Ultrasonic Fatigue Testing Research Articles

Assessment of Ultra-High Cycle Fatigue Behavior of EN-GJL-250 Cast Iron Using Ultrasonic Fatigue Testing Machine

High cycle fatigue comprising up to 107 load cycles has been the subject of many studies, and the behavior of many materials was recorded adequately in this regime. However, many applications involve larger numbers of load cycles during the lifetime of machine components. In this ultra-high cycle regime, other failure mechanisms play, and the concept of a fatigue endurance limit (assumed for materials such as steel) is often an oversimplification of reality. When machine component design demands a high geometrical complexity, cast iron grades become interesting candidate materials. Grey cast iron is known for its low cost, high compressive strength, and good damping properties. However, the ultra-high cycle fatigue behavior of cast iron is poorly documented. The current work focuses on the ultra-high cycle fatigue behavior of EN-GJL-250 (GG25) grey cast iron. An ultrasonic (20 kHz) fatigue testing system is developed and instrumented for testing at ambient temperature, and hourglass shaped cast iron specimens with circular cross-section are designed to be excited in resonance. The temperature of the specimen is measured using a pyrometer and controlled by ventilating air. The displacement at the free end of the specimen (which is linked to stress at the reduced section) is measured using an accurate laser sensor, whose analysis requires careful test preparation and data analysis. The high resonance frequency allowed to characterize the S-N behavior of the cast iron of interest within a matter of days, and repeat the tests to quantify the natural scatter in fatigue resistance. The obtained results will be used in a research project that aims to optimize the design of machine components with respect to weight and durability.

Influence of cycling frequency and testing volume on the VHCF properties of 18Ni maraging steel

Fatigue properties of nitrided 18Ni maraging steel sheets under cyclic tension loading were investigated at cycling frequency 95 Hz by using a resonance tester and at 20 kHz using ultrasonic fatigue testing equipment. High and very high cycle fatigue (VHCF) tests have been performed with two specimen shapes with distinctly different stressed volumes. Fatigue cracks started at internal aluminate or Zr(N,C) inclusions. Inclusions are considered as initial cracks, and fatigue lifetimes as the numbers of cycles necessary to propagate the cracks to fracture. Specimen size effect is explained with volume influence on statistical distribution of inclusion sizes. Comparable mean lifetimes for the same specimen shape and similar dependence of lifetimes on stress intensity factor range were found at 95 Hz and 20 kHz, which indicates the absence of frequency effects in maraging steel.

Cruciform specimens' experimental analysis in ultrasonic fatigue testing

AbstractIn this paper, two special aluminium cruciform specimens are designed and tested in an ultrasonic fatigue machine. They were designed based on Single‐Input‐Multiple‐Output (SIMO) modal analysis to induce in‐plane biaxial stress combinations (in‐phase tension‐tension [T‐T] and out‐of‐phase compression‐tension [C‐T]) when at resonance at 20 kHz. The geometries were subjected to both numerical analysis and experimental testing to understand if they can indeed create the intended biaxial state of stresses. Both numerical and experimental results showed an impact of nearby resonant modes of noninterest on the correct functioning of the specimens, especially regarding the T‐T specimen where a large deviation from the mode of interest was measured. This means that future work includes re‐designing T‐T specimens taking into account these mode shapes. Only out‐of‐phase specimens demonstrated to work properly and tests until failure were conducted. The first failure results showed to be consistent with literature when out‐of‐phase biaxial stress is applied cyclically.

Investigation on very high cycle fatigue of PA66-GF30 GFRP based on fiber orientation

The purpose of this study is to evaluate the effects of glass fiber (GF) of PA66-GF30 orientation on very high cycle fatigue (HVCF) life and also to develop an optimized test apparatus such as measurement of dynamic elastic modulus and design of test specimens. Dynamic elastic modulus, tensile strength and fatigue strength tests were conducted at room temperature based on GF orientation (0°, 45°, 90°). Conventional fatigue (3 Hz) and ultrasonic fatigue (20 kHz) tests were performed at the same stress ratio (R = −1). Fracture surface observation showed that GF 0° exhibited a higher fatigue strength in comparison with GF 45 and 90°. Various defects such as voids and microcracks to the load direction around the GF end or debonding in glass fiber reinforced plastic (GFRP) and the slope change in very long life regime of S–N diagram.

Feasibility Study on Application of Synchrotron Radiation μCT Imaging to Alloy Steel for Non-Destructive Inspection of Inclusions

In order to examine the feasibility of applying synchrotron radiation μCT imaging to alloy steels for non-destructive inspection of inclusions for potential origins of internal fatigue damage in the very high cycle region, synchrotron radiation μCT imaging was utilized for repeated non-destructive observation of Cr-Mo steel. An ultrasonic fatigue testing machine was used in aid of the repeated observation. As a result, it was found that the synchrotron radiation μCT imaging with 70 keV was useful for non-destructive observation of inclusions of more than 10 μm, one of which may be an internal fatigue origin. No identifiable damage was observed around every inclusion, and in the base metal, at least up to 70% of fatigue life was observed in the imaging volume.

Very High Cycle Fatigue Behaviour of the 316L Weldment Fabricated By Laser Butt-Welding

In nuclear industry, 316L weldments are widely used in components fabricating, which usually suffer from very-high-cycle fatigue. Therefore, the very high cycle fatigue (VHCF) behaviour of 316L weldment fabricated by laser butt-welding joint were studied using an ultrasonic fatigue testing system. The microscopic examination indicated that an evident pore is observed in the weld seam and the fine columnar-dendrite structure is formed in the central part of the weld seam. Near the weld interface in the weld seam, the weld microstructure shows larger columnar-dendrite. The hardness distribution of the weldment is to coincide with the characteristic of the microstructure. In the central part of the weld seam, the hardness is the highest, and is the lowest in the base metal. Fatigue failure still occurred in the VHCF regime. The fatigue crack initiated from the internal pores caused by laser butt-welding. Although the fatigue strength of weld seam might be decreased due to the pore, the fatigue strength of the weldment was the same as the base metal in this research. That is, the defect with a certain size in the weld seam does not affect the strength of the weldment.

Fatigue behavior of 316 L stainless steel weldment up to very-high–cycle fatigue regime

In nuclear power plant, 316 L weldments are widely used in fabricating the primary coolant pipe and in-core components, which usually suffer from very-high-cycle fatigue. Therefore, in this study, the high-cycle fatigue behaviors of 316 L stainless steel with an argon-arc welding joint were studied up to very high-cycle regime by using an ultrasonic fatigue testing system. Microscopic examination indicated that both the base steel and welding seam showed austenitic equiaxed grains. The grain size and micro-hardness values of the welding seam were comparable to those of the base 316 L steel. And heat affect zone was not evidently observed at the weld joint. Fatigue tests showed that fatigue failure still occurred in 107–109 cycles, indicating that the conventional fatigue limit does not exist for 316 L weldment. The fatigue strengths of 316 L weldments were close to those of the base 316 L steel. Scanning electron microscopic examinations revealed that multiple fatigue cracks initiated at specimen surface in the high cycle and very-high-cycle fatigue regime. A modified Murakami’s model was suggested to take into account the effect of surface roughness and Vickers hardness on fatigue life/strength of 316 L weldment. The fatigue strength predicted using the proposed model showed good agreement with the experimental results.

Effect of Artificial Defects on the Very High Cycle Fatigue Behavior of 316L Stainless Steel

Widely used for structural materials in nuclear engineering, 316L austenitic stainless steel undergoes very high cycle fatigue (VHCF) throughout its service life. Since defects caused by service conditions are unavoidable in many engineering components during service life, the effects should be properly understood. In the present study, the effect of surface defects on the VHCF behavior were investigated on solution annealed (SA) and cold-worked (CW) 316L. Surface defects were artificially created using indentation. The VHCF test was conducted using an ultrasonic fatigue testing system. The results showed that the fatigue crack initiation was independent of the indent with the applied range of depth in this research. Furthermore, the critical depth of the indent was evaluated based on an empirical formula (Murakami’s model). In the case of SA 316L, the VHCF strength was not affected when the indent depth was less than 40 μm, which is consistent with the value obtained from the empirical formula. In the case of 20% CW 316L, the VHCF strength was not affected when the indent depth was less than 80 μm. The experimental results, i.e., the critical depth of the indent, were much larger than the results obtained from the empirical formula, and might have been caused by the plastic deformation, residual stress, and probable deformation-induced martensite transition around the indent.

Influence of rolling texture on near-threshold crack extension behavior in aluminum alloy EN AW-6082

Abstract The near-threshold crack extension mechanisms of long cracks in a commercial aluminum alloy in peak-aged and overaged condition were evaluated at a stress ratio of R = −1. Tests were performed both in a conventional resonant testing machine, using flat dog-bone specimens with through-wall cracks and on an ultrasonic fatigue testing system using hourglass-shaped samples with surface cracks. After introducing the initial crack, the crack was propagated at a nominally constant Kmax with values in the range of the initially determined threshold. The primary precipitates were found to act as microstructural barriers causing a local pinning of the crack front independent of the testing procedure and hence keeping the crack from extending continuously. This effect depends on the orientation of the initial crack with respect to the rolling texture which, in turn, determines the spatial distribution of the primary precipitates and the grain morphology. Overaging had a tendency to enhance the pinning potential of the primary precipitates.

Very high-cycle fatigue properties and microstructural damage mechanisms of selective laser melted AlSi10Mg alloy

The influence of cooling rates in selective laser melted AlSi10Mg on fatigue properties is studied. The failure mechanisms during quasistatic and fatigue loading were analyzed using electron microscopy, X-ray computed tomography and ultrasonic fatigue testing systems. It was found that even when densification mechanism was not significantly enhanced, the morphology of defects was adopting more regularly spherical shape which is less critical under structural loading. The controlled cooling introduced microstructural homogeneity and further property enhancement of the microstructure which contributed to higher quasistatic and fatigue strength in this study and compared to the literature on the same alloy. Elimination of semi-coherent Si agglomerates on the melt pool boundaries impeded crack propagation and forced it to adjust to a perpendicular orientation to the columnar dendrites formed instead. The strengthening mechanism was shown effective in quasistatic, servohydraulic and ultrasonic fatigue tests in very high-cycle fatigue. The influence of testing at ultrasonic frequencies was analyzed through the fatigue data and influence was not significant. Analysis of the interrupted fatigue specimens in the X-ray computed tomography revealed a pore-crack interaction mechanism which was hypothesized earlier in the literature. Although impedance of crack propagation and theoretical drop of stress intensity factors at crack tips was expected, it was implied that recommence of crack propagation and the end life of the specimen will still be dependent on the local strength of the microstructure in the vicinity of the pore.

Granite stone subjected to ultrasonic fatigue tests under three point bending modality

Ultrasonic fatigue tests are carried out under the modality of three points bending on the granite stone. The testing specimens present a prismatic shape with the dimensions: 12 x 3 x 0.8 centimeters, in order to be supported at the two ends (1 centimeter of support each side), and to configure the three points bending modality. Granite specimens are supported on an acrylic structure destined to fix and immobilize the specimens. Vibrating load is applied at the center of testing specimens by a self-designed and fabricated aluminum alloy awl, which dimensions have been obtained by modal numerical analysis to fit the resonance condition, necessary for the ultrasonic fatigue tests. Experimental results are obtained at room temperature and humidity comprised between 60 and 75%. The fatigue endurance is determined on this material under this modality of loading, and the fracture surfaces are analyzed to identify the principal trends related to crack initiation and propagation on this material.

Effects of mean shear stress on the torsional fatigue strength of a spring steel with small scratches

The torsional fatigue behavior of a high-strength spring steel, SWOSC-V, was investigated. Of particular interest were the influences of small scratches and superimposed static shear stress in the very high cycle fatigue regime. To achieve the research objectives, an ultrasonic torsional fatigue testing machine was developed. S-Ncurves showed the fatigue limit for the tested material condition (i.e. with no residual stresses at specimen surface). This study revealed that a notable influence of the stress ratio, R, on the torsional fatigue behavior was not explicitly recognized. Concerning the influence of small scratches, it was found that the [Formula: see text]-parameter model can predict the lower bound of the torsional fatigue limit, irrespective of the value of R.

Thermal response of iron and C-Mn steels with different ferrite/pearlite phase fraction under ultrasonic fatigue loading

This work compares the thermal responses of an α-iron and two carbon steels (C12 and C65 steel) via in-situ infrared thermography under 20 kHz ultrasonic tension-compression fatigue testing. The α-iron has only ferrite phase, and C12 and C65 steels have ferrite-pearlite dual phases. During a series of interrupted self-heating tests, the surface temperature of the three materials increases gradually with stress amplitude, while the relation is non-linear. The increase of carbon content, inducing the pearlite phase percentage increasing, results in a lower temperature increase and higher fatigue strength. For the three materials, at a certain stress amplitude level, the specimen temperature suddenly increases to hundreds of degrees. Moreover, the dissipated energy per cycle is estimated based on the 1D heat diffusion equation and a power function between dissipated energy per cycle and stress amplitude is found. Finally, the mechanisms driving the observed thermal responses are discussed.

Inclusion initiated fracture under cyclic torsion very high cycle fatigue at different load ratios

The influence of inclusions on cyclic torsion very high cycle fatigue (VHCF) is studied at different load ratios. Testing material is a high-strength spring steel, which was prepared based on the SWOSC–V composition with intentionally increased number and size of inclusions. Ultrasonic torsion fatigue tests were performed at load ratios R = –1, R = 0.1 and R = 0.35 with shot peened specimens. All specimens that were cycled at R = 0.1 or R = 0.35 and failed above 108 cycles showed crack initiation at internal aluminate or MnS inclusions. Interface failures between particles and metallic matrix generate cracks at aluminate inclusions, which predominantly grow in mode I. MnS inclusions have an elongated shape oriented in the specimens' length direction. Shearing of the MnS inclusions generates initial mode II/mode III cracks, which subsequently branch to mode I cracks. All specimens cycled at R = –1 showed surface crack initiation without inclusions, irrespective of lifetime. S–N curves are shifted towards lower shear stress amplitudes with increasing load ratio, indicating a pronounced mean stress influence on cyclic torsion strength.

A method for evaluation of weld overlays exposed to combined wear and very high cycle fatigue conditions

Key machinery components used for diminishing, transporting or processing massive volumes of material in heavy industries are severely exposed to combinations of different wear mechanisms, like abrasion and surface fatigue. The microstructure and wear properties of state of the art iron based welding alloys used as surface protection do not only depend on the nominal chemical composition of the welding consumables and their manufacturing route, but also on the base material, variations in process technology and applied welding parameters. Commonly used wear and hardness and test or microstructural investigations are either cost and time consuming or do not provide a complete understanding about the material behavior of surface coatings exposed to combined wear including fatigue. Thus the aim of this study was to prove the concept of using ultrasonic fatigue testing as new method to understand and predict the behavior of welded surface coatings in a fast and reproducible way. Two flux cored wires from different manufacturing lines, but of the same nominal chemical composition were compared applying identical welding parameters. As a result, the two wires could be distinguished clearly by applying the ultrasonic fatigue test method.

Ultrasonic fatigue on the automotive steels: AISI/SAE 4140T and 1045

Ultrasonic fatigue tests were carried out on two automotive steels: AISI/SAE 4140T and 1045, to investigate the fatigue endurance of these materials and the mechanisms of crack initiation and propagation under this modality of loading. Tests were obtained at room temperature and without control of environmental humidity; the last parameter was maintained between 50 and 65%.Ultrasonic fatigue results show that the effect of non-metallic inclusions plays an important role in the decay of fatigue endurance for the 4140T steel, together with the effect of previous heat treatments. Concerning1045 steel, non-metallic inclusions play an important role too, but this time the temperature and variation of ductility with temperature induce an effect on the ultrasonic fatigue endurance. Finally, the fracture surfaces were analyzed in order to obtain the principal trends of crack initiation and propagation on these steels, under the ultrasonic loading modality.

Ultrasonic fatigue testing under multiaxial loading conditions on a railway wheel

Ultrasonic fatigue testing is a relative recent fatigue methodology that uses resonant principles for the induction of stress cycles in a specific designed material specimen. This experimental method can apply very high cycle frequency, the most common frequency being 20 kHz, and was created with the main purpose of studying material fatigue life in the Very High Cycle Fatigue regime between 107 and 109 cycles with a higher performance of time and energy wise in comparison to conventional servo-hydraulic machines. In this study an improvement of an already built multiaxial ultrasonic fatigue machine in the Instituto Superior Técnico laboratories was carried out to specific designed specimens and afterwards a fatigue study was made for a material of a worn-out railway wheel. The particular design of the specimen was achieved by numerical and experimental analysis based on previous experiments and components. Thermographic imaging and the application of rosette strain gauges to the main throat of the specimens were conducted in order to validate the improved specimen design and to understand the real induced stresses on the specimen. Afterwards fatigue tests were conducted for several specimens for a wide range of stresses with a stress ratio R=-1 and an axial vs shear stress ratio of around 0.58. Results were analysed and fracture analysis was also carried out.

Fatigue limit in very high cycle fatigue of high-strength steel

1011 cycles fatigue tests were conducted on high-strength steel to clarify a new fatigue limit in very high cycle regions. The new fatigue limit had not been confirmed by fatigue tests up to 1010 cycles, while our previous study suggested that the new fatigue limit was probably confirmed by those up to 1011 cycles. However, the 1011 cycles fatigue testing was challenging since it took 2 months even by using ultrasonic fatigue testing at 20 kHz. In this study, 3 specimens were tested beyond 1010 cycles. Although a test on a specimen was terminated at around 5 x 1010 cycles, 2 specimens reached 1011 cycles without failure. In other word, no specimen failed above 1010 cycles. These results demonstrated the existence of the new fatigue limit. The fractured specimens below 1010 cycles revealed internal fractures originating from oxidetype inclusions. When the specimens failed in very high cycle regions, clear ODAs (Optically Dark Areas) were observed on the fracture surfaces, while the ODAs were obscure in case of failure in conventional life regions. The runout specimens up to 1011 cycles were forcibly fatigue-fractured at higher stress amplitudes in the conventional life regions. As the result, the ODA was observed on the forcibly fatigue-fractured surface. This meant that small internal cracks existed in the runout specimens since the ODA was a trace of small internal crack growth. Namely, non-propagating cracks were the mechanism of the appearance of the new fatigue limit.

Ultrasonic fatigue experiments with biaxial cruciform specimens

Fatigue studies of materials in simple or complex loading systems for any given lifetime is object of continuous research. This is due to the advancements on mechanical and structural components, as well as for new and innovative materials, which implies the knowledge of a materials response to all dynamic loads. The fatigue failure regime beyond what was once considered to be the fatigue limit (infinite life) is characterized between 107 and 109, known as Very High Cycle Fatigue regime. Due to the time consuming and wide energy consumption of conventional fatigue testing for such regime, fatigue tests under ultrasonic actuators are being used, capable of applying the dynamic loads at around 20 kHz. Nowadays, several variants of ultrasonic fatigue tests were already proposed and tested but it is still a somewhat limited fatigue test if compared to the conventional servo-hydraulic fatigue testing machines of general use. In this study, biaxial in plane stresses are induced in specially designed cruciform specimens with ultrasonic fatigue testing resonant principals. Two geometries were numerically analysed, manufactured and experimentally tested, the in-phase tension-tension (T-T) specimen and the out-of-phase compression-tension (C-T) specimen. All specific designed geometries go under a thorough numerical and several experiments analysis for their validation. The specimens showing a correct and as intended behaviour are led to failure.

Evaluation of transverse crack propagation behavior of cross-ply CFRP laminates in VHCF regime using ultrasonic fatigue testing

Evaluation of transverse crack propagation behavior of cross-ply CFRP laminates in VHCF regime using ultrasonic fatigue testing