Paulo Flores

Comparison of push-in and push-out tests for measuring interfacial shear strength in nano-reinforced composite materials

The influence of the carbon nanotubes (CNTs) content on the fiber/matrix interfacial shear strength (IFSS) in glass/fiber epoxy composites was measured by means of push-in and push-out tests. Both experimental methodologies provided equivalent values of the IFSS for each material. It was found that the dispersion of CNTs increased in IFSS by 19% in average with respect to the composite without CNTs. This improvement was reached with 0.3 wt.% of CNTs and increasing the CNT content up to 0.8 wt.% did not improve the interface strength.

Influence of aramid fibers on the mechanical behavior of a hybrid carbon–aramid–reinforced epoxy composite:

This article is focused on the study of the contribution of aramid fibers in a hybrid carbon–aramid fiber twill weave used to reinforce epoxy resin. To evaluate the influence of the aramid fibers, a comparative study between carbon and carbon–aramid woven–reinforced composites, considering the mechanical behavior of both materials under several loading conditions, is performed. The tests used in this study are meant to analyze the effect of aramid reinforcements on the composite stiffness, strength, impact, and fracture performance. Higher values of energy absorption and fracture toughness were exhibited by the carbon–aramid composite. The mechanical tests performed indicated that the aramid phase present in the hybrid carbon–aramid composite induced an important enhancement on the impact (37.9% in energy absorption) and fracture resistance (12.7% for fracture initiation and 43% for steady state regime), compared to small reductions on the material stiffness. In addition, the ultimate strain and the through thickness compression strength were favorably affected, with an increase up to 19.5% and 8.3%, respectively, by the presence of aramid fiber that presents a more ductile response with respect to the carbon reinforcement.

The influence of carbon fabric weave on the in-plane shear mechanical performance of epoxy fiber-reinforced laminates

Woven fabrics used in composite materials are designed to fulfill specific manufacturing or structural requirements. Knowledge of the influence of the weave structure on the mechanical properties of the composite is essential to properly optimize the design of structural components. The focus of this work is to investigate the influence of the type of weave used for fabric reinforcement in polymers particularly on the in-plane shear mechanical performance. The selected materials are carbon fibers and epoxy resin. The laminates are manufactured by vacuum infusion. Three woven structures are selected for manufacturing the composite laminates: (a) a plain weave with unidirectional orientation in the warp direction, (b) a plain weave with balanced properties in the warp and weft directions and (c) a 2/2 twill weave with balanced properties in the warp and weft directions. The laminates are tested according to the ASTM D 4255 standard by a two-rail shear test under quasi-static monotonic and cyclic loading conditions. The resulting stress–strain curves are used to study the initial in-plane shear modulus and its evolution (which directly correlates with material damage) and the hardening produced by plastic strain. The results show that for vacuum infusion manufacturing, the weave structure has an influence on the resulting fiber and void volume fractions and, consequently, on the mechanical performance. However, for similar fiber volumes, the weave structure is found to have little effect on the experimental results.

Model identification and FE simulations: Effect of different yield loci and hardening laws in sheet forming

The bi‐axial experimental equipment developed by Flores enables to perform Baushinger shear tests and successive or simultaneous simple shear tests and plane‐strain tests. Such experiments and classical tensile tests investigate the material behavior in order to identify the yield locus and the hardening models. With tests performed on two steel grades, the methods applied to identify classical yield surfaces such as Hill or Hosford ones as well as isotropic Swift type hardening or kinematic Armstrong‐Frederick hardening models are explained. Comparison with the Taylor‐Bishop‐Hill yield locus is also provided. The effect of both yield locus and hardening model choice will be presented for two applications: Single Point Incremental Forming (SPIF) and a cup deep drawing.

Material Identification Using a Bi-Axial Test Machine

This paper shows the identification of material parameters for a DC06 IF steel sheet of 0.8 mm by mechanical tests. The experimental equipment used consists of a tensile test machine, a bi-axial test machine able to perform plane-strain and simple shear tests separately or simultaneously and an optical strain gauge. Tensile, plane-strain and simple shear tests were performed at 0°, 45° and 90° from the sheet rolling direction in order to identify Hill 1948 and Hosford 1979 yield criteria. Two identification methods are used: one based on strain measurements (anistropy coefficients) and the other one based on stress measurements (plastic contours). The results confirm that mechanical tests applying other stress-states than tensile are required to obtain accurate material parameters identification.

Multiscale Characterization of Nanoengineered Fiber-Reinforced Composites: Effect of Carbon Nanotubes on the Out-of-Plane Mechanical Behavior

The mechanical properties of the matrix and the fiber/matrix interface have a relevant influence over the mechanical properties of a composite. In this work, a glass fiber-reinforced composite is manufactured using a carbon nanotubes (CNTs) doped epoxy matrix. The influence of the CNTs on the material mechanical behavior is evaluated on the resin, on the fiber/matrix interface, and on the composite. On resin, the incorporation of CNTs increased the hardness by 6% and decreased the fracture toughness by 17%. On the fiber/matrix interface, the interfacial shear strength (IFSS) increased by 22% for the nanoengineered composite (nFRC). The influence of the CNTs on the composite behavior was evaluated by through-thickness compression, short beam flexural, and intraply fracture tests. The compressive strength increased by 6% for the nFRC, attributed to the rise of the matrix hardness and the fiber/matrix IFSS. In contrast, the interlaminar shear strength (ILSS) obtained from the short beam tests was reduced by 8% for the nFRC; this is attributed to the detriment of the matrix fracture toughness. The intraply fracture test showed no significant influence of the CNTs on the fracture energy; however, the failure mode changed from brittle to ductile in the presence of the CNTs.

Study of the effect of amino-functionalized multiwall carbon nanotubes on dry sliding wear resistance properties of carbon fiber reinforced thermoset polymers

AbstractThis work investigates the effect of multiwall carbon nanotubes (MWCNTs) on the mechanical and tribological behavior of a fiber reinforced composite (FRC). Fiber reinforced composites and nano-engineered FRCs are manufactured by resin transfer molding. In-plane tensile tests, in-plane shear tests and through-thickness compression tests are used to assess the influence of MWCNTs on the material mechanical behavior. Pin on disk dry sliding tests are used to quantify the effect of MWCNTs on the friction coefficient and the specific wear rate. It was determined that (1) MWCNTs have an influence on the improvement on both the through-thickness compression strength and the specific wear rate, and (2) they do not influence the material stiffness, in-plane tensile and shear strengths and the friction coefficient. It is assumed that the observed improvements are due to the demonstrated positive influence of the MWCNTs effect on the matrix/reinforcement interfacial strength and on the matrix fracture toughness.

Plane Strain Test for Metal Sheet Characterization

This article shows the influence of a plane strain test specimen geometry on the measurable strain field and the influence of free edge effects over the stress computation. The experimental strain field distribution is measured over the whole deformable zone of a plane strain test specimen by an optical strain gauge. The chosen material is the DC06 IF steel of 0.8 mm thickness. The stress field is computed for several geometries at different strain levels by a Finite Element (FE) commercial code (Samcef ®). The results show that the stress field is sensitive to the specimen’s geometry and also to the tested material (strain field behavior is independent of material) and, based on results, an optimal specimen geometry is proposed in order to minimized the stress computation error.

The effect of molecular weight and hydrolysis degree of poly(vinyl alcohol)(PVA) on the thermal and mechanical properties of poly(lactic acid)/PVA blends

Comision Nacional de Investigacion Cientifica y Tecnologica CONICYT (Beca de Doctorado Nacional - Proyecto) PAI 781411004 CONICYT-REGIONAL R08C1002 Programa de Financiamiento Basal para Centros Cientificos y Tecnologicos de Excelencia PFB-27

Antibacterial Nanocomposite of Poly(Lactic Acid) and ZnO Nanoparticles Stabilized with Poly(Vinyl Alcohol): Thermal and Morphological Characterization

ABSTRACT The effect of polyvinyl alcohol (PVA) as a surface coating agent on the antibacterial and thermal properties of polylactic acid (PLA)/ZnO nanocomposites prepared by melt blending was investigated. The ZnO nanoparticles were coated and stabilized with PVA using a solvothermal method. Nanocomposites were prepared with different ZnO nanoparticle content: 1, 3 and 5 wt.%. Electron transmission microscopy and Fourier transform infrared spectroscopy showed the presence of a layer around the nanoparticles and the interaction between nanoparticles and PVA, respectively. DSC analysis revealed that the thermal properties of the nanocomposites were not affected by the coating of ZnO nanoparticles with PVA. The PLA/ZnO nanocomposites with coated nanoparticles presented better antibacterial activity than those containing uncoated nanoparticles. Graphical Abstract