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The frequency dependence of the phase velocity and attenuation of ultrasonic waves were measured as a function of time during the polymerization (curing) reaction of epoxy resins. The phase velocity and attenuation were evaluated from the amplitude and phase spectra of ultrasonic signals transmitted through a layer of curing epoxy resin. The measurements were made in the frequency range of 2–20 MHz. From the experimental data follows an important conclusion: The attenuation coefficient increases linearly with frequency at all stages of the curing reaction from the viscous liquid to the solid state. The slope of the attenuation coefficient as a function of frequency is strongly dependent on the time of cure (degree of cure). The linear behavior of attenuation versus frequency suggests that the attenuation effect cannot be explained by classical viscothermal absorption or relaxation theory. This type of behavior (so‐called hysteresis behavior) is poorly understood on the molecular level and was found previo...

In an attempt to develop models for nondestructive testing of flaw characterization we have simulated “flaws” by using metal circular reflectors in water. A broad‐band ultrasonic beam was incident on the reflectors. The scattered wave amplitudes were measured as a function of frequency for fixed angles and as a function of angles for several frequencies. In our previous work [Laszlo Adler and H.L. Whaley, J. Acoust. Soc. Am. 51, 881 (1972)], the frequency distribution of the scattered sound wave was related to the size and orientation of the reflector by an approximate theory. This theory assumed that the wavelets originating from the extreme edges of the reflector will interfere. Now we have adopted a theory from the diffraction of electromagnetic waves by aperture [Keller, Appl. Phys. 28, 426 (1957)]. An expression for the diffracted amplitude was calculated as a function of frequency and angle for several reflector sizes. The result compares favorably with our experiments. [This research was sponsored ...