IFMASS10

145

SIGNIFICANCE OF FRACTURE MECHANIC TESTING

FOR STRUCTURAL INTEGRITY OF WELDED JOINTS

Vencislav Grabulov*, Dejan Momčilović**

Institute for testing of materials IMS, Belgrade, Serbia

1. INTRODUCTION

Welding technology has had a significant impact on industrial development. Fabrica-

tion by welding is an effective method to reduce production and fabrication costs and can

be mechanized, computer controlled, and incorporated in assembly lines. Welding fabri-

cation has revolutionized many industries, including shipbuilding and automotive produc-

tion, and has resulted in the development of various products, such as pressure vessels,

that could not otherwise have achieved their present functions. Welding technology is

complex and fabrication by welding encompasses properties that should be understood to

different levels by the design engineer, the fabricator, and the welder. Some of these

properties pertinent to the theme are residual stresses, imperfections, and stress concen-

tration. Failures of engineering structures occur predominately at welded joints, even in

structures designed, fabricated, and inspected according to appropriate standards and co-

des. For example, fatigue cracking in bridges, ships, offshore structures, pressure vessels,

and buildings occurs, almost without exception, at the welded joints and attachments, like

cover plate fillet welds, stiffeners, backing bars, and seam and girth weld toes.

2. WELDING AND RESIDUAL STRESSESS

Residual stresses are those that exist in a component free from externally applied

forces. They are caused by non-uniform plastic deformations in neighbouring regions.

These regions can be small, as occurs within weldment, or large, as may occur in curving

or straightening a beam or a shell during fabrication. Furthermore, residual stresses are

always balanced so that the stress field is in static equilibrium. Consequently, wherever

tensile residual stresses occur, in neighbouring region compressive residual stresses exist.

Residual stresses can be either beneficial or detrimental to the behaviour of components.

For example, controlled thermal or mechanical residual stresses are used to curve or

straighten large components. Also, compressive residual stresses are used to minimize

environmental effects on component surfaces and to improve their fatigue initiation

resistance. Because fatigue life is governed by the stress range rather than the magnitude

of the static or steady-state, applied or residual stresses, tensile residual stresses usually

have only a secondary effect on the component fatigue behaviour. On the other hand,

excessive tensile residual stress can also initiate unstable fracture in materials of low-

fracture toughness. For that, the magnitude of unfavourable residual stresses should be

controlled, especially in thick, highly constrained weldments of low fracture toughness.

In addition, residual stresses can be induced by thermal, mechanical, or metallurgical

processes. Thermal residual stresses are caused by non-uniform permanent (plastic)

deformations when a metal is heated, then cooled under restraint. Unrestrained expansion

and contraction do not generate residual stresses. However, restrained expansion and

contraction induce permanent deformation (strains) and corresponding residual stresses.