- Email: [email protected]
Fracture mechanics and fatigue crack propagation in armor steel welds
Engineering Failure Analysis 106 (2019) 104155
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Fracture mechanics and fatigue crack propagation in armor steel welds
T
⁎
Aleksandar Cabriloa, , Aleksandar Sedmakb, Zijah Burzicc, Srdja Perkovicc a b c
Faculty of Technical Sciences, University of Novi Sad, Trg D. Obradovića 6, 21000 Novi Sad, Serbia Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, 11120 Belgrade, Serbia Military Technical Institute, Ratka Resanovica 1, 11030 Belgrade, Serbia
A R T IC LE I N F O
ABS TRA CT
Keywords: Armor steel Hardness Gas metal arc welding Fracture toughness Fatigue crack growth
The welding process of armor steel is a complex process due to possible welding faults appearing in all zones of weld metal joint in the form of cracks and pores, as results of high percentage of carbon in the base metal. Military armored vehicles, are exposed to dynamic and impact loads. Hence, it is important to know the fracture toughness and fatigue crack propagation in all zones of the weld joint. Fatigue crack propagation testing was made on SEN (B) specimens. Fatigue threshold for weld metal is the lowest ΔKth = 10.1 MPa*m1/2, and for base metal is the highest ΔKth = 13.4 MPa*m1/2, while the notch values in HAZ is ΔKth = 12.6 MPa*m1/2. Due to a significant interest in quantification of material resistance to crack initiation and propagation, the fatigue crack growth rate was measured in the base metal, HAZ and welded metal zone. Accordingly, the most important attention should be paid to the zone of the base metal, while the weld metal and HAZ are safe zones.
1. Introduction Military armored vehicles are made of armor steel. Armor steel is complicated to weld as a result of high carbon content, which leads that this steel is prone to cold cracking. Cracks appear in the fusion line, as a result of high hardness and presence of hydrogen in this zone. Large content of austenite stabilizer, manganese and nickel can lead to hot cracking owing to segregation of impurity elements which have poor solubility in austenite. Crack can appear in base metal, as result of mistakes during steel production, in the fusion line because of hydrogen presence or in the weld metal as result of welding process. During military operations, for the sake of traversed on gravel terrain types, armored vehicles are exposed to impact loading. As a result of impact loads, cracks created in the weld joint can easily propagate toward which can affect structural integrity of military armored vehicles [1,2]. Gas metal arc welding (GMAW) is widely used in production of military armored vehicles [3,4]. Shield metal arc welding has a smaller deposition rate, compared to the GMAW process [5]. The GMAW process has superior productivity compared to SMAW. The superior productivity is shown in the wire continuously fed into the weld pool preventing frequent starts and stops. Armor steel presently welded with austenite and low hydrogen consumables. Austenite consumable are using to overcoming cold cracking. The main advantage of austenite in compare with low hydrogen consumable is much lower diffusivity and much higher solubility of hydrogen, hence allowing higher hydrogen levels without the occurrence of cracking in heat affected zone (HAZ). According to ESAB [6], the disadvantage of flux cored wires is that in the case of multi-pass welding the slag needs to be removed from the bead surface before deposition of subsequent passes which has an adverse influence on both productivity to the price.
⁎
Corresponding author. E-mail address: [email protected] (A. Cabrilo).
https://doi.org/10.1016/j.engfailanal.2019.104155 Received 19 April 2019; Received in revised form 14 August 2019; Accepted 22 August 2019 Available online 23 August 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Austenite consumable presently appear as a solid and cored wire. The disadvantage of low hydrogen consumable is the presence of slag, which eliminates possibility of automatic welding. Unlike to low hydrogen, solid consumable give us opportunity of automatic welding because of slag absence. The possibility of process automation, using robots and solid wire, means that the GMAW process has potentially greater productivity, which is beneficial for the military industry. Rational dimensioning of military armored vehicles are necessitates insight in higher values of dynamic effects. Therefore, significant interest in material resistance related to fracture toughness, as well as fatigue crack propagation. Fatigue crack growth rate characteristic in the linear and threshold region of the weld metal joint shows a fault – tolerant ability of investigated part of welded joint [7]. For the reliable operation of military armored vehicles, it is important to predict structural integrity. Prediction of structural integrity will be done by assessment of existing crack type faults, which can be achieved by applying fracture mechanic concept. The characteristic of fatigue crack growth rate in linear region and in the threshold region in metal weld is considered important property as it shows fault-tolerant ability of this part of welded joint. For better understanding of structural integrity regarding crack occurrence and its growth effect in armor steel welded joints, it is necessary to quantify the parameters controlling the strain behavior in crack tip vicinity and crack resistance. Therefore, in this paper the effect of heterogeneity of microstructure and mechanical properties on fatigue crack growth rate, da/dN, and fatigue threshold stress intensity factor range, ΔKth, of armor steel weld joint constituents is experimentally investigated at room temperature. Fracture surfaces for the fracture mechanic and fatigue crack growth tests were also investigated by Scanning Electron Microscope (SEM). Also, the resistance against projectile penetration in the HAZ of armor steel is analyzed. Finally, an example of residual life assessment is given. 2. Material and experiment 2.1. Materials and welding process Austenitic stainless steel is employed for welding of Protac 500 armor steel. The chemical composition (wt%) of armor steel, was 0.637 Cr, 1.07 Si, 0.27C, 1.09 Ni, 0.706 Mn, 0.039 V, 0.3 Mo, 0.02 P and 0.01 S. Solid austenitic wire AWS ER307 was used as the filler material. Typical weld metal composition (wt%) of consumable was 6.29 Mn, 8.24 Ni, 17.76 Cr, 0.89 Si, and 0.08C. All GMAW trials was carried out employing a robot controlled process, in order to have results repeatability [8], eliminate human factor, allow a fine adjustment of parameters. Welding direction was parallel to the rolling direction. Amour plates 12 mm thick are cut to the required dimensions (250 × 100 mm) by Water Jet Device, in order to eliminate heat treatment. The “V” joint under the angle of 55° was prepared by Water Jet Device. 2.2. Microstructure, hardness and mechanical property tests The welded metal microstructural examination was performed using a “Leitz-Orthoplan” microscope, while a scanning electron microscope JEOL JSM 6460LV at 25 kV was used for microstructure examination in coarse grained, fine grained and base metal zone. The samples for microstructural investigation were polished with a diamond paste. After polishing samples were etched for 20 s, with a mixture HCl and HNO3 reagent to reveal the structure. The microhardness test in weld joint were performed according to the standard EN ISO 9015-1 [9]. The microhardness was measured at every 0.5 mm, at the distance 2 mm under the upper surface. These measurements was performed using digital Micro Vickers Hardness Tester HVS 1000. For each measurement, a force of 500 g was applied for 15 s. Radiographic examination, macro graphic and micrographic analyses were conducted to assess absence of macroscopic imperfections, especially in weld metal, at the fusion line and the heat affected zone (HAZ). Tensile testing of specimens taken from base metal, and from welded joint, were performed on machine in displacement control, at room temperature. Tensile tests were performed according to the standard EN ISO 4136 [10]. Water jet cutting machine was used for specimen preparation. Tensile strength tests were carried with Instron 8033, servo-hydraulic tension testing machine, using strain rate 0.125 mm/s. 2.3. Microstructure of fracture toughness testing specimens In investigation of fracture mechanics of the armor steel weld joint [11], was presented that the smallest fracture toughness has the base metal, 86.1 MPa∙m1/2, then the HAZ, 286 MPa∙m1/2, while the weld metal has the highest fracture toughness of 355 MPa∙m1/ 2 . The aim in this part of research is to continue the examination of fracture mechanics of armor steel weld joint in the form of metallographic investigations of fractured surfaces. Metallographic testing will help to better understand the characterization of fractures, the way how cracks grow, and the effect of fatigue behavior on microstructures in these zones. 2.4. Fatigue crack growth test The fatigue crack growth test were performed using three-points bend, 12 mm thick specimens (SENeB) [12], according to the standard ASTM E647 [13]. The technical drawing of specimen for fatigue crack growth test is shown in Fig. 1 a). Specimens were cut on a water-jet device, to eliminate possibility of armor steel thermal treatment. SEN-B specimens, fatigue pre-cracked in different welded joint constituents, Fig. 2 b) and instrumented by foil RUMUL RMF A-10, of measuring length 10 mm. Specimens were tested at 2
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 1. a) Schematic drawing of the SEN (B) specimen with dimensions, b) Specimen orientation with respect to the weld axis for fatigue crack growth test.
Fig. 2. a) Results of RT testing welding process, b) optical macrograph of AWS ER 307 filler.
room temperature under variable loading for the determination of fatigue crack growth rate, da/dN, and stress-intensity factor range at fatigue threshold ΔKth. The testing was performed in load control on the CRACTRONIC high frequency resonant pulsator. 3. Results 3.1. Microstructure and hardness The specimen was examined by radiographic inspection. The weld joint is free from imperfections Fig. 2 a). The same good result was confirmed on the five investigated weld plates, by which the joint is considered acceptable, with the grade class B. The macrographic analysis of the specimens revealed perfect fusion, full penetration and complete absence of porosity and inclusions, Fig. 2 b). The weld metal, Fig. 3 a) consist of austenite with delta ferrite. The content of delta ferrite measured by Feritoscope: in the weld upper part 3.2%, in the weld center 5.4%, in the weld root 11.7%. Coarse and fine grained zone micrograph Fig. 3 b) and c) consists of tempered and quenched martensite and fine grained ferrite. The base metal micrograph consists of tempered and quenched martensite, Fig. 3 d). Microhardness was measured in the weld metal, the coarse and fine grained and the base metal zone. Micro hardness value are presented in the Table 1. It can be concluded that the minimum value of 190 HV is in the weld metal region. Hardness do not exceed 521 HV in the coarse grained zone, most critical to create crack, which is within accepted standard, MIL-STAN-1185 [14]. After the maximum, the hardness decreases with the achieved minimum hardness of 378 HV, in the fine grained zone. It should be highlighted that the fusion line hardness is around 412 HV. 3.2. Tensile characteristics Tensile testing of specimens taken from base metal (BM), from additional metal (AM), and from welded joint (WM), were
Fig. 3. a) Optical micrograph of AWS ER 307 filler, b) SEM micrograph of coarse grained zone, c) fine grained zone and d) base metal. 3
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Table 1 Microhardness values, measured 2 mm under the upper surface. Joint type
Location Weld metal
Fusion line
Coarse grained zone
Fine grained zone
Base metal
[HV]
[HV]
[HV]
[HV]
[HV]
190
412
521
378
509
performed on a machine in displacement control, at room temperature. The specimen from BM and AM for testing at room temperature, was machined from the available material, according to standard EN 895 7. Specimen from welded joint was made according EN 895. The testing results at room temperature are given in Table 2 for BM, AM and for the specimens of welded joint WM. The fracture surfaces of the weld metal, Fig. 4. a) and b), reveals many smaller dimples mixed with several larger dimples. The formation of large dimples is related to ductile fracturing with particle–matrix decohesion. A higher nickel content 8.19 (wt%) increases the toughness in fully austenitic compositions. The first requirement in armor steel welded structures is to assure the required strength. In most welded structures this is achieved with undermatching effect, superior strength of BM compared to WM. An additional proof of undermatching is the fracture of specimens from welded joint in WM and that the difference of values of yield stress and tensile strength in Table 2 is high. Special care in tensile properties should be paid to elongation. When material is homogeneous, as here BM and WM should be considered, elongation is useful for comparison. For welded joint, the elongation value is meaningless, since in measuring length of 100 mm enter BM i WM, of different tensile properties, but also a part of HAZ is included, in which tensile properties are unknown. 3.3. Microstructure of fracture mechanic results Microstructure of investigated samples are shown in Fig. 5. Cutting position for the specimen in base metal is shown in Fig. 5 a). Fig. 5 b) shows boundary between fatigue and crack opening fracture surface. Fig. 5 c) shows the fatigue fracture surface with smooth appearance and crack opening followed by ductile fracture and when inclined, while Fig. 5 d) shows the stretch zone. After determining J-R curve for cracking in HAZ, the fracture surface was examined by scanning electron microscope (SEM). The cutting position is shown in Fig. 5 e). The boundary between fatigue fracture and crack opening fracture surface with visible stretch zone is shown in Fig. 5 f) and g). Fig. 5 h) shows the fracture surface with smooth appearance on inclined specimens and stretch zone with the deformed ductile fracture dimples. Specimen cutting position in weld metal is shown in Fig. 5 i). Fracture surface is ductile. At the beginning of crack growth, the fracture surface is ductile with clearly visible ductile dimples, Fig. 5 j) and k). At the end of crack growth, the fracture surface area is still ductile with visible ductile dimples Fig. 5 l). 3.4. Fatigue crack propagation Crack length increase diagram, a, with the number of loading cycles, N, as well as fatigue crack growth rates diagram da/dN against ΔK, in base metal, HAZ and weld metal zone are shown in Fig. 6. Central, linear part of a curve is defined by Paris law, da/ dN=C∙(ΔK)m, where the crack growth rate is in the function of material constants, C and m, as well as the stress intensity factor range, ΔK. The constants are determined by curve fitting on the basis of experimental results and are shown in Table 3. Fatigue threshold, ΔKth, is obtained on the basis of growth rate da/dN = 10−6 mm. For the purpose of comparison, fatigue crack growth rate, da/dN, is determined for ΔK = 16 MPa∙m1/2. The results are shown in Table 3. The analysis of fracture surface was carried out after the examination of fatigue crack growth rate. Cutting position of the examined specimen in base metal is shown in Fig. 7 a). Images from Fig. 7 b) to d) show the crack growth in base metal, from the place of crack initiation to the place of final rupture. Fig. 7 b) at 700 μm from the place of initiation, then Fig. 7 c) 2.16 mm from the place of initiation and Fig. 7 d) 4 mm from the place of initiation. Crack growth rate, da/dN, increases with increasing crack length, but the fracture surface does not change significantly. Fatigue scratches are visible in Fig. 7 b-d), formed at the tips of crack growth. Fracture surface shown in Fig. 7 d) with clearly visible dimples is typical for fracture surface with a tensile load when the load is too big and when the structure can't support the load. Table 2 Results of tensile testing for BM, WM and FM-filler metal of specimens investigated at 20 °C. Specimen
BM FM WM
Yield stress
Tensile strength
Elongation
[MPa]
[MPa]
[%]
1205 425 552
1572 620 833
13 35 8.5
4
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 4. a) and b) SEM images showing the fracture surfaces of the weld metal region and the corresponding void networks.
Fig. 5. Fractography of specimens investigated on fracture mechanics, a) crack growth position in base metal, b) boundary between fatigue and crack opening fracture surface, c) fatigue fracture surface, d) stretch zone, e) crack growth position in HAZ, f) and g) boundary between fatigue fracture and ductile fracture with visible stretch zone, h) stretch zone with the deformed ductile fracture dimples, i) crack growth position in weld metal, j) clearly visible ductile dimples, k) and l) ductile fracture surface area with visible ductile dimples.
The analysis of fracture surface in HAZ was carried out after the examination of fatigue crack growth rate. The examined piece is shown in Fig. 7 e). Fig. 7 f) to h) continually show the crack growth from the place of initiation almost to the place of final rupture. Fig. 7 f) and g) shows the notch in the place of initiation and Fig. 7 h), 6 mm from the place of initiation. Crack growth rate, da/dN, increases with increasing crack length resulting in the fracture surface area typical for fatigue fracture. Cutting position of the examined specimen in weld metal is shown in Fig. 7 i). Fig. 7 j) to l) show the crack growth from the place of initiation almost to the place of final rupture. Fig. 7 j and k) from the place of initiation and Fig. 7 l), 6 mm from the place of 5
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 6. Fatigue crack growth rate per cycle, da/dN, vs. stress intensity factor range, ΔK, specimens pre-cracked in base metal a) HAZ b) and c) weld metal.
Table 3 Parameter values of Paris equation for notch position in base metal, HAZ and weld metal. Specimen
Fatigue threshold
Coefficient
Exponent
Crack growth rate for ΔK = 16 [MPa∙m1/2]
ΔKth
C
m
da/dN
1/2
[MPa∙m Specimen BM Specimen HAZ Specimen WM
]
13.4 15.2 10.1
[nm/cycle] 1.0∙10−9 6.0∙10−13 1.0∙10−12
3.35 5.97 5.10
1.10∙10−5 9.26∙10−6 1.38∙10−6
initiation. Crack growth rate, da/dN, increases with increasing crack length resulting in the fracture surface area typical for fatigue fracture. 4. Ballistic performance analysis Delta ferrite in weld metal zone is uniformly distributed and has potentially good mechanical properties. The good combination of hardness and toughness is achieved by a very fine delta ferrite skeleton in the austenitic base. A precipitate transition from weld metal, the ductile area, to the HAZ region is necessary for the toughness of the military armor vehicle. The most sensitive zone of the 6
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 7. Fractography of specimens investigated on fatigue crack propagation, a) Crack growth position in base metal, b) fracture surface 700 μm from the place of initiation, c) and d) fracture surface 2.16 mm and 4 mm from the place of initiation, e) crack growth position in HAZ, f) and g) fracture surface from the place of initiation, h) 6 mm from the place of initiation, i) crack growth position in weld metal, j) and k) place of initiation, l) 6 mm from the place of initiation.
armor steel welded joint is the fusion line. Good balance between hardness and ballistic protection is good in the fusion line. Martensite and bainite microstructures are presented in the coarse-grained HAZ. Thereby, the corresponding toughness was obtained in this critical area. From the aspect of ballistic protection, [15], steeply increase of hardness in the inter-critical HAZ and overtempered zone is necessary. The increase of hardness in the intercritical zone is the result of the dominant presence of martensite in this zone. High-hardness armor steel requires careful control of the welding procedure to avoid loss of heat-affected zone hardness. Material hardness is strongly dependent on the welding temperature history. The key to controlling the amount of softening in the heataffected zone is to maintain a high peak temperature gradient close to the weld bead [16]. The microstructure of the heat-affected zone is a function of the cooling rate imposed by the welding process and the chemical composition of the base plate [5]. The microstructure of the heat-affected zone impacts the hardness level and the ballistic performance of the weld joints. However, the ballistic limit in the HAZ, has been found to be highest in the case of the heat input 0.8 kJ/mm weld with the hardness level in HAZ 541 HV [17]. The HAZ of the heat input 1.2 kJ/mm with the hardness level 502 HV was also resistant to penetration and exhibited the good ballistic limit, whilst the HAZ of 1.4 kJ/mm with the hardness level 460 HV, exhibited an intermediate performance. The heat input 1.6 kJ/mm and 1.8 kJ/mm with the hardness level 426 HV and 400 HV, respectively, was not resistant to penetration and exhibited the lowest ballistic limit. Based on the heat input and hardness level in the HAZ, ballistic performance in this zone can be predicted. A similarity between the heat input / hardness of 1.2 kJ/mm / 502 HV, where ballistic resistance was achieved [17], can be found with the result in this study, where with a heat input of 1.25 kJ/mm and hardness of 521 HV was achieved. Based on this, we can conclude that HAZ in this research will have a good ballistic performance leading to projectile shattering.
5. Predicting the remaining life of the military armored vehicle Predicting the life of a military armored vehicle is necessary during the vehicle design and testing for defining control intervals to 7
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 8. Strain gauge rosette locations on the test Vehicle Lav – 25, [18].
detect cracks caused by exploitation. In cases where armor steel is welded, cracks are likely to appear [16], hence one has to assume that the crack exist in welded joint. There are number of empirical models defining fatigue crack growth for such conditions, based on Paris law, [17]. In the simplest form, i.e. by applying Paris law, one can calculate the number of cycles as given by Eq. (1):
(1− m2 )
a 1 ΔN = ∙ 0 C [Y Δσ a ]m
m 2
(1− m2 )
− ac −1
(1)
where ΔN is number of cycles for cracks to grow from initial length, a0, to critical length, ac, and Y=Y(a/W) is the correction factor for the crack length. The aim of this study is to determine remaining life, on the basis of exploitation measurements, [18], using strain gauge rosettes at cracking sites on the hull of the test military of armored vehicle, Fig. 8. Test was conducted in two phases so that the strain data from eight initially placed gauges (Rosettes A to H) could be inspected and used to help determine the locations for the final six gauges (Rosettes I to N). This procedure also minimized the possibility of placing the majority of the gauges in low stressed regions of the hull, as explained in [18]. Forty-five degree 350 Ω rosettes, type Micro Measurements CEA.46-25OUR-350 (6.4 mm grid gauge length), were used for all measurements except for Rosette L and M which were of the CEA-06-125-UR-350 type (3.2 mm grid gauge length), [18]. The Lav-25 is a military armored vehicle with eight wheels, Fig. 8, made of armor steel with hardness 500 HB, prone to cracking, especially if welded. Stresses were measured using sensors rosette mounted at critical locations as the vehicle passed through a number of different terrain types. The results of stress measurement are shown in the Table 4, [18], indicating that some locations were under relatively high stress. Dynamic stress in the vehicle are caused by rough coarse, as well as by vibrations associated with engine operation during the vehicle braking. Significant stress has developed in the front of the military armored vehicle as a result of the presence of the engine and the braking system of the vehicle. This is the result of a vibration from the complete suspension of the engine unit that was transferred to the top plates of the military armored vehicle. The suspension of the engine unit are holders welded to the interior of the surrounding Table 4 Effective stresses measured at various engine speeds while the test vehicle was stationary, [18]. Maximum stress (MPa) Rosette
A F C N L M
First class road
Rough course
Second class road
Cross country course
[MPa]
[MPa]
[MPa]
[MPa]
3.4 2.5 11 7.1 2.9 11
4.2 8.2 31 30 16 71
4.0 6.3 16 13 11 21
4,5 11 24 17 41 49
8
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Table 5 Remaining exploitation period of the military armored vehicle in exploitation of a crack in BM, HAZ and WM. Rosette position
A
Y
1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22
F
C
N
L
M
Δσ
a
REP for rough course terrain Crack in BM
Crack in HAZ
Crack in WM
[MPa]
[m]
[Years]
[Years]
[Years]
4.2 4.2 4.2 4.2 4.2 8.2 8.2 8.2 8.2 8.2 31 31 31 31 31 30 30 30 30 30 16 16 16 16 16 71 71 71 71 71
0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006
1,778,845 422,846 99,558 20,326 3613 188,492 44,806 10,550 2154 383 2176 517 122 25 4 2429 577 136 28 5 20,013 4757 1120 229 41 135 32 8 2 0.3
338,605,050,264,230 26,266,672,431,027 2,003,244,419,228 118,538,995,762 5,481,202,293 6,237,678,635,999 483,876,602,945 36,903,155,776 2,183,689,112 100,973,032 2,223,629,273 172,494,006 13,155,365 778,449 35,995 2,704,442,824 209,792,245 15,999,939 946,772 43,778 115,317,547,762 8,945,549,543 682,238,005 40,370,415 1,866,714 15,793,535 1,225,155 93,437 5529 256
4,011,412,229,428 451,658,033,382 50,120,491,133 4,477,945,901 324,070,159 132,256,805,320 14,891,226,624 1,652,479,391 147,638,484 10,684,637 149,942,255 16,882,489 1,873,450 167,381 12,113 177,235,321 19,955,505 2,214,462 197,848 14,318 4,373,778,281 492,458,013 54,648,065 4,882,456 353,345 2,190,035 246,583 27,363 2445 177
Note: ac = 9 mm. In C = 1.0∙10−9, m = 3.35 in BM, C = 6.0∙10−13, m = 5.97 in HAZ, C = 1.0∙10−12, m = 5.10 in WM.
vehicle. The position of the two holders is defined in the rosette C and N as well as M and L. Table 4 shows that the rosettes C and M sustain the highest stress. The maximum stress is obtained on rough course terrain and on cross country terrain topography. Unlike the rosette around the C, L, M and N, rosettes at A and F, located in the middle part of armored vehicle, have low stresses. The highest stress at the cross country course were recorded with the rosettes L and M, which are located on the front of the vehicle, [18]. Fatigue remaining life for constant amplitude loads, taken as the maxim stress from Table 4, is calculated using Eq. (1), based on the following assumptions: - The initial crack, a0, is a crack that can be detected by non-destructive testing, typically taken as a0 = 1 mm, which is also reasonable value for the military armored vehicle. The critical crack, ac, was taken as 9 mm, since the thickness of the armor plate of 10 mm and remaining ligament (1 mm) is just about to fail in static mode (max. Stress value is 71 MPa, Table 4). - The time of exploitation of the military armor vehicle is considered to be 8000 km/year. - The length of the growing crack is changed by 1 mm from the initial value up to 6 mm. - The values of C and m are the constants of the Paris equation determined by examining the fatigue crack growth parameters, as presented in Table 3 for BM, HAZ and WM. - The coefficient Y is a geometric member that depends on the ratio of crack length and thickness of the armor plate. The coefficient Y is calculated from the ratio of the crack length ad and a, presented in the Table 5 according to Eq. (2).
Y = 1.12–0.231∙ (a/ W ) + 10.55∙ (a/ W )2–21.72∙ (a/ W )3 + 30.39∙ (a/ W ) 4
(2)
The results of remaining exploitation period (REP), on rough course terrain of the armored vehicle, is calculated according to Eq. (3) and presented in Table 5.
REP =
∆N 6x106
(3)
where: 9
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
- 6 × 106 - the number of impact loads per year, - ΔN – the remaining exploitation cycles, calculated according to Eq. (1). The results presented in Table 5 indicate that the REP has the smallest value for the maximum stress 71 MPa (location M) and initial crack length 6 mm, is just 0.3 Year for BM. Comparing values for REP in BM, HAZ and WM, one can say that last two are safe zones, which significantly improves structural integrity of armor vehicle, since there is no real danger of having cracks in BM. Even for the worst case scenario, i.e. crack length 6 mm and stress amplitude 71 MPa, it takes 256 and 177 Years to get to the critical crack length in HAZ and WM, respectively. 6. Conclusions On the basis of the results presented in this work, the following conclusions may be made for tested armor steel: - Fracture toughness value for BM (86 MPa∙m1/2) is slightly lower in comparison with armor steels 500 class. Anyhow, HAZ and WM have significantly higher fracture toughness than BM, providing safe welded joints in respect to brittle fracture. - Suitable combination of microstructure in welded joint provides high resistance to impact loading, i.e. good ballistic protection, enabling projectile shattering. - The remaining exploitation period, determined by the rate of fatigue crack growth, shows that with respect to the integrity and life of the military armored vehicle, the most important attention should be paid to the base metal, while the weld metal and HAZ are safe zones. Acknowledgments This study was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia through Project ON 174004. References [1] Y. Kchaou, V. Pelosin, G. Hénaffa, N. Haddar, K. Elleuch, Failure mode analysis of SMAW welded UNS N08028 (Alloy28) superaustenitic stainless steel under crack growth tests, Eng. Fail. Anal. 97 (2019) 804–819. [2] J. Toribio, V. Kharin, F.J. Ayaso, B. González, J.C.M. Franco, D. Vergara, M. Lorenzo, Failure analysis of a lifting platform for tree pruning, Eng. Fail. Anal. 17 (2010) 739–747. [3] J.S. Kim, J.Y. Hui-Jun Yi, Characteristics of GMAW narrow gap welding on the armor steel of combat vehicles, Appl. Sci. 7 (2017) 658, https://doi.org/10.3390/ app7070658. [4] M. Atabaki, J. Ma, G. Yang, R. Kovacevic, Hybrid laser/arc welding of advanced high strength steel in different butt joint configurations, Mater. Des. (2014) 573–587. [5] L. Kuzmikova, J. Norrish, H. Li, M. Callaghan, Research to Establish a Systematic Approach to Safe Welding Procedure Development Using Austenitic Filler Material for Fabrication of High Strength Steel, 16th International Conference on the Joining of Materials, (2011), pp. 1–13. [6] Advantages and Disadvantages of Metal Cored Wires, http://www.esabna.com/us/en/education/blog/advantages-and-disadvantages-of-metal-cored-wires.cfm. [7] L. Tang, C. Qian, A. Ince, J. Zheng, H. Li, Z. Han, Fatigue crack growth behavior of the MIG welded Joint of 06Cr19Ni10 Stainless Steel, Materials 11 (2018) 1336, https://doi.org/10.3390/ma11081336. [8] A. Cabrilo, K. Geric, Weldability of high hardness armour steel, Adv. Mater. Res. (2016) 79–84. [9] EN ISO 9015-1, Destructive Tests on Welds in Metallic Materials. Hardness Testing Hardness Test on Arc Welded Joints, (2011). [10] EN ISO 4136, Destructive Tests on Welds in Metallic Materials. Transverse Tensile Test, (2011). [11] A. Cabrilo, K. Geric, Fracture mechanic and Charpy impact properties of a crack in weld metal, HAZ and base metal of welded armor steel, Procedia Struct Integrity (2018) 2059–2064. [12] ASTM E1820-16, Standard Test Method for Measurement of Fracture Toughness, ASTM International, West Conshohocken, PA, 2016. [13] ASTM E647–08, Standard Test Method for Measurement of Fatigue Crack Growth Rates, ASTM International, West Conshohocken, PA, 2008www.astm.org. [14] MIL-STD-1185, Department of Defense Manufacturing Process Standard: Welding, High Hardness Armor, [SUPERSEDES MIL-W-62162] (2008). [15] A. Cabrilo, K. Geric, M. Jovanovic, L. Vukic, Weldability and impact energy properties of high-hardness armor steel, J. Mater. Eng. Perform. 27 (3) (2018) 1281–1295. [16] J. Vojvodic Tuma, A. Sedmak, Analysis of the unstable fracture behaviour of a high strength low alloy steel weldment, Eng. Fract. Mech. 71 (9) (2004) 1435–1451 (17). [17] M. Arsić, R. Karić, A. Sedmak, M. Burzić, B. Vistać, Methodological Approach to Integrity Assessment and Service Life of Rotating Equipment at Hydropower Plant – Turbine Shaft, Structural Integrity and Life, vol. 13, (2013), pp. 117–124. [18] S.J. Cimpoeru, The Measurement of Dynamic Structural Stresses in a Light Armoured Vehicle. M.Eng.Sci, GradieAust, in 5 Th Australian Aeronautica Conference, Melbourne (1993), pp. 13–15.
10
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Fracture mechanics and fatigue crack propagation in armor steel welds
T
⁎
Aleksandar Cabriloa, , Aleksandar Sedmakb, Zijah Burzicc, Srdja Perkovicc a b c
Faculty of Technical Sciences, University of Novi Sad, Trg D. Obradovića 6, 21000 Novi Sad, Serbia Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, 11120 Belgrade, Serbia Military Technical Institute, Ratka Resanovica 1, 11030 Belgrade, Serbia
A R T IC LE I N F O
ABS TRA CT
Keywords: Armor steel Hardness Gas metal arc welding Fracture toughness Fatigue crack growth
The welding process of armor steel is a complex process due to possible welding faults appearing in all zones of weld metal joint in the form of cracks and pores, as results of high percentage of carbon in the base metal. Military armored vehicles, are exposed to dynamic and impact loads. Hence, it is important to know the fracture toughness and fatigue crack propagation in all zones of the weld joint. Fatigue crack propagation testing was made on SEN (B) specimens. Fatigue threshold for weld metal is the lowest ΔKth = 10.1 MPa*m1/2, and for base metal is the highest ΔKth = 13.4 MPa*m1/2, while the notch values in HAZ is ΔKth = 12.6 MPa*m1/2. Due to a significant interest in quantification of material resistance to crack initiation and propagation, the fatigue crack growth rate was measured in the base metal, HAZ and welded metal zone. Accordingly, the most important attention should be paid to the zone of the base metal, while the weld metal and HAZ are safe zones.
1. Introduction Military armored vehicles are made of armor steel. Armor steel is complicated to weld as a result of high carbon content, which leads that this steel is prone to cold cracking. Cracks appear in the fusion line, as a result of high hardness and presence of hydrogen in this zone. Large content of austenite stabilizer, manganese and nickel can lead to hot cracking owing to segregation of impurity elements which have poor solubility in austenite. Crack can appear in base metal, as result of mistakes during steel production, in the fusion line because of hydrogen presence or in the weld metal as result of welding process. During military operations, for the sake of traversed on gravel terrain types, armored vehicles are exposed to impact loading. As a result of impact loads, cracks created in the weld joint can easily propagate toward which can affect structural integrity of military armored vehicles [1,2]. Gas metal arc welding (GMAW) is widely used in production of military armored vehicles [3,4]. Shield metal arc welding has a smaller deposition rate, compared to the GMAW process [5]. The GMAW process has superior productivity compared to SMAW. The superior productivity is shown in the wire continuously fed into the weld pool preventing frequent starts and stops. Armor steel presently welded with austenite and low hydrogen consumables. Austenite consumable are using to overcoming cold cracking. The main advantage of austenite in compare with low hydrogen consumable is much lower diffusivity and much higher solubility of hydrogen, hence allowing higher hydrogen levels without the occurrence of cracking in heat affected zone (HAZ). According to ESAB [6], the disadvantage of flux cored wires is that in the case of multi-pass welding the slag needs to be removed from the bead surface before deposition of subsequent passes which has an adverse influence on both productivity to the price.
⁎
Corresponding author. E-mail address: [email protected] (A. Cabrilo).
https://doi.org/10.1016/j.engfailanal.2019.104155 Received 19 April 2019; Received in revised form 14 August 2019; Accepted 22 August 2019 Available online 23 August 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Austenite consumable presently appear as a solid and cored wire. The disadvantage of low hydrogen consumable is the presence of slag, which eliminates possibility of automatic welding. Unlike to low hydrogen, solid consumable give us opportunity of automatic welding because of slag absence. The possibility of process automation, using robots and solid wire, means that the GMAW process has potentially greater productivity, which is beneficial for the military industry. Rational dimensioning of military armored vehicles are necessitates insight in higher values of dynamic effects. Therefore, significant interest in material resistance related to fracture toughness, as well as fatigue crack propagation. Fatigue crack growth rate characteristic in the linear and threshold region of the weld metal joint shows a fault – tolerant ability of investigated part of welded joint [7]. For the reliable operation of military armored vehicles, it is important to predict structural integrity. Prediction of structural integrity will be done by assessment of existing crack type faults, which can be achieved by applying fracture mechanic concept. The characteristic of fatigue crack growth rate in linear region and in the threshold region in metal weld is considered important property as it shows fault-tolerant ability of this part of welded joint. For better understanding of structural integrity regarding crack occurrence and its growth effect in armor steel welded joints, it is necessary to quantify the parameters controlling the strain behavior in crack tip vicinity and crack resistance. Therefore, in this paper the effect of heterogeneity of microstructure and mechanical properties on fatigue crack growth rate, da/dN, and fatigue threshold stress intensity factor range, ΔKth, of armor steel weld joint constituents is experimentally investigated at room temperature. Fracture surfaces for the fracture mechanic and fatigue crack growth tests were also investigated by Scanning Electron Microscope (SEM). Also, the resistance against projectile penetration in the HAZ of armor steel is analyzed. Finally, an example of residual life assessment is given. 2. Material and experiment 2.1. Materials and welding process Austenitic stainless steel is employed for welding of Protac 500 armor steel. The chemical composition (wt%) of armor steel, was 0.637 Cr, 1.07 Si, 0.27C, 1.09 Ni, 0.706 Mn, 0.039 V, 0.3 Mo, 0.02 P and 0.01 S. Solid austenitic wire AWS ER307 was used as the filler material. Typical weld metal composition (wt%) of consumable was 6.29 Mn, 8.24 Ni, 17.76 Cr, 0.89 Si, and 0.08C. All GMAW trials was carried out employing a robot controlled process, in order to have results repeatability [8], eliminate human factor, allow a fine adjustment of parameters. Welding direction was parallel to the rolling direction. Amour plates 12 mm thick are cut to the required dimensions (250 × 100 mm) by Water Jet Device, in order to eliminate heat treatment. The “V” joint under the angle of 55° was prepared by Water Jet Device. 2.2. Microstructure, hardness and mechanical property tests The welded metal microstructural examination was performed using a “Leitz-Orthoplan” microscope, while a scanning electron microscope JEOL JSM 6460LV at 25 kV was used for microstructure examination in coarse grained, fine grained and base metal zone. The samples for microstructural investigation were polished with a diamond paste. After polishing samples were etched for 20 s, with a mixture HCl and HNO3 reagent to reveal the structure. The microhardness test in weld joint were performed according to the standard EN ISO 9015-1 [9]. The microhardness was measured at every 0.5 mm, at the distance 2 mm under the upper surface. These measurements was performed using digital Micro Vickers Hardness Tester HVS 1000. For each measurement, a force of 500 g was applied for 15 s. Radiographic examination, macro graphic and micrographic analyses were conducted to assess absence of macroscopic imperfections, especially in weld metal, at the fusion line and the heat affected zone (HAZ). Tensile testing of specimens taken from base metal, and from welded joint, were performed on machine in displacement control, at room temperature. Tensile tests were performed according to the standard EN ISO 4136 [10]. Water jet cutting machine was used for specimen preparation. Tensile strength tests were carried with Instron 8033, servo-hydraulic tension testing machine, using strain rate 0.125 mm/s. 2.3. Microstructure of fracture toughness testing specimens In investigation of fracture mechanics of the armor steel weld joint [11], was presented that the smallest fracture toughness has the base metal, 86.1 MPa∙m1/2, then the HAZ, 286 MPa∙m1/2, while the weld metal has the highest fracture toughness of 355 MPa∙m1/ 2 . The aim in this part of research is to continue the examination of fracture mechanics of armor steel weld joint in the form of metallographic investigations of fractured surfaces. Metallographic testing will help to better understand the characterization of fractures, the way how cracks grow, and the effect of fatigue behavior on microstructures in these zones. 2.4. Fatigue crack growth test The fatigue crack growth test were performed using three-points bend, 12 mm thick specimens (SENeB) [12], according to the standard ASTM E647 [13]. The technical drawing of specimen for fatigue crack growth test is shown in Fig. 1 a). Specimens were cut on a water-jet device, to eliminate possibility of armor steel thermal treatment. SEN-B specimens, fatigue pre-cracked in different welded joint constituents, Fig. 2 b) and instrumented by foil RUMUL RMF A-10, of measuring length 10 mm. Specimens were tested at 2
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 1. a) Schematic drawing of the SEN (B) specimen with dimensions, b) Specimen orientation with respect to the weld axis for fatigue crack growth test.
Fig. 2. a) Results of RT testing welding process, b) optical macrograph of AWS ER 307 filler.
room temperature under variable loading for the determination of fatigue crack growth rate, da/dN, and stress-intensity factor range at fatigue threshold ΔKth. The testing was performed in load control on the CRACTRONIC high frequency resonant pulsator. 3. Results 3.1. Microstructure and hardness The specimen was examined by radiographic inspection. The weld joint is free from imperfections Fig. 2 a). The same good result was confirmed on the five investigated weld plates, by which the joint is considered acceptable, with the grade class B. The macrographic analysis of the specimens revealed perfect fusion, full penetration and complete absence of porosity and inclusions, Fig. 2 b). The weld metal, Fig. 3 a) consist of austenite with delta ferrite. The content of delta ferrite measured by Feritoscope: in the weld upper part 3.2%, in the weld center 5.4%, in the weld root 11.7%. Coarse and fine grained zone micrograph Fig. 3 b) and c) consists of tempered and quenched martensite and fine grained ferrite. The base metal micrograph consists of tempered and quenched martensite, Fig. 3 d). Microhardness was measured in the weld metal, the coarse and fine grained and the base metal zone. Micro hardness value are presented in the Table 1. It can be concluded that the minimum value of 190 HV is in the weld metal region. Hardness do not exceed 521 HV in the coarse grained zone, most critical to create crack, which is within accepted standard, MIL-STAN-1185 [14]. After the maximum, the hardness decreases with the achieved minimum hardness of 378 HV, in the fine grained zone. It should be highlighted that the fusion line hardness is around 412 HV. 3.2. Tensile characteristics Tensile testing of specimens taken from base metal (BM), from additional metal (AM), and from welded joint (WM), were
Fig. 3. a) Optical micrograph of AWS ER 307 filler, b) SEM micrograph of coarse grained zone, c) fine grained zone and d) base metal. 3
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Table 1 Microhardness values, measured 2 mm under the upper surface. Joint type
Location Weld metal
Fusion line
Coarse grained zone
Fine grained zone
Base metal
[HV]
[HV]
[HV]
[HV]
[HV]
190
412
521
378
509
performed on a machine in displacement control, at room temperature. The specimen from BM and AM for testing at room temperature, was machined from the available material, according to standard EN 895 7. Specimen from welded joint was made according EN 895. The testing results at room temperature are given in Table 2 for BM, AM and for the specimens of welded joint WM. The fracture surfaces of the weld metal, Fig. 4. a) and b), reveals many smaller dimples mixed with several larger dimples. The formation of large dimples is related to ductile fracturing with particle–matrix decohesion. A higher nickel content 8.19 (wt%) increases the toughness in fully austenitic compositions. The first requirement in armor steel welded structures is to assure the required strength. In most welded structures this is achieved with undermatching effect, superior strength of BM compared to WM. An additional proof of undermatching is the fracture of specimens from welded joint in WM and that the difference of values of yield stress and tensile strength in Table 2 is high. Special care in tensile properties should be paid to elongation. When material is homogeneous, as here BM and WM should be considered, elongation is useful for comparison. For welded joint, the elongation value is meaningless, since in measuring length of 100 mm enter BM i WM, of different tensile properties, but also a part of HAZ is included, in which tensile properties are unknown. 3.3. Microstructure of fracture mechanic results Microstructure of investigated samples are shown in Fig. 5. Cutting position for the specimen in base metal is shown in Fig. 5 a). Fig. 5 b) shows boundary between fatigue and crack opening fracture surface. Fig. 5 c) shows the fatigue fracture surface with smooth appearance and crack opening followed by ductile fracture and when inclined, while Fig. 5 d) shows the stretch zone. After determining J-R curve for cracking in HAZ, the fracture surface was examined by scanning electron microscope (SEM). The cutting position is shown in Fig. 5 e). The boundary between fatigue fracture and crack opening fracture surface with visible stretch zone is shown in Fig. 5 f) and g). Fig. 5 h) shows the fracture surface with smooth appearance on inclined specimens and stretch zone with the deformed ductile fracture dimples. Specimen cutting position in weld metal is shown in Fig. 5 i). Fracture surface is ductile. At the beginning of crack growth, the fracture surface is ductile with clearly visible ductile dimples, Fig. 5 j) and k). At the end of crack growth, the fracture surface area is still ductile with visible ductile dimples Fig. 5 l). 3.4. Fatigue crack propagation Crack length increase diagram, a, with the number of loading cycles, N, as well as fatigue crack growth rates diagram da/dN against ΔK, in base metal, HAZ and weld metal zone are shown in Fig. 6. Central, linear part of a curve is defined by Paris law, da/ dN=C∙(ΔK)m, where the crack growth rate is in the function of material constants, C and m, as well as the stress intensity factor range, ΔK. The constants are determined by curve fitting on the basis of experimental results and are shown in Table 3. Fatigue threshold, ΔKth, is obtained on the basis of growth rate da/dN = 10−6 mm. For the purpose of comparison, fatigue crack growth rate, da/dN, is determined for ΔK = 16 MPa∙m1/2. The results are shown in Table 3. The analysis of fracture surface was carried out after the examination of fatigue crack growth rate. Cutting position of the examined specimen in base metal is shown in Fig. 7 a). Images from Fig. 7 b) to d) show the crack growth in base metal, from the place of crack initiation to the place of final rupture. Fig. 7 b) at 700 μm from the place of initiation, then Fig. 7 c) 2.16 mm from the place of initiation and Fig. 7 d) 4 mm from the place of initiation. Crack growth rate, da/dN, increases with increasing crack length, but the fracture surface does not change significantly. Fatigue scratches are visible in Fig. 7 b-d), formed at the tips of crack growth. Fracture surface shown in Fig. 7 d) with clearly visible dimples is typical for fracture surface with a tensile load when the load is too big and when the structure can't support the load. Table 2 Results of tensile testing for BM, WM and FM-filler metal of specimens investigated at 20 °C. Specimen
BM FM WM
Yield stress
Tensile strength
Elongation
[MPa]
[MPa]
[%]
1205 425 552
1572 620 833
13 35 8.5
4
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 4. a) and b) SEM images showing the fracture surfaces of the weld metal region and the corresponding void networks.
Fig. 5. Fractography of specimens investigated on fracture mechanics, a) crack growth position in base metal, b) boundary between fatigue and crack opening fracture surface, c) fatigue fracture surface, d) stretch zone, e) crack growth position in HAZ, f) and g) boundary between fatigue fracture and ductile fracture with visible stretch zone, h) stretch zone with the deformed ductile fracture dimples, i) crack growth position in weld metal, j) clearly visible ductile dimples, k) and l) ductile fracture surface area with visible ductile dimples.
The analysis of fracture surface in HAZ was carried out after the examination of fatigue crack growth rate. The examined piece is shown in Fig. 7 e). Fig. 7 f) to h) continually show the crack growth from the place of initiation almost to the place of final rupture. Fig. 7 f) and g) shows the notch in the place of initiation and Fig. 7 h), 6 mm from the place of initiation. Crack growth rate, da/dN, increases with increasing crack length resulting in the fracture surface area typical for fatigue fracture. Cutting position of the examined specimen in weld metal is shown in Fig. 7 i). Fig. 7 j) to l) show the crack growth from the place of initiation almost to the place of final rupture. Fig. 7 j and k) from the place of initiation and Fig. 7 l), 6 mm from the place of 5
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 6. Fatigue crack growth rate per cycle, da/dN, vs. stress intensity factor range, ΔK, specimens pre-cracked in base metal a) HAZ b) and c) weld metal.
Table 3 Parameter values of Paris equation for notch position in base metal, HAZ and weld metal. Specimen
Fatigue threshold
Coefficient
Exponent
Crack growth rate for ΔK = 16 [MPa∙m1/2]
ΔKth
C
m
da/dN
1/2
[MPa∙m Specimen BM Specimen HAZ Specimen WM
]
13.4 15.2 10.1
[nm/cycle] 1.0∙10−9 6.0∙10−13 1.0∙10−12
3.35 5.97 5.10
1.10∙10−5 9.26∙10−6 1.38∙10−6
initiation. Crack growth rate, da/dN, increases with increasing crack length resulting in the fracture surface area typical for fatigue fracture. 4. Ballistic performance analysis Delta ferrite in weld metal zone is uniformly distributed and has potentially good mechanical properties. The good combination of hardness and toughness is achieved by a very fine delta ferrite skeleton in the austenitic base. A precipitate transition from weld metal, the ductile area, to the HAZ region is necessary for the toughness of the military armor vehicle. The most sensitive zone of the 6
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 7. Fractography of specimens investigated on fatigue crack propagation, a) Crack growth position in base metal, b) fracture surface 700 μm from the place of initiation, c) and d) fracture surface 2.16 mm and 4 mm from the place of initiation, e) crack growth position in HAZ, f) and g) fracture surface from the place of initiation, h) 6 mm from the place of initiation, i) crack growth position in weld metal, j) and k) place of initiation, l) 6 mm from the place of initiation.
armor steel welded joint is the fusion line. Good balance between hardness and ballistic protection is good in the fusion line. Martensite and bainite microstructures are presented in the coarse-grained HAZ. Thereby, the corresponding toughness was obtained in this critical area. From the aspect of ballistic protection, [15], steeply increase of hardness in the inter-critical HAZ and overtempered zone is necessary. The increase of hardness in the intercritical zone is the result of the dominant presence of martensite in this zone. High-hardness armor steel requires careful control of the welding procedure to avoid loss of heat-affected zone hardness. Material hardness is strongly dependent on the welding temperature history. The key to controlling the amount of softening in the heataffected zone is to maintain a high peak temperature gradient close to the weld bead [16]. The microstructure of the heat-affected zone is a function of the cooling rate imposed by the welding process and the chemical composition of the base plate [5]. The microstructure of the heat-affected zone impacts the hardness level and the ballistic performance of the weld joints. However, the ballistic limit in the HAZ, has been found to be highest in the case of the heat input 0.8 kJ/mm weld with the hardness level in HAZ 541 HV [17]. The HAZ of the heat input 1.2 kJ/mm with the hardness level 502 HV was also resistant to penetration and exhibited the good ballistic limit, whilst the HAZ of 1.4 kJ/mm with the hardness level 460 HV, exhibited an intermediate performance. The heat input 1.6 kJ/mm and 1.8 kJ/mm with the hardness level 426 HV and 400 HV, respectively, was not resistant to penetration and exhibited the lowest ballistic limit. Based on the heat input and hardness level in the HAZ, ballistic performance in this zone can be predicted. A similarity between the heat input / hardness of 1.2 kJ/mm / 502 HV, where ballistic resistance was achieved [17], can be found with the result in this study, where with a heat input of 1.25 kJ/mm and hardness of 521 HV was achieved. Based on this, we can conclude that HAZ in this research will have a good ballistic performance leading to projectile shattering.
5. Predicting the remaining life of the military armored vehicle Predicting the life of a military armored vehicle is necessary during the vehicle design and testing for defining control intervals to 7
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Fig. 8. Strain gauge rosette locations on the test Vehicle Lav – 25, [18].
detect cracks caused by exploitation. In cases where armor steel is welded, cracks are likely to appear [16], hence one has to assume that the crack exist in welded joint. There are number of empirical models defining fatigue crack growth for such conditions, based on Paris law, [17]. In the simplest form, i.e. by applying Paris law, one can calculate the number of cycles as given by Eq. (1):
(1− m2 )
a 1 ΔN = ∙ 0 C [Y Δσ a ]m
m 2
(1− m2 )
− ac −1
(1)
where ΔN is number of cycles for cracks to grow from initial length, a0, to critical length, ac, and Y=Y(a/W) is the correction factor for the crack length. The aim of this study is to determine remaining life, on the basis of exploitation measurements, [18], using strain gauge rosettes at cracking sites on the hull of the test military of armored vehicle, Fig. 8. Test was conducted in two phases so that the strain data from eight initially placed gauges (Rosettes A to H) could be inspected and used to help determine the locations for the final six gauges (Rosettes I to N). This procedure also minimized the possibility of placing the majority of the gauges in low stressed regions of the hull, as explained in [18]. Forty-five degree 350 Ω rosettes, type Micro Measurements CEA.46-25OUR-350 (6.4 mm grid gauge length), were used for all measurements except for Rosette L and M which were of the CEA-06-125-UR-350 type (3.2 mm grid gauge length), [18]. The Lav-25 is a military armored vehicle with eight wheels, Fig. 8, made of armor steel with hardness 500 HB, prone to cracking, especially if welded. Stresses were measured using sensors rosette mounted at critical locations as the vehicle passed through a number of different terrain types. The results of stress measurement are shown in the Table 4, [18], indicating that some locations were under relatively high stress. Dynamic stress in the vehicle are caused by rough coarse, as well as by vibrations associated with engine operation during the vehicle braking. Significant stress has developed in the front of the military armored vehicle as a result of the presence of the engine and the braking system of the vehicle. This is the result of a vibration from the complete suspension of the engine unit that was transferred to the top plates of the military armored vehicle. The suspension of the engine unit are holders welded to the interior of the surrounding Table 4 Effective stresses measured at various engine speeds while the test vehicle was stationary, [18]. Maximum stress (MPa) Rosette
A F C N L M
First class road
Rough course
Second class road
Cross country course
[MPa]
[MPa]
[MPa]
[MPa]
3.4 2.5 11 7.1 2.9 11
4.2 8.2 31 30 16 71
4.0 6.3 16 13 11 21
4,5 11 24 17 41 49
8
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
Table 5 Remaining exploitation period of the military armored vehicle in exploitation of a crack in BM, HAZ and WM. Rosette position
A
Y
1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22 1.43 1.79 2.38 3.42 5.22
F
C
N
L
M
Δσ
a
REP for rough course terrain Crack in BM
Crack in HAZ
Crack in WM
[MPa]
[m]
[Years]
[Years]
[Years]
4.2 4.2 4.2 4.2 4.2 8.2 8.2 8.2 8.2 8.2 31 31 31 31 31 30 30 30 30 30 16 16 16 16 16 71 71 71 71 71
0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006 0.002 0.003 0.004 0.005 0.006
1,778,845 422,846 99,558 20,326 3613 188,492 44,806 10,550 2154 383 2176 517 122 25 4 2429 577 136 28 5 20,013 4757 1120 229 41 135 32 8 2 0.3
338,605,050,264,230 26,266,672,431,027 2,003,244,419,228 118,538,995,762 5,481,202,293 6,237,678,635,999 483,876,602,945 36,903,155,776 2,183,689,112 100,973,032 2,223,629,273 172,494,006 13,155,365 778,449 35,995 2,704,442,824 209,792,245 15,999,939 946,772 43,778 115,317,547,762 8,945,549,543 682,238,005 40,370,415 1,866,714 15,793,535 1,225,155 93,437 5529 256
4,011,412,229,428 451,658,033,382 50,120,491,133 4,477,945,901 324,070,159 132,256,805,320 14,891,226,624 1,652,479,391 147,638,484 10,684,637 149,942,255 16,882,489 1,873,450 167,381 12,113 177,235,321 19,955,505 2,214,462 197,848 14,318 4,373,778,281 492,458,013 54,648,065 4,882,456 353,345 2,190,035 246,583 27,363 2445 177
Note: ac = 9 mm. In C = 1.0∙10−9, m = 3.35 in BM, C = 6.0∙10−13, m = 5.97 in HAZ, C = 1.0∙10−12, m = 5.10 in WM.
vehicle. The position of the two holders is defined in the rosette C and N as well as M and L. Table 4 shows that the rosettes C and M sustain the highest stress. The maximum stress is obtained on rough course terrain and on cross country terrain topography. Unlike the rosette around the C, L, M and N, rosettes at A and F, located in the middle part of armored vehicle, have low stresses. The highest stress at the cross country course were recorded with the rosettes L and M, which are located on the front of the vehicle, [18]. Fatigue remaining life for constant amplitude loads, taken as the maxim stress from Table 4, is calculated using Eq. (1), based on the following assumptions: - The initial crack, a0, is a crack that can be detected by non-destructive testing, typically taken as a0 = 1 mm, which is also reasonable value for the military armored vehicle. The critical crack, ac, was taken as 9 mm, since the thickness of the armor plate of 10 mm and remaining ligament (1 mm) is just about to fail in static mode (max. Stress value is 71 MPa, Table 4). - The time of exploitation of the military armor vehicle is considered to be 8000 km/year. - The length of the growing crack is changed by 1 mm from the initial value up to 6 mm. - The values of C and m are the constants of the Paris equation determined by examining the fatigue crack growth parameters, as presented in Table 3 for BM, HAZ and WM. - The coefficient Y is a geometric member that depends on the ratio of crack length and thickness of the armor plate. The coefficient Y is calculated from the ratio of the crack length ad and a, presented in the Table 5 according to Eq. (2).
Y = 1.12–0.231∙ (a/ W ) + 10.55∙ (a/ W )2–21.72∙ (a/ W )3 + 30.39∙ (a/ W ) 4
(2)
The results of remaining exploitation period (REP), on rough course terrain of the armored vehicle, is calculated according to Eq. (3) and presented in Table 5.
REP =
∆N 6x106
(3)
where: 9
Engineering Failure Analysis 106 (2019) 104155
A. Cabrilo, et al.
- 6 × 106 - the number of impact loads per year, - ΔN – the remaining exploitation cycles, calculated according to Eq. (1). The results presented in Table 5 indicate that the REP has the smallest value for the maximum stress 71 MPa (location M) and initial crack length 6 mm, is just 0.3 Year for BM. Comparing values for REP in BM, HAZ and WM, one can say that last two are safe zones, which significantly improves structural integrity of armor vehicle, since there is no real danger of having cracks in BM. Even for the worst case scenario, i.e. crack length 6 mm and stress amplitude 71 MPa, it takes 256 and 177 Years to get to the critical crack length in HAZ and WM, respectively. 6. Conclusions On the basis of the results presented in this work, the following conclusions may be made for tested armor steel: - Fracture toughness value for BM (86 MPa∙m1/2) is slightly lower in comparison with armor steels 500 class. Anyhow, HAZ and WM have significantly higher fracture toughness than BM, providing safe welded joints in respect to brittle fracture. - Suitable combination of microstructure in welded joint provides high resistance to impact loading, i.e. good ballistic protection, enabling projectile shattering. - The remaining exploitation period, determined by the rate of fatigue crack growth, shows that with respect to the integrity and life of the military armored vehicle, the most important attention should be paid to the base metal, while the weld metal and HAZ are safe zones. Acknowledgments This study was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia through Project ON 174004. References [1] Y. Kchaou, V. Pelosin, G. Hénaffa, N. Haddar, K. Elleuch, Failure mode analysis of SMAW welded UNS N08028 (Alloy28) superaustenitic stainless steel under crack growth tests, Eng. Fail. Anal. 97 (2019) 804–819. [2] J. Toribio, V. Kharin, F.J. Ayaso, B. González, J.C.M. Franco, D. Vergara, M. Lorenzo, Failure analysis of a lifting platform for tree pruning, Eng. Fail. Anal. 17 (2010) 739–747. [3] J.S. Kim, J.Y. Hui-Jun Yi, Characteristics of GMAW narrow gap welding on the armor steel of combat vehicles, Appl. Sci. 7 (2017) 658, https://doi.org/10.3390/ app7070658. [4] M. Atabaki, J. Ma, G. Yang, R. Kovacevic, Hybrid laser/arc welding of advanced high strength steel in different butt joint configurations, Mater. Des. (2014) 573–587. [5] L. Kuzmikova, J. Norrish, H. Li, M. Callaghan, Research to Establish a Systematic Approach to Safe Welding Procedure Development Using Austenitic Filler Material for Fabrication of High Strength Steel, 16th International Conference on the Joining of Materials, (2011), pp. 1–13. [6] Advantages and Disadvantages of Metal Cored Wires, http://www.esabna.com/us/en/education/blog/advantages-and-disadvantages-of-metal-cored-wires.cfm. [7] L. Tang, C. Qian, A. Ince, J. Zheng, H. Li, Z. Han, Fatigue crack growth behavior of the MIG welded Joint of 06Cr19Ni10 Stainless Steel, Materials 11 (2018) 1336, https://doi.org/10.3390/ma11081336. [8] A. Cabrilo, K. Geric, Weldability of high hardness armour steel, Adv. Mater. Res. (2016) 79–84. [9] EN ISO 9015-1, Destructive Tests on Welds in Metallic Materials. Hardness Testing Hardness Test on Arc Welded Joints, (2011). [10] EN ISO 4136, Destructive Tests on Welds in Metallic Materials. Transverse Tensile Test, (2011). [11] A. Cabrilo, K. Geric, Fracture mechanic and Charpy impact properties of a crack in weld metal, HAZ and base metal of welded armor steel, Procedia Struct Integrity (2018) 2059–2064. [12] ASTM E1820-16, Standard Test Method for Measurement of Fracture Toughness, ASTM International, West Conshohocken, PA, 2016. [13] ASTM E647–08, Standard Test Method for Measurement of Fatigue Crack Growth Rates, ASTM International, West Conshohocken, PA, 2008www.astm.org. [14] MIL-STD-1185, Department of Defense Manufacturing Process Standard: Welding, High Hardness Armor, [SUPERSEDES MIL-W-62162] (2008). [15] A. Cabrilo, K. Geric, M. Jovanovic, L. Vukic, Weldability and impact energy properties of high-hardness armor steel, J. Mater. Eng. Perform. 27 (3) (2018) 1281–1295. [16] J. Vojvodic Tuma, A. Sedmak, Analysis of the unstable fracture behaviour of a high strength low alloy steel weldment, Eng. Fract. Mech. 71 (9) (2004) 1435–1451 (17). [17] M. Arsić, R. Karić, A. Sedmak, M. Burzić, B. Vistać, Methodological Approach to Integrity Assessment and Service Life of Rotating Equipment at Hydropower Plant – Turbine Shaft, Structural Integrity and Life, vol. 13, (2013), pp. 117–124. [18] S.J. Cimpoeru, The Measurement of Dynamic Structural Stresses in a Light Armoured Vehicle. M.Eng.Sci, GradieAust, in 5 Th Australian Aeronautica Conference, Melbourne (1993), pp. 13–15.
10