Brittle failure of components weakened by cracks or sharp and blunt V-notches is a topic of active and continuous research. It is attractive for all researchers who face the problem of fracture of ...materials under different loading conditions and deals with a large number of applications in different engineering fields, not only with the mechanical one. This topic is significant in all the cases where intrinsic defects of the material or geometrical discontinuities give rise to localized stress concentration which, in brittle materials, may generate a crack leading to catastrophic failure or to a shortening of the assessed structural life. Whereas cracks are viewed as unpleasant entities in most engineering materials, U- and V-notches of different acuities are sometimes deliberately introduced in design and manufacturing of structural components.
Dealing with brittle failure of notched components and summarizing some recent experimental results reported in the literature, the main aim of the present contribution is to present a review of some local approaches applicable near stress raisers both sharp and blunt. The reviewed criteria allowed the present authors to develop a new approach based on the volume strain energy density (SED), which has been recently applied to assess the brittle failure of a large number of materials. The main features of the SED approach are outlined in the paper and its peculiarities and advantages accurately underlined. Some examples of applications are reported, as well. The present review is based on the authors’ experience over more than 15 years and the contents of their personal library. It is not a dispassionate literature survey.
I present a possible worlds semantics for a hyperintensional belief revision operator, which reduces the logical idealization of cognitive agents affecting similar operators in doxastic and epistemic ...logics, as well as in standard AGM belief revision theory. (Revised) belief states are not closed under classical logical consequence; revising by inconsistent information does not perforce lead to trivialization; and revision can be subject to 'framing effects': logically or necessarily equivalent contents can lead to different revisions. Such results are obtained without resorting to non-classical logics, or to non-normal or impossible worlds semantics. The framework combines, instead, a standard semantics for propositional S5 with a simple mereology of contents.
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•Specimens fail from defects in surfaces build facing downwards.•Size of measurable defects in fracture surface is related to the fatigue life.•A diagram relating the position of ...failure initiation site to the notch acuity is proposed.•Average strain energy density failure criteria is able to predict fatigue life for sharp notches.
Powder bed fusion based additively manufactured components are known to have poor surface quality, especially when building downward facing surfaces. These surfaces can contain defects, from which fatigue cracks can be initiated. In this work the notched fatigue behaviour of Inconel 718 specimens produced by selective laser melting is investigated. The main focus is set on the interaction between notch geometries and local defects due to the amount of overhang in the notch region. Four different geometries are considered, with different amount of notch acuities and degree of downward facing surfaces. A variation in failure sites, with respect to the notch bisector line, was fond in the specimens, and the position was found to be dependent on the amount of overhang and notch acuity. The fatigue life was found to be dependent on the size of surface defects measured in fracture surfaces. Further, the use of average strain energy density as a failure criteria in additively manufactured metals is discussed.
The present paper deals with multiaxial fatigue behaviour of severely notched components made of titanium grade 5 alloy (Ti–6Al–4V). The experimental tests have been carried out under combined Mode I ...and Mode III loadings, both in phase and out of phase. Cylindrical specimens weakened by circumferential notches have been employed. Different nominal load ratios have been applied in the tests (R = −1, 0 and 0.5). The specimens had a notch‐tip radius smaller than 0.1 mm, a notch depth equal to 6 mm and an opening angle of 90°. The results obtained by multiaxial fatigue testing are depicted in comparison with data from pure modes of loading on smooth and notched samples, characterized by a load ratio in the range −3 ≤ R ≤ 0.5. A large bulk of new fatigue data (more than 160) is summarized in the manuscript. The data are first plotted in terms of the nominal stress amplitudes, and then they are reanalysed by means of the local energy measured in the control volumes surrounding the notch tip. The dependence of the size of the control radius as a function of the loading mode is analysed. A very different behaviour is found for tension and torsion, corresponding to a different notch sensitivity.
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Additive manufacturing of industrially-relevant high-performance parts and products is today a reality, especially for metal additive manufacturing technologies. The design complexity ...that is now possible makes it particularly useful to improve product performance in a variety of applications. Metal additive manufacturing is especially well matured and is being used for production of end-use mission-critical parts. The next level of this development includes the use of intentionally designed porous metals - architected cellular or lattice structures. Cellular structures can be designed or tailored for specific mechanical or other performance characteristics and have numerous advantages due to their large surface area, low mass, regular repeated structure and open interconnected pore spaces. This is considered particularly useful for medical implants and for lightweight automotive and aerospace components, which are the main industry drivers at present. Architected cellular structures behave similar to open cell foams, which have found many other industrial applications to date, such as sandwich panels for impact absorption, radiators for thermal management, filters or catalyst materials, sound insulation, amongst others. The advantage of additively manufactured cellular structures is the precise control of the micro-architecture which becomes possible. The huge potential of these porous architected cellular materials manufactured by additive manufacturing is currently limited by concerns over their structural integrity. This is a valid concern, when considering the complexity of the manufacturing process, and the only recent maturation of metal additive manufacturing technologies. Many potential manufacturing errors can occur, which have so far resulted in a widely disparate set of results in the literature for these types of structures, with especially poor fatigue properties often found. These have improved over the years, matching the maturation and improvement of the metal additive manufacturing processes. As the causes of errors and effects of these on mechanical properties are now better understood, many of the underlying issues can be removed or mitigated. This makes additively manufactured cellular structures a highly valid option for disruptive new and improved industrial products. This review paper discusses the progress to date in the improvement of the fatigue performance of cellular structures manufactured by additive manufacturing, especially metal-based, providing insights and a glimpse to the future for fatigue-tolerant additively manufactured architected cellular materials.
The equiatomic CrMnFeCoNi high entropy alloy is additively manufactured by the laser engineered net shaping (LENSTM) process, and the solidification conditions, phase formation, as-deposited ...microstructures, and tensile behavior are investigated. The LENSTM-deposited CrMnFeCoNi alloy exhibits a single-phase disordered face centered cubic (FCC) structure, as evidenced by X-ray diffraction (XRD), and rationalized by Scheil's solidification simulation. Furthermore, microstructures at multiple length scales, i.e. columnar grains, solidification substructures, and dislocation substructures, are formed. The tensile deformation process is mainly accommodated by dislocation activities with the assistance of deformation twinning. The tensile yield strength of the LENSTM-deposited CrMnFeCoNi alloy is comparable to that of finer-grained wrought-annealed counterparts, due to the additional initial-dislocation strengthening. However, the uniform tensile elongation, by contrast, is lowered, which is attributed to the increased dynamic dislocation recovery rate and hence the weakened work hardening capability of the LENSTM-deposited CrMnFeCoNi. This study demonstrates the capability of the LENSTM process for manufacturing the CrMnFeCoNi alloy, with high performance, for engineering applications.
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•Mean stress and plasticity effects are included into a novel fatigue criterion.•This is based on the definition of 4 elastic and plastic components of SED.•Stabilization of the ...elastic-plastic response is investigated.•Criterion parameters are calibrated from experimental fatigue data.•Very accurate prediction of independent fatigue data.
A novel strain-energy–density (SED) based fatigue criterion is here proposed to account for the effect of mean stress and plasticity on the uniaxial fatigue strength of plain and notched components. It is based on the definition of four SED components: ΔW¯el, the elastic SED associated to the stress range, ΔW¯el,max, the maximum elastic SED in the stabilized cycle, ΔW¯pl, the plastic SED dissipated per stabilized cycle, W¯pl,max, the plastic SED dissipated over the cycles until stabilization. The mean stress effect is incorporated in a Walker-like expression, ΔW¯elαW¯el,max1-α, while W¯pl,max is added to the expression of the total SED to include the effect of mean stress relaxation. An energetic approach is proposed to identify the condition of cycle stabilization. The coefficients of the fatigue criterion are calibrated using experimental fatigue data. The criterion is validated by predictions of independent data.
Additive manufacturing by laser powder bed fusion allows the production of complex parts including lattice structures in popular metal alloys such as Ti6Al4V. Lattice structures are a class of ...meta-materials which hold many advantages such as the possibility for the production of lightweight parts with tailored mechanical and other properties: these have many potential applications in aerospace and medical fields. The laser powder bed fusion process can result in microporosity inside the produced material, which can affect the mechanical performance of these types of materials. In this work, different typical microporosity distributions are induced in manufactured gyroid lattice structure samples and the mechanical performance is tested by both static compression and compression-compression fatigue. X-ray tomography was used to validate the microporosity distributions and samples were tested in stress-relieved state and hot isostatic pressed state. In particular, it is found that small amounts of keyhole mode microporosity of ~0.2% make no difference while lack of fusion is critical, especially when this results in inefficient HIP pore closure. The results highlight the effect of microporosity on the mechanical performance of these materials and the results add to the knowledge base and trustworthiness of these materials.
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•Microporosity induced in gyroid lattice structures by process parameters•X-ray tomography used to confirm and quantify porosity content•Static and fatigue compression tests performed•Both stress-relieved and hot isostatic pressed states investigated•Effect of porosity found to be more detrimental for lack of fusion defects
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•Studies mixed mode I/II brittle fracture behavior in rock materials.•Examines two sets of fracture test data from two different specimens.•Traditional fracture criteria cannot ...predict test data properly.•EMTSN criterion predicts fracture angles and fracture toughness very well.
This paper investigates the results of brittle fracture in rock materials subjected to the mixed mode I/II loading using different fracture criteria. Two sets of mixed mode fracture test data in the entire range of mode mixity, from pure mode I to pure mode II, reported in the literature for semi-circular and triangular shape specimens and subjected to three point bend loading (i.e. SCB and ECT specimens) are utilized to study the brittle fracture in two marble rocks. First, the onset of fracture initiation is examined by different conventional fracture criteria including Maximum Tangential Stress (MTS) and Maximum Tangential Strain (MTSN) criteria. It is shown that these two conventional fracture criteria, which only consider singular crack tip stress/strain terms, are not able to accurately predict the mixed mode fracture test data. The experimental mixed mode fracture toughness data are then predicted by an extended version of the maximum tangential strain (EMTSN) criterion which takes into account the effect of first nonsingular strain term as well as the singular strain components. It is found that both mixed mode fracture toughness results of the investigated rock materials and the crack propagation direction can be predicted successfully by the EMTSN criterion.
•VHCF strength decreases with building orientation.•Fatigue crack origin is always from an internal defect.•Rough area and fisheye have been observed on fracture surfaces.
The effect of building ...orientation on the very-high-cycle fatigue (VHCF) response of Ti-6Al-4V specimens produced through selective laser melting (SLM) process with three different building orientations (0°, 45° and 90°) has been experimentally assessed. The fatigue performance decreases with different building orientations from 0° to 90°. The fatigue crack origin has been found to be always an internal defect both at high-cycle fatigue and VHCF regime independent of building orientations. Size of defects induced fatigue failures and the stress intensity factor range decrease with the number of cycles to failure. By considering the VHCF strength at 109 cycles, the median value decreases from 217 MPa (0°) to 201 MPa (45°) and finally to 155 MPa (90°), with a 40% reduction from 0° to 90°. The building orientation significantly influences both the defect size and the resulting VHCF response.
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