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Fracture failure of a shell structure
~
Enyineeriny Failure Analysis, Vol. 4. No. 4, pp. 27l 277, 1997
Pergamon
© 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1351~6307/97 $17.00 + 0.00
P l h S 1350-6307(97)00016-2
F R A C T U R E F A I L U R E OF A S H E L L S T R U C T U R E N. S. XI, C. H. T A O a n d H. Y A N Failure Analysis Center of Aviation Industry of China, P.O. Box 81-4, Beijing 100095, China (Received 16 June 1997)
Abstract--Severalshells were found fractured at the same position after long distance transportation. Chemical composition analysis, metallographic examination, stress calculation and fracture surface observation were performed. Stress calculation and fracture surface analysis results showed that the crack originated at the upper part of the shell structure and propagated to the two sides and lower part of the shell structure. The fracture cause and mode were discussed comprehensively and preventive actions were specified. It was concluded that multiple overload induced the cracking of the shell structure. The poor transportation method and a high stress concentration were responsible for the fracture of the shell structure. © 1997 Published by Elsevier Science Ltd.
KeywordsBrittle : fracture, cracks, fracture surfaces, overload, stress concentrations. 1. I N T R O D U C T I O N Several shells were f o u n d f r a c t u r e d at the same p o s i t i o n after long distance t r a n s p o r t a t i o n . S o m e o f the shells were s e p a r a t e d c o m p l e t e l y a n d s o m e were c r a c k e d on the u p p e r h a l f p a r t s as s h o w n in Figs 1 a n d 2. The shells were f o r m e d by casting with A I - S i - C u - M g alloy a n d the h e a t t r e a t m e n t was T5. N o defects, such as cracks, gas cavities a n d so on, were discovered in the key p a r t s o f the shells by X - r a y n o n d e s t r u c t i v e inspection. It is sure t h a t the shells were in g o o d c o n d i t i o n when they left the factory. Bearing axial l o a d in service, the f r a c t u r e d shells were manifestly worthless. In o r d e r to d e t e r m i n e the failure cause a n d to specify the preventive actions, c h e m i c a l c o m p o s i t i o n analysis, tensile tests, m e t a l l o g r a p h i c e x a m i n a t i o n , fracture surface o b s e r v a t i o n a n d stress calc u l a t i o n s were p e r f o r m e d . 2. E X A M I N A T I O N
OF METALLURGICAL
QUALITY
In o r d e r to d e t e r m i n e if there existed cracks in the lower h a l f p a r t s o f the shells whose u p p e r p a r t s were c r a c k e d , b o t h fluorescent i n s p e c t i o n a n d l o n g i t u d i n a l section m e t a l l o g r a p h i c e x a m i n a t i o n were c a r r i e d out. N o defects were found.
Fig. 1. View of completely separated shell and the fracture position. 271
N. S. XI eta/.
272
i!l
Fig. 2. Crack in the half-split shell, shown by an arrow.
Six chemical samples and 24 tensile specimens (of diameter D = 5 mm) were removed from the failure shells randomly, Table 1 shows the chemical composition analysis results. This proved that the composition was normal. Tensile testing showed that the strength data were uniform with low scatter, and that the average tensile strength (297 MPa) exceeded the tensile strength (200 MPa) required for design. Figure 3 showed the microstructure of the shell material. It can be seen that the microstructure consisted of (A1) and Si phases. This demonstrated the shell had a perfect heat-treated microstructure. A longitudinal section metallographic photograph of the fracture surface is given in Fig. 4, which exhibited large height differences and microscopic plastic deformation, implying that the applied loads were heavy during cracking.
3. OBSERVATION OF F R A C T U R E S U R F A C E
3,1. Macroscopic l~'atures The fracture surfaces were observed visually and by a stereomicroscope. All the fracture surfaces had similar features detailed as follows: (a) The surface, on which there was no trace of corrosion deposits or pitting, was clear and appeared silver gray. (b) Not only was no macroscopic plastic deformation seen near the crack, but neither were fatigue beach marks, shear lips or herringbone traces discovered on the surface. (c) The whole surface was divided into two half rings. Figures 5 and 6 presenting typical macro morphology of the upper and lower rings, respectively, showed that the crack originated at the external and internal edges. Stated thoroughly, they initiated at the external edge just at a fillet radius and at the internal edge offset from the inner fillet radius at a position where grooves were
Table I. Chemical element contents (%wt) of the shell material Sample no. I 2 3 4 5 6 Technical standard
Si
Cu
Mg
Mn
Ti
B
Zr
7.14 7.75 7.25 7.29 7.77 7.12 6.0~8.0
1.48 1.47 1.49 t.49 1.50 1.50 1.3~t.5
0.27 0.35 0.31 0.37 0.33 0.28 0.2~0.4
0.20 0.22 0.24 0.19 0.23 0.24 0.15~0.25
0.076 0.108 0.085 0.087 0.120 0.083 0.05~0.15
0.003 0.003 0.006 0.008 0.003 0.005 0.001~0.05
0.070 0.106 0.086 0.080 0.104 0.093 0.05~0.15
Fe 0.22 0.19 0.17 0.17 0.16 0.20 _<0.35
Fracture failure of a shell structure
273
Fig. 3. Microstructure of the shell material.
! O0 l.tm I I
Fig. 4. Longitudinal section profile of the surface.
observed. The fracture surface was approximately perpendicular to the shell axis. Crescent shapes, formed by fracture propagation from the internal edge to the external edge, can be also seen in Fig. 6. (d) The upper fracture surface was fine and smooth but the lower surface was rough. Thus it can be deduced that the fracture initiated in the upper part. This was also demonstrated by the fact of that some shells only cracked in the upper half and the lower part was excellent without damage. 3.2.
Microscopicfeatures
A penetrating investigation of fracture surfaces in various regions was performed by SEM (Scanning Electron Microscope). Similar microscopic features were seen as shown in Figs 7 and 8. The detailed observations were:
274
N.S. XI et al.
Fig. 5. Macro morphology of the upper ring.
Fig. 6. Macro morphology of the lower ring.
Fig. 7. SEM image of the upper part.
Fracture failure of a shell structure
275
Fig. 8. SEM image of the lower part.
(a) The cracking initiated at the external and internal edges. Multiple origins were found. (b) Irregular tearing lines formed by multiple overloading were seen on the surface. (c) The surfaces were clean and had no corrosion products. They consisted of dimples and did not show the features of stress corrosion cracking, fatigue damage or corrosion fatigue.
4. STRESS ANALYSIS AT T H E F R A C T U R E SITE Occurring at the same position, the fractures meant that the failure mode is closely related to the stress distribution and the shape of the structure.
4.1. The effect of load The shell was integrated with other components into a long assembly, which was hung upon two fixed points to support its weight G in transportation as shown in Fig. 9. No other load was transferred to the shell structure. Because of the brittleness of the shell material (i.e. cast aluminum alloy), stress analysis and tensile strength were used to analyze the failure.
PA Fracture site
L,
1 L2
I PB
L4 G
L3 L Fig. 9. Diagrammaticsketch of force acting on the assembly.
N. S. XI et al. Fracture site ~'\ I PB / C ]B D Mc ~ . . . .
276
Fig. 10. Diagrammatic sketch of force acting on the fracture site.
According to Figs 9 and 10, PB, Mc and O'max can be gained, respectively, from the balance of bending moment at support A, the balance of bending moment at C and the bending stress equation of a beam. That is: PB --
(L2 - LOG L3--LI
Mc = LDGD-- L4PB, a ...... -
Mcd 21
(1) (2) (3)
where GD is the weight of one of the two segments, I is the moment of inertia of the shell crosssection and d is the inner diameter of the shell structure. A positive value of Mc was obtained, and this indicates that the upper part bears tensile loading while the lower part bears compressive loading. The largest stress occurring at the uppermost point was about 2 MPa. The above stress result is derived under the hypothesis of no cracking. However, once the shell cracks, the moment of inertia reduces, and the stress becomes higher. The stress evaluation demonstrates that the stresses at the fracture site are far smaller than the tensile strength (297 MPa). Therefore, the shell structure cannot crack under the static stress produced by gravity if there are no material defects. Nevertheless, if the load is sufficiently big, the stress analysis shows that cracking should originate at the uppermost part first and then propagate to the two sides and the lower part. The fact that cracking did follow this path hints that the shell structure may have suffered impact loads during transportation. As can be seen from Fig. 9, the poor transportation method made the shell readily subject to the effect of impact loads. 4.2. The effect of local geometo' Figure 11 shows the geometry near the fracture site. The stress concentration factor can be estimated as that of a stepped plate under tensile loading as shown in Fig. 12. That is:
/ R
Fig. 11. Local geometry.
Fracture failure of a shell structure
2B
277
2b
Fig. 12. Steppedplate under tensile load.
l
K = I + - /~ 2.8~- - 2
Sf b]°65
(4)
The high stress concentration factor reveals that the transitional radius R is too small and the design is unreasonable. When the effects of both the poor transportation method and the high stress concentration factor are taken into consideration, the maximum stress in the fracture site may well have exceeded the tensile strength.
5. DISCUSSION
5.1. Crackin9 mechanism Examination of metallurgical quality demonstrated that the shell structure was in good condition. It is well known that stress corrosion cracking is accompanied by corrosion products, etched grain boundaries, etc., and that the typical fatigue features are beach marks, striations, etc. However, none of these features were found with detailed observations. Large height differences of the fracture surface as given in Fig. 4 and crescent shapes on the lower part imply that a large loading may have happened. The observation of the fracture surface and stress analysis showed that the cracking began at the upper part. Therefore, the dimples and the tearing lines on the fracture surface strongly suggest that the failure in this case was due to impact-type overloading. 5.2. Case study of the failure Overload fracture may occur if either the stress is too high or the material strength is too low. Examination and testing showed that the shells were free of defects and had a perfect heat-treated microstructure, and that the shell had a comparatively high resistance to tensile load. The shell structure should not crack under static loading, a fact that could be deduced from the maximum stress, which only reached 12.4% of the tensile strength considering the stress concentration. Presumably, the poor transportation method and the unreasonable design made the shell rupture.
6. C O N C L U S I O N A N D R E C O M M E N D A T I O N The metallurgical quality of the shell structure was good and accorded with the technical standard. The failure of the shell is thought to be due to multiple impact-type overloading produced by vibration in transportation. The recommendation is modifying the transportation method, as well as changing the design, for example, by increasing the transitional radius R.
Enyineeriny Failure Analysis, Vol. 4. No. 4, pp. 27l 277, 1997
Pergamon
© 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1351~6307/97 $17.00 + 0.00
P l h S 1350-6307(97)00016-2
F R A C T U R E F A I L U R E OF A S H E L L S T R U C T U R E N. S. XI, C. H. T A O a n d H. Y A N Failure Analysis Center of Aviation Industry of China, P.O. Box 81-4, Beijing 100095, China (Received 16 June 1997)
Abstract--Severalshells were found fractured at the same position after long distance transportation. Chemical composition analysis, metallographic examination, stress calculation and fracture surface observation were performed. Stress calculation and fracture surface analysis results showed that the crack originated at the upper part of the shell structure and propagated to the two sides and lower part of the shell structure. The fracture cause and mode were discussed comprehensively and preventive actions were specified. It was concluded that multiple overload induced the cracking of the shell structure. The poor transportation method and a high stress concentration were responsible for the fracture of the shell structure. © 1997 Published by Elsevier Science Ltd.
KeywordsBrittle : fracture, cracks, fracture surfaces, overload, stress concentrations. 1. I N T R O D U C T I O N Several shells were f o u n d f r a c t u r e d at the same p o s i t i o n after long distance t r a n s p o r t a t i o n . S o m e o f the shells were s e p a r a t e d c o m p l e t e l y a n d s o m e were c r a c k e d on the u p p e r h a l f p a r t s as s h o w n in Figs 1 a n d 2. The shells were f o r m e d by casting with A I - S i - C u - M g alloy a n d the h e a t t r e a t m e n t was T5. N o defects, such as cracks, gas cavities a n d so on, were discovered in the key p a r t s o f the shells by X - r a y n o n d e s t r u c t i v e inspection. It is sure t h a t the shells were in g o o d c o n d i t i o n when they left the factory. Bearing axial l o a d in service, the f r a c t u r e d shells were manifestly worthless. In o r d e r to d e t e r m i n e the failure cause a n d to specify the preventive actions, c h e m i c a l c o m p o s i t i o n analysis, tensile tests, m e t a l l o g r a p h i c e x a m i n a t i o n , fracture surface o b s e r v a t i o n a n d stress calc u l a t i o n s were p e r f o r m e d . 2. E X A M I N A T I O N
OF METALLURGICAL
QUALITY
In o r d e r to d e t e r m i n e if there existed cracks in the lower h a l f p a r t s o f the shells whose u p p e r p a r t s were c r a c k e d , b o t h fluorescent i n s p e c t i o n a n d l o n g i t u d i n a l section m e t a l l o g r a p h i c e x a m i n a t i o n were c a r r i e d out. N o defects were found.
Fig. 1. View of completely separated shell and the fracture position. 271
N. S. XI eta/.
272
i!l
Fig. 2. Crack in the half-split shell, shown by an arrow.
Six chemical samples and 24 tensile specimens (of diameter D = 5 mm) were removed from the failure shells randomly, Table 1 shows the chemical composition analysis results. This proved that the composition was normal. Tensile testing showed that the strength data were uniform with low scatter, and that the average tensile strength (297 MPa) exceeded the tensile strength (200 MPa) required for design. Figure 3 showed the microstructure of the shell material. It can be seen that the microstructure consisted of (A1) and Si phases. This demonstrated the shell had a perfect heat-treated microstructure. A longitudinal section metallographic photograph of the fracture surface is given in Fig. 4, which exhibited large height differences and microscopic plastic deformation, implying that the applied loads were heavy during cracking.
3. OBSERVATION OF F R A C T U R E S U R F A C E
3,1. Macroscopic l~'atures The fracture surfaces were observed visually and by a stereomicroscope. All the fracture surfaces had similar features detailed as follows: (a) The surface, on which there was no trace of corrosion deposits or pitting, was clear and appeared silver gray. (b) Not only was no macroscopic plastic deformation seen near the crack, but neither were fatigue beach marks, shear lips or herringbone traces discovered on the surface. (c) The whole surface was divided into two half rings. Figures 5 and 6 presenting typical macro morphology of the upper and lower rings, respectively, showed that the crack originated at the external and internal edges. Stated thoroughly, they initiated at the external edge just at a fillet radius and at the internal edge offset from the inner fillet radius at a position where grooves were
Table I. Chemical element contents (%wt) of the shell material Sample no. I 2 3 4 5 6 Technical standard
Si
Cu
Mg
Mn
Ti
B
Zr
7.14 7.75 7.25 7.29 7.77 7.12 6.0~8.0
1.48 1.47 1.49 t.49 1.50 1.50 1.3~t.5
0.27 0.35 0.31 0.37 0.33 0.28 0.2~0.4
0.20 0.22 0.24 0.19 0.23 0.24 0.15~0.25
0.076 0.108 0.085 0.087 0.120 0.083 0.05~0.15
0.003 0.003 0.006 0.008 0.003 0.005 0.001~0.05
0.070 0.106 0.086 0.080 0.104 0.093 0.05~0.15
Fe 0.22 0.19 0.17 0.17 0.16 0.20 _<0.35
Fracture failure of a shell structure
273
Fig. 3. Microstructure of the shell material.
! O0 l.tm I I
Fig. 4. Longitudinal section profile of the surface.
observed. The fracture surface was approximately perpendicular to the shell axis. Crescent shapes, formed by fracture propagation from the internal edge to the external edge, can be also seen in Fig. 6. (d) The upper fracture surface was fine and smooth but the lower surface was rough. Thus it can be deduced that the fracture initiated in the upper part. This was also demonstrated by the fact of that some shells only cracked in the upper half and the lower part was excellent without damage. 3.2.
Microscopicfeatures
A penetrating investigation of fracture surfaces in various regions was performed by SEM (Scanning Electron Microscope). Similar microscopic features were seen as shown in Figs 7 and 8. The detailed observations were:
274
N.S. XI et al.
Fig. 5. Macro morphology of the upper ring.
Fig. 6. Macro morphology of the lower ring.
Fig. 7. SEM image of the upper part.
Fracture failure of a shell structure
275
Fig. 8. SEM image of the lower part.
(a) The cracking initiated at the external and internal edges. Multiple origins were found. (b) Irregular tearing lines formed by multiple overloading were seen on the surface. (c) The surfaces were clean and had no corrosion products. They consisted of dimples and did not show the features of stress corrosion cracking, fatigue damage or corrosion fatigue.
4. STRESS ANALYSIS AT T H E F R A C T U R E SITE Occurring at the same position, the fractures meant that the failure mode is closely related to the stress distribution and the shape of the structure.
4.1. The effect of load The shell was integrated with other components into a long assembly, which was hung upon two fixed points to support its weight G in transportation as shown in Fig. 9. No other load was transferred to the shell structure. Because of the brittleness of the shell material (i.e. cast aluminum alloy), stress analysis and tensile strength were used to analyze the failure.
PA Fracture site
L,
1 L2
I PB
L4 G
L3 L Fig. 9. Diagrammaticsketch of force acting on the assembly.
N. S. XI et al. Fracture site ~'\ I PB / C ]B D Mc ~ . . . .
276
Fig. 10. Diagrammatic sketch of force acting on the fracture site.
According to Figs 9 and 10, PB, Mc and O'max can be gained, respectively, from the balance of bending moment at support A, the balance of bending moment at C and the bending stress equation of a beam. That is: PB --
(L2 - LOG L3--LI
Mc = LDGD-- L4PB, a ...... -
Mcd 21
(1) (2) (3)
where GD is the weight of one of the two segments, I is the moment of inertia of the shell crosssection and d is the inner diameter of the shell structure. A positive value of Mc was obtained, and this indicates that the upper part bears tensile loading while the lower part bears compressive loading. The largest stress occurring at the uppermost point was about 2 MPa. The above stress result is derived under the hypothesis of no cracking. However, once the shell cracks, the moment of inertia reduces, and the stress becomes higher. The stress evaluation demonstrates that the stresses at the fracture site are far smaller than the tensile strength (297 MPa). Therefore, the shell structure cannot crack under the static stress produced by gravity if there are no material defects. Nevertheless, if the load is sufficiently big, the stress analysis shows that cracking should originate at the uppermost part first and then propagate to the two sides and the lower part. The fact that cracking did follow this path hints that the shell structure may have suffered impact loads during transportation. As can be seen from Fig. 9, the poor transportation method made the shell readily subject to the effect of impact loads. 4.2. The effect of local geometo' Figure 11 shows the geometry near the fracture site. The stress concentration factor can be estimated as that of a stepped plate under tensile loading as shown in Fig. 12. That is:
/ R
Fig. 11. Local geometry.
Fracture failure of a shell structure
2B
277
2b
Fig. 12. Steppedplate under tensile load.
l
K = I + - /~ 2.8~- - 2
Sf b]°65
(4)
The high stress concentration factor reveals that the transitional radius R is too small and the design is unreasonable. When the effects of both the poor transportation method and the high stress concentration factor are taken into consideration, the maximum stress in the fracture site may well have exceeded the tensile strength.
5. DISCUSSION
5.1. Crackin9 mechanism Examination of metallurgical quality demonstrated that the shell structure was in good condition. It is well known that stress corrosion cracking is accompanied by corrosion products, etched grain boundaries, etc., and that the typical fatigue features are beach marks, striations, etc. However, none of these features were found with detailed observations. Large height differences of the fracture surface as given in Fig. 4 and crescent shapes on the lower part imply that a large loading may have happened. The observation of the fracture surface and stress analysis showed that the cracking began at the upper part. Therefore, the dimples and the tearing lines on the fracture surface strongly suggest that the failure in this case was due to impact-type overloading. 5.2. Case study of the failure Overload fracture may occur if either the stress is too high or the material strength is too low. Examination and testing showed that the shells were free of defects and had a perfect heat-treated microstructure, and that the shell had a comparatively high resistance to tensile load. The shell structure should not crack under static loading, a fact that could be deduced from the maximum stress, which only reached 12.4% of the tensile strength considering the stress concentration. Presumably, the poor transportation method and the unreasonable design made the shell rupture.
6. C O N C L U S I O N A N D R E C O M M E N D A T I O N The metallurgical quality of the shell structure was good and accorded with the technical standard. The failure of the shell is thought to be due to multiple impact-type overloading produced by vibration in transportation. The recommendation is modifying the transportation method, as well as changing the design, for example, by increasing the transitional radius R.