Categorization of the hemodynamic response to hemodialysis: The importance of baroreflex sensitivity

Intradialytic hypotension (IDH) remains an important cause of morbidity and mortality in hemodialysis (HD) patients. The baroreflex arc is under autonomic control and regulates blood pressure. This study aimed to investigate the contribution of impaired baroreflex sensitivity (BRS) to the pathophysiology of IDH. Thirty-four chronic HD (12 IDH-prone, 22 IDH-resistant) patients underwent BRS measurement during HD with relative blood volume monitoring. During analysis, patients were separated into four age-matched groups according to resting BRS≥4.5 ms/mmHg and hemodynamic stability. Resting BRS was extremely heterogenous (geometric mean BRS 5.78±1.41 [range 1.76–41.41] ms/mmHg). Relative blood volume reduction was well matched in all groups (mean reduction in relative blood volume for all patients −6.74%±0.86%, P>0.05). Thirty-seven episodes of IDH occurred in the IDH prone, reduced BRS group. Patients with impaired resting BRS and prone to IDH had markedly different responses to HD as compared to the preserved BRS group, but the total peripheral resistance response was significantly lower than in the IDH-resistant patients (15.9%±2.1% vs. 42.4%±3.0%, respectively, P<0.001). In those patients prone to IDH and with impaired resting BRS, percentage reduction in cardiac output at the end of HD highly correlated with reduction in relative blood volume (r=0.94, P=0.006). Hypotension during dialysis may be an important source of recurrent cardiac injury and early recognition of those patients prone to relative symptomatic and asymptomatic hypotension remains important. Impaired resting BRS and recognition of a suboptimal peripheral pressor response, appear to predict those patients most likely to undergo hemodynamic instability and may assist in the pursuit of this elusive goal.     

Antibiotic lock: In vitro stability of vancomycin and four percent sodium citrate stored in dialysis catheters at 37°C

Catheter-related bacteremia (CRB) is a major cause of morbidity and mortality especially among patients receiving hemodialysis (HD). Antibiotic lock therapy represents a promising technique in the treatment of CRB. Several studies have evaluated antibiotics in combination with heparin as an interdialytic locking solution as adjunctive therapy for CRB. The objective of this study was to evaluate the chemical stability of the vancomycin in 4% sodium citrate in HD catheters as an interdialytic lock. Vancomycin was prepared and diluted with sodium citrate 4% and stored in polyvinyl chloride syringes, 2 carbothane dialysis catheters (Hemostar^®) and 2 dual floating HD catheters (CardioMed^®). Syringes were stored at 4 °C or 23 °C and the catheters were stored in an incubator at 37 °C for 72 hours. Samples underwent daily chromatographic analysis and the luminal concentration of vancomycn was determined on study days 0, 1, and 3. When vancomycin is reconstituted with normal saline to achieve a concentration of 50 mg/mL, and then further diluted in 4% sodium citrate, to achieve concentrations of either 1 or 3 mg/mL, and then stored at 4 °C, room temperature, or 37 °C, solutions were observed to retain >92% of the initial concentration for the study period of 3 days. Based on the fastest degradation rate determined with 95% confidence interval, >90% is retained for 6.53 days. We conclude that vancomycin—4% citrate solutions stored in polyvinyl chloride syringes or HD catheters are not significantly affected by temperature or concentration within the 72 hours storage period. Therefore, these solutions can be anticipated to be suitable as a HD interdialytic antibiotic lock in standard HD catheters.     

Short Daily Hemodialysis vs. Short Daily Hemofiltration (Search for Optimal Prescription)

It has been shown that daily hemodialysis as well as convective transfer by hemofilitration improve the quality of extra renal treatment. Two following phases of treatment of three weeks each were tested in 2 patients: daily hemodialysis 2.5 h 6 times/week (HD*6) and daily hemofiltration 2.75 h 6 times/week (HF*6) performed according to the following modalities. Phase I, blood flow rates (QB): 300 mL/min, hemofilter 1.4 m²AN 69 dialysate flow 500 mL/min. Phase II, QB: 150 mL/min, hemofilter 1m²AN 69, exchange volume of 10 L/session; 5 L predilution and 5 L postdilution (conditions were limited by the device). We measured, during the third week of treatment of each phase, the weekly mass transfers and the predialysis plasma levels of urea (U), creatinine (C), phosphate (P), and B2 microglobulin (B2M). In the 2 phases, HD*6 and HF*6, respectively, the weekly urea Kt was: 120 vs. 60 L; std Kt/V: 3.30 vs. 2.0; npcr: 1.26 vs. 1.42 g kg^–1 day^–1.  

     

Solute kinetics with short-daily home hemodialysis using slow dialysate flow rate

"NxStage System One^™" is increasingly used for daily home hemodialysis. The ultrapure dialysate volumes are typically between 15 L and 30 L per dialysis, substantially smaller than the volumes used in conventional dialysis. In this study, the impact of the use of low dialysate volumes on the removal rates of solutes of different molecular weights and volumes of distribution was evaluated. Serum measurements before and after dialysis and total dialysate collection were performed over 30 times in 5 functionally anephric patients undergoing short-daily home hemodialysis (6 d/wk) over the course of 8 to 16 months. Measured solutes included β₂ microglobulin (β₂M), phosphorus, urea nitrogen, and potassium. The average spent dialysate volume (dialysate plus ultrafiltrate) was 25.4±4.7 L and the dialysis duration was 175±15 min. β₂ microglobulin clearance of the polyethersulfone dialyzer averaged 53±14 mL/min. Total β₂M recovered in the dialysate was 106±42 mg per treatment (n=38). Predialysis serum β₂M levels remained stable over the observation period. Phosphorus removal averaged 694±343 mg per treatment with a mean predialysis serum phosphorus of 5.2±1.8 mg/dL (n=34). Standard Kt/V averaged 2.5±0.3 per week and correlated with the dialysate-based weekly Kt/V. Weekly β₂M, phosphorus, and urea nitrogen removal in patients dialyzing 6 d/wk with these relatively low dialysate volumes compared favorably with values published for thrice weekly conventional and with short-daily hemodialysis performed with machines using much higher dialysate flow rates. Results of the present study were achieved, however, with an average of 17.5 hours of dialysis per week.     

Bioreactance: A new tool for cardiac output and thoracic fluid content monitoring during hemodialysis

Outpatient hemodialysis therapy (HD) can be associated with hemodynamic compromise. Bioreactance^® has recently been shown to provide accurate, noninvasive, continuous, measurements of cardiac output (CO) and thoracic impedance (Zo) from which thoracic fluid content (TFC) can be derived assuming TFC=1000/Zo. This study was designed to evaluate the changes in TFC in comparison with the traditional indices of fluid removal (FR) and to understand the trends in CO changes in HD patients. Minute-by-minute changes in TFC and CO were prospectively collected using the bioreactance system (NICOM^®) in HD patients of a single unit. Changes in body weight (ΔW), hematocrit (ΔHct), and amount of FR were also measured. Twenty-five patients (age 77 ± 11 years) were included. The TFC decreased in all patients by an average of 5.4 ± 7.9 kΩ⁻¹, weight decreased by 1.48 ± 0.98 kg, and FR averaged 2.07 ± 1.93 L over a 3- to 4-hour HD session. There were good correlations between ΔTFC and ΔW (R=0.80, P<0.0001) and FR (R=0.85, P<0.0001). ΔHct (4.13 ± 3.42%) was poorly correlated with ΔTFC (R=0.35, P=0.12) and FR (R=0.40, P=0.07). The regression line between FR and TFC yielded FR=1.0024−0.1985TFC; thus, a 1 kΩ⁻¹ change of Zo correlates with an ∼200 mL change in total body water. The change in CO (−0.52 ± 0.49 L/min m²) during HD did not correlate with FR (R=0.15, P=NS). Changes in TFC represented the monitored variable most closely related to FR. CO remained fairly constant in this stable patient cohort. Further studies in high-risk patients are warranted to understand whether TFC and CO monitoring can improve HD session management.     

Is there an association between intradialytic hypotension and serum magnesium changes?

Intradialytic hypotension (IDH) is the most common complication of hemodialysis (HD). The aim of this study was to investigate the significance of intradialytic changes of serum magnesium (sMg) and its relation to IDH. We considered 58 patients undergoing HD. Serum magnesium was measured at start, after 2 hours, and at the end of the HD sessions. Total sMg concentration corrected for albumin was according to Krolles proposed formula. Blood pressure was measured every 30 min. Data were analyzed by SPSS.15. A P value of less than 0.05 was considered as significant. Occurrence of IDH among HD patients was 27.6% (16/58). Serum magnesium decreased significantly during HD session (P<0.05). Comparing corrected sMg in IDH group with non-IDH group showed that: corrected sMg was 0.66 ± 0.14 mmol/L vs. 0.84 ± 0.26 mmol/L at the start of dialysis (P=0.43), 0.62 ± 0.17 mmol/L vs. 0.74 ± 0.23 mmol/L (P=0.04) at 2 hours, and 0.61 ± 0.12 mmol/L vs. 0.72 ± 0.22 mmol/L (P=0.03) at the end of dialysis. Intradialytic hypotension episodes were significantly related to a decrease in sMg during dialysis (P=0.02). There was a significant decrease in sMg levels during dialysis. Intradialytic hypotension was significantly related to lowered sMg levels during dialysis.     

Aerobic exercise increases phosphate removal during hemodialysis: A controlled trial

Previous studies have suggested that exercise during hemodialysis (HD) could increase the efficacy of solute removal, although this hypothesis has not been conclusively evaluated. The goal of this study was to compare the removal of low‐molecular weight solutes between HD sessions, with and without aerobic exercise. It was a controlled clinical trial, including HD patients in a randomly cross‐over design, such that each patient received a HD session with exercise (intervention) and the next one without exercise (control), three times each. In the exercise sessions, patients pedaled on a cycle ergometer for 60 minutes. The total mass of removed urea, potassium, creatinine, and phosphate were calculated from the solutes concentration in dialysate (continuous spent sampling of dialysate). This was evaluated in a total of 132 HD sessions of patients with a mean age of 54 ± 15 years, 75% male and HD vintage of 3 (2–13) years. Phosphate removal in dialysate during intervention sessions was significantly higher (5.6 [2.5–18.9] vs. 5.1 [1.5–11.2] mg/min) than during control sessions, P = 0.04. The median mass of phosphate removed during control HD session was 1226 (367.8–2697.2) vs. 1348.6 (613.0–4536.2) mg/session during intervention sessions. The exercise did not modify the removal of urea (control 122.6 [61.3–286.0] vs. exercise 112.4 [51.1–250.3] mg/min, P = 0.44), creatinine (control 5.6 [2.5–13.8] vs. exercise 5.6 [2.5–12.8] mg/min, P = 0.49), or potassium (control 13.3 [11.2–15.8] vs. exercise 13.8 [6.6–15.8] mEq/min, P = 0.49). Aerobic exercise during HD increases the efficacy of phosphate removal, without changing urea, creatinine and potassium removal. The implications of this finding in mineral and bone disease and cardiovascular disease need to be evaluated on future clinical trials.     

CYCLIC FATIGUE OF Al2O3 AND Al2O3-Al2TiO5 COMPOSITES IN DIRECT PUSH-PULL

Abstract— A reaction sintering route is developed to produce, "in situ", composites of alumina-aluminium titanate using alumina and titania as starting powders. Aluminium titanate, can be formed by a solid state reaction between Al₂O₃ and TiO₂ at temperatures above the eutectoid temperature of 1280°C. These composites have different grain sizes of alumina matrix and a different quantity and distribution of aluminium titanate according to the heating cycle used.
In the present work direct push-pull tests under cyclic loads have been carried out with both monolithic alumina and alumina-aluminium titanate composites. It has been found that all the samples show a decrease in tensile strength with the number of applied cycles of loading when plotted in graphical form but the slopes of these graphs for both Al₂O₃-Al₂TiO₅ composites are lower than for the alumina specimens. The role of aluminium titanate and the alumina matrix grain size in fatigue crack growth resistance has been studied during push-pull tests, where failure occurs by catastrophic propagation of small surface cracks after a very short regime of subcritical crack growth. These results have been compared with measurements of slow stable fatigue crack growth rates in Al₂O3-Al₂TiO₅ composites carried out elsewhere with pre-notched specimens of the compact tension type. These latter tests provide information about the behaviour of significantly long cracks, i.e. cracks that are several millimetres long.     

Kinetics, dialysis systems, adequacy Postdialysis rebound in a case of acute methanol poisoning

Introduction:  Methanol poisoning can lead to complications that include metabolic acidosis, visual impairment and death. Treatment options include ethanol, fomepizole, and hemodialysis (HD). Objective:  To report on the occurrence of post dialysis methanol rebound during treatment. Method and Findings:  A 40‐year‐old male with a history of schizophrenia and suicide attempts presented to the emergency room after reportedly ingesting 1 quart of windshield washer fluid. The patient presented with a preliminary blood chemistry of methanol 390 mg/dL, ethanol 48 mg/dL, glucose 93 mg/dL, Na 138 meq/L, K 3.8 meq/L, Cl 98 mmol/L, CO₂ 26 mmol/L, urea 16 mg/dL, creatinine 1.2 mg/dL, and an anion gap of 14 mmol/L. The patient was started on 1360 mg of fomepizole (12:50 AM) followed by HD for 4 hours. A second dose of fomepizole (900 mg) was administered at 8:00 AM. In addition, another HD session was started at 12:00 PM and continued for 4 hours. A third dose of fomepizole (700 mg) was administered at 8:50 PM. Finally, a third HD session was started the next day at 3:05 PM and lasted 3 hours. Table 1 illustrates methanol levels in relation to each HD session. Findings:  Methanol concentration after the first HD increased from 100 mg/dL to 127 mg/dL (27%) in 5 h 20 m. It also increased from 35 mg/dL to 50 mg/dL (43%) 14 h 45 m after the second HD. Conclusions:  Close attention must be paid to the potential for post dialysis methanol rebound. It is recommended that methanol levels continue to be monitored for several hours after HD.  

  Table 1   Methanol levels before and after each hemodialysis     

10.

Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr-1 Mo pressure vessel steel     

E. Bouyne  P. Joly†  B. Houssin  C. S. Wiesner  & A. Pineau 《Fatigue & Fracture of Engineering Materials & Structures》2001,24(2):105-116

Tests were performed on a 2¼  Cr–1  Mo steel to measure the fracture toughness at initiation, K _Ic and at arrest, K _Ia . The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure K _Ia : (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to −196 °C and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (∼500  m  s⁻¹ ) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic–plastic finite element method to determine K _Ia values. These tests reveal a greater temperature shift (∼100 °C) between K _Ic and K _Ia in 2¼  Cr–1  Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼  Cr–1  Mo steel showed that this involves the nucleation of a three-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism     

11.

Studies of phosphate dynamics during hemodialysis     

Kjellstrand C.M.  Odar&#;Cederlof I.  Ing T.S.  Blagg C.R. 《Hemodialysis international. International Symposium on Home Hemodialysis》2005,9(1):98-98

Regulation of phosphate (PO₄) in hemodialysis patients is very difficult and ideal levels are rarely maintained. A high removal and a normal phosphate level is important, as high and low levels are both associated with morbidity and a very high mortality.
We studied phosphate dynamics and its relation to other small "uremic" molecules in 48 patients by measuring pre‐ and postdialysis levels and all removed phosphate, urea and creatinine (creat) in all dialysate during 455 dialyses done at different frequencies (freq): 3.7 ± 1.2, range 3–6 treatments per week and durations of dialysis (t): mean: 196 ± 95, range 80–560 min and with high (HF) and low flux membranes.
Kt/V‐PO₄, Kt/V‐urea and Kt/V‐creat, volumes (Vr) for all solutes and their relationships to frequency and duration of dialysis, urea clearance and predialysis phosphate were calculated.  

     

12.

Fatigue strength of crack-healed Si₃N₄/SiC composite ceramics   总被引：1，自引：1，他引：0  

K. Ando  M. C. Chu  F. Yao  & S. Sato 《Fatigue & Fracture of Engineering Materials & Structures》1999,22(10):897-903

Si₃ N₄ /SiC composite ceramics were sintered and subjected to three-point bending on specimens made according to the appropriate JIS standard. A semi-circular surface crack of 110 μm in diameter was made on each specimen. By using three kinds of specimen (smooth, cracked and crack healed), crack-healing behaviour, cyclic and static fatigue strengths were determined systematically at room temperature and 1000 °C. The main conclusions are as follows: (i) Si₃ N₄ /SiC composite ceramics have the ability to heal after cracking; (ii) crack-healed specimens showed similar cyclic and static fatigue strengths as smooth specimens, this being caused by crack healing; (iii) crack-healed zones had a sufficient fatigue strength and most fractures occurred outside the pre-cracked zone in those crack-healed specimens.     

13.

Effects of citrate‐enriched bicarbonate based dialysate on anticoagulation and dialyzer reuse in maintenance hemodialysis patients     

Amanda D. Rocha  Vanessa C. Padua  Esther Oliveira  Márcia M. Guimaraes  Jocemir R. Lugon  Jorge P. Strogoff de Matos 《Hemodialysis international. International Symposium on Home Hemodialysis》2014,18(2):467-472

Systemic anticoagulation with unfractionated heparin is commonly used in maintenance hemodialysis (HD), but it increases the risk of bleeding complications. We investigated whether the use of citrate‐enriched bicarbonate based dialysate (CD) would reduce systemic anticoagulation without compromising the efficacy of reprocessed dialyzers. This is a crossover study in which half of a total of 30 patients initially underwent HD with acetate‐enriched bicarbonate based dialysate and a standard heparin dose of ～100 IU/kg (Treatment A), whereas the remaining patients were treated with CD and a 30% reduced heparin dose (Treatment B). After 12 consecutive HD sessions in each treatment, the dialysate and heparin doses were reversed, then followed for another period of 12 HD sessions. The two treatment phases were split by a washout period of six HD sessions using acetate‐enriched bicarbonate based dialysate and standard heparin dose. Systemic anticoagulation was higher in Treatment A. The activated partial thromboplastin time at the end of HD session was 68 ± 36 seconds in Treatment A and 47 ± 16 seconds in Treatment B (P = 0.005). Sixty‐eight percent of the dialyzers remained adequate until the 12th use in Treatment A and 61% did so in Treatment B (P = 0.63). Patients had three and 24 cramps episodes during Treatment A and B, respectively (P < 0.001). Nine and 26 symptomatic intradialytic hypotension episodes were seen in Treatment A and B, respectively, (P = 0.003). In conclusion, the use of CD had a favorable effect on anticoagulation in the extracorporeal circuit in patients on maintenance HD, but it was also associated with more hypotension and cramps.     

14.

Cardiovascular disease on hemodialysis: Predictors of atherosclerosis and survival     

Ahmed BALSAM  Mohsen M. EL KOSSI  Robert LORD  A. Meguid EL NAHAS 《Hemodialysis international. International Symposium on Home Hemodialysis》2009,13(3):278-285

Cardiovascular disease (CVD) is the leading cause of mortality in hemodialysis (HD) patients. This could not be explained by the known traditional CVD risk factors. In this study, we attempted to elucidate the factors influencing atherosclerosis, as measured by carotid artery intima-media thickness (IMT), in HD patients and their impact on cardiovascular mortality. A cohort of 50 patients started on HD was selected for this study. At baseline, IMT and the presence of atheromatous plaques were assessed. Plasma homocysteine (Hcy), malondialdehyde, total antioxidant capacity, von Willebrand factor, vitamins C, E, B₆, B₁₂, folate, and C-reactive protein (CRP) were also measured. Patients were followed up for 2 years to determine the impact of IMT and associated markers on mortality using survival analysis as well as Cox proportional hazard. At baseline, 40% of the patients had IMT>0.8 mm. They were older, had higher CRP (P<0.001), and lower serum albumin (P=0.03). Intima-media thickness >0.8 mm was associated with high calcium (risk ratio [RR]: 6.06; confidence interval [CI]: 0.75–12.25) and CRP (RR: 10.94 [CI: 2.56–46.74]). Fifteen patients (30%) died during the 2-year follow-up; the main cause of death was CVD (42%). The relative risk mortality was high with increased IMT (RR: 120.04 [CI: 4.18–3445.9]), Index of Coexistent Disease for CVD (RR: 4.04 [CI: 1.92–8.5]), and plasma Hcy (RR: 1.08 [CI: 1.02–1.13]). Markers of inflammation and increased serum calcium were significant predictors of increased carotid artery IMT. High IMT, Index of Coexistent Disease, and Hcy were associated with a high RR of all-cause mortality among a cohort of HD patients.     

15.

CREEP-FATIGUE CRACK GROWTH BEHAVIOR IN 1Cr-1Mo-0.25V STEELS. PART I: ESTIMATION OF CRACK TIP PARAMETERS   总被引：1，自引：0，他引：1  

N. Adefris  A. Saxena  D. L. McDowell   《Fatigue & Fracture of Engineering Materials & Structures》1996,19(4):387-398

Abstract— Non-linear finite element (FEM) analyses, involving various creep deformation laws, as well as experiments with hold times of 100 s and 15 min were performed on compact type specimens with stationary cracks. The work was aimed at developing accurate expressions for estimating the small-scale creep parameters, ( C _t)_avg, for 1Cr-1Mo-0.25V steel at 538°C (1000°F). Here ( C _t)_avg is a representative value for the small-scale creep parameter C _t, averaged over a hold period under the conditions of creep-fatigue loading. These expressions were then applied to crack growth data obtained from specimens tested under the various hold times. When an elastic-plastic-primary creep-secondary creep constitutive model was used in the FEM analyses, the calculated values of C _t compared well with the measured values. The FEM results also showed that the accumulated creep deformation during the hold time was not significantly reversed during the unloading portion of the creep-fatigue cycle for this material. Therefore, a new method of estimating (C_t)_avg is proposed on the basis of these numerical and experimental results.     

16.

Prescriptions of dialysate potassium concentration during short daily or long nocturnal (high dose) hemodialysis          下载免费PDF全文

John K. Leypoldt  Baris U. Agar  Angelito A. Bernardo  Bruce F. Culleton 《Hemodialysis international. International Symposium on Home Hemodialysis》2016,20(2):218-225

     

17.

ELEVATED TEMPERATURE FRACTURE OF PARTICULATE-REINFORCED ALUMINUM PART I: FRACTURE TOUGHNESS     

B. P. Somerday    Yang Leng  R. P. Gangloff 《Fatigue & Fracture of Engineering Materials & Structures》1995,18(5):565-582

Abstract—The plane-strain initiation and growth fracture toughnesses of powder-metallurgy-processed, SiC particulate-reinforced 2009 plate were measured at temperatures from 25°C to 316°C. Initiation toughness from electrical potential monitoring ( K _JICi) is 18 MPa°m at 25°C, and is nearly constant to 220°C before decreasing sharply to 6 MPa°m at 316°C. Growth toughness, given by the tearing modulus ( T _R), is less than 3 from 25°C to 125°C, and increases dramatically above 200°C. The magnitude and temperature dependence of initiation toughness depend on detection of the critical fracture event. Standard measures of toughness K _IC and K _JIC exceed K _JICi and increase to a plateau with increasing temperature. The fracture mode for the composite is microvoid nucleation, growth and coalescence at all temperatures. Void nucleation is associated with SiC; such particles both crack and create stress and plastic strain concentrations that rupture the interface or adjacent matrix, particularly at corners. Matrix plasticity and cavitation increase with increasing temperature. Void growth is regular at all temperatures, but limited by adjacent SiC particles. Both K _JICi and T _R are governed by the temperature-dependent crack-tip plastic stress and strain fields, and the intrinsic damage resistance of the composite microstructure.     

18.

NON-ISOTHERMAL FATIGUE CRACK GROWTH IN HASTELLOY-X     

N. J. Marchand  R. M. Pelloux  B. Ilschner 《Fatigue & Fracture of Engineering Materials & Structures》1987,10(1):59-74

Abstract— Non-isothermal fatigue crack growth tests were performed on Hastelloy-X single edge notch specimens in which strain and temperature were varied simultaneously. Conditions were selected to include nominally elastic and nominally plastic conditions and temperatures up to 925°C. The crack growth rates were first reported as a function of the strain intensity factor (δ K _ε) derived from a crack compliance analysis. Out-of-phase (ε_max at T _max) cycling showed faster crack growth rates than isothermal or in-phase (ε_max at T _max) cycling under elastic straining. Under fully plastic cycling, the opposite results was observed, i.e. crack growth rates under isothermal cycling are faster than under TMF cycling. On a δ K _ε-basis, a strain range effect was observed. All the results were rationalized using a corrected stress-intensity factor (δ K _eff) computed from the actual load, the closing bending moment caused by the increase compliance with crack length, and with the effective opening stress. Each mode of fracture was found to be characterized by a unique crack growth rate vs δ K _eff curve. On a δ K _eff-basis, the isothermal crack growth rates at T _min and T _max provide an upper and a lower bound for the TMFCG rates. The effectiveness of δ K _eff to correlate crack growth rates under fully plastic cycling is discussed in detail.     

19.

EFFECT OF WAVEFORM ON CORROSION FATIGUE CRACK GROWTH     

S. KAWAI  K. KOIBUCHI 《Fatigue & Fracture of Engineering Materials & Structures》1979,1(4):395-407

Abstract A study was made on the effects of stress rise time T ₁, maximum stress holding time T ₂, stress decreasing time T ₃ and minimum stress holding time T ₄ of a cycle on fatigue crack growth for a low alloy carbon steel in 3% NaCl solution. Measurements of the effective stress intensity range ratio U and observations of crack tip response were performed to clarify the causes of waveform effects.
The results were summarized as follows; T ₁ had a strong accelerating effect due to corrosive dissolution of the fresh surfaces of the crack which were formed during T ₁. The crack growth rate was enhanced as T ₁ increased and reached a constant value (about 3 times that in air) after T ₁= 10s. The crack growth rate at low Δ K , however, decreased as T ₁ increased more than T ₁= 1 s. T ₂, T ₃ and T ₄ decreased the crack growth rate. The extent of decrease not only depended on the period of T ₂ (or T ₃, T ₄), but also on Δ K and T ₁. A previously derived crack growth law which considered waveform and frequency effects, is also valid to a first approximation for the present results.     

20.

DETERMINATION OF THE ENERGETIC PARAMETERS CONTROLLING CLEAVAGE FRACTURE INITIATION IN STEELS   总被引：1，自引：0，他引：1  

M. A. Linaza  J. M. Rodriguez-Ibabe  J. J. Urcola 《Fatigue & Fracture of Engineering Materials & Structures》1997,20(5):619-632

Abstract— The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies γ_pm and γ_mm, which define the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the γ_pm value remains constant around 7 J/m², it has been observed that γ_mm is dependent on temperature. At −196°C the value is lower than 50 J/m² and at room temperature it is higher than 200 J/m². This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.     

Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr-1 Mo pressure vessel steel

Tests were performed on a 2¼  Cr–1  Mo steel to measure the fracture toughness at initiation, K _Ic and at arrest, K _Ia . The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure K _Ia : (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to −196 °C and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (∼500  m  s⁻¹ ) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic–plastic finite element method to determine K _Ia values. These tests reveal a greater temperature shift (∼100 °C) between K _Ic and K _Ia in 2¼  Cr–1  Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼  Cr–1  Mo steel showed that this involves the nucleation of a three-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism     

Studies of phosphate dynamics during hemodialysis

Regulation of phosphate (PO₄) in hemodialysis patients is very difficult and ideal levels are rarely maintained. A high removal and a normal phosphate level is important, as high and low levels are both associated with morbidity and a very high mortality.
We studied phosphate dynamics and its relation to other small "uremic" molecules in 48 patients by measuring pre‐ and postdialysis levels and all removed phosphate, urea and creatinine (creat) in all dialysate during 455 dialyses done at different frequencies (freq): 3.7 ± 1.2, range 3–6 treatments per week and durations of dialysis (t): mean: 196 ± 95, range 80–560 min and with high (HF) and low flux membranes.
Kt/V‐PO₄, Kt/V‐urea and Kt/V‐creat, volumes (Vr) for all solutes and their relationships to frequency and duration of dialysis, urea clearance and predialysis phosphate were calculated.  

     

Fatigue strength of crack-healed Si3N4/SiC composite ceramics

Si₃ N₄ /SiC composite ceramics were sintered and subjected to three-point bending on specimens made according to the appropriate JIS standard. A semi-circular surface crack of 110 μm in diameter was made on each specimen. By using three kinds of specimen (smooth, cracked and crack healed), crack-healing behaviour, cyclic and static fatigue strengths were determined systematically at room temperature and 1000 °C. The main conclusions are as follows: (i) Si₃ N₄ /SiC composite ceramics have the ability to heal after cracking; (ii) crack-healed specimens showed similar cyclic and static fatigue strengths as smooth specimens, this being caused by crack healing; (iii) crack-healed zones had a sufficient fatigue strength and most fractures occurred outside the pre-cracked zone in those crack-healed specimens.     

Effects of citrate‐enriched bicarbonate based dialysate on anticoagulation and dialyzer reuse in maintenance hemodialysis patients

Systemic anticoagulation with unfractionated heparin is commonly used in maintenance hemodialysis (HD), but it increases the risk of bleeding complications. We investigated whether the use of citrate‐enriched bicarbonate based dialysate (CD) would reduce systemic anticoagulation without compromising the efficacy of reprocessed dialyzers. This is a crossover study in which half of a total of 30 patients initially underwent HD with acetate‐enriched bicarbonate based dialysate and a standard heparin dose of ～100 IU/kg (Treatment A), whereas the remaining patients were treated with CD and a 30% reduced heparin dose (Treatment B). After 12 consecutive HD sessions in each treatment, the dialysate and heparin doses were reversed, then followed for another period of 12 HD sessions. The two treatment phases were split by a washout period of six HD sessions using acetate‐enriched bicarbonate based dialysate and standard heparin dose. Systemic anticoagulation was higher in Treatment A. The activated partial thromboplastin time at the end of HD session was 68 ± 36 seconds in Treatment A and 47 ± 16 seconds in Treatment B (P = 0.005). Sixty‐eight percent of the dialyzers remained adequate until the 12th use in Treatment A and 61% did so in Treatment B (P = 0.63). Patients had three and 24 cramps episodes during Treatment A and B, respectively (P < 0.001). Nine and 26 symptomatic intradialytic hypotension episodes were seen in Treatment A and B, respectively, (P = 0.003). In conclusion, the use of CD had a favorable effect on anticoagulation in the extracorporeal circuit in patients on maintenance HD, but it was also associated with more hypotension and cramps.     

Cardiovascular disease on hemodialysis: Predictors of atherosclerosis and survival

Cardiovascular disease (CVD) is the leading cause of mortality in hemodialysis (HD) patients. This could not be explained by the known traditional CVD risk factors. In this study, we attempted to elucidate the factors influencing atherosclerosis, as measured by carotid artery intima-media thickness (IMT), in HD patients and their impact on cardiovascular mortality. A cohort of 50 patients started on HD was selected for this study. At baseline, IMT and the presence of atheromatous plaques were assessed. Plasma homocysteine (Hcy), malondialdehyde, total antioxidant capacity, von Willebrand factor, vitamins C, E, B₆, B₁₂, folate, and C-reactive protein (CRP) were also measured. Patients were followed up for 2 years to determine the impact of IMT and associated markers on mortality using survival analysis as well as Cox proportional hazard. At baseline, 40% of the patients had IMT>0.8 mm. They were older, had higher CRP (P<0.001), and lower serum albumin (P=0.03). Intima-media thickness >0.8 mm was associated with high calcium (risk ratio [RR]: 6.06; confidence interval [CI]: 0.75–12.25) and CRP (RR: 10.94 [CI: 2.56–46.74]). Fifteen patients (30%) died during the 2-year follow-up; the main cause of death was CVD (42%). The relative risk mortality was high with increased IMT (RR: 120.04 [CI: 4.18–3445.9]), Index of Coexistent Disease for CVD (RR: 4.04 [CI: 1.92–8.5]), and plasma Hcy (RR: 1.08 [CI: 1.02–1.13]). Markers of inflammation and increased serum calcium were significant predictors of increased carotid artery IMT. High IMT, Index of Coexistent Disease, and Hcy were associated with a high RR of all-cause mortality among a cohort of HD patients.     

CREEP-FATIGUE CRACK GROWTH BEHAVIOR IN 1Cr-1Mo-0.25V STEELS. PART I: ESTIMATION OF CRACK TIP PARAMETERS

Abstract— Non-linear finite element (FEM) analyses, involving various creep deformation laws, as well as experiments with hold times of 100 s and 15 min were performed on compact type specimens with stationary cracks. The work was aimed at developing accurate expressions for estimating the small-scale creep parameters, ( C _t)_avg, for 1Cr-1Mo-0.25V steel at 538°C (1000°F). Here ( C _t)_avg is a representative value for the small-scale creep parameter C _t, averaged over a hold period under the conditions of creep-fatigue loading. These expressions were then applied to crack growth data obtained from specimens tested under the various hold times. When an elastic-plastic-primary creep-secondary creep constitutive model was used in the FEM analyses, the calculated values of C _t compared well with the measured values. The FEM results also showed that the accumulated creep deformation during the hold time was not significantly reversed during the unloading portion of the creep-fatigue cycle for this material. Therefore, a new method of estimating (C_t)_avg is proposed on the basis of these numerical and experimental results.     

ELEVATED TEMPERATURE FRACTURE OF PARTICULATE-REINFORCED ALUMINUM PART I: FRACTURE TOUGHNESS

Abstract—The plane-strain initiation and growth fracture toughnesses of powder-metallurgy-processed, SiC particulate-reinforced 2009 plate were measured at temperatures from 25°C to 316°C. Initiation toughness from electrical potential monitoring ( K _JICi) is 18 MPa°m at 25°C, and is nearly constant to 220°C before decreasing sharply to 6 MPa°m at 316°C. Growth toughness, given by the tearing modulus ( T _R), is less than 3 from 25°C to 125°C, and increases dramatically above 200°C. The magnitude and temperature dependence of initiation toughness depend on detection of the critical fracture event. Standard measures of toughness K _IC and K _JIC exceed K _JICi and increase to a plateau with increasing temperature. The fracture mode for the composite is microvoid nucleation, growth and coalescence at all temperatures. Void nucleation is associated with SiC; such particles both crack and create stress and plastic strain concentrations that rupture the interface or adjacent matrix, particularly at corners. Matrix plasticity and cavitation increase with increasing temperature. Void growth is regular at all temperatures, but limited by adjacent SiC particles. Both K _JICi and T _R are governed by the temperature-dependent crack-tip plastic stress and strain fields, and the intrinsic damage resistance of the composite microstructure.     

NON-ISOTHERMAL FATIGUE CRACK GROWTH IN HASTELLOY-X

Abstract— Non-isothermal fatigue crack growth tests were performed on Hastelloy-X single edge notch specimens in which strain and temperature were varied simultaneously. Conditions were selected to include nominally elastic and nominally plastic conditions and temperatures up to 925°C. The crack growth rates were first reported as a function of the strain intensity factor (δ K _ε) derived from a crack compliance analysis. Out-of-phase (ε_max at T _max) cycling showed faster crack growth rates than isothermal or in-phase (ε_max at T _max) cycling under elastic straining. Under fully plastic cycling, the opposite results was observed, i.e. crack growth rates under isothermal cycling are faster than under TMF cycling. On a δ K _ε-basis, a strain range effect was observed. All the results were rationalized using a corrected stress-intensity factor (δ K _eff) computed from the actual load, the closing bending moment caused by the increase compliance with crack length, and with the effective opening stress. Each mode of fracture was found to be characterized by a unique crack growth rate vs δ K _eff curve. On a δ K _eff-basis, the isothermal crack growth rates at T _min and T _max provide an upper and a lower bound for the TMFCG rates. The effectiveness of δ K _eff to correlate crack growth rates under fully plastic cycling is discussed in detail.     

EFFECT OF WAVEFORM ON CORROSION FATIGUE CRACK GROWTH

Abstract A study was made on the effects of stress rise time T ₁, maximum stress holding time T ₂, stress decreasing time T ₃ and minimum stress holding time T ₄ of a cycle on fatigue crack growth for a low alloy carbon steel in 3% NaCl solution. Measurements of the effective stress intensity range ratio U and observations of crack tip response were performed to clarify the causes of waveform effects.
The results were summarized as follows; T ₁ had a strong accelerating effect due to corrosive dissolution of the fresh surfaces of the crack which were formed during T ₁. The crack growth rate was enhanced as T ₁ increased and reached a constant value (about 3 times that in air) after T ₁= 10s. The crack growth rate at low Δ K , however, decreased as T ₁ increased more than T ₁= 1 s. T ₂, T ₃ and T ₄ decreased the crack growth rate. The extent of decrease not only depended on the period of T ₂ (or T ₃, T ₄), but also on Δ K and T ₁. A previously derived crack growth law which considered waveform and frequency effects, is also valid to a first approximation for the present results.     

DETERMINATION OF THE ENERGETIC PARAMETERS CONTROLLING CLEAVAGE FRACTURE INITIATION IN STEELS

Abstract— The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies γ_pm and γ_mm, which define the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the γ_pm value remains constant around 7 J/m², it has been observed that γ_mm is dependent on temperature. At −196°C the value is lower than 50 J/m² and at room temperature it is higher than 200 J/m². This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.