Resistance welding of thermoplastic composites skin/stringer joints

An experimental investigation of resistance welding of APC-2/AS4 PEEK/carbon fibre composite using a stainless steel mesh heating element is presented. A special specimen geometry, the skin/stringer configuration, was used to represent a typical reinforced aerospace structural joint. The specimens consisted of a flange, representing a stringer or frame, welded onto a skin laminate. The effects of the welding parameters such as the input power level and clamping distance on the weld quality and performance were investigated. The welding parameters were optimised using short beam shear tests, ultrasonic C-scan inspection and optical microscopy. The mechanical performance of the resistance-welded skin/stringer configuration was investigated using three- and four-point bending tests and the failure mechanisms were characterised by optical and scanning electron microscopy. Two methods were used to reduce the stress concentration at the flange tip. The first method was to machine a 20° taper angle at the edge of the flange and the second one was to create a resin fillet at the flange tip. No mechanical performance improvement was obtained with the resin fillet method but the taper angle method showed 25% mechanical performance improvement when the taper angles were machined after the welding operation.     

Influence of post-buckling behaviour of composite stiffened panels on the damage criticality

In stiffened panels with defects, such as skin delaminations or stringer debonding, buckling may occur prior to the designed critical buckling load. Depending on the damage parameters, such defects may also affect the post-buckling behaviour and consequently the structural performance. An automated finite element (FE) modelling tool has been developed to predict the post-buckling behaviour of panels. It was coupled with a linear elastic fracture mechanics approach to determine damage criticality, based on the “no-growth” principle. The structural behaviour in the post-buckling range and its interaction with the damage parameters were analysed. Local buckling occurred as a result of localised stiffness reduction in the damage region. Global buckling occurred when sufficient in-plane strain was reached. The onset of local buckling was an important factor on stringer debonding criticality as the local buckling mode had an effect on the corresponding global buckling. In comparison, the onset of local buckling for the skin delamination was lower due to the thin sub-laminate separation. However, it was less influential on the damage criticality because the local buckling slowly dissipated in the far post-buckling range. It was found that the initiation of local buckling, and the interaction between the local and global buckling mode, would determine the damage criticality.     

翼肋支撑对复合材料加筋板轴压性能的影响

为确定翼肋支撑对复合材料加筋板轴压性能的影响,对施加翼肋支撑前后的复合材料工型加筋板和帽型加筋板进行压缩试验和数值模拟研究。轴压试验中,通过应变计和影像云纹法实时监测试验件的失稳载荷及失稳模态,通过断面观测分析结构损伤破坏机制。基于ABAQUS软件建立有限元模型模拟加筋板屈曲及后屈曲过程,通过失稳节线及反节线上的应力分布变化分析加筋板破坏机制。计算结果与试验结果相吻合,表明翼肋支撑对不同筋条加筋板失稳模态有影响但均不改变结构失稳载荷,位于节线上的翼肋支撑对工型加筋板破坏载荷影响较小,但位于反节线上的翼肋支撑使帽型加筋板的承载能力提高了26.2%。试验件失稳后应力向反节线上筋条蒙皮界面集中,过高的应力导致界面脱粘,使得结构集中在反节线上破坏。      

复合材料加筋壁板长桁终止端混合连接设计分析

对缩短胶接面长度以提高起裂载荷的复合材料加筋壁板长桁终止端新型混合连接设计进行了研究。采用经试验验证的有限元方法,对长桁终止端混合连接的传载与失效机制进行分析,并研究了胶接面长度对结构起裂载荷的影响规律。研究结果表明：新型混合连接设计中紧固件提供的法向约束以及剪切载荷传递路径可显著提升终止端结构的起裂载荷,并增强终止端处界面的抑制损伤扩展能力;结构的起裂载荷随着胶接面的缩短而提高;当胶接面前缘退至连接区末排紧固件之后,可避免结构在最终破坏前发生界面失效,结构强度由蒙皮的机械连接强度决定。     

长桁-翼肋连接对复合材料单加筋板压缩性能的影响

采用试验和有限元方法研究了复合材料含翼肋单加筋板试验件的压缩性能,试验件包括带有长桁-翼肋连接和不含长桁-翼肋连接2种类型。试验和数值计算研究结果表明:与不含长桁-翼肋连接的试验件相比,带有长桁-翼肋连接的试验件具有较高的刚度和较高的临界屈曲失稳载荷,在后屈曲承载过程中具有较小的形变和较小的最终破坏载荷。试验件的最终破坏模式总是长桁与蒙皮间的界面脱粘,这表明长桁-翼肋连接对加筋板试验件的最终破坏模式无影响。在复合材料翼面结构设计中,需要综合考虑长桁-翼肋连接对加筋板初始临界失稳载荷、后屈曲变形和结构承载能力等方面的影响。     

Assessment of damage tolerance in composites

Damage tolerance to processing and normal service damage has been assessed for carbon epoxy coupons and built-up panels in uniaxial loading. In coupons, the low velocity impact damage is more severe than damage in holes, delaminations or porosity and it causes compression strength loss from 58% at the barely visible threshold to 73% at the easily visible threshold. Damaged coupon fatigue ($\text{R}\text{= 10}$) S-N curves are relatively flat with 67% strength loss at 10⁶ cycles for impact damage. In built-up panels representing multispar (M-S) and multirib (M-R) wing designs, again impact damage is more severe than delaminations and the impacted M-R design is stronger than the impacted M-S design. Damage in two adjacent M-S midbays and at the edge of the M-R stringer flange are the critical locations for impact, but damage area strongly depends on the panel configuration. Impact damage grows under constant amplitude fatigue ($\text{R}\text{= 10}$) with peak load at 65% of damaged static strength for both M-S and M-R designs. Built-up configuration of the panels provides a significant increase in impact damage tolerance over that of coupons.     

On the response of two commercially-important CFRP structures to multiple ice impacts

Carbon-fibre reinforced composites (CFRPs) are likely to feature heavily as structural elements of future aerospace vehicles due to their high stiffness and low densities. However, such components are likely to be subjected to a variety of impact-related events during their in-service lives. One area which has only received limited attention in the literature is that of ice impact on CFRP structures; e.g. hail stone impacts on aerospace components. In this study the response of two aerospace-grade CFRP structures (one woven and one uni-directional lay-up) to multiple ice impacts with cumulative impact energies in the range 72–1215 J was investigated. Six empirical damage categories were identified, ranging from no apparent surface damage (Type 1) to penetration accompanied by complete lay-up disruption (Type 6). Surface damage was found to correlate with changes in recovered panel properties; determined by ultrasonic C-Scan and compression-after-impact strength tests. With both CFRP structures sub-surface disruption and residual compressive strength varied linearly with total impact energy; suggesting that damage in such structures is cumulative in nature. Further, in line with previous studies, the woven structure consistently exhibited lower levels of damage at a given impact energy, even when damage extent was normalised by areal density.     

A parametric finite element study and an analytical model of particle distributions on post-necking deformation and failure mode in AA5754 aluminum alloy sheets

Finite element analysis of a simplified large particle field, edge constrained plane strain model is used for parametric studies of the influence of particle distributions on post-necking deformation and failure mode in AA5754 aluminum sheets. The models show that the post-necking deformation decreases with volume fraction of particles and fraction of stringers, and increases with interparticle spacing. It is to be noted that a stringer is a string of second phase particles that is frequently observed in continuous strip cast (CC) sheet aluminum alloys. The post-necking deformation initially decreases with the length of stringers, but after a critical stringer length, it increases. An analytical model to estimate the number of stringers which act as initial active damage sources is able to predict a critical stringer length for least post-necking deformation and can serve as a design tool.     

An experimental and numerical study on omega stringer debonding

Omega stringers offer interesting structural capabilities and are expected on future aircraft fuselages. In postbuckling mode, the final failure of these structures may occur by stringer debonding between stringer flanges and the skin of the fuselage. In this study, it is demonstrated that the use of fracture mechanics allows to predict skin/omega stringer separation under multiple load cases. Three different load cases and experiments are presented allowing a debonding to start at different locations: at free flange edges or at the inner radius of the omega. Firstly, a skin/stringer configuration subjected to three point bending following the longitudinal axis of the stringer was tested. For this configuration, a numerical study was made and shows the influence of a refined mesh taking into account resin fillets. Secondly, new specimens were obtained by cutting into slices the longitudinal specimen. Those specimens were subjected to four points bending. It has been shown that the upper rolls position of the test jig could modify the debonding location. Numerical models have allowed to determine accurately the debonding location and the associated load level. For some specimens, resin fillets were removed from the flange tips and their effect were assessed numerically and experimentally.     

A systematic analysis and design approach for single-span FRP deck/stringer bridges

There is a concern with worldwide deterioration of highway bridges, particularly reinforced concrete. The advantages of fiber reinforced plastic (FRP) composites over conventional materials motivate their use in highway bridges for rehabilitation and replacement of structures. In this paper, a systematic approach for analysis and design of all FRP deck/stringer bridges is presented. The analyses of structural components cover: (1) constituent materials and ply properties, (2) laminated panel engineering properties, (3) stringer stiffness properties, and (4) apparent stiffnesses for composite cellular decks and their equivalent orthotropic material properties. To verify the accuracy of orthotropic material properties, an actual deck is experimentally tested and analyzed by a finite element model. For design analysis of FRP deck/stringer bridge systems, an approximate series solution for orthotropic plates, including first-order shear deformation, is applied to develop simplified design equations, which account for load distribution factors under various loading cases. An FRP deck fabricated by bonding side-by-side box beams is transversely attached to FRP wide-flange beams and tested as a deck/stringer bridge system. The bridge systems are tested under static loads for various load conditions, and the experimental results are correlated with those by an approximate series solution and a finite element model. The present simplified design analysis procedures can be used to develop new efficient FRP sections and to design FRP highway bridge decks and deck/stringer systems, as shown by an illustrative design example.     

A plate with a crack,stiffened by a partially debonded stringer

The effect of a partially debonded infinite stringer on the stress intensity factor at the crack tip is investigated. It is assumed that the stringer is bonded to an isotropic plate through an adhesive, along a line perpendicular to the crack. Due to the high stress concentration around the crack tip and on the adhesive, partial debonding is assumed to develop between the stringer and the plate. The integral equation, obtained from the continuity of displacements along the line where the stringer and the plate are bonded, is solved numerically to give the shear stress distribution between the plate and the stringer and the stress intensity factors at both crack tips.     

Large-Deformation Constitutive Theories for Structural Composites: Rate-Dependent Concepts and Effect of Microstructure

Abstract:  Polymer-based composite materials are widely used in applications subjected to a variety of loading types, including shock and impact loading in the range of hundreds of strain per second. The behaviour of composite laminates loaded at those rates is typically nonlinear and may involve rather large strains to failure. In the present study, the large-deformation characteristics and constitutive representations of structural composites were investigated as functions of strain rate and temperature. A plain-weave vinyl ester composite material was selected for the study. Tensile tests of off-axis coupon specimens were conducted over several orders of strain rates and limited change of temperatures. A three-parameter constitutive model was proposed to model the large-deformation stress–strain relationship. The constitutive model was then used to predict the material response at different strain rates. The model predictions were verified by a different set of tests. The basic concepts and methodologies involved in reducing such data to constitutive equations that can be used in commercial computational codes to enable structural analysis in the presence of large-strain progressive damage under dynamic loading is discussed.     

复合材料机身曲板环向弯曲加载试验及失效机制

对复合材料机身曲板进行了环向弯曲加载试验,采用四点弯加载方式对考核段进行纯弯加载,设计一种加强连接方式避免加载段提前破坏,通过试验对机身曲板的环向稳定性和破坏模式进行了分析。同时,建立了基于内聚力单元的考虑长桁与蒙皮粘接界面损伤的有限元模型,分别使用Quads准则和Hashin准则作为界面和层合板的失效判据分析曲板结构的失效机制,计算结果与试验结果吻合较好。试验及有限元分析结果表明,长桁帽底蒙皮的局部屈曲引起长桁与蒙皮粘接的R区出现初始开裂,并最终扩展为长桁脱粘。随着蒙皮屈曲及长桁脱粘的扩大,蒙皮由局部屈曲变为整体失稳而失去承载能力,最终导致隔框承载过大而发生断裂。根据初始损伤模式,采取了长桁帽内全包工艺改进设计,改进后的曲板结构稳定性和承载能力分别提高了21.9%和16.8%。      

水下爆炸气泡对内加筋圆柱壳结构毁伤机理分析

以内加筋圆柱壳为研究对象,对近距离非接触爆炸作用下气泡的脉动载荷以及气泡溃破高速射流对内加筋圆柱壳结构的毁伤机理进行计算分析,并且探讨结构参数、药包位置等相关物理量对结构变形特征与毁伤模式的影响。研究结果表明,水下爆炸气泡膨胀产生的膨胀脉动冲击造成结构的整体变形,另一方面气泡溃破所造成的高速射流(速度 > 100 m/s)则会对局部区域造成严重破坏,弹塑性边界以及自由液面效应会对气泡的形状产生显著的影响。     

Carbon Nanotube Sensing Skins for Spatial Strain and Impact Damage Identification

Impact damage, excessive loading, and corrosion have been identified as critical and long-term problems that constantly threaten the integrity and reliability of structural systems (e.g., civil infrastructures, aircrafts, and naval vessels). While a variety of sensing transducers have been proposed for structural health monitoring, most sensors only offer measurement of structural behavior at discrete structural locations. Here, a conformable carbon nanotube-polyelectrolyte sensing skin fabricated via the layer-by-layer technique is proposed to monitor strain and impact damage over spatial areas. Specifically, electrical impedance tomographical (EIT) conductivity mapping techniques are employed to offer two-dimensional damage maps from which damage location and severity can be easily and accurately quantified. This study deposits carbon nanotube-based sensing skins upon metallic structural plates with electrodes installed along the plate boundary. Based on boundary electrical measurements, EIT mapping captures both strain in the underlying substrate as well as damage (e.g., permanent deformation and cracking) introduced using an impact apparatus.     

Two‐stage fatigue life evaluation of an aircraft fuselage panel with a bulging circumferential crack and a broken stringer

The article presents two‐stage fatigue life evaluation of a stiffened aluminium aircraft fuselage panel, subject to ground–air–ground pressure cycles, with a bulging circumferential crack and a broken stringer. As a worst‐case scenario, it is assumed that double cracks start at the edge of a rivet hole both in the skin and in the stringer simultaneously. In the first stage, fatigue crack growth analysis is performed until the stringer is completely broken with the crack on the fuselage skin propagating. After the stringer is completely broken, the effect of bulging crack on the fatigue life of the panel is investigated utilizing the stress intensity factors determined by the three‐dimensional finite element analyses of the fuselage panel with the broken stringer. It is concluded that bulging of the skin due to the internal pressure can have significant effect on the stress intensity factor, resulting in fast crack propagation after the stringer is completely broken.     

Experimental and numerical study of composite lightweight structural insulated panel with expanded polystyrene core against windborne debris impacts

Natural disasters such as cyclone, hurricane, tornado and typhoon cause tremendous loss around the world. The windborne debris usually imposes high speed localized impact on the building envelope, which may harm people inside the building and create dominant openings. A dominant opening in the building envelope might cause internal pressure increasing and result in substantial damage to the building structures, such as roof lifting up or even collapse. To withstand the impact of such extreme event, the penetration resistant capacity of wall or roof panels to windborne debris impact should meet the requirements specified in the wind loading codes, e.g., the Australian Wind Loading Code (AS/NZS 1170.2:2011). In this study, a composite Structural Insulated Panel (SIP) with Extended Polystyrene (EPS) core sandwiched by flat metal skins that is commonly used in building industry was investigated. To study the structural response and penetration resistant capacity of the composite panel against windborne debris impacts, a series of laboratory tests were carried out by using a pneumatic cannon testing system. The effects of various specimen configurations, impact locations and debris impact velocities on their performance were investigated. The failure modes under various projectile impact scenarios were observed and compared by using two high-speed cameras. The dynamic responses were examined quantitatively in terms of the opening size, residual velocity of projectile, deformation and strain time histories on the back skin measured in the tests. The penetration resistance capacity of the panels subjected to windborne debris impact were examined and analyzed. In addition, numerical models were developed in LS-DYNA to simulate the response and damage of the composite SIP under windborne debris impact. Laboratory tested panels were first modeled. The test data was used to calibrate the accuracy of the numerical model. The validated numerical model was then used to conduct more numerical simulations to obtain more results such as energy absorption, impact force and vulnerability curve of the SIP against windborne debris impact.     

Fatigue failure characterisation of resistance-welded thermoplastic composites skin/stringer joints

An experimental investigation characterising the fatigue failure mechanisms of resistance-welded thermoplastic composites skin/stringer joints is presented. Unidirectional (UD) and quasi-isotropic adherends were welded using stainless steel meshes as heating elements. The specimen geometry consisted of a flange laminate, representing a stringer, welded onto a skin laminate. In order to avoid current leakage to the electrically conductive adherends, a ceramic-coated heating element (TiO₂ HE) was used for welding the UD specimens and some of the quasi-isotropic specimens. The fatigue performance of the welded joints was investigated under three-point bending. An indefinite fatigue life was obtained at 40% and 35% of the static damage initiation load for the UD and quasi-isotropic specimens, respectively. The failure mechanisms were documented based on observation of the fatigue cracks initiation and growth. UD specimens failed at the weld interface while quasi-isotropic specimens showed delaminations both in the flange or skin laminates and at the weld interface. The TiO₂ HE did not show any fatigue mechanical performance reduction. However, debonding at the weld interface was shown to occur between the metal mesh wires and the TiO₂ coating instead of between the laminates and the weld.     

Acoustic emission study of impact-damaged GRP pipes

Acoustic emission monitoring techniques have been used to investigate the effects of external damage on the structural integrity of GRP pipes. The pipes were 100 mm bore and 5.5 mm wall thickness, and were of wrapped chopped strand mat glass/polyester construction. Damage caused by severe impacts, of net energies up to 43 J, exerted a marked influence on the observed acoustic emissions from pipes subsequently pressurized to failure, but even though severe microstructural damage was caused by these impacts, they did not reduce the load-bearing capacity of the pipes in any way. The presence of stress concentrators in the form of axial saw-cuts resulted in a changing response to loading roughly in accordance with fracture mechanics predictions, and the results help to explain to some extent the apparently conflicting conclusions from the impact damage studies. Thus, although AE analysis was, for these materials, able to provide a clear indication of the presence of microstructural damage, it is clear that such indications can only be of value in detecting loss of structural integrity (load-bearing ability) when considered in conjunction with other information about the materials involved. Filament-wound pipes, for example, do not behave in the same manner. Amplitude analysis of the AE signals does not appear to be of significant value in giving indications of the nature of the impact damage.     

Detecting barely visible impact damage detection on aircraft composites structures

Composites have many advantages as aircraft structural materials and for this reason their use is becoming increasingly widespread. Fragility of composite material to impact loading limits their application in aircraft structures. In particular, low velocity impacts can cause a significant amount of delamination, even though the only external indication of damage may be a very small surface indentation. This type of damage is often referred to as barely visible impact damage (BVID), and it can cause significant degradation of structural properties. If the damaged laminate is subjected to high compressive loading, buckling failure may occur. Therefore, there is the need to develop improved and more efficient means of detecting such damage. In this work a new NDT approach is presented, based on the monitoring of the nonlinear elastic material behaviour of damaged material. Two methods were investigated: a single-mode nonlinear resonance ultrasound (NRUS) and a nonlinear wave modulation spectroscopy (NWMS). The developed methods were tested on different composite plates with unknown mechanical properties and damage size and magnitude.The presence of the nonlinearities introduced by the damage was clearly identified using both techniques. The results showed that the proposed methodology appear to be highly sensitive to the presence of damage with very promising future applications.