Fatigue of Metals
Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA
The importance of studying metal fatigue in civil infrastructure projects was brought into the spotlight by the collapse of the Silver Bridge in Point Pleasant, West Virginia in 1967. The eyebar-chain suspension bridge over the Ohio River collapsed during evening rush hour, killing 46 people as a result of the failure of a single eyebar with a small 0.1-inch defect. The defect reached a critical length after repeated loading conditions and failed in a brittle fashion causing the collapse. This event garnered attention in the bridge engineering community and highlighted the importance of testing and monitoring fatigue in metals.
Under normal service conditions, a material can be subjected to numerous applications of service (or everyday) loads. These loads are typically at most 30%-40% of the ultimate strength of the structure. However, after the accrual of repeated loadings, at magnitudes substantially below the ultimate strength, a material can experience what is termed fatigue failure. Fatigue failure can occur suddenly and without significant prior deformation and is linked with crack growth and rapid propagation. Fatigue is a complex process, with many factors affecting fatigue resistance (Table 1). This complexity underscores the integral need for routine and thorough inspection of structures subjected to repeated loadings such as bridges, cranes, and almost all types of vehicles and aircrafts.
| Stressing conditions | Material properties | Environmental conditions |
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Table 1. Factors affecting fatigue
- Obtain five A572 Grade specimens with dimensions and machine configuration appropriate for the Moore rotating beam machine being used. In this case we will use a rotating cantilever setup with specimens 2.40 in long and 0.15 in. in diameter with a small necked section 0.50 in. long and 0.04 in. in minimum diameter.
- For the specimen dimensions and machine configuration, calculate the weight required to produce bending stress ranges equal to ±75%, ±60%, ±45%, ±30% and ±15% of the nominal yield stress o
The final results, in terms of stress range vs. number of cycles, should be tabulated (Table 2) and plotted, as demonstrated in Fig. 2. The actual yield stress of the specimen was 65.3 ksi and its tensile strength was 87.4 ksi so the stress ranges shown here correspond to between 23% and 92% of yield.
