UNIT 21: MATERIALS ENGINEERING - FREE STUDY

1 1 unit 21: MATERIALS ENGINEERING unit code: F/601/1626 QCF level: 4 Credit value: 15 LEARNING OUTCOME 4 TUTORIAL 1 On successful completion of this unit a learner will: Understand the in-service causes of failure of ENGINEERING MATERIALS Causes of failure : failure of material categories (metals: ceramics, polymers and composites) creep, fatigue, impact, overstressing, corrosion, temperature, thermal cycling, residual stresses, stress relaxation, degradation (composition change), radiation, electrical breakdown, or combinations of these Service life: contributory effects of service conditions to failure inappropriate maintenance, inappropriate use, faults in manufacture, material selection and design faults, changes in service conditions such as environment, loading and temperature. Estimation: methods of investigating failure and the preparation of estimates of product service life that require the use of calculations creep or fatigue failure Improving service life: recommending remedial and/or preventative measures changes to material , product design, protective systems for corrosion and degradation, adjustment loading arid working temperature RECOMMENDED READING It is impractical to think that a tutorial like this could adequately cover all the things you need to know.

2 Pictures and diagrams which cannot be copied here are available at the various web links in this tutorial. The ones below are very helpful and might be used to help you complete any assignments and assessments your tutor sets for you. Practical Plant failure Analysis: A Guide to Understanding Machinery .. By Neville W. Sachs Forensic MATERIALS ENGINEERING : Case Studies by Peter Rhys Lewis, Ken Reynolds, Colin Gagg Doug Bailey-Fatigue and failure Analysis, International Surescreen who has kindly allowed use of their pictures and it contains lots of useful material on failure analysis. This link is a dissertation on some types of failure . This is a document that you can download and might have been written for this module. It has examples and pictures of failed metal marine structures. 2 CONTENTS 1. SERVICE LIFE 2. CAUSES OF failure Reason in General Overstressing Creep Fatigue Notch Sensitivity and Brittle failure Stress Corrosion Sudden Loading and Impact Spalling Wear Thermal Shock and Stress Radiation Degradation 3.

3 FAILURES IN POLYMERS 4. FAILURES IN COMPOSITES 5. ELECTRICAL FAULTS 3 1. SERVICE LIFE Any component, machine or structure should have a designed life span. Sometimes it requires regular maintenance to achieve this (for example a motor car). Sometimes it is expected to last without maintenance ( electrical/electronic components). The life span may be reduced by improper use such as the wrong environment. On the other hand, the service life can be improved by using techniques to prevent failure such as lubrication, surface coating and anti corrosion measures. There is a statistical probability of failure of parts and assemblies due to random affects. The most likely times for equipment to fail in service are when new and when they wear out from old age. A graph of failure rate and time produces a bathtub curve typically as shown which has the shape of a section through a bathtub. The bathtub curve describes the relative failure rate of a large number of products over time.

4 It is a model more suited to mass production of components or products and it is not thought to be a good model for complex machines that can be maintained like aircraft. Early failure is sometimes called infant mortality failure . The rest are intended to last until they wear out. Some will fail during the intended life span. Early failures are highly undesirable and are nearly always caused by defects and assembly errors. A product manufacturer must assure that all specified MATERIALS are adequate to function for the intended life span. Premature failure could be due to faults in the material , a faulty batch of components or faulty assembly. Testing is essential to ensure the design is good enough and when mass production takes place, suitable testing of samples must occur to enable production to be halted if a problem is found. This is a whole new topic of STUDY and not covered here.

5 In the case of things that are not mass produced, failures are less predictable as design faults may not show up until some time after going into service (such as creep and fatigue for example). This is why structures must be inspected thoroughly. Adequate material testing and certification of quality may be vitally important. Inspection of welds for example is essential to avoid premature failure due to slag inclusions, failure to fuse the root and so on. Non Destructive Testing (NDT) is another area of STUDY not covered here but widely used on aircraft and structures during inspection. Premature failure is sometimes the result of changing environmental conditions. For example the Sea Gem disaster occurred when the legs of an oil rig in the North Sea collapsed suddenly. It was determined that the disaster was due to material failure caused by corrosion, brittle fracture due to temperature change, and cyclic loading on the legs due to the wave nature and weather conditions of the changed environment.

6 Previously the rig had operated in the Caribbean with no problem. You can download a report on this at this link. The following sections should help you decide the life span of a given component and show you the reasons why they might fail prematurely. 4 2. CAUSES OF failure GENERAL REASONS material testing is covered in outcome 1 tutorial 4 so if you have studied this you will already have a good idea about why components and structures fail in service. failure may be defined as a component or structure that is no longer able to perform its design function. Things that fail in service do so because the limitations of the material have been exceeded. This may lead to fracture, yielding, distortion, wear, corrosion and so on. This may be due to the following reasons. Design Fault - for example stress raising features like undercuts. Wrong Selection of material - for example choosing a material that corrodes. Processing Problems - for example machining marks that act as stress raisers and defects in welds.

7 Defects in material - for example slag inclusions that act as stress raisers. Assembly Errors - for example over tightening of a screw placing residual stress in the component. Improper Service Conditions - for example degradation due to corrosion or temperature. The component/structure will have a design service life and this may be increased by suitable treatment during its service life. On the other hand the service life may be reduced because of lack of treatment or repair to damage. Overloading - for example lifting loads beyond the design limit and over pressurisation of containers. Abuse for example using something for other than its intended purpose like using a screw driver as a chisel. Now we will review the reasons and types of failure and look at some examples. OVERSTRESSING Overstressing normally refers to mechanical stress but can refer to other things such as overloading an electrical device ( the burned out electronic resistor shown).

8 All material failures are due to the stress limits being exceeded for one reason or another. This could simply be that too much load has been applied or that the stress limitation has been reduced by other factors such as wear, degradation, creep and so on. The way mechanical stress is calculated is covered in other tutorials and cannot be repeated here. They are usually based on complex stress conditions and theories of failure . Overstressing could also include things like wear which occurs because stresses in the surface are exceeded. Overstressing will cause a variety of failure forms from sudden to gradual. The pictures below show examples of parts that have failed due to mechanical overstress. Fractured casing Sheared pump shaft Ruptured pressure vessel Tensile fracture of over tightened bolt Stripped screw thread Broken chain link 5 CREEP Creep is a phenomenon where some MATERIALS change shape (usually growing longer) over a period of time, when a constant stress is applied to it.

9 The material may well fail although the tensile stress is well below the ultimate value. The structure may fail because the dimensions of the component change over a long period of time. Most MATERIALS will not creep at all until a certain stress level is applied. This level is called the LIMITING CREEP STRESS. Metals like lead creep very easily at room temperatures and so do polymers. This is made much worse when the polymer is warmed. Most metals and ceramics do not suffer from creep at room temperature if they have a high melting point. PROLONGED HIGH TEMPERATURE reduces the limiting creep stress of metals and the hardness as shown in the diagram. This is important in structures like steam power plants with pipes that carry steam at high temperatures and pressures High temperatures are also found in various heat exchangers like super heaters and re-heaters (see the example below).

10 HOMOLOGOUS TEMPERATURE expresses the temperature of a material as a fraction of its melting point temperature using the Kelvin scale. For example lead (Melting point 328oC) at 20oC has a homologous temperature of (20+273)/(328+273) = 296/601 = A useful general rule is that creep starts to become significant in metals when the homologous temperature is greater than This is a very important factor in the design of TURBINE BLADES that operate as high as 1400 C. In gas turbines, the blades are subject to high temperatures and prolonged periods of centrifugal force that causes them to creep. If the tip of the blades touches the casing, a catastrophic failure will occur. The MATERIALS used to make gas turbine blades are nickel-based super alloys. These have high melting points and hence a lower homologous temperature. They also have microstructures that are creep resistant. A design feature is to keep them cool by circulating cool air through channels in the blades.