^{1}

^{2}

^{3}

^{3}

Failure analysis and fatigue life prediction are very important in the design procedure to assure the safety and reliability of rubber components. The fatigue life of a railway elastomeric pad is predicted by combining the test of material properties and finite element analysis (FEA). The specially developed chloroprene rubber material’s fatigue life equation is acquired based on uniaxial tensile test and fatigue life tests performed on the dumbbell specimens of the chloroprene rubber. The same chloroprene rubber was developed at Indian Rubber Manufacturer’s Research Association, Thane. The strain distribution contours and the maximum total principal strains of the elastomeric pad at different compressive loads are obtained using finite element analysis method. The software used for the FEA was ANSYS. The three parameter nonlinear hyperelastic Mooney-Rivlin Model and plane 182 elements were used for finite element analysis. The critical region cracks prone to arise are obtained and analysed. Then the maximum first principal elastic strain was used as the fatigue damage parameter, which is substituted in the chloroprene rubber’s fatigue life equation, to predict the fatigue life of an elastomeric pad in the number of cycles at different compressive loads. The results were compared with the technical requirements given by Indian Railway’s Research Designs and Standards Organization. These requirements were achieved up to certain extents. The results were also compared with the data available in the literature and a similarity was observed between the results acquired and literature data. In short, the proposed fatigue life prediction method can shorten the product design cycle, decrease the design and product cost remarkably and improve the quality of an elastomeric pad.

Rubbers are extensively used in many applications because of their large reversible elastic deformation, excellent damping and energy absorption characteristics. Typical applications include vibration isolators for railway wagons, household electric appliances, rubber bearings for bridges, engine mounts and tires for automobiles, etc. Most of these rubber components are subjected to static and dynamic loadings in service. To prevent failures during operation is one of the critical issues in rubber component design. Therefore, fatigue analysis and strength evaluation are very important in design procedure to assure the safety and reliability of mechanical rubber components.

R. S. Rivlin [

In this paper, the fatigue lifetime prediction methodology of rubber components was developed by incorporating the finite element analysis with material characterization of the rubber material and fatigue damage parameter determined from fatigue tests. The methodology was applied to the fatigue life estimation of an elastomeric pad used for railway wagons in order to assess the durability of the rubber components at the design stage.

Load versus maximum principal strain relationship of an elastomeric pad was obtained from the nonlinear finite element analysis using a hyperelastic three parameter Mooney-Rivlin model determined from material tests. Fatigue tests using dumbbell specimens with various strains were performed and a fatigue life curve represented by strain values and fatigue life in number of cycles was obtained. A fatigue life prediction equation was developed from the aforementioned fatigue life curve. Fatigue life of an elastomeric pad at different compressive loads was predicted from the fatigue damage evaluation at the critical location of the elastomeric pad and the fa- tigue life prediction equation.

An elastomeric pad used in railway wagons is considered in this study.

prene rubber with a metal framework.

The rubber material of an elastomeric pad was vulcanized chloroprene rubber, which has the hardness of the Shore A hardness degree 66. The chloroprene rubber can be considered as a hyperelastic material, showing highly nonlinear elastic isotropic behaviour with incompressibility. A relationship between stress and strain in the hyperelastic material, generally characterized by strain energy potentials, is essential for the FEA of rubber components. In order to define the hyperelastic material behaviour, i.e. the constitutive relation, experimental test data are required to determine material parameters in the strain energy potential.

The Mooney-Rivlin function was selected to specify the constitutive model of the chloroprene rubber material in this study. The strain energy potential of the Mooney-Rivlin [

where

In the uniaxial stress state, the principal stretch ratios

where

In this study, for more stability, the three parameter Mooney-Rivlin model expressed by Equation (6) was selected to specify the constitutive model of the chloroprene rubber.

where

There are two methods to character the fatigue properties of the material, one is the fatigue life equation and curve of stress amplitude vs. fatigue life (S-N equation and curve), the other is the fatigue life equation and curve of strain amplitude vs. fatigue life (

In order to evaluate the fatigue properties of the chloroprene rubber, fatigue tests on dumbbell specimens as per ASTM D412 standards [

Sr. No. | Strain Value (%) | Fatigue Life in No. of Cycles |
---|---|---|

1 | 25 | 7,19,987 |

2 | 50 | 3,36,003 |

3 | 65 | 1,68,078 |

4 | 75 | 88,012 |

5 | 85 | 39,992 |

6 | 100 | 32,053 |

7 | 125 | 16,007 |

The

where the

So the fatigue life equation of the chloroprene rubber can be expressed as:

Equation (8) can be transformed as:

So the fatigue life equation of the chloroprene rubber in the logarithmic co-ordinates can be expressed as:

Equation (10) was used to predict the fatigue life of an elastomeric pad in number of cycles. In this equation the maximum values of the strain was substituted from FEA analysis to get the fatigue life in number of cycles at different compressive loads.

The model of an elastomeric pad shown in

The deformed shape and strain contour for 10 tonne compressive load is shown in

showing the maximum total principal strains are the critical regions where the cracks prone to arise.

Analysed and recorded the maximum total principal strains at different compressive loads by the above FEA method.

It is considered that an elastomeric pad cracks when the critical region suffered sufficient fatigue damage i.e. when a crack of 1 mm size is observed. The maximum total principal strain at the critical region determined from the FEA was used for evaluating the fatigue damage parameter of the chloroprene rubber. The fatigue lives of an elastomeric pad at different compressive loads were derived by substituting the maximum total principal strains into the fatigue life equation of the chloroprene rubber material.

The fatigue life of an elastomeric pad was predicted using the above method incorporating test of material properties and FEA.

The fatigue life of an elastomeric pad was predicted through a combination of test of the material properties and FEA in the early design procedure to assure the safety and reliability of an elastomeric pad. The chloroprene rubber material was a newly developed rubber material for an elastomeric pad to meet the specified technical requirements; therefore it is necessary to predict the fatigue life of the pad before the prototype or actual com-

Sr. No. | Parameters | Load in Tone | ||||
---|---|---|---|---|---|---|

10 | 12.5 | 15 | 17.5 | 20 | ||

1 | Max. I^{st} principal elastic strain | 0.49897 | 0.542378 | 0.577614 | 0.605677 | 0.621768 |

2 | Fatigue life in cycles | 2,707,981 | 1,591,834 | 1,066,069 | 788,050 | 666,843 |

ponent is made. The strain analysis of an elastomeric pad is carried out by using FEA which outlines the critical regions having maximum principal strain in which the cracks prone to arise. Therefore, the first evidence of fatigue failure will be observed in these critical regions. It is observed that as the compressive load on an elastomeric pad increases; the maximum principal strain is also increases and the fatigue life in number of cycle decreases. The predicted fatigue life of an elastomeric pad at maximum 20 tonne load is greater than 5 lac cycles i.e. 666,843 cycles. This satisfies the fatigue requirement of the RDSO which states that after fatigue testing of the pad for 5 lac cycles there should not be any sign of crack or bond failure.

The authors are thankful to Indian Rubber Manufacturer’s Research Association, Thane (W) and Management of Pune District Education Association and Dr. K. R. Harne, Principal, College of Engineering, Manjari (Bk.), Pune, for granting the permission to do the research work on the aforementioned topic.