Stiff Interlayers Research Articles

Crack deflection in laminates with graded stiffness—lessons from biology

A crack propagating through a laminate can cause severe structural failure, which may be avoided by deflecting or arresting the crack before it deepens. Inspired by the biology of the scorpion exoskeleton, this study shows how crack deflection can be achieved by gradually varying the stiffness and thickness of the laminate layers. A new generalized multi-layer, multi-material analytical model is proposed, using linear elastic fracture mechanics. The condition for deflection is modeled by comparing the applied stress causing a cohesive failure, resulting in crack propagation, to that causing an adhesive failure, resulting in delamination between layers. We show that a crack propagating in a direction of progressively decreasing elastic moduli is likely to deflect sooner than when the moduli are uniform or increasing. The model is applied to the scorpion cuticle, the laminated structure of which is composed of layers of helical units (Bouligands) with inward decreasing moduli and thickness, interleaved with stiff unidirectional fibrous layers (interlayers). The decreasing moduli act to deflect cracks, whereas the stiff interlayers serve as crack arrestors, making the cuticle less vulnerable to external defects induced by its exposure to harsh living conditions. These concepts may be applied in the design of synthetic laminated structures to improve their damage tolerance and resilience.

Cold Bent Laminated Glass Application – Numerical Research

AbstractIn curvilinear and free‐form glass facade constructions, more and more cold bent laminated glass is applied. The high optical quality of cold bent glass allows for continuously smooth curved and high transparent facades. Moreover, a high performance and economical design is achieved using spatial load‐bearing behaviour. The bending in such curved glass facades is obtained by mechanical fixings upon a curved substructure at the installation on site or by lamination bending with stiff interlayers. These techniques shape the glazing but induce permanent bending stresses as well as shear stresses in the laminated glass, which significantly depend on the interlayer's material behaviour as a function of time and temperature. For the application of cold bent laminated glass in facades numerous factors must be considered ‐ achievable bending radii, glazing dimensions and geometry, load‐bearing capacities, bending forces as well as required supports. These aspects are significantly influenced by the material stiffness of the interlayer type and the glass structure. For this reason, the curved bending process with mechanical line fixings was numerically investigated. The focus lays on the influence of the stiffness of the polymer interlayer and the corresponding suitability for uniaxial cold bent laminated glass applications in different geometries. In the present paper, technical design information is introduced based on numerical simulations to assist designers in understanding the principles of uniaxial cold bent laminated glass with different glazing parameters. In conclusion, constructive recommendations for application in curvilinear glass facades are given.

Horizontal mechanical responses of single pile due to urban tunnelling in multi-layered soils

This paper presents a theoretical method for the horizontal mechanical responses of single pile due to tunnelling in multi-layered soils. Based on the homogenization theory and the integral L&P formula, ground horizontal displacements due to tunnelling in layered soils can be solved analytically. The pile-soil interaction model is established in which the pile is assumed to be an Euler-Bernoulli beam placed on the Pasternak foundation. According to the two-stage thought, the horizontal displacements and bending moments of the pile can be evaluated quantitatively if the calculated horizontal displacements are input into the interaction model. The proposed method is verified by comparing with the numerical method, field measurement and centrifugal test. Parametric analyses are also performed to investigate the influence of the parameters of the pile and excavation, especially the soil stratification on the mechanical responses of the pile. Results found that the GAP parameter, the distance of tunnel axis from pile axis, the length, the diameter and the elastic modulus of the pile all have an important influence on the horizontal displacements and bending moments of the pile. Importantly, the soil stratification has a non-negligible influence on the mechanical responses of the pile. The stiff interlayer sandwiched between the upper and lower soil layer can significantly limit the horizontal displacements of the pile but increase the bending moments.

Fabrication of 3D metal-ceramic (Al-AlN) architectures using laser-powder bed fusion process

Abstract The potential of metal additive manufacturing for producing high conductivity materials and hybrid systems for thermal management in opto-, power and microelectronics has been investigated. Using the laser-based powder-bed fusion technology, the joining of ceramics (aluminum nitride) and metals (aluminum alloy: AlSi10Mg) has been studied with a focus on the fusion zone and the interlayer. Metallization of the ceramic surface with aluminum was applied to realize a stable process for forming metal-ceramic multilayer architectures. A sputtering process proved to be able to form a stiff interlayer and prevent direct contact of the laser beam with aluminum nitride. The bonding characteristics of aluminum nitride/aluminum alloy hybrid have been assessed, using scanning electron microscopy and energy dispersive X-ray spectroscopy. Owing to residual stress evolution during laser-powder bed fusion, process-induced material damage such as cracking at the binding zone was investigated and compared to residual stress simulations by which a correlation between process parameters, part geometry and the material failure could be established. Thus, the fabrication of crack-free metal-ceramics by a stable laser-powder bed fusion process was achieved.

Examination of Laminated Glass with Stiff Interlayers – Numerical and Experimental Research

Glass is a linear-elastic and isotropic building material which allows for a quick calculation of internal forces and stress in the design stage. However, in safety applications, e.g. overhead glazing or balustrades, it is made into laminated glass using plastic interlayers. Their material properties change over time and are temperature-dependent. Additionally, larger spans and the demand to reduce the amount of material used to a minimum result in significant slenderness and a considerable impact of geometric non-linearity. Consequently, the manufacturing of laminated glass products results in the requirement of complex design strategies to generate top performance. The paper introduces current design methods for laminated glass focusing on the potential use of shear action and covering load assumptions, including the effects of time and temperature. It includes a study on potential material models for three different interlayers. This led to a refined numerical model for load combinations considering shear action. For validation, vertical laminated glass panels (linearly supported on four sides) were loaded. The 36 specimens comprised two sizes: one measured 800 mm by 1200 mm and the other 1450 mm by 2800 mm and layers of glass with a thickness ranging from 3 mm to 4 mm. A planar load was introduced stepwise according to a load-time correlation model. Time and deflection correlate as a function of the interlayer. The idea was to quantify the performance of standard-PVB, stiff PVB and ionoplast interlayers as well as a monolithic glass pane as reference. The results show good agreement in comparison with the refined numerical calculations. Therefore, the load-bearing behaviour of laminated glass can be realistically modelled and allows for an economic glass design. Additionally, the numerical model was applied in an extended parametric study on the possible reduction of self-weight by using a combination of thin glass panes and stiff interlayers in insulated glass units. A resulting “butterfly chart” shows the potential of self-weight reduction as a broader summary.

Laminated Glass with Stiff Interlayers – Glass Canopies with Spans up to 1.2 Meters

ABSTRACTStandardised systems for glass canopies, such as terrace roofs or conservatories, mostly consist of laminated glass with standard‐PVB interlayers and span up to 0.8 meters. The industry's wish, however, is to enlarge the span and to increase the transparency as well as brightness in glass canopies by using slender glass structures. To enlarge the spans, the application of stiff interlayers in laminated glass is discussed. Together with Kuraray Europe GmbH, the Technische Universität Dresden evaluated the load bearing capacities of laminated glass canopies with different interlayers and spans up to 1.2 meters in a numerical study. Loads and the resulting load combinations, for the location Germany, were analysed to enable a proper dimensioning of glazings in various spans and glass structures. The aim of the study is to highlight the performance of stiff interlayers in applications with increased span and compare the load bearing capacities to those of standard‐PVB. In this manner, a focus on laminated glass design under different load combinations is set.

Enhanced monotonic and cyclic mechanical properties of ultrafine-grained laminated metal composites with strong and stiff interlayers

The accumulative roll bonding process allows to produce multilayered composites in which substantially dissimilar metals, both in terms of Young’s modulus as well as mechanical strength, can be brought together. For the optimization of such laminated metal composites a fundamental understanding of the microstructural interactions and deformation processes at the internal material interfaces is necessary. The influence of a gradient in hardness and Young’s modulus in ultrafine-grained laminated metal composites on the monotonic and cyclic mechanical properties is investigated in this paper. Special attention is put on aluminum/steel composites, as a high gradient in hardness and Young’s modulus is present at the layer interfaces. The mechanical properties are determined in monotonic and cyclic three-point bending tests. By scanning electron microscopy, the influence of the meso- and microstructure on the fatigue properties is intensively studied. Furthermore, the internal stresses during elastic straining are calculated using finite element simulations. The results are that for the aluminum/steel composites the monotonic and cyclic properties are drastically increased compared to aluminum mono-material sheets, as well as to composites based on two different aluminum alloys, in which only a gradient in hardness at the material interface exists and the gradient in elastic properties is absent. This is related to a pronounced crack deviation at the material interface as well as to an effective load transfer in the composites.

Influence of Temperature on Post-Breakage Behaviour of Laminated Glass Beams : Experimental Approach

The assessment of the post-breakage performances of laminated glass elements used in construction need to take into account the sensitivity to the temperature of the mechanical behaviour and properties of the product, in particular of the interlayer material. A general problem statement and an overview of different experimental approaches are firstly presented. Then results of specific orientation tests on pre-cracked laminated glass beams with a stiff interlayer of DuPont carried at three different temperatures (23, 45 and 60°C) are presented and commented. A comparison of the mechanical behaviour at the different temperatures is done, aiming to give a comprehensive order of magnitude of the sensitivity to temperature of the post-breakage behaviour observed during the tests.

Deformation mechanisms of TiN multilayer coatings alternated by ductile or stiff interlayers

TiN multilayers that alternate with either titanium (ductile) or nanocomposite TiSiN (hard) interlayers were surface coated by filtered arc deposition onto stainless steel substrates. Hardness and deformation mechanisms of these multilayer coatings were investigated using depth-sensing indentation in comparison with traditional monolithic TiN coatings. A dual ion/electron beam microscope was used to analyse subsurface indentation damage. It was found that microstructural layering and the presence of interlayers in TiN multilayers jointly provided resistance to deformation by intercolumnar shear sliding, observed more evidently in the monolithic TiN coatings. This resulted in an increase in hardness, which also increased with both the number of layers and the presence of interlayers. Calculations based upon a mechanistic-based model revealed that the resistance by interlayers played a more important role than an increase in intergranular shear area due to the layered structure in resisting deformation. Compared with the titanium interlayer, the use of a hard nanocomposite interlayer increased the resistance to deformation; however, cracking occurred within both the TiN layers and the nanocomposite interlayers.