Fatigue crack growth behavior of AA7050 T7451 aluminum alloy under mode II loading condition in two orientations was investigated. Compact shear specimens were prepared in TL and LT directions. A loading frame for mode II type of loading was manufactured. Using the loading frame and the specimen, KIIC values and mode II fatigue crack growth rates were calculated. Fractographic analysis of the fracture surfaces of both mode II fracture toughness test specimens and mode II fatigue crack growth test specimens were done to examine the effects of mode II load. KIIC values were measured between 1.3 and 1.5 times the KIC values for this alloy. As for mode II fatigue crack growth rates, TL orientation shows the highest mode II fatigue crack growth resistance.

For roughly fifteen years the military has sought to use the properties of ceramics for armor applications. Current high-performance ceramics have extremely high compressive strengths and low densities. One ceramic that has been shown to be highly resistant under ballistic impact is silicon carbide (SiC). It has been found that even within the silicon carbides, those manufactured by certain methods and those with certain microstructural properties have advantages over others. In order to understand the microstructural reasons behind variations in ballistic properties, plate impact tests were conducted on two sintered silicon carbides with slightly different microstructures. Two variations of a silicon carbide with the trade name Hexoloy SA were obtained through Saint Gobain. Regular Hexoloy (RH) and Enhanced Hexoloy (EH) are pressureless sintered products having exactly the same chemistries. EH went through additional powder processing prior to sintering, producing a final product with a slightly different morphology than RH. Samples of each were characterized microstructurally including morphology, density, elastic wavespeeds, microhardness, fracture toughness, and flexure strength. The characterization revealed differences in porosity distribution and flexure strength. It was determined that the porosity distribution in EH had fewer large pores leading to an 18% increase in flexural strength over that for RH. The focus of the mechanics of materials community concerning dynamic material behavior is to pin down what exactly is happening microstructurally during ballistic events. Several studies have been conducted where material properties of one ceramic type are varied and the dynamic behavior is tested and analyzed. Usually, from one variation to the next, several properties are different making it hard to isolate the effect of each. For this study, the only difference in the materials was porosity distribution. Plate impact experiments were conducted at the Army Research Laboratory (ARL) using the gas gun facilities within the Impact Physics Branch. A VISAR was utilized to measure free surface velocities. Tests were performed on each material to determine the Hugoniot Elastic Limit (HEL) and spall strength. Spall strength was measured as a function of impact stress, and pulse duration.

No-flow underfill materials that cure during the solder reflow process is a relatively new technology. Although there are several advantages in terms of cost, time and processing ease, there are several reliability challenges associated with no-flow underfills. When micron-sized filler particles are introduced in no-flow underfills to enhance the solder bump reliability, such filler particles could prevent the solder bumps making reliable electrical contacts with the substrate pads during solder reflow, and therefore, the assembly yield would be adversely affected. The use of nano-sized filler particles can potentially improve assembly yield while offering the advantages associated with filled underfill materials. The objective of this thesis is to study the thermo-mechanical reliability of nano-filled epoxy underfills (NFU) through experiments and theoretical modeling. In this work, the thermo-mechanical properties of NFUs with 20-nm filler particles have been measured. An innovative residual stress test method has been developed to measure the interfacial fracture toughness. Using the developed residual stress method and the single-leg bending test, the mode-mixity-dependent fracture toughness for NFU-SiN interface has been determined. In addition to such monotonic interfacial fracture characterization, the interface crack propagation under thermo-mechanical fatigue loading has been experimentally characterized, and a model for fatigue interface crack propagation has been developed. A test vehicle comprising of several flip chips was assembled using the NFU material and the reliability of the flip-chip assemblies was assessed under thermal shock cycles between -40oC and 125oC. The NFU-SiN interfacial delamination propagation and the solder bump reliability were monitored. In parallel, numerical models were developed to study the interfacial delamination propagation in the flip chip assembly using conventional interfacial fracture mechanics as well as cohesive zone modeling. Predictions for interfacial delamination propagation using the two approaches have been compared. Based on the theoretical models and the experimental data, guidelines for design of NFUs against interfacial delamination have been developed.

Semi-circular specimens (SCB) under three point-bending which are commonly used for fracture testing of rocks were used here for fracture mechanics tests. A total of 65 specimens were tested by using Ankara andesite rock. Investigations including the effects of initial notch thickness, different loading span ratios (S/R), flattened loading end, and little dimensional variations when preparing the specimens were carried out. Stress intensity factors for specimens with different geometries were computed individually by using a 3D finite element program ABAQUS. Specimens with a preliminary notch thickness varying from 0.84 to 3.66 mm were tested under three point bending. For a second group of specimens loading span was changed and fracture toughness variation was studied. Another change in the specimen geometry was made by machining a flat loading end at the upper load application point. Fracture toughness values were computed using the stress intensity values computed from numerical modeling and failure loads from the experiments. It was found that up to 2 mm fracture toughness was not affected by variations in the thickness of preliminary notches. Fracture toughness was not affected by changing the loading span. For specimens with flat loading ends, fracture toughness was about 16% lower than the value found from regular SCB type specimens loaded at a point at the top by a steel roller. As a result of about 46 experiments average fracture toughness of Ankara G&ouml / lbasi andesite was found as 1.36 MPa .

Modified Ring specimens are of the shape of discs having a hole inside and flattened ends. These specimens are used for determination of Mode I fracture toughness. Finite element program, named ABAQUS, is used for numerical modeling for finding stress intensity factors. Varying disc geometries were used for the experiments and numerical modeling in which size of the flat ends, radius of the hole inside, and external radius of the specimen were varied. Experiments were done by using pink Ankara andesite. Effects of internal hole radius, external disc radius and size of the flat ends on both stress intensity factor and fracture toughness were studied. In order to compare the results, fracture tests with semi-circular specimens under three point bending (SCB) were also performed. From a similar previous study, fracture toughness values of gray andesite were recalculated and compared to the fracture toughness values of pink andesite for varying geometrical factors. Size effect studies were performed as well for varying diameter of core specimens.Fracture toughness values of andesite were found to increase with increasing specimen size. Fracture toughness of 100 mm specimens was determined as 1.11&plusmn / 0.07 MPa&amp / #8730 / m, whereas fracture toughness of 75 mm specimens was 0.96&plusmn / 0.08 MPa&amp / #8730 / m. 100 mm or larger diameter specimens were suggested for the fracture toughness determination with the modified ring tests.

Fracture toughness is an important rock property for rock fracturing and fragmentation applications. Theory and practice of opening mode (mode I) and shearing mode (mode II) fracture toughness tests are still in a developing stage for the cylindrical rock cores. A new circular plate type test specimen is used for mode II fracture toughness testing on rock cores. This involves a straight edge notched circular plate type core disc geometry under three-point bending load / new method and its associated specimen geometry is referred as straight edge notched disc bend (SNDB) specimen under three-point bending. Mode II fracture toughness results of the tests with this new geometry were compared to the results of the tests commonly employed for mode II fracture toughness testing. Specimen geometries were modeled and mode II stress intensity factors were computed by finite element modeling using ABAQUS program. For comparison purposes, mode II or shearing mode fracture toughness KIIc of two different rock types were determined by different testing methods commonly employed in recent practice. Core specimens of Ankara andesite and Afyon marble rock types were tested with cracked chevron notched Brazilian disc and cracked straight through Brazilian disc specimens under Brazilian type loading, semi-circular bend specimen and straight edge notched disc bending specimen geometries under three-point bending.For all testing groups, cylindrical cores with diameters varying from 7.5 cm to 12.5 cm were prepared with notch lengths changing from 1.5 cm to 2.6 cm. Effect of specimen thickness on mode II fracture toughness was investigated for three different testing methods. Fracture toughness values remained constant when thickness of the specimens was increased for cracked straight through Brazilian disc, semi-circular bend and straight notched disc bend methods. For cracked straight through Brazilian disc method KIIc values of Ankara andesite and Afyon marble were 0.99 MPa&radic / m and 0.86 MPa&radic / m, respectively. Mode II fracture toughness with semi-circular bend specimen was 0.43 MPa&radic / m for andesite and 0.46 MPa&radic / m for marble. When the results of the two three-point bending type tests were compared straight notched disc under three-point bending resulted in higher KIIc values (0.61 MPa&radic / m for andesite and 0.62 MPa&radic / m for marble) than the results found by semi-circular bend tests.

Over the last few decades, Shape Memory Alloys (SMAs) have been increasingly explored in order to take advantage of their unique properties (i.e., pseudoelasticity and shape memory effect), in various actuation, sensing and absorption applications. In order to achieve an effective design of SMA-based devices a thorough investigation of their behavior in the presence of cracks is needed. In particular, it is important to understand the effect of phase transformation on their fracture response. The aim of the present work is to study the effect of stress-induced as well as thermo-mechanically-induced phase transformation on several characteristics of the fracture response of SMAs. The SMA thermomechanical response is modeled through an existing constitutive phenomenological model, developed within the framework of continuum thermodynamics, which has been implemented in a finite element frame-work. The effect of stress-induced phase transformation on the mechanical fields in the vicinity of a stationary crack and on the toughness enhancement associated with crack advance in an SMA subjected to in-plane mode I loading conditions is examined. The small scale transformation assumption is employed in the analysis according to which the size of the region occupied by the transformed material forming close to the crack tip is small compared to any characteristic length of the problem (i.e. the size of the transformation zone is thirty times smaller than the size of the cracked ligament). Given this assumption, displacement boundary conditions, corresponding to the Irwin’s solution for linear elastic fracture mechanics, are applied on a circular region in the austenitic phase that encloses the stress-induced phase transformation zone. The quasi-static stable crack growth is studied by assuming that the crackpropagates at a certain critical level of the crack-tip energy release rate. The Virtual Crack Closure Technique (VCCT) is employed to calculate the energy release rate. Fracture toughness enhancement associated with transformation dissipation is observed and its sensitivity on the variation of key characteristic non-dimensional parameters related to the constitutive response is investigated. Moreover, the effect of the dissipation due plastic deformation on the fracture resistance is analyzed by using a Cohesive Zone Model (CZM). The effect of thermo-mechanically-induced transformation on the driving force for crack growth is analyzed in an infinite center-cracked SMA plate subjected to thermal actuation under isobaric mode I loading. The crack-tip energy release rate is identified as the driving force for crack growth and is measured over the entire thermal cycle by means of the VCCT. A substantial increase of the crack-tip energy release rate – an order of magnitude for some material systems – is observed during actuation as a result of phase transformation, i.e., martensitic transformation occurring during actuation causes anti-shielding that might cause the energy release rate to reach the critical value for crack growth. A strong dependence of the crack-tip energy release rate on the variation of the thermomechanical parameters characterizing the material response is examined. Therefore, it is implied that the actual shape of the strain- temperature curve is important for the quantitative determination of the change of the crack-tip energy release rate during actuation.

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A study is carried out into fracture behavior of steel/steel laminates both experimentally and through finite element analysis (FEM). The laminates produced by hot pressing consisted of low carbon and medium carbon steels with two volume fractions / 0.41 and 0.81. Fracture toughness, JIC has been measured using partial unloading technique assuming a critical value of crack extension. The technique is initially applied to monolithic material and then to the laminates in crack divider orientation. Evaluation of fracture toughness of laminates indicates that there is a substantial improvement of JIC with increase in the volume fraction. The systems under study were also evaluated by FEM modeling with the use MARC package program. To evaluate JIC, the problem has been evaluated in several steps / first two-dimensional plane strain problem is considered. This is followed by three-dimensional case and then by an artificially layered system, all for monolithic materials. Values of JIC derived were close to one another in all cases. Following this verification, the method, as implemented in layered monolithic system, was applied to laminates. This has shown that JIC of laminates can be predicted using FEM analysis, including the delamination. Values of JIC varied in the same manner as the experiment verifying that fracture toughness in the current system increases with increase in volume fraction. It has been concluded that modeling as implemented in this work can be used for useful composite systems incorporating hard/brittle reinforcements both in crack divider and crack arrester orientation.

The goal of this research was to develop new experimental techniques to quantitatively study the interfacial fracture of micro-contact thin film interconnects used in microelectronic applications under monotonic and cyclic loadings. The micro-contact spring is a new technology that is based on physical vapor deposited thin film cantilevers with a purposely-imposed stress gradient through the thickness of the film. These "springs" have the promise of being the solution to address near-term wafer level probing and long-term high-density chip-to-next level microelectronic packaging challenges, as outlined by the International Technology Roadmap for Semiconductors. The success of this technology is, in part, dependent on the ability to understand the failure mechanism under monotonic and cyclic loadings. This research proposes two experimental methods to understand the interfacial fracture under such monotonic and fatigue loading conditions. To understand interfacial fracture under monotonic loading, a fixtureless superlayer-based delamination test has been developed. Using stress-engineered Cr layer and a release layer with varying width, this test can be used to measure interfacial fracture toughness under a wide range of mode mixity. This test uses common IC fabrication techniques and overcomes the shortcomings of available methods. The developed test has been used to measure the interfacial fracture toughness for Ti/Si interface. It was found that for low mode mixity Ti/Si thin film interfaces, the fracture toughness approaches the work of adhesion which is essentially the Ti-Si bond energy for a given bond density. In addition to the monotonic decohesion test, a fixtureless fatigue test is developed to investigate the interfacial crack propagation. Using a ferromagnetic material deposited on the micro-contact spring, this test employs an external magnetic field to be able to drive the interfacial crack. Fatigue crack growth can be monitored by E-beam lithography patterned metal traces that are 10 to 40nm wide and 1 to a few µm in spacing. The crack initiation and propagation can be monitored through electrical resistance measurement. In the conducted experiments, it is seen that the interfacial delamination does not occur under fatigue loading, and that the micro-contact springs are robust against interfacial fracture for probing and packaging applications.