The main aim of the current study is to evaluate the compressive quasi-static and fatigue properties of titanium alloy (Ti6Al4V) cellular materials, with different topologies, manufactured via Laser Powder Bed Fusion (LPBF) process. The topologies herein considered are lattice based regular and irregular configurations of cubic, star and cross shaped unit cell along with trabecular based topology. The results have indicated that the effective stiffness of all configurations are in the range of 0.3 – 20 GPa, which is desirable for implant applications. The morphological irregularities in the structures induce bending dominated behavior affecting more the topologies with vertical struts. The S – N curves normalized with respect to the yield stress indicate that the behavior of star regular structures is between purely stretching dominated cubic and purely bending dominated cross based structures. Trabecular structures have shown desirable quasi-static and fatigue properties despite the random distribution of struts.
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 paper deals with the problem of ultra-light and light aircraft structural health monitoring. The phenomenological basis and engineering decisions for methods to monitor accumulated fatigue damage and to register overstress of aircraft principal structural elements have been shown. The proposed fatigue indicator is lightweight and autonomous; the output information of the indicator reflects the inhering feature of the fatigue damage – appearance and growth of surface extrusions and intrusions. The overstress indicator simplicity and correctness of the information is provided by “fuse” principle of the design.
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.
In this study, an approach utilizing a conjunction of the extended finite element method (XFEM) and the GTN micro-mechanical damage model is proposed for predicting the ductile crack propagation of a mill-annealed Ti-6Al-4V alloy. The cohesive model based XFEM approach is used to capture the continuous crack propagation process and the GTN model is applied to describe the constitutive behavior of the material. Simulations are conducted by using the standard finite element code ABAQUS following a Newton-Raphson algorithm solution with employing the user material subroutine of the GTN model. In comparison to the experimental results of the smooth, notched and cracked titanium specimens, this approach is shown to be an efficient method for simulating the ductile crack propagation process under different stress triaxialities.
The grain boundary strengthening effect (GBSE) was investigated for nickel-based superalloy bicrystals with various misorientations under different temperatures and loading directions. The results show that the GBSE is enhanced with the increase of misorientation and insensitive to the temperature. Besides, the loading direction relative to GB also has strong effect on GBSE. On the other hand, the GBSE is often accompanied by GB cracking. The trend of GB cracking is heightened with the increase of misorientation and temperature. However, the trend of GB cracking is reduced strongly when the loading direction transforms from parallel direction to vertical direction.
A novel multi-axial energy-based approach is presented and used to demonstrate the influence of different finite element (FE) modelling techniques on the prediction of the fatigue life of a rubber composite with long oriented fibres. It is shown that the simplest modelling methods using 2D elements with rebar layers, layered 2D elements or layered 3D elements do not allow for a precise determination of the critical location and damage value. In contrast, modelling methods with 3D matrix and discrete reinforcement provide much better results. The predicted critical location corresponds to the measured one, although the predicted fatigue life still differs from the measured results. The most complex microscopic modelling method shows the best agreement between the predicted and measured fatigue life. Since microscopic modelling is not suitable for modelling larger products made of rubber fibre composite, it is also noted that modelling techniques with 3D matrix and discrete reinforcing elements can be used with the same accuracy if the fatigue life curve is obtained from measurements on the specimens made of composite material rather than the specimens made of the critical base material (rubber).
In last decades, many alleviation measures were proposed in order to improve the life of fretting fatigue affected components. The aim of such palliatives is that to counteract the high stress gradients that arise near the contact surface. In such a context, the shot peening treatment is worth noting. Therefore, in the present paper, the fatigue life of shot-peened aluminium and titanium alloy specimens, subject to fretting fatigue under partial slip regime, is assessed by means of the Carpinteri et al. criterion for fretting fatigue. Firstly, according to the superposition principle, the relaxed residual stresses (due to the shot peening treatment) are combined with the stress components due to fretting fatigue loading. Then fretting fatigue assessment is performed. In such a context, a novel theoretical law for the relaxed residual stress field is here proposed, the implementation of which shows very promising results in terms of fatigue life estimation of the shot-peened specimens examined.
Superaustenitic steel Sanicro 25 has been subjected to in-phase and out-of-phase thermomechanical fatigue cycles in the temperature range from 250°C to 700°C. Both constant strain rate cycling and cycling with 10 minutes dwell at peak temperature were applied. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were evaluated. The transmission electron microscopy was used to find modifications of the internal structure and precipitation of the nanoparticles. 10 min dwell at maximum temperature modified substantially the dislocation arrangement. Various nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy dispersive X-ray spectroscopy in scanning transmission electron microscope. The damage mechanism operating under specific loading conditions was investigated on the surface as well as in the interior of the cycled specimens. Scanning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for crack nucleation and propagation. Effect of dwells on fatigue behaviour, modification of internal structure and damage mechanisms are analysed and discussed.
This paper presents a novel numerical model, based on the Finite Element (FE) method, for the simulation of a welding process aimed to make a two-passes V-groove butt joint. Specifically, a particular attention has been paid on the prediction of the residual stresses and distortions caused by the welding process. At this purpose, an elasto-plastic temperature dependent material model and the “element birth and death” technique, for the simulation of the weld filler supply over the time, have been considered within this paper. The main advancement with respect to the State of the Art herein proposed concerns the development of a modelling technique able to simulate the plates interaction during the welding operation when an only plate is modelled, taking advantage of the symmetry of the joint; this phenomenon is usually neglected in such type of prediction models because of their complexity. Problems arising in the development of this modelling technique have been widely described and solved herein: transient thermal field generated by the welding process introduces several deformations inside the plates, leading to their interaction, never faced in literature. Moreover, the heat amount is supplied to the finite elements as volumetric generation of the internal energy, allowing overcoming the time-consuming calibration phase needed to use the Goldak’s model, commonly adopted in literature. The proposed FE modelling technique has been established against an experimental test, with regard to the temperatures field and to the joint distortion. Predicted results showed a good agreement with experimental ones. Finally, the residual stresses distribution in the joint has been evaluated.
Typically, the Crack Tip Opening Displacement (CTOD) is used only to quantify the crack closure phenomenon. However, more information about crack tip phenomena can be extracted from the CTOD curves, which can be used for a better understanding of fatigue crack growth. The main objective here is the development of a numerical tool for the automatic analysis of CTOD plots, which can be obtained either numerically using the Finite Element Method (FEM) or experimentally using Digital Image Correlation (DIC). The parameters extracted are the elastic and plastic CTOD in loading and unloading regimes, the corresponding load ranges, the crack opening and closure levels and the dissipated energy. This tool is expected to promote a fast and efficient analysis of DIC and FEM results, facilitating the implementation of CTOD analysis in the fatigue community.
Stir zone (SZ) of AA6061-T6 double-side friction stir welding (DS-FSW) joint with a thickness of 6 mm had a poor fatigue performance compared with that of base metal (BM). The rolling process was employed to improve the fatigue performance of SZ. The results showed that rolling process increased the dislocation density in SZ and remarkably enhanced its fatigue performance. The crack initiation played a key role in the fatigue performance of SZ. It relied on the debonding of lamellar structure in SZ. The debonding of lamellar structure depended on the cracking of fine grain layer in lamellar structure.
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.
Orthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti-6Al-4V were fabricated. The porosity for each group was measured and the results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The S-N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack-of-fusion defects in the cases of surface and internal crack initiation.
The use of passive infrared thermography comprises great opportunities to improve understanding the fatigue damage process of crack-patched structures. Quasi-static and cyclic coupon tests are performed using metallic specimens with single-sided bonded patches and monitored with passive infrared thermography. Different test setups help to differentiate between metallic crack growth and adhesive damage on thermal images. Results show that metallic crack growth can be monitored from the patched side, also in combination with local delamination at the patch/metal interface. Thus, it is possible to analyse the overall degradation progress of the crack patched component under loading conditions and thereby to identify the driving damage mechanism of the particular repair configuration. Being able to understand the overall damage behaviour of crack patched components is essential to improve the ability of predicting its long-term behaviour.
A stress-based sectional critical plane model for multiaxial fatigue life prediction is proposed. The proposed model considers the effects of material properties and loading paths on the crack initiation and propagation behaviors. By introducing the ratio of maximum shear stress amplitude to maximum normal stress amplitude, it is divided into three sections in which the maximum normal stress plane, maximum damage plane and maximum shear stress amplitude plane are considered as the critical planes, respectively. To verify the accuracy and applicability of the proposed model, experimental data of 30CrMnSiA steel conducted by the authors and other test data of different materials from the existing literatures are utilized. For 30CrMnSiA steel, the prediction results of the proposed model demonstrate that 79.3% and 93.7% of the prediction results are within the ±2 times and ±3 times scatter band of fatigue life. For the experimental data from the existing literatures, more than 85% and 70% of the results predicted by the proposed model are within ±3 times scatter band of fatigue life for steel and aluminum alloy materials, respectively.
Droplet impingement of metallic surfaces at high impact velocities results, after some time, in erosion of the surface due to fatigue. By extending our previously published analytical model to enable the use of experimental fatigue data (S-N curves), here, for the first time, a wide range of experimental liquid droplet erosion incubation period test states for both ferrous (stainless steel AISI 316) and non-ferrous (aluminium 6061-T6) engineering metals have been investigated. To achieve this, the developed model includes additional surface hardening and a residual compressive stress state at the surface due to a water drop peening effect. As such, the interrelation of the physical and mechanical properties that follows from the model has been used to identify how changes in selected metal properties might enhance droplet impingement erosion incubation life. Model predictions for both metals, using fatigue data from S-N curves from different literature sources, showed for the droplet impact velocity range of 140 to 400 m/s an excellent agreement with results from a multi-regression equation as determined from an ASTM interlaboratory test program.
Normalization method is a practical method for determining the J-R curves and fracture toughness of steels. There is some concern, however, about the performance of this method on steels with small strain hardening exponent and yield strength due mainly to the assumption of infinite strain hardening exponent (n). This paper intends to analytically modify the normalization method by removing this assumption and incorporating the strain hardening in calculating the blunting corrected crack length. This modification enables the normalization method to be applied to steels with small strain hardening exponent and yield strength. Experiments are undertaken to prove the underperformance of the normalization method for steels with small strain hardening exponent and yield strength and to verify the modified normalization method (CNM). A comparison of fracture toughness determined by CNM with that by the unloading compliance method and normalization method corroborates the improved accuracy of the developed CNM. It is found in the paper that the developed CNM performs very well for materials with small strain hardening exponent and yield strength and performs better for specimens with smaller thickness and in accordance with all standards. The paper concludes that the developed CNM overcomes the deficiency of the normalization method for steels with small strain hardening exponent and yield strength.