Fatigue and surface spalling of cemented carbides under cyclic impact loading – Evaluation of the mechanical properties with respect to microstructural processes

Proc Appl Math and Mech

2019

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Wear affects the service life of tools and counteracts an economic operation of a tunneling project. Because of wear, tools must be replaced to ensure adequate penetration, which means downtime. Targeted planning of replacement intervals requires knowledge of precise wear prediction models. In these predictive models the abrasiveness of the soil has to be regarded, which can be evaluated by experimental experiments such as the LCPC test for soil and the CAI test for solid rock. However, the forecast models developed provide only a rough guide value and often do not correlate with the real wear conditions of the tools during a tunneling project. The main reason here is to mention the disadvantage that in the previous work not the overall tribological system has been considered sufficiently. Thus, the wear occurring on the tool is not only dependent on the abrasiveness of the soil to be penetrated, but is also determined by the material properties of the tool, the loads acting on it and the presence of any intermediate and surrounding media. From this simple consideration, it can be seen that wear is not a sole property of the soil or of the tool material, but a system property. The motivation of this short report is to show the basics of tool wear of tools used for tunneling applications as well as to show basic boundary conditions for future experimental work and simulations in the field of tool wear. First, the tribological system and the four main wear mechanisms will be presented. In a further step, the damage mechanisms are focused on the dominant wear mechanisms abrasion and surface disruption of tunneling tools. Thereby, interaction of abrasive particles or crack propagation in hard phase materials at the microstructure level will be discussed. The tribological systemIn order to understand the effective wear system, the tribological system has to be considered. This system takes into account all relevant influencing factors that have an effect on wear. A simplified tribological system is shown in Figure 1. The wear pairing is formed by the main body, the counter body and the intermediate medium. The intermediate medium can be a lubricant, wear particles or a tribo layer formed by mechanical interactions between the main and counter body. Wear can take place in an inert or reactive atmosphere, whereby the later can cause corrosion or tribochemical reaction. In addition, a load spectrum (forces, relative motions, temperature) is acting on the wear pair, thus determining the tribo-mechanical interactions between the respectively components involved in the wear pairing. From the consideration of the tribosystem, it becomes apparent that wear is not a material property but a system property.Abb. 1: Schematic representation of a tribological system 2 Wear mechanism A distinction is made between the four wear mechanisms of adhesion, tribochemical reaction, surface spalling and abrasion. Adhesion is the selective welding and breaking up of two rough bodies in contact. As a result of roughness, the two bodies are in...

Section: Hard Phase Containing Materials Against Abrasionmentioning

confidence: 99%

Wear mechanisms of hard phase containing materials

Proc Appl Math and Mech

2019

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“…Aufgrund von zyklisch schlagenden Lasten unterhalb der Bruchfestigkeit erscheint die Oberfläche der Verschleißschutzschichten rissig und zerklüftet (Bild 4). Die Zerrüttung ist auf Ermüdungsprozesse im Werkstoffgefüge zurückzuführen, die zur Bildung von Rissnetzwerken und Ausbrüchen im mm-Bereich und somit zum Materialabtrag führt [5].…”

Section: Oberflächenzerrüttungunclassified

“…Due to the cyclically impacting loads lower than the fracture strength, the surface of the wear protection layers looks cracked and fissured ( Figure 4). The degradation is due to fatigue processes in the microstructure of the material, which leads to crack networks and rupturing in the mm scale and thus to material removal [5].…”

Section: Surface Degradationmentioning

confidence: 99%

Tribological analysis of the TBM tool wear in soil from the view of material science

Küpferle

Geomechanics and Tunnelling

Theisen

et al. 2018

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Excessive wear to excavation tools leads to unplanned downtime, which results in additional costs. To predict tool wear during the planning phase of a project, an analysis of the acting tribological system is essential. The different influential factors and interactions have to be considered. The abrasive surface degradation of tools is investigated on a microstructural scale with nano‐scratch experiments. Material‐dependent variables like hard phase content and size are discussed in terms of the resulting wear‐resistance. Using the RUB Tunnelling Device, abrasive surface degradation can be investigated on a more global scale and correlated with the acting tribological system, and the different tribological system components and influential factors can be considered.

“…Using the example of cemented carbides, Küpferle et al were able to show that this cyclic stress leads to crack initiation and propagation in the wearprotection layers of tunnelling tools. 4 This successive crack propagation reduces the load-bearing cross-section, enabling fragmentation and material loss to occur due to mechanical overloading. In this case, material removal takes place by material fatigue, which is known in the field of tribology as surface spalling.…”

Section: Introductionmentioning

confidence: 99%

“…Particularly when high mechanical stresses are applied, such as soils with varying properties or in the presence of boulders, wear due to surface spalling may cause premature tool failure. 4 The crack propagation velocity in the wear-protection layers governs the wear rate that is due to surface spalling. To counteract surface spalling, it is therefore crucial to understand the influence of microstructure and mechanical properties on the crack growth resistance in hard alloys.…”

Section: Introductionmentioning

confidence: 99%

Subcritical crack growth in hard alloys under cyclic loading

Brackmann

Fatigue Fract Eng Mat Struct

Weber

et al. 2020

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In this work, the subcritical crack growth in Fe‐, Ni‐ and Co‐base hard alloys was investigated. Specimens were cyclically loaded in the pressure threshold range until a ring crack resulted as a failure criterion. Crack propagation along with the individual microstructural constituents and the associated resistance of the individual materials to crack propagation was investigated by scanning electron microscopy and by the methods adapted to it. For the Ni‐base alloys, the formation of a closed ring fracture occurred after the lowest load cycle number, followed by the Co‐ and Fe‐base alloys. Almost no crack deflection by the hard phases was detected in the Ni‐base alloys. The higher number of loading cycles to produce a closed crack ring in the Fe‐base alloys is attributed to the pronounced crack deflection by the hard phases and to the higher matrix strength. Besides, phase transformations were registered in front of the crack tip of the Co‐ and the partially austenitic Fe‐base alloy. This phase transformation counteracts crack formation in the case of the Fe‐base matrix but promotes crack propagation in the Co‐base alloy.