Evaluation of the Sensitivity of Various Reinforcement Patterns for Structural Carbon Fibers to Open Holes during Tensile Tests

This paper is devoted to the experimental study of polymeric composite specimens, with various types of reinforcement, in order to evaluate the breaking strength of specimens with open holes when undergoing uniaxial compression and tensile tests. Four types of interlaced 3D woven preforms were considered (orthogonal, orthogonal combined, with pairwise inter-layer reinforcement, and with pairwise inter-layer reinforcement and a longitudinal layer), with a layered preform used for comparison. Tensile tests of solid specimens without a hole, under ASTM D 3039, and of specimens with an open hole, under ASTM D 5766, were carried out using the Instron 5989 universal electromechanical testing system. Movements and strains on the specimen surface were recorded using a Vic-3D contactless optical video system and the digital images correlation method (DIC). For all the series of carbon fiber tension specimens, strain and stress diagrams, mechanical characteristics, and statistical processing for 10 specimens were obtained. The paper evaluated deformation fields for certain points in time; the obtained fields showed an irregular distribution of deformation and dependency on types of reinforcing fibers. A coefficient of strength variation is introduced, which is defined as a ratio of the ultimate stress limits obtained on solid samples with and without open holes. Within the framework of ASTM D 5766, when calculating the ultimate stress, the hole is not taken into account, and the paper shows that for certain structures a hole cannot be excluded. The hole size must not be neglected when calculating the ultimate stress.

Effect of Fiber Reinforcement on the Compression and Flexural Strength of Fiber-Reinforced Geopolymers

This work aimed to determine the effect of the addition of different types of reinforcing fibers on the strength properties of geopolymers such as flexural and compressive strength. Geopolymers are an attractive alternative to conventional binders and building materials; however, one of the main problems of their widespread use is their low resistance to brittle fracture. To improve the mechanical properties, reinforcement in the form of glass, carbon, and basalt fibers (as grids) was applied to geopolymers in the following work. Additionally, composites with these fibers were produced not only in the matrix of pure geopolymer but also as a hybrid variant with the addition of cement. Furthermore, basalt grids were used as reinforcement for geopolymers not only based on ash but also metakaolin. An additional variable used in the study was the molar concentration of the alkali solution (5 M and 10 M) for the different types of geopolymer samples. The mechanical properties of geopolymer materials and geopolymer–cement hybrids are the highest when reinforcement in the form of carbon fiber is used. Strength values for geopolymers reinforced with basalt mats depend on the number of reinforcement layers and the concentration of the alkaline solution used. All produced composites were tested for compressive strength and bending strength. When using basalt mesh, it was possible to achieve a bending strength of 12 MPa. The highest compressive strength that was achieved was the value of 66 MPa, while for samples not reinforced with fibers, only about 40 MPa was achieved.

Analysis of Air-Coupled Transducer-Based Elastic Waves Generation in CFRP Plates

In this paper, the analysis of non-contact elastic waves generation in carbon fiber reinforced-polymer (CFRP) plate was conducted. Full non-contact elastic waves generation and sensing methods were also analyzed. Elastic waves generation was based on an air-coupled transducer (ACT) while waves sensing was based on a laser Doppler vibrometer. The excitation frequency was equal to 40 kHz. An optimal ACT slope angle for the generation of elastic waves mode was determined with the aid of dispersion curves calculated by using a semi-analytical model. Due to the stack sequence in the composite plate (unidirectional composite), ACT slope angles were different for waves generation in the direction along and across reinforcing fibers direction. Moreover, experimental verification of the optimal ACT slope angles was conducted. It was possible to generate A0 wave mode in the direction along and across the reinforcing fibers. Optimal angles determined using ACT were equal to 16° (along fibers) and 34° (across fibers). In the case of optimal angles, elastic waves amplitudes are almost two times higher than for the case of ACT oriented perpendicularly to the plate surface. Moreover, experimental results based on ACT showed that it was possible to generate the SH0 mode in the direction across the fiber for optimal angles equal to 10°. Finally, based on the A0 wave mode propagation, the process for localization of discontinuities was performed. Discontinuities in the form of additional mass simulating damage were investigated. A simple signal processing algorithm based on elastic wave energy was used for creating damage maps. Authors compared discontinuity localization for ACT oriented perpendicularly to the plate and at the optimal slope angle. The utilization of non-contact waves excitation at optimal ACT slope angles helped to focus the wave energy in the desired direction. Moreover, in this case, elastic waves with the highest amplitudes were generated.

EQUATIONS FOR DETERMINING THE DYNAMIC CHARACTERISTICS OF ELASTIC ELEMENTS

In this article, we consider equations for determining the dynamic characteristics of inhomogeneous elastic elements using refined elastic modulus and Poisson's coefficients depending on the concentration of reinforcing fibers. The dependences of the first three natural frequencies of the seamless bellows on its thickness and depth of corrugation and the first two natural frequencies of the welded bellows on its thickness are identified and analyzed. The frequency response and frequency response of an accelerometer with a UE are obtained depending on its geometric parameters.

Calculation and Design of Building Structures Given the Variatropy of the Structure, Sections and Differentiation of Constructive Characteristics of the Materials

Состояние проблемы. Возрастающие объемы строительства требуют новых технологических, конструктивных и расчетных решений железобетонных элементов. Центрифугирование, являясь перспективной технологией производства, приводит к вариатропным, т. е. различающимся по своим характеристикам (плотности, прочности, деформативности и др.), по сечению бетонам и конструкциям, выполненным из них. Это во многих случаях необходимо учитывать в расчете и проектировании, однако исследований касательно этой темы практически не проводилось. Поэтому при расчете и проектировании строительных конструкций вариатропной структуры обычно закладывается необоснованно большой запас прочности, что приводит к их сильному удорожанию. В нормах проектирования и научной литературе отсутствуют теоретические и практические методы расчета центрифугированных железобетонных строительных конструкций с учетом вариатропности структуры и характеристик бетона по сечению. Отдельными данными подтверждена эффективность центрифугирования, но в полной мере использовать его преимущества из-за отсутствия в существующих методах расчета учета вариатропности структуры пока невозможно. Результаты и выводы. В результате обзора и анализа определены векторы развития и направления будущих исследований, заключающиеся в изучении работы сталежелезобетонных центрифугированных и виброцентрифугированных сжатых элементов с использованием фиброармирующих волокон. Предполагается совершенствовать технологию изготовления и методики расчета для еще полного и всестороннего исследования такого уникального явления, как вариатропия структуры бетона строительных конструкций. Statement of the problem. Increasing numbers of construction call for new technological, structural and design solutions for reinforced concrete elements. Being a promising production technology, centrifugation causes variatropic characteristics, i. e., differing ones (density, strength, deformability, etc.), in cross section of concrete and structures made of them. This is oftentimes essential to consider in both calculation and design, but research on this topic is literally non-existent. Thus while calculating and designing building structures of the variatropic structure, an unreasonably large margin of safety is commonly present leading a surge in their price. In design guidelines and scientific literature there are no theoretical and practical methods of calculation of centrifuged reinforced concrete building structures considering the variability of the structure and characteristics of concrete in cross section. Some data confirm the efficiency of centrifugation, but it is not yet possible to make complete use of its advantages due to the lack of existing methods for calculating the variability of the structure. Results and conclusions. Based on the review and analysis, the vectors of development and directions of future research are identified, which are to sinvestigate the operation of reinforced concrete centrifuged and vibrocentrifuged compressed elements using fiber reinforcing fibers. It is suggested that the manufacturing technology and calculation methods are improved for a more complete and comprehensive study of such a unique phenomenon as variatropy of the concrete structure of building structures.

CALCULATION AND DESIGN OF BUILDING STRUCTURES CONSIDERING THE VARIATION OF THE STRUCTURE, SECTIONS AND DIFFERENTIATION OF THE CONSTRUCTION CHARACTERISTICS OF MATERIALS

Statement of the problem. The increasing volumes of construction require new technological, structural and design solutions for reinforced concrete elements. Centrifugation, being a promising production technology, leads to variatropic - differing in their characteristics (density, strength, deformability, etc.) in the section of concretes and structures made of them. In many cases, this must be taken into account in the calculation and design, but such studies have practically not been carried out. Therefore, when calculating and designing building structures of a variatropic structure, an unreasonably large margin of safety is usually laid, which leads to their strong rise in price. In the design standards and scientific literature, there are no theoretical and practical methods for calculating centrifuged reinforced concrete building structures, taking into account the variability of the structure and characteristics of concrete over the section. Separate data have confirmed the efficiency of centrifugation, but it is not yet possible to fully use its advantages due to the lack of accounting for structure variability in existing calculation methods. Results and conclusions. As a result of the review and analysis, the vectors of development and directions of future research have been identified, which consist in studying the work of steel-reinforced concrete centrifuged and vibro-centrifuged compressed elements using fiber-reinforcing fibers. It is proposed to improve the manufacturing technology and calculation methods for a complete and comprehensive study of such an undoubtedly interesting and unique phenomenon as the variatropy of the structure of concrete of building structures.

Strength and Durability of Hybrid Fiber-Reinforced Latex-Modified Rapid-Set Cement Preplaced Concrete for Emergency Concrete Pavement Repair

The benefits of using reinforcing fibers in latex-modified rapid-set cement preplaced concrete for emergency pavement repairs were examined in terms of strength, permeability, and durability as functions of the type of fiber. Single-type fibers, including jute, poly (vinyl alcohol) (PVA), and nylon fibers, as well as hybrid fiber mixtures prepared with two of the aforementioned fibers at a 1:1 weight ratio, were evaluated. Fibers were incorporated into the concrete mixture at 1.2 kg/m3. A vibratory press compactor that simulates roller compaction was used to increase compaction and densification of the resulting pavement repair material. The hybrid fiber-reinforced latex-modified rapid-set cement preplaced concrete (HFLMC) was manufactured to satisfy the criteria for opening traffic, i.e., compressive strength of 21 MPa or higher, and flexural strength of 3.5 MPa or higher after 4 h. Pavement requiring repair was removed and replaced with coarse aggregate. The rapid-set binder, fibers, and latex were then mixed and placed onto the coarse aggregate layer. The repair was considered complete after compaction. The resulting HFLMC satisfied all of the test criteria. Furthermore, concretes made with hybrid fibers were more mechanically sound than those made with a single fiber variety. Hybrid fiber concretes made with PVA and nylon fibers exhibited the best properties for emergency pavement repair. These results indicate that HFLMC is suitable for emergency pavement repair.

Development of thermoplastic composite materials based on modified polypropylene

To improve the physical-mechanical and thermophysical properties of polypropylene-based thermoplastic composite materials, we performed modification of a polymer matrix by reactive extrusion of polypropylene in the presence of benzoyl peroxide and polysiloxane polyols. Modified polypropylene was compounded with basalt, carbon, and para-aramide reinforcing fillers in a screw-disc extruder. It was established that the reinforcement of modified polypropylene by basalt fibers ensured a 110% increase in tensile strength. The reinforcement of modified polypropylene by carbon fibers allowed fabricating thermoplastic composite materials with tensile strength increased by 14%. The maximum reinforcing effect was observed by using para-aramide fibers as reinforcing fibers for modified polypropylene with tensile strength increased by 30% as compared with initial polypropylene. It was determined that the obtained thermoplastic composite materials based on modified polypropylene can be processed into products by the most productive methods (extrusion and injection molding). The developed materials exhibited improved thermal stability. The proposed ways of modification methods provide substantial improvement in physical-mechanical and thermophysical properties of modified polypropylene-based thermoplastic composite materials as compared with initial polypropylene. In addition, they ensure a significant increase in service properties of the products prepared from thermoplastic composite materials based on modified polypropylene.

Tow Deformation Behaviors in Resin-Impregnated Glass Fibers under Different Flow Rates

With the rapid development of high-performance fibers such as carbon, enhanced glass fibers in structural applications, the use of fiber-reinforced composite (FRC) materials has also increased in many areas. Liquid composite molding (LCM) is a widely used manufacturing process in composite manufacturing; however, the rapid impregnation of resin in the reinforcing fibers during processing poses a significant issue. The optimization of resin impregnation is related to tow deformations in the reinforcing fibers. The present study therefore focuses on this tow deformation. The permeability behaviors in double-scale porous media were observed under different flow rates and viscosity conditions to examine the overall tendencies of structural changes in the reinforcement. The permeability results showed hysteresis with increasing and decreasing flow rate conditions of 50–800 mm3/s, indicating structural changes in the reinforcement. The tow behaviors of the double-scale porous media with respect to the thickness and flow rate were investigated in terms of the representative indices of the minor axis (tow thickness) and major axis. The minor axis and major axis of the tow showed decreasing and increasing trends of 2–5% and 2%, respectively, with minimum and maximum values at different positions along the reinforcement, affected by the different hydrodynamic entry lengths. Finally, the deformed tow behavior was observed microscopically to examine the behavior of the tow at different flow rates.