Mitigation Of Cracking Research Articles

Characterization of High Strength Stainless Steel/Al/Brass Composite Tri-layered Clad

Delamination is a major limitation of lamellar composite structures in general, wherein interface/interphase control is a major factor. Studying/investigating the mitigation of cracks as well as toughening mechanisms is paramount to establish the control of interface/interphase kinetics. Stainless steel 316, Al 6082, CU–Zn sandwich laminate composite-sheets, in a foil–foil processing technique, were fabricated by means of hot uniaxial pressing. A parametric study was introduced using temperature and pressure effects on the composite specimens. The composite-sheet interface and its bond strength were investigated using scanning electron microscopy and tensile test. Microstructural as well as mechanical characterization, were established via optical and scanning electron microscopy; energy dispersive X-ray spectroscopy and mechanical testing. Mechanical characterization of the interfacial shear strength is investigated using hardness testing. The interphase kinetic behavior is observed through localized micro plasticity, metal flow and delocalized Al oxide and Mg oxide. Results showed that low temperature heat treatment can improve the morphology of stainless steel 316, Al 6082, CU–Zn interface and increase its bond strength. Alloying element segregation with diffusion mechanisms induced the formation of localized metal flow along with the interface. Alloy segregation during solidification in a semisolid state processing induces directional solidification with fibrous entities and oxides on structural interphase. For the stainless steel 316, Al 6082, Cu–Zn composite, the reasonable heat treatment is approximately 823 K for 1 and 3 h.

Crack Mitigation during Dealloying of Au25Cu75

The suggested use of nanoporous gold for functional and structural applications requires uniform and specifically crack‐free monolithic bodies, ideally with a structure size in the range of 10 nm or below. Here we investigate electrochemical dealloying of two different starting alloys, Au25Ag75 and Au25Cu75, as pathways towards that goal. With an emphasis on the processes that lead to crack formation, we discuss the role of the parameters (i) lattice parameter change, (ii) dealloying potential and rate, and (iii) thermo‐mechanical treatment of the master alloys. The Cu‐based alloys are found to give superior homogeneity at very small structure sizes, provided that intermediate temperature treatments are avoided. A complete suppression of crack formation was achieved by application of a dealloying potential of 1.1 V versus Ag/AgCl, at a ligament size of 11 nm.

Mitigation of hot cracking of alloy 52M overlay on cast stainless steel CF8A

Effects of ER308L buffer layer and welding parameter slope down time on the hot cracking susceptibility of alloy 52M overlay on CF8A base metal were studied. The results indicated that Si segregation was a critical factor affecting Alloy 52M hot cracking. Applying ER308L buffer layers between CF8A and Alloy 52M can reduce the dilution of Alloy 52M weld beads and minimise the contribution of Si from CF8A into Alloy 52M, thereby alleviating Si segregation and the hot cracking susceptibility of Alloy 52M. When the Si content in the grain boundary region was lower than 0·81 wt-%, the hot cracking in the weld bead could be mitigated completely. In many cases, crater cracks occurred in the end crater of the weld bead. Increasing the number of ER308L buffer layers and extending the slope down time could reduce crater crack susceptibility. However, the Si content in the grain boundary region should be controlled to be lower than 0·63 wt-% to prevent crater cracking.

Can superabsorent polymers mitigate autogenous shrinkage of internally cured concrete without compromising the strength?

The paper “Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks” deals with different aspects of using superabsorbent polymers (SAP) in concrete to mitigate self-desiccation. The paper concludes that “Addition of SAP leads to a significant reduction of mechanical strength”. The experimental results are in contradiction with several publications and question the appropriateness of using SAP as internal curing agent. However, the observed strength loss – and possibly also other observations – seems to be caused by overestimation of SAP water absorption. This results in an increase in water/cement ratio (w/c) for concrete with SAP. It is misleading to conclude on how SAP influences concrete properties, based on comparison of concrete mixes with SAP and reference concrete without SAP, if SAP mixes have higher w/c than the reference mix.

NON-DESTRUCTIVE EVALUATION OF IN‐PLACE REHABILITATED CONCRETE PAVEMENTS

Several in‐place rehabilitation techniques have been used before placing an Asphalt Concrete (AC) overlay on Portland Cement Concrete (PCC) pavements in attempts to minimize reflection cracking. Among these techniques, “rubblization” of PCC has been recognized to be the most utilized procedure for the mitigation of reflection cracking in US. This paper presents the practices on the use of rubblization in US and discusses the performance of this technique based on field evaluation. The specifications for the rubblization procedure used by state Department of Transportation (DOT) in US were reviewed. To evaluate the performance of rubblization technique, the structural and the surface conditions of AC overlaid pavements with rubblized PCC layer are compared to those with virgin PCC layer. A series of field experiment tests were carried out on six representative AC overlaid pavements with both rubblized PCC and virgin PCC in Iowa during 2007. The field experiment tests include the Nondestructive (NDT) and destructive tests, and the distress survey. Deflection Basin Parameters (DBPs) were derived from NDT test results to characterize the structural condition of pavements. The results indicate that the rubblized PCC layer has strength comparable to that of virgin PCC layer and has better performance under that structural condition. Santrauka Siekiant nustatyti asfaltbetonio (AC) sluoksnio veiksminguma, mažinant plyšiu ir itrūkiu atsiradima portlandcemenčio betono (NKK) dangose, buvo taikyti keli kelio dangu rekonstrukcijos metodai. Vertinant ivairius plyšiu susidaryma mažinančius keliu rekonstrukcijos metodus, geriausiu buvo pripažintas regeneracijos metodas (PCC rubblization), taikomas JAV. Nagrinejama mineto regeneracijos metodo taikymo patirtis JAV ir analizuojamos jo taikymo galimybes bei veiksmingumas. Ši keliu atnaujinimo metoda taiko JAV Transporto departamentas (DOT). Siekiant ivertinti nagrinejamo regeneracijos metodo veiksminguma eksploatacinemis salygomis, AC buvo padengtas pagal naujaji metoda paruoštu PCC sluoksniu, ir gauti tyrimu rezultatai palyginti su standartiškai paruošto PCC sluoksnio rezultatais. Natūriniai eksperimentiniai bandymai buvo atlikti, taikant neardomuosius (NDT) ir ardomuosius tyrimu metodus. Ilinkiai (DBPs) buvo nustatyti NDT bandymo metodais, charakterizuojančiais konstrukcines dangu salygas. Darbo rezultatai parode, kad pagal naujaji metoda paruoštas PPC sluoksnis igijo panašu stipri kaip ir PCC sluoksnis su standartine danga, tačiau regeneruota PPC danga igijo geresniu eksploataciniu charakteristiku.

Long-Term Performance of Reflective Cracking Mitigation Techniques in Nevada

Hot-mix asphalt (HMA) overlay is one of the commonly used methods for rehabilitating deteriorated pavements. The Nevada Department of Transportation (DOT) uses HMA overlays as a rehabilitation technique for the majority of the state's flexible pavements. One major type of distress influencing the life of an overlay is reflective cracking. In the past, Nevada DOT has experimented with a number of techniques— such as cold in-place recycling, reinforced fabrics, stress relief courses, and mill and overlay—to reduce the impact of reflective cracking on HMA overlays. Several projects were constructed under each category. The long-term field performance of various Nevada DOT reflective cracking mitigation techniques was evaluated; the techniques were used on flexible pavements at 33 field projects. Performances of the various projects were analyzed by fatigue, transverse, and block cracking measurements from Nevada DOT's pavement management system data. In addition, the statistical approach called principal component analysis was used to asses the effectiveness of each of the reflective cracking techniques. The study indicated that cold in-place recycling and mill and overlay were the most effective treatments for reflective cracking of HMA overlays over HMA pavements under Nevada's conditions, except when the existing pavement experiences severe alligator cracking. In such situations, it is recommended that HMA pavement be subjected to reconstruction or full-depth reclamation.

Observations, Modeling, and Mitigation Related to Reflective Cracking on Composite Pavements in New York City

As part of a 7-year field study, experimental composite pavement sections incorporating several reflective crack control treatments were built and investigated in New York City (NYC). The primary objective of this study was to identify cost-effective materials and methods to minimize reflective cracks in NYC composite pavements. The as-constructed experiment included several reflective crack control strategies such as fabrics, membranes, composites, fiberglass laminates, and saw and seal crack control techniques. These strategies were repeated for 15- and 20-ft joint spacings in the base portland cement concrete (PCC) pavement. Control sections without any treatments were also built for each of the two PCC base joint spacings as baseline references. Evaluations in the form of 10 visual condition surveys, five falling weight deflectometer surveys, forensic coring, and materials testing were conducted over the life of the experiment. Performance was measured in terms of crack initiation, length, and severity as well as load transfer deterioration across the reflective cracks. The saw and seal crack control technique and short joint spacing, in that order, had the most effect on mitigating reflective cracking. The saw and seal technique offers a life extension of at least 18.5% and 34% for the 20- and 15-ft joint spacings, respectively, when compared with the NYC Department of Design and Construction (NYCDDC) standard reflective crack control measure–a nonwoven polypropylene fabric interlayer. On the basis of these findings, it is recommended that NYCDDC switch its standard reflective crack mitigation practice to saw and seal and abolish the practice of skewing transverse joints in the PCC base. A saw and seal specification was developed to help NYCDDC implement this recommendation. Additionally, the data from this research represent a unique body of work that can be used in mechanistic-empirical modeling of this important distress type.

Circumferential Notch Tensile Testing: Role of Imposed Electrochemical Potentials in Susceptibility of Steel to Caustic Cracking

A fracture mechanics based novel approach, i.e., circumferential notch tensile (CNT) testing has been employed for determination of threshold stress intensity factor for susceptibility of engineering materials to stress corrosion cracking . Using CNT technique, of a mild steel at an open circuit potential in NaOH at was determined to be . CNT tests have also been performed under imposed electrochemical potentials to understand the mechanistic aspects of caustic cracking as well as to develop guidelines for mitigation of caustic cracking. An imposed potential in the active-passive potential regime caused much more rapid failure (than was observed at ). The fractography of the CNT specimens tested at and showed evidence of stress corrosion cracking. At an imposed potential in the passive region , the specimen did not fail even after an order of magnitude longer testing, indicating a considerable improvement in resistance to caustic cracking as a result of this application of . The resulting fractograph confirmed the absence of caustic cracking. The study has established the use of CNT testing as a simple, relatively fast and economical approach for generating the data, and showed that the data are consistent with an electrochemical mechanism for caustic cracking.

MATERIAL RELIABILITY OF Ni ALLOY ELECTRODEPOSITION FOR STEAM GENERATOR TUBE REPAIR

Due to the occasional occurrences of stress corrosion cracking(SCC) in steam generator tubing(Alloy 600), degraded tubes are removed from service by plugging or are repaired for re-use. Since electrodeposition inside a tube does not entail parent tube deformation, residual stress in the tube can be minimized. In this work, tube restoration via electrodeposition inside a steam generator tubing was performed after developing the following: an anode probe to be installed inside a tube, a degreasing condition to remove dirt and grease, an activation condition for surface oxide elimination, a tightly adhered strike layer forming condition between the electro forming layer and the Alloy 600 tube, and the condition for an electroforming layer. The reliability of the electrodeposited material, with a variation of material properties, was evaluated as a function of the electrodeposit position in the vertical direction of a tube using the developed anode. It has been noted that the variation of the material properties along the electrodeposit length was acceptable in a process margin. To improve the reliability of a material property, the causes of the variation occurrence were presumed, and an attempt to minimize the variation has been made. A Ni alloy electrodeposition process is suggested as a primary water stress corrosion cracking(PWSCC) mitigation method for various components, including steam generator tubes. The Ni alloy electrodeposit formed inside a tube by using the installed assembly shows proper material properties as well as an excellent SCC resistance.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors (IV)

A calculation model has been developed in order to evaluate effectiveness of hydrazine and hydrogen co-injection (HHC) into reactor water for mitigation of intergranular stress corrosion cracking of structural materials used in boiling water reactors (BWRs). The HHC uses the strong reducing power of hydrazine radical, which is produced in the downcomer region under irradiation by γ-rays and neutrons. Some reactions and their reaction rate constants were determined based on experiments which were carried out in aerated water, hydrogenated water, and deaerated water. The calculated results were in good agreement with experimental data by a factor of two. The model was applied to a BWR and it was found that the HHC cut oxygen and hydrogen peroxide amounts dissolved in reactor water more effectively than hydrogen water chemistry alone. Thus, the required amount of hydrogen for hydrazine injection was much lower than that for hydrogen water chemistry. Consequently, electrochemical corrosion potential of structural materials could be lowered below–0:1V vs. SHE without any increase of MS line dose rate, which has been a limitation of the conventional hydrogen water chemistry. The HHC was predicted to decrease crack growth rate of structural materials by a factor of 10.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors (IV)

A calculation model has been developed in order to evaluate effectiveness of hydrazine and hydrogen co-injection (HHC) into reactor water for mitigation of intergranular stress corrosion cracking of structural materials used in boiling water reactors (BWRs). The HHC uses the strong reducing power of hydrazine radical, which is produced in the downcomer region under irradiation by γ-rays and neutrons. Some reactions and their reaction rate constants were determined based on experiments which were carried out in aerated water, hydrogenated water, and deaerated water. The calculated results were in good agreement with experimental data by a factor of two. The model was applied to a BWR and it was found that the HHC cut oxygen and hydrogen peroxide amounts dissolved in reactor water more effectively than hydrogen water chemistry alone. Thus, the required amount of hydrogen for hydrazine injection was much lower than that for hydrogen water chemistry. Consequently, electrochemical corrosion potential of structural materials could be lowered below–0:1V vs. SHE without any increase of MS line dose rate, which has been a limitation of the conventional hydrogen water chemistry. The HHC was predicted to decrease crack growth rate of structural materials by a factor of 10.

ICONE15-10877 NOBLE METAL INJECTION DURING POWER OPERATION FOR BWR CRACK MITIGATION AND DOSE RATE REDUCTION

NobleChem^<TM> is a technology that has been accepted by the Boiling Water Reactor (BWR) industry as a means to lower ECP with low hydrogen addition, thus satisfying the criteria needed for stress corrosion cracking (SCC) mitigation. Currently, 31 operating BWRs have applied NobleChem^<TM> and the total cumulative NobleChem^<TM> experience is about 180 reactor operating years. The conventional NobleChem^<TM> technology requires the injection of a mixture of platinum and rhodium chemicals into the reactor at a lower temperature of 115 to 143℃, and takes about 60 hours of critical path time. Recently, GE developed and implemented a more effective new generation On-Line NobleChem application (OLNC) that can be employed while the BWR is operating at full power, and hence eliminates the costly critical path time requirement. Furthermore, the OLNC application is not outage related, is transparent to plant operations, and requires the use of platinum chemical only. The first OLNC application was completed in a European BWR in July 2005 followed by a second OLNC application in the same plant in January 2006. The application was successfully completed with no increase in operating dose rates, no impact on plant operations and no unexpected changes to water chemistry parameters. This paper presents the application results, the encouraging drywell dose rate results and the shroud ultrasonic (UT) crack growth rate observations following OLNC application.

Dynamic Modulus of Asphalt Concrete with a Hollow Cylinder Tensile Tester

A hollow cylinder tensile tester (HCT) has recently been shown to be a suitable surrogate test device for the Superpave® indirect tension test. The feasibility of using the HCT to obtain the dynamic modulus of asphalt concrete was explored. Previous studies have shown the dynamic modulus of asphalt concrete in tension and compression to be similar at low temperatures and high loading frequencies and that minor differences can be expected at higher temperatures and lower loading frequencies. Here, the HCT device compared favorably with dynamic modulus measurements obtained with uniaxial compression testing apparatus at 0°C and 20°C. In addition, test results were found to be in reasonable agreement with the Witczak dynamic modulus predictive equation. The ability to simply and rapidly measure the creep compliance, tensile strength, and dynamic modulus of bituminous paving mixtures over a range of temperatures and loading conditions makes the HCT device a useful general-purpose mixture analysis tool. It appears that this portable device may serve as a practical method to satisfy testing requirements for high reliability (Level 1) pavement designs in the forthcoming AASHTO 2002 design guide being developed under NCHRP 1-37A for the mitigation of both fatigue cracking and thermal cracking. However, a broader experimental program, including tests at higher temperatures and a broader range of test frequencies, is needed to fully validate the compatibility of HCT measurements with NCHRP 1-37A models.

Equilibrium chemistry of nitrogen and potential-pH diagrams for the Fe-Cr-H 2O system in bwr water

A thermodynamic analysis of the nitrogen-water system for BWR operating conditions assesses the chemical species responsible for steam line radiation increases. Nitrogen (N 2) was the important gaseous form during hydrogen injection into BWR feedwater. The potential-pH diagram for the system Fe-Cr-H 2O was derived from thermodynamic information for BWR operating temperatures. The diagram shows that a mixed oxide (FeCr 2O 4) is stable under the reducing conditions imposed by hydrogen water chemistry (HWC) for BWR while the simple oxide Fe 2O 3 is stable for the more oxidizing conditions of normal water chemistry. The presence of Cr in the oxide may account for the mitigation of intergranular stress corrosion cracking of sensitized stainless steel during HWC.