Quasi-brittle Behaviour Research Articles

The Damage Process Monitoring in Structures with Quasi-Brittle behavior in the perspective of Complex System.

The damage process in structures with quasi-brittle behavior is characterized by typical phenomena such as Size effect sensibility, the interaction of micro-fissures, and the transition of the continuum to discontinuation throughout the so-called localization effect. Structures are generally built with concrete, rocks, ceramic, and composite structures. This type of material could be considered. There are many approaches to modeling the mechanical behavior of this type of Material/Structure, but our strategy of simulation, a version of the Discrete Element Model, is aligned with the perspective of Prof. Krajcinovic; from his point of view, the random nature of the material is a key aspect that must due to consider. To address this problem, the Acoustical Emission Technique (AET) could also be useful to characterize the damage process. The combination of the signal captured in a different place in the structure determines the spatial and temporal distribution of the events. During the damage process. With this information, it is possible to compute global parameters and their evolution during the damage process of the structure, which could be used as precursors to local or global damage. There are several global indexes, and to study their evolution, let us know when the structure will be close to global or local collapse. Let's consider that the failure of the structure could be seen as a phase transformation phenomenon. It is possible to understand this problem using the Renormalization Group Theory proposed by Wilson and the basic ideas of Anderson related to a Complex System. In this context, in the present work, we consider some applications related to the characterization of the Damage Process of Quasi-Brittle structures/materials carried out for our research group, searching for the link between these results and the ideas of Wilson and Anderson. Keywords: Acoustic Emission Techniques, Discrete element Method, Critical Phenomena.

A macroscale damage model for the tensile and bending failure of C/C-SiC structural laminates

This work presents a Finite Element approach to model the in-plane mechanical behavior of C/C-SiC fabrics at the lamina level, including the non-linear response of diverse lay-ups and the bending-to-tensile strength ratio at failure. The material model employs a decomposition into two idealized phases, which can be exploited to capture matrix- and fiber-dominated responses at a high level of abstraction. The constitutive law of the matrix phase adopts a Continuum Damage approach driven by two Tsai-Wu surfaces, while a quasi-brittle behavior is attributed to the fibers phase. This decomposition effectively represents the influence of matrix degradation on the response and failure of the laminates. Moreover, simulations reveal that a statistical distribution of the strength is required to represent some of the experimental outcomes. The correlation with experimental data that was achieved points out that the technique is a promising tool for supporting the early design phase of CMC structures.

On the use of selective Z-pinning in stiffened composite panels to prevent skin/stringer debonding

Carbon fiber reinforced plastic (CFRP) composites have been widely used in engineering. This paper investigates the effect of composite z-pin (polyimide fiber/epoxy resin) on flexural properties of skin/stringer composite structure with different z-pin densities and diameters. Compared with unpinned specimens, the peak loading and energy absorption of z-pinned specimens are increased by 98% and 9 times, respectively. Parameterized research indicates that the optimal z-pin density is 1.35%. And thin z-pins are beneficial to improve the peak loading and energy absorption, which attributes to more total contact area of z-pins. The statistical results present that z-pin pre-hole inserted (ZPI) process can significantly reduce initial damages of z-pinned specimens compared with Ultrasonically Assisted Z-fiber (UAZ) process. Specifically, the z-pin inclination angle is decreased by 60%, and the initial damages in eye-like zone are prominently reduced. The z-pin makes the specimen exhibit the quasi-brittle behavior under flexural loading by playing a bridging role, which makes the flexural properties of specimens prominently increased. In addition, the “snubbing effect” will enhance the bridging effect of z-pins, but it will lead to inter-fibers debonding, splitting, and shear failure of z-pins.

Behavior of Fibers in Geopolymer Concrete: A Comprehensive Review

Over the last decades, cement has been observed to be the most adaptive material for global development in the construction industry. The use of ordinary concrete primarily requires the addition of cement. According to the record, there has been an increase in the direct carbon footprint during cement production. The International Energy Agency, IEA, is working toward net zero emissions by 2050. To achieve this target, there should be a decline in the clinker-to-cement ratio. Also, the deployment of innovative technologies is required in the production of cement. The use of alternative binding materials can be an easy solution. There are several options for a substitute to cement as a binding agent, which are available commercially. Non-crystalline alkali-aluminosilicate geopolymers have gained the attention of researchers over time. Geopolymer concrete uses byproduct waste to reduce direct carbon dioxide emissions during production. Despite being this advantageous, its utilization is still limited as it shows the quasi-brittle behavior. Using different fibers has been started to overcome this weakness. This article emphasizes and reviews various mechanical properties of fiber-reinforced geopolymer concrete, focusing on its development and implementation in a wide range of applications. This study concludes that the use of fiber-reinforced geopolymer concrete should be commercialized after the establishment of proper standards for manufacturing.

Mechanical Performance of Cementitious Materials Reinforced with Polyethylene Fibers and Carbon Nanotubes

The cracking of cementitious materials due to their quasi-brittle behavior is a major concern leading to a loss in strength and durability. To limit crack growth, researchers have incorporated microfibers in concrete mixes. The objective of this study is to determine if nano-reinforcements can arrest cracks and enhance the material performance in comparison to microfibers. A total of 28 specimens were prepared to investigate and compare the effects of incorporating carbon nanotubes (CNTs) as a nano-reinforcement and polyethylene (PE) fibers at a macro-level and their combination. Compressive and flexural strengths were experimentally tested to assess the mechanical performance. The microstructure of the mortar samples was also examined using a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX). The ductility increased by almost 50% upon the addition of CNTs, while no significant enhancement was witnessed for the compressive strength. The flexural strength increased by 169% and the flexural strain by 389% through the addition of the combination of CNTs and PE fibers.

Physicomechanical Properties of Carbon Nanotubes Reinforced Cementitious Concrete – A Review

Though concrete is one of the most widely used construction materials, there are some concerns and shortcomings associated with it. Cementitious materials' quasi-brittle behavior, which leads to cracking and a loss of durability, is a major concern in structural applications. In this review, the latest research on reinforcing cementitious concrete with carbon nanotubes (CNTs) is reviewed, with an emphasis on the material's structural performance in building and a comparison of CNTs to other reinforcing fibers. The improvement of the macro mechanical properties of carbon nanotube-reinforced composite structures has been discussed in the form of functionally graded carbon nanotubes reinforced composites (FG-CNTRC). Several researches have, in the past, used other forms of reinforcements to enhance the properties of concrete till the implementation of nanotechnology in concrete production by incorporating CNTs into the concrete mixes. Concrete's crucial mechanical properties as a structural material and the durability of conventional cement-based building materials can both be improved by CNTs. They have drawn a lot of interest because they are an engineering material with a wide range of uses. The creation and characterization of cement-based materials reinforced with CNTs have been studied by researchers. Comparisons between the effects of CNT and other fibers on concrete have also been made. This concrete reinforcement type's environmental impact and sustainability have also been discussed. According to studies, CNT can greatly enhance the performance of cement-based materials.

A numerical model for calculating the impact-induced depression

Materials usually have a quasi–brittle behavior under impact loading, even the ductile materials when loading rate is high enough. In this paper, a new numerical model for calculating the transverse impact on a quasi–brittle target is developed and the theoretical deductions with a strict mathematic logicality are illustrated. Herein, the example of a stiff ball impacting a brittle fiber bundle is investigated and the issues of stress wave propagation, inertia force, energy transformation and stress equilibrium are considered. The numerical model that can describe the impacting procedure is derived. With timing, the deformation of brittle fiber bundle, the kinetic energy of stiff ball as well as their displacements are given. Thus, the overall perspective of the impacting procedure is calculated and analyzed. This work offers a physical illustration of impact events by developing a numerical model and displays the insights into the transient procedure, that will contribute to the development of impact mechanics for engineering.

Size scaling of failure strength at high disorder

We investigate how the macroscopic response and the size scaling of the ultimate strength of materials change when their local strength is sampled from a fat-tailed distribution and the degree of disorder is varied in a broad range. Using equal and localized load sharing in a fiber bundle model, we demonstrate that a transition occurs from a perfectly brittle to a quasi-brittle behavior as the amount of disorder is gradually increased. When the load sharing is localized the high load concentration around failed regions make the system more prone to failure so that a higher degree of disorder is required for stabilization. Increasing the system size at a fixed degree of disorder an astonishing size effect is obtained: at small sizes the ultimate strength of the system increases with its size, the usual decreasing behavior sets on only beyond a characteristic system size. The increasing regime of the size effect prevails even for localized load sharing, however, above the characteristic system size the load concentration results in a substantial strength reduction compared to equal load sharing. We show that an adequate explanation of the results can be obtained based on the extreme order statistics of fibers’ strength.

Seismic Performance of SFRC Shear Walls with Window Opening and the Substitution Effect for Steel Bars

Shear walls are important vertical and lateral bearing element in structures. While shear walls with openings are fragile due to stress concentration and the quasi-brittle behavior of concrete in tension. Therefore, additional strengthening rebars are required for the shear walls with openings. However, it aggravates the problem of dense reinforcement which increases the steel cage manufacturing and concrete compaction problem and still lacks countermeasures against concrete damage and cracking. To reduce the rebar demand and improve the damage tolerance of squat reinforced concrete (RC) shear walls with openings, an optimized steel-fiber-reinforced concrete (SFRC) was adopted to understand the seismic performance by cyclical loading test. The tested specimens included a plain RC shear wall without strengthening bar around the opening (for comparison), an SFRC shear wall, and an SFRC shear wall with a reduced distributed steel bar. This paper mainly studies the effect of using SFRC to improve the seismic performance of the open shear wall and to replace the reinforcement around the opening and the shear reinforcement. The hysteresis curves, skeleton curves, stiffness degradation, bearing capacity degradation and energy dissipation of the specimens were analyzed. The results show that the failure can be delayed and relieved, the deformation capacity and energy dissipation can considerably improve, and rebars can be partially replaced by using SFRC.

Nanoproperties of Fly Ash based Geopolymer Concrete with Polypropylene Fibres

Due of its amorphous-like properties, geopolymer concrete (GPC) possesses good quasi-brittle behavior. New studies has revealed that polypropylene fibre can also be used to concrete to increase the material's strength. In the ongoing work, a novel geopolymer concrete (FGPC) that was modified with polypropylene fibres been created as a replacement to conventional cement concrete. FT-IR, XRD, SEM, and the BET adsorption-desorption isotherm model were employed to evaluate the nanoproperties of the geopolymer concrete modified with polypropylene fibre. The maximum compressive strength will be most probably present in the developed concrete. Comparing FGPC to GPC and OPC, the pore capacity of FGPC was decreased by 0.025705 cm3/g and the water absorption was decreased. The substance in FGPC's SEM picture is denser than that in OPC and GPC. The interfacial bond between the minerals and the utilization of polypropylene fibre enhance the durability of geopolymer concrete, which was additionally supported by FTIR technique. Micro - cracks on the surface of OPC and GPC reduce compressive strength while enhancing porosity and water absorption

Experimental Model for Study of Thickness Effect on Flexural Fatigue Life of Macro-Synthetic-Fiber-Reinforced Concretes

As one of the most widely used building materials, concrete has a dominantly brittle or quasi-brittle behavior. Adding fibers to concrete affects its ductility behavior as well as some mechanical properties. Finding the relationship between the addition of fibers and the change in thickness of laboratory test samples made of concrete can help in designing the optimal thickness of real concrete layers (especially concrete pavements) to withstand dynamic loads. The purpose of this research is to provide an experimental model for investigating the effect of concrete specimen size, or the thickness effect of concrete sample, on the fatigue life of concrete. Accordingly, several concrete beams with three thicknesses (80, 100 and 150 mm), constant width, and two lengths (120 mm and 450 mm) were manufactured with fiber percentages of 0 and 4% by fraction volume. The employed fiber was twisted macro synthetic fiber. After curing for 28 days, the samples were subjected to fatigue loading at three stress levels until the onset of failure and cracking stage. Here, the experimental model of the relationship between the number of loading cycles, the stress level and the thickness of the sample is presented. The results show that increasing the specimen thickness and fiber content can enhance the fatigue life of concrete up to 68%.

Disk-shaped compact tension test for fracture analysis on pharmaceutical tablets

Pharmaceutical tablets must meet a number of requirements and among them, the mechanical strength plays an important role. The diametral compression test is generally used to evaluate it but can generate unstable failures. Thanks to load-unload cycles applied to the tablets subjected to a DCT test, it was shown that the concept of equivalent linear elastic fracture mechanics usually, can be successfully applied to the Mode I fracture behavior. Within this framework, the equivalent elastic crack growth resistance, commonly called resistance curve (R-curve), of the studied material was obtained and revealed the development of a Fracture Process Zone (FPZ) which is symptomatic of a quasi-brittle behavior. Moreover, the cyclic loading applied during the fracture test revealed the existence of a second dissipative mechanism leading to residual crack opening which seems to be mainly caused by friction phenomenon in the FPZ. • Fracture behavior of pharmaceutical tablets is a key issue for their manufacturing. • An alternative to the diametral compression test is proposed here. • No instability issues using the notch opening displacement controller. • A quasi-brittle behavior identified through equivalent LEFM theory. • Presence of dissipative mechanisms like micro-friction inside the process zone.

Durability properties of metakaolin based geopolymer polypropylene fibre reinforced concrete

As the modernization is taking place these days, adaption to the new innovations are going on, which is also true for the civil construction sector. Conventional concrete i.e. Oridinary Portland Cement (OPC) has a very high carbon dioxide (CO2) emission nearly from 5%−7% in its production, leading to the major by-product pollution as greenhouse gases. Due to these back falls, need of eco-friendly and sustainable material like Geopolymer (GP) comes into place, however, even geopolymer exhibits low tensile strength and quasi-brittle behaviour which needs to be improved by adding or replacing certain components such as Meta Kaolin (MK) and Polypropylene Fibres (PPF). Also it consumes less energy and is cost effective and also helps in low emission of greenhouse gases. The durability properties of the Meta kaolin-based geopolymer (MKG) with the inclusion of polypropylene Fibres (PP) are elaborately discussed in this research paper. This Geopolymer Fibre Reinforced Concrete (GPFR) offers myriad of advantages. Sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) are used as the alkali activator solution in this for geopolymerization process. With the inclusion of polypropylene fibres, drying shrinkage decreases and various other factors like chloride penetration and temperature resistance increases. Nevertheless, in all these years only limited study has been done in this particular area. Various tests are to be investigated like Ultrasonic Pulse Velocity test (UPV), water absorption test etc for finding its different aspects in different environmental conditions.

Influence of fibres characteristics on the engineering properties of geopolymer concrete- a review

Growing environmental consciousness in the construction sector encourages the utilization of alternate binders to partially or completely replace ordinary Portland cement (OPC), as its manufacturing generates CO2, which pollutes the environment. Geopolymer concrete (GPC) have emerged as an environmentally acceptable construction material because it can be prepared using industrial wastes, thereby minimizing CO2 emissions and utilizing the wastes which would otherwise become sources of pollution. Apart from environmental benefits, GPC is more durable and shows better mechanical properties than OPC-based concrete. However, GPC exhibits quasi-brittle behavior. To address this shortcoming, numerous studies have been carried out on the development, evaluation, and deployment of fiber reinforcement in GPC. In this article, the state-of-the-art of fiber-reinforced GPC is addressed, with an emphasis on the influence of fiber type, dosage, aspect ratio, etc., on the engineering properties. Based on the literature review, challenges and future prospects have also been discussed. The goal of this research is to develop a solid scientific and conceptual understanding of fiber-reinforced GPC, which will aid in the selection of the most efficient fiber for various GPC construction applications.

Static assessment of flawed thin AlSi10Mg parts produced by Laser Powder Bed Fusion

Several factors must be considered within the assessment of parts produced by Additive Manufacturing (AM) such as, for example, heterogeneous microstructure, process-induced defects, surface quality, residual stresses, and dependence on the material’s properties with the building orientation. All these factors severely affect the resistance to static and fatigue loadings of AMed components. Among the possible failure mechanisms, the failure promoted by static loadings when cracks are present is one of the most important failure conditions, in particular for the as-built parts, which might have reduced fracture toughness and ductility compared to the wrought alloy counterpart. In this work, we present a comprehensive approach to the static assessment of AlSi10Mg parts manufactured by Laser Powder Bed Fusion. Two benchmark fracture geometries were designed to investigate the typical AM geometrical features: i) thin walls in tension and ii) notched components in bending. Finite Element analyses of the benchmark specimens showed that an approach based on elastic–plastic fracture mechanics parameters is needed to correctly predict the experimental failures, despite the quasi-brittle behaviour shown by the AlSi10Mg alloy. In view of these results, this paper explores the applicability of the Failure Assessment Diagram (FAD), a tool used for conventional ductile materials, to static assessment of AMed parts. The results show that the assessment approach based on the FAD makes it possible to properly predict the experimental failures of the benchmark specimens.

The Effects of Magnesium Borate Whiskers on the Mechanical Performance of Oil Well Cement

Quasi-brittle behaviors of cement-based material can be reinforced through diverse methods due to its unique multiscale features. Among them, reinforcing agents are the most direct and effective means, such as slag, metakaolin, and fly ash. These materials can improve the performance of cement stone to a certain extent, but their chemical composition is still mainly silica, which does not in essence change the defects of cement. Therefore, in this experiment, magnesium borate whisker (Mg2B2O5) was used as reinforcer of 90°C class G oil well cement and magnesium borate whisker reinforced cement-based material (MBWRC) was prepared. On the one hand, the performance of mechanical strength was controlled, and the resistance to compression, traction, and bending was included. Furthermore, static stress-strain behaviors analysis and toughness behaviors analysis (dynamic multicycle loading test) were further conducted. Second, mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) were used to test the characteristics of pores and interfaces of cement-based materials. Through multiscale microstructure analysis, MBWRC was found to have excellent 90°C mechanical performances when compared to control sample’s, for which tensile strength increased to 235% of controlled sample’s, and flexural strength increased to 130%, plus a healthy developed compressive performance, and MBWRC showed much denser pore structure, in which harmless micropore increased from 13.3% to 14.40%, porosity decreased from 17.01% to 16.20%, and permeability decreased from 0.2533% to 0.0205%. Furthermore, MBWRC showed resistance capability to mechanical loading, which can be attributed to the formation of denser pore structure and more excellent mechanical performance.

Crack face friction effects in the strength scaling of composites failing by kinked shear cracks under transverse compression

Recent research showed that the failure and strength scaling of composites under transverse compression exhibits a quasibrittle behavior, characterized by a non-negligible fracture process zone (FPZ). The present work is aimed at analyzing and quantifying the contributions of crack face friction towards this quasibrittleness, by focusing on fracture plane angles and the strength scaling. The failure, frictional effects, and strength scaling are numerically predicted via the fixed crack damage model, augmented with Puck’s friction-dependent damage initiation criterion and Ragueneau’s parallel coupling model for combined damage and friction evolution. The model is calibrated and verified using available test data and then used to analyze the transverse compressive failure in unnotched specimens and strength scaling in pre-cracked specimens, with and without friction. The analyses show that the friction-augmented model can accurately predict the fracture plane angle equal to 53 degrees in unnotched specimens, and can be tuned to reproduce other angles as well. Further, in all notched specimens failure occurs via a kinked shear crack propagating at an angle to the pre-crack, and having at its tip a large FPZ. The kink angle is observed to fall in the range of 44 to 47 degrees when friction is excluded, and 54 to 58 degrees when friction is included. The specimen strengths with or without friction, scale in accordance with the energetic type II size effect law. This finding is also supported analytically using energy balance-based arguments. The results reveal that the inclusion of friction can considerably increase the strength and enlarge the FPZ size (by nearly 90%), making the fracturing behavior significantly more quasibrittle. The strength increase is size dependent for smaller sizes, and appears to become roughly constant for larger specimen sizes. The FPZ of the kinked shear cracks is seen to embody a strongly multiaxial stress state. This makes the fracture energy associated with the kinked shear crack growth much higher than the pure mode II fracture energy. The frictional dissipation can further lead to an increase in this energy, which in the present case is found to be nearly 130%. These findings serve to emphasize the importance of considering and quantifying friction effects in the transverse compressive failure of composites.

The theory of critical distances applied to fracture of rocks with circular cavities

This work presents experimental and theoretical analyses of the fracture behavior of different rocks containing circular holes subjected to uniaxial compressive uniform loads. The experimental critical loads are compared with those derived from the Theory of Critical Distances (TCD) and finite element simulations. The rocks being analyzed are Floresta sandstone, Moleanos limestone, Macael marble and Carrara marble. They were previously characterized under compressive and tensile conditions, obtaining their corresponding Poisson's ratios, Young's moduli and ultimate tensile and compressive strengths. Besides, their fracture properties, specifically their fracture toughnesses and critical distances, were also available from a previous experimental campaign on Single Edge Notched Beam specimens with different notch radii and tested under 4-point-bending conditions. In this paper, prismatic specimens (9 per each type of rock) containing a cylindrical cavity at their center were uniaxially compressed until failure. The fracture pattern was revealed and agrees with previously published cases. The TCD, specifically the Point Method, was applied with the aim of comparing the corresponding fracture load predictions with the experimental ones. Three-dimensional finite element analyses were used to calculate the stress fields. The application of the TCD on the Floresta sandstone and the Moleanos limestone provides reasonable predictions of fracture loads, given their linear-elastic behavior. For the two studied marbles, the resulting predictions are overly conservative despite their quasi-brittle behavior. This may be attributed to complex process zones with intense microcracking that are revealed as white patches prior to failure and generate dust and small fragments after failure.

Mode I and II R-curves characterization of the Maritime Pine and Spruce under the same geometry

This study provides detailed data on the quasi-brittle behaviour of two softwood species, the Maritime Pine (Pinus pinaster.) and the Spruce (Picea abies.), by using R-curves. The methodology of characterization of the R-curve used in this article is an improvement of the method presented by Lespine (2007). This method is automated and based on the measurement error made on GR(Δau). And the same geometry to characterize the crack propagation in mode I and in mode II. The influence of the crack plane orientation and the density on the fracture mechanisms is investigated for both species and shows a dependency of the R-curve parameters of the Maritime Pine in mode I to the crack plane orientation. The differences between Spruce and Maritime Pine are highlighted. This demonstrate the importance to characterize the quasi-brittle behaviour of each wood species in order to provide a useful data base for engineers.

Size-Dependent Fracture Characteristics of Intermetallic Alloys

BackgroundLightweight alloys such as intermetallic titanium aluminide (TiAl) alloys are poised to be a potential candidate for replacing heavier nickel based super alloys in an aero engine. However, before an industry wide implementation is possible, it is indispensable to develop physically accurate computational material models which account for essential deformation and fracture mechanisms. This assists the virtual prototyping required for the new product development using TiAl components.ObjectiveThe objective of this work is to determine the effect of size of tested specimens on their fracture energy and provide a physically motivated scaling law.MethodsIn this work, the quasi-brittle behavior of TiAl alloys is experimentally and numerically investigated. A total number of 29 geometrically identical TiAl specimens of three different sizes are tested in a three-point bending setup. Since the final abrupt failure of each specimen is preceded by plasticity, a theoretical and numerical framework which accounts for both elastic and plastic work densities is applied in simulations.ResultsThe fracture energy density for each tested size is calculated numerically which is found to be lower for larger volumes, thereby, confirming the size effect in intermetallic TiAl alloys. A novel size effect law is proposed which is based on two physically motivated coefficients.ConclusionsThe work concludes with the quantitative knowledge of the size-dependent fracture energy of intermetallic alloys and an empirical scaling law to predict the same. Excellent predictive capability of the proposed law is successfully established with data of various quasi-brittle materials from literature.