Fatigue response up to 109 cycles of a structural epoxy adhesive

In the present paper, the very‐high‐cycle fatigue (VHCF) response of a structural adhesive used for automotive applications, Betaforce 4600G modified with microspheres, has been experimentally assessed. Ultrasonic fully reversed tension–compression tests up to 10⁹ cycles have been carried out with the testing machine developed by the authors on adhesives without macroscopic defects and on adhesives with artificial defects, inserted during the butt‐joint preparation. Fracture surfaces have been observed with the optical microscope and the P‐S‐N curves estimated. Experimental results have shown that defect location significantly affects the VHCF strength and fracture surfaces exhibit a peculiar morphology with three distinct characteristic regions.     

Fatigue at high number of cyclic loads: Testing methods and failure mechanism

A significant proportion of machinery and equipment is operated up to a number of cycles greater 10⁸, which is in the range of conventionally fatigue limit design. For materials with a face‐centred cubic crystal lattice and for high‐strength steels with a body‐centred cubic crystal lattice fatigue failures were observed even in the Very High Cycle Fatigue (VHCF) regime of load‐cycles greater N = 10⁷. To reduce the testing time in the VHCF regime, one possibility is to perform the tests at a higher frequency. In addition to the typical servo‐hydraulic testing machines or resonant fatigue testing machine with test frequencies up to f = 400 Hz, ultrasonic fatigue testing machines with frequencies up to f = 20 kHz were used. In different comparative investigations it was shown that the testing method has a significant influence on fatigue life and fatigue strength. In this paper the influence of the testing method and test frequency on fatigue behaviour in the VHCF regime is presented using the example of steels and aluminium alloys and different hypotheses for the decrease in fatigue strength in this area are discussed.     

Effects of microstructural heterogeneity on very high cycle fatigue properties of 7050-T7451 aluminum alloy friction stir butt welds

In this study, the very high cycle fatigue (VHCF) properties of 7050-T7451 aluminum alloy and its friction stir welding (FSW) butt welds have been investigated. The results show that the failure of FSW joints still occurs at 7.0 × 10⁸ cycles. The fatigue properties of the FSW joints are superior to those of the base material, especially in the super long life regime. Most fatigue cracks initiate at the thermo-mechanically affected zone and heat affected zone on the advancing side of the FSW joints, and the susceptibility of these zones to fatigue is attributed to the metallurgical heterogeneity.     

Fatigue properties of combined friction stir and adhesively bonded AA6082‐T6 overlap joints

Even though friction stir welding (FSW) has been shown to produce high performing butt joints, stress concentration at the weld edges in overlap FSW significantly reduces the performance of these joints. By combining FSW and adhesive bonding into a friction stir (FS) weld bonding, joint mechanical performance is greatly improved. Quasistatic and fatigue strength of the proposed FS weld‐bonding joints was assessed and benchmarked against overlap FSW and adhesive bonding. The characterization of the structural adhesive is also presented, including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), as well as mechanical characterization with curing temperature. A small process parameter study was made to select proper FSW parameters for AA6082‐T6 overlap FSW and FS weld‐bonded joints. FS weld bonding achieved a significant increase in quasistatic and fatigue strength when compared with overlap FSW, with 79.9% of the fatigue strength of adhesive‐bonded joints at 10⁶ cycles, whereas FSW had 41.6%.     

A rapid scatter prediction method for very high cycle fatigue

Very high cycle fatigue (VHCF) tests are often performed by a high‐frequency fatigue test system, such as ultrasonic fatigue test machine. In the paper, simple VHCF tests and cumulative fatigue (low cycle fatigue plus VHCF) tests are performed to investigate the fatigue behaviour, respectively, for a low carbon manganese steel. The test results in Wöhler diagram show a large scatter in VHCF regime. Continuum damage mechanics model is extended to VHCF region to estimate the remaining fatigue life. A rapid fatigue failure probability prediction method is applied and extended to VHCF regime in order to evaluate the fatigue dispersion based on multi‐scales model and fatigue dissipation analysis.     

Untersuchungen zur Wechselwirkung von Versetzungen und Ausscheidungen in einer Nickelbasis‐Superlegierung im VHCF‐Bereich

Over the past years the fatigue behaviour of structural materials at very high number of cycles (VHCF) has become a widely acclaimed topic of scientific and technical interest. One of the most important application fields for nickel‐base superalloys are turbines, which demand fatigue lives exceeding 10⁷ cycles. Hence, in this study the interrelationship between dislocation slip behaviour, precipitation condition (peak‐aged and overaged) and fatigue behaviour of the nickel‐base superalloy Nimonic 80A was investigated in the VHCF range focusing on the influence of the predominant microstructural feature. Surprisingly, the overaged condition of Nimonic 80A shows a slightly higher fatigue strength in the VHCF regime as compared to the peak‐aged condition. Accordingly, the VHCF‐behaviour does neither correspond to the static strength nor to the fatigue behaviour in the conventional range. Microstructural features evoked by cyclic deformation (e. g. development of slip markings at surface grains and dislocation/precipitate interaction) were characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) providing preliminary explanations for the unexpected fatigue behaviour. A comparison of cyclic deformation curves both for the conventional and the VHCF range illustrates the pronounced difference in global and local strain range and its influence on the damage evolution during fatigue testing.     

Fatigue strength prediction of ultra high strength steel butt‐welded joints

First, fatigue tests were performed on butt‐welded joints made of novel direct quenched ultra high strength steel with high quality welds. Two different welding processes were used: MAG and Pulsed MAG. The weld profiles, misalignments and residual stresses were measured, and the material properties of the heat‐affected zone were determined. Fatigue tests were carried out with constant amplitude tensile loading both for joints in as‐welded condition and for joints after ultrasonic peening treatment. Finally, in fatigue strength predictions, the crack initiation phase was estimated using the procedures described by Lawrence et al. [Lawrence F V, Ho N J and Mazumdar P K (1981) Predicting the fatigue resistance of welds. Annu. Rev. Mater. Sci, 11, 401–425]. The propagation phase was simply estimated using S–N curves for normal quality butt welds, which may contain pre‐existing cracks or crack‐like defects eliminating the crack initiation stage.     

Nanograin layer formation at crack initiation region for very‐high‐cycle fatigue of a Ti–6Al–4V alloy

The microstructural features and the fatigue propensities of interior crack initiation region for very‐high‐cycle fatigue (VHCF) of a Ti–6Al–4V alloy were investigated in this paper. Fatigue tests under different stress ratios of R = ?1, ?0.5, ?0.1, 0.1 and 0.5 were conducted by ultrasonic axial cycling. The observations by SEM showed that the crack initiation of VHCF presents a fish‐eye (FiE) morphology containing a rough area (RA), and the FiE and RA are regarded as the characteristic regions for crack initiation of VHCF. Further examinations by TEM revealed that a layer of nanograins exists in the RA for the case of R = ?1, while nanograins do not appear in the FiE outside RA for the case of R = ?1, and in the RA for the case of R = 0.5, which is explained by the Numerous Cyclic Pressing model. In addition, the estimations of the fatigue propensities for interior crack initiation stage of VHCF indicated that the fatigue life consumed by RA takes a dominant part of the total fatigue life and the related crack propagation rate is rather slow.     

Notable size effects on very high cycle fatigue properties of high-strength steel

Very high cycle fatigue (VHCF) properties were compared between two types of specimens: enlarged specimens and our standard specimens. Fatigue tests were conducted by ultrasonic fatigue testing; the material used was commercial spring steel. All tests ended in internal fracture, with large-size effects observed, i.e., the enlarged specimens showed lower VHCF strength than the standard specimens. Most of the internal fracture origins were oxide-type inclusions that were larger in the enlarged specimens than in the standard specimens, indicating the size effect to be caused by the difference in oxide-type inclusion sizes at the origins of internal fractures. The large-size effect strongly urges the use of large specimens when conducting VHCF tests on high-strength steel. Moreover, the large-size effect implies that fatigue strength cannot in this case be determined using the conventional S-N curve approach, since the S-N curve depends on the specimen size. The evaluation of the VHCF strength thus needs two steps: an estimation of the maximal inclusion size, followed by an estimation of the VHCF strength based on the maximal inclusion size.     

Fatigue experimental analysis and numerical simulation of FSW joints for 2219 Al–Cu alloy

Fatigue properties of friction stir welding (FSW) butt joints for Al–Cu alloy 2219‐T6 were investigated by experimental analysis and numerical simulation. Microstructure characteristics of FSW butt joints for 2219 aluminium alloy were studied during different fatigue stages. Micro hardness values and grain sizes across the FSW joint at different cycles were measured to study the fatigue properties of the joint. Local mechanical performances of the FSW butt joints were investigated based on the micro tensile tests. Fatigue parameters of different regions in the FSW joints were obtained from the four‐point‐correlation method. The local stress and strain response of the FSW joints were obtained based on mechanical performances of the micro tension specimens. The comparison results between simulation and tests analysis show that the built finite element model is effective for estimating the weak areas for FSW joints.     

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

It is well‐known that the high cycle fatigue (HCF) strength of steel components is influenced by a lot of factors depending on both material, loading (including environment), specimen or component geometry (design), and manufacturing process. Based on a literature review of a lot of experimental data, a synthesis is proposed in this paper to discuss the effect of the structural and operational factors on the very high cycle fatigue (VHCF) characteristics of steels. HCF and VHCF regimes are distinguished in terms of failure mechanisms and S‐N curve shapes for high and low strength steels. Then, the effect of the microstructural and mechanical features on the VHCF resistance is debated as different parameters (microstructure, inclusion size type and depth, hydrogen, environment, maximum tensile strength, and residual stresses). Next, the influence of the loading conditions is addressed by taking into account both the frequency effect, the highly stressed volume, the loading type, and loading ratio. Finally, the influence of the testing techniques used in VHCF experiments is discussed.     

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

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

Interior failure mechanism and life prediction of surface‐treated 17Cr‐Ni steel under high and very high cycle fatigue

The understanding of very high cycle fatigue (VHCF) mechanisms is critical to the development of life prediction approach. For this purpose, high cycle fatigue (HCF) and VHCF properties of a surface‐treated 17Cr‐Ni steel were investigated under axial loading with stress ratio of 0. This steel exhibits the constantly decreasing S‐N characteristics associated with the inclusion‐fisheye induced failure under the HCF and the inclusion‐FGA (fine granular area)‐fisheye induced failure under the VHCF. The cyclic pressing plays an important role in the FGA formation process, but the FGA still can be observed for the stress ratio of zero due to the slight crack closure effect. Two life modelling approaches associated with related failure mechanisms in the HCF and VHCF regimes are proposed based on the agreement between experimental and predicted results.     

The influence of mode mixity and adhesive system on the fatigue life of adhesive joints

In real applications, adhesive joints are commonly subjected to fatigue and mixed mode loading conditions. The aim of the current work is to experimentally analyse the influence of mode mixity on the fatigue strength of joints with an epoxy‐based adhesive. Different adhesive systems (acrylic and epoxies) were considered for pure mode I fatigue loading conditions. To achieve this, Arcan joints with an epoxy adhesive were manufactured and tested at different mode mixities. Based on stiffness degradation, monitored during the tests, damage evolution was calculated for different loading conditions and for all the tested adhesives. Finally, fatigue envelopes were constructed for different fatigue life regimes. Results show that a shear loading component reduces both the static strength and fatigue life of the joints. A small reduction rate of the stiffness was found throughout the most part of the life until a sudden drop was observed, indicating a smooth damage evolution.     

VHCF properties of nitrided 18Ni maraging steel thin sheets with different Co and Ti content

High cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of two 18Ni maraging steels with different cobalt and titanium content and similar static strength are investigated. Ultrasonic fatigue tests are performed with thin sheets with nitrided surfaces at load ratio R = 0.1. The specimens are mounted on a carrier and are forced to joint vibrations at approximately 20 kHz. The increase of Co content and the elimination of Ti improved the HCF and VHCF strength of 18Ni maraging steel. TiN inclusions if Ti is present and Al₂O₃ inclusions in the Ti free material with sizes (area_INC)^1/2 smaller than 10 µm were preferential crack initiation locations. Considering inclusions as initial cracks, the minimum stress intensity range for VHCF failure is 1.2 MPam^1/2 for TiN inclusions and 1.8 MPam^1/2 for Al₂O₃ inclusions. Data scatter may be slightly reduced if lifetimes are presented versus stress amplitudes multiplied by (area_INC)^1/12 rather than in an S–N diagram.     

Gigacycle fatigue data sheets for advanced engineering materials

Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 10¹⁰ cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 10⁸ cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete.     

A local strain method for the evaluation of welded joints fatigue resistance: the case of thin main-plates thickness

Current procedures for evaluating fatigue strength of welded structures may not be consistent with the real fatigue behaviour of welded joints. A local strain method for the prediction of the WELded joints FAtigue REsistance (WELFARE), by local strain measurements at the weld toe, was recently proposed on the basis of fatigue tests on more than 10 series of welded joints (T, cruciform, angular and butt joints) in structural steel, with 10–25 mm main‐plate thickness. This paper reports fatigue test results obtained from 30 cruciform and butt welded joints (3–5 mm thick) under two load ratios (0.1 and ?1) in order to extend the applicability of the method to thin welded joints.     

Development of a new device to perform torsional ultrasonic fatigue testing

The interest in gaining experimental knowledge on fatigue strength of materials over 10⁹ cycles is rapidly increasing as evidenced for the large amount of investigations on this subject presented at the last very high cycle fatigue meeting (VHCF-3), held on September 2004. Most of the fatigue results presented at this conference were obtained under tension-compression, rotating bending, flexion and bending cyclic loading (some attaining 10¹⁰ cycles), using ultrasonic devices whose design was based on the natural frequency principles. In general, very little literature concerning the metallic alloys behavior under torsion cyclic loading using ultrasonic is available; however, in order to perform an accurate component design under multi-axial loading and VHCF, the material behavior under torsion cyclic loading is required. This investigation presents the development of a new mechanical device for testing and characterizing metallic alloys in the range of 10⁹–10¹⁰ cycles in torsional cyclic loading and the first experimental results for medium carbon steel (38MnSV5S). The new device was designed to excite the components under testing with pure torsional vibration mode at a frequency of 20 kHz.     

A notch stress intensity approach to assess the multiaxial fatigue strength of welded tube-to-flange joints subjected to combined loadings

Full penetration T butt weld joints between a tube and its flange are considered, subjected to pure bending, pure torsion and a combination of these loading modes. The model treats the weld toe like a sharp V‐notch, in which mode I and mode III stress distributions are combined to give an equivalent notch stress intensity factor (N‐SIF) and assess the high cycle fatigue strength of the welded joints. The N‐SIF‐based approach is then extended to low/medium cycle fatigue, considering fatigue curves for pure bending and pure torsion having the same slope or, alternatively, different slopes. The expression for the equivalent N‐SIF is justified on the basis of the variation of the deviatoric strain energy in a small volume of material surrounding the weld toe. The energy is averaged in a critical volume of radius R_C and given in closed form as a function of the mode I and mode III N‐SIFs. The value of R_C is explicitly referred to high cycle fatigue conditions, the material being modelled as isotropic and linear elastic. R_C is thought of as a material property, independent in principle of the nominal load ratio. To validate the proposal, several experimental data taken from the literature are re‐analysed. Such data were obtained by testing under pure bending, pure torsion and combined bending and torsion, welded joints made of fine‐grained Fe E 460 steel and of age‐hardened AlSi1MgMn aluminium alloy. Under high cycle fatigue conditions the critical radius R_C was found to be close to 0.40 mm for welded joints made of Fe E 460 steel and close to 0.10 mm for those made of AlSi1MgMn alloy. Under low/medium cycle fatigue, the expression for energy has been modified by using directly the experimental slopes of the pure bending and pure torsion fatigue curves.     

Gigacycle fatigue data sheets for advanced engineering materials

Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 10¹⁰ cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 10⁸ cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete. r 2007 Published by Elsevier Ltd.