Orientation effects in creep
Creep in materials refers to the gradual strain extension of that material under sustained load over extended time periods. In metals, creep is of particular concern because it typically only manifests over long time periods, for example ten to twenty years, and at temperatures which are in excess of approximately 40% of the melting point. However at higher temperatures or high stress levels the period in which damage manifests can reduce significantly.
After such long periods and such duration of usually satisfactory performance, the perception of any danger from an erstwhile perfectly satisfactorily operating component or system is often overlooked. However, it is exactly at this stage that creep scenarios of this sort need to be considered. At a microstructural level the creep damage is manifest by dislocation movement and agglomeration to grain boundaries, in such a manner as to attempt to alleviate the very driving stresses or forces. For example, a high pressure, thick wall pipe at sustained pressure, (and at creep related temperatures) approaching 40% of melting temperature, will be subject to such creep re-distribution of dislocations, in a manner to attempt to ameliorate the stress by changing its dimension in the direction of the principal stress. In a thick walled pipe this would be in the hoop direction, where the hoop stress is typically twice the stress in the longitudinal direction.
If the temperatures are indeed in the creep range then dislocation activity is such that dislocations from the bulk material tend to gravitate to grain boundaries and particularly those grain boundaries that are normal to the principal stress. So in the case of a high pressure pipe the largest stresses would be in the hoop direction (typically twice those of stresses in a longitudinal direction). With sufficient time at creep temperatures this damage would agglomerate in strings of local grain boundary damage located predominantly in a longitudinal direction, perpendicular to the principal stresses (in a hoop direction). If sufficiently extensive such damage would lead to a reduction in fracture toughness increasing the propensity for cracking in the longitudinal direction, as opposed to cracking in the hoop, or circumferential, direction. This deterioration in fracture toughness as a function of orientation for stresses in the hoop direction compared to the longitudinal direction, would only be significant after sustained high temperature exposure for long enough times.
The creep behaviour can be well characterised by the three well known stages of initial Primary creep (I), Secondary creep (II) (generally a linear degradation with time), leading ultimately to (III) Tertiary creep, where dimensional change increases at an exponential rate, which can lead to complete collapse and catastrophic failure. This three stage creep development is well known and illustrated in the figure below, showing primary (I), secondary (II) and tertiary (III) creep. However, of more importance perhaps, from a microstructural view point, is the agglomeration of dislocation pile up damage predominantly at grain boundaries located approximately perpendicular to the principal stresses. In the case of a creep stressed pipe, this would be cracking more in the longitudinal direction (perpendicular to high hoop stresses) than cracking in the hoop direction. In due course at its extreme such microstructural damage would manifest as complete cracks and lead to longitudinal splitting of the pipe. At an earlier stage, however, before complete splitting and catastrophic rupture, there is a change in cracking resistance, as a function of orientation, and this is most easily manifest by changes in fracture toughness of the pipe steel, as a function of orientation. This could readily be measured and ‘fitness for purpose’ quantified in such pipes exposed to such creep damage.
The message then is to be aware of those pipe (or pressure vessel) application subject to creep conditions, (sustained load at or above 40% of melting temperature, for long periods of time (several years) ) and assess them from a structural integrity/Fracture Mechanics perspective.
Note: In the figure creep damage, in the form of voids or cavities, in the secondary and particularly tertiary creep range, agglomerates mostly in the vertical direction (in this figure), perpendicular to the sustained principal stress.
Published in Technical Tips by Origen Engineering Solutions on 1 October 2017