PROBLEM TO BE SOLVED: To provide acceleration degradation testing method enabling a thermomechanical reliability estimation of a metallized heat radiation substrate to be performed in a short period of time, and a four-point fatigue testing jig suitable for use in this method, and a fatigue testing device using this jig.SOLUTION: A four-point bending fatigue test jig 1 includes: an inner pin holding part 2 having a pair of a lower inner pin 201 and an upper inner ping 202; a mobile unit 3 capable of fixing the inner pin holding part 2 while abutting against an upper end part of the upper inner pin 402; and a fixing unit 4 having a pair of a lower external pin 401 and an upper external pin 202. The mobile unit 3 is vertically movable with respect to the fixing unit 4. A stress equivalent to heat residual stress due to temperature change during a heat cycling test for a plat test piece 5 is loaded to the test piece 5 using the four-point bending fatigue testing jig 1 by the mechanical stress load, and the life of the testing piece 5 by the heat cycling test is estimated based on the life of the test piece 5 by the four-point bending fatigue test.SELECTED DRAWING: Figure 1
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