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  • 1.
    Choudhry, Jamal
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    A study of wear and load behaviour on bucket teeth for heavy-duty cable shovels2020Självständigt arbete på avancerad nivå (yrkesexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    Many of today’s engineering advancements rely on minerals such as copper, gold and iron. For this reason, the mining industry plays an important role for the development of society and technological wonders. Mining excavators are commonly used tools for extracting the minerals from the mine. Mining excavators are large machines used to breakdown, penetrate and load the rock ores onto trucks that transport the minerals. During the dynamic loading, the excavator bucket experiences significant amount of wear and tear that negatively affects the production by increasing the downtime. The bucket teeth are arguably the most worn parts of the bucket and are responsible for significant amounts of downtime. This thesis aims to provide a better understanding of the load and wear on the bucket teeth of large scale mining excavators used in Bolidens Aitik copper mine in Sweden. Because of how much wear and tear the bucket teeth are exposed to, there is a need to better understand the wear behaviour of the teeth and for the whole bucket in general. This understanding can then be used to improve the service life of the teeth and other parts of the bucket and thus increase work efficiency and reduce downtime.

    This project was divided into two parts. The first part consisted of regular field measurements to follow the wear on the bucket for about two weeks of digging and loading. The gathered data was then analysed to provide a better understand about the wear behaviour. The second part was to develop a numerical model that could predict the wear on the bucket and could be verified by the field measurements.

    The field measurements consisted of seven 3D laser scans of the bucket starting with brand new teeth. At the time of the last scan, the buckets total loaded tonnage was approximately 542 kton and the excavator had operated in total of approximately 195 hours. After the raw data from the scans was gathered and analysed, various information about the wear behaviour on the teeth was achieved. The 3D scanned data was also used to provide a complete wear development cycle which allowed to track the wear of any point in the bucket. The method could also be used to create animations of the teeth as they were being worn. From the results, it was concluded that the wear rate for the teeth slowed down and even converged as the geometry changed due to wear. When comparing all nine teeth on the bucket, it was also found that the middle teeth on the bucket were most exposed to wear. The most worn tooth was found to lose around 50 kg of weight after approximately 117 operating hours, which accounts for 40 % of the original weight. The animations from the complete wear development results also showed how the individual teeth and the whole leading edge with all nine teeth were being worn as the buckets loaded tonnage increased from 0 to 542 kton.

    The numerical model consisted of simulations of loading with the rocks being modelled with the Discrete Element Method (DEM). These were divided into four cases, the first being with the bucket with all new teeth. The second bucket with a mixture of new and worn teeth. The third bucket with all worn teeth and then finally the fourth bucket in which a new tooth geometry was tested. The numerical model showed promising results and potential for being a reliable way to predict the wear on the bucket. The results showed that both the penetration force and wear for the middle teeth was higher than the other neighbouring teeth. It also showed that the completely worn teeth had a lower wear rate than the new teeth which is in agreement with the results from field measurements. Other factors such as tooth shape and length were also observed to have a significant impact on the wear and penetration force. Lastly, the new teeth geometry also showed potential for design improvements in terms of wear resistance but can be further optimised. From the new teeth geometry, a suggestion was given for using an existing tooth system that might be more wear resistant.

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  • 2.
    Choudhry, Jamal
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Numerical models for simulating wear and friction-induced heating in rough surface contacts2024Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The study of friction and wear is a crucial element in the effort to reduce carbon footprint in technology. It is evident that friction and wear are responsible for a significant amount of global energy losses, emphasizing the need for research on the topic. However, due to the complexities associated with multi-physics phenomena and surface roughness at the micro-scale, it can become challenging to understand the tribological processes involved. Apart from friction and wear, this interaction also gives rise to phenomena like frictional heating and the generation of third-body wear particles due to both adhesion and abrasion. These phenomena can lead to reductions in performance, efficiency and durability in mechanical systems. 

    The aim of present work is the development of advanced numerical tools with the purpose of studying friction and wear processes in detail. Wear, friction and the associated heating can be found in nearly all types of sliding mechanical systems. Typical examples include, but are not limited to, bearings, gears, shafts and cams. The numerical methods which exist currently are usually simplified, using idealized assumptions and non-realistic boundary conditions. For this reason, many of the models are not able to account for the various mechanisms involved in multi-asperity contacts. 

    This research presents a multi-scale and multi-asperity thermo-mechanical model to study the temperatures at the interface due to surface roughness, while accounting for wear with Archard’s wear law. Realizing that the thermo-mechanical behavior is influenced by the post-necking behaviour of stresses and their respective states, the subsequent work focuses on incorporating these in single asperity wear simulations. The research has provided valuable insights into the wear mechanisms, revealing various issues within classical models, such as Archard’s wear law and resulting in the development of more advanced tools. Specifically, in the context of asperity-to-asperity interaction, where non-linear effects are more prominent, a linear relation between the wear volume and load may no longer hold. To address this, the research introduces thermo-mechanical models that combine the Boundary Element Method with non-linear Finite Element methods to study temperatures, deformations and wear in asperity-to-asperity contacts.

    Key findings suggests that the average interface temperature is independent of roughness, unlike the maximum temperature which increases with increasing high frequency cut-off values and decreasing Hurst exponent values. Recognizing the significant influence of strain-softening and stress-states on the thermo-mechanical behavior, subsequent studies have been directed towards addressing this aspect, while focusing on single asperity collisions. The work presents an advanced three-dimensional Finite Element and a meshfree particle method to simulate large deformations and fracture in colliding asperities, accounting for stress triaxiality and lode parameters. It is shown that the maximum temperature rise and total wear volume are both affected by the triaxiality values and strain-softening. Simulations conducted with a model based on the meshfree particle method reveals a critical parameter that signals the transition from mild to severe wear, leading to the creation of a wear particle at the interface. More importantly, the findings have reveal the limitations in Archard’s wear law, serving as motivation for improvement and resulting in the final paper. In the final study, an improved wear coefficient is presented, resulting in more accurate wear predictions than the traditional Archard’s wear law. The improved wear coefficient is deduced from the contact area and the accumulation of crack energy along the direction of frictional force, resulting in a spatially varying and non-linear relation between wear volume and load. This model is coupled with the Boundary Element Method, which assumes that the surfaces are flat and semi-infinite and that the interacting surfaces are perfectly-plastic. This advancement eliminates the necessity of resorting to large, complex, and often time-consuming finite element based methods. The work also highlights deficiencies in the classical Archard’s wear model in correctly predicting the wear particle formation.

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  • 3.
    Choudhry, Jamal
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    A Multi-scale Contact Temperature Model for Dry Sliding Rough Surfaces2021Ingår i: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 69, nr 4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A multi-scale flash temperature model has been developed and validated against existing work. The core strength of the proposed model is that it can be adapted to predict flash contact temperatures occurring in various types of sliding systems. In this paper, it is used to investigate how different surface roughness parameters affect the flash temperatures. The results show that for decreasing Hurst exponents as well as increasing values of the high-frequency cut-off, the maximum flash temperature increases. It was also shown that the effect of surface roughness does not influence the average interface temperature. The model predictions were validated against data from an experiment conducted in a pin-on-disc machine. This also showed the importance of including a wear model when simulating flash temperature development in a sliding system.

  • 4.
    Choudhry, Jamal
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Improving Archard’s Wear Model: An Energy Based ApproachManuskript (preprint) (Övrigt vetenskapligt)
  • 5.
    Choudhry, Jamal
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Validation of a Multi-Scale Contact Temperature Model for Dry Sliding Rough Surfaces2022Ingår i: Lubricants, E-ISSN 2075-4442, Vol. 10, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A multi-scale flash temperature model is validated against existing experimental work. The model shows promising results and proves itself to be a reliable tool for the accurate prediction of the flash temperature development between rough surfaces in sliding systems. Model predictions for the maximum flash temperatures as well as the bulk temperature fields were in very good agreement with the experimentally measured values. The model was also able to accurately predict the formation of hotspots as well as the temperature variations around the hotspots. From the model predictions, it is concluded that it is sufficient to only assess the flash temperatures on a small portion of the contact area and thus save both computational time and memory.

  • 6.
    Choudhry, Jamal
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Prakash, Braham
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement. Department of Mechanical Engineering, Tsinghua University, Shenzhen 518000, China.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    A Stress-State-Dependent Sliding Wear Model for Micro-Scale Contacts2023Ingår i: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 145, nr 11, artikel-id 111702Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wear is a complex phenomenon taking place as two bodies in relative motion are brought into contact with each other. There are many different types of wear, for example, sliding, fretting, surface fatigue, and combinations thereof. Wear occurs over a wide range of scales, and it largely depends on the mechanical properties of the material. For instance, at the micro-scale, sliding wear is the result of material detachment that occurs due to fracture. An accurate numerical simulation of sliding wear requires a robust and efficient solver, based on a realistic fracture mechanics model that can handle large deformations. In the present work, a fully coupled thermo-mechanical and meshfree approach, based on the momentum-consistent smoothed particle Galerkin (MC-SPG) method, is adapted and employed to predict wear of colliding asperities. The MC-SPG-based approach is used to study how plastic deformation, thermal response, and wear are influenced by the variation of the vertical overlap between colliding spherical asperities. The findings demonstrate a critical overlap value where the wear mechanism transitions from plastic deformation to brittle fracture. In addition, the results reveal a linear relationship between the average temperature and the increasing overlap size, up until the critical overlap value. Beyond this critical point, the average temperature reaches a steady-state value.

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  • 7.
    Choudhry, Jamal
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    A Stress-State-Dependent Thermo-Mechanical Wear Model for Micro-Scale Contacts2022Ingår i: Lubricants, E-ISSN 2075-4442, Vol. 10, nr 9, artikel-id 223Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wear is a complex phenomenon that depends on the properties of materials and their surfaces, as well as the operating conditions and the surrounding atmosphere. At the micro-scale, abrasive wear occurs as material removal due to plastic deformation and fracture. In the present work, it is shown that fracture is stress-state-dependent and thus should be accounted for when modelling wear. For this reason, a three-dimensional finite element model has been adopted to simulate and study the main mechanisms that lead to wear of colliding asperities for a pair of metals. The model is also fully coupled with a non-linear thermal solver to account for thermal effects such as conversion of plastic work to heat as well as thermal expansion. It is shown that both the wear and flash temperature development are dependent on the stress triaxiality and the Lode parameter.

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