Thermoplastic Composite Panels Research Articles

Development of a numerical methodology for the analysis of the post-buckling and failure behavior of butt-joint stiffened thermoplastic composite panels

This paper presents the development of a finite element (FE) numerical modelling methodology for the post-buckling and failure analysis of butt-joint stiffened thermoplastic composite panels. The structural components of the investigated panels are bonded with co-consolidated butt-joint methods - a novel joining technique used in aerospace structures. Two panels are examined in this work: the first in pristine condition and the second with a pre-inflicted damage, to evaluate the influence of the damage on the mechanical response of the panel. To assess the ability of these panels to operate safely in the post-buckling region, the nonlinear FE methodology employs large displacement analysis, progressive damage modelling (PDM) and cohesive zone modelling (CZM) approaches. The focuses are placed on the prediction of the displacement fields, the intra-/inter-laminar failure and the debonding between the components of the panel. For validation purposes, the simulation results are compared with the available experimental measurements. The correlation demonstrates the ability of the developed methodology for the modelling and simulation of panels with the same structural configuration.

On the use of a rounded sonotrode for the welding of thermoplastic composites

Continuous ultrasonic welding is an attractive welding technique for thermoplastic composite structures. In this process, a metallic sonotrode connected to a piezoelectric transducer and to a press moves along the parts to be welded applying ultrasonic vibrations and a static welding force on the welding overlap. Thus far, the research carried out on this topic makes use of sonotrodes featuring a flat contact surface with the parts to be welded, which limits the use of the process to the welding of overlaps with no curvature in the welding direction. With the final aim of assessing whether this process can also be applied to curved structures, this paper explores the feasibility of using a rounded sonotrode for continuous ultrasonic welding of thermoplastic composites. The main conclusions drawn from the results obtained in this research is that it is indeed possible to continuously weld thermoplastic composite panels with a rounded sonotrode and that high-quality welds can be obtained from such a process. Furthermore, the use of a rounded sonotrode has the positive effect of lowering the temperatures at the welding interface as well as the temperatures within the adherends. On the other hand, the use of such sonotrode leads to a decreased, although still competitive, welding speed and, potentially, an increased welding force, thereby setting boundary conditions that need to be considered for each specific application.

Skin-stringer separation in post-buckling of butt-joint stiffened thermoplastic composite panels

Two aeronautical thermoplastic composite stiffened panels are analysed and tested to investigate the buckling behaviour, the skin-stringer separation and the final failure mode. The panels are made of fast crystallising polyetherketoneketone carbon composite, have three stringers with an angled cap on one side, and are joined to the skin by a short-fibre reinforced butt-joint. The panels contain an initial damage in the middle skin-stringer interface representing barely visible impact damage. Finite element analysis using the virtual crack closure technique are conducted before the test to predict the structural behaviour. During the tests, the deformation of the panels is measured by digital image correlation, the damage propagation is recorded by GoPro cameras and the final failure is captured by high speed cameras. The panels show an initial three half-wave buckling shape in each bay, with damage propagation starting shortly after buckling. A combination of relatively stable and unstable damage propagation is observed until final failure, when the middle stringer separates completely and the panels fail in an unstable manner. The test results are compared to the numerical prediction, which shows great agreement for both the buckling and failure behaviour.

Acoustic and thermal performance of sustainable fiber reinforced thermoplastic composite panels for insulation in buildings

Acoustic and thermal insulation are major issues to be considered in buildings' construction to meet the overall comfort conditions indoors and fulfill the energy-efficiency approaches. This work aimed to investigate the acoustic and thermal insulation performance of sustainable thermoplastic sandwich composite panels to be used in floor systems in buildings. Three types of recycled nonwoven fabrics were used in composites reinforcement; jute, polyester and hybrid jute-polyester (80:20%) with polypropylene as the matrix. Compression molding technique was used in manufacturing of the composite panels with different reinforcement and matrix ratios. The acoustic insulation performance of sandwich composite panels was evaluated in terms of impact sound attenuation test. As well the thermal conductivity was determined and the thermal insulation behavior was investigated using infrared thermography. The results indicated that all composite samples showed increased impact sound reduction at low frequencies. Jute and hybrid jute-polyester composites recorded a decrease in impact sounds reduction at high frequencies, whereas polyester composites showed fluctuated behavior. Increasing the reinforcement and matrix ratios enhanced impact sound reduction for jute composites at high frequencies, and for hybrid jute-polyester composites in the mid and high frequencies. The impact sound reduction index ΔLw values of all composites ranged between 12 and 21 dB. Furthermore, samples PJ and PT produced with 1-layer of reinforcement showed the lowest thermal conductivity and higher resistivity values compared to all samples. The polyester composites were the least samples to gain heat temperature after 60 min, and they exhibited the highest temperature difference compared to jute and hybrid jute-polyester composites.

Pullout Performance of Common Construction Fasteners from Long Fiber Thermoplastic Composites

The recent increase in cyclone activity in the last 20 years has pressured the construction industry to look for alternative materials for roof construction. Although plywood is a cheap material, its mechanical properties are highly dependent on moisture content. Long fiber thermoplastics are an emerging type of plastic materials that feature enhanced mechanical properties due to high aspect ratio discontinuous fibers in a tough polymer matrix. The present study focuses on comparing the pullout strength of commonplace fasteners from plywood and long fiber thermoplastic composite panels. Specimens were tested according to ASTM standards D1761-88 and D6117-97 for mechanical fasteners in wood and long fiber thermoplastic (LFT) composites. Common construction nails like 6D, 10D and 16D and a #8 screw were used to compare the performance in terms of withdrawal. The data showed that LFT composites had withdrawal strength on average two to three times that of plywood for all tested fasteners types. Experimental data was analytically verified by APA TT-109 guidelines.

Thermal stability evaluation of sweet sorghum fiber and degradation simulation during hot pressing of sweet sorghum–thermoplastic composite panels

An understanding of the thermal behavior of natural fibers is critical in efficiently converting them into composites under heat and pressure. The objectives of this study were to evaluate the thermal stability of sweet sorghum fiber and simulate its thermal degradation during hot pressing into composite panels. Thermal stability of sweet sorghum was analyzed using thermogravimetric analysis, and its thermal degradation during the hot pressing process was simulated based on kinetic parameters calculated using the ASTM E1641 method, Friedman method, and independent parallel reaction model (IPRM). The results demonstrate that experimental testing data and IPRM prediction results are in accordance. Sorghum degradation ranged from 5.4% at the surface to 2.0% at the core of the panel after 10min of hot pressing at a platen temperature of 160°C. The results of this study will assist with consolidation of sorghum fibers into products under heat.

Self-piercing riveting of aluminum alloy and thermoplastic composites

In the present study, the feasibility of joining PA6-matrix thermoplastic composite panel and aluminum alloy is considered. Self-piercing riveting processes of PA6 panels, PA6 composite panels with reinforcing fibers (glass fiber and carbon fiber), and 5754 aluminum alloy sheets are systematically investigated. Macro characterizations of the appearance and transverse sections of the self-piercing riveting joints were conducted to provide more comprehensive understanding of the composite deformation during riveting process. In particular, the effect of reinforcing fibers on the composite deformation behaviors was characterized. Lap-shear testing of the self-piercing riveting joints was performed to evaluate joint strength, and characterization of the fractured joints as well as failure mechanics analysis was conducted to investigate the underlying failure mechanisms of the self-piercing riveting joints. The effects of reinforced fibers on the failure mechanisms and mechanical performance were also discussed. The results showed that self-piercing riveting is an effective technique for joining fiber reinforced thermoplastic composite panels and aluminum alloy sheets. In addition, self-piercing riveting joints with glass fiber reinforced composite failed in a bearing mode and exhibited improved joint strength compared with the PA6 matrix; while self-piercing riveting joints with carbon fiber reinforced composite exhibited slightly decreased joint strength but significantly reduced ductility, owing to the brittleness of the carbon fiber reinforced composite.

Tea mill waste fibers filled thermoplastic composites: the effects of plastic type and fiber loading

The objective of this study was to investigate the utilization of tea mill waste fibers (TMWF) in thermoplastic composites. For this purpose, two plastic types: polypropylene (PP) and high density polyethylene (HDPE), were used as thermoplastic material while TMWF was utilized as a lignocellulosic filler. HDPE or PP and up to 50% TMWF were compounded in the single screw extruder and extrudates were compression molded into thermoplastic composite panels. The effects of plastic type and fiber loading rate on mechanical (flexural, tensile, and impact properties), physical (water absorption and thickness swelling), thermal properties, and morphological of the produced composites were determined. Tensile modulus, flexural modulus, water absorption, and thickness swelling of the produced composites were increased with the rise of the TMWF amount in the thermoplastic matrix. On the other hand, TMWF increase in the thermoplastic matrix reduced the tensile strength, flexural strength, and impact strength of the produced composites. It should also be noted that both flexural strength and flexural modulus have satisfied the requirements of ASTM D 6662. In the case of thermal properties, addition of TMWF into the thermoplastic matrix did not change the initial degradation significantly. However, the char rate of the composites increased. It appears that tea mill waste fibers may have a potential usage as filler in the PP- and HDPE-based thermoplastic composites.

Continuous cellulose fiber-reinforced cellulose ester composites. II. Fiber surface modification and consolidation conditions

We prepared thermoplastic composite panels using solution impregnation of continuous lyocell (regenerated cellulose) fibers with a cellulose mixed-ester (cellulose acetate butyrate) matrix. We examined both fiber-matrix adhesion and melt consolidation in an effort to produce uniform panels having low void content and high mechanical strength. We characterized the effect of surface modification by acetylation on interfacial adhesion between the cellulose fiber and cellulose ester. Whereas wood fiber acetylation had previously been observed to result in significant strength gains in (discontinuous) wood fiber- reinforced composites (with the same matrix material), we did not observe a similar increase in strength in the continuous lyocell cellulose fiber system. This suggests that interfacial stress transfer is not a limitation in this system. This was confirmed by microscopic examination of the fracture surfaces, which indicated that fiber-matrix adhesion was considerable in the absence of fiber surface modification. We then systematically varied melt consolidation conditions (temperature, pressure and time) in an attempt to define optimum consolidation parameters by using design of experiments (DOE) methodology. We measured both interlaminar shear strength (ILSS) and composite void volume. We found that a minimal void content (ca. 2.83 vol. %) occurred at moderate temperatures (200°C), low consolidation pressures (81.4kPa) and long press times (13min). This was also where we maximized the interlaminar shear strength (ILSS) at a value of 16.3MPa. This agrees with the regression model predictions. We observed the highest tensile properties at the ILSS and void-volume optimal-consolidation condition: a tensile modulus of 22GPa and tensile strength of 246MPa were obtained.

Modelling of pre-heating of flat panels prior to press forming

Abstract This paper describes a mathematical model which determines the heat transfer to and the transient temperature distribution within flat panels during heating. The model was developed specifically to simulate the pre-heating of thermoplastic composite panels prior to press forming, but it will also simulate the heating of isotropic materials such as brass or steel. The heating techniques modelled are infrared, platen and convection heating. Model predictions of heat-up times and temperatures are compared to experimental data for each heating technique. These comparisons show that the model predicts accurately the heat-up curves of flat sheets heated using various heating techniques.

Buckling behavior of bead-stiffened composite panels

The integral formation of stiffeners in a panel is economically attractive in terms of a reduced number of parts and manufacturing operations. For thermoplastic composite panels stiffened by integral formed open-section beads, the effects of bead spacing and bead cross-section geometry on the initiation of buckling under uniaxial compression and uniform shear loading have been investigated. Approximate analytical relationships for buckling of flat, orthotropic plates have been verified by the use of classical solution methods. Finite element results for a range of stiffened panel sizes and bead geometries are presented and compared with closed-form solutions based on an effective flat plate size. Experimental verification of analytical predictions for four of the panels is described.

Buckling of open-section bead-stiffened composite panels

Stiffened panels are structures that can be designed to efficiently support in-plane compression, bending, and shear loads. Although the stiffeners are usually discrete elements which are fastened or bonded to a flat or continuously curved plate, manufacturing methods such as thermoforming allow integral formation of the stiffeners in a panel. Such a configuration offers potential advantages in terms of a reduced number of parts and manufacturing operations. For thermoplastic composite panels stiffened by integrally formed open-section beads, the effects of bead spacing and bend cross-section geometry on the initiation of buckling under uniaxial compression and uniform shear loading were investigated. Finite elements results for a range of stiffened panel sizes and bead geometries are presented and compared with approximate closed-form solutions based on an effective flat plate size. Experimental verification of analytical predictions for one of the shear panels and one of the compression panels is described. Compensation of the forming tool to reduce the degree of initial curvature of the panels was found to be necessary.