Tortuous Crack Research Articles

Ar[formula omitted]i-Textile composites: Role of weave architecture on mode-I fracture energy in woven composites

This paper investigates the impact of weave architectures on the mechanics of crack propagation in fiber-reinforced woven polymer composites under quasi-static loading. Woven composites consist of fabrics/textiles containing fibers interwoven at 0 degrees (warp) and 90 degrees (weft) bound by a polymer matrix. The mechanical properties can be tuned by weaving fiber bundles with single or multiple materials in various patterns or architectures. Although the effects of uniform weave architectures, like plain, twill, satin, etc. on in-plane modulus and fracture energy have been studied, the influence of patterned weaves consisting of a combination of sub-patterns, that is, architected weaves, on these behaviors is not understood. We focus on identifying the mechanisms affecting crack path tortuosity and propagation rate in composites with architected woven textiles containing various sub-patterns, hence, Arχi(ar.kee)-Textile Composites. Through compact tension tests, we determine how architected weave patterns compared to uniform weaves influence mode-I fracture energy of woven composites due to interactions of different failure modes. Results show that fracture energy increases at transition regions between sub-patterns in architected weave composites, with more tortuous crack propagation and higher resistance to crack growth than uniform weave composites. We also introduce three geometrical parameters — transition, area, and skewness factors — to characterize sub-patterns and their effects on in-plane fracture energy. This knowledge can be exploited to design and fabricate safer lightweight structures for marine and aerospace sectors with enhanced damage tolerance under extreme loads.

Fracture behavior of brittle particulate composites consisting of a glass matrix and glass or ceramic particles with elastic property mismatch

The aim of this work is two-fold: i) elaborating dense and transparent inorganic glass composites with improved fracture properties, and ii) testing the theoretical analysis proposed in [1] and based on Poisson's ratio mismatch. Particulate composites, consisting of glass or ceramic particles embedded in a soda-lime-silica glass matrix, were synthesized and their fracture behavior was studied by means of the Single Edge Precraked Beam (SEPB) and Double Cantilever Drilled (DCDC) methods, using in situ experiments with X-ray tomography where possible. An important effect of the T-stress on the fracture toughness (KIc) was observed in the case of DCDC exdperiments. KIc is increased by about 40 % by incorporating 7 vol. % amorphous silica beads or SrAl2O4:Eu,Dy ceramic particles (SAED) with a 40 μm mean particle size. It is suggested that toughening results from the crack front trapping and pinning at particle sites and from the tortuous crack path in the case of a-SiO2 particles, and from the contribution of the intrinsic fracture surface energy of the ceramic particles, which are cleaved by the propagating crack, in the case of the SAED particles. The thermally induced stress field is believed to play a major role in the case of a-SiO2 particles. Two glass grades possessing Young's moduli similar to the one of the matrix but much larger Poisson's ratios were used to produce glass beads. However, the incorporation of these latter beads in the matrix was found to have a minor incidence on the fracture behavior.

Bioinspired Bouligand-Structured Cellulose Nanocrystals/Poly(vinyl alcohol) Composite Hydrogel for Enhanced Impact Resistance.

Impact-protective materials are gaining importance because of the widespread occurrence of impact damage. Hydrogels have emerged as promising candidates owing to their lightweight and flexible nature. However, achieving soft impact-resistant hydrogels with exceptional stiffness, strength, and toughness remains a challenge. Inspired by the Bouligand structure found in the smasher dactyl club of stomatopods, we propose a straightforward multiscale hierarchical structural design strategy. This strategy integrates self-assembly and salting-out techniques to enhance the impact resistance of soft hydrogels. Rigid cellulose nanocrystals (CNCs) self-assemble into Bouligand-like structures within soft poly(vinyl alcohol) (PVA) matrix via supramolecular interactions. This rational structural design combines the CNC Bouligand structure with a cross-linked network of soft PVA crystalline domains, resulting in a composite hydrogel with impressive mechanical properties: high tensile fracture strength (30.2 MPa), elastic modulus (62.7 MPa), and fracture energy (75.6 kJ m-2), surpassing those of other tough hydrogels. Moreover, the multiscale hierarchical structure facilitates various energy dissipation mechanisms, including crack twisting, tortuous crack paths, and PVA chain orientation, resulting in notable force attenuation (80.4%) in the composite hydrogel. This biomimetic design strategy opens new avenues for developing soft and lightweight impact-resistant materials.

Dynamic splitting tensile mechanical behavior of magnesia-carbon refractories under impact loading

To enhance understanding of the dynamic tensile failure mechanism of MgO-C refractories, the Split Hopkinson Pressure Bar test combined with digital image correlation techniques were utilized. The dynamic failure behavior of specimens with different heat treatments and graphite content was investigated under various impact velocities. The MgO-C specimens exhibit severer damage and a more tortuous crack path with higher impact velocity. The SiC and forsterite phases generate during heat treatment are advantageous in inhibiting crack propagation, while the formation of oxide layer makes the material more susceptible to brittle fracture. Due to the presence of microcracks, high graphite samples lead to a reduction in tensile strength, but result in a wider fracture zone, which dissipates impact energy. Compared with the significant increase of strength under compressive impact loading, the critical tensile strain rate of MgO-C is easier to achieve after which the tensile strength decreases.

Effect of Fatigue and Precorrosion Fatigue on Fatigue Crack Propagation and Fracture Failure Behavior of 5B70 Aluminum Alloy

Aluminum alloys are ideal lightweight structural materials for spacecraft, but they are susceptible to local corrosion during service. Herein, a systematic investigation is carried out to determine the fatigue behavior of 5B70 aluminum alloy under salt spray precorrosion. The mechanism of salt spray corrosion, fatigue life, crack propagation, and failure fracture mechanism of 5B70 aluminum alloy are studied. Under the salt spray corrosion, 5B70 aluminum alloy mainly undergoes pitting corrosion via a galvanic corrosion mechanism. After precorrosion, the formation of macroscopic fatigue cracks is accelerated, which significantly reduces its fatigue life. Fracture occurs due to single crack initiation and propagation under fatigue conditions. The fatigue crack tip undergoes a mixture of intergranular and transgranular modes, but mainly transgranular mode. Multiple strain localization phenomena occur during precorrosion fatigue, and corrosion pits on the surface provide a tortuous crack propagation path; the fatigue crack tip is mainly transgranular mode.

GdAlO3-Gd2Hf2O7 with eutectic composition for thermal and environmental barrier coating materials

Hafnium-based ceramic with low thermal expansion is suitable for thermal and environmental barrier coating materials. However, the fracture toughness is relatively low. In this work, GdAlO3 is doped into Gd2Hf2O7, increasing the fracture toughness by 31 %. The increase originates from crack deflecting, crack branching and more tortuous crack route. Meanwhile, GdAlO3-Gd2Hf2O7 has lower thermal expansion, making it more compatible with environmental barrier coating systems. The thermal conductivity increases from Gd2Hf2O7 to GdAlO3-Gd2Hf2O7, which can be expected to reduce by controlling the content of GdAlO3.

Tough and Ductile Architected Nacre‐Like Cementitious Composites

AbstractEnhancing fracture toughness and ductility of brittle materials such as concrete remains a challenge. Nature offers numerous mechanisms to enhance fracture toughness using purposeful designs of materials’ architecture. Natural nacre exhibits high fracture toughness by promoting inelastic deformation and hierarchical toughening mechanisms. Here, “nacre‐like‐separated” and “nacre‐like‐grooved” cementitious composites inspired by brick‐and‐mortar arrangement of mollusk shells are proposed. These nacre‐like composites are engineered by laser processing cement paste into individual tablets and grooved patterns (as intentional defects) and laminating them with limited amounts of suitable elastomeric (polyvinyl siloxane) interlayers. It is found that interlayer deformation, tortuous crack propagation guided by the defects, and crack bridging are the main toughening mechanisms in these composites that lead to rising resistance curves. The study hypothesizes tablet sliding as an additional toughening mechanism in “nacre‐like‐separated”, preventing tablet failure and leading to the postponed onset of bulk composite failure. These mechanisms significantly enhance both fracture toughness and ductility by 17.1 and 19 folds, compared to constituent hardened cement paste, respectively. By engineering laser‐induced defects into tabulated cementitious‐elastomeric material at meso‐scale, a class of tough and ductile cementitious composites is introduced, resulting in significantly high fracture toughness values (73.68 MPa.mm1/2), comparable to Ultra‐high‐performance‐concrete without sacrificing the strength.

Stochastic fracture of concrete composites: A mesoscale methodology

The accurate prediction of concrete composites’ fracture behaviour requires precise meso-structural characterization, appropriate fracture modelling, and pivotal uncertainty assessment. Towards this goal, we adopt a dynamics approach with CT images to generate numerical concrete models using real aggregates with 30%-60% contents. An image slicing method is then developed to obtain a large number of realistic models in 2D, whose heterogeneity is characterised by aggregates, mortar and interfaces. Monte Carlo simulations of uniaxial tension are performed, incorporating cross-scale fracture evolution through a phase-field cohesive zone model. Statistical analyses reveal that the stochasticity of crack patterns and stress-displacement curves, especially post-peak softening responses, is inherently dependent on the random meso-structures. It is observed that increasing aggregate content accelerates the deterioration rate of peak stress, decreases the softening curve, and leads to more tortuous crack paths, considering the ITZ crack channels. On the other hand, the tensile strength of mortar has significant impacts on the load-carrying capacity, while the fracture energy primarily influences the ductility and has negligible effects on the peak stress and the pre-peak nonlinear part. Besides, the ratios of mortar-ITZ tensile strength and fracture energy are found to affect the crack paths. The proposed numerical framework holds promise to reveal complex fracture mechanisms of concrete with more insights into other loading or environmental conditions.

Fracture surface analysis of fracture mechanics specimens with splits: Determination of the physical crack extension

Determining ductile fracture toughness requires evaluating the physical crack extension (Δap) on the fracture surface of fracture mechanics specimens. This step is essential for both the basic and single specimen methodologies. While established procedures exist for determining Δap from various standards, measuring it on fracture surfaces with splits poses challenges due to the lack of guidelines in current standards for treating such delaminations. This investigation delves into measuring Δap on fracture surfaces with and without splits using digital image analysis. The fracture surface characteristics of specimens with splits were analyzed, revealing tortuous crack fronts whose complexity increased with the number of delaminations. Additionally, issues with applying the standard method in materials exhibiting high fracture toughness (ductile behavior) and splits were discussed, highlighting significant errors that can arise from the standard methodology in fracture surfaces with splits.Subsequently, four alternative methodologies were proposed and evaluated: the Adapted Nine-Point method (A9P), the Adapted Eighteen-Point method (A18P), the Adapted Area as Lucon’s proposed method (AL), and the Adapted Area Average method (AAA). These were validated in specimens without splits, establishing the AAA method as the reference one. The A9P exhibited poor performance, similar to the AL method. In contrast, the AAA and A18P demonstrated excellent results, recommending their application in fracture surfaces with splits. The advantages and disadvantages of each proposed method are discussed comprehensively. Furthermore, crack front straightness was assessed using ASTM and BS ISO criteria, revealing 88% and 50% rejection rates, respectively.

Mechanisms of anisotropic fracture resistance of nacreous structure and design: Essential role of cracking modes of organic interfaces

Nacreous structure holds significant promise for biomimetic design applications due to its excellent mechanical properties. In this paper, the influence of loading direction on the mechanical properties of nacreous structure was investigated to understand its mechanisms of anisotropic fracture resistance. The results revealed significant anisotropy in the mechanical properties of nacre when loaded along different directions (perpendicular to y and z axes). When loaded perpendicular to the top face, nacre exhibits better mechanical properties and greater resistance to fracture compared to loading perpendicular to the side face. This distinction can be attributed to different cracking modes of organic interfaces in nacreous structure, where the nacreous structure loaded perpendicular to the top face exhibits more tortuous crack deflection, larger fracture stepped surface area and microcrack zones. Furthermore, the theoretical analysis indicates that the differences in the volume fraction of organic interfaces and the main crack deflection along organic interfaces are important factors leading to different cracking modes of organic interfaces in nacreous structure. Based on the analysis of the mechanisms of anisotropic fracture resistance, the critical parameters to characterize the anisotropic mechanical properties of the nacreous structure were proposed. The research results provide a reference and basis for future biomimetic application design.

Enhancing Cement Paste Properties with Biochar: Mechanical and Rheological Insights

Biochar, the solid sub-product of biomass pyrolysis, is widely considered an effective water retention material thanks to its porous microstructure and high specific surface area. This study investigates the possibility of improving both mechanical and rheological properties of cement pastes on a micro-scale. The results show that using biochar as a reinforcement at low percentages (1% to 5% by weight of cement) results in an increase in compressive strength of 13% and the flexural strength of 30%. A high fracture energy was demonstrated by the tortuous crack path of the sample at an early age of curing. A preliminary study on the rheological properties has indicated that the yield stress value is in line with that of self-compacting concrete.

Toughing epoxy nanocomposites with Graphite-Nanocellulose layered framework

Epoxy nanocomposites hold immense promise for advanced high-performance applications in coatings, adhesives, and fiber-reinforced composite matrices. However, their widespread use is hindered by low fracture toughness, attributed to highly cross-linked network structure. Here, this study presents a synergistic manufacturing strategy combining ultrasonic graphite-nanocellulose assembly and bidirectional freeze casting to fabricate epoxy nanocomposites with remarkable layered architectures. Subsequent infiltration and curing with epoxy prepolymer produce nanocomposites exhibiting a fracture toughness 4.67 times higher than pure epoxy. Both experimental and theoretical analyses demonstrate the nanocomposite's superior crack propagation resistance, attributable to a range of synergistic toughening mechanisms. These include robust interfacial bonding and mechanical interlocking, which arise from fiber pull-out/flake fracture effects, effectively impeding crack growth. Additionally, the distinctive layered architecture promotes tortuous crack pathways, optimizing fracture energy dissipation. This work offers pivotal insights into structure–property relationships, paving the way for the design of next-generation nanocomposites with tailored, superior mechanical performance.

A fast approach for predicting stress intensity factors in tortuous cracks under mixed-mode loading

Realistic crack morphologies are generally sinuous rather than straight, which brings a great challenge to capture accurately the stress intensity factors at the crack tip. In this paper, a fast approximation method for predicting stress intensity factors in tortuous cracks under mixed-mode loading is proposed in terms of geometry simplification and semi-theoretical calculation. First, the finite element results show that the main and branch two-segment crack geometry approximation possesses a good capability of predicting the stress intensity factors of tortuous cracks under mixed-mode loading, whereas the simplification strategy of one straight crack cannot characterize the crack tip field of sinuous cracks. Then, an efficient semi-theoretical estimation approach is proposed to obtain the stress intensity factors of the branch crack from those of the main crack, which is validated to further reduce the computational time, resulting in a two-step accelerated operation. Moreover, the effect of main crack offset is also discussed and some recommendations are given to ensure accurate predictions within an acceptable margin of error for microstructural design or other application purposes.

Anisotropic tensile creep behavior in laser powder bed fusion manufactured Al–Mn–Mg–Sc–Zr alloy

The recently emerged Al–Mn–Mg–Sc–Zr alloys were regarded as high-strength Al alloys suitable for high-temperature applications. However, the influence of laser powder bed fusion (LPBF) fabricated anisotropic microstructure on tensile creep performance has not been explored. In this work, we reported an anisotropic tensile creep behavior, which is superior in the vertical specimens than the horizontal counterparts. The tensile creep anisotropy was attributed to the bi-modal microstructure of columnar grains within melt pool and equiaxed grains at melt pool boundary, the “fish-scale” melt pool morphology and the resulted tortuous crack path in the vertical orientation.

On the size-dependent fatigue behaviour of laser powder bed fusion Ti-6Al-4V

A sample size effect which influences the fatigue behaviour of laser powder bed fusion Ti-6Al-4V is identified and quantified. Two cylindrical samples are considered: ∅ 1.3 mm and ∅ 2.0 mm. The larger specimen demonstrates better fatigue resistance particularly in the high-cycle regime, with the differing surface roughness contributing to this effect. It is also confirmed that processing-induced porosity can compromise the fatigue performance even when the initiation sites are surface defects. The larger contribution of porosity to the fatigue fracture process of the larger specimen results in a higher scatter in the fatigue life. Differences in microstructure do not seem to contribute strongly to the variation in fatigue properties of the two specimens, but we present some evidence that the coarser microstructure of the larger specimen promotes a stronger tolerance to defects and induces more tortuous crack paths which hinders fatigue crack growth.

Effect of low dosage fiber reinforcement on cracking performance of asphalt concrete

Fiber reinforcement is one of the methods used to improve cracking resistance of quasi-brittle materials such as asphalt concrete (AC). Despite the widespread use of fibers in AC, there is a lack of fundamental understanding of fracture behavior of fiber reinforced asphalt concrete (FRAC). The main goal of this research is comprehensive fracture characterization of low dosage applications of two synthetic fibers (polyethylene-aramid and nylon) and understand the performance bias of low dosage fiber reinforcement at mixture level in different test conditions. Two laboratory prepared mixtures (fine and coarse-graded) were designed with consistent volumetrics. Two crack mouth opening displacement (CMOD) – controlled experiments (SCB-CMOD and DCT) were performed to facilitate a controlled crack propagation at low and intermediate temperatures. Alternative load-line displacement (LLD) control cracking tests (I-FIT and IDEAL-CT) with different geometries, testing parameters, and loading modes were also evaluated. It was observed that the mixes with nylon fibers had a distinctive and better fracture response in all of the experiments. Some improvements were observed with aramid fiber configuration dependent on the test performed and mixture type. The key test features that resulted in more distinction between the low dosage fiber mixes and the control appeared to be ligament length, affected damage area, and cracking patterns observed in the experiments. The tests with larger ligament length (i.e., larger affected damage zones) were sometimes visible with more tortuous cracking patterns with many branches resulted in more consistent and noticeable outcomes with the fibers. Further research is needed to identify potential test bias and develop an ideal test protocol and test geometry that can objectively evaluate the performance of low dosage fibers.

The Effect of Different Biochar on the Mechanical Properties of Cement-Pastes and Mortars

In recent years, there has been a concerning surge in CO2 emissions, with the construction and materials production sectors standing out as significant contributors to greenhouse gas pollution. To tackle this pressing environmental challenge, architectural design and civil engineering are actively pursuing strategies to mitigate their carbon footprint. These initiatives include adopting eco-friendly construction materials with reduced toxicity, rigorous energy management practices across the entire life cycle of structures, and incorporating innovative materials like biochar. Biochar is a carbon-rich byproduct generated through controlled thermochemical processes, such as pyrolysis or gasification, that stands out for its remarkable capacity to extract energy from processed biomass while delivering substantial environmental advantages. This study examines the use of biochar as a filler in cement-paste and mortar, as well as its influence on mechanical properties. In the case of cementitious pastes, results show that small amounts of biochar (1-2-5% by weight of cement) can improve the compressive and flexural strength, as well as fracture energy, thus generating a more tortuous crack path that increases the final surface area. In mortar specimens, the biochar influence does not show similar patterns or characteristics as the cement-paste in flexural and compressive strengths; nevertheless, biochar particles improve the toughness.

Effect of microstructure on impact fracture mechanism of a high-strength Ti-5Al-7.5V-0.5Si-0.25Fe-0.2O alloy

The impact property of high-strength Ti-5Al-7.5V-0.5Si-0.25Fe-0.2O alloy (Ti575) with trimodal microstructure (TM) and lamellar microstructure (LM) were investigated by combining instrumented Charpy U-notch impact test with microstructure characterization. Impact load-displacement curves illustrated that the higher impact toughness of LM (47.8 J/cm2) than TM (38 J/cm2) were attributed to both intrinsic toughening during crack initiation and extrinsic toughening during crack propagation. In crack initiation stage of LM, electron back-scattered diffraction (EBSD) evidenced the nucleation of numerous {10–12} <–1011> tensile twins in coarsened lamellar α (αl) in vicinity of the crack notch. The high intrinsic toughness of LM was due to the dynamic microstructure refinement and facilitation of prismatic/pyramidal slip induced by twins during impact. For TM in contrast, smaller-sized primary α(αp) and αl suppressed the nucleation of twins, exacerbated stress concentration and reduced plastic deformation near the crack notch. In crack propagation stage, the kinking, shearing of αl, crack deflection at αl/β interfaces and secondary crack in αl colony borders contributed to a tortuous crack path and large energy dissipation in LM, whereas crack could bypass αp and cut through thin αl in TM easily, resulting in a flat crack path and poor extrinsic toughness of TM. In summary, this work provided a basic understanding of the impact fracture mechanism of Ti575 with different microstructures.

Influence of post weld aging treatment on microstructural features and fatigue behaviour of unnotched and notched specimens of friction stir welded AA6061-T651 aluminium alloy joints under completely reversed stress cycle

A comparative investigation on rotating bending fatigue behaviour between unnotched and notched specimens of friction stir-welded (FSW) AA6061-T651aluminium alloy joints has been conducted under completely reversed cyclic loading (R = −1). Post-weld aging treatment was done for 8 h at 200°C on the welded joints. After post-weld aging treatment, the fatigue strength was significantly improved, while the grain size remained unchanged. Microstructural evolution before and after post-weld aging treatment was studied using orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). It was found that post-weld aging treatment decreased dislocation density and facilitated precipitation hardening through the reprecipitation of fine β″(Mg2Si) particles. Fine particles of post-weld aged specimens of FSW joints were sheared by dislocations generated during cyclic loading, resulting in difficult crack initiation and tortuous crack propagation. Fractographic results confirmed that Post-weld aged specimens of FSW joints have better fatigue resistance than as-welded specimens, owing to the presence of significant precipitates along the grain boundary.

Simultaneous enhancements of strength and toughness by multiscale lamellar structure in Ti2AlNb based intermetallic

Multi-scale lamellar structure significantly improves toughness of Ti2AlNb based alloys, which are inherently brittle intermetallics, without compromising their strength. This structure was achieved through-B2-transus-forging (TBTF) combined with O + B2 two-phase region heat treatments. Various types of multi-scale lamellar structures were obtained by controlling the cooling rate after TBTF. These variations were mainly attributed to differences in the distribution, content, and size of the thick lamellar O phase and the size and crystallographic orientation of B2 grain. By analyzing the microstructural characteristics and crystallographic orientation near the crack propagation path, it was found that the crack propagation resistance of thick lamellae, sub grain and grain boundaries (GBs) O phase increased sequentially, accompanied by more tortuous crack propagation path. Moreover, B2 grains with high misorientation significantly deflected the crack propagation by cleavage ridges between adjoining cleavage planes. Additionally, the development of numerous secondary cleavage ridges, resulting from the transition through varying secondary cleavage planes in distinct sub B2 grains, further hindered the quick propagation of cracks. It was clarified that the cleavage planes were dominantly belonging to {110}. These findings provided valuable guidance for the design of damage tolerance strategies for Ti2AlNb-based intermetallics.