Fatigue Life Assessment of Existing Motorway Bridge

The aim of this paper is to provide a fatigue-life assessment of an existing composite steel-concrete bridge within Switzerland. The Venoge bridge is a 219 meters length, four spans, four girder composite bridge, situ-ated on the Geneva-Lausanne Swiss motorway. Stress response influence lines at fatigue critical details are characterized with a refined three-dimensional (3-D) finite element (FE) model that takes into account the ef-fective transverse load repartition and the cracking of the concrete slab; the model is further validated using strain gauge measurements. Real traffic simulations are run to get the effective traffic histograms on the bridge. Weigh in Motion (WIM) data from the Denges station (1km from the bridge) are used as input for the simulations. The effective slow lane position over the bridge section is taken in account and future traffic in-crease is considered. The fatigue failure probability of the bridge at a critical location under the long-term ef-fect of highway traffic is calculated by defining a limit state function for fatigue reliability analysis. The re-sults show that fatigue is not a concern for the Venoge bridge, even though particular attention has to be paid to future change in traffic lane configurations which could require re-consideration of the fatigue critical de-tails.

Comportement dynamique des dalles de roulement des ponts en béton sollicités par le trafic routier

The management of a network of existing road bridges involves interventions in order to maintain safety and the priority of these interventions is often determined by safety criteria. During the evaluation of safety, the dynamic effect of traffic actions is considered using equivalent static loads determined by multiplying the effect calculated using traffic load models by a dynamic amplification factor. During the evaluation of a deck slab, the application of inappropriate dynamic amplification factors could have significant financial implications, all the more since the local dynamic effects of overloaded trucks are determinant for this type of structural element. Dynamic amplification factors defined in codes have usually been derived from the measurement of global traffic action effects in the main structural elements of bridges. Unfortunately, local dynamic effects in deck slabs have not been studied in detail until now. A better understanding of the dynamic behaviour of deck slabs will lead to the definition of more accurate dynamic amplification factors and avoid the use of values that are too conservative. The behaviour of deck slabs of six typical Swiss highway bridges has been simulated in order to study their dynamic response during the passage of trucks. The structural arrangement of the deck slab was different for each of the six bridges. A parametric study was based on the simulation of various scenarios involving the passage of trucks for various combinations of speed, path and road surface roughness. Deck slab response was obtained by numerical simulation based on models of the bridge, truck and road surface. This system was resolved using a prediction - correction algorithm that considers the dynamic interaction between a bridge and trucks. Dynamic amplification factors were subsequently calculated from strains and deflections obtained from independent static and dynamic simulations. The simulation of numerous scenarios enabled the evaluation of the influence of different parameters on the dynamic response of a deck slab: The road surface roughness was found to be one of the most important parameters, with an increase in roughness leading to an increase in dynamic amplification factors. An overloaded truck produces a lower dynamic amplification factor in a deck slab than an empty truck. The truck speed influences the dynamic interaction, but a clear relationship between speed and dynamic amplification factor could not be identified. The maximum dynamic amplification factor does not vary significantly from one point to another over a deck slab in a girder bridge. The structural arrangement of a deck slab in a girder bridge has little influence on dynamic amplification factors. Overall, the different deck slabs on the girder bridges studied were all equally sensitive to the dynamic effects of road traffic. In many cases considered for the framed slab bridge, the maximum dynamic amplification factor was found to occur over the supports. For such bridges, the sensitivity of the deck slab to the dynamic effects of road traffic is not uniform. Finally, two approaches to evaluating deck slabs of existing road bridges are proposed. The first approach is only applicable in certain situations and involves a simplified evaluation using an updated traffic load model and a dynamic amplification factor. In situations where a simplified evaluation is not applicable, the second approach is to evaluate a deck slab using numerical or experimental analyses.

EPFL1999

On the Mutual Effect of Viscoplasticity and Interfacial Damage Progression in Interfacial Fracture of Lead-Free Solder Joints

John Botsis, Joël Cugnoni, Milad Maleki

The main goal of this paper is to shed light on the effect of strain rate and viscoplastic deformation of bulk solder on the interfacial failure of lead-free solder joints. For this purpose, interfacial damage evolution and mode I fracture behavior of the joint were evaluated experimentally by performing stable fracture tests at different strain rates employing an optimized tapered double cantilever beam (TDCB) design. The viscoplastic behavior of the solder was characterized in shear, and the constitutive parameters related to the Anand model were determined. A rate-independent cohesive zone damage model was identified to best simulate the interfacial damage progression in the TDCB tests by developing a three-dimensional (3D) finite-element (FE) model and considering the viscoplastic response of the bulk solder. The influence of strain rate on the load capability and failure mode of the joint was clarified by analyzing the experimental and simulation results. It was shown how, at the lower strain rates, the normal stress generated at the interface is limited by the significant creep relaxation developed in the bulk solder and thus is not sufficiently high to initiate interfacial damage, whereas at higher rates, a large amount of the external energy is dissipated into interfacial damage development.

Springer US2011

Deformation and Damage in Lead-free Solder Joints

Milad Maleki

For environmental reasons, lead has been banned from microelectronic solder materials since the adoption of the RoHS legislation in 2006. A large number of investigations have lead to the development of new lead-free solder materials like the SnAgCu alloy which can satisfy most of the technological requirements. However, the long-time reliability of the lead-free candidates is still a crucial issue and in several cases, failure of electronic assemblies has been attributed to that of the solder joints. To overcome the present challenge in the soldering technology, the damage mechanisms occurring along the service life of lead-free solder joints like microstructure evolution, viscoplastic deformation as well as interfacial cracking must be better understood. The main scope of this thesis is to shed light on the complex interdependency of microstructures, deformation property and the interfacial fracture of solder joints in order to explain the change in the failure mode of solder joints from ductile to brittle interfacial fracture as well as the link between microstructural evolution and the change in the mechanical property of the solders. The first part of this research focused on characterization and macroscopic modeling of the fracture at the interface of SnAgCu solder and Cu substrate with a special attention on the viscoplastic nature of the bulk solder. For this purpose, the viscoplastic behavior of the solder joint was initially identified and the material parameters related to a unified constitutive viscoplastic model were determined. The interfacial fracture behavior of the joint was characterized by performing stable fracture tests at different strain rates. The results confirmed that the failure mode of the solder joint is a function of the loading rate. It was shown that an increase in the loading rate causes a decrease in the load-bearing capacity of the joint by enhancing the tendency to develop an interfacial failure. The interfacial crack propagation in the fracture tests were simulated by developing a detailed three-dimensional (3D) finite element (FE) model containing a cohesive interface and considering the viscoplastic behavior of the bulk solder. The influence of strain rate and viscoplasticity on the load-bearing capacity and failure behavior of the joint were pointed out by analyzing the simulation results. It was shown how the normal stress generated at the interface is limited by the significant viscoplastic relaxation developed in the bulk solder at low strain rates, while a large portion of the external work is transferred to the interface at higher rates which gives rise to interfacial fracture. The second part of this thesis focused on developing microstructure-based models in order to simulate the change in the macroscopic mechanical response of the SnAgCu solder caused by the microstructural evolution during thermal ageing. For this purpose, first, the 3D configurations of microstructures in different ageing conditions were visualized using a combination of synchrotron X-ray and FIB/SEM tomography techniques. Analyses showed that although the size and orientation of crystalline grains as well as morphology of dendrites in SnAgCu solder remain stable during ageing, two main morphological changes occur in the Ag3Sn and Cu6Sn5 intermetallic particles: spheroidizing of needle-shape intermetallics and coarsening due to bulk diffusion. The mechanical property of the solder and its phases at different ageing conditions were experimentally characterized. A significant decrease in the yield strength of the solder joint was identified because of thermal ageing and it was shown that the deformation behavior of the solder is governed by the morphometrics of the dispersed intermetallics. The link between the microstructures and the deformation behavior of solder was successfully simulated by developing a hybrid microstructure-based modeling approach. The average indirect strengthening of the intermetallics was modeled by incorporating the effects of geometrically necessary dislocation on the in-situ deformation behavior of the tin matrix. Subsequently, a micromechanical FE homogenization was considered to model the load-sharing behavior of the intermetallics. For this purpose, the tomographic images were directly used to generate 3D FE meshes of the actual microstructures and then the constitutive deformation behavior of the solder were determined by adopting two levels of numerical homogenization in the eutectic phase and over the whole joint. The decrease in the yield strength of solder due to ageing was explained through analyses of indicators in the models. It was demonstrated how the change in the morphometrics of intermetallics during ageing reduces the contribution of intermetallics in load-sharing as well as the in-situ strength of the matrix by causing a significant change in the strain distribution as well as particle size effects. The simulation results were experimentally verified and the capabilities as well as limitations of the developed approach were pointed out. Overall, the results of this research led to a proper understanding of the critical reliability issues in lead-free solders during isothermal ageing, i.e., microstructural coarsening and interfacial fracture. In addition, through this work, a set of methodologies for multi-scale modeling and experimental characterization of the damage and deformation in solder joints were developed and validated which can be extremely helpful for efficient use of the SnAgCu solders in electronic industry as well as designing new solder alloys with higher strength and better reliability for a safe transition to lead-free solders in critical applications.

EPFL2012

Comportement dynamique des dalles de roulement des ponts en béton sollicités par le trafic routier

EPFL1999

Deformation and Damage in Lead-free Solder Joints

Milad Maleki

EPFL2012