Effect of loading‐unloading cycles on impact‐damaged jute/glass hybrid laminates

The production of glass/plant fiber hybrid laminates is a possibility for obtaining semistructural materials with sufficient impact properties, and a better life cycle analysis (LCA) profile than fiberglass. The simplest and possibly the most effective configuration for the production of these hybrids would involve the use of a plant fiber reinforced laminate as the core between two glass fiber reinforced laminates. A main limitation to the use of composites including plant fibers is that their properties may be significantly affected by the presence of damage, so that even the application of a low stress level can result in laminate failure. In particular, it is suggested that when loading is repeatedly applied and removed, residual properties may vary in an unpredictable way. In this work, E‐glass/jute hybrid reinforced laminates, impacted in a range of energies (10, 12.5, and 15 J), have been subjected to post‐impact cyclic flexural tests with a step loading procedure. This would allow evaluating the effect of damage dissipation offered by the plant fiber reinforced core. The tests have also been monitored by acoustic emission (AE), which has confirmed the existence of severe limitations to the use of this hybrid material when impacted at energies close to penetration. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Microcellular foaming of impact‐modified rigid PVC/wood‐flour composites

Solid state microcellular foaming technology was employed to investigate the influence of impact modification on the foamability of neat rigid PVC and rigid PVC/wood‐flour composite samples. The effects of impact modifier types (crosslinked versus uncrosslinked) and concentrations on the void fraction of foamed samples were examined. The influence of impact modification on the sorption behavior of CO₂ in the samples was also studied. The experimental results indicate that impact modification accelerates the rate of gas loss during the foaming process, which impedes the growth of nucleated cells, independent of modifier type. Because of this accelerated gas loss, impact modification inhibits the potential of producing foamed samples with void fractions similar to those achieved in unmodified samples.     

Composite machining damage quantification using thermoelastic stress analysis

A new experimental method is presented for quantifying machining damage in polymer matrix composites. The method consists of capturing infrared images of machined samples and using thermoelastic stress analysis to quantify damage from the machining event. A modified stress concentration factor is presented as an easily measured and useful damage parameter. Circular holes were drilled into the center of plate specimens fabricated from a commercially available glass fiber reinforced composite. A standard drill bit, brad point drill bit, and abrasive water jet machining were the three machine tools investigated. Infrared images were used to quantify the machining damage by assigning a thermoelastic stress analysis based stress concentration factor (mSCF) to each machined hole. The mSCF was then used to rank the damage inherent to each machining method. Optical and electron microscopy were utilized to identify the types of damage associated with the three machining methods. Finally, each sample was fatigued to failure to substantiate the IR results. The ranking of damage based upon the mSCF was in good agreement with the fatigue lifetime rankings: higher mSCF is associated with shorter fatigue lifetimes.     

Mechanical properties of fire‐damaged glass‐reinforced phenolic composites

Changes to the mechanical and physical properties of a glass‐reinforced resole phenolic composite due to intense radiant heat and fire are investigated. Fire testing was performed using a cone calorimeter, with the composite exposed to incident heat fluxes of 25, 50, 75 or 100 kW/m² for 325 s and to a constant flux of 50 kW/m² for different times up to 1800 s. The post‐fire tensile and flexural properties were determined at room temperature, and these decreased rapidly with increasing heat flux and heat exposure time due mainly to the chemical degradation of the phenolic resin matrix. The intense radiant heat did not cause any physical damage to the composite until burning began on exposure to a high heat flux. The damage consisted of cracking and combustion of the phenolic matrix at the heat‐exposed surface, but this only caused a small reduction to the mechanical properties. The implication of the findings for the use of glass‐reinforced resole phenolic composites in load‐bearing structures for marine craft and naval ships, where fire is a potential hazard, is discussed. Copyright © 2000 John Wiley & Sons, Ltd.     

Steam‐exploded residual softwood‐filled polypropylene composites

Residual softwood sawdust was pretreated by a steam‐explosion technique. It was used as a natural filler in polypropylene (PP)‐based composites. Dynamic mechanical analysis and tensile properties of these materials were studied. The influence of filler loading, steam‐explosion severity, and coating the fiber with a functionalized compatibilizer, such as maleic anhydryde polypropylene (MAPP), on the mechanical behavior of the composite was evaluated. The results were analyzed in relation with scanning electron microscopy observations, and surface energy (dispersive and polar components) and apparent specific area measurements. Experimental data indicate a better compatibility between MAPP‐coated fiber and PP with respect to the untreated one. The coating treatment of the softwood fiber was found to promote interfacial adhesion between both components, and to enhance the tensile properties of the resulting composite. This reinforcing effect was well predicted from theoretical calculations based on a mean field approach (Halpin‐Kardos model). The steam‐explosion pretreatment severity increased the surface energy and apparent specific surface, and resulted in a loss of the fiber entirety. The sorption behavior of these composite materials was also performed. It was found that the composites absorb more water, as the filler content is higher. MAPP coating provided protection from water uptake in the interphase region. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1962–1977, 1999     

Failure analysis of fiber‐reinforced composites under multiaxial cyclic stress

A preliminary study on the fatigue failure of continuous fiber reinforced composites under multiaxial stress has been presented. A model used to predict the direction of the critical plane of composite laminae under multiaxial cyclic stress is developed. First, two types of planes in the unidirectional lamina are defined in order to discriminate the different failure mechanisms, i.e. fibre failure and inter‐fibre failure. Based on planes of different types, the concept of effective stress is proposed to determine the direction of the critical plane. Second, fatigue experiments of filament wound composites under biaxial loading are carried out and the failure planes are described. Results show that the fracture planes in reality are close to the critical planes predicted by the model. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Viscosity analysis of polypropylene‐kaolin composites measured using single‐screw extruder

Deviations between data for the apparent and true viscosity measurement are commonly observed in the rheological studies of thermoplastics. To validate the significance on the usage of these data on the processing side, filling analysis for the apparent viscosity and true viscosity of polypropylene‐kaolin (PP‐kaolin) composites was conducted by using CadMold® software on the dumbbell Computer Aided Design model for comparative purposes. The raw apparent viscosity data were generated from a single‐screw extruder by using different sets of die geometries. Bagley and Rabinowitsch corrections were implemented to get a corrected set of true viscosity measurements. It was found that the true viscosity curves are lower compared with the apparent viscosity curves of PP‐kaolin. By constructing the corresponding data from PP‐kaolin composite rheological results, it was discovered that the high viscosity of the apparent viscosity melt data has shown a large variation in the mold distributions filling temperature, reduces the total pressure loss and melt shear stress, and increases the melt velocity during mold filling. Although the corrections were made on the calculated results, however, the significance is relatively small and it does not prove the changes in the overall physical property of the composite. Therefore, correcting the data by using the Bagley and Rabinowitsch corrections is not necessary in mold‐filling analysis. J. VINYL ADDIT. TECHNOL., 20:275–283, 2014. © 2014 Society of Plastics Engineers     

Improvement in impact‐, tensile‐, and dynamic properties of injection molded wheat‐pulp‐polypropylene composites through fiber finishing

This study was focused on the improvement of mechanical properties of injection molded wheat‐pulp polypropylene (PP) composites through fiber surface modifications. Ten different sizing and finishing agents, including fats, starch derivatives, and polysiloxanes were used as surfactants for the cellulosic pulp. As a result of polydimethylsiloxane treatment (0.3 wt %), impact strength was increased by 85%, tensile strength by 23%, and an augmentation in tensile modulus of 12% was also achieved. In consideration of the dynamic mechanical properties, the stronger effects of the modifiers on the storage‐ modulus were observed with increasing temperature. A new approach quantifying the extent of the dispersion of the pulp fibers using image analysis through transmission light micrographs was tested. The enhancement of tensile strength, tensile modulus, and impact strength could be attributed to the improved dispersion of the cellulosic fibers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Enhanced fatigue life of carbon nanotube‐reinforced epoxy composites

The effects of carbon nanotube (CNT) inclusion on cyclic fatigue behavior and the residual mechanical properties of epoxy composites after different degrees of fatigue have been studied. Tension–tension cyclic fatigue tests were conducted at various load levels (25–50 MPa) to establish the relationship between stress and the number of cycles to failure (S–N curves). The residual strength and modulus were measured after loading at 30 MPa for 5000, 15,000, and 25,000 cycles. The incorporation of a small amount of CNTs increased the fatigue life of epoxy in the high‐cycle, low‐stress‐amplitude regime by 1550%. Micrographs indicate the key mechanisms for enhancement in fatigue life such as CNT crack‐bridging and pullout. POLYM. ENG. SCI., 52:1882–1887, 2012. © 2012 Society of Plastics Engineers     

Characterization of processing effects in HIPS‐CNF composites using thermogravimetric analysis

To completely establish the processing–structure–property relationships of polymer composites such as Carbon Nanofibers (CNFs) in high‐impact polystyrene, it is necessary to understand the effects of different processes and processing conditions on the properties of the composites. In this work, using thermogravimetric analysis, the effects of solvent processing and twin‐screw extrusion on the weight loss rates and the corresponding temperatures were studied. While there were only marginal effects of the CNF concentration, the type of processing significantly affected the thermo‐oxidative behavior. Extrusion resulted in composites that had better thermal stability compared to the solvent processed ones. Furthermore, higher shear rates in extrusion also led to composites with higher thermal stability. This has important implications in choosing the appropriate process and processing conditions for producing polymer‐CNF composites. It was also demonstrated that thermogravimetric analysis can provide a means of characterizing the degree of dispersion resulting from the processing of the composites that complements conventional microscopy techniques. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Flexural fatigue behavior and residual strength evolution of SiCnws-C/C composites

The flexural fatigue behavior and residual flexural strength evolution of SiC nanowires reinforced carbon/carbon (SiCnws-C/C) composites were investigated. Specimens were loaded at a stress level of 65% of their static flexural strength for 10⁵, 5?×?10⁵ and 10?×?10⁵ cycles, and their residual flexural strength was increased by 4.87%, 13.73% and 62.45% respectively after cyclic loading. Results indicate that the residual strength after cyclic load is affected by the formation and propagation of cracks, interfacial degradation, as well as the relief of residual thermal stress. An appropriate interfacial debonding and releasing of residual thermal stress are responsible for the large improvement of residual strength of SiCnws-C/C composites after 10?×?10⁵ fatigue cycles. Compared with carbon/carbon composites, SiCnws-C/C composites demonstrate higher mechanical strength and stronger resistance to crack propagation, which are ascribed to the strengthening effect brought by the SiC nanowires, including their pull-out, breaking and bridging.     

Fabrication of the self‐reinforced composites using co‐extrusion technique

The results of this work relate to the use of co‐extrusion technology in the preparation of monocomposite pellets. The low‐melting polypropylene copolymer was used as a matrix material. The high strength polypropylene fibers were used as a fibrous reinforcement. Research confirms the possibility to produce the pellets with fibrous structure. The prepared composite material in the form of pellets was processed and shaped using the injection molding technology. Obtained samples were subjected to mechanical testing in the static tensile test and dynamic mechanical analysis. Research complements microscopic observation of scanning electron microscopy. The measurement results confirm the reinforcing effect of the fibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41180.     

Tensile stress relaxation of short‐coir‐fiber‐reinforced natural rubber composites

The stress relaxation behavior of natural rubber (NR) and its composites reinforced with short coir fibers under tension was analyzed. The rate of stress relaxation was a measure of the increase in the entropy of the compounds: the higher the rate was, the greater the entropy was. At lower strain levels, the relaxation mechanism of NR was independent of strain level. However, the rate of relaxation increased with the strain level. Also, the strain level influenced the rate of stress relaxation considerably in the coir‐reinforced NR composites. However, the relaxation mechanisms of both the unfilled compound and the composite were influenced by the strain rate. The rate of relaxation was influenced by fiber loading and fiber orientation. From the rate of stress relaxation, we found that fiber–rubber adhesion was best in the composite containing fibers subjected to a chemical treatment with alkali, toluene diisocyanate, and NR solutions along with a hexaresorcinol system as a bonding agent. In this study, the stress relaxation curves could not be viewed as segments with varying slopes; however, a multitude of inflection points were observed on the curves. Hence, we propose neither a two‐step nor three‐step mechanism for the coir‐fiber‐reinforced NR composites as reported for some other systems. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 96–104, 2004     

Mapping of stress distribution in woven‐fabric composites

Mapping of the stress distribution in composite materials, both at the fiber/matrix interface and in the composite constituents, is important to understand the material mechanical response. Stress mapping can help predict composite behavior under certain stresses especially failure or delamination. In this work, two analytical models were proposed to map the stress distribution at fiber, matrix and fiber/matrix interface utilizing the concept of stress superposition. The first model dealt with the fiber in the longitudinal direction considering axisymmetric conditions. The second model addressed the fiber stress distribution in the transverse direction. Experimental data from four‐point flexural tests of woven fabric composites was processed using the Graphical Integrated Numerical Analysis (pcGINA) to obtain the maximum stress in the target laminate and this value was used as the input for the two analytical models. The value for the maximum interfacial shear stress was calculated using the models and results were compared to pull‐out fiber test values obtained from literature. Good agreement was observed between the model calculations and the literature data. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Morphology and crystalline properties of impact‐modified polypropylene blends

Isotactic polypropylene has been reactively blended with an ethylene‐octene copolymer so as to improve mechanical and rheological properties. Free radical polymerization of styrene and a multifunctional acrylate during melt extrusion has resulted in the formation of unique features in both amorphous and crystalline phases. Transmission electron microscopy images show that the elastomer domains are less than 200 nm in diameter, whereas grafting leads to the appearance of clusters of polymeric particles, ranging in size from several nanometers up to 200 nm. Differential scanning calorimetry (DSC) shows that grafting creates lamellar crystals that melt at much lower temperatures and recrystallize at much higher temperatures than binary blends. From wide angle X‐ray diffraction and DSC, grafting has been shown to limit the maximum crystal size and perfection, as well as broaden the size distribution of the crystals. Grafting causes significant changes in the α crystalline phase of polypropylene and promotes the formation of the β phase. Scanning electron microscopy reveals a unique cross‐hatch structure of small crystals in the reactive blend, with tangential and radial lamellae appearing as crosslinked material of about the same aspect ratio. Polarized light microscopy gives evidence that grafting and branching within this blend causes a gelation‐like recrystallization. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Effects of stress concentration and residual stress on vibration fatigue behavior of socket weld

The vibration fatigue behavior in socket weld of 304L stainless steel was investigated using experimental and numerical. The vibration fatigue test results indicate that original socket weld at a higher stress tend to occur toe failures while for the case of lower stress failures tend to originate at the root. In addition, the socket welded with groove can improve the fatigue property of weld root, but it has a bad influence on the weld toe. From the simulation results, the groove can decrease the stress concentration factor of weld root, and improve the fatigue property of root; whereas the groove can also increase the tensile residual stress of toe, resulting in having a detrimental effect on fatigue property of toe.     

Fungal degradation of wood‐plastic composites and evaluation using dynamic mechanical analysis

Small samples of two wood–polyethylene (HDPE) composite formulations were incubated with either the white‐rot fungus Trametes versicolor or the brown‐rot fungus Gloeophyllum trabeum for 24 and 77 days in an agar‐block test. Noninoculated, side‐matched controls were employed in the tests to serve as references, and solid wood samples of yellow‐poplar (Liriodendron tulipifera L.) inoculated with T. versicolor were included as positive controls. Potential changes in storage and loss moduli because of fungal colonization and moisture were determined using dynamic mechanical analysis, whereas weight loss and visual observation served as indicators of fungal decay. Severe losses in storage modulus (E′) and loss modulus (E″) following incubation of yellow‐poplar with T. versicolor were observed. However, the E′ of the two wood–plastic composite (WPC) formulations increased after 24 days of incubation with T. versicolor. The same effect was observed for G. trabeum, but only in one formulation. The increase of E′ was attributed to a reinforcing effect of the fungal hyphae present in the interfacial gaps between the wood filler and the polymer matrix. Dynamic temperature scans revealed a peak in E″ between 30°C and 63°C, depending on the frequency and fungal treatment. The peak temperature of E″ represents the α‐transition of HDPE. Increased activation energies were required for the α‐transition in WPC samples incubated with T. versicolor for 77 days as compared to controls. This observation confirmed that incubation of WPC with T. versicolor improved interfacial adhesion and reinforced the composite under the assay conditions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3138–3146, 2006     

Effects of residual stress and orientation on the fatigue of injection molded polysulfone

The tensile fatigue behavior of unnotched injection molded polysulfone specimens has been investigated. The effects of orientation and residual stress were studied by comparing asmolded specimens with annealed or annealed and quenched specimens with a known residual stress pattern. The treatments are shown to have differing effects at high stresses, where failure is by shear yielding and necking, and at intermediate stresses, where failure is by fatigue crack propagation. The geometries of fatigue cracks are described for each case. An attempt is made to separate the effects of crack and craze initiation from crack propagation, and cyclic loading from cumulative time under load.     

Experimental characterization of the impact‐damage tolerance of a cross‐ply graphite‐fiber/epoxy laminate

In this study, the impact‐damage tolerance of a graphite‐fiber/epoxy composite laminate is studied by examining the correlation between the impact force and the resulting delamination area in the laminate. The cross‐ply [0₂/90₂/0₂]_s composite laminate was made of thermosetting P7051S‐20Q‐1000 prepregs (Toray Composites America). A Hopkinson pressure bar (HPB) was employed to create the impulsive loading with varying magnitude. Transient impact force, displacement, impact power, and transmitted impact energy were calculated using the transient signals recorded from the strain gage mounted on the HPB. Impulsive loads with controllable magnitude were used to induce delamination damage with varying size in the composite samples. Nondestructive evaluation based on a novel ultrasonic pulse‐echo reflector technique was used successfully for characterizing the delamination areas in the thin composite samples with thickness ∼2 mm. The present experimental results indicate that there exists a very good linear correlation between the impact force (e.g. the peak force, impact impulse, peak impact power, and the transmitted impact energy of the first impact force pulse exerted by the HPB) and the delamination area of the composite samples. This correlation can be used to determine the threshold of the impact force that initiates the delamination damage in the composite laminate. In contrast to the weight‐drop test, the present experimental method successfully examined the impact damage tolerance of polymer matrix composites (PMCs) subjected to impulsive loading with very high force magnitude and ultra short duration such as the typical ballistic impact. The present method and results can be used for the study of impact damage tolerance of PMCs with varying lay‐ups and interface modifications. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

On the design of end tabs for quasi‐static and fatigue testing of fibre‐reinforced composites

The use of end tabs is often necessary when performing quasi‐static uniaxial tests on fibre‐reinforced composites. However, finding a suitable combination of material and geometry for these end tabs to have acceptable and reproducible results may be a problem. In this article four different geometries and four different materials of the tabs are numerically examined for the tensile testing of a carbon fabric reinforced polyphenylene sulphide. First, it is assessed if a simplified finite element model of a tensile grip is acceptable. Then, this simplified model is used to examine the proposed set‐ups. It may be concluded that, for the given material, short straight end tabs with a [(0°,90°)]_4s layup should be used and the specimen should be mounted in such a way that the end tabs are completely between the grips. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers