PSI - Issue 22

W. Zhang et al. / Procedia Structural Integrity 22 (2019) 251–258

256

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

3.2. Modified CZM for strength degradation evaluation The extent of strength degradation is evaluated by the reduction of maximum load that sealing structure can bear. Analysis in 3.1 shows that CZM is a tool that can describe the load-displacement relationship between in the process of quasi-static separation, as well as derive fracture energy. For the same batch of specimens, it is achievable to simulate the separation of sealing area, and can predict the sealing adhesive strength of the structure and the defined critical displacement of failure. This has guiding significance for the reinforcement design of the hot sealing structure. However, the heat-seal material is a kind of viscous-elastic material and the existing model does not take the constant loading effect into consideration. Therefore, the model needs to be modified by adding the parameter of constant load with its action time. In the eight sets of ADTs, results show that the separating speed is related to the load level. The displacement growth rate, as a characteristic value, is used to describe the degradation rate of the cohesive strength in the process. Residual fracture energy is characterized by the following load-displacement relationship, and lost fracture energy after constant loading degradation is characterized by monitoring the change comparing with original strength displacement CZM. The degradation rate is determined by the ADT data. As shown in Figure 8, the test results show that the displacement increasing speed is approximately steady over time, and is higher under a larger load level. The displacement growth rates at different load levels are given in Table 1, fitting result is shown in Fig 14, with a conclusion that the relationship is approximately linear. The cohesive strength degradation rate is determined based on the slope and load, shown as equation (5): 0 0 = =0.4127 V C   (5) where 1 1 (mm s N ) C     represents the fitting slope of the load-tearing speed curve, and 0 (N)  represents the load.

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Table 1. Degradation rate under different load levels. 0 ( ) N  6 ( 10 / ) V mm s   0 0 5 1.54 10 4.28 15 6.16 20 8.33

10

8

6 v/(  10 -6 mm/s)

4

experiment value fitted curve

2

v=0.4127  R 2 =0.9933

0

0

5

10

15

20

25

 /N

Fig. 12. Fitting curve of degradation rate. With the degradation by constant loading effect considered, time and applied load level are taken as degradation factors to modify the original CZM equation (3). Process of new strength-displacement relationship can be divided into three stages: reaching applied load level 0 stage, maintaining applied load level 0 stage and following failure stage. Then, the modified CZM can be written as:

 

1    f

( ), 0 

0 ( )

f

 





1

1       0 ( ) 0 ( ) f

0 , ,      t

,



f

Vt



0   

(6)

1     f

( ) ( ), f Vt 

0 ( )

f

Vt



