No abstract

The attention given to brain injury has grown in recent years as its effects have become better understood. A desire to investigate the causal agents of head trauma in athletes has led to the development and use of several devices that track head impacts. In order to determine which devices best measure these impacts, a Hybrid III headform was used to quantify the accuracy for translational and angular accelerations. Testing was performed by mounting each device into the helmet as instructed by its manufacturer, fitting the helmet on the headform, and impacting the helmet using an impulse hammer. The root mean square error for the peak translational acceleration varied with location. The worst root mean square error for a head-mounted device was 74.7% while the worst for a helmet-mounted device was 298%. Headmounted devices consistently outperformed those mounted in helmets, suggesting that future sensor designs should avoid attachment to the helmet. Deployment to a high school football team affirmed differences between two of the device models, but strongly indicated that head-mounted systems require further development to account for variation between individuals, the relative motion of the skin, and helmet-sensor interactions. Future work needs to account for these issues, refine the algorithms used to estimate the translational and angular accelerations, and examine technologies that better locate the source of the impact.

Structural health monitoring in the context of a Micon 65/13 horizontal axis wind turbine was described in this paper as a process in statistical pattern recognition. Simulation data from a calibrated model with less than 8% error in the fi rst 14 natural frequencies of vibration was used to study the operational response under various wind states as well as the effects of three types of damage in the blade, low speed shaft and yaw joint. It was shown that vertical wind shear and turbulent winds lead to different modal contributions in the operational response of the turbine suggesting that the sensitivity of operational data to damage depends on the wind loads. It is also shown that there is less than a 4% change in the wind turbine natural frequencies given a 25% reduction in the stiffness at the root of one blade. The modal assurance criterion was used to analyse the corresponding changes in modal defl ections, and this criterion exhibited nearly orthogonal changes because of the three damage scenarios suggesting that the modal defl ection determines which damage is observable at a given frequency for a given wind state. The` modal contribution is calculated as a damage feature, which changes as much as 100% for 50% reductions in blade root stiffness, but only the blade damage is detected using this feature. Operational data was used to study variations in the forced blade response to determine the likelihood that small levels of damage can be detected amidst variations in wind speed across the rotor plane. The standard deviation in measured data was shown to be smallest for the span and edge-wise measurements at 1P due to gravity, which provides the dominant forcing function at this frequency. A 3% change in the response in the span and edge-wise directions because of damage is required to detect a change of three standard deviations in contrast to the 90% change in fl ap direction response that is required to detect a similar change because of damage. The dynamic displacement in the span direction is then used to extract a damage feature from the simulation data that provides the ability to both locate and quantify the reduction in stiffness in the blade root.

One practical issue that must be addressed prior to the implementation of a vibration-based structural health monitoring system is the influence that variations in the structure's environmental and boundary conditions can have on the vibration response of the structure. This issue is especially prominent in the structural health monitoring of aircraft, which operate in a wide variety of different environmental conditions and possess complex structural components connected through various boundary conditions. However, many types of damage introduce nonlinear stiffness and damping restoring forces, which may be used to detect damage even in the midst of these varying conditions. Vibro-acoustic modulation is a nondestructive evaluation technique that is highly sensitivity to the presence of nonlinearities. One factor that complicates the use of vibro-acoustic modulation as a structural health monitoring technique is that the amount of measured modulation has been shown to be dependent on the frequency of the probing signal. The frequency dependence of the modulation was investigated and the magnitude of modulation was found to be correlated with the underlying vibration characteristics of the structure, which are influenced by environmental and boundary condition variations. To facilitate the use of nonlinear vibro-acoustics for the health monitoring of complex aerospace components in varying environments, a vibro-acoustic modulation technique utilizing a swept probing signal has been developed. The developed method was demonstrated on a steel beam in varying operational conditions. The presence of a crack in the beam was detected both through an increase in the amount of normalized modulation and without the use of historical data by utilizing generalized extreme value statistics.

This article discusses the use of frequency domain transmissibility functions for detecting, locating, and quantifying damage in linear and nonlinear structures. Structural damage affects both the system poles and zeros; however, zeros are much more sensitive than poles to localized damage. This is because zeros depend on the input and output locations whereas poles do not. It is demonstrated here that since transmissibility functions are determined solely by the system zeros, they are potentially better indicators of localized linear and nonlinear types of damage. Furthermore, excitation measurements are not required to compute transmissibility functions so damage indices can be calculated directly from response measurements. It is also demonstrated that sensor arrays can sometimes be used to yield mixed transmissibility functions that are differential in nature, that is, they are less sensitive to gross fluctuations in the dynamic loading or environmental variables.