The automotive lighting industry continues to experience significant growth as it embraces new trends focused on higher performance and smarter functionalities. Any new products in automotive must undergo severe accelerated tests in order to ensure its reliability under the various conditions that it may be confronted with during its lifetime. Due to the coefficient of thermal expansion mismatch between the LED package and the PCB substrate, the solder joint fatigue failure was revealed to be one of the major failure modes during accelerated thermal cycling. The present work focuses on analyzing three LEDs (2-pads, 3-pads and 4-pads) configurations subject to thermal cycling test. First, the damage in the solder joints during tests is observed with dye penetration at different stages of cycling. During the tests, monitoring of the variation of Delta VF was made to follow the performance evolution of LED as a function of cycles. The dye penetration observations were analyzed to follow the evolution of the damage. On the 3 configurations of LEDs (2-pads, 3-pads and 4-pads) a model predicting the damage as function of the Delta VF was defined and used to establish a damage law. The second part is focusing on a thermomechanical modeling of the strains on the solder SAC305 joint occurring during thermal cycles for the 3 configurations of LEDs mounted on the type of IMS PCB. Finally, a fatigue analysis is addressed based on the 3 configurations of LEDs on the time to failure in the thermal cycle test. A comparison between three configurations is made and a correlation with test results is performed to evaluate the accuracy of the prediction.

Cast steel is commonly used to produce structural and safety parts. Foundry processes allow producing parts from scrap steel directly to the required dimensions without any forming operation. Cast components may, however, exhibit macro- and micro-shrinkage porosities. The combined effect of macro- and micro-shrinkages on the fatigue behavior of cast steel has been characterized in the literature. Macro-shrinkages may nowadays be eliminated by adequate positioning of risers. However, micro-shrinkages will always be present in cast steel components. Present work addresses the influence of micro-shrinkage porosity on a G20Mn5 cast steel. G20Mn5 (normalized) ingots have been cast under industrial conditions, but ensuring the absence of macro-porosities. Solidification leads to two very different microstructures prior to the normalization treatment: columnar dendrites beneath the surface (Skin) and equiaxed microstructures close to the center (Core). First, metallographic observations of the whole ingot revealed the same grain size in both areas. Fatigue samples were extracted, by differentiating two sampling volumes corresponding to columnar (S) and equiaxed solidification (C), respectively. The distribution of micro-porosities was determined on all samples by Micro-CT-scans. Core samples exhibit micro-porosities with volumes 1.7 larger than Skin samples. Low cycle fatigue tests (3 levels of fixed plastic strain) were run on both sample series (C, S). Results follow a Manson–Coffin law. Core specimens exhibit lower fatigue life than Skin specimens. The differences in fatigue life have been related successfully to the differences in micro-porosities sizes.

New LED technologies & modules allow glare-free automotive lighting. Valeo solutions include the first high-definition LED lighting system. These LEDs technology endure extreme tests conditions in form of thermal shocks or mechanical vibrations to insure reliability under the lifetime of automotive usage. Strict regulations require lead-free solders. Advanced optimization of LEDs for automotive lighting led to multilayered and miniaturized stack-ups. The first lead-free solders were revealed as a weak part of the electronic components and LED assembly. Enormous efforts have been spent on optimizing the lead-free solder material, design and reflow process. However, component miniaturization can also lead to high thermal gradients. Present work analyses the relation between component architectures and thermal shock induced fatigue damage. First, damage after N1 and N2 thermal shock cycles was characterized experimentally. Genuine sample preparations by a Cross polisher followed by high-resolution SEM observations reveal void growth in the solder material and cracking in the current circuit Layer. A two-scale finite element model reproduces both damage types. Simulation results can help to provide design improvements on the component architectures and modules to avoid (minimize) both damage types.

In new high definition lighting modules, assembling more than thousand elements (pixels) with a single mm -sized motherboard is achieved by reflow soldering. Lead-free solders may constitute a weak part of electronic components and LED assemblies. During reflow, the distance between pixels and motherboard controls the solder heights (10 mu m). After assembly, all the pixels are in a single plane. To ensure constant solder heights, this plane has to be parallel to the motherboard. A small disorientation during reflow may lead to severe solder damage prior to any use of the opto-electronic component. Present work presents a novel non-destructive method for characterizing any chosen solder in the assembled component without unpacking the latter.A femtosecond motorized laser source is used for digging a hole in the LED above the selected solder. Then a Focused Ion Beam facility allows to mill around this specific solder. The same FIB facility allows making several hundred slices of controlled thickness and orientation in the solder material. During this slicing operation, high -resolution inspection is done by FEG-SEM (Field Emission Gun -Scanning Electron Microscope) imaging. 3D tomographic reconstruction of the solder is obtained by classical commercial software AVIZO. Finally, 3D damage in solder material is illustrated by example results.

Damage evolution during tensile straining of an AlSi12Ni alloy has been analyzed in situ at synchrotron source using microtomography and in the scanning electron microscope by surface imaging. It is shown that damage development in the analyzed alloy presenting an interconnected network of intermetallic phases is completely different from damage progression previously observed in materials with disperse distribution of particles. In the present material, which is typical for most eutectic structures, damage is dominated by the rupture of the brittle intermetallic phase while void growth is limited by a shielding effect of the intermetallic particles encasing the void. Primary voids exhibit a size close to the thickness of branches of the intermetallic phase. Final failure occurs by void coalescence, but without the formation of secondary voids. Damage analysis from tomographic scans was only possible by applying 3D image correlation to successive reconstructions, which thanks to its sub-voxel resolution, could satisfactorily detect the volume fraction of small-voids inaccessible by conventional thresholding. The presence of many small voids issued from the breakage of the intermetallic phase also was confirmed by scanning electron microscopy imaging performed at higher resolution.

LEDs and electronic components are taking a more important part in automotive lighting systems. With the increasing need of size reduction and higher flux performances, thermal and electronic reliability are important to maintain by deploying a validated pre-sizing tool to reduce long simulation times and assure a quick design validation in a project.In this paper, we present a new simulation method to predict the thermo mechanical fatigue of solder joints in a complex 3D stack up LED. Solder joints are essential to the assemuly of components on a substrate. Yet, thermal cycles are identified as a primary cause of solderjoint breaking. The aim is to analyze the damage caused by the thermal expansion effect during reflow of the solders and thermal shock cycling on the stack up by Hutchinson's 1966 model and 3D simulation. The predicted damages are compared to microstructure analysis done by SEM.

The trends in automotive industry are towards higher performance, improved reliability, reduced tolerances and more environmental friendly products. Wear resistance enhanced mechanical components exhibiting lower friction help achieving this major challenge. Diamond-Like Carbon (DLC) deposited on mechanical components operating under lubricated conditions at temperatures between 100 and 250 degrees C, efficiently decrease the friction coefficient and insure wear protection. Wear induced debonding and blistering of hard surface coatings is still a major problem in the automotive industry. There are several open questions. Where does debonding start? Where does the interface crack propagate? Where does it deviate leading to delamination? In this work, we show how the combination of the latest experimental techniques allows to answer these questions and thus better control wear. Blistering is influenced by the overall stress in the coating and the very local microstructure of the substrate. A combination of AFM and nano-indentation measurements, as well as FIB milling with in situ FEG-SEM observations and local chemical measurements by EDX, allows to observe closely and better understand wear-induced delamination. The new availability of such physical and chemical investigations should improve mechanical and physical-based models to predict wear and enhance coating adhesion.

Crack propagation and deviation during trimming of aluminium sheet metal have been analysed experimentally and modelled by a finite element analysis (FEA). The experimental results may be summarized as follows. Two crack propagation mechanisms operate in aluminium sheet metal, i.e. either between adjacent voids or by multiple coalescence of micro-cracks. A fracture process-zone may be identified for both mechanisms. Outside this fracture process-zone, no damage is observed. The particular crack propagation mechanism does not influence the crack path. A mechanical model based on a two steps analysis is presented. In the first step, the overall stress strain field prior to cracking is determined by an explicit Arbitrary Lagrangian Eulerian (ALE) formulation based FEA. In the second step, dedicated to crack propagation, no remeshing is used. A cumulative damage model based on the equivalent plastic strain increment is shown to be very efficient. The damage variable is interpreted as bifurcation in the direction of the maximum plastic strain rate. Finally, the applications show the possibility of trimming aluminium sheet metal without sliver generation.

La comprehension des modes de rupture des points soudes d’aciers a Tres Haute Resistance (THR) est un enjeu industriel important. Des essais de caracterisation locale sont developpes, ainsi que l’observation in situ rupture d’ un point soude. La quantification du comportement et de la rupture des differentes zones d’un point soude devra permettre une meilleure comprehension de la soudabilite des aciers THR.

We study the implant-induced hydrogenated defects responsible for the Smart Cut™ layer transfer of Si (001) films. Different experimental methods are used to quantify the time dependence of the defect evolution and interactions during isothermal annealings. An optical characterization technique was developed for the statistical analysis of the formation and growth of micrometer size microcracks in the buried implanted layer. We show that the formation of molecular hydrogen is dominated by a transient phenomenon related to the rapid dissociation of the hydrogenated point defects. The impact of the H2 formation kinetics on the microcrack evolution is described and the physical mechanisms involved in their growth are identified. A comprehensive picture of the fracture phenomenon in H implanted Si leading to the full layer transfer is proposed and discussed.

The growth and coalescence of voids nucleated by decohesion or cracking of second phase particles is a common damage process for many metallic alloys. Classical damage models, based on void growth and coalescence, predict a ductility increase if the stress triaxiality is decreased. But experiments show that the material ductility decreases at very low stress triaxialities typical of sheet metal forming operations. At very low stress triaxiality no void growth is observed in metals containing second phase particles. In the present work, a new damage model for metals containing second phase particles submitted to low stress triaxiality loading is proposed.The new model is based on the observed physical damage mechanism, i.e. strain localization by reducing the inter-particle spacing during large material rotations. A two step modelling strategy has been followed to determine the ductility at low stress triaxiality. In the first step Thomason's void coalescence model is extended to large material rotations and shearing. In the second step the principles of applying this model to damage nucleated at second phase particles are described. The large material rotations observed under low stress triaxiality loading lead to large changes in the microstructure. Thus, in the second step, first appropriate representative volume and material elements are determined and then the critical damage parameters. Finally, as an example, the trimming behaviour of two aluminium sheet alloys is analyzed by the new model and the model predictions shown to be in good agreement with the experimental data, in particular for the blade displacement to crack initiation. The main outcomes of this work are: (1) a void coalescence model valid at low stress triaxiality, (2) a damage criterion valid at small stress triaxiality and large material rotations, (3) a damage variable expressed in a simple closed form for materials containing second phase particles. The damage analysis in a small fixed volume with a representative microstructure (Eulerian approach) and the damage analysis in all the material elements of the considered structure are compared in detail. In trimming (or similar processes), the major contribution to damage the material movement bringing second phase particles closer together and the void growth may be neglected. This simplifies considerable the analysis.

Material ductility, or the failure strain, is generally characterized by simple tensile tests. However, tensile tests on sheet samples lead to higher strain gradients than those observed in axisymmetric samples, so the classical Bridgeman correction is invalid. This paper first describes how to correctly calculate local values of stress the triaxiality in tensile deformed sheet samples. Secondly, the ductility of sheet samples was also simulated numerically by a damage mechanics cell model based on the finite element method. Finally, an experimental study on two Al 6xxx sheet alloys shows that the classical, average ductility (ln(Ar/A0)≅0.67 here) strongly underestimates the true maximum material ductility, i.e. the local maximum strain to failure ɛf≅0.91. The values of the average ductility predicted by the present work are also shown to agree well with the experimental values.

Two recent methods for obtaining flow stress-strain relations up to large strains of order 1.5 by channel-die compression are presented: i) for sheet metal formability tests, composite samples have been made of glued sheet layers and deformed at room temperature in a channel-die with the compression axis directed along one of the sheet metal edge directions, i.e. RD or TD. The sheet plane is parallel to the lateral compression die face. It is shown that, using a suitable lubricant, the sample deformation is homogeneous up to strains of 1.5. Tests carried out on 5xxx and 6xxx alloys to evaluate the stress-strain relations show that a generalized Voce law gives a good quantitative fit for the data. ii) for high temperature plate processing, quantitative flow stress data can be obtained up to 500°C with a rapid quench using a hot channel-die set-up. Some new results are presented here for high strain hot PSC tests on Al-Mn and Al-Mg alloys together with microstructure analyses.

Three-dimensional images of intermetallic particles and of voids embedded in the 5182 aluminium. alloy are obtained by synchrotron X-ray absorption tomography. The images are processed and analysed in order to quantify the evolution of the morphology of the particles and pores during the complete processing of a sheet product, that is, from the as-cast to the cold deformed state. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A thoroughly tested, high-temperature channel-die compression (CDC) rig is described for simulating hot plane strain compression of metallic alloys up to 500 °C. The equipment is currently used to characterize the flow stress and microstructure evolution in hot-rolled Al alloys. It has been validated by several tests involving (1) metallographic analysis of deformed samples; (2) flow stress comparisons with the same, or similar alloys deformed in conventional uniaxial or plane strain compression; and (3) microstructure and texture measurements. The use of modern lubricants enables one to obtain accurate flow stresses and true plane strain deformations that are homogeneous over 80 pct of the sample. The equipment also features rapid heating and cooling systems to minimize thermally-induced microstructure changes. Some results on high-temperature slip systems, hot deformation textures, and microstructures, and the behavior of constituent particles are outlined to illustrate the advantages of the technique.

Ductile damage evolution and fracture of a resulfurised stainless steel (AISI 303) were analysed by high temperature tension tests. Void nucleation, growth and coalescence were investigated. The existence of two distinct void coalescence mechanisms has been demonstrated. High temperature damage models assume the existence of a critical value of the void volume fraction at the onset of void coalescence. In the present work, experimental evidence of the existence of two threshold fractions: (i) a critical value of the void volume fraction fcrit at the onset of void coalescence and of (ii) a critical value of the void volume fraction at final failure ffail is given. Values of the threshold fcrit were measured by high temperature tension tests on axisymmetric smooth and notched specimens and values of the threshold ffail on deeply notched specimens. It has been shown that fcrit and ffail are physically meaningful concepts. Two simple criteria for the determination of fcrit and ffail are given. fcrit corresponds to the sudden increase of the void volume fraction in the necking area of tensile specimen at the transition from non-catastrophic to catastrophic void coalescence. ffail corresponds to the maximum void volume fraction in front of a moving crack.