Short Beam Shear Strength Research Articles

Experimental Study on the Mechanical Properties of Basalt Fibres and Pultruded Bfrp Plates at Elevated Temperatures

Basalt fibre reinforced polymer (BFRP) composites have been widely used for structural strengthening and reinforcement in civil engineering recently. Since fire risk is unavoidable for most civil structures, the degradation of BFRP at elevated temperatures needs to be known for the safe design of its strengthened or reinforced structures under fire risk. In this study, the mechanical properties of basalt fibre roving and pultruded basalt fibre reinforced epoxy plates were investigated at elevated temperatures. As the temperatures increasing from room temperature to 200 °C, the tensile strength and modulus of the fibre roving were reduced by 8.3% and 9.7%, respectively. Meanwhile, the Weibull shape parameter (m) of the fibre roving decreased by 20.5%. As regards the BFRP plates, however, the elevated temperatures show more adverse influence. The tensile strength and modulus of the BFRP plates is reduced by 37.5% and 31% as temperature rising to 200 °C. Compared to the tensile properties, the short beam shear strength (SBS) was reduced by around 90%, more susceptible to the elevated temperatures. The glass transition temperature (∼93 °C) plays a key role on the inflexion of the variation of the mechanical properties, especially for the tensile strength. In a wide temperature range, the relationship between the tensile strength and the SBS showed a good linearity, indicating the reduction of the tensile strength comes from the deterioration of the interlaminar shear strength at elevated temperatures.

Laboratory evaluation of chemical resistance of pultruded GFRP dowels for concrete pavement

This paper presents mechanical, microstructural and physical characterization of glass fibre-reinforced polymer (GFRP) rods used as dowels for concrete pavement exposed to different types of conditionings. Two types of GFRP dowels fabricated with polyester and vinylester resins were studied. GFRP dowels were exposed to different alkaline and saline solutions at 23 °C during 90 days to simulate the effect of the concrete environment. GFRP dowels were also subjected to cyclic freezing and thawing. The measured short beam shear strengths and flexural modulus of elasticity of the GFRP dowels before and after exposure were considered as a measure of the durability performance of the specimens. In addition, Fourier transform infrared spectroscopy, differential scanning calorimetry and scanning electron microscopy were used to characterize the aging effect on the GFRP dowels. The results showed the very high long-term durability of vinylester GFRP dowels exposed to tap water, CaCl2, NaOH and CaOH2 solutions, and to freeze/thaw cycles. On the other hand, the test results have shown that polyester-based GFRP dowels present an uncertain stability in the different environments simulating the field service conditions, due to plasticizing and/or irreversible chemical degradation of the polymer matrix.

Durability study of pultruded CFRP plates immersed in water and seawater under sustained bending: Water uptake and effects on the mechanical properties

This paper presents the durability behavior of pultruded unidirectional carbon fiber reinforced polymer (CFRP) plates immersed in water and seawater at room temperature, under sustained bending strain of 30% and 50% ultimate strain. In this study, water absorption kinetics of CFRP composite and effects of moisture ingress on the mechanical properties, such as tensile properties and short beam shear strength, constitute integral parts of the investigation. The study reveals that seawater immersion leads to higher equilibrium moisture content than water immersion, due to the blister induced damages on the CFRP plate surfaces in seawater. However, diffusion coefficient in seawater immersion is shown to be lower compared to the water immersion, and is attributed to the high concentration of dissolved salts in seawater that retard water diffusion by osmosis. Increasing the bending strain reduces the free volume fraction of the resin matrix, which is responsible for the decreased water uptake and diffusion coefficient for both immersions. Immersion in both media leads to the pronounced degradation in the resin controlled property (i.e., short beam shear strength) of CFRP, but shows less or negligible effects on the fiber controlled properties (i.e., tensile strength and modulus). Both immersion media and 50% bending strain level show remarkable effects on the variation of the mechanical properties of CFRP.

The role of the interphase on the shear induced failure of multiwall carbon nanotubes reinforced epoxy nanocomposites

ABSTRACTMultiwall Carbon Nanotubes (MWCNT) with an elevated aspect ratio were chemically functionalized with amines and two types of epoxide groups. Thermogravimetric analysis and Fourier Transform‐Infrared Spectroscopy (FTIR) analysis corroborated that the functionalization degree was substantial (up to 30 wt %) and the presence of a covalent bond with the MWCNT. The functionalized MWCNT (f‐CNT) were incorporated into an epoxy matrix after its dispersion in the diglycidyl ether of bisphenol A (DGEBA) precursor. To induce a shear failure mode, a short‐beam (SB) experimental setup was implemented. The SB shear strength (SBSS) proved that the functionalization had a strong influence on its value. For the case of pristine CNT, a neutral effect was obtained. A strong detrimental effect (−17.2% ± 9.5) was measured for the amine type f‐CNT and a positive effect (up to 10.9% ± 8.9) was measured of the epoxide type f‐CNT. Fractographic analysis of each formulation was correlated with SBSS performance, proving that the surface texture of the fractured samples was strongly correlated to its value. Furthermore, dynamic mechanical analysis proved that the damping factor and the crosslink molecular weight were correlated with the SBSS performance. A lower full width at half maximum of the damping factor was associated to an improvement of SBSS. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41364.

Durability study of ramie fiber fabric reinforced phenolic plates under humidity conditions

Abstract A durability study of a ramie fiber fabric reinforced phenolic resin (RFRP) plate under 50%, 85%, and 98% relative humidity for 6 months at room temperature was performed. Water absorption and desorption, tensile and short beam shear strengths of the RFRP plates were investigated as a function of exposure time. RFRP samples show strong hydrophilic characteristics and the saturated water content varies from 0.73% to 4.5% with relative humidity ranging from 50% to 98%. After 6 months of exposure to 98% relative humidity, an abnormal extra amount of moisture was absorbed, which may have resulted from cracks in the resin matrix or from debonding between fiber and resin due to swelling of the fibers with high moisture content. It was found that the tensile modulus is more susceptible to moisture uptake, which is ascribed to the degradation of ramie fibers with the water ingress. An approximate linearity between the mechanical properties and the moisture content is observed if the abnormal extra water uptake is neglected. Both tensile and short beam shear strengths of the RFRP samples recovered remarkably when samples were fully dried at 60°C, indicating a low degree of permanent degradation occurred due to the exposure.

Effects of alternating temperatures and humidity on the moisture absorption and mechanical properties of ramie fiber reinforced phenolic plates

In this article, ramie fiber reinforced phenolic (RFRP) plates were prepared with compression molding process, and the plates were subjected to 98% humidity environment and alternating temperatures (from 25°C to 55°C in 24 h for a cycle) for 4 weeks. The resulted moisture absorption and the variation of the mechanical properties of RFRPs were studied. As found, compared to constant exposure temperatures (25°C or 60°C), alternating temperatures brought in higher moisture uptake and more serious degradation in the flexural strength, flexural modulus and short beam shear strength of the RFRP samples under the same humidity condition. The deteriorated effects of alternating temperatures is attributed to more remarkable degradation of the bonding between the fiber and resin, due to the moisture uptake and the internal cyclic stress around the ramie fibers with alternating temperatures. The flexural modulus of RFRP plates was much more susceptible to the moisture uptake than the flexural strength. After fully drying, the mechanical properties of the RFRP samples were recovered to some extent, but still less than the original values, indicating permanent damages occurred. Fiber Bragg grating sensors embedded in the RFRP plate was applied to monitor the variation of the internal strain during the exposure. As indicated, the moisture absorption and alternating temperatures bring in relaxation of the internal tension stress formed during compressing process, and decrease in the coefficient of thermal expansion of the RFRP samples. POLYM. COMPOS., 36:1590–1596, 2015. © 2014 Society of Plastics Engineers

An Experimental Investigation of Factors Considered for the Short Beam Shear Strength Evaluation of Carbon Fiber–reinforced Thermoplastic Laminates

The short beam shear method is the most common test used to determine interlaminar shear strength (ILSS). However, different test standards provide a range of values and make this method unreliable. Carbon fiber–reinforced thermoplastic laminates were used for experiments with the boundary conditions specified under International Organization for Standardization and ASTM test standards. Additional factors, such as the matrix type, thermoforming, and carbon weave orientation, were chosen to build an experimental design. The work also demonstrates the unique visualization of the shear strain during three-point bending as determined via photogrammetric measurements. The specimen failure modes are described in detail. This paper highlights and evaluates the possible influences of the material and test factors that should be considered when ILSS tests are performed on woven fabric reinforced thermoplastics under different test standards and conditions.

Mechanical Properties of Self-Healing Carbon Fiber-Epoxy Composite Stitched with Mendable Polymer Fiber

Carbon fiber composites are self-healed by advance embedding of repairing agents in the composites. However, the repairing agent will influence the mechanical properties of the carbon fiber composites. In this study, poly(ethylene-co-methacrylic acid) (EMAA) filaments were stitched into carbon fiber-epoxy laminates to create a three-dimensional (3D) self-healing fiber system. Specimens with unmodified and self-healing laminates were manufactured. The mechanical properties of the carbon fiber-epoxy composite stitched with mendable polymer fiber for self-healing and unmodified laminates were compared experimentally. Results from the double cantilever beam test revealed that the stitched EMAA fibers increased the mode I interlaminar fracture toughness of the laminate by ∼120%. However, short-beam shear (SBS) strength of the composite laminates with the healing agents was slightly degraded, with a 37% reduction in the average SBS strength. The compressive-after-impact assessment showed that the strength was reduced by 6.6%. C-Scan revealed the 3D inter-connected self-healing EMAA network within the composite laminates.

Optimization of Stamp Forming Process for Thermoplastic Composites

The present study is focused on the development of a two-dimensional stamping method for the manufacturing of fiber reinforced composites with thermoplastic matrix resins. Materials investigated are carbon fiber reinforced polyamide-6. Taguchi L16 orthogonal array is used in split-plot designs. The processing conditions include thermoforming temperature, mold temperature, pressure and time, required to establish high-quality parts. From the experimental results, we derive a set of best combination, A1 (90°), B2 (263°C), C1 (105°C), D1 (33 kg/cm2) and E2 (48 sec) and carry out an estimated equation for the short-beam shear strength. The results have described the correlations between processing parameters and shear stress. Finally, for verifying the prediction ability of the estimated equation, the confirmation experiments are conducted. The confirmation test result is 48.67 kg/mm², fall in the confidence interval. It shows that the prediction ability of estimated equation and the repetition of the experimental results has confirmed and accepted by the tests.

Long term durability of pultruded polymer composite rebar in concrete environment

Pultruded glass fibre reinforced polymer (GFRP) composite rebar was immersed in alkaline concrete environment for 0, 1, 2, 3, 4, 6, 14 and 24 months at 60 °C to evaluate its durability in concrete structure. Moisture absorption of the rebar was found to be only 0.76%. It was also found that both the glass transition temperature (Tg) and the short beam shear strength were retained by about 91.5%; and the above properties were remained almost unchanged during the ageing period from 1 month to 24 months. Design tensile strength and tensile elastic modulus of the rebar were retained by 100% after 24 months exposure in concrete environment. Degradation of GFRP rebar was not evident in the FT-IR results as supported by scanning electron micrographs and energy dispersive X-ray analysis. Moisture absorption was found to be a critical factor that controlled thermal and mechanical properties of GFRP rebar.

Strut and tie modeling for RC short beams with corroded stirrups

Corrosion of steel reinforcement is one of the major problems that shorten the service life of reinforced concrete (RC) structures. Steel stirrups, due to their location as an outer reinforcement, are more susceptible to corrosion problems and damage. However, there is limited research work in the literature on the effects of stirrup corrosion on the shear strength of RC beams. This paper attempts to evaluate analytically the residual shear strength of RC short beams with corrosion-damaged stirrups. The shear strength of short or deep beams are generally determined using the strut and tie model. The corrosion effects were implemented in the model to make it capable of predicting the residual shear capacity of RC beams with corroded stirrups. The effect of corrosion is implemented considering the reduction in geometry of the concrete cross section due to spalling and reduction in effective compressive strength of concrete due to corrosion cracks. The proposed strut and tie model which accounts for the corrosion effects was verified using the experimental data available in the literature, and good agreement was found.

Fatigue susceptibility of an endodontic fibre post material

To evaluate effects of ageing and fatigue on the elastic modulus and short-beam shear strength of a quartz fibre/epoxy resin post material. Cylindrical specimens (25 × 2.2 mm) were made. Elastic moduli were dynamically measured before immersion in water; after immersion in water; periodically during storage in water for up to 7 years; periodically during thermal cycling in water for up to 10 000 cycles to produce thermo-mechanical fatigue; and periodically during boiling in water for up to 100 h. After ageing, the specimens underwent short-beam shear strength testing. Elastic modulus was significantly decreased by thermal cycling and by immersion in boiling water, but not by water storage. Short-beam shear strength was profoundly decreased by all three ageing processes. Short-beam shear strength was much more sensitive than elastic modulus to the ageing or fatigue processes applied in this study. A representative endodontic fibre post material was susceptible to a variety of ageing and fatigue processes. The effects of ageing and fatigue had a more pronounced impact on short-beam shear strength than on elastic modulus. The effects of boiling in water and thermal cycling in water were considerably larger than those of simple storage in water.

Mechanical property enhancement of flax fibre-based green composites for civil structural application

In the present work, unidirectional flax fibre reinforced epoxy green composites were prepared with hand lay-up process. Efforts were made to improve the mechanical properties of the composites to meet the requirements for civil structural application. Mercerisation under stretch was found to be an effective way to enhance the interfacial bonding performance between flax and epoxy matrix, leading to the increased flexural strength and modulus. The incorporation of organoclay to epoxy matrix can further increase the interfacial strength and modulus remarkably. With optimised mercerisation time, the flexural strength and modulus of the unidirectional flax/epoxy composite of fibre content about 25 vol. % reach 183 MPa and 5.2 GPa respectively, while the incorporation of 2% of organoclay can further increase the flexural modulus and the short beam shear strength in transverse direction by about 30%. The preliminary study indicates the promising mechanical properties of the flax fibre-based green composites for civil structural application.

Effects of variation in autoclave pressure, temperature, and vacuum-application time on porosity and mechanical properties of a carbon fiber/epoxy composite

This article presents results from investigation of the effects of variation in autoclave pressure, temperature, and vacuum-application time on porosity, hot/wet (H/W) and room temperature/dry (RT/D) short beam shear (SBS) strength, and failure mechanism of a commercial carbon fiber/epoxy prepreg, Cycom IM7/977-2 unidirectional tape. Fourteen cure cycles were designed to study a wide range of curing pressures, curing temperatures, and two different vacuum-application durations, including vacuum vented at recommended pressure and vacuum held throughout the cure cycle. The results showed that the SBS strength did not vary significantly over a relatively wide range of curing temperatures and pressures if vacuum was vented at recommended curing pressure; however, after a certain point, a decreasing trend in the SBS strength was observed by reducing the curing temperature and pressure. The C-scan images of panels cured with the vacuum held throughout the cure cycles revealed presence of a high-porosity cross-shaped defect at the center of the panels. The observed defect became larger as the curing pressure decreased. The correlation between the SBS strength and the void content was studied using theoretical models and experimental data. The investigation of the failure modes for each panel showed a change in both the H/W and the RT/D failure mechanism as a result of variation in curing temperature and pressure.

Moisture absorption of unidirectional hybrid composites

Unidirectional hybrid composite rods were conditioned in humid air to investigate the sorption kinetics and the effects of moisture on mechanical and physical properties. Sorption curves were obtained for both hybrid and non-hybrid composite rods to determine characteristic parameters, including the diffusion coefficient ( D) and the maximum moisture uptake ( M ∞). The moisture uptake for the hybrid composites generally exhibited Fickian behavior (no hybridization effects), behaving much like non-hybrid composites. A two-dimensional diffusion model was employed to calculate moisture diffusivities in the longitudinal direction. Interfaces and thermally-induced residual stresses affected the moisture diffusion. In addition, the effect of hygrothermal aging on glass transition temperature ( T g ), short beam shear strength (SBS), and tensile strength was determined for hygrothermal exposure at 60 °C and 85% relative humidity (RH). Property retention and reversibility of property degradation were also measured. Microscopic inspection revealed no evidence of damage.

Recyclability and reutilization of carbon fiber fabric/epoxy composites

Solid carbon fiber/epoxy laminates manufactured by liquid resin infusion of twill fabric reinforcement are recycled in a bath of boiling sulfuric acid to separate the fibers from the matrix. The recycled reinforcement consists of long fibers arranged in a random, entangled mat. Using the same epoxy matrix and infusion materials and process, the recycled fibers are reutilized to manufacture solid laminates. The physical properties of the recycled laminates are evaluated by means of pulse-echo ultrasound, visual microscopy, and fiber volume content. The average fiber volume content of the recycled laminates is 33%, compared to the 62% of the twill laminates. The mechanical properties of the recycled composite include tension modulus and strength, compression modulus and strength, three-point bend flexure strength, and short beam shear strength. The properties are compared against the values of the twill reinforcement with quasi-isotropic stacking sequence. Results show that the recycled material offers promising elastic properties and strength values, similar to those of advanced carbon fiber sheet molding compounds, and therefore can be used as structural material.

Effect of filler on thermal aging of composites for next-generation power lines

The thermal aging of pultruded composite rods was investigated to determine the effects of filler on oxidation kinetics and degradation mechanisms. The unidirectional hybrid composite rods were comprised of a carbon-fiber core, a glass-fiber shell, and an epoxy matrix filled with clay particles. A reaction-diffusion model was implemented for each of the two hybrid sections to calculate the oxygen-concentration profile and the thickness of the oxidized layer (TOL) within the composite rods, and results were compared with measured oxidation kinetics. The TOL was measured for samples exposed isothermally in air and in vacuum at 200 °C for up to 13,104 h (1.5 year), and the measured values were similar to modeling predictions (within 10%). The domain validity for the reaction-diffusion model was determined from gravimetric experiments (weight-loss measurement), which showed that after prolonged thermal exposure, the degradation mechanism changed from thermal oxidation to thermal degradation. Thermogravimetric analysis (TGA) was performed to determine the thermal degradation and stability of the aged composite. In addition, the effect of thermal aging on glass transition temperature ( T g ) and short beam shear (SBS) strength was determined for isothermal exposures at 180 °C and 200 °C.

Thermal, rheological, and mechanical properties of a polymer composite cured at different isothermal cure temperatures

Thermal, rheological, and mechanical properties of a commercial carbon fiber epoxy prepreg, Cycom 977-2 UD, were obtained for isothermal cure temperatures ranging from 149°C to 182°C. For each cure profile, an encapsulated-sample rheometer (ESR) was used to measure the storage modulus. Each ESR cure profile was followed by the glass transition temperature ( Tg) test. The degree of cure ( α) during cure and the heat of reaction of the prepreg were obtained using a differential scanning calorimeter (DSC). Combined loading compression (CLC) and short-beam shear (SBS) tests were performed to obtain compressive properties and SBS strength, respectively. It was observed that the compressive properties did not vary significantly for the studied isothermal cure temperatures; likewise, the compressive failure mode was the same for all the CLC specimens. However, the SBS strength for the specimens cured at 149°C was approximately 10% less than that of those cured at isothermal cure temperatures ranging from 160°C to 182°C. Further, the failure mode of the SBS specimens cured at 149°C was also different from other specimens. The storage modulus of the ESR sample cured at 149°C also showed a 10% decrease compared to other ESR samples. The SBS strength exhibited a good correlation with the storage modulus and a weak correlation with Tg and α.

Multi-scale reinforcement of CFRPs using carbon nanofibers

In this paper, stacked-cup carbon nanofibers (CNF) were dispersed in the matrix phase of carbon-fiber-reinforced composites based on a high-performance epoxy system with and without modification by an elastomeric triblock copolymer (TCP) for increased toughness. The addition of the TCP provided an enhancement in toughness at the cost of a slight degradation in modulus and strength. The CNFs, on the other hand, provided significantly enhanced strength and stiffness in matrix-dominated configurations, including tension of quasi-isotropic composites and short beam shear strength of both quasi-isotropic and unidirectional composites. Scanning electron microscopy revealed enhanced adhesion between the matrix and carbon fibers with the addition of either TCP or CNFs. However, CNF agglomeration in the studied systems partially offset the energy dissipation processes brought about by the nanofibers, thereby limiting interlaminar fracture toughness enhancements by CNF addition. These results show good promise for CNFs as low-cost reinforcement for composites while offering insight into the codependent morphologies of multi-scale phases and their influence over bulk properties.

Characterization of Fourth-Generation High-Temperature Discontinuous Fiber Molding Compounds

Thermal, physical, and mechanical properties of discontinuous fiber molding compounds incorporating fourth-generation high temperature matrices and T650-35 carbon fiber were characterized as a function of matrix chemistry. These new, lower cost structural materials provide an alternative to traditional wet lay-up prepreg materials with out-of-autoclave processability, no volatile evolution during cure, and no toxicity. Measured maximum properties include a glass transition temperature of 456°C, flexural strength of 376 MPa, compression strength up to 236 MPa, compression modulus up to 30 GPa, and short beam shear strength of ∼50 MPa.