The relative effects of residual stresses and weld toe geometry on fatigue life of weldments

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.     

Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

An investigation of residual stresses in brazed cubic boron nitride abrasive grains by finite element modelling and raman spectroscopy

Joining cubic boron nitride (CBN) abrasive grains and tool body made of steel using brazing always creates residual stress due to thermal mismatch of the components when cooling down from the brazing temperature. A large tensile stress perhaps causes grain fracture during the grinding process with single-layer brazed CBN abrasive tools. To evaluate the residual stresses occurring in brazed CBN grains, values and distribution of residual stresses are calculated using the finite element method. Effects of bonding materials, embedding depth, gap thickness and grain size on brazing-induced residual stresses are discussed. Results show that the Cu–Sn–Ti bonding alloy always results in a larger tensile stress in the CBN grains, when compared to Ag–Cu–Ti alloy during the cooling phase of the brazing process. The maximum tensile stress is obtained at the grain–bond junction region irrespective of the choice of bonding material and embedding depth. When the grain side length is 100 μm, gap thickness is 10 μm and grain embedding depth is 30%, the maximum magnitude of the tensile stresses is obtained. The maximum stress is 401 MPa with Ag–Cu–Ti alloy and 421 MPa with Cu–Sn–Ti alloy. The brazing-induced residual stresses have been finally measured experimentally by means of the Raman spectroscopy. The current simulated results are accordingly verified valid.     

Joining phenomena and tensile strength of friction welded joint between pure titanium and low carbon steel

This paper describes the effect of the friction welding condition on the joining phenomena and the tensile strength of friction welded joint between pure titanium (P-Ti) and low carbon steel (LCS). The adjacent region of the weld interface at the P-Ti side was intensely upsetting with accompanied large deformation of itself when the joint had sparkle at both applied friction pressures of 30 and 90 MPa, although that of the LCS side was hardly upset. The temperature of the whole weld interface at a friction pressure of 30 MPa reached to 1150 K or over at a friction time of 3.0 s or longer. However, the half radius and centreline portion temperatures of the weld interface at a friction pressure of 90 MPa was not reached to 1150 K, although the periphery portion of that was reached to its temperature. The central portion of the weld interface at a friction pressure of 90 MPa was deformed to a convex shape from the viewpoint of the P-Ti side, although that of 30 MPa remained almost flat after when the friction torque reached the initial peak. When the joint was made at a friction pressure of 30 MPa, a friction time of 3.0 s or longer, and a forge pressure of 270 MPa or higher, it achieved 100% joint efficiency and the P-Ti base metal fracture with no crack at the weld interface. However, many joints at friction times of 1.2 and 1.5 s fractured at the weld interface, although those achieved 100% joint efficiency, because whole weld interface temperature was below 1150 K. On the other hand, many joints at a friction pressure of 90 MPa with high forge pressure also fractured at the weld interface, although those achieved 100% joint efficiency, because the weld interface temperature at the half radius and periphery portions was below 1150 K. Those joints did not have the intermetallic compound layer at the weld interface. The difference of the fractured portion of the joint in both applied friction pressures was due to the difference between the maximum temperature at the weld interface during the friction process and the deformation amount of the LCS side caused by applied forge pressure. To obtain 100% joint efficiency with the P-Ti base metal fracture with no crack at the weld interface, the joint should be made with high forge pressure, low friction pressure, and with opportune friction time at which the temperature at whole weld interface reached around 1150 K.     

Research on fatigue behavior of welded joint spraying fused by low transformation temperature alloy powder

Modification of spraying fused (MSF) of plasma arc as heat source was used to improve the fatigue performance of welded joint, which both fundamentally reduced stress concentration at weld toe and achieved metallurgical bond between spraying fused coating and welding. The low transformation temperature alloy powder was applied to the method of MSF. After spraying fusion, especially spraying fused joint by low transformation temperature alloy powder, the distribution of residual stress is more difficult to be obtained. Finite element (FE) simulation as an important tool was used to determine the stress field and temperature field of spraying fused joint. Simulated results show that as-welded joint and welded joint spraying fused by conventional nickel base alloy powder (Conventional-joint) present tensile stress. The stress of welded joint spraying fused by low transformation temperature alloy powder (LTT-joint) is compressive stress. Fatigue test results indicated that under the condition of 2 × 10⁶ cycles, the fatigue strength of as-welded joint is 135 MPa, while that of Conventional-joint and LTT-joint is 218 MPa and 235 MPa, respectively. The fatigue strength of Conventional-joint increases by 61.48%, and fatigue strength of LTT-joint increases by 74.07%.     

Effect of residual stresses on fatigue strength of high strength steel sheets with punched holes

The effect of residual stresses on the reverse bending fatigue strength of steel sheets with punched holes was studied for steels with tensile strength grades of 540 MPa and 780 MPa. Tensile and compressive residual stresses were induced around the punched holes. Heat treatment of the specimens with punched holes at 873 K for 1 h decreased the residual stresses around the holes and improved the fatigue strength of the sheets. This result means that the tensile residual stresses induced in the sidewalls of the holes and near the hole edges by punching reduced fatigue strength. The effect of the residual stresses on the fatigue limits of the edges was estimated by the modified Goodman relation using the residual stresses after cyclic loading and the ultimate tensile strength at the fatigue crack initiation sites.     

Experimental investigations on welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steel

Tungsten Inert Gas (TIG) welding is considered as one of the cleanest welding methods. It is generally adopted for thinner materials with moderate weld joint strengths. Welding of sintered porous materials continues to be a challenge due to the inherent porosity of the parent metals. The present research work attempts to address some of the issues relating to the welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steels under TIG welding. Rectangular strips of size 70 mm × 15 mm × 5 mm, obtained by blending, compacting and sintering of elemental powders of iron, graphite and molybdenum, were upset forged – both hot and cold in order to obtain alloy steel strips of various porosities. Two identical alloy steel strips of equal density were then welded both along longitudinal and transverse directions, by TIG welding, employing filler metal of suitable composition. The welded strips were then subjected to tensile test, hardness test, microstructural and Scanning Electron Microscope (SEM) fractography studies. Cold/hot upsetting of the sintered alloy preforms has led to enhanced density. As a result of improved density, their tensile strength and hardness values were also found to be enhanced. The welded alloy exhibited higher tensile strength compared to the un-welded base metal, due to strengthening by residual stress. Similarly, the strength and hardness of the welded alloy strips were found to be enhanced with increase in density. The tensile strength of welded joint is found to be higher compared to that of the base metal due to alloy metals segregation, rapid cooling and formation of acicular ferrite at the weldment of welded joint. No porosity was observed in the weld metal or Heat Affected Zone (HAZ) of the weld joint. However, the base metal had numerous micro pores, though pore migration towards weldment has not been observed.     

Study on the mechanical properties,environmental effect,degradation characteristics and ionizing radiation effect on silk reinforced polypropylene/natural rubber composites

Natural silk fiber (20%) reinforced polypropylene (PP) composites were prepared by compression molding. Tensile strength, tensile modulus, bending strength, bending modulus, impact strength and hardness of the prepared composite were found 54.7 MPa, 1826.2 MPa, 58.3 MPa, 3750.7 MPa, 17.6 kJ/m² and 95 shore A, respectively. To improve the biodegradable character of the composite, natural rubber (NR) was blended (10%, 25%, 50% by weight) with PP. It was found that the mechanical properties of the composite decrease with increasing NR in PP (except IS which increased rather decreasing). Environmental effect on the composite and degradation in various media were investigated in this study. Gamma radiation was used to increase the mechanical properties of the prepared composites. Increase in TS and BS were maximum at 250 krad dose for silk fiber/PP, silk fiber/PP:NR (90:10), silk fiber/PP:NR (75:25) and silk fiber/PP:NR (50:50) composites.     

Rapidly cured epoxy/anhydride composites: Effect of residual stress on laminate shear strength

The drive towards rapid cure thermosetting composites requires a better understanding of the residual stresses that develop during curing. This study investigates the impact of residual stresses on the interlaminar shear strength of resin-infused epoxy/anhydride carbon-fibre laminates. The magnitude of the residual stress was varied by changing the initial injection cure temperature between 75 °C and 145 °C. The corresponding cycle times and the final glass transition temperature of the resin were also measured. The experimentally measured chemical shrinkage and thermal expansion properties of the resin after vitrification were used as inputs to a finite element analysis to calculate the peak residual stresses in the composite. An increase in the initial cure temperature from 85 to 135 °C resulted in an increase of 25% in the residual stress, which led to an experimentally measured reduction in the composite’s short beam shear strength of approximately 16% (8 MPa), in good agreement with model prediction.     

Penetration of HSLA-100 steel with tungsten carbide spheres at striking velocities between 0.8 and 2.5 km/s

A 51 mm thick plate of high-strength low-alloy (HSLA-100) steel was impacted by 6.4 mm diameter tungsten carbide spheres traveling at velocities ranging from 0.8–2.5 km/s. The width and depth of the crater for each impact event are provided in tabulated form and graphed as a function of velocity. The impacts were simulated using an explicit Lagrangian finite element model. A residual stress map over a cross-section through the crater was also measured by the Contour Method for the 2.2 km/s impact. The predominant feature of the stress map was a peak compressive stress of 1100 MPa, which is 1.6 times the yield strength, centered approximately one crater diameter below the crater floor. Residual stresses in the as-received HSLA-100 plate were also measured and were used to evaluate the effect of initial stresses on the model prediction. Good agreement is shown between the numerical simulation of the impact event and the experimental data.     

The theoretical strength of fcc crystals under multiaxial loading

Atomistic modeling of a special triaxial loading of six perfect fcc crystals is performed by means of pseudopotential density functional method. The triaxial stress state is simulated as a superposition of axial pressure and transverse biaxial stresses. The transverse stresses are treated as adjustable parameters and their influence on the theoretical compressive strength is evaluated for the 〈1 0 0〉 and the 〈1 1 1〉 crystallographic orientations of the loading axis. The obtained results revealed that the compressive strengths are increasing linear functions of the transverse compressive stresses. On the other hand, the tensile transverse stresses lower the compressive strength. This implies that the compressive strengths of individual crystals approach a zero value when some critical (characteristic) levels of tensile biaxial stresses are reached. These stresses are then considered to be the theoretical tensile biaxial strengths.     

Numerical calculation of residual stress development of multi-pass gas metal arc welding under high restraint conditions

During welding, residual stresses build-up created by the steep thermal gradient that occurs in the weld zone from localized heating and cooling, and phase transformations appearing in low-alloyed structural steel is inevitable. Welding of rather simple test plates do not cover the actual structural effects, which have to be considered during real component welding. However, the resulting welding-induced residual stress state is highly influenced by the structural characteristics, i.e. restraint conditions, of the welded construction. Therefore, a unique large-scale testing facility providing a specific shrinkage restraint while welding and subsequent cooling was used for the present investigations. Hereby, a six bead multi-pass gas metal arc weld of 20 mm thick structural steel S355J2 + N was welded under shrinkage restraint. The residual stresses were experimentally and numerically investigated, and compared to an analysis of plates welded under force-free support and free shrinkage conditions.The experimentally determined and calculated residual stresses using both 2D and 3D numerical models are in a good agreement. Furthermore, the influence of a shrinkage restraint on the residual stress distribution is both experimentally and numerically shown for the present test set-up.     

Toughness improvement of steel X38CrMoV5-1 via alternative manufacturing process and prevention of catastrophic failure in safety parts

X38CrMoV5-1 (AISI H11-1) steel parts are often manufactured by die-casting followed by quenching and tempering and the microstructure is highly process-sensitive. In order to eliminate the presence of pores in the parts, a different though more expensive manufacturing process has been implemented, involving the hot rolling of a plate and machining of samples in parallel and perpendicular to the rolling direction. Heat treatment of the samples was optimised and the mechanical tensile properties—yield strength, ultimate strength and ductility—and Charpy toughness were determined for ten samples. The results showed that samples machined in the longitudinal direction gave a toughness of about 20 J, twice the toughness of samples machined in the transverse direction (10 J) and 3.5 times the toughness of cast samples (6 J). Ultimate strength and yield strength were 1580 MPa and 1225 MPa in the transverse direction, 1505 MPa and 1275 MPa in the parallel direction, and 1400 MPa and 1260 MPa as cast, respectively. In the latter case, the standard deviation was much higher due to the presence of pores. Using Griffith's theory, the need for pore-free machined parts to prevent catastrophic failure due to cleavage, especially in lift safety systems, is also demonstrated.     

Vacuum heat treatment of iron parts produced by selective laser melting: Microstructure,residual stress and tensile behavior

Selective laser melted parts easily accumulate a large amount of residual stress due to their rapid heating and cooling, which is deleterious to their mechanical properties and limits their applications. In this work iron parts, as the basic industry material, were produced by the new emerging additive manufacturing technology, selective laser melting (SLM). SLM-fabricated iron parts were heat-treated under vacuum to eliminate the residual stress. Results have shown that the crystalline structure of iron (α-Fe) was not modified after the selective laser melting process and after the heat treatment. The broadening of XRD spectra appeared and the micro-stain decreased after the vacuum heat treatment. Columnar grains appeared in the building direction due to the temperature gradient in the molten pool during SLM process. After the vacuum annealing treatment, the grain refinement has occurred due to the residual stress as the driving force although a residual amount of the columnar microstructural architecture could be observed. Although the as-fabricated iron part possesses a higher tensile strength even than that of bulk iron material, the elastic modulus of the annealed specimens decreased to 188 ± 10 GPa and the ultimate tensile strength was much improved from 357 ± 22 MPa up to 401 ± 23 MPa. The yield strength increased from 256 ± 17 MPa up to 352 ± 21 MPa. By means of the micro-indentation method, the tensile residual stress was found in the as-fabricated iron sample.     

Effect of sintering temperature on microstructures and mechanical properties of spark plasma sintered nanocrystalline aluminum

Organic-coated aluminum nano-powders were consolidated by spark plasma sintering technique with low initial pressure of 1 MPa and high holding pressure of 300 MPa at different sintering temperature. The effect of sintering temperature on microstructures and mechanical properties of the compact bulks was investigated. The results indicate that both the density and the strain of the nanocrystalline aluminum increase with an increase in sintering temperature. However, the micro-hardness, compressive strength and tensile stress of the compact bulks increase initially and then decrease with increasing sintering temperature. The nanocrystalline aluminum sintered at 773 K has the highest micro-hardness of 3.06 GPa, the best compressive strength of 665 MPa and the supreme tensile stress of 282 MPa. A rapid grain growth of nanocrystalline aluminum sintered at 823 K leads to a decrease in micro-hardness, compressive strength and tensile stress. After annealing, a remarkable increase in strain and a slight rise in strength were obtained due to the relief of the residual stress in nanocrystalline Al and the formation of composite structure.     

Effect of laser quenching on fatigue properties and fracture morphologies of boronized layer on Cr12MoV steel

A boronized layer of Cr12MoV steel was processed with LQ (laser quenching), and the fatigue limits of original samples before and laser quenched samples were calculated with Locati tension–tension fatigue test, and the fracture morphologies were observed with a SEM (scanning electronic microscope). The results show that the compressive residual stress of −382 MPa is introduced by LQ, the fatigue strength improves from 368 MPa to 422 MPa, increasing by 14.7%, and the fatigue crack is initiated at the subsurface after LQ. The compressive residual stress of the Cr12MoV by LQ is of the main mechanism of the improving of fatigue property.     

Microstructure and tensile strength of friction stir welded joints between interstitial free steel and commercially pure aluminium

In the present study, the joining of interstitial free steel and commercial pure aluminium was carried out by friction stir welding (FSW) technique using tool rotational speeds of 600, 900, 1200 rpm and traverse speed of 100 mm/min. The microstructure and micro-hardness of the weld interface have been investigated. Optical microscopy was used to characterize the microstructures of different regions of friction stir welding joints. The scanning electron microscopy-back scattered electron (SEM-BSE) images show the existence of the different reaction layers in the welded zone. The Al₃Fe intermetallic compound has been observed in the weld interface and their thickness increase with the increase in tool rotational speed. Tensile strength was also evaluated and maximum tensile strength of ∼123.2 MPa along with ∼4.5% elongation at fracture of the joint have been obtained when processed at 600 rpm tool rotational speed.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

Effect of fiber array irregularities on microscopic interfacial normal stress states of transversely loaded UD-CFRP from viewpoint of failure initiation

A detailed investigation has been carried out to determine the effect of local fiber array irregularities and controlling fiber distribution parameters on microscopic interfacial normal stress states for transversely-loaded unidirectional carbon fiber (CF)/epoxy composites. Linear elastic finite element analyses were carried out for two-dimensional image-based models composed of about 70 fibers. The relationship between the geometrical distribution of two adjacent fibers and the interfacial normal stresses (INSs) is investigated for all fibers in different image-based models. Three boundary conditions for loading were selected: Case A involved cooling from the curing temperature (the difference in temperature was ?155 K); Case B involved transverse loading of 75 MPa chosen as an example of macroscopic transverse fracture strength; and Case C involved both cooling from the curing temperature and transverse loading of 75 MPa. High compressive INSs due to the difference in the coefficients of thermal expansion are observed at the location of the shortest interfiber distance for Case A (cooling). High tensile INSs are observed at the location of the shortest interfiber distance and where the fiber alignment angle to the loading direction is small for Case B (loading). For Case C (cooling and loading), the high thermal residual compressive INSs and the high mechanical tensile INSs compensate each other, and the INSs at a short interfiber distance are much lower than those for Case B. These results clearly indicate the importance of the contribution of the thermal residual stresses to the transverse tensile failure initiation of CF/epoxy laminates.     

Development of a new inexpensive green thermoplastic composite and evaluation of its physico-mechanical and wear properties

In the present study an attempt has been made to use turmeric spent (TS) as reinforcing filler to fabricate polypropylene (PP) green composite for load bearing and tribological applications. PP/TS composites were fabricated using varying amounts of TS viz, 10%, 20%, 30% and 40% (w/w) by twin screw extrusion method. The fabricated PP green composites were evaluated for physico-mechanical and tribological properties. Experimentally obtained tensile values were compared with theoretically predicted values using different theoretical models. Tensile modulus of composites increased from 1041 to 1771 MPa with the increase in filler addition from 0 to 40 wt.%. Flexural strength and flexural modulus of composites were improved after incorporation of TS into PP matrix. The water absorption characteristics of composites were determined. The effect of abrading distances viz., 150, 300, 450, and 600 m and different loads of 23.54 and 33.54 N at 200 rpm on the abrasive wear behaviour were studied using dry sand/rubber wheel abrasive test rig. The TS filler lowered the abrasion resistance of PP/TS composites. The wear volume loss and specific wear rate as a function of abrading distance and load were determined. The surface morphology of tensile fractured green composites and their worn surface features were examined under scanning electron microscope.