Materials Science Forum Vols. 654-656

Abstract: The split Hopkinson pressure bar (SHPB) is the most commonly used technique to characterize the dynamic behaviour of materials at very high strain rates. However, a classic single specimen test only generates a single stress-strain curve at the average strain rate of the test. This paper proposes three arrangements on the use of double specimens in SHPB compression testing. All waves propagating along the bars have been used to analyse the dynamic behaviour of the specimens. To simulate the test and predict its dynamic performance, an axisymmetric finite element analysis using LS-DYNA was conducted for the experiment using 13 mm bar diameter. The validity of the simulations was checked with experimental data from normal SHPB testing. Based on the simulations, the modified techniques are achievable and at least two stress-strain curves of materials can be extracted without violating the requirement of a valid SHPB test.

Abstract: In order to evaluate the fracture characteristic near the crack tip of beryllium specimen, beryllium compact tension specimen with plane strain state is designed. The stress distribution near the crack tip is measured at different loading level by X3000 stress analyzer. Moreover, a finite element model for calculated the stress and strain fields in beryllium compact tension specimen has also been set up. As a result, the stress and strain distribution near the crack tip at different loading has been calculated by this model. According to the critical tension loading of beryllium specimen, the maximum plastic strain and the radius of the plastic zone near the crack tip are determined, where the maximum plastic strain near the crack tip is about 0.018 and the maximum radius of plastic zone is about 0.3mm. Altogether, the fracture toughness of beryllium is obtained, which is about 19.1MPam1/2.

Abstract: Rolling Contact Fatigue (RCF) damage on the surface of rails such a head check, squats is a growing problem. Since rail fractures can cause derailment with loss of life and property, the understanding of rail fracture mechanism is important for reducing damages on the rail surface. In this study, we have investigated RCF damage, fatigue growth and fracture surface morphology on the surface of broken rail using failure analysis and finite element (FE) analysis. The investigation indicates that the crack grows at about 20° to the depth of 8mm from the surface and branches into two cracks. One crack propagates downward at about 47°, the other propagates upward. Since the crack growth rate of the downward crack was faster than that of upward crack, rail eventually was broken. Since the downward branches lead to fracture of the rail, they are more dangerous to the integrity of rails. It has been observed that White Etching Layer (WEL) occurs within the surface of broken rail. It was found that the fatigue crack initiation and propagation was accelerated by WEL.

Abstract: Upon investigation of the damaged wheels it was determined that the cracking was caused by thermal fatigue during on-tread friction braking. The thermal cracks appear as short cracks oriented axially on the wheel tread. Severe heating of the wheel tread during braking was believed to be a contributing the variation of residual stress which is related to wheel failure. It is necessary to evaluate the residual stress due to deterioration of wheel tread in order to ensure the safety of wheel. In the present paper, the residual stress of railway wheel for deterioration using x-ray diffraction system is evaluated. The result shows that the residual stress of wheel is depend on the running distance and the residual stress needs to be inspected between the wheel diameter of 800 and 780mm.

Abstract: Wheels of the railway vehicle play the important role for driving train through wheel-rail interaction. Especially wheel profile is one of the most important design factors to rule the running stability and safety of train. Accordingly, the control of rolling contact fatigue-related defects is an ongoing concern for both safety and cost reasons. This process is referred to as ratcheting. Wear of wheel and rail surfaces occur due to a mixture of adhesive, abrasive and corrosive processes. In wheel/rail systems with little wear, such failure is manifested by the appearance of closely spaced micro-cracks. In the present paper, a evaluation of surface defects of wheel and rail for Korean high-speed railway. The main research application is the wheel-rail maintenance of Korea high-speed train.

Abstract: The degradation and failure of protective coatings (paints and sealants) is a key element influencing the service life of aircraft. Such degradation is influenced by the response of coatings to environmental factors such as high temperatures and exposure to ultraviolet radiation, as well as chemical factors. However, the effect of loading and load history on coating durability has received little attention, despite clearly being a factor in determining failure sites (such as joints) and the rate of degradation. This paper describes the key characteristics of coatings at aircraft joints, and the nature of the strains experienced by coatings in locations influenced by in-service loads. It is first step in assessing the complex strain history at joint strain concentration locations as part of developing a prognostic capability for the service life of aircraft coatings. The configuration of coating layers at different joints is important and this research has considered a simplification of a butt strap joint from a RAAF military aircraft and a generic lap joint; predictions of critical movements/displacements have been made using finite element analysis; the predictions will be tested later as part of an experimental program associated with a full-scale fatigue test.

Abstract: While many fixed-wing aircraft have adopted damage-tolerant design in recent years, helicopter design is still based predominantly on a safe life approach, in which relatively simple Stress Life (S-N) data underpins the tools used for life prediction. Due to their unique loading, helicopter structures experience a high number of loading cycles as compared to fixed-wing aircraft, and this presents a more challenging fatigue life management problem. To minimise the fatigue damage, the helicopter community tends to design components such that most of the loading experienced falls below the fatigue limit of the selected material. These materials are usually of high strength and have good fatigue properties, although the large number of cycles experienced by some components raises the possibility of fatigue in the “gigacycle” regime where the fatigue limit drops to a new, lower level. This paper discusses the suitability of a high-quality PH 13-8 Mo steel for critical helicopter usage, using a simulated application in Australian service to evaluate its fatigue performance particularly at high R ratio and other properties such as density, corrosion properties and cost in terms of the operational environment experienced in Australian helicopter operations.

Abstract: The laser surface remelting (LSR) process was successfully applied to restore localized corrosion resistance in sensitized stainless steel and also as a useful method to improve passivity of some martensitic stainless steels. The LSR process can be successfully applied to repair cracks and defects at the surface of highly thermo-mechanically loaded parts of stainless steel. The purpose of presented study was to evaluate the microstructure and properties of laser remelted surface of stainless steels. The wrought austenitic stainless steel and sintered in vacuum 316L type were studied. The laser treatment was performed with the use of high power diode laser (HPDL) and the influence of beam power of 0.7-2.1kW on the properties of the surface layer was evaluated. The geometrical characteristics and x-ray analysis of weld bead were studied as well as microhardness, surface roughness and corrosion resistance were measured. The increase of laser beam power of LSR resulted in the increase of hardness of sintered stainless steel due to the reduction of porosity and formation of fine dendritic and cellular-dendritic microstructure. The corrosion resistance of remelted surface increased for sintered materials, when remelted at 2.1kW. The wrought stainless steel revealed impairment of pitting corrosion when remelted at lower beam power rate.

Abstract: The mechanism of toughness degradation during slow cooling in the austenite range was studied in CA6NM stainless steel, 13% Cr-4% Ni soft martensitic stainless steel. The variation of toughness, fracture mode and microstructural features were examined by means of cooling rate and isothermal heating in the austenite range together with chemical composition. Toughness degradation was referred to as the increases of FATT and intergranular fracture when those steels were cooled slowly after austenitizing and isothermally heated in the austenite range. The embrittlement was found to be related the intergranular fracture and the precipitation of carbide along prior austenite grain boundaries. Its fracture surface was characterized by mosaic-like markings when the carbide precipitation got to increase. Reducing carbon, silicon and phosphorus and increasing molybdenum improve the toughness degradation.

Abstract: Hydrogen embrittlement sensitivity of austenitic stainless steels, SUS316L and SUS310S exposed to high compressed hydrogen gas atmospheres was evaluated by means of a slow strain rate testing (SSRT) in air. Hydrogen evolution behavior during tensile deformation and fracture was also investigated by using a testing machine equipped with a quadrupole mass spectrometer installed in an ultrahigh vacuum chamber. When the SUS 316L specimen with hydrogen gas charging were deformed at a very slow crosshead speed of 1.67 nm/s, local deformation was promoted as compared to the specimen without hydrogen gas charging. On the other hand, no decrease of the ductility was observed in the SUS310S specimen with hydrogen gas charging even in the SSRT. In the hydrogen charged SUS316L specimen, the amount of continuous hydrogen evolution throughout deformation was much higher than that in the specimen without hydrogen gas charging. In addition, sudden hydrogen evolutions were sometimes identified in the SUS316L specimen with hydrogen gas charging during the deformation.