Non-repairable Failures Research Articles

Endocrown restorations in premolars: influence of remaining axial walls of tooth structure and restorative materials on fatigue resistance.

To evaluate the effect of the remaining tooth structure and different CAD/CAM materials on the fatigue performance and failure mode of endodontically treated premolars restored with endocrowns. Ninety maxillary premolars were endodontically treated and assignedinto 6 groups (n = 15) according to the number of remaining axial walls (four, three, and two) and restorative materials (ultra-translucent zirconia 5Y-PSZ [KATANA UTML] and lithium disilicate [IPS e.max-CAD]). The specimens were subjected tocyclic fatigue loading test(initial load 200 N; 20Hz). An incremental step load of 100N per 10,000 cycles was applied until failure. The fatigue failure load (FFL) and number of failure cycles (CFFs) datawere statistically analyzed with two-way ANOVA and Kaplan-Meier test (α = 0.05). Failedspecimens were examined under a stereomicroscope 25 × and failure modes were determined. FFL and CFF were significantly influenced by restorative material (p < 0.05). 5Y-PSZ endocrowns showed significantly higher FFL when compared with lithium disilicate. The number of remaining walls did not affect the fatigue behavior or failure mode of the specimens. Of the lithium disilicate restorations, 51% had repairable failures, while 95% 5Y-PSZ restorations had non-repairable failures. Zirconia endocrowns showed better FFL than lithium disilicate endocrowns, regardless of the number of remaining axis walls. Lithium disilicate and 5Y-PSZ endocrowns showed FFL higher than the normal masticatory loads. Restoring endodontically treated premolars with endocrown could be a promising treatment, regardless of the remaining axial walls. However, precaution should be taken in material selection since it affects the fatigue resistance and failure mode.

MMAPs to Model Complex Multi-State Systems with Vacation Policies in the Repair Facility

Two complex multi-state systems subject to multiple events are built in an algorithmic and computational way by considering phase-type distributions and Markovian arrival processes with marked arrivals. The internal performance of the system is composed of different degradation levels and internal repairable and non-repairable failures can occur. Also, the system is subject to external shocks that may provoke repairable or non-repairable failure. A multiple vacation policy is introduced in the system for the repairperson. Preventive maintenance is included in the system to improve the behaviour. Two types of task may be performed by the repairperson; corrective repair and preventive maintenance. The systems are modelled, the transient and stationary distributions are built and different performance measures are calculated in a matrix-algorithmic form. Cost and rewards are included in the model in a vector matrix way. Several economic measures are worked out and the net reward per unit of time is used to optimize the system. A numerical example shows that the system can be optimized according to the existence of preventive maintenance and the distribution of vacation time. The results have been implemented computationally with Matlab and R (packages: expm, optim).

Reliability evaluation of generating systems considering aging processes

Equipment aging has been a major concern among electric utilities’ planners, since quality of service can be put at risk. This paper proposes a new method that allows the effects of the aging of generators to be properly included into the reliability analysis of power systems. First, a new model for the failure rate capable of representing the stages of normal operation and wear-out, replacing the adoption of constant failure rates, is described. The effects of repairable and non-repairable (i.e., catastrophic) failures on the reliability assessment of generation systems are also discussed. The proposed method is built from the previous models using a sequential Monte Carlo simulation and variance reduction techniques. To validate the proposed method, the IEEE Reliability Test System is used with modifications in which different ages are defined for existing generating units that are progressively aging over the years. Several tests are described together with strategies for adopting preventive refurbishments and the corresponding results are duly discussed.

Fracture Resistance of Three-unit Fixed Dental Prostheses Fabricated with Milled and 3D Printed Composite-based Materials

To evaluate the fracture resistance of three-unit fixed dental prosthesis (FDP) made of composite, high-density polymers (HDP), fiber-reinforced composite (FRC), and metal-ceramic (MC) using different fabrication methods. A typodont model was prepared to receive a three-unit FDP replacing a missing second maxillary premolar. The prepared model was digitally scanned using an intraoral scanner (Trios3, 3Shape, Denmark). In total, 60 FDPs were fabricated and divided into four groups (n = 15) according to the materials and fabrication method: the subtractive method was used for the FRC (Trilor, Bioloren, Italy) and the HDP (Ambarino, Creamed, Germany) groups; the HDP group was monolithic, whereas the FRC group was layered with a nanocomposite (G-aenial Sculpt, GC). The additive method was used for the 3D printed (3DP) nanocomposite (Irix Max, DWS, Italy) and the Cr-Co (Starbond CoS powder 30) infrastructure of the MC groups. The FDPs were adhesively seated on stereolithography (SLA) fabricated dies. All samples were subjected to thermomechanical loading and fracture testing. The data for maximum load (N) to fracture was statistically analyzed with one-way analysis of variance (ANOVA) followed by Games-Howell post hoc test (α = 0.05). The MC group reported the highest fracture resistance with a statistically significant difference (2390.87 ± 166.28 N) compared to other groups. No significance was noted between 3DP and HDP groups (1360.20 ± 148.15 N and 1312.27 ± 64.40 N, respectively), while the FRC group displayed the lowest value (839.07 ± 54.30 N). The higher frequency of nonrepairable failures was observed in the MC and FRC groups, while HDP and 3DP groups reported a high frequency of repairable failures. Significant differences were found in fracture resistance between the tested groups. The load-bearing capacity of the composite-based FPDs exceeded the range of maximum chewing forces. 3D printed and milled composite-based materials might offer a suitable solution for the fabrication of FPDs.

A complex multi-state k-out-of-n: G system with preventive maintenance and loss of units

In this study, a multi-state k-out-of-n: G system subject to multiple events is modeled through a Markovian Arrival Process with marked arrivals. The system is composed initially of n units and is active when at least k units are operational. Each unit is multi-state, each of which is classified as minor or major according to the level of degradation presented. Each operational unit may undergo internal repairable or non-repairable failures, external shocks and/or random inspections. An external shock can provoke extreme failure, while cumulative external damage can deteriorate internal performance. This situation can produce repairable and non-repairable failures. When a repairable failure occurs the unit is sent to a repair facility for corrective repair. If the failure is non-repairable, the unit is removed. When the system has insufficient units with which to operate, it is restarted. Preventive maintenance is employed in response to random inspection. The system is modeled in an algorithmic and computational form. Several interesting measures of performance are considered. Costs and rewards are included in the system. All measures are obtained for transient and stationary regimes. A numerical example is analyzed to determine whether preventive maintenance is profitable, financially and in terms of performance.

Fracture strengths of endocrown restorations fabricated with different preparation depths and CAD/CAM materials.

The objectives of this study were to compare the fracture strength of endocrown restorations fabricated with different preparation depth and various CAD/CAM ceramics, and to assess the fracture types. Endodontically treated 100 extracted human permanent maxillary centrals were divided into two preparation depth groups as short (S: 3-mm-deep) and long (L: 6-mm-deep), then five ceramic subgroups, namely: feldspathic-ceramic (Vita Mark II-VM2), lithium-disilicate glass-ceramic (IPS e.max CAD-E.max), resin-ceramic (LAVA Ultimate-LU), polymer infiltrated ceramic (Vita Enamic-VE) and monoblock zirconia (inCoris TZI-TZI) (n=10/subgroup). The endocrowns were fabricated by CAD/CAM and were cemented with resin cement (RelyX U200). The teeth were thermally cycled (5,000cycles) and fracture tests were performed at 45º angle to the teeth. The data were statistically analyzed (Kruskal-Wallis, Mann Whitney U), failure modes were evaluated with stereomicroscopy. Zirconia group provided the statistically highest fracture strength, but also exhibited non-repairable failures. Preparation depth has an effect on the fracture strength only for feldspathic ceramic.

Optimisation of preventive maintenance policies with replacement first and last for systems subject to shocks

This paper addresses preventive replacement policies for operating systems subject to non-homogeneous Poisson process shocks. A shock causes two types of system failures with number-dependent probability: type-I (repairable or minor) failures can be rectified by minimal repair; type-II (non-repairable or catastrophic) failures must be removed by corrective replacement. A preventive replacement model is designed to replace the system before a type-II failure occurs. Similar to conventional replacement models, our model bases replacement on type-II failures at age T or number of working times N; however, contrary to conventional models, our model replaces the system at the last occurring age or working time. Thus, we refer to our model as a preventive replacement last model and existing methods as conventional replacement first models. Comparisons between these models are discussed in detail. The optimal schedule that minimises the mean cost rate of each preventive replacement model is derived analytically and computed numerically. The proposed model provides a general framework for analysing policies and other special cases of previous models.

Reliability analysis of a single warm-standby system subject to repairable and nonrepairable failures

An n-unit system provisioned with a single warm standby is investigated. The individual units are subject to repairable failures, while the entire system is subject to a nonrepairable failure at some finite but random time in the future. System performance measures for systems observed over a time interval of random duration are introduced. Two models to compute these system performance measures, one employing a policy of block replacement, and the other without a block replacement policy, are developed. Distributional assumptions involving distributions of phase type introduce matrix Laplace transformations into the calculations of the performance measures. It is shown that these measures are easily carried out on a laptop computer using Microsoft Excel. A simple economic model is used to illustrate how the performance measures may be used to determine optimal economic design specifications for the warm standby.

A preventive maintenance policy for a standby system subject to internal failures and external shocks with loss of units

In many situations, serious damage and considerable financial losses are caused by non-repairable failures of a system. Redundant systems and maintenance policies are commonly employed to improve reliability. This paper is focused on the modelling of a complex cold standby system by analysing the effectiveness and costs of preventive maintenance, always in an algorithmic form. The online unit of the system is subject to wear failures and external shocks. The online unit can go through an indeterminate number of degradation levels before failure. This one is observed when inspections occur. Inspections are performed at random intervals, and when one takes place, the unit is taken to the preventive maintenance facility if it is necessary. The preventive maintenance time and cost is different depending on the degradation level observed. If only one unit is performing, a minimal maintenance policy is adopted in order to optimise system behaviour. Reliability measures such as the conditional probability of failure are worked out in a well-structured and algebraic form in transient and stationary regimes by using algorithmic methods. The stationary distribution is calculated using matrix analytic methods, and rewards are included in the model. An optimisation example shows the versatility of the model presented.

Discrete Repairable Systems With External and Internal Failures Under Phase-Type Distributions

We study a multi-component system subject to internal, and external failures. The external failures may be repairable, or not. The unit enters the repair channel when a repairable failure occurs and there is a repairman available. The random times involved in the study of the system are phase-type distributed. From this system, new ones are developed. General systems with internal, and external repairable, or non-repairable failures are built, generalizing the initial system. The availability, and the conditional probability of failure for the different types of failures and systems are calculated for finite, and infinite numbers of units. The results are implemented computationally with Matlab, and illustrated using examples.

Reward optimization of a repairable system

This paper analyzes a system subject to repairable and non-repairable failures. Non-repairable failures lead to replacement of the system. Repairable failures, first lead to repair but they lead to replacement after a fixed number of repairs. Operating and repair times follow phase type distributions (PH-distributions) and the pattern of the operating times is modelled by a geometric process. In this context, the problem is to find the optimal number of repairs, which maximizes the long-run average reward per unit time. To this end, the optimal number is determined and it is obtained by efficient numerical procedures.

Using Spice Circuit Simulation Program in Reliability Analysis of Redundant Systems with Non-Repairable Units and Common-Cause Failures

The effectiveness of Simulation Program with Integrated Circuit Emphasis (SPICE) in calculating probabilities, reliability, steady-state availability and mean-time to failure of redundant systems with non-repairable units and common-cause failures described by Markov models is demonstrated. General equations and procedure for constructing the equivalent circuit forNparallel units are presented. Results obtained, forN=1,2,3, using SPICE are compared with previously published results obtained using the Laplace transform method. Full SPICE listings are included.

On the Replenishment of Repair Kit Inventories for Systems with Life Distributions of Phase Type

Several models are developed for the evaluation of replenishment policies for an inventory of repair kits used by a system subject to both repairable and nonrepairable failures. At the time of a repairable failure, a repair kit is removed from inventory and used to renew the system. Nonrepairable failures are assumed to occur at some time in the future distributed as a random variable of phase type and permanently terminate the life of the system. Under general stochastic assumptions concerning the lifetime of the components and the time required to perform repair, reorder strategies are identified which maximize the expected net discounted contribution over the lifetime of the systems.

Stocking Repair Kits for Systems with Limited Life

In this paper we examine several models for systems subject to both repairable and nonrepairable failures. A repairable failure can be remedied by the application of a repair kit (spare part, standby system, etc.), whereas a nonrepairable failure terminates the life of the system. The stochastic properties of key system parameters are derived under appropriate assumptions. These results are then used to determine the optimal number of repair kits to stock in order to maintain a system having a specified cost structure. The applicability of such models to problems of systems design is illustrated in several examples. Our results make use of distributions of phase type, and demonstrate that this family of distributions provides considerable flexibility and computational tractability in reliability modeling.

Uptime of Systems Subject to Repairable and Nonrepayable Failures

Abstract Expressions are derived for the distribution and expected value of uptime for systems subject to both repairable and nonrepairable failures. The results are applicable to a wide range of situations, including the analysis of systems subject to major structural failure, product or process obsolescence, or failure of critical nonrepairable subsystems. Several examples are investigated, including multiple component series systems, systems containing standby redundant nonrepairable subsystems and systems containing standby redundant repairable subsystems.