Volume 54, Issue 1

Strand anchors are a perennial problem [1]. In many instances such anchor systems have been in service for decades. Information regarding their construction, composition or manufacturer is often uncertain. In this article the damage to a number of broken strand anchors is investigated. Using relatively simple methods of investigation, it is shown that the main cause of failure encountered is hydrogen-induced stress corrosion cracking.

In spite of the experiences in the design, material selection, fabrication and storage, fastener failures are reported frequently. While each failure leaves the user with some wisdom, analysing a failure is time consuming and affects the project schedules. In the aerospace scenario, it is all the more important to analyse the causes of fastener failures as a single failure can sometimes be catastrophic and mission critical. In the present paper, an attempt has been made to address why fasteners fail with a view to provide an insight into the modes and mechanisms. Finally, an attempt is made to bring out various modes and mechanisms of fastener failures. Four typical case studies on fastener failures are presented to highlight them.

In this contribution, the most interesting and educational failure cases are presented that the author came across during his over twenty years of laboratory practice as manager of the Materials Testing Laboratory of the Berlin Gas Turbine Plant of Siemens’ Power and Gas Division. The case studies are presented and categorised in accordance with VDI Guideline 3822, the German failure analyst’s guide to the subject of how to organise and run a root cause failure analysis. An effort was made to have each of the main four categories of failure causes represented, namely failures due to mechanical loading, corrosive failures, failures due to thermal loading, and tribological failures. Case studies include turbomachinery components that failed due to tensile overload, stress corrosion cracking, intergranular corrosion, hydrogen embrittlement, hot cracking, fretting, erosion, and galling. Affected components include valves, retaining rings, tubing and piping, burners, rotor disks, lifting lugs, and casings. Some of the presented cases were published in the new section “Failure Analysis” of Practical Metallography between October 2011 and the present time. Others were oral presentations at the Metallography conferences and at the annual failure analysis conferences “VDI Jahrestagung Schadensanalyse”, held during that time. The focus of discussion of the failure cases in this paper is the metallurgical evaluation of failure causes. This is the approach taken in many small and industrial laboratories. A holistic approach of a failure case, which includes calculation and simulation methods such as finite element analysis, and which also implies a knowledge of the service stresses intended by design as well as the actual loading situation of the failed part, is not the aim of this contribution.