Multiaxial and uniaxial vibration experiments were conducted in order to study the differences in failure modes and fatigue life for the two types of excitation. An electrodynamic (ED) shaker capable of controlled vibration in six degrees of freedom (DOF) was employed for the experiments. The test specimen consisted of six large inductors insertion mounted on a printed wiring board (PWB). Average damage accumulation rate (DAR) in the inductor leads was measured for random excitations in-plane, out-of-plane, and both directions simultaneously. Under simultaneous multiaxial excitation, the average DAR was found to be 2.2 times greater than the sum of the in-plane and out-of-plane DARs. The conclusion was that multiple-step sequential uniaxial testing may significantly overestimate the durability of large/heavy structures with high center of mass in a multiaxial dynamic environment. Additionally, a test method utilizing uniaxial vibration along a direction other than the principal directions of the structure was examined. This method was found to have significant limitations, but showed better agreement with simultaneous multiaxial vibration experiments.

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Time-domain dynamic analysis of vibratory systems becomes useful in finite element analysis (FEA) when the structure’s response can no longer be assumed linear, as frequency-domain (spectral superposition) methods require. Time-domain analysis also permits the use of cycle-counting methods when assessing the vibration durability of electronic assemblies. The analyst is often limited to simulating only the first few cycles of the vibration response in very complex models, to minimize the computational burden. However, the accuracy of time domain analysis can be questionable during these first few cycles, due to unwanted transients, unless the initial conditions are properly modeled to correctly produce the steady state response. This paper explores this sensitivity to initial conditions for undamped and damped structures. Strategies for calculating and implementing proper initial conditions within FEA are discussed. Two illustrative examples are presented for simplicity. The first consists of a simple cantilever beam so that the numerical results can be compared to known analytic solutions and the basic theory can be demonstrated. The second example is a 2D representation of a circuit card assembly containing multiple leadless chip resistor components, so that implementation details can be demonstrated for more complex structures. This paper is intended to have tutorial value to FEA users who have to conduct time-domain dynamic analysis.