José Ricardo Tarpani

Nondestructive testing with thermography

Thermography is a nondestructive testing (NDT) technique based on the principle that two dissimilar materials, i.e., possessing different thermo-physical properties, would produce two distinctive thermal signatures that can be revealed by an infrared sensor, such as a thermal camera. The fields of NDT applications are expanding from classical building or electronic components monitoring to more recent ones such as inspection of artworks or composite materials. Furthermore, thermography can be conveniently used as a didactic tool for physics education in universities given that it provides the possibility of visualizing fundamental principles, such as thermal physics and mechanics among others.

Thermal, Mechanical, and Hygroscopic Behavior of Sisal Fiber/Polyurethane Resin-based Composites

This work reports hygroscopic, thermal, and mechanical properties of biomass composites comprising sisal fiber reinforcing castor oil PU resin. The effects of reinforcement geometry and alkaline treatment of fibers were evaluated. In general, alkaline treatment improved quasi-static tensile properties of composites with short randomly oriented and long aligned sisal fibers, respectively. On the other hand, an adverse effect of alkaline treatment was observed in the mechanical behavior of the composite with bidirectional fabric architecture. The outstanding influence of moisture on thermo-mechanical properties of biomass composites was confirmed through thermogravimetric and differential scanning calorimetry techniques. Dynamical-mechanical thermal analysis showed increased storage modulus (i.e., stiffness) and decreased damping properties of biomass composites as compared to neat PU matrix. Dynamical-mechanical testing also detected unexpected decrease on glass transition temperature of composites in regard to the neat polymer resin; resin plasticization due to moisturized fibers and/or alkaline treatment residues was identified as probably the culprit.

Charpy impact toughness of conventional and advanced composite laminates for aircraft construction

A weight-based analysis was made of the translaminar Charpy impact toughness performance of conventional and advanced composite materials for aircraft fabrication. The materials were carbon-epoxy (C-Ep) and hybrid fiber-metal TiGr (Titanium-Graphite) laminates. 5 mm-thick three-point bend specimens were tested over a temperature range of -70 to 180 oC to reproduce typical in-service conditions of supersonic jetliners. The energies required for the processes of damage initiation (Ei), damage propagation (Ep), and whole fracture (Et = Ei + Ep), were evaluated at two loading rates, namely, 2.25 and 5.52 m/s in an instrumented Charpy impact testing machine. C-Ep laminates with unidirectional fiber tapes arranged in cross-ply architecture consistently showed the best performance in terms of damage initiation toughness, whereas the hybrid fiber-metal laminate TiGr excelled in terms of propagation toughness. On the other hand, the overall performance of bi-directional fabric C-Ep laminates was very disappointing. The impact behavior of composite laminates was substantiated by a qualitative analysis of topographic aspects of fracture surfaces.

Mechanical performance of carbon-epoxy laminates Part I: quasi-static and impact bending properties

In Part I of this study, quasi-static and impact bending properties of four aeronautical grade carbon-epoxy laminates have been determined and compared. Materials tested were unidirectional cross-ply (tape) and bidirectional woven textile (fabric) carbon fiber lay-up architectures, impregnated with standard and rubber-toughened resins, respectively, giving rise to 1.5 mm-thick laminates. Quasi-static mechanical properties assessed in transversal mode loading were modulus of elasticity, flexural strength and tenacity at the maximum load, whereas the net absorbed energy was determined under translaminar impact conditions. Two-dimensional woven carbon fiber reinforcements embedded in a rubber-toughened matrix presented the best mechanical performance under static loading. Under dynamic loading conditions, woven fiber fabric pre-forms were favorably sensitive to increasing impact energies regardless the nature of the employed epoxy resin. However, it was concluded that great care should be taken with this material within the low energy impact regimen.

Microstructural and Fractographic Characterization of a Thermally Embrittled Nuclear Grade Steel: Part I - Annealing

A nuclear reactor pressure vessel steel was submitted to different annealing heat treatments aimed at simulating neutron irradiation damage. The obtained microstructures were mechanically tested with subsequent metallographic and fractographic characterization. The relevant microstructural and fractographic aspects were employed in the interpretation of the mechanical behavior of the microstructures in both quasi-static (J-R curve) and dynamic (Charpy impact) loading regimes. A well defined relationship was determined between the elastic-plastic fracture toughness parameter J-integral and the Charpy impact energy for very most of the microstructures.

Microstructural and Fractographic Characterization of a Thermally Embrittled Nuclear Grade Steel: Part II - Quenching and Tempering

A nuclear reactor pressure vessel steel was submitted to different quenching and tempering heat treatments aimed at simulating neutron irradiation damage. The obtained microstructures were mechanically tested and submitted to metallographic and fractographic survey. The relevant microstructural and fractographic aspects were employed in the interpretation of the mechanical performance of the thermally embrittled microstructures. A well defined correlation was determined between the elastic-plastic fracture toughness parameter J-integral and the Charpy impact energy, which was achieved for some of the Q&T microstructures.

Grain Size Effects in the Quasi-Static Fracture Resistance of a Thermally Embrittled RPV Steel (Annealed Microstructures)

The elastic-plastic fracture toughness and crack extension behavior under quasi-static loading regimes of the as-received and several thermally embrittled states of a reactor pressure vessel (RPV) steel were assessed on the basis of microstructural parameters. Through a simple rule of mixture that is typically applied for composite materials, it was found that the equivalent grain size (EGS) of dual-phase annealed microstructures is the controlling parameter of the fracture properties. It was concluded that a Hall-Petch type relationship correlates the J-fracture mechanics criteria to the EGS.

Mechanical performance of carbon-epoxy laminates. Part II: quasi-static and fatigue tensile properties

In Part II of this work, quasi-static tensile properties of four aeronautical grade carbon-epoxy composite laminates, in both the as-received and pre-fatigued states, have been determined and compared. Quasi-static mechanical properties assessed were tensile strength and stiffness, tenacity (toughness) at the maximum load and for a 50% load drop-off. In general, as-molded unidirectional cross-ply carbon fiber (tape) reinforcements impregnated with either standard or rubber-toughened epoxy resin exhibited the maximum performance. The materials also displayed a significant tenacification (toughening) after exposed to cyclic loading, resulting from the increased stress (the so-called wear-in phenomenon) and/or strain at the maximum load capacity of the specimens. With no exceptions, two-dimensional woven textile (fabric) pre-forms fractured catastrophically under identical cyclic loading conditions imposed to the fiber tape architecture, thus preventing their residual properties from being determined.

Grain size effects in the charpy impact energy of a thermally embrittled RPV steel

The Charpy impact energies of a reactor pressure vessel (RPV) steel in the as-received and several thermally embrittled conditions have been assessed on the basis of microstructural parameters. It has been concluded that the parameters controlling the impact-absorbed energy of pre-cracked and side-grooved Charpy test specimens are the equivalent grain size of dual-phase annealed microstructures, and the bainite packet size of single-phase quenched and tempered materials. The Charpy energy has been correlated very well with the reduction in area and true fracture strain of tensile specimens, which could be inferred as grain size governed mechanical properties.

Interphase analysis of hierarchical composites via transmission electron microscopy

Abstract Observation and analysis of the interphase are essential for a detailed understanding of the global composite properties when nanofillers are incorporated as interfacial agents. Techniques such as atomic force microscopy and nano-indentation provide valuable information on interfacial properties associated with the viscoelastic behavior of each phase. However, when the morphology of this region is observed in detail, instrumental errors may regularly appear, decreasing the accuracy of measurements. In this work, the use of transmission electron microscopy (TEM) was explored to image the glass fiber-reinforced polymer GFRP interphase containing interfacial nanocellulose. TEM lamellas were prepared via a focused ion beam to observe the phases disposed within the composite arrangement. Energy dispersive X-ray spectroscopy was also performed to determine the elemental composition in each sample phase. Interphase sizes between 25 and 50 nm thick were found, highlighting the ability of this characterization route to give accurate interfacial measurements. This kind of measurement will open new routes for getting rich information on hierarchically structured composites containing a nanostructure as an interfacial agent.