Natural rubber (NR)-based elastomer, by virtue of its amenability to yield shear modulus at 3.5 kg/cm2 shear stress as low as 1.6–2.0 kg/cm2, is the most preferred elastomer for the realization of flex seals that are used for the thrust vector control of large rocket motors. Despite the utmost care taken during compounding, there exists a high degree of uncertainty in achieving required shear modulus primarily because of complexity involved in compounding to obtain high degree of homogeneity of the NR compound. Assessing the quality of NR compound is a tedious and time-taking process since it involves the preparation and testing of standard quadruple lap shear (QLS) specimens. Any non-conformance in shear modulus calls for fine-tuning of NR formulation; remaking of QLS specimens to recheck shear modulus leads to huge time delay and wastage of materials. The authors have meticulously generated oscillatory disc rheometer (ODR) data in comparison with QLS test results for various NR compounds and introduced ODR as an effective quality assurance tool. This article demonstrates how, based on ODR data, shear modulus can be predicted, thereby corrective actions can be taken in NR compound quickly and the appropriate vulcanization scheme can be prescribed for the given NR compound, which will prevent reversion and ensure optimum vulcanization, thereby eliminate the causes for shear modulus increase during storage of flex seals.

Acoustic emissions (AE) generated by a structure under stressed condition provide a passive method to understand the flaw growth phenomenon. In complex structures, such as composites, characterisation of AE signals generated by various failure mechanisms enhances such understanding. Sample level tests have been carried out on carbon epoxy unidirectional laminate in longitudinal direction of fiber to study the AE characteristics of different failure mechanisms, namely, fiber breakage and inter layer de-lamination. The AE parameters such as amplitude, energy, duration, rise time and signal strength have been acquired and analysed using various correlation plots. The fiber breakage is represented by high energy and longer duration hits with an amplitude of above 90 dB. The de-lamination mechanism is producing AE hits of medium energy of about 1000 units and long duration up to 10 6 µs. The duration per unit energy and the rise time per unit energy graphs portray a vivid picture of the occurrence of fibre breakage and delamination.

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