Brittle and Ductile Failures

Introduction

The notation used to describe the development of fracture using adjectives as ductile or brittle depends on whether one refers to the micro or the macro scale. Thus, the wording brittleness and ductility may have different meanings depending on whether it is seen from a metallurgist or a designer. A metallurgist is considering how to best achieve ductile material that shows sufficient strength and elongation at fracture and that is robust with respect to fracture when defects are present in the material. Designers most often presume that the material they are using is ductile and that their main task is to avoid a brittle structural behavior through a less favorable design. However, the post-failure behavior of some structures is such that a global brittle development cannot be avoided and for these structures a larger safety factor is normally recommended than in design with development of a ductile failure mechanism. An example of this is shell structures and structural elements where local buckling may occur; see, for example, Eurocode 3 (EN 1993-1-1 2009). The terminology used by metallurgists and designers may also be combined in a matrix as shown in Table 16.1 for illustration of development of different failure mechanisms. However, there is a gradual development from brittle to ductile along both the horizontal and the vertical axes in Table 16.1.

Design of Ductile Structures

Design standards for marine structures recommend design and fabrication of ductile structures. This requires that ductile materials are used and that sound design principles are followed in order to avoid weak sections in structural elements due to locally reduced sectional area such as at bolt-holes. A requirement for material ductility is expressed through requirements for Charpy V values tested at different temperatures for different thicknesses. Crack Tip Opening Displacement (CTOD) testing is also recommended for documentation of sufficient material toughness for larger thicknesses and more complex components. Furthermore, use of overmatch material is required in butt welds. This means that the weld deposit should have a larger yield strength than the base material. This is normally specified in Welding Procedure Specifications. It is further documented during Welding Procedure Qualification Testing, when a welded butt joint is tested by bending to a specified radius without failure in the weld.