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  • FINITE ELEMENT SIMULATIONS AND CHARACTERIZATION OF AIRBAGS

    USING DOE

    BHAGEERATHA K R 1 & C ANIL KUMAR

    2

    1 PG Student, Department of Mechanical Engineering, KSIT, Bangalore, Karnataka, India

    2 Professor, Department of Mechanical Engineering, KSIT, Bangalore, Karnataka, India

    ABSTRACT

    In the present study Low velocity impact tests have been carried out to study the energy absorption capabilities

    of airbags, the main objective is to increase the energy absorption capabilities by minimizing the acceleration levels

    and the number of bounces after the airbag impacts ground. In the present work, the effect of various parameters on the

    energy absorbing capabilities of airbag are studied. The three parameters chosen for the present study are Airbag

    Shape, Initial Pressure and the drop height.

    Finite Element Method (FEM) and Design of Experiments (DOE) approach are used in order to achieve the

    intended model objectives. The combination of both techniques is proposed to result in a reduction of the necessary

    experimental cost and effort in addition to getting a higher level of verification. It can be stated that the Finite

    Element Method coupled with Design of Experiments approach provides a good contribution in characterizing the

    airbag.

    The present work is divided into four phases, in the first phase Selection and testing of Airbag material is

    carried out, where the material required for the airbag is chosen and tested according to ASTM standards to find out

    the properties of the material. The second phase includes fabrication and testing of Airbags, Where three different

    kinds of airbags (circular, cylindrical and rectangular) having same surface area are fabricated and tested according to

    the Taguchi’s L9 Orthogonal Array experimental plan. In the third phase a Finite Element Model (FEM) which

    represents the drop tests on the airbags (carried out in the Low Velocity Impact Test Rig) is developed in order to

    evaluate the quality of the process parameters. In the fourth phase results are analyzed using Taguchi’s Signal to Noise

    ratio technique, for both experimental testing and simulation of airbags to analyze the data and for the prediction of

    optimum results. Finally the two methods are compared (FEM and Experimental) using DOE and the results are

    analyzed to get the optimal set of process parameters.

    KEYWORDS: Airbag, Shape, Pressure, Drop Height, Airbag Characterization, Finite Element Method, Design of

    Experiments

    INTRODUCTION

    Research and investigations are being carried out to understand the intentional and unintentional injuries of human

    beings in society. The injury level is not only causing unnecessary suffering for millions of victims but has also

    tremendously increased the cost of aid provided to victims. Since awareness of preventive measures is increasing in

    society, effort is focused on developing effective safety devices. Researchers, aided with information obtained by

    investigators, have found new techniques to understand the dynamics involved in accidents resulting in injuries [1]. These

    have either resulted in the design of new safety devices or helped in improving the existing ones.

    International Journal of Automobile Engineering

    Research and Development (IJAuERD)

    ISSN 2277-4785

    Vol. 3, Issue 4, Oct 2013, 23-34

    © TJPRC Pvt. Ltd.

  • 24 Bhageeratha K R & C Anil Kumar

    In order to achieve optimum occupant safety inflatable restraint technology, commonly referred to as airbags has

    been developed. Airbags are the inflatable restraints generally used to absorb the kinetic energy that is dissipated during

    high speed crashes [2]. An airbag is a three dimensional structure capable of undergoing large deformations with non-

    linear material properties, in recent years some efforts have been made to the design of airbags and improve their

    performance in the crashworthiness studies of aircraft structures in low velocity impact.

    SELECTION AND TESTING OF AIRBAG MATERIAL

    Selection of Airbag Material

    Woven fabrics have been the material of choice for materials used in safety air bag Construction. The latest

    research on potential airbag materials including polyester fiber, Nylon 6,6, and Neoprene[3]. Among the three materials

    Nylon has the advantages over the other two as it has better properties such it has

     A high strength-to-weight ratio, Good elongation properties, Minimal weight for minimal space/thickness&

    Insensitivity to temperature, It gives a good balance between the strength and elongation, hence acts as god airbag

    cushion materials.

     Thermodynamically it has high melting point and heat of fusion as compared to other common fibers like

    polyester.

    Testing of Airbag Material

    Tensile and shear tests are conducted on airbag fabric with different orientation of the fabric to know the different

    properties of fabric material. The different orientations Chosen for airbag testing are WARP, WEFT and SHEAR

    directions.

    Specimen Preparation and Testing of Airbag Fabric

    Rate of Loading 2mm/min

    Temperature 73 Deg F

    Humidity 50%

    Gauge Length of Specimen 150mm

    Width of the specimen 50mm

    Thickness of specimen 0.26 mm

    Figure 1: Test Specimen According to ASTM (ASTM D579) Standards and Test Setup Conditions

    In the present study sample specimens in the WARP (0 0 ), WEFT (90

    0 ) and SHEAR (45

    0 ) directions were

    identified and marked on the airbag material roll received from manufacturer. The size and shape of the tensile and shear

    test specimens are chosen according to the ASTM standards (ASTM D579) as shown below fig 1 and to carry out the

  • Finite Element Simulations and Characterization of Airbags Using DOE 25

    tensile and shear testing on airbag fabric INSTRON- 5500 R Series Universal Testing Machine has been used. The test was

    conducted with the following test setup conditions.

    Results of Tensile and Shear Testing of Airbag Fabric

    Based on the tensile tests, the results have are tabulated in the Table 1.

    Table 1: Tensile Test Results for Airbag Material

    Tensile Test Results for Airbag Material

    Specimen Sample Max Load

    (Pmax ) N

    Ultimate Strength

    (UTS) MPa

    Max Elongation

    mm Modulus MPa

    Warp (0º)

    1A 472.998 36.38 30.815 47.889

    53.054 1B 488.082 37.54 27.333 58.108

    1C 369.075 28.39 24.667 53.165

    Weft (90º)

    2A 764.113 58.77 38.599 86.720

    94.356 2B 814.865 62.68 40.833 96.438

    2C 818.964 62.99 41.660 99.91

    Shear (±45 o )

    3A 355.136 27.31 84.197 3.569

    3.972 3B 448.195 34.47 90.333 4.303

    3C 503.884 38.76 95.831 4.043

    AIRBAG FABRICATION AND TESTING

    Airbag Fabrication

    The airbag material was made up of nylon engineering fabric. This fabric was coated one side with Rubber

    coating. To compare the response characteristics Airbags with three different shapes are fabricated. All the three Airbags

    have same surface area. The three shapes selected are Circular Airbag, Rectangular Airbag, and Cylindrical Airbag.

    The three types of airbags (Circular airbag of φ550mm, Rectangular Airbag of 594x400 and Cylindrical Airbag of

    φ382 mm and length 205 mm) are fabricated by machine stitching and subsequent bonding around the seam. The required

    fabric size was cut from the fabric roll and stitched. Then the stitching was carried out using the sewing machine with

    nylon thread. Total three rows of stitching operation were carried out around circumferential direction.

    After that, standard epoxy based adhesive with hardener is applied on the stitched area as well as the edge of the

    fabric. After 24 hrs room temperature curing, the airbag is inverted and subsequent operations like making a compressed

    air inlet port, pressure sensor port and airbag mount were carried out.

    Once again epoxy adhesive is applied on to the seam of an inverted airbag in order to minimize the leakage

    around the seam. Second time room temperature curing of adhesive was carried out before the airbag subjected to a

    ‘breathing’ operation.

    In this process, inside volume of the airbag is filled with compressed air to avoid any wrinkles around the seam of

    an airbag.

    Airbag Testing

    The airbag drop test studies are carried out in “Low Velocity Impact Test Rig” (LVITF).

    The airbag drop test studies are carried out for the cross head mass of 8 kg (includes airbag mount mass), different

    drop heights (180 cm, 130 cm and 80 cm) and at different pressures (0.4psig, 0.6psig, 1.0psig) and with different shapes

    (Circular, Rectangular, Cylindrical).

  • 26 Bhageeratha K R & C Anil Kumar

    Experiment Set up & Experimental Plan Using Taguchi’s Philosophy

    Figure 2: Schematic Diagram of LVITF & Flowchart of the Taguchi Method

    Taguchi’s philosophy is an efficient tool for the design of high quality manufacturing systems. Dr. Genichi

    Taguchi, a Japanese quality management consultant, has developed a method based on orthogonal array experiments