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Numerical stress analysis in adhesively bonded joints under thermo-mechanical loading
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Institute of Technology, Middle Technical University, Baghdad, Iraq.ORCID iD: 0000-0002-6995-2558
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-5210-4341
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0001-9649-8621
2020 (English)In: Advances in Mechanical Engineering, ISSN 1687-8132, E-ISSN 1687-8140, Vol. 12, no 10Article in journal (Refereed) Published
Abstract [en]

The objective of this work is to evaluate the effect of residual thermal stresses, arising after assembling a single-lap joint at elevated temperature, on the inelastic thermo-mechanical stress state in the adhesive layer. The numerical analysis (FEM) employing linear and non-linear material models, with geometrical nonlinearity accounted for, is carried out. Simulating the mechanical response, the calculated thermal stresses are assigned as initial conditions to polymeric, composite and metallic joint members to reflect the loading sequence where the mechanical strain is applied to cooled-down structure. It is shown that the sequence of application matters and simulations with simultaneous application of temperature and strain give different result. Two scenarios for adhesive joints with composites are studied: joining by adhesive curing of already cured composite parts (two-step process) and curing the adhesive and the composite simultaneously in one-step (co-curing). Results show that while in-plane stresses in the adhesive are higher, the peaks of out-of-plane shear stress and peel stress (most responsible for the joint failure) at the end of the overlap are reduced due to thermal effects. In joints containing composite parts, the one-step joining scenario is more favorable than the two-step. The ply stacking sequence in the composite has significant effect on stress concentrations as well as on the plateau value of the shear stress in the adhesive.

Place, publisher, year, edition, pages
Sage Publications, 2020. Vol. 12, no 10
Keywords [en]
Composites, single-lap joint, adhesive joints, thermo-mechanical load, residual thermal stresses, similar and dissimilar adherends, co-curing
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
URN: urn:nbn:se:ltu:diva-81227DOI: 10.1177/1687814020955072ISI: 000586331100001Scopus ID: 2-s2.0-85093875678OAI: oai:DiVA.org:ltu-81227DiVA, id: diva2:1479105
Note

Validerad;2020;Nivå 2;2020-11-23 (johcin)

Available from: 2020-10-26 Created: 2020-10-26 Last updated: 2020-12-10Bibliographically approved
In thesis
1. Comprehensive numerical analysis of stress state in adhesive layer of joint including thermal residual stress and material non-linearity
Open this publication in new window or tab >>Comprehensive numerical analysis of stress state in adhesive layer of joint including thermal residual stress and material non-linearity
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Omfattande numerisk analys av spänningstillstånd i limfogar inklusive termisk restspänning och icke-linjära material
Abstract [en]

The main objective of this work is to improve understanding of the stress state in the adhesive layer of bonded joints and identify key parameters which govern performance of adhesive joints. This information is crucial for the prediction of the failure initiation and propagation with the further estimation of the durability and strength of adhesively bonded structures.

A systematic numerical analysis of stress state in the adhesive layer of a single-lap and double- lap joint under various loading conditions (thermal and mechanical loading) and an alternative methodology to predict the direction for crack propagation within adhesive layer are presented in this thesis.  To identification of the most important parameters of joints is done based on the assessment of the peel and shear stress distributions in the adhesive layer. The thermal residual stresses arising after assembling of joints at elevated temperature are accounted for in the analysis.

Initially, accurate, realistic 3D finite element model with novel boundary conditions (displacement coupling) was developed and validated. The employed boundary conditions allow to eliminate the edge effect and simulate the behavior of an infinite plate of composite laminate with off-axis layers (monoclinic materials). It is also possible to decouple the edge effects induced by the finite specimen width from the interaction with ends of the joint overlap region. Due to these advanced setting it is possible to eliminate influence of some of the parameters as well as to reduce geometry of the model without losing precision. Thus, the model is optimized with respect to the number of elements as well as element size distribution and does not require excessive computational power to obtain accurate stress distributions even near to the possible sites with stress perturbations (e.g. corners, cracks, etc). Additionally to the geometrical parameters, various material models have been employed in simulations of adhesive joints. A linear and non-linear material models (adherend and adhesive) was used for the single-lap joint, while a linear material behavior was considered for double-lap joint. The geometrical non-linearity was also included in the analysis whenever required. To make results more general and applicable to a wide range of different joints the normalized (with respect to the thickness of adhesive layer) dimensions of joints were used. 

Depending on the analyzed type of joint (single- or double- lap), combination of similar and dissimilar (hybrid) materials for adherends are considered: a) metal-metal; b) composite-composite; c) composite-metal. In case of the composite adherend (carbon and/or glass fibers) different laminate lay-ups were selected: uni-directional ([08]T and [908]T) and quasi-isotropic ([0/45/90/-45]S and [90/45/0/-45]S). 

In general, discussion and conclusions concerning the importance of various joint parameters are based on the magnitude of the peel and shear stress concentration at the ends of the overlap. In order to identify general trends with respect to the influence of mechanical properties of adherends the master curves for shear and peel stresses are constructed and analyzed. 

To simulate effect of the residual thermal stresses on the behavior of joints different methods for assembly of joints were considered (using dedicated adhesive or employing co-curing method). The results of this investigation lead to the conclusions that the one of the most important factors affecting the simulation results is the sequences of application of thermo-mechanical loading for different assembly methods. It is shown that simple superposition of thermal and mechanical stresses (most common approach) in the adhesive layer works properly only for linear material but it gives inaccurate results if non-linear material is considered. The thesis demonstrates the appropriate way to combine thermal and mechanical loads to obtain correct stress distributions for any material (linear and non-linear). The analysis of the influence of residual thermal stresses has shown that the peel and shear stress concentration at the ends of overlap joint and the shear stress within the over-lap region are reduced due to thermal effect. In case of composite adherend the co-curing assembly method is more favorable (in terms of reducing stress concentrations) than using adhesive for joining the materials.

Finally, the simulation of the crack propagation within the adhesive layer for the bi-material (steel and composite) DCB sample with thick adhesive layer was carried out. The alternative to traditional fracture mechanics approach is proposed for the prediction of the crack path in the adhesive layer: a maximum hoop stress criterion. The hoop stress on the perimeters of a relatively large circle around the crack tip is evaluated to predict the direction of the crack extension with respect to the existing crack. The fracture mechanics is used to validate this approach and it is proved that if the Mode I is dominant for the crack propagation the hoop stress criterion be successfully used to predict crack path in the adhesive layer. This methodology is much more effective (in terms of required time and resources) than energy release based criterion or even X-FEM.

The main result of this thesis is a tool to obtain accurate stress distributions in the adhesive layer of joints. This tool provided better understanding of the behavior of adhesive joints and allowed to develop new approach for prediction of crack propagation in the adhesive layer. This is definitely a development in the design of stronger, more durable adhesive joints for lighter structural components.   

Place, publisher, year, edition, pages
Luleå University of Technology, 2021
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Single-lap adhesive joint, Double-lap adhesive joint, Unbalanced bi-material DCB, Residual thermal stresses, Finite element method, Crack path, Crack closure
National Category
Composite Science and Engineering Manufacturing, Surface and Joining Technology Applied Mechanics
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-81939 (URN)978-91-7790-732-9 (ISBN)978-91-7790-733-6 (ISBN)
Public defence
2021-02-26, E632, LTU Porsön, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2020-12-14 Created: 2020-12-10 Last updated: 2023-05-06Bibliographically approved

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Al-Ramahi, Nawres JJoffe, RobertsVarna, Janis

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