2020 Volume 61 Issue 11 Pages 2222-2227
Ultrasonic testing and metal magnetic memory testing were comprehensively applied to detect the internal defects and superficial defects of engine crankshaft. A special detection method and device were designed for the testing. The results indicated that the internal defects of crankshaft could be detected quantitatively by using the special detection device and ultrasonic testing, whereas, the superficial defects of crankshaft could be detected by metal magnetic memory testing. The magnetic stress concentration coefficient KM was used to characterize the fatigue damage degree. By the comprehensive nondestructive method, the internal and superficial damage of engine crankshaft can be evaluated to effectively guarantee the safety and reliability of engine crankshaft.
Fig. 7 Automatic metal magnetic memory testing results of crankshaft R-angle.
Crankshaft is an important part of automobile engine. The main function of crankshaft is to transform the reciprocating motion of piston into the rotating motion of crankshaft through connecting rod, so as to complete the energy conversion and output. Crankshaft bearings are subject to the combined action of periodic variable gas force, reciprocating mass inertia force, rotating centrifugal force and friction force in engine operation. The main failure modes of engine crankshaft are wear and fatigue fracture.1) Especially, there is stress concentration at the R-angle of crankshaft journal and crank, which is the weakest area. Fatigue cracks are easy to occur under the action of stress, leading to fatigue fracture of crankshaft.2)
Destructive tests, such as fatigue testing and microstructure observation, are usually used to detect crankshaft, which increase the testing costs and time. Because nondestructive testing methods are convenient to detect the defects and physical properties of various mechanical components,3,4) then the nondestructive testing methods are applied to detect the defects of crankshaft. For example, the ultrasonic testing is used to detect the hardened layer of crankshaft, and the eddy current testing and magnetic particle testing are used to detect the superficial defects of crankshaft.5,6) But a variety of detection probes need to be replaced in the above conventional nondestructive testing methods due to the complex structure of crankshaft, which are easy to miss and misjudge the defects. Meanwhile, there are few reports on nondestructive evaluation of fatigue damage degree in crankshaft.7,8)
In this paper, the ultrasonic testing and metal magnetic memory testing are comprehensively applied to detect the crankshaft, considering the advantages, disadvantages and application scope of various nondestructive testing methods. Ultrasonic testing is used to quantitatively detect the internal defects, such as cracks and inclusions, in the crankshaft. Metal magnetic memory testing is used to detect the superficial defects, such as cracks, and evaluate stress concentration degree of crankshaft. The quality of crankshaft is evaluated by the two kinds of nondestructive testing methods, meanwhile, a set of special testing device and method are designed for the testing.
A sketch of the crankshaft is shown in Fig. 1. The crankshaft is composed of journal and crank. The journal is divided into crankshaft journal and connecting rod journal. There is a transition angle (R-angle) between the journal and crank.9) The tested crankshaft are used in Steyr engine, the material is 45 forged steel, which is a kind of medium carbon steel, the heat treatment process is normalizing. The connecting rod journal is connected to the piston through the connecting rod. It is easy to cause fatigue cracks on the effect of alternating stress for a long time. With the expansion of fatigue cracks, the fracture failure happens in the crankshaft, and heavy losses causes. Especially, the R-angle part belongs to the stress concentration area, where the fatigue cracks are prone to occur. The R-angle part is the key detection area in the crankshaft.6)
Sketch of the crankshaft.
The ultrasonic testing instrument used in the experiment was XZU-1 digital ultrasonic testing device, and the coupling agent was engine oil. Ultrasonic testing of R-angle is easy to be missed by common probe. In order to avoid the impediment of R-angle to ultrasonic, it is necessary to use single crystal double oblique probe to detect R-angle of crankshaft. The named single crystal double oblique probe is to add a lateral inclination angle on the basis of ordinary oblique probe in order to make the front side of the ultrasonic wave emit, which effectively avoids the obstruction of oil hole and R-angle to the ultrasonic wave, so as to realize the detection of internal defects at the journal and R-angle.10) According to the material and structure size of the crankshaft, the parameters of the probe are 2.5 MHz, the wafer size is 9 mm × 9 mm, the forward inclination angle γ is 63.4 degrees, and the lateral inclination angle ϕ is 30.0 degrees. The curvature of the lower surface of the probe should be as same as the crankshaft journal.
The wear of the probe will cause the change of zero point and incident angle in the probe, which affects the test results. Therefore, the probe should be regularly calibrated by calibration blocks. However, due to the use of double oblique probe, and the inspected surface is a curved surface, therefore, it is necessary to use a special calibration block to calibrate the probe. The special calibration block named ZF-1 type calibration block, is shown in Fig. 2, which is used to calibrate the actual forward inclination angle β, the actual lateral inclination angle β′, the material sound velocity and the zero point of the probe. The corresponding calibration methods have been described in Ref. 10). The detection parameters in this research are as follows: gain value was 70 dB, material sound velocity was 3870 m/s, zero point of probe was 2.323 µs, β was 50 degrees, β′ was 12 degrees.
ZF-1 type crankshaft probe calibration block: (a) front view, (b) side view.
EMS-2003 intelligent metal magnetic memory testing instrument produced by Eddysun Electronics Co., Ltd was used in this study. The testing probe was a two-channel pen probe, which detects the normal component Hp(y) of scattering magnetic field intensity of the sample.
The principle of ultrasonic testing of crankshaft by traditional manual method is shown in Fig. 3. The double oblique probe was placed on the journal after setting the detection parameters, and the whole journal can be detected by ultrasonic along the circumference direction.
Method and principle of ultrasonic testing of the crankshaft.
The P point in Fig. 3 is the position of defect, which is below the R-angle. In the process of detection, the specific position of defect P (lateral distance PN, forward inclination distance QN and depth EQ) can be determined by measuring the echo path EP of defect, the refraction angle β and β′. Meanwhile, the equivalent size of the defect can be detected by equivalent method. According to the geometric relationship, the formula for calculating the position of the defect P point is shown by eqs. (1)–(3) as following.
| \begin{equation} \mathrm{PN} = \mathrm{EP} \times \sin\beta' \end{equation} | (1) |
| \begin{equation} \mathrm{MP} = \mathrm{QN} = \mathrm{EN} \times \sin\beta = \mathrm{EP} \times \cos\beta' \times \sin\beta \end{equation} | (2) |
| \begin{equation} \mathrm{EQ} = \mathrm{EN} \times \cos\beta = \mathrm{EP} \times \cos\beta' \times \cos\beta \end{equation} | (3) |
Manual ultrasonic testing was still used in crankshaft at present. Human factors have a great influence on the results of ultrasonic testing. In order to improve the efficiency and accuracy of testing, a set of automatic testing device and fixture needs to be designed. The detection device consisted of crankshaft fixture and probe fixture, as shown in Fig. 4 and Fig. 5.
Automatic nondestructive testing fixture for engine crankshaft.
Fixture for testing probe of engine crankshaft: (a) front view, (b) top view.
The testing fixture for engine crankshaft was used to clamp the crankshaft and make the crankshaft rotate at a uniform speed. For the need of ultrasonic testing, the rotation speed should not be too fast, and the rotation speed should be between 5 and 10 r/min. The fixture for testing probe of engine crankshaft was used to clamp the ultrasonic probe, and the probe can scan the journal in one circle when the crankshaft rotates in one circle. The testing fixture for engine crankshaft comprises a crankshaft rotation drive motor (1), a chuck (4), a center (6), a headframe (3), a frame (2), a water supply tank (22) and a hose (23). The fixture for testing probe of engine crankshaft mainly includes the upper probe sleeve (7), the lower probe sleeve (8), the positioning slider (20), the tension spring (19), the base (11), the cantilever beam (13), the probe fixing shell (14), the compression bolt (15) and the compression spring (16). The diameter of the upper probe sleeve (7) and the lower probe sleeve (8) are the same, which are slightly smaller than the diameter of the crankshaft spindle neck by 3 mm to 5 mm. One side of the upper probe sleeve (7) and the lower probe sleeve (8) is connected by a hinge pin (9), the other side is connected by a locking bolt (10), and a probe positioning groove (18) is opened on the upper probe sleeve (7). The base (11) is fixed on the upper probe sleeve (7), the base (11) and the cantilever beam (13) are connected by bolts (12), and the cantilever beam (13) and the probe fixing shell (14) are connected by a positioning pin (17). The ultrasonic testing probe is put into the probe fixing shell (14), the probe fixing shell (14) is in the probe locating groove (18), the compression bolt (15) passes through the bolt hole through the cantilever beam (13), the probe fixing shell (14) and the compression bolt (15) are connected by the compression spring (16), one end of the tension spring (19) is connected with the lower probe sleeve (8), and the other end of the tension spring (19) is fixed. The bit slider (20) is connected. When the special fixture is used, the upper probe sleeve (7) and the lower probe sleeve (8) are sleeved on the inspected crankshaft journal. The positioning slider (20) is fixed on the dovetail groove of the seat (2) through the positioning bolt (21). The center lines of the positioning slider (20), the upper probe sleeve (7), the lower probe sleeve (8) and the tension spring (19) are in a straight line.
It is necessary to use the detection device to automatically detect the crankshaft after designing the detection device of engine crankshaft. The detection steps are illustrated by Figs. 4 and 5. Firstly, the small end of the inspected crankshaft (5) was clamped on the chuck (4), and the center (6) was moved along the guide rail to the appropriate position, and the large end of the crankshaft (5) was clamped. When the testing starts, the upper probe sleeve (7) and the lower probe sleeve (8) were opened, and the crankshaft journal to be inspected was sleeved, and the locking bolt (10) was tightened. Then, the positioning slider (20) slides along the edge of the dovetail groove on the seat (2), and when the upper probe sleeve (7), the lower probe sleeve (8), the tension spring (19) and the center line of the positioning slider (20) were in a straight line, the positioning bolt (21) was tightened to fix the positioning slider (20). Then, the ultrasonic probe was placed in the fixed housing (14) of the probe, and the angle between the cantilever beam (13) and the base (11) was adjusted so that the ultrasonic probe can be placed in the probe positioning groove (18) and the bolt (12) was tightened when the compression spring (16) can provide a certain pressure to the ultrasonic probe. Finally, the pressure was appropriately adjusted by tightening the bolt (15) so that the ultrasonic probe can closely contact on the surface of the crankshaft journal for testing. After completing the above steps, the coupling agent was continuously dripped into the probe positioning groove (18) by the feed water tank (22) and the hose (23), so that the contact surface between the ultrasonic probe and the journal can be maintained sufficiently. Then, the crankshaft rotation driving motor (1) and the ultrasonic testing instrument were simultaneously activated to detect the crankshaft.
In the process of crankshaft rotation, the ultrasonic probe can detect the internal part of the journal and the R-angle in real time. When the defect echo generated, the crankshaft rotation driving motor (1) stopped, the defect echo waveform was recorded and analyzed, the type and location of the defect were determined, and the equivalent size of the defect was calculated by the distance-amplitude curve method. When there is no defect echo, the probe scans the journal surface for one circle, then the crankshaft rotation driving motor (1) was stopped, and the above steps repeat until the whole crankshaft was detected.
The purpose of metal magnetic memory testing of crankshaft was to detect and evaluate the stress concentration and surface cracks at crankshaft R-angle, so the path of metal magnetic memory testing of crankshaft is to detect the R-angle in the circumferential direction of the journal. According to author’s previous research results,11,12) the direction of detection influences the results of metal magnetic memory testing, so it is necessary to keep the direction of metal magnetic memory probe unchanged while the crankshaft rotates. The testing requirement was basically as the same as that in ultrasonic testing of the crankshaft. Therefore, the authors improved the testing probe fixture on the basis of ultrasonic testing fixture, so that both the metal magnetic memory and ultrasonic testing fixture can be simultaneously carried out. According to the size of the magnetic memory detection probe, a metal magnetic memory probe hole (24) was machined in the part near the edge of the upper probe sleeve, as shown in Fig. 5. The metal magnetic memory probe can be fixed in the probe hole. The assembly method was as same as the ultrasonic testing.
The method of quantitative characterization of R-angle defects has been elaborated by manual ultrasonic testing with single crystal double oblique probe and the internal defects of inclusions were detected in Ref. 10). In order to facilitate the comparison of manual detection and automatic detection, the crankshaft connecting rod journal is detected by automatic detection and manual detection method, respectively, taking the oil hole with larger diameter as the manual defect. The results are shown in Fig. 6.
Automatic and manual ultrasonic detection results of engine crankshaft: (a) automatic detection, (b) manual detection.
Figure 6 shows the testing results of the echo waveform of the detected oil hole by ultrasonic at the same position of the connecting rod journal. It can be seen that the echo waveform of the oil hole is very similar, and the position of the peak value is basically the same, just the latitude of the waveform in automatic detection is lower than that in manual detection. Similar results have been obtained for other positions of connecting rod journal, and the results of automatic detection are more stable than those of manual detection. The advantage of automatic detection over manual detection is that it avoids the influence of human factors in manual detection, such as pressure, detection position, which improves the accuracy and efficiency of testing.
There will be a variation peak in the metal magnetic memory testing signal when a superficial crack appears at crankshaft R-angle, and the size of the superficial crack can be expressed by the peak value of the variation peak, according to the change rule of metal magnetic memory testing signal.11–13) The R-angle in the third connecting rod journal of one crankshaft was detected by the method, the results are shown in Fig. 7. The gradient of metal magnetic field intensity K suddenly increases, when the stress concentration or superficial defects occurs at crankshaft R-angle.
Automatic metal magnetic memory testing results of crankshaft R-angle.
The detected crankshaft was broken by fatigue bench test to verify the metal magnetic memory testing results. It is found that the location of fracture starts at the place of stress concentration, which are detected by metal magnetic memory testing. Scanning electron microscope (SEM) was used to observe the surface state of the broken shaft. It is found that there are some deep wear marks and microcracks on the surface of the broken shaft, shown in Fig. 8(a). Meanwhile, the fatigue fracture was observed by SEM. Figure 8(b) is a high magnification photo of the local area of the fatigue fracture source. It can be seen that the fatigue crack starts from the material surface, the main crack generates multiple secondary cracks during the propagation process, and there are more inclusions and holes near the crack. It is because of the wear, microcracks and stress concentration leading the magnetic memory signal changes dramatically, which can reflect the fatigue damage degree of the crankshaft.
SEM of fatigue fracture of crankshaft: (a) surface of the broken shaft, (b) the fatigue fracture source.
It is also seen that there are multiple K peaks at one R-angle from the metal magnetic memory testing signals in Fig. 7. In order to evaluate the stress concentration degree at crankshaft R-angle, the theoretical formula of magnetic stress concentration coefficient KM is put forward according to the concept of theoretical stress concentration coefficient, as shown in eq. (4).
| \begin{equation} K_{\text{M}} = k_{\text{max}}/k_{\text{ave}} \end{equation} | (4) |
The metal magnetic memory testing were used to detect the R-angle in the third connecting rod journal of fifty crankshafts, in order to further verify the practicability of eq. (4). Ten of them are new crankshafts, which are qualified product and not used. Ten of them are normal crankshafts used for 100000 km, which are within the safe use cycle 200000 km. Thirty of them are old crankshafts need to be remanufactured, which are out of the safe use cycle 200000 km. The testing values of magnetic stress concentration coefficient KM are shown in Fig. 9. The longer the crankshaft is used, the greater the damage increases. It can be seen that the magnetic stress concentration coefficient KM increases with the increase of damage degree in crankshaft, indicating that the magnetic stress concentration coefficient KM can reflect the damage degree of crankshaft. The damage degree of the workpiece can be divided into three grades according to the actual test results, which is grade I (mild damage, the crankshafts are new or normal, there is almost no wear on the surface and no superficial crack, the crankshaft can continue to be safely used), grade II (moderate damage, the crankshafts are old, there is slight wear on the surface and no superficial crack, the crankshaft can continue to be used, but need to be regularly detected) and grade III (serious damage, the crankshafts are old, there is serious wear on the surface or superficial crack, shown in Fig. 8. It may cause serious losses in the application), whose KM thresholds are 7.5 and 10.5.
Magnetic stress concentration coefficient KM of crankshaft in different states.
This work was financially supported by Guangzhou Foreign Science and Technology Cooperation Project (201907010004), Innovation Capacity Building Project of Guangdong Academy of Sciences (No. 2019GDASYL-0302009) and Project of Reform and Innovation of Scientific Research Institutions in Guangdong Province (No. 2017A070701021).
