Influence of Residual Stress on the Initiation of Fatigue Cracks at Welded Piping Joints
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Date
2004-05-21
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Abstract
Fatigue failures of small bore piping systems have historically occurred in nuclear power plants, resulting in unanticipated plant downtime and substantial financial loss. These failures have been reported with increasing frequency over the past 20 years and have motivated the research described in this thesis. Recent research at North Carolina State University (NCSU) pointed to the strain ratcheting response of the welded joint as a probable reason for fatigue failure and indicated that welding residual stresses might be responsible for inducing this strain ratcheting response. It was the primary objective in this investigation to determine what happens to residual stresses at the welded piping joint under the application of low-cycle fatigue loading and to understand how residual stresses induce strain ratcheting and thus affect the fatigue life of the welded joint.
In order to achieve stated objectives, a systematic set of residual stress measurements and low-cycle fatigue tests was conducted. Initial residual stresses near the weld toe of six welded piping specimens were measured using the technique of x-ray diffraction. The specimens were then loaded in low-cycle fatigue to intermediate points in their fatigue lives and residual stresses were measured again. Strain response data near the weld toe was gathered throughout specimen fatigue life. Residual stress data and recorded strain responses in fatigue prompted conclusions concerning the role of residual stresses in inducing strain ratcheting which were verified by additional material level experiments.
Initial residual stress measurements obtained from the welded specimens revealed that residual stresses near the weld toe—the location of fatigue crack initiation and final failure—were compressive in the overwhelming majority of measurement cases. It is widely accepted that compressive residual stresses are beneficial in fatigue. However, in this set of experiments, it was observed that compressive residual stresses induced tensile strain ratcheting and were thereby detrimental to fatigue life. Residual stress measurements obtained at intermediate points in specimen fatigue lives showed that residual stresses relaxed with fatigue cycles. The extent of relaxation at a point on the welded specimen in fatigue was dependent upon the amplitude of strain cycle experienced. At the location of maximum strain cycling in fatigue, complete relaxation of residual stresses was observed in all specimens. Strain response data gathered in the fatigue tests reiterated earlier findings at NCSU—positive axial strain ratcheting occurred at the top and bottom weld toes of all specimens subjected to displacement-controlled fatigue cycles. In light of residual stress relaxation and strain response data gathered in the welded specimen tests, it was anticipated that during residual stress relaxation, the prescribed fatigue interactions between multiaxial stresses exerted a reverse mean stress effect , which induced ratcheting response. In other words, the relaxation of the compressive residual stress in the axial direction induced a positive mean stress effect in the prescribed fatigue loading cycle. Consequently, tensile strain ratcheting in the presence of compressive residual stresses resulted. In order to gain insight into this newly observed phenomenon of reverse mean stress effect, the mechanism of multiaxial stress interaction was further investigated through additional laboratory experimentation and cyclic plasticity analysis at the material level. Results from this step more directly explain the new observation. This study reveals the mechanism of recently observed ratcheting fatigue failures of welded joints. Additional research is required to understand the failure mechanism in full and for incorporation of this mechanism in design methodology.
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Keywords
Fatigue, Welded Piping Joints, Residual Stress, Ratcheting
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Degree
MS
Discipline
Civil Engineering
