Crack Propagation In Specimens Research Articles

Fracture energy of concrete after sustained loading

The fracture energy of concrete after sustained loading is a necessary parameter to calculate the residual carrying capacity of concrete structures in service. This study investigated the fracture energy of concrete after sustained loading by combining experimental observations and theoretical analyses. Firstly, a series of creep fracture tests were conducted on 100 three-point bending beams under various sustained loads, i.e., 75 %, 80 %, 85 %, and 90 % of peak loads. After different loading time, i.e., 10 %, 30 %, 50 %, 70 %, and 90 % of creep fracture lifetimes, the static fracture tests were performed until failure. Then, based on the relationship between the work done by external forces and the energy consumed by the crack propagation of specimens, an analytical model for the fracture energy of concrete after sustained loading was developed, describing the quantitative relationship between the fracture energy of concrete and the crack opening displacement, and the loading time. Finally, the impact of the loading time and sustained load levels on the fracture energy was investigated. The results indicated that the fracture energy of concrete after sustained loading decayed rapidly, with the decay rate gradually decreasing until stability over the loading time. Additionally, specimens subjected to lower sustained load levels experienced a longer loading time for the same crack opening displacement, indicating a greater decay degree of the fracture energy. The analytical model proposed in this study can be employed for predicting and evaluating the crack propagation process and residual carrying capacity of concrete structures during their service life.

A coupled thermomechanical peridynamic correspondence model for damage prediction in a freezing rock

This study presents a coupled thermomechanical (TM) bond-associated non-ordinary state-based peridynamic (BA-NOSB PD) (correspondence) model for simulating the thermal behavior and crack propagation in a saturated rock due to freeze. Considering the change in thermal property and the latent water–ice phase transition at low-temperature conditions, the PD form of the nonlinear thermal conduction equation is derived by using the PD differential operator (PDDO). The deformation response is obtained in the framework of BA-NOSB PD model considering the pore ice pressure and thermal expansion. The accuracy of the TM BA-NOSB PD model is verified by simulating the thermal behavior of tuff and sandstone and crack propagation in specimens with pre-existing cracks at low-temperature conditions. The PD predictions of temperature and frost heaving strain are in close agreement with the experimental and previous numerical results. The present model also accurately predicts crack propagation and coalescence in saturated rocks due to the freezing frost.

Phase field fracture model for additively manufactured metallic materials

Phase field models have been extensively studied in the analysis of ductile fracture. However, few have been explored to simulate additively manufactured materials and validated by experimental data to date. This study aims to develop a phase field framework for modelling complex mechanical behaviours of laser powder bed fusion printed metallic materials. To consider the laser powder bed fusion induced microstructural orientation, transversely isotropic Hill48 and modified Mohr-Coulomb constitutive models are incorporated here to depict the plastic and fracture behaviours, respectively. Five types of material specimens representing a wide spectrum of stress states are designed to identify the material parameters of plasticity and fracture. Three groups of crack propagation specimens are also tested to demonstrate the capacity of the developed model. The numerical results divulge that, by considering the stress state-dependent crack initiation, the proposed phase field model is able to better reproduce force-displacement responses of all specimens. Remarkably, the complex cracking sequences, including crack initiation, propagation and final rupture, can be properly captured by the proposed phase field model. More importantly, it is necessary to apply a transversely isotropic fracture model to characterise the orientation-dependent fracture behaviour; otherwise, the crack path would be predicted incorrectly, resulting in unacceptable global force-displacement responses. This study offers an effective phase field modelling framework for the sophisticated constitutive characterisation of laser powder bed fusion fabricated metallic materials.

Evaluating MTS criterion in predicting mixed‐mode crack extension under different loading conditions

AbstractThe maximum tangential stress (MTS) criterion is one of the most widely used criteria for predicting the direction of crack extension. The suitability of this criterion is examined under different loading conditions using the extended finite element method (XFEM). Experimental and numerical results reported in the literature are considered to evaluate the validity and accuracy of the criterion. The results demonstrate that the MTS criterion evaluated by stress intensity factors (SIFs) can accurately predict the direction of crack propagation in specimens under direct tensile loading. This criterion overestimates the angle of crack initiation in the specimen under indirect tensile loading but underestimates the angle in the specimen subjected to 3‐point bending. It is concluded that the MTS criterion based on SIFs could not accurately predict the direction of the crack initiation, which could, however, be determined properly based on the stress distribution around the crack tip obtained by the XFEM numerical models.

Fatigue crack propagation and fracture toughness of cortical bone are radiation dose-dependent.

Cortical bone allograft sterilized with a standard γ-radiation dose of 25-35kGy has demonstrated reduced static and cyclic fracture resistance compared with unirradiated bone. To mitigate radiation damage, we recently observed a dose-dependent response of high-cycle fatigue behavior of human cortical bone from 0 to 25 kGy, with lower doses exhibiting logarithmically longer fatigue lives. The objectives of this study were as follows: (1) to determine whether fracture toughness, work-to-fracture, and fatigue crack propagation resistance of human cortical bone are also radiation dose-dependent, and (2) to determine the associations of radiation dose and a Raman biomarker for collagen disorder with fracture properties. Compact tension specimens were machined from two donor femoral pairs and allocated to four treatment groups: 0 (unirradiated control), 10, 17.5, and 25 kGy. Fracture toughness specimens were monotonically loaded to failure and the critical stress intensity factor (KC ) was determined. Work-to-fracture was calculated from the load versus displacement integral up to fracture. Fatigue crack propagation specimens were cyclically loaded under constant room-temperature irrigation and fatigue crack growth rate (da/dN) and cyclic stress intensity (∆K) were calculated. Fracture toughness, work-to-fracture, and fatigue crack propagation resistance decreased 18%, 33%, and 15-fold from 0 to 25 kGy, respectively (p < 0.05). Radiation dose was more predictive of fracture properties than collagen disorder. These findings support that quasi-static and fatigue fracture properties of cortical bone are radiation dose-dependent within this dose range. The structural alterations arising from irradiation that cause these losses in fracture resistance remain to be elucidated.

Quasi-static fracture analysis by coupled three-dimensional peridynamics and high order one-dimensional finite elements based on local elasticity.

This work investigates quasi‐static crack propagation in specimens made of brittle materials by combining local and non‐local elasticity models. The portion of the domain where the failure initiates and then propagates is modeled via three‐dimensional bond‐based peridynamics (PD). On the other hand, the remaining regions of the structure are analyzed with high order one‐dimensional finite elements based on the Carrera unified formulation (CUF). The coupling between the two zones is realized by using Lagrange multipliers. Static solutions of different fracture problems are provided by a sequential linear analysis. The proposed approach is demonstrated to combine the advantages of the CUF‐based classical continuum mechanics models and PD by providing, in an efficient manner, both the failure load and the shape of the crack pattern, even for three‐dimensional problems.

Cyclic performance of RC beam-column joints with mechanical or forging (GPW) splices; an experimental study

Beam-column joints affect the performance of structures remarkably, therefore, finding an appropriate method for their construction has always been one of the major concerns of researchers. In the present study, the influence of incorporating mechanical and forging (gas pressure welding) splices in longitudinal reinforcement bars of beams on the cyclic performance of RC beam-column joints is evaluated experimentally. Totally five RC joints with non-spliced, mechanically (threaded couplers) spliced and forging spliced bars were examined under axial and cyclic loads applied to columns and beams, respectively. Crack propagation of specimens are analyzed and load-drift hysteresis curves are provided as test outputs. Moreover, the influence of the splice method and number of spliced bars on joints’ behavior is assessed in terms of seismic parameters including displacement ductility, stiffness and cumulative absorbed energy. Results showed that forging splices can be considered as an efficient and reliable method for bar splicing and mechanical splices might affect joints’ behavior significantly.

Dynamic crack propagation in specimens with a surface irregularity

Initiation of cracks and their propagation in prescribed orientations on tubular specimens has been made possible on a dynamic fatigue testing rig developed at the University of Aberdeen. This rig was originally designed to perform experiments on single edge notched beams (SENB) [1,2]. Modifications have recently been made so as to accommodate experimental tests on tubular specimens with a range of sizes and other cross sections. Crack initiation at grooves on such specimens has been followed by crack growth. At the same time, lateral oscillations of cracked specimen have been measured, as well as accelerations of base excitation, masses above and below cracked specimen. Forces on these two masses have been observed by two load cells attached at positions close to specimen. These load cells facilitated the measurement of stresses in experiment. Crack length time histories have also been constructed by applying an alternating current potential difference (ACPD) method. Fatigue cracks were initiated at the pre-cut grooves in aluminium tubular specimens. Three specimens with different groove sizes were tested in fifteen individual experiments. A three-dimensional Finite Element model was established for each specimen so as to calculate the stress concentration factor (SCF). This formed the basis of determining the amplitude of forcing input made possible by an electromechanical shaker. The phase shifts of acquired time histories has provided some indication of energy transfer mechanism during fatigue and system dynamic response. Observations of cracked specimen displacement during experiments was compared against calculated displacement from elastic theory. Nonlinear responses were observed, suggesting nonlinear stiffness characteristics of the specimen due to discontinuities introduced by crack growth and plasticity effects. Comparison of damage combinations in experimental observation was made with predictions from BS EN 1999-1-3. All experimental observations of total damage combination were higher than predicted values by the latter code.

Dynamics of destruction of epoxy composites filled with ultra-dispersed diamond under impact conditions

Dynamics of destruction of epoxy composites with different contents of nanoparticles of ultra-dispersed diamond is investigated. Epoxy diane oligomer ED-20 was employed as the basic bonding agent in composite formation. It is characterized by its ability of providing high adhesion and cohesive strengths, easy processibility, as well as low coating shrinkage on deposition onto long-length surfaces of complex profile parts. Polyethylene polyamine (PEPA) was used for cross-linking of the epoxy composites, which made it possible to carry out the curing process at room temperature. Formation of interfacial bonds ⟨⟨polymer-filler⟩⟩ due to the improved chemical activity of dispersed particles surface is determined by the method of IR-spectral analysis. The nanoparticles introduction into epoxy binder of an optimal content of q = 0.025 to 0.075 pts.wt. allows to increase the impact resilience 2 times relative to the epoxy matrix. Speed of crack propagation in specimens in time to materials destruction on pendulum is found. Fracture surface of nanocomposite materials is investigated by method of optical and electron microscopy. Based on the analysis of REM image of nanocomposite destruction surface, materials with an ordered structure are found. That allows to state about thermodynamic equilibrium in systems after cross-linking action and about stability of their mechanical strength during the operation process.

Effects of coral, recycled and natural coarse aggregates on the mechanical properties of concrete

The effects of different types of coarse aggregates on mechanical properties of concrete were compared in this paper. Three different coarse aggregates were considered: natural coarse aggregate (NCA), recycled coarse aggregate (RCA) and coral coarse aggregate (CCA). An experimental campaign was developed involving uniaxial compression tests and the use of digital image correlation (DIC) method to obtain the deformation distribution and crack propagation of specimen. The influence of coarse aggregate on deformation development of concrete phases (cement mortar and interface transition zone (ITZ), etc.) was also studied. Test results indicated stress-strain curve (SSC) of concrete using NCA (natural aggregate concrete) was similar to that of concrete adopting RCA (recycled aggregate concrete), while SSC of coral aggregate concrete (concrete using CCA) was different from the others. Compared with coral aggregate concrete (CAC), the failures of natural aggregate concrete (NAC) and recycled aggregate concrete (RAC) were more ductile. But prismatic compressive strength of CAC was higher than other specimens. Coarse aggregate type had obvious influence on the distribution of axial deformation fields of concrete. It indicated that the axial displacements of cement mortar and ITZ were higher than that of NCA before peak point. After mixing different types of coarse aggregates, obvious changes were not found in the displacement distribution of specimen. It was also found that the large strains (axial and transverse) of NAC and RAC mainly concentrated in cement mortar and ITZs before peak stress, however, those of CAC obviously appeared in CCA. Type of coarse aggregate changed the crack propagation of concrete. Compared with NAC and RAC, CAC cracking seldom appeared before peak stress, and cracks drastically expanded to critical size after that. Finally, a normalized analytical expression for different types of concretes was suggested, which could provide a basis for the application of recycled and coral aggregate concretes.

Investigation of creep behaviours of gypsum specimens with flaws under different uniaxial loads

The aim of this study is to identify the influence of the dip angle of a pre-existing macrocrack on the lifetime and ultimate deformation of rock-like material. Prediction of lifetime has been studied for three groups of specimens under axial static compressive load levels. The specimens were investigated from 65% to 85% of UCS (uniaxial compressive strength) at an interval of 10% of UCS for the groups of specimens with a single modelled open flaw with a dip angle to the loading direction of 30° (first group), at an interval of 5% of UCS increment for the groups of specimens with single (second group), and double sequential open flaws with a dip angle to the loading direction of 60° (third group). This study shows that crack propagation in specimens with a single flaw follows the same sequences. At first, wing cracks appear, and then shear crack develops from the existing wing cracks. Shear cracking is responsible for specimen failure in all three groups. A slip is expected in specimens from the third group which connects two individual modelled open flaws. The moment of the slip is noticed as a characteristic rise in the axial deformation at a constant load level. It is also observed that axial deformation versus time follows the same pattern, irrespective of local geometry. Specimens from the first group exhibit higher axial deformation under different load levels in comparison with the specimens from the second and third groups.

Fatigue strength of hybrid welded 22MnB5 overlap joints

Abstract In order to reduce the weight of automotive parts, lightweight materials like the press hardened AlSi coated high strength steel 22MnB5 is used. This steel enables a reduction of wall thicknesses and simultaneously ensures a high stiffness of the part, e. g., the B pillar. The usage of these steels requires suitable joining technologies. Therefore, a laser GMA hybrid welding process for fillet welds in overlap configuration was designed. This process provides high welding velocities and process tolerances against gaps and misalignments. In this case a single-mode fiber laser was used in order to reduce the thermal impact. Due to the fact that the laser focus spot has only a small diameter of 25 µm, a beam oscillation was used in order to distribute the laser energy over the melt pool width. This paper is focused on the cyclic load tests which were carried out and analyzed by the staircase method. The influences of laser power and oscillation width are dominating, whereas welding velocity and wire velocity have only a minor influence. By analyzing the failure location and crack propagation of specimens after the cyclic load test, the influence of the AlSi coating can be determined. Coating agglomerations at the fusion line of the melt pool are origins of crack initiation. This is attributed to the formation of brittle intermetallic phases.

Crack analysis of pre-cracked brittle specimens under biaxial compression

Crack propagation process in rock like specimens has been studied experimentally and numerically considering both; a single crack and double cracks in the center of specimens. The rock-like specimens are specially prepared from Portland Pozzolana Cement (PPC), fine sands and water. These precracked cylindrical specimens are experimentally tested under compressive loading. Effect of confining pressure on the crack propagation path of specimens with single crack is explained first. In these experiments, it has been observed that the crack propagation path is diverted from its rout due to lateral confinement compared to that of the uniaxially loaded pre-cracked experiments. The same experiments are numerically simulated by an indirect boundary element method using higher order displacement discontinuity elements. Effect of lateral stress on the crack propagation of specimens containing single and double cracks have been studied and compared with the experimental results showing a good agreement between them. Effect of confinement on the cracks coalescence in the specimens containing two cracks is also investigated. It has been observed that wing cracks are produced at the first stage of compression and start their propagation toward the direction of uniaxial compressive loading in the absence of confinement. In the presence of confining pressure the propagating wings are diverted related to the uniaxially loaded cracks and showing a mixed mode crack propagation phenomenon due to both induced tension and shear. It is also observed that the crack coalescence path is affected a little bit (due to confinement) compared to that of wing cracks.

Atomic scale investigation of grain boundary structure role on intergranular deformation in aluminium

The role that grain boundary (GB) structure plays on the directional asymmetry of an intergranular crack (i.e. cleavage behaviour is favoured along one direction, while ductile behaviour along the other direction of the interface) was investigated using atomistic simulations for aluminium 〈1 1 0〉 symmetric tilt GBs. Middle-tension (M(T)) and Mode-I crack propagation specimens were used to evaluate the predictive capability of the Rice criterion. The stress–strain response of the GBs for the M(T) specimens highlighted the importance of the GB structure. The observed crack tip behaviour for certain GBs (Σ9 (2 2 1), Σ11 (3 3 2) and Σ33 (4 4 1)) with the M(T) specimen displayed an absence of directional asymmetry which is in disagreement with the Rice criterion. Moreover, in these GBs with the M(T) specimen, the dislocation emission from a GB source at a finite distance ahead of the crack tip was observed rather than from the crack tip, as suggested by the Rice criterion. In an attempt to understand discrepancy between the theoretical predictions and atomistic observations, the effect of boundary conditions (M(T), Mode-I and the edge crack) on the crack tip events was examined and it was concluded that the incipient plastic events observed were strongly influenced by the boundary conditions (i.e. activation of dislocation sources along the GB, in contrast to dislocation nucleation directly from the crack tip). In summary, these findings provide new insights into crack growth behaviour along GB interfaces and provide a physical basis for examining the role of the GB character on incipient event ahead of a crack tip and interface properties, as an input to higher scale models.

The role of grain boundary structure and crystal orientation on crack growth asymmetry in aluminum

Atomistic simulations have shown that the grain boundary (GB) structure affects a number of physical, mechanical, thermal, and chemical properties, which can have a profound effect on macroscopic properties of polycrystalline materials. The research objective herein is to use atomistic simulations to explore the role that GB structure and the adjacent crystallographic orientations have on the directional asymmetry of an intergranular crack (i.e. cleavage behavior is favored along one direction, while ductile behavior along the other direction of the interface) for aluminum grain boundaries. Simulation results from seven 〈110〉 symmetric tilt grain boundaries (STGBs) show that the GB structure and the associated free volume directly influence the stress–strain response, crack growth rate, and crack tip plasticity mechanisms for middle-tension (M(T)) crack propagation specimens. In particular, the structural units present within the GB promote whether a dislocation or twinning-based mechanism operates at the crack tip during intergranular fracture along certain GBs (e.g., the ‘E’ structural unit promotes twinning at the crack tip in Al). Furthermore, the crystallography of the adjacent grains, and therefore the available slip planes, can significantly affect the crack growth rates in both directions of the crack – this creates a strong directional asymmetry in the crack growth rate in the Σ 11 (113) and the Σ 27 (552) STGBs. Upon comparing these results with the theoretical Rice criterion, it was found that certain GBs in this study ( Σ 9 (221), Σ 11 (332) and Σ 33 (441)) show an absence of directional asymmetry in the observed crack growth behavior, in conflict with the Rice criterion. The significance of the present research is that it provides a physical basis for the role of GB character and crystallographic orientation on intergranular crack tip deformation behavior.

Observation and quantification of three-dimensional crack propagation in poly-granular graphite

Observations of fracture are generally restricted to the surface of test specimens; yet the fracture process occurs within the material. X-ray computed tomography (XCT) provides valuable insights into the structures within materials: when XCT is combined with digital volume correlation (DVC) the response to applied loads can be measured with high precision in the form of the three-dimensional displacement field within the material. This paper reports a study of the fracture behaviour of a short-bar chevron notch crack propagation specimen fabricated from polygranular nuclear graphite – a quasi-brittle material. A three-dimensional linear elastic finite element simulation of the specimen obtained the relations between crack length, opening displacement and stress intensity factor along the crack front. Tomographic absorption contrast images were obtained from the specimen before and after crack propagation, whilst loaded. The DVC-measured displacement field was used to measure and map the crack opening displacements in 3D; the experimentally calculated crack opening displacements were consistent with the FE-predicted values. The measurements demonstrate the existence of a cohesive fracture process zone ahead of the crack tip, which is a characteristic of quasi-brittle materials. This suggests that simulation of the fracture of non-irradiated polygranular nuclear graphite requires a material model capable of showing softening behaviour.

Numerical Study on Effects of Rock Bridge Angle in Uniaxial Compression Test

Numerical specimens with two pre-existing flaws is established by using particle flow code PFC2D and by changing the relative position of the pre-existing flaws different rock bridge angle is obtained. Though the uniaxial compression test of specimens with different rock bridge angle , it can be found that rock bridge angle have a great impact on the mode of crack propagation of specimen. The different relative position between the two pre-existing flaws led to different levels of stress shielding effect under the axial force, and it is most likely to damage when the two pre-existing flaws are about overlap.

Tensile Strength versus Toughness of Cement-Based Materials against High-Velocity Projectile Impact

This paper presents a laboratory experimental study on the effect of tensile strength and toughness on the resistance of cement-based materials to high-velocity projectile impact. Three types of cement-based materials with similar compressive strength but different tensile strengths and toughnesses were examined in terms of the penetration depth, crater diameter, and crack propagation in specimens subjected to impact of ogive-nosed projectile with a diameter of 13.35 mm and a mass of 19 grams traveling at velocities of about 650 m/s. The materials evaluated include: (i) a plain high-strength concrete (HSC) with low tensile strength and toughness, (ii) a fiber-reinforced concrete (FRHSC) with high toughness but similar flexural tensile strength as the HSC, and (iii) a polymer cement composite (PCC) with high flexural tensile strength but low toughness. The PCC was developed from a combination of inorganic cements with water-soluble polymers, and it had a 28-day flexural tensile strength of 25 MPa. Results suggest that the toughness was a more important parameter that contributes to the impact resistance of the cement-based materials. Increasing the tensile strength alone, however, did not seem to contribute to the impact resistance of the cement-based materials.

Numerical Testing for Dynamic Fracture of Three Point Bending Cement Mortar Specimen with Off-Center Edge-Crack

In this paper, RFPA－dynamic numerical simulation system was adopted to simulate the entire process of the three–point bending specimen with off–center edge-crack propagation to the specimen macroscopic crack which under the dynamic load function. And showed the whole process of the cement mortar specimen crack propagation which under the dynamic load function, as well as the whole field stress distribution picture, and has carried on analysis the specimen destruction process, has pointed out the influence of the no dimension quantity a/L change to crack expansion path and the destruction form.

Ductile Crack Propagation Characteristics in Steel Thin Single Edge Notched Tension Specimens

Static and dynamic ductile crack propagation tests were carried out using thin single edge notched tension (SENT) specimens of carbon-manganese steel, each of which had a fatigue pre-crack or a sharp V-notch as a crack initiator. The crack tip opening angle (CTOA) was measured using digital images on the surface of the SENT specimens, and the critical values of CTOA for crack propagation decreased with increasing crack length while initial crack growth was still small. After the initial crack growth up to the distance of the specimen thickness, the critical CTOA remained almost constant. These tendencies were common in static and dynamic crack propagation specimens as well as fatigue pre-cracked and sharp V-notched specimens. There was no particular difference in the static crack propagation characteristics of both fatigue pre-cracked and sharp V-notched specimens. On average, it was observed that higher crack speeds affected the constant values of the critical CTOA by slightly reducing them. The constant CTOA tends to decrease with an increasing global constraint factor, and this suggests that the factor is insensitive to a crack starter, fatigue pre-crack or a sharp V-notch, but relatively sensitive to crack speed.