Fracture Of Materials Research Articles

Peridynamic modeling of the micromechanical response of snow under high strain rates

Modeling the mechanical response of snow is a challenge due to the wide range of properties and conditions that impact the mechanics, such as temperature, loading conditions, and the geometry of the microstructure. Current modeling approaches including the finite element and discrete element methods have made great strides in modeling the micromechanical behavior of snow, but they often make simplifications to the microstructure geometry and failure process. However, it has been shown that the geometry of individual snow grains and the sintered bonds that form between them, as well as the failure of these bonds, are important factors in the micromechanical behavior of snow. We investigate a modeling framework called peridynamics that has not been applied to snow problems previously, but has been successfully applied to material fracture problems. This approach can use segmented voxels from μCT scans of snow as the model input geometry, which allows for highly accurate representations of realistic snow microstructures within the model. We provide the first analyses to determine appropriate simulation domain sizes and the domain resolutions required to capture realistic snow behavior with peridynamics. We show that the approach is able to simulate realistic brittle behavior for snow at very high strain rates (≥0.1s−1). The model naturally produces highly damaged regions within the sintered bond regions of the microstructure without any preconditioning. We then discuss next steps for further validating peridynamics for snow applications.

Modified hybrid damage criterion for the cross wedge rolling process

This paper considers the problem of material fracture in the cross wedge rolling (CWR) process. A modification of the hybrid damage criterion dedicated to crack prediction in CWR was proposed. The aim of the modification was to make the limits of the new criterion independent of the implementation temperature of the calibration tests. A rotary compression test was carried out to obtain specimens in which cracking is initiated. Based on the analysis of the FE results of the rotary compression test simulation, the final version of the modified hybrid damage criterion (MHDC) was presented. Using the results of experimental tests and FE simulations, the MHDC limit values were determined. The MHDC limits for the three different temperatures were obtained at similar levels, indicating that the MHDC limits can be made independent of the rotary compression test execution temperature. The MHDC was verified on the example of the cross wedge rolling of a railway axle forging. The verification carried out confirmed the possibility of predicting cracks in CWR-rolled forgings using MHDC. The research results obtained are of an applied nature. In particular, the criterion developed can be helpful in the design of cross wedge rolling technology.

Molding quality control with nonlinear forming method in 3D cement printing

3D cement printing is an emerging technology that can construct building elements without formwork, which has broad application prospects in the field of constructing non-linear and complex building structures. However, the high-quality printing and molding of nonlinear structure has become one of the bottleneck technologies restricting the development of 3D building printing. In this paper, variable radian 3D printing nozzle is firstly developed to verify the controllability of extruded concrete material based on self-developed 3D building printer. Then, variable radian forming theory is presented and used to evaluate the mechanism material fracture and accumulation, and improve the molding quality of nonlinear structure. Finally, simulation analysis and experiments are designed to explore the controllable mechanism of variable radian molding and verify the consistency between the experimental results and the theoretical analysis. The research results can promote the application of 3D printing in the construction industry, and provide support for the utilization of building resources and green buildings.

Thermal shock fracture of honeycomb-based porous thermoelectric materials with non-Fourier heat conduction

Compared with dense thermoelectric materials (TEMs), the auxetic honeycomb porous TEMs can enhance mechanical performance and energy conversion efficiency. The Fourier heat conduction law becomes unavailable due to the thermal wave speed in the porous TEMs is limited. Considering the non-Fourier law, this paper analyzes the thermal shock fracture characteristics of the porous TEMs bonded to an elastic substrate. A theoretical model for the effects of non-Fourier heat conduction, thickness of TME, shape and dimensions of honeycomb cell on strength failure and fracture failure is proposed. The analysis demonstrates that the maximum thermal stresses in the porous TEMs are underestimated without considering the non-Fourier heat conduction. The stress level in the auxetic honeycomb-based TEMs is much smaller than that in the traditional honeycomb TEMs with the positive Poisson's ratio architecture. It is found that a thicker TEM, a bigger relative density and a larger length ratio of the horizontal to the inclined strut can result in a smaller thermal stress. Based on the strength and fracture criterions, the envelope of safety angle between the horizontal and the inclined strut is identified. The simplified expressions of the safety angle envelope versus the applied thermal load are given.

Mechanical and morphological properties of unsaturated polyester resin composites reinforced with recycled PET fibers of varying lengths

PurposeThis study aims to deal with both the development and mechanical investigations of unsaturated polyester matrix (UPR) composites containing recycled polyethylene terephthalate (PET) fibers as new fillers.Design/methodology/approachUPR/PET fibers composites have been developed as mats by incorporating 5, 8, 13 and 18 parts per hundred of rubber (phr) of 6-, 10- and 15-mm length PET fibers from the recycling of postconsumer bottles. The mechanical and physical properties of the composites were investigated as a function of fiber content and length. A significant increase in stress at break and in ultimate stress (sr) were observed for composites reinforced with 5 and 8 phr of 15-mm length PET fibers. The Izod impact strength of UPR/mat PET fiber composites as a function of fiber rate and length showed that the 5 and 8 phr composites for the 15-mm length PET fiber have the optimal mechanical properties 13.55 and 10.50 Kj/m2, respectively. The morphological study showed that the strong adhesion resulting from the affinity of the PET fiber for the UPR matrix. The ductile fracture of materials reinforced with 5 and 8 phr is confirmed by the fiber deformation and fracture surface roughness.FindingsThis study concluded that the PET fiber enhances the properties of composites, a good correlation was observed between the results of the mechanical tests and the structural analysis revealing that for the lower concentrations, the PET fibers are well dispersed into the resin, but entanglements are evidenced when the fiber content increases.Originality/valueIt can be shown from scanning electron microscopy micrographs that the fabrication technique produced composites with good interfacial adhesion between PET fibers and UPR matrix.

Evaluation of the interface adhesion mechanism between SBS asphalt and aggregates under UV aging through molecular dynamics

The bonding characteristics between asphalt and aggregates have an important role in influencing the strength of asphalt mixtures. To clarify the bonding mechanism at the interface between SBS asphalt and aggregates under UV aging conditions, this research developed molecular models of the interface between SBS asphalt and representative aggregates including quartz (granite), augite (basalt), and calcite (limestone). Subsequently, dynamic simulations were performed on the interface models to assess the adhesive properties. Adhesion work was employed to assess the interfacial adhesion performance between SBS-modified asphalt and various types of aggregates. The results demonstrated that SBS asphalt exhibited significantly higher adhesive work with alkaline aggregates, following the order of limestone > basalt > granite in terms of adhesive strength. The dominant contributor to adhesive work was van der Waals interactions, with electrostatic interactions playing a crucial role in influencing variations in adhesive work among different aggregates. After the aging-induced fracture of SBS materials, there was a noticeable increase in the gap between them and the aggregates, resulting in a significant reduction in the adhesion effectiveness between SBS and aggregates. Aged asphaltene and colloid exhibited heightened polarity, resulting in enhanced electrostatic interactions between asphalt and aggregates, thus reducing adsorption distances and elevating adhesive work. This research provides comprehensive evaluation of the bonding mechanism between asphalt and aggregate under UV aging conditions at the molecular level, and provides guidance for the design of durable asphalt pavements.

Influence of surface effect correction on peridynamic simulation of dynamic fractures in brittle materials

Influence of surface effect correction on peridynamic simulation of dynamic fractures in brittle materials

Deep learning based initial crack size measurements utilizing macroscale fracture surface segmentation

To this date the safety assessment of materials, used for example in the nuclear power sector, commonly relies on a fracture mechanical analysis utilizing macroscopic concepts, where a global load quantity K or J is compared to the materials fracture toughness curve. Part of the experimental effort involved in these concepts is dedicated to the quantitative analysis of fracture surfaces, which is a cumbersome and time-consuming process. In addition, however, it is affected by subjective bias. Within the scope of our study, we have introduced a methodology that employs deep learning models for macroscopic fracture surface segmentation. To fight subjective bias and enable improved reproducibility, we set up three distinct and unique datasets labeled by different experts that correspond to typical isolated laboratory conditions and complex real-world (multi-laboratory) scenarios. To demonstrate the benefit of our approach for the fracture mechanics assessment, we employed the models for initial crack size measurements utilizing the area average method. Furthermore, the influence of structural similarity on the segmentation capability, differing due to the miscellaneous materials, specimen types, as well as imaging-induced variance has been analyzed. For semi-supervised learning a weak-to-strong consistency regularization was implemented. We were able to train robust and well-generalizing models that learned feature representations from images across different domains without observing a significant drop in prediction quality. Our approach reduced the number of labeled images required for training by a factor of 6. The deep learning assisted measurements proved to be as precise as manual measurements. For the multi-laboratory data, mean measurement deviations smaller 1 % could be achieved, showcasing the enormous potential of our approach.

Experimental study on particle movement and erosion behavior of the elbow in liquid–solid flow

Recent investigations into the erosion of elbow junctions predominantly focus on identifying and predicting peak erosion points. Notably, these studies rely heavily on computational fluid dynamics methods, a valid approach but limited by its lack of empirical physical data. Additionally, the majority of these studies concentrate on the extrados, or outer curve of the elbow, neglecting the intrados or the inner curve. To provide a more comprehensive understanding of particle movements and the micro-mechanics of erosion on the elbow intrados, this study utilizes advanced observational technologies. High-speed camera technology, coupled with scanning electron microscopy, is employed to capture and record particle motion and micro-erosion patterns. The erosion rate is then estimated via the weight-loss method. The findings suggest that in low-speed liquid–solid flows (2.5 m/s), particles released from the intrados side of the elbow inlet exhibit a significant trajectory deviation from the centreline at an elbow angle of 60° from the inlet. Particles released from the extrados deviate towards the intrados side at an elbow angle of 30°. Secondary flow contributes to particle acceleration, unexpected trajectory deviation within the elbow, and an upward inclination in erosion on the intrados. The presence of partially overlapping scratches and cracks suggests that continuous ploughing and material fracturing are significant contributors to the micro-mechanics of erosion. When comparing the intrados and extrados, the extrados exhibits longer and shallower scratches, indicating a smaller impact angle. This research provides a more comprehensive understanding of particle trajectories and erosion patterns within elbow junctions during liquid–solid flows, offering new insights into the mechanisms underpinning these processes.

Experimental and numerical simulation of three-point bending fracture of brittle materials with inverted T-shaped obstacle cracks

In rock materials, the interaction and coalescence of three-dimensional internal cracks play a fundamental role in material failure and instability. This study focused on the generation of three-dimensional internal cracks inside specimens using the three-dimensional internal laser-engraved crack (3D-ILC) method, without causing surface damage. Experimental and numerical investigations were conducted to examine the fracture behavior of brittle solid materials with and without obstacles during three-point bending tests. The experimental results revealed that obstacle cracks positioned horizontally in an inverted T configuration exhibited a shielding effect on the vertical cracks, resulting in reduced propagation velocity and decreased curvature at the lower tip of the vertical cracks. Crack initiation and propagation were promoted in the presence of obstacle cracks, forming distinct fracture patterns, such as pear-shaped and hook-shaped configurations. While the overall failure mode of the specimens was not significantly altered by the presence of obstacle cracks, dynamic fracture dual sources, and new Mode III cracks were observed on the fracture surfaces. The 3D-ILC method provided an effective means for studying the integration and penetration of pre-existing three-dimensional internal cracks. The findings of this study contribute to the theoretical understanding of fracture propagation through obstacles in brittle materials and provide a basis for further investigations using physical experiments.

Tracing fracture damage evolution and identifying damage patterns in cast steel using advanced acoustic emission analysis

The application of advanced acoustic emission signal analysis techniques can facilitate the identification of fracture damage modes in ductile materials and enable the provision of fracture precursor warnings. Given the vast array of acoustic emission signal characteristic parameters, there must be a systematic and comprehensive basis for selection and a deep comparison among different fracture warning indicators. This study uses G20Mn5QT cast steel material fracture acoustic emission monitoring tests under different in-plane constraints as an example. We adopted principal component analysis and k-means++ to select acoustic emission feature parameters and cluster damage modes. Additionally, we conducted a comparative analysis of several fracture precursor warning indicators and influencing factors, including fractal dimensions, b/Ib values, and natural time analysis. Our findings show that shear and tensile fracture damage modes are present in type-I fracture tests when applying the k-means++ cluster to the rise angle-average frequency-energy parameter, but tensile fracture modes dominate. Fractal dimensions, b-values, and Ib-values show a strong trend correlation with the fracture damage evolution process. Natural time analysis can quantitatively provide the moment of fracture precursor warnings, but the sliding window length inversely affects the absolute value sizes. Through acoustic emission signal analysis, our research provides a reference for identifying and examining the evolution of fracture damage in ductile materials and engineering components.

A coupling algorithm of ordinary and non-ordinary state-based peridynamic models for fracture analysis in brittle and ductile materials

In this work, a novel coupling algorithm of the ordinary state-based peridynamic (OSB-PD) model and non-ordinary state-based peridynamic (NOSB-PD) model is provided under the peridynamic framework. The salient advantages of both OSB-PD and NOSB-PD models can be simultaneously inherited in this coupling model and approach. The feasibility of coupling these two models is first analyzed by demonstrating the numerical equivalence of their constitutive formulations in isotropic elastic and elastoplastic problems, and the detailed numerical implementation strategy of coupling formulation is given as well. With the coupling model in this study, the main calculation procedure of numerical simulations is accomplished in OSB-PD, while the damage and failure analysis is conducted in NOSB-PD. Moreover, different failure criteria are introduced in this coupling model to describe the damage and failure of brittle and metallic materials. Several typical examples, including the mode I and mixed mode I/II fracture of brittle materials, and the ductile fracture and impact failure of metallic materials, are conducted to demonstrate the robustness and accuracy of the presented coupling model and approach.

An optimized unibond dual-parameter peridynamic model for deformation and fracture simulation of quasi-brittle materials

Peridynamic is a nonlocal theory, which is an effective tool for predicting fracture in solids. The unibond dual-parameter peridynamic (UDPD) model can break through the Poisson's ratio limitation compared with the bond-based peridynamic model. However, the traditional algorithm for the integrated volume is designed according to the analytical value of partial length in the one-dimensional model, which will produce non-negligible errors for the two- and three-dimensional models. Besides, the classical UDPD model is restricted to ideal brittle materials rather than quasi-brittles, and the long-range force effect is generally omitted for simplified analysis. In this work, an optimized UDPD model is developed to overcome the above drawbacks in simulating the deformation and fracture of quasi-brittle materials. First, the novel algorithm to obtain a more accurate integrated volume is introduced into the UDPD model. Second, considering the long-range force effect and the deformation characteristics, the nonlinear constitutive force function coupled with the cosine type influence function is developed for the UDPD model. Then, the corresponding bond stiffnesses are derived. The proposed model is verified through several representative numerical applications, which suggests that the predicted results match well with the experimental observations and former simulations. It is indicated that the proposed model reveals a significant improvement in the convergence and precision of results, which can not only retain the stability of the classical UDPD model, but also precisely analyze the deformation behavior and damage evolution process of quasi-brittle materials.

Sacrificial Mechanical Bond is as Effective as a Sacrificial Covalent Bond in Increasing Cross-Linked Polymer Toughness.

Sacrificial chemical bonds have been used effectively to increase the toughness of elastomers because such bonds dissociate at forces significantly below the fracture limit of the primary load-bearing bonds, thereby dissipating local stress. This approach owes much of its success to the ability to adjust the threshold force at which the sacrificial bonds fail at the desired rate, for example, by selecting either covalent or noncovalent sacrificial bonds. Here, we report experimental and computational evidence that a mechanical bond, responsible for the structural integrity of a rotaxane or a catenane, increases the elastomer's fracture strain, stress, and energy as much as a covalent bond of comparable mechanochemical dissociation kinetics. We synthesized and studied 6 polyacrylates cross-linked by either difluorenylsuccinonitrile (DFSN), which is an established sacrificial mechanochromic moiety; a [2]rotaxane, whose stopper allows its wheel to dethread on the same subsecond time scale as DFSN dissociates when either is under tensile force of 1.5-2 nN; a structurally homologous [2]rotaxane with a much bulkier stopper that is stable at force >5.5 nN; similarly stoppered [3]rotaxanes containing DFSN in their axles; and a control polymer with aliphatic nonsacrificial cross-links. Our data suggest that mechanochemical dethreading of a rotaxane without failure of any covalent bonds may be an important, hitherto unrecognized, contributor to the toughness of some rotaxane-cross-linked polymers and that sacrificial mechanical bonds provide a mechanism to control material fracture behavior independently of the mechanochemical response of the covalent networks, due to their distinct relationships between structure and mechanochemical reactivity.

An adaptive phase-field simulation for hydrogen embrittlement fracture with multi-patch isogeometric method

This work proposes a phase-field framework for hydrogen-assisted failures of brittle materials using multi-patch isogeometric modeling, considering the effects of hydrogen concentration field evolution and concentration accumulation on material fracture behavior. A phase-field fracture formulation for stress-assisted hydrogen diffusion is derived within the framework of multi-patch IGA and applied to the simulation of hydrogen embrittlement fracture phenomenons in complex structural metallic materials. Nitsche’s method imposes continuous conditions to handle the coupling between interfaces, ensuring compatibility between patches and continuity of variables on the coupling edges. In response to the excessive computational burden of the phase-field method, a refinement-correction loop adaptive strategy based on combined refinement indicators is proposed to improve computational efficiency. Several numerical examples are compared with reference cases to verify the proposed model’s effectiveness, and the adaptive strategy’s performance is tested in simulating hydrogen embrittlement failure.

A local surface roughness mapping method for post-failure interpretation of brittle fracture propagation

Tensile fractures in brittle materials may exhibit striation features running along the surface that indicate the trajectory of fracture propagation. Because of the directional persistence of these features, directional roughness characterization methods can be used on subsampled fracture surface data to map localized roughness features that follow these striations. These maps can aid in the interpretation of fracture initiation sites and crack front positions with the use of post-processing visualization techniques such as vector maps and streamlines. Tensile fractures in rock and synthetic rock analogues are used to demonstrate the various fracture propagation patterns that emerge following this analysis. This mapping method and streamline analysis can be further used to analyze elevation profiles that follow fracture propagation along non-linear paths which may inform about the mechanisms that can associate surface features with the fracture process.

Modifying dental composites to formulate novel methacrylate-based bone cements with improved polymerisation kinetics, and mechanical properties

ObjectivesThe aim was to develop bone composites with similar working times, faster polymerisation and higher final conversion in comparison to Cortoss™. Additionally, low shrinkage/heat generation and improved short and longer-term mechanical properties are desirable. MethodsFour urethane dimethacrylate based composites were prepared using tri-ethylene-glycol dimethacrylate (TEGDMA) or polypropylene dimethacrylate (PPGDMA) diluent and 0 or 20 wt% fibres in the glass filler particles. FTIR was used to determine reaction kinetics, final degrees of conversions, and polymerisation shrinkage/heat generation at 37 °C. Biaxial flexural strength, Young’s modulus and compressive strength were evaluated after 1 or 30 days in water. ResultsExperimental materials all had similar inhibition times to Cortoss™ (140 s) but subsequent maximum polymerisation rate was more than doubled. Average experimental composite final conversion (76%) was higher than that of Cortoss™ (58%) but with less heat generation and shrinkage. Replacement of TEGDMA by PPGDMA gave higher polymerisation rates and conversions while reducing shrinkage. Early and aged flexural strengths of Cortoss™ were 93 and 45 MPa respectively. Corresponding compressive strengths were 164 and 99 MPa. Early and lagged experimental composite flexural strengths were 164–186 and 240–274 MPa whilst compressive strengths were 240–274 MPa and 226–261 MPa. Young’s modulus for Cortoss™ was 3.3 and 2.2 GPa at 1 day and 1 month. Experimental material values were 3.4–4.8 and 3.0–4.1 GPa, respectively. PPGDMA and fibres marginally reduced strength but caused greater reduction in modulus. Fibres also made the composites quasi-ductile instead of brittle. SignificanceThe improved setting and higher strengths of the experimental materials compared to Cortoss™, could reduce monomer leakage from the injection site and material fracture, respectively. Lowering modulus may reduce stress shielding whilst quasi-ductile properties may improve fracture tolerance. The modified dental composites could therefore be a promising approach for future bone cements.

A shear modified GTN model based on stress degradation method for predicting ductile fracture

The Gurson–Tvergaard–Needleman (GTN) model has provided a powerful description of the nucleation growth and coalescence of micro voids, but it has limitations in simulating shear fracture due to the absence of a description of shear localization behavior. A shear improvement method is proposed for simulating the ductile fracture of materials under different stress states. The modified model not only allows for strain hardening of the matrix material, but also accounts for the stress degradation caused by shear. The strength equation of the material is described by both the shear stress state function and a decay function, making it easier for materials under shear stress state to experience material softening and further inducing shear fracture. The modified GTN model is developed by incorporating the shear stress degradation factor into the yield function, while taking into account both void growth and shear failure mechanisms. By carefully calibrating the model’s parameters, the deformation and fracture processes of tensile, plane strain, notch tensile, and compression specimens in the 7A52 aluminum alloy are simulated. The damage evolution behavior of the material under different stress states is analyzed. The results indicate that the damage include void growth mechanism and void shear mechanism. The proportions of these two mechanisms vary under different levels of stress triaxiality. Upon localizing material deformation, the shear stress state intensifies, and the shear damage mechanism assumes a critical role in fracture. The modified GTN model accurately predicts the load-displacement response and fracture path of the 7A52 aluminum alloy under a wide range of stress states.

A stabilized non-ordinary peridynamic model for linear piezoelectricity

Piezoelectric materials are used in a wide range of engineering applications as sensors, actuators, and transducers due to their intrinsic properties. However, analyzing these devices is not straightforward because of the material anisotropy and coupled electromechanical nature of the problems. In this study, a stabilized non-ordinary state-based peridynamic model for linear piezoelectricity is proposed. Fully coupled electromechanical simulation is conducted using an implicit method, and the tangent stiffness matrix is obtained via perturbation technique. Three two-dimensional electromechanical problems with combined static loading and various boundary conditions are used to validate the proposed technique. Then, the crack-opening displacements, and induced electric field on the crack-face are presented for a center-cracked PZT-5H plate. Hence, this study demonstrates that the proposed technique is capable of modeling linear piezoelectric materials and provides a promising method to study fracture and damage progression in piezoelectric materials.

Evaluation of the Microstructure and Mechanical Properties of the Butt-Welded Joints of Spiral Pipes Made of L485ME (X70) Steel.

The expansion of the gas pipeline network makes it necessary, on the one hand, to meet the requirements of standards regarding the materials used, but on the other hand, it is necessary to weld them. In the case of natural gas as a fuel, the welding process is widely used, but in the case of replacing natural gas with a mixture of this gas and hydrogen, the requirements regarding the quality of the process must be significantly increased or the process must be completely changed. This article presents the results of testing welded joints for a newly developed welding technology for the transmission of a hydrogen mixture. Material tests were carried out on a butt-circumferential-welded joint made between two spiral pipes with an outer diameter of 711 mm and wall thickness of 11 mm in the X70 grade. The developed welding technology is distinguished by a change in the beveling method of the edges, which allows the heat input to the material to be limited. The technology was developed for use in natural on-shore and off-shore gas pipelines with the addition of hydrogen. As a result, additional requirements in terms of joint plasticity had to be met during welding. The test results obtained indicate that the joints are characterized by high strength (more than 581 MPa), higher than that of the base material (fracture in the base material) and good impact strength at reduced temperature (more than 129 J). In transverse corrosion, a hardness below 250 HV and a favorable structure of ferrite with different morphologies were obtained.