Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of Ϸ1 to Ϸ0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: (i) reductions of Ϸ20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, (ii) almost complete recovery of the longitudinal diffusivity following unloading, and (iii) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity.

The thermal conductance of delamination cracks in a unidirectionally reinforced ceramic composite is investigated. A phase-sensitive photothermal technique is used to measure the crack conductance in situ under load. Special emphasis is given to the effects of the local crack opening displacement (␦). A crack conductance model that considers the contributions from both the air and the fibers within the crack is developed and compared with the measurements. Despite considerable scatter in the experimental data, the model adequately predicts the increased conductance that is associated with fiber bridging, as well as the overall trend that is observed with ␦.

The paper describes a phase-sensitive photothermal technique for the determination of the thermal conductance of an interface, a thin interlayer, or crack embedded within a plate. The technique involves sinusoidally modulated heating at one point on the surface using a focused laser beam and measurement of the phase shift of the thermal wave at some other point. The technique is demonstrated using a model system comprising two stainless steel disks, placed either in direct contact with each other or with thin polyethylene sheets between them. The use of the technique for determining the conductance of delamination cracks in fiber-reinforced ceramic matrix composite is also demonstrated. [S0022-1481(00)02801-2]

Hyperbaric oxygen therapy (HBO) is used to treat some complications of head and neck carcinoma treatment. Several patients treated with HBO have had rapid growth of a clinically cured squamous cell carcinoma. Prior studies have produced conflicting evidence about the effect of HBO on tumor growth. This study was undertaken to determine the effects of HBO on established squamous cell carcinoma. Forty Golden Syrian hamster cheek-pouch carcinomas were induced with the established chemical carcinogen 9,10-dimethyl-1,2-benzanthracene. Twenty hamsters underwent 30 HBO dives for 60 minutes each to 2.81 atm, while 20 served as controls. At necropsy, animals receiving HBO therapy had significantly smaller tumors (P < .05) and showed a trend toward fewer cervical metastases (P < .06). HBO therapy with coexistent carcinoma inhibited the established tumor's growth.