Estimation of Residual Stresses in Laminated Composites by Slitting Method Utilizing Eigen Strains

The manufacturing parameters such as curing process cause residual stresses in polymeric laminated composites. Therefore, an accurate method of measurement of residual stresses is essential for the design and analysis of composites structures. The slitting method is recently used for measurement of the residual stresses in laminated composites. However, this method has some drawbacks such as high sensitivity to noise of measurements and high scattering in the final results, which necessitate using of normalization techniques. Moreover, the form of polynomials, used in the conventional slitting method for calculation of the stiffness matrix, has a significant effect on final results. In this paper, it is shown that the major reason of the drawbacks of the slitting method in calculating the residual stresses is a direct use of the elastic released strains recorded by strain gages. In the present study, instead of direct calculation of residual stresses from the elastic released strains, eigen strain distribution as a constant and invariant field has been calculated from the recorded elastic strains. Then, by using the calculated eigen strain field in a finite-element model, the residual stress filed was obtained. Also, instead of using polynomials to calculate the compliance, a superposition method was used. The results show that the new method decreases the sensitivity of the final results to noise and scattering of the experimental data. It means that the normalization methods are not needed any more

Assessment of the thermomechanical performance of continuous glass fiber-reinforced thermoplastic laminates

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.polymertesting.2018.02.023 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The effects of temperature on the static tensile behavior of continuous E-glass/polyamide laminates were studied in order to assess the feasibility of using the material system for structural applications. Uniaxial tensile tests were conducted on [0]8, [90]8, [02/902]s and [04/904]s laminates at multiple temperatures above and below the glass transition temperature, which was measured using different methods. Optical and scanning electron microscopy were performed on the tested samples, and the effects of temperature on failure modes were investigated. The [0]8 and [90]8 laminates displayed three reduction stages in modulus versus temperature, where the largest reduction was in the glass transition region as a result of notable softening of the polyamide matrix, as confirmed by fractographic analysis. However, the [02/902]s and [04/904]s laminates displayed the largest modulus reduction prior to the glass transition temperature with little reduction beyond, which was attributed to matrix softening coupled with in situ ply constraining effects.Tarbiat Modares UniversityUniversity of Waterlo

Processability and tensile performance of continuous glass fiber/polyamide laminates for structural load-bearing applications

The final publication is available at Elsevier via https://doi.org/10.1016/j.compositesa.2017.11.010. © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The performance of continuous E-glass/polyamide 6 laminates processed using distinct hot press moulding cycles was assessed and compared with similar E-glass/epoxy and E-glass/polypropylene laminates. The effects of peak processing temperature, preheating time, and temperature dwell time on laminate consolidation and quality were observed using optical and scanning electron microscopy. Corresponding quasi-static tensile tests were performed on [0]8, [90]8, [02/902]s, [04/904]s and [±45]2s laminates. Compared to E-glass/epoxy composites, the [0]8 specimens presented a similar strength, while the [90]8 specimens exhibited a much lower strength due to weaker fiber/matrix adhesion. Conversely, the E-glass/polyamide cross-ply laminates had a markedly higher strength while exhibiting the same modulus. This is because of higher toughness; the polyamide matrix provides as was proved by higher transverse matrix cracking strain of E-glass/polyamide. These findings support the feasibility of producing cost-effective and high-quality E-glass/polyamide laminates for use in high-performance applications, which is an attractive alternative to more conventional glass/epoxy laminates.Tarbiat Modares University and the University of Waterloo are greatly acknowledged for funding in support of the study

Full-Field Measurement of Residual Stresses in Composite Materials Using the Incremental Slitting and Digital Image Correlation Techniques

Background The slitting method is a widely used destructive technique for the determination of residual stresses. Because of the rich data content of the full-field methods, optical techniques such as digital image correlation (DIC) are replacing strain gages for surface measurements. Objective The objective of the current paper is to overcome the difficulties that arise in using the DIC technique combined with the slittingmethod. The present noise, low signal-to-noise ratio, and systematic errors are themain impediments to the use of DIC in the slitting method. Methods An approach based on the eigenstrain concept was exploited to ascertain the optimum region of interest (ROI) for the analysis. After that, a robust procedure was implemented to utilize the DIC method while excluding the rigid body motion and rotation artifacts from the obtained displacements. Results Different slitting steps may cause dissimilar rigid bodymotions and rotations of the specimen. The proposedmethod was able to eliminate all of these different shears and stretches in the images simultaneously. The slitting experiment was conducted on a symmetric cross-ply composite specimen, and the slit progressed down to half the thickness. Although some rigid body motions were large, the method managed to exclude all of them for eight slitting steps. Conclusion A comparison made between the results of the current method and those of the strain gage technique shows that they are in acceptable agreement with each other, and this full-field method can be extended to smaller scales or other destructive techniques

Assessment of failure toughening mechanisms in continuous glass fiber thermoplastic laminates subjected to cyclic loading

The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.compositesb.2018.10.065 © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Tensile fatigue behaviour of glass fiber/polyamide composites, including unidirectional ([0]8, [90]8) and cross-ply ([02/902]s, [04/904]s and [904/04]s) laminates, was studied and compared to that of similar glass fiber/epoxy composites. The fatigue resistance of cross-ply glass/polyamide was greater than that of glass/epoxy while also exhibiting lower stiffness reduction. To explain this key observation, residual stiffness and residual strength fatigue tests were performed on cross-ply laminates, while optical microscopy was used to measure ply crack density during the different stages of cycling. Testing of the cross-ply laminates at lower peak stresses of 50% of the ultimate tensile strength (i.e., high cycle fatigue regime) revealed partial cracks that did not propagate completely through the width and thickness of plies due to high matrix toughness and other observed toughening mechanisms such as matrix bridging. A micromechanical finite element model with explicit ply cracks was also used to predict laminate stiffness degradation corresponding to observed ply crack densities, revealing that stiffness degradation was overpredicted when cracks were assumed to span the entire specimen width. Additional finite element simulations with partial cracks showed notably less stiffness reduction. These observations suggest glass/polyamide is inherently more damage tolerant than glass/epoxy and may be a suitable replacement for fatigue critical structures.University of Waterlo

Structure-Sensitive Mechanism of Nanographene Failure

The response of a nanographene sheet to external stresses is considered in terms of a mechanochemical reaction. The quantum chemical realization of the approach is based on a coordinate-of-reaction concept for the purpose of introducing a mechanochemical internal coordinate (MIC) that specifies a deformational mode. The related force of response is calculated as the energy gradient along the MIC, while the atomic configuration is optimized over all of the other coordinates under the MIC constant-pitch elongation. The approach is applied to the benzene molecule and (5, 5) nanographene. A drastic anisotropy in the microscopic behavior of both objects under elongation along a MIC has been observed when the MIC is oriented either along or normally to the C-C bonds chain. Both the anisotropy and high stiffness of the nanographene originate at the response of the benzenoid unit to stress.Comment: 19 pages, 7 figures 1 tabl

Topological mechanochemistry of graphene

In view of a formal topology, two common terms, namely, connectivity and adjacency, determine the quality of C-C bonds of sp2 nanocarbons. The feature is the most sensitive point of the inherent topology of the species so that such external action as mechanical deformation should obviously change it and result in particular topological effects. The current paper describes the effects caused by uniaxial tension of a graphene molecule in due course of a mechanochemical reaction. Basing on the molecular theory of graphene, the effects are attributed to both mechanical loading and chemical modification of edge atoms of the molecule. The mechanical behavior is shown to be not only highly anisotropic with respect to the direction of the load application, but greatly dependent on the chemical modification of the molecule edge atoms thus revealing topological character of the graphene deformation.Comment: 9 pages, 10 figures, 1 table. arXiv admin note: text overlap with arXiv:1301.094