The tension state--deformations in the metallic structure of a multifunctional backpack.

Filipoiu, Ioan Dan ; Carutasu, Nicoleta Luminita ; Radu, Gheorghe 等

Abstract: The paper presents a model of multifunctional backpack and the mathematic modelling of the tension state--deformations in its metallic structure. The mathematic modelling was performed using the finite element method (FEM) for various functions of the backpack, such as: backpack loaded with 500 N, chairs, chaise-long, bed, which can support loads of 1200 ... 1500 N

Key words: multifunctional backpack, tension--deformations state, finite element, metallic structure.

1. INTRODUCTION

The multifunctional backpack, according the invention, has the advantage of an increased comfort (Barbu et al., 2005) Its user can satisfy the need for rest, through a quick and simple transformation of the backpack into a chair (Fig. 3), chaise-long or bed (Fig. 4), without removing the objects in the actual bag. In order to properly dimension the backpack's structure, the mathematic modelling using the finite element method (FEM), for all its functions, was necessary. The results obtained are presented in paper.

2. THE BACKPACK DESCRIPTION AND FUNCTIONS

The backpack is made from a metallic frame (metallic structure), the luggage bag, the shoulder straps and the belt for fixing it on the carrier's body. From the backpack's metallic frame, a textile material (cloth) is attached by a cord which provides increased resistance. Moreover, the metallic frame is made more rigid by two transversal strengthening elements (1) with two spacers (2) (Fig. 1 and 4-b). The inferior end of the metallic frame is reinforced with a U-frame (5), its extremities bent at a 900 angle compared to the work surface; on the U-frame, a piece of cloth is applied. On the superior end of the metallic frame, two rods are fixed, and at their inferior end a transversal rod is reinforced (3), over which a piece of cloth is laid. At the superior end of the metallic frame, a U-frame of the same shape is attached. The two shoulder straps are placed on the reinforcing element of the metallic frame and on the frame. Also, the cloth bag and the belt for fixing on the carrier's body can be found on the metallic frame.

The backpack frame dimensions fit the values for similar products existent on the market. They are:

--the exterior / interior diameter of the frame rod: [PHI]18/[PHI]16 mm;

--the frame length / height in the compact state for the actual backpack: 450/860 mm;

--the height of the frame area that takes over the load for the chair or chaise-long position: 290 mm;--the tip angle between the support areas for the chaise-long position: [40.sup.0];

--the height / outreach between the support areas for the chaise-long position: 800/600 mm;

--the length / width for the bed position: 1860 mm, with the height for the head area: 290 mm and respectively for the body and legs area: 240 mm.

The backpack is designed to fulfil four functions on a resistance structure optimal from the mass and product costs point of view. These functions are:

--back carriage of luggage of maximum 50 kg,

--creating a chair that sustains forces of up to 1200 N,

--turning into a chaise-long that can sustain loads of up to 1200 N,

--creating a bed that can sustain a maximum mass of 150 kg.

3. APPLYING FEM OVER THE METALLIC STRUCTURE OF THE MULTIFUNCTIONAL BACKPACK

For the modelling process, a series of hypotheses were elaborated, for both stresses and deformations calculation (Theocaris & Atanasiu, 1976). It was taken into consideration that the cord and cloth influence over the metallic frame lead to an exterior charge uniformly distributed.

[FIGURE 1 OMITTED]

The elastic joint between the cord, the cloth and frame reduces the load effect on vertical, modifying severely the effects of bending and twisting. It is estimated that:

--no matter the function it fulfils, the load is totally on the metallic frame;

--the backpack's metallic structure is considered as an elastic body, no matter its function;

--the cord and the cloth from the two sides of the backpack are considered as a plate;

--the cord and the cloth from the two sides of the backpack partially take over the load effects;

--the load pressing on the frame is over the normal values both for the chair and the chaise-long position, and for the bed position;

--in the chair and/or chaise-long position, part of the load is taken over by the user's legs.

The modelling started from the premises that the support cloth, anchored by the frame with cords, is removed. Such a state is more difficult than a real state, since both the cloth and the cords provide the ensemble with additional mobility (Pahl & Beitz, 2006). The frame's loading was achieved by a uniform distribution of the load taken by the frame for each functional state. Following the analytic calculation, the optimum dimensions of the rods, straps and pin joints were established.

From the analysis of the mechanical and elastic characteristics for the various textile products (cord and cloth), one can notice influences of the resistance structure behaviour for any of the backpack functions, thus emerging possibilities to obtain efficient products (Rades, 2006).

[FIGURE 2 OMITTED]

4. CONCLUSIONS

For loading in the least favourable functional state, the stress values in the backpack's resistance structure are, in the dangerous sections, below the acceptable values of the materials used. Two constructive versions were analyzed, with the advantages for production costs and backpack mass (steel metallic structure, aluminium alloy structure).

For frame mass reduction reasons, the selected version was the aluminium alloy rod.

No matter the function, the deformation state ranges between limits that do not endanger the geometrical and functional shape of the backpack. The greatest deformations occur for the bed functional position, when the person sits down and the entire weight is taken over asymmetrically compared to the frames middle part. Even for these hypotheses, the deformations range between the accepted values, as shown in Figure 4 b).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

5. REFERENCES

*** Brevet de inven tie (Invention brevet) RO 115839 B.

Barbu, V., Filipoiu, I. D., Carutasu, N. L., s a., Rucsac multifunctional cu aplicatii si in situatii de criza (Backpack multifunctional with usage in crises situations), contract de cercetare stiintifica Nr. 207/2005, Programul INVENT.

Theocaris, P. S., Atanasiu, C., sa., Analiza experimentala a tensiunilor (Experimental analyses on stresses), Ed. Tehnica, Bucuresti, 1976, pp 117-318, ISBN

Rades, M., Finite Element Analysis, Ed. Printech, Bucuresti, 2006, pp 9-13, 47-79, 225-250, ISBN.

Pahl, G., Beitz, W., Konstruktionslehre Methoden und Anwendung, Springer-Verlag Berlin Heidelberg New York, 2006.