Gruppo Italiano Frattura (IGF)
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Effects of the manufacturing process on fracture behaviour of cast TiAl intermetallic alloys
No full textThe ? -TiAl based intermetallic alloys are interesting candidate materials for high-temperatureapplications with the efforts being directed toward the replacement of Ni-based superalloys. TiAl-based alloysare characterised by a density (3.5-4 g/cm3) which is less than half of that of Ni-based superalloys, and thereforethese alloys have attracted broad attention as potential candidate for high-temperature structural applications.Specific composition/microstructure combinations should be attained with the aim of obtaining goodmechanical properties while maintaining satisfactory oxidation resistance, creep resistance and high temperaturestrength for targeted applications.Different casting methods have been used for producing TiAl based alloys. In our experimental work,specimens were produced by means of centrifugal casting. Tests carried out on several samples characterised bydifferent alloy compositions highlighted that solidification shrinkage and solid metal contraction during coolingproduce the development of relevant residual stresses that are sufficient to fracture the castings during coolingor to produce a delayed fracture. In this work, crack initiation and growth have been analysed in order toidentify the factors causing the very high residual stresses that often produce explosive crack propagationthroughout the casting
Determination of fracture mechanics parameters on a base of local displacement measurements
No full textNew experimental technique for a determination of the stress intensity factor (SIF) and T-stressvalues is developed and verified. The approach assumes combining the crack compliance method and opticalinterferometric measurements of local deformation response on small crack length increment. Initialexperimental information has a form of in-plane displacement component values, which are measured byelectronic speckle-pattern interferometry at some specific points located near a crack tip. Required values offracture mechanics parameters follow from the first four coefficients of Williams’ series. A determination ofinitial experimental data at the nearest vicinity of notch tip is the main feature of the developed approach. Thatis why it is not necessary to involve complex numerical models, which include global geometrical parameters,loading and boundary conditions of the object under study, in a stage of experimental data interpretation. Anavailability of high-quality interference fringe patterns, which are free from rigid-body motions, serves as areliable indicator of real stress state around a crack tip. A verification of the technique is performed bycomparing experimental results with analogous data of FEM modelling. Experimentally determined mode I SIFfor DCB specimen with end crack is in 5 per cent agreement with the numerically simulated case. Proposedapproach is capable of estimating an influence of the notch radius on fracture mechanics parameters.Comparing SIF and T-stress obtained for U-notches of different radius both in actual and residual stress fieldconfirms this statement
The use of thermally expandable microcapsules for increasing the toughness and heal structural adhesives
No full textIn this research, the effect of thermally expandable microcapsules (TEMs) on mode I fracture toughness of structural adhesives were investigated. The single-edge-notch bending (SENB) test was used. Firstly, a standard toughness test was performed on adhesives with microcapsules. Secondly, since TEMs start their expansion at approximately 60ºC, the next specimens were fatigue tested expecting a local heating in the notch leading to the desired expansion before being statically loaded for fracture toughness determination. Thirdly, a manual local heating at 90ºC was applied in the notch before the fracture static test. The experimental results were successfully cross-checked through a numerical analysis using the virtual crack closure technique (VCCT) based on linear elastic fracture mechanics (LEFM). The major conclusion is that fracture toughness of the modified adhesives increased as the mass fraction of the TEMs increased
Plastic zone evolution near a crack tip and its role in environmentally assisted cracking
No full textThis paper analyzes the effects of crack tip plastic strains and compressive residual stresses, createdby fatigue pre-cracking, on environmentally assisted cracking of pearlitic steel subjected to localized anodicdissolution and hydrogen assisted fracture. In both situations, cyclic crack tip plasticity improves the behavior ofthe steel. In the respective cases, the effects are supposed to be due to accelerated local anodic dissolution ofthe cyclic plastic zone (producing chemical crack blunting) or to the delay of hydrogen entry into the metalcaused by residual compressive stresses, thus increasing the fracture load in aggressive environment
Energy dissipation and storage in iron under plastic deformation (experimental study and numerical simulation)
No full textThe work is devoted to the experimental and numerical investigation of thermodynamic aspects ofthe plastic deformation in Armco iron. Dissipation and stored energies was calculated from processedexperimental data of the surface temperature obtained by infrared thermography. An original mathematicalmodel describing the process of mesoscopic defects accumulation was used for numerical simulation of thequasistatic loading of iron samples and for calculation of theoretical value of the stored energy. Experimentaland modeled values of the stored energy are in a good agreement
Initiation and growth behavior of very-long microstructurally short fatigue cracks
No full textThe present paper describes a novel experimental technique recently presented that allows one tostudy interactions between the crack and microstructural barriers with an unprecedented level of ease and detail.The method consists in increasing the grain size of Al1050 Aluminium alloy until the centimetre scale byapplying a series of mechanical and heat treatments. Once the thermo-mechanical treatment is completed andthe desired microstructure obtained, a circular notch is machined on each specimen, and the samples aresubjected to push-pull fatigue loading. Several combinations of notch and microstructural sizes have beentested. This method provides an easy way to record and analyse the effect of the microstructure upon crackgrowth rate. It was observed that the space between successive crack-tip arrests correlates well with the materialgrain size. Another interesting observation is that in the majority of the cases studied the cracks did not initiateat the point of maximum stress concentration. This is surprising since the classical methods of notched fatiguelimit analysis clearly indicate the horizontal symmetry axis as the initiation and propagation direction for pushpullloading
Graphite nodules features identifications and damaging micromechanims in ductile irons
No full textDuctile irons mechanical properties are strongly influenced by the metal matrix microstructure andon the graphite elements morphology. Depending on the chemical composition, the manufacturing process andthe heat treatments, these graphite elements can be characterized by different shape, size and distribution. Thesegeometrical features are usually evaluated by the experts visual inspection, and some commercial softwares arealso available to assist this activity. In this work, an automatic procedure based on an image segmentationtechnique is applied: this procedure is validated not only considering spheroidal graphite elements, but alsoconsidering other morphologies (e.g. lamellae)
Near tip strain evolution under cyclic loading
No full textThe concept of ratchetting strain as a crack driving force in controlling crack growth has previouslybeen explored at Portsmouth using numerical approaches for nickel-based superalloys. In this paper, we reportthe first experimental observations of the near-tip strain evolution as captured by the Digital Image Correlation(DIC) technique on a compact tension specimen of stainless steel 316L. The evolution of the near-tip strainswith loading cycles was studied whilst the crack tip was maintained stationary. The strains were monitored overthe selected distances from the crack tip for a given number of cycles under an incremental loading regime. Theresults show that strain ratchetting does occur with load cycling, and is particularly evident close to the crack tipand under higher loads. A finite element model has been developed to simulate the experiments and thesimulation results are compared with the DIC measurements
On the overall accuracy of the Modified Wöhler Curve Method in estimating high-cycle multiaxial fatigue strength
No full textThe aim of the present paper is to systematically investigate the accuracy of the so-called Modified Wöhler Curve Method (MWCM) in estimating high-cycle fatigue strength of plain and notched engineering materials damaged by in-service multiaxial load histories. In more detail, the MWCM, which is a bi-parametrical critical plane approach, postulates that initiation and Stage I propagation of fatigue cracks occur on those material planes experiencing the maximum shear stress amplitude (this being assumed to be always true independently from the degree of multiaxiality of the applied loading path). Further, the fatigue damage extent is hypothesised to depend also on the maximum stress perpendicular to the critical plane, the mean normal stress being corrected through the so-called mean stress sensitivity index (i.e., a material constant capable of quantifying the sensitivity of the assessed material to the presence of superimposed static stresses). In the present investigation, the overall accuracy of the MWCM in estimating high-cycle fatigue strength was checked through 704 endurance limits taken from the literature and generated, under multiaxial fatigue loading, by testing both plain and notched samples made of 71 different materials. Such a massive validation exercise allowed us to prove that the MWCM is highly accurate, resulting in 95% of the estimates falling within an error interval equal to ±15%
Study of the conditions of fracture at explosive compaction of powders
No full textJoint theoretical and experimental investigations have allowed to realize an approach with use ofmathematical and physical modeling of processes of a shock wave loading of powder materials.In order to gain a better insight into the effect of loading conditions and, in particular, to study the effect ofdetonation velocity, explosive thickness, and explosion pressure on the properties of the final sample, wenumerically solved the problem about powder compaction in the axisymmetric case.The performed analysis shows that an increase in the decay time of the pressure applied to the sample due to anincrease of the explosive thickness or the external loading causes no shrinkage of the destructed region at afixed propagation velocity of the detonation wave. Simultaneously, a decrease in the propagation velocity of thedetonation wave results in an appreciable shrinkage of this region