Objective Flexible polyphenylene sulfide (PPS) filter cloths, prized for their exceptional heat resistance, chemical stability, and mechanical flexibility, are extensively utilized in critical applications such as fuel cell electrolyte filtration, aerospace, high-temperature chemical filtration, and electronic packaging. In fuel cell systems, these filter cloths play a vital role in ensuring electrolyte purity and maintaining operational stability. However, during prolonged service, they are prone to developing micro-defects such as openings, glue drops, burns, and abrasions. These defects can compromise structural integrity, reduce filtration performance, and pose risks to operational safety. Therefore, establishing a high-sensitivity, non-destructive testing (NDT) method for accurate defect detection and characterization is of significant practical and industrial importance.
Methods A shearography-based NDT platform with a Michelson interferometric optical path was established, integrating a 200 mW, 532 nm single longitudinal mode laser, adjustable shear mirrors, and phase-shifting control. Two excitation modes—tensile loading along the x-axis and infrared thermal loading—were employed to stimulate defect responses. Real-time interferometric fringe patterns were captured using a 2 464 pixel×2 056 pixel industrial CCD camera, and phase differences were extracted using a four-step phase-shifting algorithm to reconstruct out-of-plane displacement gradients. Complementary finite element simulations were conducted in COMSOL Multiphysics to model the PPS specimens (260 mm × 120 mm × 1 mm) under both loading conditions. Local mesh refinement was applied to defect zones, and the out-of-plane displacement gradient field was analyzed to reveal strain concentration and deformation behavior near defects.
Results and Discussions Through systematic experimental comparisons, the shearography response characteristics of typical defects in PPS filter fabrics under different loading modes were clarified, enabling the identification of the most suitable excitation method and corresponding fringe patterns for each defect type. Significant differences in imaging sensitivity were observed between tensile and thermal loading. Moreover, the shear amount showed a strong correlation with defect size: large defects exhibited clear responses at a shear of 10-15 mm, whereas small defects required larger shear values to enhance the signal. Considering both detection sensitivity and fringe clarity, a 20 mm shear amount provided the optimal balance under the conditions of this study, making it suitable for multi-scale defect detection. Excessively small shear resulted in insufficient signals, while excessively large shear caused fringe diffusion and blurring, reducing imaging quality. In summary, defects involving structural continuity disruption are best excited by tensile loading, while defects with abrupt thermal property variations are better suited to thermal loading.
Conclusions This study systematically analyzed the shearography response patterns of typical defects in flexible PPS filter fabrics under tensile and thermal loading. The results indicate that different defect types exhibit significant sensitivity differences to loading modes: defects involving structural continuity disruption (such as openings and glue drops) produce strong displacement disturbances under tensile loading, resulting in concentrated and distinct interference fringes; defects with abrupt thermal property changes (such as burns) achieve optimal imaging under thermal loading, with effective detection possible on both front and back surfaces. In contrast, abrasion defects, which minimally affect structural and thermal properties, exhibit weak imaging responses, limiting their detectability. Leveraging its high sensitivity to out-of-plane displacement gradients, shearography effectively amplifies subtle defect signals in flexible materials, demonstrating the advantages of full-field, high-resolution, and non-contact nondestructive testing. This provides both theoretical support and practical guidance for defect identification in flexible, low-modulus materials.
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