A membrane deflection technique has been developed to measure the isotropic residual stress in biaxiallyconstrained coatings. The technique has been demonstrated on various materials, including polyimide, latex rubber and photoresist coatings. Stress values obtained from membrane deflection compared well with results obtained from time-averaged vibrational holographic interferometry except for values obtained from samples where rigidity effects were found to be important. A criterion based on the thickness, rigidity, stress and sample radius is also discussed, establishing the applicability of the technique to samples of low rigidity.

A high‐pressure curing technique was developed to help determine the effects of solvent presence during the thermal curing of the polyimide poly (N,N'‐bis‐phenoxyphenylpyro‐mellitimide) (PMDA‐ODA). A powder form of this aromatic polyimide was produced from a polyamic acid solution using the high‐pressure thermal curing technique. Preliminary characterization of the powder indicates a high degree of crystallinity with a measured density of 1.46 ± 0.01 g/cm3 and a distinct melting point of 594°C. The addition of chemical curing agents to the polyamic acid solution prior to thermal treatment reduced the amount of crystallinity observed in the cured material. Molecular weight measurements of the polyamic acid precursor and powder suggest that the high degree of order observed in the powder is a result of degradation during cure. © 1994 John Wiley & Sons, Inc.

FRACTURE BEHAVIOR OF TYPICAL STRUCTURAL ADHESIVE JOINTS UNDER QUASISTATIC AND CYCLIC LOADINGS ABSTRACTStructural adhesive joints are expected to retain integrity in their entire service-life that normally involves cyclic loading concurrent with environmental exposure. Under such a severe working condition, effective determination of fatigue life at different temperatures is crucial for reliable joint design. The main goal of this work was thus defined as evaluation of fatigue performance of adhesive joints at their extreme working temperatures in order to be compared with their fracture properties under static loading. A series of standard double-cantilever-beam (DCB) specimens have been bonded by three structural 3M epoxy adhesives selected from different applications. The specimens were tested under monotonic and cyclic opening loads (mode-I) in order to evaluate the quasi-static and fatigue performances of selected adhesives at room temperature, 80°C and -40°C. The test results revealed that the fatigue damage occurred at relatively low load levels when compared to quasi-static fracture forces. At room temperature, the maximum cyclic fatigue forces varied between 25% and 40% of corresponding quasi-static fracture loads of selected adhesives. More significant reductions in adhesive mechanical performances were observed at 80°C. At cryogenic temperature, the adhesives had their own characterizations; mainly increasing the fatigue resistance but very sensitive to testing parameters such as loading rate or crosshead speed. In conclusion, the experimental observations showed a significant influence of fatigue loading on adhesive joints mechanical performances that should be considered in joint design, particularly at non-ambient temperatures.

Adhesive joints are normally subjected to different working conditions in their service-life. This may involve both static and cyclic loadings. In many instances, a combination of various loading conditions occurs that can be further provoked by exposure to hostile environments. This, in turn, leads to the need to characterize the joint behavior under different combinations of working conditions. Extensive experimental tests are needed in order to evaluate the joint performance under such variable working conditions. This implies the development of low cost and efficient testing techniques that reduces the extra needs to operator time and sophisticated test procedures.Taking this objective into account, a novel technique in mechanical evaluation of adhesive joints was developed in the present work. Alternative monotonic and variable-amplitude cyclic loads were applied on the same double-cantilever-beam (DCB) specimens under cleavage mode. DCB specimens were made from aluminum bars joined together by a two-part toughened structural adhesive. On one face, a series of crack detection sensors were bonded to control the testing machine for switching between monotonic and cyclic loadings. The testing machine had two aligned hydraulic actuators applying bending forces on the upper and lower arms of the DCB * Corresponding author:Tel.: +1 (418) 545-5247; Fax.: +1 (418) 545-5345; email: mojtaba.eskandarian@cnrc-nrc.gc.ca specimen. The effects of testing frequency and applied load history were also investigated within a range of 4 to 20 Hz for a nominal adhesive thickness of 0.5 mm. The fatigue performance of each configuration was represented by a power-law relationship and was compared for different testing conditions. The test results revealed that the fatigue damage occurred at relatively lower load levels (35%) when compared with monotonic fracture load. The power-law constants for the tested adhesive were influenced by testing frequency but not sensitive to loading order.

A model based on linear elasticity is presented describing the stresses arising during the processing of polymeric films on tenter frames. The model demonstrates that large in‐plane shear stresses, resulting from gradients in the line, produce anisotropic stresses in the film. The magnitude of the anisotropy is found to vary transversely across the line, becoming isotropic in the center and achieving maximum values at the edges of the line. The principal directions of the stress tend to lie at ±45° to the machine direction but may be altered by machine and transverse drawing by the frame. These results agree with orientation measurements reported in the literature on polyimide films produced on tentering frames.