Bending Tests Research Articles

Research on cracking characteristics and failure modes of semi-flexible pavement materials

Semi-flexible pavement (SFP) is designed for heavy traffic but its susceptibility to cracking hinders its development and application. This study aims to investigate the cracking characteristics and failure modes of SFP materials through macroscopic tests and image processing, thereby guiding anti-cracking technology. First, the effects of gradation and bitumen content on cracking process curves were analyzed using three-point bending tests across a wide temperature range. Second, the variations in maximum tensile strain, tensile strength, and fracture energy were comparatively examined. Finally, the morphology of specimens at crack initiation and after complete fracture was analyzed using image processing techniques to quantify the failure modes. The results indicate that the SFP material exhibits elastic development and brittle damage at the low temperature, and viscoelastic development and ductile damage at medium and the high temperature. At the low temperature, the changes in maximum tensile strain and fracture energy are consistent. The primary weak points at crack initiation include internal cement failure, cement-bitumen interface failure, and bitumen-aggregate interface failure. After complete fracture, internal cement damage and bitumen-bitumen/aggregate interface damage are dominant. To improve low-temperature cracking resistance of SFP, it is recommended to increase the maximum aggregate size, add fine aggregates, and increase the bitumen content. Additionally, enhancing the internal cohesion and external bonding properties of bitumen can mitigate the negative effects of increased bitumen consumption. This study provides guidance on designing the SFP material to enhance its crack resistance.

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.

The Additional Effect of Neurodynamic Slump and Suboccipital Muscle Inhibition to Passive Stretching of the Short Hamstring: A Single-Blind, Randomized Controlled Trial.

Hamstring shortening is a significant musculoskeletal condition affecting the posture and mobility of the spine and lower extremities. This study examined the impact of incorporating neurodynamic slump stretch and suboccipital muscle inhibition into passive static stretching on hamstring flexibility in individuals with short hamstrings. 117 female participants were classified into three groups: the control group, which received passive static stretch of the hamstring muscle; the neurodynamic slump group, which received neurodynamic slump stretch with passive static stretch; and the suboccipital muscle inhibition group, which received suboccipital muscle inhibition with passive static stretch, for three sessions a week, 10 min each, for four weeks. The outcome measures were the popliteal angle test (PAT), straight leg raising (SLR) test, and forward bending test (FBT) at baseline, immediately following the first session and after four weeks. Statistically significant differences were found within groups (p < 0.001) for all outcome measures. Between the groups, there was a more significant improvement in the PAT and the SLR tests, favoring the neurodynamic slump and suboccipital muscle inhibition groups in comparison with the passive static stretch group (p < 0.001) with no significant difference between the two groups after the first session and at four weeks of treatment. However, the FBT showed no significant differences immediately following the first session or at four weeks of treatment (p > 0.05). This study found that incorporating neurodynamic slump stretch and suboccipital muscle inhibition into passive static stretch techniques effectively treats short hamstrings in healthy individuals.

A Newly Bio-Based Material for the Construction Industry Using Gypsum Binder and Rice Straw Waste (Oryza sativa)

Construction is one of the economic sectors with the greatest influence on climate change. In addition to working procedures, the primary carbon footprint is attributed to the choice of materials and the energy required for their manufacturing. The underlying idea of this study is to minimize the effects and offer new solutions to emerging problems in the quest for materials that can be deemed as natural, such as gypsum (calcium sulphate dihydrate) and rice straw (Oryza sativa). The acquisition of these materials involves a lower carbon footprint compared to the conventional materials. It is well known since ancient times that gypsum and cereal straw can be used in construction, with numerous examples still available. Cereal straw is one of the oldest construction materials, traditionally combined with earth and occasionally with certain binders, with it continuing to be employed in construction in many countries to this day. This work showcases the feasibility of producing stable prefabricated elements from straw waste with construction gypsum, addressing a significant environmental concern posed by the alternative of having to burn such materials. In this study, for the proposed bio-based material, specific tests, such as thermal conductivity, flexural and compressive strength, and fire resistance, were carried out to evaluate the principal physical and mechanical characteristics for different compositions of water, gypsum, and straw fiber samples. The results highlighted the good performance of the proposed materials in order to spread their use in the green building industry. The addition of straw fibers improved, in different ways, some important physical characteristics of these components so as to diminish environmental pollution and to obtain better material performance. The tests highlighted the different behaviors of the proposed material with respect to the different cuts of the straw and as well as the water/gypsum ratio; this is not very well understood and probably depends on the micro structure of the straw fibers. The blocks with raw straw showed a significant improvement in the breaking mechanism (1775.42 N) compared to the blocks with cut straw (712.26 N) when subjected to bending tests, and their performance in compression tests was also acceptable. Additionally, a very interesting reduction in thermal conductivity was achieved by incorporating rice straw (0.233 W/mK), and high fire exposure times were obtained, with gypsum preventing the spread of ignition in any type of fiber.

Prediction of Femoral Bone Strength in the Presence of Tumors and Tumor-like Lesions Using Finite Element Analysis.

Patients with bone tumors in their femurs are at risk of developing pathological fractures. Tumors with high fracture risk, especially fragile malignant lesions, are treated surgically. However, it is difficult to estimate bone strength based on clinical and radiographic findings. This study aimed to determine whether finite element analysis (FEA) provides useful information on the bone strength of femurs with tumors and tumor-like lesions. Total femoral computed tomography (CT) data (slice thickness, 0.5 mm) were retrospectively obtained from 18 patients with femoral bone tumors. Three-dimensional FEA of femurs were developed using CT data. The virtual femoral head compression test and direct three-point bending test were performed on the femurs using FEA to predict bone strength and fracture location. The compression direction was parallel to the mechanical axis, whereas that of the three-point bending test was applied to the tumor itself. In the femoral head compression test using FEA, 13 out of 18 femurs with bone tumors fractured at the femoral head, while 14 out of 18 femurs fractured at the tumor site during the virtual direct three-point bending test. The median loads predicted using the femoral head compression test were significantly higher than those predicted by the direct three-point bending test. The FEA results indicated that pathological fractures are unlikely to occur during normal walking. Direct external forces applied to the tumor body may lead to fractures.

Performance of historical ferrocement specimens subjected to corrosive environments

AbstractFerrocement is a type of thin wall‐reinforced concrete composed of hydraulic cement mortar reinforced with several layers of steel wire mesh. The material is used as a low‐cost construction and retrofit solution and allows the creation of very thin elements. In the past, the material was also used to build some structural engineering masterpieces that currently face preservation challenges. Despite the use of the material, few studies have analyzed its durability with respect to corrosion and its evaluation for preservation purposes. The paper presents the results of a testing campaign on ferrocement replica specimens subjected to a corrosive environment. After an aggressive aging procedure, where the specimens are exposed to chloride ingress, the specimens are tested through a four‐point bending test to compare their performances. Results of the mechanical behavior are compared to the results from the half‐cell potential monitoring carried out during the aging process and optical microscope acquisitions regarding the cross‐sectional corrosion‐loss area in the wires. The aim is to assess the performance of historical ferrocement exposed to degradation due to corrosion; this can be useful in identifying the best procedures to protect and preserve it, as well as in evaluating its performance over time.

Mechanical Properties and Microstructural Characteristics of Steel Pipes after Long-Time Hydrogen Exposure

There are three possibilities to transport hydrogen by high pressure pipeline (i) blending hydrogen into the existing natural gas pipeline system, (ii) after checking the integrity, using the natural gas transporting pipeline system to transport hydrogen, (iii) building a new pipeline system to transport pure hydrogen. In all three cases should be known the behaviour of the pipe material and its welded joints influenced under high pressure hydrogen. For this aim pipeline sections made of P355NH base material were investigated without and with hydrogen exposure, where the exposure time was 41 days. Gas metal arc welding and hybrid tungsten arc welding / gas metal arc welding were applied for preparation of the girth weld joints. Tensile, three point bending and hardness tests, furthermore microstructural investigations were performed both on base materials and girth welds of the pipeline sections. The testing results were analysed and compared with each other (base material vs. welded joints, without exposure vs. with exposure) and with limit values can be found in the literature. Conclusions were drawn on the behaviour of the investigated pipe material and its welded joints.

Interlaboratory study of flexural strength in additively manufactured alumina

AbstractWe report the results of an international interlaboratory study of the flexural strength of alumina fabricated across six laboratories using the vat photopolymerization ceramic additive manufacturing (AM) technology. The mechanical testing of all the specimens, 142 in total, was performed at the National Institute of Standards and Technology (NIST) according to the well‐established four‐point bending method standardized for traditional ceramics. Overall, the existing ASTM standard for the four‐point bend testing proved adequate for AM ceramics, with several modifications to the specimen requirements to account for the specifics of AM processes. Critical flaws that caused failure were identified in all but two cases using optical fractography augmented with the imaging of fracture surfaces in a scanning electron microscope. The flexural strength data, analyzed following the Weibull statistics, exhibited considerable variation among the specimen sets manufactured by different laboratories. This variability correlated with the presence of many distinct critical flaws. We identified seven types of flaws that accounted for the failure of 94% of specimens. The prevalent flaws depend on the printing direction relative to the specimen's geometry. We discuss the nature of these flaws and their relation to the printing and post‐processing conditions. Removal of several types of critical flaws will significantly improve mechanical properties of ceramic parts built using vat‐photopolymerization AM.

A comparative quantitative assessment of 3D-printed PEKK and PEEK thin meshes in customized alveolar bone augmentation

ObjectiveCustomized nonabsorbable membranes are widely used in severe alveolar bone defects and provide sufficient and precise regenerated bone tissue for subsequent dental implant placement. Although 3D-printed polyetheretherketone (PEEK) meshes have confirmed successful use in clinical cases, the performance of a PEEK mesh is not satisfactory. Compared with PEEK, polyetherketoneketone (PEKK) has better mechanical and processing properties. However, whether PEKK is suitable for making customized membranes remains unclear. The objectives of this study were (1) to evaluate the printing precision, surface characteristics, mechanical characteristics and biocompatibility of the PEKK mesh and (2) to compare the properties of the PEKK and PEEK meshes.Materials and methodsBoth PEKK and PEEK meshes were designed and manufactured via additive manufacturing technology combined with computer-aided design (CAD). The printing precision was evaluated with a high-resolution extraoral scanner. The surface characteristics were evaluated with a contact angle system and three-dimensional optical microscopy. The mechanical characteristics were evaluated via three-point bending tests and tensile tests. The biocompatibility was evaluated with a CCK-8 assay, live/dead viability assay and qRT-PCT.ResultsCompared with the PEEK mesh, the PEKK mesh exhibited better control in terms of the thickness and aperture area. Both the PEKK mesh and the PEEK mesh had a hydrophobic surface, but the PEKK mesh had a smoother surface. Compared with the PEEK mesh, the PEKK mesh has better compression and tensile properties. Both the PEKK mesh and the PEEK mesh had good biocompatibility. The proliferation of cells on the PEKK mesh was slightly lower than that on the PEEK mesh.ConclusionsCompared with PEEK mesh, PEKK mesh has greater printing accuracy, smoother surfaces, better mechanical properties and similar biocompatibility and is expected to be used in the production of customized barrier membranes for the augmentation of severe bone defects. To ensure the stability of the mesh for clinical application, it is best to control the aperture diameter of the PEKK mesh to less than 2 mm with a thickness of 0.2 μm.

Modeling of elastic plastic deformation of near-surface layers of materials

According to the results of strain measurements, a kinematically hardening body model is used to describe the process of elastic-plastic deformation of 1X18H9T steel. Special attention is paid to the processes occurring in the surface layers. The model takes into account the increasing tightness of shear deformations deep into the material. The increase in the stress of the plastic flow in depth is described by a second-order polynomial. The main parameters of the surface effect were determined experimentally and by calculations using the method of reduction coefficients in the process of successive approximations: depth, coefficient of hardening of the material, stresses of plastic flow on the surface and inside the material. It is shown that in order to study the near-surface effect by the strain gauge method, it is necessary to give preference to bending tests of samples rather than stretching. The presence of the surface effect explains the following facts: the destruction of the sample during the tensile test does not begin from the surface, but from the inside, the origin of fatigue cracks occurs under the surface, the surface effect practically does not affect the deformed state of the elastic body, but very strongly affects the stress state at the surface.

Fracture Analysis of 316L Specimens Fabricated via Material Extrusion Additive Manufacturing: Influence of Building Orientation and Notch Acuity

ABSTRACTThree‐point bending tests were performed on notched specimens extracted from cuboids of 316L stainless steel produced via material extrusion additive manufacturing. The cuboids were printed vertically and horizontally on the printing platform to account for the building orientation effect on the mechanical performance. For each orientation, three notch sizes were considered. Overall, the specimens printed with building direction parallel to the loading direction outperformed the others. A significant notch size effect was observed in these specimens since the sharpest notch provoked a decrease in the peak load reached by the specimens in comparison with larger notches. On the contrary, this effect was less relevant among the other specimens, which presented a conspicuous amount of residual porosity that contributed to the premature failure. Further investigations were carried out to correlate the building orientation to the density of the parts and, ultimately, to the investigated mechanical properties. The ASED and TCD criteria were also applied to assess their accuracy in the failure prediction of the tested specimens.

Physico-mechanical properties of geopolymers after thermal exposure: Influence of filler, temperature and dwell time

Geopolymers offer increasingly better physico-mechanical properties concerning thermal exposure at high temperatures compared to ordinary Portland cements (OPC). This paper aims to comprehensively study the use of different types of fillers with different particle size distributions in terms of type (silica sands and cordierites) and surface area, loaded at different temperatures and dwell times (30 min and 180 min). After thermal exposure in the temperature range of 100–1000 °C, geopolymer samples were evaluated regarding physico-mechanical properties compared to samples without thermal exposure, using OPC as a reference material. Geopolymer samples were found to have a denser microstructure than OPC, supporting their better resistance to elevated temperature conditions. In addition, the influence of different filler compositions on the resulting internal structure and porosity was demonstrated. Samples containing fillers in two particle size ranges showed better densification than samples with one particle size range.Conversely, OPC samples showed the least favourable results. In addition, the mechanical behaviour of the geopolymers under static loading, especially in bending and compression tests, showed that the prepared geopolymers exhibited better properties than Portland cement at elevated temperatures, especially in the range of 500–1000 °C. In conclusion, appropriately designed geopolymer compositions have the potential to be a sustainable material, a high-performance alternative to traditional building materials.

Mechanical behaviour of rigid-flexible combined structures: Aluminium-inflated membrane beams for application in floating photovoltaic platform

This study aims to investigate the bending and failure behaviour of aluminium-inflated membrane beams for their applications in floating photovoltaic platforms. Four-point bending tests are conducted for a range of inflated pressures and two different deck heights to assess their effect on structural stiffness and ultimate bearing capacity. Meanwhile, the surface-based fluid cavity method is employed to develop the finite element model with the material properties determined by independent coupon-level tests. The bearing capacity of the aluminium-inflated membrane beam is positively correlated with the internal pressure and deck height. The midspan strain distribution is similar to those of the traditional four-point bending beam with the upper part undergoing compression and the lower part experiencing tension, however, the structural behaviour at the failure stage is different. Failure typically occurs due to localised depressions at the loading points on the aluminium deck, ultimately leading to structural failure. The numerical model closely matches the experimental data for the initial inflated and bending configurations, exhibiting a deviation of only 0.10 % to 0.46 % in diameter and 0.32 % to 5.57 % in equivalent bending stiffness. A parametric study shows that the loading properties of the beam are more sensitive to the internal pressure than the deck height and thickness.

Ductility and cracking behavior of UHPC-RC composite beam under bending test based on acoustic emission parameters

Ultrahigh-performance concrete (UHPC) possesses improved mechanical properties due to its high strength, durability, and toughness. Currently, there is a single method for detecting the cracking behavior and evaluating the ductility of the UHPC-reinforced concrete (RC) composite beam. As an important part of structural non-destructive monitoring techniques, the acoustic emission (AE) technique can be used to analyze the damage pattern of the structure by the variation characteristics of its individual parameters. On this basis, four-point bending tests were carried out on the RC beam and UHPC-RC composite beam, and the AE technique was used for ductility analysis and cracking behavior monitoring. The results showed that the UHPC material enhanced the flexural capacity of the UHPC-RC composite beam. The AE ring counts and energy parameters could be used to analyze the cracking process of the UHPC-RC composite beam during flexural damage. The proposed new ductility indices, AE ring counts ductility index (ARD) and AE energy ductility index (AED), differed from the displacement ductility indices within the range of 2 %, providing a new method for calculating ductility indices of concrete structures. During the failure stage, the UHPC-RC composite beam produced more tensile cracks at the early stage of cracking and the stage of damage compared to the RC beam. In addition, the b value of the RC beam had a large range of fluctuations during the failure stage, indicating that the RC beam produced more micro-cracks and macro-cracks and more damage compared to the UHPC-RC composite beam.

Flexural Behavior of 3D-Printed Carbon Fiber-Reinforced Nylon Lattice Beams.

This study investigates the flexural behavior of 3D-printed multi-topology lattice beams, with a specific emphasis on octet and cube lattice geometries created through fused deposition modeling (FDM). The mechanical properties of these beams were evaluated through quasi-static three-point bending tests. A comparative analysis of load-carrying capacity, energy absorption, and specific energy absorption (SEA) indicates that octet lattice beams exhibit superior performance to cube lattice beams. The octet lattice beam in the triple-layer double-column (TL-DC) arrangement absorbed 14.99 J of energy, representing a 38% increase compared to the 10.86 J absorbed by the cube lattice beam in the same design. The specific energy absorption (SEA) of the octet beam was measured at 0.39 J/g, which exceeds the 0.29 J/g recorded for the cube beam. Two distinct types of deformations were identified for the struts and the beam layers. Octet struts exhibit enhanced performance in stretch-dominated zones, whereas the cube system demonstrates superior efficacy in compressive-dominated regions. The results highlight the enhanced efficacy of octet lattice structures in energy absorption and mechanical stability maintenance. The investigation of sandwich lattice topologies integrating octet and cube structures indicates that while hybrid designs may exhibit efficiency, uniform octet structures yield superior performance. This study provides valuable insights into the structural design and optimization of lattice systems for applications requiring high-energy absorption and mechanical robustness.

Characterization of Interface Transition Zone in Asphalt Mixture Using Mechanical and Microscopic Methods.

An enormous surge in the pavement sector requires the evaluation of interface bonding in asphalt composite, since the assessment of bonding brings considerable cost savings. Microscopic and mechanical analyses were performed to study the status of the interface transition zone of four groups of asphalt mixtures, using thin-slice preparation to obtain asphalt mixture slices with a flat surface for microscopic analysis. The interface transition zones were characterized using good knowledge of blending or diffusion phenomena by conducting tests both at the micro and macro levels to determine mixture quality. Asphalt mixture components were observed using fluorescence microscopy imaging and evaluated by the gray value change law. Asphalt mixture groups, (virgin, recycled of 30% aged and 70% unaged, 6%, and 4% rejuvenator dosage mixtures) under the same process parameters, which are a mixing time of 270 s and a mixing temperature of 150 °C, been considered optimum for component fusion in a hot asphalt mixture were used. This study relied on the influence of morphology law, assessed through rutting tests for high temperature performance, semi-circular bending tests for low temperature performance, and pull-off tests for interface bonding strength. The relationship between interface transition zones and macro performance was studied. The self-developed pull-off method was a research innovation which can be used as an alternative to study interface transition zones in asphalt mixtures, and provides the necessary data needed with 3D surface failure mode calculations. The device measured the bonding strength of a single aggregate in distinct positions using the bitumen penetration test method. The main goals were to determine a correction factor, identify the appropriate alteration, and compute the actual fracture surface area. Using scanning electron microscopy for interface characterization and micro-morphologies of mortar transition zone, our analysis provides adequate knowledge about interface position and the components present. The applied approaches to characterize asphalt mixture interfaces proved workable and reliable, as all methods have similar trends with useful information to determine asphalt pavement quality.

Micro-Macro Coupling Study on the Mechanical Properties of Continuous Fiber-Reinforced Composites.

As a key and weak point of continuous fiber-reinforced composites (CFRCs), the interface between the fiber and the matrix is vulnerable to failure under external loads, with its performance directly affecting the overall properties of CFRCs. Hence, a micro-macro coupling method that considered the microscopic properties of the interface was utilized to analyze and predict the mechanical properties of CFRCs more accurately. The microscopic mechanical parameters of the fiber-matrix interface, which were obtained using molecular dynamics, were transferred to the representative volume element (RVE). The stiffness matrix of the CFRC, required for the macroscopic finite element model, was then calculated using a unified periodic homogenization method based on the RVE and assigned to the finite element model for a macroscopic simulation. Nylon/continuous carbon fiber specimens were fabricated through additive manufacturing, with the tensile and bending strengths of the specimens obtained through tensile and three-point bending tests. The tensile strength of the experimental specimen was 200.1 MPa, while the result of the simulation containing the interface was 205.5 MPa, indicating a difference of less than 5% between the two. In contrast, the result of the simulation without an interface was 317.7 MPa, representing a high error of 58.7% compared with the experimental results. Moreover, the bending strength, Young's modulus, and flexural modulus results with and without an interface showed the same trend as that for the tensile strength. This illustrates the effectiveness of the proposed micro-macro coupling method for analyzing and predicting the mechanical properties of CFRCs.

Impact of fluoride and aluminum co-exposure on bone growth and quality in juvenile rats: dose and duration effects

This study aimed to investigate the impact of the dosage and duration of fluoride and aluminum(F and Al) co-exposure on the skeletal growth and bone quality of juvenile rats. Forty-eight 8-week-old Sprague-Dawley rats were randomly assigned to normal control, low F and Al exposure, and high F and Al exposure groups, with 45-day and 90-day subgroups established for each. We measured body length, tibia length, conducted bone histomorphometric analysis of the proximal tibia, performed micro-CT scans and three-point bending tests of the femur. Compared to the age-matched control group, the low F and Al group at 45 days exhibited increased bone formation and stiffness; the low F and Al group at 90 days and the high F and Al group at 45 days showed increases in body length, tibia length, growth plate width, longitudinal bone growth rate, bone turnover, and improved microstructure. Notably, bone elastic stress only elevated in the high F and Al group at 45 days. Conversely, the high F and Al exposure group at 90 days experienced decreases in the aforementioned parameters, with the exception of growth plate width, and displayed abnormal hypertrophic chondrocyte morphology in the growth plate. In summary, long-term exposure to low levels of F and Al and short-term exposure to high levels of F and Al promote bone formation followed by bone resorption in juvenile rats, stimulating bone growth and enhancing bone quality. However, long-term exposure to high levels F and Al results in low bone turnover, slow bone growth, and reduced bone property.

Flexural Strength of Modern CAD/CAM Restoratives After Artificial Aging.

Before clinical trials are initiated, studying the mechanical performance of modern CAD/CAM restorative materials exposed to aging conditions would provide insights on their performance in service. This study evaluated the impact of thermomechanical aging on various resin composite, ceramic, hybrid, and nano-filled resin composite materials after two polymerization modes. Specimens (3 × 4 × 14 mm3) were fabricated using (n = 12 per group) a universal composite (Filtek Supreme XTE photo-polymerized for either 40 s or 120 s per layer), hybrid ceramics (BRILLIANT Crios, GC Cerasmart, Lava Ultimate, VITA ENAMIC), glass ceramics (IPS e.max CAD, VITA Suprinity PC, Straumann n!ce), or feldspar ceramics (VITABLOCS Mark II, GC Initial LRF). In each group, half of the specimens underwent thermomechanical aging. A three-point bending test was applied to all specimens and the results were statistically analyzed (α = 0.05). Glass ceramics and hybrid ceramics presented higher flexural strength values than feldspar ceramics and the universal composite before and after aging (p < 0.05). Thermomechanical cycling affected the flexural strength of all materials (p < 0.05) except Lava Ultimate, Straumann n!ce, and GC Initial (p > 0.05). The highest decrease in flexural strength after aging was found in the universal composite (40 s polymerization) (p < 0.001) and Vita Enamic (p < 0.001), while the lowest decrease was in the hybrid ceramics, Cerasmart and Lava Ultimate (p < 0.05). Extending polymerization duration reduced the aging effect on the universal composite tested. Thermomechanical aging affected the flexural strength of most materials tested. Universal composites and feldspar ceramics presented similar flexural strength values.

The Evaluation of Sandwich Composite Materials with Vegetable Fibers in a Castor Oil Polyurethane Matrix with Their Faces and Honeycomb Core Made in a 3D Printer.

Sandwich panels are widely used in the naval and aerospace industries to withstand the normal tensile, compressive, and shear stresses associated with bending. The faces of sandwich composites are usually made of metals such as aluminum and, in some studies with composites, using a polymeric matrix, but there are no studies in the literature using a castor oil polyurethane matrix. The core of the panel must keep the faces apart and be rigid perpendicular to them. To begin the work, a study was carried out on the influence of alkaline treatment on sisal fibers to increase the fibers' adhesion to castor oil polyurethane. There are no relevant studies worldwide on the use of this resin and the adhesion of vegetable fibers to this polyurethane. In this work, a study was carried out through a three-point bending test of sandwich panels using faces of composite material with sisal fibers subjected to an alkaline treatment of 10% by weight of sodium hydroxide and an immersion time of 4 h in the dissolution, which was the best chemical treatment obtained initially in a castor oil polyurethane matrix. The honeycomb cores were made by 3D printer and in this study two different printing filament materials, PETG and PLA, and two different core heights were compared. As a result of a traction test, it was observed that sisal fibers with chemical treatment in a castor oil polyurethane matrix can be used in composites, although the stress levels obtained are 50% lower than the stresses obtained in other matrixes such as epoxy resin. The combination of sisal faces in a castor oil polyurethane matrix and honeycomb cores made in a 3D printer showed good properties, which allows the use of renewable, sustainable and less aggressive materials for the environment. In all tests, PETG was 21% to 32% stronger than PLA. Although there was no rupture in the test specimens, the PETG cores deformed 0.5% to 3.6% less than PLA. The composites with PLA were lighter, because the core density was 13.8% lower than the PETG cores. Increasing the height of the honeycomb increased its strength.