Clotilde Berdin

Local approach of damage and fracture toughness for nodular cast iron

Abstract The damage mechanisms of two types of ductile cast iron have been investigated. Tensile and tearing tests have been performed which provide information about the mechanical behaviour of the materials. In situ tensile tests performed inside a scanning electron microscope allowed the determination of the steps of physical damage evolution. From these observations two damage mechanisms can be defined. In one of the two cast irons, graphite nodules can be considered as voids and damage evolution is only related to void growth, whereas void nucleation has to be considered in the second one. The mechanical behaviour of these materials is modelled using the modified Gurson’s potential, and tearing test results can be predicted. The difference in fracture toughness of the two cast irons can be explained by the different sizes of the process zone in front of the crack.

The influence of ductile tearing on fracture energy in the ductile-to-brittle transition temperature range

The aim of this study is to examine the relationship between fracture energy and the ductile area measured on the fracture surface. Instrumented Charpy tests and fracture toughness tests are performed in the transition temperature range, as well as at lower temperatures. Quantitative fractographic analyses of Charpy specimens reveal a certain proportion of ductile fracture even if the Charpy test is conducted at low temperatures, below the transition temperature. The ductile fracture area situated next to the notch is correlated to fracture energy for all temperatures. In the transition temperature range, fracture energy and the ductile area have a large scatter. Since the limiting event in the development of the ductile area is the initiation of cleavage, the maximum principal stress has been computed in different specimens using the finite element method. It has been shown that the propagating ductile crack does not increase the stress level, but does increase the probability of cleavage fracture through an expansion of the plastic volume where weak points can be found.

Mechanical aspects of the Charpy impact test

Abstract A comparative experimental and FEM study has been carried out, in order to investigate dynamic and constraint aspects of the Charpy test. Standard V-notch Charpy specimens were tested under dynamic and static loading conditions. 2-D plane strain and 3-D models were employed in numerical analysis. In order to incorporate strain-rate effects, an elastic-viscoplastic constitutive equation has been applied, based on actual test data obtained for a low-alloy structural steel. Fully dynamic analysis clearly indicated inertial effects. Modal analysis enabled the confirmation of the origin of the oscillations on the load–displacement curve as beam vibration of the specimen resulting from interaction with the elastic striker.

Ductile fracture of duplex stainless steel with casting defects

Abstract Experimental and numerical studies were carried out on CF8M steel with special regard to the influence of casting defects i.e. shrinkage cavities on fracture behaviour. As-received and thermally aged materials were studied. Physical steps of damage in specimens containing casting defects were observed using in-situ tensile tests in a scanning electron microscope. Mechanical properties of duplex stainless steel were characterised using three-point bend tests of three types of specimens — defect free, pre-cracked, and containing shrinkage cavities. These mechanical tests were simulated using Gursons model including crack nucleation. A modelling of the shrinkage cavities is proposed in order to study their influence on the mechanical response of the specimen.

Tensile damage stages in cast A356-T6 aluminium alloy

Abstract This paper reports an investigation of the damage stages during straining of A356-T6 alloy by means of quantitative metallography, tensile tests, in situ tensile tests, and in situ micro-extensometry experiments with a SEM. Quantitative analysis of the microstructure and the damage mechanisms have identified the eutectic spatial arrangement as the crucial feature in fracture; interdendrite fine eutectics (eutectic channels) and extended eutectic zones (eutectic clusters) play different roles, so that the Si particle breaking rate varies from one region to another. The current models of Si particle breaking were then checked and their predictions compared with the local cracking rates. None of them were consistent with the experimental results. Next, cavity growth was investigated and the experimental results compared with data reported in literature and with predictions obtained by the Rice and Tracey law. It was shown that the initial porosity generated by Si particle breaking was very low and that void growth calculations completely underestimate void growth rate; once more, the specific role of Si particles spatial arrangement was decisive. Lastly, the analysis of the fracture stage revealed that cracks mostly propagate along the interdendritic channels, and this indicates the localisation of the damage, thus invalidating the ductility predictions of current fracture models.

Charpy impact test modelling and local approach to fracture

This study aims at the establishment of a non-empirical relationship between the Charpy V-notch energy CVN and the fracture toughness KIc on the lower shelf of fracture toughness and on the onset of the ductile-to-brittle transition of a low alloy structural steel. The methodology employed is based on the ‘local approach’. Brittle cleavage fracture is modelled in terms of the Beremin model [1], whereas the ductile crack advance preceding cleavage in the transition region is accounted for with the Gurson-Tvergaard-Needleman (GTN) model [2,3,4]. Temperature and rate dependence of the material flow stress were determined from tensile and compressive tests. Numerous fracture tests on CT and Charpy V-notch specimens provided the large data set necessary for statistical evaluation. Finite element analysis was empoyed for modeling. Special consideration was taken in order to handle the dynamic effects in the Charpy impact test. On the lowe shelf, the fracture toughness could be predicted from the Charpy impact test results. In the transition region the parameters of the Beremin model were found to deviate from those established on the lower shelf. Detailed fractographic investigations showed that the fractographic and microstructural features of regions of cleavage fracture initiation change with temperature.

Secondary foundry alloy damage and particle fracture

Abstract The damage mechanisms of a hypoeutectic Al–Si–Cu foundry alloy, obtained through die casting, have been studied. Observations were made during tensile tests using a tensile test machine that was inserted into an scanning electron microscope. The microstructure features that are critical for damage were determined. The cleavage fracture of intermetallic particles was found to be the main process that limited the alloy ductility. Shrinkage cavities mainly play a role in the final fracture stage; the reduction in cross-section area induced by shrinkage cavities was assumed to be the main contribution of these defects to the fracture of the specimens. Attempts were made to determine the cleavage stress that initiated the fracture of the particles. Three-dimensional finite element computations were performed considering particular locations at the free surface of the observed fractured particles. The maximum principal stress was calculated for different locations of the particle with reference to the free surface. Different shapes and different elastic behaviour were studied. It was shown that the influence of a free surface on the stress level in a particle depends on its shape. The results were compared with those found in literature concerning the fracture strength of particles in Al alloys.

Influence of ductile tearing on cleavage triggering in ductile-to-brittle transition of A508 steel

Abstract A large quantitative fractographic study was carried out onan A508 C1.3 pressure vessel steel in the temperature range corresponding to the ductile-to-brittle transition. Fractographic analyses of fractured Charpy V-notch (CVN) and compact tension (CT) specimens revealed a certain proportion of ductile fracture preceding cleavage, even if the specimens were tested at temperatures below the DBTT. The influence of the ductile tearing on cleavage triggering was studied. In particular, the stress concentrations in CT and/or CVN specimens induced by the presence of an ellipsoidal defect representing the cluster of debonded MnS inclusions were calculated by finite element method and related to the fracture probability given by Beremin model.

Mechanical Behavior Modeling in the Presence of Strain Aging

In many fields some structural materials are subjected to strain aging, which gives rise to inhomogeneous yielding such as Piobert-Luders’ bands and Portevin — Le Chatelier instabilities. These phenomena occur in steels containing interstitial elements in solid solution such as carbon or nitrogen which segregate to dislocations during aging and pin them. Strain aging in ferritic steels induces a loss in ductility and fracture toughness at mild temperatures [1]. To study the influence of strain aging on ductile tearing, it is possible to simulate plastic strain instabilities using a local constitutive equation [2] taking into account the interaction between solute atoms and dislocations responsible for strain aging [3]. This study first requires the identification of the model parameters, which needs to correlate the experimental values obtained from tensile tests to the intrinsic behavior of the material and then requires a good interpretation of the upper and lower yield stresses in the presence of static strain aging. So we have performed a numerical study of the Luders’ plateau featuring the influence of several parameters such as meshing, boundary conditions or behavior law on the nucleation and growth of Luders’ bands. The influence of the local band development on the plateau has been investigated. Correlations between the plateau stress level and the local behavior will be discussed.

KIa Crack Arrest Toughness Assessment Using Thermal Shock on Notched Disks

The aim of the study is to validate the KIa -T curve on a thermal shock experiment performed on a notched disk (DTSE) taken from a A533-B type steel. Several experiments have been performed. Non linear thermal analyses were carried out using the finite element method in order to obtain the full thermal field within the specimen during crack propagation. The results obtained are in excellent agreement with the experimental results. The DTSE is also interpretated in static terms to compare the obtained KIa (T) values with the limit curve. Finally, dynamic F.E. simulations allow to estimate the influence of dynamic effects in the DTSE and thus validate the methodology. According to the computations, the crack arrested when dK/da>0 and dKd /da = 0. The comparison between stress intensity factor computed from elastic-static analysis (or dynamic) and RCC-M code demonstrated the conservatism of the approach. Then static analysis is sufficient to analyse the result, since no wave interference with the crack propagation was identified.Copyright