Fatigue Tests Of Specimens Research Articles

Lightweight Design and Additive Manufacturing of a Fatigue-Critical Automotive Component

<div class="section abstract"><div class="htmlview paragraph">Metal additive manufacturing is finding growing applicability in motorsports and high performance car sectors. Laser-Powder Bed Fusion (L-PBF) is the most developed AM technology for lightweight aluminum alloys producing near-net-shape components of complex geometry that achieve outstanding lightweight targets. A key issue in the widespread industrial acceptability of L-PBF is the structural integrity of these lightweight components when subjected to dynamic loading conditions because it requires in-depth knowledge of the fatigue behavior of L-PBF aluminum under the combined effect of stress gradients, residual stresses, surface condition and process-induced internal defects. This contribution overviews the presentation and application of an integrated design workflow that includes the following phases: i) a topological optimization phase to achieve lightweight targets, ii) a design for additive manufacturing phase supported by process simulation to optimize part production, iii) actual part fabrication in an industrial-grade L-PBF system using AlSi10Mg alloy powder followed by post fabrication heat treatment iv) production and fatigue testing of witness specimens providing relevant design data; v) fatigue testing of the parts to determine actual performance and vi) final assessment of the design know-how developed for fatigue-critical additively manufactured metal components.</div></div>

Fracture mechanics investigation of crack coalescence in a steel tubular T-joint specimen

Fatigue test of steel tubular T-joint specimens was performed. A beach mark (BM) technique was employed to measure the crack size. Some rare fracture patterns were found. One of the specimens generated two major surface cracks around the weld toe. They were independently propagating at the initial stages and were coalesced into a single surface crack at a certain amount of cyclic loads. Subsequently, the crack was enlarged and became a doubly curved surface crack. Crack propagation (CP) simulation is performed to investigate the fracture behaviors. Extended finite element method (X-FEM) option in an open-source software Code_Aster is used. FEM CP simulation is also performed to validate the X-FEM results. The CP phenomena are carefully examined using the fracture mechanics parameters and crack front trajectories.

Improvement of fatigue performance of laser powder bed fusion fabricated IN625 and IN718 superalloys via shot peening

This study aims at investigating the possibility of fabricating IN625 and IN718 Ni-based superalloys with acceptable room temperature fatigue lives using the laser powder bed fusion (LPBF) process. For this purpose, several test coupons were printed and compared in terms of (i) density, (ii) tensile strength, (iii) surface roughness, and (iv) residual stress to find the optimum sets of process parameters utilized to print fatigue test specimens. Fatigue test was performed on the as-built and shot-peened specimens at multiple stress levels and compared to their wrought counterparts. The fatigue results were scrutinized by discussing the contributing factors, such as (i) surface roughness, (ii) residual stress state and magnitude, (iii) grain and sub-grain structure, and (iv) non-equilibrium phases in the microstructure. The fatigue life of both superalloys in as-built condition was inferior to their unnotched wrought counterparts, though exceeding the notched ones. After shot peening, the maximum stress level at infinite life for IN625 was the same as its unnotched annealed wrought counterpart. However, for shot-peened IN718, the fatigue life was still lower than that of the solution-annealed and aged wrought specimen in unnotched condition due to the absence of γ′ and γ′′ strengthening agents. This study suggests that the LPBF fabricated IN625 in the shot-peened state can be reliably used as a substitute for annealed and wrought IN625 counterparts without the need for surface smoothening or heat treatments. Regarding the shot-peened IN718, its lower fatigue life has to be considered in the design stage upon skipping the heat treatment.

Fatigue properties and S-N curve estimating of 316L stainless steel prepared by SLM

This paper investigates the fatigue properties of 316L stainless steel manufactured by selective laser melting technology (SLM) under hot-isostatic-pressing (HIP) heat-treatment. The fatigue tests of 316L specimens were carried out under pulse tensile cyclic loading to obtain the fatigue performance. To avoid falling into local optima and improve the convergence speed, the dynamic multiswarm particle swarm optimizer (DMS-PSO) algorithm was first introduced for parameter optimization of the three-parameter Weibull distribution model, and then the S-N curves based on the model were obtained. The S-N curves were highly consistent with the metallographic and fractographic phenomena. Besides, both the S-N curves and fractographic analysis show that the resistance of high-stress fatigue fracture is significantly improved by using HIP heat-treatment.

Experimental Research and Fatigue Life Prediction of Ultra-High-Strength Steel Aermet100

This study concentrates on the fatigue performance of ultra-high-strength steel Aermet100 under different loading rates. The standard specimen measured the static mechanical properties of Aermet100 steel, based on which the basic mechanical properties and fracture characteristics of the sample before and after necking was obtained. To take the strain rate effect into account, this study uses the dynamic constitutive model Johnson-Cook. The equation parameters are fitted through dynamic mechanical tests and quasi-static tests. This model is input into ABAQUS user-defined program afterward. Referring to the work done above, along with the extended finite element method (XFEM), this study establishes the dynamic fracture finite element model of the Aermet100 steel specimen on the basis of the continuous damage mechanics. Five groups of specimen fatigue tests were carried out in the laboratory. Simulation results show the feasibility and accuracy of the integrated XFEM model with the same loading and boundary conditions. The experimental data and simulation results prove that, in the loading time range of 0.0001 ~ 1s, the life cycles increase as the loading rate increases. It is worth mentioning that when the loading time is in the order of 0.0001s, the life changes significantly.

Large-Scale Fatigue Testing Based on the Rotating Beam Method

A large-scale fatigue testing machine based on the rotating beam method in a four-point bending configuration was designed and built. With the device, high-strength metal specimens with a 32-mm gauge diameter and a 100-mm gauge length can be tested at a cyclic frequency of up to 48 Hz. In this work, particular attention was paid to evaluating the spatial and temporal uniformity of the loading within the large specimen; methods for quantitative evaluation of these effects were developed. The developed test methodology allows for the fatigue testing of specimens with size, microstructure, and surface conditions that are similar to actual machine parts.

Prediction of low-cycle crack initiation life of powder superalloy FGH96 with inclusions based on damage mechanics

The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage characterization parameter was proposed after the construction of damage evolution equations. Fatigue tests of the powder superalloy specimens with and without inclusion were conducted at 530 and 600 °C, and the model verification was carried out for specimens with elliptical, semi-elliptical, polygon and strip-shaped surface/subsurface inclusion. The stress analysis was performed by finite element simulation and the predicted life was calculated. The results showed a satisfying agreement between predicted and experimental life.

Optically Pumped Magnetometer Measuring Fatigue-Induced Damage in Steel

Uniaxial fatigue testing of micro-mechanical metallic specimens can provide valuable insight into damage formation. Magnetic and piezomagnetic testing are commonly used for qualitative characterization of damage in ferromagnetic specimens. Sensitive and accurate measurements with magnetic sensors is a key part of such a characterization. This work presents an experimental setup to induce structural defects in a micro-mechanical fatigue test. Simultaneously, the resulting piezomagnetic signals are measured during the complete lifetime of the tested specimen. The key component is a highly sensitive optically pumped magnetometer (OPM) used to measure the piezomagnetic hysteresis of a small specimen whose structural defects can be analyzed on a small scale by other metallographic characterization methods as well. This setup aims to quantify the magnetic signatures of damage during the fatigue process, which could enable non-destructive mechanical testing of materials. This paper reports the initial results obtained from this novel micro-magneto-mechanical test setup for a ferritic steel specimen.

Side-groove effect on fracture mechanical fatigue testing of PLA material

PLA polymer is probably the most used thermoplastic material in FDM technology nowadays. Besides prototyping purposes, FDM materials are also considered for functional application use, thanks to the fast fabrication of components, relatively simple workflow, and the absence of material waste that this technology offers. Except for static material properties, data concerning the expected lifetime and reliability of AM parts under cyclic loading are also necessary for functional purposes. Due to the exceptional structural complexity of FDM parts, minor modifications in material testing must be used sometimes. The subject of this paper is the fracture mechanics-based fatigue testing of CT specimens that results in crack kinetics description like the Paris’ law. One of the objectives is also to describe the effect of SGs placed on the surface of CT specimens, and the effect of different layer heights. The main intention of the SGs is to secure a straight crack propagation path, thus imposing the crack to follow the Mode I condition, predominantly. A set of regular CT specimens and specimens with SGs was prepared with a full infill density interior and with different layer heights (0.1 mm and 0.3 mm). Crack kinetics measured on these specimens are presented in this paper and discussed.

FATIGUE OF ADHESIVE BONDING: WORLD FIRST FATIGUE S-N CURVE FOR FPSO APPLICATION

The ageing FPSOs (>10 years) induces frequent and costly structural maintenance operations in hazardous environment. The challenge is to perform these offshore structural maintenance operations with no production disruption while maximizing safety even for the most stressed areas such as a mid-ship deck plating.COLDSHIELD, a co-development between COLD PAD, TOTAL and IFP Energies nouvelles, has been set up to meet this very challenge. This CLASS approved innovative alternative to “crop and renew” stops the corrosion and restores hull strength. Among the steps to be taken to demonstrate that such repair can be considered as permanent, it was mandatory to characterize the fatigue behavior of this reinforcement solution.For the time being there is – to our knowledge - no S-N curve for structural bonded reinforcements. Full scale coupon specimen fatigue tests of the structural bonded reinforcements were conducted by COLD PAD, third-partied by Bureau Veritas and in collaboration with two laboratories (private and public) in order to study the adhesive fatigue. Statistical analysis was performed according to international standards. The resulting S-N curve is fit for industrial fatigue design. It demonstrates a comfortable fatigue strength (S-N curve presenting a slope of 9).This paper presents the results of a fatigue life assessment campaign of COLDSHIELD including the experimental setup, the fatigue test results, and the numerical analyses. It details the reason why the design is compatible with a stress-based approach. It also explains the conclusions that can be derived in terms of fatigue life for a deck repair campaign.

Evaluation of the effect of MWCNT amount and dispersion on bending fatigue properties of non-crimp CFRP composites

Multi-walled carbon nanotubes (MWCNTs) have many unique features that improve composite material properties, including a large surface area, thermal and chemical stability, and thermal and electrical conductivity. Our study aimed to examine the influence of MWCNT weight percentage on the fatigue characteristics of a polymer matrix composite strengthened with carbon fiber in fabric form. This study aimed to explore the additional properties and functions of these MWCNTs. For the experiments, MWCNTs were prepared by mixing them in solution with epoxy in three different weight percentages (0.5 %, 1.0%, and 1.5%) using a surfactant, and at 0.5 wt% without a surfactant. The prepared solutions were injected into four-axis non-crimp carbon fiber fabric plates via the vacuity mixture method and then cured for 48 h to produce nanocomposite plates. Using five types of composites, fatigue tests were carried out at six different amplitudes and a fixed frequency. Fatigue test specimens were subjected to detailed SEM analysis of their fatigue fracture surfaces. The investigational results revealed that the 0.5 wt% MWCNT epoxy solution had been homogeneously dispersed and exhibited the highest absorbance spectrum. The 0.5 wt% MWCNT specimen achieved the highest value in the fatigue life experiments. In addition, electron microscopy studies of the fracture surfaces of the test specimens determined that individual COOH-MWCNTs interrelated with the fibers and formed a bridge between fiber and matrix, whereas agglomerations caused weak bonding between the reinforcement fibers and matrix, making the material more brittle.

Three-Dimensional analysis of mixed mode Compact-Tension-Shear (CTS) Specimens: Stress intensity Factors, T-stresses and crack initiation angles

In order to characterize the crack tip stress field in three-dimensional fatigue and fracture problems, stress intensity factor and T-stress solutions are usually necessary. In this paper, extensive three-dimensional (3D) finite element models are constructed for Compact-Tension-Shear (CTS) specimen, which is one of the most widely used specimens for fatigue and fracture tests under mixed-mode I&II loading. In-plane and out-of-plane geometric dimensions including varying crack depths ratios (a/W = 0.3, 0.5 and 0.7) and thicknesses ratios (B/W = 0.1, 0.3 and 0.5) are considered under different loading angles (β = 0°, 15°, 30°, 45°, 60°, 75° and 90°). The results of stress intensity factors (KI,KII and KIII) and T-stresses (T11 and T33) along the crack front are computed and analyzed. The combined effects of geometric dimensions and loading angles on these fracture parameters have been illustrated. The solutions of the parameters are then described using empirical formulae by fitting numerical results with the least-squares method. Solutions obtained have been used to predict the crack initiation angles for CTS specimens based on mixed-mode fracture criteria including the maximum tangential stress (MTS) criterion, the generalized MTS (GMTS) criterion, the strain energy density (SED) criterion and the generalized SED (GSED) criterion. Solutions are useful for fracture and fatigue problems of mixed-mode I&II, such as analysis of the constraint effects on fracture toughness and propagation path of fatigue crack.

AN EVALUATION OF PARTS’ FATIGUE RESISTANCE AFTER SURFACE THERMO CHEMICAL MACHINING

In order to raise a fatigue resistance the calculation-experimental method of parts surface thermo chemical machining efficiency has been carried out. The employment of two types of reference specimens in experiments has been substantiated. The employment of smooth reference specimens for a determination of residual stresses in a surface layer and of corset reference specimens for fatigue tests has been recommended. The results of experimental study of two types of thermo chemical machining (cementation and nitriding) influence on a fatigue resistance of corset specimens made of high-strength allows ВКС-5 и ВНС-17 have been used in the work. . The calculations of the study have been carried out by the Finite Elements Modeling method using the calculation complexes ANSYS and PATRAN/NASTRAN. The residual strained state modeling is carried out by thermo elasticity method. On grounds of conducted calculation its been stated that an axial residual stresses distribution in a dangerous (smallest) section of a corset specimen doesnt practically differ from a similar distribution in smooth specimens. An endurance limit increment due to corset specimens hardening was determined with use of the average integral residual stresses criterion. Carried out results let also maintain that the optimal regimes of thermo chemical machining are the regimes under which average integral residual stresses are maximum.

Low-cycle fatigue life and duration-of-load effect for hybrid CLT fabricated from lumber and OSB

Given the nature of cross-laminated timber (CLT), with an orthogonal arrangement and rolling shear characteristics of transverse layers, it has a more complex duration-of-load (DOL) effect than other engineered wood products, such as glued-laminated timber. Developing hybrid CLT (HCLT) with structural composite lumber, which can improve the mechanical properties and material sources of CLT, is gaining increasing interest. Construction oriented strand board (COSB) and spruce-pine-fir (SPF) dimension lumber were used to fabricate HCLT specimens for this study. The regular SPF CLT and HCLT specimens having different lay-ups and numbers of layers were subjected to low-cycle fatigue bending and shear loading tests. The Foschi and Yao damage accumulation model was calibrated using the test results. The DOL factors for the CLT and HCLT specimens were calculated using the stress ratio evaluation method. Results showed that the primary failure modes for the fatigue test specimens were the same as those observed in previous short-term ramp loading tests, and a good fit was observed between the model's predictions and the test data. The DOL factor corresponding to a load duration of 50 years, for a three-layer shear CLT specimen, was determined to be 0.53, which was lower than that recommended in CSA O86-19. The fatigue life and long-term performance of CLT were improved by using COSB panels as CLT layers. The HCLT specimens that used COSB only as the transverse layer showed advantages in fatigue life, long-term performance, and practical applications.

Prediction of strength and fatigue life for 2D plain-woven high-alumina fiber reinforced alumina matrix composites under a complex in-plane stress state

This paper presents a strength criterion and fatigue life prediction method for 2D braided alumina matrix composites under a complex in-plane stress state. The static strength of the material was obtained by in-plane tensile, compression, and pure shear tests. Considering the difference between tensile and compressive properties of materials and the influence mechanism of in-plane tensile and shear coupling on material strength, a revised Hoffman strength theory was proposed. The predicted off-axis tensile strength is consistent with the test results, and the deviation is not more than 10%. Tensile fatigue tests were carried out with the off-axis angle θ=0°, 15°, 30°, 45°, the stress ratio R=0.1, and frequency f=10 Hz. The test results show that the fatigue life decreases with the increase of off-axis angle. Due to the in-plane shear stress component, the fatigue failure is gradually changed from fiber-dominated to fiber-matrix dominated mode. Based on a combination of the uniaxial tensile fatigue life curve, the Broutman-Sahu residual strength model, which is used to characterize the variation of the residual strength with the fatigue cycles, and the modified Hoffman strength theory, the paper proposes a fatigue life prediction model under complex in-plane loading conditions. The fatigue shear damage factor is defined to characterize the effect of the normal and shear stress interaction on fatigue life. The fatigue life prediction model is used to predict the fatigue life of specimens in the off-axis tensile fatigue tests. The predicted result agrees with the test result, and the deviation is within the 1-time life span. The results indicate that the proposed fatigue life prediction model can be used to predict the fatigue life of 2D braided alumina matrix composites under the complex in-plane stress condition with the given stress ratio, temperature, and fatigue load frequency.

The Low-Cycle Fatigue Behavior, Failure Mechanism and Prediction of SLM Ti-6Al-4V Alloy with Different Heat Treatment Methods.

Selective laser melting (SLM) is a promising additive manufacturing (AM) process for high-strength or high-manufacturing-cost metals such as Ti-6Al-4V widely applied in aeronautical industry components with high material waste or complex geometry. However, one of the main challenges of AM parts is the variability in fatigue properties. In this study, standard cyclic fatigue and monotonic tensile testing specimens were fabricated by SLM and subsequently heat treated using the standard heat treatment (HT) or hot isostatic pressing (HIP) methods. All the specimens were post-treated to relieve the residual stress and subsequently machined to the same surface finishing. These specimens were tested in the low-cycle fatigue (LCF) regime. The effects of post-process methods on the failure mechanisms were observed using scanning electron microscopy (SEM) and optical microscopy (OM) characterization methods. While the tensile test results showed that specimens with different post-process treatment methods have similar tensile strength, the LCF test revealed that no significant difference exists between HT and HIP specimens. Based on the results, critical factors influencing the LCF properties are discussed. Furthermore, a microstructure-based multistage fatigue model was employed to predict the LCF life. The results show good agreement with the experiment.

Characterization and Validation of Fatigue Strains for Superelastic Nitinol Using Digital Image Correlation

Abstract Fatigue is a major challenge encountered in cardiovascular implant design. While the properly heat-treated Nitinol can exhibit up to 6–7% recoverable strains allowing for minimally invasive transcatheter delivery of cardiovascular implants, the cyclic in vivo loading can cause premature fracture of the implant if the fatigue strain is too high. Strain-based criteria have been adopted for the development of Nitinol fatigue resistance. Lacking experimental tools to characterize the local material fatigue strain, fatigue testing of Nitinol specimens has largely relied on the finite element analysis to compute the cyclic strain amplitude and mean strain based on experimentally derived constitutive parameters using phenomenological strain energy theory. Without a consistent computational standard, previous works have resulted in controversy and inconsistency in the impact of mean strain on the fatigue resistance of Nitinol in terms of strain amplitude limit at high cycle fatigue regime. In this paper, digital image correlation (DIC) technique is used to experimentally determine local material strains of Nitinol fatigue specimens using monotonic and cyclic loading conditions. These local strains are compared with strains computed from finite element analysis. It was found that strains from DIC and FEA are comparable in the single-phase states (pure austenitic or martensitic), whereas the measured strains can show significant difference from simulation computed strain during the transformation stage where both austenite and martensite phase co-exist. These observations have significant implications to nitinol fatigue testing and implant reliability assessment.

Effect of Surface Integrity on Hot Fatigue Life of Ti2AlNb Intermetallic Alloy.

The effect of surface integrity on the hot fatigue performance of Ti2AlNb alloy was investigated. A turning process was used to prepare the standard specimens for hot fatigue tests. The surface integrity characterization and axial fatigue tests were performed. The results show that the influence of surface roughness on the hot fatigue performance of the Ti2AlNb alloy is a secondary factor. The compressive residual stress and enhanced microhardness in the surface layer has a significant effect on the hot fatigue life and they are dominant in the hot fatigue behavior of the Ti2AlNb alloy. Through the investigation on the characteristics of the fatigue fractures, the fatigue propagation process was significantly suppressed because of the strong residual compressive stress and microhardness distribution on the surface layer of the Ti2AlNb specimen.

Efficient Experimental and Data-Centered Workflow for Microstructure-Based Fatigue Data

BackgroundEarly fatigue mechanisms for various materials are yet to be unveiled for the (very) high-cycle fatigue (VHCF) regime. This can be ascribed to a lack of available data capturing initial fatigue damage evolution, which continues to adversely affect data scientists and computational modeling experts attempting to derive microstructural dependencies from small sample size data and incomplete feature representations.ObjectiveThe aim of this work is to address this lack and to drive the digital transformation of materials such that future virtual component design can be rendered more reliable and more efficient. Achieving this relies on fatigue models that comprehensively capture all relevant dependencies.MethodsTo this end, this work proposes a combined experimental and data post-processing workflow to establish multimodal fatigue crack initiation and propagation data sets efficiently. It evolves around fatigue testing of mesoscale specimens to increase damage detection sensitivity, data fusion through multimodal registration to address data heterogeneity, and image-based data-driven damage localization.ResultsA workflow with a high degree of automation is established, that links large distortion-corrected microstructure data with damage localization and evolution kinetics. The workflow enables cycling up to the VHCF regime in comparatively short time spans, while maintaining unprecedented time resolution of damage evolution. Resulting data sets capture the interaction of damage with microstructural features and hold the potential to unravel a mechanistic understanding.ConclusionsThe proposed workflow lays the foundation for future data mining and data-driven modeling of microstructural fatigue by providing statistically meaningful data sets extendable to a wide range of materials.

A new method for ultrasonic fatigue testing of equibiaxial and pure shear cruciform specimens

Mechanical and material testing and analysis are of indisputable importance. As the variety of materials and/or the complexity of products grows, so does the need for new material characterisation methods. In this study, experimental results from a new method for ultrasonic fatigue testing of equibiaxial and pure shear cruciform specimens is presented and validated. The method uses specially designed cruciform specimens’ geometries for ultrasonic fatigue testing machines. Three different cruciform geometries were tested. The responses from the specimens were first studied by an adapted modal calculation method FDD. The new method allows determining fatigue strength of cruciform specimens in the VHCF regime with very satisfactory results.