Analysis of strains in cast iron joints using FE-simulations and digital image correlation techniques

Abstract: The Metalock method is a mechanical joining technique most commonly employed in cracked castings of iron, aluminum and steel. It is based on inserting custom-made “keys”, which are meant to take up tensile and shear stresses, perpendicular to the crack. The marine diesel engine designer MAN Diesel & Turbo, uses the Metalock method for crack patching in large cast iron components, such as cylinder frames, in their engines. The service life of these components can thereby be significantly extended, reducing both replacement costs and environmental impact.The purpose of this thesis was to investigate the mechanical behavior of the Metalock method and study the material properties of the components included in a Metalock joint. The material properties were determined by performing thorough material testing, and the mechanical behavior of a Metalock joint was studied by full scale testing, using a non-contact deformation measurement technique known as digital image correlation, of cast iron specimens joined together by the Metalock method. Furhermore, finite element simulations were performed, and verified by experimental results, in order to study the mechanical behavior in detail and to carry out a parametric study on some of the components included in the joint.Results from material testing show that the material of the keys are made of an iron–nickel alloy and have a very low or even negative coefficient of thermal expansion at ordinary temperatures. Differences between initial tests and simulations indicate that the installation procedure of the joint introduce residual strains which affect the behavior of a joint subjected to mechanical loading. Experimental tests show that the distance from an edge to the first key as well as the distance between keys in a joint are important parameters which affect the load carrying capacity and the location of crack initiation. In addition, numerical simulations show that an increase of the key length yields less impact on the surrounding cast iron material, if a joint is subjected to tensile loading.