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This report describes an adhesively bonded, Asymmetric Double Cantilever Beam (ADCB) fracture specimen that has been expressly developed to measure the toughness of an alumina (Al₂0₃)/epoxy interface. The measured interfacial fracture toughness quantifies resistance to crack growth along an interface with the stipulation that crack-tip yielding is limited and localized to the crack-tip. An ADCB specimen is a variant of the well-known double cantilever beam specimen, but in the ADCB specimen the two beams have different bending stiffnesses. This report begins with a brief overview of how crack-tip mode mixity (i.e., a measure of shear-to- normal stress at the crack-tip) is a distinguishing feature of interfacial fracture. Which is then followed by a detailed description of relevant design, fabrication, testing, and associated data analysis techniques. The report then concludes by presenting illustrative results that compare the measured interfacial toughness of an alumina/epoxy interface when the alumina is silane-coated and when the alumina is not silane coated. This page left blank

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TYPE SAND Report YEAR 2019

DOI OSTI

Residual Stress Developed During the Cure of Thermosetting Polymers: Optimizing Cure Schedule to Minimize Stress

Kropka, Jamie M.; Stavig, Mark E.; Jaramillo, Rex J.

When thermosetting polymers are used to bond or encapsulate electrical, mechanical or optical assemblies, residual stress, which often affects the performance and/or reliability of these devices, develops within the structure. The Thin-Disk-on-Cylinder structural response test is demonstrated as a powerful tool to design epoxy encapsulant cure schedules to reduce residual stress, even when all the details of the material evolution during cure are not explicitly known. The test's ability to (1) distinguish between cohesive and adhesive failure modes and (2) demonstrate methodologies to eliminate failure and reduce residual stress, make choices of cure schedules that optimize stress in the encapsulant unambiguous. For the 828/DEA/GMB material in the Thin-Disk-on-Cylinder geometry, the stress associated with cure is significant and outweighs that associated with cool down from the final cure temperature to room temperature (for measured lid strain, Iε_cureI > Iε_thermalI). The difference between the final cure temperature and the temperature at which the material gels, T_f-T_gel, was demonstrated to be a primary factor in determining the residual stress associated with cure. Increasing T_f-T_gel leads to a reduction in cure stress that is described as being associated with balancing some of the 828/DEA/GMB cure shrinkage with thermal expansion. The ability to tune residual stress associated with cure by controlling T_f-T_gel would be anticipated to translate to other thermosetting encapsulation materials, but the times and temperatures appropriate for a given material may vary widely.