Anchalee Manonukul

Interaction of plastic zone, pores, and stress ratio with fatigue crack growth of sintered stainless steel

The relationships between plastic zone, pores, fatigue crack growth rate (da/dN), and fracture mechanics parameters of metal injection molding (MIM) 316L stainless steel at various stress ratios (R = 0.1 and 0.4) are investigated in the present work. Using two-dimensional (2D) plane strain finite element analysis (FEA), the stress intensity factor and cyclic-plastic zone at crack tip are evaluated. The slow-growth da/dN at near-threshold regime is in brittle manner, and can be characterized by maximum stress intensity factor (Kmax). While, the mid-growth da/dN at Paris regime is in ductile manner, and can be characterized by stress intensity factor range (ΔK). As a fracture mechanics parameter that combines Kmax and ΔK, the K* successfully characterizes the stress ratio effect on FCG rate at near-threshold and Paris regimes. The transition from slow-growth da/dN to mid-growth da/dN occurs when the cyclic-plastic zone at crack tip coalesces with pore. The appearances of fracture surfaces at near-threshold and Paris regimes are in good agreement with the FEA results.

Fretting Fatigue with Cylindrical-On-Flat Contact: Crack Nucleation, Crack Path and Fatigue Life

Fretting fatigue experiments and finite element analysis were carried out to investigate the influence of cylindrical-on-flat contact on crack nucleation, crack path and fatigue life of medium-carbon steel. The location of crack nucleation was predicted using the maximum shear stress range criterion and the maximum relative slip amplitude criterion. The prediction using the maximum relative slip amplitude criterion gave the better agreement with the experimental result, and should be used for the prediction of the location of crack nucleation. Crack openings under compressive bulk stresses were found in the fretting fatigues with flat-on-flat contact and cylindrical-on-flat contacts, i.e., fretting-contact-induced crack openings. The crack opening stress of specimen with flat-on-flat contact was lower than those of specimens with cylindrical-on-flat contacts, while that of specimen with 60-mm radius contact pad was lower than that of specimen with 15-mm radius contact pad. The fretting fatigue lives were estimated by integrating the fatigue crack growth curve from an initial propagating crack length to a critical crack length. The predictions of fretting fatigue life with consideration of crack opening were in good agreement with the experimental results.

Elastic-plastic fracture mechanics approach for stress corrosion cracking of nickel aluminium bronze under ammonia-containing artificial seawater

With the growth of urbanization and industries, the seawater near coastal areas has become polluted, and the nickel aluminium bronze components around coastal areas are affected by ammonia-containing seawater. Unfortunately, the influence of the ammonia concentration in seawater on the stress corrosion cracking of thin nickel aluminium bronze components with large plastic zones at the defects has not been evaluated before. In the present work, stress corrosion cracking experiments on nickel aluminium bronze components under artificial seawater and ammonia-containing artificial seawater were conducted using a four-point bending technique. The elastic–plastic fracture mechanics parameter (J-integral) was evaluated using finite element analysis. The J-integral successfully characterized the crack growth rate under the present corrosive environments. Stress corrosion cracking was possible under both artificial seawater and ammonia-containing artificial seawater. The threshold J-integral for susceptibility to stress corrosion cracking (JSCC) and fracture toughness (JC) was the highest for stress corrosion cracking under artificial seawater and decreased as the amount of ammonium hydroxide added to the artificial seawater increased.

Use of Deep Cryogenic Treatment to Reduce Particle Contamination Induced Problem in Hard Disk Drive

In this study, the effect of deep cryogenic treatment on the generation of stainless steel particles in screw tightening process in hard disc drive assembly was investigated. During the cryogenic treatment, the specimens of both stainless steel screw and contacting tool (called as “bit”) material were quenched in a chamber containing liquid nitrogen at-196 oC with the soaking times of 33 hr. The specimens were then subjected to sliding wear tests under normal loading conditions. The experiments used for simulating dry sliding wear mechanisms were carried out by TriboGear machine. The machine consists of a stationary bit loaded against the plate containing screw. The screws used were made of martensitic 410 stainless steel and the bit was made of S2 tool steel. The experiments were carried out under both under single and multiple loading cycles under the normal load corresponding to the effective stresses higher and lower than the yield strength of screw material. The results showed that the deep cryogenic treatment led to more homogeneous distribution of fine size carbide particles in both martensitic 410 stainless steel and S2 tool steel. This lead to different failure mechanism of the stainless steel resulting in smaller and slender stainless steel particles generated. This was expected due to the effect of the change in the dimension of carbide, the stress distribution in the material and the crack propagation path.

Effect of weld line positions on the tensile deformation of two-component metal injection moulding

Knowledge of the mechanical properties of two-component parts is critical for engineering functionally graded components. In this study, mono- and two-component tensile test specimens were metal injection moulded. Three different weld line positions were generated in the two-component specimens. Linear shrinkage of the two-component specimens was greater than that of the mono-component specimens because the incompatibility of sintering shrinkage of both materials causes biaxial stresses and enhances sintering. The mechanical properties of 316L stainless steel were affected by the addition of a coloured pigment used to identify the weld line position after injection moulding. For the two-component specimens, the yield stress and ultimate tensile stress were similar to those of 316L stainless steel. Because 316L and 630 (also known as 17-4PH) stainless steels were well-sintered at the interface, the mechanical properties of the weaker material (316L stainless steel) were dominant. However, the elongations of the two-component specimens were lower than those of the mono-component specimens. An interfacial zone with a microstructure that differed from those of the mono-material specimens was observed; its different microstructure was attributed to the gradual diffusion of nickel and copper.

Investigation of Nickel Aluminum Bronze Alloy under Hot Compression Test

The characteristics of Nickel Aluminum Bronze alloy (NAB) after hot deformation were investigated. The NAB alloy have been studied by dilatometer according to study the influence of hot deformation on microstructure of NAB alloy by dilatometer in the temperature range 800 - 950 °C, strain rate 10s-1 and cooling rate 40 and 100 °C/s. The experimental results showed that peak stress in relation to the involved deformation temperature, peak stresses at a constant strain rate decreased with an increase of deformation temperature. It was found that volume fraction of the β phase significantly increased with increasing temperature and cooling rate. The variation of this phase affected macro hardness of the investigated alloy. By higher temperatures, amount of β phase increased as well as the macro hardness of the NAB alloy.

Mechanical Properties at High Temperatures and Microstructures of a Nickel Aluminum Bronze Alloy

Nickel Aluminum Bronze (NAB) alloys have been widely used in different kinds of machine parts where the superior resistance to corrosion and erosion in saltwater is needed. In this work, mechanical properties at high temperatures and microstructures of a NAB alloy were investigated. First, NAB specimens were prepared as an as-cast ingot and were subsequently heat-treated at 675°C for 6 hours in order to improve microstructure and mechanical properties. The mechanical properties at high temperatures in form of the plastic flow curves of the NAB alloy were characterized by a deformation dilatometer. The NAB samples were compressed at high temperature and rapidly cooled down to room temperature. The deformation temperatures of 825°C, 850°C and 900°C, a strain rate of 0.01 s-1, and a maximum compression strain of 0.4 were considered. The influences of the temperature on flow behavior of the NAB alloy were investigated. The plastic stress-strain curves at different temperatures were compared with regard to the rate of material strain hardening and softening. It was found that the compression stresses decreased with increasing temperatures. Additionally, the resulted hardness and microstructures of the alloy after forming at high temperatures were analyzed.