Abstract: To design and analyze the butt joint ultrasonic fatigue specimen of MB8 wrought magnesium alloy,and to study the influence of weld seam size and resonance length on natural frequency of the specimen.The size of ultrasonic fatigue specimen without weld seam was calculated by analytic method which provides the preliminary reference size for the sample of the butt joint.In the finite element software,the sample of the weld joint was simulated and analyzed by establishing solid model,meshing and modal analysis.The influence of the weld size on the natural frequency of the sample was obtained by changing the weld seam size.The influence of the resonance length on the natural frequency of the sample was acquired by altering the resonance length.According to this rule,the sample size is optimized,so that the natural frequency of the butt joint sample is close to 20kHz. The weld seam on ultrasonic fatigue will make the frequency of the specimen natural deviate from 20kHz.Increasing width and height of weld seam will lead to enlarge the natural frequency of specimen.When the height of the weld seam increases,the effect on the natural frequency of specimen becomes weakened.When the width of the weld seam increases,the effect of width on the natural frequency amplitude of specimen remains unchanged.The natural frequency of the sample will be 20.022kHz when the L₃ is adjusted to 17.4 mm.Therefore final size of MB8 wrought magnesium alloy butt joint ultrasonic fatigue specimen is as below:b₁ for 3mm,b₂ for 8mm,L₁ for 12.5mm,L₂ for 15mm,R₀ for 46.25mm,L₃ for 17.4mm.

Key words: magnesium alloy, ultrasonic fatigue, butt joint, analytic calculation, modal analysis, natural frequency