Low Residual Stress Level Research Articles

Deposition of thick doped polysilicon films with low stress in an epitaxial reactor for surface micromachining applications

Polysilicon films were deposited in an epitaxial batch reactor at a high deposition rate. The effect of the deposition temperature, total pressure, source gas flow and in situ doping on the deposition rate and morphological and electrical properties of these films was studied. Layers with thicknesses up to 12 μm were deposited on a sacrificial oxide covered with a thin polysilicon nucleation layer, which was formed in a low pressure chemical vapour deposition reactor. The thick polysilicon layers were characterized in terms of the surface roughness, grain growth and concentration and distribution of dopants. The residual stress and stress gradient were also determined. This material was found to be highly suitable for surface micromachining applications because of its low residual stress level, homogeneous doping profile and excellent mechanical properties.

Evaluation of effect of plate thickness on fatigue strength of butt welded joints by a test maintaining maximum stress at yield strength

The thickness effect on fatigue strength of transverse butt welded joints was investigated by a test method which simulates the behavior of large sized welded structures having tensile residual stresses by means of ordinary narrow specimens containing a low level of residual stresses. In the test, stress range was varied from test to test while always maintaining the maximum stress at the yield strength of the base metal. The fatigue strength of 9 and 40 mm thick specimens coincided with each other in the test condition described above. Though, the fatigue strength of 40 mm thick specimens was lower than that of 9 mm thick specimens in the test at the stress ratio of 0. It is suggested that the thickness effect appeared in R = 0 test occurred from the difference of residual stresses at the toe of weld. It is also pointed out that the estimated fatigue strength by modified Goodman or Gerber relations supposing the existence of high tensile residual stress was over conservative compared with the results by the test maintaining the maximum stress at the yield strength.

A Method for Obtaining Conservative S-N Data for Welded Structures

Abstract A method of fatigue testing is proposed to simulate the behavior of large-sized welded structures having high tensile residual stresses by means of ordinary small width specimens containing a low level of residual stresses. The method involves the varying of the stress range from test to test while always maintaining the maximum stress at the yield strength of base metal. The results obtained by the proposed method agreed with those for slit welded joints containing high tensile residual stresses fatigued at constant amplitude at a stress ratio of zero. However, the fatigue strength of small width welded specimens as determined by the proposed method was lower than that obtained by the conventional method at a stress ratio of zero. It is emphasized that the proposed method is effective in obtaining conservative S-N data to be used for design of welded structures, where local fluctuating stresses were considered to pulsate downwards from tensile yield strength regardless of the applied stress ratio. It was also found that in the presence of a high tensile residual stress the grinding of the toe of welds which contain no undercut was not effective in improving the fatigue strength of welded joints.