In this letter, we present interfacial fracture toughness data for a polymer-metal interface where tests were conducted at various test temperatures T and loading rates δ˙. An adhesively bonded asymmetric double cantilever beam (ADCB) specimen was utilized to measure toughness. ADCB specimens were created by bonding a thinner, upper adherend to a thicker, lower adherend (both 6061 T6 aluminum) using a thin layer of epoxy adhesive, such that the crack propagated along the interface between the thinner adherend and the epoxy layer. The specimens were tested at T from 25 to 65 °C and δ˙ from 0.002 to 0.2 mm/s. The measured interfacial toughness Γ increased as both T and δ˙ increased. For an ADCB specimen loaded at a constant δ˙, the energy release rate G increases as the crack length a increases. For this reason, we defined rate effects in terms of the rate of change in the energy release rate G˙. Although not rigorously correct, a formal application of time–temperature superposition (TTS) analysis to the Γ data provided useful insights on the observed dependencies. In the TTS-shifted data, Γ decreased and then increased for monotonically increasing G˙. Thus, the TTS analysis suggests that there is a minimum value of Γ. This minimum value could be used to define a lower bound in Γ when designing critical engineering applications that are subjected to T and δ˙ excursions.
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
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, IεcureI > IεthermalI). The difference between the final cure temperature and the temperature at which the material gels, Tf-Tgel, was demonstrated to be a primary factor in determining the residual stress associated with cure. Increasing Tf-Tgel 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 Tf-Tgel would be anticipated to translate to other thermosetting encapsulation materials, but the times and temperatures appropriate for a given material may vary widely.