Failure analysis and optimization design of a centrifuge rotor

Transcription

1 Engineering Failure Analysis 14 (2007) Failure analysis and optimization design of a centrifuge rotor Xuan Hai-jun *, Song Jian Institute of Chemical Process Machinery, College of Material Science and Chemical Engineering, Zhejiang University, 38 Zheda Road, Hangzhou , China Received 7 December 2005; accepted 11 December 2005 Available online 9 March 2006 Abstract Centrifuge rotors are designed for high service life duty and should stand against 2 times the maximum operation speed in a spin tester in terms of the manufacture s standard. A centrifuge rotor prototype had burst before reaching 2 times the maximum operation speed in an over-speed spin testing, therefore, failure analysis was required to determine the cause of the burst and recommendations for the structure optimization were needed to improve the rotor integrated strength. After the chemical analysis and the microstructure identity, a finite element model of 1/24 rotor under centrifugal load was adopted for mechanical stress analysis. The analysis indicated that local stress peaks occurred on the top round between the receptacle and the hub and on the edges of the tube cavities. By comparing computational and experimental results, it is revealed that the rotor fractured on two regions one after another when the peak stress exceeded the ultimate strength of the material. Consequently, some recommendations were made for the structure optimization to improve the rotor safety performance. The peak stress was reduced below the allowable material strength margin with increasing the fillet radius of the round and decreasing the depth of the liquid gather groove. The burst of the redesigned rotor was eliminated in the later over-speed spin testing. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Centrifuge rotor failure; Finite element analysis; Stress analysis; Structure optimization 1. Introduction High-speed centrifuge and ultracentrifuge are designed to meet the needs of research, medical, bio-industrial and bio-processing communities. Rotors in high-speed centrifuge and ultracentrifuge units are subjected to high centrifugal loading. The resulting stress can induce structural failures which could have serious consequences. As a special case, a laboratory centrifuge can be an important tool in the university laboratory and can also be a dangerous instrument. On December 16, 1998, a rotor running in a Beckman L2-65B ultracentrifuge at Cornell University failed due to excessive mechanical stress, and the subsequent explosion completely destroyed the centrifuge [1]. Therefore, one of the major problems in high speed centrifuge is to assure adequate rotor safety level for the operator. To maintain and even improve the rotor safety, it is often * Corresponding author. Tel.: address: marine@cmsce.zju.edu.cn (X. Hai-jun) /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.engfailanal

2 102 X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) necessary to over-speed a centrifuge rotor prototype in a high-speed spin tester with 2 times the maximum operation speed without cracking or burst failure to fulfill the demands of a new design [2]. The present paper reports the investigation of the burst failure of a centrifuge rotor that took place during spin testing for the examination of safety performance. The principal aim of this investigation was to identify the root cause of the failure so as to prevent similar failures in the redesign rotor. A complete analysis was performed with chemical analysis and microstructure observation and mechanical stress calculation to ascertain whether the burst was due to a material related problem or an excessive centrifugal stress. Finite element analysis technique was used for complex geometry to calculate the detailed stress distribution on the rotor under high centrifugal loading. The rotor integrated strength was improved with structure optimization for enduring higher stress at 2 times the maximum operation speed. 2. Rotor burst Centrifuge rotors are designed to carry a maximum load at a specific speed. The centrifugal force created by high rotational speed generates the stress on the metal of the rotor, which causes it to stretch and change in size. Every newly designed high performance centrifuge rotor needs to receive a special high stress test to insure that it will have a long and safe life. This high stress test produces stresses that are far greater than the maximum possible stresses to the rotor when it is used in normal operation. A centrifuge rotor prototype required over-speed spin testing for bearing 2 times the maximum operation speed. Over-speed spin testing was conducted using high-speed rotor spin testing facility in High-speed Rotating Machinery Laboratory, Institute of Chemical Process Machinery, Zhejiang University. The upward vertical axis, flexible shaft spin tester in the laboratory employs a 45 kw, rpm speed variable DC motor connecting the drive shaft of a two steps speed increasing gear box, covers an output speed range of up to 60,000 rpm. The test was conducted in vacuum to eliminate the high air friction loss and to reduce the friction heating created if the test was to be conducted in air. The vacuum was also an important safety feature since it reduced the risk of the explosion of metal dust or oil fog during the rotor bursting. The testing chamber was armored with two lead brick layers of 100 mm in thickness and a steel containment ring of 20 mm in thickness to protect the safety of operational members and prevent the damage of the outer structure of the spin tester. The rotor has been required to slowly accelerate to 46,600 rpm and halt 5 min at this speed, then decelerate to zero speed for flaw inspection with no occurrence of cracks. However, the rotor was burst at 44,880 rpm before it reached the rotating speed objective. Pieces of the rotor itself were widely distributed throughout the testing chamber after the burst. Post-test reassembled rotor fragments are shown in Fig. 1 with the vast majority of the original test rotor pieces as we could recover. No high speed camera video of the rotor burst event could be reviewed Fig. 1. Post-test reassembled rotor fragments.

3 to aid in the failure analysis. Therefore, it is not easy to find any additional information from post-test fragments for failure analysis. 3. Failure analysis 3.1. Chemical analysis The rotor material was 7A04 aluminum alloy with T6 solution heat treated and then artificially aged state. This material possesses excellent mechanical strength and high strength-to-weight ratio with less expense and, is widely used in the centrifuge industry. Determination of chemical composition is very important for understanding material quality. Therefore, a quantitative chemical analysis of a fragment material sample obtained by standard alloy identification technique is given in Table 1 with a comparison of standard material. The composition of the rotor material conformed to the Chinese standard specification for high strength 7A04 aluminum alloy Microstructure X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) A low magnification photo of a fracture surface of a rotor fragment is shown in Fig. 2. Here, coarseness of the fracture surface indicates that it was a typical brittle fracture surface with little plastic deformation. Similar features were observed in fracture surfaces of randomly selected fragment. The homogeneity of the fracture surfaces verifies that no initial defects were formed in the rotor before spin testing. A scanning electron microscopy (SEM) fractography of the fracture surface is shown in Fig. 3. There is no evidence of material microstructural change or degradation in the broken rotor. Therefore, the possibility of rotor burst due to the material degradation is ruled out by the chemical analysis and the microstructure observation. Table 1 Chemical composition details of 7A04 (in wt%) Material element Zn Cu Mg Mn Cr Si Fe Ni Ti Zi Al Fragment material Remainder Standard material Fig. 2. Photo of fractured surface.

4 104 X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) Mechanical analysis Fig. 3. SEM micrograph of fractured surface. It could be assumed from the metallurgical analysis that there must be an insufficient structure strength resulting in locally high stress which exceeded the ultimate tensile strength of the material and caused the rupture of the centrifuge rotor. Therefore, a mechanical analysis capable of predicting stress on the rotor under centrifugal loading is needed to find the root cause of the rupture and to increase the safety performance with structure optimization. Finite element method was selected in the subsequent analysis because of the complex geometry of the centrifuge rotor Geometry and material properties Geometry data for the rotor are necessary for finite element modeling. All geometry data were obtained from a 2-dimensions drawing supplied by the manufacturer. The rotor has 170 mm maximum outer diameter and 52 mm height, and a 21 mm diameter central through hole called as connect disconnect receptacle, and 24 angled tube cavities with 11 mm diameter and 37 mm depth in the rotor rim with substantially equal circumferential distance between two cavities. Material, physical and elastic properties were obtained for 7A04-T6 aluminium alloy from Chinese standard. Table 2 gives the required properties used to carry out the stress analysis of the centrifuge rotor. The minimum ultimate tensile strength of this material is 530 MPa Finite element model A 1/24 section model has been generated with one tube cavity for a cyclically symmetric structure of the centrifuge rotor during this study. If the model repetitively patterns 24 times around the center axis, a complete model will be yielded. A parameterized solid 3-dimensions model was generated in the Unigraphics program and read into MSC.Nastran, whereupon the finite element mesh was defined, and the symmetric boundary conditions and the centrifugal load were applied. The solid model was meshed using ten-node tetrahedral elements due to the complexity of the geometry. Each node has six degrees of freedom with three translation displacements and three rotation angles. Most of the significant geometric features were modeled Table 2 7A04-T6 material s physical and elastic properties data [3] Young s modulus (GPa) Poisson s ratio Yield strength (GPa) Ultimate strength (MPa) Density (Kg/m 3 ) P

5 and relatively finer meshes were used for the regions of rounds and cavities. The global mesh density was chosen to improve the calculation performance. Fig. 4 shows a finite element model for a 1/24 section of the rotor. Some 34,762 elements and 48,912 nodes were employed in this analysis. Cyclically symmetric conditions were maintained for the rotor by imposing appropriate displacement constraints on both symmetrical sectional surfaces. Centrifugal force under the rotor bursting speed of 44,880 rpm was simulated by applying an angular velocity to elements in the model. The developed model has been used to determine the stress distribution on the rotor. The finite element analysis was performed by using linear elastic material properties which were not influenced by the loading history Stress calculation results and failure analysis X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) The centrifuge rotor experienced large centrifugal force at the burst speed. Results for stress and strain are all not shown here for the purpose of brevity, however the Von Mises stress contour profile of the complete model at the burst speed is presented in Fig. 5. It is observed that the first highest stress occurs at the root of Fig. 4. Finite element model of a 1/24 centrifuge rotor. Fig. 5. Von Mises stress contour at speed 44,880 rpm (10 3 MPa).

6 106 X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) the round on top surface between the receptacle core and the hub and the second peak stress appears on the top edge of the tube cavities in radial direction. Figs. 6 and 7 show the stresses along the round in radial direction and on the top edge of the cavity. These stresses could contribute to the type of failure and caused the rotor burst at two regions one after the other. Re-examination of the assembled rotor fragments indicate that excessive stress exceeded the ultimate tensile strength of the material and caused a circumferential crack at the root of the round as the first region of the failure, subsequently the stresses caused cracks on the top edges of the cavities in the radial direction as the second region of the broken, therefore the rotor flew apart at rotational speed 44,880 rpm. As shown in Fig. 7, two stress peaks occurred on the top edge of the cavity at the farthest and nearest points in radial direction from the rotor center, caused the outer rim of the rotor to split apart in radial sections. The stress on the farthest point is larger than that on the nearest point. Therefore, the rotor safety will benefit from the reducing of the maximum stress at two regions with the local geometry modification by which the rotor can stand larger centrifugal loading under 2 times the maximum operation speed. Increasing the fillet radius of the round and reducing the depth of the liquid gather groove on the rim flange may reduce the peak stresses at two regions within allowable margin and eliminate the rupture of centrifuge rotor in the later over-speed spin testing. The following section will present the process of structure optimization for the centrifugal rotor. Fig. 6. Von Mises stress on round at speed 44,880 rpm. Fig. 7. Von Mises stress on tube cavity top edge at speed 44,880 rpm.

7 It is also noted that lower stresses are present on the top portion of the receptacle and this should not raise high concentrated stress in detailed structure to cause the rotor separation failure, therefore, the receptacle remained on the top end of the flexible shaft of the spin tester after the rotor burst. It is also justified on the other side that the screw thread on the top portion of receptacle can be ignored in the finite element model for a simplified analysis. 4. Structure optimization X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) The specific objectives are to reduce the maximum stress on the centrifuge rotor under 2 times the maximum operation speed rotating condition and to control the maximum stress as low as possible for the improvement of safety performance by structure optimization with geometry consideration. Fig. 8(a) and (b) illustrates which dimensions of the centrifuge rotor resulted from the structure optimization has been optimized. The fillet radius of the round is increased from 5 mm to 10 mm and the depth of the liquid gather groove is decreased from 5.5 mm to 4.5 mm with the modification of the correlative dimensions. The optimization work was done under centrifugal loading at rotating speed 46,600 rpm of 2 times the maximum operation speed larger than the burst speed of the pre-optimized rotor. As shown in Fig. 9 compared with that shown in Fig. 5, the maximum stress on the rotor is decreased from 541 MPa to 514 MPa more below the required minimum ultimate tensile strength of the material in the standard. Furthermore, the position of the maximum stress occurring is translated from the round to the top edges of the cavities. The peak stress at the round is decreased from 541 MPa to 504 MPa, and the peak stress at the top edge of the cavities is decreased from 530 MPa to 514 MPa, as shown in Figs. 10 and 11 compared with that shown in Figs. 6 and 7. Fig. 8. Effect of structure optimization at two regions.

8 108 X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) Fig. 9. Von Mises stress contour at speed 46,600 rpm on optimized centrifuge rotor (10 3 MPa). Fig. 10. Von Mises stress at speed 46,600 rpm along the round after optimization. Fig. 11. Von Mises stress at speed 46,600 rpm on cavity top edge after optimization.

9 X. Hai-jun, S. Jian / Engineering Failure Analysis 14 (2007) Results and conclusions Chemical analysis and microstructure observation were used for material inspection. It was identified that the material of the rotor was as per specified grade of high strength aluminum alloy and there was no material degradation. Therefore, mechanical stress analysis using finite element method was carried out for failure analysis after the metallurgical examination. It was found that the rupture was due to insufficient integrated strength of the rotor as maximum centrifugal stress exceeds the ultimate strength of the material. Further works were done for the structure optimization of the rotor standing centrifugal loading at 2 times the maximum operation speed. The rotor dimensions have been changed at two regions and the maximum centrifugal stress was reduced to a relative low level with an improved safety performance. The structure optimized centrifuge rotor was machined by the manufacturer and stood 2 times the maximum operation speed of 46,600 rpm with 5 min in the later over-speed spin testing. It can be concluded that 3-dimensions stress analysis and structure optimization based on finite element method are essential for the safety performance improvement of centrifuge rotors. Acknowledgement The authors acknowledge Shanghai Lishen Science Instruments Limited Company for their financial support during the course of this work. References [1] Available from: < [2] State Food and Drug Administration. High speed refrigeration centrifuge (Chinese Standard). YY [3] Standardization Administration of China. Extrusion rods and bars of aluminium and aluminium alloy (Chinese Standard). GB/T