Uncracked Region Research Articles

Understanding the influence of boron in additively manufactured GammaPrint®-700 CoNi-based superalloy

Boron is commonly added to superalloys in small amounts to enhance creep resistance, but can lead to cracking at high concentrations, especially during the additive manufacturing process. Two variants of CoNi-based GammaPrint®-700 superalloy with different B contents (0.08 at% vs 0.16 at%) were printed via laser powder bed fusion (LPBF) with the same printing parameters, with only the high B alloy exhibiting solidification cracking. Atom probe tomography (APT) revealed stronger segregation behaviors in the high B alloy compared to the low B alloy at both the inter-dendritic regions and grain boundaries (GBs). The segregation behavior at inter-dendritic regions was well captured with Scheil simulation and can correlate with the existing cracking susceptibility index (CSI) on cracking tendencies, although high angle GBs are where cracking occurs according to electron backscatter diffraction (EBSD) measurements. Additionally, the extent of GB segregation was compared between the high B and low B alloy. Higher B additions led to significantly more GB B segregation in the high B alloy compared to the low B alloy. For the high B alloy, the cracked region of one GB exhibited higher levels of B compared to the uncracked region of the same GB. However, much higher B contents were also found in two other uncracked GBs in the high B alloy, which demonstrates that higher GB B concentrations are not fully responsible for the cracking. A much larger variance in GB B segregation content was found in the high B alloy compared to the low B alloy. These phenomena were explained with a solidification model with the GB segregation content expressed explicitly by a modified Langmuir-McLean equation. This model linked the GB segregation content with solidification undercooling, which can be used as quantitative cracking criteria for future builds.

Dynamic Thermal Response of Multiple Interface Cracks between a Half-Plane and a Coating Layer under General Transient Temperature Loading.

This paper explores the thermal behavior of multiple interface cracks situated between a half-plane and a thermal coating layer when subjected to transient thermal loading. The temperature distribution is analyzed using the hyperbolic heat conduction theory. In this model, cracks are represented as arrays of thermal dislocations, with densities calculated via Fourier and Laplace transformations. The methodology involves determining the temperature gradient within the uncracked region, and these calculations contribute to formulating a singular integral equation specific to the crack problem. This equation is subsequently utilized to ascertain the dislocation densities at the crack surface, which facilitates the estimation of temperature gradient intensity factors for the interface cracks experiencing transient thermal loading. This paper further explores how the relaxation time, loading parameters, and crack dimensions impact the temperature gradient intensity factors. The results can be used in fracture analysis of structures operating at high temperatures and can also assist in the selection and design of coating materials for specific applications, to minimize the damage caused by temperature loading.

Experimental determination of critical thickness limitations of (010) β -(AlxGa1−x)2O3 heteroepitaxial films

The effect of heteroepitaxial β-(AlxGa1−x)2O3 film thickness and Al content on surface morphology was characterized to experimentally determine the critical thickness limitations of the (010) β-(AlxGa1−x)2O3/Ga2O3 heterostructure. High-resolution x-ray diffraction was used to assess the strain state of the films; reciprocal space mapping (RSM) revealed that even cracked films were still fully strained. In cracked films, diffuse scattering was observed in RSMs, indicating lattice tilting. Cracking of the films was investigated using atomic force microscopy (AFM), x-ray topography (XRT), bright-field scanning transmission electron microscopy (BF-STEM), and high-resolution transmission electron microscopy. Using both AFM and XRT, the [001] direction was observed to be the most prevalent crack direction; however, cracks were also observed in the [100] direction. In uncracked regions of the films, XRT revealed the alignment of threading dislocations along the [001] direction. Cross-sectional imaging of the crack geometry and propagation was performed using BF-STEM, and it was observed that the cracks in the [001] direction extended through the thickness of the β-(AlxGa1−x)2O3 film (∼205 nm) and a further ∼100–200 nm into the β-Ga2O3 substrate. Experimental data for critical film thickness showed good agreement with previous theoretical calculations based on the Griffith criterion for crack propagation.

Discontinuity Detection System in Marble: Analysis of the Time-Frequency Characteristics of Ultrasonic P-waves

Abstract This study attempted to determine both the presence and the appearance of the surface of a crack artificially created in a marble block without axial loading or compression by FFT analysis of ultrasonic longitudinal sound waves by scanning the surface. In such respect, the study differs from other studies in the literature. The aim of the study was to minimize marble waste by determining the most appropriate slab cut direction based on the orientation of the fracture or discontinuity within the marble block. During location-controlled scanning of the marble surface, mean longitudinal wave velocity, Young modulus, Poisson ratio and longitudinal wave velocity variation data were obtained using a V-Meter Mark IV™ ultrasonic test device. For each location, longitudinal ultrasonic velocity data was taken at 250 kHz sampling rate and 256 data points were created. Data from the computer environment were subsequently reorganized using the MATLAB software and FFT conversions for such velocity data were obtained for each location. Then, the peaks of the FFT transformation components were determined by using the “Findpeaks” function in MATLAB‘s signal processing toolbox. The number of peaks in the 50-60 kHz range was determined for each location. A single difference was determined between the peak numbers at the locations within the cracked and uncracked regions. By examining the FFT graphs, it was determined that such additional peak occurring in the cracked regions appeared as a second frequency component near the dominant frequency values. Points with secondary frequency were recorded as 1, while all other points were recorded as 0 in a new matrix. Such data matrices created for each sample were examined by visualization in the form of surface graphics. Using these graphics, the orientations of discontinuities within the marble block may be determined according to the scanning surface.

Experimental investigations on early crack development in fully restrained reinforced concrete members with active zero-displacement control

The development of restraint-induced stress in reinforced concrete members at an early age can be tested using setups applying either passive or active displacement control. To provide results relevant for the practical design of minimum reinforcement to limit the crack width, an actively controlled test setup for large specimens was developed to perform a systematic experimental campaign focusing on the early crack development under well-defined mechanical and thermal boundary conditions. The full degree of restraint achieved by actively preventing any displacement of the member allowed for the theoretical interpretation of early restraint-induced stress evolution, deformation behaviour in the cracked and uncracked regions, and reinforcement stress for a variety of parameters, including hydration rates, member height, bar diameters and subsequently reinforcement ratio. The cracking process, starting with primary cracks accompanied by secondary cracks within the effective reinforcement area, is monitored in combination with the development of the tensile strength and Young’s modulus of the concrete. Furthermore, the time of cracking and the forces released upon crack occurrence, considering the effect of self-equilibrating stresses, are determined, analysed, and compared with the results of crack-force based design rules to determine the minimum amount of reinforcement that ensures the crack width limitation.

Numerical simulation of fracture behaviour of the shot-earth 772

Earth building techniques have been around for thousands of years. Among them, it is worth mentioning: cut blocks, poured earth, superadobe, adobe, cob, rammed earth, compressed stabilised earth blocks, wattle and daub, shaped earth, and straw-clay. An innovative technique among the earth building ones, and named shot-earth technique, is the topic of the present paper. The technology employs a specific mixture of the shot-earth (named shot-earth 772) consisting of 7 parts of excavated soil, 7 parts of aggregates and 2 parts of cement (by volume): the mixture stream is projected at high velocity onto a surface, adding about 3% of water (by volume) at the spraying nozzle. The present research work deals with the numerical simulation of the fracture behaviour of the shot-earth 772. More precisely, an experimental campaign performed to compute the fracture toughness of the material is here numerically simulated. To such an aim, a homemade Finite Element numerical model, developed in standard Fortran language, is used to simulate both flexural and fracture tests on the shot-earth considered. In fact, such a model is able to properly consider. The material fracture process is described by means of a cohesive law, which is introduced within the cracked zone, whereas an elastic law is adopted for the un-cracked region of the material. Subsequently, the Modified Two-Parameter Model is used in order to compute the fracture toughness, from both the experimental fracture tests and the corresponding numerical simulations. Finally, the numerical results are compared with the experimental ones.

The boundary element method of peridynamics

AbstractThe peridynamic theory brings advantages in dealing with discontinuities, dynamic loading, and nonlocality. The integro‐differential formulation of peridynamics poses challenges to numerical solutions of complicated and practical problems. Some important issues attract much attention, such as the computation of infinite domains, the treatment of softening of boundaries due to an incomplete horizon, and time accumulation error in dynamic processes. In this work, we develop the boundary element method of peridynamics (PD‐BEM). The numerical examples demonstrate that the PD‐BEM exhibits several features. First, for nondestructive cases, the PD‐BEM can be one to two orders of magnitude faster than the meshless particle method of peridynamics (PD‐MPM) that directly discretizes the computational domains; second, it eliminates the time accumulation error, and thus conserves the total energy much better than the PD‐MPM; third, it does not exhibit spurious boundary softening phenomena. For destructive cases where new boundaries emerge during the loading process, we propose a coupling scheme where the PD‐MPM is applied to the cracked region and the PD‐BEM is applied to the uncracked region such that the time of computation can be significantly reduced.

Computation of shear strength equation for shear deformation of reinforced concrete deep beams using finite element method

<abstract> This paper gives the analytical investigation of six reinforced concrete deep beams reinforced with horizontal and vertical web reinforcement. Reinforced concrete deep beams analysis is a complex problem where there is no exact solution. The effect of reinforcement distribution has been studied and compared to experimental investigation and various codes such as ACI 318 and IS 456. A new formula is proposed to define shear strength of deep beams. The codal equations are too traditional for predicting the shear strength of RC deep beams, so an improved and simplified equation was proposed using nonlinear finite element method by ABAQUS. The FE results are compared with experimental results in terms of ultimate loads, displacements, tension stress damage. The proposed shear strength equation predicts 80% of the experimental data in the range of 66–110% of measured shear strength. FE model results accurately predicted. The stress contours suggested high stresses in the path of cracks and low stresses in the uncracked regions. </abstract>

Evaluation of two crack models for reinforced concrete one-way slabs subjected to bending by means of modal tests

Cracks can affect a reinforced concrete (RC) structure and the evaluation of damage related to them is a typical application of structural health monitoring systems. This way, two RC one-way slabs with different reinforcement ratios were tested in laboratory in a four-line static load test scheme, followed by modal testing. Using modal testing data, finite element models of the cracked structures were prepared and calibrated for several cracked stages, to evaluate which crack model would present a better performance when compared to the experimental counterparts. Cracks were simulated via a reduction of flexural stiffness by penalizing the elastic modulus. Two continuous crack models were evaluated: Model 1 which employs reduction factors based on the ratio of moment of inertia after the cracking moment is reached and the moment of inertia of the uncracked section; Model 2 which is based on the stiffness reduction in the region of influence of each crack. The evaluation of both models was made using natural frequencies and mode shapes (through Modal Assurance Criterion-MAC indicator) as indicators, and Model 2 presented better results. An additional approach to globally reduce stiffness in the early stages of load before visible cracks appear and also in uncracked regions was shown necessary, to take into account the reductions on natural frequencies observed in the tests. By employing the stiffness reduction factors obtained for Model 2 it was possible to evaluate the crack depth evolution along the load stages.

Analytical mechanics solution for measuring the deflection of strengthened RC beams using FRP plates

Partial-interaction due to sliding between the steel bars, adhesively attached FRP plates and their bordering concrete surface, accompanied with the detachment of the FRP plates due to intermediate crack (IC) debonding make the deflection of strengthened RC beams difficult to anticipate. Previous research and design rules on determining the deflection of strengthened RC beams using FRP plates have opted for a full-interaction moment-curvature design technique where the deflection was measured by either deriving average effective moment of inertia and using elastic deflection equations or integrating the curvature along the beam’s length. Therefore, IC deboning of the plate and the slip resulting from the formation and broadening of new cracks were not directly considered. In this study, a partial-interaction moment-rotation analysis of an adhesively plated beam segment was used to derive analytical equations for the rotation of individual crack faces. The analytical expressions were used to compute the rotation at a crack for a given moment; subsequently, the influence of each crack to the midspan deflection of the RC beams was calculated. As for the uncracked region of the beam, the deflection contribution was measured by integrating the curvature over the uncracked span. The deflection results from the mechanics solution seem to compare well with experimental results. The analytical mechanics solution accounts for the partial-interaction between the steel bars, externally bonded FRP plate and their bordering concrete surface, and also the detachment of the external plate through IC debonding. Further, due to its generic nature and non-reliance on empirical data, the mechanics solution can be adopted to forecast the deflection of strengthened RC beams with novel types of reinforcement materials.

Mechanics-based approach for predicting the short-term deflection of CFRP plated RC beams

Most design codes available today for predicting the deflection of adhesively plated RC beams use a full-interaction moment-curvature approach that requires the flexural rigidity to be quantified empirically. Due to their empirical nature, these design rules can only be applied within the bounds of the tests from which they were derived. Furthermore, as these design rules follow a full-interaction analysis, the slip between the reinforcement and adjacent concrete was not considered and the method does not cope with the discrete rotation of the cracks; that is, the deflection associated with crack widening was not directly considered. As an alternative, partial-interaction mechanics-based methods can be used. In this study, a mechanics-based approach for quantifying the deflection of adhesively plated RC beams was presented. The approach took into account the slip between the reinforcement and adjacent concrete, the formation and widening of flexural cracks, and the intermediate crack debonding mechanism of the externally bonded plate. The deflection from the mechanics-based approach was determined by considering the discrete rotation of individual cracks and the curvature of uncracked regions of the beam. The deflection results derived from the mechanics-based approach were compared with the experimental results of seven adhesively plated CFRP RC beams bonded to their tension face and a significant correlation between the results was observed. The mechanics-based approach does not require any components on the member level to be quantified empirically; thus, it could be useful in predicting the deflection of adhesively plated RC beams with new types of reinforcement material.

Serviceability of non-prismatic concrete beams: Combined-interaction method

Interest in the shape optimisation of concrete members is increasing alongside the availability of fabric formwork as a relatively simple technique to cast non-prismatic concrete structures. Research has shown that up 40% of concrete can be saved when shape optimised concrete beams are cast in fabric forms. However, optimisation results in members with non-uniform cross-sections and the resulting beam is less stiff than an equivalent strength prismatic beam. Serviceability, rather than strength, may govern the design of such members and therefore understanding the serviceability behaviour (deflection and cracking) of shape optimised concrete members becomes is a critical design consideration. There are many methods which can be used to evaluate serviceability behaviour of reinforced concrete beams, including the full-interaction method, which assumes no slip between the reinforcement and the surrounding concrete, and the partial-interaction method which accounts for slip. The full-interaction method is based on a smeared crack approach and so is unsuited for the prediction of cracking behaviour. The partial-interaction method, on the other hand, assumes that cracks form through bond-stress transfer only. In the case of non-prismatic concrete beams, the cracking capacity varies along the member. Therefore, cracking can occur over extended regions (full and partial bond interaction regions) and so it can be argued that neither of these models is fully suitable for the prediction of deflections and cracking of shape-optimised concrete beams. In this paper, a novel combined-interaction method is, for the first time, presented to predict the serviceability behaviour of non-prismatic statically determinate concrete beams by simulating both full and partial bond interactions at different cracked and uncracked regions along the length of the member. In order to validate this approach, two non-prismatic simply supported beams were cast and tested. The test results for deflections, crack widths and crack spacings were in good agreement with the predicted results.

Self-monitoring continuous carbon fiber reinforced thermoplastic based on dual-material three-dimensional printing integration process

This study offers a novel dual-material three-dimensional (3D) printing technology for integrating of continuous carbon fibers within 3D printed thermoplastic structures to achieve both structural reinforcement and self-monitoring. Mechanical and electrical-resistance-based self-sensing properties of different fill density specimens under tensile and flexural loading were investigated and discussed. Results showed that continuous carbon fibers integrated in the thermoplastic contributed to both structural reinforcement and structural health via in situ monitoring. With up to 7.23% improvement of tensile strength, 19.07% improvement of tensile modulus, 38.66% improvement of flexural strength, and 17.99% improvement of flexural modulus for specimens compared to the non-integrated continuous carbon fibers specimens. Changes in electrical resistance measurements of the carbon fibers allowed the detection of both strain and damage in 3D printed thermoplastic structures in a very effective and sensitive way. Gauge factors of different fill density specimens ranged from 0.119 ± 0.016 up to 1.213 ± 0.165 for the uniaxial tension test in the un-cracked region. Taken together, these results suggest that this dual-material 3D printing technology is attractive for fabrication of continuous carbon fiber thermoplastic matrix (CCFT) smart structures, and have many potential civil and military applications.

BOND AND FLEXURAL BEHAVIOR OF SELF-CONSOLIDATING CONCRETE BEAMS REINFORCED WITH GFRP BARS

Self-consolidating concrete (SCC) is widely used in the construction industry. SCC is a high-performance concrete with high workability and consistency allowing it to flow under its own weight without vibration. Despite the wide spread of SCC applications, bond behavior of FRP bars embedded in SCC beams has not been fully studied. This paper presents an experimental and analytical analysis of fifteen beams reinforced with glass fiber reinforced polymer (GFRP) bars. The test parameters were the concrete type, bar diameter, concrete cover thickness and embedment length. All beams were tested in four-point bending to failure. The average bond stresses of GFRP bars in SCC were found comparable to those in NVC. However, FRP bars embedded in SCC beams had higher bond stresses within uncracked region of the beams than those embedded in NVC beams. In contrast, GFRP bars in SCC had lower bond stresses than FRP bars in NVC within the cracked region. Results indicated that when cover concrete thickness dropped less than 2 db, the splitting bond failure is predominant.

Assessment of the effect of reduced compositional heterogeneity on fracture resistance of human cortical bone using finite element modeling

The recent reports of atypical femoral fracture (AFF) and its possible association with prolonged bisphosphonate (BP) use highlighted the importance of a thorough understanding of mechanical modifications in bone due to bisphosphonate treatment. The reduced compositional heterogeneity is one of the modifications in bone due to extensive suppression of bone turnover. Although experimental evaluations suggested that compositional changes lead to a reduction in the heterogeneity of elastic properties, there is limited information on the extent of influence of reduced heterogeneity on fracture resistance of cortical bone. As a result, the goal of the current study is to evaluate the influence of varying the number of unique elastic and fracture properties for osteons, interstitial bone, and cement lines on fracture resistance across seven different human cortical bone specimens using finite element modeling. Fracture resistance of seven human cortical bone samples under homogeneous and three different heterogeneous material levels was evaluated using a compact tension test setup. The simulation results predicted that the crack volume was the highest for the models with homogeneous material properties. Increasing heterogeneity resulted in a lower amount of crack volume indicating an increase in fracture resistance of cortical bone. This reduction was observed up to a certain level of heterogeneity after which further beneficial effects of heterogeneity diminished suggesting a possible optimum level of heterogeneity for the bone tissue. The homogeneous models demonstrated limited areas of damage with extensive crack formation. On the other hand, the heterogeneity in the material properties led to increased damage volume and a more variable distribution of damage compared to the homogeneous models. This resulted in uncracked regions which tended to have less damage accumulation preventing extensive crack propagation. The results also showed that the percent osteonal area was inversely correlated with crack volume and more evenly distributed osteons led to a lower amount of crack growth for all levels of material heterogeneity. In summary, this study developed a new computational modeling approach that directly evaluated the influence of heterogeneity in elastic and fracture material properties on fracture resistance of cortical bone. The results established new information that showed the adverse effects of reduced heterogeneity on fracture resistance in cortical bone and demonstrated the nonlinear relationship between heterogeneity and fracture resistance. This new computational modeling approach provides a tool that can be used to improve the understanding of the effects of material level changes due to prolonged BP use on the overall bone fracture behavior. It may also bring additional insight into the causes of unusual fractures, such as AFF and their possible association with long term BP use.

Thermomechanical Fatigue Behavior of SS304

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) properties of SS304 stainless steel were evaluated in the temperature range of 300–600 °C. All tests were strain controlled and the strain rate was kept constant. Material exhibited cyclic hardening followed by saturation of cyclic stress. Fatigue life reduced in OP testing compared to IP testing as the strain amplitude increased. Specimens failed with multiple cracks on the surface in OP testing whereas a single crack had predominantly grown inward and weakened the section in IP testing. Nanoindentation studies showed higher hardness around the crack tip compared to the uncracked regions. TEM showed a well developed dislocation cell structure in OP TMF while IP specimens showed planar slip bands with pile up of dislocations.

Numerical calculation of expansion induced by alkali silica reaction

This paper proposed a model to predict alkali silica reaction (ASR) induced expansion. The model can estimate the influential factors on ASR expansion, such as the aggregate sizes, mechanical properties of composite materials, and initial alkali concentration. Chemical reactions were described by solid state reaction theory. Concrete was considered to include aggregates, crack region and uncracked region. Alkali silica gel was considered to be the main reason of expansion, and the gel volume was calculated according to the alkali-silica reactions. The model was validated by an experiment of ASR-affected concrete specimens containing glass aggregate. The comparison of calculation results with experimental results showed good applicability of the model.

Integrated self-monitoring of carbon based textile reinforced concrete beams under repeated loading in the un-cracked region

This paper studies the piezoresistive behavior of self-sensory carbon/glass textile reinforced concrete beams under the effect of loading, unloading, and reloading in the un-cracked region. Continuous carbon tows knitted into the textile serve both as the structural reinforcement and as the structural health sensors. The design eliminates the need for additional sensors by taking advantage of a monitoring system that is an inherent part of the load bearing element. The paper faces the challenge of understanding of the correlation between the piezoresistive change of the electrical properties of the carbon tow and the change of mechanical properties under repeated loading representing the response to service live loads. The paper looks into this correlation and explores it through an experimental investigation. It is found that for the un-cracked stage of the beam, the carbon component of the reinforcing textile provides an electro-mechanical sensing capability with gauge factors of 2.4–5.4 for tensile strain. Since the investigation focuses on flexural elements with at least two layers of textile reinforcement, the experiments also reveal negative gauge factors of −2.7 to −6.5 for compressive strains. These observations take a step towards many applications of sensory carbon based textile reinforced concrete elements.

Prediction of load–deflection behavior of multi-span FRP and steel reinforced concrete beams

This paper presents a numerical procedure to determine the deflection of concrete members reinforced with fiber reinforced polymer (FRP) or steel bars. This procedure is implemented into the stiffness matrix to allow for general use in the structural analysis. It considers effective flexibilities of members in the cracked state using either the curvature distribution along the member or available effective stiffness models under any loading or support condition. In general, structural concrete members can be considered to have three cracked regions (two at the ends and one at midspan) and two uncracked regions along their length. In this numerical procedure, the contributions of these regions to the member stiffness matrix are computed using a numerical integration technique. Using this procedure, a software program is developed which allows for the load–deflection behavior of a member reinforced with either FRP or steel bars and subjected to any loading or support condition to be rapidly determined. This calculation procedure is evaluated using available experimental data on the load–deflection behavior of simple and two-span beams reinforced with FRP and steel bars. Through comparison of the results, it is observed that the load–deflection behaviors calculated using the proposed approach utilizing the member moment–curvature response are consistent with the experimental data. This approach can provide a useful tool for the general calculation of deflection regardless of reinforcement type and can be used throughout the entire range of member behavior up to flexural failure.

Defect-Tolerant Bioinspired Hierarchical Composites: Simulation and Experiment.

Defect tolerance, the capacity of a material to maintain strength even under the presence of cracks or flaws, is one of the essential demands in the design of composite materials, as manufacturing induced defects, or those generated during operation, can lead to catastrophic failure and dramatically reduce the mechanical performance. In this paper, we combine computational modeling and advanced multimaterial 3D printing to examine the mechanics of several different classes of defect-tolerant bioinspired hierarchical composites, built from two base materials with contrasting mechanical properties (stiff and soft). We find that in contrast to the brittle base constituents of the composites, the existence of a hierarchical architecture leads to superior defect-tolerant properties. We show that composites with more hierarchical levels dramatically improve the defect tolerance of the material. We also examine the effect of adding both self-similar and dissimilar hierarchical levels to the materials architecture, and show that the geometries with multiple hierarchical levels can retain a significant portion of their fracture strength in the presence of either large edge cracklike flaws or multiple small distributed defects in the material. We compare the stress distributions in materials with different numbers of hierarchies in both simulation and experiment and find a more uniform stress distribution in the uncracked region of materials with higher hierarchy levels. These results provide micromechanical insights into the origin of the higher defect tolerance observed in simulation and experiment.