Chapter 1 Introduction
1.2 Motivation
There are strong motivations for this study both in science and applications. Sci- entifically, studying detonation-driven tube fracture leads to insights into poorly un- derstood phenomena such as crack branching and crack curving, both fundamental problems in fracture mechanics. The experimental data obtained from the complex fluid-structure-fracture also serve as a touchstone for validation of multi-physics and multi-scale numerical simulations. Applications ranging from novel applications such as the design of pulse detonation engines (PDEs) to classical engineering applications such as pipeline explosion hazard analysis and blast response of aircraft fuselages will also benefit from this research.
1.2.1 Pulse Detonation Engines
Pulse detonation engines are novel unsteady aerospace propulsion devices that gen- erate quasi-steady thrust by high-frequency cycling of gaseous detonations. PDEs operate only on test-stands at the time of this writing. No PDE has yet flown, and it is not clear what type of material is suitable for PDEs. However, like any other light-weight, load-bearing aerospace component, pre-existing and/or propagat- ing flaws could exist as (1) cracks due to external damage, (2) voids as a result of the manufacturing process, (3) interfacial cracks between layers of composites (if a PDE
is made of layered composites), or (4) fatigue, oxidation, and corrosion as a result of the punishing operating environment. The existence of flaws necessitates a fracture mechanics approached design and safety analysis.
Figure 1.1: An air-breathing PDE cycle (Wintenberger and Shepherd, 2003).
A typical PDE cycle is shown in Fig. 1.1. The traveling impulsive loading and the high temperature excursions impose a significant challenge to the structure in the form of high-cycle thermomechanical impulsive fatigue. PDE fatigue fracture accidents occurred at a test site on the Wright-Patterson Air Force Base (Schauer, 2003), and some fatigue experiments have been performed to study fatigue crack propagation in PDE tubes (Chao et al., 2003). In PDEs, multi-cycle stress-fields are complicated and it is important to first understand the problem in simpler single-cycle fracture experiments.
1.2.2 Pipeline Explosions
Examples of explosive pipeline fracture can be found in recent nuclear power plant accidents. Two such accidents occurred in Hamaoka, Japan (see Fig.1.2) and Bruns- brettel, Germany in 2001. In both accidents, sections of the carbon steel steam pipes were fragmented due to combustion events in hydrogen-oxygen mixtures created by radiolysis. Fortunately, there were no injuries or loss of life. One of the most im- portant questions that arose during the accident investigation was whether one can deduce the type of accidental combustion (deflagation, detonation, or deflagation-to- detonation transition) from the fracture patterns. No one seemed to have sufficient knowledge to give a conclusive quantitative answer, although the pressure profiles of different combustion events are distinctly different and should cause different fracture patterns.
Figure 1.2: Two ends of a ruptured section of a steam pipe from the Hamaoka Nuclear Power Plant accident (2001).
These accidents highlighted the need to study fracture events due to traveling loads in pipes. The understanding, in addition to leading to safer piping system design in nuclear power plants, can also assist accident investigators in learning what type of combustion was responsible.
1.2.3 Blast Loading in Aircraft
Blast loading in commerical aircraft has been highlighted by recent terrorist events with enormous loss of life (e.g., Lockerbie 1989). While detection procedures can
ensure that the quantities of explosive smuggled on board will not be very large, small amounts of explosive can still have devastating effects (Kanninen and O’Donoghue, 1995). The chances of aircraft survivability can be increased if designated panels can be strategically fractured in response to the traveling load brought on by the explosive products and ensure safe decompression.
1.2.4 Dynamic Crack Curving and Branching in Tubes
Dynamic crack curving and crack branching in tubes are perhaps two of the most extensively reported yet least understood problems in fracture mechanics. The reason is perhaps that there is not much practical use of crack curving and branching in tubes, except as a means of crack arrest in long pipes or for predicting crack paths in multiple site damage (MSD) scenarios in aircraft fuselages.
In axial crack propagation of pressurized tubes, mode I loading clearly dominates, but cracks seldom run straight. Crack curving and branching have been widely ob- served in metal and polymeric pipes, but the cause behind the crack path instability in pipes is not well understood. A worthy challenge for the fracture community is to develop a complete and rigorous model that could predict the crack paths (Kanninen and O’Donoghue, 1995).
1.2.5 Validation for Multi-Physics and Multi-Scale Numeri- cal Simulations
Detonation-driven tube fracture is one of the few problems that truly involves strong coupling of combustion, fluid mechanics, structural mechanics, and fracture mechan- ics. As a validation experiment, it provides a rich variety of data that challenge simulation experts. This type of data is well suited to validating the virtual test facil- ity (VTF) that has been developed at Caltech to explore software development issues in simulating response of solids to detonation loading under the sponsorship of the U. S. Department of Energy through the Accelerated Strategic Computing Initiative (Aivazis et al., 2001).