Toughening mechanisms and damage propagation in Architected-Interfaces

We investigate fracture properties of architected interfaces and their ability to maintain structural integrity and provide stable damage propagation conditions beyond the failure load. We propose theoretical and numerical frameworks to evaluate the fracture properties of architected interfaces sandwiched between two (face) materials. The microscopic geometries of these interfaces are chosen as 2D cells--pillar, tetrahedron, and hexagon--as well as their 3D counterparts--namely, pillar array, octet truss, and Kelvin cell. Our model, both numerical and analytical, exhibits a high level of accuracy in predicting the compliance before failure and failure loads. Novel results are obtained during the damage propagation regime, indicating fulfilment of the so-called fail-safe design. Some of the cell geometries unfold during fracture, thus increasing the failure load and ensuring stable and controlled damage propagation conditions

On Micropolar Elastic Foundations

The modelling of heterogeneous and architected materials poses a significant challenge, demanding advanced homogenisation techniques. However, the complexity of this task can be considerably simplified through the application of micropolar elasticity. Conversely, elastic foundation theory is widely employed in fracture mechanics and the analysis of delamination propagation in composite materials. This study aims to amalgamate these two frameworks, enhancing the elastic foundation theory to accommodate materials exhibiting micropolar behaviour. Specifically, we present a novel theory of elastic foundation for micropolar materials, employing stress potentials formulation and a unique normalisation approach. Closed-form solutions are derived for stress and couple stress reactions inherent in such materials, along with the associated restoring stiffness. The validity of the proposed theory is established through verification using the double cantilever beam configuration. Concluding our study, we elucidate the benefits and limitations of the developed theory by quantifying the derived parameters for materials known to exhibit micropolar behaviour. This integration of micropolar elasticity into the elastic foundation theory not only enhances our understanding of material responses but also provides a versatile framework for the analysis of heterogeneous materials in various engineering applications

Testing mechanical performance of adhesively bonded composite joints in engineering applications: an overview

The development of new adhesives has allowed to expand the application of bonding into the most diverse industrial fields. This review article presents the commonly used experimental methods for the investigation of mechanical performance of adhesively bonded joints in the aerospace, wind energy, automotive and civil engineering sectors. In these sectors, due to their excellent intrinsic properties, composite materials are often used along with conventional materials such as steel, concrete and aluminium. In this context, and due to the limitations that the traditional joining techniques present, adhesive joints are an excellent alternative. However, standardized experimental procedures are not always applicable for testing representative adhesive joints in these industries. Lack of relevant regulations across the different fields is often overcome by the academia and companies’ own regulations and standards. Additional costs are thus mitigated to the industrial sectors in relation with the certification process which effectively can deprive even the biggest companies from promoting adhesive bonding. To ensure continuous growth of the adhesive bonding field the new international standards, focusing on actual adhesive joints’ performance rather than on specific application of adhesive joints are necessary.This work was supported by the European Cooperation in Science and Technology [CA 18120]

Fracture toughness of architected joints involving crack instabilities

Many complex structures such as airplanes are made of dissimilar materials, combining for instance Carbon Fibre Reinforced Polymer (CFRP) Composites and metal sheets. Manufacturing these multimaterial structures requires an efficient bonding process, and the joints must be characterized by a high fracture toughness [1]. However, adhesive joints often suffer from time and labour-intensive processes and low to moderate fracture toughness [2]. Joint with designed stop holes or meshes is one of the strategies already investigated to improve the joint fracture toughness [3]. An attractive option to improve the process efficiency is to perform bonding and composite curing simultaneously. As a first step on the road to the integrated bonding of hybrid metallic composite structures, potential toughening strategies relying on joint architecturing are investigated in this work while co-curing two composites parts. For this purpose, a tough prepatterned thermoplastic film is inserted between the composite parts. This results in a bondline made of the composite resin and the thermoplastic film that replaces conventional adhesives. The mode I fracture toughness of these joints is investigated through Double Cantilever Beam tests. A stick-slip behaviour involving three fracture toughness regimes is produced by the internal architecture associated to different crack paths and instability issues. The first regime corresponds to a moderate fracture toughness at the order of aeronautic adhesives. The second regime is characterized by an intermediate fracture toughness while the crack arrest mechanism of the third regime results in unexpected high values. The micromechanical origin of the high toughening potential is unravelled through fracture surface analysis and finite element models including the details of the joint architecture. Although, offering unexpected high fracture toughness, the remaining drawback of these joints are the crack instabilities taking place when going from a tougher to a weaker regime, resulting in large crack jumps

Architected adhesive joints with improved fracture toughness

Assembly through adhesive bonding of primary and load-carrying components often requires the use of stiff and reliable joining materials with relatively high strength. Such materials often are limited by a relatively low toughness and a high sensitivity to initial flaws and cracks. Thus, much of the focus in the structural adhesive community has been on improving the damage tolerance of bondline materials. This has been achieved through a number of strategies, including reformulating adhesives, as discussed for several adhesive families in Part I, and introducing additional phases or materials (e.g., toughening phases at the nano- and microscales that dissipate energy during failure) as discussed in Chapters 7 and 8. p0015 On the verge of the green transition, with the focus shifting toward optimized material usage, it is challenging and limiting to focus solely on adhesive chemistry and composition for improvement (e.g., because of the proprietary nature of formulations,long development times, inherent difficulty in realizing continued improvements, etc.). Hence, there is strong motivation to find extrinsic approaches based on structuring and geometry that can be used to toughen adhesive joints and applied to a broad range of materials, independent of chemistry and composition, while possibly also providing an opportunity for lightweighting. The focus of this chapter is on the use of architecture at the scale of tens to a hundred micrometers and above and is distinct from toughening strategies based on nanoscale particles and fibers [1,2]. Such architecture-based approaches have been used to realize bulk mechanical metamaterials (MM) with the goal to realize higher-performance materials, materials with unique combinations of properties, or unconventional properties (e.g., negative Poisson’s ratio). The time has come to advance such a paradigm owing to progress in manufacturing, notably digital manufacturing, in the context of adhesive bonding. Contrary to the common paradigm of material continuity and isotropy, some emerging architected materials introduce predetermined defects that allow for extra energy dissipation. These “defects,” if correctly designed, can have a beneficial effect by improving toughness without significant loss of stiffness. Note that one of the basic toughening principles is to increase plastic dissipation by designing the joint thickness to optimize the plastic zone size. This is the specific subject of Chapter 18, and the underlying principle of increasing plastic dissipation will be further exploited here through variations of this idea. In this chapter, we first review several principles that can be used to enhance toughness and then show several embodiments of these principles through architecting of joints

Ultra-tough architected adhesive joints for integrated composite processing and bonding

Deployment of advanced polymer-based composites in critical structures requires, among others, breakthroughs in adhesive bonding solutions. Indeed, available methods still suffer from limited fracture toughness of adhesives and from time-consuming bonding processes. Here, we demonstrate a novel concept of architected thermoplastic joints with exceptional fracture resistance up to 5000 J/m2, fully integrated with the composite resin transfer molding process, hence simultaneously targeting both limitations. This extreme toughness is activated through controlled 3D printed hollow pattern within a Nylon bondline. A synergetic combination of plastic dissipation, crack deflection, branching and arrest is tuned by changing the pattern characteristics. Three failure regimes are unraveled through fractographic analyses and finite element models. A stress-at-a-distance fracture criterion, identified for each constituent, quantitatively predicts the toughness variations along the crack path. This approach, amenable to dissimilar bonding between metals and composites, paves the road towards novel and higher performance structures and manufacturing approaches

Bondline thickness: Fracture mechanics perspective

The bondline thickness is a crucial parameter affecting the strength and toughness of adhesive joints. This parameter is responsible for the stress distribution, and thus, the efficiency of the stress transfer between the adherends, the failure modes, and the correspondent failure loads. However, the available literature reports different trends and relationships during testing between the thickness and strength, or thickness and toughness. In this chapter, the effect of adhesive thickness on the mechanical properties of adhesive joints is explored using the fracture mechanics framework. Two testing cases are discussed in detail: (i) soft bondlines joining rigid adherends, that is, butt joint and single end notch testing specimens, and (ii) rigid bondlines joining flexible adherends, that is, double cantilever beam geometry. These cases represent two extremes in terms of where the energy is stored while testing: in the first one, the energy is stored in the adhesive while in the second, the energy is stored in the adherends. From this discussion, the bondline thickness emerges as a critical geometrical parameter dictating transition between the two extreme cases. A single theoretical framework is provided that merges the two cases and is used to disclose the recently obtained experimental results.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Integrity & Composite

On the fracture behaviour of CFRP bonded joints under mode I loading: Effect of supporting carrier and interface contamination

This paper addresses the fracture behaviour of bonded composite plates featuring a kissing bond along the crack growth path. Double cantilever beam (DCB) experiments are carried out under a displacement controlled loading condition to acquire the load response. The experimental data are collected and analysed analytically for specimens with and without kissing bond. The following aspects are observed and discussed: effect of the adhesive carrier film, non-smooth crack growth and rising R curve. An analytical model taking into account the aforementioned effects is proposed. The kissing bond leads to unstable crack growth resulting in a loss of the load carrying capacity. The presence of the knit carrier in the adhesive film results in the crack growth process characteristic for the stick-slip phenomena and a significant increase of the resistance to fracture of the bondline by triggering a bridging phenomenon. The model shows a very good agreement with the experimental data. A sound understanding of the fracture process is gained enabling analysis and prediction of the effects of kissing bonds and supporting carrier.Structural Integrity & Composite

Composite joint toughening by multiscale architecturing through integrated manufacturing

The intensive use of Carbon Fibre Reinforced Polymer (CFRP) Composites in aerospace is motivated by their high strength to weight ratio. However, the manufacturing of complex aeronautic structures combining metallic and composite parts requires an efficient bonding process [1]. Joints obtained through bonding with adhesives suffer from low to moderate toughness [2] and the process requires multiple steps separating the manufacturing of the composite parts and the bonding. Several strategies have been proposed to enhance the joint toughness, such as introducing a mesh or cavities in the bondline [3]. One strategy to improve the efficiency of the process is to bond the metal part to the composite panels during the curing process, without using any adhesive. On the road to the integrated bonding of hybrid metallic composite structures, the present works investigates potential toughening strategies by bonding composites via co-curing of two composites parts while relying on joint architecturing principles