Resistance-curve Behavior Research Articles

Fracture and Toughening of Mycelium-based Biocomposites

This study presents a combined experimental and analytical study of the fracture behavior and toughening mechanisms of bioprocessed mycelium-based biocomposites. The composites comprise hemicellulose hemp ducts (as nutritional and reinforcing components) intertwined with increasing weight percentages of laterite particles. Single-edge notched fracture experiments and in-situ observations of crack growth were used to explore the effects of varying proportions of laterite on the composite resistance-curve behavior. The toughening mechanisms, fracture modes, and crack-microstructure interactions were also elucidated. Since crack-bridging and crack-deflection were observed to be the dominant toughening mechanisms, they were modeled using fracture mechanics approaches. Crack-bridging was shown to dominate the toughening at lower weight fractions of laterite (0–20 wt%). However, as the laterite content increases (20–40 wt%), a combination of crack-bridging and crack-deflection was observed. Finally, at higher laterite weight fractions (>40 wt%), crack-tip shielding occurred primarily via crack deflection. The fracture mechanics predictions of resistance-curve behavior are shown to be consistent with the experimental measurements. The results suggest that mycelium-based and mycelium-laterite composites can be engineered with tunable fracture toughness. The implications of the results are also discussed for the development of sustainable building materials.

Influence of Loading Rate on the Cohesive Traction for Soft, Rubber-Like Adhesive Layers Loaded in Modes I and III

To date, the fracture behaviour of soft, polyurethane-based adhesive joints has rarely been investigated. This work contributes to the experimental investigation of such joints in modes I and III by performing double cantilever beam (mode I) and out-of-plane loaded double cantilever beam (mode III) tests at various loading rates. The tests were evaluated using a J-integral method, which is well established for testing stiff adhesive layers and is conventionally used to determine the cohesive traction at the crack tip. Additionally, fibre-optics measurements were conducted to provide crack extension, process zone length, and cohesive traction from the measured backface strain of the adherends. It was found that the energy release rate seems to be largely independent of the loading mode. However, differences were observed regarding process zone length and resistance curve behaviour. Furthermore, the backface strain measurement allows the determination of the cohesive traction along with the complete adhesive layer as well as separation and separation rate, yielding rate-dependent cohesive laws. A comparison indicated that the cohesive traction obtained from the J-integral method does not match the measured benchmark from the backface strain measurements because the underlying theoretical assumptions of the J-integral method are likely violated for soft, rubber-like adhesive joints.

A step toward bio-inspired dental composites

This feasibility study aimed to develop a new composite material of aligned glass flakes in a polymer resin matrix inspired by the biological composite nacre. The experimental composite was processed by an adapted method of pressing a glass flake and resin monomer system. By pressing and allowing the excess monomer to flow out, the long axis of the flakes was aligned. The resultant anisotropic composite with silanized and non-silanized glass flakes were subjected to fracture toughness tests. We observed increasing fracture toughness with increasing crack extension (Δa) known as resistance curve (R-curve) behavior. Silanized specimens had higher stress intensity KR-Δa over non-silanized specimens, whereas non-silanized specimens had a much lower Young’s modulus, and higher nonlinear plastic-elastic JR-Δa R-curve. In comparison with conventional composites, flake-reinforced composites can sustain continued crack growth for more significant extensions. The primary toughening mechanism seen in flake-reinforced composites was crack deviation and crack branching. We produced an anisotropic model of glass flake-reinforced composite showing elevated toughening potential and a prominent R-curve effect. The feasibility of flake reinforcement of dental composites has been shown using a relatively efficient method. The use of a biomimetic, nacre-inspired reinforcement concept might guide further research toward improvement of dental restorative materials.

Toughening by interpenetrating lattices

•Separate lattices can be interwoven to significantly increase their toughness •The sub-lattice properties can be tuned via topology and optimized for toughness •The toughening effect can be explained using a crack-bridging model As structural lattice metamaterials become more accessible through 3D printing, there is a need to better understand their fracture behavior, which sets practical limits for engineered structures. Lattices face a problem of decreasing toughness as their density and cell size decrease. Recently discovered interpenetrating lattices, made by weaving two or more physically separate lattices through the same volume, offer a potential path to significantly improve fracture toughness by increasing the fracture process zone size and inducing unique contact and friction toughening mechanisms. Interpenetrating lattices possess a steeply rising resistance-curve behavior, with the final toughness associated with catastrophic fracture an order of magnitude greater than the initiation toughness needed to begin advancing a crack. Remarkably, the interpenetrating lattice’s toughness in certain topologies can be five times greater than its corresponding, fully dense solid base material, and the toughening effect can be tailored by controlling the mechanical mismatch of the constituent sub-lattices. As structural lattice metamaterials become more accessible through 3D printing, there is a need to better understand their fracture behavior, which sets practical limits for engineered structures. Lattices face a problem of decreasing toughness as their density and cell size decrease. Recently discovered interpenetrating lattices, made by weaving two or more physically separate lattices through the same volume, offer a potential path to significantly improve fracture toughness by increasing the fracture process zone size and inducing unique contact and friction toughening mechanisms. Interpenetrating lattices possess a steeply rising resistance-curve behavior, with the final toughness associated with catastrophic fracture an order of magnitude greater than the initiation toughness needed to begin advancing a crack. Remarkably, the interpenetrating lattice’s toughness in certain topologies can be five times greater than its corresponding, fully dense solid base material, and the toughening effect can be tailored by controlling the mechanical mismatch of the constituent sub-lattices.

Bioinspired PcBN/hBN fibrous monolithic ceramic: High-temperature crack resistance responses and self-lubricating performances

The high strength and toughness of natural materials are mainly determined by a combination of mechanisms operating at different length scales, which can be used as a strategy to reduce the intrinsic brittleness of ceramics. Inspired by the architectures of bamboo, the polycrystalline cubic boron nitride/hexagonal boron nitride (PcBN/hBN) fibrous monolithic ceramics with a long fiber arrangement structure was constructed with PcBN fiber cells and hBN cell boundaries, and its crack resistance responses and tribological performances were investigated. The composite ceramic failed in a non-brittle manner with the rising resistance curve (R-curve) behavior, which was attributed to multiscale crack effects in the hierarchical architecture. The maximum crack growth toughness was extremely high (approximately 21 MPa·m1/2), corresponding to a 270% increase over the crack initiation toughness. Excellent fracture resistance could be retained even above 1000 °C. Moreover, the composite ceramic exhibited low and stable friction coefficients (approximately 0.33) when paired with a Si3N4 pin at high temperature (1000 °C), owing to the lubrication function of hBN cell boundaries with weak van der Waals forces and a small amount of liquid B2O3 produced. As a result, a synergistic improvement of mechanical and tribological properties at high temperature (1000 °C) was realized by combining bionic structure and tribological design. It provides important theoretical and technical support for expanding the application of self-lubricating composite ceramics in harsh environments.

The rising crack resistance curve behavior and mechanism of La2O3 doped zirconia toughened alumina composites prepared via vat photopolymerization based 3D printing

In this study, we combined liquid precursor infiltration of high introduction amounts of bi-additives (20 wt%) and vat photopolymerization-based 3D printing to fabricate zirconia toughened alumina, and the infiltration systems consist of the four following systems Zr4+/La3+, Zr4+/Ce4+, Zr4+/Gd3+, and Zr4+/Er3+. The sample immersed with Zr4+/La3+ shows intense peaks of m-ZrO2 phase compared to the other samples while a new phase of rod-like LaAl11O18 grain occurs in the Zr4+/La3+ immersed sample, the existence of which could be confirmed by XRD and EDS. The fracture toughness of the Zr4+/Ce4+, Zr4+/Gd3+, and Zr4+/Er3+samples remained basically unchanged versus the crack size, with the fracture toughness between 4 MPa m1/2 and 5 MPa m1/2; while the measured fracture toughness values for the Zr4+/La3+ system show a rising crack resistance curve behavior, with the toughness enhanced from 6.37 MPa m1/2 to 8 MPa m1/2. The obvious enhancement of the toughness could be attributed to thermal expansion misfit and rod-like LaAl11O18 in the Zr4+/La3+ system. The toughening mechanism and the effect of residual stresses on the fracture mode are discussed on the basis of theoretical calculation and analysis. The rising crack resistance curve behavior could provide a new toughening strategy in ceramic 3D printing.

Impact of test environment on the fracture resistance of cortical bone

Water is a crucial component of bone, affecting the interplay of collagen and minerals and contributing to bone's high strength and ductility. Dehydration has been shown to significantly effect osseous mechanical properties; however, studies comparing the effects of various dehydrating environments on fracture toughness of bone are scarce. Accordingly, the crack resistance curve (R-curve) behavior of human and sheep cortical bone was characterized in a bio-bath, in ambient pressure air, and in scanning electron microscopes (SEMs) under three different environmental conditions (water vapor pressure, air pressure, and high-vacuum). The aim of this work was to better understand the impact of test environment on both intrinsic and extrinsic toughening and hence crack initiation toughness, K0 and crack growth resistance, dK/dΔa. Results show significantly lower K0 values for samples that were tested inside SEMs combined with pronounced extrinsic toughening through microcracking and crack path deflections out of the mode I plane. Importantly, all three SEM test environments gave similar results, and thus it does not matter which type of SEM is used. Ex situ testing of hydrated samples revealed similar K0 for both environments but elevated crack growth resistance for testing in ambient air relative to the bio-bath. Our data reveals the experimental difficulties to directly observe microscale crack propagation in cortical bone that resembles the in vivo situation. Ex situ testing immersed in Hanks' Balanced Salt Solution (HBSS) with subsequent crack path analysis, while tedious, is thought to presents the most realistic picture of the in vivo structure-fracture property relations in biological tissue.

The beneficial effect of rolling on the fracture toughness and R-curve behavior of pure tungsten

Selected microstructural states of pure tungsten sheets with thicknesses of 1, 0.5, 0.2 and 0.1 mm were tested at room-temperature (RT) and 200 °C regarding their fracture properties with focus on the occurrence of an R-curve (crack growth resistance curve) behavior. Through thermomechanical rolling and the resulting increase of deformation a pronounced refinement of microstructure and strengthening of the rotated cube texture was induced with decreasing sheet thickness. The fracture experiments exhibited large variations of the fracture behavior depending on the testing temperature, material thickness and microstructure: While at RT, the 1 mm material did not exhibit R-curve behavior, it was found at 200 °C irrespective of the sample thickness. With decreasing sample thickness and grain-size the RT fracture-behavior changed and showed the onset of an extended R-curve behavior for the 0.5 mm followed by an increasing extent of this characteristic for the thinner material states. The change of the fracture behavior at RT is associated with a transition of the failure behavior from transcrystalline fracture for the thicker samples to a mixture of transcrystalline with delamination fracture to uniform delamination failure for the thinnest material. These observations suggest a continuous shift of the ductile to brittle transition temperature from 200 °C to RT and even below RT with decreasing sample thickness. The fracture toughness in terms of the maximum stress intensity, Kmax, increased at RT from approximately 20 MPam1/2 for the 1 mm samples to values up to 60 MPam1/2 for the thinnest material states, whereas at 200 °C this increase was less pronounced.

Fracture resistance of AlSi10Mg fabricated by laser powder bed fusion

The combination of refined microstructures (induced by rapid cooling) and melt pool-induced mesostructures in AlSi10Mg fabricated using laser powder bed fusion (LPBF) – a widely used additive manufacturing technique – impart high strength and fracture toughness. Further exploitation of such property combinations requires a detailed understanding of how the processing conditions control the micro- and mesostructures and, in turn, the mechanical performance, especially regarding fracture resistance. Towards this end, the crack resistance curve (R-curve) behavior in different orientations of LPBF-fabricated AlSi10Mg alloys processed with different layer thickness, hatch spacing, and scan strategies was evaluated and correlated with micro- and mesostructural features such as grain size and grain orientation, texture, cell morphology, and melt pool arrangement. Results show a strong anisotropy in both tensile stress-strain behavior and fracture toughness with layer thickness and hatch spacing controlling strength and scan strategy dictating fracture resistance. In terms of tensile stress-strain behavior, the arrangement of melt pool boundaries with respect to loading direction results in anisotropy in ductility whereas strength is controlled by grain size and cellular structure. In case of fracture toughness, measurements show that failure is dominated primarily by melt pool morphology and hence the mesostructure that is controlled by scan strategy. They furthermore reveal, that, despite the pronounced anisotropy in the R-curve behavior the presence of such mesostructure enables a level of damage-tolerance in AlSi10Mg that cannot be achieved in a cast alloy.

Experimental and numerical analysis of mode-I interlaminar fracture of composite pipes

A common industrial production process for composites is filament winding, widely used for the production of axially symmetric components. In these composite components, delamination is a predominant failure mechanism. The current work focuses on investigating experimentally the effect of the initial crack and fiber bridging length on the mode-I delamination resistance curve (R-curve) behavior of various double cantilever beam (DCB) specimens. For this purpose, the magnitudes of initiation and propagation fracture toughness (GIC-init and GIC-prop) and the compliance C are calculated. DCB specimens with a stacking sequence of [± 50]6 and various initial crack lengths of a0 = 33, 37, 59 and 70 mm are manufactured from a real composite pipe using filament winding process. In order to evaluate the critical energy release rate in mode-I, various fracture tests are conducted on these specimens. The greatest bridging zone length dues to good penetration of two adjacent layers of the delamination interface. Moreover, the results indicate that the fiber bridging length has a significant effect on the GIC and the largest value of fiber bridging causes a large fracture toughness value. Finally, numerical simulations are performed using finite element (FE) method and GIC-init measurements obtained experimentally are compared to the numerical findings. The numerical displacements and GIC-init, calculated from the numerical displacements, are found to be in good agreement compared to the experimental results.

Mechanics of bioinspired lamellar structured ceramic/polymer composites: Experiments and models

Creation of super-tough ceramics is one of the main goals of materials science. Bioinspired design is shown to be the most effective method to achieve this goal. Previous studies on the mechanical performance of biological multilayered materials such as nacre have shown that their outstanding mechanical properties are direct results of the small-scale features and optimized arrangement of the elements in their microstructure. Hence, the freeze casting technique has been recently introduced as a novel method to create a new class of bioinspired polymer/ceramic composites. However, the method is cumbersome and the mechanics that controls the overall performance of these composites is not well-known. In this study, the mechanical performance of bioinspired alumina/polydimethylsiloxane (Al2O3/PDMS) and alumina/polyurethane (Al2O3/PU) composite samples with lamellar structure are experimentally and analytically investigated. Bioinspired multilayered samples with micron-size layers are fabricated using the challenging freeze casting technique. Different parameters such as solution concentration, freezing rate, and sintering temperature affect the structure, and subsequently, the mechanical performance of these multilayered materials. Moreover, in order to fully understand the underlying toughening and deformation mechanisms, a micromechanics model of the mechanical response of lamellar composites is presented. The closed-form solutions for the displacements of the layers as a function of constituent properties are derived to calculate the mechanical response of lamellar structured composites such as elastic modulus, strength, and tensile toughness. The experimental results agree well with the proposed analytical models. Fracture mechanics tests are also used to study the Resistance-Curve (R- Curve) behavior of the samples. Furthermore, important toughening mechanisms in these samples are discussed and governing equations for fiber bridging and fiber pull-out in lamellar ceramic/polymer composites are presented. Finally, detailed material design relationships are derived to identify future directions in the design of next generation structural composites.

A Novel Approach to Developing Biomimetic ("Nacre-Like") Metal-Compliant-Phase (Nickel-Alumina) Ceramics through Coextrusion.

Bioinspired "brick-and-mortar" alumina ceramics containing a nickel compliant phase are synthesized by coextrusion of alumina and nickel oxide. Results show that these structures are coarser yet exhibit exceptional resistance-curve behavior with a fracture toughness three or more times higher than that of alumina, consistent with significant extrinsic toughening, from crack bridging and "brick" pull-out, in the image of natural nacre.

Elastic–plastic fracture toughness and rising JR-curve behavior of cortical bone is partially protected from irradiation–sterilization-induced degradation by ribose protectant

ObjectiveThis study tested the hypothesis that pre-treating cortical bone with ribose would protect the rising fracture resistance curve behavior and crack initiation fracture toughness of both bovine and human cortical bone from the degrading effects of γ-irradiation sterilization. Materials and methodsA ribose pre-treatment (1.8 M for bovine, and 1.2 M for human, in PBS at 60 °C for 24 h) was applied to single-edge notched bending fracture specimens prior to sterilization with a 33 kGy dose of γ-irradiation. Fracture resistance curves were generated with a single specimen method using an optical crack length measurement technique. The effect of the treatment on overall fracture resistance behavior, crack initiation fracture toughness, and tearing modulus was compared with non-irradiated and conventionally irradiation sterilized controls. Hydrothermal isometric tension testing was used to examine collagen network connectivity and thermal stability to explore relationships between collagen network quality and fracture resistance. ResultsThe ribose pre-treatment successfully protected the crack growth initiation fracture toughness of bovine and human bone by 32% and 63%, respectively. The rising JR-curve behavior was also partially protected. Furthermore, collagen connectivity and thermal stability followed similar patterns to those displayed by fracture toughness. ConclusionsThis paper demonstrates that the fracture toughness of irradiation-sterilized bone tissue can be partially protected with a ribose pre-treatment. This new approach shows potential for the production and clinical application of sterilized allografts with improved mechanical performance and durability.

Fracture and mixed-mode resistance curve behavior of bamboo

Bamboo is a natural composite abundantly available in the world, and is a source of inspiration for design of new construction materials and structural composites for many applications. The microstructure of this Functionally Graded Material (FGM) evolves through its thickness in response to wind-induced stresses that arise in the plant during its lifetime. This paper presents the results of an experimental study on the structure and toughening mechanisms in Moso culm bamboo. The hierarchical and multi-scale structure of bamboo and the distribution of micron scale fibers are revealed via optical microscopy. Four-point bend fracture experiments are performed where the applied crack-driving forces are computed as a function of crack length. This experimental data are incorporated within a finite element framework in order to understand the fracture and deformation mechanisms of bamboo, and compute the Resistance-curves (R-curve)s. The resistance-curve behavior is shown to be dependent on the orientation of the fibers. In cases where the fibers are perpendicular to the crack growth toughening is observed to occur via crack bridging. Intermediate shielding levels are observed when toughening occurs by crack deflection for crack growth along the fiber orientation.

Toughening Behavior in Natural Fiber-reinforced Earth-based Composites

ABSTRACTThis study presents a combine experimental and analytical investigation of the toughening behavior in natural fiber-reinforced earth-based composites. A specially designed single fiber pullout apparatus was used to provide a quantitative determination of interfacial properties that are relevant to toughening brittle materials through fiber reinforcement. The parameters investigated included a specially designed high strength earth-based matrix comprising of 60% laterite, 20% clay and 20% cement. The toughening behavior of whisker-reinforced earth-based matrix is analyzed in terms of a whisker bridging zone immediately behind the crack tip and interface strength. This approach is consistent with microscopy observations which reveal that intact bridging whiskers exist behind the crack tip as a result of debonding of the whisker-matrix interface. Debonding with constant frictional stress was obtained and this formed the basis for the analytical model considered and the underlying crack-microstructure interactions associated with Resistance-curve behavior was studied using in situ/ex situ optical microscopy to account for the bridging contribution to fracture toughness. The effect of multiple toughening mechanisms (debonding and crack bridging) was elucidated and the implications of the results are considered for potential applications in the design of robust earth-based building materials for sustainable eco-friendly homes.

Strength and fracture toughness of earth-based natural fiber-reinforced composites

This paper presents the results of a combined experimental and theoretical study of the strength, fracture toughness, and resistance-curve behavior of natural fiber-reinforced earth-based composite materials. The composites, which consist of mixtures of laterite, clay, and straw, are stabilized with controlled levels of Ordinary Portland cement. The compositional dependence of compressive, flexural/bend strength, and fracture toughness are explored for different proportions of the constituent materials using composites and crack-tip shielding models. The underlying crack-microstructure interactions associated with resistance-curve behavior were also studied using in situ/ex situ optical microscopy. This revealed evidence of crack bridging by the straw fibers. The measured resistance-curve behavior is also shown to be consistent with predictions from small- and large-scale bridging models. The implications of the results are then discussed for potential applications in the design of robust earth-based building materials for sustainable eco-friendly homes.

J resistance curve behaviour of S355NL structural steel using the unloading compliance technique

Room temperature toughness measurements performed on 32mm thick flat surface and side grooved C(T) specimens of the S355NL structural steel are presented. As expected, stable ductile crack growth was observed during the tests and toughness characterization based on resistance curve (R curve) concepts was therefore used. The unloading compliance technique was chosen to estimate stable crack growth during the tests, and accurate load line displacement measurements were performed using a clip gage. Stress intensity factor and compliance calibrations as a function of crack length/width ratio, published by ASTM, were used to derive the J R-curve. The R-curve for the flat surface specimen is higher than for the grooved specimen. The estimated J at initiation of stable crack propagation is also higher for the flat surface specimen, although difference between the flat and grooved specimens is very small. Generally, a drop in fracture toughness was observed for the grooved specimens, as expected given its higher triaxiality. As an additional comparison of the two types of specimens, the maximum load J0,max value was obtained using the actuator displacement record. The maximum load measurements proved to be rather similar in both types of specimen.

Mechanical properties, modeling and design of porous clay ceramics

Porous ceramics with three different porosities were fabricated by the sintering of redart clay and woodchips (sawdust). The latter was used as the pore-forming agent in porous ceramic water. The porosity, pore size and density of the materials were characterized using Mercury Intrusion Porosimetry and Helium Pyncnometer technique, while the structure and chemistry of the materials were elucidated via X-ray diffraction (XRD), environmental scanning electron microscopy (ESEM) and energy dispersive X-ray spectroscopy (EDX). The compressive strength of the porous clay ceramics were found to exhibit a downward trend with increasing porosity. Due to the anisotropic nature of the porous material, two types of specimen (T- and S-Type) were fabricated for the measurement of flexural strength, fracture toughness and resistance-curve behavior under three point bending. The observed crack-tip shielding/toughening mechanism was then modeled using fracture mechanics concepts. The measured mechanical/physical properties, such as: elastic modulus, density and porosity, were then incorporated into finite element models for the computation of stress distributions due to hydrostatic pressures exerted on the porous clay ceramics by the water in filter with different geometries and supporting configurations. The implications of the results are discussed for potential scale-up and design of a mechanically robust porous ceramic for water filtration.

In-Situ Fracture Observation and Fracture Toughness Analysis of Zr-Based Amorphous Alloys Containing Ductile Dendrites

Effects of dendrite size on fracture properties of Zr-based amorphous alloys containing ductile β dendrites were explained by directly observing microfracture processes using an in-situ loading stage installed inside a scanning electron microscope (SEM) chamber. Three amorphous alloy plates having different thicknesses were fabricated by varying cooling rates after vacuum arc melting. The effective size of β dendrites was varied from 14.7 to 30.1 μm in the alloy plates, while their volume fraction was almost constant. According to microfracture observation of the alloy containing fine β dendrites, shear bands initiated at the amorphous matrix were connected with the notch tip as they were deepened through dendrites, which led to abrupt crack propagation. In the alloy containing coarser β dendrites, shear bands were initiated at the amorphous matrix to form a crack near the notch tip region and were expanded over large matrix areas. The crack propagation was frequently blocked by β dendrites, and many shear bands are formed near or in front of the propagating crack, thereby resulting in stable crack growth, which could be confirmed by the fracture resistance curve (R-curve) behavior. This increase in fracture resistance with increasing crack length could be explained by mechanisms of blocking of crack growth, multiple shear band formation, and crack blunting.

Fracture and resistance-curve behavior in hybrid natural fiber and polypropylene fiber reinforced composites

This article presents the results of a combined experimental and theoretical study of fracture and resistance-curve behavior of hybrid natural fiber- and synthetic polymer fiber-reinforced composites that are being developed for potential applications in affordable housing. Fracture and resistance-curve behavior are studied using single-edge notched bend specimens. The sisal fibers used were examined using atomic force microscopy for fiber bundle structures. The underlying crack/microstructure interactions and fracture mechanisms are elucidated via in situ optical microscopy and ex-situ environmental scanning microscopy techniques. The observed crack bridging mechanisms are modeled using small and large scale bridging concepts. The implications of the results are then discussed for the design of eco-friendly building materials that are reinforced with natural and polypropylene fibers.