ASME STP-PT-081:2017 pdf download

ASME STP-PT-081:2017 pdf download CYCLIC STRESS-STRAIN CURVES
1 INTRODUCTION AND DESCRIPTION OF THE PROBLEM
Cyclic stress-strain curves describe the stress-strain behavior under cyclic loads. Usually, the cyclic stress amplitude Δσ/2 is plotted as a function of the cyclic strain amplitude Δε/2 for a defined cycle. The fact that the stress-strain response of a material is usually cycle-dependent requires a reference cycle which can be considered as representative for the cyclic stress-strain curve.
In contrast to monotonic stress-strain curves which deliver a unique relationship between the stress and strain of a material, the cyclic stress-strain relationships may undergo cycle-dependent changes. The material can be:
Cyclic hardening
Cyclic softening
Cyclic stable
Combinations of cyclic softening and cyclic hardening
This means that the stress-strain relationship determined in strain fatigue tests is usually cycle dependent.
Figure 1-1 shows the change of stress amplitude with a number of cycles for a low carbon steel as an example [1].
When a cyclic stable hysteresis loop is obtained, this stabilized loop can be used as a reference. If a cyclic stable hysteresis loop is not established, the cycle at half the time to rupture (Nf/2) is taken as a reference independent of whether the material is cyclic softening, cyclic hardening or shows a mixed behavior. These cycles are usually obtained from reversed strain cycling tests on a number of companion specimens, but shortcut procedures are also used by various investigators. Such shortcut methods use only a single specimen, which is cycled a certain number of times or until saturation is reached. The levels of cyclic straining are stepwise increased (incremental step test). This means that cyclic pre-deformed samples are used, which can lead to artifacts in cases where cycle-dependent microstructural changes can happen. It is also worth mentioning that low cycle fatigue (LCF) is primarily crack growth from short cracks, which can also affect the behavior of pre-deformed material. The fact that materials can cyclically harden or soften makes the relationship between cyclic and monotonic curves important.
A cyclic stress-strain curve alone, without any relation to the monotonic properties, is only of very limited use for design or safety considerations.
An independent choice of a cyclic stress-strain curve without reference to the monotonic behavior might cause misleading results. Cyclic softening material can appear as cyclic hardening and vice-versa (an example from ASME BPVC Section VIII/2 is given in Appendix C:). Therefore, relationships between monotonic and cyclic properties are required. General trends for cyclic hardening/softening of different classes of materials are given in [3]. Figure 1-3 illustrates typical examples of monotonic and cyclic stress- strain curves. Austenitic matrices tend towards cyclic hardening, whereas ferritic/martensitic materials tend towards cyclic softening.
A few attempts for the derivation of cyclic stress-strain curves exist (e.g., [4], [5], [6]). Particularly, the relationships between monotonic and cyclic yield strengths were analyzed in these investigations. Results are shown in Figure 1-4 and Figure 1-5. Figure 1-4 shows data for ferritic/pearlitic, martensitic, and austenitic steels. Ferritic/pearlitic steels tend to behave cyclic stable to slight cyclic softening. The martensitic steels are cyclic softening. Austenitic steels show cyclic hardening behavior, however, a wide scatter of data can be seen. This will be further discussed in section 7 concerning austenitic steels. The results shown in Figure 1-5 indicate that carbon and low alloy steels start cyclic stable at low yield strength and become cyclic softening at high yield strength. These results will also be discussed later in the report.