Longitudinal Tensile Properties Research Articles

Study of the effect of void defects on the mechanical properties of fiber reinforced composites

This study is to investigate the mechanical properties of fiber-reinforced polymer (FRP) composites considering void defects by adopting a representative volume element model. The results reveal that initial damage always sprouts at the interface, the void defects affect the location of the initial damage and are the dominant factor in determining the stress required for damage sprouting. In addition, the void defects as well as the neighboring debonding interfaces together determine the direction of the main damage evolution zone. The transverse tensile properties are most affected by the void defects, while the longitudinal tensile properties are least affected. In particular, the strength is more sensitive to void defects than that of elastic constants. In addition, the mechanical properties of high Fiber Volume Fraction (FVF) FRP composites are significantly stronger affected by void defects than those of low FVF FRP composites.

Experimental and numerical study of the thermomechanical properties of flexible self-reinforced polyimide composite membrane

Polyimide membranes are favored in the aerospace field for their excellent comprehensive properties, but new application requirements demand higher strength, modulus and thermal expansion properties. Here, self-reinforced polyimide composite membranes (SRPICM) with varying fiber tow arrangement densities were fabricated by unidirectional reinforcement of polyimide fiber tows to enhance the thermomechanical properties of the membranes while maintaining the characteristics of low thickness and flexibility. A micro-scale representative volume element model with interface, and a macro-scale model containing cracks were developed based on the membranes' morphology to investigate the tensile and thermal expansion behavior of SRPICM. Experimental and numerical analyses demonstrated that fiber tows significantly improved the longitudinal tensile properties of SRPICM, with maximum increases in modulus and strength to 34.02 GPa and 1.26 GPa, respectively, over 13 times those of pure PI membranes. Further, the test results, combined with the two-scale finite element model revealed the evolution of longitudinal and transverse tensile deformation and thermal expansion behavior of SRPICM. The validity of the two-scale model was confirmed by experimental results, attributed to the practical consideration of interfacial bonding, prefabricated cracks and thermal residual stress effects during initial modeling. Notably, SRPICM with 10 fiber tows/20 mm arrangement density exhibited excellent longitudinal tensile properties (modulus and strength of 14.01 GPa and 470.89 MPa, respectively) and a longitudinal thermal expansion close to zero (0.03 μm/m°C), making it an ideal material for aerospace applications.

Deformation strengthening of uranium and dilute uranium alloys

The strength and ductility of uranium and dilute uranium alloys can be substantially improved by deformation strengthening. The yield strength of unalloyed uranium can be nearly doubled by rolling in the vicinity of 250 °C. The embrittling effect of hydrogen is also overcome, eliminating the need for vacuum outgassing to obtain good ductility. Significant differences in longitudinal versus transverse tensile properties result, in part, from texture introduced by unidirectional rolling. Warm rolling of U-2.3 %Nb results in both traditional strain hardening and a significant Bauschinger effect, resulting in differences between tensile and compressive yield strength. A similar combination of strain hardening and Bauschinger effect also occurs in deformation strengthened U-0.75 %Ti. Warm rolling of U-0.75 %Ti results in significantly better combinations of tensile yield strength and ductility than can be obtained by conventional age hardening. Warm rolling prior to aging also reduces the loss of ductility which typically accompanies conventional age hardening. An approach is developed in which the effects of prior deformation on tensile and compressive yield strengths are analyzed in terms of texture, long-range dislocation, and Bauschinger effects. The Bauschinger effect is shown to vary in sign with the type of deformation, thus providing opportunities to tailor tensile and compressive properties.

Longitudinal tensile analysis of carbon fiber/epoxy composites under the coupling effect of high and low temperature cycle-humidity-bending load

The coupling effects of high and low temperature-humidity-applied load on the longitudinal tensile mechanical properties and the durability performance of epoxy resin-based carbon fiber reinforced composites (EP-CFRP) are studied in this paper. It considers two high and low alternating temperature ranges [−40°C∼40°C]/[−40°C∼25°C], two humidity conditions (soaking in water and anhydrous), and three load levels of unstressed state or 30% and 60% of the ultimate load. The results indicate that all these three factors have a significant impact on the durability of EP-CFRP. The tensile strength varies with the high and low temperature alternating cycle, showing a trend of first decreasing, then increasing, and then decreasing; however, the peak and valley values appear in the quite different alternating cycle. The coupling effects of these factors have less influence on the tensile modulus. The microcracks generated at the interface between the resin matrix and the fiber have been proved to be the main reason for the strength reduction at the later stage. The coupling effect of humidity and load promotes the expansion of cracks and exacerbates the damage to EP-CFRP. Based on the cumulative damage theory, the residual strength damage model of EP-CFRP under the three-factor coupling action of “high and low temperature cycling-humidity-load” is calibrated by nonlinear fitting method.

Experimental evaluation of longitudinal tensile properties of ferritic stainless-steel weldment joined by metal inert gas, pulse metal inert gas, and tungsten inert gas welding

In this work, ferritic stainless steel (FSS) weldments were prepared by tungsten inert gas (TIG), metal inert gas (MIG), and pulsed-MIG welding. Pure argon and a mixture of argon + 2% oxygen were utilized as shielding gas in each type of welding. The FSS (ER430) and austenitic stainless steel (ASS) (ER309L) wires were used as filler consumable wires during all the three types of welding. The effects of varying welding processes, shielding gases, and consumable electrodes on the longitudinal tensile properties of weldments were investigated. The universal tensile testing machine was used to evaluate longitudinal ultimate tensile strength and percentage elongation experimentally in each case. The maximum tensile strength of FSS weldment was obtained in the case of (Ar + 2% O2) mixture of shielding gas and ASS filler wire in each welding process. Heat treatment was also carried out for samples prepared by MIG and TIG welding. The reduction in tensile strength and enhancement in percentage elongation was observed for both MIG and TIG welding after heat treatment.

Effect of irradiation swelling on the mechanical properties of unidirectional SiC/SiC composites: A numerical investigation at microstructural level

In order to apply SiC/SiC composites in nuclear systems, it is essential to understand the potential effects of dimensional changes induced by neutron irradiation on property degradation. A microscale model has been developed to predict the mechanical behaviour of irradiated fibre composites, including matrix cracking and interface debonding. The present work investigated the states of residual stresses and damage that may be induced by swelling mismatch between fibres and matrix, and their subsequent effects on the transverse and longitudinal tensile mechanical properties of composites. Unidirectional (UD) composites with various fibre contents and porosities were subject to different swelling mismatch to simulate irradiation-induced dimensional changes. The composites were then virtually tested in tension to determine the modulus and strength. The focus of the present work is the transverse properties, and some illustrative results for the longitudinal behaviour are also presented. The sensitivity of the composites’ properties to irradiation swelling was affected by the fibre volume fraction, and not by the pore volume fraction, though the porosity dramatically affected the initial unirradiated properties. The model correctly describes experimental trends reported in the literature, and a simple optimisation of the model parameters is demonstrated by the successful simulation of experimental data for tensile loading of a mini composite specimen.

High performance natural fiber composites from mat and UD flax reinforcements backed with a mat Binder: A study of mat fiber surface fibrillation

The natural fiber composites (NFC) market is growing at fast rate in markets such as automotive interiors, construction and wind energy. The major driving force is the rise in demand for lightweight and environmentally sustainable materials. This research aims at developing high performance NFC, made from hybridizing short flax fibers and unidirectional flax filaments in the fabrication of mat and combined UD flax-mat reinforcement, by studying the mechanical surface fibrillation of the short flax fibers to improve the fiber–matrix load transfer. The results show an increase in the longitudinal tensile properties up to about 500 mill revolutions, after which a deterioration of the fiber structure occurs with corresponding reductions on properties. However, fibrillation reduces permeability to liquid resin, except in the transverse direction of UD flax-mat reinforcement. Globally, the mechanical fibrillation has a positive impact on mechanical properties and fracture behavior while impregnation can be further improved to ease manufacturing.

Understanding the effect of hygroscopic cycling on the internal stress and stiffness of natural fibre biocomposites

This article investigates the development and the evolution of hygroscopic stresses within flax/ Polypropylene + Maleic Anhydride grafted Polypropylene (PP + MAPP) asymmetric biocomposites when exposed to various hygroscopic cycles that simulate a wide range of outdoor applications with large Relative Humidity (RH) variation (from 11 to 50% RH, 50–99% RH, 11–99% RH) and with various cycling periods (1-week and 24-hour cycles between 50 and around 90% RH). The transversal and longitudinal tensile properties of biocomposite laminates are characterised, and the weight and curvature of the asymmetric samples are tracked throughout the cycles. The study shows that the hygroscopic cycles entail a reduction of internal stress due to damage development and a relaxation mechanism that are illustrated by permanent hygroscopic deformations. Two parameters have been identified to play a role on the damage development: the magnitude of the RH level and the duration of the cycles.

Continuous fibre composite 3D printing with pultruded carbon/PA6 commingled fibres: Processing and mechanical properties

Continuous Fibre composite 3D printing (CF-3DP) technology is a newly developed Additive Manufacturing (AM) process. By employing continuous fibre as a reinforcement for the composite, the mechanical properties of the 3D printed part can be significantly improved. However, owing to the process's early stage of development, the mechanical performance of the printed material is still unable to compete with composites produced using conventional manufacturing processes. This can manly be attributed to lower fibre volume fractions and/or higher void content. The grand objective of the research project is to establish a demonstrator of practical applications, and in this paper, preparatory developments were carried out to pave the ground for the subsequent developments, including feedstock materials development, printing process development, and materials properties evaluation. Efforts has been made to increase fibre volume fracture, to reduce voids contents, to evaluate effective properties systematically. The static mechanical test results showed relatively good longitudinal tensile properties while matrix sensitive properties were low, as to be expected in UD composites with a relatively weak matrix material. The advantage of CF-3DP is to achieve fibre steering, and the steered fibre design can be achieved by the current process to realize novel composite structures in future study.

Dimensional stability and mechanical performance evolution of continuous carbon fiber reinforced polyamide 6 composites under hygrothermal environment

Abstract This study aimed to investigate moisture absorption behaviors, dimensional stability, and mechanical performance evolution of continuous carbon fibers reinforced with polyamide 6 composites (CF/PA6) under a hygrothermal environment. 3D Fickian model determined the principal three-dimensional (3D) diffusion coefficients by using an optimization algorithm. Three-dimensional changes of samples were measured to quantify the relationship between swelling coefficient and water content. After hygrothermal aging, the tensile properties of PA6 resin and CF/PA6 composites in the transverse direction decreased significantly, but the longitudinal tensile properties of CF/PA6 composites were insensitive to hygrothermal aging. DMA (Dynamic Mechanical Analysis), SEM (Scanning Electron Microscopy) and FTIR (Fourier Transform Infrared) tests were also applied to analyze the mechanism of mechanical properties degradation.

On arginine‐based polyurethane‐blends specific to vascular prostheses

AbstractPolymer blends based on Tecoflex™ and an experimental aliphatic polyurethane (HMDI‐PCL‐arginine stands for 4,4 (metylene‐biscyclohexyl) isocyanate ‐ poly (ε caprolactone) diol, SPUUR stands for segmented poly(urea)urethanes using amino acid of L‐Arginine as chain extender) were obtained by solvent casting, and further studied by fourier transform infrared (FTIR) and Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis, and X‐ray diffraction (XRD). Their biological performances were assessed in terms of hemocompatibility and Human umbilical vein endothelial cell (HUVEC) cytotoxicity. Tensile properties of dumbbell specimens were compared to longitudinal and circumferential tensile properties of tubular vascular graft. FTIR showed that as the SPUUR content increased in the blend, absorptions at 2860 cm−1 increased, carbonyl absorptions at 1724 cm−1 broaden and the small peak at 2796 cm−1, typical of Tecoflex™ disappeared. Raman spectroscopy showed that the low intensity carbonyl absorption at 1724 cm−1 also increased with SPUUR content. DSC allowed detection of PCL soft segment melting (Tm = 50°C) in agreement with X‐ray reflections at 21.3° and 23.6°, assigned to SPUUR. However, no improvements in thermal stability were detected by TGA by blending. The addition of SPUUR to Tecoflex™ improved hemocompatibility and HUVEC cytotoxicity. The vascular grafts performance showed that 40% SPUUR blends exhibited the highest force in the longitudinal test whereas 50% SPUUR blends showed the highest circumferential force. Pressure burst strength was higher than 1000 mmHg for all blends. Overall, these blends can be used for high caliber vascular grafts.

Oriented PAN/PVDF/PAN laminated nanofiber separator for lithium-ion batteries

Electrospun nanofiber separators have excellent properties in terms of large surface area and high porosity, which benefits the rate capability, electrochemical stability, and safety performance of lithium-ion batteries. Herein, a 0°PAN/PVDF/90°PAN-oriented composite nanofiber separator is prepared, which is prepared layer by layer by electrospinning technology and drum orientation collector. The results show that when the rotating speed of the drum is 600 r min−1, the 0°PAN/PVDF/90°PAN-oriented composite nanofiber separator has high porosity (about 85%), improved transverse and longitudinal tensile properties (10.33 MPa and 11.03 MPa, respectively), good dimensional stability at 160°C, and good electrochemical performance (specific charge capacity of 165 mAh g−1 at 0.5C and 142.7 mAh g−1 at 1C, capacity retention of 90% after 100 cycles).

Progressive damage simulation of 3D four-directional braided composites based on surface-interior unit-cells models

To accurately predict the longitudinal tensile mechanical properties of 3D four-directional braided composites, parameter modeling for surface and interior unit-cells mesoscopic solid models was implemented and the deviation of yarn spatial traces and squeeze deformation of yarn cross-section were considered in the surface unit-cell model. Voxel mesh was used to discrete models on which appropriate boundary conditions were imposed and damage models for each constituent of composites were added into a user-defined material subroutine (UMAT) in finite element analysis software ABAQUS. By simulation analysis of surface-interior unit-cells models for 3D four-directional carbon fiber/epoxy braided composites with 30° and 45° interior braiding angles respectively, using the volume-weighted average method, longitudinal tensile modulus and strength for braided composites specimens with different thicknesses were predicted. The progressive damage process of composites was studied by counting the number of integration points with the same damage modes. Results show that the longitudinal tensile mechanical properties predicted based on surface-interior unit-cells models for 3D four-directional braided composites agree well with experimental results, and damage analysis results reflect reasonably progressive damage process of surface and interior unit-cells.

The Longitudinal and Transverse Tensile Properties of Unidirectional and Bidirectional Bamboo Fiber Reinforced Composites

Alkali treatment on bamboo fibers were reported to improve the interface strength with epoxy resin as formed to a composite. In order to reduce the process time of alkali treatment, bamboo fibers were treated in alkali with different concentrations under the room temperature. The alkali treatment process for the bamboo fibers, which results in a higher tensile strength, was used for the subsequent studies. Unidirectional and bidirectional BF preforms were constructed in our laboratory. The unidirectional (UD) and bidirectional (BD) BF/EP composites were fabricated using the bamboo fibers treated with the selected BF alkali treatment process. Tensile properties were measured in both the longitudinal and transverse directions for the UD and BD BF/EP composites with different fiber volume fractions. The UD BF/EP composite has good reinforcement effect in the fiber direction and the tensile strength is compatible to the reported results. However, the transverse strength of UD composites is weaker than the pure epoxy. For BD BF/EP composites, tensile strengths in both the longitudinal and transverse directions all show some improvement as compared to the pure epoxy.

Tensile characterisation of bamboo strips for potential use in reinforced concrete members: experimental and numerical study

The usage of bamboo as a replacement for steel reinforcement in concrete is in the early stages of development as bamboo reinforced concrete (BRC) members involve a lot more uncertainties than steel-reinforced concrete members. The properties of bamboo vary inter- and intra-species. A prerequisite to developing a rational design of BRC members, therefore, requires two stages—(i) a statistical characterisation of the mechanical properties of bamboo, and (ii) identifying the variables contributing significantly towards the stress-strain/load-displacement behaviour. This paper first experimentally characterizes the longitudinal tensile properties of Bambusa balcoa, a commonly found bamboo species in India. Statistical analysis of the experimental results suggests that most of the mechanical properties follow a lognormal distribution. The elastic modulus is found to have a dominant contribution towards the stress-strain behaviour. A high fidelity finite element model, the results of which are validated with experiments, indicates that only the variation of elastic modulus can capture 92% of the variability in the experimental results. We also create a low-fidelity finite element model using equivalent tensile modulus, which can capture 82% of the variability in the experimental results. Lastly, a finite element model of a BRC beam is created using the low-fidelity finite element model of bamboo. A comparison with the experimental results suggests a good agreement between them.

Properties of Sleeve Joints Made from Reduced Bamboo

Bamboo is a fast-growing species in the grass family, with excellent tensile and compressive strength characteristics, in the plant kingdom. The tapered hollow thin-walled cylindrical configuration of the bamboo species, namely, Gui bamboo (Phyllostachys makinoi Hayata) and Moso bamboo (Phyllostachys pubescens) culm, adversely influences its longitudinal shear and transversal tensile strength properties for effective use in engineered joints. The objective of this study is to use the thermo–hydro–mechanical (THM) process to reduce the irregular shape of bamboo ends without damaging the culms. Samples from the two abovementioned bamboo species were used for the experiments. Pullout loads and failure modes of the sleeve bamboo joints assembled by gluing were also evaluated. Eighty-nine out of 96 tested bamboo culms were successfully reduced by the THM treatment to uniform circular cross-sections under the maximum reduction ratio of 0.15. Sleeved-joint samples made from Gui bamboo with wood fittings had the highest pullout loads and strength values. Based on the findings in this work, it appears that THM-treated reduced bamboo ends, being a sustainable resource, could have the potential to be manufactured as steel-sleeve joints to be used for different engineering applications.

Temperature dependent longitudinal tensile strength model of unidirectional fiber reinforced polymer composites considering the effect of matrix plasticity

The unidirectional fiber reinforced polymer (FRP) composites have captured intensive attention of scholars owing to their superior longitudinal tensile properties. In this work, a temperature dependent longitudinal tensile strength model for unidirectional FRP composites was established based on the Force–Heat Equivalence Energy Density Principle and the bridging model. The model considers the evolution of constituents’ properties and residual thermal stress with temperature. Especially, the effect of polymer matrix plasticity at different temperatures on the temperature dependent tensile strength of unidirectional FRP composites is considered. Compared with our previous model and the Curtin’s model, the model can more conveniently predict the temperature dependent tensile strength, and the predictions obtain a better agreement with the available experimental results. Moreover, we conducted the influencing factors analysis for unidirectional FRP composites. This work provides a feasible and convenient method to predict the temperature dependent tensile strength of unidirectional FRP composites, and could offer beneficial insights for the material evaluation and optimization.

Fabrication of multilayer tubular scaffolds with aligned nanofibers to guide the growth of endothelial cells.

Aligned electrospun fibers used for the fabrication of tubular scaffolds possess the ability to regulate cellular alignment and relevant functional expression, with applications in tissue engineering. Despite significant progress in the fabrication of small-diameter vascular grafts (SDVGs) over the past decade, several challenges remain; one of the most problematic of these is the fabrication of aligned nanofibers for multilayer SDVGs. Furthermore, delamination between each layer is difficult to avoid during the fabrication of multilayer structures. This study introduces a new fabrication method for minute delamination four-layer tubular scaffolds (FLTSs) that consist of an interior layer with highly longitudinal aligned nanofibers, two middle layers composed of electrospun sloped and circumferentially aligned fibers, and an exterior layer comprising random fibers. These FLTSs are used to simulate the structures and functions of native blood vessels. Here, thermoplastic polyurethane (TPU)/polycaprolactone (PCL)/polyethylene glycol (PEG) were electrospun to fabricate FLTSs or tubular scaffolds with completely random fibers layer (RLTSs). The surface wettability of the TPU/PCL/PEG tubular scaffold was tested by water contact angle analysis. In particular, compared with RLTSs, FLTSs showed excellent mechanical properties, with higher circumferential and longitudinal tensile properties. Furthermore, the high viability of the human umbilical vein endothelial cells (HUVECs) on the FLTSs indicated the biocompatibility of the tubular scaffolds comparing to RLTSs. The aligned and random composite structure of the FLTSs are conducive to promoting the growth of HUVECs, and the cell adhesion and proliferation on these scaffolds was found to be superior to that on RLTSs. These results demonstrate that the fabricated FLTSs have the potential for application in vascular tissue regeneration and clinical arterial replacements.

Experimental study on the mechanical properties of laminates made of thin carbon fiber plies

Thin carbon-fiber prepregs can improve the designability of the number and the angle of layup at a constant laminate thickness, and meet the requirements of the load bearing capacity of the thin-walled structure. In this work, experiments based on the MIL17 method were performed to investigate the performance of carbon-fiber laminates manufactured from thin prepregs, and compare to those of conventional prepregs. Besides the performance advantages of thin prepregs and conventional prepregs hybrid laminates were explored. It is shown that longitudinal compression properties, tensile properties and mechanical joint properties of the thin unidirectional plates have been improved to some extent. The initial damage strength of the longitudinal tensile of thin laminates is increased by 30%, which indicates that the thin prepregs can delay the generation of initial damage in laminates. It is worth noting that the basic mechanical properties of the hybrid laminates are obviously improved and the longitudinal tensile/compressive strength is approximately increased 50%. In addition, these performance improvements are accompanied by changes in failure modes. The failure modes of thin specimens, especially hybrid specimens, tend to be brittle failure. The results of present study have certain guiding significance for the engineering application of thin prepregs in thin-walled structures.

Experimental investigation on the tensile and compressive properties of 3D 4-directional braided carbon fiber/epoxy resin composites in thermal environment

The longitudinal tensile and compressive experiments on 3D 4-directional braided carbon fiber/epoxy resin composites with three different braiding angles were performed in thermal environments. The effects of temperature on the longitudinal tensile and compressive properties of 3D braided composites were discussed. Macro fracture morphology and SEM micrographs were examined to understand the damage and failure mechanism. The results show that the longitudinal tensile strength of 3D 4-directional braided carbon fiber/epoxy resin composites goes up slightly and the longitudinal compressive strength decreases obviously with the increase of testing temperature. The braiding angle has no apparent impact on the longitudinal tensile failure characteristics of composites. However, the effect of braiding angle on the longitudinal compressive failure characteristics of composites is obvious. As the temperature increases, the damage and failure patterns of 3D braided composites with three different braiding angles are obviously different than in room.