Laura Alejandra Fasce

Mechanical evaluation of propylene polymers under static and dynamic loading conditions

The present investigation is concerned with the evaluation of the impact toughness of commercial-grade Propylene polymers. Conventional impact static stress–strain and static fracture experiments were carried out. Static stress–strain experiments revealed different pattern behaviors among the materials that were reflected in the fracture behavior. Under static conditions, all materials exhibited ductile behavior and crack grew under J-controlled conditions displaying stress whitening through the whole fracture surface with the sole exception of the homopolymer, which displayed a ductile instability after some stable crack growth. Under dynamic conditions the homopolymer exhibited brittle behavior, the block copolymer exhibited some plastic deformation at the crack tip, and the random copolymer samples exhibited a whitening effect due to voiding and craze formation through the whole fracture surface, indicating that stable crack propagation was occurring. Fracture mechanics tests were analyzed by following different methods, depending on the mode of fracture presented by the polymer. The Normalization J-method was used under static conditions. The elastic method, the corrected elastic method, and the essential work of fracture methodology were used to characterize brittle, semibrittle, and ductile behavior, respectively. Fracture mechanics parameters arisen from both static and dynamic conditions are compared.

ASSESSMENT OF IMPACT FRACTURE TOUGHNESS OF PP–ELASTOMERIC POLYOLEFIN BLENDS

The present investigation is concerned with evaluation of the impact fracture toughness of novel blends based on commercial polypropylene homopolymer (PPH) and elastomeric polyolefin (POEs). At room temperature and high load rate, PPH behaves in a brittle manner while the blends exhibited semi-brittle behavior as judged from the nonlinearity in load deflection curves and fracture surface appearance. The challenge of determining reliable toughness values was faced by applying different approaches available in the literature based on fracture mechanics concepts, including corrected linear elastic fracture mechanics (LEFM), equivalent energy concept, and non-LEFM. The fracture toughness data of the blends appeared to be widely scattered in accord with the samples being in the ductile–brittle transition region. In order to provide a consistent description of the entire range of sample behavior, the statistical weakest link model was also applied to the data.

Thermal Degradation Behavior, Permeation Properties and Impact Response of Polyethylene/Organo-montmorillonite/(Ethylene Methacrylic Acid) Ternary Nanocomposites

Through this work we explored the effect of melt compounding a commercial grade of HDPE with organoclays of different precedence using EMAA as compatibilizing agent on the thermal behavior, barrier properties and biaxial impact response of composites. Morphology was examined by XRD and TEM. Crystalline structure was examined by DSC. Thermal behavior was evaluated by TGA. Barrier properties to low-molecular-weight penetrants were experimentally determined employing a gravimetric technique. Mechanical properties under impact conditions were evaluated by instrumented puncture tests. Intercalated nanocomposites were obtained. Throughout the thermal degradation of the nanocomposites in oxidant atmosphere a charring process of the PE, which is normally a non-char-forming polymer, was observed. The addition of OMMT improves barrier properties due to its contribution to tortuosity path and to the reduction of molecular mobility. Impact properties were only slightly reduced by nanocomposite formation. Results demonstrate that EMAA did not improve exfoliation, but it enhanced polymer–organoclay interactions giving rise to better thermal and permeation properties, without detriment of impact response.

Preparation, Physical and Mechanical Characterization of Montmorillonite/polyethylene Nanocomposites

In this paper, we report the preparation of polyethylene composites with organically modified montmorillonite. Three different Na+-montmorillonites were modified in order to obtain organoclays and two grades of high-density polyethylene were used as composite matrices. All composites were prepared by melt blending, and their physical and mechanical properties were thoroughly characterized. The extent of clay platelet exfoliation in the composites was confirmed by X-ray diffraction (XRD). Mechanical properties under static and impact conditions were evaluated to assess the influence of the reinforcement on the properties of polyethylene.

On the Feasibility of Identifying First Order Ogden Constitutive Parameters of Gelatin Gels from Flat Punch Indentation Tests

The feasibility of extracting First Order Ogden constitutive parameters of gelatin gels from experimental flat punch indentation curves is analyzed. Eight gelatin gel samples of different formulations are evaluated. Identification of constitutive parameters is carried out by an inverse method that combines finite element modeling simulations and numerical optimization. Parameters are compared with those obtained from uniaxial compression. A parametric study of the influence of model parameters on the shape of curves and a sensitivity analysis of parameter re-identification is performed. Accurate extraction of parameters is possible if the domain in which α parameter stands are lower than 2.

Impact Fracture Toughness Determination of Ductile Polymers by SPB Method

Fracture toughness of ductile materials is often characterized by the J parameter that was developed from the J-integral concept. The determination of a critical value of the J-integral is generally performed through the construction of the resistance curve J-Δa of the material. The commonly used method for this purpose is the multiple specimen technique [1] in which several specimens are loaded to obtain different amounts of crack growth. However, this method is very difficult to apply at high rate conditions because of the need of interrupting the test at different crack growth levels. Recently, a new extremely simple single specimen method has been developed from the separation parameter, S pb, and successfully applied in fracture toughness characterization of metals and ductile polymers [1,2]. It consists on the assumption that the load can be separated into two multiplicative functions: the geometry (G) and the deformation (H) functions. The separation parameter, S pb, is defined as the load ratio of a sharp and a blunt notched specimen of the same material, geometry and constraint. Assuming that the load falls in correspondence with stable crack propagation, the load drop after maximum load can be taken as a symptom of crack growth. From the S pb expression (Eq. 1) and counting with at least two calibration points (in order to assess the m parameter in Eq. 1), a simple relationship between the load and the crack growth length can be simply obtained and then the J-Δa curve can be evaluated.

On the interpretation of textural properties of gels through the mechanics of soft materials

\left. {S_{pb} = \left. {\frac{{P_p (a_p ,v)}} {{P_b (a_b ,v)}}} \right|_v = \frac{{G_p \left( {\frac{{a_p }} {W}} \right)}} {{G_b \left( {\frac{{a_b }} {W}} \right)}}} \right|_v = \frac{{\left( {\frac{{a_p }} {W}} \right)^m }} {{\left( {\frac{{a_b }} {W}} \right)^m }} = \left. {\left( {\frac{{a_p }} {{a_b }}} \right)^m } \right|_v

Morphology and Mechanical Properties of HDPE Reinforced with PET Microfibres

(1) The S pb method may appear very similar to the Normalization method; though, S pb method has the appealing advantage of requiring the assumption of only one hypothesis, i.e. a geometry function, which indeed is well known for several specimen configurations.