Notched Beam Specimens Research Articles

Flexural strength, fracture toughness, translucency, stain resistance, and water sorption of 3D-printed, milled, and conventional denture base materials.

To compare mechanical, optical, and physical properties of denture base materials fabricated with various 3D printing systems to reference milled and conventionally heat-processed denture base materials. Specimens of denture base materials were either 3D-printed (DLP in-office printer, CLIP laboratory printer, or material jetting laboratory printer), milled, or heat processed. 3-point bend flexural strength testing was performed after 50 hours of water storage following 1hour of drying (dry testing) or in 37°C water (wet testing). Fracture toughness was performed with a notched beam specimen after 7 days of water storage and tested dry. The translucency parameter was measured with 2mm thick specimens. Stain resistance was measured as color change following 14 days of storage in 37°C coffee. Water sorption was measured following 7 days of storage in 37°C distilled water. For dry testing, all but one of the 3D-printed materials attained higher or equivalent flexural strength as the reference materials. For wet testing, all 3D-printed materials attained higher or equivalent strength as the reference materials and dry-tested materials. For 3D-printed materials, wet testing increased displacement before fracture whereas it decreased displacement for the reference materials. Only two of the 3D-printed materials had similar fracture toughness as the reference materials. One of the 3D-printed materials was more translucent and one was more opaque than the reference materials. Only one of the 3D-printed materials absorbed more water than the reference materials. 3D-printed denture base materials have mostly equivalent mechanical, optical, and physical properties to conventional and milled denture base materials.

Experimental investigation of dynamic bond behaviors between LRS-FRP and concrete

The dynamic bonding behaviour at the interface between a large-rupture-strain fibre-reinforced polymer (LRS-FRP) and concrete was investigated via drop weight impact tests of notched beam specimens. The main experimental variables included the concrete strength grade, number of LRS-FRP layers, and loading rate. With increasing loading rate, the location of the peeling damage at the interface between the LRS-FRP and concrete shifted from the interior of the concrete layer to the concrete–binder layer interface, binder layer, and even binder layer–FRP interface. The ultimate debonding load of the LRS-FRP strips increased with increasing strain rate, reaching more than 1.7 times the static value at a strain rate of approximately 4/s, while the effective bond length did not show a clear increasing or decreasing tendency with increasing strain rate. For the local bond-slip relationship, both the peak interfacial bond stress and interfacial fracture energy increased with increasing strain rate, and the influences of the concrete strength and number of FRP layers on the strain rate sensitivity were insignificant. Based on the experimental results, improved models to predict the ultimate debonding load and effective bond length at the LRS-FRP–concrete interface under both static and dynamic loading conditions were proposed. Furthermore, dynamic increase factor (DIF) formulas for the peak interfacial bond stress and interfacial fracture energy for bond-slip modelling of the interface between a polyethylene naphthalate (PEN)-FRP and concrete were established.

Experimental study and theoretical analysis of ECC type I fatigue fracture after subjected to sub-high temperature

Three-point bending (TPB) fatigue fracture tests were conducted on Engineered Cementitious Composites (ECC) prefabricated notched beam specimens at three target temperatures (T = 20℃, 100℃, 200℃) and three stress levels (S = 0.7,0.8,0.9). The fatigue life, maximum crack opening displacement (CMODmax), and stiffness attenuation were characterized. The evolution of cracks is recorded in real time. The fatigue crack growth curve (af-N) is obtained by fitting a logistic function, and the crack growth rate is determined based on its first derivative. It is found that compared to 20℃, there is a slight increase in the fatigue life of ECC, and the critical crack growth rate decreases after 100℃. After 200℃, the fatigue life of ECC decreases, the brittleness increases, and the crack growth rate increases. This is mainly related to the degradation of PVA fibers at 200℃, with most fibers being pulled off during fatigue loading, which leads to a decrease in crack propagation resistance.

Wavelet entropy-based damage characterization and material phase differentiation in concrete using acoustic emissions

Damage evolution in concrete is a complex phenomenon involving micro-cracks that originate from different material phases (mortar matrix, ITZ and aggregate), and culminate into a major crack. The information about the individual damaged material phases can provide major insights into the global failure behaviour, cracking path, and fracture processes. The evolution of damage, damage mechanisms and the material phase of origin of micro-cracks is highly influenced by the type of concrete (Normal and high strength). In order to investigate the evolution of damage in concrete of varying strength, notched beam specimens with different water-to-cement ratios have been prepared and tested under CMOD control. The evolution and growth of damage have been continuously monitored during testing through acoustic emission techniques. Based on acoustic emission results, it has been demonstrated that the damage progression in concrete can be better described as a function of the wavelet entropy of the AE events occurring during the entire loading duration, and therefore, an analytical model for the damage evolution has been proposed. The study reveals that, under the high-strength category, the level of deterioration reduces as the w/c ratio rises, while in normal-strength concrete, a reverse trend is observed. Furthermore, a novel approach has been proposed that utilizes wavelet entropy and amplitude to differentiate amongst various damaged material phases with the help of an unsupervised clustering algorithm. Normal-strength concrete with a w/c ratio of 0.4 has been used to demonstrate the proposed methodology and has been validated with the help of experiments performed on the mortar and the parent stone of aggregate. A higher occurrence of mode-I cracks compared to mode-II cracks across all material phases has been observed for the specimen. Additionally, ITZ and aggregate exhibit a greater proportion of tensile and shear cracking events than the matrix phase. In the end, a deep learning convolutional neural network (CNN) model is devised utilizing labelled scalogram images of the AE waveforms to discriminate between the various damaged material phases. A 10-fold training cross-validation of the dataset has been carried out to achieve a stable performance. The deep learning model performs well in discriminating the damaged material phases with high training, validation, and testing accuracy.

Physio-chemical, mechanical and fracture analysis of ultra-high-performance cementitious composite

The aim of this research was to develop cost-effective and environmentally friendly ultra-high-performance concrete (UHPC) using fly ash (FA) and quartz powder (QP) as sustainable cement replacements. The performance of the various constituents was examined and their influence on fresh and hardened properties was investigated in order to determine the optimum trial mix design. A cube compressive strength of 144 MPa was achieved with 20% FA and 10% QP and other ingredients without any special heat treatment. Advanced characterisation studies, such as the heat of hydration and X-ray diffraction analysis, were performed to gain understanding of the performance of FA-blended UHPC. Various fracture properties (fracture toughness, fracture energy, brittleness etc.) were investigated considering geometrically similar notched beam specimens for UHPC mixes with steel fibre contents of 1.5% and 2.5%. The optimum UHPC mix achieved good scores in terms of the strength index and the embodied carbon dioxide emissions index, indicating that the concrete is eco-friendly and sustainable.

Influence of specimen size on granite fracture characteristics and acoustic emission phenomena under mode I loading conditions

The rock mode I fracture characteristics and the size effect are of great significance in understanding the crack initiation, nucleation, propagation, and characterizing the fracture toughness of rocks which is essential for predicting the behavior of rock masses and designing underground structures. In this paper, three-point bending fracture experiments were carried out using single-edge notched beam specimens of different sizes. The fracture characteristics of the entire loading stage, including the post peak stage, were analyzed using digital image correlation (DIC) technology and acoustic emission (AE) technology. Specifically, the calculation of the b value uses the Fisher optimal split and global search algorithm (FGS) method. The results show that both the Fracture process zone (FPZ) length and fracture toughness of granite are size-dependent parameters, and the FPZ length growth rate increases with increasing load level. The failure mode of the source during fracture is mainly tensile failure, and the proportion of tensile failure tends to increase with the increase of specimen size. The temporal b value, average AF value and RA value show corresponding stage changes with time, and the minimum b value, minimum AF value and maximum RA value can characterize the maximum of macroscopic growth rate.

Fracture Behavior of Concrete under Chlorine Salt Attack Exposed to Freeze-Thaw Cycles Environment.

Fracture behavior is one of the key properties to study concrete cracking under sodium chloride attack exposed to the freeze-thaw cycles environment, which is frequently neglected. In this paper, 24 single edge notch beam specimens and 24 cubes were poured. The corresponding freeze-thaw cycles test in sodium chloride solution, standard cube compressive strength of concrete test, and three-point-bending tests were carried out. The research revealed that the fracture toughness, fracture energy, relative dynamic modulus of elasticity, and standard cube compressive strength were decreased by increasing freeze-thaw cycles under sodium chloride attack, and the damage degree of concrete caused by sodium chloride solution was deeper than that of pure water. In particular, there existed good linear correlation between the fracture behavior and imposed freeze-thaw damage for various solution. Accordingly, a more reliable damage model using fracture control parameters as damage factors was proposed.

Evaluation of Fracture Mechanics Parameters of Light-weight Concrete by Implementing Natural Pumice Stone as Coarse Aggregate

The concrete material softening response curve is essential for accurate numerical analysis of concrete. The tensile strength, initial fracture energy, and total fracture energy are the three parameters of interest in defining such softening response. Among them, the fracture energy is usually regarded in any analytical program that attempts to model reinforced concrete (RC) structures' behavior. The brittleness and fracture characteristics of lightweight concrete (LWC) produced from 19 mm maximum size natural pumice stone aggregate were investigated via testing notched beam specimens under three-point bending test. Fracture parameters were analyzed and explained using the size effect method (SEM) and the work of fracture method (WFM), and the results were compared to those found in the published literature. The results show mean total fracture energy (GF) of 145.65 N/m and a mean characteristic length (Lch) of 590.34 mm. Later, by correlating the result obtained via this method to the SEM approach, the initial fracture energy (Gf) was determined with a mean value of 58.26 N/m and the fracture toughness (KIC) was found to be 38.47 MPa.mm0.5. Lightweight concrete was found to be more ductile and need more energy to propagate fracture across the specimen, as shown by a comparison of current data to that acquired from literature.

Experimental investigation on the flexural performance and damage process of steel fiber reinforced recycled coarse aggregate concrete

The application of recycled concrete requires the reliable information on its flexural performance especially after cracking. In this study, the three-point bending tests were carried out on 40 notched beam specimens to investigate the flexural performance and damage process of steel fiber reinforced recycled coarse aggregate concrete (SFRCAC). The effects of water-cement ratio, recycled coarse aggregate replacement ratio and steel fiber volume fraction on the residual flexural tensile strength and flexural toughness of SFRCAC were analyzed. Moreover, the flexural damage process of the SFRCAC notched beam was explored and analyzed by Acoustic Emission (AE) technology. The results show that the residual flexural tensile strength and flexural toughness of SFRCAC decrease with increased water-cement ratio and recycled coarse aggregate replacement ratio, respectively. Adding steel fibers can effectively improve the residual flexural tensile strength and flexural toughness of SFRCAC. The post-cracking behavior of SFRCAC changes from strain-softening to strain-hardening when steel fiber volume fraction increases from 0.5% to 2%. Finally, the empirical formula of residual flexural tensile strength for SFRCAC has been put forward by analyzing and fitting the test results in this study and related literature.

Using beam and ENDB specimens to evaluate fracture characteristics of wavy steel fiber‐reinforced self‐compacting concrete containing different coarse aggregate volumes

AbstractThis research has addressed the effects of coarse aggregate (CA) and wavy steel fiber (WSF) volumes on the self‐compacting concrete (SCC) fracture parameters in beam and edge notched disk bend (ENDB) specimens using size effect and work fracture methods (SEM and WFM). Mix designs were based on 30%–60% CA volumes and 0.15%–0.45% WSF volumes, and 144 different‐size notched beam specimens underwent 3‐point bending tests. Results showed that increasing the CA volume and percent WSFs increased the SCC ductility in both SEM and WFM; increasing the WSF volume affected the fracture energy‐compressive strength relationship negatively at 30% and 40% CA volumes, and increasing the WSF affected the mentioned relationship positively at 50 and 60% CA volumes. Finally, we predicted the relationship between the fracture parameters calculated with ENDB specimens and those of beam specimens calculated with SEM and WFM.

Tensile Performance Test Research of Hybrid Steel Fiber-Reinforced Self-Compacting Concrete.

Notched beam specimens were loaded by the three-point bending test device, and the effects of different volume contents and combinations of steel fibers on the tensile properties of hybrid steel fiber-reinforced self-compacting concrete (HSFRSCC) were studied. The failure law and strain field distribution of the specimens were studied by digital image correlation (DIC) technology. Moreover, the curves between the load and crack mouth opening displacement (CMOD) of 18 groups of hybrid steel fiber-reinforced concrete specimens were obtained, and the stress-strain curves of 18 groups of specimens were derived from the load-CMOD curves. The results show that both single and hybrid steel fibers can improve the crack deformation resistance and tensile properties of concrete, but hybrid steel fibers have a more significant improvement effect. Only when the content of steel fiber is more than 0.6% can it have a more obvious postpeak descending section, and hybrid steel fiber has higher postpeak deformation capacity and flexural toughness. The fundamental reason why concrete with hybrid steel fibers has better tensile properties is that micro and macro steel fibers cooperate with each other to resist cracks, improving the toughness of concrete after cracking. Finally, the mechanism of different size and volume content of steel fiber was analyzed from the micro level, which can be used as a reference for the engineering design of HSFRSCC in the future.

Development of maximum tangential strain (MTSN) criterion for prediction of mixed-mode I/III brittle fracture

A number of fracture criteria are available for predicting the cracking behavior under combined contribution of opening-tearing damage mechanism (i.e. mixed-mode I/III). But most of them have some limitations or discrepancies with the experimental results. In this research, a new form of maximum tangential strain criterion called 3D-MTSN is developed for predicting mixed-mode I/III fracture phenomenon. This criterion was used originally for analyzing the mixed-mode I/II fracture but we extended its ability for mixed mode I/III as well. Also, the influence of T-stress in addition to modes I and III stress intensity factors was also considered in driving the fracture toughness and fracture angle formulations of the 3D-MTSN criterion. According to this strain-based criterion it is shown that both sign and magnitude of T-stress can affect the fracture behavior of mixed mode I/III. In general, positive T-stress increases the mixed mode I/III fracture envelope and conversely negative sign of T-stress can decrease it. Meanwhile, the Poisson’s ratio (v) has noticeable influence on the mixed mode I/III fracture envelopes and by increasing v, the mixed mode fracture curve predicted by the generalized MTSN criterion shifts down except for those specimens having positive T-stress values and subjected to dominantly pure mode I loading conditions. The practical ability and validity of 3D-MTSN criterion is examined by predicting different sets of mixed mode I/III experimental results reported in the literature for different brittle and quasi-brittle materials tested with different test samples including edge-notch disc bend (ENDB), inclined single edge notch beam (ISENB) and semi-circular bend (SCB) specimens. A comprehensive comparison with the well-known 3D-MTS (stress-based) criterion was also performed. Compared to the other available energy-based fracture models, the presented 3D-MTSN criterion provided better estimations for the experimental results of different materials and specimens under dominantly mode III conditions.

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.

Indices-Based Healing Quantification for Bituminous Materials

AbstractIn the present study, three-point bending tests are carried out using single-edge notched beam specimens of bitumen and mastic to quantify healing. Experiments are conducted at a controlled...

Fracture behavior of a sustainable material: Recycled concrete with waste crumb rubber subjected to elevated temperatures

Recycled aggregate concrete (RAC) made from construction and demolition wastes has several environmental benefits. Fire is one of the most common disasters in buildings, and RAC is a brittle construction material; therefore, the bearing capacity of RAC structures under high temperatures should be considered. According to previous studies, crumb rubber made of waste tires can further reduce damages to RAC under high temperatures. Meanwhile, fracture behaviors are one of the key characteristics of concrete materials that need to be considered, but few studies have focused on their behavior when subjected to elevated temperatures. Rubber-modified RAC (RRAC) notched beam specimens with three recycled aggregate substitutions (0%, 50%, and 100%), and four rubber contents (0%, 2%, 4%, and 6%), exposed to high temperatures (200 °C, 400 °C, and 600 °C), were tested using the three-point bending test. The fracture behaviors of the RRAC, including the crack mouth opening displacement, fracture energy, and fracture toughness were analyzed. The results show that the effect of rubber particles on the unstable fracture toughness is greater than that on the initial cracking toughness of RAC after exposure to high temperatures. However, the enhanced effect of rubber on the fracture resistance decreases after subjecting it to a high-temperature treatment owing to the softening and eventual decomposition of rubber at high temperatures. Consequently, in order to avoid the drawbacks introduced by rubber, a rubber content of more than 4% is not recommended considering the mechanical and fracture performance of RRAC.

Effect of nano and micro SiO2 on brittleness and fracture parameters of self-compacting lightweight concrete

In the present study, 16 mix compositions with two different water to binder ratios of 0.35 and 0.45 were designed to assess the fracture behavior of self-compacting lightweight concrete (SCLC) mixes with micro and colloidal nano silica. To do so, 192 notched beam specimens of various sizes were cast. The contribution of 10% micro silica (MS), 1%, 3%, and 5% colloidal nano silica (CNS) were considered in forms of binary and ternary mixtures and then the results were compared with those of controlling specimens. In order to analyze the fracture behavior of mixes, type-II size effect law were used and all tests were conducted in accordance with RILEM recommendations. The results indicated that W/B ratio as well as the replacement of MS and CNS with cement could substantially affect the fracture behavior of SCLC mixes. For instance, the use of 10% MS in the absence of CNS has led to the increase of fracture energy by 9.8% and 16.4% for mixes with W/B ratios of 0.35 and 0.45, respectively, while at the same level of 10% MS replacement by cement, the replacement of 1%, 3%, and 5% CNS by cement, has improved the initial fracture energy of the W35 group of mixes by 11.38%, 16.58%, and 8%, respectively. This was 1.8%, 10.6%, and 4.6% for W45 group of mixes with 10% of MS. In addition, this research showed that the proposed model could conveniently predict the experimental results.

Modelling and testing of Temperature-Dependent strength and toughness of asphalt concrete from −10 °C to + 23 °C using small notched beams

A closed-form fracture model with the consideration of the average aggregate size and crack-tip damage zone is used to analyse flexural strength of asphalt concrete using notched beams. Quasi-stable micro-crack formation and quasi-brittle fracture of these small samples with highly heterogeneous aggregate structures are formulated. While the apparent strength and fracture toughness of asphalt concrete based on Continuum Mechanics are size dependent, constant asymptotic “structural” tensile strength ft and “structural” fracture toughness KIC have been determined by this composite model from experimental results of notched beam specimens of any size with any notch length. Temperature (T) dependent ft and KIC measurements of asphalt concrete from −10 to + 23 °C, relevant to the real temperature environments, are then determined. The characteristic microstructure Cch in the model is the average aggregate size. The maximum failure load Pmax of asphalt concrete under three-point-bending (3-p-b) is linked directly to its constant “structural” tensile strength ft for a given temperature environment, independent of sample size and notch/crack length. In addition, although KIC is not valid for asphalt pavement with limited thickness, its influence is reflected through the “structural strength” criterion based on ft and Cch. This simple model can also be used as a design tool for asphalt concrete property optimization.

Experimental investigations on fracture parameters of random and aligned steel fiber reinforced cementitious composites

Experimental research was carried out to evaluate the comparative fracture performance of random steel fiber-reinforced composite (SFRC) and aligned steel fiber-reinforced composite (ASFRC). The desired direction of steel fiber in the fresh cementitious mortar was achieved by the electromagnetic field. Three-point bending tests were conducted on geometrically similar notched beam specimens. Various dimensions and steel fiber contents by volume Vf were considered to study fracture performance and investigate the size effect. The fracture parameters were analyzed by nonlinear fracture models, namely the size effect model (SEM), work of fracture method (WFM), and double-K fracture criterion (DKFC). Initial fracture energy, critical fracture toughness KIC, length of fracture process zone, and critical tip opening displacement from SEM, total fracture energy, and characteristic length from WFM was studied for SFRC and ASFRC. The initial cracking load was detected with strain gages to evaluate initial fracture toughness KICini. Unstable fracture toughness KICun from DKFC with special emphasis on size effect was obtained. The results show that the rise of specimen depth has no considerable effect on the KICun while the size effect of KICini is not significant. As the increment of Vf, both the KICini and KICun show a linear ascending trend and both of them are greater for ASFRC than those of SFRC specimens. It is found that there exists a strong correlation between the fracture toughness measured by DKFC and SEM (KIC from SEM≅KICun from DKFC). Generally, the fracture performance of ASFRC relative to that of random SFRC is improved by more than 80%.

Experimental and analytical investigation of fiber alignment on fracture properties of concrete

Steel fibers are nowadays increasingly employed for improving the fracture behavior of concrete. However, the fiber alignment in fiber reinforced concrete (FRC) plays a significant role in its fracture behavior. This paper presents a method for aligning fibers in the desired direction, which is done by pouring concrete in the molds through a steel hopper, developed for this application. Both regular (without a notch) as well as notched beam specimens were cast using two steel fiber volume fractions of 0.8% and 1.2%. The beam specimens were tested in flexure under a central point load. The fracture parameters of FRC containing aligned steel fibers were compared with the FRC having randomly distributed fibers. The test results demonstrate the success of the method employed for aligning fibers. The FRC having 0.8% aligned steel fibers was almost as good as the FRC containing 50% more (i.e., 1.2%) random fibers. Mathematical analysis was carried out for the alignment of steel fibers achieved using the developed hopper and to predict the fiber orientation factor. The test results are in good agreement with the predicted values.

Development length of sand-coated GFRP bars embedded in Ultra-High performance concrete with very small cover

The development length of sand-coated glass fiber reinforced polymer (GFRP) reinforcing bars embedded into ultra-high performance concrete (UHPC) with very small cover is evaluated in this study using twenty-eight notched beam specimens tested in flexure. The parameters included the type and percentage of short fibers in the UHPC mixture, namely 2% and 4% steel fibers and 2% and 3% polyvinyl alcohol (PVA) fibers. The embedment length of a 17.2 mm diameter (db) GFRP bar, a widely used size, was varied from 4db to 16db. The study focused on a very small clear concrete cover of 1.0db to simulate applications that would optimize the use of UHPC, taking advantage of its superior strength. The specimens experienced splitting bond failure as expected. The average bond strength (τu) was higher in UHPC with steel fibers than PVA fibers. It also increased with fiber content but reduced with embedment length. Consequently, the development length (Ld) was smaller in UHPC with steel fibers than PVA fibers and was also reduced as fiber content increased. The Ld values for UHPC with 4% steel, 2% steel, 3% PVA and 2% PVA were 20db, 29db, 35db and 55db, respectively. ACI440.1R-15 and CAN/CSA S806-12 design equations provided very conservative estimations for Ld, by factors of 1.5–5.7. An alternative simple equation has been suggested for Ld.