Intrinsic Tensile Strength Research Articles

The Influence and Mechanism of Polyvinyl Alcohol Fiber on the Mechanical Properties and Durability of High-Performance Shotcrete

This study investigates the impact of polyvinyl alcohol (PVA) fibers on the mechanical properties and durability of high-performance shotcrete (HPS). Results demonstrate that PVA fibers have a dual impact on the performance of HPS. Positively, PVA fibers enhance the tensile strength and toughness of shotcrete due to their intrinsic high tensile strength and fiber-bridging effect, which significantly improves the material’s splitting tensile strength, deformation resistance, and toughness, and the splitting tensile strength and peak strain have been found to be increased by up to 30.77% and 31.51%, respectively. On the other hand, the random distribution and potential agglomeration of PVA fibers within the HPS matrix can lead to increased air-void formations. This phenomenon raises the volume content of large bubbles and increases the average bubble area and diameter, thereby elevating the pore volume fraction within the 500–1200 μm and >1200 μm ranges. Therefore, these microstructural changes reduce the compactness of the HPS matrix, resulting in a decrease in compressive strength and elastic modulus. The compressive strength exhibited a reduction ranging from 10.44% to 15.11%, while the elastic modulus showed a decrease of between 8.09% and 12.67%. Overall, the PVA-HPS mixtures with different mix proportions demonstrated excellent frost resistance, chloride ion penetration resistance, and carbonation resistance. The electrical charge passed ranged from 133 to 370 C, and the carbonation depth varied between 2.04 and 6.12 mm. Although the incorporation of PVA fibers reduced the permeability and carbonation resistance of shotcrete, it significantly mitigated the loss of tensile strength during freeze–thaw cycles. The findings offer insights into optimizing the use of PVA fibers in HPS applications, balancing enhancements in tensile properties with potential impacts on compressive performance.

Determining the Cohesive Length of Rock Materials by Roughness Analysis

In this research, the cohesive length of various rock types is measured using quantitative fractography alongside a recently developed multifractal analysis. This length is then utilized to gauge material cohesive stress through the theory of critical distances. Furthermore, the fracture process zone length of different rings sourced from identical rocks is assessed as a function of ring dimensions and experimental measurements of fracture toughness, in accordance with the energy criterion of the finite fracture mechanics theory. Subsequently, employing the stress criterion within coupled finite fracture mechanics, the failure stress corresponding to the fracture process zone is determined for various rings. Ultimately, through interpolation, the critical stress corresponding to the cohesive length, quantified via quantitative fractography, is approximated. Remarkably, the cohesive stress values derived from both methodologies exhibit perfect alignment, indicating the successful determination of cohesive length for the analyzed rock materials. The study also delves into the significant implications of these findings, including the quantification of intrinsic tensile strength in quasi-brittle materials and the understanding of tensile strength variations under diverse stress concentrations and loading conditions.

An alternative method to evaluate the micromechanics tensile strength properties of natural fiber strand reinforced polyolefin composites. The case of hemp strand-reinforced polypropylene

Micromechanics models allow the prediction of a composite material's properties by adding their phases' contributions to such properties. The models can be used to obtain the intrinsic properties of the reinforcements because are difficult to obtain experimentally. This paper explores a simplified model to obtain the intrinsic strength of natural fibers. This model allows obtaining the value directly from the experimental strength of a composite and the matrix. Other models like the Kelly and Tyson equation have three unknowns, needing the use of mathematical methods to obtain a solution, and the obtained solution sometimes deviates from the expected values for natural fiber-reinforced composites. The proposed equation has been able to evaluate the intrinsic strength of hemp fibers as polypropylene composites at a mean value of 600 MPa. This value agrees with the literature. The proposed method simplifies the obtention of the intrinsic tensile strengths of natural fiber reinforcements and does not need morphologic properties of such reinforcements to obtain a solution, decreasing the costs in time and equipment in comparison to usual models like Kelly and Tyson's. furthermore, the obtained results are like those obtained with other micromechanics approaches and reveal the same information about the intrinsic tensile strength of the reinforcements and the strength of the interface.

Quantifying the cohesive strength of rock materials by roughness analysis using a domain based multifractal framework

Cohesive strength or intrinsic tensile strength as well as cohesive length are two important unknowns for cohesive modelling of fracture and failure analysis of quasi-brittle materials including rocks. There is no direct method for measuring these parameters and their quantification is always challenging and controversial. In this study, a novel multifractal framework is employed to quantify the cohesive length of four different rock types including sandstone, marble, fine-grained granite and coarse-grained granite by analysing the roughness of their fracture surfaces in a wide range of length scales. On the one hand, microstructural heterogeneities of rock material at small enough length scales will cause multifractality of the roughness of its fractured surface. On the other hand, this intrinsic heterogeneity together with extrinsic features including loading and environmental conditions as well as geometrical features including shape and size of a quasi-brittle specimen or structure are forming a fracture process zone (FPZ) in front of any stress concentrators before crack propagation. Therefore, it is proposed that locating the transition from multifractality to mono-fractality of a rough rock fractured surface using the employed statistical mechanics method leads to quantifying the effective length of FPZ of a sharp crack or the cohesive length. This length is quantified for the studied rocks ranging from 0.4 to 1.1 mm. Moreover, by employing the theory of critical distances, the cohesive strength σc of these rocks is also quantified ranging from 15 to 36 MPa.

Evaluation of the Interface Strength in the Abaca-Fiber-Reinforced Bio-Polyethylene Composites.

Bio-based polymers, with any of their constituents based on nonrenewable sources, can answer the demands of society and regulations regarding minimizing the environmental impact. The more similar such biocomposites are to oil-based composites, the easier the transition, especially for companies that do not like the uncertainty. A BioPE matrix, with a structure similar to that of a high-density polyethylene (HDPE), was used to obtain abaca-fiber-reinforced composites. The tensile properties of these composites are displayed and compared with commercial glass-fiber-reinforced HDPE. Since the strength of the interface between the reinforcements and the matrix is responsible for the exploitation of the strengthening abilities of the reinforcements, several micromechanical models were used to obtain an estimation of the strength of the interface and the intrinsic tensile strength of the reinforcements. Biocomposites require the use of a coupling agent to strengthen their interface, and once an 8 wt.% of such coupling agent was added to the composites, these materials returned tensile properties in line with commercial glass-fiber-reinforced HDPE composites.

Thickness effect on flexural strengths of laminar carbon fibre composites

The thickness dependence of flexural strengths of laminar carbon fibre reinforced polymer (CFRP) plates was investigated experimentally and analytically in this study. This thickness effect was linked to the ply thickness by a simple composite formula. Four different sample thicknesses of 1.48, 2.08, 2.48 and 2.96 mm were tested, and thickness dependent flexural strengths were evaluated by the Strength of Materials (SOM) formula and modelled by the simple composite formula containing the ply thickness. The ASTM standard D7264, recommending 4 mm thick CFRP samples, was justified analytically, i.e., the thickness effect is negligible if W≥ 4 mm for CFRP plates with typical ply thickness around 120–130μm. The intrinsic tensile strength, independent of the sample thickness, was deduced from all flexural tests by the composite formula, which compared well with the tensile strength from unidirectional tensile tests and the tensile strength estimated from notched flexural samples. The specimen and notch/crack size requirements in ASTM standards for the fracture toughness were also explained analytically.

Effect of post weld artificial aging and water cooling on microstructure and mechanical properties of friction stir welded 2198-T8 Al-Li joints

Friction stir welding (FSW) under both air cooling and water cooling conditions with welding parameters of 800–1200 rpm rotation rates and 50–200 mm/min welding speeds was carried out on 2198-T8 Al-Li alloys, and post weld artificial aging was performed on the air cooled joints. No welding defects other than lazy S were observed in the nugget zone (NZ) for all joints. Under air cooling condition, the lowest hardness zone (LHZ) occurred in the heat affected zone (HAZ). FSW resulted in gradual dissolution of original T1, θ′ and δ′/β′ from the base material (BM) to the thermo-mechanically affected zone (TMAZ), and complete dissolution of all precipitates in the NZ with δ′/β′ and Guinier-Preston zones precipitating during cooling. The air cooled joints exhibited no noticeable changes in intrinsic tensile strength with a joint strength reaching 81.3% of the BM, but varied elongation with welding parameters, which was closely related to failure in the NZ and fracture along lazy S. Post weld artificial aging led to the largest hardness recovery in the TMAZ but smaller hardness recovery in the initial LHZ and the NZ. Different aging kinetics across the joint was determined by volume fraction of both original precipitate dissolution during welding and coarse particles formed during aging, and by dislocation density inherited from welding. Post weld artificial aging greatly enhanced the joint strength with the ultimate tensile strength reaching 87.3% of the BM. As compared to air cooling condition, water cooling hardly affected the NZ hardness and did not improve the joint strength, and the reason was discussed in light of precipitates, hardness changes and fracture behavior.

A new methodology inspired from the Theory of Critical Distances for determination of inherent tensile strength and fracture toughness of rock materials

Measuring the intrinsic fracture properties of quasi-brittle materials like rocks is of great importance and at the same time a major issue for engineers. In this study, we explore the ability of the Theory of Critical Distances (TCD) to determine accurately both the tensile strength and fracture toughness. To this end, we conduct ring tests and semi-circular bend tests on four rock types including a red sandstone, a white coarse-grained marble, a fine-grained granite and a coarse-grained granite. This selection covers sedimentary, metamorphic and igneous rock types with different grain sizes. The experimental data are analysed using a new methodology developed from the so-called Point Method (PM), a particular form of the TCD, from which we infer the intrinsic tensile strength and the fracture toughness of the studied rock materials. Our results are compared with those obtained from ISRM suggested methodology that is modified to take into account the finite notch root radius used in our experiments. The comparison is successful, supporting that the newly developed methodology is suitable to determine the intrinsic tensile strength and fracture toughness of rock materials.

Micromechanics of Tensile Strength of Thermo-mechanical Pulp Reinforced Poly(lactic) Acid Biodegradable Composites

ABSTRACT Development of materials that are biobased and environmental sound is one of the goals within the current bioeconomy. This goal comes from an increasing conscientious society that pushes manufacturers and regulators toward a sustainable development. However, to be a feasible alternative, biobased materials should also match or outperform the mechanical performance of fossil-based materials. In this study, wood pulp fiber-reinforced polylactic acid (PLA) biocomposites were prepared, tested, and compared with glass fiber reinforced polypropylene. Pre-extrusion with a kinetic mixer and subsequent injection processing ensured correct dispersion of the reinforcement. The biocomposites showed mechanical properties in line with commercial materials, comparable to composites reinforced with 20% w/w of glass fiber. Micromechanics of PLA-based biocomposites showed the existence of strong interphase between the matrix and the pulp fibers. The interfacial shear strength was around 29 MPa and with a intrinsic tensile strength of the fibers 729 MPa. These materials offer a reliable alternative to oil-based matrices reinforced with mineral fibers.

Fracture and strength of single-atom-thick hexagonal materials

The fracture toughness has been theoretically and experimentally estimated for few of various two-dimensional (2D) materials. Relationship between the fracture toughness and other mechanical properties of 2D hexagonal materials is extremely needed for a quick estimation in engineering applications, as well as for a better comprehension of their fracture properties. The present work investigates through molecular dynamics simulations at room temperature the mode-I fracture of 25 single-atom-thick hexagonal materials with the buckling height-bond length ratio in the range from 0 through ∼0.6, including 6 planar sheets (graphene, boronitrene, SiC, GeC, AlN and InN) and 19 buckled sheets (silicene, InP, SiGe, SnSi, SnGe, SnO, GeO, SiO, blue-P, arsenene, GeS, SnS, antimonene, bismuthene, SiTe, SnSe, SiSe, GeTe and SnTe). Fracture mechanism is considered. With this large data set and based on dimensional analysis, an empirical formula is established to estimate the mode-I fracture toughness from the elastic modulus, intrinsic tensile strength, bond length and buckling height of a single-atom-thick hexagonal material with reasonable accuracy. This simple formula provides quick estimation of the fracture toughness and is useful for engineering applications.

Dynamic flexural failure of rocks under hydrostatic pressure: Laboratory test and theoretical modeling

Dynamic flexural failure of rocks is commonly encountered near deep underground openings due to the bending load derived from earthquakes or production blasts in rock engineering. Meanwhile, deep underground rock is also generally subjected to static hydrostatic pressure. Thus, it is desirable to investigate dynamic flexural failure characteristics and mechanisms of rocks under static hydrostatic pressure. In this study, the dynamic flexural tensile strengths of Laurentian granite (LG) under five hydrostatic pressures (i.e., 0 MPa, 5 MPa, 10 MPa, 15 MPa and 20 MPa) were measured by using a triaxial split Hopkinson pressure bar (SHPB) system in combination with the semi-circular bend (SCB) method. The experimental results indicate that dynamic flexural tensile strength under a certain hydrostatic pressure rises with the loading rate, and dynamic flexural tensile strength at a certain loading rate increases with the hydrostatic pressure due to the closure of microcracks in rocks. In addition, an empirical formula is proposed to successfully predict both the rate dependence and the hydrostatic pressure effect on the dynamic flexural tensile strength. Moreover, the parameter in terms of the hydrostatic pressure is first introduced in the two-parameter tensile strength model, which is improved in this study to quantitatively rationalize the discrepancy between the dynamic intrinsic tensile strength and the dynamic flexural tensile strength for rocks under hydrostatic pressures, and to further establish the relationship between the hydrostatic pressure and the characteristic material length δ for rocks. Meanwhile, the characteristic material length is optimized by the particle swarm optimization method and the value of δ decreases with the hydrostatic pressure due to the closure of microcracks and pores. Besides, an equation is established to describe the relation between the hydrostatic pressure and the characteristic material length. In addition, the morphology of the main fracture in the tested SCB specimens is digitized and the fractal dimension is applied to quantitively describe the fracture surfaces roughness. There is no clear trend to indicate the influence of hydrostatic pressure on the surface roughness, whereas there is generally a decrease of fractal dimension with increasing loading rate for LG under each hydrostatic pressure.

A new approach for understanding the mechanism of wellbore strengthening theory

To achieve fast, safe and economical drilling operations through formations containing natural fractures or vugs, all fractures and vugs must be perfectly sealed using proper fractures seal materials in a process called wellbore strengthening. Wellbore strengthening may elevate the overall tensile strength of the treated formation to a value approaches the intrinsic tensile strength of unfractured rock.Numerous studies have been conducted for testing fractures seal materials suitability for wellbore strengthening application in various formations. However, there is no solid and simple theoretical explanation on how wellbore strengthening process can recover the tensile strength and hence the fracturing pressure of the treated naturally fractured formation.In this study, test specimens cored from an artificial (building) sandstone bricks were used to measure the unconfined compressive strength (UCS), Brazilian indirect tensile strength (BTS), and establishing the multi-stage triaxial compression (MS-TCS) test. Core samples from the same sandstone were previously used for wellbore strengthening studies using crushed dates palm seeds. Crushed dates palm seeds succeeded to completely stop mud losses at a pressure equal to 6.9 MPa.Data obtained from unconfined compression, multi-stage triaxial compression, Brazilian indirect tensile, and wellbore strengthening tests were collectively used to develop a clear theoretical understanding of the mechanism of tensile strength recovery caused by wellbore strengthening effect.It was found that wellbore strengthening effect in sealing natural fractures and vugs is similar to the confining pressure effect in closing the induced shear fractures in the test specimen during a multi-stage triaxial compression test.For the tested sandstone, using crushed dates palm seeds as a fracture seal material has improved wellbore strengthening from zero tensile strength (complete loss of mud circulation) to 2.56 MPa tensile strength at which there was a complete fracture seal (zero mud loss). The recovered tensile strength (2.56 MPa) is approaching the intrinsic tensile strength (BTS = 2.7 MPa) of the same sandstone in its initial unfractured state.

Topography of the Interfacial Shear Strength and the Mean Intrinsic Tensile Strength of Hemp Fibers as a Reinforcement of Polypropylene.

The strength of the interphase between the reinforcements and the matrix has a major role in the mechanical properties of natural fiber reinforced polyolefin composites. The creation of strong interphases is hindered by the hydrophobic and hydrophilic natures of the matrix and the reinforcements, respectively. Adding coupling agents has been a common strategy to solve this problem. Nonetheless, a correct dosage of such coupling agents is important to, on the one hand guarantee strong interphases and high tensile strengths, and on the other hand ensure a full exploitation of the strengthening capabilities of the reinforcements. The paper proposes using topographic profile techniques to represent the effect of reinforcement and coupling agent contents of the strength of the interphase and the exploitation of the reinforcements. This representation allowed identifying the areas that are more or less sensitive to coupling agent content. The research also helped by finding that an excess of coupling agent had less impact than a lack of this component.

Determination of Mean Intrinsic Flexural Strength and Coupling Factor of Natural Fiber Reinforcement in Polylactic Acid Biocomposites.

This paper is focused on the flexural properties of bleached kraft softwood fibers, bio-based, biodegradable, and a globally available reinforcement commonly used in papermaking, of reinforced polylactic acid (PLA) composites. The matrix, polylactic acid, is also a bio-based and biodegradable polymer. Flexural properties of composites incorporating percentages of reinforcement ranging from 15 to 30 wt % were measured and discussed. Another objective was to evaluate the strength of the interface between the matrix and the reinforcements, using the rule of mixtures to determine the coupling factor. Nonetheless, this rule of mixtures presents two unknowns, the coupling factor and the intrinsic flexural strength of the reinforcement. Hence, applying a ratio between the tensile and flexural intrinsic strengths and a defined fiber tensile and flexural strength factors, derived from the rule of mixtures is proposed. The literature lacks a precise evaluation of the intrinsic tensile strength of the reinforcements. In order to obtain such intrinsic tensile strength, we used the Kelly and Tyson modified equation as well as the solution provided by Bowyer and Bader. Finally, we were able to characterize the intrinsic flexural strengths of the fibers when used as reinforcement of polylactic acid.

Study on the Tensile Strength and Micromechanical Analysis of Alfa Fibers Reinforced High Density Polyethylene Composites

In the recent years, there has been a growing interest in eco-friendly composites, based on natural fiber reinforcements. In this work, Alfa fibers were obtained from Alfa grass (Stipa tenacissima) through an intensive cooking with sodium hydroxide and anthraquinone. These fibers, with contents ranging from 20 to 50 wt%, were used to reinforce a high density polyethylene. The composites were prepared in a kinetic mixer and later on tensile tested under standard conditions. The results showed that the tensile strength of the composites increased with the amount of fiber in the composite. Aditionally, a morphology and chemical composition analysis of the fibers surface were carried on, in order to further discuss the micromechanics analysis results. The average orientation factor and intrinsic tensile strength of the fibers were also determined during the micromechanics analysis.

Interface and micromechanical characterization of tensile strength of bio-based composites from polypropylene and henequen strands

The contribution of a reinforcement to the tensile strength of a composite can be evaluated by different micromechanics models. Nonetheless, one of the main difficulties is the evaluation of the intrinsic properties of the reinforcements. The literature shows experimental and model-based methodologies to estimate such intrinsic properties, and few are based on single fiber tensile test in combination with a Weibull analysis. This paper proposes using henequen strand to prepare polypropylene-based composites. Henequen fibers show a high cellulose content that allows obtaining strong interfaces when a coupling agent is added to the composite formulation. The novelty of this work is based on a simplified methodology to evaluate the intrinsic tensile strength of the reinforcements and its contribution to the tensile strength of the composite. A percentage of coupling agent that returns the highest tensile strength is identified with a strong interface. Then, typical values for a coupling factor and interfacial shear strength are used with a modified rule of mixtures and a modified Kelly and Tyson models to obtain the orientation factors. The prediction of composite behavior from fiber properties is necessary to anticipate the correlation between experimental and the back-calculated parameters.

Tensile strength of graphene nanoribbons: An experimental approach

Atomically perfect graphene is one of the strongest materials ever measured. However, there remains a basic challenge to experimentally determine the inherent tensile strength of graphene nanoribbons, providing a benchmark for design of a variety of graphene-based structures. Using the atomic-scale characterization and in situ high-field tests in a field-ion microscope, it is shown that the inherent strength is 99.34 GPa and represents the intrinsic tensile strength of statistically defect-free graphene nanoribbons. The experimental results were compared with the theoretical data and found to be reliable. Mechanical failure of graphene ribbonss under tensile stress near the ideal strength determines a fundamental upper limit of internal stress in electromechanical graphene devices.

Topology and polarity of dislocation cores dictate the mechanical strength of monolayer MoS2

In contrast to homoelemental graphene showing common dislocation dipole with pentagon-heptagon (5|7) core, heteroelemental MoS2 is observed to contain diverse dislocation cores that tune the chemical and physical properties. Yet, how the inevitable dislocation cores in MoS2 affect the mechanical behaviours remains virtually unexplored. Herein, we report direct atomistic simulations of mechanical characteristics of isolated dislocation-embedded MoS2 monolayers under tensile load. All isolated dislocation cores in MoS2 monolayer rise polar stress-concentration, while those with larger Burgers vector are less energetically-favorable configurations but show local wrinkling behaviour. It is revealed that the intrinsic tensile strength of MoS2 is dictated by topology and polarity of dislocation cores. There is a strong inverse correlation between the maximum residual stresses induced by the dislocation cores and the strength of MoS2 monolayers. Mechanical failure initiates from the bond at dislocation polygon on which side there is a missing atomic chain. Armchair-oriented 4|8 dislocation exhibits sole brittle failure, however, dual brittle/ductile fractures occur in zigzag-oriented dislocations; Mo-S-Mo angle-oriented crack is brittle, while the S-Mo-S angle-oriented crack becomes ductile. Our findings shed sights on mechanical design of heteroelemental 2D materials via dislocation engineering for practical application.

Non-local failure theory and two-parameter tensile strength model for semi-circular bending tests of granitic rocks

Several indirect tension tests have been proposed and widely used to quantify rock tensile strength due to difficulties associated with experimentation in direct tension tests. However, discrepancies exist between rock tensile strengths obtained from the direct tension tests and those from indirect tension tests. The non-local theory has been used to reconcile such differences. However, the physical meaning of the characteristic material length δ in the non-local theory is still unclear. We investigate the physical meaning of the characteristic length δ by testing three typical granitic rocks featuring different grain sizes. An indirect tension method - semi-circular bending (SCB) test is applied to obtain the flexural tensile strength for these rocks. The specimens are perfectly split into two quadrants by a main crack through the center of the specimen. The flexural tensile strengths of three rocks generally decrease with the span distance. From flexural tensile strengths under different testing conditions, the optimized values of the characteristic material length δ and the rock intrinsic tensile strength are determined for these three rocks. It is found the tensile strengths of three rocks predicted by the non-local theory match well with those obtained from independent experiments using the Brazilian disc method. The non-local failure theory is used to explain the observation that the flexural tensile strength of three rocks decreases with the span distance. Moreover, the material characteristic length δ correlates well with the rock grain size, and the physical meaning of material characteristic length can be considered as the scaled average grain size of rocks. Based on the successful application of the non-local theory, a two-parameter tensile strength model for rocks is proposed.

Mechanical and electronic properties of lateral graphene and hexagonal boron nitride heterostructures

Owing to the promising applications in two dimensional devices, lateral graphene/h-BN (G/BN) heterostructures have attracted significant attentions. Here, we investigate the mechanical and electronic properties of lateral G/BN with different types of interfaces using density functional theory. For G/BN heterostructure with armchair (AC) or zigzag (ZZ) type interfaces, the intrinsic tensile strength is close to that of pristine graphene. For the heterostructure with the misorientated interface, the intrinsic tensile strength highly relies on the misorientation angle. G/BN heterostructure with Clar’s interface is found to be mechanically more stable than those with non-Clar’s interface. Moreover, with the uniaxial tensile strain increasing from 0% to 10%, the band gap of the G/BN with ZZ interfaces are nearly unchanged while the G/BN with AC interfaces shows a non-monotonic variation of band gaps. These theoretical results provide a useful picture for the interface effects on the mechanical and electronic behaviors of lateral G/BN heterostructures.