Earl David Reedy

Comparison of butt tensile strength data with interface corner stress intensity factor prediction

Abstract The butt tensile strength of a joint that bonds two stainless steel rods together with an unfilled epoxy adhesive (Epon 828/T-403) has been determined for a wide range of bond thicknesses. The measured joint strength shows a pronounced bond thickness dependence; joint strength increases by a factor of 2 as bond thickness is reduced from 2.0 to 0.25 mm. A failure criterion, based upon a critical interface corner stress intensity factor, accurately predicts the observed bond thickness effect. This fracture criterion suggests that the strength of an adhesively bonded butt tensile joint of one bond thickness can be estimated from strength data for a joint with a different bond thickness by the simple relation σ ult t 2 = σ ult t 1 ( h 1 / h 2 ) 1/3 , where 2 h i is bond thickness, σ ult ti is the nominal butt tensile strength, and subscript i = 1, 2 identifies the two joints with differing bond thickness. This relation applies to thin bonds when the adhesives Poissons ratio is between 0.3 and 0.4, the adherends are relatively stiff, and small scale yielding conditions hold at the interface corner.

Asymptotic interface corner solutions for butt tensile joints

In previous work, the intensity of the stress singularity of type rδ (δ < 0) found at the interface corner between a thin elastic adhesive layer and one of a pair of rigid adherends was fully determined for a butt tensile joint. This stress intensity factor, referred to here as the free-edge stress intensity factor Kf, can be applied to both plane strain and axisymmetric geometries. This study investigates the potential application of a Kf-based failure criterion to butt tensile joints. Detailed elastic-plastic finite element calculations for adhesive properties representative of a high strength epoxy indicate that when residual cure stress can be neglected (1) the region dominated by the interface corner singularity is reasonably large relative to adhesive layer thickness, (2) the plastic yield zone is contained within the singular field at nominal failure loads, and (3) the plastic zone size is characterized by kf and displays the expected load level and layer thickness dependence. The way uniform adhesive shrinkage (thermal contraction) during cure alters interface corner stress fields is also discussed. When adhesive shrinkage is present, both constant and singular terms must be included in the asymptotic solution to get good agreement with full field finite element results. In general, there is no unique relation between the size of the interface corner yield zone and Kf, although for a prescribed shrinkage strain, Kf does characterize the extent of plastic yielding. Calculated results suggest that the presence of residual stress can have a considerable effect on the relation between bond thickness and joint strength.

Connection between interface corner and interfacial fracture analyses of an adhesively-bonded butt joint

Abstract Interfacial crack growth in a tensile-loaded, adhesively-bonded butt joint with rigid adherends is analyzed. First, the asymptotic, small-scale cracking solution for a short interfacial crack originating at a sharp interface corner is presented. Then the asymptotic, steady-state solution for a long interfacial crack is discussed. These asymptotic results are compared with full finite element solutions of a butt joint containing a 0.001 to 10 bond thickness long interfacial crack. Finally, the applicability of both interface corner and interfacial fracture mechanics approaches to failure analysis is discussed. The small-scale cracking solution indicates that when one can apply an interface corner failure analysis, one can also apply an interfacial fracture mechanics approach with a suitably chosen inherent flaw. Although the two methods are equivalent, it should be emphasized that the inherent flaw and corresponding toughness may have limited physical significance.

Mechanical properties of Hysol EA-9394 structural adhesive

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.

Use of self assembled monolayers at variable coverage to control interface bonding in a model study of interfacial fracture: Pure shear loading

Abstract The relationships between fundamental interfacial interactions, energy dissipation mechanisms, and fracture stress or fracture energy in a glassy thermoset/inorganic solid joint are not well understood. This subject is addressed with a model system involving an epoxy adhesive on a polished silicon wafer containing its native oxide. The proportions of physical and chemical interactions at the interface, and the in-plane distribution, are varied using self-assembling monolayers of octadecyltrichlorosilane (ODTS). The epoxy interacts strongly with the bare silicon oxide surface, but interacts only weakly with the methylated tails of the ODTS monolayer. The fracture stress is examined as a function of ODTS coverage in the napkin-ring (nominally pure shear) loading geometry. The relationship between fracture stress and ODTS coverage is catastrophic, with a large change in fracture stress occurring over a narrow range of ODTS coverage. This transition in fracture stress does not correspond to a wetting transition of the epoxy. Rather, the transition in fracture stress corresponds to the onset of large-scale plastic deformation within the epoxy. We postulate that the transition in fracture stress occurs when the local stress that the interface can support becomes comparable to the yield stress of the epoxy. The fracture results are independent of whether the ODTS deposition occurs by island growth (T dep = 10°C) or by homogeneous growth (T dep = 24°C).

Rigid square inclusion embedded within an epoxy disk : asymptotic stress analysis

Abstract The asymptotically singular stress state found at the tip of a rigid, square inclusion embedded within a thin, linear elastic disk has been determined for both uniform cooling and an externally applied pressure. Since these loadings are symmetric, the singular stress field is characterized by a single stress intensity factor Ka, and the applicable Ka calibration relationship has been determined for both a fully bonded inclusion and an unbonded inclusion with frictionless sliding. A lack of interfacial bonding has a profound effect on inclusion-tip stress fields. When the inclusion is fully bonded, radial compression dominates in the region directly in front of the inclusion tip and there is negligible tensile hoop stress. When the inclusion is unbonded the radial stress at the inclusion tip is again compressive, but now the hoop tensile stress is of equal magnitude. Consequently, an epoxy disk containing an unbonded inclusion appears to be more likely to crack when cooled than a disk containing a fully bonded inclusion. Elastic–plastic calculations show that when the inclusion is unbonded, encapsulant yielding has a significant effect on the inclusion-tip stress state. Yielding relieves stress parallel to the interface and greatly reduces the radial compressive stress in front of the inclusion. As a result, the encapsulant is subjected to a nearly uniaxial tensile stress at the inclusion tip. For a typical high-strength epoxy, the calculated yield zone is embedded within the region dominated by the elastic hoop stress singularity. 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.

Nucleation and propagation of an edge crack in a uniformly cooled epoxy/glass bimaterial

Abstract An epoxy/glass bimaterial beam test configuration has been used to study cooling-induced crack nucleation and propagation. This effort extends a nucleation criterion, previously applied to tensile-loaded, adhesively bonded butt joints, to another geometry and type of loading. Loading by thermally induced straining complicates the application of a nucleation criterion based upon parameters defining the asymptotic stress fields at the interface edge (i.e. at the edge discontinuity defined by the intersection of the interface and stress-free boundary). In contrast to the tensile-loaded butt joint, where the magnitude of asymptotic stress state is fully characterized by a single interface-edge stress intensity factor K a , an additional, non-negligible r -independent regular term K a0 always exists for thermally induced strains. In the present work, a direct extension of the previously used nucleation criterion is applied: crack nucleation occurs when K a = K ac , but with the stipulation that interface-edge toughness K ac depends on K a0 .

Effect of nanoscale patterned interfacial roughness on interfacial toughness.

The performance and the reliability of many devices are controlled by interfaces between thin films. In this study we investigated the use of patterned, nanoscale interfacial roughness as a way to increase the apparent interfacial toughness of brittle, thin-film material systems. The experimental portion of the study measured the interfacial toughness of a number of interfaces with nanoscale roughness. This included a silicon interface with a rectangular-toothed pattern of 60-nm wide by 90-nm deep channels fabricated using nanoimprint lithography techniques. Detailed finite element simulations were used to investigate the nature of interfacial crack growth when the interface is patterned. These simulations examined how geometric and material parameter choices affect the apparent toughness. Atomistic simulations were also performed with the aim of identifying possible modifications to the interfacial separation models currently used in nanoscale, finite element fracture analyses. The fundamental nature of atomistic traction separation for mixed mode loadings was investigated.

Molecular-To-Continuum Fracture Analysis of Thermosetting Polymer/Solid Interfaces

This report focuses on the relationship between the fundamental interactions acting across an interface and macroscopic engineering observable such as fracture toughness or fracture stress. The work encompasses experiment, theory, and simulation. The model experimental system is epoxy on polished silicon. The interfacial interactions between the substrate and the adhesive are varied continuously using self-assembling monolayer. Fracture is studied in two specimen geometries: a napkin-ring torsion geometry and a double cantilevered beam specimen. Analysis and modeling involves molecular dynamics simulations and continuum mechanics calculations. Further insight is gained from analysis of measurements in the literature of direct force measurements for various fundamental interactions. In the napkin-ring test, the data indicate a nonlinear relationship between interface strength and fracture stress. In particular, there is an abrupt transition in fracture stress which corresponds to an adhesive-to-cohesive transition. Such nonlinearity is not present in the MD simulations on the tens-of-nanometer scale, which suggests that the nonlinearity comes from bulk material deformation occurring on much larger length scales. We postulate that the transition occurs when the interface strength becomes comparable to the yield stress of the material. This postulate is supported by variation observed in the fracture stress curve with test temperature. Detailed modeling of the stress within the sample has not yet been attempted. In the DCB test, the relationship between interface strength and fracture toughness is also nonlinear, but the fracture mechanisms are quite different. The fracture does not transition from adhesive to cohesive, but remains adhesive over the entire range of interface strength. This specimen is modeled quantitatively by combining (i) continuum calculations relating fracture toughness to the stress at 90 {angstrom} from the crack tip, and (ii) a relationship from molecular simulations between fracture stress on a {approx} 90 {angstrom} scale and the fraction of surface sites which chemically bond. The resulting relationship between G{sub c} and fraction of bonding sites is then compared to the experimental data. This first order model captures the nonlinearity in the experimentally-determined relationship. A much more extensive comparison is needed (calculations extending to higher G{sub c} values, experimental data extending to lower G{sub c} values) to guide further model development.

Process-Based Quality Tools to Verify Cleaning and Surface Preparation

A test method, the Tensile Brazil Nut Sandwich (TBNS) specimen, was developed to measure mixed-mode interfacial toughness of bonded materials. Interfacial toughness measured by this technique is compared to the interfacial toughness of thin film adhesive coatings using a nanoindentation technique. The interfacial toughness of solvent-cast and melt-spun adhesive thin films is compared and found to be similar. Finally, the Johnson-Kendall-Roberts (JKR) technique is used to evaluate the cleanliness of aluminum substrates.