Robert S. Salzar

Pressure-induced mechanical stress in the carotid artery bifurcation: A possible correlation to atherosclerosis

A possible correlation between regions of high intramural wall stress and the development of atherosclerotic lesions in the carotid artery bifurcation is investigated. The bifurcation geometry is determined through in vivo studies, as well as the analysis of cadaver specimens. Having compiled accurate geometric data, two representative finite element models were created in order to determine the areas of localized stress concentrations that occur in the bifurcation. The artery is assumed isotropic and is mechanically loaded with an incremental pressure of 40 mmHg. A highly localized stress concentration of approximately 9 to 14 times the proximal circumferential wall stress occurs at the point of bifurcation. A lower stress concentration of approximately 3 to 4 times the proximal circumferential stress occurs over a large area of the sinus bulb. Acknowledging that these two regions of the carotid bifurcation are highly susceptible to atherosclerotic lesions, it appears possible that a correlation between wall stress and atherosclerosis may exist.

Pulmonary Injury Risk Assessment for Short-Duration Blasts

BACKGROUND Blast injuries are becoming more common in modern war and terrorist action. This increasing threat underscores the importance of understanding and evaluating blast effects. METHODS For this study, data on more than 2,550 large animal experiments were collected from more than 50 experimental studies on blast. From this dataset, over 1,100 large animal experiments were selected with positive phase overpressure durations of 30 milliseconds or less. A two variable nonlinear logistic regression was performed on the experimental data for threshold injury and lethality in terms of pressure and duration. The effects of mass, pressure, and duration scaling were all evaluated. RESULTS New injury risk assessment curves were analyzed for both incident and reflected pressure conditions. Position dependent injury risk curves were also analyzed and were found to be unnecessary, at least for prone and side on conditions. CONCLUSIONS The injury risk assessment showed good correlation to some of the existing injury assessments. It also showed good correspondence to a reported human case of blast exposure. Pressure scaling was analyzed to be unnecessary for these short duration exposures. Recommended injury assessments for various orientations relative to the incoming blast wave are included.

Survival Risk Assessment for Primary Blast Exposures to the Head

Many soldiers returning from the current conflicts in Iraq and Afghanistan have had at least one exposure to an explosive event and a significant number have symptoms consistent with traumatic brain injury. Although blast injury risk functions have been determined and validated for pulmonary injury, there is little information on the blast levels necessary to cause blast brain injury. Anesthetized male New Zealand White rabbits were exposed to varying levels of shock tube blast exposure focused on the head, while their thoraces were protected. The specimens were euthanized and evaluated when the blast resulted in respiratory arrest that was non-responsive to resuscitation or at 4?h post-exposure. Injury was evaluated by gross examination and histological evaluation. The fatality data from brain injury were then analyzed using Fishers exact test to determine a brain fatality risk function. Greater blast intensity was associated with post-blast apnea and the need for mechanical ventilation. Gross examination revealed multifocal subdural hemorrhages, most often near the brainstem, at more intense levels of exposure. Histological evaluation revealed subdural and subarachnoid hemorrhages in the non-responsive respiratory-arrested specimens. A fatality risk function from blast exposure to the head was determined for the rabbit specimens with an LD(50) at a peak overpressure of 750?kPa. Scaling techniques were used to predict injury risk at other blast overpressure/duration combinations. The fatality risk function showed that the blast level needed to cause fatality from an overpressure wave exposure to the head was greater than the peak overpressure needed to cause fatality from pulmonary injury. This risk function can be used to guide future research for blast brain injury by providing a realistic fatality risk to guide the design of protection or to evaluate injury.

Brain Injury Risk from Primary Blast

BACKGROUND Military service members are often exposed to at least one explosive event, and many blast-exposed veterans present with symptoms of traumatic brain injury. However, there is little information on the intensity and duration of blast necessary to cause brain injury. METHODS Varying intensity shock tube blasts were focused on the head of anesthetized ferrets, whose thorax and abdomen were protected. Injury evaluations included physiologic consequences, gross necropsy, and histologic diagnosis. The resulting apnea, meningeal bleeding, and fatality were analyzed using logistic regressions to determine injury risk functions. RESULTS Increasing severity of blast exposure demonstrated increasing apnea immediately after the blast. Gross necropsy revealed hemorrhages, frequently near the brain stem, at the highest blast intensities. Apnea, bleeding, and fatality risk functions from blast exposure to the head were determined for peak overpressure and positive-phase duration. The 50% risk of apnea and moderate hemorrhage were similar, whereas the 50% risk of mild hemorrhage was independent of duration and required lower overpressures (144 kPa). Another fatality risk function was determined with existing data for scaled positive-phase durations from 1 millisecond to 20 milliseconds. CONCLUSION The first primary blast brain injury risk assessments for mild and moderate/severe injuries in a gyrencephalic animal model were determined. The blast level needed to cause a mild/moderate brain injury may be similar to or less than that needed for pulmonary injury. The risk functions can be used in future research for blast brain injury by providing realistic injury risks to guide the design of protection or evaluate injury. (J Trauma Acute Care Surg. 2012;73: 895–901. Copyright

The temperature-dependent viscoelasticity of porcine lumbar spine ligaments.

Study Design. A uniaxial tensile loading study of 13 lumbar porcine ligaments under varying environmental temperature conditions. Objectives. To investigate a possible temperature dependence of the material behavior of porcine lumbar anterior longitudinal ligaments. Summary of Background Data. Temperature dependence of the mechanical material properties of ligament has not been conclusively established. Methods. The anterior longitudinal ligaments (ALLs) from domestic pigs (n = 5) were loaded in tension to 20% strain using a protocol that included fast ramp/hold and sinusoidal tests. These ligaments were tested at temperatures of 37.8°C, 29.4°C, 21.1°C, 12.8°C, and 4.4°C. The temperatures were controlled to within 0.6°C, and ligament hydration was maintained with a humidifier inside the test chamber and by spraying 0.9% saline onto the ligament. A viscoelastic model was used to characterize the force response of the ligaments. Results. The testing indicated that the ALL has strong temperature dependence. As temperature decreased, the peak forces increased for similar input peak strains and strain rates. The relaxation of the ligaments was similar at each temperature and showed only weak temperature dependence. Predicted behavior using the viscoelastic model compared well with the actual data (R2 values ranging from 0.89 to 0.99). A regression analysis performed on the viscoelastic model coefficients confirmed that relaxation coefficients were only weakly temperature dependent while the instantaneous elastic function coefficients were strongly temperature dependent. Conclusions. The experiment demonstrated that the viscoelastic mechanical response of the porcine ligament is dependent on the temperature at which it is tested; the force response of the ligament increased as the temperature decreased. This conclusion also applies to human ligaments owing to material and structural similarity. This result settles a controversy on the temperature dependence of ligament in the available literature. The ligament viscoelastic model shows a significant temperature dependence on the material properties; instantaneous elastic force was clearly temperature dependent while the relaxation response was only weakly temperature dependent. This result suggests that temperature dependence should be considered when testing ligaments and developing material models for in vivo force response, and further suggests that previously published material property values derived from room temperature testing may not adequately represent in vivo response. These findings have clinical relevance in the increased susceptibility of ligamentous injury in the cold and in assessing the mechanical behavior of cold extremities and extremities with limited vascular perfusion such as those of the elderly.

Failure properties of cervical spinal ligaments under fast strain rate deformations.

Study Design. The failure responses of the anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum were examined in vitro under large strain-rate mechanical loading. Objective. To quantify the failure properties for 3 cervical spinal ligaments at strain rates associated with traumatic events. Summary of Background Data. There exists little experimentation literature for fast- rate loading of the cervical spine ligaments. The small amount of available information is framed only in extensive experimental coordinates, and not in the context of strains. Methods. Bone-ligament-bone complexes were strained at fast rates, in an incrementally increasing loading protocol using a servohydraulic mechanical test frame. Failure loads and displacements were converted to engineering and true stress and strain values, and compared for the different ligaments (anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum), spinal levels (C3–C4, C5–C6, and C7–T1), and for male versus female specimens. Results. There were no significant differences in force or true stress for gender or spinal level. There was a significant difference in force and true stress for ligament type. A difference was found between the posterior longitudinal ligament and ligamentum flavum for failure force, and between the ligamentum flavum and both the anterior and posterior longitudinal ligaments for failure true stress. No significant differences were found in true strain for ligament, gender, or spinal level. The mean ligament failure true strain was 0.81. Failure true strains were approximately 57% of the failure engineering strains. Conclusions. Once the injury mechanisms of the cervical spine are fully understood, computational models can be employed to understand the potentially traumatic effects of clinical procedures, and mitigate injury in impact, falls, and other high-rate scenarios. The soft tissue failure properties in this study can be used to develop failure tolerances in fast-rate loading scenarios. Failure properties of the anterior and posterior longitudinal ligaments were similar, and the same properties can be used to model both ligaments.

Pulmonary injury risk assessment for long-duration blasts: a meta-analysis

BACKGROUND Long-duration blasts are an increasing threat with the expanded use of thermobaric and other novel explosives. Other potential long-duration threats include large explosions from improvised explosive devices, weapons caches, and other explosives including nuclear explosives. However, there are very few long-duration pulmonary blast injury assessments, and use of short-duration exposure injury metrics is inappropriate as the injury mechanism for long-duration exposures is likely different from that of short-duration exposures. METHODS This study develops an injury model for long-duration (>10 milliseconds positive overpressure phase) blasts with sharp rising overpressures. For this study, data on more than 2,730 large animal experiments were collected from more than 55 experimental studies on blast. From this dataset, nearly 850 large animal experiments were selected with positive phase overpressure durations of 10 milliseconds or more. Various models were evaluated to determine the best fit of injury risk as a function of pressure and duration. A linear logistic regression was performed on the experimental data for threshold injury and lethality in terms of pressure and duration. The effects of mass, pressure, and duration scaling were all evaluated, and two goodness-of-fit indicators were used to assess the different models. RESULTS AND CONCLUSIONS New injury risk assessment curves were determined for both incident and reflected pressure conditions for reflecting surface and free-field exposures. Position dependent injury risk curves were also determined. The resulting curves are an improvement to existing assessments, because they use actual data to demonstrate theoretical assumptions on the injury risk.

An evaluation of a new approach for the thermoplastic response of metal-matrix composites

Abstract The utility of a recently developed analytical solution methodology to the micro-mechanics concentric-cylinder model for the inelastic response of metal-matrix composites under thermal loading is illustrated by comparison with the results generated using the finite-element approach. The model is based on the concentric-cylinder assemblage consisting of an arbitrary number of elastic or elastoplastic sublayers with isotropic or orthotropic, temperature-dependent properties. The elastoplastic boundary-value problem of an arbitrarily layered concentric cylinder is solved using the local/global stiffness matrix formulation (originally developed for elastic layered media) and Mendelsons iterative technique of successive elastic solutions. These features of the model facilitate efficient investigation of the effects of various microstructural details, such as functionally graded architectures of interfacial layers, on the evolution of residual stresses during cool down. The available closed-form expressions for the field variables can readily be incorporated into an optimization algorithm in order to efficiently identify optimal configurations of graded interfaces for given applications. Comparison of residual stress distributions after cool down generated using finite-element analysis and the present micromechanics model for four composite systems with substantially different temperature-dependent elastic, plastic and thermal properties illustrates the efficacy of the developed analytical scheme.

MUSCLE TETANUS AND LOADING CONDITION EFFECTS ON THE ELASTIC AND VISCOUS CHARACTERISTICS OF THE THORAX

Thoracic deformation under an applied load is an established indicator of injury risk, but the force required to achieve an injurious level of deformation currently is not understood adequately. This article evaluates how two potentially important factors, loading condition and muscle tensing, affect the structural response of the dynamically loaded thorax. Structural models of two human cadaver thoraxes and two porcine thoraxes were used to quantify the effects. The human cadavers, which represent anthropometric extremes, were subjected to anterior loading from (1) a 5.1-cm-wide belt oriented diagonally (i.e., seatbelt-like loading), (2) a 15.2-cm-diameter rigid hub, and (3) a 20.3-cm-wide belt oriented laterally (i.e., a distributed load). A structural model having the mathematical formulation of a quasilinear viscoelastic material model was used to model the elastic and viscous response, with ramp-hold tests used to determine the model coefficients. The effect of thoracic musculature was assessed using similar ramp-hold tests on the porcine subjects, each with and without forced muscle contraction. Even maximally contracted thoracic musculature is shown to have a minimal effect on the response, with similar elastic and viscous characteristics exhibited by each subject regardless of muscle tone. The elastic response is shown to be approximately a factor of three stiffer for diagonal belt loading and for this distributed loading condition than for the hub loading, indicating that the response is influenced most by the particular anatomical structures that are engaged and, secondarily, by the area of load application. Specifically, shoulder involvement is shown to have a strong influence. The force relaxation is found to be pronounced, but insensitive to the loading condition, with long-time force relaxation coefficients (G∞) in the range of 0.1 to 0.3. The findings of this study provide restraint-specific guidelines for the force-deflection characteristics of both physical and computational thoracic models.

Functionally graded metal matrix composite tubes

Abstract Current advanced manufacturing techniques allow the continuous variation of fiber or inclusion volume fraction in metal matrix composites. With this technology, it is now possible to tailor a composite to the expected loads by using the constituent materials to redistribute the stress and strain states through the material. Lighter and more structurally efficient components will be obtained through this grading process. The focus of this paper is the evaluation of the effects of material property and fiber volume grading on the overall mechanical response of metal matrix composite tubes subjected to mechanical loadings. This is accomplished through the development of a fully elastic-plastic axisymmetric generalized plane strain tube model. This analytical model incorporates a micromechanics algorithm in order to determine the elastic-plastic response of a heterogeneous fiber-reinforced composite cylinder. An arbitrary number of heterogeneous concentric cylinders can be included in the model, each with independent material properties. The inelastic analysis is performed through the method of successive elastic solutions. The optimization algorithm used in conjunction with this solution procedure utilizes the method of feasible directions and accepts any combination of design variables, constraints, and objective functions. As an example of the effectiveness of this grading, it is possible to vary the fiber volume fraction in an SiC/Ti-24Al-11Nb tube in such a way that the effective stress at the critical inner surface of an internally pressurized tube is reduced. For a 50.8 mm (2in) thick tube with an internal radius of 25.4 mm (1 in) and an internal pressure of 206.8 MPa (30 ksi), a uniform 40% fiber volume fraction distribution results in a tube that begins to plastically yield at the inner radius. By grading the fiber volume fraction, the tube now behaves elastically under the same pressure loading, allowing the tube to have a wall thickness of 25.4 mm (1 in) before plastic yielding begins. This grading results in a 60% weight saving.