Failure-survivability

Study Patients Adjuvant therapy a Local failure Survival... 

These data clearly linked the ACE D allele with signiflcantly poorer transplant-free survival. This effect was primarily evident in patients not treated with P-blockers and was not seen in patients receiving adrenergic therapy. These findings suggest a potential pharmacogenetic interaction between the ACE D/I polymorphism and therapy with P-blockers in the determination of heart failure survival [16]. 

Pitt B, Poole-Wilson PA, Segal R, et al. Effect of losartan compared with captopril on mortality in patients with symptomatic heart failure randomised trial—the Losartan Heart Failure Survival Study ELITE II. Lancet 2000 355 1582-1587. 

Northridge D 1996 Frusemide or nitrates for acute heart failure [see comments] Lancet 347 667-668 Peters N S et al 2002 Atrial fibrillation strategies to control, combat and cure. Lancet 359 593-603 Pitt B et al 2000 Effect of losartan compared with captopril on mortality in patients with symptomatic heart failure randomised trial — the Losartan Heart Failure Survival Study ELITE II. [see comments] Lancet 355 1582-1587 Podrid P J 1999 Redefining the role of antiarrhythmic drugs. New England Journal of Medicine 340 1910-1912... 

Deedwania PC, Singh BN, Ellenbogen K, Fisher S, Fletcher R, Singh SN. Spontaneous conversion and maintenance of sinus rhythm by amiodarone in patients with heart failure and atrial fibrillation observations from the veterans affairs congestive heart failure survival trial of antiarrhythmic therapy (CHF-STAT). The Department of Veterans Affairs CHF-STAT Investigators. Circulation 1998 98 2574-9. 

Almost all patients with cocaine rhabdomyolysis without renal failure survive and are discharged after an average hospital stay of 5 days. The patients with acute renal failure require hemodialysis, and have both a lower survival rate and a more prolonged hospitalization. Of patients with acute renal failure who die, most did so between 2 to 15 days after admission with associated disseminated intravascular coagulation and severe liver dysfunction. Autopsies on these patients showed no evidence of pre-existing renal disease or underlying glomerulopathy. 

In high-integrity control and protection systems, some failure-survival capability is derived from the use of redundancy ie the replication of the system hardware by constructing a number of active identical subsystems and combining and voting on the subsystem outputs. 

Failure-survivability, ensures that a failure does not result in a significant loss of performance. This is usually achieved by some form of separate back-up or multiple redundancy. 

The expected life is sometimes used as an iadicator of system rehabihty however, it can be a false iadication and should be used with caution. In most test situations the chance of surviving the expected life is not 50% and depends on the undedyiag failure pattern. For example, considering the exponential as used ia equation 10, the expected life would be... 

The ha2ard function can be interpreted as the instantaneous failure rate. The quantity b(i)At for small A/ represents the probabiUty of failure in the interval At, given that the device was surviving at the beginning of the interval. 

Conditional Failure Probability. The concept of conditional piobabihty of faiuie is useful to predict the chances of survival for a device that has been in operation for a period of time and is not in a failed state. Such information is helpful for maintenance planning. 

Example 3. A centrifugal pump moving a corrosive Hquid is known to have a time-to-failure that is well approximated by a normal distribution with a mean of 1400 h and a standard deviation of 120 h. A particular pump has been in operation for 1080 h. In order to plan maintenance activities the chances of the pump surviving the next 48 h must be deterrnined. 

From equation 8 it was shown that the chance of surviving the mean life was 36.8% for the exponential distribution. However, this fact must be used with some degree of rationaHty in appHcations. For example, in the above situation the longest surviving MPU that was observed survived for 291.9 hours. The failure rate beyond this time is not known. What was observed was only a failure rate of A = 1.732 x lO " failures per hour over approximately 292 hours of operation. In order to make predictions beyond this time, it must be assumed that the failure rate does not increase because of wearout and... 

The chalk with which I write on the blackboard when I teach is a brittle solid. Some sticks of chalk are weaker than others. On average, I find (to my slight irritation), that about 3 out of 10 sticks break as soon as I start to write with them the other 7 survive. The failure probability, Pf, for this chalk, loaded in bending under my (standard) writing load is 3/10, that is... 

This, then, is our final design equation. It shows how the survival probability depends on both the stress (rand the volume V of the component. In using it, the first step is to fix on an acceptable failure probability, Pp 0.3 for chalk, 10 for the cutting tool, 10 for the vacuum-chamber window. The survival probability is then given by P = 1 -. ... 

First, one pump running in a parallel system tends to suffer from cavitation because operation to the right of the BF.P indicates that the NPSHr of the pump rises drastically. To survive this condition, you should use dual mechanical seals on these pumps. Dual or double mechanical seals can withstand cavitation better than a single seal. There is a discussion on this in the mechanical seal chapter of this book. Many engineers perceive that parallel pumps are problematic becau.se they appear to suffer a lot of premature seal failure. Parallel pumps deserve double seals even if it s only a cold water system. 

Figure 4.41 shows the Stress-Strength Interference (SSI) diagrams for the two assembly operation failure modes. The instantaneous stress on the relief section on first assembly is composed of two parts first the applied tensile stress,. v, due to the pre-load, F, and secondly, the torsional stress, t, due to the torque on assembly, M, and this is shown in Figure 4.41(a) (Edwards and McKee, 1991). This stress is at a maximum during the assembly operation. If the component survives this stress, it will not fail by stress rupture later in life. 

However there are NV such defects in the specimen, where N is the number of pores per unit volume and V is the specimen volume. Each has a survival probability of one minus its failure probability, i.e.,... 

The total survival probability of the volume V under stress a, containing 2NV crack tips, is thus (Ps), and the total probability of failure of the specimen or stressed volume may be written as ... 

However, examples of the failure of thioketals to survive lithium aluminium hydride reduction have been reported (see ref. 134). 

For example, consider tlie case of a standby redundancy system consisting of one operating pump and two on standby so that tlie system can survive two failures. Assume tlutt tlie pumps are independent with respect to failure and tliat each lias a probability of failure of 0.1. Thus, n = 3, r = 2, 1 - p = 0.9, and tlie reliability of the system is found from Eq. (20.5.3)... 

Tliis is tlie probability if n or fewer failures in the time period specified by t. For example, consider tlie case of a standby redundancy system with one operating unit and one on standby (i.e., a system tliat can survive one failure). If the failure rate is 2 units per year, then tlie 6-nionth reliability of the system is... 

For each of the 36 bus sections tliat had not already failed, the Weibull distribution was used to detennine tlie probability of failure before tlie next outage. Under assumption (a), tliis probability is P(T < 3301T > 209) i.e., tlie conditional probability of failure before 330 days, given tliat tlie bus section lias survived 209 days. Under assumption (b), tlie corresponding probability is P(T < 330 T > 230). For part (b), tlie estimates of the Weibull distribution parameters used in part (a) were modified to take into consideration tlie absence of failures for 3 additional weeks. 

In medical follow-up studies on the length of survival of patients after treatment, contact with some patients may be lost or some may die from causes unrelated to the disease treated. Similarly, in engineering studies, units may be lost or fail for some reason unrelated to the failure mode of interest for example, a unit on test may be removed from a test or destroyed accidentally. 

Maraging steel welds are somewhat less resistant than base plate. U -bend exposure of 1 240 MNm strength welds survived for up to 2 years in seawater while at 1 380 MNm failures occurred in 2-18 months. ... 

Two conclusions can be drawn from an inspection of the S-N curve (1) the higher the applied material stress or strain, the fewer cycles the specimen can survive and (2) the curve gradually approaches a stress or strain level called the fatigue endurance limit below which the material is much less susceptible to fatigue failure. Different materials may... 

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