Melody A. Verges

Investigation of Microcrack Growth in [0/90]s Composite Laminates

Techniques such as optical microscopy and X-radiography have provided useful information regarding damage in composite laminates, particular in therms of microcracking behavior in individual plies. This focuses on the investigation of microcracking and damage evolution in loaded composite laminates via X-ray computed microtomography. The main advantage in the use of such a technique is that damage within the composite can be assessed in three-dimensions without destruction of the composite. In this work, IM7/977–2, IM7/5555, and IM7/5276-1 coupons were uniaxially tested in a tensile substage, Graphs that convey microcracking density information as a function of applied load were created for [0/90/90/0] laminates. The three dimensional geometry and connectivity of microcracks and other damage in these samples were investigated through microtomographic reconstruction.Copyright

Indentation Testing of Bulk Zr 0.5 Hf 0.5 Co 1-y Ir y Sb 0.99 Sn 0.01 Half-Heusler Alloys

Indentation tests were performed to assess the influence of compositional changes on the mechanical properties of several half-Heusler compounds with the general composition Zr 0.5 Hf 0.5 Co 1- x Ir x Sb 0.99 Sn 0.01 ( x =0.0,0.1,0.3,0.5,0.7). These samples were synthesized by high temperature solid-state reactions and were consolidated by hot-pressing. Indentation measurements were obtained using both microhardness testing (Vickers) and depth-sensing nanoindentation. These measurements were used to determine the microhardness and the elastic modulus of each half-Heusler compound. The Vickers hardness values were found to range between 876 and 964. A slight increase in hardness was observed with the addition of iridium. The elastic stiffness values ranged from 229 GPa to 246 GPa. Here, a slight decrease in stiffness was observed with the addition of iridium.

Identification of Critical Surface Displacement Parameters for Characterization of Subsurface Flaws

Several NDE methods provide accurate techniques for measuring surface displacements. While these techniques have been successful in identification of near-surface embedded flaws, they generally offer little in terms of characterization of the flaws. The ability to characterize embedded flaws from measurements of surface displacements would offer substantial benefits, especially in terms of remaining life predictions. This paper focuses on the identification of critical out-of-plane surface displacement parameters, and assessment of the potential to characterize subsurface flaw geometry based on these parameters. Finite element models of a homogeneous material have been created that vary the embedded flaw size and edge distance. The results suggest that there is potential to characterize the subsurface geometry from the surface displacement parameters.Copyright

Young’s Modulus and Hardness of Zr0.5Hf0.5CoxRh1−xSb0.99Sn0.01 and Zr0.5Hf0.5CoxIr1−xSb0.99Sn0.01 Half-Heusler Alloys

Mechanical testing was performed to determine the influence of compositional changes on the Young’s modulus and hardness of half-Heusler compounds of the base composition Zr0.5 Hf0.5 CoSb0.99 Sn0.01 . In the efforts to decrease the thermal conductivity of the composition toward the development of thermoelectric materials with high thermal conversion efficiencies, specimens were fabricated with varying amounts of rhodium and iridium at the cobalt site. In addition to the general Zr0.5 Hf0.5 CoSb0.99 Sn0.01 composition, six hot-pressed samples of the Zr0.5 Hf0.5 Cox Rh1−x Sb0.99 Sn0.01 (0.0≤x≤1.0) composition and four hot-pressed samples of the Zr0.5 Hf0.5 Cox Ir1−x Sb0.99 Sn0.01 (0.0≤x≤0.7) composition were synthesized. Indentation measurements were obtained using both microhardness testing and depth-sensing nanoindentation. The general Zr0.5 Hf0.5 CoSb0.99 Sn0.01 composition was observed to have a hardness and elastic modulus around 896HV0.2 and 247GPa, respectively. For all of the compositions tested the hardness range was observed to lie between 347HV0.2 and 951HV0.2. The elastic moduli for these compositions were found to range between 97GPa and 247GPa. The effects of the rhodium substitution and iridium substitution at the cobalt site on the elastic stiffness and hardness are examined.© 2010 ASME

The Influence of Underfill Properties on the Fatigue Life of Ball Grid Array Packages

The influence of underfill material properties on the fatigue life of a Ball Grid Array (BGA) package in the presence of thermal cycling is investigated in this study. The underfill material properties that are varied include Young’s modulus, Poisson’s ratio, and the coefficient of thermal expansion. The range in values are in accordance with typical underfills used in packages today. A finite element model is created using general purpose Ansys code by assuming that there exists an infinite array of solder interconnects, cylindrical in shape, surrounded by underfill material. The finite element geometry generated consists of a unit cell of concentric cylinders, with the inner being solder material and the outer being underfill material. The interconnect material is modeled as eutectic solder that behaves elastically-perfectly plastic. The Mode I cyclic stresses in the solder are determined as a temperature loading is applied. These stresses are then compared to the residual compressive stresses that are induced as a result of underfill shrinkage upon curing. Results suggest that Mode I stresses induced in the interconnects upon thermal cycling are not negligible in comparison to the beneficial compression that they provide upon curing. Even in the presence of this residual compression, for several material combinations a substantial amount of tension is induced in the connections while being cycled.Copyright