Initial Fracture Energy Research Articles

Micromechanical Properties of Microstructured Elastomeric Hydrogels.

Local, micromechanical environment is known to influence cellular function in heterogeneous hydrogels, and knowledge gained in micromechanics will facilitate the improved design of biomaterials for tissue regeneration. In this study, a system comprising microstructured resilin-like polypeptide (RLP)-poly(ethylene glycol) (PEG) hydrogels is utilized. The micromechanical properties of RLP-PEG hydrogels are evaluated with oscillatory shear rheometry, compression dynamic mechanic analysis, small-strain microindentation, and large-strain indentation and puncture over a range of different deformation length scales. The measured elastic moduli are consistent with volume averaging models, indicating that volume fraction, not domain size, plays a dominant role in determining the low strain mechanical response. Large-strain indentation under a confocal microscope enables the visualization of the microstructured hydrogel micromechanical deformation, emphasizing the translation, rotation, and deformation of RLP-rich domains. The fracture initiation energy results demonstrate that failure of the composite hydrogels is controlled by the RLP-rich phase, and their independence with domain size suggested that failure initiation is controlled by multiple domains within the strained volume. This approach and findings provide new quantitative insight into the micromechanical response of soft hydrogel composites and highlight the opportunities in employing these methods to understand the physical origins of mechanical properties of soft synthetic and biological materials.

Influence of cement content and maximum aggregate size on the fracture parameters of magnetite concrete using WFM, SEM and BEM

Fracture behavior is an important issue that should be taken into account in analysis and design of important concrete structures such as nuclear power plants. The fracture behavior of concrete is dependent on its constituents. Thus, due to the application of heavyweight aggregates with high densities in heavyweight concrete (HWC), the structure and fracture behavior of this type of concrete will be different from normal concrete. In this experimental research, the effects of maximum aggregate size (MAS = 12.5, 19, 25 mm), cement content (350, 400 kg/m3) and water to cement ratio (w/c = 0.45, 0.65) on the fracture parameters of heavyweight magnetite concrete were studied using three methods: Work of Fracture Method (WFM), Size Effect Method (SEM) and Boundary Effect Method (BEM). The results show that with increasing MAS, fracture energy and ductility increase and this trend is more obvious for lower cement contents and water to cement ratios. In addition, peak loads of heavyweight concrete beams can be acceptably estimated using initial fracture energy (Gf). It was also found that size independent fracture energy values obtained by BEM and Elices et al. methods are close to each other.

Investigation of size effect and smeared crack models in ordinary and dam concrete fracture tests

This study aims to investigate the size effect and smeared crack models in concrete fracture tests, and explore whether such non-linear models can show the effects of size or not. In the present analysis, the fixed smeared crack model (FCM) was developed for cracked concrete specimens of laboratory size under three-point bending and wedge splitting, and the different fracture parameters of the size effect model were evaluated using the data obtained from the laboratory and numerical analysis. According to the analysis, the need for a separate examination of the effects of material properties of ordinary concrete and dam concrete was shown. Besides, even the creation of specific equations for them which was due to the effect of the fracture energy, the effective length of the fracture process zone (FPZ) and the fracture toughness was clearly demonstrated in the size effect and the related numerical equations. It is concluded that the fracture parameters obtained from Bažant’s size effect model and other nonlinear fracture models including the FCM are influenced by the specimen size. The results indicate the relative weakness and decay of this type of the FCM described in this study can be used to better describe the effects of the size, especially when the concepts of the initial and total fracture energy are not considered.

Effect of magnetic field intensity on fracture behaviours of self-compacting lightweight concrete

The effects of water subjected to an electromagnetic field on the fracture parameters and mechanical properties of self-compacting lightweight concrete (SCLC) were investigated. The test variables were the magnetic field intensity (MFI) used to treat the water and the water/cement (w/c) ratio. Eight mix compositions with various MFIs and two w/c ratios (0·37 and 0·42) were considered. For each w/c ratio, the nominal maximum aggregate size and all mix designs were constant, while four magnetic fields were considered. Three-point bending tests were conducted on 96 notched beams and the results were analysed by means of the size effect method. Satisfactory results were achieved in terms of the fracture parameters and the mechanical properties of the SCLC. By increasing the MFI, the results indicated that: (a) the mechanical properties, initial fracture energy and fracture toughness increased at different rates, which can be attributed to the positive effects of the magnetic field on water clusters; (b) the effective length of the fracture process zone increased, illustrating an improvement in ductility of the SCLC specimens; (c) the desired design criterion of the SCLC samples approached the strength criterion.

Opposite effects of ground granulated blast-furnace slag and silica fume on the fracture behavior of self-compacting lightweight concrete

In this study, an experimental investigation is presented on the fracture behavior of self-compacting lightweight concrete (SCLC) including different contents of cement replacement materials. Portland cement was substituted with silica fume (SF) and ground granulated blast-furnace slag (GGBFS) in different proportions with and without magnetic water (MW) for this purpose. The results displayed that cement replacement materials in SCLC decreased the rheology of concrete. At hardened state, fracture behavior and mechanical properties were investigated at the age of 28 days. For determination of SCLC fracture parameters three-point bending tests were carried out on 120 notched beams. The test results showed that when the SF content increased from 0 to 5 and 10% of the cement weight, the initial fracture energy improved by 19 and 24%, and the fracture toughness increased by 14 and 18%, respectively. However, when the GGBFS content increased from 0 to 10 and 20%, the initial fracture energy decreases by 11 and 19%, and fracture toughness decreased by 7 and 13%, respectively. Moreover, it was observed that the SCLC incorporating 10% of SF and MW was the best mix design. This mix improved the initial fracture energy, fracture toughness, tensile strength and compressive strength of SCLC up to 31%, 22%, 25% and 34%, respectively.

Combined effects of steel fibers and water to cementitious materials ratio on the fracture behavior and brittleness of high strength concrete

This experimental program investigated the effect of steel fibers and water to cementitious materials ratio (w/c) on the fracture characteristic of plain high strength concrete (HSC) and high strength steel fiber reinforced concrete (HSSFRC). In this research, the w/c ratios of 0.24 and 0.35 as well as four volumetric steel fiber contents of 0.2%, 0.4%, 0.6%, and 1% were examined. Two methods including size effect method (SEM) and work of fracture method (WFM) were applied to determine the fracture properties of concrete. Based on the experimental results, by increasing the steel fibers and decreasing the w/c ratio in HSSFRC, the total fracture energy (GF), characteristic length of concrete (Lch), initial fracture energy (Gf) and fracture toughness (KIC) were increased. Also, for steel fiber content greater than 0.2%, the effective length of fracture process zone (Cf) was increased; however, the brittleness number (β) was reduced. The GF/Gf ratio from 2.53 and 2.5 in HSC was increased to 11.88 and 10.26 in HSSFRC when w/c ratio was increased from 0.24 to 0.35.

An experimental investigation on fracture parameters and brittleness of self-compacting lightweight concrete containing magnetic field treated water

In this study, an experimental research on the fracture parameters of self-compacting lightweight concrete (SCLC) using tap and magnetic water (MW) is presented. For all SCLC mixtures, common fracture parameters were specified by means of both work of fracture method (WFM) and the size effect method (SEM). Three-point bending tests were carried out on 90 notched beams with six mix compositions. The beams were made from mixes with three different water to cement (w/c) ratios of 0.37, 0.47 and 0.52; also, both tap and magnetic water were consumed in the concrete mixes, separately. The results of this research showed that by using MW in SCLC and decreasing w/c ratio from 0.52 to 0.37: (a) both initial fracture energy Gf and total fracture energy GF increased; (b) the fracture toughness and the mechanical properties of SCLC improved. Additionally, the achieved data exhibited that there was a reasonable relation between the total fracture energy obtained by WFM and the initial fracture energy achieved through SEM. The average of GF to Gf ratio in SCLC was 2.70. Furthermore, by using MW in SCLC mixtures, compressive strength, splitting tensile strength and modulus of elasticity increased up to 18%, 9% and 10%, respectively.

Relationships Between Stretch-Flangeability and Microstructure-Mechanical Properties in Ultra-High-Strength Dual-Phase Steels

To clarify the direction of microstructure design for improving stretch-flangeability, relationships of stretch-flangeability to microstructure and mechanical properties of ultra-high-strength dual-phase (DP) steels were investigated. Microstructure of relatively simple ferrite-martensite DP steels was modified by intercritical annealing, then the effects of microstructure modification on stretch-flangeability, tensile properties, and fracture resistance of the DP steels were systematically quantified. The hole-expansion ratio (HER) increased linearly with an increase the apparent fracture initiation energy, but was not significantly correlated with any individual microstructural properties of DP steels, which have been reported to correlate with HER (e.g., the fraction of martensite, the carbon content of martensite, or the hardness difference between ferrite and martensite). To increase the stretch-flangeability of an ultra-high-strength DP steels, its microstructure should be designed to increase its fracture toughness (i.e., microstructure with low mechanical heterogeneity).

Effect of Polymer Volume Fraction on Fracture Initiation in Soft Gels at Small Length Scales.

The influence of polymer volume fraction, ϕv on fracture initiation via puncture is studied in self-assembled triblock copolymer gels. Spherically tipped indenters of radii varying over a wide range were used to characterize puncture at length scales on the same order of magnitude as the elasto-capillary length (∼μm) and significantly below the elasto-fracture length (∼mm) for ϕv = 0.12-0.53. Critical energy release rate, Gc for ϕv = 0.12-0.30 was found to be in agreement with the predicted scaling from the classical Lake-Thomas model modified for gel fracture via the failure mechanism of chain pull-out and plastic yielding of micelles (Gc ∼ ϕv2.2). Interestingly, we demonstrate that fracture initiation energy, o, from puncture scales as o ∼ ϕv, thus, indicating the role played by different fundamental mechanisms governing fracture initiation in soft gels. Additionally, gels with ϕv = 0.53 show deviation from experimental scalings for Gc and o, likely due to a change in micellar morphology leading to anomalous fracture behavior.

Cohesive zone models to understand the interface mechanics of thin film transfer printing

Competing fracture in the transfer of thin films from a relatively rigid host substrate to a flexible polymer substrate is studied using finite element simulations with cohesive zone models. Cohesive zone models for delamination based on traction-separation relations with a maximum stress criterion for damage initiation and mode-independent fracture energy for complete separation are explored to identify important parameters that affect transfer printing. Successful transfer of a thin film to a relatively compliant polymer substrate from a stiffer substrate depends on relative crack lengths, interface strengths, and fracture energies. Interface selection occurs where the mode-mix at the crack tip is predominantly due to normal stresses, despite the interface toughness being mode-independent. The observations and the fracture maps developed here predict the interface selection directly with material properties of the interfaces, substrates, and films.

Tensile properties of plastic concrete and the influence of temperature and cyclic loading

The cracking of plastic concrete remains a problem in the construction industry due to its complexity and incomplete understanding. One of the reasons for this is the lack of knowledge regarding the tensile material properties of the plastic concrete. This paper describes and builds the appropriate test setup needed for the challenging tensile testing of plastic concrete as well as discusses the measured tensile properties. The tensile strength and Young's modulus increase exponentially near the final setting time and initial setting time respectively. The strain capacity and initial fracture energy decrease significantly between the initial and final setting times. The tensile properties develop faster and with higher values the greater the ambient temperature. Plastic concrete proved to be resilient and capable of withstanding multiple loading and ceased loading cycles, while a solid but still weak concrete could not. The tests also highlighted the need for further research regarding the influence of hydrostatic pressure and the relaxation of stresses in plastic concrete.

Weld metal fracture characterization of API X65 steel using drop weight tear test

Measurement of fracture energy using drop weight tear test (DWTT) specimen with large dimensions (as an alternative to small Charpy test sample) gives more realistic results. Although large amounts of test data are available from DWTT experiments on different pipeline steels (base metal), apparently no information has been reported for fracture energy of welded seam of these pipe steels. In the current study, the fracture energy of spirally welded steel pipe was measured using DWTT specimens with two different notches (i.e. pressed and with chevron notch) and compared with base metal (BM) fracture energy. The test samples were machined from an actual gas pipe with a spiral seam having 1219 mm outside diameter and 14.3 mm wall thickness. The DWT tests were conducted on an impact machine with 30 kJ capacity equipped with Wheatstone load-cell. The measured total fracture energy in pressed-notch (PN) specimen (6211 J) was around 5% higher than that of chevron-notch (CN) specimen (5890 J). Based on the measurements, in both specimens a correction factor of 1.8 was suggested for welded seam (the fracture initiation energy was around 43% of the total fracture energy). Since the test samples were taken from a real pipe with original thickness, this result was more realistic than those previously reported for conventional Charpy fracture test specimens for similar steel material. The fracture energies for weld metal (WM) specifications were less than that of the base metal (7085 J). Therefore, immediate attention must be paid to DWTTs of WM in pipelines, which had not been investigated before, and the results indicate the importance of urgent improvement of the toughness of welded zone in manufacturing processes, which requires further research. Since, there is no standard for WM in the DWTT, the results of this research can lead to the development of new standards based on WM DWTT.

Size Effect Analysis for the Characterization of Marcellus Shale Quasi-brittle Fracture Properties

Mechanical characterization of shale-like rocks requires understanding the scaling of the measured properties to enable the extrapolation from small scale laboratory tests to field study. In this paper, the size effect of Marcellus shale was analyzed, and the fracture properties were obtained through size effect tests. A number of fracture tests were conducted on Three-Point-Bending (TPB) specimens with increasing size. Test results show that the nominal strength decreases with increasing specimen size, and can be fitted well by Bazant's Size Effect Law (SEL). It is shown that SEL accounts for the effects of both specimen size and geometry, allowing an accurate identification of the initial fracture energy of the material, Gf, and the effective Fracture Process Zone (FPZ) length, cf. The obtained fracture properties were verified by the numerical simulations of the investigated specimens using standard Finite Element technique with cohesive model. Significant anisotropy was observed in the fracture properties determined in three principal notch orientations: arrester, divider, and short-transverse. The size effect of the measured structural strength and apparent fracture toughness was discussed. Neither strength-based criterion which neglects size effect, nor classic LEFM which does not account for the finiteness of the FPZ can predict the reported size effect data, and nonlinear fracture mechanics of the quasibrittle type is instead applicable.

Describing Snow Instability by Failure Initiation, Crack Propagation, and Slab Tensile Support

AbstractSnow instability is a generic term describing the propensity of a snow slope to avalanche. In need of a concise mechanics‐based concept we suggest a framework based on failure initiation, crack propagation, and slab tensile support. Following these three steps we modeled three metrics from mechanical data, which we derived from snow micropenetrometer signals. Verifying the metrics with field measurements confirmed that slab thickness and weak layer strength typically influence failure initiation, elastic modulus and weak layer fracture energy largely control crack propagation, and slab thickness and tensile strength provide the required tensile support. For all three metrics, considering slab layering was essential. Validation with signs of instability showed that the most accurate model includes all three steps – suggesting that snow instability can be described by failure initiation, crack propagation, and slab tensile support. Further validation is needed to assess the framework's potential for operational use.

Parametric analysis of dynamic crack propagation of concrete bending beam based on the extended finite element method

The fracture properties of concrete are affected by various factors, including material parameters and crack geometry parameters. In the paper, pure mode I fracture and mixed mode fracture of concrete bending beams under impact load were studied experimentally and numerically. Numerical models by applying extended finite element method based on the cohesive crack model were employed to simulate pure mode I fracture and mixed mode fracture of concrete bending beams, an experimental program was conducted to validate the numerical models. The effects of material parameters as well as crack geometry parameters on the fracture properties of concrete bending beams were discussed and the most sensitive parameter to affect the fracture properties of concrete bending beams was determined based on the numerical simulations. The results show that the simulation results match reasonably well with the experimental results. For pure mode I fracture, concrete strength grade is proved to have less influence on the fracture properties of concrete bending beams under relative high strength grade condition; Elastic modulus was determined to have the greatest impact on the fracture properties of concrete bending beams, followed by tensile stress, initial crack length and fracture energy; however, Poisson ratio have almost no effect on the fracture properties of concrete bending beams. For mixed mode fracture problem, the influences of initial notch position on crack propagation path can be expressed as a competition between mode I fracture and mixed mode fracture, with offset parameter increasing from 0.0 to 1.0, the fracture failure mode of concrete bending beams changes from pure mode I fracture to mixed mode fracture, followed by both mode I and mixed mode fractures, and finally translates to mode I fracture.

Stretch-flangeability of twinning-induced plasticity steel-cored three-layer steel sheet

In this study, the stretch-flangeability of twinning-induced plasticity (TWIP)-cored three-layer steel (TWCLS) sheet was evaluated. Although the mechanical properties of TWCLS sheet follow the force-based rule-of-mixtures (ROM), the experimental hole expansion ratio (HER) is less than the predicted HER values. The poor HER in TWCLS sheet originates from crack initiation at the fracture zone of the TWIP steel-core. The fracture initiation energy vs. HER in TWCLS sheet reveals that the layered material follows the behavior of the monolithic material when the sound interface is conserved. If the initial defects are removed by a reaming process in the hole, the HER of TWCLS sheet follows the force-based ROM. This result shows that the stretch-flangeability of the layered material can be controlled by managing the volume fraction of parent materials when the sources of extrinsic damage (i.e, interface delamination and defects) are removed.

COHESIVE FINITE ELEMENT SIMULATIONS OF CONTRACTION AND SHAPE EFFECTS ON CELL DE-ADHESION

Cohesive-interface-based finite element simulations were conducted to investigate the critical shear stress required for cell de-adhesion from extracellular substrates. The interface ligand–receptor bonds are modeled by a cohesive interface model with initial stiffness, interface strength, and fracture energy as the governing parameters. The ratio of the cell modulus to the interface stiffness defines a length scale. If this length is much less than the contact size, the de-adhesion process can be modeled by the linear elastic fracture mechanics, while the opposite limit leads to the concurrent sliding of the cell or, equivalently, debonding of all the interface ligand–receptor pairs. Since it generates additional shear-stress concentration along the interface, cell contraction generally reduces the critical de-adhesion stress. Cell de-adhesion is more prone to take place for three-dimensional irregular cell shapes because of the much easier failure in the anti-plane Mode III shear, as well as the additional stress concentration in these geometric irregularities.

Crack growth and fracture toughness of amorphous Li-Si anodes: Mechanisms and role of charging/discharging studied by atomistic simulations

Fracture is the main cause of degradation and capacity fading in lithiated silicon during cycling. Experiments on the fracture of lithiated silicon show conflicting results, and so mechanistic models can help interpret experiments and guide component design. Here, large-scale K-controlled atomistic simulations of crack propagation (R-curve KI vs. Δa) are performed at LixSi compositions x=0.5,1.0,1.5 for as-quenched/relaxed samples and at x=0.5,1.0 for samples created by discharging from higher Li compositions. In all cases, the fracture mechanism is void nucleation, growth, and coalescence. In as-quenched materials, with increasing Li content the plastic flow stress and elastic moduli decrease but void nucleation and growth happen at smaller stress, so that the initial fracture toughness KIc≈1.0MPam decreases slightly but the initial fracture energy JIc ≈ 10.5J/m2 is similar. After 10 nm of crack growth, the fracture toughnesses increase and become similar at KIc≈1.9MPam across all compositions. Plane-strain equi-biaxial expansion simulations of uncracked samples provide complementary information on void nucleation and growth. The simulations are interpreted within the framework of Gurson model for ductile fracture, which predicts JIc=ασyD where α ≃ 1 and D is the void spacing, and good agreement is found. In spite of flowing plastically, the fracture toughness of LixSi is low because voids nucleate within nano-sized distances ahead of the crack (D ≈ 1nm). Scaling simulation results to experimental conditions, reasonable agreement with experimentally-estimated fracture toughnesses is obtained. The discharging process facilitates void nucleation but decreases the flow stress (as shown previously), leading to enhanced fracture toughness at all levels of crack growth. Therefore, the fracture behavior of lithiated silicon at a given composition is not a material property but instead depends on the history of charging/discharging. These findings indicate that the mechanical behavior (flow and fracture) of lithiated Si must be interpreted within a fully rate- and history-dependent framework.

Investigation of fatigue cracks on RAP mixtures containing Sasobit and crumb rubber based on fracture energy

Fatigue cracking, rutting and low temperature cracking are three major distresses of asphalt mixtures. In last years, different methods based on fracture mechanic have been developed to measure performance of asphalt mixtures due to cracking. In this study, fracture mechanic method developed by Roque et al. has been employed to evaluate fatigue cracking performance of modified asphalt mixtures. Samples containing Reclaimed Asphalt Pavement (RAP) with different percentages of Sasobit (as a Warm Mix Asphalt additive) and crumb rubber were built. Resilient Modulus, static creep and Indirect Tensile (IDT) tests also conducted on samples and Resilient Modulus, creep compliance and Indirect Tensile Strength were measured. Based on fracture mechanic, Dissipated Creep Strain Energy (DCSE) was calculated for each sample, and fatigue strength of mixtures were investigated. The results indicated that the use of RAP, crumb rubber and Sasobit simultaneously caused desirable fatigue performance of modified mixtures. These mixtures had better Resilient Modulus and more loading capacity, as well as, lower cumulative damage rates rather than virgin sample. The ITS results also indicated more appreciable improvement in crack initiation threshold, Fracture Energy (FE) and Energy Ratio (ER).

Key factors of stretch-flangeability of sheet materials

Stretch-flangeability evaluated using hole-expansion testing represents the ability of sheet materials to resist edge fracture during complex shape forming. Despite a property imperative for automotive part applications of advanced high-strength steels, factors governing stretch-flangeability are not yet well understood. In this study, the mechanical properties of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion tests to find the factor governing the stretch-flangeability that is universally applicable to a variety of metallic materials. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability. We verified that fracture toughness is the key factor governing stretch-flangeability, showing that the hole-expansion ratio could be well predicted using finite element analysis associated with a simple ductile damage model, without explicitly taking into account the microstructural complexity of each specimen. This validates the use of the fracture toughness as a key factor of stretch-flangeability.