Evaluation of embedded optical fiber sensors in composites: EFPI sensor response to fatigue loading

Extrinsic Fiber Fabry‐Perot Interferometric (EFPI) sensors were fabricated and embedded within a 16‐layer cross‐ply composite. The composites with and without the embedded EFPI sensors were subjected to tension/compression loading. The presence of the embedded sensor was not found to have adverse effect on the tension/compression fatigue properties. However, the performance of the EFPI sensor was found to degrade with fatigue cycles, with the introduction of a compressive element in the loading regime; samples were tested using stress ratios of −1, −2.5, and −3. Although the reasons for this observed degradation in the response of the sensor to applied strain is not known at present, it is speculated that this may be due to debonding of the key components of the sensor. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Quasi‐static and dynamic experiment investigations on the crashworthiness response of composite tubes

Crashworthiness performance of carbon and glass composite tubes have been comprehensive investigated under quasi‐static and dynamic axial crush testing. In this study, collapse modes and specific energy absorption (SEA) of different ply orientation of carbon fabric composites and unidirectional glass tubes were analyzed. For the weaker tensile strength and bending strength of glass composites, crack propagated approximately perpendicular to the fiber direction when the ply angle was small. Large amount of fibers breakage made the specific energy absorption over 80 kJ/kg under dynamic load. Thickness effect had inverse influence on SEA under different impact rate. The specific energy absorption declined as tube thickness increased under dynamic crush tests, however, increased under quasi‐static tests. Hybridization of glass/carbon tubes and carbon/carbon composites were analyzed by increased the axial fiber content. It was found that hybridization tubes of G803/3234 fabric and G827/3234 axial tapes with higher G827/3234 content present excellent energy‐absorption capability under dynamic and quasi‐static tests for all specimens tested. POLYM. COMPOS., 34:1099–1109, 2013. © 2013 Society of Plastics Engineers     

Anisotropic continuum damage model for prediction of failure in flax/polypropylene fabric composites

An anisotropic continuum damage modeling approach was applied to model failure of a composite of unidirectional flax in a polypropylene matrix under quasi‐static tensile loading. Tensile, compressive and shear stiffness, and strength values of the composite were characterized according to ASTM standards, and the damage was quantified by optical microscopy. Based on the experimental strength and damage values, an anisotropic strain‐dependent material damage model was developed and implemented in the finite element program ABAQUS. This was combined with geometric models of the fabric composites incorporating the yarn geometry. Good agreement was observed between the experimental and numerical stress–strain curves, and the failure strength prediction by the model was within 3.1% of the experimental value. This study shows that combining a geometric model closely incorporating the actual geometry of a fabric composite with an experimentally determined material degradation model can yield good predictions of the mechanical behaviour of the composite. POLYM. COMPOS., 37:2588–2597, 2016. © 2015 Society of Plastics Engineers     

Effect of geometry and loading fractions on energy absorbing properties of fiberglass polymer composite under quasi‐static and low‐velocity impact loadings

The focus of this study is to experimentally investigate the mechanical properties of fiberglass reinforced composite with various aspect ratios and loading fractions in the quasi‐static and low‐velocity impact loading conditions. In this study, short fiberglass reinforced polycarbonate composite materials were fabricated via a solution mixing method and characterized for their tensile properties by varying both fiberglass loading fraction and aspect ratio. The tensile properties including tensile toughness of the fiberglass reinforced composites were characterized and compared. It was observed in this study that the toughness of the composite was dramatically improved whereas the tensile strength and Young's modulus were moderately enhanced over the neat polymer, which were measured to be only up to 15% and 70% increase, respectively. The low‐velocity impact behaviors of the fiberglass composites were also investigated and compared to the tensile toughness of the corresponding composites. Besides, the effect of thickness on their low‐velocity impact properties was investigated. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40821.     

Effect of compressive loading speed and stacking sequence on mechanical characteristics of satin weave E‐glass/epoxy composites

This article experimentally investigated the in‐plane loading speed dependent mechanical properties and failure modes of satin weave E‐glass/epoxy composite laminates [45/−45/0/90]_ns. Two types of E‐glass fabric/epoxy pre‐impregnated tapes were used to manufacture the composite laminates specimens. The low strain rate tests were conducted with an INSTRON™ testing machine, and the high strain rate tests done using a pulse shape modified compressive Split Hopkinson Pressure Bar apparatus. From the experimental result, it was concluded that under different strain rate loading, compressive strength, modulus, and strain at peak stress were rate sensitive. Optical and microscopic photos of the specimens were taken to determine operative failure modes. Within the studied strain rate regimes, the failure mode changed from splitting followed by fiber kink buckling to predominantly delamination and shear fracture as strain rate increases from quasi‐static to high strain rates. Compressive properties and failure modes were severely affected by strain rate, stacking sequence, and fabric material. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

短纤维对橡胶发泡复合材料增强机理的细观分析

分别制备未增强的橡胶发泡体、未处理短纤维增强的橡胶发泡体和预处理短纤维增强的橡胶发泡体,考察气体泡孔和短纤维在橡胶发泡复合体中的微观形态,细观分析短纤维对橡胶发泡复合材料拉伸行为和压缩行为过程各阶段的增强机理。结果表明:气体泡孔在3种发泡复合材料基体中分布均匀,模压工艺使得短纤维在基体中呈现为平面分布,未处理短纤维周围有气泡包围,而预处理短纤维与橡胶间粘合良好;短纤维能有效地提高发泡复合材料在初始阶段的拉伸模量和100%定伸应力,特别是预处理短纤维表面与橡胶之间具有良好的粘合,有利于传递应力,限制橡胶基体的变形,而对拉伸强度却影响不大。在压缩弹性阶段,由于主要承载的泡壁中纤维呈平面取向,短纤维对压缩模量影响不大,但能有效地限制橡胶发泡复合材料在后屈曲阶段的压缩变形,提高其在高应变下的压缩强度,预处理短纤维增强效果明显高于未处理短纤维。     

Tensile properties of plant fibre‐polymer composites in fire

The structural performance of polymer composites reinforced with plant fibres when exposed to fire was experimentally evaluated and compared against an E‐glass fibre laminate. Fire testing under combined one‐sided radiant heating and static tensile loading revealed that flax, jute, or hemp fibre composites experience more rapid thermal softening and fail within much shorter times than the fibreglass laminate, which is indicative of vastly inferior structural performance in fire. The plant fibre composites soften and fail before the onset of thermal decomposition of the plant fibres and polymer matrix, whereas the E‐glass fibres provide the composite with superior tensile properties to higher temperatures and higher applied tensile stresses. The tensile performance of the three types of plant fibre composites in fire was not identical. When exposed to the same radiant heat flux, the flax fibre composite could withstand higher tensile stresses for longer times than the hemp and jute laminates, which showed similar performance.     

Physical and mechanical behavior of unidirectional banana/jute fiber reinforced epoxy based hybrid composites

During the last few years, natural fiber composites are replacing synthetic fiber composites for practical applications due to their advantages like low density, light weight, low cost, biodegradability and high specific mechanical properties. In this connection, the present investigation deals with the fabrication and mechanical properties of unidirectional banana/jute hybrid fiber reinforced composites and compares with the single natural fiber reinforced composites. The physical and mechanical properties of the natural fiber composites were obtained by testing the composite for density, tensile, flexural, inter‐laminar shear, impact, and hardness properties. The composite specimens with different weight percentages of fibers were fabricated by using hand lay‐up technique and testing were carried out as per ASTM standards. Incorporation of both the fibers into epoxy matrix resulted in an increase in mechanical properties up to 30 wt% of fiber loading. It is found that the hybrid composite give encouraging results when compared with the individual fiber composites. The morphologies of the composites are also studied by scanning electron microscope. POLYM. COMPOS., 38:1396–1403, 2017. © 2015 Society of Plastics Engineers     

The tensile behavior of off‐axis loaded plant fiber composites: An insight on the nonlinear stress–strain response

Composites in load‐bearing applications are often exposed to off‐axis loads. For plant fiber composites (PFCs) to be seriously and readily considered in structural applications, knowledge and reliable prediction of their response to off‐axis loads is critical. This article (i) characterizes the stress–strain response, (ii) investigates the tensile properties, and (iii) analyses the fracture modes, of unidirectional flax‐polyester composites subjected to off‐axis tensile loading. A key finding of this study is that due to the nonlinear stress–strain response of PFCs, the apparent stiffness of the composite reduces by ∼30% in the strain range of 0.05 to 0.25%. In addition, through cyclic tests on the composites, the elastic strain limit is found to be only ∼0.15%. This has major implications on the strain range to be used for the determination of the composite elastic Young's modulus. Consequently, it is proposed that the tensile modulus for PFCs should be measured in the strain range of 0.025 to 0.100%. Through comparison with experimental data, conventional composite micromechanical models are found to be adequate in quantitatively describing the tensile behavior of off‐axis loaded PFCs. The application of such models has also enabled the determination of, otherwise difficult to measure, material properties, such as fiber shear and transverse modulus. Off‐axis loaded PFCs fail by three distinct fracture modes in three different off‐axis ranges; each fracture mode produces a unique fracture surface. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

Improved strain sensing performance of glass fiber polymer composites with embedded pre-stretched polyvinyl alcohol–carbon nanotube fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers differing on their pre-stretching condition were embedded in glass fiber reinforced plastic (GFRP) composites and used as strain sensors for damage monitoring of the composite. Strain sensing of the composite was made by the in situ measurement of the embedded fiber’s electrical resistance change during the mechanical tests. Four glass fiber composite plates were manufactured; each one had embedded a different type of produced PVA–CNT fibers. The multi-functional materials were tested in monotonic tensile tests as well as in progressive damage accumulation tests. The electrical resistance readings of the PVA–CNT fibers were correlated with axial strain values, taking into account the induced damage of the composite. It has been demonstrated that increasing the fiber’s pre-stretching ratio, its electrical resistance response increases due to higher degree of the CNTs alignment in the PVA matrix. Higher fiber pre-stretching degree enables the better strain monitoring of the composite due to higher measured electrical resistance change values noticed for the same applied axial strain values. To this end, it enables for the better monitoring of the progressive damage accumulation inside the composite.     

Research on the mechanical properties prediction of carbon/epoxy composite laminates with different void contents

The influence of the porosity on the static mechanical strength of the carbon fiber fabric reinforced epoxy composites laminates was investigated. The tensile, compressive, bending, and interlaminar strength test on the CFRP laminates with porosity of 0.33% and 1.50% were conducted and simulated by a finite element analysis model. The article proposes the failure criterion of the static mechanical strength of the fabric fiber reinforced composites based on the improved Hashin failure criterion that is suitable for the undirectional composite laminates. The basic composite strength parameters are used to evaluate the mechanical properties of CFRP laminates with different porosities. A finite element analysis model is established by using software ABAQUS™ combined with the sudden stiffness degradation model. The experiment results show that the tensile, compressive, bending, and interlaminar strength decrease with the increasing porosities. The tensile, compressive, bending, and interlaminar strength of the fabric carbon fiber reinforced epoxy composites laminates are simulated accurately by the finite element model. POLYM. COMPOS., 14–20, 2016. © 2014 Society of Plastics Engineers     

Measurement of low velocity and quasi‐static failure modes in PMMA

An investigation of the low velocity impact and quasi‐static failure of polymethylmethacrylate (PMMA) based on global and local strain measurements was conducted. Local strains were obtained from surface‐mounted fiber Bragg grating (FBG) sensors, and they were combined with global measurements from quasi‐static indentation and low‐velocity impact experiments to obtain detailed maps of how failure evolves. For both loading regimes, the interactions between the host PMMA specimens and the sensors played a crucial role in the evolution of residual strains. A mapping of the strains clearly shows that strains decrease radially, from high values near the point of impact to far‐field values. Sensors located in critical locations had the highest residual strains prior to PMMA fracture. Furthermore, it was determined that strain transfer to the sensor is strongly influenced by the bonding conditions at the specimen's surface. Because of the debonding of the sensor and the frictional effects associated with the bonding agent, compressive residual strains occurred on the rear‐surface. Hence, a detailed understanding of how strain evolves due to sensor–host interactions and catastrophic fracture can be obtained, which can potentially be used to mitigate damage in PMMA for a range of strain rates. POLYM. COMPOS., 28:381–391, 2007. © 2007 Society of Plastics Engineers     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Experimental and finite element modeling of vinyl ester nanocomposites under blast and quasi‐static flexural loading

Materials used in blast, penetration, and impact loaded structural applications require high strength and toughness under high strain rate loading. 510A‐40 brominated bisphenol‐A‐based vinyl ester resin was developed and reinforced with different loadings of nanoclay and exfoliated graphite platelet to produce composites with optimal flexural rigidity, vibration damping, and enhanced energy absorption. As these reinforced polymeric materials are viscoelastic in principle, the mechanical behavior was characterized under two extremes of strain rate loading. In this article, the macroscopic response of brominated vinyl ester reinforced with 1.25 and 2.5 wt % nanoclay and exfoliated graphite platelet is considered. Air‐blast experiment was conducted by subjecting these specimens to a high‐transient pressure in a shock‐tube with flexural loading configuration. The axial response was investigated quasi‐statically in a uniaxial tension/compression test and dynamically in a compression Split‐Hopkinson bar test. The servo‐hydraulic MTS system was used to simulate the shock‐tube testing in a flexural quasi‐static loading configuration. High strain rate properties obtained from shock‐tube experiment are compared with that of characterized under the simulated quasi‐static flexural loading. Further, a computational finite element analysis model was developed in ANSYS LSDYNA to predict with reasonable accuracy the dynamic response of shock‐loaded nanoreinforced specimens. Drop in both failure strain and energy absorption was observed with the addition of nanoparticles to pristine vinyl ester. However, an improvement in energy absorption was observed in case of shock‐tube loading at high strain rates as compared to that loaded quasi‐statically. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci., 2012     

Investigation of mechanical properties of unidirectional steel fiber/polyester composites: Experiments and micromechanical predictions

The article introduces steel fiber reinforced polymer composites, which is considered new for composite product developments. These composites consist of steel fibers or filaments of 0.21 mm diameter embedded in a polyester resin. The goal of this investigation is to characterize the mechanical performance of steel fiber reinforced polyester composites at room temperature. The mechanical properties of unidirectional steel fiber reinforced polyester composites (SFRP) are evaluated experimentally and compared with the predicted values by micro‐mechanical models. These predictions help to understand the role of material and process parameters on material properties. Two types of SFRP were studied: polyester resin reinforced by both steel fabric containing unidirectional fibers and steel fibers wound on a metal frame with 0° orientations. The effects of the fiber volume fraction and the role of polymer yarns (weft) on mechanical properties were analyzed through tensile, compressive, and shear tests. These tests were performed as per the standard test procedures. In particular, issues related to processing difficulties, polymer yarns effect on properties, standardized testing, and properties under various loading conditions were addressed. Microscopic observations were analyzed to assess the laminate quality and the macroscopic fracture surfaces of shear test specimens were studied by standard techniques. POLYM. COMPOS., 37:627–644, 2016. © 2014 Society of Plastics Engineers     

Compressive behavior of biaxial spacer weft knitted fabric reinforced composite at various strain rates

The in‐plane and out‐of‐plane compressive properties of biaxial weft knitted E‐glass fabric reinforced vinyl ester composite at quasi‐static strain rate of 0.001/s and high strain rates from 700/s to 2200/s were tested to investigate the strain rate effect on the compressive behavior. The compressive tests were conducted on split Hopkinson pressure bar at high strain rate and on MTS 810.23 system at quasi‐static state. The experimental results indicated the strain rate sensitivity of compressive stiffness, failure stress, and strain of the composite in both out‐of‐plane and in‐plane compressive direction. The compressive stiffness and failure stress linearly increased with the increase of strain rate. The failure strain linearly decreased with the increase of strain rate. As the strain rate increased, the main failure mode at out‐of‐plane compression is the interlaminar shear failure and at in‐plane direction is the delamination. At the high strain rate of 2200/s, the composite coupon was compressed into debris with the shear or delamination failure. POLYM. COMPOS., 28:224–232, 2007. © 2007 Society of Plastics Engineers     

Mechanical properties of unidirectional continuous fiber self‐reinforced polyethylene graded laminates

The aim of this study is to prepare of self‐reinforced polyethylene graded composite laminates (SrPEGCL) by adopting both concepts of “graded” and “self‐reinforced” and analyze their mechanical properties under tensile loading. Three different kinds of fiber volume fractions were employed to prepare continuous fiber unidirectional symmetry SrPEGCL with two graded directions. Tensile experiments were carried out to investigate tensile properties of SrPE composites in longitudinal, transverse, and 45‐bias direction. The microscopic failure mechanism of SrPEGCL were studied and observed by Scanning Electron Microscope (SEM). Laminate stress analysis with ply‐by‐ply discount method was adopted to investigate the damage mechanism using failure criteria and parallel spring model. Observations and conclusions about the effect of graded structure and graded direction on mechanical properties of SrPEGCL under tensile loading were discussed. Compared to common self‐reinforced polyethylene composites, SrPEGCL with the same or even less overall fiber volume fraction exhibited 10–20% higher tensile strength under longitudinal, transverse and 45‐bias loading direction, while graded direction had an effect on the mechanical strength of SrPEGCL as well. POLYM. COMPOS., 36:128–137, 2015. © 2014 Society of Plastics Engineers     

Comparison of tensile and compressive characteristics of intra/interply hybrid laminates reinforced high‐density flexible foam composites

The current study focused on fabrication and mechanical evaluation of intra/interply hybrid laminates – reinforced high‐density flexible foam composites. The effects of composite thickness and expansion factor on the tensile and compressive characterization of the hybrid ‐ laminated composites were experimentally investigated. Double face sheets were made of high‐strength intra/interply hybrid laminates containing recycled Kevlar nonwovens and glass woven fabric. The results revealed that the hybrid laminates face sheet apparently promoted the tensile strength and tear resistance of the high ‐ density flexible polyurethane foam. Tearing resistance in perpendicular direction exceeded more than twice the value in parallel direction. In terms of dynamic cushioning properties, cushioning force increased with the increase in composite thickness and the decrease in expansion factor , whereas the cushioning capacity loss, however, showed a different trend with the variation of the parameters. Most samples buffered more than 95% incident force under dynamic loading. Composite thickness and expansion factor exhibited significant influence on compression and indentation properties, including hardness, initial hardness factor , and indentation modulus. Except the composites with 10 mm thickness, the intra/interply hybrid laminated composites exhibited hysteresis loss of indentation force deflection ranging from approximately 30 to 38%, which was due to the fiber and thermal bonding point failure of hybrid laminates as unrecoverable damage. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41438.     

Compressive properties of soybean oil‐based polymers at quasi‐static and dynamic strain rates

Quasi‐static and dynamic compressive properties of three soybean oil‐based polymeric materials, which were made through the reaction of epoxidized soybean oil with diamine compounds, have been determined. Quasi‐static properties were determined with an MTS 810 hydraulically driven testing machine, whereas dynamic experiments were conducted with a split Hopkinson pressure bar (SHPB) modified for low‐impedance material testing. All three materials were capable of deforming to very large strains, with significant nonlinear stress–strain response. Their compressive behaviors were strain‐rate sensitive with distinctive rate sensitivities. On the basis of the experimental results at various strain rates, a compressive one‐dimensional stress–strain material model with strain‐rate effects was developed to describe the experimental results for all three materials under both quasi‐static and dynamic loading conditions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 2006     

Alumina nanoparticle filled epoxy resin: High strain rate compressive behavior

This study demonstrates high strain rate behavior of neat epoxy and epoxy resin reinforced with 2 wt% and 5 wt% alumina nanoparticles. The present study investigates effect of strain rate as well as effect of alumina nanoparticle dispersion on the compressive stress – compressive strain behavior of epoxy resin. Studies were carried out on compressive split Hopkinson pressure bar apparatus in the strain rate range of 700–3200 per sec. Significant enhancement of compressive strength of epoxy resin up to 147% was noted at high strain rate compared with that at quasi‐static loading. The corresponding increase for alumina nanoparticle filled epoxy resin was up to 142%. It was also observed that the compressive strength of epoxy resin increases up to 8.6% with the addition of alumina nanoparticles at a strain rate of 3000 per sec. Synthesis of alumina nanoparticle filled epoxy resin, high strain rate and quasi‐static testing and main findings of the study are presented in this article. POLYM. ENG. SCI., 54:2896–2901, 2014. © 2014 Society of Plastics Engineers