Abstract: The development of aerospace technology has promoted the research of low temperature performance of materials. The fracture toughness of high-performance aluminum alloy materials at low temperatures is gradually receiving attention. In this paper, the commonly used methods and criteria for measuring fracture toughness are introduced. The status quo of fracture properties of aluminum alloys and their joints at home and abroad are analyzed, and the evaluation of fracture toughness evaluation of 2219 aluminum alloys is proposed. Later pointed out that China's lack of assessment of low-temperature fracture performance and the need for improvement.
Keywords: Aluminum alloy Welded joints Low temperature Fracture toughness
Preface
With the rapid development of aerospace technology, the demand for ultra-low temperature materials has become increasingly urgent. Such as rocket liquefier containers, liquefaction chillers, research cryostats, etc., accompanied by the material requirements for ultra-low temperature is increasingly strict [1]. Among various materials, high-strength aluminum alloy materials have low density, non-magnetic properties, stable alloy phases at low temperatures, low specific resistance in a magnetic field, good airtightness, and rapid decay of induced radiation energy, and thus are an important type of low temperature. Materials are studied and applied [2,3].
In aerospace materials, the structure is required to be very compact, with no parasitic weight, but also to ensure safety and reliability, the traditional mechanical strength and toughness index requirements have been difficult to meet the requirements, reliability evaluation technology based on fracture mechanics has gradually become the development trend of structural evaluation. However, due to the limitations of low-temperature experimental conditions and technologies, the evaluation standards for low-temperature properties of aluminum alloys are still not perfect. Therefore, the research on low-temperature properties of high-strength aluminum alloy materials and reliability evaluation techniques are not commensurate with the practical application of low-temperature materials. Fracture properties are less studied.
Welding is an important processing method for high-performance aluminum alloy structures. Welded joint is a structural body with mechanical and geometric inhomogeneity. Cracks and other defects are easy to appear in three different positions of the weld, fusion line and heat affected zone. As the weak link in the entire welded structure, the welded joints require low temperature performance is an important indicator of the safety and reliability of the entire structure. This article focuses on the research work on the low-temperature fracture properties of aluminum alloy base metal and welded joints, and evaluates the fracture toughness of 2219 aluminum alloy.
Fracture mechanics theory
1.1 Fracture Mechanics Criterion
With the development of fracture mechanics in recent years, a more appropriate measure has become fracture toughness in evaluating structural performance. The ability of a material to resist crack propagation in fracture mechanics is called fracture toughness. In actual engineering applications, we use the fracture mechanics damage criteria? How to use fracture mechanics to guide selection and determine fracture toughness? These are the problems that must be solved first. At present, there are many fracture mechanics fracture criteria, and their characteristics and starting points are different. For example, linear elastic fracture mechanics (KIC) can be considered as the stress criterion, crack open displacement (COD) can be considered as the displacement criterion, J integral can be considered as the energy criterion, and the size Ï of the plastic zone can be considered as the strain criterion, etc. .
What are the criteria for assessing structural problems? Which is more appropriate? To this end, it is necessary to understand the characteristics, constraints, advantages, and disadvantages of these criteria. Linear elastic fracture mechanics applies to plane strain or small-range yield conditions; for large range yield ,
,
Criteria for the full yield
No longer valid, only used
with
;but
The theory is not yet perfect, and the J-integral method is a promising method in elastoplastic fracture mechanics [4].
1.2 fracture toughness test method
Now we will introduce the test of small sample in fracture toughness test.
(1) Plane Strain Fracture Toughness Test (KIC Test)
It is a static bending test, using a special clamp strain gauge to find the displacement of the notch, and then find the KIC value according to the relationship with the load. However, in this method, the lateral contraction of the crack tip must be a plane strain state. In order to satisfy this condition, there are problems such as testing at a lower temperature than the use temperature of the product, or the necessity of using extremely large-sized specimens. This method uses three-point bending specimens, compact tension specimens, arched three-point bending specimens. The standard test method for fracture toughness of plane strain is that all fracture toughness test methods have high accuracy and complete data. However, the size of the sample is large, the test period is long, and the cost is high.
(2) COD test
It is original by Cottrell and Wells and is not limited by the plane strain state. The current COD criteria have been widely used in the assessment of crack resistance of welded structures. Although the sample shape and loading method of this method are similar to those of the KIC test, the width of the sample is taken as the thickness of the material to be tested, and the definition of the load value (PQ) used for fracture toughness calculation is not introduced. The test becomes easy. In addition, only the section that is brittle after the test is regarded as effective, and the converted value of COD can be calculated from the displacement (Vc) of the clip-type strain gauge at the time of occurrence of the fracture.
(3) JIC test
Corresponding to the British COD test is the United States proposed JIC test. Since JR Rice proposed J integral, J integral has been widely used in fracture mechanics. Begley and Landes proposed J-breaker criteria earlier based on experiments, and EPRI (American Electric Power Research Institute) further pointed out J-integral engineering calculation methods and evaluation criteria. With the J-integral, the thickness of the test specimen can be greatly reduced.
1.3 Experimental criteria for fracture mechanics
The test of KIC has always used the standard of ASTM E399-72 in the United States. In 1979, the standard of YB947-78 “Test method for planar strain fracture toughness of metallic materials†was established in China and was widely tried in China. In 1984, China established the equivalent standard in the United States, which is GB4161-84 “Fracture Toughness KIC Test Method for Metallic Materialsâ€.
In 1972, the British Mechanical Engineering Industry Standards Conference issued the draft DD19 crack open displacement (COD) test program. In China, 80 years have also formulated relevant standards, GB/T 2358-80 "Cracking Displacement (COD) Test Method". Related standards include the United States ASTM E1290 -02e1. In China JB/T4291-86, test methods for crack open displacement (COD) of welded joints were developed.
For the JIC test, China has the standard GB/T2038-91 “Metallic ductility fracture toughness JIC test methodâ€. The United States ASTM E 813-1989 “Test method for fracture toughness of JIC†was later supplemented and refined. The newer version is ASTM E1820-2006e1.
With the development and application of fracture mechanics, many countries have formulated and issued test standards for fracture mechanics parameters KIC, COD, and JIC. The International Organization for Standardization has also established relevant standards, such as ISO “Unified Test Method for Determination of Quasi-Static Fracture Toughness of Metal Materialsâ€. In recent years, the British Welding Research Institute has proposed the BS7448 standard, “Measurement of KIC, Limit COD, and Limit J Integral Values ​​for Metallic Materials [5]â€, which standardizes the testing of three fracture mechanics parameters: KIC, COD, and JIC. It has received the attention of the International Welding Society and has been promoted and applied. It has now been adopted by the International Bureau of Standards and numbered ISO/TC164/SC4-N400 [6].
The various fracture parameters are linked as follows:
use estimate
The formula is:
(plane stress state);
(plane strain state)
These relationships are only available under online elasticity conditions. Equal to energy release rate
It was strictly established. In this area, there is an appropriate limit to the size of the pattern.
(plane strain state) expression is more appropriate.
use estimate
The formula is
parameter Is the constraint factor, and 1<
<2.
So by the above formula can be drawn
In the formula For dimensionless constants, for large range yields, 1<
<2.
The previous tests KIC, COD, and JIC were also carried out dynamically in addition to static loads. These tests are called dynamic fracture toughness tests, but the test equipment is more complicated. In addition, there are many other test methods, such as the Lzod test and the Schneider test similar to the Becker test, but they are rarely used. In addition, there have been popular methods such as Kahen, Tipper, Van Derven, Comal, and Lehi [7].
2. Research status of fracture toughness
Many aluminum alloys work at low temperatures and therefore must know their fracture toughness at low temperatures. Table 1 shows the test data of fracture toughness of 2024 and 2124 alloys by an organization in Russia [8].
Table 1 Fracture toughness parameters of semi-finished products of 2024 and 2124 alloy at room temperature and low temperature
alloy
Semi-finished product type
Sampling direction
Test temperature °C
KIC kg/mm ​​3/2
2024T
Extrusion strip
(65×200mm)
Longitudinal
20
-196
120.0
180.0
Wide
Same as above
99.0
116.0
Gao Xiang
Same as above
94.0
98.0
2124
Extrusion strip
(65×200mm)
Longitudinal
20
-196
148.0
197.0
Wide
Same as above
105.0
138.5
Gao Xiang
Same as above
96.0
105.0
2024T1
Extrusion strip
(65×200mm)
Longitudinal
20
-196
140.0
169.0
Gao Xiang
Same as above
64.7
62.5
2024
Unrecrystallized strip
(12×75mm)
Longitudinal
20
-196
121.0
143.0
Recrystallization strip
(12×75mm)
Longitudinal
20
-196
135.0
161.0
2024-T851
Thick plate
(B=35mm)
Longitudinal
twenty four
-80
-196
71
77
78
In this test, in order to ascertain the true variation of KIC with temperature drop, two or three samples were taken for each state of the alloy, and the method of measuring the fracture toughness of a sample multiple times was tested twice. The KIC was first measured at room temperature until fracture, and fatigue cracks were recreated in the sample, and then tested in liquid nitrogen at -196°C.
From the results shown in the table, it can be seen that the type of semi-finished product and the pressure processing method (the extruded strip of 65×200 and 12×75 mm in cross-section, 35 mm thick plate) and the purity of the alloy (Fe, Si impurities are each less than 0.01 ), heat treatment methods (artificial and natural aging), sampling direction (longitudinal, widthwise, and high direction), and tensile straightening (T851) prior to quenching after aging are not relevant, KIC values ​​increase with decreasing temperature.
The fracture toughness KIC of commonly used aluminum alloy structural materials can generally be found in the manual (usually at room temperature), while the KIC for the weld center, heat affected zone, and fusion zone material must be determined experimentally.
The literature [9] has tested and studied the fracture toughness JIC of the tank plate LD10 aluminum alloy and its weldment. Since the thickness of the aluminum alloy plate to be measured is 13 mm, JIC is determined using the J-integration method because the plate is not thin enough to satisfy the plane strain state. The authors used three-point bending specimens. The cracks were cut from the wire and opened in the base metal, weld and HAZ. The location of cracks in the welds and heat affected zone is referred to BS7448: 1997-Part II. The experimental process is carried out according to GB/T 2038-1991. After loading and unloading, the sample is pressed off and the crack propagation amount Δa and fracture toughness are calculated from the load displacement curve. Then, according to the empirical formula J=C1ΔaC2, the intersection point of the Δa=0.20mm bias line is the JIC to be measured. JIC's effectiveness is judged later. The results show that a large range of passivation occurs in the crack tip of the specimen in the heat affected zone of LD10 aluminum alloy, and the tear resistance is excellent. The fracture toughness JIC is 1.7 times that of the base metal. This is due to the influence of heat during welding. Material structure changes. The fracture toughness of the LD10 aluminum alloy weld is lower than that of the base metal, and there are defects such as impurities and pores in the weld.
In [10], for the unpenetrated cracks in the propellant tank structure, the fracture front-end stress intensity factor KI is determined by fracture theory, and then the weld center, fusion line, and heat affected zone are respectively selected for the welding sample. Typical location prefabricated surface cracks, find KIC, compare sizes.
In [11], the surface cracking method was used to study the fracture performance of aerospace aluminum alloys at low temperature (20K) using a self-developed low temperature multi-sample tensile device. The sample is an elliptical notch on the surface of the weld, and by controlling the fatigue process, a suitable surface crack is obtained. Then after loading, temperature control, acquisition and other parts. What is obtained later is the relationship curve between elongation and stress of the test piece, and the relationship between crack open displacement and stress cannot be obtained directly.
The literature [12] analyzed the fracture toughness of the high and low mating weld joints with all parent and total welds. The J-integral test results show that for the 9Cr-1Mo, the J-integral parameters of the low-grade welded joints with 2-1/4Cr-1Mo and BX52 as the parent metal are reduced in descending order of the total parent metal, welded joints, and total welds. The high-grade matching is just the opposite of the low-matching, and as the width of the weld increases, the difference between the J-integral value of the welded joint of the material set and its total parent material increases, and the difference with the total weld material result gradually decreases.
The study of the fracture toughness of welded joints is not thorough enough, especially at low temperatures, for further study.
3.2219 aluminum alloy welded structure low temperature fracture toughness test program
The heat-treated, strengthened 2219 aluminum alloy is a lightweight, high-strength structural material for aerospace products, with an operating temperature range of -250°C to +250°C. As early as the 1960s, the United States began to study the use of 2219 aluminum alloy as a low-temperature fuel tank for launch vehicles. The structural material of Russia's "Energy" launch vehicle tank is the 1201 aluminum alloy (Russian aluminum alloy No.) with similar composition and performance to 2219 aluminum alloy. In the aerospace field, reliability and safety are more important indicators. Only by fully grasping the mechanical properties data of the alloy and analyzing it can we guarantee safety. For the time being, China still lacks comprehensive test data on the mechanical properties of 2219 aluminum alloy. Therefore, it is necessary to determine the mechanical and fracture mechanical properties of the low temperature material 2219 aluminum alloy and its welded joints.
At present, 2219 aluminum alloys are used for the storage tanks of rockets. The welding methods mainly include welding methods and friction welding methods. Fracture mechanics evaluation is performed for 2219 aluminum alloy base materials and welded joints in different states.
For the above test work, a variety of mechanical performance parameters for the various temperatures of the plate and the welded joint should be determined under certain conditions of certain materials and certain welding methods. There are several steps for evaluating its low temperature fracture toughness:
(1) Select Reference Standard
For the assessment criteria of fracture toughness, our country has not developed very well. For the aluminum alloy base metal, reference can be made to the national standard GB/T 2038-1991 "JIC test method for ductile fracture toughness of metal materials" and GB 4161-84 "Fracture toughness KIC test method for plane strain of metal materials". For the determination of welded joints, China has not established relevant standards, and there is no standard for fracture toughness testing at low temperatures. The British Standard BS 7448-1997 “Measurement of KIC, Limit COD and Limiting J Integral Values ​​of Metallic Materials†provides relevant regulations for the fracture toughness test of welded joints at room temperature and is included in ISO.
(2) Select test plan
Since the aluminum alloy plate to be tested is thin and does not meet the plane strain condition, the JIC for the base material and the welded joint can only be tested by the J-integral method. As for the value of its KIC, the KIC can be calculated by referring to the relationship between JIC and KIC in the BS7448 standard. Determination of the JIC of the base metal at low temperature can be referred to GB/T 2038-1991, but there is no provision in this standard for the applicable temperature. For the determination of JIC for weld joint welds, heat affected zone and fusion zone, there is no reference standard in China. The reference standard is British Standard BS 7448-1997, although this standard still does not specify that it can be applied at low temperatures.
(3) Data Analysis Methods
After the fracture toughness data of the parent metal and welded joints are measured, the data needs to be analyzed. We can calculate an average value in the test results of multiple samples, but this does not truly reflect the fracture mechanics properties of aluminum alloy materials and their welded joints. In terms of mathematical theory, there is only 50% reliability. In the aerospace field, the reliability of materials is extremely demanding. The 50% confidence level can only satisfy our basic understanding of materials. Therefore, for the analysis of the fracture properties of the 2219 aluminum alloy of the carrier rocket tank, we need to master the confidence of 95% or even 98.5%. Therefore, the data needs to be analyzed by mathematical statistics.
Conclusion
Mechanical performance testing is a must for any type of welded structure before it is used, especially for welded structures used in aerospace. The traditional strength index and toughness index of mechanical properties can not meet the increasingly stringent requirements of modern materials. The fracture toughness test has been paid more attention with the development of fracture mechanics.
The test of the tensile properties of metal structural materials and welded joints was developed in China as early as the 1980s and has been refined in recent years. However, the development of standards for the fracture toughness testing of metallic structural materials is not perfect. With the wide application of cryogenic technology in aerospace, nuclear physics, and electronic engineering, China should strengthen the assessment techniques for fracture toughness testing of cryogenic materials. In order to better promote the wide application of low temperature materials. As an important processing method, welding should also be put on the agenda for the evaluation of welding joints that are prone to defects. At present, there is no relevant standard for the fracture toughness test of welded joints in China. The British BS 7448 standard does not describe the matters that should be noted in the determination work at low temperatures. Therefore, Chinese scientists and researchers should strengthen their study of fracture toughness knowledge and work out their own standards as soon as possible.
references
[1] Yeno Chair Macro [Japanese], translated by Zhang Xingren. Development of high-strength and high-toughness aluminum alloy for ultra-low temperature [J]. In 1991, 37-48.
[2] Shigeoki SAJI et al. Mechanical properties of aluminum alloys at very low temperature. Light Metal (Japan), 1989:39(8):574.
[3] Yoshimitsu. Miyagi et al. Characteristics and application of aluminum alloys at cryogenic temperature. R & D Kobe Steel Engineering. Reports (Japan), 1984:34(3):67.
[4] Begley, J. A. Landes, Fracture and Toughness, ASTM STP 514 (1972), 1-20.
[5] BS 7448: Part 4, 1997, Method for determination of fracture resistance curves and initiation values ​​for stable crack extension in metallic materials [S].
[6]Hao Lixing. Fracture behavior and evaluation of welded structures [M]. Beijing: Mechanical Industry Press, 2000.
[7] Inagaki Inada, Kadada Nakumi [Day]. Low temperature material standard and fracture toughness test [J]. Foreign Technology, 90-99
[8] Kurdishov, Smolentsev [Soviet Union], Gao Yunzheng, et al. Aluminum alloy fracture toughness [M]. Beijing: Metallurgical Industry Press, 1980.116-118.
[9] Yang Haisheng, Chang Xinlong. Determination of fracture toughness of LD10 aluminum alloy by three-point bending test JIC [J]. Physical and chemical inspections - Physics Volume, 2005, 41(5): 226-229.
[10] Yuan Jiehong, Tang Guojin, Zhou Jianping, et al. The application of fracture mechanics in the safety assessment of propellant tanks [J]. Strength and Environment, 1999,(1):30-36.
[11] Tu Zhihua, Zhang Zhong, Zhao Lizhong, et al. Aluminum alloy welds with surface cracks at low temperature fracture properties [J]. Experimental Mechanics, 1996, 11(1): 84-89.
[12] Zhang Min, Lin Xiangzhu, Xu Shizhen, et al. Experimental study on fracture properties of welded joints [J]. Mechanical strength, 2003, 25(1): 85-89.
About the author: Peng Xingna (1983-), female, major in materials processing engineering, graduated from Beijing University of Aeronautics and Astronautics. The main research direction is the advanced connection technology of materials. Tel:, E-mail:
Study of Fracture Toughness of Aluminum Alloy and Its Welding Joint at Cryogenic Temperature
Peng Xingna Zhang Guohua Qu Wenqing
School of Mechanical Engineering & Automation, Beihang University, Beijing, China,100083
ABSTRACT: As the development of aeronautic and astronautic techniques, the mechanical properties of cryogenic temperature are raised and more. People pay more attention to the fracture toughness of Al alloy at cryogenic temperature. This paper introduces manners of measuring fracture toughness, The analyzed the present evaluation of fracture toughness of aluminum alloy and its welding joint, and proposed scheme of evaluation of the fracture toughness of 2219 Al alloy. In the end, it was stated that there were a lot of deficiencies in evaluation of the fracture toughness at Cryogenic temperature in our country.
KEYWORDS: Aluminum alloy; Welding Joint; Cryogenic temperature; Fracture toughness