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ABSTRACT In fatigue evaluation of welded structures, explicit weld representations in finite element (FE) models are needed for reliably capturing stress or strain concentration behaviors at critical weld locations, for example, weld toe or weld root, in using widely accepted traction structural stress or extrapolation hot‐spot stress methods. The laborious efforts needed for generating weld geometry have been a major challenge for fatigue evaluation of complex structures containing many welds. In this paper, we present a user‐defined fillet‐weld element formulation and its numerical implementation for computing traction mesh‐insensitive structural stresses. The fillet‐weld element is formulated by connecting several linear four‐nodes Mindlin shell elements around weld region as a user‐defined element. The resulting elements can be directly used with major commercial FE codes through an available user subroutine interface. A number of well‐documented fillet‐welded components are then used for validating the accuracy and robustness of the developed fillet‐weld elements. 

Abstract With the rising importance of virtual engineering in an increasingly competitive marketplace, there is a growing need for simplified representations of finite element (FE) modeling for spot joints in lightweight structures without losing accuracy in structural life evaluation. For this purpose, this paper presents a spot weld element with an implicit weld representation and its numerical implementation as a user element for deployment in commercial FE code for reliably computing traction structural stress in a mesh‐insensitive manner. The spot weld element is formulated by degenerating conventional first‐order four‐nodes shell elements by imposing kinematic constraints with respect to a series of virtual nodes placed in the region around a spot weld. The simplicity and effectiveness of the spot weld element have been validated by comparing with the explicit weld representation for computing mesh‐insensitive structural stresses and fatigue life correlation of welded components. 

Polypropylene (PP) and its composites are one of the hardest to directly join with metals due to their inherent chemical incompatibility. This paper presents a simple, efficient, and cost-effective method for joining PP composite to aluminum alloy in spot welding configuration by seeding the functional groups via an insert layer of PA6 thin film without requiring surface or material pre-treatment. The resulting joint loading capacity is shown to be sufficiently high to consistently develop failures in PP substrates in lap shear tensile tests away from the bonded area. Joint interface microstructure features are examined in detail. Bonding mechanisms are then described based on the detailed observations obtained in this study. 

A hybrid structural stress method is presented for significantly simplifying spot weld representations in fatigue evaluation of complex spot-welded structures while retaining a high degree of accuracy in structural stress computation. The method is formulated by extracting nodal forces and moments around a group of domain elements connected to a spot weld represented by a regular beam element. Through a systematic decomposition technique, existing closed-form solutions, previously only valid for modeling single-spot weld test specimens, can now be used for calculating the relevant structural stresses under complex loading conditions in structures, as validated its ability in correlating fatigue test data. 

Adhesive-bonding has become increasingly adopted for multi-material lightweight applications (e.g., automotive structures). There is a growing interest in understanding the fatigue behaviors in this type of joint for supporting structural durability modeling in practice. In this paper, an analytical fracture mechanics modeling procedure is presented in the context of a generalized sandwich specimen. Its closed form stress intensity factor solutions were then derived and applied for the correlating fatigue test data obtained from the lap-shear and coach-peel test specimens with demonstrated effectiveness. Some important implications of these analytical solutions on joint design are also discussed.