Ultrasonic Fatigue Testing System Research Articles

Accelerated estimation of the very high cycle fatigue strength and life of polymer composites under ultrasonic cyclic three-point bending

Ultrasonic fatigue testing (UFT) offers a strong potential to accelerate the testing time of fatigue experiments of materials up to very high cycle (VHCF) and giga-cycle (GC) fatigue regimes. This can especially be useful during the materials selection processes when designing load-bearing structures. In this study, one increasing- and three constant-amplitude fatigue experiments were conducted using a UFT system on three different composites consisting of woven Carbon fibers with Poly-Ether-Ketone-Ketone (CF-PEKK), Polyphenylene Sulfide (CF-PPS), Epoxy (CF-Epoxy) as polymer matrix materials. The surface temperature data obtained from these experiments were used to evaluate the heat dissipation rate and fracture fatigue entropy (FFE) range of the three composite materials. The fatigue strength estimated using this FFE range for CF-PEKK (normal stress), CF-PPS (shear stress), and CF-Epoxy (shear stress) composites were 26.3 MPa, 4.7 MPa, and 15.1 MPa, respectively. The fatigue life comparison between the estimation model adopted in this study and the experimental results found in the literature was in reasonable agreement for all three composites. The FFE concept is adapted to the UFT method to rapidly estimate the VHCF properties of composite materials used in load-bearing structures within three weeks.

Online and Ex Situ Damage Characterization Techniques for Fiber-Reinforced Composites under Ultrasonic Cyclic Three-Point Bending.

With ultrasonic fatigue testing (UFT), it is possible to investigate the damage initiation and accumulation from the weakest link of the composite material in the very high cycle fatigue (VHCF) regime in a shorter time frame than conventional fatigue testing. However, the thermal influence on the mechanical fatigue of composites and the scatter in fatigue data for composites under ultrasonic cyclic three-point bending loading still need to be investigated. In this study, we conducted interrupted constant-amplitude fatigue experiments on a carbon-fiber satin-fabric reinforced in poly-ether-ketone-ketone (CF-PEKK) composite material. These experiments were carried out using a UFT system, which operates at a cyclic frequency of 20 kHz with a pulse-pause sequence. Various parameters, such as the CF-PEKK specimen's surface temperature, acoustic activity, and the ultrasonic generator's input resonance parameters, were measured during cyclic loading. During experiment interruption, stiffness measurement and volumetric damage characterization in the CF-PEKK specimens using 3D X-ray microscopy (XRM) were performed. The locations of damage initiation and accumulation and their influence on the changes in in situ parameters were characterized. Under fixed loading conditions, damage accumulation occurred at different locations, leading to scattering in fatigue life data. Further, the damage population decreased from the surface to the bulk of the composite material.

Test Data Evaluation of Very High Cycle Fatigue Based on Maximum Likelihood Method

The very high cycle fatigue tests of TC4 alloy were accomplished through ultrasonic fatigue experiment which were conducted on the ultrasonic fatigue testing system at frequency range of 20000 ±1000 Hz. In contrast to the commonly used Basquin model, a Maximum Likelihood Method to fit the three-parameter nonlinear model was used to verify the adaptability in very high cycle fatigue regime. These two methods were used to estimate S-N curves of three types of materials which were TC4 alloy, GH4169 alloy and TC17 alloy in the very high cycle fatigue regime and the results were compared with test data. The evaluation result of the MLM model based on the three-parameter nonlinear model shows a good agreement with test data. The mean and variance of relative error of the MLM model are less than those of the Basquin model, providing a suitable evaluation method for predicting very high cycle fatigue limit which often has higher scatter.

Damage assessment during ultrasonic fatigue testing of a CF-PEKK composite using self-heating phenomenon

The knowledge and experience with the very high cycle fatigue (VHCF) behavior are limited compared to the low cycle and high cycle fatigue behavior of fiber-reinforced composite materials. Accelerated cyclic loading using an ultrasonic fatigue testing system is one potential solution to evaluate the longevity of composite materials within a reasonable time. Insufficient sampling rates and the inherent inaccuracies of non-contact measurement devices pose critical challenges in the adaptation of accelerated fatigue experiments. This study aims to overcome these challenges by combining the temperature response of the composite specimen and the input parameters for the dynamic response of the in-house developed ultrasonic fatigue testing system to detect damage before failure. Increasing- and constant-amplitude fatigue experiments were conducted on a carbon fiber satin fabric-reinforced poly-ether-ketone-ketone (CF-PEKK) composite material. These experiments were carried out under three-point bending loading conditions at a cyclic frequency of about 20 kHz. Various parameters, such as the surface temperature, the cyclic displacement, and the input resonance parameters of the ultrasonic generator, were recorded during these experiments. The VHCF behavior of the CF-PEKK material system was characterized based on these results and through digital light optical microscopy.

Very High Cycle Fatigue Properties and Damage Evolution in Al-Si Cast Alloys

Abstract The steady progress in electro mobility increases the demand for lightweight space frame and engine solutions. Advanced aluminum-silicon cast alloys with additions of magnesium, copper, or both for precipitation hardening are promising candidates for lightweight structures because of their high strength-to-weight ratio. For a reliable design, the fatigue performance in the very high cycle fatigue (VHCF) regime must be understood. Therefore, this study deals with the influence of frequency on VHCF properties as well as fatigue damage evolution in AlSi7Mg and AlSi10Mg sand-cast alloys. The VHCF tests were performed using a resonant (70 Hz), a high-frequency resonant (1 kHz), and an ultrasonic (20 kHz) fatigue testing system. Thereby, a significant frequency effect could be determined that resulted in an increased fatigue lifetime by 1 decade at 1 kHz and by 2 decades at 20 kHz, whereas no significant change in fatigue limit could be determined. Moreover, the fatigue crack initiation mechanisms change from surface crack initiation at 70 Hz and 1 kHz to volume crack initiation at 20 kHz. Using the defect-based Murakami approach, including a lightweight extension of Noguchi, a defect-induced (size, location) frequency effect could be excluded. Based on damage monitoring, the effect could be related to the crack propagation phase, which is accelerated at low frequencies (<100 Hz) because of a humidity reaction with the fatigue crack surface and the formation of atomic hydrogen resulting in a saturated hydrogen embrittlement per cycle. At high frequencies of 1 kHz and 20 kHz, no saturated hydrogen embrittlement per cycle can be reached.

Fatigue behavior and lifetime assessment of an austenitic stainless steel in the VHCF regime at ambient and elevated temperatures

AbstractWhile the LCF behavior of austenitic steels used in nuclear power plants is already well investigated, the VHCF regime has not been characterized in detail so far. For this, fatigue tests on the metastable austenitic steel AISI 347/1.4550 were performed with a servo‐hydraulic testing system at test frequencies up to 980 Hz and with an ultrasonic fatigue testing system at a test frequency of 20,000 Hz. To compare these test results to the ASME standard fatigue curve (total strain amplitude vs. load cycles to failure), a fictitious‐elastic and an elastically plastic assessment method was used. The elaborated elastic–plastic assessment method generates good results, while a purely elastic assessment in the VHCF regime, commonly used in literature, leads to significantly nonconservative results. Moreover, phase transformation from metastable austenite into stable α′‐martensite can take place, and no specimen failure occurs in the VHCF regime. Consequently, for this material, a real endurance limit exists.

Investigation of Self-Heating During Ultrasonic Fatigue Testing and Effect on Very High Cycle Fatigue Behavior of Titanium 6Al-4V

Abstract Very high cycle fatigue (VHCF) data and experiments, 107–109 cycles to failure, have traditionally been both a cumbersome and costly task to perform. However, characterizing VHCF behavior of material systems is critical for the design and sustainability of turbine engines as outlined in the turbine engine structural integrity program (ENSIP). With recent advancements, ultrasonic fatigue (UF) test systems have become increasingly available to generate VHCF fatigue data. A primary consideration for ultrasonic fatigue testing is the frequency of loading, the resulting thermal evolution, and its effect on fatigue life. To mitigate the heat generation within the specimen during experiments, cooling air is directed to the specimen and cyclic loading is performed by selecting an appropriate test frequency or defining a duty cycle rather than continuously subjected to fatigue. However, standardization of experimental test procedures remains ongoing and continues to be developed. In this study, a Shimadzu USF-1000A ultrasonic fatigue test system is used to characterize VHCF behavior of Ti-6Al-4V to understand the effect of duty cycle and thermal evolution on fatigue life for ultrasonic fatigue testing. Titanium 6Al-4V test specimens are subjected to fully reversed axial fatigue at 20 kHz exciting resonance in an axial mode to better characterize the experimental process. Three duty cycle-cooling air configurations and their effect on fatigue life due to self-generated heat during the experiment are investigated. Heat generation is monitored in situ via a single-point optical pyrometer, and in situ mechanical and thermal data are collected and compared to standardized servohydraulic fatigue test data performed in this study as well as from data found in the literature.

Very High Cycle Fatigue Behavior of Austenitic Stainless Steels with Different Surface Morphologies

The fatigue behavior of the two austenitic stainless steels AISI 904L and AISI 347 with different surface morphologies, (i) conventionally turned and finally polished, (ii) cryogenic turned using CO2 snow, as well as (iii) cryogenic turned and finally polished, was investigated using an ultrasonic fatigue testing system up to the very high cycle fatigue regime using an ultrasonic fatigue testing system. The AISI 904L is stable against deformation-induced phase formation while the AISI 347 is in the metastable state and shows martensite formation induced by cryogenic turning as well as mechanical loading. For the detailed characterization of the surface morphology, confocal microscopy, scanning electron microscopy, and X-ray diffraction methods were used. The specimens from stable austenite failed in the high cycle fatigue and very high cycle fatigue regime. Opposed to this, the metastable austenite achieved true fatigue limits up to load cycle N = 1 × 109 and failed only in the high cycle fatigue regime. Furthermore, due to surface modification, an increase of fatigue strength of metastable AISI 347 was observed.

Evaluation of very high cycle fatigue properties for transverse crack initiation in cross‐ply carbon fiber‐reinforced plastic laminates

AbstractThis paper investigates the transverse crack initiation behaviors of carbon fiber‐reinforced plastics (CFRPs) in the high‐cycle and very high cycle regimes. To accelerate the fatigue test up to the very high cycle fatigue (VHCF) regime, an ultrasonic fatigue testing system that can apply the specimen ultrasonic load at a frequency of 20 kHz was used. The fatigue behavior of cross‐ply CFRP laminates formed with T800S/2592 prepreg was investigated. The fatigue behaviors at N = 103–107 cycles and N = 108–109 cycles were obtained by hydraulic and ultrasonic fatigue tests, respectively. The slope of the S–N diagram for transverse crack initiation showed a linear decrease in the region of N = 103–106 cycles and leveled off over N = 106 cycles. Furthermore, no cracks occurred in any specimen tested under the 90°‐layer stress σ90°max = 44 MPa up to a maximum of N = 6.56 × 109 cycles.

The effect of laser shock peening on surface integrity and high and very high cycle fatigue properties of 2024-T351 aluminum alloy

Laser shock peening (LSP) as an emerging surface peening technology that is widely used in aerospace manufacturing. The effect of laser shock peening on surface integrity and high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of 2024-T351 aluminum alloy was investigated in this study. Accordingly, the surface roughness, surface topography, microstructural, microhardness and residual stress of specimens treated with two different LSP pulse energy levels (10 J and 20 J) were analyzed. Additionally, the HCF and VHCF test was performed by ultrasonic fatigue test system. The results showed that surface roughness Ra value and microhardness of LSP treated specimens increased by a maximum of 89.37% and 16.3% comparing to the untreated specimen, respectively. X-ray diffraction analysis also showed that subgrain sizes were refined to the nanoscale. Furthermore, high-level compressive residual stresses (the maximum value about −210 MPa with 700 μm thickness) were introduced to the surface layer of the specimens. The S-N curve results showed that LSP treatment reduced the fatigue life of specimens, and the higher the laser pulse energy, the more obvious the effect of reduction. The fatigue fracture of specimens was analyzed via scanning electron microscope and residual stress distribution map. It is observed that fatigue cracks are predominantly initiated at fractured secondary phase particle locations in a VHCF range for untreated specimens. However, for LSP specimens, surface and internal crack initiation mechanisms were both observed. Under low and intermediate stress levels, the tensile residual stresses (which coexists with the compressive residual stresses) initiated fatigue cracks from the interior of the specimens; under high-stress levels, fatigue cracks were initiated at the material surface.

Fatigue lifetimes of 1.4306 and 1.4307 stainless steels subjected to ultrasonic loading

The contribution brings results of high-frequency fatigue tests of the 1.4306 and 1.4307 steels. The S304L austenitic stainless steels are the most versatile and widely used stainless steels. They are used in food processing, chemical industry, petrochemicals, but also in mechanical and civil engineering. The 1.4306 steel is more highly alloyed and more corrosion resistant than 1.4307. A new trend in bridge structures is the use of stainless steels for their corrosion resistance and mechanical properties. The bridge structures are loaded cyclically and are designed for decades of operation. Thus, long-term fatigue properties are an important issue. The results of experimental studies serve as inputs for a reliable assessment of the service life of cyclically loaded structures. Measurements of both steel types were performed on ultrasonic fatigue testing systems. Cyclic loading was performed in fully reversed push-pull mode with the stress ratio R = - 1. Experimental results plotted in the S-N curves are shown. From the results, it can be concluded that 1.4306 steel shows a slightly higher fatigue resistance than 1.4307 steel. The failure in the gigacycle region (around 108 and 109 cycles) only occurs in the case of 1.4306 steel. Fracture surfaces were analyzed and only surface crack initiation was observed.

Cyclic deformation behavior of metastable austenitic stainless steel AISI 347 in the VHCF regime at ambient temperature and 300 °C

Very high cycle fatigue (VHCF) tests on metastable austenitic stainless steels using an ultrasonic fatigue testing (USFT) system are challenging due to the transient material behavior and associated pronounced self-heating effects. Because the conventional measurement of stress-strain hysteresis is not possible in USFT, the cyclic deformation behavior can’t be described in a conventional manner. Hence, an energy-based approach is proposed for the characterization of cyclic deformation behavior of austenitic stainless steels in the VHCF regime at ambient temperature (AT) and elevated temperature. Therefore, in-situ dissipated energy and temperature measurements were performed. At AT both values underwent a change during cyclic loading, while at 300 °C only the dissipated energy changed. The investigated metastable austenitic stainless steel AISI 347 (X6CrNiNb1810, 1.4550) showed cyclic softening in the high cycle fatigue (HCF) regime (Nf < 107) at both testing temperatures, which led to specimen failure. In the VHCF regime, cyclic hardening was observed, which resulted in reaching the limiting number of load cycles Nl = 2 × 109 without specimen failure. This change in the cyclic deformation behavior by a small reduction of the stress amplitude led to a horizontal course of the Woehler curve in the VHCF regime and resulted in the true endurance limit for the investigated material. At AT, ferromagnetic α′-martensite was detected in all fatigued specimens using magnetic measurements. At 300 °C only specimens which reached the limiting number of cycles without failure exhibited a small amount of α′-martensite.

Very High Cycle Fatigue Investigations on the Fatigue Strength of Additive Manufactured and Conventionally Wrought Inconel 718 at 873 K

The fatigue lives of additively manufactured (AM) Inconel 718 (IN718) produced by selective electron beam melting and conventional wrought material as reference conditions were studied in the very high cycle fatigue regime under fully reversed loading (R = −1) at the elevated temperature of 873 K using an ultrasonic fatigue testing system. The fatigue lives of the AM material were significantly reduced compared to the wrought material, which is discussed in relation to the microstructure and a fractographical analysis. The additively manufactured material showed large columnar grains with a favoured orientation to the building direction and porosity, whereas the wrought material showed a fine-grained structure with no significant texture, but had Nb- and Ti-rich non-metallic inclusions. Crystallographic crack initiation as well as crack initiation from the surface or internal defects were observed for the AM and the wrought IN718, respectively.

Small‐sized specimen design with the provision for high‐frequency bending‐fatigue testing

AbstractThe testing and study of emerging materials—such as additively manufactured materials—demands for specimen designs that are cost effective and time saving. The design of a small‐sized bending‐fatigue test specimen for an ultrasonic fatigue testing system is reported in this paper. The design is optimized based on the finite element analysis and analytical‐solution results to achieve the proper vibration shape and stress distribution. The proposed design is evaluated in the high‐ and very‐high‐cycle fatigue regimes under 20‐kHz frequency. Both simulation and testing results confirm that the desirable vibration mode occurs and the specimen fails at the designated test (gauge) section, where the maximum stress exists. The stress–life (S–N) curve is obtained for Inconel alloy 718 and indicates an expected trend.

Comparison of high‐ and low‐frequency fatigue properties of structural steels S355J0 and S355J2

AbstractThe paper brings results of high‐cycle fatigue and very‐high‐cycle fatigue tests of the steel S355 tested during low‐ and high‐frequency loading. Two grades of structural steel (S355J0 and S355J2) were tested to gain fatigue properties that serve as inputs to reliable finite element calculations of cyclically loaded structures. Experimental measurements of both steel grades were performed on low‐frequency hydraulic and high‐frequency ultrasonic fatigue testing systems. Both low‐ and high‐frequency loading showed higher lifetime for the grade S355J2. The difference between the two studied steel grades was more apparent during low‐frequency loading. The significant difference between the results of low‐ and high‐frequency loading tests can be explained by the fact that the resistance to plastic deformation increases with an increasing rate of deformation. Fracture surfaces were studied by electron microscopy, where they exhibited both surface and internal crack initiation.

Very High Cycle Fatigue Properties of 2024 Aluminum Alloy Samples in Three Sizes

An ultrasonic fatigue testing system was used to carry out very high cycle fatigue (VHCF) tests on 2024 aluminum alloy in three different sizes of specimens. The influence of specimen size and optimization on the experimental results was discussed and the VHCF properties of 2024 aluminum alloy were obtained at the same time. The results show that: The S-N curves of all specimens show a continuous decreasing tendency and the curve of the dog-bone-shaped specimen (the half-length of the testing volume L0 > 0) is lower than that of the hourglass-shaped specimen (L0 = 0) at low-cycle and high-cycle fatigue stages, which is opposite at the VHCF stage; The optimized specimen solves the problem of fracture position deviation and effectively improves the accuracy of experimental data; 2024 aluminum alloy will still fracture when the fatigue life exceed 107, and there exist no fatigue limit; All cracks initiate at surface and the fracture morphologies show characteristics of the crack initiation and propagation.

Giga-Cycle Fatigue Behavior of the Nuclear Structure of 316L Weldments

Some components made of 316L stainless steel in nuclear reactors are connected by welding, and these are under giga-cycle fatigue loading. Therefore, the giga-cycle fatigue behavior of 316L weldments, which are fabricated by Laser Beam Welding (LBW) and Gas Tungsten Arc Welding (GTAW), were investigated using an ultrasonic fatigue testing system. The results indicate that the fatigue strength of LBW-made weldments is almost the same as that of GTAW-made weldments even though the microstructure and mechanical properties of the weldments are different. For the LBW-made specimens, the LBW-induced internal pores with a diameter range of about 89–270 μm were observed in the fracture surface. However, an obvious decrease in fatigue life was not observed in such cases. For the GTAW-made specimens, the quality requirement of the weld seam has to be more strict to prevent fatigue strength from decreasing. The fatigue failure mode of the GTAW-made specimens is the same as that of LBW-made specimens in the high-cycle fatigue regime but different in the giga-cycle fatigue regime.

Fatigue property and failure mechanism of TC4 titanium alloy in the HCF and VHCF region considering different forging processes

Based on the self-built three point bending ultrasonic fatigue test system, fatigue behavior of TC4 titanium alloy after different forging processes in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes was discussed in this paper. The experimental results showed that the fatigue S-N curves of TC4 titanium alloy with three different structures presented different characteristics: continuous decline; double platform; linear decline. The fatigue property of TC4 titanium alloy is closely related to the content of primary α phase and β-transformed microstructure. For TC4 titanium alloy, grain refinement and certain volume fraction of the primary α phase contributed to enhancing fatigue property. The fatigue performance of bimodal structure by near β forging was obviously better than two structures by α + β forging in HCF and VHCF regime. It was found that failure mode shifted from the surface at relatively high stress to the subsurface at relatively low stress. For three kinds of structures, crack initiations were observed at the surfaces of specimens in HCF regime. Meanwhile, cracks of the three structures all originated from primary α cleavage planes in the interior. The fatigue life of VHCF is dominated by the crack initiation stage.

Effect of Turning on the Surface Integrity and Fatigue Life of a TC11 Alloy in Very High Cycle Fatigue Regime

In this work, the effect of a turning process on fatigue performance of a Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) titanium alloy is studied in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. For this purpose, the surface characteristics including surface morphology, surface roughness and residual stress were investigated. Moreover, axial fatigue tests were conducted with an ultrasonic fatigue testing system working at a frequency of 20 kHz. The results show that the turning process deteriorated the fatigue properties in both HCF and VHCF regimes. The fatigue strength at 1 × 108 cycles of turned samples is approximately 6% lower than that of electropolished ones. Fracture surface observations indicate that turning marks play a crucial role in the fatigue damage process, especially in the crack initiation stage. It was observed that the crack of all the turned samples originated from turning marks. In addition, the compressive residual stress induced by the turning process played a more effective role in resisting crack propagation in the VHCF regime than in the HCF regime.

High cycle fatigue deformation mechanisms of a single phase CrMnFeCoNi high entropy alloy

High entropy alloys (HEAs) with exceptional mechanical properties have gained a considerable attention in the recent years. Some of these alloys, including the equiatomic, single-phase, face-centered-cubic CrMnFeCoNi alloy with an excellent combination of strength, ductility and fracture toughness at room and cryogenic temperatures have been studied extensively. Still, further studies are required to explain the fatigue behavior and the encountered deformation mechanisms of this class of alloys. In this study, high cycle fatigue behavior of the single-phase CrMnFeCoNi HEA, known as Cantor alloy, has been investigated. S–N curve has been obtained in the range of 106 to 109 loading cycles by using small-scaled samples and an ultrasonic fatigue test system. Microstructural investigations and fracture surface analyses suggest that cyclic plastic deformation occurs due to the combined interaction of planar slip and deformation twinning at low-stress amplitudes in the very high cycle regime.