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Determining the initiation of adhesive failure at a surface buried deep within the bulk of an epoxy is qualitatively different from measuring the propagation of an existing surface crack. Most current tests are shown to be unsuitable for assessing the critical traction at initiation. A new test geometry is presented that initiates failure away from an air interface, produces a slowly varying stress distribution near the initiation site and minimal contributions from thermal residual stresses, and enables tests with mixed modes of loading. This new geometry is used to examine temperature-dependent adhesive failure in tensile, shear, and mixed modes of loading for both smooth and rough surfaces. Some of the experimental results are unexpected. As examples, the critical traction at initiation of adhesive failure is apparently insensitive to surface roughness, and the critical normal traction is independent of temperature while the critical tangential traction tracks the shear yield stress.

In situ transmission electron microscopy study of the electric field-induced transformation of incommensurate modulations in a Sn-modified lead zirconate titanate ceramicThe ferroelectric-to-ferroelectic phase transformation between the high temperature (FE RH ) and the low temperature (FE RL ) rhomobohedral phases in lead based perovskite under the dc bias conditions was investigated. Dielectric measurements show that an external electric field stabilizes the FE RL phase and moves the phase transformation to a higher temperature. The observation has been further verified by an in situ microfocused x-ray study where an external field can effectively induce the oxygen octahedral tilting in the crystalline lattice and extends the thermal stability region of the FE RL phase to a higher temperature. An analysis based on the combination of the Clausius-Clapeyron relationship with lattice dynamic principles suggests that the transformation from FE RH to FE RL is driven by a short-range interaction in the crystalline lattice. The origin of this short-range interaction is proposed, based on the structural evolution during the phase transformation. Experimental evidence suggests that such interaction driving the structural instability can be exploited by an external electric field near the phase transformation temperature and leads to an unusual, transient field-enhanced deformation near the FE RH /FE RL phase transformation.

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Physical stress relaxation in rubbery, thermoset polymers is limited by cross-links, which impede segmental motion and restrict relaxation to network defects, such as chain ends. In parallel, the cure shrinkage associated with thermoset polymerizations leads to the development of internal residual stress that cannot be effectively relaxed. Recent strategies have reduced or eliminated such cure stress in thermoset polymers largely by exploiting chemical relaxation processes, wherein temporary cross-links or otherwise transient bonds are incorporated into the polymer network. Here, we explore an alternative approach, wherein physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into the backbone of the polymer network. A standard epoxy resin is cured with a diamine derivative of ferrocene and compared to conventional diamine curing agents. The ferrocene-based thermoset is clearly distinguished from the conventional materials by reduced cure stress with increasing cure temperature as well as unique stress relaxation behavior above its glass transition in the fully cured state. The relaxation experiments exhibit features characteristic of a physical relaxation process. Furthermore, the cure stress is observed to vanish precipitously upon deliberate introduction of network defects through an increasing imbalance of epoxy and amine functional groups. We postulate that these beneficial properties arise from fluxional motion of the cyclopentadienyl ligands on the polymer backbone.