Failure Mechanism Maps Research Articles

Fabrication and failure mechanisms of all-composite honeycomb sandwich cylinder under the axial compression

Winding-based method, an innovative technology, is proposed to successfully fabricate the all-composite honeycomb sandwich cylinder (CHSC). Excellent mechanical properties of CHSC can be achieved owning to continuous prepreg winding. The critical force of intracell buckling for exact hexagonal boundaries is proposed based on the energy principle, which has small difference between the simulation and experimental results. Three-dimensional failure maps are derived to reveal failure mechanisms of fabricated CHSC. Multi-failure mechanisms of CHSC under axial compression are explored by failure mechanism maps, which can more intuitively express the influence of dimensionless geometric parameters on failure modes. Failure modes of intracell buckling and face crushing were obtained by typical experiments under axial compression. The honeycomb sandwich cylinder prepared in this paper shows excellent bearing capacity compared to other types of sandwich cylinders. Winding-based method is expected to provide an important support for the fabrication of curved multifunctional honeycomb.

Failure of carbon fiber composite sandwich cylinders with a lattice core under axial compressive loading

Sandwich cylinders with a low-density pyramid-like truss core made of plain-woven carbon fiber fabrics are manufactured using a sequential hot press method. Then, the structural compressive strengths corresponding to four possible failure modes (Euler buckling, axisymmetric buckling, local buckling and face panel crushing of the sandwich cylinders) are obtained by theoretical analysis. The 3D failure mechanism map is accordingly generated to guide the design and preparation of the cylinder specimens. Thereafter, axial compressive tests are performed on the specimens to investigate the failure mechanisms of these structures and to validate the theoretical predictions. The errors between theoretical and experimental results, including both the axial compressive ultimate loads and failure modes, are acceptable. The developed 3D failure mechanism map provides an intuitive guide for the design of sandwich cylinders with a pyramid-like lattice truss core against buckling failure.

Three-point bending behaviors of the foam-filled CFRP X-core sandwich panel: Experimental investigation and analytical modelling

To enhance the energy absorption capacity of the sandwich structures, the foam filled composite X-core sandwich panels were fabricated by a hot-press molding technology. The bending behaviors of the empty and hybrid composite sandwich panels were investigated by the three-point bending tests. Analytical models and finite element models were developed to characterize the bending behaviors and the interaction effect between the foams and composite members. Generally, the specific bending modulus and strength of composite X-core sandwich structures are comparable to that of the competing cellular structures. The theoretical expressions can provide useful predictions of the bending stiffness and initial failure load, whilst the numerical models had a higher accuracy. A failure mechanism map was constructed to highlight the effect of geometry parameters on the failure modes. A minimum weight design was employed to provide a guideline for obtaining the weight-efficient hybrid composite sandwich panels.

Failure analysis of brazed sandwich structures with square honeycomb-corrugation hybrid cores under three-point bending

In this work, the bending stiffness, initial failure load, initial failure modes and minimum mass design of square honeycomb-corrugation hybrid core sandwich structures (SHCH) subjected to bending were explored by a combined theoretical analysis, experimental testing, and numerical prediction method. Samples were fabricated by brazing process and measured under three-point bending. To theoretically obtain the initial failure mechanism map, six different initial failure modes were taken into account, which agree well with associated experimental data and numerical results. It is found that honeycomb filler not only changed the failure mode of corrugated core sandwiches but also enhanced dramatically its bending resistance. Then the minimum weight design is achieved as a function of load index, and the effect of key factors, including honeycomb relative density, loading platen width, material parameters, and inclination angle are quantified. Furthermore, the mechanical performances of SHCHs are compared with the competing structures.

Effect of accelerated curing and layer deformations on structural failure during extrusion-based 3D printing

Recent experimental research by Reiter et al. (Cem. Concr. Res., 132:106047, 2020) indicates that the buildability of fresh concrete used in extrusion-based 3D printing processes can be significantly enhanced by chemically accelerating the curing process. In the present contribution the effect of accelerated curing on failure by plastic collapse and elastic buckling during 3D concrete printing is explored by incorporating a power-law curing function in the parametric 3D printing model developed by Suiker (Int. J. Mech Sci, 137:145–170, 2018). A structural yield criterion is derived for the case of accelerated curing, and the main advantages on the resistance against plastic collapse are demonstrated through a comparison of the predicted failure characteristics to those for linear curing and exponentially-decaying curing. Subsequently, the elastic buckling behaviour under accelerated curing is derived for a free wall configuration, and the competition between elastic buckling and plastic collapse of the free wall structure is assessed via the construction of failure mechanism maps. In addition, a modelling recipe is proposed for consistently accounting for the vertical deformations of layers in the prediction of structural failure during 3D concrete printing. The modelling of this effect may further increase the accuracy of the prediction of the number of layers at structural failure. For failure under plastic collapse, results are computed for linear curing, exponentially-decaying curing and accelerated curing. The model outcome for linear curing is used for a comparison with results from 3D concrete printing experiments recently presented in the literature, showing an excellent agreement. It is further demonstrated that the effect of vertical wall deformations on the prediction of failure by elastic buckling typically is minor, so that for this failure mechanism this contribution may be left out of consideration. All design graphs presented in this communication are generic, in a sense that they are not restricted to concrete, but can be applied for other printing materials as well.

Three-point bending of physically asymmetric metal sandwich beams with aluminum foam core: Failure behavior and optimal design

The failure behavior of physically asymmetric metal foam core sandwich beams under three-point bending is studied experimentally and analytically. Five failure modes including indentation, face yield, core shear (Type A), core shear (Type AB) and core shear (Type A-AB) were observed in the three-point bending tests. Analytical formulae for the critical loads of different failure modes are presented, and failure mechanism maps are constructed based on the formulae. There is a reasonable agreement between the analytical predictions and the experimental results. The effects of face material strain hardening and exchanging two faces, on the failure behavior of physically asymmetric metal sandwich beams are studied. Optimal designs are performed for the load-carrying capacity of physically asymmetric sandwich beams under three-point bending. Based on the optimization results, a method to compare the performances of physically asymmetric sandwich beams with various material combinations is proposed.

Composite honeycomb sandwich columns under in-plane compression: Optimal geometrical design and three-dimensional failure mechanism maps

In this paper, three-dimensional failure mechanism maps are developed to investigate the in-plane compressive characteristics of all-composite honeycomb sandwich columns and optimize their load-weight efficiency. Analytical models are derived based on five possible failure modes, including shear macro-buckling, intracellular dimpling, face wrinkling, face fracture and debonding. The dominant failure mode can be determined by three dimensionless geometrical parameters, i.e., the length-height ratio of the sandwich column, the height ratio of face and core, and the relative density of honeycomb core. A series of three-dimensional failure mechanism maps are constructed based on the fiber orientations in the face sheet and honeycomb core. It is observed that failure mode transforms with dimensionless geometrical parameters in a spatial region. A typical three-dimensional failure mechanism map is experimentally validated. Sectional views in different directions are used to locate the boundaries of failure modes. All-composite honeycomb sandwich columns are designed and fabricated using the tailor-folding method. In-plane compressive experiments are carried out to verify the analytical models and three-dimensional failure mechanism map. The experimental results agree well with analytical predictions. Optimal geometrical design is performed to obtain the geometrical relations of honeycomb sandwich columns with the highest load-weight efficiency under in-plane compression. The novelty of this work lies in broadening the failure mechanism map to spatial region and optimal geometrical design.

Fabrication and mechanical behaviors of quartz fiber composite honeycomb with extremely low permittivity

The mechanical properties and electromagnetic parameter of quartz fiber composite honeycombs (QFCH) with different equivalent densities were studied. Equivalent permittivity of the QFCH was inversed according to the transmission efficiency, which was tested by the Naval Research Laboratory Arch (NRL-arc) method. The strengths of QFCH under out-of-plane compressive and in-plane shear loading were predicted and measured. The failure mechanism map was established to predict the main failure modes of QFCH. Meanwhile, the numerical simulation was used to verify the accuracy of failure mechanism map. The QFCH designed in this study has extremely low permittivity and excellent mechanical properties, which has immense potential to be used as a 3-D metamaterial mechanical framework and more competitive in the vigorous development of new generation radomes.

Theoretical prediction model and failure mechanism map of human skull porous structures with quasi-static bending performance

A theoretical model has been developed to predict the bending properties and failure mechanism of the human skull. The structure of the skull bone, which is naturally a porous material, was obtained via non-destructive 3D micro-computed tomography. By considering the combined influence of curvature and porosity of the skull, the mid-span deflection of the skull bone loaded in three-point bending was calculated using theoretical prediction. The cranial bending peak load corresponding to several possible failure modes was also analyzed, and the relevant failure mechanism map was generated. The models also showed that the curvature is a critical consideration for accurate precisions to be achieved. To verify the reliability of the theoretical results, as well as to gain a good understanding of the intrinsic connections between internal geometric features and mechanical properties of the cranium, 3D finite element (FE) models, based on posteriorly reconstructed by 2D cranial scanning images, were developed. The bending properties of various reconstructed sections were studied in detail using the FE simulation models, leading to an understanding of the bending failure mechanism and crack propagation path. The numerical results of deflection and peak loads agree well with the theoretical results. In addition, the failure modes of the numerical models have been verified using published experimental results. This work has overall demonstrated that the theoretical and numerical models would provide a reliable basis for predicting cranial bending performance.

Failure Modes of a Laminated Composite With Complaint Interlayers

Abstract Composites comprising a high-volume fraction of stiff reinforcements within a compliant matrix are commonly found in natural materials. The disparate properties of the constituent materials endow resilience to the composite, and here we report an investigation into some of the mechanisms at play. We report experiments and simulations of a prototype laminated composite system comprising silicon layers separated by polymer interlayers, where the only failure mechanism is the tensile fracture of the brittle silicon. Two failure modes are observed for such composites loaded in three-point bending: failure under the central roller in (i) the top ply (in contact with the roller) or (ii) the bottom ply (free surface). The former mode is benign with the beam retaining load carrying capacity, whereas the latter leads to catastrophic beam failure. Finite element (FE) simulations confirm this transition in failure mode and inform the development of a reduced order model. Good agreement is shown between measurements, FE simulations, and reduced order predictions, capturing the effects of material and geometric properties on the flexural rigidity, first ply failure mode, and failure load. A failure mechanism map for this system is reported that can be used to inform the design of such laminated composites.

Enhanced design of hourglass truss sandwich structures for compressive resistance

In this article, the design of the hourglass truss sandwich structure is improved by optimizing the number of layers to enhance the compressive strength of both the core and the face sheet and then its mechanical performance. The hourglass truss structures characterized by three different numbers of layers are manufactured using an interlocking and vacuum brazing method. The effect of the layer number of the hourglass core panels on their out-of-plane compression and in-plane compression performance is investigated, and the results from calculations and experiments are in reasonable agreement. The results show that as the layer number of the hourglass core increases, the out-of-plane compressive strengths show little change, but their energy absorption properties are effectively increased. The in-plane compressive failure mechanism maps are constructed, and the specimens are designed to examine the local elastic and inelastic buckling failure modes of the face sheets. The results suggest that as the number of layers of the hourglass core increases, its maximum in-plane compressive load increases. The maximum in-plane compressive loads of the two-layer hourglass truss panels are 57%–70% higher than those of the single-layer panels. It can also be concluded that the out-of-plane and in-plane compression mechanical properties of the multilayer hourglass truss outperform those of the pyramidal truss. Furthermore, the number of layers of the hourglass core is optimized in consideration of both mechanical properties and fabrication cost.

Crack channelling mechanisms in brittle coating systems under moisture or temperature gradients

Crack channelling is predicted in a brittle coating-substrate system that is subjected to a moisture or temperature gradient in the thickness direction. Competing failure scenarios are identified, and are distinguished by the degree to which the coating-substrate interface delaminates, and whether this delamination is finite or unlimited in nature. Failure mechanism maps are constructed, and illustrate the sensitivity of the active crack channelling mechanism and associated channelling stress to the ratio of coating toughness to interfacial toughness, to the mismatch in elastic modulus and to the mismatch in coefficient of hygral or thermal expansion. The effect of the ratio of coating to substrate thickness upon the failure mechanism and channelling stress is also explored. Closed-form expressions for the steady-state delamination stress are derived, and are used to determine the transition value of moisture state that leads to unlimited delamination. Although the results are applicable to coating-substrate systems in a wide range of applications, the study focusses on the prediction of cracking in historical paintings due to indoor climate fluctuations, with the objective of helping museums developing strategies for the preservation of art objects. For this specific application, crack channelling with delamination needs to be avoided under all circumstances, as it may induce flaking of paint material. In historical paintings, the substrate thickness is typically more than ten times larger than the thickness of the paint layer; for such a system, the failure maps constructed from the numerical simulations indicate that paint delamination is absent if the delamination toughness is larger than approximately half of the mode I toughness of the paint layer. Further, the transition between crack channelling with and without delamination appears to be relatively insensitive to the mismatch in the elastic modulus of the substrate and paint layer. The failure maps developed in this work may provide a useful tool for museum conservators to identify the allowable indoor humidity and temperature fluctuations for which crack channelling with delamination is prevented in historical paintings.

Bending characteristics of all-composite hexagon honeycomb sandwich beams: experimental tests and a three-dimensional failure mechanism map

The bending characteristics of all-composite honeycomb sandwich beams were investigated by a three-dimensional failure mechanism map and verified by three-point bending tests. In this paper, analytical models were used to predict the three-point bending stiffness, failure load and failure modes of all-composite hexagon honeycomb sandwich beams. A three-dimensional failure mechanism map was generated to characterize the dominant failure mechanism based upon failure load criteria for shear buckling, shear fracture, debonding, intracellular dimpling and face fracture. The tailor-folding method was used to fabricate the all-composite honeycomb sandwich beams. To verify the analytical models and three-dimensional failure mechanism map, three-point bending tests were carried out on the sandwich beams with different core relative densities, face thicknesses and loading spans. It was observed that the analytical predictions were in good agreement with the experimental results. A single case with the supreme load-weight ratio on the failure mechanism map has been identified and verified. Besides, the paths of maximum load design for a series of geometrical parameters were also traced. The study provides insights into the role of the physical dimension in tuning the flexural property of the sandwich structure and expands the application envelope of the failure mechanism map by effectively increasing the structural analytical dimensionality.

Flexural performance of composite grid panels with deep ribs

This paper reports on the flexural performance of an innovative composite grid panel composed of glass fiber-reinforced polymer face skins and deep glass fiber-reinforced polymer ribs with a trapezoidal cross-section. Three-point and four-point bending experiments were performed to demonstrate the feasibility of the composite grid panels under concentrated loads. Compared with the composite grid panels without skins, maximum increases in the ultimate load, and initial bending stiffness of the composite grid panels of approximately 68.2% and 306.7%, respectively, were achieved with the existence of both upper and lower skins. Furthermore, an analytical analysis was carried out to predict the initial bending stiffness and mid-span deflection of the composite grid panels. A comparison of the analytical and experimental results showed that the analytical model accurately predicted the flexural performance of the composite grid panels subjected to three-point and four-point bending. Failure mechanism maps were constructed to predict the mechanical response and failure modes of the composite grid panels. Moreover, the validated model was used in a parametric analytical study to further estimate the effects of various parameters on the flexural performance of the composite grid panels. The results demonstrated that the initial bending stiffness can be significantly improved by increasing the trapezoidal section ratio, face skin thickness, and grid height.

Bending behavior of lightweight C/SiC pyramidal lattice core sandwich panels

Due to the excellent mechanical and chemical properties at ultra-high temperature, ceramic matrix composite structures have potential applications for thermal protection systems of hypersonic vehicles. This paper presented a PIP method to fabricate C/SiC pyramidal lattice core sandwich panels (LCSPs). Their bending behaviors with different core angles were experimentally studied firstly. Then the critical failure loads of different failure modes, including facesheet crushing or wrinkling, core member crushing, core shear failure and interlayer delamination, were theoretically established to construct the failure mechanism maps. The influence of geometric parameters, such as the length and the core angle of the core strut, and the thickness of the facesheet, on the failure mode was also analyzed. In addition, further failure process under bending load was investigated by finite element analysis (FEA) method. Elastic-damage constitutive model and cohesive element were applied to modeling the behavior of C/SiC composite and interlayer delamination between the sheet of the core and the facesheet. Theoretical and FEA results agreed well with experimental data. This work is believed to be helpful to understand the bending mechanism C/SiC pyramidal LCSPs.

Fabrication and mechanical behaviors of an all-composite sandwich structure with a hexagon honeycomb core based on the tailor-folding approach

The tailor-folding method is proposed to make an all-composite sandwich panel with a carbon fiber reinforced polymer (CFRP) hexagon honeycomb core. Using this method, a CFRP honeycomb core with continuous fibers is fabricated automatically from a continuous plain woven prepreg to reinforce the constraints between adjacent cell walls. The analytical expressions were derived for predicting the stiffness and strength of the CFRP hexagon honeycomb sandwich panels for out-of-plane compression and shear loadings. The corresponding failure mechanism maps were also constructed for estimating the dominant failure mode of the honeycomb core, including elastic buckling and fracture under out-of-plane compressive and shear loadings. Selected geometries of the sandwich panels were tested to illustrate these failure modes, with a reasonable agreement between the analytical predictions and experiments. It was observed that the CFRP hexagon honeycomb exhibit good specific energy absorption ability. The tailor-folding method has the potential to provide new opportunities for lightweight multifunctional honeycombs.

Load transfer within the bolted joint of a laminate made from ultra-high molecular weight polyethylene fibres

The mechanism of load transfer within the bolted joint of a laminate sheet made from ultra-high molecular weight polyethylene (UHMWPE) plies is investigated both experimentally and by an analytical model. The nature of load transfer and the active failure mechanisms are obtained as a function of joint geometry and of the lateral clamping force on the faces of the laminate (by pre-tensioning of the bolt). A combination of X-ray tomography and optical microscopy reveal that the dominant failure mechanism in the clamped joint is shear failure involving splits of the 0° plies and sliding at the interface between the 0° and 90° plies. A simple analytical model is developed for this shear failure mechanism and, upon noting the competing failure mechanisms of bearing failure, bolt shear and of tensile failure of the 0° plies, a failure mechanism map is constructed in terms of the geometry of the bolted joint, for the case of no pre-tension of the bolt. The analytical model for shear failure suggests that the enhancement in joint strength with increased pre-tensioning of bolt is due to the fact that the shear strength of the UHMWPE increases with increasing hydrostatic pressure.

Low-velocity impact response of sandwich beams with a metal foam core: Experimental and theoretical investigations

Dynamic response of fully clamped sandwich beams with a metal foam core under low-velocity impact is investigated experimentally and theoretically. Failure modes of metal sandwich beams are identified and competing initial failure mechanism map depending on the geometries and material properties is constructed to characterize the initial failure mechanism. A fracture criterion of the maximum allowed deflection is proposed based on the maximum tensile strain of face sheets to evaluate the fracture-resistance of sandwich beams. Effects of loading location, geometries, and material properties are also considered for both initial failure and final fracture. Good agreement is achieved between the experimental results and theoretical predictions. It is shown that the loading location, geometries, and material properties have significant effects on low-velocity impact response of sandwich beams.

Fabrication and mechanical behaviors of carbon fiber reinforced composite foldcore based on curved-crease origami

A carbon fiber-reinforced composite foldcore based on curved-crease origami was designed and fabricated using a hot press molding method. The fabrication process based on curved-crease origami reduces the abrupt change in fiber direction and is more suitable for continuous fiber-reinforced composites. The analytical models based on differential and integral method for predicting the compressive stiffness and strength of curved-crease origami foldcores were developed first and a three-dimensional failure mechanism map was constructed. The compressive properties and failure modes of carbon fiber reinforced composite curved-crease origami foldcores were investigated through experiments and finite element analysis (FEA). A reasonable agreement among the analytical models, experiments, and FEA results was observed. The dominant failure mode in the curved-crease origami foldcores could change from buckling to crushing with the wall thickness of foldcores increasing when subjected to compressive loads. The novel curved-crease origami design with open core channels has potential to applications for lightweight multifunctional structures, such as fuselage panels of airplane.

Lattice materials with pyramidal hierarchy: Systematic analysis and three dimensional failure mechanism maps

Sandwich core materials that offer superior mechanical properties at minimum weight are essential in designing high-performance sandwich structures. Hierarchical materials are ideal templates for this purpose. In this paper, we investigate the mechanical performance of a pyramidal–pyramidal hierarchical lattice material to highlight its potential as the core material for sandwich structures. Three-dimensional failure mechanism maps for the pyramidal–pyramidal hierarchical lattice material are developed under different loading conditions and the results are compared to finite element simulations. Next, we study the mechanical response and failure modes of a sandwich panel with self-similar pyramidal lattice core construction subjected to in-plane compression and three-point bending. The current study indicates that the pyramidal–pyramidal hierarchical configuration can improve the load bearing capacity and core buckling resistance of the sandwich structures at low density. The study provides insights into the role of structural hierarchy in tuning the mechanical response of the lattice materials and expands the application envelope of lightweight sandwich structures by effectively increasing the structural buckling resistance.