Residual Flexural Properties Research Articles

Post impact flexural behavior investigation of hybrid foam-core sandwich composites at extreme Arctic temperature

This study explores the post-impact bending behavior and failure mechanisms in hybrid sandwich composites made of Carbon Fiber Reinforced Polymer (CFRP) and Glass Fiber Reinforced Polymer (GFRP). Flexural tests conducted at both ambient room temperature and low temperature Arctic conditions reveal a significant enhancement in flexural performance when GFRP layer is incorporated on the outer side of the hybrid composite. The investigation utilizes images from testing to elucidate damage modes, including fiber and matrix cracking in the composite facesheet, as well as core shearing and debonding in the Polyvinyl Chloride (PVC) foam core. Residual flexural properties are notably influenced by stacking sequence, facesheet compressive properties, pre-existing impact damage and temperature conditions. Analytical predictions, validated experimentally, highlight the effect of stacking sequence, low temperature, and impact energy on flexural collapse modes, with competing failure modes such as indentation and core shear. Collapse maps indicate that room temperature specimens predominantly collapse through indentation, while diverse collapse mechanisms emerge due to facesheet thickness, rigidity, and degraded tensile strength. The study aims to provide fundamental insights for future composite designs tailored for Arctic applications.

Assessment of ply thickness and aluminum foils interleaving on the impact response of CFRP composites designed for cryogenic pressure vessels

Carbon fiber reinforced polymer (CFRP) proved to be the best choice to produce cryogenic pressure vessels coupling a reduced weight with superior mechanical performance. Despite this, CFRPs are prone to fuel leakage due to interconnected matrix cracks resulting from mechanical and thermal stresses. Two strategies were combined in this work to reduce fuel permeability, i.e., increase of plies number and metallic film interleaving, evaluating the cryogenic impact response of these new configurations. At first, the only ply thickness effect was assessed and the hybrid sandwich-like (SL) configuration with traditional CFRP (RC) skins and a thin-ply (TP) core displayed impact and residual flexural properties close to RC ones. Then, the effect of aluminum foils interleaving was investigated, and the interleaved SL configuration displayed higher residual flexural properties than interleaved RC for all temperatures, e.g., the residual flexural strength was 30.3 % higher at room temperature and 14.6 % higher at −70 °C, respectively.

Extreme temperature influence on low velocity impact damage and residual flexural properties of CFRP

AbstractIn this work, the behavior of carbon fiber reinforced polymer composites (CFRPs) interleaved with electrospun veils under low velocity impact (LVI) conditions and extreme environmental temperatures was investigated. 2/2 twill carbon fiber/epoxy laminates were subjected to LVI at three energy levels (10, 20, and 30 J), and three temperatures (−50°C, room temperature, and 100°C). Two interleaved configurations were explored (six veils placed symmetrically with respect to the middle plane of the laminate and with respect to the external layers of the laminate). Particularly at room temperature and up to 20 J, nanofibrous interlayers effectively reduced localized deformation (by about 13.0%) and delamination (by about 12.2%) when positioned in the outer ply interleaved configuration compared to the reference laminate. At 100°C, this effect is maintained at 10 J, preventing an increase in the delaminated area. At −50°C and 10 J, the promotion of delamination prevented back surface fiber failure. Regarding post‐impact flexural properties, the presence of nanoveils ensured superior mechanical properties compared to the corresponding reference laminate impacted at the same conditions, demonstrating their efficacy in enhancing the damage tolerance of the overall laminate.Highlights Electrospun veils were interleaved in 20 layers of 2/2 twill carbon/epoxy laminate. Three configurations were tested under LVI at 10 J, 20 J, 30 J, and −50°C, RT, and 100°C. Observed damage modes include delamination, indentation, and back surface fiber cracks. Veils symmetrically placed in external layers limit delamination at 20 J (RT) and 10 J (100°C). Electrospun veils enhanced CFRP bending and residual post‐impact properties at RT and 100°C.

Low-Velocity Impact and Post-Impact Residual Flexural Properties of Kevlar/EP Three-Dimensional Angle-Interlock Composites.

In this study, five three-dimensional angle-interlock fabrics with different warp and weft densities were fabricated using 1000D Kevlar filaments. The Kevlar/EP composites were prepared by vacuum-assisted molding techniques. The low-velocity impact property of the composite was tested, focusing on the effects of the warp and weft densities, impact energy, impactor shape, and impactor diameter. The damage area, dent depth, and crack lengths in the warp and weft direction were used to evaluate the impact performance, and the specimens were compared with plain-weave composites with similar areal densities. The dominant failure mode of the conical impactor was fiber fracture, while the dominant failure mode of the hemispherical impactor was fiber-resin debonding. The cylindrical impactor showed only minor resin fragmentation. The residual flexural strength of the composite after impact was tested to provide insights into its mechanical properties. The study findings will provide a theoretical basis for the optimization of the design of impact-resistant structures using such materials and facilitate their engineering applications.

Hybrid effects and failure mechanisms of carbon/Kevlar fiber composite laminates under the bending-after-impact loading

The hybrid fiber design concept is widely used in the aerospace industry to improve the mechanical properties (stiffness, strength, elongation, energy absorption ability) of composite laminates. In the present work, an experimental method was employed to investigate the hybrid effects on the failure mechanism of carbon-Kevlar hybrid fiber-reinforced polymer (HFRP) composite laminates under bending-after-impact (BAI) loading conditions. Carbon/Kevlar FRP specimens with five different hybrid ratios and three different stacking sequences were designed and fabricated by the compression molding method. A series of three-point bending (3 PB) tests and BAI tests were conducted to characterize the degradation of the residual flexural properties of the laminates under low-energy impacts. Among the laminates designed with different hybrid ratios, the [K3C3] structure combined the advantages of the high strength of carbon fibers and the high toughness of Kevlar fibers; thus, it exhibited excellent residual bending properties. Among the laminates designed with different stacking sequences, the outermost Kevlar fiber layer in the [KCC]S structure effectively protected the inner carbon fiber layers on the compressed and stretched sides; thus, it yielded excellent residual flexural properties.

Using z-pinning to improve the low-velocity impact performance of composite skin/stringer structures

This paper investigates the effect of the z-pin pre-hole inserted (ZPI) process on the impact resistance and post-impact flexural properties of skin/stringer composite structure with different z-pin parameters. The responses of the specimens are characterized in terms of impact denting & cracking length, load-carrying capacity, energy absorption and so on. Experimental results demonstrate that the z-pin cannot inhibit the initiation of debonding cracks at the skin/stringer interface. However, the z-pin forms a bridging traction zone across the cracking and prevent it from developing, which ensures the skin/stringer structure preserve higher peak loading and impact resistance. The specimens with the highest z-pin density have better impact resistance and outstanding residual flexural post-impact properties. Moreover, z-pins play a bridging role through its own failure. One is the inter-fiber debonding and splitting or shear failure of z-pins due to “snubbing effect,” the other is the sliding friction of z-pins and z-pins’ elastic deformation. These results could provide some guidance for designing z-pinning connection of composite skin/stringer structure.

Cell-filling reinforced materials for improving the low-velocity impact performance of composite square honeycomb sandwiches: Polymethacrylimide foam vs. aluminum foam

In this study, PMI foam and aluminum foam are used as reinforcement materials for filling the cells of carbon fiber-reinforced composite square honeycomb sandwiches (CSHSs), forming two new sandwich structures, namely, PMI foam-reinforced CSHS (PRCSHS) and aluminum foam-reinforced CSHS (AFRCSHS). The impact resistance and residual flexural performance of the composites is compared by using low-velocity impact (LVI) and three-point bending (3 PB) experiments. The different impact energy levels of 25 J, 50 J, 65 J, and 100 J are applied. The impact and bending failure process and failure mechanism are analyzed using a combination of industrial computed tomography (ICT) and scanning electron microscopy (SEM). PRCSHS and AFRCSHS are compared for various enhancement effects and their resulting lightweight features. The residual flexural and impact resistance properties of the two sandwiches improved significantly. Compared with CSHS, the maximum impact load of PRCSHS and AFRCSHS increased by 115.6% and 78.9%, respectively. Moreover, the impact energy absorption, residual flexural load, and residual flexural energy absorption increased by 30.9% and 21.3%, 80.7% and 58.5%, and 173.1% and 84.3%, respectively. The PRCSHS was noticeably lightweight, but AFRCSHS was not sufficiently lightweight. The qualitative results of comparison of the two reinforcement materials in this study can be extended to other carbon fiber-reinforced all-composite 2D honeycomb sandwiches.

Effect of fire exposure on the bonding behavior of hybrid engineered composite systems

When used with self-compacting concrete (SCC) in a fresh state, engineered cementitious composites (ECC) can enhance flexural properties and durability. There is, however, some uncertainty as to whether the composites and their related interfacial bonds will endure high temperatures. This article investigates how fire exposure affects hot-jointed concrete systems (CS) that combine SCC and ECC. Residual flexural and tensile properties and possible degradation/debonding of the interfacial zone after exposure to 800 °C were examined, considering the effect of three different fiber types (i.e. polyvinyl alcohol (PVA), steel (SF), and hybrid PVA/steel fibers) in the tension ECC layer. Visual, microstructural, ultrasonic pulse velocity and mass loss properties were assessed for air- and water-cooled specimens. Also, mathematical models were proposed based on the results of each concrete composite. ECC added to SCC at fresh-to-fresh state significantly increased its flexural and splitting tensile resistance, as well as thermal integrity. All composites were negatively affected by fire, but composite systems maintained their interfacial bond without excessive degradation. It was also determined that fibers at the interface area of composite systems are crucial to preserving their mechanical and physical bonding under fire, thus SF resulted in higher thermal resistance compared to PVA.

The use of pre-hole Z-pinning (PHZ) method to improve the impact resistance and post-impact flexural performance of composite skin/stringer joints

Carbon fiber reinforced plastic (CFRP) composites have been widely used in the last thirty years. The main issue in using CFRP composite joints is the bond strength between skin and stringer. This paper presents the results of an experimental investigation into the effect of pre-hole z-pinning technique (PHZ) on the impact resistance and post-impact flexural properties of skin/stringer composite joint with different z-pin configuration. Unpinned and pinned carbon/epoxy specimens are subjected to low-velocity impacts resulting in barely visible damage. The responses to impact of the laminates are characterized in terms of damage evolution, load-carrying capacity, and energy absorption. It is found that z-pinning maintains the integrity of the skin/stringer structure, and it improves the impact resistance and is capable of delaying the onset of delamination. Z-pin shifts the impact damage from interface debonding to matrix cracks on the impacted surface. Z-pin significantly improve the residual flexural properties. Skin crush occurs to the specimens with the highest z-pin Vol., as the bonding strength gets so improved that the weakness of the joint transfers from interface bonding strength to skin in-plane strength. These results can provide some guidance for designing z-pin reinforced joints.

Low-velocity impact and residual flexural properties of intraply hybrid laminates reinforced with glass and flexible fibers

In this study, Low-velocity impact and residual flexural properties of intraply hybrid epoxy composites reinforced with E-glass, aramid, high tenacity polyester (HTPES), and ultra-high molecular weight polyethylene (UHMWPE) fibers were investigated. Flexural properties of the samples were measured before and after applying a 5 J impact by performing the three-point bending test. The Charpy impact test was also performed on the samples to measure their impact resistance. Based on the results, the pure glass fiber composite had 85–112% higher flexural strength and 55–127% higher flexural energy than hybrid samples, but 18% and 27–34% lower flexural modulus than hybrid glass/aramid and glass/UHMWPE composites, respectively. However, hybrid samples were considerably less affected by the impact loads. The loss in flexural strength and flexural energy after the impact for hybrid samples were 2–20% and 2.5–34% and for pure glass sample 63% and 71%, respectively. Scanning electron micrographs of the fractured surface of samples after the Charpy impact showed that the flexible fibers can contribute to the impact strength of the composite with such mechanisms as elongation, pull-out, and folding.

Temperature effect on impact response of flax/epoxy laminates: Analytical, numerical and experimental results

Polymer composite materials are widespread in most industrial fields, such as automotive, civil engineering, marine, wind energy, packaging and sports, and their demand is increasing since they have the potential of combining high mechanical properties with the lightness of the components. However, due to their non-biodegradable nature, they are not easy to dispose of at the end of the components’ life cycle. To overcome this issue, vegetable fibers represent prospective substitutes to traditional reinforcements since they are able to provide good mechanical properties, together with low cost, renewability, and biodegradability. From the experimental point of view, the present work provides a complete characterization of the impact behavior of twill flax/epoxy composites in a wide range of temperatures and assesses the effect of the aforementioned impact temperatures on the residual flexural properties of these composites. Moreover, for the impact tests, analytical and numerical models are used to predict a threshold impact load for the damage onset and extent, and to draw an approximation of the load-displacement curve. Good agreement was found when comparing the derived analytical and numerical load curves with the experimental ones.

Impact Performance and Bending Behavior of Carbon-Fiber Foam-Core Sandwich Composite Structures in Cold Arctic Temperature

This study investigates the impact performance, post-impact bending behavior and damage mechanisms of Divinycell H-100 foam core with woven carbon fiber reinforced polymer (CFRP) face sheets sandwich panel in cold temperature Arctic conditions. Low-velocity impact tests were performed at 23, −30 and −70 °C. Results indicate that exposure to low temperature reduces impact damage tolerance significantly. X-ray microcomputed tomography is utilized to reveal damage modes such as matrix cracking, delamination and fiber breakage on the CFRP face sheet, as well as core crushing, core shearing and debonding in the Polyvinyl Chloride (PVC) foam core. Post-impact bending tests reveal that residual flexural properties are more sensitive to the in-plane compressive property of the CFRP face sheet than the tensile property. Specifically, the degradation of flexural strength strongly depends on pre-existing impact damage and temperature conditions. Statistical analyses based on this study are employed to show that flexural performance is dominantly governed by face sheet thickness and pre-bending impact energy.

Influence of moderate/high temperatures on the residual flexural behavior of pultruded GFRP

This work aims to investigate the influence of the exposure to moderate/high temperatures on the residual flexural properties of commercial pultruded profiles of glass fiber-reinforced polymer (GFRP) composite having different formulations. The experimental program included flexural and interlaminar shear tests for four different GFRP formulations at room temperature and after exposure to temperatures up to 320 °C. To investigate the materials’ thermal stability, thermogravimetric analyses (TGA) were performed and dynamic mechanical analyses (DMA) were carried out to evaluate the changes in the glass transition temperature. Scanning electron microscopy (SEM) and x-ray microtomography techniques were also used to analyze the microstructure and the damages after exposure to temperature. The results from bending tests showed that all but polyester-based composites showed a slight increase of modulus for intermediate temperatures, followed by a decrease as it approached the degradation temperature. All specimens failed in interlaminar shear due to porous interface between layers and a large deviation was obtained for the strength. For interlaminar shear properties also seemed to be influenced by the competing effects of post-curing and matrix degradation. For this latter test, vinyl ester matrix composite had the best performance whereas the polyester matrix composite had the lowest.

Effect of water absorption on the mechanical properties of bamboo/glass-reinforced polybenzoxazine hybrid composite

In this study, bamboo and glass fibers were successfully added into bisphenol A–aniline-based benzoxazine resin by hot pressing method. The effect of water absorption on the mechanical properties of the neat benzoxazine and its composites was studied. The composites prepared in this study has been proved to exhibit better water resistance than some conventional natural fiber/polymer composites due to good hydrophobicity of the benzoxazine matrix. From the mechanical tests, the tensile performance of both neat benzoxazine and the composites degraded seriously after prolonged immersion in water. However, neat benzoxazine and the composites maintained relatively higher residual flexural properties (residual mechanical properties refer to the ratio of strength and modulus of the saturated samples to that of the nonaged samples) after 20 days of water immersion. The effect of water molecules on the fracture surface morphologies of the composites was studied using a scanning electron microscope (SEM). The SEM images showed that the interfacial bonding between fibers and the matrix degraded seriously due to the attack of water.

Residual performance of HPFRCC exposed to fire – Effects of matrix strength, synthetic fiber, and fire duration

This study investigates the effects of matrix strength, synthetic fiber content, and heating duration on the explosive spalling resistance and residual compressive and flexural properties of high-performance fiber-reinforced cementitious composites (HPFRCCs). Three different matrix strengths, ranging from 100 to 180 MPa; four different volume fractions of synthetic fibers, i.e., polypropylene (PP) and nylon (Ny) fibers, ranging from 0% to 0.6%; and three different fire durations of 1, 2, and 3 h under the ISO standard fire curve were adopted. Scanning electron microscope (SEM) images were obtained to evaluate the microstructural states of HPFRCCs and a mercury intrusion porosimetry (MIP) analysis was conducted to determine pore size distribution characteristics before and after fire exposure. The test results indicate that the addition of PP and Ny fibers is effective at enhancing the explosive spalling resistance, and higher amounts are required for higher strength matrices. The compressive strength of HPFRCCs decreased by more than 75% after the fire (≥950 °C), and higher residual compressive strengths were obtained with shorter fire durations and higher amounts of added PP and Ny fibers. The toughness was decreased by the fire more than the flexural strength, and the residual flexural performance were enhanced with higher amounts of synthetic fibers and matrix strengths. In particular, the residual flexural performance significantly decreased when the fire duration increased from 1 to 2 h, and only a minor change was observed thereafter. The effectiveness of higher amounts of synthetic fibers at improving the residual performance diminished with longer fire durations.

Properties of Aging Pentachlorophenol-Treated Douglas-Fir Crossarms

Abstract Wooden crossarms play a major role in supporting electric distribution lines in North America, but relatively few data exist on their condition as they age. The residual capacity of Douglas-fir crossarms in service in western Oregon for 45 to 60 years was investigated. Arms were sampled for residual preservative retention, the presence of visible decay fungi, and residual flexural properties; these results were then compared with three nondestructive tools. A majority of the arms tested had preservative levels well below those required for new arms, but only one decay fungus was isolated, and only five arms removed and dissected had any evidence of visible internal decay. Moduli of rupture for the arms were generally below the minimum levels required by national standards, but most still retained at least 67 percent of this value. Nondestructive evaluation tools were generally poorly correlated with flexural properties, possibly because of the heavily weathered and checked exterior condition.

Flexural fatigue properties of a polyvinyl alcohol-engineered cementitious composite

The flexural fatigue properties of a recently developed polyvinyl alcohol-engineered cementitious composite (PVA-ECC) using local dune sand were investigated. Fatigue flexural tests were conducted under a four-point bending setup. Normal concrete beams were also tested as a benchmark. To determine the stress range versus fatigue life relationship, five and four stress ranges were applied to the PVA-ECC and the normal concrete specimens, respectively. It was found that, compared with normal concrete specimens, the PVA-ECC specimens showed much superior flexural fatigue performance in terms of fatigue strength and deformation capacity, with a more ductile failure mode. Furthermore, typical fatigue performance indicators of the PVA-ECC specimens, including evolution of the midspan deflection, total crack mouth opening displacement, crack width and crack depth of the critical crack that led to final failure, with number of cycles as well as total dissipated energy, were recorded and compared with those of normal concrete specimens. It was observed that the PVA-ECC specimens tested under a stress range of 3 MPa (32% of the flexural strength) did not fail by fatigue after 5·6 million cycles; subsequent static tests demonstrated that they still had good residual flexural properties, including residual flexural strength, deformation capacity, stiffness and energy absorption capacity.

Impact and bending analysis of composite scarf adhesive joints modified with MWCNTs at room and hot temperatures

Scarf adhesive joints (SAJs) have attracted an increasing attention in joining/repairing of carbon-fiber reinforced epoxy (CFRE) composite structures due to their zero eccentricity, which provides better aerodynamic surfaces. This study evaluated for the first time, the performance of SAJs in CFRE composites under thermo-mechanical impact loads. The adhesive was modified with optimum percentage of multi-walled carbon nanotubes (CNTs). The impact tests were performed at room temperature of +25°C, +50°C and +75°C. The residual flexural properties of the unfailed impacted joints were measured using three-point bending test. Results from impact tests at temperature of 25°C, 50°C and 75°C showed improvement in the impact bending stiffness of the modified SAJs with CNTs by 8.3%, 7.4% and 11.8% and maximum contact force by 15.6%, 21.3% and 18.9%, respectively. The energy at failure of the CNT-SAJs was increased by 15.2% and 16.4% respectively at temperatures of 25°C and 50°C. At test temperature of 75°C the SAJs have hysteresis load-displacement behavior and energy-time curve with rebound energy of 35% and absorbed (damage) energy of 65%. The residual flexural strength of the modified and unmodified SAJs is 98.2% and 86.1% respectively, while their residual moduli have remarkable decrease to 71.7% and 81.3%.

Experimental analysis of composite scarf adhesive joints modified with multiwalled carbon nanotubes under bending and thermomechanical impact loads

Scarf adhesive joints have attracted an increasing attention in joining/repairing of carbon fiber reinforced epoxy composite structures due to their zero eccentricity, which provides lower stress distribution across the adhesive layer and better aerodynamic surfaces compared to other bonded joints. The main objective of this study is to evaluate the performance of the scarf adhesive joints in carbon fiber reinforced epoxy composites under thermomechanical impact loads, which is very important for the aerospace and automotive industries. The adhesive was modified with optimum percentage of multiwalled carbon nanotubes. The impact tests were performed at 25 ℃, 50 ℃, and 75 ℃. The residual flexural properties of the unfailed impacted joints were measured using three-point bending test. Results from impact tests at 25 ℃, 50 ℃, and 75 ℃ showed improvement in the impact bending stiffness of the modified scarf adhesive joints by 8.3%, 7.4%, and 11.8% and maximum contact force by 15.6%, 21.3%, and 18.9%, respectively. The energy at failure of the modified scarf adhesive joints with multiwalled carbon nanotubes was improved by 15.2% and 16.4% respectively at 25 ℃ and 50 ℃. At test temperature of 75 ℃, the scarf adhesive joints have hysteresis load–displacement behavior and energy–time curve with rebound energy of 35% and absorbed (damage) energy of 65%. The residual flexural strength of the modified and unmodified scarf adhesive joints is 98.2% and 86.1% respectively, while their residual moduli have remarkable decrease to 71.7% and 81.3%.

Residual flexural properties of CFRP sandwich structures with aluminum honeycomb cores after low-velocity impact

This paper aims to evaluate the influences of impact-induced damage on the residual flexural strength of honeycomb core sandwich panels with different structural configurations by combining the experimental, numerical and theoretical methods. Low-velocity impact tests and three-point bending tests after impact are carried out to determine and quantify the effects of structural configuration and impact energy on the impact damage and residual flexural strength of such structures. Subsequently, an integrated FE model with the VUMAT subroutine is developed to further investigate the damage states and failure mechanisms for the impact and bending simulation. The numerical results match well with the experimental ones in terms of impact load, absorbed energy, residual flexural strength and failure mechanisms. Results indicate that increasing cell wall thickness or decreasing side length of honeycomb core has significant effects on peak load, while increasing core height has little effect. Specimens with lower core stiffness fail through core buckling and crushing under the bending load, while specimen with higher core stiffness fails by top face sheet fracture. The residual flexural strength reduces markedly even through the impact damage is barely visible, indicating that it has a strong correlation with impact energy and structural configuration of cores.