Fracture Mechanics Investigation Research Articles

Comparison of the structural behaviour of monolithic and masonry obelisks: Obelisk of Theodosius and Walled Obelisk in Istanbul

In this paper, the structural behaviour of the Obelisk of Theodosius and Walled Obelisk, located near the Sultan Ahmed Mosque in Istanbul, is investigated. The Obelisk of Theodosius is made of an integrated and solid red granite that stands on four pieces of bronze cubes and large pieces of stone cubes. The Walled Obelisk is made of small pieces of limestone that stand on only one large piece of stone cube, without bronze cubes. The purpose of this study is the comparison of the structural behaviour of these two obelisks and the effect of their supporting base on their performance under seismic loading. The nonlinear three-dimensional finite element method is used for modelling and the Willam-Warnke failure criterion is adopted for investigation of brittle fracture mechanism. The results of analysis show that the Walled Obelisk is more flexible than the Obelisk of Theodosius. However, the existence of four bronze cubes and four red granite cubes at the corners of the supporting base increases the flexibility of the Obelisk of Theodosius.

Influence of Microstructure on Static and Cyclic Fracture Toughness of EN-GJS-600-3 Nodular Cast Iron

In this paper, the effect of microstructure of a thick-walled rotor shaft for wind turbines on fracture toughness properties has been investigated. The relevant nodular cast iron grade EN-GJS-600-3 was processed using chill casting technology. Due to different solidification conditions over the wall thickness, heterogeneous microstructures were formed. To illustrate the influence of the microstructure gradient caused by chill casting technology, specimens were taken from different sample positions in the cross section of the casting component. A detailed metallographic analysis revealed essential differences in microstructure. The crack growth resistance under quasi-static loading conditions and the fatigue crack propagation under cyclic loading conditions were measured. The results of the static fracture mechanics investigations revealed that fracture toughness is strongly influenced by the microstructure of this pearlitic ductile iron grade. On the other hand, cyclic fracture mechanics analysis showed that the complex formation of the microstructure has only a minor effect on the fatigue threshold value, but microstructure has a significant effect on the stable crack growth. For the assumed load cases, it was shown that microstructure can be a dominant factor on the mechanical and fracture toughness properties.

Atomistic investigation of fracture mechanisms in phosphorus-functionalized epoxy resins

Modifying epoxy resin molecules using phosphorus-containing functional groups can enhance the liquid oxygen compatibility (LOC) of polymeric matrix composites, but will significantly change the mechanical and fracture properties of the functionalized thermosets due to the increased complexity of the molecular architecture. The underlying mechanisms responsible for these property changes are not well understood. In this work, we unveiled the molecular-scale fracture mechanisms of epoxy resins modified by 10-(2,5-dihydroxyphenyl) -10-hydrogen-9-oxa1-phenanthroline-10-oxide (ODOPB) using molecular dynamics (MD) simulations. We adopted a two-step reaction scheme to prepare the modified cross-linked networks of the epoxy thermosets. Then, a fracture simulation approach was developed based on hybrid use of non-reactive and reactive force field parameters, which enables accurate and efficient bond scission representation during tensile deformation. Efforts were made to explore the impact of varying the P content (proportional to ODOPB amount) on the molecular architectures and mechanical performance. It was found that the chain length distribution was a crucial determinant of the mechanical and fracture properties. More intriguingly, the simulation results showed that at a fixed P content, properties such as the fracture energy could be enhanced by regulating the chain length. This study offers valuable insights into the design and fabrication of high-performance aerospace composites with remarkable tolerance to harsh engineering environments.

Fracture mechanics investigation of crack coalescence in a steel tubular T-joint specimen

Fatigue test of steel tubular T-joint specimens was performed. A beach mark (BM) technique was employed to measure the crack size. Some rare fracture patterns were found. One of the specimens generated two major surface cracks around the weld toe. They were independently propagating at the initial stages and were coalesced into a single surface crack at a certain amount of cyclic loads. Subsequently, the crack was enlarged and became a doubly curved surface crack. Crack propagation (CP) simulation is performed to investigate the fracture behaviors. Extended finite element method (X-FEM) option in an open-source software Code_Aster is used. FEM CP simulation is also performed to validate the X-FEM results. The CP phenomena are carefully examined using the fracture mechanics parameters and crack front trajectories.

A computational investigation of applicability of nonlinear fracture mechanics in nano-scale fracture of graphene

One of the main concerns for large-scale applications of graphene is unwanted fracture due to its low fracture toughness, close to that of an ideally brittle solid. However, graphene fracture properties and criteria have not been fully understood yet and the related understanding at the continuum-mechanics level is mostly limited to the linear elastic fracture mechanics (LEFM) investigations. In this paper, the applicability of nonlinear fracture mechanics (NLFM) to describe the fracture behavior of graphene is investigated through modeling the graphene sheets, including a center crack along the armchair and zigzag directions under mode I and mode II loading. A hyperelastic high-order continuum description is developed to be used in the nonlinear finite element simulations at the continuum level, by expanding the elastic strain energy in terms of the Lagrangian strains up to the seventh-order terms. The proposed continuum model is parameterized using the molecular dynamics (MD) simulation results to accurately reproduce the nonlinear mechanical behavior of graphene in uniaxial strain, equi-biaxial strain, and simple shear deformations. Finally, the applicability of NLFM, compared to LEFM, is investigated for graphene fracture through the continuum mechanics and MD fracture simulations. © 2001 Elsevier Science. All rights reserved

Effect of Epoxy Structure on Properties of Waterborne Coatings and Electrical Steel Laminates.

Epoxy varnishes are of high relevance to advanced steel laminates for the transformation of electric energy. Structure–property correlations of epoxy varnishes, coil coatings and electrical steel laminates are poorly described. Hence, the main objective of this paper was to develop, implement and evaluate well-defined waterborne model epoxy varnishes for electrical steel laminates, and to elucidate structure–property correlations. Adhesives with systematically varied equivalent epoxy weight (EEW) based on bisphenol-A-diglycidyl ether (DGEBA) were investigated and used to formulate waterborne varnishes. Crosslinking agent dicyandiamide (DICY) was added in an over-stoichiometric ratio. The waterborne model varnishes were prepared by shear emulsification at elevated temperatures. The model varnishes in the A-stage were applied to electrical steel using a doctoral blade. At a peak metal temperature of 210 °C, the coatings were cured to the partly crosslinked B-stage. Coated steel sheets were stacked, laminated and fully cured to C-stage at 180 °C for 2 h. For laminates with an epoxy adhesive layer in the C-stage, glass transition temperatures (TG) in the range of 81 to 102 °C were obtained by dynamic mechanical analysis in torsional mode. Within the investigated EEW range, a negative linear correlation of EEW and TG was ascertained. Presumably, higher EEW of the varnish is associated with a less densely crosslinked network in the fully cured state. Roll peel testing of laminates at ambient and elevated temperatures up to 140 °C confirmed the effect of EEW. However, no clear correlation of roll peel strength and glass transition temperature was discernible. In contrast, fatigue fracture mechanics investigations revealed that hydroxyl functionality and crosslinking density were affecting the crack growth resistance of laminates in a contrary manner. The energy-based fracture mechanics approach was much more sensitive than monotonic peel testing.

Investigation of fracture mechanics, physical and dynamic properties of UHPCs containing PVA, glass and steel fibers

Fracture energy, non-destructive testing, some physical and mechanical tests and microstructural investigations were performed on 18-month 0.7 V%, 1.3 V% and 2.0 V% PVA fiber, glass fiber (GF) and steel fiber (SF) reinforced Ultra High Performance Concretes (UHPCs). Fracture test was performed on Single Edge Notched Beams (SENB) to investigate fracture energy, dissipated energy, flexural strength, crack resistance index (CRI), and flexibility index (FI) of UHPCs. Dynamic modulus of elasticity and damping ratios of the UHPCs were calculated by performing resonance frequency tests on the SENB samples by ASTM C215 standard. In addition, compressive strength, static modulus of elasticity, electrical resistivity, ultrasonic pulse velocity (UPV), Leeb hardness, Schmidt hammer, and density tests were also performed on the cylindrical samples. Field Emission Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-Ray (EDX) analyzes were performed to examine the interaction between the UHPC matrix and different fibers. Among all the mixtures, an exceptionally positive effect of SFs on the all mechanical properties of the UHPCs was observed. Dynamic test results showed that the increase in fiber ratio increased the damping ratio of UHPCs irrespective of fiber type. The UPV and electrical resistivity results offered promising results regarding the internal structure of UHPCs.

Investigation of the transverse fracture mechanisms of bamboo by the finite element method

Investigation of the transverse fracture mechanisms of bamboo by the finite element method

Investigation of microscale fracture mechanisms in glass–ceramics using peridynamics simulations

AbstractGlass–ceramics (GCs), obtained by controlled crystallization of a specially formulated precursor glass, are interesting materials that show great promise in obtaining superior properties compared to those of the precursor glass. Controlled crystallization enables creation of a microstructure with multiple phases which impacts macroscale properties in interesting ways. The present work develops microstructure‐scale computational models using the theory of peridynamics to investigate the increase in fracture toughness of GCs compared to traditional glass. Computational modeling is a promising tool to probe microstructural mechanics, but such studies in the literature are scarce. In this work, the theory of peridynamics, a non‐local theory of continuum mechanics, is applied to simulate crack propagation through microstructural realizations of a model lithium‐disilicate glass–ceramic. The crystalline and glassy phases within the microstructure are explicitly considered, with the size and shape of crystals inspired by experimental data. Multiple toughening mechanisms are revealed, which are functions of crystallinity and morphology, and the impact on fracture toughness is demonstrated. Crack path tortuosity is studied, and it is found that an optimum level of crack path tortuosity can be obtained in the range of 0.6–0.8 crystallinity. Numerical results are shown to agree well with previously published experimental and modeling results.

In situ EBSD/DIC-based investigation of deformation and fracture mechanism in FCC- and L12-structured FeCoNiV high-entropy alloys

This study investigated the plastic deformation behavior of a polycrystalline L12-structured FeCoNiV high-entropy alloy (HEA) using in situ electron backscatter diffraction (EBSD) and digital image correlation (DIC) methods. The different deformation mechanisms in two HEAs, which affected their mechanical performance, were explored using a face-centered cubic (FCC)-structured sample for comparison. Using slip traces and lattice rotation path analysis, {111}<110> slip systems were found to be activated in the L12-structured FeCoNiV HEA. In addition, a lower average lattice rotation rate was estimated for this sample compared to that of the FCC specimen; this macroscopically verified the existence of additional obstacles to dislocation slip caused by the ordered structure during plastic deformation, and was found to contribute to the high strength of the L12-structured FeCoNiV HEA. Furthermore, these additional obstacles blocked the formation of deformed substructures in the L12-structured sample and aggravated the intergranular incompatibility, which enabled crack initiation at the grain boundaries. These findings are important for understanding the deformation behavior and fracture mechanism in L12-structured HEAs and for designing new high-performance ordered HEAs.

Fracture mechanical investigation of preformed metal sheets using a novel CC-specimen

Joining is used in many areas of manufacturing where components are formed from individual parts or sheets into complex structures. A simple and common method applied in industry to join dissimilar and coated materials is clinching. This joining process does not require additional joining parts, which saves weight to reduce energy requirements and harmful emissions due to moving masses. In addition, the joining process is carried out without pre-punching and is therefore time-efficient. Damage caused by the joining process or by operational loading can lead to cracks near the joint, to crack propagation due to time-varying loading and subsequently to the complete failure of the component. To avoid these damage cases, knowledge about the fracture mechanical behavior of the clinched connection is required.Within the scope of this paper, a novel CC-specimen for fracture mechanical investigation of preformed metal sheets is developed and investigated. The new specimen geometry is taken directly from a clinched joint in order to identify a possible influence of the outer forming area of the joint on the fatigue crack growth rate. The clinched joint is placed at three different positions and is moved from the radius to the center of the C-shape at 3 mm intervals. In order to investigate the crack growth rate of this new specimen, it is necessary to numerically determine the geometry factor function for the stress intensity factor as well as the calibration curve for the crack length measurement with the direct current potential drop method beforehand. Finally, the results of the crack growth rate tests using the CC-specimen with different positions of the clinched joint are compared with the results of the base material.

Fracture Mechanical Analysis of Gleeble Simulated Heat Affected Zones in High Strength Steels

During the research work the fracture mechanical investigation of heat affected zones of thermomechanical rolled high strength steels (Voestalpine Alform 960M) were carried out. For production of appropriate heat affected zones Gleeble 3500 physical simulator was applied, with different heating cycles and specific cooling times. Following the simulation, fracture mechanical investigations were performed, in favor of determination crack tip opening displacement (CTOD or δ) values.

Numerical study of long-time growth of hydraulic fractures in a line drive

This work concerns detailed numerical modelling of injection fracturing in chalk formations under 2–3 km burial. The conditions and properties are chosen to resemble field conditions as typically found in oil fields subject to water injection in the Central Graben in the North Sea off-shore Denmark, Norway and the UK, but the studied phenomena are general to injection fracturing irrespective of geographical location and technological application. The complexity of reality is a challenging factor when mathematical models of hydraulic fractures in the subsurface are formulated. To this end the finite element method and a multi physics approach is instrumental, and data from fields developed and operated under conditions prudent for modelling are equally important. In the present study, a poroelastic finite element model is applied and used under plane strain conditions for a linear fracture mechanics investigation. This approach is suitable for analysing fracturing in arrays of parallel and horizontal wells as can be found in many oil fields undergoing water-flooding. Based on realistic field data, e.g. reservoir properties and rate and pressure scenarios representative for typical fields in the North Sea region, fracture initiation and fracture growth are analysed in details in a realistic field setting. Using a formulation of the J-integral, that includes Functionally Graded Material and poroelastic effects, it is demonstrated that formation fracture toughness determines fracture initiation and fracture height. The fracture propagation speed is controlled by continuity, e.g. the injection rate must balance the leak-off from the fracture wall. By combining the fields of geotechnical engineering, petroleum engineering and mechanical engineering with realistic data measurements, this study provides a novel realistic study of how the producer and injector pressure influence the fracturing process for induced hydraulic fractures in a line drive.

Investigation of Microstructure and Fracture Mechanism of Al-5.0Mg Alloys Fabricated by Wire Arc Additive Manufacturing

Investigation of Microstructure and Fracture Mechanism of Al-5.0Mg Alloys Fabricated by Wire Arc Additive Manufacturing

Timely and atomic-resolved high-temperature mechanical investigation of ductile fracture and atomistic mechanisms of tungsten

Revealing the atomistic mechanisms for the high-temperature mechanical behavior of materials is important for optimizing their properties for service at high-temperatures and their thermomechanical processing. However, due to materials microstructure’s dynamic recovery and the absence of available in situ techniques, the high-temperature deformation behavior and atomistic mechanisms of materials are difficult to evaluate. Here, we report the development of a microelectromechanical systems-based thermomechanical testing apparatus that enables mechanical testing at temperatures reaching 1556 K inside a transmission electron microscope for in situ investigation with atomic-resolution. With this unique technique, we first uncovered that tungsten fractures at 973 K in a ductile manner via a strain-induced multi-step body-centered cubic (BCC)-to-face-centered cubic (FCC) transformation and dislocation activities within the strain-induced FCC phase. Both events reduce the stress concentration at the crack tip and retard crack propagation. Our research provides an approach for timely and atomic-resolved high-temperature mechanical investigation of materials at high-temperatures.

Investigation of Ductile Fracture Mechanism in Multipass Drawing of Hollow Specimen

Investigation of Ductile Fracture Mechanism in Multipass Drawing of Hollow Specimen

Investigation of the Effect of Inhomogeneous Material on the Fracture Mechanisms of Bamboo by Finite Element Method.

Bamboo is a remarkably strong and sustainable material available for construction. It exhibits optimized mechanical characteristics based on a hollow-inhomogeneous structure which also affects its fracture behavior. In this study, the aim is to investigate the effect of material composition and geometrical attributes on the fracture mechanisms of bamboo in various modes of loading by the finite element method. In the first part of the investigation, the optimized transverse isotropy of bamboo to resist transverse deformation was numerically determined to represent its noticeable orthotropic characteristics which prevail in the axial direction. In the second part of this study, a numerical investigation of fracture mechanisms in four fundamental modes of loading, namely bending, compression, torsion, and shear, were conducted by considering the failure criterion of maximum principal strain. A crack initiation stage was simulated and compared by implementing an element erosion technique. Results showed that the characteristics of bamboo’s crack initiation differed greatly from solid geometry and homogeneous material-type models. Splitting patterns, which were discerned in bending and shear modes, differed in terms of location and occurred in the outside-center position and inside-lowermost position of the culm, respectively. The results of this study can be useful in order to achieve optimized strength in bamboo-inspired bionic designs.

Investigation of Fracture Behavior and Mechanism in High-Speed Precise Shearing for Metal Bars with Prefabricated Fracture-Start Kerfs.

A laser-assisted high-speed shearing (LAHSS) method has been proposed for metal bars, which prefabricates equally spaced fracture-start kerfs by Nd:Yag laser to make stress concentration, and applies a high-speed load to complete fracture separation. Comparative tests were conducted for Q235, 40Cr, and 304 steel bars, and the effects of fracture-start kerfs and axial clearance were investigated on the fracture section. Moreover, the fracture behavior was demonstrated by numerical simulation, and the micro-fracture mechanism was revealed by fractographic analysis. The numerical simulation results show that the material damage concentrates along with the kerf tips with peak equivalent plastic strain, and the corresponding stress triaxiality drops to almost zero at the kerf tip, which reveals that the material is subjected to pure shearing at kerf tip; the Max. loading force is reduced by 15.2%–29.6%, and the impact energy is decreased by 29.8%–46.9% for the three types of bar material. The experimental results showed that the fracture-start kerfs effectively inhibited the plastic deformation stage, and higher precision blanks were obtained in the LAHSS test: roundness error improved from 2.7%–10.9% to 1.1%–2.6%, Max. bending deflection decreased from 1.3–3.4 mm to 0.4–1.0 mm, and flatness error dropped from 0.9–3.3 mm to 0.3–0.7 mm. The fractographic analysis reveals that the crack initiation is related to alternative V-shape micro-notches at the laser-affected zone; the predominant fracture mechanism involves mode II microvoid coalescence at the main fracture plane; smaller and less elongated dimples were formed in 40Cr steels due to higher number density of grains and pinning effect of second-phase particles compared to Q235 and 304 steel bars.

Elastic plastic fracture mechanics investigation of toughness of wet colloidal particulate materials: Influence of saturation

HypothesisPrevious use of linear elastic fracture mechanics to estimate toughness of wet particulate materials underestimates the toughness because it does not account for plastic deformation as a dissipation mechanism. Plastic deformation is responsible for the majority of energy dissipated during the fracture of wet colloidal particulate materials. Plastic deformation around the crack tip increases with saturation of the particulate body. The toughness of the body increases with increasing saturation. ExperimentsElastic plastic fracture mechanics using the J-integral approach was used for the first time to measure the fracture toughness (JIC) of wet micron sized alumina powder bodies as a function of saturation. The samples were prepared by slip casting. The saturation was controlled by treatment in a humidity chamber. The elastic modulus (E) and the energy dissipated by plastic flow (Apl) were measured in uniaxial compression. The critical stress intensity factor (KIC) was measured using a diametral compression sample with a flaw of known size. The fracture toughness (JIC) was calculated from these measured quantities and the geometry of the specimen. FindingsElastic plastic fracture mechanics was used for the first time to quantitively account for plastic deformation of wet particulate materials. The linear elastic fracture mechanics approach previously used accounted for less than 1% of the total energy dissipated in fracture. Toughness (JIC) was found to increase with increasing saturation due to plastic deformation that increased with saturation level. The improved understanding of toughness as a function of saturation will aid in providing quantitative analysis of cracking in drying colloidal films and bodies.

Fracture mechanical analyses of high strength steels applying experiments and simulation

The paper contains information about the experimental and simulation test results of thermomechanically rolled high strength steel. The S960TM material grade was produced by a modern technological process, which provide longer estimated life time, better performance, and due to higher strength enable smaller applied wall thickness, which require less welding activity. The only disadvantage of this material grade is that they can contain material discontinuities in their microstructure. The characterisation of the response of these materials against load can be analysed by fracture mechanical investigation. Fracture mechanical experiments were carried out with three-point bending method, calculating fracture toughness value of crack-tip opening displacement (CTOD). To reduce the complicated real experiment in fracture mechanics, the simulation possibility of fracture mechanical behaviour of the investigated material was applied, using DEFORM software, which gave an acceptable narrowing to the measured phenomenon.