Issue 49

A. Kumar et alii, Frattura ed Integrità Strutturale, 49 (2019) 515-525; DOI: 10.3221/IGF-ESIS.49.48 524 R EDUCTION IN LENGTH OF WAVEGUIDE he main purpose of waveguide is to transmit signal from the vibrating body to which it is attached at one end (ALIP in present case) to the accelerometer mounted at its other end. An insulation of thickness 150 mm covers the ALIP body, and hence, the minimum length of waveguide must be greater than this. Initially it has been proposed to use a 300 mm waveguide for which fatigue life has been estimated. With the objective of obtaining lower stresses and hence, higher fatigue life, a 200 mm long waveguide has been analyzed. This decrease in length is assumed to have no appreciable increase in the temperature at the accelerometer region as compared to original length. Analysis The bending stress at the weld location for 200 mm long waveguide was found to be 47.84 MPa, whereas the natural frequency of this waveguide is shown in following table: Mode Frequency (Hz) 1 56.20 2 876.21 Table 4: Different mode shape frequency of the waveguide Stress due to vibration For finding out the stress response, 3 cycles were analyzed in this case at a frequency of 10 Hz and maximum amplitude of 0.5 m/s 2 . The stress response is shown in the figure below. Figure 12: Stress at the weld location as a function of time Estimation of fatigue life of the weld In this case the mean stress has reduced to about 48 MPa while the stress range has become 5.8 MPa. Under such condition and neglecting the exponentially decaying amplitude region the fatigue life of the weld has been estimated to be N = 1.483 × 10 9 cycles i.e. the welded joint can withstand a continuous operation for 1716 days or approximately 4.7years, if it continues to vibrate at 10 Hz at 200°C. With a decrease in stress range as well as mean stress the number of start up which the weld can withstand, due to initial exponentially decaying stress amplitude, would be higher than those calculated for 300 mm length. T