PSI - Issue 22

Yihua Dou et al. / Procedia Structural Integrity 22 (2019) 33–42

36 4

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

2.1. Strain gauge arrangement scheme As shown in Fig. 2, the strain and stress near the hole and the casing are measured by arranging strain gauge near the hole and the casing according to a certain rule by using the experimental stress test method. Then the concentrated stress at the edge of the hole is obtained by extrapolation. The stress concentration coefficient around the perforated hole is obtained by comparing the stress on the edge with that in the pipe.

Fig. 2 strain gauge scheme on perforated casing The specific layout scheme is shown in Fig. 2. It is a triangle formed by three adjacent perforated holes. Due to the burr around the perforated hole and the size limit of strain gauge, it is impossible to attach the strain gauge to the edge of the hole completely. Therefore, four strain gauges 1, 2, 3 and 4 are attached along the ring. Attach four strain gauges 5, 6, 7 and 8 along the axial direction; From the strain stress values of 1, 2, 3 and 4 points, the circular concentrated stress at the edge of the perforation can be extrapolated. From the strain stress values of 8, 7, 6 and 5 points, the axial concentrated stress at the edge of the perforation can be extrapolated. The strain gauge 11 at the center of the triangle is used to measure the stress of the pipe body. The strain gauge at the middle point of the three sides of the triangle monitors the strain and stress values between the two adjacent holes. Although a strain gauge is accurate, it should be noted that the strain gauge accumulates the measured values along its length, and the stress gradient is large at the stress concentration, so a very small strain gauge should be used. In the vicinity of the hole, although it is possible to get as close to the edge as possible, it is not strictly possible to reach the edge of the hole completely. Three strain gauges are used to extrapolate the analytic curve, and three measured values determine a parabola. It can be assumed that the strain value changes in accordance with the parabola. The following formula (2) can be used to approximate the maximum stress at the hole edge.

2 2 d d e m e  ( )

( )( 2 ) d e d e  

( 2 ) d d e 

M

m

m







3

1

2

2

2

2

e

e

(2) where, m1, m2 and m3 is measured values of three strain gauge; d is the distance from the first strain gauge to the boundary; e is the distance from one strain gauge to each other.

2.2. Sample preparation

As shown in Fig. 3, the casing with about 9.8 meters long and 5 1/2 "x 9.17 mmP110 is evenly cut for 7 pipe samples. In the experiment well the commonly used perforating cartridges are combined for test. With 45 °, 60 °, 90