Low Fatigue Property Research Articles

Effects of microstructure coarsening and casting pores on the tensile and fatigue properties of cast A356-T6 aluminum alloy: A comparative investigation

A356 alloys processed mostly with the low pressure die casting method are always known to contain casting pores and inhomogeneous microstructure. In this study, the effects of microstructure coarsening and casting pores on the tensile and high-cycle fatigue properties of A356-T6 alloy are comprehensively investigated. The deformation behavior and fracture mechanisms are systematically analyzed by combining tensile tests and crystal plasticity finite element method (CP-FEM) simulations with distinctively initial microstructure features (fine microstructure, coarse microstructure, and coarse microstructure with pores). The results show that microstructure coarsening is presented in the notably grown α-Al dendrites, which leads to the connection and thickening of eutectic regions accompanied by the aggregative distribution of eutectic Si particles. With the coarsening microstructure, microcrack is easier to initiate in those thick eutectic regions, due to the higher stress concentration caused by the aggregated eutectic silicon. Particularly, the dominant crack propagation path changes from the trans-dendrite for fine microstructure to along eutectic regions with coarse microstructure. In addition, it confirms that the casting pores have larger detrimental effects on the tensile and fatigue properties than the microstructure coarsening in the aspects of crack initiation. Stress concentration is prone to be induced at the local edge of pore at the initial loading stage, resulting in the micro plastic deformation and lower fatigue properties. With the rapid strain accumulation in later stage, the microcracks can initiate prematurely at the local edge of pore, and thus lead to obvious decrease in tensile elongation. Moreover, a modified fatigue model considering the effects of casting pores and the microstructure difference is proposed to describe the fatigue performance of A356-T6 alloy.

A systematic review of fatigue behaviour of laser welding titanium alloys

In the existence era, the usage and need for Titanium (Ti) alloys has been increased for manufacturing weight-less products of machine-made automobile and aerospace tools with increased application of multi-material design. When compared to conventional manufacturing techniques, titanium and its alloys are categorized through better machinability, enhanced melting of temperature, great strength-to-weight ratio, lower thermal conductivity, and increased reaction to oxygen. In recent years, numerous assemblage systems have been pondered to manufacture Ti alloys but Laser Beam Welding (LBW) opens a worthwhile possibility towards other techniques for joining of titanium and alloys in industries due to its adaptability, great and explicit heat input, tractability. In optimal process circumstances, however there occurs a problem such as poor elongation and resistance to corrosion joined with lower fatigue properties, the strength-to-weight ratio of laser welded titanium alloys found to be approaching to unique material. In this study, a small review on the fatigue behaviour of laser welding titanium alloys has been analysed to enhance its mechanical properties, influence of process parameters, fracture mechanisms.

Effective Reduction of Cyclic Stresses in Coated Samples

Two condensed electron-beam copper coatings, one consisting of copper and the other of Hf/Ag/Ni/Cr microlayers, were deposited onto a titanium alloy (Ti−6Al−4V) substrate to study the energy dissipation and fatigue resistance in the uncoated and coated samples under nonbreaking and breaking cyclic stresses. The microlayer coating is peculiar in that a material with a higher elastic modulus is used in each subsequent layer (from the substrate to the outer layer) and the coating layers and submicron twins in the binder sublayer have a columnar microstructure peculiar to electron-beam evaporation and vacuum condensation. Having layered macro- and microstructure, the coating absorbs the energy of vibrations through both internal friction (between the coating layers and between the coating and substrate) and dissipation of vibration energy on numerous defects in the columnar structure (intercolumnar porosity). Resonant first- and secondmode vibrations were excited in cantilevered samples to determine how the vibration decrement depended on their maximum stresses and the breaking fatigue stresses on the number of load cycles. In addition, vibration transmission coefficients for cyclic stresses and vibration energy were experimentally defined and justified for use. One coefficient is equal to the ratio of difference in stresses between the uncoated and coated samples to stresses in the uncoated sample, relative machine power being the same in various tests. The other coefficient is equal to the ratio of difference in energy between the uncoated and coated samples to the energy of vibrations induced in the uncoated sample, relative machine power being the same in various tests. The Hf/Ag/Ni/Cr coating is shown to provide greater energy dissipation but lower fatigue properties than the Cu coating and substrate. The vibration transmission coefficients are more sensitive to energy dissipation than the vibration decrement is.

A novel brace with partial buckling restraint: An experimental and numerical investigation

For the immediate damage evaluation of a buckling-restrained brace (BRB) after earthquakes, a novel brace with partial buckling restraint was proposed. In the novel brace called the partially buckling-restrained brace (PBRB), only the edge parts of the core member are restrained, and the middle portion of the core member, the unrestrained part, is designed for visual inspection. This paper focuses on the partial restraining mechanism of the PBRB, and three PBRB specimens were tested and compared with two conventional BRB specimens. The unrestrained ratio of the width of the unrestrained part to the thickness of the same part was studied as one of the key parameters that affects the behaviour of the PBRB. Even if the PBRB shows relatively lower fatigue properties and higher compression strength adjustment factors compared with the conventional BRB, the behaviour of the PBRB is still acceptable. The PBRB with a large unrestrained ratio fails at the end of the yielding core plate, where the unrestrained buckling deformation of the core plate is significant, but the fracture of the PBRB with a small unrestrained ratio occurs around the stopper, similar to the failure mode of the BRB. Numerical results show that the maximum plastic strain concentrates in the wave valley of the ends of the yielding core plate for the PBRB with the large unrestrained ratio. To make sure that the behaviour of the PBRB is comparable to that of the conventional BRB, it is recommended that the unrestrained ratio should be less than 5.

Further Study on the Effect of Environment on Fatigue Crack Growth Behavior of 2000 and 7000 Series Aluminum Alloys

The 7000 series alloys have the highest strength in the aluminum alloys, but lower fatigue properties than 2000 series alloys. Thus, 7000 series alloys are not applied to a large proportion of the aircraft components. However, the mechanism for this has not been elucidated yet. In humid air, hydrogen embrittlement based on intergranular cracking has been known to occur in 7000 series alloys. To date, in order to explain the difference in the fatigue crack growth behavior in the two series alloys, the effect of the test environment on the fatigue crack growth of the two series alloys has been investigated in this study. A 7075-type as well as 2024-type alloy with relatively coarse equi-axed grains was T6-tempered and subjected to fatigue crack growth test in humid and dry environments. Crack growth rate at low ΔK level seemed to be larger in the 7075-type alloy than the 2024-type alloy in the humid air, when assessed by means of gradually decreasing K method. In order to clarify this result, crack growth rate of the two alloys was assessed by means of gradually increasing K method as well as decreasing K method. Crack growth rate of the 7075-type alloy in moist air was concluded to be the largest in consistent with the previous study. Thus, the large fatigue crack growth rate of the 7075-type alloy is attributable to hydrogen embrittlement.

1000N級鋼（950N/mm2級鋼）の低サイクル疲労特性と溶接H形鋼柱梁継手の構造性能

Recently, ultra high strength steel plate such as 1000N (950N/mm2 Class) class strength is focused as material of various type of steel structure. 1000N class steel has already applied in hydropower plant in order to enjoy its effect on weight and cost reduction. However, in building construction field, high strength steel remains unfamiliar material. There are only a few example of application of 780N/mm2 class strength steel in building field. Authors have already developed 1000N class steel plate and welding consumables for building. However, from viewpoint of actual application, structural performance under massive earthquake must be evaluated. Local area of column-to-beam connection seems be subject to large amount of strain when seismic event. So, in this report, low cycle fatigue properties of 1000N class steel and its weld was evaluated and structural experiments were conducted. As a result, it is revealed that Low fatigue property of 1000N class steel is equivalent to that of 780N class and can maintain the loading capacity under 26∼51 times of strain amplitude at cδp. Furthermore, it is also shown that structural performance can be estimated by test results of small specimen with strain calculation by FE analysis and finishing of edge of beam is effective for extension of lifetime through decrease of strain.

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65a and a small amount of incoherent η (MgZn2). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35a. Aging the 7050 alloy to produce particles larger than 50a gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δep/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.