Igor Telichev

A new load transfer index () with considering six degrees of freedom and its application in structural design and analysis

ABSTRACT Load paths analysis is important in design process of load bearing structures. U* index was introduced as the indicator of load paths based on finite element analysis. However, the U* index was only formulated based on three translational degrees of freedom (DOFs) without considering the rotational DOFs. Hence, the U* index cannot be used together with plate and shell theories, which are very important in engineering computations of thin-walled structures. A coupling issue in finite element analysis is that shell elements which are governed by plate and shell theories are invalid for U* calculations. This paper proposed a new load transfer index () with considering both translational and rotational DOFs (i.e. six DOFs). To demonstrate the effectiveness of the new index in structural design and analysis, a case study was performed to a complex thin-walled structure. The stiffness to mass ratio of the sample structure is raised by 18.3% based on the load transfer analysis using the new index.

Demonstration of the effectiveness of U*-based design criteria on vehicle structural design:

A basic function of vehicle structures is bearing the load. To design high efficient vehicle structures, it is crucial to know how the applied forces are transferred in the structure. The U* index was introduced as the indicator of the main load path in the structure. U*-based design criteria were developed to promote the ability of the U* index theory for vehicle structural design. However, the effectiveness of these U*-based design criteria on improving the structural performance is still unknown. In this paper, an improved design of a vehicle component was proposed based on the U* governed design criteria. Compared to the original design, the weight of the modified structure is reduced by 10% while the maximum displacement and stress are declined by 5% and 26%, respectively. The paper proves that the application of the U* driven design criteria can effectively increase the structural performance.

Experimental validation of the U* index theory for load transfer analysis

Engineering structures are designed to transfer external loads to their supports. Therefore, it is necessary to study the pattern of load transfer. The U * index method has been introduced to follow the load path in the structure. The U * value at each point corresponds to its significance in the load carrying process. The U * index theory has been used as a new design approach in lightweight vehicle structures but there has not been any experimental validation of this theory. In this study, two experiments are presented to show that the U *index is a true indicator of load transfer in the structure and areas with higher U * index values indeed carry more load. The results also prove the insensitivity of the U * index to stress concentrations. This study presents the first experimental validation of the U * index theory which is important to this new approach for light-weight vehicle structural design.

Experimental Study of U* Index Response to Structural and Loading Variations

Load transfer analysis tracks the path, on which the imposed load is being carried through the structure. Recently, vehicle structure designers have paid growing attention to this aspect of structural analysis for designing lighter vehicle structures that can efficiently carry the imposed loads with minimum weight. There are two main procedures for load transfer analysis in automotive engineering: 1) Stress trajectory method and 2) U* index theory. The former method faces some difficulties in following load path in structures with stress concentrations made by geometrical irregularities. As a result the U* index theory has been utilized more frequently in this area. This theory has shown exceptional capacities in following load transfer in the structure and has provided innovative tools for design modification in automotive industry. Although it can be shown mathematically that U* index quantifies the internal stiffness of the structure there has not been an experimental validation for that. Moreover, the term internal stiffness itself is not an easy concept to follow and it can be easily mistaken for the structural stiffness of the structure. As a result in the current paper two experimental testing procedures are presented to distinguish the internal stiffness, that can be quantified with U* index and the structural (conventional) stiffness of the structure. Through these experiments, for the first time, physical evaluation of U* index response to loading and structural variations can be performed.Copyright

Load Transfer Index for Composite Materials

Load transfer analysis is a new paradigm for lightweight vehicle design. U* index has been proved to be an effective indicator for the load path. The U* theory indicates that the external loading mainly transfers through the parts with higher U* values in the structure. However, the fundamental equations of the theory are based on isotropic, homogenous, and linear elastic assumptions for the materials. Consequently, U* index is inadequate for composite materials which are increasingly used in automotive structures. In this study, a new load transfer index for composite structures, U*O, is proposed for the first time inspired by the basic U* theory. The U*O index considers the composite material as orthotropic instead of isotropic and eliminates the limitation of the basic U*. The effectiveness of the new U*O index on load path prediction is demonstrated by a case study for a general Graphite-epoxy lamina. The U*O index is capable to evaluate the accurate load path for the composite specimen. By contrast, the basic U* analysis shows the incorrect results.Copyright

Extension of U* Index Theory to Nonlinear Case of Load Transfer Analysis

Load transfer analysis has been proved to be an effective approach for designing light weight vehicle structures in last two decades. There are two main procedures for predicting the load path in a vehicle: The stress trajectory method and the U* index theory. The first approach has shown some shortcomings in dealing with geometrical irregularities. As a result, automotive industries have mainly applied the U* index as a design tool to study the load transfer behavior in the vehicle structure. The U* index, is an indicator for the load transfer in the structure, i.e. higher U* index value indicates more significant role in the load transfer process. So, the distribution of the U* index in the structure can be used to predict the main load path in the structure. Nevertheless, this foundation of this theory is based upon the linear elasticity equations and consequently, it has always been limited to linear elastic problems in static or quasi static conditions. Eradicating this limitation and extending the U* Index theory to nonlinear elastic problems is the main objective of this study. An extension to nonlinear criteria for U* index theory is proposed in this paper. It is shown, for the very first time, that the extended nonlinear load transfer index (U*NL) is a true measure for the load transfer in the structure in a nonlinear elastic problem.Copyright

Numerical parametric study on factors affecting passenger safety in motorcoach frontal collision

ABSTRACT The paper presents the results of a numerical parametric study on the factors affecting passenger safety in a motorcoach frontal collision. The study is performed using the numerical model of the motorcoach sled test validated by the experimental data. The numerical model implements the rigid-body dynamics approach coupled with the finite-stiffness joints and deformable finite-element models of dummies. Three factors are parametrically studied: seating and restraint layout, seat stiffness and the shape of the input acceleration pulse. The positive effect of the seatbelt is demonstrated; the head injury criterion values for belted passenger do not exceed the safety limit for all considered seating layouts. The most dangerous seating layout is found to be the 50th percentile occupant with a 95th unbelted occupant sitting behind. Seat stiffness variation studies have shown the positive effect of plastic deformation in the seat structure. By varying the shape of the input acceleration pulse, the probability of a head injury is demonstrated to increase with the increase in the duration of maximum acceleration and time of its occurrence.

Fragmentation of Millimeter-Size Hypervelocity Projectiles on Combined Mesh-Plate Bumpers

This numerical study evaluates the concept of a combined mesh-plate bumper as a shielding system protecting unmanned spacecraft from small (1 mm) orbital debris impacts. Two-component bumpers consisting of an external layer of woven mesh (aluminum or steel) directly applied to a surface of the aluminum plate are considered. Results of numerical modeling with a projectile velocity of 7 km/s indicate that, in comparison to the steel mesh-combined bumper, the combination of aluminum mesh and aluminum plate provides better fragmentation of small hypervelocity projectiles. At the same time, none of the combined mesh/plate bumpers provide a significant increase of ballistic properties as compared to an aluminum plate bumper. This indicates that the positive results reported in the literature for bumpers with metallic meshes and large projectiles are not scalable down to millimeter-sized particles. Based on this investigation’s results, a possible modification of the combined mesh/plate bumper is proposed for the future study.

Conceptual Design of an “Umbrella” Spacecraft for Orbital Debris Shielding

Current protection techniques leave spacecraft vulnerable to objects between approximately 1 and 10 cm. This paper summarizes the conceptual design of a space vehicle with the objective of shielding spacecraft from objects in this range of sizes, which was made to study the feasibility of such a method for spacecraft protection. The design was divided into three stages: first, using SPH simulations, a multi-layer shield capable of defeating large projectiles was designed; next, a deployment mechanism that allowed the shield to be stored compactly for launch was designed and analyzed using a vector-based kinematics and dynamics method; finally, a general design of the service module was made. The final design has feasible dimensions for a spacecraft to be placed in Low Earth Orbit (LEO) and consists of an eight-layer shield with an umbrella-inspired deployment mechanism.

Development of an engineering methodology for non-linear fracture analysis of impact-damaged pressurized spacecraft structures

Motivated by the dramatic worsening and uncertainty of orbital debris situation, the present paper is focused on the structural integrity of the spacecraft pressurized modules/pressure vessels. The objective is to develop an engineering methodology for non-linear fracture analysis of pressure wall damaged by orbital debris impact. This methodology is viewed as a key element in the survivability-driven spacecraft design procedure providing that under no circumstances will the “unzipping” occur. The analysis employs the method of singular integral equation to study the burst conditions of habitable modules, although smaller vessels containing gases at higher pressures can also be analyzed.