3-D AE visualization of bone-cement fatigue locations

Hyperbolic source location of crack related acoustic emission in bone

Little work has been done on the localization of microcracks in bone using acoustic emission. Microcrack localization is useful to study the fracture process in bone and to prevent fractures in patients. Locating microcracks that occur before fracture allows one to predict where fracture will occur if continued stress is applied to the bone. Two source location algorithms were developed to locate microcracks on rectangular bovine bone samples. The first algorithm uses a constant velocity approach which has some difficulty dealing with the anisotropic nature of bone. However, the second algorithm uses an iterative technique to estimate the correct velocity for the acoustic emission source location being located. In tests with simulated microcracks, the constant velocity algorithm achieves a median error of 1.78 mm (IQR 1.51 mm) and the variable velocity algorithm improves this to a median error of 0.70 mm (IQR 0.79 mm). An experiment in which the bone samples were loaded in a three point bend test until they fractured showed a good correlation between the computed location of detected microcracks and where the final fracture occurred. Microcracks can be located on bovine bone samples using acoustic emission with good accuracy and precision.

An innovative multi-component variate that reveals hierarchy and evolution of structural damage in a solid: application to acrylic bone cement

A major limitation of solid mechanics is the inability to take into account the influence of hierarchy and evolution of the inherent microscopic structure on evaluating the performance of materials. Irreversible damage and fracture in solids, studied commonly as cracks, flaws, and conventional material properties, are by no means descriptive of the subsequent responses of the microstructures to the applied load. In this work, we addressed this limitation through the use of a novel multi-component variate. The essence of this variate is that it allows the presentation of the random damage in the amplitude spectrum, probability space, and probabilistic entropy. Its uniqueness is that it reveals the evolution and hierarchy of random damage in multi- and trans-scales, and, in addition, it includes the correlations among the various damage features. To better understand the evolution and hierarchy of random damage, we conducted a series of experiments designed to test three variants of a poly (methyl methacrylate) (PMMA) bone cement, distinguished by the methods used to sterilize the cement powder. While analysis of results from conventional tension tests and scanning electron microscopy failed to pinpoint differences among these cement variants, our multi-component variate allowed quantification of the multi- and trans-scale random damage events that occurred in the loading process. We tested the statistical significance of damage states to differentiate the responses at the various loading stages and compared the damage states among the groups. We also interpreted the hierarchical and evolutional damage in terms of the probabilistic entropy (s), the applied stress (σ), and the trajectory of damage state. We found that the cement powder sterilization method has a strong influence on the evolution of damage states in the cured cement specimens when subjected to stress in controlled mechanical tests. We have shown that in PMMA bone cements, our damage state variate has the unique ability to quantify and discern the history and evolution of microstructural damage.

Nondestructive evaluation of bone cement and bone cement/metal interface failure

To quantify the failure mechanisms related to the loosening of cemented hip joint replacements, novel techniques, capable of monitoring, nondestructively, the initiation and progression of failure during in vitro fatigue tests, were employed. Fatigue testing of model cement and cement-stem test pieces was monitored using acoustic emission (AE) sensors. Once damage was detected, an ultrasonic imaging system was used to obtain an image of the damage site and to measure the stiffness of the affected region. This method of examination provided a detailed insight into the internal crack propagation and delamination patterns. Initial work was conducted on bulk cement specimens subjected to bending and tension. The second stage of the work examined a model stem-cement interface under tensile opening loading conditions. A novel ultrasonic technique was used to measure the bond quality at the cement-metal interface. Progressive delamination was identified over time, and the AE technique was able to identify critical areas of delamination before they could be identified conclusively by ultrasonic imaging. The work has demonstrated the potential of the AE technique as a tool for the preclinical assessment of total hip replacements.

Defect-induced fatigue microcrack formation in cement mantle

Acoustic emission (AE) was used to monitor the progress of the fatigue damage process in the cement mantles of two cemented femur stem constructs that contained naturally occurring defects. After the fatigue tests, morphological features of the defects were investigated using an environmental scanning electron microscope. It showed that the regions with no visible defects were mainly microcrack free, whereas the defect regions were the main sources generating microcracks. Two types of microcracks were identified: type I and type II. Signal energies associated with type I microcracks were about an order of magnitude higher than that of type II. The microstructural investigations of the defects and the areas in the vicinity of the defects suggested their categorization into stable and unstable. The accumulative energy-time relationships revealed that stable and unstable microcrack curves had convex [formula: see text], and concave [formula: see text] shapes, respectively. The progress of fatigue microcrack formation occurred over three distinct phases: initiation, transition, and stableness.

3D real time methodology monitoring cement failures in THA

The present work proposed a methodology to monitor cement microcrack formation in the cemented femoral stem construct using the acoustic emission technique. This technique provides a unique means to automatically tally the number of microcracks, to visualize microcrack distribution, and to animate the progress of crack formation in a given time window of a fatigue test. In this work, the formulation of microcrack source location was derived and a computer program was developed specifically for the proposed application. The program was validated using computer simulation and standard pencil lead break tests. It was found that the mathematical errors complied with the acceptable minimal error. Based on the pencil lead break tests, the average technical error used to estimate the resolution of this technique was 4.7 mm at the present stage. The program was then used to monitor the fatigue damage in precoated cemented femoral hip constructs loaded for a total of more than five million cycles. Two types of microcrack activities were observed in the experiments: Type I and Type II microcracks. A Type I microcrack was a crack that was captured by four or more sensors, and therefore its location was defined uniquely by a set of coordinates. A Type II microcrack was a crack that was captured by three or less sensors, therefore it was unlocatable. Both counts of Type I and Type II microcrack were tallied with respect to the day of fatigue tests. Acoustic emission microcrack graphs were used to visualize the distribution of Type I microcracks in the construct. It was found that the Type I microcracks distributed mainly over the proximal third of the stem. The amount of microcrack events decreased significantly as the number of loading cycles increased.