Simplified HTHA Evaluation Using Larson Miller Parameter Concepts

Researchers have been developing mechanistic approaches describing High Temperature Hydrogen Attack (HTHA) damage for quite some time. Although there are a variety of approaches, all of them make use of describing HTHA as a time and temperature dependent phenomena that is sensitive to methane pressure. HTHA research shows the damage process is a phenomenon that is very similar to creep damage which has an exponential relationship to the applied stress and temperature. Based on these observations, the authors propose an HTHA damage assessment procedure that uses the familiar Larson Miller Parameter (LMP) approach and employs the well-known Linear Life Fraction Rule for evaluating operating condition variations in hydrogen partial pressure and temperature.

Effects of Fatigue Damage on the Microscopic Modulus of Cortical Bone Using Nanoindentation

Alterations to the bone structure from cycle loadings can undermine its damage resistance at multiple scales. The accumulation of fatigue damage in a bone is commonly characterized by the reduction in the elastic modulus. In this study, nano-indentation was used for investigating microscopic damage evolution of bovine tibia samples subjected to fatigue loading. Indentation tests were conducted in the same 60 μm × 120 μm area with different degrees of damage, including fracture, and the evolution of reduced modulus was observed. The results showed that bone’s reduced modulus decreased significantly during the initial 40% of the life fraction, whereas it proceeded slowly during the remaining period. As the size of the residual indentations was about 4 μm in length, the degradation of bone’s reduced modulus reflected the accumulation of fatigue damage at smaller scales.

Dynamic Coercivity of Tempered Ferritic Steel Subjected to Creep-Fatigue for Nondestructive Evaluation by Reversible Permeability

This study aims to characterize nondestructively the creep-fatigue damage of tempered ferritic steel by dynamic coercivity measured by reversible permeability. The creep-fatigue test was achieved under load control with a hold time of 60 s and 600 s. The dynamic coercivity based on the reversible magnetic permeability profiles was successfully obtained on top of the test specimen. Based on these results, we attempted to evaluate the creep-fatigue of tempered ferritic steel with the support of microstructural evaluation such as dislocations and precipitates. The dynamic coercivity decreases with fatigue life fraction and closely related to the number of Cr23C6 precipitates with N0.8 and the dislocation density with ρ0.4 In addition, Vickers’s hardness continuously decreased up to the failure deducing the softening of mechanical strength. In conclusion, it can nondestructively characterize the influence of the precipitates and dislocation density on the dynamic coercivity of ferritic steel during creep-fatigue.

Critical lifetime of HDPE pipes through damage and reliability models

Damage models are not directly applicable on high-density polyethylene (HDPE) pipes. In this paper, static and strain-unified theory damage models are adapted to fit the HDPE case by substituting the dynamic tests’ endurance limits by preloading simulation through notch and stiffness evaluation. Then, tensile and burst tests are following up to evaluate the specimens’ residual life. Compared to virgin specimens, the rupture limit of old HDPE pipes’ specimens had dropped significantly and their elongation decreased from 275 mm to about 26 mm. The degradation of the seven categories of specimens are different. Indeed, the degradation is too noticeable, disappearance of the plastic phase, for the categories 6 and 7, which are in the bottom of the pipe. Then, a reduced plastic phase on the lateral categories 4 and 5 showing an important impact of degradations. Finally, a larger plastic phase for the categories 1 and 2 taken from the top of the pipe, showing a medium impact of degradation. Thus, the use of the stiffness factor, reflecting the variability of degradation of the different categories of specimens, and the thickness reduction as life fractions for both aged and neat HDPE specimens was possible. The developed strains damage model compared to static burst pressures’ one confirmed the damage stages and the critical life fraction of HDPE pipes. By comparing these models, the drastic change of HDPE pipes’ behavior, from a ductile to a brittle one, have been proved. These findings allowed us to find out the critical life fraction of neat and old HDPE pipes, which has been confirmed by comparing the burst pressure curves of a notched and an old pipe. The presented approach is cost effective allowing a deep analysis of HDPE pipes failure and damage quantification through simply made models based on static tensile and burst test instead of tedious and very costly dynamic ones.

The Experimental Prediction of the Lifetime of Acrylonitrile Butadiene Styrene (ABS) by the Energetic Method

In this paper, we are dealing with the study of the mechanical behavior of an amorphous polymer, acrylonitrile butadiene styrene "ABS". In fact, uniaxial tensile tests on rectangular specimens containing a combined defect, with simple and double notches, has been done. The proposed approach develop a method, based on energy parameter, to calculate the evolution of damage over the materials’ life. This method can be used to predict quantitatively the risk of sudden rupture in a structure. Therefore, the damage evaluation based on the residual energy method was compared to the unified theory one for different loading levels. The prediction of damage by experimental models has led to the determination of the three stages of damage evolution, which are the initiation, propagation and complete deterioration of the material. Thus, the concept of reliability is used to specify the critical life fraction, which is similar to the notch depth (βc) of a modeled defect as a combined defect on an ABS sample. In addition, the unified theory was used in this work, to define on the one hand, the parameter of damage which is the internal variable which describes the failure level of the structure in function of life fraction, on the other hand, for the theoretical evaluation of the level of damage. Finally, we have proved that the theoretical and experimental results show a good agreement.

Prediction of the Lifetime of the Chlorinated PVC Thermoplastic Material Subjected to Thermomechanical Tests - Tensile Test under the Influence of Temperature

In this paper we are studying the effects of temperatures ranging from-10 to 70°C of behavior on chlorinated PVC (CPVC), whose direct consequences are the strong modifications of the mechanical and physical characteristics of this polymer [1]. The purpose of this paper is to predict the damage of non-weld CPVC, based on simple tensile tests on non weld specimens. Thereafter and with the establishment of the relation Damage-Reliability, the critical life fraction βc is identified that predicts the time at which the damage becomes sudden and it is necessary to calculate the reliability and predictive maintenance of the system.

An Investigation into Creep Cavity Development in 316H Stainless Steel

Creep-induced cavitation is an important failure mechanism in steel components operating at high temperature. Robust techniques are required to observe and quantify creep cavitation. In this paper, the use of two complementary analysis techniques: small-angle neutron scattering (SANS), and quantitative metallography, using scanning electron microscopy (SEM), is reported. The development of creep cavities that is accumulated under uniaxial load has been studied as a function of creep strain and life fraction, by carrying out interrupted tests on two sets of creep test specimens that are prepared from a Type-316H austenitic stainless steel reactor component. In order to examine the effects of pre-strain on creep damage formation, one set of specimens was subjected to a plastic pre-strain of 8%, and the other set had no pre-strain. Each set of specimens was subjected to different loading and temperature conditions, representative of those of current and future power plant operation. Cavities of up to 300 nm in size are quantified by using SANS, and their size distribution, as a function of determined creep strain. Cavitation increases significantly as creep strain increases throughout creep life. These results are confirmed by quantitative metallography analysis.

Short fatigue crack behavior of carbon structural steel under tension–torsion loading

Fatigue short crack replica tests of a carbon structural steel, i.e., LZ50 steel, were carried out under tension–torsion loading using MTS electro hydraulic servo machine. Totally eight specimens with smooth axial hourglass shape were tested. The results show that short fatigue cracks generally initiate from the ferrite grain boundary or in the ferrite grain. In the microstructural short crack (MSC) stage, the crack grows slowly, but the growth rate will decrease when the crack encounters microstructural barriers, such as the ferrite grain boundary and the rich pearlite banded structure. In the physical short crack (PSC) stage, the crack breaks through the rich pearlite banded structure and meanwhile the dominant effective short fatigue crack (DESFC) has established its dominant position. As a result, the size and the growth rate of the crack increase rapidly. Based on the fatigue failure mechanism and the safety in tail region predictions, the statistical evolution of the DESFC size and the life fraction was analyzed respectively by comparing six commonly used statistical distributions. The statistical results show that the extreme minimum value (EMV1) distribution is a good assumed distribution for both the DESFC size and the life fraction.

Fitness-for-Service Evaluation of a Gr 91 to Gr 22 Dissimilar Metal Weldment Subject to Creep

A fitness-for-service (FFS) evaluation was performed on a cogeneration power plant high pressure steam line. The piping system included a Gr 91 to Gr 22 dissimilar metal weld (DMW) between an SA-335 Gr P91 pipe and an SA-335 Gr P22 reducer. This unit had accumulated more than 100,000 service hours. The FFS project included as-built and as-found piping stress analyses, redistribution of creep stresses, weld residual stress estimates, review of the nondestructive examination (NDE) results, evaluation of the plant information data, and a fracture mechanics evaluation. Due to tapering of the Gr P22 pipe to match the wall thickness of the Gr P91 pipe at the DMW interface, the Gr P22 pipe wall thickness was not in compliance with the ASME B31.1-2000 Code minimum wall thickness (MWT). In addition, an inspection of the single groove girth DMW revealed ultrasonic indications along the fusion face of the weld. The lack of side wall fusion (LSWF) can be considered as a planar and crack-like indication, so a fracture mechanics evaluation was performed. The piping stress analysis revealed that the DMW was subject to low axial stresses. A creep life fraction evaluation was performed for the material that does not have the LSWF indication, considering conservative creep rupture data, weld performance degradation, redistributed creep stresses, and multiaxial stresses in the DMW. Due to the low stresses, the life fraction analysis estimated more than 50 years of remaining creep life. Failure Assessment Diagrams (FADs) were developed to evaluate the component integrity for various combinations of crack depths and lengths. The reported indications should not compromise the integrity of the component subject to the typical plant operation during the next 12 years and the component is fit for continued operation. The crack propagation evaluation determined that the DMW has a predicted minimum safe operating life of at least 12 years if operated as in the past. Because the wall thickness of the tapered reducer is below the ASME B31.1 Code allowable MWT, it was recommended that the DMW be examined during a scheduled outage within 5 years and the component be replaced with Gr P91 material during a major outage within 8 years.

Crude Furnace Creep Assessment and High Temperature Degradation

Creep calculations indicate a crude furnace radiant section carbon steel tubes exceeding their life fraction due to flame impingement reaching up to 700°C for a year. The ambiguity of the temperature and material data means the life fraction of creep calculations were based on limited inspection data and infra-red scanning giving a conservative indication of end of life. Due to unavailable tubes in stock, a planned pit stop cannot be arranged due to economic and safety reasons as the furnace may not be started back up safely. To safeguard the integrity of the furnace until the planned outage, the temperature on the furnace tube was stabilized to a current limit of 540°C through improvements in burner operations. The crude diet was also maintained within the crude acceptance envelope. Visual checks at every shift were done to ensure no observation from tube bulging or uneven flame pattern. A decision tree was created to facilitate quick decision making using a go/no go criteria of which tubes to replace during the August 2015 planned turnaround. The criteria set for the decision tree required tube wall thickness, surface hardness test, tube outer diameter ring gauge to be examined. Failing any of the criteria will require the tube to be replaced. The replaced tubes (one worst and one representative) will also be lab tested through destructive examination to identify the degradation mechanism and high temperature properties of the worst tubes to quantitatively define the high temperature properties and life fraction of the tubes that are left in the furnace. The lab test will provide results after a year of creep testing and can give assurance of continued furnace operation for 4 more years until the next outage. The final decision after the examination based on the decision tree was made required 17 tubes to be replaced in this turnaround. The worst degraded tubes were found to be at the vicinity of the initial observed location around the flame impingement zone.