Som R. Soni

A single-loop deterministic method for reliability-based design optimization

Reliability-based design optimization (RBDO) is a technique used for engineering design when uncertainty is being considered. A typical RBDO problem can be formulated as a stochastic optimization model where the performance of a system is optimized and the reliability requirements are treated as constraints. One major challenge of RBDO research has been the prohibitive computational expenses. In this research, a new approximation approach, termed the single-loop deterministic method for RBDO (SLDM_RBDO), is proposed to reduce the computational effort of RBDO without sacrificing much accuracy. Based on the first order reliability method, the SLDM_RBDO method converts the probabilistic constraints to approximate deterministic constraints so that the RBDO problems can be transformed to deterministic optimization problems in one step. Three comparison experiments are conducted to show the performance of the SLDM_RBDO. In addition, a reliable forearm crutch design is studied to demonstrate the applicability of SLDM_RBDO to a real industry case.

The Application of Memetic Algorithms for Forearm Crutch Design: A Case Study

Product design has normally been performed by teams, each with expertise in a specific discipline such as material, structural, and electrical systems. Traditionally, each team would use its members experience and knowledge to develop the design sequentially. Collaborative design decisions explore the use of optimization methods to solve the design problem incorporating a number of disciplines simultaneously. It is known that such optimized product design is superior to the design found by optimizing each discipline sequentially due to the fact that it enables the exploitation of the interactions between the disciplines. In this paper, a bi-level decentralized framework based on Memetic Algorithm (MA) is proposed for collaborative design decision making using forearm crutch as the case. Two major decisions are considered: the weight and the strength. We introduce two design agents for each of the decisions. At the system level, one additional agent termed facilitator agent is created. Its main function is to locate the optimal solution for the system objective function which is derived from the Pareto concept. Thus to Pareto optimum for both weight and strength is obtained. It is demonstrated that the proposed model can converge to Pareto solutions.

PZT behavior in cyclic strain environments

This paper presents results of an experiment designed to determine the impact of repeated strain cycles on lead ziconate titanate (PZT) transducers affixed to an aluminum test specimen. The goal of this research effort is to determine the impact of three cyclic strain levels on PZTs affixed with two different glue types. PZT transducers are evaluated because they are one of the leading health monitoring technologies used in aircraft structures due to their ability to transmit and receive Lamb Waves. Analysis of changes in the received signals can indicate the presence of structural damage. This monitoring paradigm can only be successful if signal changes due to exposure to aircraft environmental factors (temperature/strain/pressure cycles, etc) over time can be clearly identified and characterized. This paper presents the results and initial analysis of experiments to determine the changes in signal responses due to cyclic mechanical strain. Results indicate cyclic strain at 800 με has no effect to 510K cycles, while cyclic strain at 1700 and 2600 με both cause signal loss to varying degrees.

Experimental Structural Health Monitoring of Z-Fibre Reinforced, Co-Cured Composite Pi-Joints using Lamb wave Propagation

The goal of this research is to establish a methodology for damage detection in unreinforced and z-fiber reinforced cocured composite pi- joints using lamb wave based structural health monitoring technique. Because of the lack of natural reinforcement in the thickness direction, delamination has been a predominant failure mode besides other failure modes in laminated composites. Z-fiber reinforcement is one of the ways of controlling or delaying delamination and thus, delaying the failure. Here, DCB (Double Cantilever Beam) and Pi-Joint specimens, with and without z-fiber reinforcement, are considered for experimental analysis. Damage is experimentally induced in the specimen under static loading. Lamb wave propagation based structural health monitoring is performed using PZT sensors in a pitch-catch arrangement. Amplitude vs. time and amplitude vs. frequency response are plotted for various excitation frequencies. At lower frequencies (particularly at 20 KHz), pure A0 mode is generated, which is confirmed by out of phase response of simultaneous PZT sensors. From the response data analysis, presence of damage in unreinforced, z-fiber reinforced DCB and pi-joint specimens is confirmed.

Lamb wave propagation in Z-pin reinforced co-cured composite pi-joints

This paper presents an initial study on Lamb wave propagation characteristics in z-pin reinforced, co-cured composite pi-joints for the purposes of structural health monitoring (SHM). Pi-joint test articles were designed and created to replicate a co-cured, all composite skin-spar joint found within a typical aircraft wing structure. Because pi-joints exhibit various complex damage modes, formal studies are required if SHM systems are to be developed to monitor these types of joints for potential damage. Experiments were conducted on a undamaged (healthy) and damaged test articles where Lamb waves were excited using one lead zirconate titanate (PZT) transducer. A three-dimensional (3D) scanning laser Doppler vibrometer (LDV) was used to collect high-density scans of both the in-plane and out-of-plane velocity measurements. In the damaged test article, where delamination, matrix cracking, and fiber breakage can clearly be seen, changes in both the fundamental antisymmetric A0 and symmetric S0 Lamb wave modes are apparent. In both test articles, the effects of narrow geometry, discontinuity due to the attachment of the web, and thickness has detectable effects on Lamb wave propagation. From the comparisons between Lamb waves propagating through the undamaged and damaged test articles, it is clear that damage can be detected using Lamb waves in z-pin reinforced, co-cured composite pi-joints for this case of extensive damage.

Lamb wave propagation in a restricted geometry composite pi-joint specimen

Abstract : The propagation of elastic waves in a material can involve a number of complex physical phenomena, resulting in both subtle and dramatic effects on detected signal content. In recent years, the use of advanced methods for characterizing and imaging elastic wave propagation and scattering processes has increased, where for example the use of scanning laser vibrometry and advanced computational models have been used very effectively to identify propagating modes, scattering phenomena, and damage feature interactions. In the present effort, the propagation of Lamb waves within a narrow, constrained geometry composite pi-joint structure are studied using 3D finite element models and scanning laser vibrometry measurements, where the effects of varying sample thickness, complex joint curvatures, and restricted structure geometries are highlighted, and a and a direct comparison of computational and experimental results are provided for simulated and realistic geometry composite pi-joint samples.

Effects of Z-pins on Lamb waves in composite plates

This experimental research investigates the effects of adding z-pins to a carbon fiber reinforced plate (CFRP) on Lamb wave propagation, such as mode conversion and reflections. The motivation for this study is derived from the current and expected future use of z-pins in aircraft structures coupled with the requirement to design structural health monitoring (SHM) systems for detecting damage in regions of composite structures with z-pins. This experimental study is conducted on two 4.8 mm thick CFRP test articles, where one plate has a 20 by 279 mm2 band of z-pins and the other does not. The z-pins have an average diameter of 0.28 mm and are inserted through the thickness of the panel with an area density of 4% before curing. A three-dimensional (3D) laser Doppler vibrometer (LDV) was employed to collect velocity measurements over a 1 mm uniformly-spaced grid of 17,899 scan points. Time-sequenced 3D LDV scans are presented to show that adding this relatively small amount of z-pins to a 4.8 mm thick CFRP has few measureable effects on Lamb wave propagation.