PSI - Issue 17

V.P. Matveenko et al. / Procedia Structural Integrity 17 (2019) 363–370 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

365

3

considered. Assume that with external influences that may be different from each other by a constant factor, there is a set of sensors, that measure the strain i  in a given number of points, where i – is the number of sensor ( ) 1, 2, , i n = K . Then for the considered set of external influences, the measured strains will be characterized by the following number ( ) 1 2 n n − of relative values:



1,2, , , , i s n = K K ;

(1)

i

1, 2, , , j i i s n = + + K K



j

The defect appearance in the vicinity of point s will lead to a change in the strain s  and, accordingly, to a change in the relative values i j   1,2, , 1 i s = − K ; j s = и i s = , 1, 2, , j s s n = + + K . Thus, the change in monitoring of the relative values i j   signals the appearance of the defect and its development. The practical implementation of the considered technique becomes real when using a limited number of sensors. To select the sensors location, the results of numerical analysis of the stress-strain state are used for the considered variant of external loads, in particular, external forces specified on the surface of the monitored object: Here S  − the part of the external surface of the object on which external forces are specified; ij  − stress tensor components; j n − surface normal vector components; i x − coordinates of surface points; j P − components of the surface forces vector; k – constant. Based on the obtained numerical results, the sensors are proposed to be placed in the stress concentration zones, where the probability of a defect (destruction) appearance is naturally high and also in the zone without stress concentration. The results of the numerical analysis of the stress-strain state are also used to obtain the initial values i j   for the corresponding option of external influences. The reliability of the relative values i j   obtained in a numerical experiment is estimated experimentally at the initial stage of monitoring, when there are no defects (damages) in the material. 3. Experimental registration of defects in a structurally similar element As an object, which illustrates the method of damage detection on the basis of experimental strain data, a structurally similar element (specimen) of VPS-48 glass fiber reinforced plastic in the shape of an angle with a loading scheme, shown in figure 1, is considered. The mechanical characteristics of the material are given in table 1. x S   ( ) j i n P x k  =  ; ( ij j ) 1, 2,3; 1, 2,3 i j = = (2)

Table 1: Mechanical properties of GFRP VPS-48 X E , GPa Y E , GPa Z E , GPa XY  YZ 

YZ G , GPa

XZ G , GPa

XY G , GPa

XZ  0.14

22

22

8.7

0.14

0.14

3

3

3

In figure 2 various stages of the tensile test of the specimen on the testing machine are shown. According to the given loading scheme, the lower part of the large shelf of the specimen is fixed, and a tensile force is applied to the short (upper) shelf of the specimen. This loading scheme allows to ensure the defects occurrence in the stress concentration zone of a specimen. The possible changes in the boundary conditions in the area of the bolted joint under loading process are not taking into account. Th at’s why only qualitative correspondence of numerical simulation and experimental results can be achieved.