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1、基于电磁的裂缝检测 课程论文 BASED ON MAGNETIC CRACK DETECTION Abstract: This paper summarized recent diagnosis methods of marine diesel crankshaft crack and introduced the fundamental principle, development and application of metal magnetic memory testing (MMMT) diagnosis technology. Compared with other methods,
2、 a scheme developed from MMMT combined with torsional signal to make on-line diagnosis of crankshaft crack was proposed, and the on-line diagnosis experimental platform of diesel was designed. Finally, comments were also made on the on-line MMMT method. Keywords: marine diesel; crankshaft; crack; me
3、tal magnetic memory 1 Introduction Most of the crankshaft cracks on ocean marine diesel are deeply latent and difficult to detect artificially. In general, while the crack develops to a certain extent it will rupture and cause destructive breakage1. If inchoate stress concentration and tiny crack of
4、 crankshaft are detected on-line by Non- Destructive Testing (NDT) methods, the diesel can be stopped before the full development of the crack and thus rupture can be avoided. A new NDT method, MMMT developed rapidly recent years with advantages in such areas as cheap and simple sensors, measuring w
5、ithout impacting diesel operation and long-term on-line detection. Therefore, there will be a great foreground to use MMMT in crankshaft crack detection. 2 Materials and Methods 2.1 Static Diagnosis of Crankshaft Crack Crankshaft crack is one of the fatal faults in diesels. There are some static NDT
6、 methods to diagnose crankshaft crack2 such as Ultrasonic Testing, Radiographic Testing, Magnetic Particle Testing, Penetrate Testing, Eddy Current Testing, Optical Holography, and Microwave Testing. These methods have taken an important part in crankshaft quality control and accident prevention. Ho
7、wever, on the other hand there are several disadvantages of these methods. Firstly, all of them must find cracks already formed and can not detect unexpected ones. Secondly, they are unable to make on-line detection and fault diagnosis. Thirdly, most of them need surface pretreatment of the faciliti
8、es before inspection. Finally, the accuracy of detection is affected by crankshaft configuration and personal skill. 2.2 Dynamic Diagnosis of Crankshaft Crack At present there are some methods to diagnose shaft cracks on-line: rotor engine vibration displacement method, train wheel shaft temperature
9、 testing, acoustic emission and tortional vibration signal testing. However, vibration displacement method is unfit for diesel crankshaft crack diagnosis due to the strong vibration from reciprocating movement which makes it difficult to survey displacement and acceleration. Although there is local
10、high temperature on the cracks, it is hard to inspect the change of the crankshaft temperature due to the high temperature of the lube oil in the diesels. Acoustic emission method is still in developing as great noise from diesel operation makes it difficult to measure high frequency acoustic wave e
11、mitted by crack. Therefore at present it is feasible to diagnose crankshaft crack on-line by using torsional vibration signal. 2.3 Crankshaft Crack Diagnosis Using Torsional Vibration Signal Literature 3 brought forward a method using torsional vibration signal detected real-time from diesel to diag
12、nose crankshaft fault. It studied crack fault diagnosis though measuring the characteristics of the torsional vibration of the shafting flywheel system, and presented that model frequency and damp were sensitive parameters of cracks. The torsional vibration measuring system of the diesel had several
13、 advantages such as: easy measuring, no direct contact, cheap sensor and longevity. It was a long-term reliable alarm method for diesel shafting crack. Research group in Marine Engine Institute of Jimei University has done the research of internal-combustion engine score fault with intelligent torsi
14、onal vibration diagnosis system4, and it was a successful case of on-line fault diagnosis based on torsional vibration signal. 3 The Improved Method 3.1 Method one Marine diesel engine crankshafts are mainly made up from ferromagnetic material. According to MMM theory, stress concentration zone on c
15、rankshaft will produce maximum tangential component HpX of the magnetic field intensity and zero normal component HpY with alternation of symbol. With scanning inspection of the surface magnetic field of crankshaft by MMM testing sensor and instrument, the stress concentration zone will be found out
16、 according to the zero normal component of the field intensity, and then the existence of crack on the crankshaft can be estimated indirectly. With this method, the location of cracks can be found out without interrupting normal operation of the diesel, and give an alarm in time to remind engineers
17、of operation condition of the diesel. In the design, the sensor of the instrument will be placed into the body of the diesel through the guide door of the diesel, and it can directly scan zones with the most probable existence of stress concentration, such as locations near oil bore and fillet. In c
18、ase the normal component of magnetic field intensity of the location shows a change from positive to negative or from negative to positive, it means that there may be tiny crack or stress concentration taking place, and at the same time, the instrument will record in a log and give an alarm. The met
19、hod is simple, effective and feasible to on-line testing on operating diesel on ships. 3.2 Method two In order to put MMM crankshaft crack testing method into practical application to ships, simulation experiment should be done, and the model experiment platform is built from a two cylinders diesel.
20、 The platform takes shaft accessory as the research object, and uses standard ball bearings to support shaft. By means of replacing shaft and bearings, the platform is able to do simulate experiment for shafts in different sharp or length such as roll shaft and crankshaft. The shaft is connected to
21、electromotor by adapting flange, and the motor controlled by transducer can drive the shaft with different rotate speed to simulate running of uniform velocity and accelerated velocity. The bearings are fixed on steel foundation by two bearing seats, and the shell on the steel foundation is made up
22、from organic glass to make the observation and the installation of the MMM sensor easy. There is lube oil deposited in the steel foundation and heated up by electro thermal coil. Heated lube oil is pumped to the top of the shell by a small gear oil pump and injected to shaft and bearings to lubricat
23、e them and simulate high temperature in diesel. Then the oil flows into oil sump forming a cycle. The MMM sensor is installed in the sides of the platform body through a probe hole, and its installation angle, depth and horizontal location can be adjusted to scan the magnetic field intension of the
24、whole shaft to find the stress concentration zone. At the same time, stress distribution of diesel crankshaft with crack will be analysized by finite element method software ANSYS and the magnetic field intension model varied with time and temperature will also be constructed for calculation. All of
25、 these simulations are fundamental to MMM crack testing of crankshaft9. 4 Conclusion MMM crankshaft crack testing method is a new technique used for on-line inspection, and there are some issues to deal with in further study. The first one is quantificational relation between leak magnetic field int
26、ension near crack and stress, surrounding magnetic field, and the depth and width of crack10. Secondly, there are several factors having certain impacts to the MMM signal such as temperature, residual magnetic field and surface roughness, therefore, data processing should be done to eliminate these
27、impacts. At present, marine diesel crankshaft crack testing is still being done by means of static method in China. MMM testing may solve the problem of early stress concentration inspection. This method has the advantages as cheap and simple sensors, measuring without affecting diesel operation and
28、 long- term on-line detection. It may forecast stress concentration zone and avoid hidden trouble of crankshaft rupture accident. Combined with torsional signal, MMM testing on-line diagnosis of crack will be achieved. References 1. Songbo Xia and Ruiyan Liu, Theory, Technique and Application of Equ
29、ipment Fault Diagnosis, Beijing, Academic Publisher, 1996 (in Chinese). 2. Zhenxiong Cai, Zhiqiu Wang and Hongxing Hua, Diagnosis of Crankshaft Crack in Diesel Using Torsional Vibration. Journal of Shanghai Jiaotong University, 2004(11), 1028-1031 (in Chinese). 3. Weichang Zhong, Theoretical Fundame
30、ntals of the Metal Magnetic Memory Diagnostics: Spontaneous Magnetization of Ferromagnetic Materials by Elastic-Plastic Strain. NDT, 2001(10), 424-426 (in Chinese). 4. Junming Lin, Xingzeng Yu and Chunjing Lin, Software M3DPS for Metal Magnetic Memory Diagnostic Instrument. NDT, 2001(7), 286-288 (in
31、 Chinese). 5. Qilin Qian, Hailiang Gao, Huihua Zhang and Junming Lin, Theory and Application of Hull Structure Steel Plate Welding Based on Metal Magnetic Memory Diagnosis. Ship Building Technique, 2002(1), 25-27 (in Chinese). 6. Jilin Ren, Stress Analysis and Magnetic Memory Testing of Ferromagnetic Items. NDT, 2001(7), 286-288 (in Chinese).