Flaw Distribution Research Articles

High-ductility strain-hardening cementitious composites at subzero temperatures by recycling waste crumb rubber: Air entrainment and flaw distribution optimization

The ductility of strain-hardening cementitious composites (SHCC) is known to degrade when exposed to subzero temperatures, compromising the resilience and sustainability of civil infrastructures. To address this issue, a preliminary attempt is conducted to explore the potential of employing an air-entraining agent (AEA) or waste crumb rubber to weaken the SHCC matrix and enhance the ductility of SHCC. The effects of incorporating AEA and crumb rubber (fine crumb rubber and coarse crumb rubber, referred to as FR and CR) on the fresh, physical, and mechanical properties of SHCC at various subzero temperatures are thoroughly examined and compared. The experimental results demonstrate that the air-entraining effect of incorporating waste crumb rubbers is similar to that of AEA, resulting in an approximate 9 % air content within the SHCC. Furthermore, the incorporation of crumb rubber significantly improves both the tensile strength and ductility of SHCC at subzero temperatures. Remarkably, the CR-modified SHCC exhibits a tensile strength of up to 10.50 MPa and a strain capacity of 4.30 % at −60 °C. The addition of crumb rubbers introduces uniformly distributed spherical bubbles, optimizing the size and distribution of flaws within the SHCC matrix and enhancing the strain potential of SHCC. This study proposes a cost-effective and environmentally friendly SHCC formulation by recycling waste crumb rubber and proposes a promising approach for developing high-ductility and sustainable SHCC in cold regions. By leveraging the potential of solid waste, this research contributes to the advancement of durable and resilient infrastructure solutions.

Evaluation of 3-Point and 4-Point Bending Tests for Tensile Strength Assessment of GFRP Bars.

Glass Fiber Reinforced Polymer (GFRP) bars are used as reinforcement for structural concrete, especially in cases where corrosion of traditional steel reinforcement is a problem. The tensile strength of these reinforcing bars is the primary characteristic on which the design of concrete members reinforced with GFRP bars relies. Determination of the tensile strength of the bars using a direct tensile test is a time and resource-intensive task and therefore is not routinely conducted for quality control. The tensile strength can also be measured from flexure tests, which are much simpler than direct tensile tests, and use appropriate correlation formulations. In this paper, the applicability of flexure testing for the identification of bars' tensile strength is investigated by conducting and analyzing both 3-point and 4-point flexure testing. The correlation formulations are presented that allow the determination of tensile strength from the modulus of rupture. The Weibull weakest link model and the assumption of the same flaw distribution in tensile and flexure tests is adopted. The results of the 3-point and the 4-point bending are presented and compared. Comparisons are also conducted to select direct tensile test results. The work shows that 3-point and 4-point bending tests provide very similar results, with the difference between the results being 2% to 10%, suggesting both tests can be used for tensile strength determination of GFRP bars.

Simultaneous enhancement of ductility and sustainability of high-strength Strain-Hardening Cementitious Composites (SHCC) using recycled fine aggregates

In this study, recycled fine aggregates (RA) were successfully adopted to produce strain-hardening cementitious composites with a compressive strength of over 120 MPa. A multiple-scale investigation was conducted to study the tensile performance and cracking mechanism of the developed cement-based composites. Compared to the fine silica sand-based counterpart, significantly higher tensile strain capacity (>5%), more distinguished multiple cracking, and smaller average crack width (<100 μm) were achieved in RA-based strain-hardening cementitious composites. Supported by microscopic and mesoscopic results, the old mortar of RA with loose microstructures and weak RA/matrix interfaces were found to work as “additional flaws” in the matrix, which effectively tailored the distribution of active flaws and promoted multiple cracking behavior. In addition, the developed RA-based composites presented higher sustainability and lower material costs compared to fine silica sand-based ones. This study provides a new avenue for the upcycling of construction wastes in developing greener high-performance fiber-reinforced cementitious composites.

Failure behaviour of frozen rock with double ice-filled flaws under Brazilian splitting tension

Geotechnical engineering in cold regions frequently involves dealing with frozen fractured rock masses, particularly in the activities of strip mining and tunnel support design, where tensile failure predominates as the main failure mode in stress-driven failures. Seasonal freezing can adversely affect the performance of rock masses. To investigate the tensile characteristics of frozen rock masses, marble specimens with double parallel ice-filled flaws were artificially frozen and subjected to Brazilian splitting experiments at a loading rate of 0.001 mm/s. The influence of the angle of the double coplanar flaws and the rock bridge angle of ice-filled flaws on the tensile behaviours were numerically studied by RFPA3D (3D Rock Failure Process Analysis), a finite element method specifically developed for simulating rock failure. Results showed that as the angle of the double parallel ice-filled flaws increases, tensile strength initially decreases and then increases. Furthermore, in specimens with the double coplanar ice-filled flaws, an increase in the angles of the flaws and rock bridge leads to a gradual decrease in tensile strength. The spatial distribution of ice-filled flaws is a crucial factor that influences the failure mode after split cracking occurs. The ice filling and bonding mechanism controls the tensile strength and cracking behaviour of frozen rock masses. The findings of this study provide valuable insights and serve as a theoretical foundation for designing blasting parameters and disaster prevention in strip mines located in cold regions.

Effect of high temperature annealing on microstructure and mechanical properties of Morlion735 alumina fiber

Continuous alumina fiber-reinforced alumina (Al2O3/Al2O3) composite is a very promising candidate for thermal structural materials in aerospace vehicles and other applications. Alumina fibers used in Al2O3/Al2O3 composites play a crucial role in enhancing their mechanical properties and thermostability. In this article, the effect of annealing on the tensile strength and microstructure of Morlion735 alumina fiber was investigated. The results showed that the phase transformation occurs at 1147 °C, resulting in a mullite phase with an orthogonal lattice structure. As the annealing temperature increases, the crystal domain size of the mullite gradually increases. Both the phase transformation and the increasing crystal domain size lead to more lattice defects, which significantly affect the tensile strength and tensile modulus of alumina fibers. A 0.67 GPa reduction in tensile strength (from 1.14 GPa to 0.47 GPa) was observed when the annealing temperature increased from 1200 °C to 1400 °C. Weibull statistical analysis revealed that as the annealing temperature increased, the tensile strength dispersion of Morlion735 fiber also increased. This can be attributed to the volume-dependent flaw distributions. These results provide an in-depth understanding of the tensile strength evolution mechanism under high temperatures.

Deploying Machine Learning for Radiography of Aerospace Welds

Artificial intelligence is providing new possibilities for analysis in the field of industrial radiography. As capabilities evolve, there is the need for knowledge concerning how to deploy these technologies in practice and benefit from the new automatically generated information. In this study, automatic defect recognition based on machine learning was deployed as an aid in industrial radiography of laser welds in an aerospace component, and utilized to produce statistics for improved quality control. A multi-model approach with an added weld segmentation step improved the inference speed and decreased false calls to improve field use. A user interface with visualization options was developed to display the evaluation results. A dataset of 451 radiographs was automatically analysed, yielding 10037 indications with size and location information, providing capability for statistical analysis beyond what is practical to carry out with manual annotation. The distribution of indications was modeled as a product of the probability of detection and an exponentially decreasing underlying flaw distribution, opening the possibility for model reliability assessment and predictive capabilities on weld defects. An analysis of the indications demonstrated the capability to automatically detect both large-scale trends and individual components and welds that were more at risk of failing the inspection. This serves as a step towards smarter utilization of non-destructive evaluation data in manufacturing.

Effect of welding speed on microstructure, nano-mechanical and nano-tribological characteristics of dissimilar friction stir welded AA6061-T6 and AZ31 alloy

The microstructural, tribological, and mechanical performance of friction-stir welded AA6061-T6 and AZ31 alloys were investigated using electron back scattered diffraction, scanning electron microscope, X-ray diffraction, and electron dispersed spectroscopy methods. The non-destructive X-ray microcomputed tomography test was used to analyse weld porosity. The distribution of volumetric and surface flaws (pores) was found to be confined across the weldment. Using a novel atomic force microscopy method, the nano-tribological characteristics of the base metal and localized regions of welded sample were studied for the first time. The significant difference in wear and friction behaviour was observed in all weld zones and base metal of magnesium side, which was correlated with the hardness of these regions. AFM results showed strong variation in tribological properties in correlation with the hardness trends of different weld zones, i.e. higher hardness leads to low wear. In weldments, intermetallic compounds such as Al12Mg17 and Al3Mg2 were produced, resulting in nanohardness differences as measured by the nanoindentation test. The joint efficiency of 65%, 84%, and 91% were obtained at welding speeds of 16 mm/min, 20 mm/min, and 25 mm/min, respectively.

Stability of Ice Shelves and Ice Cliffs in a Changing Climate

The largest uncertainty in future sea-level rise is loss of ice from the Greenland and Antarctic Ice Sheets. Ice shelves, freely floating platforms of ice that fringe the ice sheets, play a crucial role in restraining discharge of grounded ice into the ocean through buttressing. However, since the 1990s, several ice shelves have thinned, retreated, and collapsed. If this pattern continues, it could expose thick cliffs that become structurally unstable and collapse in a process called marine ice cliff instability (MICI). However, the feedbacks between calving, retreat, and other forcings are not well understood. Here we review observed modes of calving from ice shelves and marine-terminating glaciers, and their relation to environmental forces. We show that the primary driver of calving is long-term internal glaciological stress, but as ice shelves thin they may become more vulnerable to environmental forcing. This vulnerability—and the potential for MICI—comes from a combination of the distribution of preexisting flaws within the ice and regions where the stress is large enough to initiate fracture. Although significant progress has been made modeling these processes, theories must now be tested against a wide range of environmental and glaciological conditions in both modern and paleo conditions. ▪Ice shelves, floating platforms of ice fed by ice sheets, shed mass in a near-instantaneous fashion through iceberg calving.▪Most ice shelves exhibit a stable cycle of calving front advance and retreat that is insensitive to small changes in environmental conditions.▪Some ice shelves have retreated or collapsed completely, and in the future this could expose thick cliffs that could become structurally unstable called ice cliff instability.▪The potential for ice shelf and ice cliff instability is controlled by the presence and evolution of flaws or fractures within the ice.

Numerical study on the failure evolution of rock slopes containing multi-flaws by strength-based fracture method

In this study, we adopt the proposed strength-based local maximum stress (SLMS) criterion to calculate the crack propagation of tensile and shear cracks in a plate with two parallel closed flaws at different rock bridge inclinations (β) under compression. The effect of β on the failure mode of rock bridges was investigated. Then, we modeled the three-section landslides caused by crack propagation under different friction coefficients to validate the effectiveness of SLMS in modeling crack propagation in slopes. Finally, the formation of sliding surfaces in four specific rock slopes with multi-flaws was modeled to investigate the effect of flaw distribution on the depth and shape of sliding surfaces of slopes. Results indicate that flaws located in the middle of the slope have a significant effect on the failure mode and the depth of the sliding surface at this location. Due to the small increments of displacement during the failure process of the slope, which are difficult to monitor, displacement monitoring methods are ineffective for landslide early warning. The instability of the rock slope is caused by the initiation, propagation, and coalescence of cracks, therefore, real-time monitoring of the micro-failure accumulation in the slope is beneficial for early warning of landslides. Moreover, the variation of contact status of crack surfaces during the formation of slope sliding surfaces was analyzed, and the effect of sliding surface formation on contact sliding distance and contact gap distance of crack surfaces was investigated.

Influence of flaw distribution on tensile behavior of Engineered Cementitious Composites (ECC) based on flaw probability model

The high ductility of Engineered Cementitious Composites (ECC) is attributed to the internal fibers’ bridging effect during matrix cracking, where the distribution of strength in components is crucial for ductility design. Starting from fracture mechanics, this paper proposes a method based on the bridging model to convert flaw sizes into fracture strength in an ECC system. The distribution of flaws in the components is implemented using the Weibull function. By incorporating a crack tracing algorithm, the tensile behavior of components under different flaw distributions is calculated. The effectiveness of the proposed model, which considers flaw distribution, is verified through the comparison between the simulated and experimental data. Furthermore, the simulation results indicate that moderate adjustment on the flaw distribution in components can greatly improve the corresponding tensile ductility without compromising the strength. The research outcomes benefit to provide technical supports for the optimization design of material ductility.

Effect of the Number and Distribution of Circular Holes on the Damage and Fracture of Limestone Using PFC2D

The common occurrence of various number and distribution of hole flaws complicates the mechanical behavior and fracture mode of the rock masses that contain them. This study develops seven numerical models of limestone samples with different numbers and distributions of circular hole flaws using 2-D particle flow discrete element code (PFC2D) to investigate their impact on the mechanical properties of limestone while maintaining the same flaws area. In addition, it analyzes the effect of these factors on the mesomechanical features of each model, including the characteristic stress values (peak stress, crack initiation, and damage stress), crack evolution, stress, and displacement field. The results showed that the peak stress, crack initiation, and damage stress of the single-hole model are between those of multihole models. As the arrangement of dip angle increases, the peak stress, crack initiation, and damage stress of models with the same number of multihole flaws exhibit a V-shaped change. The characteristic stress values are the largest when the holes are vertically aligned. Model differences in crack development path, shape, and number, as well as stress concentration area and failure mode, are primarily due to the number and distribution of holes. The circular holes are arranged at approximately 45°, and the greater the number of defects, the more likely the model is to fail. The study’s findings can provide support and reference for the research system of deformation and failure of rock mass with hole flaws.

Probability of failure and weibull size-scaling parameters for thin glass subjected to two-point bending

The two-point bending test is a valuable test for estimating the strength of the thin and flexible glass/ceramic parts that are increasingly used in next-generation flexible electronics devices. We present closed-form expressions for determining Weibull material scale parameters and probability of failure for the two-point bending test. The presented solution, tested on Corning® Willow® glass samples, eliminates the need for iterative and often time-consuming approaches (e.g., Finite Element Analysis) for estimating scale parameters. Considering the strength-controlling flaw distribution on a surface or inside a volume, the solution assumes that the Weibull modulus (m) and scaling modulus (mAor mV) are distinct quantities to ensure the scale parameter is independent of the applied load. This condition reveals a special relationship between the moduli that is unique to two-point bending given its highly non-uniform state of stress, which contrasts with other uniaxial bending tests (such as three and four-point bending) in which both moduli have the same numerical value.

Investigation of Specimen Size Effects on P-Quantile Diagrams and Normal Distributions of Critical Flaw Strengths in Fiber Tows

The present paper proposes a model of the specimen size effect on the critical flaw strength distribution in fiber tows for composite reinforcement. The model is based on the basic assumption of brittle fracture that the failure probability at a given strength increases with specimen size in the p-quantile vs. strength relation and on the normal distribution. Empirical results derived from force–strain curves determined on tows made of 1000 and 500 Nicalon SiC filaments and with various gauge lengths show some discrepancy with predictions using the model. The empirical p-quantile diagrams and cumulative distributions of critical flaw strengths exhibited excellent reproducibility at longer gauge lengths, which suggests the absence of a size effect above a critical tow size. The reproducibility of flaw strength distributions at gauge lengths above 60 mm and the higher strengths obtained at lower gauge lengths despite structural effects were related to the features of the critical flaw distribution in tows of parallel fibers.

Editorial: Dental Ceramics: From Science and Technology to Clinical Application

Editorial: Dental Ceramics: From Science and Technology to Clinical Application

Monte Carlo Simulations of Strength Distributions of Brittle Materials – Type of Distribution, Specimen and Sample Size

Abstract Strength data of brittle materials have a large scatter. Therefore, probabilistic methods have been developed for designing with brittle materials. In general, a Weibull-type strength distribution function is determined by strength testing of a sample of about 30 specimens. This function is then used to predict a tolerable load level for a high component’s reliability. Curved structures in Weibull plots are often attributed to special features of the strength distribution function, e. g. to a threshold strength for failure or to bi- and multimodal flaw distributions. For such strength distributions the usual extrapolation is impossible. By means of Monte Carlo simulations it is shown that on the basis of small samples (containing around 30 experiments) real structures in strength distributions can hardly be distinguished from artefacts arising from the statistical nature of the data. On the basis of a small sample it is also not possible to decide whether a distribution is of a Weibull-type or any other distribution. Requirements for the determination of strength distribution functions and possible problems are discussed. Safe data extrapolations are possible if the flaw size distribution, which causes the strength distribution, is known. This knowledge can be acquired by testing a sample containing several subsamples with specimens of different size.

In-situ detection of redeposited spatter and its influence on the formation of internal flaws in laser powder bed fusion

The pervasive adoption of laser powder bed fusion (LPBF) as an industrial manufacturing technique relies on the improvement of its repeatability, currently limited by the stochastic formation of flaws. Considering that large flaws can form randomly and despite the optimization of process parameters, an in-situ monitoring technique suitable for detecting deviations that originate these critical flaws is paramount. The redeposition of spatters on the build area has previously been identified as one of the factors responsible for the rise of internal flaws, but so far limited are the efforts towards their detection. This study aims to detect spatter redeposits via in-situ monitoring and to couple the detections to lack of fusion. For that, long-exposure near-infrared in-situ monitoring associated with image analysis is employed to determine the exact locations and quantify the incidence of spatter redeposits across three full builds performed at varying layer thicknesses. The existence and distribution of internal flaws is verified ex-situ by means of ultrasonic inspection and metallography. The formation of internal flaws is attributed to spatter redeposits after detailed characterization of size, particle and surface morphology of spatter and identification of particles with identical characteristics on the fracture surface in the adjacencies of lack of fusion. It is found that spatters preferentially redeposit on the adjacencies of the gas outlet, but that the affected portion of the build area and the prevalence of detections is heavily dependent on the powder layer thickness employed in the manufacturing process. The monitoring system setup preferentially acquires signal from spatters redeposited on print regions, making it particularly suitable for flaw detection.

Impact of Experimental Protocols on the Flexural Strength Testing of Lithium Disilicate-Based Dental Glass-Ceramics

The transition of dental ceramics and glass-ceramics (for fixed all-ceramics and metal-ceramics restorations and prostheses) from laboratory to clinic requires the materials to pass through a series of tests following the protocols and requirements discussed in the standard ISO 6872:2015. However, the literature reveals that the researchers in the dental ceramics community usually do not adhere to the ISO standard’s specifications when measuring their materials’ flexural strength, assuming that these deviations from the protocols exhibit minimal or no impact on the materials’ strength. Therefore, the present study aims to fill that gap by elucidating the impact of the following experimental protocols – (i) polishing, (ii) edge chamfering and (iii) adhesive tape on the non-tensile side – on three-point flexural strength of lithium disilicate based glass-ceramics. The result reveals that polishing increases flexural strength and tends to introduce edge or corner flaws in the specimen. Though edge chamfering of specimens did not help remove the edge defects in our study, it leads to a tighter flaw distribution, increasing the reliability of the flexural strength results. The application of adhesive tape (on the compressive side) exhibits an insignificant impact on the glass-ceramic specimens’ flexural strength. Accordingly, revisions have been suggested in the ISO 6872 standard.

Failure Mechanisms in Vertically Aligned Dense Nanowire Arrays.

Nanowires are an increasingly prevalent class of nanomaterials in composites and devices, with arrays and other complex geometries used in various applications. Little investigation has been done regarding the mechanical behavior of micron-sized nanowire structures. We conduct in situ microcompression experiments on vertically aligned dense microbundles of 300 nm diameter single-crystalline zinc oxide nanowires to gain insights into their structural failure. Experiments demonstrate that bundles containing approximately 10-130 nanowires experience two failure regimes: (1) localized noncatastrophic interfacial splitting and (2) global structural failure. Utilizing Weibull statistics and experimental results, we develop a technique for analyzing flaw distribution and use it to predict the expected range of bundle failure stress. This analysis provides guidelines for nanowire arrays' susceptibility to failure, sensitivity to flaw size, interfacial interactions of constituents, and degree of alignment. This work develops insights to understand and predict fundamental failure mechanisms in highly aligned, dense structures.

Distribution distortion in the statistical analysis of strength data when using multiple specimen batches

The strength distribution of Zerodur® (Schott AG) prepared by grinding and etching is determined through statistical analysis to be best described as normal. Although a 3-parameter Weibull appearance is exhibited when multiple specimen batches are combined and analyzed together, this appearance is the result of mixing of components of variation. As a result of the particular type of flaw population, little effect of scale is exhibited. Caution is advised when assuming a 3-parameter Weibull distribution and associated threshold solely based on distribution shape. Fractography to understand the flaw distribution should be employed.

Continuous and discrete methods based on X-ray computed-tomography to model the fragmentation process in brittle solids over a wide range of strain-rates - application to three brittle materials

In the present work, two methods, named “continuous” and “discrete”, are proposed to model the fragmentation process in brittle solids. Both methods rely on a preliminary analysis of the existing flaws population in scanned samples with X-ray micro-Computed Tomography (microCT). By converting the size of defects into critical stresses, the density of critical defects versus the applied stress level is deduced and used as an input of both a continuum and a discrete method. To do so, the concept of critical defects obscuration is implemented. Introduced in the DFH (Denoual-Forquin-Hild) micromechanics model, this concept consists in describing how cracks propagating from triggered flaws prevent neighbouring flaws from being activated. This obscuration phenomenon is implemented in the present work by using the flaws population determined via microCT analysis as an input. In the continuous method, the differential equation of the obscuration probability provided in the DFH model is integrated. In the discrete method, a cubic sub-volume of the scanned volume is considered and the growth of obscuration volumes is numerically simulated considering the real location of each critical defect and their stress of activation. Both methods provide predictions for the material dynamic strength and final cracking density according to the applied strain-rate. These two methods are applied to three types of brittle materials: an Ultra-High Performance Concrete (UHPC), a porous polycrystalline ice and a silicon carbide with spherical “fuse-flaws”. Finally, the obtained predictions are compared to the closed-form solution of the DFH model, which is based on a Weibull distribution of the critical flaws identified from bending tests. Whereas the three approaches match very well at low strain-rates, the continuous and discrete methods diverge from the DFH closed-form solution at high strain-rates, due to the activation of smaller and more numerous defects that could not be activated in the quasi-static bending tests.