Ultrasonic fatigue testing of concrete

Cyclic compression fatigue properties of concrete are studied with the ultrasonic fatigue testing method with cycling frequency 19 kHz and are compared to servo-hydraulic tests performed at 60 Hz. Ultrasonic testing was found applicable for rapid generation of very high cycle fatigue (VHCF) data of concrete. Fatigue cracks can be initiated, however specimens do not rupture, since cyclic stresses decrease with increase of compliance in displacement controlled ultrasonic tests. Observation of resonance frequency, analysis of higher order harmonics of vibration, and computed tomography of specimens are successful methods to analyse fatigue damage. Calorimetric evaluations can be used to calculate the cyclic irreversible strain, which is about 1% of the elastic strain in the ultrasonic VHCF test.

Constant and variable amplitude ultrasonic fatigue of 2024-T351 aluminium alloy at different load ratios

Ultrasonic fatigue testing equipment is presented that is capable of performing constant amplitude (CA) and variable amplitude (VA) experiments at different constant load ratios. This equipment is used to study cyclic properties of aluminium alloy 2024-T351 in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime at load ratios R=-1 and R=0.5. CA loading does not reveal a fatigue limit below 10¹⁰ cycles. Cracks leading to VHCF failure start at broken constituent particles. Specimens that survived more than 10¹⁰ cycles at R=-1 contain non-propagating cracks of lengths below grain size. Resonance frequency and nonlinearity parameter β(rel) show changes of vibration properties of specimens at low fractions of their VHCF lifetime. VA lifetimes are measured in the HCF and VHCF regime and compared with Miner calculations. Damage sums decrease with decreasing load (and increasing mean lifetimes) and are lower for R=0.5 than R=-1.

Analysis of fatigue properties and failure mechanisms of Ti6Al4V in the very high cycle fatigue regime using ultrasonic technology and 3D laser scanning vibrometry

Accelerated fatigue tests with Ti6Al4V were carried out using a 20kHz ultrasonic testing facility to investigate the cyclic deformation behavior in the Very High Cycle Fatigue (VHCF) regime in detail. Beside parameters like the ultrasonic generator power and the displacement of the specimen, a 3D laser scanning vibrometer was used to characterize the oscillation and fatigue behavior of the Ti-alloy. The course of the S-N(f) curve at the stress ratio R=-1 shows a significant decrease of the bearable stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 10⁷ cycles. Microscopic investigations of the distribution of the α- and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. High resolution vibrometry was used to visualize the eigenmode of the designed VHCF-specimen at 20 kHz in the initial state and to indicate local changes in the eigenmodes as a result of progressing fatigue damage. Non-contact strain measurements were realized and used to determine the stress amplitude. The determined stress amplitudes were correlated with strain gauge measurements and finite element analysis.

Influence of specimen geometry on temperature increase during ultrasonic fatigue testing

During the last decades the interest in fatigue properties of materials subjected to more than 10(7) cycles has strongly increased. To reach such high numbers of cycles the use of ultrasonic fatigue testing systems is a well-established method due to their very high fatigue frequencies. However particular considerations about the specimen design have to be made for ultrasonic fatigue testing. In order to investigate the influence of the specimen design on the fatigue behavior in more detail, two different gauge lengths were chosen. The influence of specimen design was found to be very prominent on the characteristic frequencies which are used for operating ultrasonic fatigue testing systems. As it is already known, with increasing specimen mass the characteristic frequency decreases. Also the amplitude has an influence on the characteristic frequency during an ultrasonic fatigue test. With increasing test amplitude micro-plasticity becomes more pronounced which leads also to a decrease in frequency. Most prominent, a distinct influence of the specimen geometry on the temperature increase during cycling was found. Specimens with a shorter gauge length showed a higher increase in temperature then specimens with the longer gauge length. It is suggested that these observations are caused by an easier and more equal distribution of heat in the specimens with the longer gauge length design. The influence of specimen shape was accomplished by simulation using a finite element method.

Application of in situ thermography for evaluating the high-cycle and very high-cycle fatigue behaviour of cast aluminium alloy AlSi7Mg (T6)

The present paper illustrates the application of infrared thermal measurements for the investigation of crack initiation point and crack propagation in the high-cycle and the very high-cycle fatigue range of cast AlSi7Mg alloy (A356). The influence of casting defects, their location, size and amount was studied both by fractography and thermography. Besides internal and surface fatigue crack initiation as a further crack initiation type multiple fatigue crack initiation was observed via in situ thermography which can be well correlated with the results from fractography obtained by SEM investigations. In addition, crack propagation was studied by the development of the temperature measured via thermography. Moreover, the frequency influence on high-cycle fatigue behaviour was investigated. The presented results demonstrate well that the combination of fractography and thermography can give a significant contribution to the knowledge of crack initiation and propagation in the VHCF regime.

Influence of environment on the fatigue crack growth behaviour of 12% Cr steel

In the present work, the influence of different environments on the fatigue crack growth behaviour of 12% Cr steam turbine blade steel is investigated. Fatigue crack growth rates (FCGRs) in the near threshold regime are measured with ultrasonic fatigue testing technique. Fatigue tests are performed in vacuum, air and different aqueous environments with defined chloride and oxygen content. Furthermore, the influence of different stress ratios is investigated. It is found that crack propagation is not necessarily enhanced with increasing corrosiveness. In the aqueous environments, the FCGRs below 10⁻⁸ m/cycle are lower than in air. The threshold stress intensity factor ranges are higher or equal. Observation of the fracture surfaces shows oxide formation and partly intergranular fracture for specimens tested in aqueous environments. Crack closure effects seem to be responsible for this unexpected behaviour.