Bridging Ligaments Research Articles

Fracture toughness of cemented carbides obtained by electrical resistance sintering

The unique combination of hardness, toughness and wear resistance exhibited by WC-Co cemented carbides (hardmetals) has made them a preeminent material choice for extremely demanding applications, such as metal cutting/forming tools or mining bits, in which improved and consistent performance together with high reliability are required. The high fracture toughness values exhibited by hardmetals are mainly due to ductile ligament bridging and crack deflection (intrinsic to carbides). In this work two WC-Co grades obtained by using the electric resistance sintering technique are studied. The relationships between the process parameters (cobalt volume fraction, sintering current and time, die materials, etc.), the microstructural characteristics (porosity, cobalt volume fraction, carbide grain size, binder thickness and carbide contiguity) and mechanical properties (Vickers hardness and fracture toughness) are established and discussed. Also the presence of microstructural anisotropy and residual stresses is studied. The sintering process at 7 kA, 600 ms and 100 MPa, in an alumina die, followed by a treatment of residual stress relief (800 °C, 2 h in high vacuum), allows to obtain WC-Co pellets with the best balance between an homogeneous microstructure and mechanical behaviour.

Extremely toughened Ti-based solid-solution carbide cermets

A new cermet with extremely high toughness of ∼22 MPa m1/2 is designed. The high toughness of the cermet is mainly attributed to its characteristic microstructure, i.e., the even distribution of highly tough ceramic particles, unusual core/rim structure with a low internal strain, and strong bridging ligament. The microstructure and toughness of (Ti,W)C-Ni solid-solution cermets are strongly influenced by the addition of WC, i.e., by the W concentrations in the rim and binder phases. As the WC content increases, the shape of the carbide particles changes from faceted to rounded, and the toughness of the cermets increases sharply. In addition, crack propagation induced by an indentation is altered from the trans-granular mode to the inter-granular mode when the WC content is increased.

Effect of hydration and crack orientation on crack-tip strain, crack opening displacement and crack-tip shielding in elephant dentin

ObjectivesTo quantify the extent of crack-tip plasticity, crack opening displacement (COD) and crack bridging for crack growth perpendicular (HAH) and parallel (RAR) to the tubules in elephant dentin under both hydrated and dry conditions to better understand their influence on intrinsic and extrinsic toughening during crack growth. MethodsCompact tension test-pieces were prepared from a tusk of African elephant ivory. Crack-tip strain mapping and COD measurements by digital image correlation (DIC) technique were made under incremental loading and unloading of cracks for hydrated and dry dentin of different orientations. ResultsFor the RAR test-piece the plastic zones were significantly larger in the hydrated condition compared to when dry. By contrast, the plastic strains in the HAH test-piece were negligible in both wet and dry conditions. In the RAR condition the crack front was broken up into overlapping longitudinal ‘fingers’ with crack bridging regions in between, the ligaments extending 400μm behind the crack front in the dry case. This could only be seen in 3D by X-ray CT. Extrinsic shielding reduces the crack-tip stresses by 52% and 40% for hydrated and dry RAR test-pieces respectively. No significant bridging was found in the HAH case. SignificanceFor crack growth parallel to the tubules, collagen plasticity determines the intrinsic toughening, whereas microcracking from the tubules governs extrinsic shielding via ligament bridging, which is maintained further behind the crack in the hydrated case. For cracks grown perpendicular to the tubules, neither toughening mechanisms are significant.

Fracture behavior of heterogeneous nanostructured 316L austenitic stainless steel with nanotwin bundles

The fracture behavior was investigated of a bulk nanostructured 316L austenitic stainless steel with embedded nanotwin bundles incorporated by dynamic plastic deformation. The nanotwin bundles were demonstrated to be critical to strengthen and toughen the as-deformed samples with mixed microstructures of nano-grains and nano-twins. With increment in strength, the fracture toughness decreases due to the generation of increasingly more nano-grains. Additional controlled thermal annealing that makes the nano-grains recover or recrystallize leads to reduced strengths but more remarkably improved fracture toughness. The enhanced strength−fracture toughness synergy can be attributed to the nanotwin bundles that constrict the damage development in the matrix of either nano-grains or recrystallized grains, and that resist crack propagation via acting as ductile crack bridging ligaments.

Effects of oxygen-related damage on dwell-fatigue crack propagation in a P/M Ni-based superalloy: From 2D to 3D assessment

Effects of oxygen-related damage (i.e. oxidation and dynamic embrittlement) on fatigue crack propagation behavior in an advanced disc alloy have been assessed in air and vacuum under dwell-fatigue conditions at 725°C. The enhanced fatigue crack propagation is closely related to oxygen-related damage at/ahead of the crack tip, which is determined by the testing environment, the dwell period and the crack propagation rate itself based on two dimensional (2D) observation of the crack tip in an optical microscope and scanning electron microscope. X-ray computed tomography has also been employed to examine the differences between three dimension (3D) crack morphology in air and vacuum conditions, and the crack features have been quantified in terms of crack opening displacements, secondary cracks and uncracked bridging ligaments. The results show that the fatigue crack propagation rate is related to the amount of secondary cracks, and the crack length increment in a loading cycle is related to the breaking/cracking of the uncracked bridging ligaments within the discontinuous cracking zone ahead of the crack tip as oxygen-related damage preferentially occurs in these highly deformed regions. By combination of 3D X-ray computed tomography and traditional 2D observation, a deeper understanding is provided of the mechanisms of oxygen-enhanced fatigue crack propagation behavior.

Phase stability, swelling, microstructure and strength of Ti3SiC2-TiC ceramics after low dose neutron irradiation

Mn+1AXn (MAX) phase Ti3SiC2 materials were neutron irradiated at ∼400, ∼630, and 700 °C to a fluence of ∼2 × 1025 n/m2 (E > 0.1 MeV). After irradiation at ∼400 °C, anisotropic c-axis dilation of ∼1.5% was observed. Room temperature strength was reduced from 445 ± 29 MPa to 315 ± 33 MPa and the fracture surfaces showed flat facets and transgranular cracks instead of typical kink-band deformation and bridging ligaments. XRD phase analysis indicated an increase of 10–15 wt% TiC. After irradiation at ∼700 °C there were no lattice parameter changes, ∼5 wt% decomposition to TiC occurred, and strength was 391 ± 71 MPa and 378 ± 31 MPa. The fracture surfaces indicated kink-band based deformation but with lesser extent of delamination than as-received samples. Ti3SiC2 appears to be radiation tolerant at ∼400 °C, and increasingly radiation resistant at ∼630–700 °C, but a higher temperature may be necessary for full recovery.

Inclined Fiber Pullout from a Cementitious Matrix: A Numerical Study.

It is well known that fibers improve the performance of cementitious composites by acting as bridging ligaments in cracks. Such bridging behavior is often studied through fiber pullout tests. The relation between the pullout force vs. slip end displacement is characteristic of the fiber-matrix interface. However, such a relation varies significantly with the fiber inclination angle. In the current work, we establish a numerical model to simulate the entire pullout process by explicitly representing the fiber, matrix and the interface for arbitrary fiber orientations. Cohesive elements endorsed with mixed-mode fracture capacities are implemented to represent the bond-slip behavior at the interface. Contact elements with Coulomb’s friction are placed at the interface to simulate frictional contact. The bond-slip behavior is first calibrated through pull-out curves for fibers aligned with the loading direction, then validated against experimental results for steel fibers oriented at 30 and 60. Parametric studies are then performed to explore the influences of both material properties (fiber yield strength, matrix tensile strength, interfacial bond) and geometric factors (fiber diameter, embedment length and inclination angle) on the overall pullout behavior, in particular on the maximum pullout load. The proposed methodology provides the necessary pull-out curves for a fiber oriented at a given angle for multi-scale models to study fracture in fiber-reinforced cementitious materials. The novelty lies in its capacity to capture the entire pullout process for a fiber with an arbitrary inclination angle.

Crack path simulation in a particle-toughened interlayer within a polymer composite laminate

With recent advances in computational resources and the development of arbitrary cracking methods, such as the Augmented Finite Element Method (A-FEM), more complex simulations can now be represented featuring multiple interacting cracks. It has been established that Mode I crack propagation in particle-toughened interlayers within some toughened Carbon Fibre Reinforced Polymer (CFRP) laminates involves a discontinuous process zone, rather than a distinct crack tip. This results from multiple cracks forming ahead of the main crack that subsequently coalesce, leaving behind bridging ligaments that may then provide traction across the crack flanks. An idealised two-dimensional A-FEM model is presented in this work, which represents the ‘particles’ as one-dimensional cohesive regions. The model shows that variables such as particle spacing, distribution, strength and toughness, and fibre interface strength can be tailored in order to maintain the crack path within the interlayer. This competition between crack paths is important, as a reduction in composite toughness is reported when the crack path migrates to the fibre interface. The simulations are complemented by time-resolved Synchrotron Radiation Computed Tomography (SRCT) data, which identify the chronology of the damage processes, along with the effects of particle distribution on the crack path and the formation of bridging ligaments.

Self-Assembled, Iridescent, Crustacean-Mimetic Nanocomposites with Tailored Periodicity and Layered Cuticular Structure.

Natural high-performance materials inspire the pursuit of ordered hard/soft nanocomposite structures at high fractions of reinforcements and with balanced molecular interactions. Herein, we develop a facile, waterborne self-assembly pathway to mimic the multiscale cuticle structure of the crustacean armor by combining hard reinforcing cellulose nanocrystals (CNCs) with soft poly(vinyl alcohol) (PVA). We show iridescent CNC nanocomposites with cholesteric liquid-crystal structure, in which different helical pitches and photonic band gaps can be realized by varying the CNC/PVA ratio. We further show that multilayered crustacean-mimetic materials with tailored periodicity and layered cuticular structure can be obtained by sequential preparation pathways. The transition from a cholesteric to a disordered structure occurs for a critical polymer concentration. Correspondingly, we find a transition from stiff and strong mechanical behavior to materials with increasing ductility. Crack propagation studies using scanning electron microscopy visualize the different crack growth and toughening mechanisms inside cholesteric nanocomposites as a function of the interstitial polymer content for the first time. Different extents of crack deflection, layered delamination, ligament bridging, and constrained microcracking can be observed. Drawing of highly plasticized films sheds light on the mechanistic details of the transition from a cholesteric/chiral nematic to a nematic structure. The study demonstrates how self-assembly of biobased CNCs in combination with suitable polymers can be used to replicate a hierarchical biological structure and how future design of these ordered multifunctional nanocomposites can be optimized by understanding mechanistic details of deformation and fracture.

FIB/FESEM experimental and analytical assessment of R-curve behavior of WC–Co cemented carbides

Exceptional fracture toughness levels exhibited by WC–Co cemented carbides (hardmetals) are due mainly to toughening derived from plastic stretching of crack-bridging ductile enclaves. This takes place due to the development of a multiligament zone at the wake of cracks growing in a stable manner. As a result, hardmetals exhibit crack growth resistance (R-curve) behavior. In this work, the toughening mechanics and mechanisms of these materials are investigated by combining experimental and analytical approaches. Focused Ion Beam technique (FIB) and Field-Emission Scanning Electron Microscopy (FESEM) are implemented to obtain serial sectioning and imaging of crack–microstructure interaction in cracks arrested after stable extension under monotonic loading. The micrographs obtained provide experimental proof of the developing multiligament zone, including failure micromechanisms within individual bridging ligaments. Analytical assessment of the multiligament zone is then conducted on the basis of experimental information attained from FIB/FESEM images, and a model for the description of R-curve behavior of hardmetals is proposed. It was found that, due to the large stresses supported by the highly constrained and strongly bonded bridging ligaments, WC–Co cemented carbides exhibit quite steep but short R-curve behavior. Relevant strength and reliability attributes exhibited by hardmetals may then be rationalized on the basis of such toughening scenario.

Bone: Bone as a Structural Material (Adv. Healthcare Mater. 9/2015)

On page 1287, E. A. Zimmermann and R. O. Ritchie discuss how human cortical bone's hierarchical structure resists deformation and crack growth through physics-based mechanisms at small and large length-scales, which are essential to resist fracture. Using synchrotron micro-computed tomography, the three-dimensional structure of human cortical bone, with its vascular canals at the center of each osteon (in green), can be observed resisting the propagation of a crack (in yellow) through crack-tip shielding mechanisms, such as uncracked ligament bridging.

The role of organic proteins on the crack growth resistance of human enamel

With only 1% protein by weight, tooth enamel is the most highly mineralized tissue in mammals. The focus of this study was to evaluate contributions of the proteins on the fracture resistance of this unique structural material. Sections of enamel were obtained from the cusps of human molars and the crack growth resistance was quantified using a conventional fracture mechanics approach with complementary finite element analysis. In selected specimens the proteins were extracted using a potassium hydroxide treatment. Removal of the proteins resulted in approximately 40% decrease in the fracture toughness with respect to the fully proteinized control. The loss of organic content was most detrimental to the extrinsic toughening mechanisms, causing over 80% reduction in their contribution to the total energy to fracture. This degradation occurred by embrittlement of the unbroken bridging ligaments and consequent reduction in the crack closure stress. Although the organic content of tooth enamel is very small, it is essential to crack growth toughening by facilitating the formation of unbroken ligaments and in fortifying their potency. Replicating functions of the organic content will be critical to the successful development of bio-inspired materials that are designed for fracture resistance.

In situ synchrotron tomographic evaluation of the effect of toughening strategies on fatigue micromechanisms in carbon fibre reinforced polymers

Abstract Micromechanisms of fatigue failure in double-notch cross-ply carbon/epoxy coupons have been investigated using synchrotron radiation computed tomography (SRCT). The fatigue behaviour of toughened and untoughened matrices has been compared, highlighting similarities and differences of damage modes in terms of fatigue initiation and propagation. Results show that damage does not propagate evenly for the toughened systems: the presence of resin rich regions constrains crack propagation, which is shown to suppress damage growth. In contrast, untoughened material is characterized by more uniform crack progression. The presence of toughening particles in the resin system favours complex local crack geometries, such as crack deflection and the formation of bridging ligaments along the crack wake due to particle debonding events. A distinctive aspect of fatigue loading is identified in the degradation of crack bridging ligaments associated with increasing number of cycles.

How Does Crack Bridging Change at Cryogenic Temperatures?

The crack bridging behaviors of silicon nitride (Si3N4) ceramics based on the advancing cracks have been evaluated at cryogenic temperatures in contrast with those happened at ambient temperatures. R‐Curve behavior of Si3N4 ceramics was determined to compare the cumulative effect of bridging at 77 and 293 K. The detailed changes in crack morphologies and crack opening displacement profiles at different temperatures were investigated. The bridging stress maps around a bridging ligament were recorded by in situ Raman spectroscopy. It was found that the highest tensile stress measured at 77 K (~1.0 GPa) is much higher than that at 293 K (~0.7 GPa). The experimental results suggested that Si3N4 ceramics at cryogenic temperatures possessed a higher probability of crack bridging with a higher closing force, which could make them a promising candidate for cryogenic applications.

Fatigue micromechanism characterisation in carbon fibre reinforced polymers using synchrotron radiation computed tomography

In situ synchrotron radiation computed tomography (SRCT) has been used to evaluate fatigue damage micromechanisms in [90/0]s carbon fibre reinforced epoxy double-edge notched specimens. Interactions between cracks and toughening particles have been identified within the epoxy, particularly: particles de-bonding ahead of the main crack tip, creating a preferential damage path, and the bridging of cracks by un-failed ligaments. The critical mechanism of fatigue crack growth appears to be the degradation of bridging ligaments in the crack wake. Damage has been quantified in terms of crack opening and shear displacements, and the results have been compared with corresponding damage occurring due to quasi- static loading of the same materials. The removal of bridging ligaments in fatigue loading results in higher, more uniform crack opening (and shear) displacements and less serrated crack fronts. These observations have potential implications for material development, damage resistant and damage toler- ant structural design approaches

Fracture and fatigue of rock bit cemented carbides: Mechanics and mechanisms of crack growth resistance under monotonic and cyclic loading

In an attempt to improve the material selection, design and reliability of rock bit WC–Co cemented carbides (hardmetals), an extensive and detailed study is conducted with the main goal of characterizing the fracture and fatigue crack growth (FCG) behavior of four hardmetal grades. Work includes basic microstructural and mechanical characterization of the materials, assessment of fracture toughness and FCG kinetics. It is found that rock bit cemented carbides exhibit relatively high fracture toughness values (between 17 and 20MPa√m) in direct association with their specific microstructural characteristics, i.e. medium/coarse carbide grain size and medium cobalt content. The influence of microstructure on the measured crack growth mechanics under monotonic loading may be accounted by considering the effective operation of ductile ligament bridging and crack deflection as the prominent toughening mechanisms. Regarding FCG behavior, it is observed to exhibit a significant Kmax influence. Furthermore, relative increments in toughness are maintained, in terms of crack growth threshold, under cyclic loading. As a consequence, fatigue sensitivity for rock bit cemented carbides is found to be lower than that extrapolated from data reported for fine-grained grades. Crack growth resistance under cyclic loading for the hardmetals studied may be understood on the basis that prevalent toughening mechanisms (ductile ligament bridging and crack deflection) show distinct susceptibility to fatigue degradation and are thus critical in determining fatigue sensitivity.

An in-situ transmission electron microscopy study on room temperature ductility of TiAl alloys with fully lamellar microstructure

An in-situ transmission electron microscopy study was conducted at room temperature in order to understand an underlying mechanism on room temperature ductility of TiAl alloys. From in-situ straining transmission electron microscopy experiments, it was revealed that the crack path is different between the TiAl alloys with/without room temperature ductility. The crack in TiAl alloys having room temperature ductility interacted with lamellae by forming bridging ligaments between the two α2 lamellae and the γ lamellae. In contrast, the cracks in TiAl alloys without room temperature ductility propagated along grain (colony) boundaries showing brittle intergranular fracture. Finally, we proposed the important microstructural factors to have room temperature ductility of TiAl alloys.

Micromechanical modeling of R-curve behaviors in human cortical bone

The risk of bone fracture increases with age because of a variety of factors that include, among others, decreasing bone quantity and quality due to increasing porosity and crack density with age. Experimental evidence has indicated that changes in bone microstructure and trace mineralization with age can result in different crack-tip strain field and fracture response, leading to different fracture mechanisms and R-curve behaviors. In this paper, a micromechanical modeling approach is developed to predict the R-curve response of bone tissue by delineating fracture mechanisms that lead to microdamage and ligament bridging by incorporating the influence of increasing porosity and crack density with age. The effects of age on fracture of human femur cortical bone due to porosity (bone quantity) and bone quality (crack density) with age are then examined via the micromechanical model.

Linking Crack Tip Morphology to Tear Toughness of Hot Rolled AA7050 Alloys Using X‐Ray Computed Tomography

AbstractA lab scale X‐ray tomography scanner is used to quantify the morphological details of crack tip regions of Kahn tear tested AA7050 aluminum samples subjected to different hot rolling conditions. The 3D crack tip images show distinctive differences in crack tip morphology (such as orientation and shape of the crack tip, bridging ligaments, crack opening distance, and plastic deformation around the crack tip) for samples with different fracture toughness. Quantification of the observations showed that the fracture toughness of AA7050 sample is directly related to the crack tip opening angle measured with computer tomography method.

Three-dimensional measurement of intervertebral range of motion in ossification of the posterior longitudinal ligament: are there mobile segments in the continuous type?

In this paper, the authors' goals were to determine the extent of the effect of continuous-type ossification of the posterior longitudinal ligament (OPLL) of the cervical spine on intervertebral range of motion (ROM) and to examine the relationship between the 3D morphology of OPLL and intervertebral ROM. The authors evaluated 5 intervertebral segments in each of 20 patients (11 men and 9 women) with continuous-type OPLL, for a total of 100 intervertebral segments, using functional CT in anteroposterior (AP) flexion and right and left axial rotation. Three-dimensional kinematics were evaluated using the voxel-based registration method. Ossification was classified on the basis of 3D kinematics and morphology. The authors found 49 ossifications that were obviously of the continuous type. They were divided into 2 types: 1) bridging (13 instances), with thick, continuous ossification of the anterior or posterior longitudinal ligament bridging intervertebral segments and with an ROM of 0.3° in AP flexion and 0.2° in rotation; and 2) nonbridging (36 instances), with a minute gap in the ossification itself or between the ossification and vertebra and with an ROM of 4.9° in AP flexion and 4.0° in rotation. There were 8 stalagmite-type ossifications in the nonbridging group that had the unique kinematics of restricted AP flexion and normal axial rotation. The authors' findings indicate that most continuous-type ossifications that are categorized using the conventional radiographic classification system have mobile segments. The discrimination between bridging and nonbridging on CT scans can be a useful predictive index for dynamic factors.