Numerical prediction of the low-velocity impact damage and compression after impact strength of composite laminates

Low-velocity impact damage can drastically reduce the residual mechanical properties of the composite structure even when there is barely visible impact damage. The ability to computationally predict the extent of damage and compression after impact (CAI) strength of a composite structure can potentially lead to the exploration of a larger design space without incurring significant development time and cost penalties. A three-dimensional damage model, to predict both low-velocity impact damage and compression after impact CAI strength of composite laminates, has been developed and implemented as a user material subroutine in the commercial finite element package, ABAQUS/Explicit. The virtual tests were executed in two steps, one to capture the impact damage and the other to predict the CAI strength. The observed intra-laminar damage features, delamination damage area as well as residual strength are discussed. It is shown that the predicted results for impact damage and CAI strength correlated well with experimental testing.

Crush responses of composite cylinder under quasi-static and dynamic loading

Despite the abundance of studies investigating the performance of composite structures under crush loading, disagreement remains in the literature regarding the effect of increased strain rate on the crush response. This study reports an experimental investigation of the behaviour of a carbon-epoxy composite energy absorber under static and dynamic loading with a strain rate of up to 100s^-1. Consistent damage modes and measured force responses were obtained in samples tested under the same strain rate. The energy absorption was found to be independent of strain rate as the total energy absorption appeared to be largely associated with fibre-dominated fracture, which is independent of strain rate within the studied range. The results from this study are beneficial for the design of energy absorbing structures.

Predicting low velocity impact damage and Compression-After-Impact (CAI) behaviour of composite laminates

Low-velocity impact damage can drastically reduce the residual strength of a composite structure even when the damage is barely visible. The ability to computationally predict the extent of damage and compression-after-impact (CAI) strength of a composite structure can potentially lead to the exploration of a larger design space without incurring significant time and cost penalties. A high-fidelity three-dimensional composite damage model, to predict both low-velocity impact damage and CAI strength of composite laminates, has been developed and implemented as a user material subroutine in the commercial finite element package, ABAQUS/Explicit. The intralaminar damage model component accounts for physically-based tensile and compressive failure mechanisms, of the fibres and matrix, when subjected to a three-dimensional stress state. Cohesive behaviour was employed to model the interlaminar failure between plies with a bi-linear traction–separation law for capturing damage onset and subsequent damage evolution. The virtual tests, set up in ABAQUS/Explicit, were executed in three steps, one to capture the impact damage, the second to stabilize the specimen by imposing new boundary conditions required for compression testing, and the third to predict the CAI strength. The observed intralaminar damage features, delamination damage area as well as residual strength are discussed. It is shown that the predicted results for impact damage and CAI strength correlated well with experimental testing without the need of model calibration which is often required with other damage models.

Predicting the crushing behaviour of composite material using high-fidelity finite element modelling

The capability to numerically model the crushing behaviour of composite structures will enable the efficient design of structures with high specific energy absorption capacity. This is particularly relevant to the aerospace and automotive industries where cabin structures need to be shown to be crashworthy. In this paper, a three-dimensional damage model is presented, which accurately represents the behaviour of composite laminates under crush loading. Both intralaminar and interlaminar failure mechanisms are taken into account. The crush damage model was implemented in ABAQUS/Explicit as a VUMAT subroutine. Numerical predictions are shown to agree well with experimental results, accurately capturing the intralaminar and interlaminar damage for a range of stacking sequences, triggers and composite materials. The use of measured material parameters required by the numerical models, without the need to ‘calibrate’ this input data, demonstrates this computational tool's predictive capabilities

Numerical prediction of the low-velocity impact damage and compression after impact strength of composite laminates

Low-velocity impact damage can drastically reduce the residual mechanical properties of the composite structure even when there is barely visible impact damage. The ability to computationally predict the extent of damage and compression after impact (CAI) strength of a composite structure can potentially lead to the exploration of a larger design space without incurring significant development time and cost penalties. A three-dimensional damage model, to predict both low-velocity impact damage and compression after impact CAI strength of composite laminates, has been developed and implemented as a user material subroutine in the commercial finite element package, ABAQUS/Explicit. The virtual tests were executed in two steps, one to capture the impact damage and the other to predict the CAI strength. The observed intra-laminar damage features, delamination damage area as well as residual strength are discussed. It is shown that the predicted results for impact damage and CAI strength correlated well with experimental testing.

Insight of the interface of electroless Ni-P/SiC composite coating on aluminium alloy, LM24

Electroless nickel composite coatings with silicon carbide, SiC, as reinforcing particles deposited with Ni–P onto aluminium alloy, LM24, having zincating as under layer were subjected to heat treatment using air furnace. The changes at the interface were investigated using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to probe the chemistry changes upon heat treatment. Microhardness tester with various loads using both Knoop and Vickers indenters was used to study the load effect clubbed with the influence of second phase particles on the coating at the vicinity of the interface. It was observed that zinc was absent at the interface after elevated temperature heat treatment at 400–500 °C. Precipitation of copper and nickel with a distinct demarcation (copper rich belt) along the coating interface was seen with irregular thickness of the order of 1 μm. Migration of copper from the bulk aluminium alloy could have been the factor. Brittleness of the coating was confirmed on heat treatment when indented with Vickers. However, in composite coating the propagation of the microcrack was stopped by the embedded particles but the microcracks continue in the matrix when not interrupted by second phase particles (SiC).

An efficient genetic algorithm for the design optimization of cold-formed steel portal frame buildings

The design optimization of a cold-formed steel portal frame building is considered in this paper. The proposed genetic algorithm (GA) optimizer considers both topology (i.e., frame spacing and pitch) and cross-sectional sizes of the main structural members as the decision variables. Previous GAs in the literature were characterized by poor convergence, including slow progress, that usually results in excessive computation times and/or frequent failure to achieve an optimal or near-optimal solution. This is the main issue addressed in this paper. In an effort to improve the performance of the conventional GA, a niching strategy is presented that is shown to be an effective means of enhancing the dissimilarity of the solutions in each generation of the GA. Thus, population diversity is maintained and premature convergence is reduced significantly. Through benchmark examples, it is shown that the efficient GA proposed generates optimal solutions more consistently. A parametric study was carried out, and the results included. They show significant variation in the optimal topology in terms of pitch and frame spacing for a range of typical column heights. They also show that the optimized design achieved large savings based on the cost of the main structural elements; the inclusion of knee braces at the eaves yield further savings in cost, that are significant.

Characterisation of phase composition, microstructure and microhardness of electroless nickel composite coating co-deposited with SiC on casting aluminium LM24 alloy substrate

Electroless Ni-P (EN) and composite Ni-P-SiC (ENC) coatings were developed on cast aluminium alloy, LM24. The coating phase composition, microstructure and microhardness were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and microhardness tester, respectively, on as-plated and heat-treated specimens. The original microstructure of the Ni-P matrix is not affected by the inclusion of the hard particles SiC. No formation of Ni-Si phase was observed upto 500°C of heat treatment. The microhardness is increased on incorporation of SiC in Ni-P matrix. The hardening mechanism is the formation of intermetallic phase Ni3P on annealing at elevated temperature. Overall, the composite coating (ENC) was found to be superior as compared to particles free (EN) coating in both as-deposited and heat-treated conditions.

Understanding aerospace composite components' supply chain carbon emissions

This paper examines a large structural component and its supply chain. The component is representative of that used in the production of civil transport aircraft and is manufactured from carbon fibre epoxy resin prepreg, using traditional hand layup and autoclave cure. Life cycle assessment (LCA) is used to predict the component’s production carbon emissions. The results determine the distribution of carbon emissions within the supply chain, identifying the dominant production processes as carbon fibre manufacture and composite part manufacture. The elevated temperature processes of material and part creation, and the associated electricity usage, have a significant impact on the overall production emissions footprint. The paper also demonstrates the calculation of emissions footprint sensitivity to the geographic location and associated energy sources of the supply chain. The results verify that the proposed methodology is capable of quantitatively linking component and supply chain specifics to manufacturing processes and thus identifying the design drivers for carbon emissions in the manufacturing life of the component.

Element-Free Galerkin Modelling of Crack Migration in Composite Laminates

The development of a virtual testing environment, as a cost-effective industrial design tool in the design and analysis of composite structures, requires the need to create models efficiently, as well as accelerate the analysis by reducing the number of degrees of freedom, while still satisfying the need for accurately tracking the evolution of a debond, delamination or crack front. The eventual aim is to simulate both damage initiation and propagation in components with realistic geometrical features, where crack propagation paths are not trivial. Meshless approaches, and the Element-Free Galerkin (EFG) method, are particularly suitable for problems involving changes in topology and have been successfully applied to simulate damage in homogeneous materials and concrete. In this work, the method is utilized to model initiation and mixed-mode propagation of cracks in composite laminates, and to simulate experimentally-observed crack migration which is difficult to model using standard finite element analysis. N

Improving composite damage modelling through automatic placement of cohesive elements

Numerous experimental studies of damage in composite laminates have shown that intralaminar (in-plane) matrix cracks lead to interlaminar delamination (out-of-plane) at ply interfaces. The smearing of in-plane cracks over a volume, as a consequence of the use of continuum damage mechanics, does not always effectively capture the full extent of the interaction between the two failure mechanisms. A more accurate representation is obtained by adopting a discrete crack approach via the use of cohesive elements, for both in-plane and out-of-plane damage. The difficulty with cohesive elements is that their location must be determined a priori in order to generate the model; while ideally the position of the crack migration, and more generally the propagation path, should be obtained as part of the problem’s solution. With the aim of enhancing current modelling capabilities with truly predictive capabilities, a concept of automatic insertion of interface elements is utilized. The consideration of a simple traction criterion in relation to material strength, evaluated at each node of the model (or of the regions of the model where it is estimated cracks might form), allows for the determination of initial crack location and subsequent propagation by the insertion of cohesive elements during the course of the analysis. Several experimental results are modelled using the commercial package ABAQUS/Standard with an automatic insertion subroutine developed in this work, and the results are presented to demonstrate the capabilities of this technique.

Optimising postbuckling composite panels for damage resistance

The design of current composite primary aerostructures, such as fuselage or wing stiffened panels, tends to be conservative due to the susceptibility of the relatively weak skin-stiffener interface. This weakness is due to through-thickness stresses which are exacerbated by deformations due to buckling. This paper presents a finite-elementbased optimization strategy, utilizing a global-local modelling approach, for postbuckling stiffened panels which takes into account damage mechanisms which may lead to delamination and subsequent failure of the panel due to stiffener debonding. A genetic algorithm was linked to a finite element package to automate the iterative procedure and maximize the damage resistance of the panel in postbuckling. For a given loading condition, the procedure optimized the panel’s skin layup leading to a design displaying superior damage resistance compared to non-optimized designs

Of no fixed form: The Irish Pavilion at Venice 2014 and beyond

This paper explores the theme of exhibiting architectural research through a particular example, the development of the Irish pavilion for the 14th architectural biennale, Venice 2014. Responding to Rem Koolhaas’s call to investigate the international absorption of modernity, the Irish pavilion became a research project that engaged with the development of the architectures of infrastructure in Ireland in the twentieth and twenty-first centuries. Central to this proposition was that infrastructure is simultaneously a technological and cultural construct, one that for Ireland occupied a critical position in the building of a new, independent post-colonial nation state, after 1921.

Presupposing infrastructure as consisting of both visible and invisible networks, the idea of a matrix become a central conceptual and visual tool in the curatorial and design process for the exhibition and pavilion. To begin with this was a two-dimensional grid used to identify and order what became described as a series of ten ‘infrastructural episodes’. These were determined chronologically across the decades between 1914 and 2014 and their spatial manifestations articulated in terms of scale: micro, meso and macro. At this point ten academics were approached as researchers. Their purpose was twofold, to establish the broader narratives around which the infrastructures developed and to scrutinise relevant archives for compelling visual material. Defining the meso scale as that of the building, the media unearthed was further filtered and edited according to a range of categories – filmic/image, territory, building detail, and model – which sought to communicate the relationship between the pieces of architecture and the larger systems to which they connect. New drawings realised by the design team further iterated these relationships, filling in gaps in the narrative by providing composite, strategic or detailed drawings.

Conceived as an open-ended and extendable matrix, the pavilion was influenced by a series of academic writings, curatorial practices, artworks and other installations including: Frederick Kiesler’s City of Space (1925), Eduardo Persico and Marcello Nizzoli’s Medaglio d’Oro room (1934), Sol Le Witt’s Incomplete Open Cubes (1974) and Rosalind Krauss’s seminal text ‘Grids’ (1979). A modular frame whose structural bays would each hold and present an ‘episode’, the pavilion became both a visual analogue of the unseen networks embodying infrastructural systems and a reflection on the predominance of framed structures within the buildings exhibited. Sharing the aspiration of adaptability of many of these schemes, its white-painted timber components are connected by easily-dismantled steel fixings. These and its modularity allow the structure to be both taken down and re-erected subsequently in different iterations. The pavilion itself is, therefore, imagined as essentially provisional and – as with infrastructure – as having no fixed form. Presenting archives and other material over time, the transparent nature of the space allowed these to overlap visually conveying the nested nature of infrastructural production. Pursuing a means to evoke the qualities of infrastructural space while conveying a historical narrative, the exhibition’s termination in the present is designed to provoke in the visitor, a perceptual extension of the matrix to engage with the future.

Central corneal thickness and corneal hysteresis associated with glaucoma damage.

PURPOSE:

We sought to measure the impact of central corneal thickness (CCT), a possible risk factor for glaucoma damage, and corneal hysteresis, a proposed measure of corneal resistance to deformation, on various indicators of glaucoma damage.

DESIGN:

Observational study.

METHODS:

Adult patients of the Wilmer Glaucoma Service underwent measurement of hysteresis on the Reichert Ocular Response Analyzer and measurement of CCT by ultrasonic pachymetry. Two glaucoma specialists (H.A.Q., N.G.C.) reviewed the chart to determine highest known intraocular pressure (IOP), target IOP, diagnosis, years with glaucoma, cup-to-disk ratio (CDR), mean defect (MD), pattern standard deviation (PSD), glaucoma hemifield test (GHT), and presence or absence of visual field progression.

RESULTS:

Among 230 subjects, the mean age was 65 +/- 14 years, 127 (55%) were female, 161 (70%) were white, and 194 (85%) had a diagnosis of primary open-angle glaucoma (POAG) or suspected POAG. In multivariate generalized estimating equation models, lower corneal hysteresis value (P = .03), but not CCT, was associated with visual field progression. When axial length was included in the model, hysteresis was not a significant risk factor (P = .09). A thinner CCT (P = .02), but not hysteresis, was associated with a higher CDR at the most recent examination. Neither CCT nor hysteresis was associated with MD, PSD, or GHT "outside normal limits."

CONCLUSIONS:

Thinner CCT was associated with the state of glaucoma damage as indicated by CDR. Axial length and corneal hysteresis were associated with progressive field worsening.