Finite element modelling and high speed testing of countersunk composite aircraft joints

Bolted joints feature extensively in composite aircraft structures, facilitating disassembly for maintenance while offering a cost effective joining method. However, inadequate tools for predicting failure of composite bolted joints have led to extensive physical testing programs and conservative design solutions. As usage of composites in airframe design has increased, the optimisation of composite bolted joints has become a key priority for aircraft manufacturers. The phasing out of metal in large commercial aircraft has now reached the fuselage structure, with next-generation aircraft such as the Airbus A350 featuring all-composite designs. Bolted fuselage skin joints tend to be single-lap and incorporate countersunk fasteners for aerodynamic reasons. In order to investigate the mechanical behaviour of single-lap countersunk composite joints, detailed 3D FE models have been developed. Initially, this difficult contact problem was studied elastically using the implicit FE solver, Abaqus/Standard. The mechanical response and hole boundary stress distributions were studied for clearance levels both inside and outside typical aerospace tolerances. Increased clearance delayed load take-up, reduced stiffness and altered hole boundary stresses. Significant convergence issues in these elastic implicit analyses prompted the choice of an explicit solver for challenging simulations of bearing failure in single and multi-bolt joints. A composite damage model was developed for the simulaton of bearing failure in 3D Abaqus/Explicit joint models. Physically-based failure criteria and a crack band model ensured accurate and objective solutions. Predictions of bearing failure in quasistatically loaded, carbon-epoxy fuselage joints correlated well with experiment and gave a novel insight into the development of failure in countersunk composite joints. As part of a dynamic experimental test series, these fuselage joints were shear loaded at 5 m/s and 10 m/s. The final failure mode significantly affected energy absorption, which governs crashworthiness. The 3D explicit approach was finally applied to study issues of concern to industry, which included dynamic simulations. The quasi-static mechanical behaviour of large, panel-level joints was also investigated. An automated Python scripting approach, developed to create 3D composite bolted joints in Abaqus, was presented as a GUI which dramatically reduces model preparation times

FE modelling of countersunk composite joints for next-generation aircraft

FE modelling of countersunk composite joints for next-generation aircraf

Finite element analysis of catastrophic failure of dynamically-loaded countersunk composite fuselage joints

Accurate models of dynamic structural failure are important for crashworthiness studies. To date, catastrophic failure of dynamically-loaded composite bolted joints has been studied using global or stacked shell element models. In this paper, high-fidelity (three-dimensional solid) explicit FE models are used to simulate catastrophic failure of countersunk composite fuselage joints. While current state-of-the-art 3D modelling approaches focus almost exclusively on the prediction of composite damage, this study also investigates the treatment of fastener damage. Fastener fracture is a common catastrophic joint failure mode, particularly in joints designed to initially fail in bearing. A Johnson-Cook material model and cohesive elements were used to predict plasticity, damage and fracture of the titanium (Ti-6Al-4V) fastener. Although a model calibration was required, due to the complex interaction of model parameters, numerical results demonstrate key trends of experiments and provide a starting point for the development of more predictive approaches for simulating fastener failur

Stress analysis of single-bolt, single-lap, countersunk composite joints with variable bolt-hole clearance

Single-lap, carbon-epoxy joints with countersunk fasteners were modelled using the nonlinear finite element code Abaqus. A highly-detailed analysis of the stress distribution at the countersunk hole boundary is provided. Bolt-hole clearance, which arises due to limitations in manufacturing capabilities, is modelled extensively. Clearance levels both inside and outside typical aerospace fitting tolerances are studied and the finite element model is validated with experimental data. Plots of radial stress in each ply of the countersunk laminate show the load transfer to be severely localised, with only a few plies bearing the majority of the load. The inclusion of clearance in the model was shown to result in far higher radial stresses compared to those in the neat-fit joint model. An associated loss in joint stiffness of more than 10% was recorded for the highest clearance considered (240 mu m). Finally compressive through-thickness stresses are shown to be present at the damageable region of the countersunk hole, and increase with bolt-hole clearance. These compressive stresses, which are an indicator of lateral constraint, are seen to suppress "brooming" failure in the countersunk laminate. (C) 2011 Elsevier Ltd. All rights reserved

Static and high-rate loading of single and multi-bolt carbon-epoxy aircraft fuselage joints

Single-lap shear behaviour of carbon epoxy composite bolted aircraft fuselage joints at quasi-static and dynamic (5 m/s and 10 m/s) loading speeds is studied experimentally. Single and multi-bolt joints with countersunk fasteners were tested. The initial joint failure mode was bearing, while final failure was either due to fastener pull-through or fastener fracture at a thread. Much less hole bearing damage, and hence energy absorption, occurred when the fastener(s) fractured at a thread, which occurred most frequently in thick joints and in quasi-static tests. Fastener failure thus requires special consideration in designing crashworthy fastened composite structures; if it can be delayed, energy absorption is greater. A correlation between energy absorption in multi-bolt and single-bolt joint tests indicates potential to downsize future test programmes. Tapering a thin fuselage panel layup to a thicker layup at the countersunk hole proved highly effective in achieving satisfactory joint strength and energy absorption

An Alternative Approach to ChIP-Seq Normalization Enables Detection of Genome-Wide Changes in Histone H3 Lysine 27 Trimethylation upon EZH2 Inhibition

<div><p>Chromatin immunoprecipitation and DNA sequencing (ChIP-seq) has been instrumental in inferring the roles of histone post-translational modifications in the regulation of transcription, chromatin compaction and other cellular processes that require modulation of chromatin structure. However, analysis of ChIP-seq data is challenging when the manipulation of a chromatin-modifying enzyme significantly affects global levels of histone post-translational modifications. For example, small molecule inhibition of the methyltransferase EZH2 reduces global levels of histone H3 lysine 27 trimethylation (H3K27me3). However, standard ChIP-seq normalization and analysis methods fail to detect a decrease upon EZH2 inhibitor treatment. We overcome this challenge by employing an alternative normalization approach that is based on the addition of <i>Drosophila melanogaster</i> chromatin and a <i>D</i>. <i>melanogaster-</i>specific antibody into standard ChIP reactions. Specifically, the use of an antibody that exclusively recognizes the <i>D</i>. <i>melanogaster</i> histone variant H2Av enables precipitation of <i>D</i>. <i>melanogaster</i> chromatin as a minor fraction of the total ChIP DNA. The <i>D</i>. <i>melanogaster</i> ChIP-seq tags are used to normalize the human ChIP-seq data from DMSO and EZH2 inhibitor-treated samples. Employing this strategy, a substantial reduction in H3K27me3 signal is now observed in ChIP-seq data from EZH2 inhibitor treated samples.</p></div

EZH2 inhibition reduces global H3K27me3 levels, however standard ChIP-seq methods do not reveal the reduction.

<p><b>(A)</b> Western blot showing reduced global H3K27me3 levels in KARPAS-422 cells treated with 1.5 μM CPI-360 for 4 and 8 days. Whole cell extracts were resolved by SDS page and immuno-blotted with anti-H3K27me3. Anti-H3 immuno-blots show equal levels of total H3. <b>(B)</b> Western blot showing reduced global H3K27me3 levels in PC9 cells treated with 1 μM of GSK126 for 5 days. Whole cell extracts were resolved by SDS page and immuno-blotted with anti-H3K27me3. Anti-H3 immuno-blots show equal levels of total H3. <b>(C, D)</b> Representation of H3K27me3 ChIP-seq data using IGV. No obvious differences are detected in CPI-360 (C) and GSK126 (D) treated KARPAS-422 and PC9 cells when compared to vehicle-treated controls. <b>(E, F)</b> Genome-wide data from H3K27me3 ChIP-seq experiments under different treatment conditions are represented as scatter plots.</p

Schematic representation of the ChIP-seq spike-in protocol.

<p>ChIP-seq spike-in reactions are set up by adding the test chromatin of interest (human or other), the target antibody of interest, a small portion of <i>D</i>. <i>melanogaster</i> chromatin and the <i>D</i>. <i>melanogaster-</i>H2Av-specific antibody. The <i>D</i>. <i>melanogaster</i> spike-in chromatin is added in equal amounts and the H2Av antibody functions to pull down a small portion of the <i>D</i>. <i>melanogaster</i> chromatin in each reaction. After sequencing, tags are mapped to the genome corresponding to the test chromatin as well as to the <i>D</i>. <i>melanogaster</i> genome. The total number of tags uniquely mapping to the <i>D</i>. <i>melanogaster</i> genome are counted for each sample and used to generate correction factors (DMSO tags/inhibitor tags). The test chromatin tag counts are then normalized using the correction factors.</p

Reduced H3K27me3 binding is detected by ChIP-qPCR.

<p><b>(A)</b> ChIP was performed using chromatin from KARPAS-422 cells treated with the EZH2 inhibitor CPI-360. qPCR using the positive control primer <i>MYT1</i> showed reduced H3K27me3 occupancy in the presence of the inhibitor. <b>(B)</b> ChIP was performed using chromatin from PC9 cells treated with the EZH2 inhibitor GSK126. qPCR using the positive control primer <i>MYT1</i> showed reduced H3K27me3 occupancy in cells treated with the inhibitor. (<b>C</b>) Libraries were generated from KARPAS-422 cells using 15 cycles of PCR amplification. Library DNA was diluted and qPCR was performed using positive control primers for <i>MYT1</i> and <i>CCND2</i>. (<b>D</b>) Libraries were generated from PC9 cells as described in (C) and library DNA was used for qPCR using positive control primers for <i>MYT1</i> and <i>CCND2</i>. All experiments are represented as the mean of two independent experiments with qPCRs performed in triplicate ±SD. The <i>ACTB</i> promoter served as a negative control for all experiments.</p

<i>D</i>. <i>melanogaster</i> tag counts from H3K27me3 ChIP-seq reactions are elevated in EZH2 inhibitor treated samples.

<p>H2Av bound regions of the <i>D</i>. <i>melanogaster</i> genome were determined using the H2Av antibody in ChIP-seq reactions containing <i>D</i>. <i>melanogaster</i> S2 or OSS chromatin. <i>D</i>. <i>melanogaster</i> tags from ChIP-seq spike-in reactions were mapped only to these pre-defined H2Av regions. <b>(A)</b> H3K27me3 ChIP-seq reactions with <i>D</i>. <i>melanogaster</i> spike-in in KARPAS-422 cells have a substantial increase in <i>D</i>. <i>melanogaster</i> tags in spike-in libraries prepared from CPI-360 treated cells both at 4 days and 8 days after treatment. <b>(B)</b> The increase was not observed in the control H3K9me3 reactions. <b>(C)</b> H3K27me3 ChIP-seq reactions with <i>D</i>. <i>melanogaster</i> spike-in in PC9 cells have a substantial increase in <i>D</i>. <i>melanogaster</i> tags in spike-in libraries prepared from GSK126 treated cells. <b>(D)</b> The substantial increase in tags was not observed in the control H3K4me3 ChIP-seq spike-in reactions.</p