Failure Analysis & Characterization: Fatigue & Failure II
Program Organizers: Andrew Havics, PH2 LLC; James Lane, Rimkus Consulting Group Inc; Burak Akyuz, Applied Technical Services Inc; Pierre Dupont, UMONS, Faculté Polytechnique de MONS (FPMs)
Wednesday 2:00 PM
October 2, 2019
Location: Oregon Convention Center
Session Chair: Erhan Ulvan, Acuren Group Inc.; Robert O'Shea , Engineering Systems Inc; Daniel Baker , General Motors; Pierre Dupont , Schaeffler Belgium Sprl/bvba
Fatigue Crack Threshold Analysis with TiAl SENT and CC Specimens – Influence of Starter Notch and Pre-cracking: Sven Eck1; Jürgen Maierhofer1; Christian Tritremmel1; Stefan Marsoner1; Nigel Martin2; 1Materials Center Leoben Forschung GmbH; 2Rolls-Royce plc
Due to their low specific weight and beneficial properties at high temperatures, new TiAl alloys are used as components in commercial aero-engines. Fatigue crack threshold analysis is a safety critical input to the component design. Fatigue properties of a TiAl alloy with lamellar microstructure were investigated by means of cyclic fatigue testing. Two different types of fatigue specimens (SENT – single edge notch tension and CC – corner crack) were subjected to cyclic loading temperatures between room temperature and 700°C. The work examines the influences of sample geometry, starter notch preparation and the effect of compression pre-cracking on the measured threshold stress intensity factor range ΔKth. The results show to which extent the different specimen geometries and starter notch preparations yield the same fatigue crack growth threshold results and which setup yields in the most conservative results, i.e. lowest crack growth thresholds for the same material and load condition.
The Influence of Monotonic Damage on Metal Fatigue at High Stress-ratios: Peter Huffman1; 1John Deere
Fatigue damage is almost universally accounted for using the concept of cycles or reversals, and typically the initial monotonic loading which precedes the cyclic loading is neglected under the assumption that the damage is negligible compared to the damage from thousands or millions of cycles. As the mean stress approaches the ultimate tensile strength of a material this assumption may become invalid, as the material approaches a condition of failure before the cyclic loading begins. This work proposes a method for accounting for the damage accumulated during monotonic loading that often occurs prior to high stress-ratio cyclic loading, and describes how the remaining specimen life is influenced in terms of stress-amplitude versus life and stress-amplitude versus mean-stress at a constant life. Results are compared to data from the literature.
The Effect of Microstructural Features and Composition on the Damage Behaviour of Tool Steels: Alexandra Rose1; Robyn Marentette1; Daniel Green1; Ahmet Alpas1; 1University of Windsor
Tool steels with increased wear-resistance are being developed for use in trimming operations to improve tool life and final part quality. This study investigates a 5% chromium tool steel and an 8% chromium tool steel, both exhibiting microstructures with small, evenly distributed carbides. The microstructure and mechanical properties of these steels are compared to D2, a conventional 12% chromium tool steel with bands of much larger carbides. Tests were conducted using miniature tensile specimens. Digital image correlation (DIC) strain measurements were used to develop a flow curve. Fractographic analyses carried out using scanning electron microscopy (SEM) revealed facets and cleavage planes characteristic of a dominantly brittle fracture. The new tool steels showed increased fracture stresses and strains compared to the conventional D2. The 5% chromium tool steel also showed plastic deformation when compared to the purely elastic deformation in D2 and the 8% chromium tool steel.
Comparative Analysis of Convertor Tubes Failed due to Thermo-mechanical Fatigue and Creep: Kaushal Kishore1; Manashi Adhikary1; Goutam Mukhopadhyay1; Sandip Bhattacharyya1; 1Tata Steel Limited
Hood tubes of steel making convertors ensure safe passage to hot by-product gas of steel making. However, these water cooled tubes can fail during service due to several reasons, viz., improper cooling, inferior material quality of the tube, sub-optimal design etc. Present work is focused on comparative study of two distinct failures of hood tubes in thermo-mechanical fatigue and creep modes. Investigation consisted of comparison of operational parameters, visual observations, chemical analysis and detailed microstructural and scale characterization using scanning electron microscope coupled with energy dispersive x-ray spectroscopy and Raman spectroscopy. It was observed that no gross microstructural changes occurred in thermo-mechanical fatigue. In contrast, a number of phase transformations like pearlite disintegration, formation of bainite and acicular ferrite took place in case of creep. Root causes of both the failures have been pinpointed and preventive measures are put forward. Key words: Tubes; Fatigue, Creep, Bainite, Raman Spectroscopy
3:20 PM Break
Failure Analysis and Remaining Life Assessment of Reformer Outlet Manifold: Abdulmohsen Alsahli1; 1Saudi Basic Industries Corp. (Sabic)
A thorough failure investigation was conducted into a failure of an outlet manifold in a steam methane reformer to determine the failure mechanism, and contributing factors. The investigation was performed utilizing different characterization techniques, including optical light microscopy, scanning electron microscopy, mechanical testing and chemical analysis techniques. The investigation findings showed that the outlet manifold failure is attributed to creep. Details of the failure’s contributing factors, and recommendations to avoid similar failures, are discussed in this article. Further, life estimation of the outlet manifold was done based on thorough bulging measurements at previous inspections.
Torsional Fatigue Failure of Pump Shaft of a Gas Cleaning Plant(GCP) in Blast Furnace: Ankita Roy1; 1Tata STeel
Centrifugal pump shaft of GCP failed after 2.5 years of service at mid-impeller location leading to operational breakdown for 10 hours. Visual and stereoscopic observation of failed shaft revealed fatigue beachmarks along the peeled off region and cracks along keyway seat of shaft. Keyway seat dimensions were non-compliant with ANSI B17.1-1967 (R1998) standards and root radius was found to be sharper. Microstructural characterisation via optical, SEM and EDS revealed annealed structure with chromium carbide strewn in the matrix while mechanical characterisation via hardness, tensile test revealed lower values than expected from quenched and tempered AISI 410SS-A276 grade.Crack initiated due to combined effect of sharp radius at keyway corner and inferior heat treatment of material. Morphology of crack and its propagation revealed that shaft failed in torsional fatigue due to stress concentration at root radius and lower strength of material due to improper heat treatment aggravated initiation and propagation of crack.
Brittle Crack Arrest Behavior of a Heavy-gauge Steel Plate: Ohjae Lee1; Seok Gyu Lee1; H.Y. Bae2; H.C. Jeong2; S.H. Lee1; N.J. Kim1; 1POSTECH; 2POSCO
Recently, there has been a large increase in the size of container ships, requiring the use of steel plates with heavier gauge and higher strength. However, fracture toughness of steel plate decreases with increasing its thickness and strength, which may increase the probability of unstable catastrophic fracture of container ships. Therefore, several studies have been conducted on the brittle crack arrestability of steels and it has been shown that the crack arrest morphology changes with a change in the plate thickness; a unique W shape (split-nail shape) in 95-100 mm thick plates and a U shape (thumbnail shape) in less thick plates. The present study is aimed at identifying the factors that are responsible for the occurrence of a unique W shaped crack arrest morphology found in heavy-gauge steel plate and the relationship between microstructure, mechanical properties, texture, and crack arrest behavior will be discussed.