Damage detection essential to ensuring reusability of cryogenic tanks Published April 11, 2007 By Dr. Kumar V. Jata and Dr. Peter S. Meltzer, Jr. AFRL Materials and Manufacturing Directorate WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- In-house researchers at the Air Force Research Laboratory Materials and Manufacturing Directorate's Nondestructive Evaluation Branch, together with University of Dayton Research Institute and Purdue University's School of Mechanical Engineering, have made significant advances in the development of an improved capability for monitoring the structural health of cryogenic tanks for space missions. The researchers conducted high resolution nondestructive evaluation and structural health monitoring experiments using elastic wave propagation (in the form of surface and guided waves) and were able to quantify microstructure and mechanical damage in friction stir welded aluminum-lithium alloys (prominent candidates for cryogenic tank construction). Their research is essential to ensuring the reusability of cryogenic tanks for hundreds of missions. Improved ability for detecting incipient damage in these structures enhances safety, enables quicker turn around time, and reduces the operating and material costs. The research is also applicable to other types of unitized structures. Cryogenic tanks or "cryotanks" are used to store very low temperature elements such as liquid oxygen and liquid hydrogen, and monitoring their structural health is very important to the successful planning and execution of future space programs. A critical part of improving this capability is validating mechanical systems diagnostics and prognosis, the principal elements of SHM, because when cryotanks are pressurized and reused for several missions, a potential for leakage and microstructure degradation exists. In the past, NDE and SHM diagnostic research has focused primarily on damage mechanisms such as cracks, corrosion, debonds, and delaminations. Today, in an effort to improve the reliability and reusability of generic structures, such as cryotanks, scientists are exploring ways to identify incipient damage, the earliest stages of damage. AFRL, UDRI, and Purdue University used acoustic-impedance and wave-propagation sensing and data interrogation methodologies to detect these early forms of damage in friction stir welded aluminum-lithium test plates. The researchers examined the dynamical behavior of the alloy plates and determined nodal and anti-nodal points. They then applied both high (elastic wave propagation) and low frequency (vibration) acoustic waves to interrogate the damage in the plates. High frequencies were imposed using piezo electric patches/transducers and low frequencies were introduced using a dynamic shaker system. Burst and swept waveforms were used for the high frequencies. Different forms of damage were then simulated using either localized temperature gradients or mass placement along and away from the weld. Damage studies were also conducted by introducing cracks in the plates at various locations. Sensors were placed at different locations to understand their effectiveness in capturing the damage data. The time domain data obtained from the sensors were then analyzed using various signal processing methods, including conversion to frequency-domain data, time-frequency analysis, and harmonic wavelet analysis (each wavelet level corresponding to an octave band of traveling plate wave modes). Statistically significant features for damage detection were also assessed and damage locations were identified using beam forming methods. The researchers were able to show throughout the course of many experiments that the vibro-acoustic (low and high frequency combination) method is an effective and appropriate way to interrogate damage in both non-welded and welded test plates. Structural health monitoring of friction stir welded structures is a key to meeting three of the primary goals of future space programs: increased safety, quicker turn around time between missions, and lower costs. Al-Li alloys are prominent candidate materials for cryotank construction. They contribute to substantial weight savings and have several major advantages when friction stir welded over conventional variable polarity plasma arc welded tanks currently in use. Friction stir welding has been demonstrated as an effective tool for welding difficult-to-weld aluminum alloys, resulting in fewer welding defects, lower residual stresses, better post-welding material properties, and improved dimensional stability. Future studies will involve the application of SHM and NDE methods in situ in combined-environment testing of high-strength Al alloys to develop a capability for detecting incipient damage in the form of gradients in dislocation density. These methods will then be demonstrated at the structural component level to show both detection and location of precursors to damage.