The launch vehicle tank bottom, as a critical load-bearing component of the propellant storage tank, directly determines the reliability of the flight mission due to the mechanical properties of its welded joints. This paper proposes a performance testing method for in-service components based on the TIME5150 Leeb hardness tester for aluminum alloy tank bottom structures using a hybrid welding process of friction stir welding (FSW) and variable polarity tungsten inert gas welding (VP-TIG). By analyzing the hardness distribution characteristics of typical weld areas on the tank bottom, the mechanical response laws of weak points such as “T” joints are revealed. The study shows that the TIME5150 Leeb hardness tester, with its portability, wide measurement range, and customizable material functions, can achieve in-situ hardness testing of the tank bottom under in-service conditions, providing key technical support for structural integrity assessment and life prediction.
Engineering Challenges of Launch Vehicle Propellant Tank Bottom Welding Technology
The bottom of a launch vehicle propellant tank is a typical ellipsoidal thin-walled structure, usually welded together from multiple aluminum alloy components such as forked rings, lobes, top covers, and center flanges. This structure withstands complex pressure loads during service, and its welding quality directly affects the success or failure of the launch mission.
In recent years, friction stir welding (FSW) technology has become a development trend in the manufacturing of aluminum alloy propellant tanks for launch vehicles due to its advantages of fewer welding defects, smaller deformation, and higher joint performance. However, limited by technical equipment conditions, actual production often involves a combination of friction stir welding and fusion welding (such as VP-TIG). The coexistence of these two welding processes leads to uneven weld microstructure, especially in the “T”-shaped joint area formed by the intersection of longitudinal and circumferential seams, which becomes a weak point in the load-bearing capacity of the tank bottom.
Technical Requirements for In-Service Inspection
For launch vehicle tank bottom structures already in use, accurately evaluating the mechanical properties of their welded joints without disassembly or damage is a pressing problem for the engineering community. Traditional destructive sampling and testing methods are unsuitable for in-service components, while conventional non-destructive testing methods (such as radiography and ultrasound) can detect macroscopic defects but are difficult to directly evaluate the degree of degradation of material mechanical properties.
Hardness testing, as a mechanical property indicator closely related to material strength and toughness, provides a new technical approach for the performance evaluation of in-service components. In particular, the advent of the Leeb hardness tester has made on-site hardness testing of large, non-removable components possible.
TIME5150 Leeb Hardness Tester Technical Characteristics
Working Principle and Technical Parameters
TIME5150 Leeb hardness tester uses the Leeb dynamic hardness testing principle: a specified mass impactor impacts the sample surface at a certain velocity under elastic force. The Leeb hardness value (HL) is calculated by measuring the ratio of the rebound velocity of the impactor at a distance of 1 mm from the sample surface to the impact velocity. ### 2.2 Product Features and Applicability
The TIME5150 series hardness tester boasts the following technical advantages:
Portability and Operability: The main unit measures only 177mm × 38mm × 29mm and weighs approximately 120g. It features a 128×64 graphic dot matrix OLED display, supporting one-handed operation and facilitating testing in confined spaces with curved surfaces.
Material Adaptability: It includes built-in hardness conversion tables for various materials such as steel, cast steel, alloy tool steel, stainless steel, gray cast iron, ductile iron, and cast aluminum alloys, covering commonly used aerospace materials like 2A14 aluminum alloy.
Custom Material Function: Users can generate their own custom hardness conversion tables through comparative tests, effectively solving the problems of inaccurate measurements and large errors in pre-installed conversion tables. This is particularly suitable for testing new aerospace materials and processes.
Data Management Capabilities: It can store 100 average measurements, supports real-time output via Bluetooth printer connection, and has a reserved mobile app data transmission function for easy database creation.
Method for Hardness Testing of Welded Area at Box Bottom
Structural Characteristics and Layout of Testing Points at Box Bottom
The test object is an ellipsoidal box bottom with a module of 1.6, made of 2A14 aluminum alloy, with a welded area thickness of 4.5 mm. The box bottom is constructed from a fork-shaped ring, eight lobed sections, a top cover, and a central flange, and includes three circumferential seams and eight longitudinal seams.
Based on the strain monitoring results during the hydraulic test, the strain concentration areas on the bottom of the test chamber are mainly concentrated in two locations:
- The “T”-shaped joint formed by the intersection of the longitudinal seam of the melon-shaped section and the circumferential seam of the top cover
- The outer area of the fork-shaped circumferential seam
Therefore, hardness testing points should be primarily located at:
- The intersection of the longitudinal seam of the melon-shaped section and the circumferential seam of the top cover (8 points)
- The intersection of the longitudinal seam of the melon-shaped section and the fork-shaped circumferential seam (8 points)
- The pure fusion weld area of the top cover circumferential seam (away from the longitudinal seam)
- The pure fusion weld area of the fork-shaped circumferential seam (tested on both the inner and outer sides)
Preparation and Parameter Settings Before Testing
Surface Treatment: According to the TIME5150 requirements, the surface roughness Ra of the test points should be ≤1.6μm. For the aluminum alloy bottom of the test chamber, the surface oxide layer and contaminants should be removed by polishing with a nylon brush (standard configuration) or fine sandpaper to ensure that the test points are clean and flat.
Material Selection: Select the corresponding aluminum alloy conversion table from the instrument’s material list for either “M01 – Steel and Cast Steel” or “M03 – Stainless Steel”. Since 2A14 aluminum alloy is an Al-Cu-Mg alloy with a hardness range of approximately 80-150 HB, it should be within the instrument’s measuring range.
Direction Setting: The TIME5150 has an automatic function to identify common impact directions. During testing, ensure the impact device is perpendicular to the surface being tested. For curved surfaces at the bottom of the test chamber, a special-shaped support ring (optional) can be used to ensure vertical positioning.
Hardness Testing and Data Acquisition
Perform 3-5 effective impacts at each test point and record the average value. Precautions include:
- The spacing between measuring points should be no less than 3mm to avoid overlap of hardened areas at adjacent measuring points.
- Check the indentation morphology after each measurement to ensure the correct impact direction.
- Regularly calibrate the instrument using a standard Leeb hardness block (high value 790±40HLD).
Analysis of Test Results and Engineering Applications
Hardness Characteristics of the “T” Joint Area
Hydraulic test strain measurement results show that the “T” joint formed by the intersection of the longitudinal seam of the melon lobes and the circumferential seam of the top cover is the first to enter the yield state under a pressure of 0.7MPa, with a longitudinal strain value reaching 6000με, significantly higher than the 2000με of the pure weld zone.
Combining hardness testing further reveals the intrinsic mechanism of performance degradation in this region:
- Due to undergoing two thermal cycles (FSW and VP-TIG), the microstructure of the “T”-shaped joint region experiences recrystallization and coarsening of age-induced precipitates, resulting in a 15%-25% decrease in hardness compared to the base material.
- The low-hardness zone and high-strain zone in this region highly overlap spatially, verifying the effectiveness of hardness testing as a mechanical property evaluation indicator.
Hardness Distribution Pattern of the Fork-Shaped Ring Joint
Strain measurements in the fork-shaped ring joint region exhibit a significant asymmetry: the outer measuring point yields under 0.65 MPa pressure, while the inner measuring point remains elastic throughout the test. This phenomenon is closely related to the rigidity distribution of the fork-shaped ring structure.
Hardness testing results show:
- The hardness value of the outer weld area is relatively low, approximately 75-85 HB.
- The hardness value of the inner weld area is relatively high, approximately 95-105 HB.
- The hardness value of the base material area is approximately 115-125 HB.
This hardness distribution pattern provides a quantitative basis for understanding the non-uniform load-bearing behavior of the fork-shaped ring joint. Engineering Application Value
The application value of the TIME5150 Leeb hardness tester in the in-service inspection of launch vehicle box bottoms is reflected in the following aspects:
In-situ inspection capability: No cutting or sampling is required; inspection can be performed directly on the assembled box bottom structure, enabling pinpoint monitoring of critical weld areas.
Batch inspection efficiency: The instrument can preset upper and lower hardness limits and provides out-of-tolerance alerts during testing, facilitating rapid screening of abnormal areas.
Data traceability: Measurement data can be printed out via Bluetooth or stored in a mobile app, providing a data foundation for establishing a full life-cycle structural health record.
Process optimization feedback: By comparing the hardness distribution characteristics under different welding process parameters, quantitative basis can be provided for the matching optimization of friction stir welding and fusion welding processes.
Conclusion:
(1) The TIME5150 Leeb hardness tester is characterized by its small size, light weight, wide measurement range, and simple operation, making it suitable for on-site hardness testing of in-service components of launch vehicle box bottoms. (2) The T-joint area and the outer side of the forked annular seam at the bottom of the box are key areas for hardness testing, and their hardness values show a good correlation with the strain response of the hydraulic test.
(3) Through systematic point-based testing and data analysis, a hardness distribution map of the weld area can be established, providing a basis for identifying structural weak points and assessing remaining service life.
(4) In accordance with the requirements of the aerospace industry standard ISO 10786 regarding the testing and inspection of space system structural components, the TIME5150 hardness testing method can serve as an important component of the box bottom structural integrity evaluation system.
With the development of reusable launch vehicle technology, the demand for performance monitoring of in-service components is becoming increasingly urgent. The TIME5150 Leeb hardness tester, with its portable, efficient, and non-destructive technical characteristics, will play an increasingly important role in the field of aerospace structural health monitoring.
