Toughened Resin System Research Articles

Fractographic Investigation of Cryogenic Temperature Mode-II Delamination Behavior of Filament Wound CFRP Laminates with Varied Resin Systems

This study investigates the mode-II delamination performance of filament-wound unidirectional composites with different types of epoxies as their matrix phase under room and cryogenic temperatures. A typical vacuum infusion resin, an aerospace-grade cold-curing resin and crack-resistant toughened resin systems were wet-wound with 12K carbon fiber tows to manufacture the composite samples. Test samples with a (0)16 ply sequence were tested according to ASTM D7905-19. The tested samples were investigated via microscopic analysis to assess the failure mechanisms associated with varying the matrix material and temperature. ENF tests at room temperature were found to be susceptible to the inherent variance in the fiber architectures along with resin-viscosity-driven fiber wetting. Cryogenic conditions induce a shift in the mode-II delamination behavior from a rather complex failure mechanism to a consistent fiber/matrix debonding mode with diminishing GIIc values except for the toughened resin system. The provided comprehensive fractographic analysis enables an understanding of the various causes of fracture, which determines the laminate performance. The combined evaluation of the distinctive damage modes reported in this study provides guidance on the conventional wet-winding process, which still remains a volumetrically dominant and viable option for cryogenic applications, particularly for vessels with limited operational durations like sounding rockets.

Triblock Copolymer Toughening of a Carbon Fibre-Reinforced Epoxy Composite for Bonded Repair.

Epoxy resins are the most widely used systems for structural composite applications; however, they lack fracture toughness, impact strength and peel strength due to high cross-linking densities. Use of conventional toughening agents to combat this can lead to reductions in mechanical, thermal and processability properties desirable for bonded composite applications. In this work, an asymmetric triblock copolymer of poly(styrene)–b–poly(butadiene)–b–poly(methylmethacrylate) was used to modify an epoxy resin system, with the materials processed using both vacuum bag and positive pressure curing techniques. Interlaminar fracture toughness testing showed improvements in initiation fracture toughness of up to 88%, accompanied by a 6 °C increase in glass transition temperature and manageable reductions in gel-time. Shear testing resulted in a 121% increase in ultimate shear strain with only an 8% reduction in shear strength. Performance improvements were attributed to nano-structuring within the toughened resin system, giving rise to matrix cavitation and dissipation of crack front strain energy upon loading.

Analysis of composite T-joint designs for enhanced resistance to hydrodynamic ram

This study is concerned with the feasibility of different T-joint designs for application in aircraft composite fuel tanks which may be subjected to hydrodynamic ram loading. In a first step, the response of a hybrid metallic/composite T-joint is compared against a state-of-the-art composite T-joint. In a second step, two different resin systems are compared using a third T-joint design – one state-of-the-art aerospace resin and one novel, toughened system. Experimental work is done under both, quasi-static and under high-rate loading conditions. The hybrid metallic/composite T-joint showed superior post-failure behavior compared to the state-of-the-art composite T-joint. The use of the toughened resin system results in a significant increase of joint strength. The post-failure behavior is however not affected by the resin system.

Dynamic Response of Nano‐Scale Core‐Shell Rubber Toughened Sandwich Composites

ABSTRACTHigh‐pressure shock performance of sandwich composites consisting of nano‐scale core‐shell rubber (CSR) toughened E‐glass Vinyl‐Ester face‐sheets and CorecellTM A500 (Gurit (USA) Inc., Bristol, Rhode Island, USA) foam was studied using a shock tube apparatus. The core material and thickness, as well as overall specimen dimensions, were held constant, with the only difference arising in the resin system used during the infusion. The non‐core‐shell rubber toughened resin system (Non‐CSR) consisted of a vinyl‐ester resin only, while the CSR toughened resin consisted of the same vinyl‐ester resin, but with Kane Ace MX 153 CSR particles added to the vinyl‐ester resin system. Prior to the shock tube experiments, the quasi‐static and dynamic constitutive behaviour of the face‐sheets (tensile/compressive) and foam (compressive) was evaluated. During the shock tube testing, a high‐speed photography system coupled with the optical technique of digital image correlation was utilized to capture the real‐time deformation process, as well as mechanisms of failure in the sandwich composites. Post‐mortem analysis was also carried out to evaluate the overall shock performance of these configurations. Results indicated that adding CSR particles to sandwich composites aids in dispersing the initial shock wave loading, thus reducing the overall deflection, strain, and velocity and improving the overall blast resistance of the structure.

Automatic Damage Detection and Monitoring of a Stitch Laminate System Using a Fiber Bragg Grating Strain Sensor

There are numerous aspects and questions related to the use of composite materials for primary structures in aircraft where the structural integrity is the most important factor. This is especially true if the main concerns are that the material should have good reliability and durability for the primary structural application. Composite laminates are highly sensitive to out-of-plane failure due to their low inter laminar fracture toughness. An alternate method to increase the damage resistance is through three-dimensional fibrous reinforcement such as through-the-thickness stitching with a single fiber as the thread. Recent studies have shown that the stitching of standard laminates can enhance damage tolerance to levels obtainable with toughened resin systems. However, for next-generation aircraft, material improvement alone is not enough to assure or increase the safety and reliability of the structure. Continuous damage monitoring during operation will become an important issue in aircraft safety. Embed ding fiber Bragg grating (FBG) technology into the composite structure as strain sensors could potentially solve the above problem because the FBG can be used to detect and characterize the damage before it reaches a critical stage. The model used to represent this problem is a 6 × 6 Vectran stitched carbon/ epoxy laminate under tension loading, and the real-time monitoring using the FBG strain sensors is combined with acoustic emissions that were conducted during the test. A numerical laminate theory using a rebar element and first-ply failure criterion is performed to determine the preferred area on the specimen for the placement of the FBG before manufacturing and testing. Experiments are presented to determine the damage growth that was quantified with an ultrasonic (water immersion) c-scan. In this research, the FBG successfully detected and characterized the damage in the carbon/epoxy stitch laminate caused in tension loading cases. The FBG is enhanced with acoustic emission data and can quantitatively predict the damage growth.

Enhanced toughness and shape memory behaviors of toughed epoxy resin

This paper reports on an approach to enhance the toughness of shape memory epoxy by using polypropylene glycol diglycidyl ether (G) as the toughening agent. The mechanical properties and shape memory behavior of the toughened resin systems with different loading level of G were studied, respectively. Results of the torsional braid analysis (TBA) test indicated that G had good compatibility with the epoxy resin matrix and induced a decrease in the glass transition temperature, Tg, of the toughened systems when compared to that of the neat resin system; and the decrease in Tg scaled with the content of G added in the system. Impact strength tests showed that the impact strength was improved significantly by adding G into the resin system and it increased by a factor of 13.7 for the system with 13 wt.% content of G. In addition, the toughened systems were found to yield during the impact strength test whereas brittle fracture occurred for the neat epoxy resin system; this behavior could be further confirmed by the results of scanning electron microscopy (SEM). In the shape memory behavior tests, strain fixity ratio reached as high as 98.9% for toughened systems with 7, 9, 11, 13, and 15 wt.% of G. Toughened systems also displayed changed shape recovery behavior that was comparable with that of the neat epoxy resin system during shape memory process.

Out-of-autoclave cure cycle study of a resin film infusion process using in situ process monitoring

Liquid resin infusion (LRI) of textile tailored reinforcements (TRs) is increasingly applied in new processing technologies for manufacturing carbon fibre composites. This work presents a cure cycle study of an out-of-autoclave toughened resin film infusion (RFI) process as part of the examination of an alternative manufacturing process for composites. To successfully produce laminates using resin film infusion in combination with a fast-curing process, the flow behaviour of the selected resin material under changed processing conditions was investigated. The effect of processing parameters, specifically heating rates and dwell times, on resin viscosity and laminate infiltration was evaluated through experimental work and supported by in situ process monitoring. A DC-resistance sensor system was applied to track the change in resin viscosity during cure. Results showed that cure cycles with a relatively short dwell time and higher heating rate compared to an autoclave cure led to enhanced flow properties of the toughened resin system. High quality laminates, comparable to autoclave panels, were manufactured with vacuum pressure only by modifying the original vacuum bagging arrangement.

Assessment of Residual Tensile Strength of Carbon/Epoxy Composites Subjected to Low-Energy Impact

This paper presents the results of an experimental study to investigate the effects of impact loading on the residual tensile strength of woven graphite epoxy laminates with a toughened resin system. In this study, both cross-ply, [0∕90]6, and angle-ply, [±45]6, laminate lay-up configurations were studied and compared. Test specimens were subjected to various levels of impact loading, after which tensile pull tests were performed to determine the residual tensile strength properties. The study results demonstrated that impact damage causes a significant reduction in tensile strength properties of woven cross-ply [0°/90°] laminates. For example, cross-ply laminated composite specimens subjected to the lowest impact level, 6.8J, exhibited a 25% decrease in ultimate tensile strength. Angle-ply woven laminates [±45°], however, exhibited an 18% increase in ultimate tensile strength after being subjected to 6.8(J) impact. This characteristic of increasing tensile strength in [±45°] specimens is an example of increased fiber reorientation in composite laminates with limited fiber damage.