Resistance fail strain gage technology as applied to composite materials

Existing strain gage technologies as applied to orthotropic composite materials are reviewed. The bonding procedures, transverse sensitivity effects, errors due to gage misalignment, and temperature compensation methods are addressed. Numerical examples are included where appropriate. It is shown that the orthotropic behavior of composites can result in experimental error which would not be expected based on practical experience with isotropic materials. In certain cases, the transverse sensitivity of strain gages and/or slight gage misalignment can result in strain measurement errors

Advances in test and measurement of the interface adhesion and bond strengths in coating-substrate systems, emphasising blister and bulk techniques

In this paper, recent advances in the minimum-destructive testing of the adhesion of coating-substrate systems are reviewed, focusing on key techniques such as micro- and nano-scale levels of indentation, scratching, laser-induced wave shock, as well as the blister and buckle approach. Along with adhesion failure tests, the latest and most extensive applications of the adhesion test methods in nano-, micro- and bulk-coating technology and the associated techniques to determine the minimum damage defects left on the coatings are discussed and their use reviewed

J-Integral analysis of the mixed-mode fracture behaviour of composite bonded joints

The adhesive technology has been constantly growing and expanding into industrial environments, not only for traditional applications but also for high-end applications, where it has been competing fairly with the conventional connection technologies, such as welding, brazing, bolting and riveting. Its unique key features allow it to raise the type of technology to unreachable levels, for certain applications, by its competitors. Some of the advantages are the lightness of the adhesively-bonded joints, good behaviour under cycling and fatigue loading conditions, flexibility in bonding several types of materials and low stress concentrations. However, in order to design and develop efficient adhesively-bonded joints, the strength prediction must be accurate for the assessment of the fracture properties, mainly the critical energy release rate for tensile (JIC) and shear (JIIC), associated to the mode I and II, respectively. For most of the adhesively-bonded joints applications, the loading conditions under operational service feature a combination of different stresses, for instance tensile and shear stresses, from which the concept of mixed-mode came to exist. For this reason, the assessment of fracture properties under those conditions is essential, especially the energy release rates related to different mode-mixities. The fracture properties are related to Fracture Mechanics and are obtained through energetic analyses, from which three methods are often used: models based on the measurement of the crack length during the damage propagation, models based on an equivalent crack length and methods based on the Jintegral formulation. In the specific case of the J-integral it is furthermore possible to obtain the cohesive laws of the adhesive, which can be later used in the design of adhesively-bonded joints. This current work presents an experimental and numerical analysis of a Single-Leg Bending (SLB) adhesively-bonded joint where the specimens were bonded with three distinct adhesives, in order to assess and compare their behaviour under mixed-mode load conditions, fracture properties and cohesive laws. For that purpose, the J-integral formulation of Ji et al. [1] was considered to obtain the energy release rate for mode I and II, tensile (JI) and shear (JII), respectively, whereas the cohesive laws are attained through direct differential operation of the JI-w0 and JII-δ0 curves, where w0 and δ0 are the local normal separation and local tangential slip between the two adherends at the cross-section of the crack tip, respectively. Afterwards, the fracture analysis was performed, where the experimental results were compared through load-displacement (P-δ) curves. The JI and JII values, obtained through correlation between experimental and numerical results incorporated into the J-integral formulation, were addressed by R curves and fracture envelopes. These latter were used to establish which criterion was more suitable for each adhesive type. For last, the tensile and shear stresses were determined through the cohesive laws, attained by the direct method. Overall, a good agreement on the fracture properties was obtained between the specimens of the same adhesive. Moreover, the cohesive laws also presented a good correspondence between specimens, and further enabled the design of adhesively-bonded joints with arbitrary geometry.A tecnologia adesiva tem vindo a evoluir significativamente, expandindo-se para ambientes industriais, não apenas para aplicações convencionais, mas também para aplicações de elevada exigência, onde compete justamente com outras tecnologias de conexão tradicionais, como a soldadura, brasagem e ligações aparafusadas e rebitadas. As suas características únicas permitem elevar esta tecnologia para níveis inacessíveis, para certas aplicações, relativamente às suas concorrentes. Algumas das vantagens são o baixo peso das juntas adesivas, bom comportamento sob condições de cargas cíclicas e à fadiga, flexibilidade na construção da junta, possibilidade para ligar materiais diferentes e também baixa concentração de tensões. Contudo, a fim de projetar e desenvolver juntas adesivas eficientes, a previsão da resistência deve ser precisa para a avaliação das propriedades de fratura, principalmente a taxa crítica de libertação de energia em tração (JIC) e corte (JIIC), associada ao modo I e II, respetivamente. Na maioria das aplicações de ligações adesivas, as condições de carga cujas juntas estão sujeitas, sob condições de serviço operacional, consistem numa combinação de esforços distintos, como por exemplo tração e corte, a partir dos quais o conceito de modo misto foi criado. Por essa razão, é essencial a avaliação das propriedades de fratura sob essas condições, especialmente as taxas de libertação de energia relacionadas a diferentes modos mistos. As propriedades de fratura estão relacionadas com a Mecânica da Fratura e são obtidas através de análises energéticas, das quais são frequentemente utilizados três métodos: modelos baseados na medição do comprimento de fenda durante a propagação do dano, modelos baseados no comprimento de fenda equivalente e métodos baseados na formulação do integral J. No caso específico do método do integral J, é ainda possível obter as leis coesivas do adesivo, que podem ser utilizadas posteriormente no projeto de juntas adesivas. Nesta dissertação é apresentada uma análise experimental e numérica realizada a uma junta adesiva de configuração Single-Leg Bending (SLB) onde os provetes foram colados com três adesivos distintos, de modo a avaliar e comparar o seu comportamento sob condições de carga em modo misto, as suas propriedades à fratura e as respetivas leis coesivas. Para esse efeito, considerou-se a formulação proposta por Ji et al. [1] do método do integral J, de modo a determinar a taxa de libertação de energia para os modos I e II, tração (JI) e corte (JII), respetivamente, enquanto as leis coesivas foram obtidas por derivação direta das curvas JI-w0 e JII-δ0, onde w0 e δ0 correspondem à separação normal local e deslizamento tangencial local entre os dois aderentes na secção transversal da ponta da fenda, respetivamente. Posteriormente, foi realizada uma análise de fratura onde os resultados experimentais foram comparados, através de curvas carga-deslocamento (P-δ). Os valores de JI e JII, obtidos através da correlação de dados experimentais e numéricos incorporados na formulação do integral J, foram analisados pelas curvas R e envelopes de fratura. Estes últimos foram utilizados para estabelecer qual o critério mais apropriado para cada tipo de adesivo. Por fim, as tensões de tração e corte foram obtidas das leis coesivas, estimadas pelo método direto. No geral, foi conseguido um bom acordo entre as propriedades à fratura entre os provetes colados com o mesmo adesivo. Além disso, as leis coesivas apresentaram uma boa correspondência entre os provetes, possibilitando assim o projeto de justas adesivas de geometria arbitrária

Application of adhesive joints on a tensegrity floor: verification of technology and mechanical performance

openIl lavoro svolto ha avuto come obiettivo la verifica l’applicabilità della tecnologia adesiva su componenti edilizi innovativi. L’impiego degli adesivi strutturali rappresenta la traduzione costruttiva del principio della “semplificazione tecnologica”, ovvero della possibilità di realizzare prodotti industrializzabili con elevate prestazioni ed un numero limitato di componenti, riducendo le emissioni ambientali nelle loro fasi di vita. In particolare, è stata verificata la fattibilità tecnico-costruttiva di un solaio tensegrale in acciaio e vetro, sviluppando l’idea brevettuale del brevetto n. 00014426973, inventore Prof. P. Munafò. L’attività di ricerca si è sviluppata con la sperimentazione di diverse tipologie di giunti adesivi con aderendi in acciaio, alluminio, vetro e adesivi sia dopo maturazione in condizioni di laboratorio che dopo invecchiamento artificiale accelerato. È stato poi definito costruttivamente il giunto adesivo tra sottostruttura in acciaio e l’impalcato in vetro per rendere strutturalmente collaborante l’impalcato con la sottostruttura tensegrale. Costruito il prototipo di solaio si è passati alla verifica delle prestazioni meccaniche con prove di carico. Parallelamente all’attività sperimentale sono state condotte analisi numeriche sull’elemento costruttivo, per verificare le ipotesi assunte. Il risultato ottenuto ha validato l’assunto alla base dell’idea brevettuale validando le previsioni dell’analisi numerica. In-fatti, dalle prove di carico si è riscontrato un significativo incremento della rigidezza del solaio grazie alla giunzione adesiva tra impalcato in vetro e sottostruttura tensegrale.The aim of the present work was to examine the applicability of adhesive bonding technology to innovative building components. The use of structural adhesives represents the constructive implementation of the principle of “technological simplification”, that is, the possibility of assembling industrially manufactured products with high performance and a limited number of components, reducing environmental emissions during their life phases. In particular, the technical-constructive feasibility of a steel and glass tensegrity floor was verified, developing the idea of Patent No. 00014426973 (inventor Prof. P. Munafò). The research activity developed with the testing of different types of adhesive joints with steel, aluminium, glass, and adherends both after curing in laboratory conditions and after artificial accelerated ageing. The adhesive joint between the steel substructure and the glass deck was then determined by design to allow the deck to structurally cooperate with the tensegrity substructure. The prototype of the floor was assembled, and the mechanical performance was verified by load tests. In parallel with the experiments, numerical analyses were performed on the structural element to verify the hypotheses adopted. The results obtained confirmed the assumption underlying the patent idea by validating the predictions of the numerical analysis. In fact, the load tests showed a significant increase in the stiffness of the floor thanks to the adhesive joint between the glass deck and the tensegrity substructure.INGEGNERIA CIVILE, AMBIENTALE, EDILE E ARCHITETTURAopenMarchione, Francesc

Modelling degradation in adhesive joints subjected to fluctuating service conditions

Adhesive joining is an attractive alternative to conventional joining methods, such as welding and mechanical fastening. The benefits of adhesive bonding include: the ability to form lightweight, high stiffness structures; joining of different types of materials; better fatigue performance, and reduction in the stress concentrations or the effects of the heat associated with welding. However, concerns about the durability of adhesive joints still hinder their widespread use in structural applications. Moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying moisture conditions throughout their useful life. The aim of this research is to develop models to predict degradation in adhesive joints under varying moisture conditions. This was achieved by a combination of experimental and numerical methods. Experiments were carried out to characterise the moisture uptake and mechanical properties of the single part epoxide adhesive, FM73-M. Single lap joints were manufactured from aluminium alloy 2024 in heat treated (T3) and non heat treated (O) states using the FM73-M, BR127 adhesive-primer system. Tensile testing of the single lap joints was carried out after the joints had been exposed to hot-wet conditioning environments. Models were developed for predicting moisture concentration in the adhesive under cyclic moisture absorption and desorption conditions. A finite element based methodology incorporating moisture history was developed to predict the cyclic moisture concentration. In the next step, a novel finite element based methodology, which was based on moisture history effects, was developed to determine stresses in bonded joints after curing, conditioning and tensile testing. In the final step, a moisture history dependent cohesive zone element based damage and failure criterion was introduced to predict damage initiation, crack growth and failure under variable moisture and temperature conditions. The methodology proposed in this work and its implementation by finite element method provides a systematic approach for determining the degradation in adhesive joints under varying environmental conditions and accomplishes the aim of this research

Modelling degradation in adhesive joints subjected to fluctuating service conditions

Adhesive joining is an attractive alternative to conventional joining methods, such as welding and mechanical fastening. The benefits of adhesive bonding include: the ability to form lightweight, high stiffness structures; joining of different types of materials; better fatigue performance, and reduction in the stress concentrations or the effects of the heat associated with welding. However, concerns about the durability of adhesive joints still hinder their widespread use in structural applications. Moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying moisture conditions throughout their useful life. The aim of this research is to develop models to predict degradation in adhesive joints under varying moisture conditions. This was achieved by a combination of experimental and numerical methods. Experiments were carried out to characterise the moisture uptake and mechanical properties of the single part epoxide adhesive, FM73-M. Single lap joints were manufactured from aluminium alloy 2024 in heat treated (T3) and non heat treated (O) states using the FM73-M, BR127 adhesive-primer system. Tensile testing of the single lap joints was carried out after the joints had been exposed to hot-wet conditioning environments. Models were developed for predicting moisture concentration in the adhesive under cyclic moisture absorption and desorption conditions. A finite element based methodology incorporating moisture history was developed to predict the cyclic moisture concentration. In the next step, a novel finite element based methodology, which was based on moisture history effects, was developed to determine stresses in bonded joints after curing, conditioning and tensile testing. In the final step, a moisture history dependent cohesive zone element based damage and failure criterion was introduced to predict damage initiation, crack growth and failure under variable moisture and temperature conditions. The methodology proposed in this work and its implementation by finite element method provides a systematic approach for determining the degradation in adhesive joints under varying environmental conditions and accomplishes the aim of this research

Solid rocket motor internal insulation

Internal insulation in a solid rocket motor is defined as a layer of heat barrier material placed between the internal surface of the case propellant. The primary purpose is to prevent the case from reaching temperatures that endanger its structural integrity. Secondary functions of the insulation are listed and guidelines for avoiding critical problems in the development of internal insulation for rocket motors are presented

Progress of Fiber-Reinforced Composites

Fiber-reinforced composite (FRC) materials are widely used in advanced structures and are often applied in order to replace traditional materials such as metal components, especially those used in corrosive environments. They have become essential materials for maintaining and strengthening existing infrastructure due to the fact that they combine low weight and density with high strength, corrosion resistance, and high durability, providing many benefits in performance and durability. Modified fiber-based composites exhibit better mechanical properties, impact resistance, wear resistance, and fire resistance. Therefore, the FRC materials have reached a significant level of applications ranging from aerospace, aviation, and automotive systems to industrial, civil engineering, military, biomedical, marine facilities, and renewable energy. In order to update the field of design and development of composites with the use of organic or inorganic fibers, a Special Issue entitled “Progress of Fiber-Reinforced Composites: Design and Applications” has been introduced. This reprint gathers and reviews the collection of twelve article contributions, with authors from Europe, Asia and America accepted for publication in the aforementioned Special Issue of Applied Sciences

A new mixed model based on the enhanced-Refined Zigzag Theory for the analysis of thick multilayered composite plates

The Refined Zigzag Theory (RZT) has been widely used in the numerical analysis of multilayered and sandwich plates in the last decay. It has been demonstrated its high accuracy in predicting global quantities, such as maximum displacement, frequencies and buckling loads, and local quantities such as through-the-thickness distribution of displacements and in-plane stresses [1,2]. Moreover, the C0 continuity conditions make this theory appealing to finite element formulations [3]. The standard RZT, due to the derivation of the zigzag functions, cannot be used to investigate the structural behaviour of angle-ply laminated plates. This drawback has been recently solved by introducing a new set of generalized zigzag functions that allow the coupling effect between the local contribution of the zigzag displacements [4]. The newly developed theory has been named enhanced Refined Zigzag Theory (en- RZT) and has been demonstrated to be very accurate in the prediction of displacements, frequencies, buckling loads and stresses. The predictive capabilities of standard RZT for transverse shear stress distributions can be improved using the Reissner’s Mixed Variational Theorem (RMVT). In the mixed RZT, named RZT(m) [5], the assumed transverse shear stresses are derived from the integration of local three-dimensional equilibrium equations. Following the variational statement described by Auricchio and Sacco [6], the purpose of this work is to implement a mixed variational formulation for the en-RZT, in order to improve the accuracy of the predicted transverse stress distributions. The assumed kinematic field is cubic for the in-plane displacements and parabolic for the transverse one. Using an appropriate procedure enforcing the transverse shear stresses null on both the top and bottom surface, a new set of enhanced piecewise cubic zigzag functions are obtained. The transverse normal stress is assumed as a smeared cubic function along the laminate thickness. The assumed transverse shear stresses profile is derived from the integration of local three-dimensional equilibrium equations. The variational functional is the sum of three contributions: (1) one related to the membrane-bending deformation with a full displacement formulation, (2) the Hellinger-Reissner functional for the transverse normal and shear terms and (3) a penalty functional adopted to enforce the compatibility between the strains coming from the displacement field and new “strain” independent variables. The entire formulation is developed and the governing equations are derived for cases with existing analytical solutions. Finally, to assess the proposed model’s predictive capabilities, results are compared with an exact three-dimensional solution, when available, or high-fidelity finite elements 3D models. References: [1] Tessler A, Di Sciuva M, Gherlone M. Refined Zigzag Theory for Laminated Composite and Sandwich Plates. NASA/TP- 2009-215561 2009:1–53. [2] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. Assessment of the Refined Zigzag Theory for bending, vibration, and buckling of sandwich plates: a comparative study of different theories. Composite Structures 2013;106:777–92. https://doi.org/10.1016/j.compstruct.2013.07.019. [3] Di Sciuva M, Gherlone M, Iurlaro L, Tessler A. A class of higher-order C0 composite and sandwich beam elements based on the Refined Zigzag Theory. Composite Structures 2015;132:784–803. https://doi.org/10.1016/j.compstruct.2015.06.071. [4] Sorrenti M, Di Sciuva M. An enhancement of the warping shear functions of Refined Zigzag Theory. Journal of Applied Mechanics 2021;88:7. https://doi.org/10.1115/1.4050908. [5] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses, Ibiza, Spain: 2013. [6] Auricchio F, Sacco E. Refined First-Order Shear Deformation Theory Models for Composite Laminates. J Appl Mech 2003;70:381–90. https://doi.org/10.1115/1.1572901