The present paper is dedicated to the mechanical and fracture characterisation of a specific earthen material, that is, the shot-earth 772. Although such a material has been recently characterised from a microstructural, chemical and physical point of view, the knowledge of its mechanical and fracture properties (essential for extending its use in construction industry) is still lacking. Such characterisations are here performed both experimentally, through laboratory tests, and numerically, through a FE model. The experimental tests (i.e. flexural, compression and fracture tests) are carried out on shot-earth specimens according to Recommendations available for concrete and a method proposed by some of the present authors, named Modified Two-Parameter Model. The numerical analyses are performed by employing a micromechanical model (implemented in a non-linear 2D FE homemade code), which allows to simulate both flexural and fracture behaviour of the shot-earth examined. Finally, the obtained numerical results are compared with the experimental ones.
In order to provide a sufficient theoretical basis for the fatigue resistant design of the aircraft wheels, strain-controlled low-cycle fatigue (LCF) tests were carried out on specimens machined in the extrusion direction (ED) and transverse direction (TD) of die-forged 2014 aluminum alloy wheel. Although the TD specimens show lower tensile strength and yield strength, the fatigue test results reveal that the TD specimens show superior fatigue life compared with the ED specimens at total strain amplitudes of 0.5% ~ 0.8%. This is predominantly caused by the Al 12(MnSi) 2(FeCu) intermetallic particles near the surface layer lead to a relatively short crack initiation stage for the ED specimens. In contrast, TD specimens with finer and more uniform recrystallized grains have better resistance to fatigue crack initiation (FCI) and propagation (FCP).
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 2×10 5 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 σ w R = σ w R = - 1 ∙ 1 - R 2 0.63 and τ w R = τ w R = - 1 ∙ 1 - R 2 0.63 .
The synergistic combination of carbon nanotubes (CNTs) and ductile thermoplastic resin has shown large potential in the improvement of fracture resistance for the epoxy matrix composites using the interleaving toughening method in recent years. The hybrid structure of CNTs and thermoplastic resin in interlayers affects directly the interlaminar structure and the resultant crack propagation path of the interleaved composites. In this work, the CNTs and thermoplastic polyetherketone-cardo (PEK-C) were used to prepare the interlayer with different hybrid structures to interleave the carbon fiber reinforced epoxy composites and the influence of hybrid structure on the interlaminar structure and the fracture toughness was investigated. The results showed that PEK-C/CNT/PEK-C sandwich interlayer produced the best toughening effect in mode I interlaminar fracture toughness (GIC) and the GIC was 446.76 J/m 2, increased by 138.11% compared to blank composites, which benefited from the multilayered structure in the interlaminar region formed during curing process and the resultant tortuous crack propagation.
In order to make full use of the potential fatigue crack growth resistance provided by layered architectures, a validated crack path simulation algorithm for crack propagation through different elements of the layered architectures was established. The crack path approaching a material interface was predicted by using the maximum tangential strain (MTSN) criterion and the crack behaviour at the interface was simulated by a developed two-step method (a modified stress-and-energy-based cohesive zone method considering the change in direction of an interface penetrating crack). The crack path simulation by using this algorithm in layered example architectures indicates: 1) there are two criteria zones for the transition between crack deflection and penetration in terms of the relationship between interfacial strength and toughness; 2) the likelihood of a crack deflecting out of the interface will increase with the propagation of an interfacial crack and 3) the architecture difference which affects shielding or anti-shielding behaviour has a significant effect on crack deflection or penetration events.
The stress ratio ( R) effect, especially at elevated temperatures, is associated with the fatigue crack growth (FCG) prediction in aero-engine hot-end components under complex operation conditions. The FCG experiments with three R were conducted on Ni-based superalloy GH4169 at room temperature (RT), 550 °C, and 700 °C. The results indicate that the R-effect on the FCG of GH4196 is temperature-dependent. Therefore, efforts were made to identify the R-effect at various temperatures to describe the FCG. The concept of the crack-closure or the two-driving-force was examined to quantify the R-effect considering the temperature influence. Fractographic analyses on the fracture surface were performed to discuss the underlying mechanism responsible for the temperature influence. The study can contribute to the R-dependent FCG modeling at various temperatures.
The fatigue behaviour of continuous wire polymer composite (CWPC) fabricated by fused filament fabrication was investigated. Four compositions were examined: polylactic acid (PLA), PLA with copper wire (Cu), thermoplastic polyurethane (TPU), and TPU with Cu wire. Residual properties were measured after different sets of number of cycles (10 2, 10 4, 10 5). Residual strengths were 89.736% and 70.464% of the ultimate tensile strength of the original material after 10 5 cycles for PLA CWPC and TPU CWPC, respectively. A one-way analysis of variance (ANOVA) statistical tests showed insignificant changes in the residual strengths of PLA-based materials after an increasing number of cycles and significant changes for the TPU-based materials. CWPC electromechanical properties under fatigue test demonstrated reverse piezoresistivity behavior. A strain-controlled fatigue life analytical model was compared to the experimental results showing good agreement. This study demonstrates the applicability of FFF technique to print sensors with continuous integrated wire with tunable properties.
The maximum tangential stress (MTS) criterion is one of the most widely used criteria for predicting the direction of crack extension. The suitability of this criterion is examined under different loading conditions using extended finite element method (XFEM). Experimental and numerical results reported in the literature are considered to evaluate the validity and accuracy of the criterion. The results demonstrate that the MTS criterion evaluated by stress intensity factors (SIF) can accurately predict the direction of crack propagation in specimens under direct tensile loading. This criterion overestimates the angle of crack initiation in the specimen under indirect tensile loading, but underestimates the angle in the specimen subjected to three-point bending. It is concluded that the MTS criterion based on SIF could not accurately predict the direction of the crack initiation, which could, however, be determined properly based on the stress distribution around the crack tip obtained by XFEM numerical models.
The process of fatigue spalling in the rings of ball bearings at durability exceeding 10 8 cycles under in-service loading conditions is analyzed on the basis of fractography and the slices prepared in radial planes of rings. The cracks are shown to originate at subsurface from carbides inherent in the bearing steel or inclusions permissible by sizes for the material. Subsequently, the development of cracks perpendicular to the ring raceway surface takes place similarly as in the VHCF regime with the elliptical “fish-eye” formation. The subsequent crack growth was demonstrated step-by-step up to the ring material fragment separation. The total crack path by alternating stops of propagation and new crack nucleation under conditions of mixed-mode I+II+III mechanisms with the crack branching was discussed. In the final stage, the crack grows towards the ring raceway and either appears on the raceway surface or coalesce with a similar adjacent crack followed by fatigue spalling formation.
This paper contains the in-service fatigue fracture analysis for the first stage low-pressure compressor disk of the aircraft engines D30KU-154. Based on the results of the fractographic investigation on collapsed compressor disks the fatigue crack initiation and propagation mechanisms were established. It is shown that the crack initiation in the rim part of the compressor disk is due to a high frequency loading that leads to very high cycle fatigue fracture. The total fatigue life of the compressor disk is determined by simultaneous action of low amplitude loading due to blades vibration, and high amplitude loading due to flight cycle (centrifugal forces). To study the in-service fracture of the compressor disk the numerical simulation of the stress state in the damaged zone under corresponding loading conditions was evaluated. The estimation of fatigue life and crack path predictions were performed based on the multi-regime fatigue fracture model proposed by the authors.
The current study aims to find suitable fatigue assessment methods for welded structures (cover plates and T-joints) subjected to axial and bending loading. For this purpose, the Hot Spot Stress method (HSS), 1-mm stress (OM) method, Theory of Critical Distances (TCD) method, Stress Averaging (SA) method, and Effective Notch Stress (ENS) method are evaluated in terms of accuracy and reliability. The evaluation is based on fatigue test data extracted from the literature and fatigue testing carried out in the current study; cover plate joints subjected to axial loading and T-joints subjected to bending. The notch stress methods have higher estimation accuracy for the cover plate joints subjected to axial loading than the structural stress methods. It is found that the SA method can be used to assess the fatigue strength of cover plate joints with good accuracy and low scatter, followed by the ENS method. The fatigue strength assessment of T-joints subjected to bending using the HSS method, TCD method, SA method, and ENS method and corresponding FAT class is conservative, whilst the accuracy is low. Fatigue design curves applicable for T-joints under bending are introduced and described, which can be used in the TCD method, SA method, and ENS method.
The recently developed Edge Tracing (ET) method allows to estimate the radial deformation in axisymmetric tensile specimens via analysis of digital images recorded during the experiments. Images are processed to detect the sample’s contours and therefore, estimate the minimal cross–section diameter. This technique was mainly developed to characterize the elastic–plastic behavior well beyond the necking strain. The aim of this work is to extend the ET method to two case studies. Firstly, the post–necking behavior and failure of a low ductility Al–alloy are investigated. Low ductility alloys tend to fail brutally after reaching the maximum load. The major result is the capture of the sharp load drop which allowed to calibrate parameters of a GTN damage model. Secondly, the anisotropic elastic–plastic behavior of a “vintage” line pipe steel is characterized by a direct measurement of the Lankford coefficient. Assembled experimental data allowed to model the anisotropic plasticity in different loading directions.
Titanium is a versatile biocompatible metal that is desirable in additively manufactured medical implant devices. However, additively manufactured parts have particular microstructures, porosity, residual stress and surface conditions which can have a strong impact on fatigue performance. Implants have an added complexity from the saline operating environment and the associated impact on the safe design life. Equally, direct energy deposition induces a complex thermal history which, if not carefully controlled, can significantly alter the mechanical/material properties of the component. This study investigates the decrease in fatigue life, in an in-vitro body fluid simulation using Ringer’s solution, observed in Ti-6Al-4V specimens extracted from coupons manufactured by directed energy deposition. An interrupted deposition strategy was employed to control build regularity, which appeared to influence certain mechanical properties, including corrosion fatigue life. An ≈50% decrease in fatigue life was observed in Ringer’s solution at 6 Hz loading frequency, clearly important in designing implants.
A multilayer overlay coating system containing an intermediate intermetallic layer (designated 2IML) is an architecture expected to show good fatigue resistance. Experimental characterisation and modelling simulations were carried out to classify the different crack initiation mechanisms occurring during fatigue of this coating system and to reveal how changes in the layer architecture lead to fatigue improvement. Fatigue improvement is achieved by decreasing the IML-Top layer thickness due to the increased surface crack initiation resistance. However subsurface initiation mechanisms inhibit the improvement (dominated by surface initiation mechanism) achieved by locating the IML-Top layer closer to the top surface.
To exploit the full material potential of short fibre reinforced PA6T/6I, specific component calculations including aniso- tropic material behaviour is necessary. For this, different failure criteria and fatigue models are used to describe the behaviour during a component service life. This paper deals with the determination and consideration of fibre orientations for failure criteria and fatigue calculations. Therefore, a novel method to determine fibre orientation (FO) distributions across injection moulded plates, is proposed. The developed method allows a forecast of FOs for different specimen extraction positions and angles on injection moulded plates by using only a few measured reference points. As a result, fatigue models can be calibrated with the strength values and the corresponding FO, calculated for fracture position. The performed tests show a non-negligible influence of failure positions, due to fibre orientation distributions along the specimens. So, the FO determination method delivers an improvement in strength values estimation.
In this study, a combined low and high cycle fatigue (CCF) life prediction model, which considers the crack closure effect (CCE) of micro-defects, is proposed based on the continuous damage mechanics. The CCF life prediction model is decomposed into three sub-models: the low cycle fatigue (LCF), high cycle fatigue (HCF) under the maximum stress of LCF (HCFLM), and their coupled damage models. The CCE is considered by taking one CCE parameter into the HCFLM sub-model. The experimental CCF data of K403 full-scale turbine blades under different vibration stresses is used to verify the accuracy of the proposed model to compare with other life prediction models. The prediction life from the proposed model falls within the 2 times of scatter band compared with the experimental results. Further, there are the different damage evolution forms at different vibration stresses. When the vibration stress is below 64.48MPa, the CCF damage mainly is caused by the LCF damage. However, while the vibration stress is higher than 64.48MPa, the HCFLM damage plays a major role in the CCF damage accumulation, and it is predicted that the CCF damage of the first stage serration on the K403 turbine blades is mainly from LCF.
In this work the boundaries of small-scale yielding (SSY) and large-scale yielding (LSY) have been experimentally evaluated from the analysis of crack tip opening displacement (CTOD) measured by Digital Image Correlation (DIC). The approach published in a previous numerical work  has been used to define the boundaries of SSY and LSY. According to this approach, CTOD must be resolved into its elastic and plastic components, analysing the ratio between the elastic CTOD range and the total CTOD range ( Δδe/ Δδt) to define the boundary where SSY conditions can be established. Three materials have been studied, commercially pure titanium and 2024-T3 and 7050-T6 aluminium alloys, tested at different stress ratio values (0.1 and 0.6 for titanium, and 0.1, 0.3 and 0.5 for the aluminium alloys). SSY conditions are shown to dominate when Δδe/ Δδt≥79% and ≥78% for titanium and the two aluminium alloys, respectively. In addition, LSY can be established when Δδe/ Δδt≤66.3% and ≤67.2% for titanium and for 2024-T3 and 7050-T6 aluminum alloys, respectively. Transition or LSY conditions are more probable in fatigue tests conducted at low R-ratio than in tests at high R-ratio. In addition, crack lengths above 40% with respect to the width of the specimen promote transition or LSY conditions. The results obtained in this work can assist to a better understanding of the mechanisms driving fatigue crack growth.