changes such as an increase in the volume fraction of α-Cr precipitate particles in the coating as well as a marginal decrease in the grain size of the matrix. In addition, Pt alters the defect chemistry of the B2-NiAl structure, directly affecting its elastic properties. A strong correlation was found between the fracture toughness and the initial Pt layer thickness associated with the bond coat. As the Pt layer thickness was increased from 0 to 5 µm, resulting in increasing Pt concentration from 0 to 14.2 at.% in the B2-NiAl matrix and changing α-Cr precipitate fraction, the initiation fracture toughness (KIC) was seen to rise from 6.4 to 8.5 MPa·m1/2. R-curve behavior was observed in these coatings, with KIC doubling for a crack propagation length of 2.5 µm. The reasons for the toughening are analyzed to be a combination of material's microstructure (crack kinking and bridging due to the precipitates) as well as size effects, as the crack approaches closer to the free surface in a micro-scale sample.

Bulk metallic glasses (BMGs) exhibit high yield strength but little tensile ductility. For this class of materials, damage tolerance is a key mechanical design parameter needed for their engineering use. Recently we have discovered a correlation between the local structural characteristics in the glass and the propensity for shear transformations. Based on the dependence of glass structure on alloy composition, zirconium (Zr)-rich Zr–titanium (Ti)–copper (Cu)–aluminum (Al) compositions are predicted to be more prone to spread-out plastic deformation and hence profuse shear banding. This structural perspective has guided us to locate a Zr61Ti2Cu25Al12 (ZT1) BMG that exhibits a record-breaking fracture toughness, on par with the palladium (Pd)-based BMG recently developed at Caltech. At the same time, the new BMG consists of common metals and has robust glass-forming ability. Interestingly, the ZT1 BMG derives its high toughness from its high propensity for crack deflection and local loading-mode change (from mode I to substantially mode II) at the crack tip due to extensive shear band interactions. A crack-resistance curve (R-curve) has been obtained following American Society for Testing and Materials (ASTM) standards, employing both “single-specimen” and “multiple-specimen” techniques as well as fatigue precracked specimens. The combination of high strength and fracture toughness places ZT1 atop all engineering metallic alloys in the strength–toughness Ashby diagram, pushing the envelop accessible to a structural material in terms of its damage tolerance.

A new physical model of plastic deformation in nanocrystalline (NC) materials with finest grains (whose grain size is 2–4 nm) is suggested and theoretically described. The model represents the effect of the finest grains located at triple junctions on the fracture toughness of NC materials in the case that there are multiple dislocations pile-up at grain boundaries (GBs). The maximum number n of the pile-up dislocations is determined by both the capacity of dislocations emitting associated with the crack propagation and the capacity of dislocations pile-up due to the existence of the finest grains. The calculation indicates that the parameter n increases with increment of the grain size and decreases with the finest grain size increasing. The results theoretically reveal that the triple junctions with finest grains can significantly improve the fracture toughness of NC materials compared with the normal triple junctions in wide ranges of their structural parameters.

Nanoparticles of ∼55 nm in diameter have been found to significantly increase the fracture toughness of nanocomposites with a ductile polymer matrix. This paper presents a comparative study of the effectiveness of micrometer-sized and nanosized elastomeric particles in improving the fracture toughness and strength of polymeric adhesives. Particular focuses are on the effects of particle size, matrix ductility, and adhesive thickness on the shear strength and fracture toughness. The results reveal that for an epoxy adhesive cured with a J400 hardener, nanosized particles can produce nearly 18 times more increase in fracture toughness than what can be achieved using micrometer-sized particles at the same volume fraction of 5.0 vol%. This huge improvement of adhesives' fracture toughness by nanosized particles is similar to that observed in bulk nanocomposites, indicating that the superior toughening mechanism of nanosized elastomeric particles is equally effective in thin adhesives constrained by stiff adherends.

les différentes méthodes qui utilisent l’essai d’indentation pour estimer la ténacité sont revues et commentées. Pour les matériaux fragiles, les formules sont principalement basées sur la mesure des fissures formées par l’indenteur ou, si l’essai est instrumenté, sur les décrochements (pop-in) observés sur la courbe charge-déplacement. D’autres méthodes moins connues peuvent utiliser la mesure de l’ouverture en fond de fissure, une équivalence entre la longueur de fissure formée et l’accroissement de pénétration de l’indenteur ou le clivage provoqué par une indentation proche du bord du matériau étudié. Pour les matériaux ductiles aucune fissure n’est visible à la surface, même pour les fortes charges appliquées. Dans ce cas la ténacité est reliée par la mécanique continue du dommage à un dommage critique mis en évidence par la diminution du module d’élasticité avec l’augmentation de la profondeur d’indentation. Toutes ces méthodes permettent de calculer la ténacité avec une incertitude pouvant aller au maximum à 50 %.

Three D.C.B. tests (Double Cantilever Beam) under different loading histories have shown that the applied crack opening rates can have an effect on the KIC fracture toughness values computed on the basis of linear elastic behavior of concrete. This assumption can be well justified for a certain range of crack opening rates, outside of which viscous phenomena must be taken in account. The concepts of ductile and brittle fracture appear then as related to the crack opening rate.