Random glass mat reinforced thermoplastic composites. Part I: Phenomenology of tensile modulus variations

The mechanical properties of random continuous glass mat reinforced composites, as determined by standard tensile tests, are known to have a very large scatter. To understand this scatter, new test procedures were developed to map the local tensile elastic moduli in a large plaque at 12.7-mm (½-in) intervals. Surprisingly, the tensile modulus in these materials can vary by a factor of two over the 12.7-mm distance. The elastic modulus is shown to vary by a factor of three in a 150 × 305-mm (6 × 12 in) plaque. Expressions have been obtained for the average moduli measured by tensile and bend tests. These expressions have been used to compare measured flexural moduli with values predicted by using measured tensile moduli.     

Mechanical properties of rigid thermoplastic foams—Part II: Stiffness and strength data for modified polyphenylene oxide foams

The through-thickness variation in the porosity of structural foam material is known to result in different “material properties” when mechanics based on homogeneous materials is used to interpret data from standard tensile and bend tests. Procedures for determining the mechanical properties of rigid thermoplastic structural foams and for the application of these properties to the design of load-bearing components were developed in a companion paper. This paper reports the mechanical properties of modified polyphenylene oxide foams, such as elastic moduli, ultimate stress and strain, as determined by tests on specimens cut from large, edge-gated foam plates. Tests were conducted to study plate-to-plate variations in properties and to evaluate the effect of specimen thickness. Correlations of tensile and flexural data with the average specimen density are also discussed.     

Importance of Mullins effect in commercial silicone elastomer formulations for soft robotics

Silicone elastomers are used in a wide range of applications, including artificial muscles, biomedical devices, and soft robotics, for which chemical, thermal, and mechanical stability are important requirements that these elastomers must fulfill. However, to ensure that silicone elastomers' properties and performance remain constant under long-term deployment, it is necessary to examine and account for the Mullins effect, which has the potential to significantly alter certain elastomer properties of interest. In this article, the mechanical properties of soft and hard commercial silicone elastomers and two blends of commercial silicone elastomers are investigated—specifically their softening behavior due to the Mullins effect. Ultimate stresses, ultimate strains, and Young's moduli are obtained from uniaxial tensile tests. Results show that the point of softening greatly depends on both the elastomer type and its strain history. Furthermore, a significant permanent set is observed in the softest commercial formulations.     

Effect of temperature on mechanical properties of ultra-high performance concrete

High temperature mechanical property data are needed for evaluating fire resistance of structural members. Being a relatively new construction material, there is a lack of temperature-dependent mechanical property data on ultra-high performance concrete (UHPC). To address this knowledge gap, this paper presents results from an experimental study on the effect of temperature on mechanical properties of UHPC. Specimens made of two UHPC mixes: one with only steel fibers (UHPC-S) and the other with hybrid fibers, that is, both steel and polypropylene (UHPC-H), were tested under different heating conditions in 20 to 750°C temperature range. Compressive strength, tensile strength, stress-strain response, and elastic modulus of UHPC were evaluated at various temperatures. Results generated from these property tests on UHPC were compared with property relations specified in design codes for conventional normal strength concrete (NSC) and high strength concrete (HSC). The comparisons show that UHPC experiences faster degradation in compressive strength and elastic modulus as compared to conventional concrete. However, UHPC exhibits slower degradation in tensile strength and ductility at elevated temperatures due to the presence of steel fibers. Data generated from these property tests were utilized to propose relations for expressing the mechanical properties of UHPC as a function of temperature and these relations can be used as input to numerical models for evaluating fire resistance of structures made of UHPC.     

Macroscopic effective moduli and tensile strength of saturated concrete

Many concrete structures such as dams, abutment piers of bridges, offshore platforms, costal and port structures, etc., are often submerged in water. The water within concrete pores or cracks has a great influence on the macroscopic mechanical properties of concrete, especially its global modulus of elasticity and strength. The present study investigates the quantitative influence of water content, i.e. concrete porosity, on the global mechanical properties of saturated concrete. By a three-phase spherical model and a hollow cylindrical rod model, the effect of porosity on the effective bulk and shear moduli of saturated concrete is studied in a quantitative manner. Based on the assumption that the pore-water has no shear capacity, the effective elastic modulus and Poisson's ratio of the saturated concrete are obtained using the theory of elasticity subsequently. Furthermore, according to the maximum tensile stress failure criterion, the quantitative relationships between the porosity and the global tensile strengths as well as their corresponding tensile peak strains of concrete in dry and saturated states are established. Finally, a comparison between the theoretical results and experimental data is made to verify the rationality and the accuracy of the present approach. The present results are comparable to the experimental observations of Yaman et al. [4,23], indicating that the present approach is applicable to predict the effective mechanical properties of concrete in both dry and saturated states. Moreover, it is found that compared with dry concrete, the water within saturated pores limits the surrounding concrete matrix deforming into pores, causing the enhancement of the global elastic modulus and Poisson's ratio of concrete. The effect of pore-water on concrete tensile strength is significant and should not be neglected in design.     

Influence of elevated temperatures on epoxy adhesive used in CFRP strengthening systems for civil engineering applications

This paper presents experimental investigations about the influence of elevated temperatures on the mechanical behaviour of an epoxy adhesive typically used in carbon fibre reinforced polymer (CFRP) strengthening systems and numerical investigations about the influence of changes underwent by the adhesive on the response of bonded joints between CFRP strips and concrete. The experiments included shear and tensile tests at elevated temperatures (up to 120 °C) on a commercial epoxy adhesive. In both types of tests, the mechanical response of the adhesive at different temperatures was assessed, namely in terms of stress vs. strain curves, stiffness, strength and failure modes. The results obtained highlighted the considerable reduction of both shear and tensile properties with increasing temperatures: at 70 °C the shear and tensile strengths are both reduced to around 15% of the corresponding ambient temperature strengths, while the tensile and shear moduli can be considered negligible. Analytical formulae were fit to the test data, describing the reduction with temperature of the adhesive's tensile and shear properties. In the numerical investigations, three-dimensional finite element models were developed to simulate previous double-lap shear tests performed on concrete blocks strengthened with CFRP strips according to either the externally bonded reinforcement (EBR) or the near surface mounted (NSM) techniques, using the epoxy adhesive characterized in the present study. Two distinct modelling strategies were adopted for the concrete-CFRP bond in order to assess the relative importance of the adhesive distortion and interfacial slippage at the concrete-adhesive-CFRP interfaces in the overall slip between concrete and CFRP: (i) to explicitly simulate the adhesive, considering the mechanical properties determined in the tests and assuming a perfect bond at all interfaces; and, alternatively, (ii) to simulate the CFRP-concrete interaction by means of global bilinear bond-slip laws for different temperatures. Comparison between numerical results and test data allowed quantifying the relative importance of the adhesive distortion and of the interfacial slippage at the bonded interfaces as a function of temperature, providing a better understanding of the contribution of these two mechanisms to the CFRP-concrete bond at elevated temperature. While the former effect is the most relevant at ambient temperature, with elevated temperature the interfacial slippage at the bonded interfaces becomes the most relevant mechanism.     

Modeling of the tensile moduli of mechanical,thermomechanical, and chemi‐thermomechanical pulps from orange tree pruning

Stiffness is one of the most relevant properties of composite materials. Although fiberglass has been traditionally used as reinforcement, natural fibers are seen as possible replacements due to concerns for environmental protection. In this work fibers from orange tree prunings were prepared and converted into mechanical, thermomechanical and chemi‐thermomechanical pulps, to be used as reinforcement for polypropylene. Polypropylene composite materials with 20–50% of reinforcing fibers were prepared and mechanically characterized. The intrinsic Young's modulus of the fibers was back calculated by means of the Hirsch model. The moduli were also obtained by Halpin‐Tsai equations with Tsai‐Pagano methods and then compared to establish the influence of the aspect ratio. Finally, a fiber tensile modulus factor was defined in order to characterize the contribution of the fibers to the Young's moduli of the composites. POLYM. COMPOS., 34:1840–1846, 2013. © 2013 Society of Plastics Engineers     

Rheological,thermal, and mechanical characterization of fly ash‐thermoplastic composites with different coupling agents

Composite materials were prepared by mixing fly ash obtained from biomass combustion as filler and isotactic polypropylene (PP) as matrix. Three silane‐type coupling agents mainly differing in the size of their functional groups were used to improve the compatibility between both components. Uniaxial tensile tests showed that the incorporation of untreated ash into PP led to stiffer but also more brittle and weaker materials, as Young's modulus significantly increased and tensile strength and elongation at break decreased. Furthermore, an enhancement in storage and loss moduli as well as in composite viscosity was observed with the addition of fly ash. Hardness tests and thermal and fracture surface analyzes revealed tensile test results similar to those mentioned earlier. In summary, after analyzing the effects of the three silanes on mechanical, thermal, morphological, and rheological properties, the silane containing the vinyl functional group (XL10) was selected as the most appropriate for the PP/ash composites investigated. POLYM. COMPOS., 31:1722–1730, 2010. © 2010 Society of Plastics Engineers.     

Fracture and Stiffness Characteristics of Particulate Composites of Diamond in Zinc Sulfide

Diamond particles have been embedded in hot-pressed zinc sulfide (ZnS) ceramic to improve various mechanical properties while preserving special optical properties. Roomtemperature mechanical tests on small specimens have shown that adding 10 wt% diamond to ZnS has no effect on the yield stress, but increases the tensile strength and the elastic moduli ∼20%, and increases the fracture toughness ∼100%. The doubling in fracture toughness can be explained by elastic interaction of the diamond particles with the crack-tip stress field. The results and the interpretation presented here are believed to represent a class of composite materials where both constituents are brittle but the dispersed phase has a much higher elastic modulus than the matrix.     

Elastic modulus of viscoelastic magnetic silicone gel body

This paper describes various viscoelastic magnetic silicone gel bodies developed to create a new type of viscoelastic magnetic material. This material is capable of undergoing substantial changes in mechanical properties due to the large deformation caused by magnetic traction force under the application of a moderate strength magnetic field. This study performed tensile tests for various viscoelastic magnetic silicone gel bodies under a uniform steady magnetic field. The elastic moduli of the gel bodies were measured under different controlled experimental conditions. The experimental results showed that, under the applied magnetic field, the elastic moduli of the viscoelastic magnetic silicone gel bodies increased and were largely dependent upon the magnetic properties of the magnetic particles. The magnetic particle size and the material properties of the dispersant and the silicone gel also had significant effects on the moduli of the gel bodies. This paper also discusses the most appropriate combination of the materials used in this study from the standpoint of gaining a large magnetic traction force. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Simulation and Validation of Thermo‐mechanical Stresses in Planar SOFCs

To study possible failure modes of the Hexis Galileo solid oxide fuel cell stack, various stack components such as nickel/yttria stabilised zirconia anodes, lanthanum strontium manganese cathodes, 3 mol%‐yttria stabilised zirconia electrolytes and chromium alloy metallic interconnectors have been characterised with respect to their thermo‐mechanical properties. Specifically, coefficients of thermal expansion, Young's moduli, bending strengths, Poisson's ratios and fracture toughnesses have been measured. Furthermore, the temperature‐dependent warpage of complete cells has been investigated by video analysis. All experimental data were taken as input parameters for a set of finite element models to analyse various thermo‐mechanical phenomena on different length scales. The simulations offer an explanation for the often observed ‘saddle‐like‘ deformations of cells at room temperature. They also show that cracks that first develop within the anode induce local tensile stresses within the electrolyte and hence represent a weakening mechanism for the cells. It is shown that the induced electrolyte stresses depend on the anode crack density. The electrolyte stresses decrease as the distances between the anode cracks become smaller.     

A novel,efficient route for the crosslinking and creep improvement of high modulus and high strength polyethylene fibres

A new route is presented for the chemical crosslinking of solution‐spun, ultra‐drawn Ultra‐High‐Molecular‐Weight Polyethylene (UHMW‐PE) fibres. UHMW‐PE fibres with a range of draw ratio's, Young's moduli and tensile strengths were impregnated with a radical initiator using supercritical carbon dioxide as a carrier. After impregnation, the drawn fibres were crosslinked with ultra‐violet light and fibres with a high gel content (> 90%) were obtained. It was found that the chemical crosslinking strongly reduces the plateau creep rate of the fibres and that the threshold stress for irreversible creep is enhanced. Simultaneously, the high Young's modulus and the high tensile strength of the drawn fibres are preserved which illustrates that the long term properties of the fibres (i. e. creep) are improved without a large sacrifice short term mechanical properties such as Young's modulus.     

Bulk and surface mechanical properties of clay modified HDPE used in liner applications

High‐density polyethylene (HDPE)/clay nanocomposites were prepared by melt blending process. The HDPE was mixed with different organoclays and polyethylene‐grafted‐maleic anhydride was used as a compatibiliser. A masterbatch procedure was used to obtain final organoclays concentrations of 1, 2.5 and 5 wt%. The effects of various types of nanoclays and their concentrations on morphological, thermal and mechanical properties of nanocomposites were investigated. Surface mechanical properties such as instrumented nanohardness, modulus of elasticity and creep were also measured using a nanoindentation technique. Young's, storage and loss moduli, were found to be higher than that of the neat polymer at low loading (2.5 wt%) for clay Cloisite 15A and at higher loading (5 wt%) for clay Nanomer 1.44P. The ultimate strength and the toughness decreased slightly compared to pure HDPE. The differential scanning calorimetry analysis revealed that the peak temperature of the nanocomposites increased with increased clay content while the crystallinity decreased. Also, dynamic mechanical analysis revealed the storage and loss moduli are enhanced by addition of nanoclay. Both mechanical and thermal properties of HDPE/Nanomer 1.44P nanocomposite showed interesting trends. All properties first dropped when 1 wt% of the clay was added. Thereafter, a gradual increase or decrease then followed as the loading of Nanomer was increased. These trends were observed for all mechanical properties. The results obtained from nanoindentation tests for surface mechanical properties also showed similar trend to that of bulk measurements. Based on these measurements a nanoclay additive for a liner grade HDPE was selected. © 2011 Canadian Society for Chemical Engineering     

UV-cured clay/based nanocomposite topcoats for wood furniture. Part II: Dynamic viscoelastic behavior and effect of relative humidity on the mechanical properties

Topcoat constituting multi-layer coatings for wood furniture used in high humidity environments, like bathrooms, must have not only good barrier properties, but also good mechanical properties. Three different types of commercial organoclays, namely Cloisite 10A (C10A), Cloisite 15A (C15A) and Cloisite 30B (C30B), were chosen in this study as reinforcing agents. These nanoparticles were dispersed (1 and 3 wt% into the formulation) into a commercial epoxy acrylate oligomer by means of a three roll mill. Samples obtained from free standing UV-cured coatings were used for mechanical assessments. Mechanical tests were performed in both dynamic and static mode in order to investigate the viscoelastic behavior and tensile properties of coatings. Results from dynamic mechanical analysis have shown that all nanocomposite coatings have higher (72–75 °C) glass transition temperature compared to that observed (71 °C) in unreinforced coatings. The restriction of polymer chains mobility, due to the presence of layered silicate nanoparticles, has been used to explain the increase of glass transition temperature related to the decrease of the free volume. The storage modulus for nanocomposites containing 3 wt% of C10A, C15A and C30B was found to be slightly higher than that observed in pure coatings. The analysis of tensile stress–strain curves has revealed that tensile properties are affected by relative humidity (RH) due to the plasticization effect of humidity. In fact, results have shown that regardless of the organoclay type, the increase of RH decreases both Young's modulus and tensile strength while increasing maximum strain. We believe that low interfaces between photocrosslinked polymer chains and organoclays explain the lack of any effect of organoclays on both storage and Young's moduli. Among samples from each type of UV-cured coating tested at 0, 20 and 80% of RH, regardless of the organoclay type and content, only samples tested (tensile tests in static mode) at RH = 80% were broken. SEM images obtained from the fractured surface of these samples have shown that unreinforced UV-cured coatings and nanocomposite coatings are respectively characterized by smooth and rough fracture surface.     

Cationically cured natural oil‐based green composites: Effect of the natural oil and the agricultural fiber

Green composites were produced from various cationically cured natural oil‐based resins and agricultural fibers. The natural oils and agricultural fibers of interest included corn, soybean, fish, and linseed oils and corn stover, wheat straw (WS), and switchgrass fibers. The effects of the types of natural oil and agricultural fiber on the structure and thermal and mechanical properties of the composites were studied using Soxhlet extraction, thermogravimetric and dynamic mechanical analysis, and tensile testing. The green composites, with agricultural fiber loadings of 75 wt %, have thermal stabilities up to 275°C. The Young's moduli and tensile strengths of the composites ranged from 1590 to 2300 and 5.5 to 11.3 MPa, respectively. In general, an increase in the degree of unsaturation of the natural oil resulted in improvements in the thermal and mechanical properties of the composites. The WS fibers tended to give composites with the best thermal and mechanical properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Production and mechanical characterization of Titanium Carbide ISOL target disks fabricated by direct ink writing

Titanium carbide boasts a large variety of high-temperature applications from aerospace to electronics and is also employed as Isotope Separation On-Line target for the production of Radioactive Ion Beams, which are employed in numerous research and technological fields, ranging from nuclear physics to medical applications. High working temperature, open and tailored porosity and resistance to thermal stresses are fundamental characteristics for this kind of targets. In this work, an extrusion-based additive manufacturing technique (Direct Ink Writing) was used for the fabrication of complex three-dimensional macro-porous structures in the shape of disks with dimensions compatible for their use as targets. An ink containing a suspension of TiC powders with solid loading of 47.5 vol% was prepared and its rheological properties were investigated. Afterwards, single filaments with an average diameter of 0.36 mm were produced and characterized with four-point bending tests to determine the bulk material tensile strength and Young's modulus. TiC targets were then manufactured and their mechanical properties were characterized with the Ball on Three Balls approach, a biaxial flexural test suitable for disk-shaped samples. For both flexural tests, a Finite Element model was developed representatively reproducing the experimental results. The calculated tensile strength values for both filaments and disks were analyzed with Weibull's statistical approach to provide reference stress limit, corresponding to a survival probability of 99.9%.     

Effect of Titanate Coupling Agent on the Mechanical Properties of Flyash Filled Chloroprene Rubber

Effects of treatment of coupling agent [(neopentyl (diallyl) oxy, trineodecanonyl titanate) (LICA 01)] on mechanical properties of composites made from chloroprene rubber and flyash is reported here. A 1% coupling agent was selected for the treatment of the flyash filler. The treatment resulted in enhancement of mechanical properties of composites of the said rubber when compared with untreated flyash composites separately. The properties under consideration were tensile strength, moduli at 100% and 400%, Young's modulus, hardness, etc. Tensile strength was improved by 1365%, modulus at 400% was found to improve by 1410%, while Young's modulus was improved by 1216%.     

Elastic properties of adhesive polymers. I. Polymer films by means of electronic speckle pattern interferometry

The elastic modulus and Poisson's ratio of seven different polymers frequently used as wood adhesives and/or matrix polymers in wood‐ and natural‐fibre‐reinforced composites, respectively, were determined by means of tensile tests. Specimen deformation during testing was measured by means of a mechanical extensometer and an electronic speckle pattern interferometry system, respectively. The results from both methods show an excellent correlation for the elastic modulus. The elastic moduli of the studied polymers cover a wide range from 0.47 GPa for polyurethane to 6.3 GPa for melamine–urea–formaldehyde, whereas Poisson's ratios show less variability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3936–3939, 2007     

Evaluation by various experimental approaches of the crosslink density of urethane networks based on hydroxyl‐terminated polybutadiene

Crosslink density (CLD) is an important characteristic for elastomeric polymer networks. The mechanical and viscoelastic properties of the elastomers are critically dependant on the CLD. Several methods have been adopted for its determination, but swelling and stress–strain methods continue to be more popular because of the convenience associated with these techniques. In this article, the determination of CLD of allophanate–urethane networks based on hydroxyl‐terminated polybutadiene and toluene diisocyanate with swelling and stress–strain methods is reported. The Flory–Rhener relationship was applied to calculate CLD from the swelling data. CLDs were also calculated from the initial slope of the stress–strain curve (Young's modulus), Mooney–Rivlin plots, equilibrium relaxation moduli, and dynamic mechanical properties. A comparison was drawn among the values obtained with the various methods. Although the CLD values obtained from Mooney–Rivlin plots were slightly lower than those obtained from swelling data, the values obtained with Young's modulus and storage modulus were considerably higher. The values obtained with swelling and equilibrium relaxation moduli data were very close to each other. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3129–3133, 2007     

About the influence of temperature and environmental relative humidity on the longitudinal and transverse mechanical properties of elementary alfa fibers

Composites reinforced with plant-based fibers present a high potential for valorization in new industrial applications due to their good specific mechanical characteristics, renewability, and recyclability. In order to accelerate their wide industry adoption, it is critical to assess their behavior and durability in heat and humid environments. This article aims at investigating the effects of temperature and relative humidity (RH) on the longitudinal and transverse mechanical properties of the lignocellulosic fibers extracted from alfa plant (Stipa tenacissima L). For this purpose, tensile and nanoindentation tests were performed on elementary alfa fibers subjected to a thermal cycle of 200°C. The fibers were held at various periods of 15, 30, 60 and 120 min. The test results showed that the longitudinal and transverse Young's moduli are moderately affected by short thermal cycles having duration of 15–30 min. However, for longer thermal cycle (i.e., 2 hr), a degradation of 21% for the transverse modulus was recorded. This degradation doubled for the longitudinal modulus (43 vs. 21%). A similar trend was observed for the breaking strength. This study also showed that the RH strongly affects the mechanical performances of alfa fibers.