Transverse Direction Specimens Research Articles

High‐Cycle‐Fatigue Anisotropy of an Aluminum Alloy Superthick Plate

The effect of anisotropy on high‐cycle fatigue (HCF) property along the rolling direction (RD) and the transverse direction (TD) of a 7××× aluminum alloy super thick plate is mainly investigated in this study. The results show a similar microstructure at different thicknesses, and the HCF properties are close as well. The fatigue fracture morphologies show that the fatigue crack sources are all located on the surfaces of the RD and TD specimens. By considering the differences in the HCF properties, there is only a slight anisotropy effect at the center thickness of the plate, which can be attributed to the tensile anisotropy caused by textures. Besides, the fatigue damage degree along the TD is slightly higher than that along the RD at the center thickness. The reason can be explained by two aspects: one is the higher probability of defect occurrence along the TD compared with that along the RD which is analyzed by the previously proposed Yield strength‐Tensile strength‐Fatigue strength model, and the other is the influence of the textures. The combination of hard and soft orientations may cause fatigue damage localization easily in the soft‐oriented grains which accelerates the fatigue failure.

Role of loading direction on high-temperature tensile behavior of Fe-13Cr-4Al-2Mo-0.65Nb alloy

The Fe-13Cr-4Al-2Mo-0.65Nb alloy is being considered as a promising accident-tolerant cladding fuel material for light water reactors design. The present work investigated the mechanical properties and serration behavior of the hot-rolled FeCrAl alloy in the rolling direction (RD) and transverse direction (TD) under various temperatures. The presence of the high-density dislocation bands parallel to RD and the smaller grain size in TD are recognized as leading causes of the lower ductility of the TD specimens. Additionally, both RD and TD specimens show identical serrated flow behaviors at each temperature, suggesting the serration behavior is independent of the loading direction.

Micro-deformation behavior of laser melting deposited multi-gradient TC4/TC11 alloy under tension–tension cyclic loading

The microstructure of additive manufacturing titanium alloy is closely related to its mechanical properties. In this study, a TC4/TC11 titanium alloy with 20 layers was prepared by laser melting deposition (LMD) technology. Microstructure analysis revealed a transformation of the columnar grains into equiaxed grains along the build direction (BD), accompanied by the presence of heat affected zone (HAZ) bands and pore defects. The defect families of specimens in different directions were characterized by X-ray micro-computed tomography (micro-CT). The porosity of BD specimens was 0.0021%, and the porosity of the transverse direction (TD) specimens was 0.0001%, indicating that the material was dense. Specimens with different directions were subjected to tension–tension cyclic loading, and their deformation was analyzed by digital image correlation (DIC). The results show that the presence of noticeable pore defects leads to crack initiation around these regions due to local strain concentration. Moreover, the dispersion and anisotropy of mechanical properties are attributed to the influence of porosity and microstructure deformation, the latter being the dominant factor. In addition, the deformation mechanism and failure mode of the microstructure in LMD TC4/TC11 are proposed in this paper. The deformation of the α laths of TD specimen is hindered by the columnar grain boundary, resulting in improved resistance against crack nucleation and propagation compared to the BD specimen.

Anisotropic deformation mechanisms of rolling-textured Zircaloy-4 alloy by a crystal plasticity model

This study focuses on investigating the anisotropic deformation behavior of rolling-textured Zircaloy-4 alloy under uniaxial tension along the rolling direction (RD) and transverse direction (TD). A dislocation density-based crystal plasticity finite element method is used to study the associated mechanisms. The simulation accurately reproduces the experimental results on stress–strain curves and strain hardening rate. The relationship between relative activities in different slip systems and anisotropic plastic deformation is established. It is revealed that in the tensile deformation, prismatic slip dominates plastic deformation in the RD, while both prismatic slip and basal slip dominate plastic deformation in the TD. The difference in the dominant slip modes leads to a lower yield stress and a higher strain hardening rate in the RD. Furthermore, the stress and strain distributions in the tensile direction are also statistically analyzed. It is found that a certain proportion of hard grains results in more severe stress concentration and lower uniform elongation in the TD specimen compared with the RD specimen. In addition, the strain rate sensitivity of the yield stress is investigated. It is shown that Zircaloy-4 alloy has almost the same yield anisotropy (TD > RD in terms of the yield stress) at different strain rates in the RD and TD.

Low‐cycle fatigue characteristics and fracture behavior of the die‐forged 2014 aircraft wheel

AbstractIn order to provide a sufficient theoretical basis for the fatigue‐resistant design of the aircraft wheels, strain‐controlled low‐cycle fatigue (LCF) tests were carried out on specimens machined in the extrusion direction (ED) and transverse direction (TD) of die‐forged 2014 aluminum alloy wheels. Although the TD specimens have lower tensile strength and yield strength, the fatigue test results show that the TD specimens have superior fatigue life compared with the ED specimens at total strain amplitudes of 0.5–0.8%. This is predominantly caused by the coarse Al12(MnSi)2(FeCu) intermetallic particles close to the surface layer, which results in a relatively short crack initiation stage for the ED specimens. In contrast, TD specimens with finer and more uniform recrystallized grains exhibit more excellent resistance to fatigue crack initiation (FCI) and propagation (FCP). Moreover, the fatigue life of alloys could be accurately predicted via a Manson–Coffin–Basquin (MCB) model based on total strain.

Quantifying the effect of macrozones on the cold-dwell fatigue response of UD-rolled Ti-6Al-4V using high-energy X-ray diffraction

High-energy X-ray diffraction is used to investigate the evolution of elastic lattice strains in rolled Ti-6Al-4V specimens during cyclic loading in-situ with and without the inclusion of a 120s dwell period. A 1mm segment of the gauge section is monitored throughout the first 100 cycles for specimens extracted along the rolling direction (RD), the transverse direction (TD) and 45° between the two, to explore the effects of texture on the evolution of the micromechanical response. Five families of lattice planes from the hexagonal α phase are analysed with emphasis on lattice strain measured at the peak of each cycle, while macroscopic strain accumulation is simultaneously monitored via Digital Image Correlation. In cyclic loading conditions including a dwell period at load, a prominent increase in elastic strains is observed in prismatic and basal lattice planes with the specimen loaded 45° from the rolling direction. In the absence of dwell, both RD and TD specimen orientations exhibited subtle cyclic hardening in all families of lattice planes probed despite negligible evolution in accumulated macroscopic plastic strain. Estimations of lattice orientation-dependent stresses are also presented using directional moduli to examine redistribution of load across sets of grains with the increasing cycle count.

Effect of deformation modes on continuous dynamic recrystallization of extruded AZ31 Mg alloy

Herein, uniaxial compression experiments, with a series of true strains at different temperatures (573–673 K) and strain rates (3 ×10−3 - 3 ×10−1 s−1), are conducted on AZ31 magnesium alloy with different initial textures. The deformation modes and dynamic recrystallization (DRX) behavior are investigated using electron back-scattered diffraction /transmission electron microscopy and crystal plasticity finite element modeling (CPFEM). A unitive constitutive equation is established for different initial textures, and the activation energies of extrusion direction (ED) and transverse direction (TD) samples are found to be 134.5 kJ/mol and 138.4 kJ/mol, respectively. The microstructural investigations and CPFEM simulations are performed to identify the characteristics of DRX processing during compression and categorize according to the deformation modes originating from the difference in initial texture of alloy. Moreover, it is revealed that basal<a> slip prevails in the TD sample at all strain rates under consideration, whereas prismatic<a> slip dominates the initial deformation of the ED specimen. Consequently, more pronounced kink bands and superior compressed peak stress are observed, corresponding to the higher density of prismatic<a> slip in the ED sample than that in the TD sample. Accordingly, these kink bands along with elevated deformation energies accelerate the development of an early continuous dynamic recrystallization with a higher volume fraction and a smaller grain size of the ED sample compared to the TD specimen. The improvement in strength and ductility of the ED sample after being deformed at a strain of 1.2 is attributed to the smaller grains and the weaker texture than those of the TD specimen. For the first time, the role of prismatic slips and kink bands in the recrystallization process is discussed in detail.

Nanoindentation and Hierarchy Structure of the Bovine Hoof Wall.

The bovine hoof wall with an α-keratin structure protects the bovine foot from impact loads when the cattle are running. Reduced modulus, hardness and creep behavior of the bovine hoof wall have been investigated by a nanoindentation technique. The average reduced modulus of the Transverse Direction (TD) specimens from the outside to inside wall is 3.76 and 2.05 GPa, respectively, while the average reduced modulus of the Longitudinal Direction (LD) specimens from the outside to inside wall is 4.54 and 3.22 GPa, respectively. Obviously, the orientation and the position of the bovine hoof wall have a significant influence on its mechanical properties. The use of the generalized Voigt–Kelvin model can make a good prediction of creep stage. Mechanical properties of the LD specimens are stronger than those of the TD specimens. The bovine hoof wall has a layered structure, which can effectively absorb the energy released by the crack propagation and passivate the crack tip. Therefore, a kind of structural model was designed and fabricated by three-dimensional printing technology, which has a 55% performance improvement on fracture toughness. It is believed that the reported results can be useful in the design of new bionic structure materials which may be used in motorcycle helmets and athletes’ protective equipment to achieve light weight and improved strength at the same time.

Experimental Investigation of Fatigue Crack Growth Behavior in Banded Structure of Pipeline Steel

The effect of a ferrite/mixed phase banded structure on the fatigue crack growth behavior of pipeline steel has been investigated in this work. Constant stress-controlled fatigue tests were carried out on unilateral notch specimens in the longitudinal direction (LD) and the transverse direction (TD). The strain accumulation was measured by digital image correlation (DIC) method and the fatigue fracture morphology was analyzed. The results show that the banded structure in the LD would cause a wide range of crack deflections, especially in the early stage of fatigue crack growth, which results in a lower fatigue crack growth rate compared to the TD. At the later stage, due to the increase of the plastic zone of the crack tip, more ferrite in the crack growth path of the TD specimen is also beneficial for crack tip blunting, thereby slowing down the increase of the crack growth rate.

Anisotropic Electroplastic Effects on the Mechanical Properties of a Nano-Lamellar Austenitic Stainless Steel

Effects of electropulsing treatment (EPT) on the microstructural evolution and mechanical properties of a cold-rolled 316L austenitic stainless steel with nano-lamellar structure were investigated. The EPT experiments were carried out with the electric current direction along the rolling direction (RD) and the transverse direction (TD) of the samples, respectively. Significant anisotropic electroplastic effects for the RD and TD specimens, i.e., reduced hardness/strength and enhanced ductility, were obtained owing to the different recrystallization behaviors of the RD and TD specimens during EPT. The RD specimens after EPT with larger recrystallized grain size and higher volume fraction of recrystallization show lower strength and higher elongation than that of the TD specimens. The main reason might be attributed to the change of the current direction in the two kinds of samples, which results in the different sensitivity of the microstructures to thermal and athermal effects during EPT.

Texture Evolution Behavior and Its Triggered Mechanical Anisotropy of CP Ti During Severe Cold Rolling and Subsequent Annealing

The texture evolution behavior and its triggered mechanical anisotropy of commercially pure titanium (CP Ti) during severe cold rolling and subsequent annealing are discussed based on the optical microscopy and the electron backscattered diffraction analyses. Some enlightening results are found. It is shown that planar textures exist under all treatments, namely the {11–29}<10–10> under rolling state, the {11–27}<10–10> under 300 °C annealing state and the {11–24}<10–10> under 500 °C annealing state. This indicates that the crystal plane indices of planar texture change toward {− 12–10} with increasing annealing temperature, which is a result of crystal lattice rotation. Planar texture triggers anisotropy of the mechanical properties for CP Ti sheets under all treatments. In particular, CP Ti sheets exhibit severe and similar anisotropy behavior under rolling and 300 °C annealing states. Generally speaking, the rolling direction (RD) specimens get relatively low yield strength, high ultimate tensile strength and good plasticity, and RD + 45° specimens show relatively high yield strength, low ultimate tensile strength and good plasticity. The transverse direction specimens, however, usually exhibit high yield strength and low plasticity. It is proved that the above anisotropy behavior is mainly determined by the Schmid factor distribution of the (10–10)[11–20] prismatic slip system in different directions. Due to the non-negligible influence exerted by the (0001)[11–20] basal slip system after 500 °C annealing, the anisotropy behavior under this state is obviously different.

Unusual texture formation in Mg–3Al–1Zn alloy sheets processed by slope extrusion

A novel extrusion process, namely, slope extrusion (SE), was proposed to produce Mg–3Al–1Zn alloy sheet, thereby tailoring the strong basal texture of conventional extruded (CE) sheet. The grain morphology and texture were examined, and the tensile tests were performed along 0°, 45°, and 90° with extrusion direction. Moreover, the finite element simulation was performed to have a better understanding on the extrusion processes. An elongated weak double-peak texture titled from extrusion direction to transverse direction about 48° was obtained for the SE sheet and meanwhile the two peaks titled away from normal direction by ~ 35°. Therefore, the SE sheet showed lower yield strength and higher ductility compared with CE sheet. Especially, in terms of the transverse direction specimen of the SE sheet, the ductility reached 27.3%, which was considered as an excellent value for the wrought AZ31 sheet.

Effect of initial texture on fatigue properties of extruded ZK60 magnesium alloy

AbstractThe effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED−9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED−9.4% specimens are similar, and they experience twinning exhaustion → &lt;c + a&gt; slip and detwinning exhaustion → &lt;c + a&gt; slip during each loading cycle. At a medium strain amplitude of 1%, twinning‐detwinning is involved in the cyclic deformation, but different deformation mechanisms were observed in the 3 different specimens. Partial twinning‐complete detwinning mode dominates the cyclic deformation in the ED specimen, while partial detwinning‐retwinning mode occurs in the ED−9.4% specimen. For the TD specimen, both &lt;a&gt; basal slip and tension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED−9.4% specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens.

Deformation of extruded ZK60 magnesium alloy under uniaxial loading in different material orientations

The deformation behavior of an extruded ZK60 magnesium (Mg) alloy was investigated experimentally under uniaxial loading in the extrusion direction (ED) and the transverse direction (TD). X-Ray diffractometer (XRD) and electron back scattered diffraction (EBSD) were employed to examine the microstructure evolution using companion specimens. The results indicated that the flow stress and strain hardening rate were strongly affected by the loading directions relative to the initial material orientations. Strong tension-compression asymmetry was exhibited when the loading was applied along the ED. Practically identical stress-strain responses were obtained when the tension/compression was applied in the TD. The difference in tension-compression responses between the ED and TD specimens arises from the distinctive twinning activities, which are inherently related to the initial basal texture. When loading was applied in the ED, copious tension twinning can attained only under compression. However, in the TD, either tension or compression can impose similar amounts of grains under c-axis tension, thus activating tension twinning in both directions. Detailed analyses of strain hardening rate responses under different loading paths were made. The deformation mechanisms associated with different stages of strain hardening rate were discussed with a particular emphasis on the roles of deformation twinning.

Fracture behavior of magnesium alloys – Role of tensile twinning

In this work, Mode-I fracture experiments are conducted using notched compact tension specimens machined from a rolled AZ31 Mg alloy plate having near-basal texture with load applied along rolling direction (RD) and transverse direction (TD). Moderately high notched fracture toughness of JC∼46N/mm is obtained in both RD and TD specimens. Fracture surface shows crack tunneling at specimen mid-thickness and extensive shear lips near the free surface. Dimples are observed from SEM fractographs suggesting ductile fracture. EBSD analysis shows profuse tensile twinning in the ligament ahead of the notch. It is shown that tensile twinning plays a dual role in enhancing the toughness in the notched fracture specimens with reduced triaxiality. It provides significant dissipation in the background plastic zone and imparts hardening to the material surrounding the fracture process zone via operation of several mechanisms which retards micro-void growth and coalescence.

Research Difference of Strain Distribution and Microstructure Evolution between Rolling Direction and Transverse Direction of AM50 Mg Alloy Plate by Digital Image Correlation

The rolling direction (RD) and transverse direction (TD) of AM50 Mg alloy plate were drawn by tensile test while Digital Image Correlation (DIC) was applied to record local strain distribution and evolution. Then, the microstructure in the maximum strain position was observed so as to find the reason causing the difference between RD and TD specimen. Finally, the fracture surface morphology of the broken specimen was observed by scanning electron microscope (SEM) to determine the crack types. Through the analysis, it is found that the critical failure equivalent strains in the RD specimen and TD specimen are 0.1675 and 0.118, respectively, and the maximum equivalent strain position is the crack source; more twinning is generated in the RD specimen than that of the TD specimen, which indicates that twinning plays an important role in affecting its plasticity. The fracture surface morphology indicates that the RD specimen fracture is quasi-cleavage fracture while the TD specimen shows character of cleavage fracture.

Acoustic emission study of fatigue crack propagation in extruded AZ31 magnesium alloy

The fatigue crack propagation behavior and corresponding AE characteristics of extruded AZ31 magnesium (Mg) alloy were investigated in this study. The effects of specimen orientation and loading frequency were considered. By combining the AE parameter and waveform analysis with the micro-structural and fractograph observations, the findings of the study showed that crack extension and twinning at the crack tip were two major AE source mechanisms during fatigue crack propagation in the Mg alloy. More twinning events were observed in the transverse direction (TD) specimens than in the extruded direction specimens, which contributed to more cyclic deformation irreversibility and cumulative fatigue damage, leading to worse fatigue performance and higher AE counts in the TD specimens. The results also indicated that increasing the loading frequency could slightly increase the fatigue life, and significantly decrease the AE counts in TD specimens due to the frequency or strain rate dependence of twinning activity. These results suggest that twinning plays an important role in the fatigue process in this Mg alloy, and that the AE technique is capable of detecting crack propagation and twinning events during fatigue.

Fatigue Strength of Conventionally Cast Tool Steels and its Dependence of Carbide Microstructure

AbstractThe axial fatigue strength at two million cycles was experimentally determined for two conventionally cast tool steels and successfully compared with results from a fatigue limit model. Specimens were tested both in the rolling and transverse direction and showed large differences in fatigue properties due to the segregated carbide microstructure. Rolling direction specimens experienced higher fatigue strength than the transverse direction specimens. This is due to smaller carbides present in the load affected cross section of the rolling direction fatigue test bars compared to the cross section of the transverse direction fatigue test bars. Fractographic analysis of failed specimens showed that large carbides had caused fatigue failure, which was also predicted by the model. Measured size distributions of carbides and inclusions were used as input data in the model. The probability that at least one particle will be present in the material volume having a size larger than the threshold value for crack propagation was calculated.

Mechanical properties of an extruded wood plastic composite

The focus of this study has been to fully describe the mechanical properties and behaviour of an extruded Wood Plastic Composite product. The use of plasturgy technologies for the elaboration of lignocellulose reinforced composites raises fundamental questions about the behaviour of these materials. Mechanical properties of a composite depend generally on fibre content, fibre/matrix interface link or added coupling agents. Processes play an important role on the reinforcement distribution in the matrix. The main purpose of this paper is to describe mechanical behaviour of WPC composites which are manufactured using an extrusion process. So, complete set of experiments has been realised: tensile, compressive, bending and shear tests. Longitudinal direction refers to extrusion process directional flow and transverse direction specimens have been tested. Mechanical performances of Wood Plastic Composites depend also on the link of fibre/matrix interface, fracture surface has been analysed using an optical microscope.

Processing maps for hot deformation of rolled AZ31 magnesium alloy plate: Anisotropy of hot workability

Processing maps on rolled AZ31 magnesium plate have been developed in the range 300–550 °C and 0.0003–10 s −1 by hot compression of specimens parallel to the rolling direction (RD), the transverse direction (TD), or the normal direction (ND) with a view to examine whether the hot workability is anisotropic. The processing map for RD specimens exhibited a single wide domain of workability in the temperature range 350–550 °C and strain rate range 0.0003–0.3 s −1 in which dynamic recrystallization (DRX) occurs. The apparent activation energy in this domain is estimated to be 143 kJ/mole, which is close to that for self-diffusion in magnesium. On the other hand, the maps for the TD and ND specimens exhibited two DRX domains, one at lower strain rates (<0.01 s −1) and the other at higher strain rates (>1 s −1). The apparent activation energies estimated in the lower strain rate DRX domain of TD and ND maps are 180 and 168 kJ/mole, respectively, suggesting cross-slip occurs in this domain. In the higher strain rate domain, the apparent activation energy estimated in both TD and ND specimens is 105 kJ/mole, which is close to that for grain-boundary diffusion. The results suggest that the hot deformation behavior is anisotropic and a simple correlation with the known primary rolling texture ( { 0 0 0 2 } 〈 1 0 1 ¯ 0 〉 ) revealed that both the first order and second order pyramidal slip systems are favored in the RD orientation resulting in higher workability. The other two orientations may be considered “harder” as regards hot workability since either the first order or the second order pyramidal slip is likely to dominate. In all the three orientations and within the DRX domains, the grain size varies linearly with the Zener-Hollomon parameter.