Cyclic Fatigue Loading Research Articles

Evaluation of Fatigue Life of Recycled Opaque PET from Household Milk Bottle Wastes.

Polyethylene terephthalate (PET) is among the most used thermoplastic polymers in large scale manufacturing. Opaque PET is increasingly used in milk bottles to save weight and to bring a glossy white aspect due to TiO2 nanoparticles. The recyclability of opaque PET is an issue: whereas the recycling channels are well established for transparent PET, the presence of opaque PET in household wastes weakens those channels: opaque bottles cannot be mixed with transparent ones because the resulting blend is not transparent anymore. Many research efforts focus on the possibility to turn opaque PET into resources, as one key to a more circular economy. A recent study has demonstrated the improvement of the mechanical properties of recycled PET through reactive extrusion. In the present work, the lifespan of recycled opaque PET has been evaluated throughout tensile–tensile fatigue loading cycles at various steps of the recycling process: The specimens are obtained from flakes after grinding PET wastes (F-r-OPET), from a subsequent homogenization step (r-OPET-hom) and after reactive extrusion (Rex-r-OPET). Virgin PET is also considered as a comparison. First, tensile tests monitored by digital image correlation have been carried out to obtain the elastic modulus and ultimate tensile stress of each type of PET. The fatigue properties of reactive REx-r-OPET increase, probably associated with the rise of cross-linking and branching rates. The fatigue lifespan increases with the macromolecular weight. The fracture surface analysis of specimens brings new insight regarding the factors governing the fatigue behavior and the damaging mode of recycled PET. TiO2 nanoparticles act as stress concentrators, contributing to void formation at multiple sites and thus promoting the fracture process. Finally, the fatigue life of REx-r-OPET is comparable to those of virgin PET. Upcycling opaque PET by reactive extrusion may be a relevant new route to absorb some of the growing amounts of PET worldwide.

Fatigue analysis of high-pressure hydrogen storage vessel based on optimum autofrettage pressure

To improve the fatigue performance of high-pressure hydrogen storage vessels, a method is proposed to determine the optimal autofrettage pressure of the vessel using numerical simulation and experimental verification. First, a finite element model considering the actual structure of the vessel is established based on grid theory. Then, the effect of autofrettage pressure on the equivalent alternating stress amplitude of liner under cyclic fatigue load is studied by using the modified equation of Smith–Watson–Topper mean stress. Lastly, the optimum autofrettage pressure is determined according to the DOT-CFFC and CGH2R standards, and the fatigue life is predicted. The results show that under the optimum autofrettage pressure, the bearing capacity of the liner under working pressure and the fatigue life under fatigue cycle load are increased by 74% and 12 times, respectively. The experimental results are in good agreement with the numerical simulation results, with an average error of 6.3%. This research has important reference significance for the preliminary design stage of composite vessels.

Thickness and internal adjustment of monolithic resin composite milled crowns: Effect on the load-bearing capacity under fatigue

ObjectivesTo evaluate the fatigue failure load (FFL), cycles for fatigue failure (CFF) and survival rates of different occlusal thicknesses of resin composite simplified crowns with internal adjustments (IA) or without as control (C). Methods30 monolithic simplified crowns of CAD/CAM resin composite (Tetric CAD, Ivoclar) were milled in three different occlusal thicknesses (0.5 mm, 1.0 mm, and 1.5 mm). Half of the crowns were submitted to restricted adjustment with diamond burs on the crown's inner surface and half remained milled without internal adjustment. The samples were treated and adhesively luted onto a prosthetic preparation made of epoxy resin reinforced by glass-fiber substrate. The sets were subjected to a fatigue test (cyclic fatigue: initial load of 200 N; step-size of 50 N; 10,000 cycles/step; 20 Hz; maximum load: 2800 N).Microscopic analysis of tested representative samples was performed. The fatigue data were statistically analyzed (α= 0.05) and the micrograpic images were qualitatively evaluated. ResultsAll specimens from groups 1.0C and 1.5C survived the cyclic loads, while all 0.5C samples failed during the test. The 1.0IA and 1.5IA crowns showed no statistical difference for FFL (2530 N= 2670 N) and CFF (471,000 cycles= 499,000 cycles) between them, and they were both statistically superior to the 0.5IA (FFL= 1812.50 N; CFF= 327,500 cycles). The 1.0 mm and 1.5 mm crowns presented superior fatigue behavior (2530 N–2800 N) compared to the 0.5 mm crowns (1812 N–2140 N), whether internal adjustment was performed or not. A statistically significant difference was found for FFL and CFF of 0.5 mm crowns with and without internal adjustment, with a deleterious impact of the adjustment for both parameters (FFL and CFF: 0.5C > 0.5IA). SignificanceResin composite milled crowns can bear high cyclic fatigue loads despite thickness, although thicker crowns had better mechanical behavior; however, the internal adjustment in the composite resin crown leads to a detrimental effect on its fatigue behavior, which presents a risk of mechanical failure due to premature fatigue.

Phase field modeling of brittle fracture in large-deformation solid shells with the efficient quasi-Newton solution and global–local approach

To efficiently predict the crack propagation in thin-walled structures, a global–local approach for phase field modeling using large-deformation solid shell finite elements considering the enhanced assumed strain (EAS) and the assumed natural strain (ANS) methods for the alleviation of locking effects is developed in this work. Aiming at tackling the poor convergence performance of standard Newton schemes, a quasi-Newton (QN) scheme is proposed for the solution of coupled governing equations stemming from the enhanced assumed strain shell formulation in a monolithic manner. The excellent convergence performance of this QN monolithic scheme for the multi-field shell formulation is demonstrated through several paradigmatic boundary value problems, including single edge notched tension and shear, fracture of cylindrical structure under mixed loading and fatigue induced crack growth. Compared with the popular alternating minimization (AM) or staggered solution scheme, it is also found that the QN monolithic solution scheme for the phase field modeling using enhanced strain shell formulation is very efficient without the loss of robustness, and significant computational gains are observed in all the numerical examples. In addition, to further reduce the computational cost in fracture modeling of large-scale thin-walled structures, a specific global–local phase field approach for solid shell elements in the 3D setting is proposed, in which the full displacement-phase field problem is considered at the local level, while addressing only the elastic problem at the global level. Its capability is demonstrated by the modeling of a cylindrical structure subjected to both static and fatigue cyclic loading conditions, which can be appealing to industrial applications.

Rockburst characteristics of gypsum-like rocks after fatigue cyclic loading

Rockburst characteristics of gypsum-like rocks after fatigue cyclic loading

Novel porous structures with non-cubic symmetry: Synthesis, elastic anisotropy, and fatigue life behavior

Natural porous structures are often anisotropic in their elastic properties, i.e., they have directional variations that are related to their topology and geometry. This paper presents the synthesis and simulation framework of novel families of non-cubic porous structures based on implicit modeling of cosine surfaces in the three-dimensional Euclidean space. The synthesis was performed using Field-Driven Design (FDD). An in-depth study of the elastic properties and simulated fatigue compression–compression behavior of a selection of five structures from the orthotropic anisotropy family is presented as case studies exposing the stretching-dominated mechanisms. The apparent properties characterized include Young’s modulus , Poisson’s ratio , shear modulus , bulk modulus , and relative density . A systematic approach to the characterization of average apparent properties was performed according to the schemes of Voigt, Reuss, and Hill. We show the anisotropic variation of the cosine surface–based porous structures using the Universal Elastic Anisotropy Index (AU) and compare it with six well-known triply periodic minimal surface (TPMS) structures by analyzing the stiffness tensor to validate and discuss which individual property (stiffness, rigidity, compressibility) has a higher impact on the final anisotropy value. We also provide a formal curvature analysis, based on the notions of mean and Gaussian curvatures to evidence the ridge-shaped surface in the structures. The proposed porous structures showed advantages when compared to cubic TPMS structures. From the data processing of the five analyzed porous structures, two synthesized structures have AU = 0.394 and 0.478, which are lower values than the structures based on Neovius and Schwarz’s Primitive surfaces with AU = 0.529 and 0.604. In addition, simulation results of cyclic compressive fatigue loading indicate that the fatigue resistance properties of the non-cubic porous structures are higher than the TPMS structures.

Does adhesive luting promote improved fatigue performance of lithium disilicate simplified crowns?

The present study aimed to determine the influence of adhesive luting on the fatigue mechanical properties of simplified lithium disilicate crowns luted to dentin analogues preparations, and to compare two different approaches of simulation of a non-adhesive luting procedure. To this end, 30 prosthetic preparations of glass fiber-filled epoxy resin used as dentin analogues were milled and lithium disilicate crowns were pressed (1.5 mm thickness), which were allocated into three different groups: resin cement (RC), resin cement isolated by paraffin oil (RCI) and zinc phosphate cement (ZP). For luting procedures, the ceramic crowns of the RC and RCI groups had their internal surface treated with 5% hydrofluoric acid and universal primer. Afterwards, a thin layer of paraffin oil was applied onto the ceramic crown intaglio surface in the RCI group. The dentin analogues were treated with 10% hydrofluoric acid, followed by primer application and luting as recommended by the manufacturer. No previous surface treatments were performed for the ZP group, and the cement was applied as recommended. The fatigue test was run under cyclic fatigue (load 400–2000 N, step-size 100 N, 15,000 cycles/step, frequency 20 Hz) until failure detection (radial crack) by transillumination. The fatigue failure load (FFL), number of cycles until failure (CFF), and survival rate in each testing step were recorded. The RC group presented a higher FFL, CFF and survival rate compared to the other groups (p < 0.001), which were statistically equal to each other (p > 0.05). Weibull analysis detected no difference between groups for the Weibull module. Therefore, loss of adhesion between the evaluated substrates induces a significant reduction in load-bearing capacity under fatigue of the lithium disilicate crowns, strengthening the strong-importance of long-lasting adhesion for crown's survival. In addition, different methods for the simulation of non-adhesive conditions induce the same performance in the explored outcomes, confirming that both are valid approaches for laboratory studies.

Numerical simulation and experimental investigation of laser joining Ti6Al4V alloy and CFRTP with embedded Ti6Al4V alloy

Laser joining technology shows a great application prospect in the connection of Carbon Fiber Reinforced Thermoplastic Composite (CFRTP) and titanium alloy with the advantages of high production flexibility and strong heat input controllability. In this paper, a laser joining of the Ti6Al4V alloy and the CFRTP with embedded Ti6Al4V alloy is proposed to enhance the mechanical properties of laser joining CFRTP and titanium alloy. The CFRTP with embedded Ti6Al4V alloy is prepared by hot-pressing. Numerical simulations of laser joining the Ti6Al4V alloy and the CFRTP with embedded Ti6Al4V alloy are performed by applying the finite element method. To better understand the process parameters, the joining process under different laser powers and scanning speeds are simulated, and the temperature field and the shape of molten pool are calculated. In addition, the shape of molten pool calculated using the simulation model exhibits good agreement with experimental results to substantiate the established model. The microstructure characteristics and energy dispersive spectroscopy results are analyzed. The failure load, fatigue life, and failure mode for laser joining CFRTP/Ti6Al4V alloy are studied. The results indicate that the tensile failure load of the joint exceeds 10195.6 N, obtained by using the optimal process parameters based on simulation. The tensile fracture occurs in CFRTP base material. Moreover, the specimen ruptures at the overlap region of Ti6Al4V alloy under fatigue cyclic loading and the fatigue failure mode is edge through crack growth failure.

Interface wear effects in ceramic composite crack opening

For fiber-reinforced ceramic-matrix composites (CMCs), crack opening behavior controls oxidation atmosphere ingress into the internal composite, which affects the operation reliability and durability of engineering CMC components. Under cyclic fatigue loading, interface wear affects the crack opening behavior in CMCs. In this paper, the stress/cyclic-dependent crack opening behavior in CMCs is analyzed considering interface wear effect. The analytical crack opening displacement (COD) and related damage parameters (e.g., interface debonding ratio (IDR)) are determined based on the microstress analysis in the fiber and the matrix. Experimental CODs of different matrix cracks in SiC/SiC minicomposite are predicted under tensile loading. Considering cyclic-dependent interface wear effect, CODs and related damage parameters are analyzed for different cycles. Theoretical relationships between the COD, peak stress level, cycle number, and composite constitutive properties are established. Effects of constitutive properties, peak stress level, and damage state on cyclic-dependent COD and related damage parameters are discussed. Under tensile loading, with increasing tensile stress, COD first increases nonlinearly and then linearly with applied stress corresponding to change of the interface debonding from partial to complete debonding. The increasing rate of COD curve is relatively low upon initial loading due to short interface debonding range. However, the increasing rate of COD curve decrease again at high stress level at the condition of complete interface debonding. Under cyclic fatigue loading, the change of interface debonding condition with increasing applied cycles affects the COD and IDR curves.

Assessment of dynamic mode-I delamination driving force in double cantilever beam tests for fiber-reinforced polymer composite and adhesive materials

The double cantilever beam (DCB) tests are widely used to assess the interfacial delamination properties of laminated composites. For quasi-static loads, the DCB tests are standardized based on the beam mechanics; for dynamic loads, however, such as high-loading-rate impact and cyclic loads, there is no established analytical theory. This presents a significant obstacle preventing the research community from assessing the delamination behavior of composites or adhesives for their application under complex in-service loads. In this paper, the theory of evaluating dynamic mode-I delamination driving force for DCBs under general displacement loads is developed for the first time, accounting for structural vibration effects. The developed theory is demonstrated by two examples: high-loading-rate split Hopkinson bar impact and cyclic fatigue loads. The analytical solutions are validated by published experiment results and in-house tests. This work provides a fundamental analytical tool to study and assess the fracture behavior of fiber reinforced-polymer composite and adhesive materials under various loading conditions.

Coupled sliding–decohesion–compression model for a consistent description of monotonic and fatigue behavior of material interfaces

A thermodynamically consistent model proposed in this work introduces a generative symbolic framework for computational components, which can capture the inelastic behavior of interfaces with 3D kinematics in response to monotonic, cyclic, and fatigue loading histories. It contributes to a long list of existing cohesive zone models by introducing novel forms of potentials for free energy, threshold function, and dissipation to deliver the following model properties: (i) the evolution equations derived from the potentials are linked in such a way that damage develops along with the cumulative measure of plastic sliding, (ii) the convex, continuous, and smooth shape of the threshold function introduced in the effective stress domain enables a general and efficient time integration that automatically handles unloading and reloading, and (iii) the non-associative flow potential includes a scalable degree of coupling between normal and tangential damage evolution. The studies performed in the paper demonstrate that these model ingredients lead to a consistent coverage of arbitrary non-proportional loading scenarios in tangential and normal directions. For monotonic loading, the presented examples illustrate the ability of the model to reproduce the shear dilatancy, pressure sensitivity including the vertex effect, stiffness recovery in compression, and abrasion due to sliding. For cyclic fatigue loading, the studies show that the model reflects the link between the hysteretic behavior and the fatigue damage evolution. This link constitutes the basis for a physically sound description of fatigue degradation along material interfaces. It manifests itself in the presented energy breakdown with distinguished fractions of energy release due to damage and plasticity ascribed to normal and tangential directions. Two calibration and validation examples using pull-out tests exposed to monotonic and cyclic loading are provided to show the applicability of the model for the characterization of the bond between steel bars and concrete. A publicly available implementation using a computer algebra system (CAS) is provided in the form of an interactive Jupyter notebook to document the reproducibility of the results.

Research on fatigue performance and optimal design of steel-UHPC composite slab

To study the crack resistance of UHPC (Ultra-High Performance Concrete) in the negative moment region of the steel-UHPC light composite bridge deck and the performance of fatigue-prone details around the arc-shaped notch, a full-scale two-span model with double U-ribs was fabricated according to the prototype of a steel box girder and subjected to six million fatigue load cycles. In parallel with the fatigue test, a numerical model (M1) was developed in ANSYS. Using the numerical model M1, three factors affecting the composite deck mechanical performance were analyzed, including the thickness of the UHPC layer, the amount of reinforcement placed in the UHPC layer, and the shear studs spacing. Combined with the reliability theory and Miner’s cumulative damage model, the fatigue reliability of the arc-shaped notch welding details was analyzed using another segmental numerical model (M2). The test results revealed no fatigue cracks around the arc-shaped notch of the diaphragm and the surface of UHPC layer after six million loading cycles. Furthermore, there was no delamination between the UHPC and the steel plate. The numerical simulation results indicate that the upper surface tensile stress for thin UHPC increases with the reduction in the shear studs spacing, however, there is an opposite trend for thicker UHPC. Moreover, the thicker the UHPC is, the more significant the influence of the reinforcement mesh spacing on the extreme tensile stress of the UHPC upper surface is. The calculation results of the segmental numerical model showed that the fatigue detail of the arc-shaped notch welded joints on the transverse rib (clear height of 0.9 m) base metal could not meet the design requirements. Based on the analysis of the relationship between the reliability index and UHPC thickness and the transverse rib clear height, it has been found that the detail of the arc-shaped notch welded joints on the transverse rib base metal could achieve infinite fatigue life without changing other design parameters except for the transverse rib clear height.

New anterior controllable antedisplacement and fusion surgery for cervical ossification of the posterior longitudinal ligament: a biomechanical study.

The traditional anterior approach for multilevel severe cervical ossification of the posterior longitudinal ligament (OPLL) is demanding and risky. Recently, a novel surgical procedure-anterior controllable antedisplacement and fusion (ACAF)-was introduced by the authors to deal with these problems and achieve better clinical outcomes. However, to the authors' knowledge, the immediate and long-term biomechanical stability obtained after this procedure has never been evaluated. Therefore, the authors compared the postoperative biomechanical stability of ACAF with those of more traditional approaches: anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF). To determine and assess pre- and postsurgical range of motion (ROM) (2 Nm torque) in flexion-extension, lateral bending, and axial rotation in the cervical spine, the authors collected cervical areas (C1-T1) from 18 cadaveric spines. The cyclic fatigue loading test was set up with a 3-Nm cycled load (2 Hz, 3000 cycles). All samples used in this study were randomly divided into three groups according to surgical procedures: ACDF, ACAF, and ACCF. The spines were tested under the following conditions: 1) intact state flexibility test; 2) postoperative model (ACDF, ACAF, ACCF) flexibility test; 3) cyclic loading (n = 3000); and 4) fatigue model flexibility test. After operations were performed on the cadaveric spines, the segmental and total postoperative ROM values in all directions showed significant reductions for all groups. Then, the ROMs tended to increase during the fatigue test. No significant crossover effect was detected between evaluation time and operation method. Therefore, segmental and total ROM change trends were parallel among the three groups. However, the postoperative and fatigue ROMs in the ACCF group tended to be larger in all directions. No significant differences between these ROMs were detected in the ACDF and ACAF groups. This in vitro biomechanical study demonstrated that the biomechanical stability levels for ACAF and ACDF were similar and were both significantly greater than that of ACCF. The clinical superiority of ACAF combined with our current results showed that this procedure is likely to be an acceptable alternative method for multilevel cervical OPLL treatment.

Effects of Non-Metallic Inclusions and Mean Stress on Axial and Torsion Very High Cycle Fatigue of SWOSC-V Spring Steel

Inclusion-initiated fracture in high-strength spring steel is studied for axial and torsion very high cycle fatigue (VHCF) loading at load ratios of R = −1, 0.1 and 0.35. Ultrasonic S-N tests are performed with SWOSC-V steel featuring intentionally increased numbers and sizes of non-metallic inclusions. The fatigue limit for axial and torsion loading is considered the threshold for mode I cracks starting at internal inclusions. The influence of inclusion size and Vickers hardness on cyclic strength is well predicted with Murakami and Endo’s area parameter model. In the presence of similarly sized inclusions, stress biaxiality is considered by a ratio of torsion to axial fatigue strength of 0.86. Load ratio sensitivity is accounted for by the factor ((1 − R)/2)α, with α being 0.41 for axial and 0.55 for torsion loading. VHCF properties under torsion loading cannot appropriately be deduced from axial data. In contrast to axial loading, the defect sensitivity for torsion loading increases significantly with superimposed static mean load, and no inclusion-initiated fracture is found at R = −1. Size effects and the stress gradient effective under torsion loading are considered to explain smaller crack initiating inclusions found in torsion ultrasonic fatigue tests.

Mean-stress sensitivity of an ultrahigh-strength steel under uniaxial and torsional high and very high cycle fatigue loading.

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = -1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 105 cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by and .

Non‐destructive fatigue damage detection of carbon fiber‐reinforced thermoplastics using positron annihilation method

AbstractTensile axial fatigue tests were carried out using carbon fiber‐reinforced thermoplastics (CFRTP) plate with a hole notch up to very high cycle fatigue (VHCF) regime, where number of loading cycles was 2 × 107. The fatigue test was periodically interrupted, and fatigue damage accumulation around the hole notch was non‐destructively detected by two positron annihilation methods. Positron annihilation lifetime spectroscopy was applied to obtain positron lifetime and Doppler broadening method to obtain S‐parameter. Both parameters increased with increasing fatigue loading cycles. Fatigue damage detected by two positron annihilation methods was attributed to the expansion of free volume in the polymer matrix. The estimated size of free volume was sub‐nanoscale according to the experimental value of the positron annihilation lifetime. SEM and μCT scan revealed that the macroscopic fatigue damages, such as surface cracking, transverse cracking and delamination, increased with increasing fatigue loading cycles, which indicated that sub‐nanoscale and macroscopic fatigue damages progressed simultaneously. Consequently, it can be concluded that the positron annihilation methods can be used for the non‐destructive fatigue damage detection of CFRTP.

A Digital Image Correlation (DIC) prototype system for crack propagation monitoring in aircraft assemblies

Background: In the Clean Sky 2 project DIMES, the cyclic loading of a section of an A320 wing with pre-existing damage was carried out. Methods: We present a Digital Image Correlation (DIC) prototype system to monitor crack propagation in the aircraft wing. This system includes a mount for easy installation and adjustment in a confined space. Results: Strain localization and evaluation due to crack propagation was successfully observed in the Region-of-Interest (ROI) during cyclic fatigue loading. The results from the DIC prototype system were supported by conventional contact Resistance Strain Gauge (RSG) sensors acting as a far-field monitor. Conclusions: Future improvements, the combination of two DIC modules for a stereo DIC system and the potential of the DIC system for ground-based tests and Structural Health Monitoring (SHM) applications are also discussed.

An adaptive floating node based formulation for the analysis of multiple delaminations under high cycle fatigue loading

A novel efficient numerical formulation for the analysis of multiple fatigue-driven delamination cracks is presented. A cohesive zone model is used in combination with an Adaptive Refinement Scheme (ARS) and an Adaptive Floating Node Method (A-FNM) element that refine the model effectively during the analysis. Novel techniques are proposed to track the positions of multiple crack tips and calculate the mode decomposed energy release rates for the individual crack tips using the J-integral. The method has been implemented in a Matlab finite element code and validated with single and multiple delamination cases with varying mode mixities. Comparisons with theoretically based predictions and available experimental data showcase the high accuracy of the method. The presented method lowers the computational time compared to standard, fully refined finite element models by a factor of 4–5.

Comparison of wrought and additively manufactured IN718 concerning crack growth threshold and fatigue crack growth behaviour

Rotary components in turbine machinery such as gas turbine blades and discs or turbocharger wheels need to be designed so that high cycle fatigue (HCF) loads are well below the fatigue endurance limit to ensure safe operation. This requires an accurate measurement of the fatigue endurance limit and the related intrinsic fatigue crack growth (FCG) threshold. Furthermore, knowing the FCG threshold is crucial in order to assess the criticality of initial defects (e.g. cavities, carbide nests, etc.). In this study, the influence of the manufacturing route on the FCG threshold and the FCG behaviour at 650 °C in air of the nickel based superalloy IN718 is investigated. For this, a per Laser Powder Bed Fusion (LPBF) process manufactured version is compared to a conventionally wrought IN718 material state, both in the short and long crack regime. To measure crack growth increments of about 1 μm, an alternative pre-cracking and threshold test procedure is proposed for high temperature testing. To assess the behaviour of both material states in the short crack growth regime, cyclic R-curves have been generated taking the influence of two different R ratios into account. A further comparison of the FCG behaviour of the LPBF and the wrought material state is made on the basis of da/dN-ΔKI plots, with a focus on the Paris regime created from both classical FCG tests as well as continued threshold tests. The results observed are discussed in relation to the initial characterization of the two material states, which includes tensile tests at room temperature and at 650 °C as well as microstructural investigations.

Advancement on optical methods in stress dead-zone characterisation and SIF evaluation

The work described in this paper is a study of the stress dead zone (SDZ) concept with advanced full-field optical methods. The SDZ is a region located around the fatigue crack flanks, adding a negligible contribution to internal stresses on the global deformation energy calculation. In the analytical modelling, this area can be removed from the structural member, resulting in a geometry simplification, and in the possibility of using analytical formulations in the Stress Intensity Factor (SIF) calculation associated to the existing fatigue crack. In a fatigue-testing program, aluminium middle tension (MT) specimens were prepared and then subjected to a cyclic fatigue loading, in order to generate different crack lengths for each specimen. Then, the fatigued specimens are monotonically tested under a uniaxial tensile loading to assess the SDZ concept.The experimental data is collected and analysed through two full-field high-resolution optical techniques with different system resolutions, Digital Image Correlation (DIC), and Electronic Speckle Pattern Interferometry (ESPI). As a result, the SDZ is geometrically characterized through a set of mathematical formulations based on the compliance function for the studied specimens, and is experimentally identified through deformation fields obtained from both DIC and ESPI techniques. The obtained results from both high-resolution methods verify the validity of the concept and demonstrate that it can be used to simplify the analysis.