Adaptation of a highly compressible elastomeric material model to simulate compressed expanded graphite and its application in the optimization of a graphite-metallic structure

Abstract

The paper presents simulations of mechanical properties of compressed expanded graphite. To model the stress and strain states of compressed expanded graphite, a hyperelastic material model formulated by the Blatz-Ko was adapted. The idea of using this model resulted from the assumptions that compressed expanded graphite exhibits similar mechanical properties to polyurethane foam with high density. The material constant of the Blatz-Ko model was determined by numerical calculations where deformation energy equation was parameterized with the shear modulus. Such material model used in a compressed expanded graphite simulation was experimentally validated. Subsequently, the Blatz-Ko model was used in a numerical simulation of the graphite-metallic structure in the form of a spiral wound gasket which is composed of metallic and elastic tapes. This structure was subjected to an optimization process which included the reduction of stress in the elastic graphite tape obtaining the appropriate axial stiffness of the gasket. In the final stage, the optimal design of the gasket was experimentally tested. The experimental results were compared with the data obtained numerically, which allowed to verify the adapted material model and boundary conditions of the numerical gasket model.

Basic Design Parameters Influencing on Axial Stiffness of the Spiral Wound Gasket

“…The deformation of the filler described by Equation (12) or (13) allows the location of the point on the compression curve ( Figure 12 ) to be determined and the derivative determining the local value of the modulus of elasticity (with a known analytical notation of the equation that models the course of the compression curve of the filler) to be calculated. In paper [ 24 ], a slightly different method of calculating this parameter was proposed, and it was shown that the modulus of elasticity is directly proportional to the area under the stress–strain curve. The modulus of elasticity (calculated in this way) is the average value from the entire deformation range.…”

“…The modulus of elasticity (calculated in this way) is the average value from the entire deformation range. In the method presented below, the determination of the average value of the modulus of elasticity is based on the model presented in [ 24 ] but takes into account the effect of the initial compression/deflection of the material that fills the windings of the spiral gasket. If the filler is initially compressed, the area under the stress–strain curve (representing the local modulus of elasticity) will get smaller.…”

“…Murali Krishna et al [ 23 ] performed a study on the sealing efficiency of bolted flange connections with spiral wound gaskets using finite element analysis. Jaszak investigated a spiral gasket placed between a pair of hydraulic press plates using a highly compressible elastomeric material model [ 24 ] and an elastic–plastic bilinear model [ 25 ]. Nelson [ 26 ] analysed the stress in spiral wound gasket with graphite filler using a micromechanical approach and axisymmetric finite element analysis.…”

Optimized design of a semimetal gasket operating in flange-bolted joints

“…In the analyzed solution, the parameters that limit the sought area of the optimal gasket's structure were the geometrical dimensions of the core's cross-section, the strength of the used materials, as well as the ensuring of a minimum pressure exerted on the sealing surface. A similar optimization model was presented in [15]. The limitations of the gasket's dimensions were the maximum outer diameter dz = 90mm, the minimum inner diameter dw = 49mm, and the maximum height h = 10 mm.…”

Effect of tightening process parameters on the T-F curve of bolts in composite structures

“…Experimental and simulation results showed that these constructions have a larger effective contact area than conventional gaskets. Jaszak 34 studied the mechanical properties of compressed, expanded graphite used in bolted joint structures. The feasibility of a hyperelastic material model was developed and verified, and the connection structure was optimised.…”