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.

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.

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.

In this work, the effect of residual stress on mixed-mode crack propagation behavior in friction stir welded (FSW) 7075-T6 panel under biaxial loading was investigated. The cruciform sample was designed and manufactured by FSW. Residual stress profiles across the welded sample were measured by the X-ray diffraction technique. Crack propagation behaviors were simulated with five different biaxial loading ratios. Stress intensity factors (KΙ and KΙΙ) were evaluated by finite element method (FEM) and used to study the effects of residual stress on crack behaviors. It was observed that residual stress has a considerable effect on the mixed-mode crack growth. In most of the cases, the crack deflection is mainly affected by residual stress at the beginning of crack propagation. The variation of crack propagation path is strongly linked with the residual stress as well as the biaxial loading ratio. In addition, KΙ and KΙΙ are susceptible to residual stress under biaxial loading conditions. Residual stresses contribute to a higher proportion of KΙΙ compared to that of KΙ. KΙ and KΙΙ in the retreating side are more affected by the residual stress.

The consideration of realistic load assumptions is important for the fatigue design of highly stressed nodular cast iron components for wind energy application. Especially in case of overloads causing elastic-plastic deformation, residual stresses may have a strong impact on fatigue life. In strain-controlled fatigue tests with constant and variable amplitudes, the influence of overloads on the lifetime was investigated. The overload was applied with the objective to create high tensile residual stresses. During fatigue testing the transient material behavior, cyclic hardening, cyclic relaxation of the residual stresses as well as quasi static creep effects, of the EN-GJS-400-18-LT was recorded and evaluated. To quantify the influence of the transient material behavior on the calculated lifetime, fatigue analyses are carried out with the strain-life approach, both with and without consideration of the transient material behavior. The results show that conservative damage sums are derived if the transient material behavior, especially the relaxation of tensile residual stresses, is neglected.

A concept of initial cracking strength of concrete is elaborated in this study. A fracture model and associated methods for determining independent initial cracking strength and initial fracture toughness by using the three-point-bending (3-p-b) and wedge splitting (WS) concrete specimens are present. The initial cracking strength and initial fracture toughness can be simultaneously determined using a curve-fitting method from the proposed fracture model. All of the initial fracture curves can be obtained using the determined concrete materials. The initial loads of the 3-p-b and WS specimens can be predicted on the basis of the curves with ±15% ranges. Furthermore, analytical functions are used to obtain and determine the initial cracking strength and the initial fracture toughness of concretes directly. The determined values with ±15% ranges cover the most of initial loads of the 3-p-b and WS specimens.

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.

An experimental protocol has been set up in order to study the Low Cycle Fatigue (LCF) damage micromechanisms in a Lost Foam Casting (LFC) A319 alloy at room temperature. The microstructure of the studied material was characterized by using X-ray Computed Tomography (X-ray CT) prior to the LCF tests performed with surface in-situ observations, which allow crack initiation and propagation being tracked in real-time. The mechanical fields measured by Digital Image Correlation (DIC) method allowed establishing the relations between strain localizations, damage evolutions and microstructure while a developed etching method, which gives a natural texture to the surface, makes DIC feasible to an acceptable resolution without masking the microstructure. The results showed that crack initiation is ascribed to strain localizations induced by large pore and/or the propagation of a previously nucleated crack. Cracks propagate along hard inclusions but the orientation of hard inclusions has also an influence on crack path.

The present investigation is concerned with high-cycle axial fatigue testing of a 2 mm AA6060-T6 HYB butt weld produced in the solid state using AA6082 filler metal addition. The results complement the three-point bend testing and the tensile testing done in two previous studies. In this study, optical microscope and scanning electron microscope examinations have been carried out to reveal the joint macro/microstructure and document possible surface and root defects deemed to affect fatigue life. In the as-welded condition, the HYB weld suffers from surface irregularities at the weld face and “kissing” bond formation in the root region. Despite of this, the subsequent testing shows that the fatigue properties exceed those reported for comparable AA6082-T6 gas metal arc butt welds and matching those reported for corresponding high-strength laser beam and friction stir weldments.

The influence of pre-strain on the tensile and fatigue properties of a dual phase DP600 was studied. The material was pre-strained by uni-axial tension in rolling and transverse direction. Thereafter, specimens were cut from the deformed plates in parallel or orthogonal to pre-strain direction. It was found that pre-strain increases yield and tensile strength. Results suggested that strain path change primarily affects the elastic-plastic transition during early stage of reloading. Pre-strained specimens showed an increase in high cycle regimes as a consequence of yield strength increment, irrespective of imposed pre-straining direction. A modified stress life equation which accounts for pre-strain was proposed and showed good agreement with experimental data. Bake hardening enhanced both tensile and high cycle fatigue resistance. Walker equation was successfully fitted to account tensile mean stress. In low cycle fatigue, negligible influence of pre-strain was observed due to cyclic softening and residual stress relaxation.

This paper develops a data-driven autonomous method for detection of fatigue damage and classification of the associated damage risk in mechanical structures, based on ultrasonic signal energy. The underlying concept is built upon attenuation of the signal and stability of the attenuation process. The attenuation provides pertinent information for damage quantification, whereas the stability represents resistance towards the fatigue damage growth. The proposed neural network (NN) model has been trained using the scaled conjugate-gradient back-propagation method. The NN model is capable of damage detection and damage classification into five classes of increasing risk. The Daubechies wavelet transform has been used to reduce the noisy pattern of the ultrasonic signal energy by using the associated approximation coefficients. The results show that the proposed method of approximation signal energy can detect and classify the damage with an accuracy of up to ∼ 9 8 . 5 % .

Aiming at the structural safety problems caused by the coupling and randomness of factors such as the geometric characteristics, load status, service characteristics and failure mechanism of the in-service bridge structure in time and space, a real-time prediction method of fatigue life of bridge structure under digital twin is proposed. Based on the physical entity of the general bridge crane, the status information acquisition system is used to get the current service status information of the crane equipment, and the obtained current service status information is combined with inherent information and historical service information to construct a fuzzy information database, which is clearly quantified to form twin data; based on the characteristics of the equipment structure and the technology cycle process, the load, strength, defect and life analysis models are determined to form a multi-theoretical calculation model of the bridge structure through information transmission; using the informational data of the active service state, the multi-theoretical computational model is modified to obtain the main factors affecting the fatigue life, and fuzzy comprehensive evaluation theory is combined to determine the comprehensive influence factor; the Kriging proxy model is constructed through experimental design to obtain the response relationship between inherent and service information and the fatigue life of the bridge structure, and the whole life cycle process of the equipment is reflected by completing the real-time prediction of the fatigue life of the bridge structure in the virtual space. Taking the QD20/10t×43m×12m general bridge crane as an example, the feasibility and applicability of the proposed method are verified by comparison, which provides a strong theoretical basis for the safe service and timely scrapping of the crane.

The influence of the hole surface integrity on the fatigue life of 2024-T351 aluminium drilled parts was investigated. Fatigue tests were conducted on open-hole specimens and a large campaign was carried out to characterize the hole surface integrity (roughness measurements, hardness measurements, metallographic observations and residual stress analysis). An innovative technique, the Hole Opening Comparative Technique, was set up in order to analyse the residual stress state of the parts. In this study, strain hardening of the hole subsurface seems to be the main factor influencing the fatigue behaviour, associated with a residual stress state of the part.

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.

Low-cycle fatigue testing of a lead-free solder (InnoLot) based on Sn-3.8Ag-0.7Cu (SAC387) with three simultaneous additions of bismuth, nickel and antimony was conducted using miniature-sized fatigue specimens at different temperatures and strain amplitudes. The experiments show a decline of the load capacity of the solder alloy with the number of loading cycles. The fatigue life of the solder is also decreased by the level of imposed temperature. The modified Coffin-Manson and Morrow models were used to analyze the behavior under fatigue and predict lifetime. The parameters in the two fatigue models which were determined by considering different temperatures and total strain amplitudes. Compared to other reference lead-free solders, the InnoLot solder shows much better fatigue strength. The better fatigue strength is found to result from the effect of BiNiSb elements. Also, lifetime predictions were made with both models for the solder alloy under different conditions.

The interface fracture toughness of SnSb11Cu6/20steel was measured by calculating the critical energy release rate and stress phase angle of the interface crack. A three-point bending test was used to introduce cracks into the bonding interface, and the cohesion model of the bonding interface was established through experimental data. Through finite element analysis of load-deflection curves with and without interface crack propagation, the crack initiation point is found. Then the energy calculation model of crack propagation is established, and the critical energy release rate is obtained using the virtual crack growth criterion. The calculation results of the stress phase angle show that the crack propagation is greatly affected by the normal stress after the babbitt alloy layer fractures. If the strength of the substrate material is weaker, the crack will continue to expand in the tangent perpendicular to the crack tip.

This study is focused on the anisotropic fatigue crack growth (FCG) behaviour of an aluminium AA7050-T7451 plate. L-T and S-T orientations were studied in M(T) samples with W=50 mm, in mode I loading, with R-ratio of +0.05. A numerical approach was used, assuming that crack tip plastic strain is the crack driving force. A purely kinematic elastic-plastic model was calibrated using experimental data from low cycle fatigue tests of smooth specimens in L and S orientations. The predicted FCG rates agree well with experimental trends in the Paris’ regime, suggesting that cyclic plastic deformation is the main damage mechanism. The numerical model was used to estimate the stress ratio effect for both orientations, which was found to be linked with crack closure variations. However, the closure free predicted trends for both microstructural orientations at R=0.05 are not overlapped, suggesting an effect of microstructure not linked to crack closure.

In order to raise the hardness and strength of the surface layer of mechanical components and induce favourable residual compressive stresses, case-hardening procedures have become established in the heat treatment of steel. In this work, a calculation concept for the fatigue strength of components that have been case-hardened through carburizing heat-treatment is being developed. The residual stress and the load stresses in complex-shaped, carburized materials are determined using a finite element (FE) model. The fatigue limit of the components is derived using probabilistic methods and taking into account hardness gradients, residual stresses, and non-metallic inclusions. The model is validated with available axial bending fatigue test data and then used to predict the rotating bending fatigue limit of samples with various geometries and heat-treatment conditions. This work demonstrates the capability of combining probabilistic and FE-based modelling to represent complex interactions between variables that affect the fatigue of heat-treated components, such as steel cleanliness, notch shape, case-hardening depth, or loading conditions.