International Journal of Fatigue 35 (2012) 37–44

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International Journal of Fatigue

journal homepage: www.elsevier .com/locate / i j fa t igue

Study on the fracture reason and fatigue life for guide pillar of injection machine

Jinguo Li a,b,⇑, Xiaogui Wang a, Lijuan Lu a, Zengliang Gao a,1

a College of Mechanical Engineering, Zhejiang University of Technology, Hangzhou 310014, Chinab College of Mechanical and Electrical Engineering, Taizhou Vocational & Technical College, Taizhou 318000, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 May 2010Received in revised form 4 November 2010Accepted 9 December 2010Available online 14 December 2010

Keywords:IGPNumerical simulationExperimental researchFatigue life prediction

0142-1123/$ - see front matter Crown Copyright � 2doi:10.1016/j.ijfatigue.2010.12.003

⇑ Corresponding author at: College of Mechanical Esity of Technology, Hangzhou 310014, China. Tel.: +8

E-mail addresses: lijinguo_2002@126.com (J. Li), z1 Tel.: +86 571 88320763.

Some injector guide pillars (IGP) used in injection machine were fractured only after 1.5-year service. Inorder to determine the cause of the fractures, the numerical simulation technology was applied to ana-lyze the mechanical properties of the IGP. To model the contact between the mating surfaces of theclamping mechanism, nonlinear multi-region contacts of surface–surface were applied to establish thecontact model of FEA. The constraint of tie was used for modeling thread joint. The simulated results indi-cated that the smaller area of contact surface yielded higher value of stress in the neck of IGP. The value ofmaximum stress concentration factor (SCF) was obtained by FEA. The stresses at outer wall of the IGPwere measured by strain gages, in order to check the results of FEA and examine the load balancebetween each IGP. The experimental data agree with the simulated results very well because the relativeerror of them is no more than 3.7%. The experimental results also proved that the load of the IGP wasequal with each other, which means there was no partial load between each IGP. Based on the numericalanalysis and experimental study, the structure of the IGP was improved by adopting a smoother double-round neck. The fatigue life of the improved structure is longer than that of the original machine.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Injection machine is a kind of important fast production device,which was shown in Fig. 1. It can make different shape products bymold, including vertical rubber injection, vertical plastic injection,and vertical die-casting. As a direction-guiding and load-bearingpart, the injector guide pillar (IGP) endured high clamping forceto provide injection pressure and packing pressure for mould. Be-cause the clamp mechanism serves in the very severe conditions,such as the heavy clamping force, high temperature and cyclichigh-level pulse loads, several cases of the IGP have fractured after1.5-year service, as shown in Fig. 2. When one IGP of injection ma-chine fractured, others would have been damaged. So all of the IGPshould be replaced meantime, that led to enormous economic lossfor manufacturers.

Glinka et al. [1] employed a mixed finite element analogousmethod to analyze the stress at the teeth of a threaded connec-tion. They ignored nonlinear behavior of contact surfaces. Chenet al. [2] studied comprehensively the IGP on composition,microstructure, fractures and crack. They found out that the mis-use of the materials of the IGP led to the fatigue failure. How-ever, no detailed studies have been done on the fatigue

010 Published by Elsevier Ltd. All r

ngineering, Zhejiang Univer-6 138 576 93100.lgao@zjut.edu.cn (Z. Gao).

strength of the IGP. Cao et al. [3] employed SEM and opticalmicroscopy to study the fracture of press frame pillar. The re-sults shown that spot-welding on the surface of the press framepillar mainly resulted in the fracture. Chen et al. [4] employedANSYS to analyze fracture failure of the drag link and presentedtwo simple and reliable structures of the screws, which cangreatly improve the reliability of the drag link. Placido and Vala-da [5] conducted some experiments on full and reduced scalesamples of IGP under cyclic bending and constant tensile loadsto investigate their fatigue mechanism. Stress analysis resultsof this study were the same as the previous researches.

In the present study, optical microscope was employed toanalyze the microstructure of the IGP, including material defects,crack initiation location, crack propagation area and the fast frac-ture area. Numerical simulation and experimental research wereused to analyze the stress field of clamping mechanism of theIGP. Based on the obtained numerical and experimental results,an improved structure for the IGP was designed. The fatigue lifeof the improved structure is longer than that of the originalmachine.

2. Fracture analysis of the IGP

XZL-1000 � 2000 injection machine was chosen as study sub-ject, which fractured frequently. Material of the IGP of XZL-1000 � 2000 is AISI4140 (35CrMo) and the surface is quenchedand tempered and chrome-plated.

ights reserved.

38 J. Li et al. / International Journal of Fatigue 35 (2012) 37–44

2.1. Macro analysis of fracture

The fracture section and location are shown in Fig. 2. The frac-ture morphology is shown in Fig. 3, which appears multi-origin fa-tigue fracture [6,7]. The crack origins are in the zone A, and the fastfracture surface is zone D.

Fig. 1. Injection machine.

Fig. 2. Crack location.

Fig. 3. Fracture morphology.

Fig. 4. Crack morphology of zone A.

Fig. 5. Morphology of crack propagation in zone B.

Fig. 6. Morphology of crack propagation in zone C.

Fig. 7. Finite element mesh of 1/4 model.

Fig. 8. Finite element mesh of IGP.

J. Li et al. / International Journal of Fatigue 35 (2012) 37–44 39

2.2. Metallographic examination

Metallurgical microscope was employed to analyze the fracturesurface. It was found that the initial cracks were originated in smallholes at the edge of zone A, as shown in Fig. 4.

Crack propagated from zone A to zone B gradually. Figs. 5 and 6show the morphology of crack propagation in zone B and zone C,respectively. The metallographic examination result shows thatthe fracture surface was rough, which illustrated the rapid crackpropagation.

Fig. 9. Loadi

3. Finite element analysis of the IGP

3.1. FE modeling

The main structure of rubber injection machine, XZL-1000 � 2000, was shown in Fig. 1. Because of symmetry of theloads and the structure, 1/4 geometry model was used in the anal-ysis of finite element (ABAQUS). The meshed analytical model wasgiven in Figs. 7 and 8. There are 64345 C3D8I solid elements in thewhole model. Because fracture occurred in the neck of the IGPwhere there is stress concentration, fined mesh was adopted tosimulate the high gradient of stress and strain. Material of nutand the IGP is 35CrMo whose Young’s modulus is 210 GPa andPoisson ratio is 0.3. Material of fixed mould plate is nodular castiron whose Young’s modulus is 200 GPa and Poisson ratio is 0.28.Material of mould is P20 whose Young’s modulus is 210 GPa andPoisson ratio is 0.22.

3.2. Loading and boundary condition

According to the working properties of the injection machine,the load curve of the oil cylinder is shown in Fig. 9. Line AB repre-sents the process of clamping, load increased gradually toward thepoint B. Line BC is mold-filling process, and the point C representsthat the mold cavity has been full of plastic. The point D meansinjecting has been finished. Peak load takes place in line DE, whichis the molding packing process. Product is fully formed at point E,and then the load decreased gradually to the point F. One loadingcycle takes 8 min with nearly 4-min-duration of peak load. Themaximum load of the oil cylinder is 200 tons. The injection ma-chine works 20 h a day and 300 days a year. The load can be re-garded as cyclic load. The load is calculated by Eq. (1). Themaximum pressure load used for numerical simulation is

p ¼ F � 103 � 9:8A� B

; ð1Þ

where F is the load, A and B the side length of the mould contactsurface, C the thickness of the mould (Fig. 10). In case 1, the contactsurface is A � B = 500 � 500 mm2, so p = 8.22 MPa. In case 2, thecontact surface is A� B ¼ 300� 300 mm2, so p = 31.36 MPa.

Symmetric constraints are applied to the surfaces at symmetricboundary. Six degrees of freedom were constrained at the root ofthe IGP. The interaction of sliding surfaces was modeled with aCoulomb friction law [8]. Nonlinear multi-region contact of sur-face-to-surface was applied to establish contact model of FE be-tween mating surfaces of the clamping mechanism, such as theIGP and the fixed mould plate, nuts and fixed mould plate, mould

ng path.

Fig. 10. Dimension of the mould.

Table 1Relationship of contacts.

Contact body IGP Nut Fixed mould plate Mould

IGP � Tie Contact �Nut Tie � Contact �Fixed mould plate Contact Contact � ContactMould � � Contact �

Fig. 11. Boundary conditions.

Fig. 12. Distribution of s1 in case 1 (MPa).

Fig. 13. The location of s1 in case 1 (MPa).

Fig. 14. Strain measurement points of FEA in the neck of the IGP.

Test

poi

nts

in th

e ne

ck o

f th

e IG

P

S1 (MPa)

Fig. 15. The principal stresses curve of test points on the neck of IGP.

40 J. Li et al. / International Journal of Fatigue 35 (2012) 37–44

upper-surface and fixed mould plate. Friction coefficients of thecontact surfaces were assumed to be 0.15. It was assumed thatthe screw thread teeth are engaged together under the preload,so the constraint of tie was used to simplify contact of the threadjoint in the FE modeling [9]. The detailed applied boundary condi-tions were given in Table 1 and Fig. 11.

Contact analysis is a highly nonlinear iterative solution process,so it is difficult for convergence when the relationship of contact is

instability [10]. The clamp mechanism of injection machine en-dured high pulse load, if the first load step is too big in Abaqus6.4, it would be led dramatic changes in contact model and theiterative solution would be incorrect. Therefore, multi-step loadwould be efficient to solve the multi-region contact problem. Forexample, it was very easy to set up contact relationship amongthe various parts by imposing a small load on the first step, andthen loaded full load to the model, the method is effective to im-prove the convergence rate.

Side

-Len

gth

of m

ould

(m

m)

S1 (MPa)

Fig. 16. s1 under variation of different contact surface.

Table 2Principal stress and stress intensity amplitude.

Cases Location (N3371) s1 ðMPaÞ s3 ðMPaÞ salt ðMPaÞ

Case 1 Neck of IGP 345.7 �23.67 184.68Case 2 Neck of IGP 456.3 �26.32 241.31

Fig. 18. Stress measurement of the fixed plate.

J. Li et al. / International Journal of Fatigue 35 (2012) 37–44 41

3.3. Simulation results

Fig. 12 showed the distribution of principal stress in the clampmechanism under case 1. Fig. 13 showed the stress concentrationon the neck of the IGP, the s1 (maximum principal stress) is346 MPa. The simulation results of case 2 showed that the locationof s1 is similar to case 1, and the s1 of case 2 is 456 MPa. Obviously,the maximum stress of case 2 is higher than that of case 1.

Fig. 14 showed the test points on the neck of the IGP and Fig. 15showed the curve of s1 on the neck of the IGP. The test point 8 lo-cated in the inside of the assemble hole of IGP and fixed mouldplate, which led the greatest moment, so s1 at point 8 larger thanthat at other points.

For more comprehensive investigation of effect of the area ofcontact surface of the mould on the stress concentration factor,several cases for different side lengths of the mould are considered.The analysis results are shown in Fig. 16. It is evident that thesmaller the area of contact surface is, the higher stress concentra-tion on the neck of the IGP is.

Fig. 17. Stress measurement of the IGP.

In order to provide valid data for the following fatigue assess-ment, two conditions of stress amplitude was calculated, as shownin Table 2. Case 1 and case 2 have the same load, but there are dif-ferent contact surface of mould. In case 1, the contact surface isA� B ¼ 500� 500 mm2. In case 2, the contact surface is A� B ¼300� 300 mm2. According to the FEA results, the maximum stresslocation appeared in the neck of IGP near the node 3371. s1, s3

(min-principal stress) and salt ¼ jðs1 � s3Þ=2j (stress intensityamplitude) during the load cycle are shown in Table 2.

4. Experimental study

In order to verify the FEA results, the stresses at the neck of theIGP and the fixed mould plate were measured by strain gages [11].Three directional 0�, 45� and 90� strain gages were used in thestress concentration area at the corner, and two directional straingages were used in the other places. The strain measurements ofthe IGP and the fixed mould plate were shown in Figs. 17 and 18.The Eqs. (2) and (3) was employed to transfer strain into stress.

s1 ¼E2

e0� þ e90�

1� l þ 11þ l

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðe0� � e90� Þ2 þ ð2e45� � e0� � e90� Þ2

q� �;

ð2Þ

s3 ¼E2

e0� þ e90�

1� l� 1

1þ l

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðe0� � e90� Þ2 þ ð2e45� � e0� � e90� Þ2

q� �;

ð3Þ

test

poi

nts

in th

e ne

ck o

f IG

P

S1 (MPa)

Fig. 19. Stress on the neck of the IGP.

Fig. 20. Stress measurement of the pillar.

Fig. 21. The location of the strain gage on the pillar.

Table 3Data of measurement (MPa).

Pillar Test 1 Test 2 Test 3 Test 4 Average

No. 1 78.3 80.3 77.5 81.2 79.3No. 2 79.3 80.9 78.5 82.1 80.2No. 3 78.7 81.2 77.6 82.4 79.9No. 4 78.9 80.6 78.1 81.6 79.8

42 J. Li et al. / International Journal of Fatigue 35 (2012) 37–44

where e0� , e45� and e90� are the strain in direction 0�, direction 45�,direction 90�, respectively. E and l are young modulus, Poisson ra-tio, respectively.

The measured results of stresses and simulation results undercase 2 were shown in Fig. 19. It can be found that the stress distri-bution obtained by FE analysis and experimental measurement arevery similar. However, s1 by FE analysis is slightly different fromthe experimental results, and the maximum measured stress inthe neck of the IGP is lower than the maximum calculated stressby 13 MPa. The relative error of them is no more than 3.7%, sothe experimental data agree with the simulation results very well.The error arises from two aspects: In the one hand, the FE meshesused in the simulation are so fine that the stress gradient at the IGPneck can be captured; in the other hand, it was difficulty to deter-mine the exact location of maximum stress in advance in theexperimental measurement. So, the difference between experi-mental data and simulated results arises.

Two directional strain gauge was employed to the axial dis-placement test of IGP, the purpose was to obtain the partial loadconditions among the IGPs. the location of the strain gauge wasshown in Figs. 20 and 21. Table 3 showed the data of measurement.

Measurement results showed that the maximum stress is80.2 MPa and the minimum stress is 79.3 MPa. The difference isabout 1.12% of the maximum stress, so it was negligible.

5. Fatigue assessment

Because IGP works under cyclic loading, fatigue life is assessedby Chinese pressure code JB4732-1995 [12].

There are S–N curves for materials with ultimate tensilestrength 552 MPa and 793 MPa in JB4732-1995. The ultimate ten-sile strength of the IGP material sb fall within 552–793 MPa, the fa-tigue life is calculated by linear interpolation

N ¼ sb � sb1

sb2 � sb1

� �ðN2 � N1Þ � N1; ð4Þ

where sb, sb1 and sb2 the ultimate tensile strength, N1 and N2 the fa-tigue cycle for the ultimate tensile strength sb1 and sb2 .

The ultimate tensile strength of the IGP material issb ¼ 600 MPa, and the fatigue life of the IGP was calculated byEq. (4). N1 and N2 can be obtained by fatigue design curve, asshown in Fig. 22. Table 4 shows the calculation fatigue life whenIGP working under case 2. The calculated fatigue life is1:66� 104 cycles.

6. Structure improvement of the IGP

Results of FE analysis illustrated that stress concentration onthe neck of IGP is one of the major reasons for IGP fatigue failure.Therefore, it’s very important to decrease the stress concentrationfactor on the neck of the IGP. The smoother double-round neck wasdesigned for improving the structure of the IGP, as shown inFig. 23.

FE software is used to analyze the mechanical properties of theimproved IGP again. The results were shown in Fig. 24. While theradius (R2Þ of the round is fixed, the principal stress decreasesquickly with the increasing of the radius (R1) of the round in theneck of the IGP. However, while the value of R1 is larger than25 mm, the chamfer can not be machined unless decreasing the va-lue of R2. As a result, an inverse tendency of principal stress wasobtained. Therefore, the radius of the round in the neck is selectedto be 25 mm. Table 5 showed the fatigue life of the IGP of modifiedstructure. Fatigue life of the modified IGP is four times as long asthat of the original structure. With the structural improvements,the IGP has been in operation for 4 years without fracture.

7. Conclusion

FE analysis and experimental investigation were conducted onthe fracture of the IGP. Based on the FE analysis and experimentalresults, the following conclusions can be drawn.

Fig. 22. Fatigue design curve of carbon steel, low alloy steel, high strength steel under 375 �C.

Table 4Calculation fatigue life of IGP.

s1 ðMPaÞ s3 ðMPaÞ salt ðMPaÞ N1 ðcycleÞ N2 ðcycleÞ N ðcycleÞ

Case 2 456.3 �26.32 241.31 1:34� 104 2:25� 104 1:66� 104

Fig. 23. Modified structure of the IGP.

Rad

ius

of r

ound

in n

eck

of th

e IG

P (m

m)

S1 (MPa)

Fig. 24. s1 versus different radius of round.

J. Li et al. / International Journal of Fatigue 35 (2012) 37–44 43

Table 5Fatigue life of IGP after modified the structure.

s1 ðMPaÞ s3 ðMPaÞ salt ðMPaÞ N1 ðcycleÞ N2 ðcycleÞ N ðcycleÞ

Value 305.6 �32.8 169.2 4:13� 104 1:68� 105 7:06� 104

44 J. Li et al. / International Journal of Fatigue 35 (2012) 37–44

(1) The metallurgical microscope was employed to analyze thefracture surface of the IGP. It was found that the fatigue isthe main reason for fracture of the IGP.

(2) The numerical simulation results showed that there washigh stress concentration in the round of the neck of the IGP.

(3) The structure of IGP was improved by adopting a smootherdouble-round neck. The fatigue lives of the original andimproved IGP were assessed by Chinese pressure codeJB4732. Fatigue life of the modified IGP is four times as longas that of the original structure.

Acknowledgments

The research work was supported by the National Natural Sci-ence Foundation of China (50975260) and the Natural ScienceFoundation of Zhejiang Province of China (Z1091027).

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