CONSTRUCTION MANAGEMENT

Construction Management

Construction Management

Introduction

The fatigue damage of structural materials is the most frequent cause of the threshold state of materials. Fatigue properties of structural materials have been investigated for more than 150 years. The S-N curve (also known as Wohler curve) including fatigue limit (referred to N = 2 × 106-107 cycles) is known since 1858. According to the valid standards the fatigue data (S-N curves) are generally investigated up to N = 107 cycles of loading (steel and cast iron). It is assumed, that the fatigue limit can be determined as a horizontal line below which no failure occures. With modern developments in industry, the design lifetime of many components now often exceeds the conventional fatigue limit However, the fatigue fracture was observed also after more than billion cycles. These facts encourage the fatigue lifetime investigation in very-high-cycle (VHC) region (107 < N < 1010 cycles). Fatigue initiation mechanism in high-cycle (HC) and VHC region is the topic of increasing importance due to rising demands on lifetime of engineering components. In contrast to low-cycle (LC) and conventional HC fatigue, where the mechanisms of fatigue damage, crack initiation and propagation are more explored, progress in VHC fatigue is highly desirable and the S-N curve in VHC life region must be determined in order to guarantee the real fatigue strength and the safe life of components.

Bearing steels and austempered ductile iron (ADI) are very essential structural materials due to their enhanced mechanical and technological properties. Bearings are important machine parts, which have to sustain extremely high number of loading cycles during their lifetime. ADI is used for strongly dynamically loaded machine parts, e.g. gear and traversing wheels, crankshafts of motor-cars, vans and trucks, rail brakes, pressure pipes in oil industry, in civil and military area. Fatigue lifetime of both materials is therefore very important from the viewpoint of reliability and safety during the operation.

Task 1:

Fatigue properties of high strength materials such as bearing steels and ADI generally depend strongly on the surface state and on structural defects. Defects on the surface—small notches, scratches, impressions and roughness can be usually successfully eliminated by suitable technological processes. On the other hand, various microstructural defects in the bulk of the material are always present and determine the fatigue crack initiation and consequently the fatigue lifetime. Fatigue cracks initiate predominantly in their vicinity.

Fatigue lifetime behaviour of bearing steel 100Cr6 obtained using rotating bending loading. It may be described by a typical “two-stage” or “stepwise” S-N curve. Similar behaviour was reported also by other authors. They observed “two-stage” S-N curves having a horizontal plateau in the vicinity of a conventional fatigue limit in the range between 106 and 107 cycles. The main reason of the “two-stage” S-N curve seems to be the transition from the surface crack initiation in the LC region to the internal crack initiation in the HC and VHC region.

On the other hand, experimental results obtained during tension-compression loading, , do not exhibit any plateau. Despite of the large scatter of experimental data the decrease of stress amplitude seems to cause the ...