Papers by Author: K. Houri

Abstract: Fatigue failure of steel occurs when cracks form in a component and continue to grow to a size large enough to cause rupture. In order to understand the crack initiation and propagation phenomena, non-destructive evaluation methods that can be correlated to in-situ measurements around the crack tips are necessary. In the present work, we developed a scanning Hall probe microscope (SHPM) equipped in a GaAs film sensor to observe fatigue cracks at room temperature in air while they were growing. Medium carbon low alloy steels specimens (JIS S45C) were used in the experiments. Only the area around the crack tip was magnetized and the changes at the crack tips were observed.

Abstract: Fatigue failure of steel occurs when cracks form in a component and continue to grow to a size large enough to cause fracture. In order to understand the strength of a steel component, it is important to locate these cracks. We developed a scanning Hall probe microscope (SHPM), equipped with GaAs film sensors to observe fatigue cracks at room temperature in air while they were growing. In our previous works [1,2], the correlation between crack growth and magnetic field in high carbon tool steels (JIS SKS93 and JIS SUJ2) were determined. We also reported the sensitivity of the SHPM equipped with a three-dimensional line-probe that was developed to decrease the sensor gaps. By using the line-probe sensor we succeeded to measure the magnetic flux density distributions in very close proximity to the specimen’s surface. However, in order to further understand the relation between magnetic flux density and crack growth, other materials, microstructures and fatigue test conditions should be evaluated. In the present work, we focus on the effect of stress ratios on the changes of the magnetic flux density in annealed carbon tool steel.

Abstract: Radial ball bearings made of metal, ceramics and plastics are commonly used as important components in industrial machinery. Usage of high performance engineering plastic polymers is increasing progressively as a replacement for metal components due to the latest markets demands. Poly-ether-ether-ketone (PEEK) is a promising material for precision-machined custom bearings, products that are expected to suit special market needs. In the present study, PEEK radial ball bearings were manufactured by lathe machining under different parameters and their rolling contact fatigue (RCF) resistance under water lubricated conditions was investigated. We observed the surface of the bearings prior and after testing by laser confocal microscope. The wear loss was measured by weighing the bearings before and after test. Cracks and/or flaking failures were identified on the bearing surface after testing. From the RCF tests results, we found that, at water lubricated conditions, crack initiation occurred later in the material that was machined at slower feed rate while at dry condition, the feed rate had little influence on the wear loss and cracking. Wear loss in the case of bearings tested under water was much less severe than that of bearings tested at dry conditions.

Abstract: In this work, two SAE 52100 grades with different oxygen contents were repeatedly quenched and the influence of this cyclic heat treatment as well as the role of the oxygen content level on the mechanical properties of the material were investigated by rolling contact fatigue tests. The repeated quenching process increased the retained austenite content and had little influence on the material’s hardness. The prior austenite grain size was decreased and consequently, refinement of the martensitic phase in both types of materials occurred. The higher content of the retained austenite (higher fracture toughness) and the refinement of the microstructure accounted for the higher fatigue properties of the repeatedly quenched material. Flaking failure was present in the material quenched only once, after 4.3x107 cycles (higher oxygen content) and 108 cycles (lower oxygen content). Flaking failure did not occur in the samples treated by repeated quenching. The results show the importance of both optimized heat treatment and alloy cleanness for the lifetime of bearing steel.