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

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

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

Failure of laminated composite pinned connections

In this investigation the behavior of pin-loaded composite plates is studied analytically. A progressive damage model is presented which is capable of predicting the three different mechanisms of failure: bearing, shearout, and net tension. The model consists of three major parts: stress analysis, and material property degradation rules.Based on the model a computer code is developed. The computer code is capable of assessing damage, evaluating residual strength, and predicting ultimate strength of pin-loaded composite plates. Predicted results are compared with available experimental data. Excellent agreement between the predicted and the experimental data was found.The computer code is used to study geometric parameters that influence joint strength. Such studies are useful in designing mechanical fastened joints using advanced composites

An Estimation of Longitudinal Strength Reduction of Unidirectional E-glass/Epoxy Composite Exposed to Sulfuric Acid Using a Micromechanics Model

A new model is proposed in this research to calculate the longitudinal strengthof unidirectional E-glass reinforced polymer composites exposed to sulfuricacid environment, using a micromechanics model. In the proposed method,it is assumed that the residual strength of the degraded composites under acidic environment can be calculated by knowing the degraded strength properties of the constituent materials. In order to measure the properties of the degraded epoxy resins and E-glass fibers, corrosion tests are performed on them when exposed to 5% sulfuric acid for different immersion times. Acid penetration in composites is a time consuming phenomenon. Thus, before the acid reaches to inside region of composites, the degraded composites cross-section can be divided to two regions, namely intact and degraded regions. In this stage, a simple model is suggested to estimate the acid penetration depth in the degraded composites. Based on the corrosion mechanisms of glass fibers, the energy dispersive x-ray microanalysis (EDX) results of different points of composites cross-section are used to estimate the acid penetration depth in composites. Both the acid penetration depth model and micromechanics model are used to calculate the longitudinal strength of intact and degraded regions for different immersion times. Thus, the longitudinal strength of degraded composites can be calculated. Moreover, some similar unidirectional E-glass/ epoxy composites exposed to sulfuric acid for different immersion times are tested to measure the longitudinal strength of them. The theoretical results are in good agreements with those experimentally measured

Experimental Investigation on Fatigue Behavior of Epoxy Resin under Load and Displacement Controls

The mechanical properties of epoxy resin including tensile and flexural modulus, tensile and flexural strength for static conditions are currently studied. The frequency effect as significant parameter at room temperature is investigated and fatigue behavior of the epoxy resin in tension-tension loading conditions for different frequencies of 2, 3 and 5 Hz are obtained. The epoxy resin has been taken under flexural bending fatigue loading and fatigue life is investigated. The results of the experiments show the values of 2.5 and 3 GPa of tensile and flexural modules and 59.98 and 110.02 MPa of tensile and flexural strengths for the resin, respectively. To achieve a linear load-deflection relationship in a three-point bending experiment, a maximum allowable deflection of 5 mm is acquired. The relationship between the frequency and fatigue life shows higher frequency results in lower fatigue life. Loading with frequency of 2 Hz has provided 5.8 times more fatigue life compared with 5 Hz loading. For a tension-tension fatigue loading condition, the variation of tensile module of epoxy resin shows no noticeable change during the fatigue loading condition. This module decreases significantly only in the primary and failure cycles close to the fracture point. In further experiments, fatigue behavior of epoxy resin was tested under flexural bending fatigue loadings with controlled deflection at room temperature. Maximum applied normalized stresses versus the number of cycles to failure curve are illustrated and it can be performed in order to predict the number of cycles to failure for the resin in arbitrary applied normal stresses as well

Determination of the Appropriate Gradient Elasticity Theory for Bending Analysis of Nano-beams by Considering Boundary Conditions Effect

Abstract In the present paper, a critique study on some models available in the literature for bending analysis of nano-beams using the gradient elasticity theory is accomplished. In nonlocal elasticity models of nano-beams, the size effect has not been properly considered in governing equations and boundary conditions. It means that in these models, because of replacing of the size effect with the inertia gradient effect, the size dependency has been ignored in bending analysis of nano-beams. Therefore, as the beam dimensions increase in comparison to its material length scale parameter, the obtained solution based on the gradient elasticity theory (either in the nonlocal elasticity theory or the strain gradient elasticity theory) should converge to the classical elasticity solution. Hence, satisfying of boundary conditions is a crucial point. In this paper, governing equations and boundary conditions are presented based on two gradient elasticity theories (i.e., nonlocal elasticity and strain gradient elasticity theories). Also, boundary conditions in strain gradient elasticity theory are modified based on a dimensional analysis approach. The results indicate that the strain gradient elasticity theory captures the size effect more sensitive in comparison with the nonlocal elasticity theory in bending analysis. In addition, modified boundary conditions in strain gradient elasticity theory can lead to converge the classical solution at large scales. To prove that the boundary conditions of nano-beam have the direct effect on mechanical behavior of structure, the size-dependent Young modulus of carbon nanotube (CNT) is investigated and the results show that the prediction of strain gradient elasticity theory with modified boundary conditions is in a good agreement with experimental results