STP-PT-017

PROPERTIES FOR COMPOSITE MATERIALS IN HYDROGEN SERVICE

STP-PT-017

PROPERTIES FOR COMPOSITE MATERIALS IN HYDROGEN SERVICE

Prepared by:

Craig Webster, P.Eng. Powertech Labs Inc.

Date of Issuance: March 19, 2008

This report documents the work sponsored by National Renewable Energy Laboratory (NREL) and the ASME Standards Technology, LLC (ASME ST-LLC).

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TABLE OF CONTENTS

Foreword vii

Abstract ix

1. PURPOSE AND USE 1

2. HYDROGEN EMBRITTLEMENT RESISTANCE OF AA6061-T6 AT 103 MPa (15,000 PSI) . 2 2.1 Summary 2

   1. Applicable Standards 2

   2. Material 2

      1. Chemistry 2

      2. Mechanical Properties 3

   3. Test Procedure 3

      1. Specimen Preparation 3

      2. Specimen Loading 4

      3. Exposure to Hydrogen Gas. 4

      4. Post-Test Procedure 4

   4. Macroscopic Results 5

   5. Scanning Electron Microscopy (SEM) Results 5

   6. Conclusion 5

3. INVESTIGATION OF FILAMENT WINDING LARGE DIAMETER THIN-WALLED

LONG LENGTH VESSELS 11

1. Background 11

2. Measurements on Tanks 11

3. Test Data on Tanks 11

4. Manufacturing Considerations 14

   1. Dynetek Industries 14

   2. Lincoln Composites 14

5. Conclusion 14

1. EFFECT OF COMPOSITE WALL THICKNESS ON FIBER STRESS 15

   1. Background 15

   2. Tank Manufacture 15

   3. Description of Tests 18

   4. Discussion 21

   5. Conclusion 21

2. TEST METHODS FOR NON-METALLIC MATERIALS IN HIGH PRESSURE

   HYDROGEN 22

   1. Summary 22

   2. Applicable Standards 22

   3. Background 22

   4. Materials 23

   5. Test Procedure 23

      1. Tensile Test Specimens 23

      2. Environment Exposure 25

      3. Tension Tests 25

   6. Results and Discussion 25

   7. Conclusions 42

   8. Recommendations 42

References 44

Acknowledgements 45

Abbreviations and Acronyms 46

LIST OF TABLES

Table 1 - Chemistry of Aluminum Alloy 6061 (ASTM B210) 2

Table 2 - Measured Mechanical Properties of 6061-T6 Alloys 3

Table 3 - Summary of Test Conditions 4

Table 4 - Results of FTIR Testing 23

Table 5 - Initial Geometry Measurements 24

Table 6 - Tensile Test Results for Material G 28

Table 7 - Tensile Test Results for Material K 31

Table 8 - Tensile Test Results for Material Q 35

Table 9 - Tensile Test Results for Material T 38

LIST OF FIGURES

Figure 1 - Compact Tension Specimen and Taper Pin Geometry 6

Figure 2 - Fatigue Pre-Cracking of Compact Tension Specimen 7

Figure 3 - Pin-Loading of Compact Tension Specimen 7

Figure 4 - Pin Loaded Sample. 8

Figure 5 - Fracture Surface of Specimen 1A 8

Figure 6 - Sample 1B Fracture Surface 8

Figure 7 - Sample 1C Fracture Surface 9

Figure 8 - Sample 1D Fracture Surface 9

Figure 9 - Sample 1A Fracture Surface (22x) 10

Figure 10 - Sample 1A Boundary between Cracks (110x) 10

Figure 11 - Sample 1B Boundary between Cracks (110x) 10

Figure 12 - Straight Edge Illustrating Profile on a Type 3 Design of 2800 mm (110 in) Length 12

Figure 13 - Detail of Profile on a Type 3 Design of 2800 mm (110 in) Length 12

Figure 14 - Straight-Edge Profile on Type 4 Tank of 3058 mm (120 in) Length 12

Figure 15 - Detail of Profile Variation on Type 4 Tank of 3058 mm (120 in) Length 13

Figure 16 - Straight-Edge Profile on Type 3 Tank of 1811 mm (71.3 in) Length 13

Figure 17 - Straight-Edge Profile on Type 2 Tank of 6000 mm (236 in) Length 13

Figure 18 - Detail of Strain Gage Used on Internal Fiber Layers 16

Figure 19 - Installation of Strain Gage on Sidewall during Winding (Note White Plastic Liner Showing under Carbon Fiber Wrap Layers) 17

Figure 20 - Detail of Strain Gage Installed on Innermost Fiber Layers 17

Figure 21 - Pressure Vessel Label 18

Figure 22 - Strain Measurements on Innermost Sidewall Fiber Layers – 241.3 MPa 19

Figure 23 - Strain Measurements on Innermost Dome Fiber Layers - 241.3 MPa. 19

Figure 24 - Appearance of Tank after Pressurization to 241.3 MPa - Note Also Strain Gages

Attached to External Dome and Sidewall 20

Figure 25 - Comparison of Internal and External Strain on Dome and Sidewall - 103 MPa 20

Figure 26 - Rectangular Tensile Test Specimen Dimensions 23

Figure 27 - As-Machined Plastic Tensile Specimens 24

Figure 28 - Bubbles Formed in Material T During Hydrogen Exposure 25

Figure 29 - Optical Micrograph of Bubble in Hydrogen-Exposed Sample T8 (7x) 26

Figure 30 - Optical Micrograph of Pore in as-Received Sample T2 (45x) 26

Figure 31 - High Temperature Ageing of Liner Materials 27

Figure 32 - Material Q Discoloration from High Temperature Ageing 27

Figure 33 - Stress-Strain Curves for as-Received Samples G1-3 28

Figure 34 - Fractured Samples G1-3 29

Figure 35 - Stress-Strain Curves for High-Temperature Aged Specimens G4-6 29

Figure 36 - Fractured Samples G4-6 30

Figure 37 - Stress-Strain Curves for Hydrogen-Exposed Specimens G7-9 30

Figure 38 - Fractured Samples G4-6 31

Figure 39 - Stress-Strain Curves for as-Received Samples K1-3 32

Figure 40 - Fractured Samples K1-3 32

Figure 41 - Stress-Strain Curves for High-Temperature Aged Specimens K4-6 33

Figure 42 - Fractured Samples K4-6 33

Figure 43 - Stress-Strain Curves for Hydrogen-Exposed Specimens K7-9 34

Figure 44 - Fractured Samples K4-6 34

Figure 45 - Stress-Strain Curves for as-Received Samples Q1-3 35

Figure 46 - Fractured Samples Q1-3 36

Figure 47 - Stress-Strain Curves for High-Temperature Aged Specimens Q4-6 36

Figure 48 - Fractured Samples Q4-6 37

Figure 49 - Stress-Strain Curves for Hydrogen-Exposed Specimens Q7-9 37

Figure 50 - Fractured Samples Q4-6 38

Figure 51 - Stress-Strain Curves for as-Received Samples T1-3 39

Figure 52 - Fractured Samples T1-3 39

Figure 53 - Stress-Strain Curves for High-Temperature Aged Specimens T4-6 40

Figure 54 - Fractured Samples T4-6 40

Figure 55 - Stress-Strain Curves for Hydrogen-Exposed Specimens T7-9 41

Figure 56 - Fractured Samples T4-6 41

FOREWORD

Commercialization of hydrogen fuel cells, in particular fuel cell vehicles, will require development of an extensive hydrogen infrastructure comparable to that which exists today for petroleum. This infrastructure must include the means to safely and efficiently generate, transport, distribute, store and use hydrogen as a fuel. Standardization of pressure retaining components, such as tanks, piping and pipelines, will enable hydrogen infrastructure development by establishing confidence in the technical integrity of products.

Since 1884, the American Society of Mechanical Engineers (ASME) has been developing codes and standards (C&S) that protect public health and safety. The traditional approach to standards development involved writing prescriptive standards only after technology has been established and commercialized. With the push toward a hydrogen economy, government and industry have realized that they cannot afford a hydrogen-related safety incident that may undermine consumer confidence. As a result, ASME has adopted a more anticipatory approach to standardization for hydrogen infrastructure which involves writing standards with more performance based requirements in parallel with technology development and before commercialization has begun.

Today, ASME codes and standards are used for hydrogen storage, transmission and distribution. The anticipated requirements of the hydrogen economy will require local refueling stations with the capability to fill gaseous hydrogen vehicle tanks rapidly to pressures as high as 15,000 psig (100 MPa). Although current standards could be used to build pressure tanks, piping and pipelines meeting these operating requirements, it is likely that the resulting components would not, as a practical matter, enable commercialization of the technology.

ASME has worked closely with the Department of Energy (DOE), national laboratories and other standards developing organizations (SDOs) to identify lead organizations to address the need for standards for hydrogen applications. ASME was selected to lead the efforts for pressure tanks, piping and pipelines for storage, transportation and distribution of hydrogen. Initial work of the ASME Hydrogen Steering Committee led to the formation of volunteer task forces under the ASME Board on Pressure Technology Codes and Standards (BPTCS) to explore the standardization requirements for storage tanks, transportation tanks, portable tanks, piping and pipelines for hydrogen-specific applications. The task forces submitted their recommendations at the end of 2003 and these recommendations led to initiation of standards actions, formation of project teams and commencement of supporting research.

The ASME Boiler and Pressure Vessel (BPV) Standards Committee appointed a project team to develop new Code rules in the Boiler and Pressure Vessel Code Section VIII (pressure vessels) and Section XII (transport tanks) for hydrogen storage and transport tanks to be used in the storage and transport of liquid and gaseous hydrogen and metal hydrides. Rules for gaseous storage tanks with maximum allowable working pressures (MAWPs) up to 15,000 psig (100 MPa) will be needed. Research activities are being coordinated to develop data and technical reports concurrent with standards development and have been prioritized per Project Team needs. The Project Team may identify additional needs and gaps as drafts are developed.

The Technical Reports to be developed will establish data and other information to be used to support and facilitate separate initiatives to develop ASME standards for the hydrogen infrastructure. These reports will target specific disciplines and fill the gaps identified by ASME’s hydrogen task forces. An initial report, developed under the sponsorship of the National Renewable Energy Laboratory (NREL), Hydrogen Standardization Interim Report for Tanks, Piping and Pipelines was issued on May 3, 2005. This interim report addressed priority topical areas within each of the four pressure technology applications for hydrogen infrastructure development: storage (stationary) tanks, transport tanks, piping and pipelines and vehicle fuel tanks.

The present report builds on the work of the interim report and investigates properties of composite materials in hydrogen service.

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ABSTRACT

Studies were conducted to address three specific questions related to the use of composite-reinforced pressure vessel designs for the transportation of compressed hydrogen at pressures up to 103 MPa (15,000 psi). These studies involved determining the hydrogen embrittlement resistance of AA6061- T6 aluminum alloy material typically used as a liner in composite-reinforced cylinder designs; determining whether composite-reinforced pressure vessels using plastic or thin-wall metallic liners were subject to distortion during the filament winding process; and identifying test methods that can be used to establish the long-term performance of non-metallic materials exposed to high-pressure hydrogen environments.

Long-term hydrogen embrittlement tests were conducted on AA6061-T6 samples using compact tension specimens according to ISO 11114-4, Method C. Specimens were fatigue pre-cracked, following which the fatigue cracks were pre-loaded to various stress intensity factors. The specimens were then inserted into a pressure vessel containing hydrogen at 103 MPa (15,000 psi). After 1,000 hours exposure, there was no evidence observed of any hydrogen-induced crack growth in the aluminum.

A variety of composite-reinforced pressure vessels that use plastic liners and thin-walled aluminum liners, and having lengths up to 3058 mm, were inspected. There was no evidence of any axial distortion. In addition, pressure cycle and burst test data between composite-reinforced pressure vessels of relatively short length and relatively long length were compared, confirming that the designs of different length had the same performance.

Plastic liner materials cut from four high-pressure hydrogen storage tanks of different design were tested for effects of high-temperature ageing and of long-term exposure to high-pressure hydrogen. Specimens were tensile tested in the as-received condition, after one-month exposure to 70 MPa (10,000 psi) hydrogen and after one-month exposure to 85˚C atmosphere. The 70 MPa hydrogen exposure for 30 days had no noticeable effect on the strength of the materials but did create some bubbles in the surface. On average, ageing three of the materials for 30 days at 85˚C caused an increase in tensile strength. It was concluded that more samples needed to be tested to develop a more acceptable statistic average of the mechanical properties, and that full-scale testing should be performed on complete pressure vessels at both high and low service temperatures with hydrogen pressure.

INTENTIONALLY LEFT BLANK

1. PURPOSE AND USE

   This report is intended to answer three specific questions related to the use of specific materials and composite-reinforced pressure vessel designs for the transportation of compressed hydrogen at pressures up to 103 MPa (15,000 psi). The individual studies involve:

   • Determining the propensity for hydrogen embrittlement to occur in AA6061-T6 aluminum alloy material typically used as a liner in composite-reinforced cylinder designs.

   • Determining whether the manufacture of composite-reinforced pressure vessels of relatively long length over thin-wall metallic liners or plastic liners can result in sagging of the vessels, thus affecting the tension in the filament winding process.

   • Identifying test methods that can be used to establish the long-term performance of non-metallic materials exposed to high-pressure hydrogen environments.

   
   

2. HYDROGEN EMBRITTLEMENT RESISTANCE OF AA6061-T6 AT 103 MPA (15,000 PSI)

   
   

   1. Summary

      Long-term hydrogen embrittlement tests were conducted according to ISO 11114-4, Method C [1]. Compact tension (CT) specimens were withdrawn from the sidewall of a 6061-T6 aluminum alloy cylinder liner. Four samples were machined from the material with the direction of crack propagation longitudinal to the cylinder liner axis. Each specimen was fatigue pre-cracked and then a pin was inserted into a tapered hole to impart a stress intensity of 21.4 MPa-m for 6061-T6. After loading, the specimens were placed in a cylinder and pressurized to 103 MPa (15,000 psi) with 100% hydrogen.

      After 1000 hours of exposure, the samples were visually examined for obvious signs of crack propagation. To ensure that the crack mouth opening displacement (CMOD) had not changed drastically during the test, their values were measured and corresponding stress intensities were calculated. Since crack extension was not visible, the specimens were cyclically loaded to create fatigue striations, marking the end of any hydrogen induced crack propagation. After breaking apart the specimens, it was determined that the cracks did not propagate any further. This was confirmed by taking high magnification micrographs with a scanning electron microscope (SEM).

      
      

   2. Applicable Standards

      The test was performed according to Method C of ISO 11114-4, “Transportable Gas Cylinders– Compatibility of Cylinder and Valve Materials with Gas Contents–Part 4: Test Methods for Selecting Metallic Material Resistant to Hydrogen Embrittlement.” Specimen dimensions and the fatigue pre- cracking procedure were taken from ISO 7539-6, “Corrosion of Metals and Alloys–Stress Corrosion Testing” [2] and ASTM E1820-01, “Standard Test Method for Measurement of Fracture Toughness” [3].

      
      

   3. Material

2.3.1 Chemistry

The 6061-T6 aluminum alloy was removed from the liner of a hoop-wrapped cylinder manufactured by the CNG Cylinder Co. The design chemistry is described in Table 1.

Table 1 - Chemistry of Aluminum Alloy 6061 (ASTM B210)