Deformation and failure of polymer glasses

This thesis is composed of six papers in the field of deformation and failure of polymer glasses. Prediction of deformation and failure behaviour of polymers has become very important. In the last two decades considerable effort is addressed to the development of 3D constitutive models that were able to capture the visco-elastic and post-yield behaviour of glassy polymers. The compressible Leonov model, as developed in our group, proved to be a suitable model which provides an adequate description of this behaviour, including rate- and temperature-dependent yield, strain softening and strain hardening. However a failure criterion is still lacking. Previous studies indicated that macroscopic deformation behaviour is dominated by the intrinsic post-yield behaviour. Improving the ductility should hence focus on avoiding! r localisation of strain by elimating strain softening and promoting the contribution of the strain hardening. Although it is quite well established that strain hardening originates from the contribution of the entangled polymer network, the high strain hardening modulus compared to the rubber-modulus and its temperature dependence requires further investigation. The physical origin of strain softening is less well known, although it is reported that strain softening can be altered by thermal treatments and can even be eliminated by mechanical rejuvenation. The limited resistance to void nucleation and the build-up of high dilative stresses under certain loading conditions, show that decreasing strain softening and increasing strain hardening are not sufficient to achieve tough deformation behaviour. To circumvent these problems heterogeneity should be introduced in the structure to relieve the build-up of high hydrostatic stresses. For materia! ls like polycarbonate and polyamide this results in a transition from crazing to shear yielding. For polystyrene this is only the case if the thickness of the ligaments within the structure are sufficiently small. The concept of a critical thickness suggests that an absolute length-scale is encountered. An absolute length-scale of the same order of magnitude as is found in mechanical tests, is also reported in polymer physics where a Tg-depression is found in thin polystyrene films. In chapter 2 the influence of the network density on the strain hardening modulus is investigated. An increase in network density of polystyrene, achieved by cross-linking and blending with polyphenylene oxide results in a proportional increase in strain hardening modulus. It is discussed that the maginitude of the strain hardening modulus and its negative temperature dependence might orginate from the time-scale of the stress-induced segmental mobility and tha! t, on this time-scale, the secondary interactions still play a significant role. The transient deformation behaviour of mechanically rejuvenated polstyrene in studied in chapter 3. Although the recovery of yield stress and strain softening is independent of the molecular weight, the time to re-embrittlement proves to increase with increasing molecular weight. This is rationalised by the fact that the tensile strength of the material, and hence the recovered yield stress at which this strength is exceeded in a localised plastic zone, depends on the molecular weight. The post-yield behaviour dominates the macroscopic deformation behaviour of amorphous polymers. In chapter 4 it is shown that polycarbonate with its moderate strain softening and strong strain hardening results is stable neck growth during deformation. By annealing the strain softening increases, leading to more severe localisation of strain and even brittle failure. The deformatio! nmode can be be predicted in a straight-forward manner using a stability analysis. The pronounced strain softening and weak strain hardening of polystyrene lead to extreme localisation of strain and explain that standard polystyrene can never be ductile. Elimination of strain softening by mechanical rejuvenation inhibits localisation of strain and results in (temporary) ductile deformation behaviour. Additional finite element simulations illustrate the route to improve ductility. Since a failure criterion was still lacking in the finite element simulations employing the compressible Leonov model, micro-indentation experiments are used to generate crazes in a reproducible way. By evaluation of the local stress and strain distribution by finite element simulations, a critical hydrostatic stress of 40 MPa was found in polystyrene (provided that this event is preceded by plastic deformation) as a criterion for void nucleation. This criterion prov! ed to be independent of thermal history and strain rate but proved to increase with network density. By means of micro- and nano-indenations on polystyrene the influence of an absolute length-scale, as reported in other areas of polymer science, is investigated. For large indenters and indentation depth the experiments compare well to the length-scale independent finite element simulations, using bulk properties. For the smallest indenter (2.2 ??m) and shallow indentation depth (100 nm) the resistance to indentation is much less than expected from the simulations, indicating that the mechanical properties near a free surface in polystyrene might differ from the bulk properties. Using the criterion for void nucleation, as identified in chapter 5, brittle-to-ductile transitions (BDTs) were predicted by the deformation of a representative volume element (RVE). By increasing the temperature in the RVE, the overall stress level lowers in such a ! way that at 70ÆC the critical level of 40 MPa is not exceeded anymore in the simulations and hence a transition from crazing to shear yielding is achieved in polystyrene. The length-scale which is encountered experimentally and numerically in chapter 6 was incorporated in the RVE by assuming a gradient of increased temperature near free surfaces. At an interparicle distance of less than approximately 15 nm the critical value of 40 MPa is not exceeded anymore and crazing is hence inhibited. Both brittle-to-ductile transition compare well to experimental observations

Analytical and Experimental Determination of Mechanical Properties of Irradiated Materials

Determination of bulk properties of in-service irradiated material from micro specimen testing is proposed in the work. Due to complex degradation process of aging experimental reproduction of such aging condition is difficult to obtain. Moreover the availability of such aged material is limited due to high cost. Miniaturized specimens are used for this purpose due to many advantages i.e. they can be directly taken out of in-service materials, ease of irradiating using an external source, less dose to personnel during post irradiation testing etc. So both experimental and numerical method is employed to study the aged material response. For numerical analysis, porous solid Gurson-Tvergaard -Needleman model is used and effect of different parameters of G-T-N model is studied. The G-T-N model is calibrated to fit the Stress-Strain curve of unirradiated and irradiated curves and Gurson parameters are found. Effects of irradiation on Gurson parameters are studied. These calibrated Gurson parameters are used to simulate a standard CT specimen. Much experimentation is carried out on fabrication and testing of nonstandard miniature tensile specimen and results are compared with standard sub-tensile specimens (ASTM-370). Proton irradiation of miniature specimen is carried out at DAE facility VECC with irradiation doses of 0.01, 0.02, 0.04 and 0.06dpa. Same methodology is applied to the irradiated specimen to determine their properties to test the correctness of the proposed methodology

Constraint-Based Master Curve Analysis of a Nuclear Reactor Pressure Vessel Steel

This report presents the outcome of four fracture test series, addressing the ductile-to-brittle toughness behaviour of a nuclear reactor pressure vessel steel. Each test series corresponds to a specific test specimen geometry, tensile or three-point-bend, with a given degree of crack-tip constraint. A brief overview is given of available constraint-based fracture mechanics methodologies in the ductile-to-brittle transition range, including both engineering and local approach procedures. The obtained experimental data are analysed by means of the Master Curve standard ASTM E1921. Variability of the resulting reference temperature, T0, is successfully confirmed by a selection of constraint-based methodologies.JRC.DG.F.4-Safety of future nuclear reactor

Crystal plasticity modeling of the effects of crystal orientation and grain-to-grain interactions on DSA-induced strain localization in Al–Li alloys

International audienceWe develop a crystal plasticity model to investigate the coupled actions of crystal orientation, grain neighborhood and grain-to-grain elasto-plastic interactions on dynamic strain aging (DSA) and the onset and development of associated plastic strain localization in Al-Li alloys. Considering simple model multilayered microstructures with preferred orientations representative of rolled alloys, the aim is to identify grain orientation couples that can limit dynamic strain aging induced strain localization without compromising the flow stress and strain hardening properties. To this end, a slip system-based formulation of dynamic strain aging is implemented in a crystal plasticity finite element framework. The model validity is first checked with the simulation of a tensile specimen loaded at quasi-static applied strain rate. The introduction of dynamic strain aging allows predicting complex propagation of intense plastic localization bands. We further investigate the influence of crystal orientations on early strain localization in Al-Cu-Li-Mg alloys, by performing simulations representative of the early stage of a Kahn Tear test for single crystals and layered polycrystals. Using experimentally reported crystal orientations for rolled microstructures, the simulation results show that in both single and multilayered crystals, there is a strong influence of dynamic strain aging on localization patterns, as well as a significant orientation dependence. In multilayered crystals, the nature of strain localization can be remarkably modified when stand-alone crystals of a certain orientation are coupled with other orientations: strain localization may intensify or fade away depending on the coupling with neighboring orientations

An investigation of hot forming quench process for AA6082 aluminium alloys

This thesis is concerned with the mechanical properties and microstructure evolution during the novel solution Heat treatment Forming cold die Quenching (HFQ) process. HFQ is a hot sheet forming technology which incorporates the forming and quenching stages to produce high strength and high precision Al-alloy sheet parts. The work in the thesis divided into three main sections: Firstly, viscoplastic behaviour of AA6082 at different deformation temperatures and strain rates was identified through analysis of a programme of hot tensile tests. Based on the results from the hot tensile tests, a set of unified viscoplastic-damage constitutive equations was developed and determined for AA6082, providing a good agreement with the experimental results. SEM tests were carried out to investigate the damage nucleation and failure features of the AA6082 during hot forming process and the results are discussed. Secondly, the viscoplastic-damage constitutive equations were implemented into the commercial software ABAQUS via the user defined subroutine VUMAT for the forming process simulation. An experimental programme was designed and testing facilities were established for the validation of the FE process modelling results. A fairly good agreement between the process simulation and the experimental results was achieved. This confirms that the established FE process simulation model can be used for hot stamping of AA6082 panel parts. Further process modelling work was carried out to identify the optimal forming parameters for a simplified representation of a panel part. Finally, a precipitation hardening model was developed to predict the post-ageing strength of AA6082 panel parts, having varying amounts of forming-induced plastic strain. The model was tested against results of experiments which were carried out to investigate the effect of pre-deformation on the ageing kinetics of AA6082. The model is shown to fit and can be used to explain changes in the strength of the material. This set of equations was implemented in the VUMAT, in combination with the viscoplastic damage constitutive equation set, to model the whole HFQ process. The FE model was tested with experimental ageing and hardness results providing good agreements, which are discussed in light of the future development of the HFQ process

Oxidation Assisted Intergranular Cracking in Alloy 718: Effects of Strain Rate and Temperature

Alloy 718 is the most widely used superalloy in industry due to its excellent mechanical properties, as well as its oxidation and corrosion resistance over a wide range of temperatures and solicitation modes. Nevertheless, it is a well-known fact that this alloy is sensitive to oxidation assisted intergranular cracking under loading in the temperature range encountered in service. The mechanisms resulting in such degradation are not well-understood, but it has been well established that a relation exists between a change in fracture mode and the apparition of plastic instability phenomena over a wide range of temperatures. Quantification and characterization of the damaging process provide important information leading to a better understanding of the degradation mechanisms involved in the oxidation assisted intergranular cracking of this alloy. These observations allow various domains to be defined in the strain rate - temperature plane, where the damaging process characteristics are different: a high strain rate / low temperature domain in which instabilities occur and where the fracture mode is systematically transgranular ductile, an intermediary domain where numerous intergranular crack initiations can be observed, and a slow strain rate / high temperature domain where crack propagation is enhanced. These results lead to the proposal of consistent scenarii to explain grain boundary opening due to applied intergranular normal stress and critical decohesion stress changes

Fracture Characterization of Rolled Sheet Alloys in Shear Loading: Studies of Specimen Geometry, Anisotropy, and Rate Sensitivity

The final publication is available at Springer via http://dx.doi.org/10.1007/s11340-016-0211-9Two different shear sample geometries were employed to investigate the failure behaviour of two automotive alloy rolled sheets; a highly anisotropic magnesium alloy (ZEK100) and a relatively isotropic dual phase steel (DP780) at room temperature. The performance of the butterfly type specimen (Mohr and Henn Exp Mech 47:805–820, 16; Dunand and Mohr Eng Fract Mech 78:2919-2934, 17) was evaluated at quasi-static conditions along with that of the shear geometry of Peirs et al Exp Mech 52:729-741, (27) using in situ digital image correlation (DIC) strain measurement techniques. It was shown that both test geometries resulted in similar strain-paths; however, the fracture strains obtained using the butterfly specimen were lower for both alloys. It is demonstrated that ZEK100 exhibits strong anisotropy in terms of failure strain. In addition, the strain rate sensitivity of fracture for ZEK100 was studied in shear tests with strain rates from quasi-static (0.01 s−1) to elevated strain rates of 10 and 100 s−1, for which a reduction in fracture strain was observed with increasing strain rate.Cosma International, Automotive Partnership CanadaOntario Research FundNatural Sciences and Engineering Research Council of CanadaCanada Research Chairs SecretariatCanada Foundation for Innovatio

Integrated modeling of friction stir welding of 6xxx series Al alloys: Process, microstructure and properties

International audienceCompared to most thermomechanical processing methods, friction stir welding (FSW) is a recent technique which has not yet reached full maturity. Nevertheless, owing to multiple intrinsic advantages, FSW has already replaced conventional welding methods in a variety of industrial applications especially for Al alloys. This provides the impetus for developing a methodology towards optimization, from process to performances, using the most advanced approach available in materials science and thermomechanics. The aim is to obtain a guidance both for process fine tuning and for alloy design. Integrated modeling constitutes a way to accelerate the insertion of the process, especially regarding difficult applications where for instance ductility, fracture toughness, fatigue and/or stress corrosion cracking are key issues. Hence, an integrated modeling framework devoted to the FSW of 6xxx series Al alloys has been established and applied to the 6005A and 6056 alloys. The suite of models involves an in-process temperature evolution model, a microstructure evolution model with an extension to heterogeneous precipitation, a microstructure based strength and strain hardening model, and a micro-mechanics based damage model. The presentation of each model is supplemented by the coverage of relevant recent literature. The "model chain" is assessed towards a wide range of experimental data. The final objective is to present routes for the optimization of the FSW process using both experiments and models. Now, this strategy goes well beyond the case of FSW, illustrating the potential of chain models to support a "material by design approach" from process to performances

Correlation of fractography, microstructure and fracture toughness behavior of high strength alloys.

Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.Vita.Includes bibliographical references.Ph.D