Improving Reliability of Electrical Connectors with the method of SeparationVourtsis Pavlos 17038650Programme:Module: P04791 – MSc ProjectsSubmission: Project progressYear: 2017/18Word count:(should be less than 3000 words)School of Engineering, Computing and MathematicsContentsSummaryIntroductionMethodologyInterim results and discussionDiscussion on the progress against the planReferencesSummaryAs an aim this project has to improve the reliability of electrical connectors by using the method of separation. There are many devices that suffer from the malfunction of the electrical connectors.
For that reason we are aiming to improve their reliability, to extend their overall time function and reduce the failures which cause breakdown of the devices.The objectives of the project are to identify past failure mechanisms associated with the electrical connectors, to conduct the necessary stress and strain analysis calculations, fatigue calculations and demonstrate reliability improvement by at least 30%. Using the method of separation.The methods that are going to be used are mainly numerical calculations and not experimental using aspects from mechanical, electrical engineering, mathematics and physics.
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From the research and the progress so far a very important finding has been highlighted. All the scientific papers until now in order to deal with the improvement of separation are using material alteration and coating the electrical connector. For that reason by using a recently founded method will be used to improve the reliability.IntroductionMany studies have been made in order to improve the reliability of electrical connectors. All of them are focusing on coating the electrical connector with other materials or by changing the materials of the connector such as polymers.
The major contact resistance failure mechanism has been found to be from fretting by many authors. That’s why they are focusing in that specific area, coating in order to minimize that phenomenon, but none of them has been focusing at separating the materials, the components or the properties. A typical luxury car has in excess 400 electrical connectors with over 3000 individual connectors.
Humanity has reached a level where we have full electric cars but we are using the same type of planting the electrical connectors which is vulnerable to fretting corrosion leading to irregular high contact resistance.This research will approach the reliability from a different angle. By using the method of separation we will improve the reliability or extend the fatigue life. Looking into the future we can see that there will be only electric cars which will require high reliability electrical connectors.The method of separationThe method of separation on properties is an efficient technique for compensating the drawbacks associated with homogeneous properties. Separation is the act of disuniting risk-critical factors.
There have been many examples such as barriers to reduce crash impact, TRIZ methodology for inventive problem solving etc. which are using the method separation but they have never been connected to that. It is only recently that the method of separation has been suggested in (Todinov, 2015) as a potential risk reduction tool. The logical separation can be an efficient and low-cost risk reduction technique. At the method of separation, also exists the separation of functions subsist of assigning different functions to various parts of a component or assembly.
Improving reliability and decreasing risk through several distinct mechanisms is the result form separating crucial functions among various parts. There are many different ways in order to use the separation of functions resulting to improving the reliability, and these are separating functions:To different parts and make it easier to optimize the separate partsTo reduce load magnitudesSeparation of a single function into multiple componentsFor a mutual compensation of deficienciesTo prevent unwanted interactionsTo improve reliability of an estimated valueSeparation of InformationSeparation of duties to reduce the risk of errors and fraudSeparation of reliability across components according to their cost of failureReliabilityIs the capability of a component or a system to operate at a required function failure under specific conditions for an accustomed time interval. Reliability is often measured by the probability of endure a specified time intervalFailureIs the state or condition when a component or assembly are not stands up to the desirable or intended operation which basically is the opposite of success.
Failure ModeThe way a component or system is unsuccessful to function as it was intended to. It is the effect by which failure is noticed.Failure MechanismThe physical process which provokes a particular failure modeCommon Technical Failure Modes and Failure Modes of Electrical ConnectorsThe most common failure modes are:CorrosionExcessive wearFatigue fractureOverloadingOverheatingStress corrosion crackingShort circuit Yielding BucklingSparks in the contacts could significantly increase the resistance of the contactCorrosion of the contact materialFretting corrosionWear due to frictionPressure reductionOpen circuit Excessive elastic deformation CreepLeakage, Brittle fracture Stress relaxationBending Plastic deformation contact damage Fretting corrosion is the wear and the corrosion damage at the roughness of the contact surfaces and due to the fact that is the decline at the coherence between the surfaces it is normal to appear at tight-fitting parts.
Designing having in mind reliability is about preventing failure modes from taking place during the specified lifetime of a product.FMEAFailure Mode and Effect Analysis (FMEA) is a systematic procedure to determine all possible failure modes in a structure, calculating their impact and implying mitigating measures. FMEA is designed to recognize, give priority and restrain these failure modes. There are 2 different categories the Design FMEA and the Process FMEA.
The Design FMEA investigates the possibility of product failure, shortened product life, safety and managerial concerns that comes from geometry, tolerances, material properties, engineering noise and interfaces with other components.The Process FMEA uncovers the failures that affect the product quality, that diminish the reliability they create customer complaints and safety which are results from methods followed while processing, machines utilized, human factor, materials used, environment factors on process work and the measurement systems that affect the approval.Electrical ConnectorsAn electrical connector is an electro-mechanical device which has be applied in order to join electrical terminations and generate am electrical circuit. The connection can be either permanent or temporary, as for movable equipment tool for assembly and removal is required. An adapter is an example which connect together dissimilar connectors.
Electrical connectors can be defined by their pin out and physical construction, contact resistance, size, ruggedness and resistance to vibrations, insulation between pins, resistance of insertion of water and other contaminants, reliability, resistance to pressure, lifetime ( number of connection/disconnection, operation before failure)and ease of connecting and disconnecting. Usually it is wanted for a connector to be easy to identify visually, very quickly and assemble, desirable to use only one tool and to be inexpensive. Many connectors are keyed with mechanical part, which exist in order to forbid joining except with a correctly oriented connector. AC power plugs and sockets started becoming available in the 1880s. Plugs are assembled of many components, for that reason we have to considerate the overall function of the plug as a system Important Design QuestionsIn order to have great reliability when we design a component or an assembly there are some important which have to be made and answered.For each component (mechanical, electrical, ergonomic, aesthetic etc) what are the functions and requirements that have to be meet?Every part of assembly, from which material is made of and why?What is the function of the plug and how does it work?In order to make-create each part, what manufacturing methods were used and why?Are there alternative materials or designs in use or can you recommend enhancements?Failure Mode vs Failure MechanismsFailure mode is the specific way in which a component would fail functionally or with respect to defined acceptance criteria. Failure mechanism is the reason physical, chemical, electrical or mechanical stresses may induce failure.
These mechanisms may act in combination or individually.CopperCopper is a chemical element with symbol Cu and atomic number 29. It is malleable, uctile and soft metal with very high electrical and thermal conductivity.
Due to the fact of the above and given that it does not react with atmospheric oxygen to form a layer of brown-black copper oxide which protects the underlying metal from further corrosion it has been selected as the most usable material at the electrical connectors.MethodologyMethods and LimitationsIn this project the methods that are going to be used in order to improve the reliability are: Stress Analysis Calculations, Finite Element Analysis, Castigliano’s formula, Fatigue Calculations, Paris Law, Corrosion Calculations, Failure Mode and Effect Calculations.The limitations of this project is that there will be only theoretical and numerical calculations and not experimental values.Friction calculationsIn order to see the impact of the friction at the contact it is very important to calculate the friction of the surfaces. The formula to calculate the friction force is quite simple: F=?*? where F is the force of friction in newton, ? is the dimensionless coefficient of friction and N is normal force, There are 2 different types of friction, the static friction which acts when the component is stationary and we have the kinetic friction which exist when the component is on move such as a car.
Stress analysis calculationsStress analysis is an engineering field that uses a variety of ways to figure out the stresses and the strain in materials, components and assemblies which are dealing with forces. There are 3 different type of stresses, tensile stress, compressive stress and shear stress.Basic stress formula ? = F/A ? = Stress, measured in N/m^2 or Pascals (Pa). Instead of ? use ? for shear stress.
A = Area (this can be either sectional or axial)Basic strain formula ? = ?L/L ? = Strain?L = Change in lengthL = Initial lengthModulus of elasticity (Youngs modulus) ? = ?/? E = Modulus of elasticity? = Stress? = StrainPoisson’s Ratio ? = – ?t / ?l? = Poisson’s ratio?t = Transverse strain?l= Longitudinal or axial strainBending stress ?b = 3 FL/2wt2 F = ForceL = Length of the structural elementw = Widtht = ThicknessBending modulus Eb = FL3/4wt3y F = ForceL = Length of the structural elementw = Widtht = Thicknessy = Deflection at the load pointFactor of Safety (FoS) fs = Ys / Ds fs = Factor of Safety (FoS)Ys = Yield strength of the materialDs = Design stressMaximum allowable stress UTS/fsUTS = Ultimate Tensile Strengthfs = Factor of Safety (FoS)Finite element analysisFinite element analysis is an arithmetic approach for solving problems of engineering and mathematical physics. It also is known as Finite element analysis Analysis (FEA). Some common problem areas of interest are heat transfer, fluid flow, structural analysis, mass transport and electromagnetic potentialCastigliano’s FormulaIs a method for determining the displacements of a linear-elastic system based on the partial derivatives of the energy. He is known for his two theorems.
The primary approach is that a change in energy is equivalent to the causing force times the resulting displacement. Thus, the causing force is equal to the change in energy divided by the resulting displacement. Alternatively, the resulting displacement is equal to the change in energy divided by the causing force. Partial derivatives are needed to relate causing forces and resulting displacements to the change in energy.
Castigliano’s Theorem is given as: Where ? is the deflection, U is the strain energy and P is the force (or torque) at a certain point.Different loading conditions require different strain energies. For axial loading we have:Where E is the material’s young’s modulus, A is the cross sectional area, L is the length, P is the Load.For the material in bending we have:Where I is the area moment of inertia, M is the moment appliedFor the material in torsion:Where G is the modulus of Rigidity, J is the polar moment of inertia and T is the torque applied.Fatigue CalculationsIn materials science, fatigue is the weakening of a material caused by assigning loads over and over again.
It is the gradual and localized structural damage that arise when a material is subjected to periodic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the yield stress limit or the ultimate tensile stress limit. Fatigue occurs when we have repeated loading and unloading subjected to a material.Paris LawParis’ law describes the stress intensity factor range to sub-critical crack growth under a fatigue stress system. As such, it is the most popular fatigue crack growth model used in fracture mechanics and materials science.
The basic formula is: Where ? is the crack length and d?/ dN is the crack growth rate. C and m are constants which are dependent from the stress ratio, environment and material. ?? is the range of the stress intensity factor during the fatigue cycle.
??= ?max-Kmin.Furthermore Paris Law can be used to quantify the residual life given a specific crack size. Where Y is a dimensionless parameter that depends on the geometry, is the range of cyclic stress amplitude. Also Y takes values from 1 for a center crack of length 2a in a infinite sheet. For short cracks, C can be assumed as independent of a and the differential equation can be solved via separation of variables.And subjected integration Where Nf is the remaining number of cycles to fracture, ?c is the critical crack length at which instantaneous fracture will occur and ?i is the initial crack length at which fatigue crack growth starts for the given stress range ??.
Corrosion calculationsWhen most metals come into contact with certain element in the water or air, they experience a chemical change which decreases the sincerity of the metal. This mechanism is called corrosion. Sulfur, oxygen, salt and other materials can all lead to corrosion. When a metal erodes or deteriorates, it cannot withstand the same loads as it did before the corrosion had begun. At a certain point, corrosion can lead to threatening conditions.In order to calculate the corrosion rate from metal loss data we use:Rcorr (mm/yr) = 87.6 x (W/DAT)Where W is the weight loss in milligrams, D is metal density in g/cm^3, A is the area of sample in cm^2, T is the time of exposure of the metal sample in hoursFailure Mode and Effects Analysis (FMEA)It is a systematic procedure for identifying all possible failure modes in a system and evaluating their impact.
It was one of the first highly structured, systematic techniques for failure analysis. It was developed by reliability engineers in the late 1950s to study problems that might arise from malfunctions of military systems. A FMEA is often the first step of a system reliability study.
Interim results and discussion All the chargers that exist right now trying to cope with the 2 aspects at the same time. Conductivity and pressure, when for example a plug is being inserted at the power plug, they have almost the same dimension from where friction occurs between the surfaces resulting to the pressure between them and contact in order to have electricity, but at the same time we have fretting corrosion which means after several times the contacts will fail to operate. That is where the method of separation comes in order to separate the properties.
With a new design we can have both properties from different components, reducing the failure mode. Figure SEQ Figure * ARABIC 2: FEMALE PART Figure SEQ Figure * ARABIC 1 MALE PART At the above scetch we have a typical power plug and how it works, the male part gets into the female part and by having the same diameter it creates friction occuring to contact and pressure at the same time in order to avoid the the fatigue a new design has been created.Discussion on the progressThe project is going according to the initial estimation. Research about the electrical connectors in order to understand in depth operation has been made and the materials that have been used. Furthermore research has been made in order to figure out all the failure modes and mechanisms of the electrical connectors and where improvement has to be made. Undertaking is the research and designs to apply the method of separation on the electrical connectors in order to improve the reliability by minimizing the failure modes.ReferencesAnderson, K.
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