Ductile Materials Research Articles

What are the “dispersive shear bands” on the surfaces of layered heterostructured materials?

Hetero-deformation-induced (HDI) strengthening mechanism has been well investigated in heterostructured materials including layered/gradient materials prepared by surface attrition or other processing techniques. While the roles played by the geometrically necessary dislocations (GNDs) and the forward/back stresses on the grain scale have been a focus in nearly all these studies, some latest works reveal the peculiar formation of “dispersive shear bands” or strain localizations on the surfaces of layered/gradient materials. Feature sizes of these “shear bands” are commensurate with the macroscopic sample size, but not on the microstructural length scales, thus excluding the HDI strengthening as the primary mechanism. In this work, using a sandwich structure as an illustrative example, we demonstrate that the origin of these shear bands be localized necking with intermediate wavelengths and inclined orientations, which are critically dictated by the hardening behavior of the constituent layers, the geometric parameters, and the initial morphological perturbations. The layered structure under tension may not neck with an infinite wavelength (i.e., the Considère mode), but neck at intermediate wavelengths which actually correspond to a much larger uniform ductility than the commonly observed Considère necking mode. The arrangements of these shear bands can be further classified as I, X, and W types. Findings in this work not only resolve the origin of recent unusual experimental observations, but also suggest an alternative way of understanding and improving the ductility in heterostructured materials.

Effect of the Degree of Filling on Mechanical Properties of Polymeric Specimens from Polyethylene Terephthalate Glycol and Polylactic Acid Produced by 3D Printing

Based 3D printing has become very popular in recent years due to the emergence of projects for low-cost machines, making the technology very accessible. In view of this, some polymers, in general, in thermoplastic filaments, are placed on the market for application in this type of printing technique, making it increasingly necessary to develop research for the characterization of materials to provide information on physical, thermal and mechanical properties. For the development of this work, the polymer poly(ethylene glycol terephthalate) (PETG) was used for a comparative study in relation to poly(lactic acid) (PLA). PETG is obtained by adding modified glycol to the material composition during the polymerization process. It consists of a polymer with a glass transition temperature close to 80°C, with mechanical properties similar to those of PET, with the advantages of notable tenacity, flexibility, and high processing capacity, and PLA is a polymer synthesized from corn sugar, potatoes, and sugar cane, through bioconversion and polymerization. PLA presents biocompatibility, biodegradability, and biological absorption, presenting good mechanical properties, processability, thermal stability and low environmental impact. Mechanical tests of compressive strength and flexural strength were carried out. In the compressive strength test, the specimen with 100% filling presented a deformation 76% greater than the specimen with 50% filling. This can be attributed to the mechanical property of the PETG polymer, as it is very ductile, thus facilitating the processability of this artifact. The mechanical flexural strength tests carried out with the PLA polymer with the highest filling percentages (100% and 50%) showed less deformation until failure, characterizing them as more ductile materials. On the other hand, specimens with 30% filling showed ~215% greater deformation than specimens with 100% filling. With this, it can be seen that PLA has greater flexibility and tenacity for fillings of low percentages, due to the internal spacing absorbing the impact of loads. The PLA polymer showed better mechanical properties, such as Young's modulus, ductility and more satisfactory resistance when compared to the PETG polymer. As well as the synthesis of PLA, it characterizes the process in a more sustainable way, as it is a biopolymer, in addition to its excellent processability.

Molecular Design of Organic Ionic Plastic Crystals Consisting of Tetracyanoborate with Ultralow Phase Transition Temperature.

Organic ionic plastic crystals (OIPCs) are a ductile soft material where the composing ions are in isotropic free rotation, while their positions are aligned in order. The rotational motion in its plastic phase promotes ion conduction by decreasing the activation energy. Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]-) and various organic cations. In particular, the OIPC composed of [TCB]- and spiro-(1,1')-bipyrrolidinium ([spiropyr]+) cations can transform into its plastic phase at ultralow temperature (Tp = -55 °C) while maintaining a high melting point (Tm = 242 °C). Replacement of the cation with either tetraalkylammonium or phosphonium and comparing their phase behavior, the high Tm was attributed to the relatively small interionic distance between [spiropyr]+ and [TCB]-. At the same time, the low Tp was realized by the restricted vibrational mode of the spirostructure, allowing the initiation of isotropic rotational motion with less thermal energy input.

Hysteretic energy dissipation capacity of dual-steel welded-plate preloaded bolted T-stubs

With the high- and ultrahigh-strength steels applied in the construction industry, the seismic safety of their structural components is very concerned due to the inferior material ductility of those steels. The current studies on bolted high-strength steel T-stubs mainly focus on their monotonic loading behavior. There is limited research work conducted to study their cyclic loading behavior. In this study, cyclic tests of bolted dual-steel T-stubs were conducted. Their T-elements were produced by Q690 high-strength steel flanges and Q355 normal-strength steel webs, and class 10.9 high-strength bolts with pretension were used to connect the groove-welded T-elements. Failure modes, force-deformation hysteretic and backbone curves, cumulative plastic deformation and energy dissipation capacities were tested and analyzed. The test results indicated that an optimal β ratio between the plastic resistances of Mode 1 (neglecting the influence of bolt action on a finite contact area) and Mode 3 in Eurocode 3 Part 1–8, could be 0.3 for those bolted T-stubs to maximize hysteretic energy dissipation. Accordingly, predictive formulas with good precision were proposed for the low-cycle fatigue failure criterion and hysteretic energy dissipation capacity of those bolted T-stubs, respectively.

A numerical model for calculating the impact-induced depression

Materials usually have a quasi–brittle behavior under impact loading, even the ductile materials when loading rate is high enough. In this paper, a new numerical model for calculating the transverse impact on a quasi–brittle target is developed and the theoretical deductions with a strict mathematic logicality are illustrated. Herein, the example of a stiff ball impacting a brittle fiber bundle is investigated and the issues of stress wave propagation, inertia force, energy transformation and stress equilibrium are considered. The numerical model that can describe the impacting procedure is derived. With timing, the deformation of brittle fiber bundle, the kinetic energy of stiff ball as well as their displacements are given. Thus, the overall perspective of the impacting procedure is calculated and analyzed. This work offers a physical illustration of impact events by developing a numerical model and displays the insights into the transient procedure, that will contribute to the development of impact mechanics for engineering.

Microstructural origin of the superior strength–ductility synergy of γ′-strengthened high-entropy alloy with heterogeneous grain structure and discontinuous precipitation configuration

The pursuit of a superior combination of strength and ductility in metallic materials for structural applications has long been a challenge. Recently, L12 (γ′) precipitation-strengthened high-entropy alloys (HEAs) that undergo direct aging from cold-rolled states have exhibited exceptional strength-ductility synergy. However, the origins of this synergetic effect in direct aging are still a subject of controversy. Herein, we aim to unravel the fundamental factors contributing to the improved mechanical properties of directly aged HEAs. Two distinct aging processes, namely cold-rolling and direct aging (CA), and recrystallization and aging (RA), are employed on the Cr18Fe18Co28Ni28Al4Ti4 alloy. The RA process induces continuous precipitation (CP) behavior, while the discontinuous precipitation (DP) is confined near the grain boundaries. On the contrary, the CA process leads to DP-dominant reaction accompanied by simultaneous recrystallization. Additionally, the inherent heterogeneity of the cold-rolled alloy results in a heterogeneous structure in the CA alloys. The CA alloy aged at 600 °C exhibits significantly increased strength and ductility compared to the cold-rolled state, with a remarkable yield strength of 1649 MPa, tensile strength of 1741 MPa, and total elongation of 19.5%. This ultra-high strength is attributed to the precipitation strengthening and heterogeneous deformation-induced (HDI) strengthening resulting from the heterogeneous grain structure. Furthermore, the fine grain and ultra-fine grain regions promote additional HDI strengthening during plastic deformation, leading to sustained strain hardening. The findings of this study establish a correlation between microstructural features and mechanical behavior in directly aged γ′ precipitation-strengthened HEAs, providing valuable insights for the design of such alloys.

On the predictive capabilities of non‐local models for ductile crack propagation under different levels of stress triaxiality

AbstractDuctile materials are used in many applications such as hydrogen storage and transport, energy plants and additively manufactured components. High safety standards are vital for such applications, which underline the necessity of thoroughly investigating ductile failure to ensure safety and increase components efficiency. Ductile failure is mainly prompted by the evolution of the so‐called ductile damage, characterized by the nucleation, growth and coalescence of microvoids due to plastic deformation. Moreover, the plastic zones formed at the crack tip of ductile materials exhibit high sensitivity to the stress triaxiality level, which in turn distinctly depends on the geometry of the considered component. The quantification of the stress triaxiality at the crack tip is therefore essential to better understand and predict ductile crack propagation and failure. For that reason, a non‐local ductile damage model is employed in this work to simulate the ductile crack propagation under different stress triaxiality conditions. Different geometries are considered, such as constrained geometries of notched bending specimens and unconstrained geometries of center cracked tension specimens, which characterize the different triaxiality levels. To address the effects of thickness and initial crack length, three‐dimensional geometries are simulated, which account for the out‐of‐plane crack‐tip constraints. Finally, to evaluate the prediction quality of the simulations, corresponding experiments have been carried out and direct comparisons are conducted, with respect to the crack length, ductile crack propagation and resistance curves.

Three-dimensional computational characterization of grain size and texture effects in magnesium alloys

This work systematically investigates the microstructure-property relationship in Mg alloys. Emphasis is placed on understanding, through high resolution crystal plasticity modeling, how grain size and texture collectively impact material strengthening and hardening, net plastic anisotropy, and tension-compression asymmetry. To achieve this, 528 fully three-dimensional finite element calculations are performed, which comprise eleven textures, four grain sizes, six loading orientations, and two uniaxial loading states (tension and compression). The grain size effect follows Hall-Petch relation that depends on both, loading orientation and initial texture. The reduction in extension twinning with grain size refinement is influenced by texture as well. Below a threshold textural strength, grain size refinement leads to an appreciable reduction in the net plastic anisotropy at yield, quantified using Hill anisotropy, and reduced tension-compression asymmetry. Using a micromechanical basis, the effect of grain size and texture on material ductility is predicted to be non-monotonic. The computational predictions serve as synthetic data sets for experimental validation and reduced-order modeling.

Friction Sensitivity Test Experiment and Desensitization Mechanism of Nitrocellulose-Coated DNTF Explosive Crystals

In response to the problem of the high friction sensitivity of 3,4-bis(3-nitrofurazan-4-yl) furoxan (DNTF) in solid propellants, the inherent component of solid propellants, nitrocellulose (NC), was used to coat DNTF explosives via the water suspension method. The coated samples were characterized by scanning electron microscopy (SEM) and friction sensitivity tests at a fixed 66° swing angle, and molecular dynamics calculations were performed to study the friction sensitivity and desensitization mechanism of NC-based DNTF coatings. The results show that NC, when used as a coating layer, can form a white gel on the surface of DNTF crystals, which can effectively reduce the friction sensitivity of DNTF. The coating effect becomes more obvious as the NC content increases. When the NC content is 5%, the friction sensitivity decreases by 72%, and a prediction formula for the exponential decay of DNTF friction sensitivity is obtained. MD calculation data show that NC can weaken the stiffness, enhance the elastic-plastic properties, increase the ductility and toughness of DNTF materials, and effectively improve the mechanical properties of DNTF. By combining experiments and simulation calculations, while considering the compatibility of new components and changes in propellant energy and other unpredictable new issues, NC can be referred to as a better coating layer for DNTF, as it has a certain feasibility for improving the friction sensitivity of DNTF.

Cement-based ductile rapid repair material modified with self-emulsifying waterborne epoxy

Ordinary silicate cement repair materials are slow to set and harden, have low early strength, and cannot meet the demands of rapid repair, and the disadvantage of brittleness after repair is obvious. Calcium sulfoaluminate cement (CSA) is more suitable for use as a cementitious material for rapid repair materials due to its fast hardening and early strength characteristics, but this cementitious material still suffers from low bond strength and poor toughness, which cannot be effectively resolved. In recent years, polymer-modified cementitious materials have attracted a great deal of attention in the field of repair and reinforcement. Among the many polymer options, epoxy resins have become the most promising choice due to their excellent compatibility and coupling effect with cement hydration and their excellent overall performance. In this paper, a novel waterborne epoxy resin-CSA cement composite restoration material was prepared by using a self-synthesized waterborne epoxy resin (WEP). The mechanism by which WEP improves the performance of CSA cementitious restorative material was also elucidated. The results show that by adjusting the amount of WEP, the workability of the repair material can be regulated. WEP significantly improved the interfacial bond strength of the repair material, and the toughness of CSA cement repair material with 10% WEP increased by 32.2% and the tensile ductility increased by 93.8% compared to the control group. The fully cured WEP and CSA cement hydration products form a dense cross-linked network-like structure, which refines the pore structure of the repair material and improves the disadvantages of the highly brittle and low ductility of the repair material.

Collaborative improvement of strength and ductility over Mo composites enabled by WC and La2O3

The low ductility of refractory Mo-based materials limits their further processing and wide applications. To address this challenge, a composite strengthening and toughening strategy was proposed by the simultaneous addition of WC and La2O3. And a collaborative improvement of strength and ductility is achieved over the as-sintered Mo-WC-La2O3, exhibiting a total elongation (TE) of 33.63% and an ultimate tensile strength (UTS) of 611.33 MPa, about 50.27% and 24.76% larger than those of pure Mo, respectively. Furthermore, the corresponding microstructural features and tensile fracture behavior were also discussed to explore the improvement mechanism. And it is found that the enhancement should be attributed to the high relative density, grain refinement, generated substructures and secondary phase particle refinement. Moreover, the additional toughening mechanisms of crack deflection, particles pull-out and crack halt can also prevent crack extension, which helps to give full play to the potential ductility of Mo composites. This work provides a new pathway for manufacturing advanced Mo-based products with excellent strength-ductility balance.

Controlling anisotropy and brittle-to-ductile transitions by varying extrusion width in short fibre reinforced additive manufacturing

PurposeExtrusion width, the width of printed filaments, affects multiple critical aspects in mechanical properties in material extrusion additive manufacturing: filament geometry, interlayer load-bearing bonded area and fibre orientation for fibre-reinforced composites. However, this study aims to understand the effects of extrusion width on 3D printed composites, which has never been studied systematically.Design/methodology/approachFour polymers with and without short-fibre reinforcement were 3D printed into single-filament-wide specimens. Tensile properties, mechanical anisotropy and fracture mechanisms were evaluated along the direction of extruded filaments (F) and normal to the interlayer bond (Z). Extrusion width, nozzle temperature and layer height were studied separately via single-variable control. The extrusion width was controlled by adjusting polymer flow in the manufacturing procedure (gcode), where optimisation can be achieved with software/structure design as opposed to hardware.FindingsIncreasing extrusion width caused a transition from brittle to ductile fracture, and greatly reduced directional anisotropy for strength and ductility. For all short fibre composites, increasing width led to an increase in strain-at-break and decreased strength and stiffness in the F direction. In the Z direction, increasing width led to increased strength and strain-at-break, and stiffness decreased for less ductile materials but increased for more ductile materials.Originality/valueThe transformable fracture reveals the important role of extrusion width in processing-structure-property correlation. This study reveals a new direction for future research and industrial practice in controlling anisotropy in additive manufacturing. Increasing extrusion width may be the simplest way to reduce anisotropy while improving printing time and quality in additive manufacturing.

Ballistic performance of double-layered plates Q235 and 45 steel subjected to impact by projectiles of middle strength

The ballistic resistance behavior of double-layer steel plates incorporated with different materials is experimentally and numerically investigated by using blunt- and ogive-nosed projectiles of middle strength. These materials are Q235 steel (low strength and high ductility) and 45 steel (high strength and low ductility). Experimental results showed that the ballistic resistance is better for the double-layer plates with the front layer of low ductility and high strength and the back layer of high ductility and low strength material than that of the opposite layer order impacted by ogive-nosed projectiles, but the situation is opposite for blunt-nosed projectiles. The ballistic limit of H + A (753.85 m/s) is increased by 29.5% when compared to that of A + H (582.19 m/s) for ogive-nosed projectiles of 45 steel. In the case of blunt projectiles, a 2.8% decrease in the ballistic limit of middle-strength projectile. Ogive-nosed projectiles are more sensitive than blunt-nosed projectiles when it comes to the effect of the layer order of the double-layer plates. Moreover, the ballistic limit velocities of blunt-nosed projectiles are obviously bigger than that of ogive-nosed projectiles for the double-layer plates of the front layer of low strength, while the situation is opposite for the double-layer plates of the front layer of high strength. Furthermore, the projectiles lose some mass and length after impact. The deformation of projectiles is also concerning the nose shape and velocity of projectiles, as well as the layer order of the double-layer plates. Finally, numerical simulation results are obtained and compared using the ABAQUS subroutine VUMAT-modified Johnson-Cook material model. And these results fairly agree with the experiment.

A finite crack growth energy release rate for elastic-plastic fracture

The concept of energy release rate proposed by Griffith originally used for elastic brittle material was extended for ductile material by Irwin and Orowan using the sum of the surface energy and the work of plastic dissipation as fracture resistance. This fracture resistance, however, depends on the load type and specimen geometry, which hinders its wide application. In contrast, in this paper, a finite crack growth energy release rate is proposed by assuming a finite crack growth, which is conveniently applicable to finite element analysis. The effect of the plastic dissipation on the crack growth is considered in the driving force. Elastic-plastic finite element analysis is used to apply the present energy release rate. Compared to the experiment results, it is shown that for a fixed finite crack growth, a constant critical energy release rate can well predict the stable crack growths and residual strengths of sheets with single and various collinear cracks subjected to different types of loads. The present finite crack growth energy release rate is conceptually simple, has a solid physical foundation, and would be appealing for ductile crack growth analysis.

Radially and axially graded cellular tibial stems for total knee replacement

In spite of the high success rate of existing total knee replacement implants, the aseptic loosening of components, particularly the tibial one, is a matter of concern. To control the short- and long-term loosening mechanisms, the tibial stem material shall have diverse stiffness properties in different regions and directions, such that it encourages osseointegration and prevents bone resorption. The previous studies in the literature have failed to provide designs which could satisfy these two requirements concurrently. The purpose of this study was to design heterogeneous and anisotropic porous titanium tibial stems, based on radial and axial functionally graded cellular (FGC) structures, with precisely tailored stiffness properties. A 3D voxel-based finite element model of the bone-implant construct was developed to predict the nodal micromotion at the interface, assuming a frictional contact, and the stress-shielding effect in the periprosthetic bone, assuming a perfectly bonded connection. An exponential FGC material model was assigned to the implant along each of the radial and axial directions, with the gradient parameters considered as design variables. A design of experiment (DOE) -based multi-objective optimization procedure was then employed to maximize the osseointegration potential and minimize the bone resorption index. Equivalent FGC lattices, composed of Cartesian and cylindrical arrangements of gyroid unit cells, were produced for the optimal designs and fabricated by selective laser melting (SLM) technique. In comparison with the solid stem, the radial graded design resulted in a substantial improvement in the bone resorption behavior (19.4%), but a moderate reduction in the osseointegration potential (9.4%). The axial graded design, on the other hand, was associated with a moderate improvement in the bone resorption characteristics (12.0%) and slight improvement in the osseointegration index (2.2%). The dimensional accuracy of the SLM-fabricated specimens was reasonably high (strut thickness: 0.032 to 0.076 mm RMSE, pore size: 0.029 to 0.057 mm RMSE). The radial and axial graded specimens behaved as brittle and ductile materials, respectively, in axial compression tests and failed due to buckling and excessive plastic deformation. The cylindrical lattice arrangement demonstrated a lower elastic modulus (radial graded: 1945 vs. 3053 MPa, axial graded: 779 vs. 2324 MPa) and yield strength (radial graded: 77 vs. 114 MPa, axial graded: 17 vs. 43 MPa) than the Cartesian arrangement. It was concluded that an optimal bi-directional axial and radial gradient structure with a cylindrical lattice arrangement would provide the most favorable biomechanical performance for the tibial stem.

Effect of incorporating Lode angle into fracture criterion on predicting ballistic resistance of 6061-T651 aluminum alloy plates with different thicknesses struck by blunt projectiles

According to existing research, in addition to the second deviatoric stress invariant, Lode angle parameter (hereinafter abbreviated as Lode angle), which is related to the third deviatoric stress invariant, is also required to be taken into account when studying plastic flow and failure of metal materials. In order to study the effect of incorporating Lode angle into fracture criterion on predicting ballistic resistance of 6061-T651 aluminum alloy plates, a one-stage gas gun system was used to conduct ballistic tests of 6061-T651 aluminum alloy plates with different thicknesses when struck by the blunt projectile with a diameter of 12.66 mm and a mass of 34.0 g. Meanwhile, Abaqus/Explicit finite element software with user-defined material subroutine (VUMAT) was used to establish corresponding finite element models. Under the same constitutive model, numerical simulations were carried out by using Lode independent MJC fracture criterion and Lode dependent WMJC fracture criterion. It is found that when using MJC fracture criterion, not only the ductility of material is overestimated in low stress triaxiality state, resulting in significantly higher predicted ballistic limits; but also the predicted material fracture strain in high stress triaxiality state is negative, sometimes causing irrational predictions. Therefore, some modifications to MJC fracture criterion are needed to reasonably predict the impact process. By contrast, the predicted results of Lode dependent WMJC fracture criterion are more consistent with experiments. Besides, as the thickness of the target increases, the ballistic limit shows an “S” shaped growth trend. Overall, for ballistic tests of targets with different thicknesses, WMJC criterion has a better predictive effect than MJC criterion, indicating the importance of incorporating Lode angle into fracture criteria.

A non-local damage model-based FFT framework for elastic-plastic failure analysis of UD fiber-reinforced polymer composites

A novel computational framework combining fast Fourier transform (FFT) with the non-local damage model is proposed to simulate the nonlinear mechanical behaviors of unidirectional composites. Integral-type non-local damage model and gradient-regularized non-local damage model are integrated to analyze brittle materials and ductile materials, respectively. Under transverse tensile load, the failure of matrix in the composites is characterized by non-local maximum principal stress damage model or the elastic-plastic damage model obeying the parabolic yield criterion, respectively. It is verified that predicted stiffness, strength, and ultimate crack morphologies obtained by the proposed model are in good agreement with traditional finite element method (FEM) and phase field method (PFM) in the literature. The effects of microstructure features on the mechanical behaviors and the suitability of multiple crack propagation are further studied effectively and accurately.

A Study on the Attachment Effect of CFRP Plates to Internal Reinforcements of Curtain Wall Frames Using GFRP

This study is for the development of internal reinforcing materials using fiber-reinforced plastics that can be used for curtain wall frames. The main material of the reinforcement is light and inexpensive GFRP. However, GFRP has characteristics of low elastic modulus and brittle fracture. Therefore, CFRP is attached to supplement the disadvantages. In this study, a flexural experiment was performed to confirm the performance improvement effect of GFRP reinforced with CFRP. The results of the flexural experiment showed that the total flexural stiffness increased by about two times when the carbon fiber reinforced plastic plate was attached compared to the specimen made of a single material of glass fiber reinforced plastic. Also, the pattern of the load-displacement graph was found to reflect the characteristics of the ductile material. However, it was observed that adhesion failure first occurred on the attachment surface before the member yielded. Therefore, it was determined that it was necessary to increase the adhesive strength or set the minimum length in order to prevent poor adhesion when manufacturing actual stiffeners. Therefore, in this study, additional specimens were prepared using the specimen length, adhesive strength, and adhesive rate as variables, and then the experiment was conducted. As a result of the experiment, the maximum load decreased in the long specimen, but the overall stiffness was similar, and adhesive failure did not appear conspicuously before the member was destroyed. In the case of adhesive strength, the case using epoxy was more stable than the specimen using resin, and the maximum load and stiffness were slightly higher when the adhesive width of carbon fiber reinforced plastic was the whole.

A new criterion based on the ratio of strain energy density for mixed-mode crack initiation prediction in ductile materials

In consideration of the different mechanisms of microscopic separation governed by dilation and microscopic shearing controlled by distortion on crack initiation, a new criterion based on the ratio of two strain energy densities (RSED) is proposed by differentiating the dilatational and distortional parts in strain energy density. The influence of the weights of two strain energy densities under different stress states on fracture resistance of crack initiation is also considered in the RSED criterion. The size and shape of the core region do not affect the direction and fracture resistance of crack initiation predicted by the RSED criterion. Moreover, the direction predicted by the RSED criterion is theoretically demonstrated to be identical to the orientations predicted by the maximum stress triaxiality (MST) criterion and the T criterion on crack initiation. The crack initiation directions which are obtained by the experiments for different materials and predicted by various criteria are found to be mostly coincident near the mode-I crack, and so are the fracture resistances. In the vicinity of the mode-II crack, the direction of crack initiation predicted by the RSED criterion shows a superior consistency for the experimental results of the ductile materials reported by Theocaris et al. (1982) and Kong et al. (1995) over the three classical criteria, such as the maximum tangential stress (MTS), the maximum energy release rate (MERR) and minimum strain energy density (MSED). The RSED criterion also has the potential to amend the issue of the underestimation for KIIC/KIC in some typical materials, e.g., the face-centered-cubic (f.c.c) metals. Therefore, the RSED criterion prefers to predict the direction and fracture resistance of crack initiation in ductile materials.

Functionally graded IWP reinforced cementitious composites: Design, fabrication, and the enhanced ductility

The Schoen I-graph-wrapped package (IWP) structures have many promising applications for their high specific mechanical properties. In this work, functionally graded IWP (FG-IWP) architectures were developed and constructed to enhance the ductility of the produced cementitious composites. The FG-IWP architectures were designed using two grading strategies: relative-density-based and cell-size-based. The flexural test was used to investigate the mechanical behavior of the proposed composites, including flexural strength and deflections. The failure patterns and strain distributions of the specimens were measured using the Digital image correlation (DIC) approach. The results reveal that the designed IWP architectures noticeably improve the flexural strength and ductility of the composites. Comparing the FG-IWP to the uniformly configured IWP architectures, the grading strategies exhibit an increased ductility. Furthermore, the two grading strategies have different effects on ductility improvement. Cell size gradient strategy yields greater enhancement in flexural strength, whereas relative density strategy leads to better enhancement in ductility. The variations of cell size and relative density parameter ranges also result in distinct improvement on mechanical properties. Overall, the outcomes of this study present an FG-IWP-based novel approach for improving the ductility of cementitious materials.