Abstract
The application of composite pressure vessels is of great interest due to tremendous weight saving. Nevertheless, ageing of CFRP (carbon fiber reinforced plastic) composites pressure vessels underlies complex interactions between metallic liner and composite and is not fully understood yet. One main challenge is the application of appropriate testing methods for detecting the influences of pressure cycles and creep behavior on the material. This contribution presents results achieved by using conventional eddy current testing (ET) and high frequency ET. Here, we could show that also conventional ET with low frequencies are useable to investigate pressure vessels of different material combinations like aluminum-CFRP and synthetics-CFRP.
Introduction
UN Recommendations for the Transport of Dangerous Goods are regulating the test of pressure vessels used for storage of gases and liquids [1]. Hydraulic internal pressure test is the standard method for testing pressure vessels and must be performed before use and every 5 or 10 years. This procedure only detects leakage when there is a fully-grown crack through the liner. Slight tearing cannot be found with this method. Therefore, a forecast of the further useful life is not possible, especially in case of composite pressure vessels.
Conventional pressure vessels are made of aluminum or steel. Their long-time behavior is well known and they underlie strict orders regarding life time and period of inspection [2]. However, the weight of these vessels affects their application adversely. Therefore, a technology transfer of the NASA Firefighter’s Breathing System Program in the 1970s led to the development and commercialization of composite pressure vessels [3]. Subsequently, the use of composite pressure vessels to store dangerous goods like hydrogen or liquefied petroleum gas (LPG) rose sharply over the last decades due to tremendous weight saving. Several studies were conducted in order to investigate degradation due to temperature and load cycles, life time, and to predict safe service life [2,4–6]. Nevertheless, the long-time behavior of these vessels concerning ageing due to filling and discharging cycles over lifetime is not fully understood.
For this purpose, BAM investigates ageing of pressure vessels. Different non-destructive testing methods can be used to examine degradation of the CFRP like eddy current testing (ET). Normally, the applied frequencies ET of CFRP are in the order of 1 MHz up to 100 MHz due to the low conductivity of the carbon fibers (0.01 MS/m) and capacitive coupling between the fibers–also called high-frequency (HF-)ET. Nevertheless, conventional ET techniques can also be applied and provide enough information about the structural behavior even at a frequency of f = 5 kHz depending on the CFRP itself. Furthermore, low-frequency ET-techniques are capable to detect cracks in the metallic liner from the outside, which was successfully shown by Hatsukade et al. [7] who used a SQUID-ET-system for the detection of cracks in an aluminum liner.
Experimental methods and test samples
For the ET-measurements we used conventional technique and probes. The data acquisition for testing pressure vessels in the high frequency range was done by a vector network analyzer which works in a frequency range between 300 kHz and 2 GHz. In case of low frequency application, we used the conventional ET-system “B320” (Rohmann GmbH, Frankenthal, Germany). Two different types of probes were deployed–one type with cup cores (SK 14 TA-2 and SK 9 TA-2) and another type designed at BAM especially for high-resolution eddy current imaging (AN 05 and AN 16). All probes were manufactured at BAM.
(a) Setup for testing pressure vessels from the inside. The mounting for the ET-probe was designed at BAM. (b) Facility for high-frequency ET. The available frequencies range from 300 kHz up to 2 GHz. Data acquisition is done by a vector network analyzer.
Figure 1 shows two different test setups at BAM. Figure 1(a) presents the facility for internal testing of pressure vessels [8]. It consists of a special probe mounting to test the liner from the inside. The applied sensor is a absolute transformer probe. The second setup for high-frequency testing in Fig. 1(b) comprises a vector network analyzer (National Instruments Corp., Austin, Texas) with a frequency range from 300 kHz to 2 GHz used as test device. For HF-ET we use a direction-independent absolute sensor optimized for high frequencies. In this way, all fiber orientations are recorded equally. Depending on the direction, a direction-dependent evaluation is carried out after eddy current testing with image processing.
Ageing tests were carried out for a high number of pressure vessels (see Table 1) with two different liners –aluminum liner (type A) and plastic liner (type B). The thickness of the CFRP woven around the liners was for both types 8 mm. These vessels were brand new before investigation and initially tested before artificial ageing. Artificial ageing was performed with filling/discharging cycles till leakage.
Preliminary tests were performed before ageing investigation. Here, main task was to identify the capabilities of conventional ET for testing composite pressure vessels. For this purpose, we investigated composite pressure vessels with different liners and known defects/leakage. It was found out that conventional ET is suitable for composite pressure vessels. Defects could be detected in both steel and aluminum liners. In case of aluminum also slight tearing are detectable from the outside because the penetration depth is large enough to test through the thickness (4 mm) of the aluminum liner at applied frequency of f = 5 kHz. In addition, the CFRP texture could be resolved from the outside using this low frequency. The preliminary tests showed that low-frequency ET could be an alternative to iterative hydraulic pressure tests which is state of the art. For more details concerning preliminary test see [9].
Figure 2(a) and (b) show ET scans of a composite pressure vessel with aluminum liner of the inner wall and from the outside before starting the artificial ageing process, (c) and (d) show ET scans after artificial ageing until leakage. Reference marks were used to correlate the indications between scanning from the inside and outside as well as before and after ageing. In case of testing from the inside, we used a permanent magnet (black spot in Fig. 2(a) and (c)). At the same position an aluminum strip was placed for measurement from the outside (see Fig. 2(b) and (d)). The applied frequency was f = 300 kHz for internal testing. Here, penetration depth is of no importance since defects normally grow from the inside through the liner and are open to the inner surface. In case of testing from the outside using a frequency of f = 5 kHz we could resolve clearly the CFRP texture (Fig. 2(b) and (d)). This result somewhat surprised since normally frequencies in the MHz-range must be used to excite eddy currents in the CFRP due to capacitive coupling. This leads to the assumption that in case of the pressure vessels investigated in this study the fibers are galvanically coupled which increases eddy currents in the CFRP structure.
List of pressure vessels used for evaluation of ageing
List of pressure vessels used for evaluation of ageing
(a) and (b) presents testing results for a pressure vessel with aluminum liner before ageing from the inside (f = 300 kHz) and from the outside (f = 5 kHz), respectively. The black spot in (a) and (c) indicates the position of a permanent magnet at the outside used as 0° reference mark. In (b) and (d) an aluminum strip was placed at the same position. In (c) and (d) the results after ageing (5434 load cycles) are shown. Besides the reference marks a clear signal indicates a crack fully grown through the aluminum liner.
To investigate ageing due to filling cycles and pressurized storing we performed hydraulic load changes and internal pressure creep test. The load changes lead to a degradation of the aluminum-liner. After 5434 load cycles a leakage was detected which could be resolved with conventional ET from the outside through the CFRP as well as at the inner surface of the liner (see Fig. 2 (c) and (d)). The crack detected at the inner wall is relatively long. In case of testing from the outside, the leakage was more localized. Nevertheless, slight tearing could be resolved in Fig. 2(d) due to the high penetration depth of the low frequency applied.
Further investigation involved ageing of composite pressure vessels which completed a fully life cycle before they were separated. Here, testing results show a different defect behavior between “natural” and artificial ageing. Figure 3 presents four scans from the inside for different pressure vessels with aluminum liner. Two pressure vessels (no. 216 and 161) were separated at end of life time. After separation, they were aged with additional load cycles until leakage (Fig. 3 left). Two vessels, which are shown in Fig. 3 right, were brand new before ageing. In case of artificial ageing only one relatively long crack is detectable (see results in Fig. 3 right for pressure vessel A01 and A02). The results of the “natural” aged vessels show two different defect patterns. In case of vessel “216” a field of many cracks at the inner surface is visible beside additional indications. We assume that this vessel was stored over a certain amount of time. Water inside the pressure vessel could lead to this field of tearing due to corrosion. Pressure vessel “161” show a more statistical distribution. Here, a continuous use of the vessel could prevent the development of a field of defects.
Results of four pressure vessels tested from the inside. Left: both pressure vessels (no. 216 and no. 161) were used until end of life time and then aged until leakage. Cracks and tearing are visible in both cases. Right: Pressure vessels A01 and A02 were brand new before investigation also aged until leakage (cracks are clearly visible). Test frequency was 300 kHz for all measurements.
The results of Fig. 3 show that contrary to metallic pressure vessels “artificial” ageing of composite pressure vessels does not seem to fully represent “natural” ageing during life cycles.
Figure 4 shows the results of high frequency ET (f = 303 MHz) before (left) and after artificial ageing (middle) as well as the difference of both tests (right). Not only the texture of the carbon fiber is clearly visible. There are also some dark spots caused by inhomogeneities of the resin content. The interpretation of the small differences between the tests before and after ageing is very difficult. The slight differences may be caused by degradation of the composite but inaccuracy of the tests cannot be excluded. Damage of the CFRP like delamination or breaking of fiber strands was not detected. Further investigations like destructive testing are still pending.
High frequency eddy current testing results for a pressure vessel with plastic liner. Left: initial test. Middle: Test after ageing until leakage. Right: Difference between initial test and test after failure.
It was shown that conventional eddy current technique is capable to test composite pressure vessels having a metallic liner. For that reason, ET can be an alternative to common testing procedure since hydraulic internal pressure tests do not detect cracks till they are fully grown through the metallic liner. Here, early damage detection can be provided by an internal testing of the inner wall using eddy current technique.
Ageing behavior can be investigated with conventional and high-frequency ET. The CFRP texture was visible in both. In case of conventional ET, this behavior is not common. Normally, only in the high-frequency regime eddy currents introduced in the fibers are high enough to detect. Also, signals coming from the resin are more detectable in the high-frequency application. Here, capacitive effects get stronger. Nevertheless, the interpretation of the test results with HF-ET is more difficult and ageing has to be investigated in more details using destructive testing methods.