Fracture Toughness of Structural Flaw Butt Joint Weld of Low Carbon Steels Chandrahas Bekal 1, Dr. C. G Ramachandra 2, Dr. Shrinivasa Mayya D 3 Sathyaprakash A 4 1 Asst. Prof, Department of Mechanical Engineering,, Srinivas Institute of Technology, Mangaluru. 2 Head, Department of Mechanical/Marine Engineering, Srinivas Institute of Technology, Mangaluru 3 Principal, Srinivas Institute of Technology, Mangaluru 4 Asst. Prof, Department of Mechanical Engineering,, Srinivas Institute of Technology, Mangaluru Abstract Fracture is one of the greatest problems faced in structures as a small crack is sufficient to cause catastrophic failure if it is not controlled. Failure due to fracture has increased in the present day as the system is becoming more and more complex day by day. Advances in the field of fracture mechanics has reduced the potential threats to the structures caused due to fracture. Determination of fracture toughness has become a vital factor in the field of fracture mechanics. As welded joints are subjected to high thermal stresses and external loads, the strength of the weld will be much different than the actual parent material. A small external crack in the welded joint will cause the failure in the joint before it reaches its yield strength. Hence determination of fracture toughness of a welded joint is required in critical structure design. Traditional method of fracture is applied to determine the fracture toughness and finally real time video analysis is done to cross check the obtained value of crack tip opening displacement. Keywords Butt joints, critical stress intensity factor, crack propagation, fracture toughness, fracture mechanics, real time video analysis, Welded joints. I. INTRODUCTION Fracture mechanics is one of the subject which deals with the analysis of crack and its propagation. Any specimen which consists of an external crack will fail before it reaches the yield stress. The crack will act as a stress concentration point where the stress will be maximum as the area to which the load is subjected is practically very less. Traditional strength of mechanics approach is not applicable as this approach doesn t include the presence of crack in the material in its basic assumptions. A welding process is a very basic form of joining process in which the 2 materials are joined permanently. The commonly used welding method is electric arc welding method in which two metals are joined by passing electric current through a coated electrode and subjected to high temperature and melting the electrode and adding it as a flux between the metal parts. When the metal is welded large amount of thermal stress are induced in the heat affected zone which is the weakest part in the welded joint. During welding process, due to improper welding most of the time, cracks or irregularities will be formed on the surface of the welded joint which will be act as stress inducers. Traditional strength of materials approach cannot be used as the material will not be homogenous in nature practically. Hence fracture mechanics approach is applied to determine the stress intensity factor of the welded joint. Prof. Rohit Jha, Dr. A.K. Jha has derived the conclusion that At the welding current of 110amp the tensile strength was maximum for single V joint design in comparison with weld carried out of 90amp, 100amp and 120amp (UTS 387.74MPa, YS 297.81MPa). F. V Lawrence and W.H Munse conducted experiments on low carbon steel a crack initiation period of about half the total fatigue life, the latter being severely reduced by joint penetration defects. M. Parry, H. Nordberg and R. W. Hertzberg conducted experiments on A514 base plate material and associated weldment and concluded that the fatigue crack growth rate was primarily a function of applied stress intensity range. II. EXPERIMENTAL PROCEDURE Eight metal pieces of mild steel of following has been cut to a dimension using power hacksaw to a common length of 200 mm. Two mild steel flats is welded using E6013 electrode. A gap of 4 mm is kept between the two flat pieces and was joined by butt joint. The welding machine Model-WS-400 Time Inverter, MMA/TIG Welding Machine with a 400A capacity was used. Shielded metal arc welding technique was used to weld the two plates. The mild steel flats were placed on the welding table to avoid the unwanted distortion where the welding process was carried out. Four specimen was prepared with slightly varying the value of width of the weld and keeping all the other variables constant. The other variables include length of plates, thickness of plates, current, voltage and welding speed. Welding was performed on both sides of the plate. The welded joints were allowed to cool for 48 hours in open space. After 48 hours a V-grove of 5 mm was cut and cracks of different length was made in the ISSN: 2231-5381 http://www.ijettjournal.org Page 95
welded joint at two different locations. The first location was directly at the centre of the welded joint and the second location was at the junction of weld and the base material which is usually termed as the heat affected zone. The crack length of 4 mm, 6.5 mm, 7 mm were made in different specimen. The specimens were tested in 40 ton universal testing machine for tensile loading. The video recording of the testing was done by using Sony Hdr pj-410 model handy cam recorder with data rate 9100 kbps, bitrates 256 kbps, frame width 720X576 pixels. The time for the corresponding load was noted in seconds. The video was analysed using the Kinovea video analysis software and open source physics to validate the results. The chemical composition of mild steel is given in table number 1. E6013 welding electrodes were used in welding process and the composition of the electrode is given in table no. 2 Table 1: The Chemical Composition of the Base Metal Fig 1. Preparing the Specimen using Power Hacksaw Material C Mn S Si P percentage 0.14 0.76 0.013 0.28 0.01 Table 2: Chemical Composition of the Welding Electrode. Yield strength Ultimate strength % elongation CVN Impact value At 0 C 390 460 24 70 J Table 3: Allowable Current Conditions Size 2.50 X 3.25 X 4.00 X 5.00 X (mm) 350 350 450 450 Current amps 60-90 100-140 140-180 180-230 Table 4: Mechanical Properties of the Weld Metal Fig 2. Welded Joint Material C Mn Si S P percentage 0.07 0.40 0.22 0.02 0.02 Fig 3. Cracking of Welded Joint ISSN: 2231-5381 http://www.ijettjournal.org Page 96
Specimen No. Width of the welded plate (W) mm Initial Crack length(a) mm Width Of weld in mm Critical load kn Critical Crack length in mm National Conference on Advances in Mechanical Engineering Science (NCAMES-2016) The tensile tests were conducted and the results are as shown in table no. 5. Table 5. Tensile Test Results Spe cim en No. Table 6. Theoretical Results. Critica l stress kn/m m 2 Fracture toughnes s kn/mm 1/ 2 Fractu re toughn ess (practi cal) Crack tip opening displace ment mm 1 7.22 67.34 60.40 3.044 2 7.67 55.56 56.58 2.43 3 6.36 46.43 41.47 2.04 4 7.22 53.69 48.27 2.42 1 48.3 4.2 7.6 124 17.7 2 48.3 7.00 7.9 100 16.7 3 49.1 4.2 7.9 107 16.9 III. EXPERIMENTAL RESULTS AND DISCUSSIONS A. Analytical solution 4 50.2 6.5 8.2 115 17.6 Equations for stress intensity factor for a finite plate subjected to tensile load with edge crack 1) According to Irwin/feddersew/Iside equation K C = Critical stress =critical load / resisting area Resisting area is a function of crack length i.e area = T X ʃ (W-a)dw Fig 4. Initial crack length v/s Critical load 2) Stress intensity factor for practical case: K C = y and y= f( = 1.99 0.4[ ] 3) Crack tip opening displacement CTOD max = The theoretical values were evaluated and the results are presented in the table no. 6. Fig. 5 Initial crack length v/s Critical crack length ISSN: 2231-5381 http://www.ijettjournal.org Page 97
Fig. 6 Initial crack length v/s Critical stress Fig.7 Initial crack length v/s Fracture toughness length. It was observed that immediately after the critical crack had appeared the load starts decreasing in the UTM and there is a slow propagation of crack. When the breaking load has reached, the crack immediately opens and leads to the failure of the weld. From fig 6, the critical stress appears to remain within the range of 6 kn/mm 2 to 7.5 kn/mm 2 for the entire specimen. After applying the obtained experimental values, the value of critical fracture toughness is calculated and the values fall between the ranges of 50 to 65 MPa/mm -2. Fig 7 shows the variation of critical stress intensity factor or the fracture toughness. Fig 8 shows the value of opening of crack of different specimen. It is termed as CTOD. The crack tip opening displacement was also found to lie between 2.04 mm to 3.04mm. This value also matched with the results obtained from the video analysis. The final crack tip opening angle was found to be 72. B. Video analysis technique The experiment conducted for specimen 3 and 4 was recorded live streaming using Sony handy cam and the time in seconds for every corresponding load was noted down. Kineova video analysis software was used for the video analysis. The actual critical crack opening displacement was determined using the video analysis software and was compared with the theoretical value and the crack opening angle was also observed and noted down. Data s obtained from video analysis for specimen no 3 Crack length= 4.2mm Initial crack width = 1 mm Initial angle = 52 Critical crack width = 2.2mm Final angle = 77 Fig. 8 Initial crack length v/s CTOD From fig 4 it is observed that the critical load taken up by the welded joint varied from 100 kn to 125 kn for all the values of the thickness of the welding plate and thickness of weld. From fig 5, we can notice that the critical crack length remains almost same, i.e. within 16 mm to 18 mm for all specimens having different value of initial crack IV. CONCLUSIONS 1. It was observed that the crack propagated along the junction of the weld and the base metal. I.e. The head affected zone is the weakest point in the welded joint. 2. The critical crack tip opening angle was observed to be 32 3. The welded joint with structural flaw has a fracture toughness of 55kN/mm 1/2 to 75kN/mm 1/2 4. Video analysis was found to be more easier and useful technique to observe the crack propagations. 5. The mean critical stress was found to be 7.02kN/mm 2. 6. The mean crack opening displacement was found to be 2.06 mm (theoretical) and 2.95 (video analysis) ISSN: 2231-5381 http://www.ijettjournal.org Page 98
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