Gruppo Italiano Frattura (IGF)
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Effects of surfaces nanocrystallization induced by shot peening on material properties : a Review
No full textA brief description of surface nanosrystallization process via severe plastic deformation is presented. To come to the point different shot peening methods which have proved to be able to create nanocrystalline layers are demonstrated clarifying the actual state of the art. Then the influence of the process is reviewed on material behavior and a wide range of affected properties are investigated. On this basis some possible addresses for future research in this field are drawn and underlined
Multi-parameter crack tip stress state description for estimation of fracture process zone extent in silicate composite WST specimens
No full textFor wedge splitting test specimens, the stress and displacement fields both in the vicinity and alsoin larger distance from the crack tip are investigated by means of numerical methods. Several variants ofboundary conditions were modeled. The stress intensity factor K, T-stress and even higher-order terms ofWilliam series were determined and subsequently utilized for analytical approximation of the stress field. A goodfit between the analytical and numerical solution in dependence on the distance from the crack tip was shown,compared and discussed. Presented approach is considered as suitable for estimation of the fracture processzone extent in silicate composite materials
The application of the infrared thermography on titanium alloy for studying fatigue behavior
No full textThe infrared thermography is an attractive tool for studying the fatigue behavior of materials. Basedon two theoretical models of fatigue damage indicators, this work studied the fatigue properties of the virgin Ti-6Al-4V alloy. According to the two damage indicators and the energy theory, the relationship between themacro-phenomenon and the micro-structural evolution during fatigue process was discussed. The fatigue limitof the titanium alloy was rapidly determined based on the measured temperature increment signals. Thecapability of the infrared thermographic method on the evaluation of fatigue properties was validated
Analisi termica per la valutazione del comportamento a fatica di provini soggetti a successive serie di carichi
No full textPartendo dalla osservazione che provini precedentemente danneggiati hanno un limite di faticadifferente (vedi Miner) viene esaminato il comportamento di un acciaio soggetto a serie di carichi affaticanti alfine di verificare la conseguente risposta termica.L’esame, mediante analisi termica, delle prove di fatica su provini di acciaio C40 danneggiati a diverso grado, dainteressanti indicazioni sul ruolo dei carichi applicati in relazione alla possibilità di produrre danno nel materiale.Viene messo in evidenza come la temperatura, conseguenza dell’energia consumata dal provino, possa essereelemento indicativo dello stato del materiale. Vengono riportate le curve di fatica e le curve di temperatura diprovini lisci sottoposti a storie di carico diverse. Da esse si può evincere che mentre è valida la legge di linearitàdel danno (?ini/Ni= cost), la curva definita secondo la regola del Miner-Manson (?ini/Ni=1), che tiene soloconto del numero di volte in cui la tensione supera il limite di fatica del materiale (provini lisci), invece, nonsempre rispecchia il reale stato di danno. Questo specialmente quando si è in presenza di carichi prossimi allimite di fatica che, per la sequenza dell’applicazione, possono diventare carichi affaticanti
Fractal statistics of brittle fragmentation
No full textThe study of fragmentation statistics of brittle materials that includes four types of experiments ispresented. Data processing of the fragmentation of glass plates under quasi-static loading and the fragmentationof quartz cylindrical rods under dynamic loading shows that the size distribution of fragments (spatial quantity)is fractal and can be described by a power law. The original experimental technique allows us to measure, apartfrom the spatial quantity, the temporal quantity - the size of time interval between the impulses of the lightreflected from the newly created surfaces. The analysis of distributions of spatial (fragment size) and temporal(time interval) quantities provides evidence of obeying scaling laws, which suggests the possibility of selforganizedcriticality in fragmentation
Misure di tenacità a frattura su acciai utilizzando velocità di deformazione elevate
No full textThe knowlegde of dynamic mechanical properties is useful in all cases where the strain ratesensitivity of metallic materials is an issue, and whenever the actual loading conditions for a structure (eitherin normal operation or under accidental circumstances) are different from static. Furthermore, in someinvestigations increasing the loading rate is used to simulate other embrittling mechanisms such as thermalaging or neutron exposure.This paper provides an overview of SCK•CEN experience on measuring fracture toughness of steels at elevatedloading rates, with specific emphasis on instrumented impact tests on precracked Charpy (PCVN)specimens.After briefly dwelling on the basic mechanisms which explain loading rate effects on cleavage and ductilefracture toughness, the experimental and analytical procedures for measuring fracture toughness at elevatedloading rates are addressed, both in terms of official ASTM and ISO test standards and considering standardizationefforts currently in progress under SCK•CEN coordination: revision of ASTM E1921 (MasterCurve methodology for measuring fracture toughness in the ductile-to-brittle transition region) and a future ISO standard on instrumented PCVN testing. This latter document is examined in more detail, focussing theattention on the dynamic evaluation of brittle fracture toughness (Impact Response Curve) and the determinationof crack resistance curves using multiple and single-specimen techniques.Finally, selected examples from SCK•CEN database of dynamic toughness measurements will be illustrated,mainly relevant to reactor pressure vessel (RPV) steels
Gradient enriched linear-elastic crack tip stresses to estimate the static strength of cracked engineering ceramics
No full textAccording to Gradient Mechanics (GM), stress fields have to be determined by directlyincorporating into the stress analysis a length scale which that takes into account the material microstructuralfeatures. This peculiar modus operandi results in stress fields in the vicinity of sharp cracks which are no longersingular, even though the assessed material is assumed to obey a linear-elastic constitutive law. Given both thegeometry of the cracked component being assessed and the value of the material length scale, the magnitude ofthe corresponding gradient enriched linear-elastic crack tip stress is then finite and it can be calculated by takingfull advantage of those computational methods specifically devised to numerically implement gradient elasticity.In the present investigation, it is first shown that GM’s length scale can directly be estimated from the materialultimate tensile strength and the plane strain fracture toughness through the critical distance value calculatedaccording to the Theory of Critical Distances. Next, by post-processing a large number of experimental resultstaken from the literature and generated by testing cracked ceramics, it is shown that gradient enriched linearelasticcrack tip stresses can successfully be used to model the transition from the short- to the long-crackregime under Mode I static loading
Fracture mechanics analyses of the slip-side joggle regions of wing-leading-edge panels
No full textThe Space Shuttle wing-leading edge consists of panels that are made of reinforced carbon-carbon. Coating spallation was observed near the slip-side region of the panels that experience extreme heating. To understand this phenomenon, a root-cause investigation was conducted. As part of that investigation, fracture mechanics analyses of the slip-side joggle regions of the hot panels were conducted. This paper presents an overview of the fracture mechanics analyses
Extension of the CJP model to mixed mode I and mode II
No full textThe present authors have previously proposed a novel ‘plastic inclusion’ approach for dealing withthe local plasticity which occurs at the tip of a growing fatigue crack. This meso-scale model provides amodified set of crack tip stress intensity factors that include the magnitude of plastic wake-induced crack tipshielding and which have the potential to help resolve some long-standing controversies associated withplasticity-induced closure. The present work extends the CJP model to deal with the case of mixed Mode I andMode II loading and thus opens up enhanced possibilities for testing it on inclined cracks in metallic specimens.This extension requires the addition of only one new force parameter to the model, i.e. an anti-symmetric shearforce on either side of the crack
Research on borehole stability of shale based on seepage-stress-damage coupling model
No full textIn oil drilling, one of the most complicated problems is borehole stability of shale. Based on thetheory of continuum damage mechanics, a modified Mohr-Coulomb failure criterion according to plasticdamage evolution and the seepage-stress coupling is established. Meanwhile, the damage evolution equationwhich is based on equivalent plastic strain and the permeability evolution equation of shale are proposed in thispaper. The physical model of borehole rock for a well in China western oilfield is set up to analyze thedistribution of damage, permeability, stress, plastic strain and displacement. In the calculation process, theinfluence of rock damage to elastic modulus, cohesion and permeability is involved by writing a subroutine forABAQUS. The results show that the rock damage evolution has a significant effect to the plastic strain andstress in plastic zone. Different drilling fluid density will produce different damage in its value, range and type.This study improves the theory of mechanical mechanism of borehole collapse and fracture, and provides areference for the further research of seepage-stress-chemical-damage coupling of wall rock