Studies on fracture toughness of cold wire addition in narrow groove submerged arc welding process

Due to the faster joint completion rates coupled with good mechanical properties, narrow gap submerged arc welding (NGSAW) is widely used for fabrication of thick-walled pressure vessels. Several researchers are working on further increase in productivity in NGSAW. In this paper, we propose to increase the quality and productivity in NGSAW through cold wire addition without addition of heat input. Further toughness at sub-zero temperature is also enhanced. The improvement in toughness in cold wire NGSAW is demonstrated through different tests such as impact energy test, fracture toughness tests, plane strain fracture toughness test K _Ic, and crack tip opening displacement test.     

Characteristics Evaluation of Plasma Sprayed Ceramic Coatings by Nano/Micro-Indentation Test

The most controversial topics in plasma sprayed ceramic coating systems recently are mechanical properties such as bond strength, cohesive strength and toughness. In our research, critical fracture load (P _c) and interface toughness (K _c) of four-coating materials were computed from the applied load, crack length and Young's modulus data that were measured by micro-Vickers and a nano-indentation test. It is reasonable to consider the P _c value as comparison data of bond strength, and the K _c value, considering a trace of indent in the interface, was computed by a modified (E/H)_I ^1/2ratio. Also, we knew that P _c decreased as the hardness of the coating increased. In the case of the high load (9.8 and 19.6N) in the Al₂O₃+13%TiO₂ coating, the critical point (P _co) was found at which the coating was broken. Used by a XRD phase analysis, we checked changes of the coating's properties and predicted a possible change of the phases in plasma-sprayed coating.     

Effect of fiber orientation on interfacial fracture toughness for adhesively bonded composite plates

In this study, interfacial fracture toughness was investigated experimentally and numerically in laminated composite plates with different fiber reinforcement angles bonded with adhesive. The composite plates are four-layered and the layer sequence is [0º/θ]_s. DCB test was applied to composite plates reinforced with epoxy resin matrix and unidirectional carbon fiber. The experimental sample model for the DCB test was made using the ANSYS finite element package program. In the numerical study, four layered composites were prepared in three dimensions. Under critical displacement value; mode I fracture toughness at the crack tip was calculated using VCC (virtual crack closure) technique. Numerical values consistent with experimental results have presented in graphical forms. At 60o and 75° the greatest fracture toughness was obtained. In addition, numerical results have shown that fiber orientation prevents the uniform distribution of stress on the interface crack tip and causes stress accumulation, especially at the edge of the plate.

     

Comparative study of toughness between the AH 40 fatigue crack arrester steel and its weld metal in the case of robotic metal-cored arc welding

The current investigation aims to scrutinize the impact and fracture toughness of the AH 40 fatigue crack arrester (FCA) steel and its weld metal, when welded with the metal-cored arc welding technique (MCAW). Initially, macroscopic observation and microstructural characterization were carried out in the areas of interest. Subsequently, the impact toughness was determined with the use of the Charpy V-notch test (CVN) at various temperatures, while the values of the absorbed energy (KV), the percentage of shear fracture (PSF), and the lateral expansion (LE) were recorded. Moreover, the ruptured surfaces were examined with a scanning electron microscope (SEM). Finally, the crack tip opening displacement (δ) parameter was estimated at room temperature by fracture toughness testing. The obtained data led to the quantification of the toughness parameters, when dynamic or quasistatic load is applied, while the combined effect of several factors to the degradation of the weld metal toughness was elucidated. The ductile to brittle transition curve and the crack tip opening displacement in the weld metal appeared to be lower than inside the unaffected material. Nevertheless, fracture toughness properties were evaluated within acceptable limits in all cases.     

Influence of Deposition Positions on Fretting Behaviors of DLC Coating on Ti-6Al-4V

Abstract

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a fretting wear test rig with a cylinder-on-flat contact. The results indicated that, for tests without coating (Ti-6Al-4V–Ti-6Al-4V contact), the friction (Q_max/P) was high (0.8–1.2), wear volumes were large (0.08–0.1?mm³) under a large displacement amplitude of ±40 µm and small (close to 0) under a small displacement amplitude of ±20 µm, and the wear debris was composed of Ti-6Al-4V flakes and oxidized particles. For tests with the DLC coating, under low load conditions, the DLC coating was not removed or was only partially removed, Q_max/P was low (≤0.2), and the wear volumes were small. Under high load conditions, the coating was entirely removed, Q_max/P was high (0.6–0.8), and the wear volumes were similar to those in tests without coating. The wear debris was composed of DLC particles, Ti-6Al-4V flakes, and oxidized particles. The DLC coating was damaged more severely when deposited on a flat surface than when deposited on a cylindrical surface. The DLC coating was damaged more severely when sliding against a DLC-coated countersurface than when sliding against the Ti-6Al-4V alloy.     

Mechanical properties and scratch test studies of new ultra-hard AlMgB14 modified by TiB2

Nanoscale composites of AlMgB₁₄ with AlN, TiC or TiB₂ were prepared by mechanical alloying in a vibratory mill, followed by compression molding at 1400 °C. Determination of microhardness and fracture toughness indicated that TiB₂ was an effective addition for increasing both quantities. The optimum percent TiB₂ addition corresponding to the highest hardness and fracture toughness was 60–70 wt%. The abrasion resistance of AlMgB₁₄ composites with varying amounts of TiB₂ has been studied using single-point diamond scratch tests with loads ranging from 20 to 70 N in 10 N increments. The scratch width, as measured by stylus profilometer, increased almost linearly with the applied load and decreased with increasing TiB₂ proportion up to 70 wt%. Furthermore, macroscopic abrasion resistance increased with both mean hardness and fracture toughness. Energy dispersive spectroscopy (EDS) revealed the presence of Al₂MgO₄ (spinel) and FeB₄O₇ in AlMgB₁₄. Cracks were observed on the surface of AlMgB₁₄−70 wt% TiB₂ when scratched under 70 N load, but there was no cracking in the absence of TiB₂.     

Effect of temperature on the fracture toughness of A516 Gr70 steel

Fracture toughness JIC and KIC tests were performed on A516 Gr70 carbon steel plate at the temperature ranging from −160°C to 600°C, and test results were analyzed according to ASTM E 813 and ASTM E 399. Unloading compliance J-integral tests were performed on ITCT specimens. The relation between the J_IC value and the test temperature was obtained. It was concluded that the temperature ranging from −15°C to 600°C is the upper shelf region of ductile-brittle transition temperature, and in this temperature range, fracture toughness J_IC values decreased with increasing temperature. The ductile brittle transition temperature of the material may be around −30°C. In the region near −30°C, the tendency of J_IC to decrease with decreasing temperature was significant.     

Mechanical behavior and numerical estimation of fracture resistance of a SCS6 fiber reinforced reaction bonded S13N4 continuous fiber ceramic composite

Continuous fiber ceramic composites (CFCCs) have advantages over monolithic ceramics: Silicon Nitride composites are not well used for application because of their low fracture toughness and fracture strength, but CFCCs exhibit increased toughness for damage tolerance, and relatively high stiffness in spite of low specific weight. Thus it is important to characterize the fracture resistance and properties of new CFCCs materials. Tensile and flexural tests were carried out for mechanical properties and the fracture resistance behavior of a SCS6 fiber reinforced Si₃N₄ matrix CFCC was evaluated. The results indicated that CFCC composite exhibit a rising R curve behavior in flexural test. The fracture toughness was about 4.8 MPa m^1/2 , which resulted in a higher value of the fracture toughness because of fiber bridging. Mechanical properties as like the elastic modulus, proportional limit and the ultimate strength in a flexural test are greater than those in a tensile test. Also a numerical modeling of failure process was accomplished for a flexural test. This numerical results provided a good simulation of the cumulative fracture process of the fiber and matrix in CFCCs.     

Indentation of Polytetrafluoroethylene (PTFE) Thin Film: Simulations by Using Continuum Damage Mechanics

Thin compliant films on relatively hard substrates have a wide range of applications. In this work, continuum damage mechanics is used to simulate indentation of a 10-μm-thick polytetrafluoroethylene (PTFE) film deposited on glass for different load levels by finite element analysis. The results, compared to experiments, are useful in investigating the mechanics of wear and friction of soft thin films. The material model is elastic–plastic before damage initiation and includes linear damage progression thereafter. The effects of ductile and shear damage criteria and two parameters pertinent to the damage model, the equivalent plastic strain for damage initiation and the bulk fracture toughness, on the indentation are investigated. It is shown that the shear damage model is more suitable to characterize the indentation of the PTFE thin film. The bulk fracture toughness has greater significance with regard to damage compared to equivalent plastic strain at the onset of damage initiation. Comparison of simulation and experimental results shows that bulk fracture toughness of the thin PTFE film is approximately 20 J/m². This value is lower than that for the bulk PTFE, and the difference is attributed to the thin-film nature of the case considered here.     

Dry sliding wear behaviors of La2O3–WSi2–MoSi2 composite against alloy steel

A molybdenum disilicide (MoSi₂) matrix composite with the addition of WSi₂ and La₂O₃ (RWM) was fabricated as a wear resistant material by self-propagating high temperature synthesis (SHS) and hot pressing (HP). This composite was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The wear resistance of MoSi₂ against steel is significantly improved by the addition of both WSi₂ and La₂O₃, and it is attributed to the increase in hardness and toughness of the composite. It is found that the wear behavior of the RWM is sensitive to sliding speed, load and hardness of the counter-face material. When worn against a steel with a lower hardness (A), the wear rate of RWM increases with an increase of sliding speed, and increases initially and then decreases with an increase of load. The material removal mechanisms varied from ploughing wear at low load and speed to serious adhesive wear at high load and speed. When worn against a steel with a higher hardness (B), the wear resistance of the RWM improved and the material removal mechanism were brittle fracture wear at low speed and adhesive wear at high speed.     

Analysis of a composite double cantilever beam with stitched reinforcements under mixed mode loading: Formulation (I)

Several methods for improving the lnterlaminar strength and fracture toughness of composite materials are developed Through-the-thickness stitching is considered one of the most common ways to prevent delamination Stitching significantly increases the Mode I fracture toughness and moderately improves the Mode II fracture toughness An analytical model has been developed for simulating the behavior of stitched double cantilever beam specimen under various loading conditions For z-directional load and moment about the y-axis the numerical solutions are compared with the exact solutions The derived formulation shows good accuracy when the relative error of displacement and rotation between numerical and exact solution were calculated Thus we can use the present model with confidence in analyzing other problems involving stitched beams     

An evaluation of cast stainless steel (CF8M) fracture toughness caused by thermal aging at 430°C

Cast stainless steel may experience embrittlement when it is exposed approximately to 300°C for a long period. In the present investigation, the three classes of the thermally-aged CF8M specimen were prepared using an artificially-accelerated aging method. After the specimens were held for 300, 1800 and 3600hrs. at 430°C, respectively, the specimens were quenched in water which is at room temperature. Load versus load line displacement curves andJ- R curves were obtained using the unloading compliance method,J _IC values were obtained using the ASTM E 813–87 and ASTM E 813–81 methods. In addition to these methods,J _IC values were obtained using the SZW (stretch zone width) method described in JSME S 001–1981. The results of the unloading compliance method areJ _Q = 543.9kj/m² for virgin materials. The values ofJ _IC for the degraded materials at 300, 1800 and 3600hrs. are obtained 369.25kJ/m², 311.02kJ/², 276.7kJ/², respectively. The results obtained by the SZW method are compared with those obtained by the unloading compliance method. Both results are quite similar. Through the elastic-plastic fracture toughness test, it is found that the value ofJ _IC is decreased with an increase of the aging time.     

Friction and wear of carbon steel near T1-transition under dry sliding

The wear phenomenon of metals under dry sliding is, generally, divided into two modes of severe and mild wear. A discontinuous transition between the wear modes often takes place in a certain load range. The T₁-transition is usually observed at lower levels of load or sliding velocity. There is a great difference in wear rate between severe and mild wear. This indicates that the occurrence of severe wear should be avoided, especially in the field of machine design to prevent energy loss, occurrence of noise and vibration, and life reduction of machines and their components. Therefore, it is important for machine designers to know the relationship between friction and wear and the difference in properties of the wear surfaces in the two wear modes. In this study, wear tests of 0.35% C steel in contact with itself under constant load were conducted in moist air at various contact loads under dry sliding. The friction and wear were measured continuously throughout each test. After the tests, the relationship between friction and wear and the difference in properties of the wear surfaces were investigated in each wear mode. From the results, the upper and lower critical loads (P_acr and P_Acr) appeared between severe and mild wear. The phenomenon of zero wear has been newly found in the early period at very low loads. The zero wear continued for a long sliding distance and then changes to mild wear. The critical load between zero wear and mild wear is defined as P_zerowear. The load was changed once in a step-wise manner from low to high levels in process of test. Since the rubbing history under mild wear condition at the low load in the first stage affected the properties of wear surface, the wear mode at the high load in the second stage changed from ‘mild wear’ to ‘quasi-mild wear’ having a low rate. From the relationship between sliding distance necessary for the appearance of quasi-mild wear and contact load in the first stage, the boundary curve between severe wear and quasi-mild wear in the second stage is hyperbolic. This curve gradually approaches P_zerowear with decreasing contact load. Thus, P_zerowear is one of the important critical loads for elucidating the test results under varying load.     

Investigation of the fracture characteristics of the interfacial bond between bone and cement: experimental and finite element approaches

This study integrated the finite element method, fracture mechanics, and three-point bending test to investigate the fracture characteristics of the interfacial bond between bone and cement. The fracture tests indicated that the interfacial fracture toughness of the bone/cement specimens was 0.34 MN/m^3/2, with a standard deviation of 0.11 MN/m^3/2, which was in good agreement with the experimental data available in the literature. A finite element model of the experimental testing specimen was used to predict the critical stress intensity factor (SIF) at the fracture load by the proposed fracture analysis method. The critical SIF of the opening mode of the interface crack was 0.392 MN/m^3/2, which was slightly higher than the fracture toughness obtained in the experiment. Additionally, considering the coupled effects of the crack opening mode and shearing mode, the critical effective SIF was 0.411 MN/m^3/2, with a phase angle of 17.2°. Comparisons of the results obtained from the bending test and numerical analysis made it obvious that the fracture characteristics of the bonded interface between the bone and cement could be accurately predicted by the proposed model. With this analysis model, a realistic investigation on the debonding behavior of cemented artificial prosthetic components is highly expected.     

Influences of equal biaxial tensile loads on the stress fields near the mixed mode crack

A hybrid method for photoelasticity is introduced and applied to the plane problems of isotropic polycarbonate plates with a central crack under uniaxial and equal biaxial tensile loads. Also, the influences of equal biaxial tensile loads on the isochromatic fringes, stress fields and stress intensity factors near the mixed mode crack-tip have been investigated. The results show that, when an equal lateral tensile load is added to the specimen under uniaxial tensile load, the asymmetric isochromatic fringes about the line of crack gradually become symmetric, and the slope of the isochromatic fringe loop near the crack-tip is inclined towards the crack surface according to the increasing of the inclined angle of crack. Furthermore, the shapes of distribution of all stress components are changed from asymmetric to symmetric. In the equal biaxial tensile load condition against the uniaxial tensile load condition, the values of stress intensity factors are changed little, and only the region of compressive stress of σ _x /σ _O is changed when β = 0°, but the values of K _I /K ₀ are increased and those of K _II /K ₀ become almost zero, namely, we have the mode I condition when β = 15°∼45°. This paper was recommended for publication in revised form by Associate Editor Chongdu Cho Dong-Chul Shin received the B.S., M.S. and Ph.D. degrees in Mechanical Engineering from Yeungnam University in 1995, 1997 and 2001, respectively. Dr. Shin studied at the University of Tokyo, Japan, for three years (from April, 2005 to January, 2008) as a Post-Doctoral fellow (supported by Korea Research Foundation (KRF) and Japan Society for the Promotion of Science (JSPS)). Dr. Shin is currently a Research Professor at the School of Mechanical Engineering at Pusan National University, Korea. His research interests include the static and dynamic fracture mechanics, stress analysis, and fracture criteria of piezoelectric ceramics, etc. Jai-Sug Hawong received a B.S. in Mechanical Engineering from Yeungnam University in 1974. Then he received his M.S. and Ph.D. degrees from Yeungnam University in Korea in 1976 and from Kanto Gakuin University in Japan in 1990, respectively. Prof. Hawong is currently a professor at the School of Mechanical Engineering at Yeungnam University, in Gyeongsan city, Korea. He is currently serving as vise-president of Korea Society Mechanical Engineering. His research interests are in the areas of static and dynamic fracture mechanics, stress analysis, experimental mechanics for stress analysis and composite material etc.     

Theory of Friction with Applied Load

We present a molecular dynamics simulation for the static friction under the effect of load. We chose a system formed by slabs of molecules of N ₂ deposited on a (111) surface of Pb. In contrary to many calculations, we assume that the Pb atoms are not kept fixed in the lattice positions, but can vibrate in their own phonon’s field. This has the important consequence that the upper and lower block can exchange energy and momentum. During the molecular dynamics simulation, the two systems can reach a thermodynamical equilibrium. When in our molecular dynamics simulation the equilibrium is reached the N ₂ plane at the interface reconstructs. The unit cell is still hexagonal but it contains 16 molecules in disordered positions. These positions of the N ₂ molecules are strongly modified by the presence of load. For small load there is a small increase of the disorder that produces a small reduction in the static friction. For larger loads the formation of clusters begins, and for high loads the size of the clusters increases and there is a tendency to the formation of vacancies. These effects are producing a large increase in the force friction. We can then distinguish different regions that characterize the behaviour of the static friction as a function of the load.     

The Griffith-Orowan fracture theory revisited: The T-Criterion

In the present paper a new fracture criterion (the T-criterion) is described. It is based on the Griffith fracture theory, as modified by Orowan, according to which failure is the result of the mutual action of both fracture and yielding mechanisms. The main idea of the T-criterion is that fracture is caused by normal stresses, whose contribution is included in the dilatational part T_V of the strain-energy density. It is then postulated that fracture will initiate outside the plastic region surrounding the crack-tip, when T_V reaches a critical value. On the other hand, plasticity depends on the distortional part of strain-energy density and its extent is defined by the Mises yield locus, which is used for the calculation of the elastic-plastic boundary, along which T_V is computed. Then, the T_V = f() curve around the crack constitutes a physically sound curve of the distribution of the total elastic strain-energy density there.Then, the critical T_V,0- and T_D,0-parameters control the failure behaviour of the material and, thus, the T-criterion can distinguish between failure by fracture or yielding. A study is made on the relative validity of the expected angle of crack propagation, especially in ductile materials. Different predictions are obtained from the T-criterion, as they are compared to those of an up-to-now popular criterion suitable only for brittle fractures. Experimental data show good agreement with the predictions of the T-criterion, especially for the critical stress for fracture.     

On cropping and related processes

Two distinct aspects of cropping and related processes are considered, (i) the maximum in punch force caused by plastic instability and (ii) the initiation and propagation of cracks after the onset of plastic flow.An expression for punch travel at the peak load is derived in terms of the work hardening index of the workpiece, its thickness and state of prestrain. Good agreement is found with experimental results from a variety of sources. The expression is also modified to include cases where cracks appear before the peak in load.A re-examination of experimental autographic punch load-punch penetration traces shows that it is possible, within the limitations of a single shear plane model, to partition the total work of deformation into flow and fracture components. It would seem that crack propagation in cropping or blanking starts when the incremental energy consumed by cracking plus flow is smaller than the alternative process of carrying on flowing over a larger plastic volume with no cracking. Estimates for fracture toughness are possible by this method, e.g. 500–600 lbf-in/in² for brass and aluminium and 170 lbf-in/in² for lead from the results of Chang and Swift[1]. A criterion for the occurrence of multiple cracking on the cropped faces is derived in terms of toughness, flow stress and workpiece/punch geometry and is shown to agree with experimental observations.An appendix highlights some more general aspects of the area of combined flow and fracture, and discusses how cracks in plastic flow fields may be identified and dealt with analytically.     

Evaluation of Monkman-Grant parameters for type 316LN and modified 9Cr-Mo stainless steels

The Monkman-Grant (M-G) and its modified parameters were evaluated for type 316LN and modified 9Cr-Mo stainless steels prepared with minor element variations. Several sets of creep data for the two alloy systems were obtained by constant-load creep tests in 550-650°C temperature range. The M-G parameters,m, m’,C, andC’ were proposed and discussed for the two alloy systems. Them value of the M-G relation was 0.90 in type 316LN steel and 0.84 in modified 9Cr-Mo steel. Them’, value of the modified relation was 0.94 in type 316LN steel and 0.89 in 9Cr-Mo steel. Although creep fracture modes and creep properties between type 316LN and modified 9Cr-Mo steels showed a basic difference, the M-G and its modified relations demonstrated linearity quite well. Them’ of modified relation almost overlapped regardless of the creep testing conditions and chemical variations in the two alloy systems, and the parameterm’ was closer to unity than that of the M-G relation.     

Development of cleavage fracture toughness locus considering constraint effects

In this paper, the higher order terms in the crack tip stress fields are investigated macroscopically for more realistic assessment of structural material behaviors. For reactor pressure vessel material of A533B ferritic steel, effects of crack size and temperature have been evaluated using 3-point SENB specimens through a series of finite element analyses, tensile tests and fracture toughness tests. The T-stress, Q-parameter andq-parameter as well as theK andj-integral are calculated and mutual relationships are investigated also. Based on the evaluation, it has proven that the effect of crack size from standard length (a/W=0.53) to shallow length(a/W=0.11) is remarkable whilst the effect of temperature from - 20°C to-60°C is negligible. Finally, the cleavage fracture toughness loci as a function of the promising Q-parameter orq-parameter are developed using specific test results as well as finite element analysis results, which can be applicable for structural integrity evaluation considering con-straint effects.