Stress cracking and photodegradation behavior of polycarbonate. The combination of two major causes of polymer failure

This work investigates the effects of photodegradation on the environmental stress cracking resistance of polycarbonate (PC). Injection molded samples were exposed to the ultraviolet (UV) light for various times in the laboratory prior to solvent contact. The bars were then stressed with two different loads in a tensile testing machine under the presence of ethanol. During this period, the stress relaxation was monitored and, after unloading, the ultimate properties were evaluated. Complementary tests were done by size exclusion chromatography, UV-visible spectroscopy, scanning electron microscopy, and light microscopy. The results indicated that ethanol causes significant modification in PC, with extensive surface crazing as well as reduction in mechanical properties. The previous degraded samples showed a higher level of stress relaxation and a greater loss in tensile strength in comparison with the undegraded ones. The synergist action of photodegradation and stress cracking in PC may be a consequence of the chemical changes caused by oxidation.

Resposta térmica de um compósito PEEK+PTFE+Fibra de carbono+grafite

Composites based on PEEK + PTFE + CARBON FIBER + Graphite (G_CFRP) has increased application in the top industries, as Aerospace, Aeronautical, Petroleum, Biomedical, Mechanical and Electronics Engineering challenges. A commercially available G_CFRP was warmed up to three different levels of thermal energy to identify the main damage mechanisms and some evidences for their intrinsic transitions. An experimental test rig for systematize a heat flux was developed in this dissertation, based on the Joule Effect. It was built using an isothermal container, an internal heat source and a real-time measurement system for test a sample by time. A standard conical-cylindrical tip was inserted into a soldering iron, commercially available and identified by three different levels of nominal electrical power, 40W (manufacturer A), 40W (manufacturer B), 100W and 150W, selected after screening tests: these power levels for the heat source, after one hour of heating and one hour of cooling in situ, carried out three different zones of degradation in the composite surface. The bench was instrumented with twelve thermocouples, a wattmeter and a video camera. The twelve specimens tested suffered different degradation mechanisms, analyzed by DSC (Differential Scanning Calorimetry) and TG (Thermogravimetry) techniques, Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Rays (EDX) Analysis. Before and after each testing, it was measured the hardness of the sample by HRM (Hardness Rockwell M). Excellent correlations (R2=1) were obtained in the plots of the evaporated area after one hour of heating and one hour of cooling in situ versus (1) the respective power of heat source and (2) the central temperature of the sample. However, as resulting of the differential degradation of G_CFRP and their anisotropy, confirmed by their variable thermal properties, viscoelastic and plastic properties, there were both linear and non-linear behaviour between the temperature field and Rockwell M hardness measured in the radial and circumferential directions of the samples. Some morphological features of the damaged zones are presented and discussed, as, for example, the crazing and skeletonization mechanism of G_CFRP

In vitro fracture strength and thermal shock resistance of metal-ceramic crowns with cast and machined AuTi frameworks

STATEMENT OF PROBLEM: AuTi alloys with 1.6% to 1.7% (wt%) Ti provide sufficient bond strength to veneering ceramics, but the strength of entire metal-ceramic restorations fabricated from these alloys is not known. However, this information is important to assess the clinical performance of such materials. PURPOSE: This in vitro study evaluated the fracture strength and thermal shock resistance of metal-ceramic crowns with AuTi frameworks produced by milling or casting. MATERIAL AND METHODS: Frameworks of the alloy Au-1.7Ti-0.1Ir (wt%) (Esteticor Vision) were produced by milling or casting (test groups). A high-gold alloy (Esteticor Special) was used as the control. The frameworks were veneered with ceramic (VMK 95). Specimens (n=7) were loaded until fracture. Loads at failure (N) were recorded and the mean values statistically evaluated using 1-way analysis of variance and a post hoc Dunnett test (alpha=.05). To assess the crazing resistance of the veneering ceramic, 6 additional crowns of each group were subjected to a thermal shock test. Fractured surfaces were documented by scanning electron microscopy. Coefficients of thermal expansion of the materials used were measured (n=2) to assess the thermal compatibility between alloys and ceramic. RESULTS: The mean fracture strength of the crowns with machined AuTi frameworks (1294 +/- 236 N) was significantly lower (P=.012) than that of the cast AuTi frameworks (1680 +/- 150 N), but statistically not different than the high-gold alloy (1449 +/- 159 N). Bonding failure to the AuTi alloy predominantly occurred at the alloy-oxide interface. For the high-gold alloy, more ceramic residues were observed. In the thermal shock test, crowns with milled AuTi frameworks showed significantly higher thermal shock resistance compared to the other groups. The coefficients of thermal expansion (Esteticor Vision cast: 14.5 microm/m.K; Esteticor Vision milled: 14.3 microm/m.K; Esteticor Special cast: 13.7 microm/m.K) did not correlate with the results of the thermal shock test. CONCLUSION: The in vitro fracture strength of crowns with milled AuTi frameworks is lower than that obtained with cast AuTi frameworks, but comparable to those crowns produced with a high-gold alloy.

Simple room temperature method for polymer optical fibre cleaving

In this paper, we report on a new method to cleave polymer optical fibre. The most common way to cut a polymer optical fibre is chopping it with a razor blade; however, in this approach both the fibre and the blade must be preheated in order to turn the material ductile, and thus, prevent crazing. In this paper, we make use of the temperature-time equivalence in polymers to replace the use of heating by an increase of the cleaving time and use a sawing motion to reduce fibre end face damage. In this way, the polymer fibre can be cleaved at room temperature in seconds with the resulting end face being of similar quality to those produced by more complex and expensive heated systems.