Donatus C. Ohanehi

A High Precision Experimental Method to Determine Poisson’s Ratios of Encapsulant Gels

A high precision technique for measuring Poissons ratios ofelastomers was experimentally implemented. Test fixtures consisted of hollow metal cylinders fitted with matching pairs of pressure sensors at the top and bottom. Evaluation of a preliminary low-pressure fixture led to the development and testing of a high pressure version to investigate potential nonlinearities in the response. A series of elastomeric gels was evaluated by casting them into the cylinders and pressurizing the free end. The pressure at the closed end of the gel was attenuated by the resulting deformation in the gel. The resulting pressure profile obeys equations similar to classical shear lag behavior. Poissons ratio was easily calculated from the ratio of the two pressures and the aspect ratio of the gel specimen. A series of seven encapsulant gels was characterized. Consistent results for most gels were obtained to the fifth significant digit, confirming the high precision capabilities of this simple technique. Accurate information on Poissons ratio may be required for elastomeric materials that are used in constrained configurations where volumetric changes can be important.

Coupled Experimental and Computational Analysis of Fracture Path Selection in PMMA Blocks

While developing experimental and computational tools for analyzing crack path selection and failure loci in adhesively bonded joints, we have initially applied these tools for studying crack paths in pre-notched monolithic blocks of polymethyl methacrylate (PMMA), a common material for conducting brittle fracture experiments. Specimen configurations similar to the compact tension specimen but of varying length/width ratios were used to explore the effect of the T-stress on destabilizing the crack from growing straight along its original direction. Asymmetric versions of this geometry were also used to determine the effect of imposed mode mixity on crack path selection. These test configurations provided useful data for checking the robustness of the computational software based on a meshless local Petrov-Galerkin formulation of the boundary-value problem. The PMMA was assumed to be linear elastic, homogeneous and isotropic. A crack was assumed to initiate when the maximum principal tensile stress reached a critical value and propagate in the direction of the eigenvector of this stress. Effects of the mode-mixity on the crack propagation have been studied.

Observations of Rate-Dependent Fracture of Locally Weakened Interfaces in Adhesive Bonds

Studies of the fracture behavior of adhesive joints can provide scientific understanding of failure processes as well as properties required for engineering design purposes. The focus of the present paper is to discuss the role that locally weakened interfaces in adhesive bonds can have on the fracture behavior of double cantilever beam specimens loaded in both mode I conditions and in mixed (mode I/II) conditions in a dual actuator load frame that permits independent control of the applied loads. Locally weakened areas are created by several methods of contamination of one aluminum adherend, including physical vapor deposition of copper through a mask perforated with the desired size, spacing, and pattern. Results from this experimental study have provided evidence of the size of a weakened zone that is required to be detected by a growing cohesive crack for a commercial adhesive system. The detection size depends on the mode mixity applied, with opening shear conditions rendering detection of smaller weakened zones and closing shear conditions detecting only larger weakened zones. In addition, an interesting rate dependence will be described in which rapidly growing cracks are more likely to detect locally weakened zones than more slowly growing cracks for several systems studied. Possible mechanisms will be suggested.

Developing a Simple Damage Model for the Long-Term Durability of Acrylic Foam Structural Glazing Tape Subject to Sustained Wind Loading

AbstractThis article presents a simple linear damage accumulation model that may have applicability for predicting damage from sustained winds in double-sided acrylic foam tape used to attach curtain wall glazing panels to buildings. The purpose of this model is to investigate the possible cumulative effects of years of wind-induced stresses that are less than the peak stress expected during a 3-s gust, as specified in current design guidelines established by the structural glazing industry and adopted by the manufacturer of these structural glazing tapes. Several representative wind histories are selected to provide input data for the model. These wind histories provide multiple years of average wind speeds over either 10-min or 1-h recording intervals, depending on the source. Each entry in a wind speed history is converted to stress on the glazing adhesive tape on assumed window dimensions using the standard wind loading design equations of ASCE 7-05. A creep rupture prediction equation, developed from...