Fatigue Testing Technique Research Articles

Very high cycle fatigue assessment at elevated temperature of 100 μm thin structures made of high-strength steel X5CrNiCuNb16-4

Many components and structures are exposed to very high number of cycles and challenging environmental conditions during operation. This study contributes to a better understanding of the very high cycle fatigue (VHCF) properties of high-strength steel X5CrNiCuNb16-4 at room temperature (RT) and 350 °C. For this purpose, conventional specimens and thin-walled structures are extensively examined with novel high-frequency fatigue testing techniques at elevated temperature. Tests with unnotched specimens at 350 °C show a 21.7% reduction in fatigue strength for 107 cycles and a different failure mechanism compared to RT. In contrast, no temperature influence is observed for mildly notched specimens and even a higher local fatigue strength is found for sharply notched specimens at 350 °C. The decrease in fatigue strength for 109 cycles is more pronounced at 350 °C (−10%) than at RT (−5%), and it is proven that notched specimens adequately represent the VHCF behaviour of structures. The transferability of specimen results to components and structures is given great attention. A new proposal for the VHCF strength assessment of structures with high stress gradients is presented, which is based on specimen results, an extended material-mechanical support factor and a VHCF reduction factor. The prediction model gives conservative fatigue strength estimates for 109 cycles with a maximum deviation of 5.8%. This demonstrates that even complex shaped structures can be successfully evaluated with suitable specimens and methods.

Application of SBF Test to Fatigue Damage Assessment of Type 316 Steel

Abstract A new fatigue testing technique, the small bulge fatigue (SBF) test, was recently developed by some of the present authors. In this technique, a cyclic oil pressure is alternatively applied to both surfaces of a small disk-type specimen (8 mm in diameter) with flat and concave surfaces. This SBF test is a kind of biaxial fatigue test as well as the disk bending fatigue (DBF) test. In this study, damaged specimens were subjected to the SBF test to examine the adaptability of this test to the fatigue damage assessment of actual components. To prepare these damaged specimens, Type 316 austenitic stainless steel was subjected to the DBF test using large disk specimens (87 mm in diameter) by considering that most of components were generally operating under multiaxial loading condition. The test was stopped at several stages of the fatigue life. Then, the SBF test specimens were taken from the surfaces of the interrupted DBF test specimens. It was expected that fatigue life of the SBF tests corresponded to the degrees of fatigue damage induced by the DBF test. The SBF test results showed that the fatigue life of the damaged SBF specimen, which contained microcracks formed by the DBF test, was only one tenth as long as that of the undamaged specimen. It was noteworthy that some microcracks introduced by the DBF test were opened and became clearly visible after the SBF test, although they were closed and not visible just after the DBF tests.

In situ corrosion fatigue life of 2198-T8 Al–Li alloy based on tests and DIC technique

Abstract The advanced aluminum–lithium alloy is used in aircrafts fuselage. Based on the combination of fatigue test and digital image correlation technique, the influence of pre-corrosion and in situ corrosion damage on the fatigue performance of 2198-T8 aluminum–lithium alloy was investigated. The integrated evolution process of fatigue crack and strain distribution fields of two types of the damaged sheets were captured and evaluated by the digital image correlation technique. The results reveal a declining tendency of in situ corrosion fatigue life with the rise of solution temperature, as well as the decrease of flow rate. But, at increasing the NaCl concentration, the fatigue life tends to decrease first and then increase. The observed fatigue behavior and related phenomena are directly associated with fracture morphology such as micro-cracks and corrosion pits. The analysis indicates a competition mechanism between fatigue and corrosion, the fatigue damage dominates the failure process under lower NaCl concentration on the contrary to the higher one that the corrosion damage is the dominant factor. Considering the effect of flow rate on the surface adhesion, the crack tends to initiate at the position of low flow rate. The simulation on the flow field presents an attractive similarity with experimental results.

Fatigue properties of wood at different load ratios tested at 50 Hz and 20 kHz

AbstractFatigue properties of sycamore maple (Acer pseudoplatanus) under cyclic tension, cyclic compression and tension‐compression loading at 50 Hz are investigated. Furthermore, for the first time, fatigue test results of wood generated with the ultrasonic fatigue testing technique at 20 kHz under tension‐compression loading are presented. Cyclic properties are in line with the static properties – with the tensile strength being more than two times the compressive strength. Mean lifetimes of 106 cycles are found if the maximum tensile stress for cyclic tension is 69 %±8 % of the tensile strength and if the maximum compressive stresses for cyclic compression and tension‐compression are 75 %±7 % and 54 %±4 % of the compressive strength, respectively. Tension‐compression loading at 20 kHz delivers lifetimes comparable to servo‐hydraulic tests at 50 Hz, and no pronounced frequency effect on fatigue lifetime is found. Ultrasonic fatigue testing is therefore, in principle, appropriate for accelerated fatigue testing of wood up to the very high cycle fatigue regime. Tests up to 109 cycles show no evidence of a fatigue limit for sycamore maple.

Effect of heat treatment conditions on the fatigue resistance of press hardened 22MnB5 steel evaluated through rapid testing technique

Fatigue strength is considered as a crucial parameter for automotive applications subjected to cyclic loads during their long service life, as chassis parts. The high yield stress of press hardened steels poses them as good candidates for lightweight solutions with improved fatigue resistance. However, their high strength leads to an increase in notch sensitivity which can ruin the whole part’s integrity. This behaviour was observed in previous works on press hardened steels, where their high fatigue strength was significantly affected by the surface conditions and by heat treatment conditions. Nevertheless, press hardening steels are still good candidates to manufacture complex geometry parts reaching high performance.Aiming at increasing the existing knowledge on the fatigue behaviour of press hardened steels, this paper analyses the fatigue performance of boron steel (22MnB5) under different time austenitizing times. Fatigue resistance is evaluated using a novel rapid fatigue testing technique based on the stiffness evolution. The method permits a fast and reliable determination of the fatigue limit. Based on results obtained with this rapid testing method, the most suitable heat treatment to mitigate fatigue notch sensitivity and then achieving the best fatigue performance for chassis applications is discussed.

Testing Techniques and Fatigue of Additively Manufactured Inconel 718 – A Review

Additive Manufacturing (AM) of metallic components shows unfavorable properties in their as-built state; surface roughness, anisotropy, residual stresses, and internal /surface defects are common issues that affect dynamic properties of AM metals. This paper reviews traditional fatigue testing techniques, summarizes published fatigue data for wrought and additively manufactured metals with focus on Inconel 718. Surface and volume defects of AM metals were presented and how post processing techniques could improve fatigue performance were shown. Different methods for normalizing fatigue data were explored due to varying results of different fatigue testing techniques.

Characterizing the damage behavior of thin sheets for fuselage based on in situ corrosion fatigue test and digital image correlation technique

The effect of salt spray and salt solution on the fatigue properties of 2024-T3 aluminum alloy plate used for aircraft fuselage structure have been investigated by combining Digital Image Correlation (DIC) technology with in-situ corrosion fatigue test. For this purpose, an in-situ corrosion fatigue platform was designed for gas and liquid phases. Fatigue experimental investigation was carried out in salt spray and salt solution environments respectively, the crack propagation process was monitored by the DIC technology and the influences of NaCl concentration on fatigue life and crack growth rate were analyzed under the two kinds of corrosive environments. Moreover, the failure mechanism is discussed based on the micro-morphology of fracture and the composition of corrosion products. Experimental results demonstrated that for the corrosion fatigue in salt spray environment, there is a competition between the hydrogen embrittlement and the crack tip closure effect caused by corrosion products, which results in fatigue life decreasing with the increase of NaCl concentration first and then increasing. The fatigue life (with 3.50 wt.% NaCl salt spray) is the lowest, which is 82.9% of that in air, while the fatigue life (with 5.00 wt.% NaCl salt spray) is 112.6% of that in air. But for the corrosion fatigue in the NaCl salt solution environment, there is no obvious crack tip closure effect. In the 5.00 wt.% NaCl salt solution environment, fatigue life is 74.2% of that in air. The microfractures and strain nephogram of specimens revealed the stress concentration caused by corrosion pits, which might be the main factor for the decrease of fatigue life.

High Speed In Situ Synchrotron Observation of Cyclic Deformation and Phase Transformation of Superelastic Nitinol at Ultrasonic Frequency

The near equi-atomic intermetallic Ni Ti alloy Nitinol is used for medical implants, notably in self-expanding stent grafts and heart valve frames, which are subjected to several hundred million load cycles in service. Increasing the testing frequency to the ultrasonic range would drastically shorten the testing times and make the very-high cycle regime experimentally accessible. Such tests are, however, only meaningful if the material response at ultrasonic frequency is identical to that observed in conventional fatigue tests. A novel fatigue testing setup where superelastic Nitinol dog bone specimens are loaded at ultrasonic cycling frequency is presented. Loading conditions resemble in vivo loading (i.e., repeated cyclic loading with relatively small strain amplitudes, specimens in a pre-strained multi-phase state). Strains and phase transformations during ultrasonic frequency cycling are quantitatively measured in an X-ray diffraction (XRD) synchrotron experiment and compared to the material response at low frequency. The XRD experiment confirms that forward and reverse stress-induced phase transformation from austenite to martensite via the intermediate R-phase occurs during low frequency (0.1 Hz, strain rate dot{varepsilon}approx 10−3 s−1) and ultrasonic frequency (20 kHz, dot{varepsilon}approx 102 s−1) cycling. Since the same deformation mechanisms are active at low and ultrasonic frequency, these findings imply a general applicability of the ultrasonic fatigue testing technique to Nitinol.

Development of small bulge fatigue testing technique using small disk‐type specimen

AbstractA new fatigue testing technique, the small bulge fatigue (SBF) test using a small disk‐type specimen with flat and concave surfaces, was developed in this study. In the technique, a cyclic oil pressure could be alternatively applied to both specimen surfaces at the frequency of 10 Hz. After some verification tests for the displacement and strain measurements, type 316 austenitic stainless steel specimens were subjected to a preliminary test using this newly developed testing technique. As a result, the SBF test results (S‐N curve) were in good agreement with those of conventional fatigue tests by defining fatigue life as the number of cycles to the sudden drop in oil pressure because of fracture.

Fatigue of metals at nanoscale: Metal thin films and conductive interconnects for flexible device application

Abstract As electrodes and electrical interconnects in flexible electronic devices, metal films are one of the weakest components in the system against mechanical deformation in daily use. Fatigue reliability at nanoscale becomes a practical concern for these flexible electronic devices. This review introduces state-of-the-art fatigue testing techniques and evaluation methods for thin metal films and conductive interconnect materials constrained by a substrate. Then, experimental results about fatigue damage behaviors, fatigue properties and fatigue life prediction are summarized. Furthermore, fundamental insights into fatigue mechanisms of metals at the nanoscale and the size effects on fatigue properties are elucidated. Finally, the perspectives of studies on fatigue of thin metal films constrained by a substrate are proposed.

Investigation on fatigue performance of T800 composites structural component

This paper presents a practical method for investigating fatigue performance of the typical structural component of carbon fiber reinforced plastic. Servo-hydraulic fatigue test machine is properly refitted to conduct the fatigue test under load controlled method. Distribution location of strain gauges is determined by finite element method (FEM). Strain values on surface are recorded with the increase of the fatigue cycle number. In the process of fatigue testing, the cracks are observed under the optical microscope. The experiment results show that failure modes of the structural component are mainly characterized by delamination which occurred in the thickness transition area. Strain will redistribute with the cracks propagating. And the suddenly decreased strain values will return to a stable level. Moreover, the application of Digital Image Correlation (DIC) technique for fatigue testing of structural component is discussed.

Design and Evaluation of an Experimental Technique for Mechanical and Fatigue Testing of Sub-Sized Samples

Some recently developed processing techniques allow fabrication of small specimens. The fatigue behavior of these small samples cannot be characterised according to the existing standards due to size limitations. The main objective of the present work is to develop an original technique for both mechanical and fatigue testing of sub-sized specimens produced by a high pressure torsion (HPT) method. The fatigue behavior of the sub-sized Ti-6Al-4 V samples, made from HPT-processed disks, is compared with that of the standard samples prepared according to the ASTM standards. Finite element method (FEM) analysis of the stress distribution under applied static load, for both sub-sized specimens and grips designed for their testing, is performed. Good agreement between the outcomes of tensile testing of the standard ASTM samples and sub-sized samples is observed. Fatigue data, represented by Wohler curves for both types of samples, showed significant divergence of high cycle fatigue properties caused by incompliance of specimen geometries and surface/defect size ratio. The obtained results can be used to scale up the fatigue behavior from sub-sized specimens to the standard ones, as well as to predict the fatigue behavior of the bulk nanostructured metals and alloys produced by HPT.

A new high frequency testing method for steels under tension/compression loading in the VHCF regime

Nowadays there are several fatigue testing techniques operating in a frequency range from 2Hz up to 20kHz. A lot of effort is put into the understanding of very-high-cycle fatigue failures. A new testing technique, which is capable of operating at 834Hz located in between conventional testing machines and ultrasonic testing machines, is presented. An analytic approach, which was extended by FEA simulations to design the final setup is shown. The actual setup provides resonant frequencies at min. 732Hz, max. 1052Hz (analytic), 780Hz (FEA) and at 834Hz (experimental). Additional to that, the experimental’s damping factor (D=0.013) is defined. It was shown, that depending on the level of acceleration, a certain load (up to 689MPa) at the specimen’s cross section could be applied. Furthermore the tests showed a linear behaviour between load level and amount of acceleration.

Fatigue testing of thin CoNiCr wire up to 1010 cycles

The CoNiCr alloy MP35N in the form of thin wires is commonly used as conductor in cardiac leads. The application requires high fatigue strength, in particular in the very high cycle fatigue (VHCF) regime. High frequency fatigue testing techniques are of great interest for material characterization and development since existing techniques require very long testing times.In this study, adapting the ultrasonic fatigue testing method to sub-millimeter wires has been accomplished for the first time. Fatigue tests of as-drawn low-Ti MP35N wire with 100μm diameter have been performed with a novel technique at 20kHz cycling frequency at load ratio R=0.3. Measured lifetimes show good agreement with conventional servo-hydraulic tension-tension fatigue data of the same material at 30Hz. Ultrasonic tests were run up to 2×1010cycles, and no fatigue limit was observed.Crack initiation occurs at Al2O3-inclusions at the surface, where the interfaces between particles and matrix fail. Sizes of the crack initiating inclusions, areainc. are between 1.9μm and 3.5μm. Fatigue lifetimes can be approximated with a single S-N curve between 104 and 1010 cycles. No change of crack-initiation mode into the interior is observed when lifetimes extend into the VHCF regime. Inclusions can be considered as initial cracks, and a fracture mechanics concept is used to describe fatigue lifetimes. On average a stress intensity range as low as 1.0MPam can propagate the crack to fracture at lifetimes above 108cycles.

Fatigue and fracture characterization of fiberglass grid-reinforced beam specimens using four-point bending notched beam fatigue test and digital image correlation technique

The purpose of this research is to develop an experimental and analytical framework for describing, modeling, and predicting the reflective cracking patterns and crack growth rates in GlasGrid®-reinforced asphalt pavements. In order to fulfill this objective, the effects of different interfacial conditions (mixture type, tack coat type, and grid opening size) on reflective cracking-related failure mechanisms and the fatigue and fracture characteristics of fiberglass grid-reinforced asphalt concrete beams were studied by means of four-point bending notched beam fatigue tests (NBFTs). Also, the digital image correlation technique was utilized to determine the lengths of the interfacial and vertical cracks during each test. The stress intensity factors (SIFs) at the crack tips were backcalculated using a finite element model and applying linear elastic fracture mechanic principles to the crack lengths. The local effect of the reinforcement on the stiffness of the system at a vertical crack-interface intersection or the resistance of the grid system to the deflection differential at the joint/crack (joint stiffness) for GlasGrid®-reinforced asphalt concrete beams was determined by implementing a joint stiffness parameter into the finite element code. The crack length and SIF test results were used to find the Paris’ law parameters for each interlayer condition. According to the NBFT results, GlasGrid® reinforcement retards reflective crack growth by stiffening the composite system and introducing joint stiffness. The results also show that the higher the bond strength and interlayer stiffness values, the greater the joint stiffness and retardation effects.

Special Testing Equipment and Validation of Measurement Methodologies for High Temperature Low Cycle Fatigue Testing of Miniature Metallic Specimens

A technique for high temperature low cycle fatigue testing of metallic materials has been developed, to determine fatigue behaviour through the testing of miniature specimens. The miniature specimen geometry was specifically designed, such that it could be manufactured from a small volume of material removed by chain-drilling extraction. An extensometry method to measure and control strain at the specimen shoulders during testing was adopted. This was undertaken to minimise the deleterious contact effects that can occur via extensometry attached at the gauge length of specimens, hence leading to premature failure and inaccurate fatigue data. By the application of this technique, the high temperature low cycle fatigue behaviour of 2.25Cr-1Mo steel was successfully characterised at 540 °C, under a fully reversed strain-controlled regime. The fatigue properties of the steel obtained from testing miniature specimens were shown to correlate well with existing literature for the material under comparable conditions, as determined by the testing of conventional standard-sized specimens.

Influence of intrinsic and artificial defects on the VHCF properties of 17-4PH stainless steel

In the present work, fatigue test results in the very high cycle fatigue (VHCF) regime are discussed. Experiments were performed with smooth specimens in which crack initiation mostly originated at non-metallic inclusions. Additionally, the influence of small artificial defect, i.e. circumferential notches, drilled holes and corrosion pits, was investigated. Fatigue tests in the VHCF regime were conducted using ultrasonic fatigue testing technique. Further experiments in the high cycle fatigue regime were performed using rotating bending and servo-hydraulic testing machines.For smooth specimens, fatigue fracture was observed even at more than 1010 number of cycles. In contrast, no fatigue failure occurred above approximately 107 cycles in the presence of artificial defects, i.e., a pronounced fatigue limit was determined. Furthermore, it was found that the fatigue limit is not only depending on the defect size but also on the notch root radius of a defect. Consequently, different approaches were used to predict the fatigue limit in the presence of different defects.

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.

Development of disc bending fatigue test technique for equi-biaxial loading

The disc bending fatigue test technique was developed to investigate the fatigue life under an equi-biaxial loading condition. In this test, a uniform thickness disc specimen was subjected to a bending load by applying air pressure on the specimen surface. Eleven specimens made of Type 316 stainless steel were tested in a room temperature ambient environment. The crack initiation and growth behaviors during the test were observed through a transparent window. The fatigue life was defined when the peak pressure measured near the specimen surface was reduced to 95% of the supplied air pressure. The fatigue life obtained by the disc bending fatigue test was shorter than that obtained by the uniaxial and plate bending fatigue tests for the same principal strain range. It was confirmed that the equi-biaxial loading condition reduced the fatigue life. The finite element analysis together with test results revealed that the crack was initiated at the edge of the specimen when the specimen thickness was less than 1.0mm. The specimen thickness should be 1.2mm in order to maximize the strain range at the specimen center. It was concluded that the disc bending fatigue test can derive the fatigue life under an equi-biaxial loading condition, for which strain range is measured at the specimen center.

Role of intermetallics on the mechanical fatigue behavior of Cu–Al ball bond interfaces

The mechanical fatigue behavior of Cu–Al interfaces occurring in thermosonic ball bonds –typically used in microelectronic packages for automotive applications – is investigated by means of a specially designed fatigue test technique. Fully reversed cyclic shear stresses are induced at the bond interface, leading to subsequent fatigue lift off failure and revealing the weakest site of the bond. A special focus is set on the role of interfacial intermetallic compounds (IMC) on the fatigue performance of such interfaces. Therefore fatigue life curves were obtained for three representative microstructural states: The as-bonded state is compared to two annealed states at 200 °C for 200 h and at 200 °C for 2000 h respectively. In the moderately annealed state two IMC layers (Al2Cu, Al4Cu9) could be identified, whereas in the highly aged state the original pad metallization was almost entirely consumed and AlCu is formed as a third IMC. Finally, the crack path is traced back as a function of interfacial microstructure by means of electron microscopy techniques.Whereas conventional static shear tests reveal no significant decrease of the bond shear force with increased IMC formation the fatigue tests prove a clear degradation in the cyclic mechanical performance. It can be concluded that during cycling the crack deflects easily into the formed intermetallics, leading to early failure of the ball bonds due to their brittle nature.