Preparation and Preservation of Fracture Specimens

Abstract

FRACTURE SURFACES are fragile and subject to mechanical and environmental dam- age that can destroy microstructural features. Consequently, fracture specimens must be care- fully handled during all stages of analysis. This article will discuss the importance of care and handling of fractures and what to look for during the preliminary visual examination, fracture-cleaning techniques, procedures for sectioning a fracture and opening secondary cracks, and the effect of nondestructive inspec- tion on subsequent evaluation. faces contact each other, the surfaces will be mechanically damaged. Crack closure during fatigue cracking is an example of a condition that creates mechanical damage during the frac- ture event. Chemical damage of the fracture surface that occurs after the fracture event is the result of environmental conditions present after the frac- ture. Any environment that is aggressive to the base metal will cause the fracture surface to be chemically damaged. Humid air is considered to be aggressive to most iron-base alloys and will cause oxidation to occur on steel fracture surfaces in a brief period of time. Touching a fracture surface with the fingers will introduce moisture and salts that may chemically attack the fracture surface. Mechanical damage of the fracture surface that occurs after the fracture event usually results from handling or transporting of the fracture. It is easy to damage a fracture surface while opening primary cracks, sectioning the fracture from the total part, and transporting the fracture. Other common ways of introducing mechanical damage include fitting the two frac- ture halves together or picking at the fracture with a sharp instrument. Careful handling and transporting of the fracture are necessary to keep damage to a minimum. Once mechanical damage occurs on the frac- ture surface, nothing can be done to remove its obliterating effect on the original fracture mor- phology. Corrosive attack, such as high- temperature oxidation, often precludes success- ful surface restoration. However, if chemical damage occurs and if it is not too severe, cleaning techniques can be implemented that will remove the oxidized or corroded surface layer and will restore the fracture surface to a state representative of its original condition.