Massimiliano Avalle

Casting defects and fatigue strength of a die cast aluminium alloy: a comparison between standard specimens and production components

Abstract The influence of casting defects on static and fatigue strength is investigated for a high pressure die cast aluminium alloy. Defects exist in gas and shrinkage pores as well as cold fills, dross and alumina skins. For the three batches of specimens, differing for the sprue–runner design, the influence was straightforward, while no significant variation in the fatigue strength was observed when looking at batches of “acceptable” and “non-acceptable” components, as judged within the foundry quality control. In this case, defects count for their size and location, while quality control often takes no account for component working conditions. The Haigh diagram shows a good matching between the specimen reference material and the component fatigue data.

Experimental evaluation of the strain field history during plastic progressive folding of aluminium circular tubes

Abstract The axisymmetric collapse by plastic progressive folding of a circular tube submitted to axial loading is considered by an experimental approach. The strain field history is measured by means of electric strain gages properly placed on the external surface of the tube so that more than one fold is covered and both axial and circumferential strains are measured. The measured strains are examined both as time-histories and as a deformation field. The formation and development of circumferential plastic hinges are pointed out. The strain histories, reported as a function of the displacement of the testing machine cross-head, are then correlated with the crushing force diagram, leading to a better understanding of the folding mechanics. In particular, the formation of each fold develops through three subsequent phases: the initialization at the closure of the previous fold, the flattening of the upper conical surface, and the flattening of the lower conical surface. While most of the tube wall is pushed outwards of the original cylindrical surface, a portion is pushed inwards of that surface. Moreover, there is a small portion of the wall that is pulled inward during the fold initialization and then pushed outward during the fold closure. The analysis of these histories lead to the validation of the basic assumption of our and other recent kinematical models of the plastic progressive folding.

AlSi7 metallic foams – aspects of material modelling for crash analysis

Metallic foam samples of matrix alloy AlSi7 have been produced and mechanically tested under quasi-static and dynamic load. Model parameters for the Deshpande–Fleck and the ABAQUS ‘crushable foam’ material model were determined covering a density of 0.3–0.8 g/cm3. Yield surface determination uses uniaxial hydrostatic compression test results, extended by tensile test results for the latter model. Strain hardening was described on the basis of uniaxial compression by fitting a Rusch model to the experimental data, deriving its parameters as function of density. The predictive capabilities of the parameterised models were evaluated using experimental data gathered for load cases characterised by superimposed uniaxial and hydrostatic compression. Analyses show good agreement between simulation and experiment. Further uniaxial compression tests performed at varying strain rates over 4 orders of magnitude revealed no significant strain rate dependency of material properties and thus qualify the material model parameters determined for crash simulation.

Static lateral compression of aluminium tubes: Strain gauge measurements and discussion of theoretical models

Abstract This paper illustrates experimental results obtained by means of strain gauges applied on circular tubes during static lateral crushing. The aim of the work is to obtain an experimental insight regarding the strain field during lateral compression and to verify the known theoretical models. The experimental activity involved aluminium tubes of 80 and 100 mm diameter and 2 mm wall thickness. Up to thirty strain gauges were applied on the internal and external surfaces of each tube. Additional theoretical results were obtained by means of numerical simulation performed using a non-linear finite element code. The measurements are well fitted, both qualitatively and quantitatively, by the Reid-Reddy theoretical model.

A theoretical approach to the optimization of flexural stiffness of symmetric laminates

Abstract A theoretical study is performed to optimize the design of symmetrically laminated plates with respect to fiber orientation and layer thickness. Rectangular plates simply supported along the edges and with transverse pressure load are considered. The expression for plate flexural stiffness is obtained using approximate energy approaches, such as Ritz-Rayleigh method, using different shape functions. Results of analytical solution are compared with results from finite element analysis, which show a good correlation between theoretical and numerical results. Optimality criteria are applied to the analytical expression of flexural stiffness to determine the optimality condition. A closed form solution is obtained for the fiber orientation and it is demonstrated that the optimal orientation of the fiber is unique and independent of the stacking sequence. Results are also obtained for the optimal values of thickness for each layer.

Numerical Analysis of Hybrid Joining in Car Body Applications

Nowadays, there are two fundamental issues in car body design. On the one hand, there is the need for weight reduction in order to reduce fuel consumption and, consequently, pollutant emissions, and on the other hand, there are ever more stringent safety requirements. To meet these targets, the trend is towards using hybrid structures made of unconventional materials, like aluminium, polymeric and composite materials. The use of these materials brings about some problems, one of them is associated with the joining techniques because the traditional resistance spot-welding, used in the assembly of a common steel chassis, cannot be used. Among the different alternative solutions, the most promising is the use of structural adhesives. From this perspective, this work aims to model the behaviour of simplified crash box elements made of different types of materials joined together by structural adhesives. In particular, the attention was focused on the adhesive joint modelling with a cohesive element formulation. Starting from experimental results for the characterization of the adhesive, the cohesive parameters were identified. The results were then applied to model the crushing of simplified crash boxes. The crushing axial compression of these components was investigated considering geometrical and loading conditions. The models were developed and verified by comparing the numerical with experimental results on these components. A good correlation was found in all loading conditions and with different geometries and substrate combinations.

Experimental investigation of the energy absorption capability of bonded crash boxes

In the design of vehicle structures for crashworthiness there is a need for rigid subsystems that guarantee an undeformable survival cell for the passengers and deformable subsystems able to efficiently dissipate the kinetic energy. The front rail is the main deformable component dissipating energy in a frontal impact, which is the most dangerous crash situation. In frontal impact these rails have the greatest influence on vehicle crash performance. The design of the front rail, usually consisting of a thin walled prismatic column, requires definition of the geometry, that is, of the shape and dimensions of the cross section, of the thickness of the material, and of the material itself. In this work the analysis of the effect of different cross sections of the front rail and of the joining system is carried out. Furthermore, the collapse during crash is influenced by the loading rate since the loading speed has substantial influence on the mode of collapse and on the material behaviour. In fact, the structural materials used in this application are known to be strain-rate sensitive. Within the work, different types of sections are compared. The ground-breaking joining technology of bonding is examined: three different types of adhesive are compared, an acrylic, a one component epoxy and a two component epoxy. Adhesives can be used as a substitute to the widely used spot-welding to improve the structure performance mainly because of the continuous joint. The effects of the loading speed are taken into account by comparing quasi-static crush tests with dynamic impact tests. Dynamic tests have been performed under a drop tower testing apparatus built within the Vercelli campus of the II Faculty of Engineering of the Politecnico di Torino.

Static and fatigue strength of a die-cast aluminium alloy under different feeding conditions:

Abstract This paper reports investigations of the influence of porosity and casting defects on the static and constant-amplitude fatigue strength of a die-cast aluminium alloy. Three batches of specimens, differing in the sprue-runner design and consequently in content and types of defects, were tested in ‘as-cast’ conditions. Defects consisted of gas and shrinkage pores as well as cold fills, dross and alumina skins. Casting defects were observed to reduce significantly the static and fatigue properties of the material. Whereas for the static characteristics the decrease was progressive with the porosity range, for the fatigue strength the decrease was most significant from the lowest to the middle porosity range. The batches were classified with regard to the porosity level, as the metallurgical defects were not detectable through X-ray examination. The content and size of metallurgical defects were observed to increase together with the porosity level. Scanning electron microscopy (SEM) observation of the fracture surfaces demonstrated the important role played by dross, alumina skins and, above all, cold fills on the fatigue fracture.

Optimization of a Passive Safety Device by Means of the Response Surface Methodology

The design of new cars requires the satisfaction of ever stricter safety standards defined by certification regulations and to match the continuously growing attention of the customer for safety. Hence, it is necessary to improve every component of the vehicle to achieve better performance. An effective component in reducing loads transmitted to the driver chest in frontal crashes is the deformable steering wheel column, capable of collapsing with high energy absorption while reacting with little axial loads. The controlled collapse of the column can be achieved by inserting a corrugated tube. To achieve optimum performance it is necessary to find the proper value of its geometrical parameters. Aim of the work is to show how the use of the structural optimization approach based on the response surface methodology is an effective strategy to optimize such a component.

Incidences of various passenger vehicle front-end designs on pedestrian lower limb injuries

The present study aims to investigate the influence of various passenger vehicle front-end designs on knee ligaments and tibia injuries of pedestrian lower limbs by finite element simulations. Using a detailed finite element model of the lower limb, this work was focused on tibia fractures and knee ligament ruptures of lower limbs during vehicle–pedestrian impacts. The influences of vehicle front-end structures on the risk of these two injury occurrences were investigated using super mini, small family car, executive car, and multipurpose vehicle passenger vehicle types. The overall results show that the bumper beam design play an important role in inducing pedestrian lower limb injuries. Its height to the road surface, width in the height direction, and the deformable depth between it and the bumper fascia should be all considered in the vehicle design to protect pedestrian lower limbs.