Formation Of Multiple Cracks Research Articles

Tensile and Low‐Cycle Fatigue Behavior of Laser Powder Bed Fused Inconel 718 at Room and High Temperature

This study investigates the room‐ and high‐temperature (650 °C) tensile and low‐cycle‐fatigue behavior of Inconel 718 produced by laser powder bed fusion (PBF‐LB/M) with a four‐step heat treatment and compares the results to the conventional wrought material. The microstructure after heat treatment is characterized on different length scales. Compared to the wrought variant, the elastic and yield properties are comparable at both test temperatures while tensile strength, ductility, and strain hardening capacity are lower. The fatigue life of the PBF‐LB/M variant at room temperature is slightly lower than that of the wrought material, while at 650 °C, it is vice versa. The cyclic stress response for both material variants is characterized by cyclic softening, which is more pronounced at the higher test temperature. High strain amplitudes (≥0.7%) at room temperature and especially a high testing temperature result in the formation of multiple secondary cracks at the transitions of regions comprising predominantly elongated grain morphology and columns of stacked grains with ripple patterns in the PBF‐LB/M material. This observation and pronounced crack branching and deflection indicate that the cracks are controlled by sharp micromechanical gradients and local crystallite clusters.

Simulating progressive failure in laminated glass beams with a layer-wise randomized phase-field solver

Laminated glass achieves improved post-critical response through the composite effect of stiff glass layers and more compliant polymer films, manifested in progressive layer failure by multiple localized cracks. As a result, laminated glass exhibits greater ductility than non-laminated glass, making structures made with it suitable for safety-critical applications while maintaining their aesthetic qualities. However, such post-critical response is challenging to reproduce using deterministic failure models, which mostly predict failure through a single through-thickness crack localized simultaneously in all layers. This numerical–experimental study explores the extent to which progressive failure can be predicted by a simple randomized model, where layer-wise tensile strength is modeled by independent, identically distributed Weibull variables. On the numerical side, we employ a computationally efficient, dimensionally-reduced phase field formulation – with each layer considered to be a Timoshenko beam – to study progressive failure through combinatorial analysis and detailed Monte Carlo simulations. The reference experimental data were obtained from displacement-controlled four-point bending tests performed on multi-layer laminated glass beams. For certain combinations of the glass layer strengths, results show that the randomized model can reproduce progressive structural failure and the formation of multiple localized cracks in the glass layers. However, the predicted response was less ductile than that observed in experiments, and the model could not reproduce the most frequent glass layer failure sequence. These findings highlight the need to consider strength variability along the length of a beam and to include it in phase-field formulations.

Micromechanical study on polypropylene-bicomponent fibers to improve mechanical interlocking for application in strain-hardening cement-based composites

Polyproplylene (PP) fibers find application in strain-hardening cement-based composite to enable the formation of multiple fine cracks for high energy absorption. Often, however, the mechanical performance of such composites suffers from insufficient fiber-matrix interaction. In the research at hand, bicomponent PP single fibers with rough surfaces for improved mechanical interlocking are produced using an IPF (Leibniz-Institut für Polymerforschung Dresden e. V.), an in-house designed and built, laboratory-scale piston fiber spinning device. The melt-spun fibers consist of a shell component composed of PP and various volume percentages of different inorganic particles of calcium carbonate (CaCO3), aluminum oxide (Al2O3), and of a core component made of the same polymer as in the shell. The bicomponent fibers, with shell diameters between 20 and 45 μm, were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to understand their morphology and to study the fiber surfaces after composite failure. Tensile strength and Young's modulus of the fibers were evaluated using tension tests. Furthermore, the single fiber pullout (SFPO) test was used to investigate the interfacial interaction between fiber and a cement-based matrix. Significant improvements in fiber-matrix bonding were achieved due to the rough surface employed in connection with the particles incorporated in the outer shell. Still further, the fiber's strength, attained using an offline step in the drawing process, contributes to enhanced energy adsorption under dynamic pullout loading. To evaluate the performance of the newly developed bicomponent fibers, they were compared to a self-spun monocomponent PP fiber and a commercial PP fiber. This comparison revealed slip-hardening induced by increasing surface roughness for enhanced mechanical interlocking and plastic polymer deformation in the fiber-matrix contact zone.

Gas gun driven dynamic expansion of 3D-printed AlSi10Mg rings

This paper presents a new experimental setup to study dynamic fragmentation of metallic rings manufactured by 3D-printing technology. The ring is inserted over a ductile thin-walled tube which is impacted axially by a conical-nosed cylindrical projectile fired by a single-stage light–gas gun. The diameter of the projectile is larger than the internal diameter of the thin-walled tube, which expands as the projectile advances, pushing radially outwards the metallic ring, that eventually breaks into multiple fragments. The impact velocities considered in the present experimental campaign range from 197 m/s to 385 m/s. The AlSi10Mg ring specimens are printed by Selective Laser Melting technique, with an inner diameter of 14 mm and square cross section of 2 × 2 mm2. To obtain time-resolved information on the mechanics of fragmentation, the number of fractures, and the size of the fragments, the tests have been recorded with two-high speed cameras. A tunnel-shaped soft casing made of polymer foam is placed around the specimen to softly recover the ejected fragments, that have been weighted and sized to determine the statistics of the fragments size distribution. In addition, the predictions of the fragmentation theory of Kipp and Grady (1985) for the evolution of the number of fragments with the impact velocity have been compared with the experimental evidence, and an excellent quantitative agreement has been found within the whole range of loading rates investigated. Compared to other available techniques for dynamic fragmentation of rings, in which the expansion of the specimen is driven by electromagnetic or explosive loading, this experimental setup stands out for its simplicity, fast operation, quick assembly, and flexibility to test different engineering materials, which facilitates performing extensive experimental campaigns (34 successful tests have been carried out in this research). To the authors’ knowledge, this paper presents the most comprehensive data set to date on the fragmentation of dynamically expanded printed rings, including the first high-resolution video recordings of the formation of multiple cracks throughout the circumference of the samples, and scanning electron microscopy images of the fractures showing the porous microstructure on the cracks surface.

Standardized RC beam tests for modeling the fiber bridging effect in SFRC

Fiber reinforcement is a promising solution to cracking problems and improving the concrete’s structural performance. The residual strength of the cracked concrete can characterize the reinforcement efficiency. However, quantifying the residual performance of steel fiber-reinforced concrete (SFRC) is challenging. The existing methodologies provide empirical formulas for estimating the residual strength using test results of SFRC elements in which a predominant crack governs the mechanical resistance. For instance, the 0.5 mm crack width determines the minimum value considered in the RILEM standard formulas. Thus, the SFRC strength evolution at earlier cracking stages remains unknown. At the same time, such a crack approximation is irrelevant to structural cases when reinforcement bars stimulate the formation of multiple cracks, and the 0.1–0.2 mm crack typically corresponds to the yielding of the steel bars. This study describes an alternative approach for quantifying the average residual stresses in SFRC elements with multiple cracks. It hypothesizes the possibility of separating the mechanical resistance components, corresponding to tension stiffening and fiber bridging effects characteristic of SFRC elements with bar reinforcement, using standardized small-scale specimens to estimate the fiber contribution. The laboratory tests of the plain concrete and SFRC beams with bar reinforcement illustrate the proposed technique. The Rilem standard three-point bending tests and the numerical simulation of full-scale beams verify the analysis’s adequacy. The developed model is suitable for finite element simulations (employing the smeared crack model); the capability of separating the tension stiffening and fiber bridging effects ensures its versatility.

Micromechanics-based model of single crack propagation in Engineered cementitious composites (ECC)

Engineered Cementitious Composites (ECC) are composites exhibiting strain-hardening behavior accompanied by the formation of multiple cracks. A special feature of ECC is the ability to tune the material composition to meet the different needs of various structural components, which benefits from the special micromechanics-based design theory of ECC. However, these existing micromechanics-based design theories are specific to the case of direct tension and has not been fully discussed and developed for the case where the ECC is subjected to bending load. In this study, a micromechanics-based crack propagation model for ECC under bending load is developed to guide the design of flexural performance of ECC. The entire crack propagation process is simulated by considering the microstructure of ECC and the crack propagation criterion. For ECCs with different types of fiber, the simulated load-crack mouth opening displacement curves agree well with experimental results and the dispersion of the ECC specimens can be captured by imparting randomness to the model. Based on this model, parametric studies of several representative parameters are performed to illustrate the applicability of the model. The results indicate that the material has a higher ultimate flexural bearing capacity near a fiber aspect ratio of 448 for the given material property parameters; and the fiber/matrix bonding should not be excessively strong. Insights from this model can facilitate the design of ECC members with excellent bending properties through fiber tailoring and surface treatment.

Crack Shape Coefficient: Comparison between Different DFOS Tools Embedded for Crack Monitoring in Concrete.

The article presents research on the performance of different distributed fibre optic sensing (DFOS) tools, including both layered cables and monolithic composite sensors. The main need for the presented research was related to the growing applications of the DFOS techniques for the measurements of cracked concrete structures. There are no clear guidelines on the required parameters of the DFOS tools, which, despite their different designs, are offered for the same purpose (strain sensing). The state-of-the-art review and previous experiences show noticeable differences in the quality of the results depending on the applied DFOS tool. The technical construction of selected solutions was described with its theoretical consequences, and then laboratory tests on full-size reinforced concrete beams were discussed. Beams equipped with embedded tools were investigated in four-point bending tests, causing the formation of multiple cracks in the tension zone along the beams' length. The results in the form of strain profiles registered by selected DFOS tools were analysed regarding the qualitative (crack detection) and quantitative (width estimation) crack assessment. The comparison between crack-induced strain profiles was based on a new parameter called crack shape coefficient CSC, which could be applied to assess the effectiveness of the particular DFOS tool in crack detection and analysis. It was one of the world's first research allowing for such direct comparison between the layered and monolithic sensing tools. The summary indicates practical guidelines referring to the preferable design of the tools best suitable for crack measurements, as well as the field proofs based on data from two concrete bridges in Germany.

Effect of Fiber Type and Volume Fraction on Fiber Reinforced Concrete and Engineered Cementitious Composite Mechanical Properties

Engineered cementitious composites (ECC) are an ultra-ductile cement-based composite material reinforced with short randomly distributed fibers. It differs from fiber reinforced concrete (FRC) in that it has a distinct ductile behavior. The study aims to assign mechanical properties, such as tensile, flexural, and compressive strength using locally available fiber rather than polyvinyl alcohol (PVA) fiber, which is not widely available in many countries, to ECC. PVA fiber is also very expensive. Instead of PVA, lightweight fibers, such as polypropylene, polyolefin, and glass fiber, as well as heavyweight fibers, such as steel fiber, were used. To assess the mechanical properties, the influences of curing, fiber volume fraction (2%, 4%, and 6%), fiber type, and fiber hybridization were adjusted in this study. The formation of multiple cracks along the specimen is the governing factor in ECC formation. The test results show that increasing the fiber volume fraction improves flexural and tensile strength. Water curing increased compressive, tensile, and flexural strength. Lightweight fiber hybridization has no effect on compressive strength, whereas heavyweight fiber hybridization improves compressive strength. For tensile and flexural strength, hybridization was associated with an improvement in all mechanical properties. The hybridization of lightweight fiber achieved ECC behavior at a lower volume fraction than the use of a single fiber volume. Relationships between tensile strength and flexural strength depending on the compressive strength of ECC were driven by demonstrating high performance.

Crack initiation mechanisms in Ti-6Al-4V subjected to cold dwell-fatigue, low-cycle fatigue and high-cycle fatigue loadings

Although Ti-6Al-4V is the most commonly used titanium alloy in the aerospace industry, the mechanisms governing crack initiation in the different fatigue regimes are not well understood yet. This situation partly pertains to a competition between multiple crack initiation mechanisms. In particular, applied loading conditions were identified as a key parameter governing the transition between mechanisms. The fatigue behavior of Ti-6Al-4V with a bi-modal microstructure was investigated in the present study using different waveforms, load ratios and frequencies to clarify this feature. A detailed characterization of the main crack initiation sites was carried out to identify microstructural configurations governing crack nucleation in low-cycle dwell-fatigue, low-cycle fatigue and high-cycle fatigue regimes. While lifetimes and fracture surfaces showed a good consistency with data from prior studies, a single microstructural configuration was found involved with the formation of crack initiation facets. All investigated cracks leading to specimen failure were nucleated along (0001) twist grain boundaries exhibiting similar features. Based on this finding, a criterion is proposed to identify candidates for crack initiation. This also demonstrates no major sensitivity of the critical microstructural configurations to environment, free surface, loading conditions and, as shown in previous studies, microstructure and composition. However, conventional fatigue failure results from one, or a few, surface or subsurface crack initiation sites while dwell-fatigue failure involves the formation of multiple internal cracks. This is accompanied by a significant dwell-fatigue life debit at peak stress magnitudes close to the yield strength. Features of microstructural configurations prone to crack nucleation are finally compared with data reported in existing literature to propose a mechanism of crack formation in Ti alloys.

Tensile properties of cement-stabilised clays and their contribution to seawall design

Deep cement mixing is a typical ground improvement technique, and has recently been introduced in some large-scale reclamation projects in Hong Kong. There is a lack of internationally recognised testing standard for determining the tensile strength of cement-stabilised soil. In this study, tensile properties of cement-stabilised clays have been investigated using direct tension test and Brazilian test. The interpretation of tensile strength in Brazilian test is not straightforward due to the formation of multiple cracks during loading. As such, focus is placed on the validation of the fundamental assumption on crack initiation mechanism. A consolidated database of tensile strengths of cement-stabilised soils is compiled. A constitutive model in the finite-element program Plaxis, namely concrete model, has been studied. This model, originally aiming to simulate the behaviour of concrete and shotcrete, duly considers the cracking and the strain-softening characteristics, and is able to reasonably simulate the fundamental tensile behaviour of cement-stabilised soil. Numerical simulations have been carried out using the concrete model to assess the stability of seawall founded on cement-stabilised clay. Tensile properties are found to have an important bearing in maintaining the stability of the seawall when column-pattern stabilisation is adopted.

Age-related skull fracture patterns in infants after low-height falls.

Prior research and experience has increased physician understanding of infant skull fracture prediction. However, patterns related to fracture length, nonlinearity, and features of complexity remain poorly understood, and differences across infant age groups have not been previously explored. To determine how infant and low-height fall characteristics influence fracture patterns, we collected data from 231 head CT 3D reconstructions and quantified length and nonlinearity using a custom image processing code. Regression analysis was used to determine the effects of age and fall characteristics on nonlinearity, length, and features of fracture complexity. While impact surface had an important role in the number of cracks present in a fracture, younger infants and greater fall heights significantly affected most features of fracture complexity, including suture-to-suture spanning and biparietal involvement. In addition, increasing fracture length with increasing fall height supports trends identified by prior finite-element modeling. Finally, this study yielded results supporting the presence of soft tissue swelling as a function of fracture location rather than impact site. Age-related properties of the infant skull confer unique fracture patterns following head impact. Further characterization of these properties, particularly in infants <4 months of age, will improve our understanding of the infant skull's response to trauma. Younger infant age and greater fall heights have significant effects on many features of fracture complexity resulting from low-height falls. Incorporating multiple crack formation and multiple bone involvement into computational models of young infant skull fractures may result in increased biofidelity. Drivers of skull fracture complexity are not well understood, and skull fracture patterns in real-world data across infant age groups have not been previously described. Understanding fracture complexity relative to age in accidental falls will improve the understanding of accidental and abusive head trauma.

A Unified Model for Fibers With Divergent Pullout Behaviors in Strain Hardening Cementitious Composites (SHCC)

Strain Hardening Cementitious Composites (SHCC) are materials exhibiting high tensile ductility with the formation of multiple cracks. Since the mechanical properties of SHCC members are governed by the interfacial characteristics between fibers and matrix, understanding the mechanism of single fiber pullout behavior is crucial for SHCC material design. Existing model was set up based on Polyvinyl Alcohol (PVA) fibers, making it inapplicable to other kinds of synthetic fibers those exhibit divergent pullout behaviors. As a result, the simulated curves cannot agree well with the tested results of SHCC made with other fibers or hybrid fibers. In this study, a unified single fiber pullout model was proposed to take divergent kinds of fiber pullout behavior into account. Five parameters were defined to describe the single fiber pullout behavior, where the frictional strength during the pullout stage and the blocking length of fiber under pulley force were for the first time introduced. For verification purpose, fiber-pullout samples with Polyethylene terephthalate (PET) and PVA fibers were tested and the results agree well with the simulated curves from proposed model. The stochasticity of each parameters were then analyzed and described by normal distribution, through which fibers with various random pullout behaviors can be incorporated into a larger scale modelling. Therefore, the fiber-bridging constitutive law for a single crack was calculated and compared with the single crack test results, which confirmed again the validity of the proposed model.

Characterization of toughening mechanisms in UHPC through image correlation and inverse analysis of flexural results

The nature of cracking in flexural specimens made using ultra-high-performance concrete (UHPC) is studied in the context of mechanisms of enhanced strength and ductility. The effect of fibers on the suppression of cracking and the attendant toughening mechanisms are addressed at two different fiber contents to elucidate the strain-softening and strain-hardening responses. Results indicate that the first crack strength is directly related to the fiber content, with fibers extending the stable crack growth region at high fiber contents such that flexural strengths as high as 20 MPa are obtained. Fiber bridging results in resistance to crack growth, stabilization, as well as multiple crack formation, all of which contribute to toughening. Using a combination of experimental results, digital image correlation (DIC) measurements of crack extension, and an analytical model for the flexural response, the stress and strain distributions across the depth of the specimens are determined, leading to stress-crack width relationships, and consequently, the relationship between tensile and flexural stresses in fiber-reinforced UHPCs. The results are also used to determine the tensile and compressive strain and stress distributions. Using back-calculated tensile properties from the flexural response and the 3-D DIC results, crack width profile and the stress-crack width relationship in tension are obtained for UHPCs, leading to a comprehensive understanding required for the structural design using such high-ductility materials.

Creep of pre-cracked sisal fiber reinforced cement based composites

In the present paper, the creep behavior of sisal fiber cement based composites was investigated. The composites were manufactured using a matrix with low calcium hydroxide content, obtained by partially replacing the cement by fly ash and metakaolin. Two types of composites were developed, each with three layers of long and unidirectional sisal fibers, varying the condition of the fibers in saturated with water and with natural humidity. The mechanical behavior was evaluated through direct tension and four-point flexural tests. The composites reinforced with saturated fibers showed higher strain capacity in tension due to the properties of the fiber–matrix interface and lower ultimate bending strength. Cracking mechanisms were studied through photographs obtained during the tests, as well as analyzes by a stereoscopic microscope. All composites presented strain/deflection hardening behavior with the formation of multiple cracks. Tensile and flexural creep tests were performed on pre-cracked composites in order to study time-dependent responses. The evolution of cracks was monitored throughout the creep test with a stereoscopic microscope. The composites showed a reduction in strength and strain capacity after the creep tensile and flexural test, which resulted in a decrease in toughness of 36.9% and 48.5% for the composites with saturated fibers and natural humidity, respectively. In addition, for the flexural creep test, the composites with saturated fibers showed a decrease in toughness of 48.2% while the composite with natural humidity had a decrease of 35.3% for the load equivalent to 50% of their strength.

Challenges in the fatigue crack growth characterization of metal/composite joints: A compliance-based investigation of a Ti/CFRP joint

The challenges and issues that arise in the fatigue crack growth characterization of dissimilar joints due to multiple crack formation, suitable compliance and strain energy release rate definitions are thoroughly investigated in the present work. The framework is set by the need to characterize the fatigue behavior of a new Ti/CFRP adhesive joint. A new method for measuring the experimental compliance is proposed and critically compared to the dynamic compliance concept. The compliance calibration method while taking into account secondary cracking is addressed. The fatigue crack growth curves are extracted and discussed under the prism of the aforementioned challenges.

Surface energy evaluation using a modified 3-D M-integral for multiple surface cracks

In the present paper, we develope a 3-D Msrf-integral to evaluate the surface energy that is corresponding to formation of multiple surface cracks. The Msrf-integral is formulated by modifying the conventional 3-D M-integral with deliberate origination of the coordinate system and proper delimitation of the integral surface. The integral is demonstrated to be surface-independent in a modified manner and so can be accurately evaluated provided an integration surface remote from the crack fronts is selected. With this feature, the theory of integration can be straightforwardly incorporated into the finite element method. In fact, it is not necessary to simulate the near-front regions with very fine FE simulations to obtain accurate solutions of Msrf.

Flexural behavior of stone slabs strengthened with prestressed NSM steel wire ropes

In order to enhance the flexural behavior of the stone building structures, a technique of strengthening stone slabs using prestressed near-surface mounted (NSM) steel wire ropes is proposed. Six strengthened stone slabs were tested under four-point bending by considering the reinforcement ratio, level of prestressing and bonding agent as testing parameters. Additionally, a bare stone slab was also involved in the test program for comparison purpose. It was found from the experimental investigation that, the strengthened stone slabs exhibited ductile failure mode with visible deflection. Formation of multiple cracks in both constant bending zone and flexural-shear zone was observed in the strengthened stone slabs. As expected, abrupt fracture occurred in the bare stone slab as soon as the cracking moment was attained. Besides the improved failure mode, the flexural strength of the strengthened specimens was improved significantly and tended to enhance with the increase of prestressing level. No end slip of the steel wire ropes was observed highlighting the favourable bond characteristics between the constituent elements (steel wire ropes and stone slab). A simplified theoretical model to be used to predict the flexural strength of the strengthened stone slabs was developed and assessed.

Shear strengthening of reinforced concrete beams with high strength strain-hardening cementitious composites (HS-SHCC)

Strain-hardening cementitious composite (SHCC) was widely recognized as a material that possesses great potential in the repair and strengthening of reinforced concrete (RC) structures. SHCC can usually achieve high ductility with multiple cracking behavior and it also has excellent durability. Moreover, high tensile strength can also be achieved with proper mix design. In this study, conventional RC beams with shear span-to-depth ratio of 1.5:1 and 2.5:1 were cast first, while high-strength SHCC were then cast as thin patches on lateral surfaces of the RC beams to serve as the strengthening layers. The beams were subject to four-point bending test to obtain their ultimate shear capacities. It was observed from the test results that the ultimate shear capacity of strengthened RC beams increases evidently compared to the reference beams. Upon ultimate shear failure, no spalling of surface concrete occurred as it was restrained by the strengthening layers with the formation of stable multiple fine cracks. A finite element model was constructed to simulate the experimental tests. The results of FEM analysis correlated well with experimental results. A numerical parametric study was then carried out to evaluate the influence of the thickness of HS-SHCC layer and the shear span-to-depth ratio. This study concludes the feasibility of the use of high strength SHCC in shear strengthening of RC structures.

Evaluation of surface energy for formation of multiple edge cracks using Medg-integral

A novel contour integral approach termed Medg is introduced for computation of the surface energy required for the formation of multiple edge cracks. The method is developed by reinterpretation of the conventional M-integral with deliberate delimitation of integration contour and selection of coordinate origin. Due to path independence, this method is efficient, easy to implement by using finite elements, and does not require a complicated mesh around the crack tips for good accuracy. Attention is also addressed to discussion of the size effects on proper interpretation of its physical meaning. The adequacy of the numerical results computed for the finite size corrections has been validated by using some of the available empirical formulations. It is observed that the size effect can be neglected when the crack size remains under one-tenth of the structure size. The results of a specific multi-cracked geotechnical structure suggest that, the damage state such as degradation of the structural stiffness due to the presence of edge cracks can be properly inspected by using Medg.

Biaxial flexural fatigue behavior of strain-hardening UHPFRC thin slab elements

The biaxial flexural fatigue behavior of thin slab elements made of strain-hardening Ultra High Performance Fiber Reinforced Cementitious Composite (UHPFRC) is investigated experimentally by means of the ring-on-ring test method. Fourteen flexural fatigue tests under constant amplitude fatigue cycles up to the Very High Cycle Fatigue domain (20 million cycles) are conducted with varying maximum fatigue stress level S ranging from 0.50 to 0.68. Digital Image Correlation (DIC) technology is applied to capture the 3D full-field strain contours on the tensile surface through the entire fatigue test. Test results presented in the S-N diagram reveal a fatigue endurance limit under biaxial flexural fatigue at S = 0.54. Fatigue tests exhibiting failure show four distinct phases of damage evolution, while only the first two phases are observed in the case of run-out tests. DIC analysis reveal formation and propagation of multiple fine fictitious cracks that dominate the stable fatigue propagation phase with slow rate, representing the longest part of fatigue life of the UHPFRC specimen. Finally, the secant modulus of deflection and fictitious crack opening with respect to fatigue cycles is found to characterize quantitatively fatigue damage evolution.