The functionality and reliability of the separator are crucial to the abuse tolerance of a battery since the separator serves as the physical barrier to prevent any contact (short circuit) between the positive and negative electrodes. Therefore, understanding the mechanical behavior, especially the deformation and fracture behaviors of the separator are of great importance for battery design and manufacturing. Here we report the deformation behaviors of five commercially available microporous polymer separators investigated by conventional tensile testing coupled with in situ tensile testing under an atomic force microscope. Morphological models were developed to elucidate the tensile deformation mechanisms. For anisotropic separators (Celgard 2325 and 2400) made by the dry process, material direction dictates the significant diversity in overall mechanical integrity of the separator: they have limited mechanical properties when stretched in the transverse direction (TD), whereas they are rather robust when pulled in the machine direction (MD). The anisotropy of these separators is a result of the distinct deformation mechanism of the stacked lamellae in the separator. Separators manufactured by the wet technique (Toray V20CFD and V20EHD, Teijin Lielsort) behaved more biaxially – all mechanical properties were nearly identical in both MD and TD. Moreover, in order to evaluate the fracture properties of these separators, the essential work of fracture (EWF) approach was adopted. The EWF results show that the fracture properties for the dry processed separators also present orientation dependence. When stretching in the MD, the MD-oriented slit-like pores serve as crack tip blunters to inhibit the propagation of cracks whereas the TD-oriented pores exactly facilitate the crack propagation by linking up the pores with the crack tip when stretching in the TD. The same toughening mechanism (tip blunting) was also found in the case of wet processed separators.

Ti-6Al-4V alloy is extensively used in aerospace, biomedical and automotive industries for processing the precise and ultra-precision parts because of its excellent material properties. However, the ultra-precision processing of Ti-6Al-4V alloy has always been a challenge. In this paper, a series of experimental investigations was performed to study the ultra-precision processing feasibility of Ti-6Al-4V alloy with different ultrasonic vibration-assisted cutting modes. The surface defect, surface roughness and chip formation under conventional cutting (CC), ultrasonic linear vibration assisted cutting (ULVC) and ultrasonic elliptical vibration-assisted cutting (UEVC) of Ti-6Al-4V alloy were analyzed. The experimental results revealed that, in the CC process, many surface defects like adhered particles, chip fragments and grooves appeared, and the long continuous ribbon chips were produced. In the ULVC process, the side flow appeared, which greatly deteriorated the surface integrity. Furthermore, the long helical chips were obtained. In contrast, an almost flawless-machined surface was obtained in the UEVC process, and its roughness value was 39.2 nm, which was decreased nearly 31% than the roughness value in CC process and 40% in ULVC process. Moreover, the shorter, helical and controllable chips were obtained. Experimental results indicated the UEVC was applicable for ultra-precision cutting of Ti-6Al-4V alloy.

Anisotropy is one central influencing factor on achievable ultimate machined surface integrity of metallic materials. Specifically, grain boundary has a strong impact on the deformation behaviour of polycrystalline materials and correlated material removal at the microscale. In the present work, we perform molecular dynamics simulations and experiments to elucidate the underlying grain boundary-associated mechanisms and their correlations with machining results of a bi-crystal Cu under nanocutting using a Berkovich tool. Specifically, crystallographic orientations of simulated bi-crystal Cu with a misorientation angle of 44.1° are derived from electron backscatter diffraction characterization of utilized polycrystalline copper specimen. Simulation results reveal that blocking of dislocation motion at grain boundaries, absorption of dislocations by grain boundaries and dislocation nucleation from grain boundaries are operating deformation modes in nanocutting of the bi-crystal Cu. Furthermore, heterogeneous grain boundary-associated mechanisms in neighbouring grains lead to strong anisotropic machining behaviour in the vicinity of the grain boundary. Simulated machined surface morphology and machining force evolution in the vicinity of grain boundary qualitatively agree well with experimental results. It is also found that the geometry of Berkovich tool has a strong impact on grain boundary-associated mechanisms and resultant ploughing-induced surface pile-up phenomenon.