Publications

Polymer stresses around sharp corners and in constrained geometries of encapsulated components can generate cracks leading to system failures. Often, analysts use maximum stresses as a qualitative indicator for evaluating the strength of encapsulated component designs. Although this approach has been useful for making relative comparisons screening prospective design changes, it has not been tied quantitatively to failure. Accurate failure models are needed for analyses to predict whether encapsulated components meet life cycle requirements. With Sandia's recently developed nonlinear viscoelastic polymer models, it has been possible to examine more accurately the local stress-strain distributions in zones of likely failure initiation looking for physically based failure mechanisms and continuum metrics that correlate with the cohesive failure event. This study has identified significant differences between rubbery and glassy failure mechanisms that suggest reasonable alternatives for cohesive failure criteria and metrics. Rubbery failure seems best characterized by the mechanisms of finite extensibility and appears to correlate with maximum strain predictions. Glassy failure, however, seems driven by cavitation and correlates with the maximum hydrostatic tension. Using these metrics, two three-point bending geometries were tested and analyzed under variable loading rates, different temperatures and comparable mesh resolution (i.e., accuracy) to make quantitative failure predictions. The resulting predictions and observations agreed well suggesting the need for additional research. In a separate, additional study, the asymptotically singular stress state found at the tip of a rigid, square inclusion embedded within a thin, linear elastic disk was determined for uniform cooling. The singular stress field is characterized by a single stress intensity factor K{sub a} and the applicable K{sub a} calibration relationship has been determined for both fully bonded and unbended inclusions. A lack of interfacial bonding has a profound effect on inclusion-tip stress fields. A large radial compressive stress is generated in front of the inclusion-tip when the inclusion is well bonded, whereas a large tensile hoop stress is generated when the inclusion is unbended, and frictionless sliding is allowed. Consequently, an epoxy disk containing an unbended inclusion appears more likely to crack when cooled than a disk containing a fully bonded inclusion. A limited number of tests have been carried out to determine if encapsulant cracking can be induced by cooling a specimen fabricated by molding a square, steel insert within a thin, epoxy disk. Test results are in qualitative agreement with analysis. Cracks developed only in disks with mold-released inserts, and the tendency for cracking increased with inclusion size.

While surface cleaning is the most common process step in DOE manufacturing operations, the link between a successful adhesive bond and the surface clean performed before adhesion is not well understood. An innovative approach that combines computer modeling expertise, fracture mechanics understanding, and cleaning experience to address how to achieve a good adhesive bond is discussed here to develop a capability that would result in reduced cleaning development time and testing, improved bonds, improved manufacturability, and even an understanding that leads to improved aging. A simulation modeling technique, polymer reference interaction site model applied near wall (Wall PRISM), provided the capability to include contaminants on the surface. Calculations determined an approximately 8% reduction in the work of adhesion for 1% by weight of ethanol contamination on the structure of a silicone adhesive near a surface. The demonstration of repeatable coatings and quantitative analysis of the surface for deposition of controlled amounts of contamination (hexadecane and mineral oil) was based on three deposition methods. The effect of the cleaning process used on interfacial toughness was determined. The measured interfacial toughness of samples with a Brulin cleaned sandblasted aluminum surface was found to be {approximately} 15% greater than that with a TCE cleaned aluminum surface. The sensitivity of measured fracture toughness to various test conditions determined that both interfacial toughness and interface corner toughness depended strongly on surface roughness. The work of adhesion value for silicone/silicone interface was determined by a contact mechanics technique known as the JKR method. Correlation with fracture data has allowed a better understanding between interfacial fracture parameters and surface energy.

The main goal of this research was to develop degradable systems either by developing weaklink-containing polymers or identifying commercial polymeric systems which are easily degraded. In both cases, the degradation method involves environmentally friendly chemistries. The weaklinks are easily degradable fragments which are introduced either randomly or regularly in the polymer backbone or as crosslinking sites to make high molecular weight systems via branching. The authors targeted three general application areas: (1) non-lethal deterrents, (2) removable encapsulants, and (3) readily recyclable/environmentally friendly polymers for structural and thin film applications.

Wind turbine blades are often fabricated with composite materials. These composite blades are frequently attached to a metallic structure with an adhesive bond. For the baseline composite-to-steel joint considered in this study, failure typically occurs when the adhesive debonds from the steel adherend. Previous efforts established that the adhesive peel stresses strongly influence the strength of these joints for both single-cycle and fatigue loading. This study focused on reducing the adhesive peel stresses present in these joints by tapering the steel adherends. Several different tapers were evaluated using finite element analysis before arriving at a final design. To confirm that the selected taper was an improvement to the existing design, the baseline joint and the modified joint were tested in both compression and tension. In these axial tests, the compressive strengths of the joints with tapered adherends were greater than those of the baseline joints for both single-cycle and low-cycle fatigue. In addition, only a minor reduction in tensile strength was observed for the joints with tapered adherends when compared to the baseline joints. Thus, the modification would be expected to enhance the overall performance of this joint.

The nucleation and growth of cracks at critical interfaces can degrade electrical and mechanical performance of electronic assemblies. Sandia National Laboratories is working to develop a fracture mechanics-based approach for assessing the reliability of components containing interfaces and subjected to thermal/mechanical fatigue. Models are being developed to predict the nucleation of a crack-like flaw in the vicinity of an interface, the path of crack propagation (along interface or into substrate), and the conditions for crack propagation. In addition, interfacial fracture toughness data are being generated to support model development. This paper summarizes an experimental study aimed at measuring the fracture toughness of epoxy-to-substrate interfaces that are representative of those found in bonded and encapsulated electronic components.

Two polyurethane systems, EN-7 and L-100, have a long history as encapsulants and coatings in Sandia programs. These materials contain significant amounts of toluene diisocyanate (TDI), a suspect human carcinogen. As part of efforts to reduce the use of hazardous materials in the workplace, PET-90A, a polyurethane with less than 0.1% free TDI, was identified as a candidate for new applications and as a replacement for the more hazardous polyurethanes in selected programs. This report documents the results of a two-year accelerated aging study of PET-90A, EN-7, and L-100 polyurethane elastomers to characterize the effect of 135{degrees}F isothermal aging on selected physical, electrical, mechanical and thermal properties. In general, there was very little change in properties over the two year period for the three elastomers. The largest changes occurred in EN-7, which is the polyurethane with the longest service history in Sandia applications.

This report describes an experimental study that supported the LDRD program ``A General Approach for Analyzing Composite Structures``. The LDRD was a tightly coupled analytical / experimental effort to develop models for predicting post-yield progressive failure in E-glass fabric/polyester composites subjected to a variety of loading conditions. Elastic properties, fracture toughness parameters, and failure responses were measured on flat laminates, rings and tubes to support the development and validation of material and structural models. Test procedures and results are presented for laminates tested in tension, compression, flexure, short beam shear, double cantilever beam Mode I fracture toughness, and end notched flexure Mode II fracture toughness. Structural responses, including failure, of rings loaded in diametral compression and tubes tested in axial compression, are also documented.

Dextor`s Hysol EA-9394 is a room temperature curable paste adhesive representative of the adhesives used in wind turbine blade joints. A mechanical testing program has been performed to characterize this adhesive. Tension, compression stress relaxation, flexural, butt tensile, and fracture toughness test results are reported.

Tensile properties were measured for nineteen different formulations of epoxy encapsulating materials. Formulations were of different combinations of two neat resins (Epon 828 and Epon 826, with and without CTBN modification), three fillers (ALOX, GNM and mica) and four hardeners (Z, DEA, DETDA-SA and ANH-2). Five of the formulations were tested at -55, -20, 20 and 60C, one formulation at -55, 20 and 71C; and the remaining formulations at 20C. Complete stress-strain curves are presented along with tables of tensile strength, initial modulus and Poisson`s ratio. The stress-strain responses are nonlinear and are temperature dependent. The reported data provide information for comparing the mechanical properties of encapsulants containing the suspected carcinogen Shell Z with the properties of encapsulants containing noncarcinogenic hardeners. Also, calculated shear moduli, based on measured tensile moduli and Poisson`s ratio, are in very good agreement with reported shear moduli from experimental torsional pendulum tests.

A combined experimental and analytical study of strains that develop in encapsulated assemblies during casting, curing, and thermal excursions is described. The experimental setup, designed to measure in situ strains, consisted of thin,’ closed-end, metal tubes that were instrumented with strain gages and thermocouples before being over-potted with an encapsulant. Three epoxy-based materials were studied. After cure of the encapsulant, tube strains were measured over the temperature range of —55°C to 90°C. The thermal excursion experiments were then numerically modeled using finite element analyses and the results were compared to the experimental results. The predicted strains were overestimated (conservative) when a linear, elastic, temperature-dependent material model was assumed for the encapsulant and the stress free temperature was assumed to correspond to the cure temperature of the encapsulant. Very good agreement was obtained with the linear elastic calculations provided that the stress free temperature corresponded to the onset of the glassy-to-rubbery transition range of the encapsulant. Finally, very good agreement was obtained when a viscoelastic material model was utilized and a stress free temperature corresponding to the cure temperature was assumed. © 1993 by ASME.