5.2 Mechanics

Fatigue and wear, degradation of materials in complex work environments

Despite all our current knowledge and breakthroughs in material and mechanical engineering, up to 90% of fractures and defects in machine parts are due to material fatigue, or the malfunction of the part during oscillating operating conditions in a complex work environment (the influence of temperatures, work environment, technological condition of the part, etc.). The introduction of a wide range of applications for new types of materials (e.g. fibrous composites) and new technologies (e.g. additive technologies) not only uncovered new possibilities in design direction and application, but also opened up a host of new, hitherto unsolved problems with mechanical and thermomechanical fatigue and degradation of the newly produced product parts under operating conditions.

Further development of knowledge and expertise in material mechanics is essential in the field of mechanical and thermal materials degradation (high- and low-cycle), concerning the spread and formation of defects due to fretting (corrosion by friction), or the creation of phenomenological models. In the field of additive production processes, despite the development of surface treatment methods, the long-standing problem is the existence of sub-surface defects such as pores and other imperfections, often in combination with significant residual tensile stresses. A deciding mechanism in fatigue damage of materials with internal defects typical with additive manufacturing, is the initiation and growth of physically short cracks out of these defects. In this area it is crucial to develop expertise and sensitive measurement methods allowing for a characterisation of the process of fatigue crack growth, leading to the design of optimal procedures (appropriate conventional and novel technologies of surface treatment) to increase material resistance. To evaluate the surface technologies used for example in railway axles, railway car wheels or the discs of SUVs, it is necessary to develop expertise in the interactions of specific applications for a given steel and nature of load (such as complex loads on railway axles under stress, fatigue damage as a result of fretting).

Competencies and provided expertise:

• High-cycle fatigue testing and tests to determine the fatigue limit beyond frequencies around 100 Hz, allowing for faster testing under more affordable conditions than when using hydraulic machines.
• Multiaxial tests in a tension / torsion regime with separately controlled load channels with the target area primarily in low-cycle fatigue.
• Research and tests of durability in conditions of multiaxial thermomechanical fatigue for various thermomechanical load history.
• Gathering of experimental data necessary for the determination of durability criteria and for the calibration of constitutive models needed for the simulation of stress-deformation feedbacks.
• Research into the mechanisms behind deterioration and the development of microstructures using electron microscopy, and their integration into proposed models.
• Research into strength under monotonic loading for various object geometries and temperature loads.
• Research into the fatigue strength of materials in fretting conditions during various contact ratios (regimes), followed up by metallographic and fractographic analyses mapping the micromechanisms of damage and the development of unique structural durability prediction methods.
• Preparation of benchmarks for the verification of various computational techniques in the area of high-cycle fatigue.
• Use of experimental data from previously existing sources included in the database of experiments.
• Analysis of specific cases of fatigue loading and findings in the database of experiments, determination of which data can be used for analyses in the project pre-development phase, or the direction of future attention in planned fatigue experiments.
• Qualitative and quantitative structural and phase analysis of materials.
• Determination of material hardness: macro-, micro- and nanohardness.
• Fractographic analyses of components and determination of the causes of damage using light and electron microscopy.
• Evaluation of the serviceability of machine parts.
• Evaluation of the material degradation due to operational activities.
• Crystallographic analyses using the EBSD method (diffraction using backscattered electrons).
• Determination of material inputs for numerical simulations.
• Chemical microanalyses using EDS (energy dispersive spectroscopy).
• Complex analysis of the chemical composition on a micro and macro level including the quantitative analysis of undesirable elements (O, H, N).
• Sensitive methods for measuring the speed of growth and conditions for halting of physically short cracks, typically shorter than 1 millimetre for parts produced with additive technologies.
• Understanding of fatigue crack growth mechanisms and resulting application of technologies to increase resistance to them.
• Experimental methods of the evaluation of mutual shifts between the axle and wheel under pressure, which are crucial for the initiation and growth of fatigue damage via fatigue friction corrosion mechanisms (fretting).
• Evaluation of the conditions of subsequent surface treatment of components and parts of railway axles with conventional (shot peening) as well as advanced methods (for example using lasers, induction hardening, additive cladding).

Prediction of material properties in critical applications

A key factor of material properties are the conditions of the specific work environment, most importantly in critical applications (high mechanical or thermal load), i.e. in the components of energy devices, turbines and motors. Also, applications with high-load components, such as time-varying loading of machines working under increased temperatures, but also significantly below freezing point. This requires expertise and unique devices for mechanical tests (material fatigue, creep, the interaction of creep and fatigue, fracture and material failure, all within a broad range of temperatures), characterisation of material structure from a macro to a nano level including load-dependent changes. Modern materials are used in demanding engineering applications for safe and environmentally-friendly electricity generation. The latest findings have brought about a new generation of high-strength steels for the automotive and aerospace industries, nanomaterials with unique properties, high-temperature superalloys for turbines, materials used in medical implants, functional materials, ceramic materials and composites or materials and components produced using additive technologies.

Competencies and provided expertise:

• Material creep tests with continual measurement of deformation in a range from room temperature to 1000°C in a controlled atmosphere / air; in a vacuum or Ar atmosphere with a thermal capacity up to 1400°C; outside up to 1000°C.
• Creep tests with extremely low creep rates of 10^-13 s^-1 (typical for good engineering practice) with a capacity of up to 1000°C.
• Torsional stress on specimens at a temperature range up to 1200°C.
• Unique laboratory tests and evaluations of experimental data with the goal of establishing cyclical hardening / softening curves, cyclical deformation curves, Manson-Coffin durability curves, S-N durability curves, places and mechanisms for the origin of fatigue cracks, the propagation speed of short fatigue cracks.
• Cyclic curves of stress deformation, Goodman diagrams, fatigue crack growth velocity curves including threshold values for propagation with an evaluation of the surrounding conditions.
• Fractographic analysis of fracture surfaces and an assessment of the causes of damage to machine components.
• Quantitative description of the influence of defects and stress concentrators on the fatigue life of materials.
• Prediction of fatigue life, numerical simulation of the fatigue crack growth, numerical prediction of the useful life of components under a random loading spectrum.
• Assessment of the strength of composite and polymer materials based on a fracture-mechanics approach.
• Analysis of the influence of microstructures on the mechanical properties of materials and an assessment of microstructure changes resulting from fatigue load with the use of the state-of-the-art technologies of electron microscopy (SEM, EBSD, TEM) with the goal of determining the damage mechanisms.
• Determination of strength (tension, compression, bending) under various loading conditions including analysis of deformation and fracture processes including above all fracture mechanics in the temperature range between -196°C and 1200°C in the open air and from 20°C to 1600°C in a vacuum or inert argon atmosphere.

Tribology

Tribology is a relatively young multidisciplinary scientific field concerned with the study of friction, lubrication and wear. An understanding and use of the findings and processes of tribology brings significant savings in the automotive, aerospace and consumer goods industries, in sports, medicine and even in personal hygiene products. Recent studies have shown that roughly 23% of worldwide energy consumption (~ 119 EJ) is accounted for by tribological contact, in the form of overcoming friction losses (20%) and the replacement of worn parts (3%). Tribology has brought to light a number of phenomena in lubricated contacts, especially concerning the influence of surface roughness or lubricant rheology. This allows for the design of specific applications in areas of lubricant design, abrasion-resistant surface coatings, optimisation of the geometry and topography of friction surfaces and more. Tribological results have critical applications in the design of machine parts for the automotive and machine industries, such as rolling and plain bearings and gears, leading to the optimal choice of lubricant, friction surface geometry and materials.

Competencies and provided expertise:

• Research into the problems of the friction, wear and lubrication of flexible and rigid contacts. Visualisation and evaluation of the creation of ultrathin lubricating films.
• The construction of unique tribometers using optical measuring methods (colorimetric interferometry, infrared radiometry, fluorescent method, optical monitoring of particle movement).
• Experimental study of lubrication flow inside and around contact areas, the rheological properties of lubricants, changes in friction surface topographies, the influence of the size, shape and orientation of aberrations on the lubricating film, the behaviour of the individual components of synovial fluids in contact.
• Analysis of lubricating films in tribological systems. Use of optical tribometers in the resolution of problems related to thin lubricating films based on mapping of thickness and other physical properties under given operating conditions (such as aging of lubricants and their influence on the life expectancy of bearings, the tribological properties of coolants, the influence of lubricant contaminants with for example water in transmission parts of machines and transport systems; the development of traction lubricants; influence of lubrication on joint replacement implants; the development of hydrogel as a joint cartilage substitute; actuator components for satellites; lubricants for rail transport, etc.).
• Monitoring particle movement in lubricated contacts and their visualisation using optical tribometers to determine the influence of friction surface topographies and operating conditions (for example the targeted input of foreign particles into a contact, negative effects of foreign particles, development of nanoparticle-based lubricants).
• Quantifying tribological properties of films, layers and base materials. Determining the degree of friction and wear based on operating conditions (useful for instance in the development of films and layers with the required level of friction and wear; optimisation of sliding layers, lining and covering components of machine tools, tyre wear, etc.).
• Monitoring of topographical changes in friction surfaces as an indicator of tribological processes with the possibility of directly influencing them through targeted modifications of the topography and an assessment of the influence of the production process (for example through the method of targeted topography for the elimination of friction and wear, intermediate machine parts, analyses of the lubrication of sintered materials, etc.).

Fluid mechanics and heat and mass transfer

The movement of fluids and mass transfer often occur under conditions of 3D turbulent (transitional), transient flow, or with a mutual interaction between phases. Turbulent flow is also one of the causes of vibration and noise. Transport phenomena include a broad range of issues in the area of multi-phase fluid mechanics and heat and mass transfer that may act independently or mutually interact. Results are applied to ensure thermal management of machine products, heat transfer  in living organisms, quality fuel dispersion, cleaning, cooling and coating of surfaces, cleaning flue gas or air, medicine administration, airways protection or gas filtration. These results are used in the aerospace and automotive industries, in engineering, the energy industry, medicine, pharmacology, agriculture, the food industry and ecology.

Competencies and provided expertise:

• Research into a broad spectrum of devices for the atomisation and spraying of liquids, study of droplet and aerosol dynamics, interactions with the surrounding environment, detailed micro and macro characterisation of sprays and aerosols with optical and visualisation methods.
• Evaluation of liquid mass flows, spray geometry, trajectory and velocity of droplets and their interactions.
• Complex imaging and photogrammetry analysis of fluid structures and data reduction for input/boundary conditions in computer models.
• Predictions of outflow parameters of two-phase liquid-gas mixtures from jets using developed software, application of two-phase flow diagram series for the prediction of the properties of the investigated systems, measurement and evaluation of the stability of internal flow, jet outflow and resulting spray.
• Preparation of liquid aerosols and solid particles with various rheological and optical properties and chemical composition to trace the movement of fluids, for calibration and testing of devices or study of mixing.
• Research into the flow and spread of medical and toxic aerosols. One unique capability is the recording of microfibres in channels based on flow parameters.
• Research into the flow and settlement of various types of inhaled particles (fibrous, porous, pharmaceutical, solid and liquid) in respiratory tract models.
• Research into principles of comfort perception and thermoregulation of the human body.
• Research into physical phenomena associated with the transfer, distribution and management of heat in the human body as well as machine products.
• Testing of various types of spray jets, injectors and nebulisers including a broad range of operational pressures, flows and operational liquids.
• Design of liquid-spraying devices, for example for engine fuel systems, gas cleaning and pollutant removal technologies, high viscosity liquids, waste-derived fuel suspensions.
• Execution of experiments in fluid mechanics on scalable models, preparation of model liquids, design of optically-accessible models.
• Research into the acoustic and vibrational properties of fluid systems and their optimisation.
• Measurement of flow rates, flow velocity and flow fields in fluids (liquids, gasses, vapours and mixtures) using a number of classical (intrusive) as well as modern (nonintrusive) methods.
• Visualisation of one- and multi-phase flow via a method of introducing smoke / fog / bubbles lit up by a so-called laser knife and captured by a high-speed camera.
• Filtration efficiencies of various types of materials, cloth or other masks, further testing and development of scrubbers and air purifiers.
• Testing of inhalators for the transport of medicines, taking into account specific lung diseases.
• Research into the energy efficiency of HVAC systems for the creation of a comfortable work and technological environment.
• Climatic testing of machine products and their durability under extreme climatic loads.
• Development and testing of special protective equipment for dangerous environments with chemically and biologically pathogenic substances.

Dynamic machine / machine mechanism behaviours

Gathering and analysis of real data from dynamic machine behaviour is a necessary contribution to each stage of development, that is during initial state evaluation to determine weak points, gain input parameters for computational models, their subsequent verification and tuning and the following testing of functional models and prototypes. For a complex evaluation of the dynamic behaviour of machines and mechanisms it is necessary to synchronously measure a number of physical quantities such as acceleration, strength, lift, wattage, temperature, momentum, angular velocity, noise, vibration and others. This necessitates high demands on device and sensor facilities, appropriate spaces, analytical software and the development of measurement methods (for example digital angle measurement). Structural and material modifications or designs that correspond to the results of high-level diagnostics and are applied in various types of machines and mechanisms (machining, printing tools, glass and textile machines, aerospace and automotive systems and mechanisms, etc.).

Competencies and provided expertise in the field:

• Measurement and analysis of the dynamic behaviour of machines and machine nodes ensured by a combination of mathematical and measurement methods including the evaluation and interpretation of data and recommendations for alterations in the materials or structures used.
• Measurement and analysis of controlled servo drives already integrated into machinery, measurement of the overall level and discovery of the main sources of vibrations during steady and transitional state operation and start-up and run-down of machines.
• Measurement of the transmission and attenuation characteristics of oscillating systems, carrying out experimental modal analyses and discovery of oscillation shapes under operation, special correlational methods to compare experimentally determined modal properties with calculated ones.
• Study of the temperature dependencies of various quantities on machines, thermodynamic analysis.
• Evaluation of rotary machine properties, of their components, for example cam mechanisms, gearboxes, belt transmissions, torsional oscillators and controlled servo drives, dynamic balancing mechanisms.
• Structural dynamic analyses, causation analysis for noise and vibrations and the means of their spread and relation to modal and dynamic properties of systems in modified laboratories (semi-reflective, reverberant).
• Research and hygienic measurements of levels of acoustic pressure and sound intensity, mapping of sound fields and determination of acoustic power.
• The analysis of machine noise emissions is done using the newest methods of noise source identification using microphone arrays (acoustic holography – near field acoustic holography, beamforming etc.) for the full range of audible frequencies.
• Measurement of sound absorption and determination of acoustic power in a diffuse field.
• Formulation of recommended measures for the construction and utilised materials with the goal of a minimisation of noise and vibrations, as a result of complex measurements.