Vibration based diagnostics on rolling contact fatigue test bench

The paper presents the first results of a study on vibrations associated with a rolling contact fatigue test bench and how this mechanical behavior may be correlated to the fatigue damage of the specimens. In particular, the aim of this study was to evaluate the possibility to detect and quantify, thanks to vibration analysis, the damage on two discs subjected to rolling contact fatigue. The first part of this work regards a description of the bench with a focus on the results acquired by its static and modal fem analyses. Then, some pure rolling and sliding condition tests were carried out and a procedure to monitor both the specimens damage state and to record accelerometric data was implemented by placing a set of piezoaccelerometers on the machine and developing a virtual instrument for automatic data handling and analysis. Tests were also periodically stoppedand the rolling contact surface profile was acquired by means of a linear video camera in order to evaluate its progressive damage. Data acquired were analyzed, considering also the results from the first part of work, both using a standard approach, such as a spectral analysis (FFT, PSD and waterfall), and by implementing custom digital weighting filters for a windowed RMS in order to estimate, realtime during the measurement, a good estimator for the specimen damage state development

Application of a failure assessment diagram under rolling contact to components with hardness variable along the depth

Abstract A development of a failure assessment diagram for the evaluation of the safe working area of components subjected to rolling contact loading is here presented regarding the application to components whose hardness varies along the depth. The approach takes into account the influence of inherent defects in determining subsurface rolling contact fatigue, depending on working conditions, material properties and hardness profile along the depth. For this aim, crack propagation from inherent defects is assessed in terms of applied stress intensity factor range normalized with respect to short crack growth threshold, defect – free fatigue is assessed in terms of Dang Van stress normalized with respect to shear fatigue limit, where the material quantities depend on hardness variable with depth. These two normalized quantities are the coordinates of the points of a "reference curve" in the failure assessment diagram, which location indicates whether and where failure is expected to occur. By analysing different combinations of loading condition, inclusion dimension and hardness profile, it was possible obtaining a design diagram of general validity, which allows a fast prediction of safety against subsurface rolling contact fatigue

Influence of micro-notches on the fatigue strength and crack propagation of unfilled and short carbon fiber reinforced PEEK

Short carbon fiber reinforced (SCFR) PEEK is a highly attractive material for lightweight structures; improving knowledge about the influence of local imperfections on its fatigue behavior is essential for the design of real components. To this aim, fatigue strength and crack propagation of two grades of SCFR PEEK and neat matrix were investigated by testing at different stress levels specimens with a micro-notch consisting of a small blind hole (range diameter 0.1–1 mm). Overall, the presence of a micro-notch resulted in a decrease of fatigue strength compared to un-notched condition, but with different sensitivity and crack propagation patterns; while a higher fiber volume fraction enhanced fatigue strength and resistance to crack propagation, the combination of a lower fiber content and inclusion of additive particles had a negative effect. Crack propagation in the notched region was also evaluated. The average values of Paris' law exponential coefficients were similar and within the range of literature values, without apparent correlation with reinforcement type. Preliminary investigations in the presence of the smallest micro-notches seem to indicate the presence of a threshold size below which the influence of a small notch is comparable with that of material inherent defects, but further testing is necessary

Estimation of Fatigue Limit of a A356-T6 Automotive Wheel in Presence of Defects

The automotive wheel is a critical safety component in the vehicle and, for such a reason, it has also to meet strict requirements about technological properties. This component is produced by low pressure die casting technique and the casting defects related to the process have to be properly considered having a high effect in decreasing both static and dynamic resistance of the component. Effectively, casting defects like porosities influence the fatigue crack initiation and strongly affect the fatigue life too. One of the most common problem in the real component is the mismatch between the experimental data and literature. In fact, many scientific researches were carried out on small samples produced in a controlled condition and therefore it is difficult to direct transfer the laboratory results to a real cast component with a well-defined shape and different thicknesses. In the present study, an aluminum alloy A356-T6 wheel was analyzed in order to correlate the fatigue performance taking in to account the casting defects. The fatigue limit of the component was studied by rotating bending fatigue tests executed on the whole wheels. Microfractographic analyses on the broken wheels were carried out on the fracture surfaces using a Scanning Electron Microscope in order to identify the crack initiation zone: it was recognized that the crack always started from shrinkage porosities. The statistical population of these defects was therefore investigated on samples taken from the wheel in crack nucleation positions of the spoke and the maximum expected defect size on the component was estimated by the statistics of extreme values. The experimental fatigue limit was finally compared with the theoretical value predicted with the Murakami’s method

Fatigue behavior and cyclic damage of peek short fiber reinforced composites

Fatigue strength and failure mechanisms of short fiber reinforced (SFR) PEEK have been investigated in the past by several research groups. However some relevant aspects of the fatigue behavior of these materials, like cyclic creep and fatigue damage accumulation and modeling, have not been studied yet, in particular in presence of both fillers and short fibers as reinforcement. In the present research these aspects were considered by carrying out uni-axial fatigue tests in load control (cycle ratio R = 0) on neat PEEK and PEEK based composites reinforced either with short carbon fibers only or with addition of fillers (graphite and PTFE). For each material stress-life curves were obtained and compared. Fatigue fracture surfaces were analyzed to identify failure mechanisms in presence of different reinforcement types. The evolution of cyclic creep strain was also monitored as a function of the number of cycles, thus allowing investigation on the correlation between cyclic creep parameters and fatigue life. The evolution of cyclic damage with loading cycles was then compared by defining a damage parameter related to the specimen stiffness reduction observed during the tests. Progressive cyclic damage evolution of short fiber reinforced PEEK composites presented significantly different patterns depending on applied stress level and on the presence of different reinforcement typologies. In order to reproduce the different fatigue damage kinetics and stages of progressive damage accumulation observed experimentally, a cyclic damage model was finally developed and implemented into a finite element code by which a satisfactory agreement between numerical prediction and experimental data at different stress levels for each examined material

Evaluation of the residual stresses induced by shot peening on some sintered steels

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Predictive maps for the rolling contact fatigue and wear interaction in railway wheel steels

Rolling contact fatigue (RCF) and wear are competing phenomena in railway wheels, but they are typically studied as independent events. In this work, preliminary tests are carried out with a twin-disc bench to explore the relationship between RCF and wear. First, the collected tests are reported on the shakedown maps to assess the cyclic response and calculate the fatigue index parameter used to predict the surface-initiated fatigue damage. Second, the fatigue index and material cyclic yield strength are related to the experimental wear rate. This work proposes maps to predict how the wear rate and cyclic response change by varying the contact conditions (clean or contaminated with water or sand), slip ratio and extraction position of specimens. Furthermore, the estimated wear rate is superimposed to the shakedown map to investigate a possible relationship between RCF and wear. The proposed approach allows to understand how a railway wheel steel behaves as a function of the working conditions and whether some working parameters can be changed without compromising the material performance

A failure assessment diagram for components subjected to rolling contact loading

A failure assessment diagram for the evaluation of the safe working area of components subjected to rolling contact loading is proposed. Rolling contact fatigue limitation is treated in terms of non-propagation condition of inherent defects, following the El-Haddad model for the short-cracks growth threshold. Static fracture and ratchetting limitations are also added to the diagram. In this way, the approach gives an overview of the possible damage mechanisms, automatically indicating which is expected for a specific case. In particular way, the diagram presents different areas: a safe zone (infinite life), a rolling contact fatigue zone almost independent on defects content, a rolling contact fatigue zone dependent on defects, a ratchetting zone and a static fracture zone. Depending on material properties, operating conditions and inclusion content, a reference point can be drawn on this diagram, indicating in which area the component is working and, consequently, if it is safe or which damage mechanism is expected. Some experimental evidences referring to rolling contact tests carried out in the past where re-interpreted and verified by this approach, highlighting the role of working conditions, material properties and inclusion content in determining the damage mechanism