Residual stress and strain in MIG butt welds in 5083-H321 aluminium: As-welded and fatigue cycled

This paper presents single-line residual stress profiles for 8 mm 5083-H321 aluminium plates joined by gas metal arc (MIG) welding. The data were obtained by synchrotron diffraction strain scanning. Weld metal stresses (up to ～7 mm either side of the centreline) are quite scattered and unreliable because of the large epitaxial grain size in the fusion zone. Peak magnitude of the transverse stresses varies between +50 MPa (19% of parent plate proof strength) at the HAZ boundary to ?150 MPa (57% of PP proof strength) at the weld centreline. Equivalent values for longitudinal stresses are +90 MPa (34% of PP proof strength) some 22 mm from the weld centreline to ?120 MPa (45% of PP proof strength) at the weld centreline. Plate-to-plate variation in the as-welded transverse and longitudinal residual stress values across the weld toe region is around 40 MPa. The effect on residual stress and strain values of a sequence of applied fatigue loads was also considered and reported.     

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.     

Off-axis tensile fatigue assessment based on residual strength for the unidirectional 45° carbon fiber-reinforced composite at room temperature

The tensile fatigue behavior of unidirectional carbon fiber-reinforced thermoplastic and thermosetting laminates was examined at room temperature. Tension-tension cyclic fatigue tests were conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limits of carbon fiber-reinforced thermoplastic laminates (CF/PA6) and thermosetting laminates (CF/Epoxy) were found to be 28.0 MPa (48% of the tensile strength) and 56.2 MPa (63% of the tensile strength), respectively. Two types (in constant and incremental loading way) of loading-unloading low cycle fatigue tests were employed to investigate the modulus history of fatigue process for announcing the fatigue mechanism. The residual tensile strength of specimens that survived fatigue loading maintained with the increase of fatigue cycles and applied stress. Examination of the fatigue-loaded specimens revealed that the more flexible/ductile trend of resins and the formation of micro-cracks at the interface between fiber and matrix was facilitated during high fatigue loading (⩾fatigue limit stress), while no interfacial/matrix damage in resins was detected during low fatigue loading (<fatigue limit stress), which was consider to be the governing mechanism of strength maintain during fatigue loading.     

Tensile and fatigue behavior of superelastic shape memory rods

The tensile and fatigue behavior of superelastic shape memory alloy (SMA) bars heat-treated at three different temperatures were examined. Low cycle fatigue tests at variable load rates were carried out to determine the effect of stress and frequency on residual strain and energy dissipation in a fatigue cycle. The mechanism of energy dissipation was studied by monitoring the temperature changes in the fatigued samples as a function of applied stress and frequency of testing. Results from the tensile tests revealed that the stress for the Austenite to Martensite transformation decreased from 408 MPa to 204 MPa with an increase in temperature of heat treatment from 300 to 450 °C. The ultimate strength of the SMA increased from 952 MPa to 1115 MPa when the heat treatment temperature was increased from 300 to 450 °C. Fatigue testing prior to conducting the tensile test decreased the ultimate strength of the SMA and also reduced the failure strain. The energy dissipation in fatigue tests was found to decrease as test frequency increased from 0.025 Hz to 0.25 Hz and the change in sample temperature during the test at the lower test frequency was found to be considerably higher than at the higher frequency.     

Evaluation of residual stress relaxation and its effect on fatigue strength of AISI 316L stainless steel ground surfaces: Experimental and numerical approaches

This paper is aimed at evaluating the residual stress relaxation and its effect on the fatigue strength of AISI 316L steel ground surfaces in comparison to electro-polished surfaces. An experimental evaluation was performed using 3-point and 4-point bending fatigue tests at R_σ = 0.1 on two sets of notched specimens finished by electro-polishing and grinding. The residual stress fields were measured at the notch root of specimens, before and after fatigue tests, by means of the X-ray diffraction technique. It was found a degradation of about −35% for the 4-point bending fatigue limit at 2 × 10⁶ cycles of the ground specimens in comparison to the electro-polished ones. This degradation is associated with a slight relaxation of the grinding residual stresses which remain significant tensile stresses at the stabilized state. While under the 3-point bending test, these residual stresses relax completely and provoke a noticeable increase of the fatigue limit estimated at about 50% in comparison to the 4-point bending fatigue test. The numerical evaluation of residual stress relaxation was carried out by FE analyses of the cyclic hardening behaviour of the ground layer. The isotropic and nonlinear kinematic model proposed by Chaboche was used and calibrated for the base material and the ground layer. The results show that residual stresses relax to a stabilized state characterized by elastic-shakedown response. This stabilization is occurred after the first cycle of the 4-point bending test corresponding to the higher stress concentration (K_t-4p = 1.66), while it requires many cycles under the 3-point bending test corresponding to the lower stress concentration (K_t-3p = 1.54). The incorporation of stabilized residual stress values into the Dang Van’s criterion has permitted to predict with an acceptable accuracy the fatigue limits under both bending modes.     

Tension–compression fatigue of a SiC/SiC ceramic matrix composite at 1200 °C in air and in steam

Tension–compression fatigue behavior of a non-oxide ceramic composite with a multilayered matrix was investigated at 1200 °C in laboratory air and in steam. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. Tension–compression fatigue behavior was studied for fatigue stresses ranging from 80 to 200 MPa at a frequency of 1.0 Hz. Presence of steam significantly degraded the fatigue performance. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Improvement of yield strength of LM24 alloy

In this study, Al₂O₃ particles were employed to improve the microstructure of LM24 and therefore, to increase the yield strength and tensile strength of this kind of alloy. In situ Al₂O₃ particles were obtained by direct reaction between oxygen and Al melt at 750–800 °C. Microstructure examination shows that the size of in situ formed Al₂O₃ particles was about 1–2 μm, and interestingly, with addition of in situ Al₂O₃ particles, the coarse primary Si phase was disappeared completely. More important, the yield strength and the tensile strength of Al₂O₃/LM24 are increased by 52 MPa, 16 MPa than that of LM24 alloy with 0.1% Sb addition. The value of 181 MPa and 315 MPa is for yield strength and tensile strength of Al₂O₃/LM24 respectively. Besides, the yield strength and tensile strength are 180 MPa and 314 MPa respectively for Al₂O₃/LM24 alloy after remelting and casting. This verifies that the improvement of mechanical properties of such kind of material possesses stability and reliability.     

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 dispersion of sepiolite in sepiolite-NBR composite on the tensile strength

The effect of dispersion of sepiolite in a sepiolite-NBR composite on the tensile strength of the composite was examined. Fibrous sepiolite was ultrasonically treated in distilled water with various types of dispersant. The ultrasonicated sepiolite was mixed with NBR latex to form a sepiolite-NBR complex. The tensile strength of the complex was examined to evaluate the degree of dispersion of sepiolite. The tensile strength results for the composites confirmed that a lower ultrasonication frequency produced a high strength in the composite. The tensile strength actually increased from 3 to 8 MPa as a result of ultrasonication at 28 kHz. To improve and stabilize the dispersion state, five kinds of dispersant were used. The addition of two dispersants in particular, ammonium polycarboxylate and amino alcohol polyphosphate, resulted in an increase of the strength from 8 to 15–16 MPa. When the composites were heated at 450 °C, the strengths of the composites produced with and without the dispersant were approximately 1.4–1.7 and 0.85 MPa, respectively. Thus, the dispersant still affected the strength of the heated samples.     

Determination of residual stresses in SiC monofilament reinforced metal-matrix composites using Raman spectroscopy

Raman spectroscopy has been used to follow the deformation of chemical vapour deposition type SCS-6 and Sigma 1140+ SiC monofilaments and to determine residual stresses in these SiC monofilaments reinforced metal-matrix composites. Raman bands at 1330 and 1600 cm⁻¹ due to carbon have been observed on the monofilament surface and it has been shown that both bands shift linearly to lower wavenumbers during tensile deformation. The residual stresses in SiC monofilament reinforced composites arising from thermal expansion mismatch have also been determined by measuring the shifts of carbon bands from the same monofilaments embedded in a Ti–6Al–4V matrix. The axial residual stresses in the carbon coating are found to be around −850 MPa for the SCS-6 composite and −540 MPa for the Sigma 1140+ composite.     

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.     

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%.     

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.     

Cold expansion of holes and resulting fatigue life enhancement and residual stresses in Al 2024 T3 alloy – An experimental study

This paper presents the experimental results of fatigue life enhancement and the residual stresses around the cold expanded holes in Al 2024, a widely used aerospace alloy. Two techniques for cold expansion of holes, namely split-sleeve with taper pin technique and split-sleeve with ball technique were considered for comparison, as the former involves surface contact and the latter has line contact during expansion. The techniques were compared based on the fatigue life enhancement in the expanded holes, the induced and the residual stresses due to expansion. The holes were expanded by 2%, 3%, 4%, 5%, and 6% using INSTRON machine in both the techniques. While both the techniques resulted in improvement in fatigue life of the expanded holes, the taper pin technique yielded 200% higher fatigue life improvement than that obtained by ball technique. The induced residual stresses were measured by mounting strain gages of 0.2 mm gage length. These are drawn as a function of induced strain. In both the techniques residual stresses increased with increase in percentage of expansion until 5% and then decreased for 6% expansion. The increase in fatigue life at 5% expansion was 1.88 times and 5.3 times higher than that of the non-expanded holes for ball and tapered method, respectively. The corresponding improvement in taper method was greater than the non-expanded holes. While, it was observed that the residual stresses decreased with respect to the distance from the hole in both the techniques, the ball technique resulted in lower residual stresses than that of taper pin technique.     

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.     

Base material fatigue data for low alloy forged steels used in the subsea industry. Part 2: Effect of cathodic protection

In air S–N fatigue data for forged low alloy steels as used in the subsea industry are presented in Part 1 of this paper. The test scope in Part 1 included testing to quantify the effect of the surface roughness, mean stress and material strength on the high cycle fatigue strength of low alloy steels with a tensile strength in the range of 600–800 MPa. A method for estimating the in air S–N curve from the tensile strength (material grade), surface roughness (machining) and mean stress (such as residual stresses, pressure testing, pre-load and external loads) is presented in Part 1. In this Part 2, fatigue test results for low alloy steels and one carbon steel tested in seawater with cathodic protection with a potential of −1050 mV versus an Ag/AgCl reference electrode are presented. The fatigue testing has been performed using smooth specimens. The tested smooth specimens have (actual) tensile strengths in the range from 627 to 790 MPa. Penalty factors for the tested smooth specimens in seawater with cathodic protection with respect to in air performance (Part 1) are presented and compared with penalty factors used in fatigue design codes such as DNVGL-RP-0005 (former DNV-RP-C203) and BS 7608. The obtained environmental reduction factors are found to be in accordance with the penalty factors used in BS 7608 provided that the maximum stress in the cycle is less than 94% of the yield stress for the material. The penalty factors used for forged steels in DNVGL-RP-0005 are non-conservative compared to the test outcome for the steel tested in an artificial 3.5% NaCl seawater solution. For higher stress levels, larger penalty factors than used in BS 7608 are required. It is found that the obtained S–N based environmental reduction factors are of similar magnitude as BS 7910 fatigue crack growth based reduction factors for CP.     

Influence of defects on fatigue strength and failure mechanisms in the VHCF-region for quenched and tempered steel and nodular cast iron

Investigations are presented in this paper on quenched and tempered steel 42CrMoS4 from two batches, with two different tensile strengths (R_m = 1100 MPa, 1350 MPa) but with similar microstructure, and a nodular cast iron EN-GJS-900-2 (R_m = 930 MPa). Fatigue tests with smooth (K_t = 1) and notched (K_t = 1.75) specimens were performed at R = −1 and R = 0 up to the number of cycles N = 2·10⁹ in order to determine the fatigue strength behaviour and failure mechanisms, especially in the VHCF-region. Failure in smooth specimens often initiated at material defects such as oxides in the quenched and tempered steel and shrinkage holes in the nodular cast iron. Firstly, a fatigue strength analysis was performed that did not consider these defects. A possibility of analysis of experimental data including VHCF-results has been discussed. Next, a linear elastic fracture mechanics analysis was performed in order to describe the defect behaviour, assuming that the defects act like cracks. The results showed that there are lower limit or threshold values of the stress intensity factor range ΔK for crack propagation in both materials. Analysis of defects and defect distribution in run-out specimens confirmed this conclusion. From the comparison of the results with an S–N curve from the design code FKM-Guideline Analytical strength assessment of components, recommendations for design and assessment of components have been derived.     

Fatigue strength and fracture mechanism of steel modified by super-rapid induction heating and quenching

Four kinds of surface hardened-specimens (ordinary structural steel with carbon content of 0.45% C) having hardened thicknesses of 0.7–1.8 mm were prepared using a ‘super-rapid induction heating (SRIH) system’. Rotation bending fatigue tests were performed with special focus on the effect of a hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 10⁷ cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000 MPa. This is because the fatigue crack originating from inside the hardened layer leads to the final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved on the specimen with thick hardened layers, such as those about 1.8 mm thick. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to final fracture (hardened-layer fracture mode).     

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.     

Experimental study on the fatigue failure mechanism of QP-16 type ball-eye under asymmetric load

The ball eye (BE) is a key connecting component between the insulator and transmission tower, whose fatigue characteristics concern the safety of transmission lines. To understand the fatigue mechanism and characteristics of it, the fatigue test was conducted based on the following data: r = 0.25, S = 500 MPa,then plotting of SN and Δε_axis − N, to analyze the fatigue failure of the test specimen from the macro and micro point of views. The research results show that: the life of BE significantly reduces with the increase of the stress amplitude, but the relative reduction in life is not the same; softening and strain amplitude of the specimen change differently before and after the stress amplitude of 300 MPa; when S ≤ 300 MPa, the fracture is more smooth, the fatigue crack propagation is slow; when S > 300 MPa, the rate of fatigue crack growth is faster, and the fatigue crack growth zones are not obvious. The cracks are easily detectable appear at the joint of the BE and insulator cap, and the cracks along the fracture cross section are constantly expanding, showing multiple fatigue sources and fatigue steps. The number of fatigue steps increases as the magnitude of the tensile stress increases. When S = 500 MPa, the yield strength decreases during the lifetime, the decrease rate of the tensile strength and microstructure strength in each stage are different. Axial lengthening and section shrinkage ratio decrease with the development of fatigue, fatigue evolution process is accompanied by phenomenon of crystalline slip, deformation, dislocation, at the same time, dissipation and decomposition of pearlite occur, and carbide precipitates from the matrix, growing and moving to the grain boundaries, the specific phenomenon of grain growth appears.