Failure of a Cast Aluminum Engine Head in a High Performance Application

The investigation described in this article examined a cast aluminum head used in a high performance (drag racing) application. These engine heads had been failing within a few seconds of startup. A sample head was examined to determine the cause. Visual inspection found two cracks on the exhaust side of the head. One crack initiated at the bottom of a spark plug well. The head was then sectioned for metallographic examination and fracture analysis. Metallographic examination revealed a large number of shrinkage pores throughout the casting. The fracture surface intersected many of these shrinkage pores. A significantly sized pore was located near the bottom of the suspect spark plug well near a stress-concentrating feature. This pore acted as the crack-initiating defect, which caused the engine head to fail.     

High Gd Concentration GaGdN Grown at Low Temperatures

GaGdN layers were grown at temperatures below 300°C by radio-frequency plasma-assisted molecular beam epitaxy on sapphire substrates. GaGdN samples with high Gd concentration as high as 12.5% were obtained by lowering the growth temperature. X-ray diffraction results showed no obvious secondary phase, which means that the phase separation can be suppressed by the growth at low temperatures. All samples, including those grown at room temperature, showed ferromagnetic characteristics. Photoluminescence emission was observed, though spectra exhibit broad and sharp luminescence bands related to many kinds of defects. It is suggested that electrons coming from defects, especially, nitrogen vacancy, stabilize ferromagnetism, and that the carrier-induced ferromagnetism occurs in the low-temperature-growth GaGdN.     

道路与匝道中线自动绘制的算法与实现

道路与立交匝道平面中线的计算机自动绘制是道路路线CAD软件的核心功能之一,在道路工程计算机动态交互式设计、计算机辅助成图方面具有基础性作用.依据道路中线线形的曲率变化特点,推导了道路中线坐标计算的通用数学模型,并根据一个工程实例在通用图形系统AutoCAD环境下实现了道路与匝道中线的自动绘制.