Experimental results and fatigue life evaluation of magnesium laserbeam‐welded joints under proportional and non‐proportional multiaxial fatigue loading with variable amplitudes

Fatigue life of magnesium laserbeam‐welds (AZ31 and AZ61 alloys) was assessed experimentally under variable amplitude loadings. The specimens were subjected to load‐controlled cyclic loadings. The tests were carried out using a Gauss‐distributed amplitude sequence of length L_S = 5 · 10⁴ cycles and loading ratio R = –1 under pure axial, pure torsion as well as in‐phase and out‐of‐phase combined loadings. The notch stresses were obtained from a linear‐elastic FE‐model using the reference radius approach with r_ref  = 0.05 mm. The stress‐based hypotheses were applied: Effective equivalent stress hypothesis (EESH), shear stress intensity hypothesis (SIH), Findley, and modified Gough‐Pollard. A non‐proportionality factor is introduced and steps required for computing are presented in order to improve fatigue life assessment under non‐proportional loadings.     

Fretting fatigue behaviour of shot-peened Ti-6Al-4V at room and elevated temperatures

Fretting fatigue behaviour of shot‐peened titanium alloy, Ti‐6Al‐4V was investigated at room and elevated temperatures. Constant amplitude fretting fatigue tests were conducted over a wide range of maximum stresses, σ_max= 333 to 666 MPa with a stress ratio of R= 0.1 . Two infrared heaters, placed at the front and back of specimen, were used to heat and maintain temperature of the gage section of specimen at 260 °C. Residual stress measurements by X‐ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Elevated temperature induced more residual stress relaxation, which, in turn, decreased fretting fatigue life significantly at 260 °C. Finite element analysis (FEA) showed that the longitudinal tensile stress, σ_xx varied with the depth inside the specimen from contact surface during fretting fatigue and the largest σ_xx could exist away from the contact surface in a certain situation. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from FEA results to characterize fretting fatigue crack initiation behaviour. It showed that stress relaxation during test affected fretting fatigue life and location of crack initiation significantly. MSSR parameter also predicted crack initiation location, which matched with experimental observations and the number of cycles for crack initiation, which showed the appropriate trend with the experimental observations at both temperatures.     

The effect of cerium on high-cycle fatigue properties of die-cast magnesium alloy

The effect of cerium (Ce) on high‐cycle fatigue behaviour of die‐cast magnesium alloy AZ91D was investigated. Mechanical fatigue tests were conducted at the stress ratio, R= 0.1 on specimens of AZ91D alloys with different Ce additions. The microstructure and fatigue fracture surfaces of specimens were examined using a scanning electron microscope (SEM) to reveal the micromechanisms of fatigue crack initiation and propagation. The results show that the grain size of AZ91D is refined, and the amount of porosity decreases and evenly distributes with the addition of Ce. The fatigue strength of AZ91D evaluated by the up‐and‐down load method increases from 96.7 MPa to 116.3 MPa (1% Ce) and 105.5 MPa (2% Ce), respectively. The fatigue cracking of AZ91D alloy initiates at porosities and inclusions of the alloy's interior, and propagates along the grain boundaries. The fatigue fracture surface of test specimens shows the mixed fracture characteristics of quasi‐cleavage and dimple.     

Evaluation of fatigue life of AZ31 magnesium alloy fabricated by squeeze casting

Cyclic deformation behavior and fatigue life of squeeze-cast AZ31 magnesium alloy was studied under stress amplitude-control at room temperature. Low and high cycle fatigue tests with engineering stress amplitudes in the range from 40 to 110 MPa were conducted. Analysis of hysteresis curves was performed. Tension–compression asymmetry of hysteresis loops was not observed; the alloy exhibited cyclic hardening in tension and compression. The fatigue life in the low cycle fatigue region was expressed by Wöhler and derived Manson–Coffin curves. Experimental data in both, the low and high cycle fatigue regions were fitted by means of regression functions. S–N curves exhibited a smooth transition from the low to the high cycle fatigue regions and significant scattering of experimental points was observed. Furthermore, metallographic and fractographic analyses were performed. Crack initiation occurred from the specimen surface or on clusters of secondary particles; the region of final fracture was characterized by a transgranular ductile fracture.It can be concluded that the fatigue properties of squeeze cast magnesium alloy AZ31 are significantly improved comparing to materials prepared by common methods of casting. Squeeze casting also enables the cost-effective fabrication of complicatedly shaped parts.     

Cyclic deformation behaviour and fatigue crack propagation in AZ91HP and AM50HP

Abstract

The purpose of the present work was to investigate room temperature cyclic deformation and crack propagation behaviour in the most widely used die casting magnesium alloy AZ91HP with different heat treatments. In addition, examination of the low cycle fatigue properties of solid solution treated alloy AZ91HP-T4 was emphasised in comparison with AM50HP. Obvious cyclic strain hardening was found in low cycle fatigue tests, especially for AZ91HP-T4 at high cyclic strain amplitudes. Nevertheless, it was very difficult to evaluate differences in low cycle fatigue behaviour between die casting alloy AZ91HP-F, artificially aged alloy AZ91HP-T6, solution treated alloy AZ91HP-T4, and AM50HP(-F) because of the scatter of test data. However, it may be concluded that the last two alloys had greater plastic strain components during cyclic deformation, and AZ91HP-T4 exhibited a longer fatigue life than that of AM50HP at the highest strain amplitude. According to results of tests carried out on AZ91HP compact tension (CT) specimens, it was concluded that solution treatment could reduce the fatigue crack propagation rate, and plasticity induced crack closure was considered to have a predominant effect on fatigue crack propagation.     

Effects of neodymium rich rare earth elements on microstructure and mechanical properties of as cast AZ31 magnesium alloy

Abstract

The effects of neodymium rich rare earth elements [RE(Nd)] on microstructure and mechanical properties of as cast AZ31 magnesium alloy were investigated. The microstructures of as cast AZ31–xRE(Nd) alloys display a dendrite configuration, and the secondary dendrite spacing of the α-Mg phase was decreased with the increasing Nd content. The addition of RE(Nd) resulted in the formation of Al₂Nd and Mg₁₂Nd phases. Mechanical properties were improved significantly due to grain refinement and precipitation of intermetallic phases. When the amount of RE is 1·0 wt-%,The as cast AZ31 alloy reached its maximum tensile strength of 249 MPa at room temperature, yield strength of 169 MPa and elongation of 9·0%.     

Influence of shot peening on notched fatigue properties of magnesium alloy ZK60

Abstract

The influence of shot peening on high cycle fatigue performance of notched specimen was investigated for ZK60 and ZK60-T5 magnesium alloys. The results show that the notched fatigue strengths (at 10⁷ cycles) for ZK60 and ZK60-T5 alloys increase from 150 and 155 MPa to 220 and 240 MPa at the optimum Almen intensity of 0·30 and 0·40 mmN respectively. In comparison to ZK60 alloy in extruded condition, higher notched fatigue performances of both unpeened and peened specimens were observed for ZK60-T5 alloy.     

Investigation of uniaxial low-cycle fatigue failure behavior of hot-rolled AZ91 magnesium alloy

The uniaxial low-cycle fatigue behavior of hot-rolled AZ91 magnesium alloy was investigated by asymmetric cyclic stress-controlled experiments at room temperature. The effects of the sampling direction, peak stress and stress ratio on the fatigue life were discussed. The fatigue life increases with increasing the stress ratio or decreasing the peak stress. Due to the anisotropic property, the specimen in transverse direction shows superior fatigue resistance. Considering the effects of mean stress on the fatigue strength coefficient and fatigue strength exponent, a modified Basquin model was proposed and validated to evaluate the fatigue life of AZ91 magnesium alloy.     

Fatigue behaviour of welded joints from magnesium alloy (AZ31) according to the local strain concept

In the present study, the results of fatigue tests with the magnesium alloy AZ31 (ISO‐MgAl3Zn1) in the material states base metal, heat affected zone and weld metal obtained under strain control at room temperature within a range from 2·10² to 5·10⁶ cycles are presented. The fatigue behaviour was characterized by the Coffin–Manson–Basquin equations and the stress – strain behaviour by the Ramberg–Osgood equation. The data can be used to assess welded magnesium joints according to the local strain concept.     

Influence of nano-sized carbon nanotube reinforcements on tensile deformation,cyclic fatigue,and final fracture behavior of a magnesium alloy

Magnesium alloy (AZ31) based metal matrix composite reinforced with carbon nanotubes (CNTs) was fabricated using the technique of disintegrated melt deposition followed by hot extrusion. In this research paper, the microstructure, hardness, tensile properties, tensile fracture, high cycle fatigue characteristics, and final fracture behavior of CNTs-reinforced magnesium alloy composite (denoted as AZ31/1.0 vol.% CNT or AZ31/CNT) is presented, discussed, and compared with the unreinforced counterpart (AZ31). The elastic modulus, yield strength, tensile strength of the reinforced magnesium alloy was noticeably higher compared to the unreinforced counterpart. The ductility, quantified both by elongation-to-failure and reduction in cross-section area of the composite was higher than the monolithic counterpart. A comparison of the CNT-reinforced magnesium alloy with the unreinforced counterpart revealed a noticeable improvement in cyclic fatigue life at the load ratios tested. At all values of maximum stress, both the reinforced and unreinforced magnesium alloy was found to degrade the cyclic fatigue life at a lower ratio, i.e., under conditions of fully reversed loading. The viable mechanisms responsible for the enhanced cyclic fatigue life and tensile behavior of the composite are rationalized in light of macroscopic fracture mode and intrinsic microscopic mechanisms governing fracture.     

Influence of transverse load on the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy

A new testing procedure, employing transverse load was adopted to investigate the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy. The tests were conducted with an electro dynamic shaker system by employing specimens fabricated as per ASTM standard. S–N plot was generated from the test results and compared with that of gravity cast AZ91 alloy tested in identical ambience. The influence of transverse load on the fatigue behaviour of these alloys is discussed. As fatigue cracks were found to have initiated in pores in most of the tested samples, pores were assumed as initial cracks as per linear fracture mechanics and the critical stress intensity amplitude (K_cr) was estimated. Structure–fatigue property correlations are discussed using fractographs. Mean stress effect on the fatigue properties and effects of alloying constituents are also discussed.     

Foreword – Materialwissenschaft und Werkstofftechnik 10/2011

Load controlled fatigue tests were performed up to 10⁷ cycles on flat notched specimens (K_t = 2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferritic‐bainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain softening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (R_σ = –1), while they have no influence under pulsating loading (R_σ = 0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependent.     

An experimental analysis of fatigue behavior of AZ31B magnesium alloy welded joint based on infrared thermography

The fatigue behavior of AZ31B magnesium alloy welded joint during high cycle fatigue test was investigated by infrared thermography. Five stages of superficial temperature evolution were observed: an initial temperature increase, a temperature decline, a temperature equilibrium, an abrupt temperature increase and a temperature drop after the failure. The theoretical models were formulated to explain the observed temperature evolution. The mean temperature decline caused by thermoelastic effect was observed and discussed when the maximum stresses were below 30 MPa. The influence of weld reinforcement on fatigue behavior was also investigated. A good precision was achieved in fatigue strength prediction by means of infrared thermography.     

Fatigue behaviour of dissimilar Al 5052 and Mg AZ31 resistance spot welds with Sn‐coated steel interlayer

The fatigue property of dissimilar spot welds between an aluminium alloy (AA5052) and a magnesium alloy (AZ31) was studied in this research. The AA5052 and AZ31 coupons were resistance spot welded together by using an interlayer of Sn‐coated steel between the two coupons. The fatigue test results revealed that the Mg/Al joints had the same level of fatigue strength as Mg/Mg resistance spot welds. It was found that within the life range of N_f < 10⁵ cycles, Mg/Al welds degraded faster than Mg/Mg joints. This was attributed to the larger bending moment on the plane of fatigue failure in the Mg/Al welds. Three failure modes were observed under different cyclic loading regimes: Al/steel interfacial failure, Mg coupon failure and Al coupon failure. Fatigue fracture surface of Mg/Al welds consisted of two distinct regions: crack propagation region with brittle morphology and final rupture with ductile morphology.     

Improving low-cycle fatigue properties of rolled AZ31 magnesium alloy by pre-compression deformation

The effect of pre-compression deformation on the low-cycle fatigue properties and cyclic deformation behavior of as-rolled AZ31 alloy was investigated by performing the stress-controlled low-cycle fatigue tests at room temperature. Fatigue properties and cyclic damage process should be closely related to the twins. The present work aimed to investigate the deformation mechanism and fatigue life caused by the introduced {1 0−1 2} twinning–detwinning from the viewpoint of stress amplitude. The results reveal that the twins contribute to the fatigue properties and cyclic damage process of AZ31 alloy. There were noticeable changes in hysteresis loops, microstructures and fatigue lives when the stress amplitude increased from 120 to 150 MPa. The fatigue life of pre-compressed samples was more superior to that of the as-rolled sample under different stress amplitudes, especially under the stress amplitude close to the tensile yield strength of the as-rolled sample.     

The effect of load ratio on fatigue life and crack propagation behavior of an extruded magnesium alloy

Fatigue experiments were carried out in laboratory air using an extruded magnesium alloy, AZ31, to investigate the effect of load ratio on the fatigue life and crack propagation behavior. The crack propagation behavior was analyzed using a modified linear elastic fracture mechanics parameter, M. The relation crack propagation rate vs. M parameter was found to be useful in predicting fatigue lives at different R ratios. Good agreement between the estimated and the experimental results at each stress ratio was obtained.     

Fatigue life estimation after crack repair in 6005 A-T6 aluminium alloy using the cold expansion hole technique

In this paper, the hole drilling (HD) and the cold expansion (CE) processes, which were used as a technique for crack repair, were investigated in order to estimate the beneficial effects on fatigue crack initiation (FCI). The FCI life is defined as the number of cycles to initiate a new crack of 0.2 mm on the surface of the specimen. Three hole radii and three degrees of cold expansion (DCE%) values were tested after a crack propagation period. Crack retardation after the CE process was observed. This phenomenon is due to two mechanisms: retardation owing to both geometric and mechanical effects, which is produced by the stress concentration at the drilled hole, and the large strain‐induced compressive residual stresses around the hole. In this report, the influence of the loading conditions was studied. For high values of the stress intensity factor range ΔK_ρ around the hole (based on the pseudo crack length a + ρ), the number of cycles corresponding to crack initiation N_i is low. At the edge of the hole, the maximum stress range can be approximated by the following formula: Δσ_max = 2ΔK_ρ /√πρ , where ρ is the hole radius and ΔK_ρ is the related stress intensity factor range.The FCI life extension, defined by the number of cycles corresponding to crack re‐initiation N_i , is related to the relative maximum stress range ratio R_σ = [(Δσ_max )/(Δσ_max )_th ] where (Δσ_max )_th is the value of the threshold maximum stress range obtained when N_i = 2 × 10⁶ cycles. The relationship between N_i and R_σ may be written as a power function.     

Effect of strain ratio on cyclic deformation and fatigue of extruded AZ61A magnesium alloy

Strain-controlled fatigue experiments were conducted on an extruded AZ61A magnesium alloy at three strain ratios (R_ɛ = −∞, −1, 0) using smooth tubular specimens. As the strain ratio decreased, stronger cyclic hardening, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed. These phenomena were associated with twinning in the compressive phase and detwinning in the tensile phase during cyclic deformation. At the same strain amplitude, fatigue life increased with decreasing strain ratio. The strain-fatigue life curve at each strain ratio exhibited a distinguishable kink. Such a kink point represents a demarcation point above which persistent twinning–detwinning occurs under cyclic loading. Two Smith, Watson, and Topper (SWT) fatigue criteria can predict the fatigue lives of the material at all strain ratios satisfactorily.     

Small fatigue crack growth behavior of titanium alloy TC4 at different stress ratios

In this paper, the small fatigue crack behavior of titanium alloy TC4 at different stress ratios was investigated. Single‐edge‐notch tension specimens were fatigued axially under a nominal maximum stress of 370 MPa at room temperature. Results indicate that fatigue cracks in TC4 initiate from the interface between α and β phases or within α phase. More than 90% of the total fatigue life is consumed in the small crack initiation and growth stages. The crack growth process of TC4 can be divided into three typical stages, ie, microstructurally small crack stage, physically small crack stage, and long crack stage. Although the stress ratio has a significant effect on the total fatigue life and crack initiation life at constant σ_max, its effect on crack growth rate is indistinguishable at R = ?0.1, 0.1, and 0.3 when crack growth rate is plotted as a function of ?K.     

ELEVATED TEMPERATURE FATIGUE IN Ni3Al-BASED ALLOYS

Both high-cycle and low-cycle fatigue properties of hot-extruded powders of a Ni₃Al-based alloy, IC218, have been evaluated. High cycle fatigue measurements were performed under stress controlled conditions at temperatures ranging from 25°C to 850°C. Tests were made in both laboratory air and vacuum environments. Low cycle fatigue tests were conducted under total strain control in a laboratory air environment at 650°C. In high cycle fatigue, high ratios of the fatigue limit (Δσ at 10⁶ cycles) to monotonic yield strength (σ_ys), of approximately Δσ/σ_ys～1, were obtained in the powder extruded IC218 alloy for temperatures ranging from 25°C to 650°C. In low cycle fatigue, a substantial decrease in fatigue life occurred at 650°C, compared to results obtained previously at 25°C. High cycle fatigue performance at low stress/strain amplitudes is better than expected when compared to precipitation strengthened superalloys. The improved performance is explained in terms of the cyclic hardening behavior of the alloy.