Improvement of the impact energy absorbtion efficiency for deformable safety barriers made from composites.

Jiga, Gabriel ; Pastrama, Stefan Dan ; Dobrescu, Tiberiu 等

1. INTRODUCTION

The efficiency of absorption of impact energy in the case of collision depends on each of the components of the deformable safety barrier. It is obvious that the working element--the slide bar has the greatest importance, an adequate construction of this element being essential for the main purpose of a safety barrier--to take up the impact and to steer the vehicle on the carriageway (Caldwell, 1965; www.highwayguardrail.com, 2007). A solution which has not been applied till now is to insert a supplementary elastic element between the slide bar and the bracket pole. In this way, the pole could have the necessary stiffness able to guide the vehicle on the carriageway, the impact energy being absorbed successively first by the slide rod and then by the elastic element, this component having initially an elastic deformation followed by a plastic one or by complete failure.

2. THE PROPOSED VARIANTS

A constructive variant for manufacturing such an element is given by the tubular elastic element. The circular shape compressed on the diameter direction represents an acceptable compromise between the elasticity necessary for the damping of medium amplitude impacts and the plasticity necessary to dissipate the energy of strong impacts, action governed by the flattening of the section.

From the point of view of the material, the tubular element could be obtained from metallic or non-metallic (P.V.C., polypropylene or composite materials) barrels.

The assemblies of the safety barriers slide bars made from composite materials through an elastic element increase their functional efficiency. A constructive and, in the same time, a feasible variant consists in an "in situ" wrapping of yarns or reinforced fabric tapes on a metallic plunger die with a cylindrical mandrel shape. The resulted product is a tubular element with required geometric and mechanical characteristics. This technology (Fig. 1) is currently used for the manufacturing of tubular products, with different destinations (posts for flare-path lighting, craft masts etc.).

The tubular elastic elements could be mounted either segmented (individually on each bracket pole) (Fig. 2) or continuously, by alignment of a tubular non-segmented element with the slide bar on all barrier length (Fig. 3).

[FIGURE 1 OMITTED]

If the metallic pipe is not indicated or the plastic pipes have a poor strength, the tubular element is the most feasible one. The second variant could be improved from the point of view of the stiffness by filling up the cavity of the tubular element with polyurethane auto expandable foam, the supplement of stiffness having an important effect in taking-up of small amplitude impacts or to re-direct the vehicle on the carriageway, vital requirements in this case.

A comparative evaluation of the barrier strip strength could be obtained starting from the stresses which occur in the elastic domain as well as from the ultimate elastic and plastic bending moments. The transverse sections of the strip could have different shapes. If the normal stresses are obtained in a very easy way--irrespectively of the section shape, the shear stresses are quite difficult to be calculated, especially in the case of open profiles.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The knowledge of shear stresses is absolutely necessary especially for the correction of the normal yield stress, in such a way that in the ultimate plastic moment evaluation the shear effect should be included.

3. CALCULUS MODEL

The geometric characteristics of the two above mentioned solutions and a new one--the continuous thrie-beam were considered for the calculus of the geometric characteristics and the stresses, using a numerical code (Anghel & Modiga, 1999). The section of the continuous thrie-beam safety barrier is obtained by the addition of a tubular elastic element at the W--beam section (Modiga & Olaru, 1996, Anghel & al., 2005).

The geometric characteristics and main values of the stress distribution for the thrie-beam safety barrier are presented in Figure 4 and Tables 1 and 2 respectively. In Table 3, comparative results for the three considered types of safety barriers profiles are presented.

[FIGURE 4 OMITTED]

4. CONCLUSIONS

Starting from the analysis of mechanical characteristics, the manufacturing technological variants as well as the economic efficiency of the performances of medium heavy safety barriers made from composite materials, one could conclude the following:

a) The mechanical characteristics specific to composite materials (high elasticity and excellent dissipation of impact energy by an initial delamination between layers and further fracture by flexion of the fibers) recommend their use in manufacturing of the deformable barriers.

b) No matter the material or variant of barrier, the shape of the profile section must allow an easier pulling out from the die, without supplementary operations (injection with compressed air or vibrations). In that sense, the metallic slide bars profiles correspond in a good manner to the requirements.

d) The presence of a tubular elastic element for the thriebeam profile improves not only the absorption properties of the impact energy but also the mechanical strength of the safety barrier.

e) The normal and shear stresses decrease radically from the N2 profile, to the W-beam and finally for the Thrie-beam.

f) The section symmetry of the W-beam has an unfavorable effect on the barrier behavior in comparison with the N2 profile.

5. REFERENCES

Anghel, L.; Dimache, A. & Modiga M. (2005). Ultimate longitudinal strength of large box beam, Proceedings of the Romanian Symposium of Fracture Mechanics, ISSN 145365-36, pp. 123-130, Ploiesti, 21 Oct. 2005

Anghel, L. & Modiga, M. (1999). Programme for the calculus of shear flux and reduction in nodes of the sheer force in the transversal sections of the ship, Bulletin of the SECOMAR '99 Scientific Symposium, Navy Academy "Mircea cel Batran", vol. 1, pp. 191-196, Constanta, Romania

Caldwell, J. B. (1965). Ultimate Longitudinal Strength, Trans. RINA, Vol. 107, pp. 411-430, ISSN 1479-8751

Modiga, M., Olaru, V.D. (1996). A New Approach for Shear Flow Calculus On Thin Walled Beams, The Annals of "Dunarea de Jos" University of Galati, Romania, Fascicle X, year XIV(XIX), pp.15-20

*** (2007) www.highwayguardrail.com--Trinity Highway Products, Accessed on:2009-03-15

Tab. 1. Geometric characteristics of the thrie-beam section

A [[mm.sup.2]] [I.sub.y] [I.sub.z]
 [[mm.sup.4]] [[mm.sup.4]]

4245.583 793.31 x [10.sup.4] 2892.19 x [10.sup.4]

A [[mm.sup.2]] [Z.sub.G] [y.sub.G]
 [mm] [mm]

4245.583 18.48 0

Tab. 2. Stress values for the thrie-beam section

 [[sigma].sub. [[tau].sub.xy]
[[sigma].sub.top] bottom max
[MPa] [MPa] [MPa]

27.589 35.697 -0.001

 [[tau].sub.xz]
[[sigma].sub.top] max Ultimate yield
[MPa] [MPa] bending moment

27.589 3.499 -0.053

Tab. 3. Comparative results for three different types of safety
barriers profiles

Mechanic. Stresses
 char. [MPa]

 [[sigma].sub. [[sigma].sub.
Profiles [[tau].sub.max] top] bottom]

N2 10.9 120 192
W-beam 4.1 62.5 60.5
Thrie-beam 3.5 27.6 35.7

Mechanic. Ultimate
 char. moments
 [MNm]

Profiles [M.sub.elastic] [M.sub.plastic]

N2 0.006 0.01
W-beam 0.019 0.025
Thrie-beam 0.032 0.053