Search Results for Crystallization

Housings (being tested as part of a material conversion) from an electrical appliance failed during an engineering evaluation. They had been injection molded from a commercial polycarbonate/PET blend. Parts produced from the previous material, a nylon 6/6 resin, had consistently passed the testing regimen. Grease was applied liberally within the housing assembly during production. Investigation included visual inspection, 24x SEM images, micro-FTIR in the ATR mode, and analysis using DSC. No signs of material contamination were found, but the thermograms showed a crystallization of the PET resin. The grease present within the housing assembly, analyzed using micro-FTIR, was composed of a hydrocarbon-based oil, a phthalate-based oil, lithium stearate, and an amide-based additive. The conclusion was that the appliance housings failed through environmental stress cracking caused by a phthalate-based oil that was not compatible with the PC portion of the resin blend. Thus, the resin conversion was the root cause of the failures. Additionally, during the injection molding process the molded parts had been undercrystallized, reducing their mechanical strength. More importantly, the resin had been degraded, producing a reduction in the molecular weight and reducing both the mechanical integrity and chemical-resistance properties of the parts.

This article describes the visual, fractographic, and metallographic evidence typically encountered when analyzing stress rupture of turbine airfoils. Stress-rupture fractures are generally heavily oxidized, tend to be rough in texture, and are primarily intergranular and/or interdendritic in appearance compared to smoother, transgranular fatigue type fractures. Often, gross plastic yielding is visible on a macroscopic scale. Commonly observed microstructural characteristics include creep voiding along grain boundaries and/or interdendritic regions. Internal voids can also nucleate at carbides and other microconstituents, especially in single crystal castings that do not possess grain boundaries.

This article provides practical information and data on property development in engineering plastics. It discusses the effects of composition on submolecular and higher-order structure and the influence of plasticizers, additives, and blowing agents. It examines stress-strain curves corresponding to soft-and-weak, soft-and-tough, hard-and-brittle, and hard-and-tough plastics and temperature-modulus plots representative of polymers with different degrees of crystallinity, cross-linking, and polarity. It explains how viscosity varies with shear rate in polymer melts and how processes align with various regions of the viscosity curve. It discusses the concept of shear sensitivity, the nature of viscoelastic properties, and the electrical, chemical, and optical properties of different plastics. It also reviews plastic processing operations, including extrusion, injection molding, and thermoforming, and addresses related considerations such as melt viscosity and melt strength, crystallization, orientation, die swell, melt fracture, shrinkage, molded-in stress, and polymer degradation.

This article presents tools, techniques, and procedures that engineers and material scientists can use to investigate plastic part failures. It also provides a brief survey of polymer systems and the key properties that need to be measured during failure analysis. It describes the characterization of plastics by infrared and nuclear magnetic resonance spectroscopy, differential scanning calorimetry, differential thermal analysis, thermogravimetric analysis, thermomechanical analysis, and dynamic mechanical analysis. The article also discusses the use of X-ray diffraction for analyzing crystal phases and structures in solid materials.

Rivets from the longitudinal seam of the terminal shell ring of a 12 year old Lancashire boiler broke off easily during examination. Cleavage fractures indicated a brittle material. Microstructure of a sectioned rivet head was typical of a normal rimming steel except the ferrite crystals contained numerous nitride needles. Their existence indicated an abnormally high nitrogen content. If such a steel is heated for a lengthy period to a temperature of that prevailing in a boiler, precipitation of the nitrides may be expected, with consequent embrittlement. In this case, embrittlement of this type was the primary cause of the breaking off of the type rivet heads. Nothing was observed in the course of the examination that suggested caustic cracking.