223,730 research outputs found

    An Evaluation of Analysis Methods to Eliminate the Effect of Density Variation in Property Comparisons of Wood Composites

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    The objective of this research was to evaluate commonly used data analysis methods in property comparisons of wood composites to eliminate the effect of the density variation among board test specimens and to suggest a more reasonable and robust method. The methods reviewed included average, specific strength, and analysis of covariance. The indicator variable method was also applied to the property comparison and compared to the other methods. The modulus of rupture of wood fiber/polymer fluff composites manufactured with different material combinations and press temperatures was tested in the experiment for evaluation of the different analysis methods. The results of this study indicated that the statistical analysis method employed was very important in the study of the physical and mechanical properties of wood composites. The specific strength method is limited to the analysis of strength comparison for the high density composites. The analysis of covariance can be applied to all the property comparisons for either high or low density composites in eliminating the density variation effect. However, error exists in the property comparison using the analysis of covariance method when the slopes of the regression lines of property vs. specific gravity (SG) are different for the different composites being tested. The indicator variable method is shown to be more reliable than the specific strength and analysis of covariance methods because it compares the linear regression lines of property vs. SG by testing both the intercept and slope based on the data in the whole specific gravity range of test specimens

    Predicting Mortar Compressive Strength Using HYDCEM

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    The compressive strength of mortar is a significant property that will influence its performance in concrete or masonry. Being able to accurately model and predict the mortar compressive strength would be of great benefit to suppliers and end users alike that could possibly reduce the need for multiple physical testing. A section of the original HYDCEM cement hydration model (amoungst others) has been partitioned to focus on predicting the compressive strength of Portland cement and cement-limestone mortars, entitled HYDCEM_CompressiveStrength. The model uses the cement/binder oxide composition along with other inputs to predict the compressive strength development over time. This paper presents a study into how accurately the HYDCEM_CompressiveStrength model can predict the mortar’s compressive strength over time for European cements. Experimental results of mortar cube’s and bar’s compressive strength in accordance with ASTM C 109 for a CEM I + 10% limestone binder and EN196-1 for a CEM I and CEM II cement are presented along with predictions from the model following a parametric study. Comparisons have shown reasonably good agreement between measured and predicted values over time

    Macroscopic Anisotropic Bone Material Properties in Children with Severe \u3cem\u3eOsteogenesis imperfecta\u3c/em\u3e

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    Children with severe osteogenesis imperfecta(OI) typically experience numerous fractures and progressive skeletal deformities over their lifetime. Recent studies proposed finite element models to assess fracture risk and guide clinicians in determining appropriate intervention in children with OI, but lack of appropriate material property inputs remains a challenge. This study aimed to characterize macroscopic anisotropic cortical bone material properties and investigate relationships with bone density measures in children with severe OI. Specimens were obtained from tibial or femoral shafts of nine children with severe OI and five controls. The specimens were cut into beams, characterized in bending, and imaged by synchrotron radiation X-ray micro-computed tomography. Longitudinal modulus of elasticity, yield strength, and bending strength were 32–65% lower in the OI group (p \u3c 0.001). Yield strain did not differ between groups (p ≥ 0.197). In both groups, modulus and strength were lower in the transverse direction (p ≤ 0.009), but anisotropy was less pronounced in the OI group. Intracortical vascular porosity was almost six times higher in the OI group (p \u3c 0.001), but no differences were observed in osteocyte lacunar porosity between the groups (p = 0.086). Volumetric bone mineral density was lower in the OI group (p \u3c 0.001), but volumetric tissue mineral density was not (p = 0.770). Longitudinal OI bone modulus and strength were correlated with volumetric bone mineral density (p ≤ 0.024) but not volumetric tissue mineral density (p ≥ 0.099). Results indicate that cortical bone in children with severe OI yields at the same strain as normal bone, and that their decreased bone material strength is associated with reduced volumetric bone mineral density. These results will enable the advancement of fracture risk assessment capability in children with severe OI

    Replacement of MDA with more oxidatively stable diamines in PMR-polyimides

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    Studies are performed to investigate the effect of substituting 4,4'-oxydianiline and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane for the 4,4'-methylenedianiline in PMR polyimide matrix resin. Graphite fiber reinforced composites are fabricated from unsized Celion 6000 and PMR-polyimide matrix resins having formulated molecular weights in the range of 1500 to 2400. The composite processing characteristics are investigated and the initial room temperature and 316 C (600 F) composite mechanical properties are determined. Comparative 316 C composite weight losses and 316 C mechanical properties retention after prolonged 316 C air exposure are also determined

    Surface protection of graphite fabric/PMR-15 composites subjected to thermal oxidation

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    Graphite fabric/PMR-15 laminates develop matrix cracks during long-term exposure in air at temperatures in the range of 500 to 600 F. This study was performed to demonstrate the effectiveness of incorporating graphite mat surface plies as a means of reducing the developing of matrix cracks. Celion 3000 graphite fabric/PMR-15 laminates were fabricated with graphite or graphite mat/325-mesh boron powder surface plies. Laminates without mat surface plies were also fabricated for control purposes. Composite flexural strength, flexural modulus, and interlaminar shear strength were determined at 288 C before and after long-term exposure (up to 1500 hr) in air at 316 C. The results of this study showed that the incorporation of graphite mat surface plies reduces matrix cracking and improves the elevated temperature mechanical property retention characteristics of the composites

    Mechanical property characterization of intraply hybrid composites

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    An investigation was conducted to characterize the mechanical properties of intraply hybrids made from graphite fiber/epoxy matrix (primary composites) hybridized with varying amounts of secondary composites made from S-glass or Kevlar 49 fibers. The tests were conducted using thin laminates having the same thickness. The specimens for these tests were instrumented with strain gages to determine stress-strain behavior. Significant results are included

    The effect of inorganic fillers on the properties of wood plastic composites

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    The effect of inorganic fillers including precipitated calcium carbonate (PCC), glass fiber (GF), and nano-clay on properties of structured WPCs was investigated. In PCC-bamboo-polymer hybrid composites, tensile and flexural moduli were improved with increasing PCC content. After silane treatment of bamboo, RBF-filled hybrid composites showed better mechanical properties compared to those of GBP-filled hybrid composites. The hybrid composites showed 3-4 times higher modulus than those of PCC-filled composites at high PCC levels. Various property differences were observed between weak- and strong-core coextruded systems with shell composition changes. While the weak-core systems showed improved flexural strengths compared to their core-only control, the strong-core systems had lowered flexural strengths. In both systems, impact strengths increased at low shell filling levels but decreased at high shell filling levels. Impact fracture types varied with core quality and shell filling composition. Coextruded composites with treated PCC-filled shell showed better water absorption (WA) property compared to core-only controls and coextruded composites with high WF-filled shell. Plastic-only shell increased overall coefficient of thermal expansion (CTE) of coextruded composites, but filled shells led to the CTE decreases of coextruded composites. GF in shell behaved as an effective reinforcement for coextruded composites. The comparisons of flexural property among different core systems show that GF reinforcements were optimized at high GF loadings in a shell layer and GF alignments in the shell layer also played an important role. In coextruded composites with different shell thicknesses, the flexural property enhanced with the increase of shell bending modulus and strength at a given shell thickness. When the flexural property of shell was less than that of core, the increase of shell thickness led to reduced flexural property. On the other hand, when the flexural property of shell was higher than that of core, the opposite was true. In sound transmission loss (TL) testing, the stiffness and surface density were major factors influencing the sound insulation property of materials. The experimental TL results showed that the addition of clay or PCC and/or wood fiber (WF) fillers led to the increases of general resonance frequencies and TL in filled composites. However, at high filling levels, composite stiffness decreases led to TL reduction. The experimental TL curves of filled HDPE and WPCs were well approximated with the combined TL predictions from their corresponding stiffness-1 and stiffness-2 TL for S-region and mass law TL for M-region

    The effect of elevated temperature exposure on the fracture toughness of solid wood and structural wood composites

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    This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Springer and can be found at: http://www.springer.com/life+sciences/forestry/journal/226.Fracture toughness of wood and wood composites has traditionally been characterized by a stress intensity factor, an initiation strain energy release rate (G[subscript init]) or a total energy to fracture (G[subscript f]). These parameters provide incomplete fracture characterization for these materials because the toughness changes as the crack propagates. Thus for materials such as wood, oriented strand board (OSB), plywood and laminated veneer lumber (LVL), it is essential to characterize the fracture properties during crack propagation by measuring a full crack resistant or R curve. This study used energy methods during crack propagation to measure full R curves and then compared the fracture properties of wood and various wood-based composites such as, OSB, LVL and plywood. The effect of exposure to elevated temperature on fracture properties of these materials was also studied. The steady state energy release rate (G[subscript SS]) of wood was lower than that of wood composites such as LVL, plywood and OSB. The resin in wood composites provides them with a higher fracture toughness compared to solid lumber. Depending upon the internal structure of the material the mode of failure also varied. With exposure to elevated temperatures, G[subscript SS] for all materials decreased while the failure mode remained the same. The scatter associated with conventional bond strength tests, such as internal bond (IB) and bond classification tests, renders any statistical comparison using those tests difficult. In contrast, fracture tests with R curve analysis may provide an improved tool for characterization of bond quality in wood composites
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