Unidirectional Specimens Research Articles

Experimental and numerical study on polypropylene fibers reinforced concrete slabs with corroded reinforcement under monotonic loading

The degradation of structural performance due to reinforcement corrosion in reinforced concrete (RC) structures and the subsequent measures to improve the mechanical properties of corroded RC structures have garnered significant interest. This study investigates the structural performance of corroded polypropylene fiber RC slabs through both experimental and numerical approaches. Specimens were subjected to accelerated corrosion via electrochemical methods to simulate rebar corrosion. Key experimental parameters include polypropylene fiber volume fractions of 0% and 3%, design corrosion levels of 0%, 5%, and 10%, and both one-way and two-way constraints. The Digital Image Correlation (DIC) technique was employed for data acquisition. Load, deflection, strain, and crack morphology data were collected during loading. Results indicate that reinforcement corrosion increases the maximum crack width during loading, while a 3% polypropylene fiber volume fraction reduces this width by 20% to 48% at the same load level. Polypropylene fibers also enhance the peak loads and corresponding deflections of unidirectional and bidirectional specimens, with a more pronounced increase in unidirectional specimens of 11.3% and 15.4%, respectively. Although corrosion reduces the load-carrying capacity of the specimens, the inclusion of 3% polypropylene fibers significantly mitigates this reduction. A numerical model, incorporating rebar corrosion and polypropylene fibers through appropriate material constitutive models, was developed and validated against experimental results. The model accurately simulates the mechanical behavior of polypropylene fiber RC slabs with corroded reinforcement.

Tensile properties of carbon fabric-reinforced cementitious matrix (FRCM) at high temperatures

Despite many advantages of externally bonded FRP systems, the rapid loss of strength at high temperatures is a concern for fire scenarios. Fabric-reinforced cementitious matrix (FRCM) is deemed to perform superior at high temperatures. In this study, 84 carbon FRCM coupons were tested in tension at temperatures from 25 to 400 °C using both steady-state and transient-state test protocols. The main parameters considered in this study are the number of fabric layers and FRCM thickness, fabric orientation, target temperatures, and heating protocol. Results showed that at room temperature, the ultimate strength and cracked elastic modulus decreased by about 18% to 20% when the number of layers was increased from one to three. At 400 °C, the ultimate strength and cracked elastic modulus were reduced by up to 60 to 70% of their original values. The bidirectional FRCM specimens presented a slightly higher elastic modulus and strength than unidirectional specimens in most tests. In transient-state tests and under the same sustained load level, the failure temperature of 30 mm-thick specimens were approximately 100 °C higher than 20 mm-thick specimens. The test results showed that the carbon FRCM composites tested in this study were able to resist non-negligible tensile forces at elevated temperatures although they are significantly reduced.

The application of cycle merging and an extension of a fatigue spectrum simplification methodology from unidirectional to woven composite materials

This work details the application of cycle merging on a fatigue spectrum applied to a unidirectional specimen. The specimen design has previously been the subject of a ground up approach to simplify a loading spectrum using truncation. The extension of this methodology to a woven specimen is also detailed with truncation demonstrating its ability to remove low damaging cycles, achieving a reduction of up to 74%. The combined application of truncation and cycle merging to achieve a 95% load spectrum reduction is also presented, with reduced spectra demonstrating similar experimental scatter in fatigue tests to the original uncompressed load spectrum.

Tensile properties of FDM 3D-printed wood flour filled polymers and mathematical modeling through classical lamination theory

PurposeThe architecture of 3D-printed parts made through fused deposition modelling (FDM) with raster infill resembles that of composite laminates. Classical lamination theory (CLT), the simplest model for composite laminates, has been proved successful for describing the stiffness properties of FDM parts, while strength modeling so far has been limited to unidirectional lay-ups. The aim of this paper is to show that CLT can be used to predict also FDM part failure.Design/methodology/approachWood flour-filled polyester has been chosen as a model material. Unidirectional specimens oriented at 0°, 90° and ± 45° have been first characterized in simple tension to obtain the properties of the single layer. Next, two quasi-isotropic lay-ups, possessing different layer sequences, have been tested again in simple tension for CLT validation.FindingsThe measured properties are in good agreement with theoretical predictions, both for stiffness and strength, and an even better agreement can be achieved if a correction for taking the contour lines into account is implemented.Originality/valueThe paper shows that also the tensile strength of FDM parts can be predicted by using a mathematical model based on CLT. This opens up the possibility of using CLT for studying optimization of raster filled lay-ups, for example in terms of the best raster angles sequence, to better resist applied external loads.

Continuous Fiber-Reinforced Aramid/PETG 3D-Printed Composites with High Fiber Loading through Fused Filament Fabrication

Recent development in the field of additive manufacturing, also known as three-dimensional (3D) printing, has allowed for the incorporation of continuous fiber reinforcement into 3D-printed polymer parts. These fiber reinforcements allow for the improvement of the mechanical properties, but compared to traditionally produced composite materials, the fiber volume fraction often remains low. This study aims to evaluate the in-nozzle impregnation of continuous aramid fiber reinforcement with glycol-modified polyethylene terephthalate (PETG) using a modified, low-cost, tabletop 3D printer. We analyze how dimensional printing parameters such as layer height and line width affect the fiber volume fraction and fiber dispersion in printed composites. By varying these parameters, unidirectional specimens are printed that have an inner structure going from an array-like to a continuous layered-like structure with fiber loading between 20 and 45 vol%. The inner structure was analyzed by optical microscopy and Computed Tomography (µCT), achieving new insights into the structural composition of printed composites. The printed composites show good fiber alignment and the tensile modulus in the fiber direction increased from 2.2 GPa (non-reinforced) to 33 GPa (45 vol%), while the flexural modulus in the fiber direction increased from 1.6 GPa (non-reinforced) to 27 GPa (45 vol%). The continuous 3D reinforced specimens have quality and properties in the range of traditional composite materials produced by hand lay-up techniques, far exceeding the performance of typical bulk 3D-printed polymers. Hence, this technique has potential for the low-cost additive manufacturing of small, intricate parts with substantial mechanical performance, or parts of which only a small number is needed.

Prediction of tensile strength of fused deposition modeling (FDM) printed PLA using classic laminate theory

The application of Fused Deposition Modeling (FDM) is restricted due to limited information about the mechanical properties of printed parts. Therefore, it is required to determine the mechanical properties of the FDM properties to avail the full benefit of the FDM process. In the present study, Classic Laminate Theory (CLT) has been employed at the different configurations of layer thickness and raster width. The required elastic constant of material for CLT has been experimentally obtained through FDM printed Polylactic Acid (PLA) unidirectional specimens at 0°, 45° and 90° for different combinations of layer height and raster width. For these different combinations of layer height and raster width, constitutive models were developed to predict the tensile properties of the PLA parts. Tensile strength of the FDM printed bi-directional specimens has been experimentally obtained to validate the proposed CLT model results. The experimental tensile strength data is in good agreement with the data predicted by the proposed CLT model. Higher tensile strength and modulus were achieved with 0° raster angle compared to 90° raster angle. In the case of a bi-directional printed specimen, higher tensile strength was obtained with 45°/-45° raster angle followed by 30°/-60° and 0°/90° raster angle.

Mechanical and dynamic responses of unidirectional/woven carbon fiber reinforced thermoset and thermoplastic composites after low velocity impact

It is highly important to determine how mechanical and dynamic properties of composite materials will change after impact loads considering the coupled effects of composite design parameters. For these reasons, three-point bending and vibration tests have been carried out for the carbon fiber reinforced thermoset and thermoplastic composites with various stacking sequences before and after low velocity impact, and it is expected that these results achieved from the current study will be beneficial for applications where high damping and impact resistance are demanded together. In this context, vibration tests were carried out under free-free boundary conditions, and their natural frequencies, flexural moduli and structural damping were obtained. Furthermore, three-point tests were conducted in the elastic region with 1 mm/min crosshead speed using a universal test machine, and thus flexural moduli of the composite specimens were obtained. The results were validated by comparing the flexural moduli obtained from the both vibration and three-point bending tests, found to be reliable and comparable. As a result of the current study, it was concluded that woven fabric reinforced composite specimens exhibited 50% higher specific damping capacity (SDC) but 70% lower flexural modulus than unidirectional specimens thanks to biaxially fiber alignment. On the other hand, specific damping capacities of the thermoset and thermoplastic composites with different stacking sequences have been examined, and it was observed that thermoset specimens exhibited unexpectedly 192% higher SDC compared to the thermoplastics. This was interpreted as even though thermoplastics are normally expected to exhibit more damping than thermosets, stacking sequence being more effective on damping responses. Apart from that, although there were slight changes in material properties due to degradation in structural integrity after 2 m/s and 3 m/s low-velocity impacts, it was not found to be significantly effective due to the limited damage areas.

Influence of interface ply orientation on delamination growth in composite laminates

The standard experimental procedures for determining the interlaminar fracture toughness are designed for delamination propagation in unidirectional specimens. However, in aerospace structural components, delamination usually occurs between plies at different orientations resulting in different damage mechanisms which can increase the value of the fracture toughness as the delamination propagates. Generally, numerical analyses employ the value measured at the delamination onset, leading to conservative results since the increase resistance of the delamination is neglected. In this paper, the fracture toughness and the R-curves of carbon/epoxy IM7/8552 are experimentally evaluated in coupons with delamination positioned at 0°/0° and 45°/−45° interfaces using Double Cantilever Beam (DCB) and Mixed-Mode Bending (MMB) tests. A simplified numerical approach based on the Virtual Crack Closure Technique (VCCT) is developed to simulate variable fracture toughness with the delamination length within a Finite Element code using a predefined field variable. The results of the numerical analyses compared with the experimental data in terms of load-displacement curves demonstrate the effectiveness of the proposed technique in simulating the increase resistance in delamination positioned between plies at 45°/−45° interface.

Effect of High Velocity Deformation on Strength of ArmoredComposite Materials

This paper considers the problem of determining the strength of carbon fiber reinforced plastic with straight and curved fibers under high-speed loading. High-speed tests of unidirectional CFRP specimens with rectilinear and wavy structure have been carried out. The influence of the structure and high-speed loading on the ultimate strength and ultimate deformation of the material is investigated. For the first time, a detailed study of the effect of fiber curvature on the properties of CFRP under high-speed deformation has been carried out. As a result of dynamic tests, it was shown that the ultimate strength in unidirectional laying is higher than in wavy laying. The effect of increasing the ultimate deformations of specimens with bent fibers was established, which was noted earlier for the case of tensile tests.

A Versatile Methodology for the Fatigue Life Prediction of Unidirectional Carbon Fiber-reinforced Plastic Specimens by the Micro-stress Analysis of Resins

A Versatile Methodology for the Fatigue Life Prediction of Unidirectional Carbon Fiber-reinforced Plastic Specimens by the Micro-stress Analysis of Resins

The Off-Axis IBII Test for Composites

Composite components regularly experience dynamic loads in service. Despite this, it is still difficult to obtain accurate mechanical properties of composite materials under high strain rate conditions. In this study, a new application of the Image-Based Inertial Impact (IBII) test methodology was developed, to generate an accurate in-plane transverse and shear moduli dataset from unidirectional (UD) off-axis composite specimens. The obtained dataset was consistent across different sample configurations, where results from the UD45^{circ } off-axis specimens agreed well with the UD90^{circ } values. Validation of the shear modulus identification was also undertaken by comparing the results from the UD90^{circ } and UD45^{circ } specimens with a multi-directional (MD) configuration. Here, it was found that MD±45^{circ } specimen shear modulus values where marginally lower than that from the UD specimens, in accordance with the lower fibre volume fraction of the MD laminate. Low strain rate sensitivities in the 0.5-2times10^{3} hbox {s}^{-1} regime evidenced in this work suggest previously published data (often from split-Hopkinson bar tests) may include both a material and system i.e. testing apparatus response.

Influence of Temperature and Moisture on the Compressive Strength of Carbon Fiber Reinforced Polymers

The effect of moisture absorption and high temperature on the compressive strength of unidirectional IM7/977-2 carbon/epoxy resins have been investigated experimentally. The specimens were divided into 4 groups, and tested under 4 different conditions by varying the testing temperature and moisture parameters. The fiber orientation selected were 0o, ±45o and 90o. The reported results showed that the compressive strength, fracture load and compressive modulus of the specimens were degraded under the influence of moisture absorption and high temperature. The largest compressive strength degradation was observed in the unidirectional specimens. Furthermore, the most severe case was noted for the specimens that were immersed in water and tested at 80°C. The observed reduction in the strength varies depending on the fiber orientation, immersion time and test temperature. The results indicate the importance of considering environmental parameters in designing the composite structures for compression loadings.

Study of the Fatigue Behavior of the Unidirectional Zylon/epoxy Composite Used in Pulsed Magnets

The fatigue behavior of the Zylon/epoxy composite has been tested under quasi-static tensile and tensile fatigue loads. The distribution of the fatigue life at different stress was determined. The results show that the statistical characteristic of the static strength and the fatigue life can be satisfactorily described by the normal, lognormal or Weibull distribution model. The measured S-N curve exhibits a pronounced nonlinear behavior. Strains of the unidirectional specimens were monitored with a mechanical extensometer during the test. Three distinct stages of the strain evolutionary process were observed. The evolution curves show a sharp increase in the first stage, an approximately linear slow increasing in the middle stage and an unstable acceleration in the third stage before failure. Similar to the strain evolution, the three-stage stiffness of degradation was also observed. The maximal cycle strain is chosen as the indicator of damage evaluation. The experimental data of the damage parameters are fitted with the existing analytical model. The presented models have a good agreement with the tested results.

Numerical tool with mean-stress correction for fatigue life estimation of composite plates

In this paper a numerical fatigue assessment method is presented for the cyclic life estimate of composite plates via continuous strength and stiffness degradation calculation combined with mean stress correction and multiaxial fatigue failure criterion check. The most value-adding features of this methodology are the simultaneous monitoring of mean stress dependent strength and stiffness degradation over successive cycles using empirical interaction type degradation rules. The procedure has also been coded in a form of a complex automated workflow where the actual stress state for each ply is evaluated by appropriate Finite Element assessment and the residual strength and the ply-based stiffness analysis combined with failure check is executed in an external Python routine. The practical use of the automated workflow is presented via the virtual analysis of load- and strain-controlled tension-tension type fatigue tests of unidirectional specimens as well as a cross-ply open-hole test piece. Based on the results the expected damage mechanism is discussed.

In-situ observation of damage in unidirectional CMC laminates under tension

We observe microscopic damage progression in un-notched unidirectional SiC–SiC ceramic matrix composite laminates loaded monotonically in tension. In situ tensile testing was conducted in air at room temperature and combined with X-ray tomography imaging. The unidirectional specimens tested were novel double-reduced dogbones designed to develop matrix cracks in the synchrotron field of view. Multiple matrix cracks that rapidly traversed the section of each specimen were observed through the volume until the specimens broke into two pieces. The details of each matrix crack were tracked at each stress increment to better understand how the cracks evolve under load. One of the unidirectional specimens contains a matrix crack that displays a sharp 90° turn along the fiber direction in its final crack path. Fiber breaks were observed and several fibers in the unidirectional specimen showed multiple breaks. Fiber break opening was measured for each fiber break and the average fiber break opening for each stress increment is reported. The location and pullout distance of fiber breaks surrounding a matrix crack, as well as matrix crack spacing, are of modeling interest and are reported.

Experimental Study of Stepped-Lap Scarf Joint Repair for Spar Cap Damage of Wind Turbine Blade in Service

The objective of this paper was to design configuration parameters for a stepped-lap scarf joint repair, which can be used for spar cap damage of a wind turbine blade in service and to realize the post-repair monitoring. Two experimental studies were included. First, tensile test for the unidirectional tape specimens with a large aspect ratio repaired using a multiple stepped-lap scarf joint method was carried out. The results showed that the reinforcement layer could effectively improve the load-carrying capacity of the repaired zone. The stepped-lap joint surface was identified as the weak part of the spar cap repair, which should be monitored. Second, by embedding carbon nanotube buckypaper sensors on the stepped-lap joint surface of the repaired specimens, quasi-static tensile tests and fatigue tests were carried out. According to the resistance response of the sensors, the quasi-static tensile test confirmed the failure processes, namely the stiffness turning point, damage evolution, crack propagation, and fracture. The fatigue test could accurately identify the progressive failure, namely the initial damage, damage accumulation, initial cracking, and crack propagation to structural failure. The above tests provided an important configuration parameter basis for evaluating the spar cap repair scheme and presented a promising method for the health monitoring of a spar cap after repair.

A new FE modeling procedure to investigate the effects of toughening mechanisms on the fracture toughness of laminated composites

The aim of this study is to estimate the mode-I critical strain energy release rate (GIc) extracted from a cross-ply laminated composite with 0//90 interface by using the GIc extracted from a unidirectional laminated composite with 0//0 interface. It is claimed that the GIc is a material property and toughening mechanisms such as fiber bridging and zigzag crack propagation acting in the fracture process zone (FPZ) play a prominent role in the change of this value from unidirectional (UD) laminated composites with 0//0 interface to cross-ply ones with 0//90 interface. For this purpose, a bilinear cohesive law is employed to predict the initiation and propagation values of the fracture toughness. Toughening mechanisms are then included in the finite element analysis using a new modeling procedure. It is revealed that the fiber bridging and the zigzag crack propagation account for 230% and 10% of the increase in GIc from unidirectional specimens to cross-ply ones, respectively. Finally, a tri-linear cohesive law is implemented in order to predict the toughening behavior of the cross-ply specimen and achieve lower runtimes.

Energy dissipation characteristics of additively manufactured CNT/ABS nanocomposites

PurposeThis study aims to discuss the effect of carbon nanotubes (CNTs) on the mechanical properties of acrylonitrile–butadiene–styrene (ABS) composites fabricated by additive manufacturing (AM). Insight into the energy-dissipation mechanisms introduced and/or enhanced by the addition of CNTs is presented in this study.Design/methodology/approachABS/CNT filaments were fabricated with different concentrations of CNTs. Using a fused deposition modeling approach, unidirectional specimens were printed using a MakerBot Replicator 2X (MakerBot Industries, Brooklyn, NY, USA). Specimens were tested under static and dynamic conditions, with the loading coinciding with the printing direction, to determine elastic modulus, strength and viscoelastic properties.FindingsA CNT reinforcing effect is evident in a 37 per cent increase in elastic modulus. Likewise, the strength of the composite increases by up to 30 per cent with an increase in weight fraction of CNTs. At low dynamic strain amplitudes (0.05 per cent), a correlation between dissipated strain energy of the butadiene phase and strength of the composite is found such that less dissipation, from constraint of the butadiene particles by the CNTs, leads to higher strength of the composite. At higher dynamic strains, the presence of a high concentration of CNT leads to increased energy dissipation, with a maximum measured value of 24 per cent higher loss factor compared to baseline specimens. Because the trend of the composite behavior is similar (with a higher absolute value) to that of neat ABS, this study’s results indicate that well-established polymer/CNT dissipation mechanisms (such as stick-slip) are not significant, but that the CNTs amplify the dissipation of the ABS matrix by formation of crazes through stress concentrations.Originality/valueThis study provides knowledge of the dissipation behavior in additively manufactured ABS/CNT composites and provides insight into the expansion to new printable materials for dynamics applications.

The effect of the addition of carbon nanoparticles and methods for combining them with an epoxy matrix on the properties of polymeric composite materials

The influence of the methods of introducing carbon nanoparticles into the epoxy binder and their amount on the complex of physicomechanical characteristics of unidirectional organic and carbon plastics was evaluated. An increase in the breaking load in microplastic made of carbon filaments up to 86% due to the introduction of nanoparticles is shown. An increase in the compressive strength of unidirectional carbon-fiber specimens modified with nanoparticles using a wave machine, up to 32% in comparison with unmodified, has been established.

Generation of mixed mode I/II failure criteria from MMB specimens: an experimental study

Characterization of resistance to delamination is of great importance in selection of composite material. Mixed mode delamination is one of the most serious failure modes in aerospace composites. This article examines the delamination behaviour of mixed mode bending (MMB) test specimens made of glass/epoxy and carbon/epoxy composites. Literature published test data on different composites have been utilzed to identify the best suited failure criterion. Benzeggagh-Kenane (B-K) criterion as well as the polynomial criteria are used to generate failure envelopes from the fracture toughness data of composite specimens. Experiments were conducted on unidirectional fiber reinforced epoxy MMB specimens to analyze the failure trend of laminated composites and to validate the proposed polynomial criteria. Present investigations reveal that both B-K and polynomial criteria fit well for unidirectional specimens and cubic polynomial criterion fits well for angle-ply specimens.