C. Pinna

Microstructure evolution and strain localization during shear deformation of an aluminium alloy

Abstract Strain localization by shear banding during shear tests of a commercial aluminium alloy is described at different scales using optical microscopy, SEM and TEM. The evolution of the dislocation microstructure is correlated to the global mechanical behaviour. Two initial states of the material—heavily cold-rolled and recovered after cold-rolling—having the same crystallographical texture are compared. The localization occurs only for the as-rolled samples, and its consequence on damage and fracture depends on the angle between the initial rolling direction and the shearing direction. The discussion focuses on the predominant role of the microstructure, rather than the crystallographic texture, in the localization phenomena. Macroscopic arguments for localization are also evoked.

Modeling and optimal design of machining-induced residual stresses in aluminium alloys using a fast hierarchical multiobjective optimization algorithm

The residual stresses induced during shaping and machining play an important role in determining the integrity and durability of metal components. An important issue of producing safety critical components is to find the machining parameters that create compressive surface stresses or to minimize tensile surface stresses. In this article, a systematic data-driven fuzzy modeling methodology is proposed, which allows constructing transparent fuzzy models considering both accuracy and interpretability attributes of fuzzy systems. The new method employs a hierarchical optimization structure to improve the modeling efficiency, where two learning mechanisms cooperate together: the Nondominated Sorting Genetic Algorithm II (NSGA-II) is used to improve the models structure, while the gradient descent method is used to optimize the numerical parameters. This hybrid approach is then successfully applied to the problem that concerns the prediction of machining induced residual stresses in aerospace aluminium alloys. Based on the developed reliable prediction models, NSGA-II is further applied to the multiobjective optimal design of aluminium alloys in a “reverse-engineering” fashion. It is revealed that the optimal machining regimes to minimize the residual stress and the machining cost simultaneously can be successfully located.

Interface Cohesive Elements to Model Matrix Crack Evolution in Composite Laminates

In this paper, the transverse matrix (resin) cracking developed in multidirectional composite laminates loaded in tension was numerically investigated by a finite element (FE) model implemented in the commercially available software Abaqus/Explicit 6.10. A theoretical solution using the equivalent constraint model (ECM) of the damaged laminate developed by Soutis et al. was employed to describe matrix cracking evolution and compared to the proposed numerical approach. In the numerical model, interface cohesive elements were inserted between neighbouring finite elements that run parallel to fibre orientation in each lamina to simulate matrix cracking with the assumption of equally spaced cracks (based on experimental measurements and observations). The stress based traction-separation law was introduced to simulate initiation of matrix cracking and propagation under mixed-mode loading. The numerically predicted crack density was found to depend on the mesh size of the model and the material fracture parameters defined for the cohesive elements. Numerical predictions of matrix crack density as a function of applied stress are in a good agreement to experimentally measured and theoretically (ECM) obtained values, but some further refinement will be required in near future work.

Damage Assessment of Composite Structures Using Digital Image Correlation

The steady increase of Carbon-Fiber Reinforced Polymer (CFRP) Structures in modern aircraft will reach a new dimension with the entry into service of the Boeing 787 and Airbus 350. Replacement of damaged parts will not be a preferable solution due to the high level of integration and the large size of the components involved. Consequently the need to develop repair techniques and processes for composite components is readily apparent. Bonded patch repair technologies provide an alternative to mechanically fastened repairs with significantly higher performance, especially for relatively thin skins. Carefully designed adhesively bonded patches can lead to cost effective and highly efficient repairs in comparison with conventional riveted patch repairs that cut fibers and introduce highly strained regions. In this work, the assessment of the damage process taking place in notched (open-hole) specimens under uniaxial tensile loading was studied. Two-dimensional (2D) and three-dimensional (3D) Digital Image Correlation (DIC) techniques were employed to obtain full-field surface strain measurements in carbon-fiber/epoxy T700/M21 composite plates with different stacking sequences in the presence of an open circular hole. Penetrant enhanced X-ray radiographs were taken to identify damage location and extent after loading around the hole. DIC strain fields were compared to finite element predictions. In addition, DIC techniques were used to characterise damage and performance of adhesively bonded patch repairs in composite panels under tensile loading. This part of work relates to strength/stiffness restoration of damaged composite aircraft that becomes more important as composites are used more extensively in the construction of modern jet airliners. The behaviour of bonded patches under loading was monitored using DIC full-field strain measurements. Location and extent of damage identified by X-ray radiography correlates well with DIC strain results giving confidence to the technique for structural health monitoring of bonded patches.

Damage in dual phase steel DP1000 investigated using digital image correlation and microstructure simulation

Microstructure failure mechanisms and void nucleation in dual-phase (DP) steels during deformation have been studied using a combination of in situ tensile testing in a scanning electron microscope (SEM), digital image correlation (DIC) and finite element (FE) modelling. SEM images acquired during in situ tests were used to follow the evolution of damage within the microstructure of a DP1000 steel. From these images, strain maps were generated using DIC and used as boundary conditions for a FE model to investigate the stress state of martensite and ferrite before the onset of the martensite phase cracking. Based on the simulation results, a maximum principal stress of about 1700 MPa has been estimated for crack initiation in the martensite of the investigated DP1000 steel. The SEM image observations in combination with the FE analyses provide new insights for the development of physically-based damage models for DP-steels.

Void growth and coalescence modelling in AA2050 using the Rousselier model

In this study, the damage evolution process in an AA2050 aluminium alloy is studied under various triaxiality levels using notched tensile specimens. The performance of the Rousselier model is assessed and it is shown that modifications are required both in the hardening curve and the damage evolution parts of the model in order to match the experimental results. The original model is corrected by adding the effect of hydrostatic pressure on the yield surface as well as implementing the Thomason coalescence model. The transferability of the Rousselier model parameters to other triaxiality levels is investigated and a procedure to identify these parameters is proposed.

Measurement of deformation gradients in hot rolling of AA3004

In this paper we describe an experimental technique developed to measure the deformation gradients and temperature in a single hot rolling pass of an AA3004 sample that was fitted with an insert. The insert had been previously hand engraved with a 1×1 mm grid pitch, and the analysis of the data digitally captured from the image of the deformed grid enabled the calculation of the components of the deformation gradient tensor. Four steel pins prevented relative motion between the insert and the rest of the sample. No detachment was observed between insert and sample after rolling. The temperature was measured during rolling using two embedded thermocouples, one close to the surface and the other in the centerline. The commercial finite element code ABAQUS was used to create a three-dimensional model of the rolling process. The recorded temperature was compared to the numerical values evaluated after tuning the heat transfer coefficient. The shape of the grid after rolling was checked against the deformed mesh using different fricition coefficients in order to obtain the optimum match. The unusually large length of the insert enabled the rolling process to be stopped halfway so that a picture of the roll-gap area could be obtained. This provided a partially deformed grid that represented the transient state during rolling. The experimentaily determined deformation gradient in this area as well as in the steady-state area agreed well with the finite element oredictions.

An optical method for measuring surface roughness of machined carbon fibre-reinforced plastic composites:

Characterization of the damage induced by machining of fibre-reinforced composites is usually performed by measuring surface roughness. Contact-based surface profilometers are the most used equipment in industry; however, it has been found that there are performance limitations which may result when used to measure machined heterogeneous composite surfaces. In this research, surface roughness is characterised using a commercial non-contact optical method, and compared with a conventional stylus profilometer. Unidirectional and multidirectional carbon fibre laminates were edge trimmed and slot milled. The variation in surface roughness was compared using different tool types, fibre orientations and cutting parameters. Surface damage and cutting mechanisms were assessed by using scanning electron microscope images, and the suitability of roughness parameters were also analysed including: Sa, Skewness and Kurtosis. Using the optical system allowed accurate roughness calculation of individual plies on a multidirectional laminate with different fibre orientations. The research has also shown that the optical system, including the use of areal roughness parameters, can increase the accuracy of roughness measurement for machined fibrous composite surfaces and is less sensitive to measurement position than the stylus.

Prediction of Machining Induced Residual Stresses in Aluminium Alloys Using a Hierarchical Data-Driven Fuzzy Modelling Approach

The residual stresses created during shaping and machining play an important role in determining the integrity and durability of metal components. An important aspect of making safety critical components is to determine the machining parameters that create compressive surface stresses, or at least minimise tensile surface stresses. These machining parameters are usually found by trial and error experimentation backed up by limited numerical modelling using Finite Element Methods (FEM) and guided by expert knowledge. The shortcomings of FEM approaches are the length of time needed for the solution of complex models and the inability to learn from data. To solve these problems, a fuzzy modelling approach is presented in this paper and is shown to be successful in modelling machining induced residual stresses.

Effect of roll pass schedule on through thickness texture development in Al–Mn alloy

Abstract During hot rolling a texture gradient is developed through the thickness of the slab. This is directly related to the different strain paths experienced by the material between the surface and the centre plane. The difference in strain path not only affects the texture, but can also give differences in stored energy though the thickness, which in turn affects the recrystallisation kinetics and ultimately the recrystallisation texture. The strain path is further complicated when a number of roll passes are involved, since the material is subjected to more complex strain paths. In the current investigation the effects of roll pass schedule (rolling direction, i.e. reverse rolling or continuous rolling) on the texture development during deformation and subsequent annealing have been characterised for an Al-1%Mn alloy. The study has shown that the texture of the surface region of the slabs is dependent on the roll pass schedule. This effect is at a maximum in the near surface region, although the effect of roll pass schedule on the recrystallisation kinetics is at a maximum 20% of the half thickness below the surface of the slabs.