Plane Strain Fracture Toughness Determination for Magnesium Alloy

Transcription

1 Plane Stra Fracture Toughness Determation for Magnesium Alloy Plane Stra Fracture Toughness Determation for Magnesium Alloy Mohd Ahadl Mohd Daud 1, Mohd Zulkifli Selamat 2, Zaudd Sajuri 3 1 Faculty of Mechanical Engeerg, Universiti Teknikal Malaysia Melaka, Locked Bag 1752, Pejabat Pos Durian Tunggal, Durian Tunggal, Melaka. 3 Faculty of Engeerg and Built Environment, Universiti Kebangsaan Malaysia, Bangi, Selangor 1 ahadl@utem.edu.my, 2 zulkiflis@utem.edu.my, 3 sajuri@eng.ukm.my ABSTRACT A stress tensity factor K was used as a parameter to determe true material property, i.e. plane stra toughness K IC of AZ61 magnesium alloy usg a sgle edge notch bend (SENB) specimen accordance to ASTM E399 testg method. Five different specimen thicknesses of 2 to 10 mm were used test. A sharp fatigue pre-crack was itiated and propagated to half of specimen width at a constant crack propagation rate of about 1 x 10-8 m/cycle before specimen was loaded tension until stress is reached and n rapid occurred. The toughness K C values obtaed for different thicknesses showed that KC value decreased with creasg specimen thickness. The highest KC value obtaed was 16.5 MPa m for 2 mm thickness specimen. The value of K C became relatively constant at about 13 MPa m when specimen thickness exceeds 8 mm. This value was n considered as plane stra toughness K IC of AZ61 magnesium alloy. Calculation of mimum thickness requirement for plane stra condition and size of shear lips of validate obtaed K IC value. KEYWORDS: Stress tensity factor, Fracture Toughness, Thickness, Shear lips Magnesium alloy. 1.0 INTRODUCTION In recent decades, magnesium alloys have gaed great attention by automotive dustry players for ir promisg application as structural materials. The major application benefit is weight reduction due to ir low density which consequently lead to fuel savg. Or advantages of magnesium alloy are high specific strength, good castg, machg and recyclability (Mordika, 2001). European car maker such as Volkswagen and 45

2 Journal of Mechanical Engeerg and Technology BMW have troduced application of wrought magnesium alloys several automotive components [Duffy and Schumann et.al., 2003]. For structural application, it is important to ensure mechanical properties of magnesium alloys satisfy both reliability and safety requirement. The ma mechanical properties of AZ61 such as tensile strength and modulus of elasticity are well known, but some or important parameters such as toughness are still unknown. There are some data on toughness K ΙC value for magnesium alloy that was reported by (Hidetoshi et.al., 2005) and (Barbagallo et.al., 2004) respectively. They reported that toughness K ΙC value for as-extruded AZ31 was 15.9 MPa m and for AZ91C T6 condition was 11 MPa m. However, to best authors knowledge, re is no detail work done to determe plane stra toughness of AZ61 magnesium alloy. It is very important for engeers to know parameter before use material real applications. Therefore, objective of this study is to determe plane stra toughness for extruded AZ61 magnesium alloy usg several specimens thickness. 2.0 EXPERIMENT PROCEDURE The specimen used for toughness test was sgle edge notch bend (SENB or 3 pot bendg) specimen as shown Fig. 1. Specimen geometry was selected accordg to ASTM E399 standard. The specimen was n polished with 500 to 1500 grit emery papers to obta smooth. The precrackg was attaed at pre-crackg growth rates less than 10-8 m/cycle until crack reaches half of width of specimen. Pre-crackg were carried out on a pneumatic fatigue testg mache (14 kn maximum capacity). The pre-crackg were performed at frequencies 10 Hz and usg susoidal loadg form. Stress ratio R=0.1 was applied pre-crackg procedure at room temperature. The pre-crackg was performed at a constant ΔK level to obta constant crack growth rate. The stress tensity factor value for SENB specimen was calculated accordg to followg equations: 3PS K Q = 2 BW 3/ 2 a f W (1) Geometrical factor, f a W = 1/ 2 3/ 2 5/ 2 7 / 2 9 / α 3.07α α 25.11α α (2) Where, P = load, [N], S = span length, [40 mm], B = specimen thickness, [mm], W = specimen width, [10 mm] and a = crack length, [mm]. 46

3 Plane Stra Fracture Toughness Determation for Magnesium Alloy P mak max P P Q W Beban, Load, P (N) S a o B Displacement, Anjakan, s s (mm) FIGURE 1 FIGURE 2 Geometry of SENB specimen Types I of load-used test as per ASTM. displacement records (ASTM E399, 2008) A secant le through orig with slope of 95% of itial elastic loadg slope was used to determe conditional maximum load value. For validation of maximum load P max, prciple type of load displacement record as shown Fig. 2 was used for comparison as recommended by ASTM E399. To identify validity of plane stra toughness value, Eq. (3) was referred. Here, a is a crack length, B is mimum thickness that produces a condition where plastic stra energy at crack tip is mimal, K C is toughness of material and σ y is yield stress., 2.5 K a B C σ y 2 (2) 3.0 RESULT AND DISCUSSION Figure 3 showed load-displacement curves for 2, 4, 6, 8 and 10 mm thickness specimens. All load-displacement curves exhibited type I loaddisplacement record as shown Fig. 2. P Q is determed to be valid value of maximum load for calculation of toughness. The mode-1 stress tensity factor at K Q was calculated usg Eq. (1) based on P Q value obtaed. The calculated values Table 1 were n plotted a K C versus thickness relation curve as shown Fig. 4. The results showed that highest K C value obtaed was 16.5 MPa m for 2 mm thickness specimen. The value of K C became relatively constant at about 13 MPa m when specimen thickness exceeds 8 mm. This value was n considered as plane stra toughness K IC of AZ61 magnesium alloy. The shear lip ratio value for validation of K ΙC is below 0.1 (Anderson, 2005). The shear lip ratio for 8 and 10 mm specimen thickness were 0.1 and 0.08, respectively. Therefore, plane stra toughness K ΙC value was valid for specimen thickness more than 8 mm. 47

4 Journal of Mechanical Engeerg and Technology TABLE 1 Fracture toughness value for different thickness of magnesium alloy Specimen thickness, B (mm) P max (kn) P Q (kn) Fracture toughness, K C (MPa m) Shear lip ratio Condition Plane- Stress Plane-Stress Plane- Stress/Mixed Mode Plane-Stra Plane-Stra mm 4 mm 6 mm 8 mm 10 mm Load, P (N) Displacement, s (mm) FIGURE 3 Load-displacement curve for 2, 4, 6, 8 and 10 mm thickness of AZ61 magnesium alloy. Fracture Toughness, Kc, MPa 1/ Pla-Stress Experimental data Fittg curve Plane-Stra Specimen thickness, B (mm) FIGURE 4 Effect of thickness on toughness for AZ61 magnesium alloy. 48

5 Plane Stra Fracture Toughness Determation for Magnesium Alloy Macroscopic observation of s of specimens clearly showed two discrete regions. These two distct regions are shown optical micrograph of Fig. 5. The boundaries of se regions are well distguished between fatigue region and rapid region. The direction of crack propagation was clearly determed. The fatigue crack itiated from notch and propagated parallel on both side. The fatigue region dicated gradual crack propagation due to fatigue while rapid region with shng appearance shows unstable crack propagation and characterized by fast crack features. For 8 and 10 mm thickness samples, of fatigue region looks rough and shy with limited shear lip zone. This dicates that plane stra conditions are achieved. For 2, 4 and 6 mm thickness samples rapid region looks rough and shy with large amount of shear lip zone which dicated that samples were plane stress condition. Notch Fatigue Rapid Crack propagation direction 2 mm 4 mm 6 mm 8 mm 10 mm FIGURE 5 Overview of for sample 2 mm, 4 mm, 6 mm, 8 mm and 10 mm. Figure 6 shows of 2 mm and 8 mm thickness samples after toughness test. The images were taken from middle of sample thickness. Figure 6(a) showed of 2 mm thickness sample were relatively rough feature with presence of many ductile dimples which may resulted from shear loadg. Overview of revealed that almost half of area was domated by shear lip. Detail observation of shear lip area is shown Fig. 7. It is suggested that plastic zone size developed durg loadg of 2 mm thickness sample was very big. For pla stra toughness validation, plastic zone size should be not more than allowable area of (1/6π)( K ΙC /σ y ) 2 or 2% of crack length. 49

6 revealed that half of7. area wasthat domated by shear lip. Detaildurg observation of shear lip area is almost shown Fig. It is suggested plastic zone size developed loadg area is shown Fig.very 7. It big. is suggested plastic zonetoughness size developed durg plastic loadg of shear 2 mmlipthickness samplewas For plathat stra validation, of Mechanical Engeerg andfor Technology ofjournal 2size mm thickness was veryallowable big. plaofstra plastic )2 or 2% ofvalidation, crack length. zone should be notsample more than area (1/6π)( KΙC/σytoughness zone size should be not more than allowable area of (1/6π)( KΙC/σy)2 or 2% of crack length. (a) (a) (b) (b) FIGURE 6 FIGURE 6 FIGURE SEM observation on 6 middle of tested samples SEM on middle of tested samples SEMobservation observation on middle of tested samples FIGURE7 7 FIGURE SEM observation shear area tested FIGURE SEM observation on on shear lip7lip area of of tested SEM observation on shear lip area of tested sampes. sampes. SEMshowed observation on middleassociated of tested samples Figure 6(b) cleavage with was river pattern Figure 6(b)SEM showed cleavage associated with tested river pattern observed observation on middle samples was Figure observed domated ofof 8thatmm thickness sample. 6(b) showed cleavage associated with river patternwas wasfailed observed domated of 8 mm thickness sample. This shows sample domated of deformation. 8 mmwas thickness sample. This shows sample wassample failed This shows that sample failed brittle manner with limited plastic brittle manner with limited plastic Small ratio of shear lipthat area validates brittle manner with limited plastic deformation. Small ratio of shear lip area validates sample pla stra condition. deformation. Small ratio of shear lip area validates sample pla pla stra condition. stra condition. 4.0 CONCLUSION Pla stra toughness of AZ61 magnesium alloy was vestigated. Based on results obtaed, fdgs are concluded as follows: Fracture toughness value, KC for AZ61 magnesium alloy decreased with creasg of specimen thickness. The KC value became relatively constant at specimen above 8 mm. The critical plane stra toughness, KΙC of extruded AZ61 magnesium alloy was 13.0 MPa m. ISSN: Vol. 4 No. 2 July-December 2012

7 Plane Stra Fracture Toughness Determation for Magnesium Alloy 5.0 ACKNOWLEDGEMENTS The author would like to thank Prof. Dr. Y. Mutoh of Nagaoka University of Technology, Japan for supplyg magnesium alloy and Faculty of Mechanical Engeerg, Universiti Teknikal Malaysia Melaka for providg frasture and supportg for this research. 6.0 REFERENCES ASTM E Standard Test Method for Lear Elastic Plane Stra Fracture Toughness K IC of Metallic Materials. B.L. Mordika and T. Ebert Magnesium. Properties-applications-potential. Material Science and Engeerg. A302. pp Duffy Magnesium Alloy: The light choice for aerospace. Materials World. pp S. Schumann and H. Friedrich Current and Future Use of Magnesium Automobile Industry. Material Science Forum. Vol pp S. Hidetoshi and M. Toshiji Effect of texture on toughness extruded AZ31 magnesium alloy. Scripta Materialia. 53. pp S. Barbagallo and E. Cerri Evaluation of K IC and J IC parameter a sand cast AZ91 magnesium alloy. Engeerg Failure Analysis. 11. pp T. L. Anderson, PhD Fracture mechanics, fundamental and application. 2 nd ed., CRC Press: Boca Raton, New York. 51

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