Ultrasonic Fatigue Testing Research Articles

고장력 강판의 초음파 피로시험

The demand of high cycle fatigue behavior on plate material is increasing because of its various applications. However, the high-cycle fatigue life data of the plate material is very rare compared to the rod material. Thus, in this study, a plate specimen is designed for the ultrasonic fatigue test because it is time efficient as compared to the conventional fatigue test. To apply the ultrasonic fatigue test, the specimen design is required to resonate at 20 kHz. Therefore, the dynamic elastic modulus was determined by measuring the resonance frequency with a piezoelectric element and laser doppler vibrometer (LDV). As a result, the plate specimen is designed and demonstrated using the ultrasonic fatigue testing machine. The ultrasonic fatigue test results were compared with the hydraulic fatigue test results.

Fatigue Strength, Crack Initiation, and Localized Plastic Fatigue Damage in VHCF of Duplex Stainless Steels

The fatigue strength of two‐duplex stainless steel grades, 2304 SRG and LDX 2101, with austenitic–ferritic microstructure is tested using ultrasonic fatigue testing equipment operating at 20 kHz. The testing is conducted in tension‐compression mode with the load ratio R = −1. The fatigue strength is evaluated at 107, 108, and 109 load cycles and the estimates of fatigue strength are higher for the LDX 2101 grade. The fatigue crack initiation mechanisms are analyzed using a scanning electron microscope. The fatigue cracks, in all cases, appear to initiate due to accumulation of plastic fatigue damage at the surface. In the 2304 SRG grade, accumulation of fatigue damage occurs at the external surface of fatigued specimens in the form of extrusions at the grain/phase boundaries and in the form of individual slip lines in the austenite phase. Meanwhile, in the LDX 2101 grade accumulation of plastic fatigue damage in the form of extrusions and intrusions occurs mainly within the ferrite grain. When the crack is microstructurally short, the crack growth appears to be crystallographic in nature and the crack appears to change its direction propagating from one grain into another.

Recent developments in ultrasonic fatigue

AbstractThe recently increased interest in very high cycle fatigue properties of materials has led to extended use and further development of the ultrasonic fatigue testing technique. Specimens are stimulated to resonance vibrations at ultrasonic frequency, where the high frequency allows collecting lifetime data of up to 1010cycles and measuring crack propagation rates down to 10−12 m per cycle within reasonable testing times. New capabilities and methods of ultrasonic testing and outstanding results obtained since the year 1999 are reviewed. Ultrasonic tests at load ratios other than R = −1, variable amplitude tests, cyclic torsion tests and methods forin situobservation of fatigue damage are described. Advances in testing at very high temperatures or in corrosive environments and experiments with other than bulk metallic materials are summarized. Fundamental studies with copper and duplex steel became possible and allowed new insights into the process of very high cycle fatigue damage. Higher cyclic strength of mild steels measured at ultrasonic frequency because of plastic strain rate effects are described. High‐strength steels and high‐alloy steels are less prone to frequency influences. Environmental effects that can lead to prolonged lifetimes in some aluminium alloys and possible frequency effects in titanium and nickel and their alloys are reviewed.

Very high cycle fatigue properties of Cr–Mo low alloy steel containing V-rich MC type carbides

The present study was attempted to investigate the very high cycle fatigue (VHCF) properties of Cr–Mo low alloy steel containing V-rich MC carbides by using ultrasonic fatigue testing machine. All the specimens were quenched and tempered at 500°C and 600°C and part of them were electrochemical hydrogen charged. Mainly owing to the precipitation of fine V-rich MC particles at 600°C, the introduced hydrogen content of the 600°C tempered specimen is much higher than that of the 500°C tempered specimen. Both the hydrogen-charged and uncharged specimens revealed internal fractures in the long-life region and no notable fracture surface difference were observed. Both fatigue strength and fatigue strength ratio increase with increasing tempering temperature from 500°C to 600°C for both the hydrogen-charged and uncharged specimens, though the 600°C tempered specimen possessed higher hydrogen content. Increasing hydrogen content was definitely detrimental to the fatigue properties. However, compared to other high strength steels investigated previously, the detrimental effect of hydrogen on fatigue properties of the tested steel was comparatively small, which was ascribed mainly to the presence of fine V-rich MC carbides acting as hydrogen trapping sites. In is concluded that appropriate controlling of fine V-rich MC carbides is a promising way to alleviate the detrimental effect of hydrogen on VHCF properties of high strength steels.

The potential significance of microalloying with niobium in governing very high cycle fatigue behavior of bainite/martensite multiphase steels

We elucidate here the effect of microalloying with niobium (Nb) on very high cycle fatigue (VHCF) behavior in high-strength C–Mn–Si–Cr bainite/martensite (B/M) multiphase steels studied through ultrasonic fatigue testing. The tensile strength (Rm) and fatigue limit strength after 109 cycles (σw9) and in the non-failure condition of the steel microalloyed with Nb were 1640MPa and 900MPa, respectively. Thus, the value of σw9/Rm exceeded in comparison to conventional steels and was approximate 0.55. Three types of failure modes were observed in Nb-bearing steels depending on the surface condition, inclusion, and the matrix microstructure, i.e., surface defect-induced failure mode (S-mode), inclusion-induced failure mode (I-mode), and non-inclusion induced failure mode (N-mode). Only two failure modes were observed in Nb-free steels, the S-mode and the N-mode. The study clearly suggests that Nb had a distinct effect on the VHCF properties of B/M steels. The VHCF limit of Nb-bearing steel was enhanced by 200MPa because of refinement of the microstructure and pinning of dislocations by randomly distributed nanometer-sized Nb(C, N) precipitates. It is underscored that microalloying with Nb is a potential approach to enhance VHCF properties in advanced high-strength steels.

Crack initiation and propagation on the polymeric material ABS (Acrylonitrile Butadiene Styrene), under ultrasonic fatigue testing

Crack initiation and propagation have been investigated on the polymeric material ABS (Acrylonitrile Butadiene Styrene), under ultrasonic fatigue testing. Three controlled actions were implemented in order to carry out fatigue tests at very high frequency on this material of low thermal conductivity, they are: a) The applying load was low to limit heat dissipation at the specimen neck section, b) The dimensions of testing specimen were small (but fitting the resonance condition), in order to restraint the temperature gradient at the specimen narrow section, c) Temperature at the specimen neck section was restrained by immersion in water or oil during ultrasonic fatigue testing. Experimental results are discussed on the basis of thermo-mechanical behaviour: the tail phenomenon at the initial stage of fatigue, initial shear yielding deformation, crazed development on the later stage, plastic strain on the fracture surface and the transition from low to high crack growth rate. In addition, a numerical analysis is developed to evaluate the J integral of energy dissipation and the stress intensity factor K, with the crack length

Multiscale study of fracture in aluminum-magnesium alloy under fatigue and dynamic loading

In this paper we investigated the influence of consecutive dynamic and gigacycle fatigue loads on the lifetime of aluminum-magnesium alloy AlMg6. Preloading of samples was achieved during dynamic tensile tests in the split-Hopkinson bar device. Fatigue tests were conducted on Shimadzu USF-2000 ultrasonic fatigue testing machine. This machine provides 109-1010 loading cycles with the amplitude from 1 to several dozens of microns and frequency of 20 kHz, which reduces dramatically the testing time in the comparison to the classical fatigue testing machines. The New-View 5010 interferometer–profiler of high structural resolution (resolution of 0.1 nm) was used for qualitative fracture surface analysis, which provided the data allowing us to find correlation between mechanical properties and scale-invariant characteristics of damage induced roughness formed under dynamic and gigacycle fatigue loading conditions. Original form of the kinetic equation was proposed, which links the rate of the fatigue crack growth and the stress intensity factor using the scale invariant parameters of fracture surface roughness. The scale invariance characterizes the correlated behavior of multiscale damage provides the link of crack growth kinetics and the power exponent of the modified Paris law.

Ultra-High Cycle Fatigue Behavior of DZ125 Superalloy Used in Turbine Blades

The high temperature ultra-high cycle fatigue (UHCF) behaviors of DZ125 superalloy used in aero-engine turbine blades were systematically studied. The results show that the fatigue fracture still occurs above 108 at the frequency of 20kHz, R=-1 and 700°C. There is a negligible frequency effect for the DZ125 superalloy, therefore, it is proposed that the ultrasonic fatigue testing could be expected as an accelerated fatigue testing method. Fatigue cracks originate from the subsurface of the specimens, where have no metallurgy defects or “fish eye” character. The crystal orientation change of the alloy is very little after fatigue.The maximum value changed for the elastic modulus of the alloy is about 30GPa after fatigue compared with that before fatigue.

Effect of Shot Peening on the Long Life Fatigue Properties of Ti6Al4V with Different Heat Treatment

In order to investigate the effect of shot peening on the long life fatigue properties of Ti6Al4V (TC4), ultrasonic fatigue tests were performed with a frequency of 20 kHz. According to different heat treatment, two groups of specimens were surface modified by shot peening with an Almen intensity of 0.10 mmA~ 0.15 mmA and an overlapping rate of 100%. One group was stress-relief annealed at 650 °C, and the other was then treated with solution-aging. With the shot peening, both the depth of the residual stress layer and the maximum compressive residual stress are increased. Surface hardness is also increased. The fatigue strength is increased, but enhancement is no more than 5% because of the increase of surface roughness. Noticeably, all of the TC4 specimens after shot peening show inner crack initiations. Oxide inclusion is always the core of cracks, and the small crack propagation in crack initiation area acts as the slip of α-phase.

Effect of Temperature and Loading Frequency on the Fatigue Behavior of Ti-17

A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of Ti-17. Ultrasonic (20 kHz) fatigue tests were performed at room temperature, 200°C and 350°C, respectively. The dynamic Young’s modulus and fatigue endurance limit decrease with increasing temperature linearly. Rotating bending (50 Hz) tests were performed to evaluate the influence of loading frequency at room temperature, 200°C and 350°C, respectively. There is an obviously loading frequency effect at elevated temperature, although no loading frequency effect at room temperature.

VHCF Behavior and Word Hardening of a Ferritic-Martensitic Steel at High Mean Stresses

Low-pressure steam turbine blades undergo VHCF-loadings induced by inhomogenous flow behind the vanes resulting in excitation frequencies of ≈ 2 kHz for rotational speeds of 50 Hz and a typical number of stator vanes of ≈ 60. The VHCF loading is superimposed by considerable mean stresses caused by centrifugal forces. In the present study, the VHCF-behavior of the ferritic-martensitic turbine blade steel X10CrNiMoV12-2-2 is investigated using an ultrasonic fatigue testing system up to cycle numbers of 5∙109 at stress ratios from R = -1 up to 0.7, i.e. up to very high mean stresses. Generally, crack initiation changes from the surface to internal inclusions at fatigue lives around 4∙107. The transition between fatigue failure and run-outs is shifted to higher lifetime with increasing R, and fine grained areas (FGAs) at the crack initiation sites only occur at R < -0.1. However, the fracture mechanics approach proposed by Murakami consistently describes the lifetime behavior for all load ratios over 4 decades of lifetime. At R up from 0.5 considerable cyclic creep occurs, even for lifetimes above 108 cycles, resulting in cyclic hardening which was proved by microhardness measurements at longitudinal sections. This effect at least partially explains the high maximum stresses close to the tensile strength of the material occurring in the VHCF regime at load ratios ≥ 0.5.

On the Research and Application of Ultrasonic Fatigue Testing Technology

In the present paper, the characteristic and the application of ultrasonic fatigue testing technology is illuminated. The main problems i.e. the size effect, the thermal effect and frequency effect due to the high frequency are discussed. The results show that: 1. As there is a size effect, a uniform specimen size should be adopted in the very-high cycle fatigue standard and for special designed specimen the designed size should be noted along with the fatigue test results; 2.the heat generation attributes mainly to the low yield strength and the high applied stress, as a result, ultrasonic fatigue testing technology can be mainly applied to the ultra-high cycle fatigue test of high-strength steel; 3.the frequency effect is related to the crystal structure of metallic materials, however, ultrasonic fatigue testing technology can be applied to conduct the comparison of the fatigue properties of the same steel grade before and after the smelting process.

Through thickness property variations in friction stir welded AA6061 joint fatigued in very high cycle fatigue regime

Ultrasonic fatigue tests were performed on friction stir welded AA6061 joint to investigate very high cycle fatigue (VHCF) behaviors. As a result, almost all the fatigue cracks are initiated from local plastic slip markings around the boundary between thermo-mechanically affected zone and heat affected zone. The fatigue strength decreases from the top to root of the welded joint, owing to the variation of plastic deformation history and temperature distribution through the thickness. In fractography, the fatigue crack initiation site is surrounded by a semicircular flat zone, of which the formation in VHCF regime accounts for more than 98% of the total fatigue life.

A new piezoelectric fatigue testing machine in pure torsion for ultrasonic gigacycle fatigue tests: application to forged and extruded titanium alloys

AbstractThis article briefly discusses the history of the development of ultrasonic fatigue testing methods, with respect to industrial needs. The development of ultrasonic techniques and the progress made in the computer industry have led to improvements in ultrasonic testing techniques. It has been shown, that existing ultrasonic testing systems have limitations that lead to the need for a new ultrasonic fatigue testing machine operating in pure torsion. This paper introduces the development of a new piezoelectric machine working in the continuous regime (i.e. without pulse‐pause). This machine has been used to investigate the Very High Cycle Fatigue (VHCF) properties of the VT3‐1 alpha beta aeronautical titanium alloy (which is similar to Ti–6Al–4V) produced by two manufacturing processes: forging and extrusion. The extruded titanium alloy has a higher VHCF strength in torsion compared to the forged one. Despite the maximum shear stress occurring at the specimen surface under torsion loading, internal fatigue crack initiation can be observed in both the forged and extruded alloys.

Gigacycle fatigue behaviour of austenitic stainless steels used for mercury target vessels

A mercury enclosure vessel for the pulsed spallation neutron source manufactured from a type 316L austenitic stainless steel, a so-called target vessel, suffers the cyclic loading caused by the proton beam induced pressure waves. A design criteria of the JSNS target vessel which is defined based on the irradiation damage is 2500 h at 1 MW with a repetition rate of 25 Hz, that is, the target vessel suffers approximately 109 cyclic loading while in operation. Furthermore, strain rate of the beam window of the target vessel reaches 50 s−1 at the maximum, which is much higher than that of the conventional fatigue. Gigacycle fatigue strength up to 109 cycles for solution annealed 316L (SA) and cold-worked 316L (CW) were investigated through the ultrasonic fatigue tests. Fatigue tests were performed under room temperature and 250 °C which is the maximum temperature evaluated at the beam window in order to investigate the effect of temperature on fatigue strength of SA and CW 316L. The results showed that the fatigue strength at 250 °C is clearly reduced in comparison with room temperature, regardless of cold work level. In addition, residual strength and microhardness of the fatigue tested specimen were measured to investigate the change in mechanical properties by cyclic loading. Cyclic hardening was observed in both the SA and CW 316L, and cyclic softening was observed in the initial stage of cyclic loading in CW 316L. Furthermore, abrupt temperature rising just before fatigue failure was observed regardless of testing conditions.

Application of full-surface view in situ thermography measurements during ultrasonic fatigue of cast steel G42CrMo4

In the present investigation a full-surface view in situ thermography method is adapted to an ultrasonic fatigue testing system. Full-surface view in situ thermography measurements were successfully performed in the high cycle fatigue and in the very high cycle fatigue regime on cast steel G42CrMo4in the quenched and tempered state. The method enables the monitoring of the entire cylindrical specimen circumference during fatigue testing by infrared temperature field measurements with one thermocamera and two mirrors. Moreover, by correlating fractography and thermography the precise determination of the location of the crack initiation site and the time of final crack growth is possible. The technique is applied to study crack initiation at non-metallic inclusions in the investigated cast steel specimens. Moreover, the effect of a novel carbon-bonded metal melt filter coated with a functionalized spinel (MgAl2O4) coating is evaluated by ultrasonic fatigue testing in combination with the full-surface view in situ thermography technique and subsequent scanning electron microscopy.

Fatigue crack initiation behaviors throughout friction stir welded joints in AA7075-T6 in ultrasonic fatigue

This paper presents the results of experimental investigation on fatigue behaviors of friction stir welded joints in AA7075-T6 with ultrasonic fatigue test system (20kHz). Two kinds of particles, Fe-rich intermetallic compounds and Mg2Si-based particles, governed the fatigue crack initiation. The plastic deformation and recrystallization during welding process led to the changes in particle size and micro crack occurrence between thermo-mechanically affected zone (TMAZ) and nugget zone (NZ). Therefore, the fatigue crack initiation sites leaned to be located at the TMAZ in short fatigue life, or at the NZ in very high cycle fatigue regime.

Gigacycle Fatigue Behaviour of Sintered WC-Co Hardmetals Investigated by Ultrasonic Resonance Testing

Hardmetals, manufactured from powders by pressing and sintering, are the most important tool materials in service today. In many applications, such as milling or percussion drilling, they are subjected to fatigue with considerable loading cycle numbers. In the present study, the fatigue behaviour of hardmetals in push-pull loading was investigated up to Nmax= 1010using ultrasonic resonance fatigue testing. It showed that with all hardmetal grades investigated there is no fatigue “limit”, i.e. a horizontal branch of the S-N curve, but a consistent drop of the curve up to maximum N. Crack initiation was found to occur predominantly microstructure-controlled, as compared to defect controlled as typical for powder metallurgy tool steels.Keywords: gigacycle fatigue, WC-Co hardmetals, ultrasonic fatigue testing, fatigue limit

Cyclic Deformation Behaviour of Cu Multilayered Films on Si Substrates

In the present study the high cycle fatigue behaviour of multilayered Cu films with different thicknesses and microstructure on silicon substrate was investigated. An ultrasonic resonance fatigue testing systems was used to study the isothermal mechanical fatigue behaviour of the Cu multilayers at room temperature and 150°C. Investigations of fatigue damage on the surface of the samples showed distinct slip band formation on preferentially oriented grains of the Cu metallization. The degree of fatigue damage, which could be related to the thickness, grain size and orientation of the Cu films, was evaluated by determination of the slip band density as a function of loading cycles. It was found that with increasing film thickness and grain size the density of deformed area strongly increases, with the majority of surface deformation occurring in the grains with ⟨111⟩ orientation with respect to the film surface. Furthermore increasing the testing temperature resulted in a significant degradation ofthe multilayered film stack due to a considerable increase in the degree of plastic deformation of theCu metallization layer.

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior (107 – 109 cycles) of 304L austenitic stainless steel was studied with ultrasonic fatigue testing system (20 kHz). The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation. It was found that micro-plasticity (slip markings) could be observed on the specimen surface even at very low stress amplitudes. The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure. By detailed investigation using a scanning electron microscope and an infrared camera, slip markings appeared at the large grains where the fatigue crack initiation site was located. The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack. Finally, the coupling relationship among the fatigue crack propagation, appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.