The importance of understanding loading, stresses and strains in structures

APRIL 2015

Poor understanding of the loading and resulting stresses / deflections in a structure may lead to damage or premature failure.

Correct understanding of the applied loads, and stresses induced by these loads in a structure, is vital when designing or modifying structures. 
Both simple analytic and numerical (FEA) analyses of the stress provide valuable insight into the distribution of stresses and strains throughout a structure.  Such analyses, undertaken by suitably qualified engineers/specialists are undoubtedly valuable tools, but are only as good as assumptions of how/where loading is applied. 

In reality actual stresses may differ significantly from those predicted by the stress analysis as the actual vs. 'as modelled' correlation is heavily dependent on the validity of the modelling assumptions.  These assumptions include, inter alia i) how, where and what loads are applied, ii) the magnitude of these loads, iii) the boundary conditions (are 'simply supported', 'built in' or 'point load' assumptions valid), iv) the extent of modelling (which areas of the component are modelled), v) how joints are modelled (contact , friction), vi) differences between actual  and as manufactured geometry and vii) actual loading applied. 

Full understanding of the applied loads is non trivial and should not be overlooked.  Ideally sensitivity analyses and testing the effect of a number of potential loading scenarios should be undertaken.  Not only do such analyses allow calculation errors to be highlighted, they also allows a feel for the structure/component to be developed.  'First principle' type analyses should not be discounted as these allow the various parameters influencing the effect of changing variables to be assessed (especially when solutions are solved symbolically).

Experimental load/stress measurement (temperature measurement, strain gauging and DIC) provide the opportunity to analyse and determine local stresses and strains at predetermined positions as a function of the actual loading that is applied. 

Such measured temperatures/strains analysed using FEA techniques, allow the actual performance to be determined and will highlight if there are areas of high stress or unforeseen loading conditions.

Published in Technical Tips by Origen Engineering Solutions on 1 April 2015