Ultrasonic Fatigue Testing Research Articles

Effect of direct metal laser sintering build parameters on defects and ultrasonic fatigue performance of additively manufactured AlSi10Mg

AbstractThe high cycle fatigue behaviour of additively manufactured AlSi10Mg is evaluated using ultrasonic fatigue as a means to accelerate fatigue testing. Build parameters during the additive manufacturing process are varied, and their effect on defect type, size, and distribution is determined. These defects are further found to influence fatigue behaviour, which is analysed using a Murakami model. Finally, the ultrasonic fatigue test results are interpreted in the context of applied stress intensity factor and an optimized fatigue limit fit. Two different kinds of physical behaviour, representing Murakami dependence and a long crack regime, are found to better correlate the fatigue life behaviour than the Murakami model alone. With this information, we can tailor defect size, type and distribution, within the context of an optimized processing route, to obtain necessary high cycle fatigue properties.

Failure analysis on pre-corroded specimens of Inconel alloy 718, under ultrasonic fatigue tests at room temperature

This work deals with the pre-corrosion effects on the ultrasonic fatigue endurance failure of Inconel alloy 718 (I-718), and on crack initiation and propagation at room temperature. Pre-corrosion was induced using the international standard ESA ECSS-Q-ST-70-37C of the European Space Agency. The base and pre-corroded specimens were tested under the ultrasonic fatigue technique at frequency of 20 kHz, room temperature, without control of environmental humidity and with the load ratio R = -1. The results of ultrasonic fatigue endurance show that at higher levels of stresses, up to 746 MPa, the pre-corroded specimens show a lower fatigue endurance with a factor of 10 approximately, compared with the base material. In addition, the principal trends of pitting at the specimen neck section were reproduced by numerical models in order to investigate the stress concentration factors and its impact in fatigue endurance. Finally, the stress intensity factor in mode I was evaluated numerically at the specimen neck section from crack initiation to fracture and the stress intensity factor range threshold ΔKTH was obtained for the two type of tested specimens, together with the number of cycles in the “Stable Zone of Crack Growth”, for both types of specimens.

Comparison of Very High Cycle Fatigue Properties of 18CrNiMo7-6 Steel after Carburizing and Pseudo-carburizing

The very high cycle fatigue (VHCF) properties of case hardening steel 18CrNiMo7-6 after carburizing and pseudo-carburizing have been investigated by means of ultrasonic fatigue tests. Results showed that the pseudo-carburized specimens with the hardness of 442 HV did not show VHCF fracture, and fatigue fracture did not occur even after 109 cycles when the applied stress amplitude was lower than the high cycle fatigue limit of 650 MPa obtained at 107 cycles. When carburized thoroughly with the hardness of 700 HV, the experimental steel showed VHCF phenomenon, and it might fracture even at nearly 109 cycles when the stress amplitude was about 600 MPa. Observations of the fracture surfaces of the fatigue specimens showed that after carburizing, the fatigue crack initiation sites changed from martensite matrix to nonmetallic inclusions, forming granular bright facet (GBF) and fish-eye before final abrupt fracture. The stress intensity factor at GBF area, which was calculated to be 3.5 MPa m1/2, could be used as a critical value for VHCF fracture for the carburized specimens of the experimental steel. The VHCF fracture of the carburized specimens of the experimental steel could be related to the high hardness and large amount of retained austenite as a result of carburizing.

On the Influence of Control Type and Strain Rate on the Lifetime of 50CrMo4

In this study, we investigate the influence of control type and strain rate on the lifetime of specimens manufactured from 50CrMo4. This influence is described by a strain rate dependent method that uses cyclic stress strain curves to correct displacement-controlled cyclic test results. The objective of this correction is to eliminate the stress related differences between displacement-controlled cyclic test results and force-controlled cyclic test results. The method is applied to the results of ultrasonic fatigue tests of six different combinations of heat treatment, specimen geometry (notch factor) and atmosphere. In a statistical analysis, the corrected results show an improved agreement with test results obtained on conventional fatigue testing equipment with similar specimens: the standard deviation in combined data sets is significantly reduced (p = 4.1%). We discuss the literature on intrinsic and extrinsic strain rate effects in carbon steels.

On the Influence of Control Type and Strain Rate on the Lifetime of 50CrMo4

In this study, we investigate the influence of control type and strain rate on the lifetime of specimens manufactured from 50CrMo4. This influence is described by a strain rate dependent method that uses cyclic stress strain curves to correct displacement-controlled cyclic test results. The objective of this correction is to eliminate the stress related differences between displacement-controlled cyclic test results and force-controlled cyclic test results. The method is applied to the results of ultrasonic fatigue tests of six different combinations of heat treatment, specimen geometry (notch factor) and atmosphere. In a statistical analysis, the corrected results show an improved agreement with test results obtained on conventional fatigue testing equipment with similar specimens: the standard deviation in combined data sets is significantly reduced (p = 4.1%). We discuss the literature on intrinsic and extrinsic strain rate effects in carbon steels.

An Ultra-High Frequency Vibration-Based Fatigue Test and Its Comparative Study of a Titanium Alloy in the VHCF Regime

This paper proposes an ultra-high frequency (UHF) fatigue test of a titanium alloy TA11 based on electrodynamic shaker in order to develop a feasible testing method in the VHCF regime. Firstly, a type of UHF fatigue specimen is designed to make its actual testing frequency reach as high as 1756 Hz. Then the influences of the loading frequency and loading types on the testing results are considered separately, and a series of comparative fatigue tests are hence conducted. The results show the testing data from the present UHF fatigue specimen agree well with those from the conventional vibration fatigue specimen with the loading frequency of 240 Hz. Furthermore, the present UHF testing data show good consistency with those from the axial-loading fatigue and rotating bending fatigue tests. But the obtained fatigue life from ultrasonic fatigue test with the loading frequency of 20 kHz is significantly higher than all other fatigue test results. Thus the proposed ultra-high frequency vibration-based fatigue test shows a balance of high efficiency and similarity with the conventional testing results.

Fatigue endurance limit and crack front evolution in metallic glass

Fatigue endurance limit is a threshold stress-amplitude under which a solid subjected to a cyclic loading can sustain infinite life. Such a limit has been confirmed in ferrous materials but remains controversial in many advanced new materials, including bulk metallic glasses with superb strength. By using a combination of ultrasonic fatigue (USF) test and conventional fatigue test, we obtain the stress amplitude vs. loading cycle curve of the Vitreloy 1TM metallic glass, with loading cycles up to 109. There is a clear fatigue endurance limit (FEL) which is about 320 MPa, and is 17% of the strength of Vitreloy 1TM. The residual strength of survivals after 109 cycles is nearly the same as those of intact samples. We demonstrate that all fatigue cracks initiate from spherical pores which are inevitable and intrinsic amid casting and fast cooling, and the fatigue crack front resembles an ellipse arc till final rapid rapture of all tested samples. The size and position of a pore can be utilized to extract the fatigue initiation threshold K0th, and the elliptical fatigue crack front is used to obtain the mode I fracture toughness KIC, which are K0th=2.0±0.5MPam and KIC=18-20MPam for Vitreloy 1TM, respectively. The fatigue endurance limit and its intrinsic origin of Vitreloy 1TM pave the way to understand fatigue in other metallic glasses.

Influence of processing parameters of selective laser melting on high‐cycle and very‐high‐cycle fatigue behaviour of Ti‐6Al‐4V

AbstractOrthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti‐6Al‐4V were fabricated. The results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high‐cycle fatigue (HCF) and very‐high‐cycle fatigue (VHCF) regimes. The S‐N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Then, the tests on the validation group were performed to verify the optimal combination of SLM processing parameters. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack‐of‐fusion defects in the cases of surface and internal crack initiation.

Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy

To investigate the influence of shot peening (SP) on very high cycle fatigue (VHCF) performance of 2024-T351, the specimens with three surface conditions were performed under ultrasonic fatigue tests: mechanicallypolished without peening (NP), ceramic shot peening (SP1), steel and glass mixed shot peening (SP2). The roughness, microhardness, residual stress, fractography measurement and scanning electron microscopy (SEM) were applied before fatigue test to characterize the effective layer induced by the peening treatment. For the failed specimens, the fracture surface were analysed using SEM to study the mechanisms of fatigue crack propagation. In addition, the fatigue life curve in ultra-high cycle region continuously decreased in the three series of specimens. However, the experimental results revealed that fatigue strength improvement resulting from shot peening treatment was negligible in very high cycle regime. Furthermore, the stress intensity factor for the surface crack initiation (SCI) and interior crack initiation (ICI) was discussed based on quantitative analysis on the fracture surface. The average values of ΔKfish-eye for NP, SP1 and SP2 specimens are about 2.22, 1.48 and 1.61 MPa · m1/2, respectively.

Recent Advances in Very High Cycle Fatigue Behavior of Metals and Alloys—A Review

We reviewed the research and developments in the field of fatigue failure, focusing on very-high cycle fatigue (VHCF) of metals, alloys, and steels. We also discussed ultrasonic fatigue testing, historical relevance, major testing principles, and equipment. The VHCF behavior of Al, Mg, Ni, Ti, and various types of steels were analyzed. Furthermore, we highlighted the major defects, crack initiation sites, fatigue models, and simulation studies to understand the crack development in VHCF regimes. Finally, we reviewed the details regarding various issues and challenges in the field of VHCF for engineering metals and identified future directions in this area.

Novel method for in situ damage monitoring during ultrasonic fatigue testing by the advanced acoustic emission technique

Ultrasonic fatigue testing (USFT) is an effective method for the rapid characterisation of the high cycle fatigue properties of structural materials. However, the process of initiation and progression of fatigue damage remains uncertain in this way of testing due to the limitations of existing measuring techniques. The acoustic emission (AE) method was developed in the present work to pave a new way to monitor the fatigue process during USFT. The proposed new methodology revealed the AE activity related to fatigue damage, allowing to distinguish between surface and internal fatigue crack initiation and to follow the development of fatigue damage.

Effects of Geometric Parameters and Axial Tension on Vibration Characteristics

The vibration frequency of the aero-engine blade is affected by its own mass characteristics and steady-state centrifugal force, which can be used to adjust the blade resonance frequency to avoid the exciting source and prolong the fatigue life of the blade, and can also be used to approach the exciting source frequency and guide the design of ultrasonic fatigue specimens. Through the combination of mechanical analysis and finite element simulation, the influence mechanism of geometric parameters and axial tension on modal frequency is analyzed from the point of view of vibration and energy, the influence law is obtained, and the optimal design of two kinds of three-point bending specimens is completed. The resonant frequency error calculated by finite element method is controlled within ±5Hz, and the frequency is 20.03 kHz verified by ultrasonic fatigue test, which meets the accuracy requirements, which provides a reliable theoretical support for the design of simulated blade ultrasonic fatigue specimens.

Ultrasonic fatigue testing of cast steel G42CrMo4 at elevated temperatures

AbstractThe fatigue life of cast steel G42CrMo4 in two different heat treatment conditions was investigated at room temperature (RT), 473 K and 773 K up to the range of very high cycle fatigue (VHCF), that is, 109 cycles. The fatigue life is determined by casting defects, the hardness of the steel matrix and by temperature. Fatigue life data were discussed in correlation with crack‐initiating defects analysed on fracture surfaces. The SN curves obtained at RT and at 473 K show a large scatter. However, the SN curve at 773 K exhibits a larger slope parameter and a significantly reduced scatter. It is shown that the fatigue behaviour of the cast steel G42CrMo4 changes from 473 to 773 K in the range of VHCF. The fatigue lives of the specimens tested at 773 K were described with a crack growth model.

Enhanced extra-long life fatigue resistance of a bimodal titanium alloy by laser shock peening

Ultrasonic fatigue tests were performed on notched specimens to investigate high and very high cycle fatigue behaviors in the laser shock peened region of a compressor blade titanium alloy (Ti–8Al–1Mo–1V). The plastic deformation and compressive residual stress, induced by the laser shock peening (LSP), present gradient distributions along the depth direction. High-angle grain boundaries (HAGB, >15°) are increased by deformation twins, which are introduced into primary α phases during the plastic deformation. The enhancement in fatigue resistance is ascribed to the comprehensive retardation effects of the compressive residual stress, high-density dislocations, and HAGBs.

Ultrasonic and Conventional Fatigue Endurance of Aeronautical Aluminum Alloy 7075-T6, with Artificial and Induced Pre-Corrosion

Ultrasonic and conventional fatigue tests were carried out on the AISI-SAE AA7075-T6 aluminum alloy, in order to evaluate the effect of artificial and induced pre-corrosion. Artificial pre-corrosion was obtained by two hemispherical pitting holes of 500-μm diameter at the specimen neck section, machined following the longitudinal or transverse direction of the testing specimen. Induced pre-corrosion was achieved using the international standard ESA ECSS-Q-ST-70-37C of the European Space Agency. Specimens were tested under ultrasonic fatigue technique at frequency of 20 kHz and under conventional fatigue at frequency of 20 Hz. The two applied load ratios were: R = −1 in ultrasonic fatigue tests and R = 0.1 in conventional fatigue tests. The main results were the effects of artificial and induced pre-corrosion on the fatigue endurance, together with the surface roughness modification after the conventional fatigue tests. Crack initiation and propagation were analyzed and numeric models were constructed to investigate the stress concentration associated with pre-corrosion pits, together with the evaluation of the stress intensity factor in mode I from crack initiation to fracture. Finally, the stress intensity factor range threshold ΔKTH was obtained for the base material and specimens with two hemispherical pits in transverse direction.

Fatigue Strength and Fracture Mechanisms in the Very‐High‐Cycle‐Fatigue Regime of Automotive Steels

Very‐high‐cycle‐fatigue (VHCF) strength properties are of interest to several technical applications assessed globally at different laboratories with long‐life fatigue testing capabilities. Also, VHCF failure mechanisms are a scientific topic with remaining open research questions. Herein, three automotive bar grade steels are studied with respect to VHCF strength and initiation mechanisms. A microalloyed ferritic–pearlitic steel (38MnSiV5, 870 MPa tensile strength), a quenched and tempered martensitic steel (50CrV4, 1410 MPa tensile strength), and a carburizing steel (16MnCr5, 1180 MPa core structure tensile strength) are studied to reveal characteristics regarding initiation and VHCF failure mechanisms. A 20 kHz ultrasonic fatigue testing instrument is used to obtain fatigue lives up to and above 109 load cycles in uniaxial loading. Hour‐glass specimens, smooth or notched, are tested at R = −1 and R = 0.1. Fatigue strength and stress life (SN)‐diagram data are achieved, and crack initiation and growth mechanisms are studied using primarily field‐emission gun–scanning electron microscopy (FEG–SEM). Fatigue strengths are explained by a modified life‐dependent Murakami‐expression, the Haigh diagram, and notch sensitivity. Interior and surface crack initiations by surface defects, triple points, and inclusions are found. The fine granular area (FGA) to fish‐eye crack growth transition conditions are explored and schematic descriptions are given.

A method for the strain rate dependent correction for control type of fatigue tests

In this paper, we investigate the influence of the control type of cyclic tests on the lifetime of the specimens tested. We present a strain rate dependent method to compare results of displacement controlled fatigue tests to results of force controlled fatigue tests. This method is successfully used to explain longer lifetimes observed in ultrasonic fatigue tests compared to conventional fatigue tests on specimens manufactured from quenched and tempered SAE 4150. This method may also be used to normalize displacement controlled fatigue tests in conventional testing, which allows parallel testing of multiple specimens in a single testing machine.

Review of Multiaxial Testing for Very High Cycle Fatigue: From ‘Conventional’ to Ultrasonic Machines

Fatigue is one of the main causes for in service failure of mechanical components and structures. With the development of new materials, such as high strength aluminium or titanium alloys with different microstructures from steels, materials no longer have a fatigue limit in the classical sense, where it was accepted that they would have ‘infinite life’ from 10 million (107) cycles. The emergence of new materials used in critical mechanical parts, including parts obtained from metal additive manufacturing (AM), the need for weight reduction and the ambition to travel greater distances in shorter periods of time, have brought many challenges to design engineers, since they demand predictability of material properties and that they are readily available. Most fatigue testing today still uses uniaxial loads. However, it is generally recognised that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. By combining both Ultrasonic Fatigue Testing with multiaxial testing through Single-Input-Multiple-Output Modal Analysis, the high costs of both equipment and time to conduct experiments have seen a massive improvement. It is presently possible to test materials under multiaxial loading conditions and for a very high number of cycles in a fraction of the time compared to non-ultrasonic fatigue testing methods (days compared to months or years). This work presents the current status of ultrasonic fatigue testing machines working at a frequency of 20 kHz to date, with emphasis on multiaxial fatigue and very high cycle fatigue. Special attention will be put into the performance of multiaxial fatigue tests of classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low cost piezoelectric transducers. Together with the description of the testing machines and associated instrumentation, some experimental results of fatigue tests are presented in order to demonstrate how ultrasonic fatigue testing can be used to determine the behaviour of a steel alloy from a railway wheel at very high cycle fatigue regime when subjected to multiaxial tension/torsion loadings.

On the formation of ridges and burnished debris along internal fatigue crack propagation in 42CrMo4 steel

AbstractA detailed microscopic analysis of fracture surfaces of 42CrMo4‐hardened steel after ultrasonic fatigue testing revealed globular and cylindrical particles located in ridges along the crack propagation direction. Observed particles could be easily taken for non‐metallic inclusions; however, chemical analysis showed that they are of the same composition as the steel matrix. The formation of such round‐shaped debris was found to be a result of cutting out the matrix fragments by ‘en passant’ cracks interaction and their subsequent fretting burnishing. Possible correlation of the parameters of ridges and debris with cyclic plastic zone width and martensite structure is discussed.

Structure characterization within the vicinity of the fine granular area by synchrotron radiation nano‐CT

AbstractThe three‐dimensional (3D) microstructures within the vicinity of the fine granular area (FGA) were investigated with synchrotron radiation nano‐computed tomography (CT). In this investigation, the very high cycle fatigue test of a high‐strength steel was carried out with an ultrasonic fatigue testing machine. Then, focused ion beam cutting was used to cut a part near the surface of the FGA. Finally, the 3D structure of the sample was obtained with the nano‐CT imaging. The results indicated that the base material was composed of various subgrains that are tens to hundreds nanometres in size. Through analysis of the slices, small granular areas consist of several subgrains. The statistical analysis of the subgrain size indicated that the subgrain size near the surface of the FGA is similar to that far from the surface of the FGA and near the fracture surface except the FGA. It indicated that the refinement has not been found by nano‐CT.