Applied Mechanics and Materials Vols. 7-8

Abstract: This paper describes the development of a software simulation tool of discrete elements which has been developed for the purpose of investigating the dynamic response of multilayer sandwich structures that incorporate highly nonlinear crumple (buckling) elements. These structures are to be optimised as cushions in order to minimise the transmission of shocks when exposed to transient excitation, such as in a free fall. Presented results are for multilayer corrugated paperboard. A single layer was modelled as a nonlinear 2-DOF system with an additional elastoplastic element to reflect contact conditions. Numerical models of the platen and the exciter with either acceleration or displacement control were developed and applied to perform numerical compression tests of the sandwich layer at various strain rates to validate the model of a single layer. Sandwich structures were then numerically assembled and subjected to simulated impacts. The model predicted inter- and intralaminar forces, displacements, velocities and accelerations. The shock attenuation characteristics were obtained and presented as the time-acceleration-static stress maps. A postprocessor was developed to produce animations to reveal complex dynamic interactions within modelled sandwich structures.

Abstract: This paper considers the development of an active vibration regulation (AVR) system on a convertible car equipped with a hydraulic actuation system. Experimental modal analysis is used to identify a model from the measured dynamic response of the car body and the hydraulic actuators. Two different methods are used to design the AVR system in the presence of process and measurement uncertainties, namely optimal feedback regulation and proportional-integralderivative (PID) compensation. With the aim of reducing the amplitude of the first eigenmode of the system considered, the AVR algorithms are digitally implemented and real-time experiments involving a hydro-pulse four poster rig are conducted. The results from these are evaluated and it is shown that the AVR systems achieve a significant vibration reduction in the first eigenmode, which is important for passenger comfort.

Abstract: There are two main characteristics of cushioning materials that are required to design a robust cushioning system for the protection of critical element against enviro-mechanical hazards: the attenuation of shocks as a function of the static load and the vibration transmissibility. The effect of a shock on a hypothetical critical element is normally evaluated by the Shock Response Spectrum (SRS) whereas the Frequency Response Function (FRF) performs similar function in relation to vibrations. This paper is concerned with the latter. Cushioning materials are generally nonlinear and, together with the interacting mass, form a nonlinear dynamic system. This paper shows how the Reverse Multiple Input-Single-Output (R-MISO) method can be used to describe the nonlinear characteristics of cushioning systems by generating a series of FRF terms. However, this creates ambiguity in relation to the effect of transmitted vibration on the critical element. This paper proposes to resolve this, by analogy to the SRS, through a numerical calculation of the Vibration Response Spectrum (VRS) for a hypothetical critical element, using as the excitation of the critical element either experimental cushion response data or data synthesised via R-MISO FRFs. Values of the VRS are defined as the ratio of acceleration rms of the critical element to the rms of the cushion excitation, although other descriptors of critical element's exertion can also be considered. The VRS can be considered as the true transmissibility. It is shown that the R-MISO method is superior over the Single Input-Single-Output (SISO) method in determining the transmissibility of polystyrene cushions. Since the cushion system is nonlinear, the excitation of the linear critical element will in general be non-Gaussian, although this paper has shown that it is near- Gaussian in the vicinity of cushion resonance. A chosen hypothetical critical element can be linear or, if its characteristics is known in advance, nonlinear. Results presented in this paper demonstrate how the R-MISO and the VRS can be used to determine the dynamic characteristics of EPS as a nonlinear cushioning material.

Abstract: The main objective of this study is to measure and characterize the mechanical properties of the thermoplastic syntactic foams at the intermediate and high strain rates. The syntactic foam consists of the elastically deformable microballoons in the polypropylene matrix. The four types of syntactic foams and one polypropylene bulk specimen are prepared at same manufacturing process: 0, 20, 30, 40 and 50 volume percent of microballoons. Tensile tests are conducted at nominal strain rates ranging from 10-1 to 102 (1/sec). Elastic modulus, yield stress and rupture strain are measured and the effects of microballoons on the mechanical properties are studied. In addition, fracture surfaces are observed with ESEM (Environmental Scanning Electron Microscopy). Finally, the changes of fracture mode due to microballoons are discussed.

Abstract: This paper presents a practical approach for health monitoring of an aircraft fuselage using vibration measurements with piezoelectric transducers. For the test specimen, a fuselage element of an Airbus A320, the feasibility of health monitoring of stringers, frames and panels, is studied. The proposed structural health monitoring (SHM) system consists of three major components: vibration measurement, signal processing and damage diagnosis. By using applied piezoelectric patch actuators, flexural waves are excited which propagate along the monitored component. The used signal for the actuator is a broadband sine-sweep signal with an upper frequency of 100 kHz. The structural response is measured with piezoelectric patch transducers, which generate a charge proportional to the induced strain due to the vibration. These sensors are positioned on different locations around the exciting piezoelectric actuator. The sampled time signal is used to compute the complex frequency response functions between actuator and sensor. After measuring the vibration behaviour of the undamaged structure, the structure is artificially damaged by saw cuts to simulate cracks. After that the vibration behaviour is measured again in the same way. The measurements of the damaged and undamaged state are now evaluated with different mathematical algorithms, like root mean square value, norm, energy content, analysis of the phase and correlation coefficient. The aim is to find a method that needs very small computation effort and that even might be implemented by an analogue circuit. Finally the data are analysed to find appropriate damage metrics. It can be shown that some of the tested methods are a sensitive damage metric. With only a few actuators and sensors cracks in the outer panel with a length of 30-60 mm and in the stringer profile with a length of 10 mm can be found.