Fracture mechanics

The objective of the research presented in this thesis is to develop analysis and test procedures for the characterization of disbonding crack growth in a honeycomb (HC) core sandwich structure. Face sheet-to-core disbonding are of particular interest to aircraft certification authorities due to several in-service occurrences. Experimental investigation was initially focused on the mode I dominated Single Cantilever Beam (SCB) test method. Various data reduction methodologies were employed to determine the fracture toughness. The MBT method produced the most consistent and conservative results. Finite element analysis (FEA) a double periodic array of hexagonal cells was conducted to determine the effective in-plane extensional modulus and Poisson ratio of the HC core. It was shown that deformation constraints on the core, due to attachment of the core to rigid face sheets, will drastically change the behavior of the HC core. The response changes from being governed by bending to stretching which substantially elevates the effective in-plane modulus. Fracture mechanics analysis of a face/core interface crack in a HC core SCB specimen was performed using FEA. The influence of in-plane properties of the constrained core on energy release rate and mode mixity phase angle was examined. Use of plane strain conditions and an elevated modulus of the constrained core in the analysis is recommended. The approach is substantiated by testing of HC core SCB sandwich. Test results showed good agreement with FEA prediction of compliance and kink angle.

This work details the development of tools and controllers for station keeping
control of twin screw vessels. A fundamental analysis is conducted of the dynamics of
twin screw displacement hull vessels and their actuator systems, where the response
characteristics and maneuverability are quantified through a series of full scale trials
conducted in different environmental conditions while recording the environmental
conditions, actuator states, and geodetic and inertial measurements. The data from these
maneuvers were repeatable from run to run and thus provide valuable benchmarks for
several maneuvers and the measured actuator response provides valuable set points of
performance characteristics/limitations for control development. A comprehensive
general simulation of small twin screw displacement hull boats is developed as a tool to
estimate ship and actuator responses in support of developing and tuning of control
systems. The model and computer simulation is capable of modeling a wide range of the
surface vessels, including their actuators and environmental conditions. This model
proved to be accurate, when compared to the sea trial data, and model estimates have rms velocity errors for the various steady maneuvers of 1.2-4.6% for surge, 12.6-17.9% for
sway, and 7.6-20.2% for yaw.
A path following station keeping controller is developed that uses Lyapunov
stability analysis to determine the path the vessel should follow to effectively eliminate
position error. This controller showed good performance for several different
environmental conditions. Encouraged by these finding, three additional station keeping
control methodologies are developed for twin screw surface ships. All four of these
controllers are examined for their robustness to environmental conditions, as well as their
sensitivity to sensor precision, sensor update rates, and actuator limitations. All
controllers are evaluated in sea state 4 yielding rms position errors from 3.3 to 16.2 m,
the rms surge and sway accelerations are under 0.62 m/s , and the engine shifting
frequencies are between 0.011 and 0.145 Hz. These four controllers are then tested over a
wide range of environmental conditions, sensor precisions and update rates, and actuator
response rates. The results from these tests give quantitative data that will aid in selecting
the appropriate controller for a specific application, and will assist in selecting
appropriate sensors.

Sandwich composites provide excellent structural integrity for a variety of
applications. In this study pristine and functionalized 30 nrn Silicon Carbide
nanoparticles are infused into a low density polyurethane foam used for the inner core
of the sandwich structure. The mechanical properties are characterized using
compressive, tensile, and flexural tests. A plane-strain fracture test and a TSD (Tilted
Sandwich Debond) test characterize the fracture properties of the foam and the coreskin
interface. Thermal characterization is carried out using Dynamic Mechanical
Analysis (DMA) and Thermo-Gravimetric Analysis (TGA). FTIR spectral analysis
reveals changes in molecular bonding due to pristine and functionalized nanoparticle
infusion. The fracture resistance of the foam is improved and the delamination
strength of the sandwich construction with nanophased cores is dramatically
improved. The TSD testing indicated that the G1c value rose from 0.14 kJ/m^2 in the
neat foam to 0.56 kJ/m^2 with just 0.1 wt% of SiC nanoparticle inclusion reflecting an
enhancement of almost 300%.

This thesis explores the feasibility of using morphing rudders in autonomous
underwater vehicles (AUVs) to improve their performance in complex current
environments. The modeling vehicle in this work corresponds to the Florida Atlantic
University's Ocean EXplorer (OEX) AUV. The AUV nonlinear dynamic model is
limited to the horizontal plane and includes the effect of ocean current. The main
contribution of this thesis is the use of active rudders to successfully achieve path
keeping and station keeping of an AUV under the influence of unsteady current force.
A constant ocean current superimposed with a sinusoidal component is considered.
The vehicle's response is analyzed for a range of current frequencies.

Debond failures in structural sandwich may lead to severe reductions in load-bearing capability of the structure because of impartial transfer of shear and tensile forces between facing and core due to the lack of interfacial bonding. Analysis of interfacial bonding in sandwich specimens subjected to transverse tensile and shear forces is presented. Stress intensity factors computed based on the near-tip displacement field are related to experimental crack growth observation on the sandwich beams with aluminum skins on a wide range of PVC foam cores. Experimentally it was found that the crack tends to grow at the interface between the bondline and core as opposed to skin/bondline interface. In shear dominated fields, a pre-existing flow tended to deflect into the core rather than grow along the interface. The tendency for kinking and the direction of the kink is examined experimentally and analyzed using the finite element method.

The deformation and fatigue crack growth behavior of amorphous metals are presented. A discussion of magnetic domains, and their influence on magnetostriction is also included. Testing of Metglas 2605SC, utilizing magnetostriction to generate DeltaK, indicated near-threshold (Region I) fatigue crack growth behavior for as-cast and annealed specimens. A Delta Kth on the order of 0.6-1.0 kg/mm^3/2 was also indicated. The best crack growth rate behavior was obtained for transverse field-annealed specimens. Significant variability in the data, however, prevented a determination of Delta Kth for this condition. A life prediction model based on a Paris equation was developed. Crack growth data were consistently lower than predicted by the Region II model, which would be expected for near-threshold data. Fractographic analyses supported the influence of domains on crack growth behavior. The scatter in the Region I data appears to be contributed by the effects of domains and domain walls.

Mode I interlaminar fracture toughness, G IC, of interleaved graphite/epoxy has been investigated with DCB specimens, beam theory, and finite element analysis. Finite element modeling aimed to investigate the influence of interleaf thickness on compliance and energy release rate and possible mixed mode loading in the case of asymmetric interfacial crack. Another objective was to compute crack tip yield zone dimensions as a function of thickness and elastic properties of the interleaf material. The analysis is correlated with experiments. Thermoplastic interleaves enhanced G IC to a great extent. The toughness increased sharply with film thickness to a maximum at 16 mu m and decreased for the thicker interleaves. On the other hand, inadequate adhesion preempted the toughness potential of thermoset interleaves.

This thesis presents the experimental investigation of durability and fracture toughness (K IC) of fly ash concrete in the marine environment. The findings indicate that the deterioration rate of durability parameters, such as compressive strength, weight loss, and dynamic modulus of elasticity, due to 450 wet and dry cycles exposure (the Accelerated Durability Testing), was inversely proportional to the amount of fly ash replacement. On the other hand, tensile strength properties, such as modulus of rupture and fracture toughness, were independent of fly ash replacement, but increased with the period of accelerated testing. The mean K IC values of fly ash concrete mixes showed that they are closely related to their compressive strengths and size effects. According to AE, unstable crack propagation initiated at 93-97% maximum load. With SEM observations, it was found that crystallized particles were precipitated in the void spaces due to chemical reaction between the cement paste and seawater.

Interlaminar mode II fracture toughness, GIIC, of thermoset and thermoplastic interleaved (TSI and TPI) composites were investigated over a wide range of interleaf thickness. TPI specimens had four to about seven times larger GIIC than those without an interleaf. Poor adhesion observed for some TPI specimens were likely to be due to contaminated film materials. Thermoset interleaves were less effective in enhancing the mode II fracture toughness. However, even 0.043 mm thermoset interleaves gave three times larger G$\sb{\rm IIC}$ than those without an interleaf. Estimates of the volume of the yielded material around the crack tip based on a quasi-elastic finite element approach and Irwin's model showed that the yield zone height reaches a peak value for increasing interleaf thickness for both TSI and TPI specimens. Furthermore, fracture toughness data correlates well with yield zone heights.

A study of the stress distribution in and fracture behavior of the hermetic glass seal in a typical Integrated Circuit package is presented herein. Finite Element Analysis and Fracture Mechanics approaches were found effective for this investigation. A prescribed load or displacement applied at the tip of the lead protruding from the package causes high stresses at the lead-glass interface, which can lead to cracking and fracture of the seal. An approach for finding the value of the allowable load or displacement applicable at the lead tip is discussed. A correlation with a standard crack shape is presented for the 3-D model of the package. An extension of the problem revealing the effects of crack propagation on the stress intensity factor for the glass material is presented in later chapters. The J-integral method from Fracture Mechanics is found to be extremely useful for this investigation. A decline in the stress intensity factor with crack growth was observed from this study.