Ultrasonic Fatigue Analysis on Steel Specimen with Temperature Control : Evaluation of Variable Temperature Effect

Non peer reviewe

Modelling and empirical development of an anti/de-icing approach for wind turbine blades through superposition of different types of vibration

The generation of green, safe and inexpensive energy by wind turbines is often decreased or interrupted in severe climate areas during cold weather. When the blades are even partially covered by different types of ice, their efficiency drops suddenly due to degradation of the blade profile from the ideal. The present study presents a new approach using ultrasonic guided waves as an anti/de-icing technique supplemented by low-frequency vibrations to effect shedding of the ice from the turbine blades. The study consists of a series of steps including initial theoretical studies and finite element simulation of representative plates and turbine blades, followed by a number of experimental validations concluded by tests of the complete approach in an icing wind tunnel. The results show the efficacy of the developed approach in tackling the different types of ice which can form on the blades, using very low power compared to available thermal techniques

Instumentation of Ultrasonic High-Frequency Machine to Estimate Applied Stress in Middle Section of Specimen

Paper Ref. 2807The objective of this paper is to describe the instrumentation of a new developed ultrasonic fatigue testing machine at 20 kHz working frequency and compare the possible solutions to estimate the stress applied in the middle section of the specimen. Stress is estimated by measuring the displacement on bottom of the specimen and their strain on middle section. The monitoring of the displacement, considered here in the bottom face of the specimen, is carried out using a high resolution laser, the strain using a strain gauge in middle of specimen and field temperature using a pyrometer and thermographic camera. A LabVIEW routine is used to control the initial parameters of the transducer, such as the frequency reset, the frequency seek, the load amplitude and to record data of all sensors. To manage and process the data, a data acquisition device working at 400 kHz from National Instruments is used.Final Accepted Versio

Identification of fatigue damage evolution in 316L stainless steel using acoustic emission and digital image correlation

One of the main objectives of Acoustic Emission (AE) monitoring is to identify approaching critical stage of damage in the structure before it fails. State-of-the-art AE analysis is done on the features in both the time and frequency domains. Many features such as centroid frequency, duration, rise-time, count and energy are dependent on acquisition settings; threshold and timing parameters. Incorrect acquisition settings may result in inaccurate classification of the AE source. This work proposes a new feature in the time domain signal based on 2nd order Renyi’s entropy, which proves to be efficient in identifying different stages of damage. Renyi’s entropy is a measure of uncertainty or randomness of the signals and is directly derived from the distribution of signal amplitude. Therefore, it is independent of threshold and timing parameters. The validity of the proposed parameter is investigated by performing AE monitoring during fatigue endurance test of 316L stainless steel. Digital Image Correlation (DIC) and global strain monitoring was carried out to relate material damage with AE activity. The result shows Renyi’s entropy to be an effective measure to identify critical stages of damage in the material

Automation in Strain and Temperature Control on VHCF with na Ultrasonic Testing Facility

The necessity of increased safety and reliability in mechanical components or structures became a subject of prime importance over the years. Therefore, to have a proper understanding of the damage and rupture mechanisms in materials subjected to fatigue at Very High Cycle Fatigue (VHCF) regimens is extremely important nowadays. However, using conventional fatigue testing machines to carry out VHCF tests can be very time-consuming and expensive; for instance, making a fatigue test at 30 Hz as working frequency would take more than one year to reach 109 cycles. Ultrasonic fatigue testing machines are being used to perform materials testing in the range of 108 to 1010 fatigue cycles. The so-called VHCF regimen is now under intense studies in which concerns the performance of ultrasonic fatigue testing machines themselves. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain-rate, temperature and frequency) is still a matter of concern and continuous development. Due to the high frequencies involved in VHCF testing, the heat generated on the specimens greatly affects the parameters that influence fatigue behavior. The objective of this paper is to describe the design, construction, instrumentation and operation of an ultrasonic fatigue testing machine of 20 kHz working frequency, with automatic strain and temperature control. In order to achieve fully automated tests, a closed loop control system was developed to use monitored temperature and displacement amplitude of the specimen to set the length of the powered and the cooling periods of the machine. The monitoring of the displacement, measured here at the bottom face of the specimen, is carried out using a high resolution laser. The specimen´s temperature is monitored online through a pyrometer and an infrared thermography camera. Data is acquired, managed and processed with a National Instruments DAQ device working at 400 kHz sampling frequency. The software was developed in-house using the LabView® package. The present paper describes the advantages and drawbacks of metal fatigue testing at very high frequencies with special emphasis on the strain and temperature control issues. Comparison of tests carried out with and without both displacement and temperature control are made. Fatigue tests were carried out on two metallic materials, copper 99% and carbon steel, with the determination of S-N curves.Non peer reviewe

Identification of fatigue damage evolution in 316L stainless steel using acoustic emission and digital image correlation

One of the main objectives of Acoustic Emission (AE) monitoring is to identify approaching critical stage of damage in the structure before it fails. State-of-the-art AE analysis is done on the features in both the time and frequency domains. Many features such as centroid frequency, duration, rise-time, count and energy are dependent on acquisition settings; threshold and timing parameters. Incorrect acquisition settings may result in inaccurate classification of the AE source. This work proposes a new feature in the time domain signal based on 2nd order Renyi’s entropy, which proves to be efficient in identifying different stages of damage. Renyi’s entropy is a measure of uncertainty or randomness of the signals and is directly derived from the distribution of signal amplitude. Therefore, it is independent of threshold and timing parameters. The validity of the proposed parameter is investigated by performing AE monitoring during fatigue endurance test of 316L stainless steel. Digital Image Correlation (DIC) and global strain monitoring was carried out to relate material damage with AE activity. The result shows Renyi’s entropy to be an effective measure to identify critical stages of damage in the material