Principal Investigators: Dr. Kenneth Cunefare( Integrated Acoustics Laboratory, Associate Professor Woodruff School of Mechanical Engineering), Dr. Robert Guldberg(Orthopaedics Biomechanics Laboratory, Assistant Professor Woodruff School of Mechanical Engineering, Institute of Bioengineering and Bioscience.
Graduate Students: Gaylon Hollis
Acoustic emission(AE) is one of the most sensitive techniques to non-invasively monitor deformation, fatigue, and fracture of many materials. Acoustic emission is the phenomenon of acoustics wave generated by the release of energy from localized sources in a material subjected to an externally applied stimulus. The principal difference between AE and the conventional non-invasive techniques of ultrasound or dual electron x-ray absorption (DEXA) is that no energy is introduced into the system. That feature is a strong factor when the system is comprised of living tissue. DEXA introduces the harmful radiation associated with x-rays. The energy deposited by ultrasound can cause cavitation within tissues. In comparison to the other damage detecting techniques, AE has certain advantages and disadvantages.
Its primary disadvantage is that it is not easily calibrated to a physical parameter, so successive testing can not be repeated with tolerable precision and accuracy.
The image is a typical output to the AE software once an AE event has been detected. Upon establishing the testing parameters, it was necessary to optimize the Threshold voltage with pre-gain on the transducer amplifier.
Theory of an AE signal.
In processing an AE signal it is important to consider the voltage out put. The output from a transducer excited by AE leads to the expression
Vo is the initial signal amplitude; g is the decay constant, and ω is the signal frequency.
The electrical energy E present in a transient AE event is defined as:
R is the resistance of the measuring circuit.
The
World Health Organization defines osteoporosis as bone density
of 2.5 standard deviations below the mean of young white adult
women.
The medical field estimates health care cost
to be $10 to $15 billion annually. As high as that figure
is, it does not include the indirect costs of lost wages or productivity.
Non-invasive techniques used to analyze bone density would
substantially alleviate some of the financial and physical effects
of osteoporosis because of the earlier diagnosis. 
The image shows a longitudinal cross-section of the human femur. The outer shell of the bone is formed by cortical bone which posses a 10% porosity. Trabecular bone has a porosity of 50% to 90%. Trabecular bone serves as the shock absorber for human joints. Thus severe osteoporosis can leave sufferer with much pain and fragility
The image on the left represents a 10micron resolution of healthy trabecular bone. The right is a section of osteoportic bone.
The specimens used in testing are 9mm cross-sectional cylinders with an approximate length of 12mm. The specimens are extracted from cow femurs and tibias. Final preparation involves placing each cylindrical specimen in end-caps. The experimental setup of the research has two main the systems. The first system is A hydraulic mechanical testing system machine will be used to load trabecular regions from cow femur and tibias in the initial stages; the second is the acoustic emission equipment. The AE equipment includes the transducers to detect the AE events and a software package to analyze the data on a DOS compatible CPU.
As indicated by the test setup diagram, all of the information proceeds back to the Acoustic Emission software so that it can be processed and analysis can be made.
I. To evaluate acoustic emission from trabecular bone samples loaded to failure under quasi-static compressive loading.
II. To quantify the relationships between AE signals (cumulative counts, amplitude, count rate) with modulus degradation during cyclic compressive loading.
III. To correlate acoustic emission detection of trabecular bone damage to 3-D Microcomputer Tomagraphy(Micro CT) imaging.
IV. To correlate the acoustic emission to histology reports from some of the trabecular bone specimens