PSI - Issue 22

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

252

2

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

are packaged by aluminum-plastic compounding films rather than hard shells, In an EV, safety of its energy storage system is crucial, failure of the cell level could escalate to battery module, then to battery pack and ultimately to vehicle level . Gas generation associated with swelling issues is a common phenomenon for pouch cells. It can be caused by electrolyte decomposition during service, and there is a probability of gas generation even the battery is only stored. When the inner gas pressure increases to some extent, it will damage the package container, especially reduce the strength of sealing structure. Therefore, the sealing strength of a pouch cell is a key factor that seriously affect the reliability and safety of the pouch cell. For the heat-seal structure of soft packages, tensile strength tests are used to evaluate the strength, and the influence of technological and environmental factors are studied for industrial manufacture. In the area of polymer strength degradation, Shimokawa investigated the effect of thermal cycles on microcracks and degradation of polymer-matrix composite materials. K.Liao studied the influence of moisture-induced stress on unidirectional polymer composite. However, the heat-seal strength degradation of pouch cell still needs to be extensively investigated for the development of new-energy vehicles. Based on elastic and plastic fracture mechanics, the CZM is a mathematical method used to study the elastic and plastic region of the crack tip. And there are examples using CZM for crazing in polymers. Needleman proposed a unified theoretical framework for the cohesion zone from initial degumming to complete failure. Kent used the trapezoidal cohesion model to investigate the composite cracking process of the film adhesive layer. Zhao simulated the peeling process between metal film and ceramic substrate using the CZM. However, these methods only describe the instantaneous strength-displacement relationship, and they do not take the strength degradation caused by viscous deformation into account. Therefore, it is deserved to develop a method based on CZM for evaluating the adhesive strength degradation of sealing. ADT is adopted in this study to measure the degradation. ADT accelerates performance degradation by increasing the stress level and collects performance degradation data without changing the degradation mechanism. These data are then used to estimate the degradation rate at normal stress levels. Nelson firstly analyzed the ADT data of the insulation material and evaluated the service life under the normal stress. The constant stress accelerated degradation test has been widely applied in product life assessment, the operation of which is simple and whose data statistics method is mature. This paper is devoted to studying the degradation of sealing adhesive strength, and developing an effective testing and modeling method for residual strength evaluation. Firstly, the behavior and mechanism of sealing cohesive strength degradation are analyzed, considering inner gas generation only. And a constant stress ADT is designed and implemented. Then, a modified CZM is developed to evaluate strength degradation using test data, and the validation of model is verified. Finally, some conclusions are given based on the current research. 2. Testing This study only focuses on how inner gas pressure influence the heat-seal area by cohesive strength degradation. Based on that gas generation is a very slow process, which indicates that the influencing process of gas pressure is quasi-static, and the aim is to investigate relationship between degradation rate and residual fracture energy. We set up a group of constant stress tests at room temperature to simulate this process. 2.1. Load-displacement test The material tested in this study is outsourcing aluminum-plastic film of the pouch cell-PA / AL / CPP composite film. Figure 1 shows the product. The standard specimens are cut out from the sealing side with an adhesive overflow area as shown in Figure 2. The specimen is fixed on the testing machine in T-type peeling as shown in Figure 3. The tests were carried out using an in-situ tensile test system, as shown in Figure 4. Several tests were performed at different loading rates, and the load-displacement curves can converge when the tensile rate drops to 0.1N/s. Therefore, the loading rate is defined as 0.1N/s. A relationship between the load and separating displacement of the heat-seal area is obtained.