Ultrasonic Testing Machine Research Articles

Very High Cycle Fatigue and Ultrasonic Testing in Multiaxial Loading

Abstract Fatigue damage has special relevance on the lifespan of mechanical components and structures, as it takes responsibility for the majority of registered structural failures. Most fatigue testing today still uses uniaxial loads, nonetheless it is generally recognized that multiaxial stresses occur in many full-scale structures. Nowadays, the growing need for greater lifespans forced the study of material behavior under what is nowadays very high cycle fatigue (VHCF) regime, fatigue research beyond 10E07 up to 10E10 cycles. However, most classic fatigue machines are often very costly, both in terms of time and energy, for such an elevated number of cycles. The high costs of equipment and time to conduct such experiments have seen a massive improvement with the introduction of high frequency ultrasonic machines. Presently, it is already possible to characterize materials VHCF response under axial and multiaxial loading conditions in a fraction of the time. Ultrasonic testing machines were first introduced for simpler uniaxial loading and more recently adapted for multiaxial loading conditions. The present work reviews the current background of ultrasonic fatigue testing machines working at 20-kHz frequency, with emphasis on multiaxial fatigue and VHCF. A review of a series of fatigue tests are presented, respectively, axial, shear, axial/shear, and in-plane bi-axial fatigue tests with different combining dimension specimens allowing different shear/axial stress ratios. Special attention will be put into the performance of multiaxial classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low-cost piezoelectric transducers. By comparing all so far developed and successfully reached multiaxial specimens and respective methodologies, two base approaches were possible to be established. When designing new specimens or ultrasonic machines, or both, one of the two methods must be followed. To have a well-comprehended methodology and successful design approach will undoubtedly guide future multiaxial ultrasonic method ideas.

Innovative tensile test for brittle materials: Validation on graphite R4550

This paper proposes an innovative ultrasonic tensile test methodology for the assessment of quasi-static uniaxial mechanical properties and tensile strength of brittle materials. An ultrasonic testing machine, commonly employed for very high cycle fatigue tests, had its control and data acquisition systems adapted to induce specimen failure in about 100 cycles, avoiding cyclic load damage. The mechanical properties are thereafter estimated with a finite element model which simulates the experimental test. The proposed method has been validated by assessing the tensile strength of graphite R4550, characterized by brittle behaviour. The mechanical properties for graphite R4550 estimated with this experimental-numerical approach were found to be close to literature values obtained in different quasi-static testing configurations, confirming the effectiveness of the proposed method and the feasibility of employing the ultrasonic fatigue testing machine for studying the behaviour of brittle materials. Furthermore, the proposed methodology eliminated issues caused by mechanical fixtures and tensile testing machine alignment, while also allowing a considerable increase of the material loaded volume when compared to that of traditional test methods. The experimental and numerical approaches are successfully validated on graphite R4550, also highlighting its capability of characterizing the material nonlinear behaviour, including viscoelasticity and asymmetrical response in tension and compression.

VHCF ultrasonic tests on EN AW‐6082 aluminum alloy samples over a wide dimensional range

AbstractIn the present paper, a very high cycle fatigue test campaign carried out with an ultrasonic testing machine up to 1010 cycles is presented. The aluminum alloy hourglass and dog‐bone samples have dimensions spanning over a very wide range, with the diameter in the middle cross‐section comprised between 3 and 30 mm. From the experimental results, the influence of structural size can be clearly detected, and it is possible to observe a transition between small scales, where the size effect is more pronounced, and larger scales, where the size effect is vanishing. This phenomenon can be explained in an effective way by adopting a multifractal formalism, which is equipped with the probabilistic treatment of the statistical dispersion of experimental data.

The effect of risk volume on the fatigue life of stainless steels subjected to high-frequency loading

In this study, fatigue lifetime of two types of specimens with different volumes (given by the gauge length and the diameter of the central part of the specimen) subjected to high-frequency loading is investigated. Studied materials are austenitic stainless steels 1.4306 and 1.407, both commonly known as AISI 304L. An ultrasonic testing machine working at frequency of 20 kHz was used to reach very high cycle fatigue region. Fatigue tests were carried out under uniaxial symmetric tension/compression loading (R = - 1) at room temperature. The study includes a microstructural description of investigated materials, fracture surface analysis, and conclusions.

Cracking of S355J2+N steel in the high-cycle and very-high-cycle fatigue regimes

The aim of this paper is to present the results of HCF and VHCF tests of the S355J2+N steel performed on the ultrasonic testing machine at 20 kHz load frequency. The test method consists of three parts: preliminary tests aimed at identification of basic properties of the tested material, specimen geometry analysis using the finite element method (FEM), main fatigue test using ultrasonic system. The fatigue tests consisted of two stages. In the first one, the fatigue limit was determined using the Staircase (Up-And-Down) Method. In the second stage, the fatigue tests were carried out above and below the fatigue limit. Based on the fractographic analyses of the fatigue fracture surfaces, three types of them were distinguished. The analyses of the cracking made it possible to determine the upper load limit for tests of this material in both VHCF and HCF regimes on the ultrasonic testing machine. This level is almost 20 % lower than the range of the linear relationship between stress and strain in tensile test.

Interior crack initiation during the very‐high‐cycle fatigue of railway wheel steel under axial loading and rolling contact loading

AbstractIn this paper, a comparative study of the very‐high‐cycle fatigue (VHCF) behavior of railway wheel steel under axial loading and rolling contact loading was conducted. Fatigue tests were performed with an ultrasonic fatigue test machine under axial loading, and the fracture surfaces from the fatigue tests and shattered rims taken from the failed railway wheels were observed. The wheel steel under axial loading presents a VHCF behavior with Mode I crack, and that under rolling contact loading is a VHCF behavior with mix Mode II–III crack. For the VHCF behavior with Mode I crack, surface and interior crack initiation occurred with equal probability at both low and high stress levels and produced a dual linear S–N curve since the value of fatigue limits for the surface and interior crack initiation are close. For the VHCF behavior with mix Mode II–III crack, cracks were initiated from the interior Al2O3 inclusion, and the fatigue life was beyond 107 cycles. Fatigue bands were observed on the fracture surface under rolling contact loading. The ferrite nanograins formed due to the stress state of shear plastic strain with a large compressive stress. The formed nanograins were softer than the matrix caused by the redistribution of the carbon.

Damage-failure transition under consecutive dynamic and very high cycle fatigue loads

The Foreign Object Damage problem is considered as the damage tolerance statement for consecutive loading tests, which include the shock-wave loading (by explosive generator) of massive planar targets (aluminum alloy AlMg6) to provide a billet for the specimen machining with controlled damage that is the analogous material of the fan blades subjected to high-speed collision with solid particles. These samples were used to perform high cycle fatigue and very high cycle fatigue tests with the ultrasonic testing machine, which allows fatigue loading for 108–1010 cycles with an amplitude of up to several tens of micrometers and a frequency of 20 kHz. It is shown that the fatigue strength of AlMg6 alloy specimens pre-loaded by shock in the 109 cycle regime reduces by 24%. The fatigue damage-failure transition and crack initiation were studied by the amplitude–frequency analysis of higher harmonics associated with the influence of defects on the effective elastic properties. The structural study of the fracture surface for the specimens after consecutive loading was conducted using the profilometry data to identify the roughness scale invariants induced by defects for corresponding areas responsible for the staging of fatigue damage-failure transition. The scale invariants and corresponding lengths were used for the formulation of the generalized Paris law for the crack advance in the damaged material.

Acoustic Response of Reference Notches: Derivation of Acceptance Criteria for Automated Ultrasonic Testing Based on the Correlation Between Defect Quality and Effective Defect Size

This paper presents a detailed study of the acoustic response of artificially manufactured notches, which are established reference defects in common standards for ultrasonic testing (UT). Measurements were performed with standard tubes manufactured for the commissioning of automated UT machines. Included reference notches are at the inner and outer tube surface and have different lengths and orientations (longitudinal, oblique and transverse). From the acoustic reflection profile, i.e. the measured defect amplitude along the notch axis, the effective defect length and the defect homogeneity are introduced as measured quantities of the reference defect quality. The large data sample of 320 evaluated notch profiles gives insight to the statistics of the achieved reference defect quality. Key approach of the analysis is the correlation between both measured quantities. It reveals a universal behaviour, which can be described by a sigmoidal function converging for high quality defects towards the theoretical limits. Based on the obtained parametrization it is possible to define criterions for the required defect quality and the test speed as function of the specified repeatability. This solves the current dilemma how to distinguish between variations due to the reference quality from those related to the testing machine. Furthermore, the presented measurement procedure can be used for a reference certification, which complements existing standards for the ultrasonic system and the probes.

Very high cycle fatigue behaviour of S690 structural steel

Testing beyond 10 millions of cycles in a conventional fatigue machine is a very time consuming task, so the fatigue limit has been settled around this number of cycles. However, during the service life, several engineering components experience a number of cycles larger than 10 million cycles. For that reason, the region of very high cycle fatigue (VHCF), and the concept of fatigue limit has been matter of study and interest in the last decades. In order to overcome the time effort required, ultrasonic fatigue testing machine has being developed and widely used. Nevertheless, ultrasonic testing system and the VHCF region brought new challenges such as the frequency effect and the internal crack initiation. Thus, this research work intends to characterize the fatigue behaviour of S690 structural steel in the VHCF region. An experimental campaign of twenty specimens was performed in ultrasonic testing machine at a frequency of 20 kHz and a mean curve was determined. This work also approaches a design methodology of smooth fatigue specimens to be tested in ultrasonic fatigue machine.

Damage-failure transition in VT6 titanium alloy under consecutive dynamic and VHCF loads

The lifetime of VT6 (Ti-6Al-4V) titanium alloy under consecutive dynamic and very-high cycle fatigue loads is studied. The relevance of the problem is determined by the applications to the prediction of the endurance fatigue limit for materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. The titanium alloy specimens were used to perform High Cycle Fatigue (HCF) and Very High Cycle Fatigue (VHCF) tests on the ultrasonic testing machine, which allows fatigue loading for 108-1010 cycles with amplitude up to several tens of micrometers and frequency of 20 kHz. It is shown that VT6 alloy specimens in the 108 cycle regime subject to dynamic preloading reduces the fatigue strength by 70%. Structural study of the fracture surface for the specimens after consecutive loading was conducted using the profilometry data to identify the roughness scale invariants induced by defects for corresponding areas responsible for the staging of fatigue damage-failure transition. The scale invariants and corresponding lengths were used to formulate the generalized Paris law governing the crack growth in damaged materials.

Multi-Fractal Scaling Law applied to VHCF with an emphasis on statistical fluctuations

In recent decades, ultrasonic testing machines have allowed to investigate the very-high cycle fatigue (VHCF) range, featuring feasible gigacycle fatigue testing in a very short time, although the influence of frequency is still partly controversial. The fatigue behaviour depends on the random distribution of initial defects, whatever the stress range amplitude is. As a consequence, the structural size comes into play and the so called ”size-effects” are usually observed. On the other hand, the scaling is not uniform when sufficiently large dimensional ranges are analysed. To this aim, the multi-fractal formalism was adopted to explain the observed specimen-size effect in the VHCF regime when a wide dimensional range is investigated. In addition, the statistical dispersion of experimental results was accounted by adopting different probabilistic functions. Among them, the Generalized Extreme Value Distribution Type-I was selected. In this way, it was possible to derive the analytical relationship for probabilistic-scale dependent P-S-N-b curves.Subsequently, the proposed model was used to analyse the experimental data obtained in an experimental campaign carried out at Politecnico di Torino by the present authors. More in detail, ultrasonic fully reversed tension–compression fatigue tests in the very-high cycle fatigue (VHCF) range were conducted on a set of hourglass and dog-bone specimens made of EN AW-6082 aluminium alloy, with a diameter in the middle cross-section ranging from 3 mm up to 30 mm.The comparison between the theoretichal model and the experimental data allowed to demonstrate the ability of Multi-Fractal Scaling Law (MFSL) to provide objective values of the fatigue strength of full-size components subjected to VHCF and to guarantee safe structural design.

Investigation of Fatigue Damage of Tempered Martensitic Steel during High Cycle Fatigue and Very High Cycle Fatigue Loading Using In Situ Monitoring by Scanning Electron Microscope and High‐Resolution Thermography

Herein, the fatigue damage mechanisms of a low‐alloyed 0.5 wt% carbon steel (50CrMo4) are examined in the high cycle fatigue and very high cycle fatigue regime, taking into account different strength conditions. For this purpose, the heat treatment of the material is conducted using two different batches at two different tempering temperatures, which lead to hardnesses of 37HRC and 57HRC, respectively. The fatigue tests are conducted accounting for different test frequencies of f = 95 Hz and f = 20 kHz, as well as size effects. It is found that the fatigue behavior of the tempered steel 50CrMo4 can be subdivided into type I (surface cracks) and type II (internal cracks) VHCF damage behavior depending on the material strength condition. With the aid of in situ test equipment, the influence of microstructural banding on the local fatigue strength is demonstrated by high‐resolution thermography. By implementing an ultrasonic testing machine in a high‐resolution scanning electron microscope, the development of a fine granular area (FGA) at an artificial defect in vacuum atmosphere is demonstrated.

Prediction of titanium alloy Ti-6A1-4V lifetime under consecutive shock-wave and gigacycle fatigue loads

The influence of consecutive shock-wave and gigacycle fatigue loads on the lifetime of titanium alloy VT6 (Ti-6A1-4V) was investigated. An experimental methodology for estimating the ultra-high cycle lifetime was developed with reference to accidental high-speed collision of solid particles with fan blades and subsequent fatigue failure under the flight cycle conditions normal for operating aircraft engines. The program of ultra-high cycle loading (the number of cycles is 107-109) of specially made specimens was implemented experimentally. In these experiments, massive flat targets made of titanium alloy VT6 after shock-wave loading were impacted by a flat striker (explosive generator method), which initiated controlled damage in the material. Ultra-high cycle loading tests were carried out on the Shimadzu USF-2000 ultrasonic testing machine, which made it possible to conduct fatigue loading tests up to 108-1010 cycles and frequency of 20 kHz.The "in situ" method of determining the accumulation of irreversible fatigue damage was applied. It is based on the analysis of nonlinear features of the feedback signal in a closed system of an ultrasonic fatigue installation, which allowed us to relate the microscopic mechanisms of fatigue with model representations, as well as to consider the stages of damage evolution based on the nonlinear kinetics of defect accumulation during cyclic loading in high - and gigacycle fatigue modes. In the mode of ultra-high cycle (gigacycle) fatigue, the material exhibits abnormalities of elastic properties, which lead to the effect of non-linearity in the oscillation amplitude.

Modal and strain experimental analysis to an improved axial-axial cruciform specimen for ultrasonic fatigue testing

Fatigue behavior for any material or composite is an ever-evolving complex subject of big engineering importance. Within the very high 106 to 109 cycle regime the use of ultrasonic fatigue has been a big research topic in recent years, ranging from all metals and composite studies, new experimental methods and induced stress states, to the frequency effect and associated fracture mechanics. Following the associated cruciform ultrasonic fatigue testing research in development, new and improved cruciform specimens were acquired and subjected to experimental analysis. The conducted fatigue cruciform testing method induces an axial-axial in phase stress state through the use of a piezoelectric ultrasonic testing machine built at Instituto Superior Técnico laboratories. A modal response analysis was performed within the frequency working range of the used piezoelectric transducer using a Polytec laser vibrometer. A modal calculation method termed Frequency Domain Decomposition (FDD) was adapted to the measured cruciform vibration displacements. The points of measure were in correlation to a previous published numerical research. The results were then compared to previous cruciform specimens’ behavior and numerical results. Strain gauges were also used for the strain and stress study at the specimen’s center. With both conducted experiments showing promising results, the new and improved specimens were led to fatigue failure and a preliminary fracture analysis made.

Effect of Ultrasonic Scaling on Debonding of Lithium Disilicate Glass Ceramic Laminate Veneer Restorations Cemented to Tooth Structures: in Vitro Study

Background: This in vitro study was conducted to investigate the effect of ultrasonic scaling on debonding of lithium disilicate glass ceramic laminate veneers restorations cemented to tooth structure. Materials and Methods: 16 extracted sound upper anterior teeth were prepared to receive laminate veneer restorations. They were randomly assigned into 2 groups (n=8), as in group Non-US , the cemented fabricated laminate veneers were not exposed to ultrasonic scaling and in the ultrasonic group (US) the cemented fabricated laminate veneers were exposed to ultrasonic scaling. The GC Initial LiSi Press Lithium disilicate glass ceramics laminate veneers were fabricated according to manufacturer instructions. The debonding force test was applied after cementation and after ultrasonic scaling using Instrone testing machine. The obtained data were tabulated and analyzed statistically. Differences were considered significant at P<0.05. Results: the recorded values of the debonding force mean ±SD for Non-US group were (79.5288 ±16.5228N) and mean ±SD values recorded for US group were (54.0713 ±8.2647 N). The T test showed significant difference between 2 groups (p value=0.0016) and 95% CI (29.45±14.0057). Conclusion(s): Within the limitations of this study, it was found that ultrasonic scaling significantly affect the debonding force of cemented laminate veneers.

Numerical analysis of vhcf cruciform test specimens with non-unitary biaxiality ratios

With the development of new materials, it is now known that there is no such thing as a fatigue endurance limit, i.e., materials do not have infinite life when the stress level is such that there is no fracture up to 10 million (1E7) cycles. The problem of testing materials above this number of cycles is that most testing equipment operates well below 150 Hz, making testing up to 1 billion (1E9) cycles or above an impracticality. The recent developments of ultrasonic testing machines where frequencies can go as high as 20 kHz or above enabled tests to be extended to these ranges in just a few days. This is now known as Very High Cycle Fatigue (VHCF). On the other hand, critical components used in Engineering applications are usually subjected to multi-axial loads, as is the case of the fuselage and wings of aircrafts which are subjected to biaxial states of stress. In this paper, VHCF cruciform test specimens purposely designed to develop orthogonal biaxial stresses with different biaxiality ratios will be analysed. The specimens are composed from Aluminium 6082-T651, a medium strength alloy used in many highly stressed engineering applications, including trusses, cranes, bridges and transportation. The specimens work as tuning forks with determined mode shapes at 20±0.5 kHz, where maximum principal stresses are developed at the centre of the specimen. Finite Element Analysis (FEA) is used to assess the dynamic behaviour of the specimens. The framework on how to design and manufacture cruciform specimens with different biaxiality ratios will be explained in a clear way so it can be used by other engineers in the field.

Study of Elastic Modulus Determination of Polymers with Ultrasonic Method

Elastic modulus is one of the mechanical properties of the material that often used in industry or research field as a benchmark to determine materials’ performance in term of withstanding load without being deformed. Destructive testing is perpetually used to determine this property. However, destructive testing needs a sample, and in situ testing is implausible. Various types of materials are used in the production process of the industry, e.g., polymers. Polymers have time-dependent properties, which can result in high variety value. By principle, ultrasonic inspection, which depends on sound velocity and density of materials, can be used to determine elastic modulus. Ultrasonic test with through-transmission method has been studied to determine elastic modulus and dynamic elastic modulus for polymers. For the sake of quality control and engineering design, ultrasonic pulse-echo contact is preferable. Ultrasonic testing was conducted with GE USM 35X device, which is a Pulse-Echo method and also contact method type of ultrasonic testing machine. Experiments were conducted on several types of polymers with frequency and thickness as experiment parameters. With an input of specimens’ thickness, materials’ sound velocity (ν) could be obtained. Thus results of ν were counted to attain the elastic modulus. Comparison between ultrasonic testing results and mechanical testing results of polymers’ elastic modulus were performed to analyze the data. In this research, elastic modulus value obtained from the ultrasonic test has a profound error, up to 65% (minimum) and 388% (maximum), especially for a polymer with an eminently low density. Further research should be conducted because of the attenuation effect. Also, lower probe frequency eases the detection of alternating ultrasonic wave. Specimens’ thickness adjusted with near-field calculation can eliminate the near-field effect, which is a natural phenomenon of the ultrasonic wave. However, it would not have yielded an accurate value because an excessive thickness will give an attenuation effect.

Self-Similar Patterns of Damage Development and Reliability Assessment of AMg6 and D16T Aluminum Alloys under Consecutive Dynamic and Gigacycle Loading

In the paper, we study the kinetics of fatigue crack growth in MMg6 and D16T aluminum alloys in the gigacycle fatigue mode under dynamic preloading. The relevance of the problem statement is determined by the critical applications—life estimation of materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. Specimens were preloaded by dynamic tension using the split Hopkinson (Kolsky) pressure bar at strain rates up to ~103 s−1, with consecutive gigacycle loading on the Shimadzu USF-2000 ultrasonic testing machine. Quantitative fractography of fracture surfaces was performed using profilometry and scanning electron microscopy data. We propose an original form of the kinetic equation, which relates the fatigue crack growth rate to a change in the stress intensity factor. The scale invariance of defect structures responsible for the formation of the fracture surface relief under gigacycle fatigue loading is found to be related to the power exponent of the Paris law. The fracture surface morphology of an aluminum-magnesium alloy under consecutive dynamic and gigacycle loading is studied by the multifractal detrended fluctuation analysis method. It is found that a transition from the stage of formation of a fish-eye zone of localized deformation is caused by the generation of fracture sites and accompanied by a qualitative change of the nonlinear dynamics of the system—a transition from monofractal to multifractal dynamics characterized by broadening of a multifractal spectrum at the final crack growth stage, which leads to macrofracture.

Scale-invariant patterns of damage development in AMg6 alloy under consecutive dynamic and gigacycle loading

A study of fatigue crack growth kinetics in aluminum alloy AMg-6 in gigacycle fatigue mode at preliminary dynamic deformation has been carried out. The relevance of the statement is determined by the critical applications of estimating materials and structural elements resource of aircraft gas turbine engines in terms of flight cycle with random dynamic effects. Preliminary loading of samples was carried out by the dynamic tension on Hopkinson-Kolsky rod split at strain rates up to ~103 s-1 subsequent gigacycle loading on the ultrasonic testing machine Shimadzu USF-2000 quantitative analysis and fractography of fractures based on the data from profilometry and scanning electron microscopy. The authors propose an original form of kinetic equation which relates the rate of the fatigue crack growth with changes in the stress intensity factor. The scale invariance of defect structures responsible for the formation of the fracture surface relief under gigacycle fatigue loading was found out to be related to the power exponent of the Paris law.

Notch Effect on the Fatigue Behavior of a TC21 Titanium Alloy in Very High Cycle Regime

The very high cycle fatigue (VHCF) property of TC21 titanium alloy blunt-notched specimens were investigated by using an ultrasonic fatigue test machine with a frequency of 20 kHz. S–N of blunt-notched specimens illustrated a continuous decrease characteristic with a horizontal line over the 105–109 cycle regimes. However, the fatigue life showed a large scatter for blunt-notched specimens. Blunt-notch significantly reduced the fatigue property in the high cycle and very high cycle regimes compared with that of smooth specimens. The crack initiation modes for blunt-notched specimens in the very high cycle regime can be divided into three types: (i) surface initiation, (ii) subsurface with flat facet, and (iii) subsurface with “facet + fine granular area”. The crack initiation mechanism of blunt-notched specimens is discussed in view of the interaction of notch stress gradient distribution and heterogeneous microstructure. Furthermore, the fatigue limit model based on the theory of critical distance (TCD) was modified for the very high cycle regime, and the scatter of the fatigue property of the blunt-notched specimens were well predicted by using this model.