•Unifying damage processes of Al–Si cast alloys in high and low plastictiy regimes.•Novel in-situ microstructural test system links HCF and LCF by crack growth behaviour.•Correlation of fatigue ...lifetime and the in-situ damaging process.•Mathematical approach based on in-situ damaging improves with higher correlation.
In this publication, ambient condition fatigue investigations with different types of Al–Si–Cu and Al–Si–Mg cast alloys in rotating-bending high-cycle fatigue (HCF) and push–pull low-cycle fatigue (LCF) regimes have been performed with varying Si, Cu, Fe and Sr contents. The cast alloys investigated here are common used in cylinder heads for automotive application. Because the cylinder head is one of the most fatigued parts in combustion chamber engines, the microstructural knowledge of the damage process provides a tool of construction and its material selection. The investigations were also supported with an in-situ microstructural crack observation in high plasticity rotating-bending regimes. The specimens were directly processed out of serial produced T79 heat-treated cylinder heads to provide the equal microstructure for testing as under operational conditions.
The observations clearly identified the effects of the individual alloying elements both under low- and high-cycle fatigue. The crack propagation speed and the crack paths were majorly influenced by the eutectic silicon. Additional, the precipitation hardening due to copper affected significantly the fatigue endurance, too. In high plasticities the silicon’s influence got almost lost and only the matrix strength was crucial. Thus, increased fatigue strength in high loaded LCF regimes was observed for alloys with less copper content, thus higher ductility. By contrast, improved HCF and low loaded LCF endurance was only achieved when the matrix strength was increased by copper’s precipitation hardening. Crack branching and deflections strongly influenced the microstructural damage of the ductile AlSi7Mg(Sr) and hence, gained its fatigue strength. Iron phases could not identified as harmful inclusions, since the phases were similar in size of other hard phase elements like the other primary intermetallic phases like Al2Cu and β-Si phases under notch stress aspects, by the well defined solidification process in the test section. Because the crack nucleation mainly occurred on Si particles, strontium as a refinement agent influenced the early crack onset and accordingly the fatigue in total. Thus, the AlSi6Cu4(Sr) had increased lifetimes compared to AlSi6Cu4 both in HCF and LCF. Further, the presented results provide a modification of the Manson–Coffin approach to describe the relationship between plastic strain and lifetime, valid for all proposed alloys with only one set of parameters. Thus, it was possible to perform the fatigue calculation with a reduced range of scatter.
Acoustic emissions from tribological contacts have become an interesting field of science in recent years. This study focuses on predicting the friction power of a given system (lubricated ...ring-on-disc geometry), independently of the used sliding material and lubricant, from the acoustic emissions emitted from the system. The low-frequency (1 Hz), continuously measured RMS value of the acoustic data is combined with short-duration and high-frequency (850 kHz) signal data in form of the power spectra and hit rate with three prominence levels. The classification system then predicts the friction power of the test system continuously over the whole test time. Prediction is achieved by four different machine learning methods (tree-type, support vector machine, K-nearest-neighbor, neural network) trained with data from 54 ring-on-disc tests with high variation in material and oil combinations. The method allows for the quantifiable and step-free prediction of absolute values of friction power with accuracy of 97.6% on unseen data, with a weighted K-nearest-neighbor classifier, at any point in time during an experiment. The system reacts well to rapid changes in friction conditions due to changes in load and temperature. The study shows the high information degree of acoustic emissions, concerning the actual friction mechanisms occurring and the quantitative, and not only qualitative, information that one can gain about a tribological system by analyzing them.
The measurement of acoustic emission data in experiments reveals informative details about the tribological contact. The required recording rate for conclusive datasets ranges up to several ...megahertz. Typically, this results in very large datasets for long-term measurements. This in return has the consequence, that acoustic emissions are mostly acquired at predefined cyclic time intervals, which leads to many blind spots. The following work shows methods for effective postprocessing and a feature based data acquisition method. Additionally, a two stage wear mechanism for bearings was found by the described method and could be substantiated by a numerical simulation.
The main objective of the present paper is the validation of new methods concerning experimental investigations of journal bearings under ordinary and extraordinary operational conditions. Derived ...results should facilitate the determination of damage mechanisms and a qualitative ranking for a prospective coating development. Prior investigated scenarios like start/stop and emergency running behavior were extended by starved and unlubricated experiments for an aircraft application. Focusing on thermal stability and reproducibility, two new subscale test strategies were developed derived from generalized operational conditions of journal bearings. For an improved ex-situ wear measurement of bearing shells, a new device was developed successfully. The results showed that the newly designed test methodologies are suitable to investigate unlubricated coatings leading to conclusive and repetitive results.
Acoustic signals (AE) have become a valuable tool for monitoring various tribological systems. This study investigates three key tribological behaviors: the initial running-in process, severe wear, ...and the lubrication state of journal bearings using four AE postprocessing methods. These methods are friction power-scaled quadratic mean (SRMS), power spectrum (PSD), normalized power spectrum (nPSD), and a statistical hit-rate (HR) analysis of the raw AE signal. The research draws on data from four tests conducted in three set-ups: Ring-on-Disc (RoD), Ring-on-Liner (RoL), and Journal-bearing-adapter (JBA) test configurations. AE measurements were taken at both 1 Hz and 1 kHz for the quadratic mean value (RMS), as well as at 900 kHz for short time intervals during each test to capture the raw signal data. In conjunction with other measured parameters like friction coefficient, temperature, contact potential, and wear, we establish correlations between the acoustic parameters and tribological behaviors. The findings indicate that SRMS and nPSD are valuable for identifying the end of the initial running-in phase. All four methods are effective in recognizing high wear states, with nPSD and HR even distinguishing between different wear phenomena. The lubrication state of journal bearings is best assessed using nPSD, allowing users to monitor certain geometric changes in the bearing.
The primary focus of this work is to characterise the tribological behaviour of physical vapour deposition (PVD) coated silver bearing coatings under representative operational conditions in aircraft ...engines. Silver is a promising and well-known material for lead free journal bearings due to its excellent chemical and physical properties, such as heat conduction, ductility and its non-toxicity, but not comprehensively investigated for many potential tribological applications. The silver coating was tested in mixed friction, hydrodynamic friction and deficient lubrication against polished steel counterparts. In comparison to reference bearing materials silver showed a good damage tolerance in emergency operation scenarios by virtue of its ductility and heat conductive properties.
•Silver showed an excellent wear resistance during repeated start-stop cycles.•The protective mechanism of silver under deficient lubrication was found.•Up to a load level of 2 MPa silver showed no significant wear under starved lubrication.
Acoustic emission in the ultrasonic range in tribology and from friction-based events in general are an interesting field of study, because it provides information classical parameters can’t. ...Therefore, it is very important to develop good working and unified methodologies for recording the acoustic signals. In this work two methodologies for recording acoustic emissions for sliding contacts are presented. The validation of the signals from the sensor to the digital data in the computer is shown. To gain good information it is necessary to reduce noise and disturbances in the data as much as technically feasible. Electrical insulation, shielding, cable management, filtering and blind-testing are introduced. Signal processing of the data happens via various methods, such as Root-mean-square (RMS), filtering (band-pass, filter bank), short-time-Fourier-transformation (STFT), wavelet-transformation and combined techniques. Gathered information with these methods is shown for an example of a plain sliding surface Ring-on-disc test. The RMS works good as overall running indicator, while the spectral methods give insights in the different tribological effects and changes on the surface of the sample over the course of the test.
copyright information to be updated in production process
It is nowadays widely recognized that good and profound knowledge of the deformation behavior and/or crack propagation behavior is a vital part of high performance simulation for advanced high stress ...fatigue life prediction. Concerning of those two different approaches, one must, however, be aware that additional and detailed information of fatigue induced crack nucleation, its further propagation path and the associated crack growth rate is important for the intrinsic understanding of the material damage process.
This paper investigates how the fatigue process works in high stressed Al–Si–Mg and Al–Si–Cu fatigue samples starting from crack nucleation to sample rupture by rotating bending loads at room temperature. The samples themselves were taken directly from serial casted and heat treated cylinder head components and were processed to conventional hourglass specimens. Because no suitable test method which fulfils all the necessary test requirements was commercially available, a new rotating bending testing machine which directly operates in a confocal light microscope was developed. This test method allowed us to easily compare every change of the crack growth rate (especially for microstructural small cracks) with the microstructure and its crack path. At high stress regimes eutectic phases became very important for fatigue induced crack nucleation and early crack propagation. In the first third of the lifetime, both alloys cracked on multiple spots by interface cracking between hard phase particles (β-phase) and eutectic matrix. Depending on the crack length and stress intensity factor ΔK proceeding cracks were also able to move into the dentritic matrix (α-phase). At the end of the lifetime only a fraction of all nucleated cracks of each specimen became large enough to exert influence for fatigue failure.
Furthermore, using this test method as an integral element of stress gradient fatigue investigation makes it possible to identify and define major microstructural crack relevant objects like particles during material fatigue. Hence, it not only increases the knowledge for high performance fatigue simulation but also provides a tool for material optimization by fatigue processes themselves.
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