Mechanical performance of composite flat specimens and pressure vessels produced by carbon/epoxy towpreg dry winding

Abstract

The main purpose of this study is to evaluate the effects of certain processing parameters on the mechanical performance of carbon/epoxy towpreg wound composite structures. For this purpose, composite sample productions and their evaluations were conducted in two steps. In the first step, dry winding of carbon/epoxy towpregs was used to produce flat composite plates. Their evaluation was performed by rheological analysis, interlaminar shear tests, and unidirectional tensile tests. In the second step, towpreg dry winding was used to produce composite pressure vessel samples. Their performance was evaluated by observing the effects of various winding process parameters on the safety of the vessels via hydrostatic burst pressure tests. Compared to the traditional wet filament winding, the main difficulty observed was maintaining the “straight towpreg path” necessary for efficient winding operations. This problem was prevented by applying higher tension forces during dry winding. Evaluation of the hydrostatic burst tests in terms of burst pressure, hoop strain and safe failure mode revealed that the optimum pressure vessel performance could be obtained in the vessel samples with “helical-hoop-helical winding layer sequence.” On the other hand, use of “complex helical pattern” resulted in no advantages at all, due to basically higher number of undulation zones acting as stress concentration zones.

Keywords

Carbon/epoxy towpreg wet filament winding dry winding hydrostatic burst pressure mechanical properties

Introduction

Among other composite manufacturing techniques, today filament winding is the most efficient one used for many hollow structures. These hollow structures could be axisymmetric cylindrical geometries such as various pipes used in civil engineering applications, bicycle frames, or water–oil–gas pipelines. Of course, filament winding is also used to produce non-cylindrical hollow structures such as certain parts of aircraft wings and radomes.

Technologically, the most significant application area of filament winding is the production of various forms of “pressure vessels” used for many purposes. Due to mainly weight saving, many metallic pressure vessels (including those with a high level of internal pressure) have been replaced with filament wound composite pressure vessels.

Since continuous fiber forms are wetted (impregnated) with a liquid thermoset matrix resin during the winding operation, the traditional filament winding technique is also named as “wet winding process.” Although there are many advantages, traditional wet winding has several drawbacks and limitations.

One serious challenge is the slippage of fibers. As the fibers are impregnated during the process, it is challenging to control the amount of resin sticking on fiber bundle surfaces. Over-wet fiber bundles tend to slip, especially during the formation of complex hollow geometries and lower helical winding angles. Likewise, difficulty in controlling resin ratio can result in non-homogeneous structures and resin accumulated zones which may form voids during curing. In-situ fiber impregnation also makes resin mixture solution more apt to collect dirt and inclusions from the environment, which might reduce part quality.

Therefore, to overcome these shortcomings of traditional wet filament winding, progress in the “towpreg” technology today is offering “dry filament winding” alternative.

Towpregs are basically continuous fiber bundles pre-impregnated with a matrix resin. After partial curing, these fiber bundles could be cold stored and used in many composite manufacturing techniques, including fiber placement processes and filament winding. Due to the controlled impregnation process, towpreg rovings contain a steady and homogenous fiber/resin ratio. Thus, consistent product performance can be achieved due to the uniform properties obtained. Towpregs have the potential to solve reproducibility, reliability, and homogeneity problems occurring in the traditional wet winding process.

Another significant advantage in the use of towpregs is that compared to the wet filament winding technique, “the load transfer mechanism from the matrix to the fibers” would be much more effective in the dry filament winding process, that is, the degree of effective fiber strength in the composite structure would be higher. This is simply named as “fiber strength translation ratio.” This higher translation ratio is a significant advantage, especially for the structures where weight saving is critical such as rocket motor cases. Thus, it would be possible to obtain equal performance vessels by winding fewer materials. Then, in terms of handling, process speed, cleaning, scrap rate, labor health/safety, and simplicity, dry winding becomes very advantageous compared to traditional wet winding.

Literature review revealed that there are extensive numbers of studies investigating influences of parameters on the performance of tubular structures,^1–11 including mechanical testing and characterization^12–16 aspects of traditional “wet filament winding” technique. For instance, in their very comprehensive studies, Cohen et al.^5,6 focused on the influences of especially winding tension, laminate stacking sequence, and winding time. They indicated that winding tension directly increases performance of wet wound pressure vessels because increasing the winding tensions increases the carbon fiber content with lower possibility of void formation. Their modeling also revealed a very strong relation between the fiber volume fraction and hoop strength of the composite pressure vessels. They concluded that use of dry filament winding via towpregs could improve vessel performance significantly due to their ability to have more fiber content.

There are only a limited number of research on the use of towpregs in “dry filament winding” techniques. These limited numbers of studies are summarized below in two categories: the first group being studies on “flat specimens,” that is, coupon specimens, and the second group being on “hollow specimens,” that is, pressure vessel specimens.

In one of the studies, Reddy et al.¹⁷ compared the mechanical performance of the specimens produced by glass fiber/epoxy towpreg dry winding and carbon fiber/epoxy wet winding. Testing of flat coupon specimens produced by these two different techniques indicated that even though glass fibers have much lower fiber strength than carbon fibers, towpreg glass/epoxy dry wound samples had almost the same performance as wet wound carbon fiber composite samples, basically due to higher degree of load transfer mechanism seen in towpregs which is named as “strength translation ratio.”

Reddy et al.¹⁸ conducted another study to determine several properties of a carbon/epoxy towpreg by using “flat coupon specimens” produced by dry winding. The aim is to reveal towpregs mechanical and thermal performance for the design and analysis of real-life parts by analyzing the material’s micromechanical behavior. Test plates were produced by the towpreg dry winding method using a flat steel mandrel. In order to obtain flat coupons, dry wound plates were removed from the mandrel and then cured in an autoclave instead of curing the mandrel and the wound part together. Interlaminar shear strength and tensile strength, modulus, and strain values were acquired with corresponding mechanical tests. DSC analysis was conducted to understand the curing behavior of the resin system used in the towpreg.

Almeida et al.¹⁹ especially investigated certain problems of dry filament winding during the manufacturing of carbon/epoxy towpreg flat specimens, such as fiber slippage, angle deviations, and thickness variations. One critical observation was the level of damage in the tab region during unidirectional tensile tests due to the relatively thin tabs used. Their recommendation was to optimize tab thickness prior to mechanical tests.

In their another comprehensive study, Reddy et al.²⁰ conducted a detailed characterization of carbon fiber/epoxy towpreg structures to validate the complete design process of a space propulsion rocket motor case step by step. After the determination of mechanical properties using flat coupon specimens, a buckling model was developed with the material properties obtained. To confirm the reliability of the developed FEA model and obtain experimental data which meets structural specifications for the rocket case, carbon/epoxy towpreg hollow cylinders were dry wound and tested accordingly. It was found that for both flat specimens and cylindrical hollow specimens, towpreg material performance was within the safe limits defined for their motor case.

Alam et al.²¹ investigated another comprehensive study on the carbon/epoxy towpreg dry filament wound Type IV pressure vessels with finite element model verification by conducting hydrostatic burst tests. Effects of winding angle, layer sequence, and layer numbers on the burst performance of the pressure vessels were determined by using Digital Image Correlation (DIC) method to quantify failure strains of burst vessels in various directions as a replacement for strain gauge measurements. An optimum winding sequence of carbon/epoxy towpreg material was found as a polar–hoop–polar sequence with 17° polar winding and 88.5° hoop winding angles. The burst test and FEM results showed that vessels failed in the cylindrical region by hoop or shear stresses, which were significantly affected by ply thickness and winding angles. DIC analysis also showed that during burst tests, fiber hoop strain in the cylindrical region became non-linear just before bursting, which indicates a cylindrical region hoop ply failure.

One of the advantages of using towpregs in dry filament winding is the ability to control band overlaps by minimizing slippage risk due to their tacky nature. Park et al.²² used this concept to model the fiber angle variations along polar regions of a composite rocket motor case where fiber angle and thickness variations are generally high in traditional wet filament wound vessels. Pressure vessels were dry wound by carbon fiber/Novalac resin towpregs and were burst tested for verification. They indicated that angle changes along a fiber bundle in the dome–polar boss region might increase fiber stresses up to 40 times, resulting in premature vessel failure if the matrix cracks near polar boss openings propagate to the entire dome part.

Literature survey revealed that compared to the vast number of studies on the traditional wet filament winding of carbon/epoxy composite structures, there are only limited number of research investigating the performance of towpreg dry wound composite structures. Therefore, the main purpose of this study is to contribute to the related literature by evaluating the effects of certain processing parameters on the performance of carbon/epoxy towpreg wound composite structures.

For this purpose, composite sample productions and their evaluations were conducted in two steps. In the first step, dry winding of carbon/epoxy towpregs was used for the production of flat composite plates. Their evaluation was performed by rheological analysis, interlaminar shear tests, and unidirectional tensile tests.

In the second step, carbon/epoxy towpreg dry winding was used for the production of composite pressure vessel samples. Their performance was evaluated by observing the effects of various winding process parameters on the safety of the vessels with hydrostatic burst pressure tests.

Experimental work

Experimental works conducted in this study mainly consist of two steps. In the first step, mechanical, thermal, and physical properties of towpreg flat coupon specimens produced by carbon/epoxy dry winding were determined. In the second step, performance and failure modes of pressure vessel samples produced by the same dry winding process were investigated.

Carbon fiber/epoxy towpreg material used

In this study, during dry winding operations, epoxy pre-impregnated carbon fiber towpreg material produced by SGL Carbon Inc. (Germany) with the trade name of SIGRAPREG C [TP24/6-5.0/270-E910/35%] was used. It contains standard modulus high-strength carbon fiber yarns having 24K filaments. The resin system used in the towpreg is a modified epoxy resin with the trade name of E910, having controlled flow properties, excellent tackiness, and a glass transition temperature of around 120°C. Properties of this towpreg given in its technical data sheet are given in Table 1. Winding speed for the towpreg is limited to 200 m/min by the manufacturer, while the processing temperature is advised in the range of 12–35°C depending on the resin tackiness required.

Table 1.

Certain properties given in the technical data sheet of carbon/epoxy towpreg used.

Property	Unit	Value
Fiber tensile strength	MPa	4400
Fiber tensile modulus	GPa	250
Fiber elongation at break	%	1.65
Towpreg tensile strength	MPa	2400
Towpreg tensile modulus	GPa	170
Towpreg interlaminar shear strength (ILSS)	MPa	75
Towpreg yarn fineness	Tex (grams/1000 m)	1600
Towpreg bandwidth	mm	6.4
Resin mass content	%	35
Resin glass transition temperature	°C	120

Production of flat specimens by towpreg dry winding

Flat composite plates for coupon specimens were dry wound onto a flat steel mandrel manufactured according to the ISO 1268-5²³ standard requirements. For all dry winding operations, a 4-axis CNC-controlled filament winding machine system was used. After certain trials, optimum dry winding process parameters (Table 2) determined were programmed into Winding Expert software embedded in the system. Test plates were manufactured using the same set of parameters which were monitored using a computer-controlled interface.

Table 2.

Towpreg dry winding parameters used during flat plate productions.

Dry winding parameter	Value (unit)
Winding tension	50 (N)
Winding angle	90 (°)
Towpreg bandwidth	5 (mm)

Main steps used during dry winding process for the production of flat composite plates are illustrated in Figure 1. First, the steel flat mandrel was positioned onto the winding machine system (Figure 1(a)). Then, towpreg winding operations on the flat mandrel started (Figure 1(b)). During the process, winding tension, relative humidity, and workshop temperature were kept in control, and deviations were not permitted. After the winding is completed, before curing, separate side plates were placed on both mandrel/composite plate surfaces to apply compressive pressure for better layer consolidation (Figure 1(c)). Then, the mandrel/composite plate was placed into a convection furnace for curing, using the parameters recommended by towpreg manufacturer (Figure 1(d)), that is, 120°C for 2 h. Finally, flat composite plates were removed from the mandrel using a saw-cutter (Figure 1(e)).

For the preparation of coupon specimens in accordance with the dimensions given in the related standards, further machining of the produced composite plates into dimensions was achieved by using a wet machining system Extec Labcut® 5000 Series Advanced Precision Composite Plate Saw (Figure 1(f)) to minimize specimen damages and fiber misalignments.

Tests and analysis conducted for flat specimens

In order to characterize towpreg filament wound flat specimens, various mechanical tests and certain other analysis were conducted as listed in Table 3 indicating the related standards used.

Table 3.

Various tests and analysis used to characterize towpreg dry wound flat specimens.

Tests and analysis	Properties obtained	Standards used
UD tensile tests	Longitudinal tensile strength	ISO 527-5
	Longitudinal tensile modulus	ISO 1268-5
	Longitudinal tensile strain	ISO 1268-5
Short-beam tests	Interlaminar shear strength	ASTM D2344
Determination of fiber content	Fiber volume percent	ASTM D3171
	Density	ASTM D2734
	Void percentage	ASTM D2734
Rotational rheometer analysis	Glass transition temperature	ASTM D7028
Rotational rheometer analysis	Shear storage modulus	ASTM D7028

Determination of fiber content

It is known that amount of the reinforcing fibers in the polymer matrices is the most crucial parameter in improving all mechanical and other properties of the composite materials. Therefore, in this study, ASTM D3171 standard²⁴ was used to acquire the fiber content of the specimens with sulfuric acid and hydrogen peroxide solution digestion method by using the Procedure B in the standard. First, specimens were weighted using a precision scale before holding them in the acid solution until no epoxy matrix was left. The remaining carbon fibers were washed, dried, and weighted precisely to determine their weight percentage values. By using the following relation, the weight percentage of carbon fibers were transformed into volume percentages

V_{r} = (M_{f} / M_{i}) \times 100 \times (ρ_{c} / ρ_{r})

where V_r is the reinforcement volume percent, M_i and M_f are the initial and the final mass of the specimen in grams, while ρ_r and ρ_c are the density of reinforcement and the specimen in g/cm³, respectively.

Rotational rheometer analysis

Rheometric analysis was conducted for the specimen size of 55 × 12 × 2 mm via Scientific Ares Rheometer 6A device with its rectangular torsion apparatus. The heating rate was set to 5°C/min with 0.01% strain rate and 1 Hz frequency from room temperature up to 200°C. After obtaining shear storage modulus, loss modulus, and tan θ curves of the specimens, glass transition temperature of the epoxy matrix resin and the values of storage modulus (G′) at different temperatures were determined.

Short-beam tests

Interlaminar shear strength (ILSS) values of the flat coupon specimens were obtained by using the short-beam test method given in ASTM D2344 standard.²⁵ Universal Instron testing machine with 200 kN capacity is used for the specimen dimensions of 24 × 8× 4 mm with the span length of 16 mm. Specimens failed with compression or tension fail modes in the center were excluded, and only specimens with mid-plane interlaminar failure modes were counted. Interlaminar shear strength (ILSS) values of the specimens were determined by using the following relation

I L S S = F^{s b s} = 0.75 \times \frac{P_{m}}{b \times h}

where F^sbs is the short beam strength in MPa, P_m is the maximum load observed during test in N, while b and h are the specimen width and thickness in mm, respectively.

Unidirectional (UD) tensile tests of the flat specimens

In this study, the use of “unidirectional tensile test” of the flat specimens was conducted because in the design of rocket motor case structures where reinforcing fiber directions can be adjusted according to prime load directions, tensile strength parallel to the fiber axes is the most critical material property.

Under uniaxial tensile loading, the cross-sectional area, that is, “thickness” of the specimens is critical. Thus, 1 and 2 mm thickness values are selected as the specimen geometry parameters for investigation. After several trials, the number of required winding layers for 1 mm and 2 mm thick specimens were determined as four hoop layers and six hoop layers, respectively.

During unidirectional tensile tests of flat coupon specimens, it is known that “tabbing” between the machine grips and specimen upper and lower end surfaces are always necessary to protect the test specimen from any gripping damage. In this study, Nema Grade G10 tab material composed of woven fiberglass sheets was used. Film adhesive material used to bond the tabs onto the specimen end surfaces was an epoxy-based 0.25 mm thick film.

After preparing the flat coupon specimens, UD tensile tests were conducted using Instron 4481 universal testing system with 100 kN load capacity. At least eight specimens were tested for each different specimen thickness value. Fiber content determined for each specimen was checked to make sure that they have approximately 60% fiber volume content, as required in rocket motor case applications.^6,21

Production of pressure vessels by towpreg dry winding

In the second step of this study, small-scale pressure vessel samples were produced. Then, their performance was observed by hydrostatic burst tests. Details of these procedures are explained below.

Geometry of the pressure vessel samples

In order to represent rocket motor case applications, a typical cylindrical pressure vessel geometry with two domes and polar openings was selected. As shown in Figure 2, the inner diameter and the length of cylindrical region is 146 mm and 56 mm, respectively. The diameter of the two openings is 94 mm.

Preparation of mandrels

Just like in the traditional wet filament winding, the first fundamental step in towpreg dry winding of pressure vessels is mandrel preparation. In the industry, various types of mandrels can be used depending on the geometry, size, and cost. In this study, a water-soluble sand mandrel was selected due to its cost efficiency for small-scale laboratory productions (Figure 3). Mandrel material was a ceramic-based powder mixture, and a solution was formed with water at first. Then, this slurry mixture was poured into tubular metal molds having 20 mm diameter shafts in the center. These metallic shafts were carefully centered to avoid any deflection during rotation in dry winding operations. At this stage, slurry mixture becomes a green body ceramic structure requiring further drying before machining to reduce the risk of machining-induced damages on the mandrel surfaces (Figure 3(a)). After partial solidification, tubular metal molds were removed, and the mandrels were sintered for 56 h at 135°C. Finally, these rough and stiff mandrel structures were machined into their final geometry using a CNC-controlled turning machine (Figure 3(b)).

Preparation of the liners

It is known that in order to have no leakage problems, composite pressure vessels require a liner material that acts as a barrier layer. In this study, a rubber-based liner form was purchased having the same geometry and size of the pressure vessel samples to be produced. These forms are available in “two halves” (Figure 3(c)). After applying an epoxy-based adhesive (Figure 3(d)) on the edges of these halves, they are placed over the prepared mandrel carefully (Figure 3(e)). In order to obtain sufficient degree of bonding between these two halves, curing was applied with vacuum bag approach (Figure 3(f)).

Dry winding operations

Towpreg dry winding system, used for the production of flat plates in the first step, was used also for the production of pressure vessel samples. After inputting all the necessary geometrical parameters, carbon/epoxy towpreg material properties, and various winding parameters into the embedded software, Winding Expert, dry winding operations were started by using two main winding types as “hoop winding” and “helical winding” (Figure 4(a) and (b)).

It is known that, just like traditional wet winding, towpreg dry winding has many significant winding parameters influencing the burst pressure performance of the pressure vessels. Note that effects of four different important winding parameters would be discussed in Results and Discussion section, in detail.

Curing and mandrel removal

According to the data given in the technical sheet of the towpreg producer, the same curing cycle in a convection heated furnace was applied for all samples. In the furnace, there was a rotation system which is critical for homogenous curing of the vessels (Figure 4(c)). When vessels were fully cured, they were slowly cooled inside the furnace to avoid possibility of thermally induced residual stresses.

After the curing operation, the ceramic slurry–based mandrel must be removed from the structure to get a hollow pressure vessel geometry. For this purpose, pressured water stream was used to disintegrate the mandrel inside the pressure vessel. Following the mandrel removal, all vessels were dried at 40°C for 48 h. Additionally, X-ray radiographic inspections were carried out for each produced vessel. The purpose was to make sure that there were no major manufacturing defects such as voids inside or between towpreg bands and the presence of delamination. Figure 4(d) shows final appearances of the pressure vessels after mandrel removal.

Hydrostatic burst tests of dry wound pressure vessels

It is known that the best way to determine “burst pressure” performance of all pressure vessel structures is the “hydrostatic burst test” which applies extremely high levels of internal pressure usually via a liquid medium. In this study, an advanced set up with water medium was used. Components and main procedures used were as follows.

In order to observe the behavior of the vessel samples during the test, a chamber with transparent PMMA windows and aluminum frames was used (Figure 5(a)). The chamber was protected with a steel plate roof having several holes for water inlets and strain gauge cables. The capacity of the water pressurization sub-system was 10 kpsi with a pressure rate of up to 250 lbf/in²/s that can be controlled within the interface (Figure 5(b)).

For the observation of failure initiation and propagation during the tests, a high-speed camera system was installed in front of the protective transparent chamber (Figure 5(c)). External light sources were used to enhance the image quality captured during tests. Moreover, a mirror was placed to the inner back wall of the test chamber so that failures forming on the backside of the vessel could also be captured.

Hoop strain values developed during the tests were recorded by conventional strain gauges bonded to the cylindrical region of the vessels (Figure 5(d)).

Since carbon fibers have black color, it was difficult to observe initiation and propagation of failure during the tests. Thus, after strain gauge bonding, all the vessels were painted with an acrylic white spray paint (Figure 5(e)). Images were adjusted by using the related software (Figure 5(f)).

Results and discussions

In this study, use of carbon fiber/epoxy towpregs during dry winding was conducted in two stages, the first one being “flat specimens” and the latter being “pressure vessels.” Therefore, results of these stages are discussed in the following two sections.

Behavior of the towpreg wound flat specimens

In this section, before discussing the various mechanical performances of the flat specimens, certain problematic issues observed during winding operation and the values of the fiber content obtained will be presented.

Problematic issues observed during towpreg winding

Compared to the traditional wet filament winding, the first difficulty observed was obtaining a very “straight towpreg path” just before the winding operations (Figure 6(a)). If a straight towpreg path could not be maintained, then towpreg bundles would be subjected to torsion, twisting, folding, or other kinds of motion leading to certain problems, such as formation of spaces between towpreg bundles (Figure 6(b)). Forming a straight pathway is not a problem in wet winding because in wet winding, fibers are initially dry, not impregnated with the resin yet. Thus, when winding tension is applied, dry fiber rovings become very straight along their path. However, in the case of towpregs, presence of partially cured resin leads to difficulties. In this study, after several trials, to ensure straight towpreg pathways during the winding operation, higher tension forces were applied.

Another problematic issue observed was during the peeling of towpregs from their spools. If towpreg bundles were peeled-off vertically, then fiber damages were observed (Figure 6(c)). After several trials, this fiber damage problem was prevented by peeling-off towpreg bundles tangentially with a certain angle.

It is known that spools of thermoset towpregs must be stored in subzero cold rooms or refrigerators in order to prevent further curing reaction of their partially cured structures, so that the shelf-life of these towpregs would be longer. Therefore, towpreg spools must be properly conditioned at room temperature before the winding operations. In the present study, after several trials, it was observed that at least 18 h of conditioning was necessary to get a uniform structure without problems.

Fiber content of the flat specimens

It is known that performance of the composite materials mainly depends on the fiber amount in the structure. Therefore, fiber content of the 1 mm and 2 mm thick flat specimens was determined by using a large number of samples according to the procedure given in the experimental part. For both 1 mm and 2 mm thick specimens, Table 4 indicates that the average fiber content was around 60 vol%, which is considered as an optimum value for filament wound structures, in the literature.²¹ Due to towpregs uniform nature, a steady fiber/resin ratio throughout the part was generally achievable, which is not easy in traditional wet wound parts.

Table 4.

Values of fiber and void content and density of flat specimens.

Specimen thickness (mm)	Fiber content (vol %)	Void content (vol %)	Density (g/cm³)
1	59.5 ± 1.3	0.8 ± 0.4	1.52 ± 0.06
2	61.2 ± 0.9	3.2 ± 0.7	1.54 ± 0.04

Table 4 also revealed that 2 mm thick specimens have higher void content than the 1 mm thick specimens. This can be attributed to the size effect phenomenon, that is, when the volume of the structure increases, the possibility of void formation during processing would also increase.

Rotational rheometer analysis of the flat specimens

Thermomechanical performance of the flat specimens was determined by using the rotational rheometer analysis explained in the experimental part. The analysis was conducted for several specimens. Since their thermomechanical behavior was very identical, only one example is given in Figure 7. The results of this analysis were especially evaluated in terms of “storage modulus” (G′) at different temperature levels (25, 50, 100, and 120°C) as tabulated in Table 5. It is seen that the average storage modulus of the flat specimens at room temperature is 4.38 GPa. There is almost no decrease at 50°C. The decrease in the storage modulus value at 100°C is only 7%, while this decrease reaches to 26% at 120°C.

Table 5.

Storage modulus values of the flat specimens at various temperatures.

Temperature (°C)	G′ storage modulus (GPa)
25	4.38 ± 0.14
50	4.28 ± 0.08
100	3.77 ± 0.03
120	2.74 ± 0.12

Figure 7 also shows that the temperature at the “tan θ” peak was around 137°C. On the other hand, if glass transition temperature T_g is measured as the start temperature of maximum decrease in storage modulus curve, it is in the range of 115–120°C, which is the level given in the technical data sheet of the towpreg producer.

Interlaminar shear strength of the flat specimens

It is known that in the multilayered composite structures, the degree of bonding between the layers could be measured by interlaminar shear strength (ILSS) tests. For this purpose, ILSS tests for the 4 mm thick flat specimens were conducted in accordance with the standard given in the experimental section. After testing 16 specimens, it was observed that their “load versus deflection curves” are identical. Hence, five example curves are given in Figure 8. The average ILSS value determined was 73 MPa with the standard deviation of ±2 and coefficient of variation of 2.93%. As shown in Figure 8, it was also observed that specimens were failed with “interlaminar shear mode.”

Unidirectional tensile tests of flat specimens

Mechanical performance of the flat specimens was determined by unidirectional (UD) tensile test of the 1 mm and 2 mm thick specimens in accordance with the related standards explained in the experimental part. For each thickness, at least five specimens were tested.

Figure 9 shows that typical linear stress–strain curves of the specimens were identical. Then, average values of the mechanical properties, that is, tensile strength, tensile modulus, and tensile strain of the specimens were determined according to the procedures given in the related standards. These mechanical properties are tabulated in Table 6 together with the values of standard deviation (±SD) and coefficient of variation (%CV). Note that %CV values for all properties were less than 8%, which is considered as an acceptable level for material properties acquired during the design of filament wound composite structures.^20,26

Table 6.

Tensile strength, tensile modulus, and tensile strain properties of the flat specimens with ± standard deviation and % coefficient of variation values.

Properties	Thickness, mm	Value	±SD	% CV
Tensile strength (MPa)	1	2290	72	3.6
Tensile strength (MPa)	2	2065	45	2.5
Tensile modulus (GPa)	1	162	5	3.4
Tensile modulus (GPa)	2	143	3	2.7
Tensile strain (%)	1	1.50	0.09	5.9
Tensile strain (%)	2	1.30	0.05	3.6

Table 6 revealed that 1 mm thick specimens had higher mechanical properties than 2 mm thick specimens because as discussed in the Fiber Content section before, increasing the thickness, that is, increasing the volume of the specimen under load, increases the amount and sizes of the void formation in the structure. Thus, mechanical properties might decline due to this “size effect” phenomenon.

Failure modes of the UD tensile test specimens were investigated by visual examination of each specimen. As expected, the primary failure mode observed was “tensile fiber fracture” in the “gage-length zone” (Figure 9(a)). Homogenous stress concentration in the gage-length zone resulted in an “explosive” character. It is stated in the literature that explosive gage-length failure of UD tensile test specimens are considered as safe and normal failure mode.²⁷

Other unwanted failure modes were also observed in a few specimens due to anomalous stress concentration in the tab region and the formation of transverse stresses or strains.²⁸ As shown in Figure 9(b) and (c), these failure modes were named as “failure in the tab region” and “longitudinal fiber splitting” modes. Of course, data obtained from these unwanted failure modes were discarded during the determination of mechanical properties.

Comparison of the flat specimen performance with other studies

In order to compare the performance of the flat specimens produced by carbon fiber/epoxy towpreg dry winding, specific properties such as ILSS, UD tensile strength, and modulus values were evaluated in the same table with the data of very limited published study from the literature. Table 7 revealed that these properties obtained in the present study were very close to the properties obtained in the other studies.^19,20,29 Slight differences might arise due to certain differences in the type of carbon/epoxy towpreg and winding operations, leading to possibly different void content and other manufacturing defects formed in the specimens.

Table 7.

Comparison of the certain mechanical properties obtained in this study with other studies published.

Studies	ILSS (MPa)	Tensile strength (MPa)	Tensile modulus (GPa)
This study	73	2290	162
Reference [20]	74	2435	141
Reference [29]	85	2317	189
Reference [19]	—	1410	130

Behavior of the towpreg wound pressure vessels

After gaining experience in the production of flat specimens by carbon/epoxy towpreg winding, the second step of this study was the production and testing of pressure vessels. In this part, first of all, studies were conducted to determine effects of “winding layer sequence” on the performance of pressure vessel samples. After the determination of an optimum layer sequence, the effects of three other “towpreg winding parameters” were investigated according to their performance during burst pressure tests.

Effects of winding layer sequence

The schematic geometry of the pressure vessel samples produced in this study is given in Figure 10. It is known that the optimum layer sequence should provide sufficient vessel thickness to have high burst pressure performance with the “hoop layer failure mode” in the “cylindrical region” of the vessel, which is named as “safe failure mode.”^21,30

The netting theory is a common approach for determining composite pressure vessel wall thickness. The theory assumes that fibers carry the generated loads only, and the contribution of the matrix is neglected. In this study, only one aspect of netting theory is used: the stress factor ratio.³¹ The stress factor ratio is generally used to predict the burst failure region of the composite pressure vessels. It is described as the fraction of fiber stresses that occur in helical layers to fiber stresses on hoop layers. As hoop layers are only applicable to flat cylindrical region, stress factor ratio calculations represent the windings in the vessel’s cylindrical region. Generally, a stress factor ratio between 0.60 and 0.85 is recommended for achieving cylindrical hoop bursts.³² However, this recommendation is not valid for all cases as the failure tendency depends on many factors, such as the material set to be used, vessel geometry, and production method. As explained previously, the main goal of this study was to acquire a cylindrical region hoop failure burst mode. Therefore, three different layer sequences with smaller stress factor ratio values to reduce the amount of stress on the helical layer compared to the stresses formed on hoop layers and one sequence having a relatively larger stress factor ratio were selected.

By reinforcing the vessel domes with helical layers, the probability of obtaining predominantly hoop burst modes can be increased since stress carried by hoop fiber layers becomes higher. A general expression is that helical stresses in the vessel must be significantly lower than hoop fiber stresses to avoid dome dominant unsafe failure modes. One critical factor is creating a hoop/helical layer sequence that provides the required burst pressure values. Vessels with a stress factor ratio greater than one generally fail with axial failure mode, where the dome region fails with polar boss ejection.³² Therefore, a theoretical stress factor ratio smaller than one was calculated for several different layer sequences. The equation below represents the simplified definition of stress factor ratio, which is found by solving vessel loading conditions according to Classical Lamination Theory (CLT)

S F = \frac{t_{h}}{t_{α} (2 \cos^{2} α - \sin^{2} α)}

In this relation, SF is the stress factor ratio, t_h is total thickness of hoop layers, t_α is the total thickness of helical layers, and α is the helical winding angle.³¹

In this study, after determining layer thickness values of the hoop and helical windings via Winding Expert software, four different winding layer sequence candidates, designated as XOX, XXO, XXOX, and XOOX were obtained. In these designations, “X” represents a single “helical winding” layer while “O” represents a single “hoop winding” layer. For example, one of the winding layer sequence candidates, “XOX” represents a total of three layers with the winding sequence of “helical–hoop–helical.”

Both polar openings of the vessel samples have the same diameter value as described in the experimental section. Therefore, modified version of Clairaut’s equation given below can be used to determine the helical winding angle (α), which represents a geodesic winding trajectory³²

α = a r c \sin (\frac{r_{o p e n i n g}}{r_{c y l i n d r i c a l}})

where r_opening is the polar opening radius and r_cylindrical is the cylindrical region radius, resulting in winding angle (α) value of 40°.

After producing pressure vessel samples with four different winding layer sequences (XOX, XXO, XXOX, and XOOX), burst pressure tests were applied for all of them to determine the effects of the winding layer sequence. For each winding layer sequence, three pressure vessels were tested. Data obtained during tests and failure modes observed after the tests are tabulated in Table 8 with ± standard deviations while burst pressure and hoop strain values are compared in Figure 10. Images of the vessels showing their final “failure modes” are also given in Figure 11.

Table 8.

Burst pressure testing results and observed failure modes for the vessels having four different winding layer sequences.

	Winding layer sequence
	XOX	XXO	XXOX	XOOX
Stress factor ratio (σ_helical/σ_hoop)	0.33	0.33	0.22	0.66
Burst pressure (MPa)	27.1 ± 0.6	23.0 ± 1.0	29.6 ± 0.5	31.2 ± 0.9
Hoop strain (%)	1.55 ± 0.04	1.28 ± 0.02	1.38 ± 0.02	1.21 ± 0.01
Hoop strain/burst pressure (με/MPa)	572.5	527.8	486.1	407.3
Burst failure mode	Cylindrical region	Cylindrical region	Near dome region	Polar boss region

Apart from the importance of “burst pressure,” “hoop strain,” and “hoop strain/burst pressure ratio,” the “burst failure mode” of the pressure vessels are considered as another significant criterion. That is, failure in the “dome regions” and “polar boss regions” should be avoided; instead, pressure burst failure of the filament wound vessels should be preferably in the “cylindrical region.”

Table 8 and Figure 10 show that vessels with XXOX and XOOX winding layer sequences had higher burst pressure values (30 and 31 MPa) than the vessels with XOX and XXO sequences (27 and 23 MPa). However, for higher safety, having high levels of burst pressure would not be sufficient. It was expected that the winding layer sequence should also result in a higher level of hoop strain in the cylindrical region.

Even though vessels with XOOX and XXOX layer configurations resulted in higher overall burst pressures, measured hoop strain values were comparably lower. Compared to the XOX vessels, the given layer sequences resulted in a stiffer vessel. Especially for XOOX vessels, the extra hoop layer wound in the middle significantly increased the hoop stiffness and burst pressure of the samples, which was noticeable in normalized strain differences. However, fibers in the cylindrical hoop region could not achieve complete loading, resulting in lower hoop strains. Samples failed with an apparent dome failure indicated performance of polar boss and dome regions, not mechanical performance of towpreg.

From this aspect, maximum hoop strain in the cylindrical region (1.55%) was obtained only in the vessels with XOX layer sequence. Remember that, in the first step of this study, during UD tensile tests of the flat specimens, the tensile strain value determined was 1.50%. This means that when vessels were wound with XOX layer sequence, the maximum hoop strain value would be similar to the tensile strain value of the carbon/epoxy towpreg material used.

In the design of a safe pressure vessel, the “hoop strain/burst pressure” ratio is also important.²¹ Compared to the higher values of this ratio, lower values represent increased “hoop stiffness” levels resulting in lower vessel safety. Table 8 indicates that hoop strain/burst pressure ratio was highest (being 573 µε/MPa) for the vessel with XOX winding layer sequence. Additionally, failure mode shifted from relatively safer regions (i.e., cylindrical) toward unsafe regions (i.e., dome/polar boss) with increasing vessel stiffness.

It is stated that^21,30 stresses concentrated in the cylindrical regions cause fiber breakage in the hoop layer, and other composite damage modes such as delamination or matrix cracking have minor influences on the burst mode. In this region, hoop layer failure is frequently favored since it is easier to assess material strength data by conventional UD tensile tests. Thus, cylindrical region hoop failure indicates that winding efficiency is sufficient to reach the maximum strength of the composite material. On the contrary, helical failure mode in the dome region is considered unsafe because metallic polar bosses are ejected in this failure mode and can result in catastrophic consequences. This type of failure generally occurs due to fiber breakages in helical layers near polar boss and dome regions; still, other damage modes, mainly matrix cracking, can influence the burst pressure in this failure mode.

Figure 11 revealed that pressure vessel samples having XOX and XXO winding layer sequences had “cylindrical region” failure mode. XXO vessels having a similar stress factor ratio with XOX samples failed with lower pressure and hoop strain amounts. One reason for this behavior could be couplings in bending, twisting, and shearing responses of the non-symmetric laminates.^33,34 Another reason could be free-surface effect existing on the outer hoop layer. Burst tests revealed that outer hoop layers failed early at lower hoop strains before the complete vessel failure. High shear deformation and matrix damage between outer hoop layer and in-plane helical layers can easily weaken the outer hoop layer due to the free-surface effect.³⁵

Therefore, when all the parameters were considered, the pressure vessel sample with XOX winding layer sequence appeared to be the optimum vessel for optimal performance. Thus, this vessel was chosen as the “Reference Vessel” for the investigation of the effects of three other processing parameters discussed in the next section.

Effects of three other winding parameters

Although “winding layer sequence” is considered as the most significant parameter, it is known that other towpreg dry winding parameters might influence the burst performance of the pressure vessels. In this study, after determining the optimum winding layer sequence (i.e., XOX), the effects of three other winding parameters were also investigated by keeping the XOX sequence constant. These selected parameters are “winding tension,” “helical band overlap,” and “helical pattern.” In the previous step, optimum values of these three parameters for the reference vessel were determined after several experimental trials. As listed in Table 9, values of these three parameters were selected as “lower” and “higher” values and “more complex” forms, respectively.

Table 9.

Values of the three other winding parameters used. Note that all vessels have the same winding layer sequence of XOX.

Pressure vessels	Winding tension (N)	Helical band overlap (%)	Helical pattern
Reference vessel	50	20	3/1
Lower winding tension	20	20	3/1
Higher helical band overlap	50	45	3/1
Complex helical pattern	50	20	17/1

After producing three pressure vessel samples for each parameter group, burst pressure tests were applied. Results compared to the Reference Vessel are tabulated in Table 10 with ± standard deviations, while burst pressure and hoop strain values are compared in Figure 12. Just after the burst tests, images of each group were examined to observe their failure modes as shown in Figure 12(a)–(c).

Table 10.

Effects of three other winding parameters on the burst performance and failure modes of the vessels.

	Reference vessel	Lower winding tension	Higher helical band overlap	Complex helical pattern
Burst pressure (MPa)	27.1 ± 0.6	20.8 ± 1.4	29.1 ± 0.7	25.7 ± 0.6
Hoop fiber strain (%)	1.55 ± 0.04	1.15 ± 0.06	1.60 ± 0.04	1.50 ± 0.05
Hoop strain/burst pressure (με/MPa)	572.5	514.3	560.4	578.7
Burst failure mode	Cylindrical region	Near dome region	Near dome region	Cylindrical region

Effects of lower winding tension

In the optimum winding layer sequence studies mentioned above, the applied tensile force during towpreg dry winding operations was 50 N. In order to reveal the effects of winding tension, a lower value, that is, 20 N tensile force, was applied.

Table 10 and Figure 12 indicated that the use of lower winding tension during towpreg winding operations decreased all burst performance values considerably. For example, the decrease in burst pressure was from 27 MPa down to 20 MPa, that is, a decrease of 23%. Similarly, the detrimental effect in hoop strain was a decrease of 26%, while this decrease in hoop strain/burst pressure ratio was 10%.

Moreover, as shown in Figure 12(a), the use of lower winding tension resulted in the shifting of the “safe cylindrical region” failure mode to the “unsafe near dome region” failure mode. Thus, it could be stated that for the burst performance of towpreg wound pressure vessels, the value of winding tension used during the process should not be much less than 50 N.

Effects of higher helical band overlap

During all types of filament winding processes, fiber “bundles” or “bands” always overlap with a certain amount. It might be thought that increasing the level of overlap might increase the burst pressure performance. On the other hand, increased fiber band overlap also increases the thickness of the vessel leading to weight increases, which might be considered a disadvantage in light-weight applications.

Although band overlap amount can be considered a pre-determined design variable for filament wound pressure vessels, the ability to inspect band overlap amount precisely during the process can be challenging, specifically for wet filament winding operations. Even though it is possible to control the bandwidth overlap amounts, dynamic and delicate fiber tension control becomes necessary. On the contrary, for dry towpreg winding, it is much easier to control the overlap percentage between the fiber bundles thanks to its tacky nature, controlled resin content, and less slip tendency.

In this study, normally, 20% band overlap was used during all towpreg winding operations. To observe the effects of using higher overlap values, 45% band overlap was used in the helical layers of towpreg winding operation. This overlap was not used for the hoop layers because higher levels of overlap in hoop winding trials resulted in significant thickness variations leading to non-uniform, low-quality hoop layer surfaces.

Table 10 and Figure 12 revealed that using a high band overlap of 45% in the helical layers resulted in certain increases in the two values of burst performance. The burst pressure value increased from 27 MPa to 29 MPa (an increase of 8%), while the increase in hoop strain value was only 3%.

On the other hand, two detrimental effects of using higher helical band overlap were also observed. One of them was a slight decrease of 2% in the hoop strain/burst pressure ratio, while the other one, as shown in Figure 12(b), was the shifting of “safe cylindrical region” failure mode into “unsafe near dome region” failure mode.

Thus, a trade-off analysis should be conducted before increasing the level of helical layer fiber band overlap, considering the advantages and disadvantages of the burst performance of towpreg wound pressure vessels.

Effects of complex helical pattern

During towpreg dry filament winding operations, the pattern used in the helical layers would cover the mandrel surface, forming different areas with different grid structures. What is important is the formation of “fiber undulation zones” in these helical layers because the number of fiber undulation zones might influence the burst performance of the pressure vessels.

Several studies highlighted possible adverse effects of undulation zones and mosaic pattern structure on the burst pressure performance of the wet filament wound vessels.^36,37 This study aimed to examine the impact of undulation zone quantity on the towpreg wound pressure vessel burst performance.

In this study, a typical “3/1 helical pattern” was used during all towpreg helical layer winding operations because the 3/1 helical pattern leads to only three different areas with only two undulation zones. In this section, to evaluate the effects of a more complicated pattern, “17/1 helical pattern” was used for comparison. As shown in Figure 13, using a complex 17/1 helical pattern resulted in many different grid structure areas with so many fiber undulation zones.

After the burst tests of these vessels, it was observed that there was no change in the “safe cylindrical region” failure mode (Figure 12(c)). Although the burst failure was still initiated from the cylindrical region at the center, a progressed hoop layer damage along the radial direction also contributed to the final failure of the vessel. This damage mechanism took place throughout the fiber undulation zones inside the interwoven structure.

On the other hand, the burst performance results given in Table 10 and Figure 12 indicated slight decreases in the values of both burst pressure and hoop strain; the decreases were 5% and 3%, respectively. Rousseau et al.³⁶ observed similar results for wet filament wound closed-end tubular structures. They indicated that damage growth in interweaving zones increased. The main reason for this effect was the presence of voids or fiber-free resin pockets that could occur in undulation zones, that is, crack initiation zones.

Thus, it can be pointed out that due to the very high number of fiber undulation zones acting as stress concentration zones, the use of complex helical patterns for the better burst performance of towpreg wound pressure vessels is not advantageous.

Conclusions

Compared to the traditional wet filament winding, the main difficulty observed was maintaining the “straight towpreg path” necessary for efficient winding operations. This problem was prevented by applying higher tension forces during dry winding. Other specific conclusions acquired from the two steps of this study can be summarized as follows:

(i) Carbon/Epoxy Towpreg Wound Flat Specimens

• Average fiber content of the samples was determined as 60 vol%, which is not easy to obtain that level homogenously in traditional wet wound parts because towpregs have a steady fiber/resin ratio.

• Rotational rheometer analysis revealed that storage modulus value (4.38 GPa) of the samples was almost not affected up to 100°C. The reduction was 26% at 120°C.

• Interlaminar shear strength tests indicated that ILSS value of 73 MPa is also consistent with the samples produced by conventional wet winding.

• Tensile strength (2290 MPa) and elastic modulus (162 GPa) values determined by unidirectional tensile tests revealed typical mechanical properties of carbon fiber/epoxy composite specimens.

(ii) Carbon/Epoxy Towpreg Wound Pressure Vessels

• Evaluation of the hydrostatic burst tests in terms of burst pressure, hoop strain, and safe failure mode revealed that the optimum pressure vessel performance could be obtained in the vessel samples with “helical–hoop–helical winding layer sequence.”

• Burst tests also indicated that other towpreg winding parameters might influence performance of the pressure vessels. For instance, the value of “winding tension” used during the process should not be less than 50 N. Use of lower winding tensions resulted in considerable decreases in burst pressure and hoop strain values with an unsafe failure mode.

• Use of “higher helical band overlap,” for example, 45% instead of typical overlap of 20%, might lead to certain advantages and disadvantages; thus, a trade-off analysis would be necessary.

• On the other hand, use of “complex helical pattern,” for example, 17/1 instead of typical pattern of 3/1, resulted in no advantages at all, due to basically higher number of undulation zones acting as stress concentration zones.

As a final remark, it can be stated that when the process parameters are appropriately determined, towpreg dry filament winding technique has the potential to enhance the filament wound parts in terms of both part quality and process convenience by overcoming the inherent challenges of the conventional wet filament winding process.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Cevdet Kaynak

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