This study was experimentally carried out through the Fine Dot Grid Strain Measurement Method using the Fine Grid Image Processing System to obtain a new and effective dynamic parameter, ΔA which would dominate the fatigue crack propagation behavior. In this experiment, the local fatigue strain distribution was examined at real time in the local area near the crack tip where the fatigue fracture happended actually.
Fatigue crack propagation rate, da/dN was newly evaluated by using the new parameter of fatigue strain intensity factor, ΔA. And also, the experimental constant, C in the evaluation equation can be easily estimated by the material constants. Therefore, the newly proposed evaluation equation can be used to estimate the fatigue crack propagation behavior of the structure or members in random load.
The results obtained are summarized as follows :
1. The Fine Grid Image Processing System was designed to estimate quantitatively the fatigue strain distribution in the local crack tip area.
2. The Fine Dot Grid Strain Measurement Method using the Image Processing System could give the powerful and useful means to measure the fatigue strain distribution in the local crack tip area.
3. The fatigue strain state in the local crack tip area(r≤100 ㎛) can be expressed by the following equation ;
ΔAεeq=ΔA · f(θ) · r-1
f(θ)=cos(θ/2) · (1+3/2sin²θ)
4. The fatigue strain intensity factor, ΔA, was an effective and unique parameter to indicate the fatigue strain state of the local crack tip area regardless of the fatigue conditions and the materials. And its value depended upon the type of materials and the fatigue load level.
5. The result correlating the fatigue crack propagation rate(da/dN) with the fatigue strain intensity factor(ΔA) was different to those with AK or in that it was not influenced by stress ratio, and in that all the data points lay on a single line.
6. Regardless of the materials or material properties, the relationship between the fatigue crack propagation rate(da/dN)and the fatigue strain intensity factor (ΔA) showed the heavily reduced scattering to present its mechanical usefulness.
7. The experimental equation showing the linear relation between the da/dN and the ΔA in log-log scale can be proposed as below ;
da/dN=C · (ΔA)n
where C, n : experimental constants
8. The n in the above equation was about 2.0 regardless of the materials and stress ratio. And a simple equation was established to estimate C from the material properties.
Finally, the proposed equation using ΔA can give the powerful and unique means to evaluate the fatigue crack propagation rate.
C=235X10-5 · (σys/JIC)0.35
da/dN=2.35X10-5 · (σys/JIC)0.35 · (ΔA)2n