Analysis of mechanically fastened composite joints by boundary element methods

This article focuses on the boundary element formulation development for analyzing a mechanically bolted composite. Boundary equations are formulated for all the member panels of the composite joints. These equations are solved together with the fastener equations to get the resultant contact forces for all the fasteners involved. The fasteners are then modeled as 1D springs that are governed by linear relationship between the fastener forces and the displacements of member panels at the respective fastener centers. After obtaining all the fastener forces from the global analysis, detailed stress analysis is performed for region around an individual fastener. The stress distributions around fastener holes are then used to evaluate the margin of safety of the composite panels. The numerical predictions on the fastener forces, failure modes and failure loads of two typical bolted composite joints using the proposed method agree well with that of the experimental results.     

Contact analysis of mechanically fastened joints in composite laminates by linear complementarity problem formulation

The contact phenomena and stress distributions in the vicinity of the hole on mechanically fastened joints are investigated in composite laminates exhibiting nonlinear elastic behavior. The effects of fastener stiffness, friction force between the fastener and the composite laminates, geometrical nonlinearities due to concentrated fastener load around the hole edge for the snug and clearance-fit in the [0 °]_S, [± 45 °]_S, [0°/± 45 °]_S and laminates are also considered. An efficient numerical procedure is developed for the solution of the contact problems with Coulomb friction between the fastener and the hole based on a linear complementarity problem (LCP) formulation in an incremental form. A nonlinear finite element code for stress analysis of laminated composites using Lemke's complementary pivoting algorithm is developed for solving the LCP. The nonlinear analysis is composed of the combination of updated Lagrangian formulation of the frictional contact problem and the nonlinear shear stress-strain relation in each ply. The accuracy, applicability and computational efficiency of the proposed method are confirmed by comparison with previous researches.     

Damage mode characterization of mechanically fastened composite joints using carbon nanotube networks

The practice of upgrading metal parts with composites in large structures has led to an increased use of composite joints, particularly mechanical fastenings, due to ease of assembly and replacement. A drawback of mechanical joints is that damage is difficult to detect visually. In this research, an embedded carbon nanotube network has been used to modify the conductivity of bolted composite joints. In situ electrical resistance measurements in conductive composites have potential to provide quantitative evidence of damage as well as insight into the type of damage which occurs during tensile loading. We demonstrate that the electrical resistance of a bolted composite joint is more sensitive to certain modes of damage (e.g., matrix cracking and delamination) than others (bearing and shear-out), due mostly to the varying amount of void space created, thus proving the potential of an embedded carbon nanotube network in the health monitoring of mechanically fastened cross-ply composites.     

A mechanically fastened composite laminate joint and progressive failure analysis

A comprehensive procedure for a mechanically fastened composite laminate joint (ASTM D5961 Proc. A, B) is demonstrated from fixture design to analysis of test results. The ASTM tests are applied to evaluate the standard laminate properties and the composite joints. Composite laminate mechanical joints were analyzed using the finite element method (FEM), and the results were compared to test results. A progressive failure analysis (PFA) was applied to the FEM to predict the overall failure behavior of the test specimens. Three laminate failure theories – maximum stress, maximum strain, and Tsai–Wu – were applied to the PFA to predict the test failure load, displacement and strength. The PFA method was suitable to predict the initial test range of test and maximum test load except for the excessive failure area.     

Thermo-mechanical fatigue analysis of liquid shim in mechanically fastened hybrid joints for aerospace applications

As resin-infused composite laminates continue to replace alloy components in modern commercial aircraft, liquid-shim is often necessary to compensate for assembly tolerances occurring in mechanically-fastened primary structures. This paper evaluates the structural performance of aerospace-grade liquid-shim in double-bolt composite-aluminium alloy ‘hybrid’ joints representative of multi-fastener joints found in the primary structures of modern commercial aircraft. Hybrid joints with liquid-shim were subjected to simultaneous thermal and mechanical-fatigue (TMF) loading conditions representative of those experienced over the lifetime of a commercial aircraft. The joints were subjected to 200,000 fatigue load cycles at −59 °C and 100,000 fatigue load cycles at +85 °C to simulate service conditions. Key variables investigated were shim material and shim thickness. The main conclusions from this study are: (i) there was no degradation of the liquid-shim in terms of a loss of mechanical stiffness and, secondly, no significant damage was observed on the bearing-plane; (ii) a reduction in joint stiffness due to the presence of liquid-shim was observed using high magnification 2D digital image correlation (DIC) and the reduction was dependent on the thickness of liquid-shim employed; and (iii) joints with second generation liquid-shim exhibited marginally lower stiffness relative to joints manufactured with third generation liquid shim.     

Elastic analysis of hybrid bonded joints and bonded composite repairs

The authors extend the closed-form bonded joint linear elastic analysis method of Delale et al. [Delale F, Erdogan F, Aydinoglu MN. Stresses in adhesively bonded joints: a closed-form solution. J Compos Mater 1981;15:249–71] and Bigwood and Crocrombie [Bigwood DA, Crocombe AD. Elastic analysis and engineering design formulae for bonded joints. Int J Adhes Adhes 1989;9(4):229–42] to include the composite deformation mechanisms and the thermal residual strains that arise in hybrid metal-composite joints such as those presented by bonded composite repairs applied to metallic aircraft structures. The analytical predictions for the adhesive stresses and the compliance are compared to the results of a linear elastic finite element model that has itself been validated by comparison with experimental results. The results are applied to the problem of coupled linear extension and bending of a bonded composite repair applied to a cracked aluminum substrate. The resulting stress intensity factor and crack-opening displacement in the repaired plate are compared to the results of a three-dimensional finite element analysis, and also exhibit excellent results. Throughout the text, observations are made regarding the practical application of the results to failure prediction in hybrid joints, whereby the authors demonstrate the need for consistency in the analytical methods used to determine the fatigue and failure of composites from the coupon level to the analysis of the final structural details.     

Experimental investigation of composite lockbolt fastened joints under in-plane low velocity impact

Two different composite fastened configurations, i.e. the filled hole and the single-lap double-fastener joint, are experimentally investigated in tensile mode through different loading rates. The composite material system is the UD carbon/epoxy AS4/8552 and the coupons are fastened with titanium countersunk lockbolts. The experiments are performed in a range from quasi-static to 2.8 m/s impact velocity, using an innovative testing device developed and adapted in a drop tower machine. The main experimental observations are the limited loading rate sensitivity in terms of strength for both tested configurations, the elevated absorbed energy values in the dynamic tests of the lap joint samples, as well as the differences in their failure evolution and modes between quasi-static and impact loading.     

Numerical investigation on strength design and curling effect of mechanically fastened joints in cold-formed austenitic stainless steel

A variety of parametric studies utilizing the finite element analysis (FEA) have been performed by Kim et al in order to predict the mechanical behavior (ultimate strength) of single shear bolted joints in cold-formed (thin-walled) stainless steel sheet. Strength equations considering the strength reduction by curling effect in bolted joint with long end distance and edge distance have been suggested. The applicability of numerical fracture analysis has been already verified through the comparisons of ultimate strength and curling occurrence between the test results conducted by Kuwamura et al. and FEA results. The precedent researches have been conducted with respect to the mechanical behaviors of single shear bolted joints fabricated with four bolts (2 columns × 2 rows bolt arrangement) and two bolts (1 column × 2 rows bolt arrangement) for extended variables such as plate thickness, end distance parallel to the direction of load and edge distance perpendicular to the direction of load. Subsequently, this study has been focused on the ultimate behaviors and the influence of curling on ultimate strength in bolted joints fabricated with one bolt. FE models are assumed with varying end distance, edge distance and plate thickness. It is found that the design manual of Stainless Steel Building Association of Japan (SSBA) is more reasonable for estimating the ultimate strengths of bolted joints with no curling compared with ultimate strengths predicted by American Society of Civil Engineers (ASCE) standard or American Iron and Steel Institute (AISI) standard (North American specification). Whereas, for fastened joints with severe local buckling or out of plane deformation(hereafter, curling) like previous results about other bolt arrangements, even SSBA manual tended to overestimate the ultimate strength of joint. Also, it has been known that the conditions of curling occurrence differ from the above stated variables. Therefore, revised strength equations considering the strength reduction by curling for one bolted joint were suggested in this paper.     

The development of mode I, linear-elastic stress intensity factor solutions for cracks in mechanically fastened joints

Accurate representation of crack tip stress intensity is an essential part of the assessment of the damage tolerance capability of aerospace structures. In typical applications, mode I stress intensity solutions are taken as factorial combinations of the fundamental form and various boundary correction factors for any given geometry and applied load. Total solutions are then developed by superimposing individual solutions for various load conditions. This technique is discussed for typical structure involving loaded fastener holes. Tabulated and plotted results are presented for cracks emanating from both open and loaded holes in finite width plates, lugs and multi-fastener joints. In the case of the open hole and lug solutions, the results are compared with published finite element values. The expressions presented herein lend themselves to programming on a digital computer. Once established, they afford the engineer the ability to develop accurate K_I solutions for a wide range of applications which are typical of the geometry/loading configurations encountered in the damage tolerance analysis of mechanically fastened joints. Equally as important, they offer accuracy comparable to that obtained with finite element modeling at a fraction of the cost.     

复合材料缠绕接头设计

复合材料接头由于其轻质、高强及可设计性好等特点,被越来越多地运用于国外大型民用飞机的结构设计中.本文通过有限元建模,对一种新型的复合材料缠绕接头在拉伸和压缩载荷下的力学响应进行研究.主要围绕基础型、梭型和锥型三种典型的接头形式,比较了不同构型对缠绕接头力学性能的影响.结果表明,三种典型构型的复合材料缠绕接头均具有较好的...     

Inverse identification of a bearing-stress-interface-slip relationship in mechanically fastened FRP laminates

The use of mechanically-fastened fibre-reinforced polymer (MF-FRP) strips has been recently proposed as a possible alternative solution to the most common externally-bonded (EB) sheets and laminates. Although several applications of MF-FRP strengthening on reinforced concrete (RC) structures are already available, further experimental and theoretical studies are needed for both achieving a thorough knowledge of their mechanical behaviour and formulating sound design rules.     

Slip effects in reinforced concrete beams with mechanically fastened FRP strip

This investigation concerns the flexural behavior of reinforced concrete (RC) beams strengthened with a mechanically fastened pultruded FRP strip (MF-FRP beams). Twelve small size MF-FRP beams and two control RC beams were tested under flexural loading. The main failure mode observed in this experimental program was nail rotation and bearing damage under increasing flexural load, which resulted in FRP slip with respect to the soffit of the RC-beam and loss of stress transfer. Strain gage data and visual observations obtained during the experiments provided useful insight for developing a new procedure for estimating the nominal moment capacity of the MF-FRP beams. The proposed method is guided by experimental evidence pointing to the significance of nail rotation associated with flexural cracking in RC beams. The developed procedure, based on a “strain reduction factor” of 24%, is able to estimate the nominal moment capacity of the MF-FRP beams with good accuracy.     

Design and validation of composite patch repairs to cracked metallic structures

This paper describes an approach to a patch design for a crack repair, based largely on the simple closed-form analyses. For simplicity, the design approach does not include the effect of the out-of-plane bending nor polygon-shaped patch. However, once the basic design was brought out by that design approach, it can be re-analysed by using other advanced analytical models to account for those latter complexities. These advanced analytical methods are therefore also reviewed briefly in the paper. In addition, test data to validate the mentioned design and analysis methods are also presented and discussed.     

Modeling of flexural behavior of RC beams strengthened with mechanically fastened FRP strips

An alternative to fiber reinforced polymer (FRP) materials adhesively bonded to the concrete substrate is the implementation of mechanically fastened FRP (MF-FRP) systems using steel anchors to secure the laminate to the substrate. The benefit of MF-FRP, compared to adhesive bonding for FRP flexural strengthening, is the speed of installation with unskilled labor, minimal or absent surface preparation under any meteorological condition and immediate use of the strengthened structures. Some of the potential shortcomings are: possible concrete damage during anchoring and limited opportunity of installation in the presence of congested internal reinforcement in the members to be strengthened. Laboratory testing and a number of field applications have shown the effectiveness of the MF-FRP method. In this paper, an analytical model is discussed for reinforced concrete (RC) members strengthened with MF-FRP strips. The model accounts for equilibrium, compatibility and constitutive relationships of the constituent materials; in particular, it accounts explicitly for the slip between the substrate surface and the FRP strip due to the behavior of the fasteners. The proposed flexural model, coupled with the computation algorithm, is able to predict the fundamentals of the behavior of RC flexural members strengthened with MF-FRP strips, in terms of both ultimate and serviceability limit states. Comparisons between the analytical predictions and the experimental results have been successfully performed.     

Fatigue crack closure analysis of bridged cracks representing composite repairs

This article presents an analytical and numerical study of the fatigue crack‐closure behaviour of a bridged crack representing a crack that has been repaired by a composite patch. It is shown that, provided that the plate stress beneath the patch is less than 40% of the material’s yield stress, the crack‐closure stress of a patched crack is approximately equal to that of an unbridged crack under small‐scale yielding, depending only on the stress ratio. Furthermore, it is shown that the transient crack‐closure behaviour of a patched crack subjected to variable amplitude loading can be determined by analysing an unpatched crack subjected to the same stress intensity factor history. Based on these findings, it is proposed that the fatigue crack closure of a patched crack can be determined by analysing an unpatched centre crack subjected to an adjusted stress, for which an explicit expression is given. Predictions based on the proposed method are shown to correlate very well with experimental results obtained under two aircraft loading spectra.     

Analysis and design of RC structures strengthened with mechanically fastened FRP laminates: A review

The use of Mechanically Fastened Fiber Reinforced Polymer (MF-FRP) laminates is emerging as a viable alternative to adhesively bonded FRP laminates for the rehabilitation of reinforced concrete (RC) members such as beams and slabs. A recently published state-of-the-art review of the experimental research has demonstrated the viability and effectiveness of MF-FRP systems. This paper provides a state-of-the-art review of the analytical and numerical studies performed over the last decade with the aim of: (a) predicting the strength, the load-deformation response and the failure mode of rehabilitated RC members, and (b) accounting for the interfacial behavior between the concrete and the MF-FRP laminate. Ultimate strength models and constitutive models are critically reviewed based on their key assumptions and formulations and compares the analytical predictions with previously reported experimental results.     

XFEM fracture analysis of cracked pipeline with and without FRP composite repairs

Abstract

The effect of fiber reinforced polymer composite (FRPC) repair on crack propagation in thin-walled steel pipes is examined. The extended finite element method is used in this study to simulate a pressurized cylindrical pipe with longitudinal crack in two conditions: the original cracked pipe and the pipe repaired with a composite patch. Carbon/epoxy or E-glass/epoxy FRP in two different fiber orientations are assumed for cracked pipe repair. Performance of four types of FRP repair systems are investigated by CTOA, COD and COA fracture criteria for both the pipe integrity assessment and the potential age of leak before break criterion.     

A review of the strength of joints in fibre-reinforced plastics: Part 1. Mechanically fastened joints

Published work, mostly experimental, relating to all aspects of screwed, riveted and bolted joints in glass and carbon fibre-reinforced epoxy and polyester resin is reviewed. The work is subdivided into the effects of material parameters, fastener parameters and design parameters. Numerical data are collated and, where possible, general principles and design guidelines presented.     

Development of life extension strategies for Australian military aircraft, using structural health monitoring of composite repairs and joints

The DSTO Centre of Expertise for Structural Mechanics (COE-SM) has recently developed methodologies for simulating structural health monitoring (SHM) systems for adhesively bonded composite repairs to Australian military aircraft. System design, interrogation strategy, and sensor placement are discussed, with particular emphasis on the development of techniques for embedding optical fibre sensors for optimal SHM system response.     

Adhesively-bonded joints and repairs in metallic alloys, polymers and composite materials: Adhesives, adhesion theories and surface pretreatment

In the present paper, the following topics are reviewed in detail: (a) the available adhesives, as well as their recent advances, (b) thermodynamic factors affecting the surface pretreatments including adhesion theories, wettability, surface energy, (c) bonding mechanisms in the adhesive joints, (d) surface pretreatment methods for the adhesively bonded joints, and as well as their recent advances, and (e) combined effects of surface pretreatments and environmental conditions on the joint durability and performance. Surface pretreatment is, perhaps, the most important process step governing the quality of an adhesively bonded joint. An adhesive is defined as a polymeric substance with viscoelastic behavior, capable of holding adherends together by surface attachment to produce a joint with a high shear strength. Adhesive bonding is the most suitable method of joining both for metallic and non-metallic structures where strength, stiffness and fatigue life must be maximized at a minimum weight. Polymeric adhesives may be used to join a large variety of materials combinations including metal-metal, metal-plastic, metal-composite, composite-composite, plastic-plastic, metal-ceramic systems. Wetting and adhesion are also studied in some detail in the present paper since the successful surface pretreatments of the adherends for the short- and long-term durability and performance of the adhesive joints mostly depend on these factors. Wetting of the adherends by the adhesive is critical to the formation of secondary bonds in the adsorption theory. It has been theoretically verified that for complete wetting (i.e., for a contact angle equal to zero), the surface energy of the adhesive must be lower than the surface energy of the adherend. Therefore, the primary objective of a surface pretreatment is to increase the surface energy of the adherend as much as possible. The influence of surface pretreatment and aging conditions on the short- and long-term strength of adhesive bonds should be taken into account for durability design. Some form of substrate pretreatment is always necessary to achieve a satisfactory level of long-term bond strength. In order to improve the performance of adhesive bonds, the adherends surfaces (i.e., metallic or non-metallic) are generally pretretead using the (a) physical, (b) mechanical, (c) chemical, (d) photochemical, (e) thermal, or (e) plasma method. Almost all pretreatment methods do bring some degree of change in surface roughness but mechanical surface pretreatment such as grit-blasting is usually considered as one of the most effective methods to control the desired level of surface roughness and joint strength. Moreover, the overall effect of mechanical surface treatment is not limited to the removal of contamination or to an increase in surface area. This also relates to changes in the surface chemistry of adherends and to inherent drawbacks of surface roughness, such as void formations and reduced wetting. Suitable surface pretreatment increases the bond strength by altering the substrate surface in a number of ways including (a) increasing surface tension by producing a surface free from contaminants (i.e., surface contamination may cause insufficient wetting by the adhesive in the liquid state for the creating of a durable bond) or removal of the weak cohesion layer or of the pollution present at the surface, (b) increasing surface roughness on changing surface chemistry and producing of a macro/microscopically rough surface, (c) production of a fresh stable oxide layer, and (d) introducing suitable chemical composition of the oxide, and (e) introduction of new or an increased number of chemical functions. All these parameters can contribute to an improvement of the wettability and/or of the adhesive properties of the surface.