Tensile Fracture Toughness Research Articles

Experimental analysis of mechanical properties of in-situ formed MMC from Al/TiB2/Cu and Al/TiB2

The In-situ formed Metal Matrix composites 94% of Al & 6% of TiB2 and 90% of Al, 6 % of TiB2 and 4 % of Cu are examined in this study. Both composites underwent testing after being produced using the stir casting technique. To make the composite, a melt of A356 Aluminum is combined with potassium tetra fluoro boreate (K2TiF6) and potassium hexafluoro titanate (KBF4), the precursor salts at the temperature of 820 °C, stirring condition of 300 rpm, and a 30 min choking time. The stoichiometric ratios of these salts translate into 6% of TiB2 particles by weight. The composite melt was added with 4 wt% of copper powder and the poured into the permanent mould. Utilizing XRD and optical microscopy, the TiB2 and Cu particles in the metal matrix composite were studied. Mechanical properties tensile strength, hardness, and fracture toughness are tested according to ASTM standards. The mechanical properties of the In- situ formed metal matrix composite with Copper Al/6 wt% TiB2/4 wt% Cu are higher than that of the composite Al/6 wt% TiB2.

The interplay between constituent material and architectural disorder in bioinspired honeycomb structures

The advent of additive manufacturing has enabled the precise manufacturing of lattice materials at different length scales. While honeycombs are usually constructed from a tessellated array of identical unit cells, most structures in nature tend towards stochastic or disordered tessellation. In this work, we explore the effect of disorder properties of honeycombs, in particular, stiffness, strength, fracture toughness, and crack propagation paths using finite element models. The honeycombs are created using Voronoi tessellations with different levels of disorder measured by a regularity parameter δ, where δ=1 corresponds to a regular hexagonal honeycomb and decreasing values of δ correspond to increasing levels of disorder. We consider three constituent materials: very brittle, quasi-brittle, and very ductile (0.0007–0.25 in failure strain). The results show that pseudo-order realizations with 0.5≤δ≤0.8show the best strength-toughness trade-off. With increased plasticity, the improvement in fracture toughness compared to regular honeycombs can be as high as 50% at the expense of a 20% decrease in strength. This increase in fracture toughness is achieved both at crack initiation with crack blunting intrinsically and extrinsically during crack growth by a combination of crack deflection and crack bridging. Also, a single mathematical variable δ usually referred to as a “regularity parameter” to control cell stochasticity cannot account for the variability in tensile strength and fracture toughness parameters for brittle or quasi-brittle constituent materials and significantly more for a ductile constituent material. Thus, a local parameter for disorder is required to explain the large statistical variance seen in the mechanical properties of disordered honeycomb structures across the same irregularity value.

Prediction of interfacial tensile bond strength in 3D printed concrete based on a closed-form fracture model

Interfacial bond strengths in 3D printed concrete may affect the deformation compatibilities among the extruded filaments and the anisotropy of concrete. However, the previous predictions were size dependent inevitably based on the assumption of interfacial continuity and homogeneity. This work aims to propose a fracture model capable of predicting the realistic tensile bond strength (ft) and fracture toughness (KIC) of interfaces between filaments and layers in 3D printed concrete. To reflect the discontinuity and heterogeneity of interfaces, a discrete number (β) and a microstructure characteristic parameter (G) were proposed. A simple linear relationship of the maximum fracture load (Fmax) related to ft or KIC was established, and thus, the realistic interfacial ft and KIC can be determined after obtaining Fmax from the tests on the specimens with any sizes. Subsequently, three-point bending tests were performed to study the interfacial fracture properties of printed concrete with various contents of manufactured sand. The results show that the addition of manufactured sand improved the interfacial ft and KIC between filaments but had little effect on the interfacial fracture parameters between layers. The anisotropic behavior of the 3D printed concrete was clarified by evaluating the predicted ft and KIC along different directions. This research brings novel insights into the interfacial behaviors in 3D printed concrete.

Influence of Retrogression Time on the Fatigue Crack Growth Behavior of a Modified AA7475 Aluminum Alloy

This paper investigates the effect of retrogression time on the fatigue crack growth of a modified AA7475 aluminum alloy. Tests including tensile strength, fracture toughness, and fatigue limits were performed to understand the changes in properties with different retrogression procedures at 180 °C. The microstructure was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The findings indicated that as the retrogression time increased, the yield strength decreased from 508 MPa to 461 MPa, whereas the fracture toughness increased from 48 MPa√m to 63.5 MPa√m. The highest fracture toughness of 63.5 MPa√m was seen after 5 h of retrogression. The measured diameter of η' precipitates increased from 6.13 nm at the retrogression 1 h condition to 6.50 nm at the retrogression 5 h condition. Prolonged retrogression also increased the chance of crack initiation, with slower crack growth rate in the long transverse direction compared to the longitudinal direction. An empirical relationship was established between fracture toughness and the volume fraction of age-hardening precipitates, with increasing number density of precipitates seen with increasing retrogression time.

Mechanical anisotropy and fracture behaviour of bulk ultra-fine grained AA2099 alloy

ABSTRACT In this work, the effect of cryo-deformation and post-deformation annealing process on tensile, anisotropic and fracture behaviour of AA2099 has been investigated. The development of ultrafine-grained (UFG) microstructure is achieved initially by cryorolling (CR) process on the solution-treated (ST) alloy by subjecting it to a true strain value of 2.3. Cryorolled samples are then followed by annealing process at temperatures of 150°C (CR + 150°C) and 200°C (CR + 200°C). A notable improvement in yield strength (~342 MPa), ultimate tensile strength (~472 MPa), hardness (~138 HV) and fracture toughness parameters (K Q (31.8 MPa √ m ), K ee (31 MPa √ m ) and J integral (57.67 kJ/m2)) is recorded, whereas anisotropy increases in the rolling direction and ductility show a significant drop at CR sample condition in comparison with coarse grains at ST condition. When annealing was executed at 150°C and 200°C, notable improvements in tensile, anisotropic and fracture behaviour of UFG AA2099 were observed. There has been a significant enhancement in tensile strength, anisotropy and fracture toughness until temperature reaches 150°C being the highest (~506 MPa), whereas after 150°C, these properties have been shown to decrease notably.

Fracture Behavior of Polybenzoxazine Toughened by Polyrotaxane Molecules and Core–Shell Rubber

The inherent brittleness of polybenzoxazines (PBas) makes them difficult for high-performance microelectronics and composite applications. Conventional toughening approaches are either ineffective or cause a reduction in the glass transition temperature (Tg) of PBa. Polyrotaxane (PR), a mechanically interlocked supramolecular polymer, is an effective toughening agent to improve the tensile strength and fracture toughness of various polymers due to its unique topological structure. In this study, the effect of the molecular weight and functional groups of PRs on the morphology and mechanical properties of PR-modified PBa was investigated. PBa toughened by well-dispersed epoxide-functionalized PR (EPR) and benzoxazine-functionalized core–shell rubber (BCSR) was found to greatly enhance fracture toughness without compromising Young’s modulus or Tg. The fracture toughness is significantly improved from 0.82 MPa m1/2 for neat PBa to 1.49 MPa m1/2 for PBa/EPR/BCSR. The synergistic toughening effect is explained by the molecularly dispersed EPR in the PBa network, which results in higher network chain mobility, which promotes more cavitation of BCSR and triggers larger matrix plastic deformation. It is believed that the knowledge gained from the present study can help develop tougher polymers for vast engineering applications, especially in electronics, automotive, and aerospace applications.

Fracture toughness and high-cycle fatigue behaviour of Al–Mg and Al–Mg–Sc–Zr alloys

Tensile, fracture toughness and high-cycle fatigue tests were carried out on Al–Mg and Al–Mg–Sc–Zr alloys. Fracture morphology and the microstructure of the alloys were observed by SEM and TEM, etc. Al–Mg–Sc–Zr alloy with fibrous fine grain structure showed higher tensile strength and fracture toughness than Al–Mg alloy, which had a larger grain size and obvious recrystallisation. In high-cycle fatigue tests, Al–Mg–Sc–Zr alloy presented a higher fatigue limit owing to its finer grains and nanosized precipitates, which increased the difficulty of fatigue crack initiation. Al–Mg alloy presented a lower fatigue limit because crack initiation caused by slip is more likely to occur. However, its rougher fracture surfaces may be beneficial to hindering the propagation of macrocracks.

Natural fibrous sepiolite-reinforced emulsion polymer isocyanate for timber structural adhesive

ABSTRACT The natural fibrous sepiolite is introduced into emulsion polymer isocyanate (EPI) consisting of aqueous vinyl acetate copolymer-based main component (MC) and polymeric methylene diisocyanate (pMDI) for the first time to prepare a high-performance, environmentally friendly, and cost-effective timber adhesive. We have studied the effect of the sepiolite content on the tensile stress-strain curves, dynamical mechanical properties, essential work of fracture of the cured glue layer, and the EPI bonding properties. In addition, we have also investigated the chemical group, fractional free volume, and morphology of EPI polymers with various sepiolite contents. The results indicate that adding sepiolite can significantly increase the tensile strength, elastic modulus, dynamic stiffness, and fracture toughness of cured EPI films. Remarkably, EPI-bonded structures exhibit superior resistance to compressive shear damage upon ageing in boiling water. For MC with 6.0–9.0 wt% sepiolite, only the addition of 10.0 wt% general pMDI crosslinker is required to achieve the JIS K6806 structural adhesive. The underlying enhancement mechanism is the significant hydrogen-bonding coupled chemical reaction capability of sepiolite for the available groups in the EPI system.

Microscopic Study of Shale Anisotropy with SEM In Situ Compression and Three-Point Bending Experiments

The microscopic anisotropy of shale has an important impact on its mechanical properties and crack behavior, so it is essential to understand the microscopic origin of anisotropic growth with a more effective laboratory work scheme. Uniaxial compression test and three-point bending test are considered to be efficient means to study the elastic properties and crack behavior of rocks. In this paper, uniaxial compression experiments and three-point bending experiments were conducted on shale outcrops in the Changqing area using field emission scanning electron microscopy (SEM) and in situ tensile testing, and the microscopic deformation and crack processes were quantitatively characterized by the digital image correlation (DIC) method. For the compression experiments, the observation of the first principal strain fields indicated that the microscopic anisotropy of shale was related to the bedding planes, and the microscopic deformations were mainly concentrated in the clay mineral accumulation area and at the microcracks. Elastic moduli and compressive strengths of specimens with different bedding angles were affected by the strong shear stress effects. The specimens with a bedding angle of 30° showed lower peak loads and compressive strengths, and the specimens with a bedding angle of 60° had lower elastic moduli. Three-point bending experiments were conducted for studying the effects of crack-bedding orientation relationships on cracking processes, and four critical fracturing mechanical properties were calculated. The short transverse-type cases were prone to break and had a lower peak load, tensile strength, fracture toughness and elastic-bending modulus. The divider-type cases were more difficult to break, formed a more tortuous crack and had a higher tensile strength, fracture toughness and elastic-bending modulus. The arrester-type cases had a middle range of mechanical parameters but developed the longest cracks. This study provides a feasible experimental and analysis method for understanding the microscopic anisotropy of shale samples. The small specimen size also makes the requirements of core samples easier to be satisfied considering the field application. Furthermore, the anisotropy of cracking processes can be understood better by building the connections between microstructural characteristics and mechanical performances.

Meso-scale fracture modelling and fracture properties of rubber concrete considering initial defects

With the massive incorporation of rubber, the fracture damage mechanism of rubberized concrete is more complex than that of ordinary concrete. This paper studies the damage fracture process of rubberized concrete. Based on the panoramic microstructure scanning images of rubberized concrete, mesoscale parameters such as microcracks and pore initial defects inside rubberized concrete are collected. The numerical model of rubberized concrete three-point bending beam is built based on the real mesoscale structure. The fracture process of rubberized concrete containing initial cracks is simulated under quasi-static loading. The results indicate that the number of microcracks at the aggregate-mortar interfacial transition zone (ITZ) decreased and the pore content increased with the increase of rubber content. The mesoscopic numerical model of rubberized concrete based on the real mesoscopic structure of rubberized concrete has good applicability and accuracy. The fracture toughness of concrete shows a trend of increasing and then decreasing with the increase of rubber content, and the fracture toughness reaches the maximum at 10% of rubber admixture. The simulation results yielded dimension-independent tensile strength and fracture toughness values.

Evaluation of age-hardening time on the mechanical behavior of Al-Mg-Si (Al 6063) alloy composites reinforced with alumina particles

This research attempts to evaluate the effects of age-hardening on the mechanical behavior of aluminum (6063) alloy reinforced with alumina (Al2O3) particles. Aluminum (6063)/Al2O3 composites containing 0, 5, 7, 9 and 10 wt% Al2O3 particles respectively were fabricated using double stir casting technique and representative samples from each composition were subjected to age-hardening treatment at 180 °C for 60 min, 120 min and 180 min respectively. The microstructures were evaluated using SEM while fracture toughness and tensile properties were used to evaluate the mechanical behavior of both the as-cast composites and their age-hardened counterparts. The microstructure evaluation reveals formation of coherent precipitates of Mg2Si evenly distributed in the age-hardened composites. Significant improvement in tensile strength values of 129.8, 88.6, 90.6, 137.9 and 170.6 MPa were obtained for the age-hardened composites compared to their as-cast composites with 42.2, 72.9, 81.2, 80.84 and 71.5 MPa across the series respectively. Similar trend was observed for fracture toughness with KIC values of 7.5, 5.8, 4.5, 7.8 and 9.6 MPam1/2 for age-hardened composites compared with 4.68, 4.64, 4.07, 4.06 and 3.59 MPam1/2 obtained for their as-cast composites. Age-hardening time of 180 min impart better tensile strength and fracture toughness.

Specific application method for determining the strength and fracture toughness of metal materials

The elastic–plastic fracture of metal components with cracks is an interesting structural behavior because of the sizable crack-tip plastic zone and its strong interactions with the structural boundaries and load conditions. These complex interactions lead to constant elastic–plastic fracture phenomenon that does not exist normally. Currently, there are several restrictions, such as specific height, thickness, initial crack length, loading fixture, and loading method, on the test specimens to determine the fracture toughness of metal materials. Considering the plastic zone at the crack tip, it is proposed that the elastic–plastic fracture phenomenon of metals can be predicted entirely by two basic material properties, tensile yield strength and fracture toughness. A 40 mm wide plate is used to test ordinary carbon steel Q235B, low alloy carbon steel Q345B and aluminum alloy 6061 under tension. The initial cracks introduced through wire-cutting were 4, 8, 12, 16, 20, 24, and 28 mm in dimensions with the initial notch length to the width ratios of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7. Based on the elastic–plastic fracture model, the specific application method for determining the strength and fracture toughness of metal materials was realized, and the complete failure curves of three metal materials were determined.A cruciallinkbetween the structural behavior and material parameters of metal materials was established, which can predict the yield load of actual large-scale metal structures based on the elastic–plastic fracture model.

Experimental and Numerical Investigations of Fracture Behavior for Transversely Isotropic Slate Using Semi-Circular Bend Method

Slate with inherently transverse isotropy is abundant in metamorphic rock, in buildings, and in geotechnical engineering worldwide; the tensile and shear fracture behavior of layered slate is vital to know for engineering applications. In this paper, the Brazilian and semi-circular bend (SCB) tests of layered slate were performed. The fracture characteristics of the slate were investigated by numerical simulations developed by the hybrid finite and cohesive element method (FCEM). Results showed that the measured experimental tensile strength, and mode I fracture toughness of layered slate all showed a typical V-type trend as the bedding angle increased from 0° to 90°, and with divider type. The developed empirical relationship between tensile fracture toughness and tensile strength KIC = 0.094σt + 0.036 fitted experimentally and strongly correlated. The mechanical response and fracture patterns predicted by FCEM agreed well with those of the laboratory experiments. Moreover, the shear fracture behavior and mode II fracture toughness of the layered slate were explored by systematic numerical simulations. Research results provide potential insights for further prediction and improvement of the complex fracture behavior of anisotropic rock masses for rock engineering.

In vitro and practical guide for the analysis of bond strength to ceramics

The purpose of this study was to characterize different test designs applied for analysis of the bond strength between ceramic and resin cement. One lithium disilicate (LD, IPS Emax CAD®, Ivoclar) and one zirconia (YZ, Ceramill, Amann-Girrbach) ceramic were used for specimens’ preparation (n = 10) according to the proposed test: Microtensile, Tensile, Microshear, Shear, Micropush-out, Push-out, and Interfacial Fracture Toughness. A resin cement (Multilink Automix dualcure, Ivoclar) was used to bond the ceramic to the substrate (composite resin, Tetric N-Ceram, Ivoclar) selected according to the test design. During preparation and testing of the specimens, the time required, amount of material and level of difficulty were assessed. Bond strength data were analyzed using one-way ANOVA and Tukey test (P < 0.05). The homogeneity and magnitude of the bond strength values were verified by calculating the Weibull modulus. Failure mode was analyzed by stereomicroscope for all tests. Microshear resulted in the highest (LD: 41.00 ± 14.96 MPa; YZ: 48.95 ± 6.64 MPa) and Interfacial Fracture Toughness in the lowest (LD: 3.54 ± 0.94 MPa; YZ: 4.10 ± 1.65 MPa) bond strength values, regardless of the ceramic. The analysis of the mode of failure showed 100% adhesive failure for Microtensile and Micropush-out samples for both, LD and YZ. Interfacial Fracture Toughness samples of LD also presented 100% adhesive failure. Most of the samples presented mixed failure modes, and only Tensile and Push-out tests presented cohesive failure in the resin cement layer. Overall, the test design affected bond strength results. Weibull modulus indicated that a smaller sample size may be adequate for the analysis of bond strength to ceramic materials. Microtensile test was classified as the most difficult test in terms of preparing the specimen and performing the test.

The Influence of Fiber on the Mechanical Properties of Geopolymer Concrete: A Review

Geopolymer is widely used as a supplement to cementitious composites because of its advantages of low carbon and environmental protection, and geopolymer concrete is also broadly used in practical engineering. In recent years, geopolymer concrete has attracted increasing interest owing to its superior mechanical properties, and a series of research results have been obtained. In this paper, from the preparation of geopolymer concrete, based on the characteristics that geopolymer concrete is brittle and easy to crack, the types and basic properties of fibers to enhance the toughness of concrete are analyzed, the advantages and disadvantages of different fibers used as a material to enhance the toughness of concrete are summarized, and we review the effects of type, shape, volume rate, aspect ratio, and hybrid fiber combinations on the static mechanical properties. The results indicate that fibers have significant potential to enhance the compressive strength, splitting tensile strength, flexural strength, and fracture toughness of geopolymer concrete, and the optimal fiber volume rate seems to be related to the fiber type. Whereas the effect of aspect ratio and hybrid fiber combinations on the properties of geopolymer concrete seems to be obvious. This paper reviews the influence of fiber on the basic mechanical properties of geopolymer concrete, which provides a solid foundation to promote the further development and application of the research on the toughness of fiber-reinforced geopolymer concrete and provides recommendations for future research.

Strength and cracking resistance of concrete containing different percentages and sizes of recycled tire rubber granules

In order to use recycled materials in the composition of greener and cleaner concrete products, it is vital to ensure that the produced concrete have the appropriate level of strength. This paper investigates the effect of gradation (size) and percentage of recycled rubber granules on the physical parameters (slump and density), strength parameters (tensile and compressive strength values), and in particular the crack initiation and propagation characteristics (under the opening, shearing, and tearing modes). The tensile strength, compressive strength, fracture toughness (KIc, KIIc, and KIIIc), and fracture energy values at the initial crack initiation and ultimate failure propagation stages were determined by testing specimens containing 0, 1, 2, 4, and 12% of fine and coarse recycled rubber granules. Rubber particles were graded in two distinct ways so that they could serve as both filler and granules. In contrast to fine rubber granules, the addition of coarse granules had a positive effect on mode-II and mode-III fracture toughness. But, the addition of fine and coarse rubber granules reduces mode I fracture toughness. The analysis of fracture values indicates that coarse rubber granules have a positive effect on the post-peak failure energy absorption (i.e., the crack propagation stage) of all fracture modes. However, this increase in fracture energy for the fine rubber combination was only observed for mode-II fracture stress. The fracture toughness and fracture energy values of the similar concrete were empirically determined based on their corresponding tensile strength. The link between fracture toughness and fracture energy values was also discovered. According to the experimental results, the concrete containing 4% coarse rubber granules provides good mechanical and cracking resistance properties.

An Innovative Method to Analyze the Hydraulic Fracture Reopening Pressure of Hot Dry Rock.

This paper focuses on a new test method and theoretical model for measuring and evaluating the reopening pressure during hot dry rock hydraulic fracturing. Firstly, rock blocks of four lithologies were collected from the hot dry rock strata. Hydraulic fracturing tests at high temperatures in real-time were conducted using drilled cubic specimens and drilled cubic specimens with a pre-crack. Breakdown pressure, reopening pressure, and fracture toughness were measured, respectively. In addition, Brazilian splitting tests at high temperatures in real-time were performed using Brazilian disc specimens to measure tensile strength. Secondly, an empirical equation for evaluating the reopening pressure during hot dry rock secondary fracturing was developed based on fracture mechanics and hydraulic fracturing theory. Third, the values calculated by the new equation, considering breakdown pressure, fracture toughness, and tensile strength, were compared to the values determined by the classical equation and to measurement results. It was found that the new equation predicted closer reopening pressure to the measurement results, regardless of the lithology of the hot dry rock. Moreover, with increasing temperature in the specimens, the error between the value calculated by the new equation and the measurement value remained low. In contrast, the difference between the classical equation predictions and the measurement results was widened. In addition, the reopening pressure was positively correlated with tensile strength and fracture toughness. Variations in lithology and temperature affected tensile strength and fracture toughness, which then changed the hot dry rock reopening pressure.

High-performance thermoplastic polyaryletherketone/carbon fiber composites: Comparison of plasma, carbon nanotubes/graphene nano-anchoring, surface oxidation techniques for enhanced interface adhesion and properties

This investigation highlights the development and performance comparison of carbon fiber (CF)-polyaryletherketone (PAEK) thermoplastic laminates prepared using CF, surface-modified with plasma, multi-walled carbon nanotubes (MWCNT), graphene, acid, and rare earth elements. Analysis of surface-treated CF by Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) revealed development of functional groups and changes in elemental composition on the CF surface. Significant lowering of contact angle and improvement in wettability was achieved by plasma and MWCNT/graphene treatment on CF. Field emission scanning electron microscopy (FESEM) analysis gave insight into the surface irregularities caused by various surface treatment techniques. The CF-PAEK composites were prepared by film stacking and compression moulding. The plasma-treated PAEK-CF composites exhibited the highest tensile properties, flexural strength, fracture toughness, and interlaminar shear strength. The superior properties are credited to better interfacial adhesion between CF and PAEK arising from mechanical interlocking over the roughened surface and covalent bonds by functional groups grafted on CF surface by plasma. MWCNT and graphene-anchored CF-PAEK composites also showed significant improvement in mechanical properties attributed to the bridging effect and interfacial interaction arising from increased surface area and functional groups. The surface-treated CF-PAEK composites exhibited a positive shift in the glass transition temperature, which is evidence of better adhesion. In dynamic loading conditions, MWCNT anchored CF-PAEK composite presented the highest storage modulus and energy absorption characteristics. Examination of the tensile fracture surface and fiber pull-out gave insight into the failure pattern and interfacial adhesion mechanism for the individual surface modification techniques.

Effects of microstructure characteristics on the tensile properties and fracture toughness of TA15 alloy fabricated by hot isostatic pressing

Effects of microstructure characteristics on the tensile properties and fracture toughness of TA15 alloy fabricated by hot isostatic pressing

Doubled strength and ductility via maraging effect and dynamic precipitate transformation in ultrastrong medium-entropy alloy

Demands for ultrahigh strength in structural materials have been steadily increasing in response to environmental issues. Maraging alloys offer a high tensile strength and fracture toughness through a reduction of lattice defects and formation of intermetallic precipitates. The semi-coherent precipitates are crucial for exhibiting ultrahigh strength; however, they still result in limited work hardening and uniform ductility. Here, we demonstrate a strategy involving deformable semi-coherent precipitates and their dynamic phase transformation based on a narrow stability gap between two kinds of ordered phases. In a model medium-entropy alloy, the matrix precipitate acts as a dislocation barrier and also dislocation glide media; the grain-boundary precipitate further contributes to a significant work-hardening via dynamic precipitate transformation into the type of matrix precipitate. This combination results in a twofold enhancement of strength and uniform ductility, thus suggesting a promising alloy design concept for enhanced mechanical properties in developing various ultrastrong metallic materials.