Gruppo Italiano Frattura (IGF)
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High-cycle fatigue strength of a pultruded composite material
No full textDealing with composites in polymeric matrix, the pultruded ones are among the more suitable for large production rates and volumes. For this reason, their use is increasing also in structural applications in civil and mechanical engineering.
However, their use is still limited by the partial knowledge of their fatigue behaviour; in many applications it is, indeed, required a duration of many millions of cycles, while most of the data that can be found in literature refer to a maximum number of cycles equal to 3 millions. In this paper a pultruded composite used for manufacturing structural beams is considered and its mechanical behaviour characterized by means of static and high-cycle fatigue tests. The results allowed to determine the S-N curve of the material and to assess the existence of a fatigue limit. Observations at the scanning electronic microscope (SEM) allowed to evaluate the damage mechanisms involved in the static and fatigue failure of the material
Damage tolerance analysis of aircraft reinforced panels
No full textThis work is aimed at reproducing numerically a campaign of experimental tests performed for the development of reinforced panels, typically found in aircraft fuselage. The bonded reinforcements can significantly reduce the rate of fatigue crack growth and increase the residual strength of the skin. The reinforcements are of two types: stringers and doublers. The former provides stiffening to the panel while the latter controls the crack growth between the stringers. The purpose of the study is to validate a numerical method of analysis that can predict the damage tolerance of these reinforced panels. Therefore, using a fracture mechanics approach, several models (different by the geometry and the types of reinforcement constraints) were simulated with the finite element solver ABAQUS. The bonding between skin and stiffener was taken either rigid or flexible due to the presence of adhesive. The possible rupture of the reinforcements was also considered. The stress intensity factor trend obtained numerically as a function of crack growth was used to determine the fatigue crack growth rate, obtaining a good approximation of the experimental crack propagation rate in the skin. Therefore, different solutions for improving the damage tolerance of aircraft reinforced panels can be virtually tested in this way before performing experiments
Tests in tension-torsion conditions with descending sections of strain curve construction
No full textThe paper is devoted to theoretical and experimental investigation of materials behavior onpostcritical deformation stage (strain softening). Tests results, which confirm theoretical justification ofspecimen configuration impact on the possibility of creating descending sections of strain curve, are given. It isshown that, with sufficient stiffness of loading system, equilibrium postcritical deformation of materials ispossible. It is confirmed in uniaxial tension tests with unloading on postcritical deformation stage. Test resultson tension with torsion of thin-walled tubular specimens in strain softening conditions are adduced
A consistent use of the Gurson-Tvergaard-Needleman damage model for the R-curve calculation
No full textThe scope of the present work is to point out a consistent simulation procedure for the quasi-staticfracture processes, starting from the micro-structural characteristics of the material. To this aim, a local nineparametersGurson-Tvergaard-Needleman (GTN) damage law has been used. The damage parameters dependon the micro-structural characteristics and must be calculated, measured or opportunely tuned. This can bedone, as proposed by the author, by using an opportunely tuned GTN model for the representative volumeelement simulations, in order to enrich the original damage model by considering also the defect sizedistribution. Once determined all the material parameters, an MT fracture test has been simulated by a FE code,to calculate the R-curve in an aeronautical Al-based alloy. The simulation procedure produced results in a verygood agreement with the experimental data
Structural transitions in a NiTi alloy: a multistage loading-unload cycle
No full textNiTi shape memory alloys (SMAs) are increasingly used in many engineering and medical applications, because they combine special functional properties, such as shape memory effect and pseudoelasticity, with good mechanical strength and biocompatibility. However, the microstructural changes associated with these functional properties are not yet completely known. In this work a NiTi pseudo-elastic alloy was investigated by means of X-ray diffraction in order to assess micro-structural transformations under mechanical uniaxial deformation. The structure after complete shape recovery have been compared with initial state
Use of a gray level co-occurrence matrix to characterize duplex stainless steel phases microstructure
No full textDuplex stainless steels are widely used in industry. This is due to their higher strength compared to austenitic steels and to their higher toughness than ferritic steels. They also have good weldability and high resistance to stress corrosion cracking. These steels are characterized by two-phase microstructures composed by almost the same level of ferrite and austenite. Duplex steel 2205 samples evaluated are: as received, cold rolled (33%) and heat-treated at 800°C for 10 hours. A metallographic etching with 10% oxalic acid has been carried out to highlight the phases morphology. Some photos have been taken by SEM microscope and submitted to image analysis. The analysis carried out is based on the determination of co-occurrence matrix and on the following interpretation of appropriate indicators. Through these indicators is possible to estimate the features of images objectively
Variation of stress intensity factor and elastic T-stress along the crack-front in finite thickness plates
No full textNon-singular terms in the series expansion of the elastic crack-tip stress fields, commonly referred to as the T-stress. The T-stress is as an additional stress field characterizing parameter to stress intensity factor (K) in the analysis of cracked bodies. T-stress is used as an important constraint parameter in the fracture analysis. In this investigation, three-dimensional finite element analyses have been conducted to compute the elastic T-stress considering a single edge notched tensile (SENT) specimen with varied thickness and a/W ratio.
The results indicate that the T-stress depends on the specimen thickness and significantly varies along the crackfront from surface to centre of the specimen. The T-stress results obtained in the present analysis together with corresponding KI values can be used for analysis of constraint effects in a fracture specimen
High magnification crack-tip field characterisation under biaxial conditions
No full textThis work presents a novel methodology for characterising fatigue cracks under biaxial conditions.The methodology uses high magnification Digital Image Correlation (DIC) technique for measuringdisplacement and strain crack-tip fields. By applying micro-speckle pattern on the metal surface it is possible toachieve high magnification for DIC technique. The speckles were created by electro-spray technique. Thevalidity of this novel technique is demonstrated by direct comparison with standard extensometermeasurements, under tension-compression and torsion conditions. In order to image the correct region, thenotch effect on the fatigue life was also evaluated
Fracture failure analysis of baseplates in a fluidic amplifier made of WC-11Co cemented carbide
No full textA fluidic amplifier is a crucial automatic control component in a liquid jet hammer used to drillhard formations in the oil industry. This study aims to determine the true causes of the fracture failure ofbaseplates in a fluidic amplifier made of WC-11Co cemented carbide in a very short period of time.Computational fluid dynamics (CFD), theoretical estimation, and finite element analysis (FEA) were employedto analyze the effect of static and dynamic loads on the strength of the baseplates. Fractographic,metallographic, and processing defect analyses were also carried out. The FEA results showed that the staticand dynamic loads caused stress concentrations at the actual fracture locations, and the effect of working loadson material strength was allowable and safe. Fracture surfaces exhibited typical characteristics of a brittlefracture. The metallographic analysis revealed that a specific amount of brittle eta-phase (?-phase) was presentin the material. The microstructure of the processing cutting zone was inspected and the results revealed thatsome voids, pores, and microcracks were formed on the processing surface. The manufacturing and processingdefects resulted in low stress fracture failure of the baseplates
Is notch sensitivity a stress analysis problem?
No full textSemi–empirical notch sensitivity factors q have been widely used to properly account for notcheffects in fatigue design for a long time. However, the intrinsically empirical nature of this old concept can beavoided by modeling it using sound mechanical concepts that properly consider the influence of notch tip stressgradients on the growth behavior of mechanically short cracks. Moreover, this model requires only wellestablishedmechanical properties, as it has no need for data-fitting or similar ill-defined empirical parameters.In this way, the q value can now be calculated considering the characteristics of the notch geometry and of theloading, as well as the basic mechanical properties of the material, such as its fatigue limit and crack propagationthreshold, if the problem is fatigue, or its equivalent resistances to crack initiation and to crack propagationunder corrosion conditions, if the problem is environmentally assisted or stress corrosion cracking. Predictionsbased on this purely mechanical model have been validated by proper tests both in the fatigue and in the SCCcases, indicating that notch sensitivity can indeed be treated as a stress analysis problem