No description available.

No description available.

M.S.

Although bond-normal stresses have been shown to be responsible for the failure of most laboratory adhesive joint geometries, the measurement of these stresses has been accomplished only through the use of very sophisticated optical techniques. In order to develop a more versatile measurement technique, poly(vinylidene fluoride) film was used to develop piezoelectric stress sensors. The sensitivities of the film to normal stresses in the three principal material directions of the orthotropic film were accurately measured using a charge amplifier and a storage oscilloscope. These measured sensitivities comprised the calibration constants of the film. In order to reduce the detrimental effect on bond strength caused by embedding the low surface energy film into adhesive bondlines, surface treatment methods were investigated using contact angle studies, XPS analysis and 1800 peel and tapered double cantilever beam adhesion specimens. An acid etch using a mixture of acetic, phosphoric and nitric acids was found to greatly improve the bond strengths to an epoxy adhesive without reducing the piezoelectric activity of the film. The bond-normal stresses in both the elastomeric butt joint and the single lap shear joint were measured using the developed stress sensors. Comparison of the measured stresses with calculated values obtained from closed-form analytical solutions and finite element analysis for the stresses was excellent. The piezoelectric sensors do have several important limitations. The piezoelectric activity of the film is lost at temperatures above 100°C (210°F). Also, the sensors are only sensitive to dynamic loads. Nonetheless, the sensors provide an accurate means of measuring peel stresses in many adhesive joints of practical interest. / Ph. D.

Prediction of the performance of adhesively bonded joints is essential to the acceptance of this mode of fastening. In ideal situations where the bonding is uniform throughout the joint the stress distribution depends on the material properties and the joint geometry. Knowledge of the bond properties and the stress distribution and magnitude can then be used to determine failure initiation, damage growth, subsequent stress distributions, and final failure. However, few bonds can be characterized as "ideal", as the bonding is generally not uniform throughout the joint and even the properties of the adhesive may vary. This paper describes work that addresses this situation. Acoustic microscopy has been used to provide a detailed image of the bonded joint, while acousto-ultrasonic measurements have been used to evaluate the stress transfer capability of modified lap shear joints. Knowledge of the stress distribution in the joint has provided a means of identifying critical areas of interest in the joint. The adhesively bonded composite specimens were mechanically tested and performance correlated with NDE results. Wave mode filtering is offered as a phenomena describing the basis for the correlation. / Master of Science