FRACTURE MECHANICS FOR A PLATE HEAT EXCHANGER GASKET-1

17-04-2020

Abstract

A plate heat exchanger consists of a number of thin, corrugated plates with openings for two heat exchanging fluids. The plates are put together in a stack with a rubber gasket between each plate. Due to assembly pressure and high operating temperature in the plate heat exchanger, the gaskets sometimes tend to crack. 

This work evaluates and implements fracture models for rubber in FE-applications. This includes performing laboratory tests to determine material characteristics for two rubber materials, supporting and also verifying FE-simulations. Experiments using pure shear test specimens are presented and the pure shear test method reflects the true condition in the gasket well. 

The tearing energy criterion is thoroughly evaluated and concluded not valid for crack lengths less than 5mm. Hence, the presence of small scale cracks (material irregularities) of approximately 50μm are not supported by the tearing energy criterion or by any other fracture criterion evaluated in this work. 

Stress analyses of an EPDM gasket are performed in ABAQUS, showing that the maximum principal Cauchy-stress reaches a level of 9.5MPa at a temperature of 130°C. Hence, the material strength is exceeded and fracture mechanics is ruled out as a major factor influencing rupture of gaskets. 

1 Background

In a plate heat exchanger, the gasket and plate material along with the geometric shape of the gasket and the gasket groove geometry are critical factors for the performance of the plate heat exchanger. In order to improve sealing characteristics and also reduce time in the design process, Alfa Laval has started to use finite element analysis as a tool in development of new designs and for modification of existing products. 

This master thesis is a continuance of a previous master thesis, FE-analysis on a plate heat exchanger gasket [6]. Concluding remarks in that work established that the levels of stresses not alone could make the gaskets collapse. Explanations of gasket failure are therefore sought elsewhere, investigating whether fracture mechanics is a prime factor in the solution to the problem.

2 Rubber elasticity

This chapter is founded on the thesis Modeling of Elasticity and Damping for Filled Elastomers written by P. -E. Austrell, [2]. 

Rubbers are highly non-linear materials and the simple linear elastic stress-strain relation with a constant Young's modulus E cannot be applied. It is thus necessary to describe the material behaviour using some other mathematical model, particularly the elastic property. The constitutive relation for a hyperelastic material is, as well as a linear elastic material, defined as a relationship between total stress and total strain. 

The strain energy density then plays a central role in defining the constitutive relation for rubber materials. Stresses are determined by derivatives of the strain energy density function W, which is a function of the strain invariants 

plate heat exchanger

Two common forms of the strain energy function W, implemented in most of the general finite element programs are the Neo-Hooke model, i.e. 

plate heat exchanger gasket

and the Yeoh model, i.e.

heat exchanger

The elastic parameters C10, C20, C30 are the constants that describe the hyperelastic material behaviour in FEanalyses.


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