Structural Analysis

Stress Analysis

Performed to characterize stress levels and verify that a mechanical component or assembly will safely perform, from a strength standpoint, throughout its design service life. Stress analysis tools include classical “hand” analysis as well as Finite Element Analysis (FEA).

Dynamic Analysis

MODAL ANALYSIS:
Modal Analysis is performed to determine the natural resonant frequencies and mode shapes of a structure. Resonant frequencies are an important design consideration since response to oscillating forces at or near resonant frequencies can result in high deflections and stress levels. The fundamental natural frequency is also a measure of stiffness versus mass.

RANDOM RESPONSE ANALYSIS:
Random Response Analysis is used to determine the response of a structure to input forces defined in terms of a Power Spectral Density (PSD) function. Generally, in the analysis of spacecraft, input PSD curves are based on vibration acceleration levels in the launch vehicle at the spacecraft mounting location. These launch environment PSD functions are used as input in the spacecraft FE model, and in turn, the response levels of components within the spacecraft can be obtained. RMS accelerations, velocities, displacements, stresses and strains can be calculated at any point in the FE Model.

PEI is experienced in Force Limiting Random Response Analysis, which is a method of shaping the input PSD curve to prevent over-stressing critical components during experimental random response testing.

SINE SWEEP ANALYSIS:
Frequency response analysis, or since sweep analysis, is often used to show compliance with a transportation and handling environment specification. Sine sweep analysis entails calculating structural response due to a harmonic force, where the frequency of the oscillating force is swept through a range of frequencies.

SHOCK RESPONSE ANALYSIS:
Shock Response Analysis is used to determine structural response to high energy but short time duration events. It is often used in the analysis of spacecraft to address loading due to pyrotechnic fasteners or devices that are used to trigger the separation of rocket stages and the spacecraft from the launch vehicle as well as to trigger deployable structures such as antenna and solar arrays. Shock Response Analysis is generally defined in terms of a Shock Response Spectrum (G’s vs. Frequency). Shock input can also be defined in the time domain as a displacement or acceleration event, which is more common in the analysis of shipping and handling events.

Fatigue Analysis

Structures can fail under cyclic loads that produce stress levels that are often much lower than the material’s yield or ultimate stress. Fatigue failures occur due to the development and propagation of flaws (“cracks”) in the material. Fatigue analysis is employed to calculate the useful life of a structure when subject to cyclic loading. Fatigue analysis generally applies to metallic structures since S-n data (stress vs. cycles, “fatigue-life curves”) is more readily available than for composite materials. S-n data is based on testing that determines the allowable number of cycles for a given stress level. Under simple sinusoidal-type loads, stress analysis is performed and compared to fatigue-life curves to determine either the allowable useful life or the stress level below which the structure will not fail. For more complex loading events that can be defined in terms of a fatigue spectrum (a list of number of cycles at various loads), fatigue analysis generally entails the calculation of stress levels at a given load such that cumulative fatigue damage can be determined.

Fracture Analysis

Fracture analysis is used to calculate the safe service life of a component when subject to cyclic loading. Unlike fatigue analysis, fracture analysis assumes a crack of a specific size exists and determines the propagation of the crack over service time (per stress cycle). The existing flaw size is usually the minimum detectable size based on inspection equipment to be used in the quality control and maintenance program. Fracture analysis is also used to specify areas of inspection on a component based on critical points of failure should a crack exist.

Nonlinear Analysis

When ductile metals are loaded past their elastic limit, the instantaneous modulus decreases, resulting in larger deflections and strains than linear analysis would predict. Also, very flexible structures whose stresses remain in the elastic range may require nonlinear solutions since stiffness may change as a function of displacement. Often, non-linear analysis can result in less conservative and more realistic stress results since loads can redistribute through redundant load paths.

Composites Analysis

The use of composite materials such as carbon/epoxy (carbon fibers in epoxy resin) are ideal when weight or electromagnetic transparency is critical to a design. Composites allow for structures to take on complex shapes with minimal part counts and connections, and for reinforcing fiber directions and content to be optimized for best structural performance. The analysis of composite structures entails development of effective material properties using classical lamination theory so that laminate stiffness properties can be calculated. Then, using FEA or hand calculations, laminate failure indices can be determined. Failure indices consider calculated stresses versus allowable stresses for all applicable failure modes of the laminate.

Thermal Analysis

Thermal analysis is performed to solve heat dissipation problems and characterize temperature distributions within a mechanical system. Understanding transient and steady-state heat flow is important when electrical or structural performance can be compromised by material or component temperatures being outside of their allowable range. Also, significant internal stresses can develop due to temperature differentials in a structure, especially if the structure is comprised of materials with different coefficients of thermal expansion.