6.3 Residual stress
Residual stresses are those that reside in a component or structure in the absence of an applied load. They generally form as a result of a local mismatch in shape, such as thermal gradients or local deformation. When considering the structural integrity or mechanical performance of a component, residual stresses act in addition to applied loads. Therefore, residual stresses should be treated in the same way as applied stresses when assessing a component's fitness for purpose.
Most of the AM techniques include some form of local thermal gradient and, if not, local phase or volume change may well be occurring. As a result, residual stress is a problem with most AM techniques. This is not a new problem because it was a problem for RP systems, however although significant research went into trying to solve it, little has been achieved.
To illustrate the issue with residual stress, take FDM as an example. Assume you are trying to produce a tall, thin wall consisting of a single bead of polymer. Half-way through the process you will have something that resembles Figure 27.
Assume the uppermost layer has cooled to room temperature. As a new, hotter layer is deposited onto the cooler layer, there is a temperature difference and therefore a difference in density. However, because the new layer is laid across the entire previous layer and is fused, its contraction will now be constrained (Figure 28). The contraction results in the generation of residual tensile stress.
Click on ‘View interactive version’ on Figure 28 to compare what happens to a new layer that does fuse and one that doesn’t fuse when the temperature decreases.
This example of a thin wall is considerably simpler than most true applications. It is easy to imagine more complex stress fields with a more complex part. Several attempts have been made by different manufacturers to mitigate the formation of residual stress. Evidence of the effectiveness of these processes is limited and complete mitigation of residual stress is unlikely. Whenever there is a local change in temperature, material or phase, it is almost certain there will be a residual stress.