Delamination Buckling Research Articles

Post-buckling and delamination propagation behavior of composite laminates with embedded delamination

Delamination occurred due to poor manufacturing process or in-service actions significantly affects the mechanical and failure behavior of laminated composite structures. In this study, the buckling and post-buckling delamination behavior of laminated composite with an embedded initial delamination under in-plane compression was studied experimentally and numerically. First, compression tests for laminated composite specimens with embeded initial delamination were performed and the buckling and delamination responses were obtained. Then the experimental test was numerically simulated using finite element methods with the progressive failure accounted for by using cohesive zone modeling. The load-displacement curve, strain behavior and delamination shapes of experimental specimens obtained from load cells, strain gages installed at different locations, and C scan images, respectively, were compared with the FEM results, and good agreements were attained. The effect of the buckling modes, laminate stacking sequence and shape of initial delamination on the buckling load and propagation behavior was studied by considering different ply stacking and shapes of initial delaminations. It was found that the buckling mode determined the growth direction of the delamination propagation, and the stacking sequence influenced the extent of the propagation area, while the orientation of the delamination affected the buckling loads.

Radial buckle delamination around 2D material tents

Microscale tents, formed when transferring two-dimensional (2D) materials over nanoparticles or nanopillars, or when indenting a suspended 2D material drumhead with an atomic force microscope (AFM) tip, emerge to be a useful structure for the strain engineering of 2D materials. In the periphery of the tents, where the 2D materials are supported by the substrate, radial buckle delamination can often be observed, yet the formation mechanism and the profile characteristics remain unclear. Here, we suggest that the tent-induced buckles result from the 2D material-substrate interface sliding radially inward, and their profiles and extent are controlled by the interface adhesion and friction. We experimentally characterized that the crest curvature of the buckles is proportional to a characteristic length that compares the elastic bending energy of the 2D material with its adhesion energy to the substrate. We then obtain theoretical predictions for the extent of those buckles by exact closed-form solutions to Föppl–von Kármán (FvK) equations under both near-threshold and far-from-threshold conditions. Our results are highly analytical, provide a direct means to estimate the interfacial shear and adhesive properties of the 2D material-substrate system based on simple topological characterizations of buckles. Our theoretical understandings also establish a fundamental base for the rational design of 2D material tents.

Influence of interface steps on the buckle delamination of thin films

The mechanics of thin film buckle delamination has been widely studied, with the assumption of perfect (flat) interfaces. This work is reporting a study of the evolution of an initially straight-sided buckle propagating in a thin film lying on a substrate in the presence of a step at the film/substrate interface, resulting from the plastic deformation of the substrate. Assuming the interface toughness depends on the mode mixity, it has been found using finite elements calculations that depending on the step orientation and height, the initially straight propagation of the buckle can be perturbed so that the buckle eventually propagates along the step. The critical parameters (orientation and height) of the step have been then determined, which characterizes the transition from a straight propagation perpendicular to the compression axis to a propagation along the inclined step. A “behavior” diagram has been finally presented for the buckle, where the propagation, either perpendicularly to the step or along it have been displayed versus the step height and orientation.

Delamination Buckling and Crack Propagation Simulations in Fiber-Metal Laminates Using xFEM and Cohesive Elements

Simulation of fracture in fiber-reinforced plastics (FRP) and hybrid composites is a challenging task. This paper investigates the potential of combining the extended finite element method (xFEM) and cohesive zone method (CZM), available through LS-DYNA commercial finite element software, for effectively modeling delamination buckling and crack propagation in fiber metal laminates (FML). The investigation includes modeling the response of the standard double cantilever beam test specimen, and delamination-buckling of a 3D-FML under axial impact loading. It is shown that the adopted approach could effectively simulate the complex state of crack propagation in such materials, which involves crack propagation within the adhesive layer along the interface, and its diversion from one interface to the other. The corroboration of the numerical predictions and actual experimental observations is also demonstrated. In addition, the limitations of these numerical methodologies are discussed.

Templated solid-state dewetting of single-crystal iron films and specifically localized buckle delamination of surface oxide

Morphological evolution during the dewetting of single-crystal Fe(100) patches is shown to strongly depend on the patch alignment and annealing ambient owing to the anisotropies of the edge retraction rate and surface energy of iron films. It is also observed that iron films dewet in the presence of continuous iron oxide layers when the hydrogen flow rate is sufficiently low. The oxide layers in the downhill area of thickening rims are shown to be buckle-delaminated, resulting in the formation of oxide membrane patterns on top of the dewetted structures.

Adhesion energy and related plastic deformation mechanism of Cu/Ru nanostructured multilayer film

Sputter-deposited Cu/Ru nanolaminate composites with individual layer thickness as small as 1.5 nm were tested by nanoindentation probing to initiate and drive delamination. Focused ion beam (FIB) observations show the layer buckling and interface delamination with lateral crack along the interface between the multilayer and substrate. The buckling behaviors, dependent on the critical length scale, are rationalized in the light of the repeating structural unit in the coherent multilayer. In addition, due to the presence of the interfaces and constraints between the hetero-layers, the condition for plastic dissipation in multilayers shifts significantly from those of single layer films, therefore a modified energy-dissipative model has been employed to obtain the quantitative adhesion energy for Cu/Ru multilayer, which agrees well with previous reports on the Cu-based films adherent on rigid substrates. The present result provides meaningful adhesion energy estimates and helps to understand the underlying deformation mechanism in metallic multilayers.

Modelling Cracked Cross-Ply Laminates with Delamination Buckling

The mechanical behavior of cross-ply laminates loaded under in-plane compression containing matrix cracks and delaminations is investigated in order to study their influence on the structural stability behavior. This is done by employing a semi-analytical modelling approach which comprises an analytical framework for a structural stability analysis of damageable structures and the Equivalent Constrained Model for deriving reduced stiffness properties of the cracked layers. Cross-ply laminates with varying delamination depths as well as varying matrix crack densities are studied.

Fracture mechanics analysis of delamination buckling of a porous ceramic foam coating from elastic substrates

Ceramic foams are ideal materials for thermal protection systems such as those used as a thermal shield on the space shuttle. The working temperature difference between the outer and inner surfaces of the ceramic foam is extremely high. Under this adverse condition, the ceramic foam coating could buckle from its underneath structures. Therefore, the general problem of damage due to buckling delamination of a ceramic coating on an elastic substrate is investigated in this paper. The delamination buckling amplitude and the stress intensity factor at the tip of buckling region are derived in closed form in terms of the porosity of the ceramic foam. Based on the maximum amplitude and the critical stress intensity factor criteria, critical temperatures of coating buckling from the substrate are established. A fitting formula of maximum buckling amplitude as a function of outer surface temperature is given. It is found that higher porosity and length-to-thickness ratio of the coating will result in a smaller stress intensity factor. However, it seems that the buckling amplitude is independent of the porosity of ceramic foams.

Compressive failure analysis for low length-width ratio composite laminates with embedded delamination

Abstract For low length-width ratio composite laminates with embedded delamination under compressive loading, different failure modes occur and interact tightly, which catastrophically decrease the load carrying capacity of the laminates. In order to systematically investigate the compressive behaviour of these laminates, a thorough progressive failure model taking into account the interaction among intralaminar failure, interlaminar failure and buckling failure was established. In this model, the intralaminar failure was evaluated with the progressive damage method (PDM), and the interlaminar delamination growth was simulated with the virtual crack closure technique (VCCT). Compression tests of low length-width ratio composite laminates with different embedded circular delaminations were conducted. Good agreements between numerical predictions and experimental results, including failure loads, load-central deflection curves and shapes of delamination growth, were obtained. It follows that under compressive loading, low length-width ratio laminates with small embedded delamination fail due to the buckling and intralaminar failure, while more severe laminate failure is induced by the buckling, intralaminar failure and additional delamination growth for low length-width ratio laminates with large embedded delamination.

Compression and shear response of carbon fiber composite sandwich panels with pyramidal truss cores after thermal exposure

ABSTRACTThe composite pyramidal truss core sandwich panels with reinforced joints are manufactured by the water jet cutting and interlocking assembly method. The effect of thermal exposure on the out-of-plane compression and shear performances of composite sandwich panels is investigated experimentally. In particular, the out-of-plane compression and shear moduli and strengths of the structures after thermal exposure are predicted theoretically. Experimental results show that the out-of-plane compression and shear performances of composite pyramidal truss core sandwich panels first increase and then decrease with thermal exposure temperature at the thermal exposure of 6 h. The failure modes depend on the particular temperature and time of the applied thermal exposure. As well, the compression and shear performances first increase and then decease with thermal exposure time at 150°C. However, their compression and shear performances have a decreasing tendency with time for a given 230°C exposure temperature. In addition, the destruction responses of composite sandwich panels are also investigated and their possible failure modes (including the crushing, delamination and local buckling of struts) are complemented with the analytic results.

Buckling analysis of variable angle tow composite plates with a through-the-width or an embedded rectangular delamination

Variable angle tow (VAT) composite laminates, in which fibre orientation varies spatially in-plane in a continuous fashion yet is piecewise constant through-thickness, have been made possible by advanced automated fibre placement technology. Such designs have shown considerable potential to improve the performance of lightweight composite structures. In the present study, an analytical model is developed to study the buckling behaviour of VAT composite plates with a through-the-width or an embedded rectangular delamination under compression loadings. The proposed model can accurately capture the global, local and mixed buckling response of delaminated VAT composite plates. Both free and constrained modes are assumed in the delamination buckling analysis. A constrained point approach is employed to analyse the buckling response when contact occurs between delaminated layers. The accuracy and reliability of this proposed delamination buckling model is validated by finite element analysis and with prior results. The influence of delamination size, position and varying fibre orientation angles on the buckling response of delaminated VAT composite plates is studied by numerical examples. It is shown that the buckling loads decrease with an increase of the delamination size. The VAT laminates with an off-midplane delamination may lead to the delamination opening up, which further reduces the buckling loads. Finally, the mechanism of taking advantages of VAT laminates to improve the buckling performance of delaminated composite plates is thoroughly investigated in a parametric study. This study also shows that the residual buckling resistance of the delaminated composite plates can be significantly improved through using the VAT design concept.

Multi-failure analysis of composite Isogrid stiffened cylinders

Adopting filament winding and co-curing technique, carbon fiber reinforced composite (CFRC) Isogrid stiffened cylinders (ISCs) were designed and manufactured. Revealed by experiments, CFRC ISC has multi-failure modes, including material failure, global instability, local buckling, rib crippling and end delamination. Failure criteria for the first four failure modes were proposed and applied to predict the failure load of specific ISC. Skin thickness, cell dimension, rib height, rib thickness and end strengthening scheme jointly decide the failure pattern and failure load of the ISC. Failure maps were deduced to figure out the optimizing route for lightweight design of ISC. Wrapped ends were suggested to restrict the end delamination failure mode which results in rather low and uncertain load carrying capacity.

Experimental and numerical investigation of buckling and delamination of Ti/TiN coatings on depleted uranium under compression

The cracking evolution and failure mechanisms of Ti/TiN coatings on depleted uranium (DU) deposited by high power impulse magnetron sputtering were investigated by combing a compression-to-fail test and a numerical fracture modeling technique. The coating buckling and interface delamination were induced by unaxial compression tests, meanwhile the cracking pattern and the buckling characteristics were tracked and measured by an optical microscope (OM). A three dimensional (3D) cohesive fracture model considering both detailed coating cracking and interface cracking was developed to analyse the coating failure mechanism and to evaluate the interfacial adhesive properties. Our simulation results agree well with experiments. It is found that the coupled interacting of ridge crack and a pair of fringe cracks in coatings lead to a spontaneous self-replication propagation for the buckle; mode I fracture played a dominant role in the initiation and propagation of the ridge crack and fringe cracks; the top, the tails and the remained part of the curved interface crack front experienced pure mode I, pure mode II and mixed mode fracture, respectively. The interfacial adhesive strength of Ti/TiN coatings on DU was in the range of 80–100Mpa, and the critical interfacial adhesive energy was in the range of 2–3J/m2.

Failure of flight feathers under uniaxial compression

Flight feathers are light weight engineering structures. They have a central shaft divided in two parts: the calamus and the rachis. The rachis is a thinly walled conical shell filled with foam, while the calamus is a hollow tube-like structure. Due to the fact that bending loads are produced during birds' flight, the resistance to bending of feathers has been reported in different studies. However, the analysis of bent feathers has shown that compression could induce failure by buckling. Here, we have studied the compression of feathers in order to assess the failure mechanisms involved. Axial compression tests were carried out on the rachis and the calamus of dove and pelican feathers. The failure mechanisms and folding structures that resulted from the compression tests were observed from images obtained by scanning electron microscopy (SEM). The rachis and calamus fail due to structural instability. In the case of the calamus, this instability leads to a progressive folding process. In contrast, the rachis undergoes a typical Euler column-type buckling failure. The study of failed specimens showed that delamination buckling, cell collapse and cell densification are the primary failure mechanisms of the rachis structure. The role of the foam is also discussed with regard to the mechanical response of the samples and the energy dissipated during the compression tests. Critical stress values were calculated using delamination buckling models and were found to be in very good agreement with the experimental values measured. Failure analysis and mechanical testing have confirmed that flight feathers are complex thin walled structures with mechanical adaptations that allow them to fulfil their functions.

Growth from buckling to buckling-driven delamination in a film/substrate system

Compressing a stiff film bonded to a compliant substrate with finite thickness can lead to various instabilities, including global buckling, local wrinkling, delamination or their concomitant buckling. This paper proposed an analytical model, which integrates global and local interactive effects due to the finite thickness, to reveal the growth from buckling to buckling-driven delamination. The resulting governing non-linear equations (non-autonomous fourth-order ordinary differential nonlinear equations with integral conditions) are then solved by introducing a continuation algorithm, which offers considerable advantages to detect multiple bifurcations and trace a complex post-buckling path. The critical conditions for global buckling, local wrinkling and buckling-driven delamination are carefully studied. Two different growth processes from destabilization to restabilization (snap-back) are captured in the post-buckling range. Moreover, it is found that the interface toughness and the pre-existing delamination crack length dominates the critical strain for the onset of buckling-driven delamination, and further decide the initial instability mode. Finally, two phase diagrams are plotted to predict both initial and advanced instability modes in such a bilayer system. The phase diagrams can be used to guide the design of film/substrate systems to achieve desired modes of instabilities.

How soft substrates affect the buckling delamination of thin films through crack front sink-in

As the development of stretchable devices advances, engineering components with rigid films on soft substrates are becoming more numerous. We propose to analyse the buckle delamination of a film on a soft substrate, under a biaxial compressive stress state. This problem has already been investigated by an Euler column buckling analysis. In this paper, experiments on soft substrates are presented, which demonstrate that the buckle shape is, in some cases, better approximated by a “Mexican hat” shape. A model using a non-linear plate bonded to an elastic medium by a cohesive interaction is used to describe the delamination process. It is demonstrated that the “Mexican hat” shape modifies the crack propagation behavior for a soft substrate.

Delamination-Based Measurement and Prediction of the Adhesion Energy of Thin Film/Substrate Interfaces

With the rapid emerging of two-dimensional (2D) micro/nanomaterials and their applications in flexible electronics and microfabrication, adhesion between thin film and varying substrates is of great significance for fabrication and performance of micro devices and for the understanding of the buckle delamination mechanics. However, the adhesion energy remains to be difficult to be measured, especially for compliant substrates. We propose a simple methodology to deduce the adhesion energy between a thin film and soft substrate based on the successive or simultaneous emergence of wrinkles and delamination. The new metrology does not explicitly require the knowledge of the Young's modulus, Poisson's ratio, and thickness of the 2D material, the accurate measurement of which could be a challenge in many cases. Therefore, the uncertainty of the results of the current method is notably reduced. Besides, for cases where the delamination width is close to the critical wrinkle wavelength of the thin film/substrate system, the procedure can be further simplified. The simple and experimentally easy methodology developed here is promising for determining/estimating the interface adhesion energy of a variety of thin film/soft substrate systems.

Experimental and numerical characterization of delamination buckling behavior of a new class of GNP-reinforced 3D fiber-metal laminates

The delamination buckling behavior of graphene Nano platelets (GNP) reinforced 3D fiber-metal laminates (3DFMLs) is investigated experimentally and numerically. In this study, the resin used to bond the metallic layers to the 3D fiberglass fabric (3DFGF) is reinforced with GNP, with the aim of improving the delamination resistance at the interface, thus enhancing the overall stability response of the 3DFMLs. For that, four different weight-percentages of GNP are used to establish the effect of GNP content on the stability (buckling) response of the 3DFMLs. In total, four groups of specimens with four different delamination lengths are used to investigate the effects of GNP on the buckling capacity of the 3DFMLs. In addition to the experimental investigation, a nonlinear finite element model is developed, using the commercial finite element code ABAQUS, to simulate the delamination buckling response of the 3DFMLs. The numerically obtained critical buckling capacities and failure modes are compared to the experimental results. Good agreements between the finite element (FE) and experimental results are obtained.

Visible-range hollow waveguides by guided buckling of Ta2O5/SiO2 multilayers.

Hollow waveguides operating near 550nm wavelength were fabricated by guided formation of delamination buckles within Ta2O5/SiO2 multilayers. The fabrication process employed a pair of sequentially deposited 10-period Bragg mirrors separated by a patterned, low-adhesion fluorocarbon layer. Propagation loss as low as a few dB/cm was measured, consistent with theoretical predictions.

Bistability in buckled dome microcavities.

We describe optical bistability in monolithically integrated, curved-mirror Fabry-Perot microcavities. The cavities were fabricated by controlled formation of circular delamination buckles within sputtered Si/SiO(2) multilayers. The dominant source of the bistability is heating due to residual absorption in the mirror layers, which leads to out-of-plane deflection of the buckled mirror. Hysteresis occurs for submilliwatt input powers.