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acase@tulane.edu / The application of continuum mechanics principles to biological tissues is paramount to understanding (patho)physiological changes in tissue structure and function. Experimental and mathematical approaches can be utilized to quantify tissue mechanical behavior. In particular, planar biaxial mechanical testing of soft tissues (i.e. applying loads or deformation along two axes in the same plane) has proven to mimic physiologically relevant conditions for most soft tissues. Constitutive relations can then be formulated based on biaxial data to describe and predict soft tissue mechanical behavior. These mathematical tools could aid in delineating underlying mechanisms of and evaluating treatments for various clinically relevant issues. Therefore, the overall objective of this thesis is to build a custom planar biaxial mechanical testing device to characterize the mechanical properties of soft biological tissues to identify appropriate constitutive relations. A custom planar biaxial mechanical testing device was successfully built and validated. A LabVIEW program was written to interface with the stepper motors and load cells of the device to control their movements. A mechanical testing protocol was developed and incorporated to enable the characterization of a variety of soft tissue structure-function relationships. Foundations were laid for studies using the planar biaxial device for research in a tissue-engineered nipple-areolar complex (NAC), pelvic floor disorders, and age-specific tendinopathy. The planar biaxial device has the potential to impact many areas of clinical research. / 1 / Taylor McCrady

acase@tulane.edu / Soft tissue mechanics attempts to describe biological tissues such as skin, tendon, and the reproductive organs using concepts found in mechanical engineering. By approaching soft tissues using this framework, the complex biomechanical response of such tissues, which have been implicated in the development of disease and injury, can be ascertained and quantified. Robust mechanical tests, in which tissue stress-strain behavior is characterized, are needed in order to inform constitutive models of healthy and diseased tissue. The overall objective of this thesis was to design, construct, program, and validate a planar biaxial device capable of testing soft tissues. Improvements and redesigns were made to the device to better suit the nature of testing required for soft tissue. Custom grips, modules, and software were developed and fabricated to facilitate accurate biaxial mechanical tests. Optimized for testing of small soft tissues, the biaxial device is an evolution of the standard approach towards mechanical testing. The overall device and the individual systems were validated internally and externally. Pilot studies were conducted on murine skin, compared to existing data from literature, and observed to correspond with known stress-strain and load-displacement properties. Further, experimental protocols were developed to evaluate the biaxial behavior of soft tissues, including cervical, uterine, vaginal, and uterosacral ligament tissue. Studies were described in which experimental data could be used to establish structure-function relationships describing reproductive tissue. Results from these studies could be used to elucidate the underlying mechanical etiologies of preterm birth and pelvic organ prolapse. / 1 / Jonathan Nguyen

Clinical thermal therapies are widespread and gaining in appeal due to improved technology of heating devices and promising results. Outcomes of thermal treatment are often unpredictable and suboptimal, however, due in part to a lack of appreciation of the underlying biothermomechanics. There is a pressing need, therefore, to understand better the role of clinically-controllable parameters on the thermal damage processes of tissue. Heretofore, researchers have primarily sought to understand this process through various uniaxial experiments on tissues containing collagen as their primary constituent. Most biological tissues experience multiaxial loading, however, with complex boundary constraints inclusive of both isotonic and isometric conditions. The primary focus of this work is on the isothermal denaturation of fibrillar collagen subjected to a biaxial isometric constraint. Results from our tests reveal a complicated process, the kinetics of which are not easily measured. Evolving isometric contraction forces during heating do not correlate with resultant mechanical behaviors, as thermal shrinkage does in biaxial isotonic tests. Furthermore, resultant mechanical behaviors at variousdurations of heating reveal a two phase process with a rate dependent on the amount of isometric stretch. For tissues heated at 75oC for 15 minutes, at which point the first phase of mechanical alteration dominates for all constraints herein, resultant mechanical behaviors correlate well with the amount of isometric stretch. The correlation is similar to that between isotonic loads and resultant mechanical behaviors from previous studies. In light of the need for a better measure of thermal damage in isometric tests, we performed a histological analysis of tissues heated under varying constraints. Results show a good correlation between the level of isometric constraint and thermally-induced histological aberrations. Finally, we demonstrate that our seemingly limited and qualitative knowledge can be applied well to a specific clinical application: namely, the use of glycerol as a clearing agent for laser therapies. Our results suggest that glycerol is safe to use for such therapies because it increases the thermal stability of fibrillar collagen, and its hyperosmotic effects on mechanical behavior are fully reversed upon rehydration.

Research Doctorate - Doctor of Philosophy (PhD) / Masonry walls will almost invariably be required to resist lateral out-of-plane loads due to the action of wind or earthquakes; less commonly walls may be subjected to water or earth pressure or blast loading. Of particular interest is the common case which arises when the walls are supported on two or more adjacent edges. Under these conditions the masonry is subjected to a complex state of biaxial (two-way) out-of-plane bending combined with vertical in-plane compression due to the self weight of the wall and any superimposed loads. Different approaches currently exist for the design of masonry wall panels subjected to out-of-plane loads. However, these approaches are all empirical and often yield widely varying design recommendations and there has been significant criticism by proponents of the different methods regarding the use of alternative approaches. In this study an extensive program of laboratory testing in parallel with numerical analysis was conducted to examine the bending, biaxial bending in particular, behaviour of masonry walls. The aim was to provide a better understanding of the behaviour at the fundamental level towards ultimately developing a fully rational biaxial-bending failure model that can predict behaviour under any simultaneous combination of bending moments in the two principal directions, along with a superimposed compression force on the bed joints. Experimentally, 'single joint' four brick unit specimens were studied comprehensively, using a newly commissioned test rig, by subjecting them to various vertical and horizontal bending moments both separately and in combinations, along with a superimposed compression force on the bed joints. These tests provided important information about the flexural behaviour of mortar joints and the torsional behaviour of bed joints. In addition, a complete set of characterization tests was also performed to study the fundamental material properties of masonry, which were important input parameters in the numerical modelling. Numerically, a 3D non-linear finite element micro-model with cohesive contact was proposed and implemented in the ABAQUS/Standard software package. Numerical viii analyses were performed to provide rational explanations to the bending behaviours observed in the four brick unit specimen tests and evaluate a newly proposed torsion shear test method. A simplified 3D non-linear finite element micro-model was also proposed to simulate the bending behaviour of small walls. Its effectiveness was clearly demonstrated in its application to masonry walls, with or without openings, subjected to both in-plane and out-of-plane loads.

Peyronie's disease (PD) affects the tunica albuginea of the penis with a formation of dense and fibrous plaques. The plaques can cause a variety of symptoms in the patient such as pain during erection, erectile dysfunction, and penile disfigurement. There are many treatment options available, though none without disadvantages. Quantification of the material properties of the tunica tissue can inform potentially improved treatments. In this study tunica tissue (n=5) from donors were tested biaxially along the circumferential and longitudinal directions to a specified load magnitude and maintained at a fixed displacement for 30 minutes to allow for stress relaxation. The loading and relaxation were then repeated for a second time. Stiffness, Young's Modulus, strain at inflection point between the toe region and linear region on the stress-strain curve, as well as percent stress relaxation after 30 minutes were compared between anatomic directions and ramp number. Only stiffness and percent relaxation showed a statistically significant difference between ramps 1 and 2, regardless of direction (p = 0.012). The results from this study report the baseline data of the biaxial mechanical properties of the tunica albuginea to be used for future comparisons to the properties of PD and graft tissue. / Master of Science / Peyronie’s disease (PD) affects a layer of tissue in the penis known as the tunica albuginea. Dense and fibrous plaques form on this layer which can cause pain during erection, erectile dysfunction, and penile disfigurement. There are many treatment options available, though none without disadvantages. In this study tunica tissues from donors were tested in order to find the material properties of the tissue to improve treatments. The tissue samples were pulled in tension in two directions simultaneously to a specified load magnitude and then held at a fixed displacement for 30 minutes. This protocol was then repeated a second time. The material properties of the tissue were compared between the anatomic directions of the tissue was well as protocol number. The results from this study report the baseline data of the biaxial mechanical properties of the tunica albuginea to be used for future comparisons to the properties of PD and graft tissue.

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This thesis presents the results of an experimental investigation into the strength of brickwork under biaxial tension-compression. Since there is insufficient experimental evidence available on the strength of brickwork under biaxial stress to explain the behaviour of brick masonry walls under in-plane loads, experiments were carried out on one-sixth scale model brickwork panels under uniform stress conditions. An idealized failure surface is suggested based on experimental results, and the effect of shear bond strength and tensile bond strength on the results is discussed. An iterative plane stress finite element computer programme incorporating the above information is used to simulate the in-plane behaviour of brickwork. Brickwork is treated as an elastic, isotropic material with limited capacity when stressed in a state of biaxial tension-compression. The model reproduces the non-linear behaviour of masonry produced by progressive cracking. Shear wall tests have been used to test the validity of the analytical model. Sensitivity analysis of the elastic constants used in the model are performed to illustrate their influence on the calculated stresses. The influence of the stress distribution on shear wall behaviour, and the derivation of a failure criterion for local failure in masonry shear walls, are described. This criterion, in terms of the vertical stress and shear stress at a point, has been derived for particular values of horizontal stress from the three dimensional surface mentioned above. The effect of the shape of the specimen, testing technique, and boundary conditions on the shear strength of masonry panels is discussed.

Pseudo-plane strain compression tests are carried out on rolled plate Zircaloy-2 using different combinations of loaded and constrained sample directions relative to the plate principal directions. Lattice strains are measured for 17 out of 18 possible measureable sample directions. The inability to obtain true plane strain led to little effect of the compression rig on deformation during elastic loading; however noticeable differences are seen when compared to similar uniaxial data for Zircaloy-2 in the plastic region. Work hardening increased with increased constraint and was affected by the configuration of loaded and constrained sample directions. Constraint showed significant effects on twinning when twinning was present. For the RD loaded cases the initiation of twinning occurs at -318 MPa for the RD/ND case (RD loaded, ND constrained direction) and -420 MPa for the RD/TD case. Intensity profiles of the (0002) and {101 ̅ 0} indicate that more twinning occurs in the RD/TD case than the RD/ND case. For TD/YD an amplification of twinning was seen in the TD/RD when compared to the TD/ND. This is indicated both by texture results as well as the intensity profiles of the (0002) and {101 ̅ 0}. Using the experimental data an elastic-plastic self-consistent (EPSC) code was used to probe the micromechanical processes that are occurring when the compression rig is operated. The experimental data was used further to constrain the hardening parameters of the EPSC code using an inverse approach. The EPSC code was able to capture the relative activity of the twinning characteristics found by the experimental data but unable to truly capture the evolution of the (0002) lattice strains when twinning occurs. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2014-01-13 11:24:21.692