Problem

Multi-material joining techniques are becoming increasingly important and have applications in a wide variety of industries from oil and gas, physical security, automotive, aerospace, and energy systems to consumer goods and medical devices. The demands for improved system performance, weight reduction and reduction in manufacturing operations for components and structures are increasing. To meet these demands, materials with a high strength-to-weight ratio are becoming increasingly attractive and the joining of these materials presents challenges. Traditional welding techniques are not capable of joining materials that are dissimilar in nature (e.g. metals to polymers). Thus, joining technologies such as adhesive bonding and mechanical fastening have to be considered to accommodate these challenges and enable more widespread use of multi-materials in structures.

The advancement of joining technologies is critical to expand the use of new multi-material assemblies. Currently, process modelling and the simulation of multi-material joining techniques are not ready for industrial applications and the mechanical properties of complex joined structures cannot be predicted. Research on different physical scales is required to obtain a more fundamental understanding of deformation, damage and fracture, and other failure mechanisms.

In the design of structures or products, large shell elements are used for computational efficiency which hampers an accurate representation of the connections and their failure modes. Engineering approaches that increase the prediction accuracy, but do not affect the overall computational time have to be developed based on a fundamental understanding of the behaviour of the structural joint. 

Objective and scope

The aim of this research programme is to provide validated computational models for multi-material joints applicable in large-scale finite element analyses. The scope is limited to the behaviour and modelling of structural joints made with screws, adhesive bonding and self-piercing rivets - as well as possible combinations of these. The considered materials are steel, aluminium and reinforced polymers.

Research tasks

Two main research tasks have been identified in this programme, and these are briefly described below.

1.     Structural bonding: Adhesive bonding of dissimilar materials like reinforced polymers to high-strength steel or aluminium alloys will be considered. A multi-scale approach will be designed to enable virtual characterization of bonded structures with the purpose of replacing physical testing in the calibration of macroscopic models. As structural adhesives are polymeric materials, the constitutive and fracture models developed in the Polymer structures programme will be employed with fine solid element meshes to understand the physical mechanisms behind fracture initiation and propagation. As fracture in these assemblies can arise from the interface between the adhesive and the metal/reinforced polymer to be joined, lower scale approaches such as molecular dynamics will be considered. Such lower scale models will form the basis for continuum-based modelling and subsequent engineering techniques based on cohesive element formulations. The proposed models will be validated against experimental data covering a large range of loading rates, deformation modes and temperatures. The development of novel experimental techniques is also included here.

2.     Mechanical fasteners: Mechanical fasteners such as flow-drilling screws and self-piercing rivets used for multi-material and multi-layered assemblies will be considered. A deep understanding of their behaviour will be established based on both experimental and numerical techniques. As part of a multi-scale approach, the joining processes will be simulated by means of accurate finite element models and history variables will subsequently be mapped to 3D representations of the connections. These models will be employed to increase the understanding of the behaviour of these assemblies, replace expensive and time-consuming testing for the calibration of engineering models, and develop new generation engineering models incorporating more physics than the existing ones. A validation strategy will be worked out involving different loading paths, deformation rates and operating temperatures. As mentioned in the previous research task, new experimental techniques have to be developed.