DEPARTMENT OF ENGINEERINGHNC/D AEROSPACE ENGINEERING

Unit 23 NQF MATERIALS ENGINEERING

Assignment 4: Diagnose causes of failure of materials Tasks For a product, of you choice, appropriate to your vocational experiences, describe the component and its function. Then: 1. Identify potential causes of failure that may occur in service. 2. Discuss the factors that may influence the service life of the component. 3. Describe and illustrate the typical microscopic features that would be present in

failures associated with:

. Ductile fracture

. Brittle fracture

. Fatigue Fracture

. Corrosive attack

1. Evaluate the methods that could be used to provide an estimate of the life of the component.

2. Suggest suitable methods for improving the service life of the component.

Your completed assignment should be: . Technically and professionally presented, together with various calculations and diagrams etc. . Word processed . Contain a bibliography . Contain between 1800 and 2000 words.

Ray Gibbons April 2009

Assignment 4: Diagnose causes of failure of materials.

For a product, of you choice, appropriate to your vocational experiences, describe the component and its function.

The component I have chosen to investigate is the HP Turbine blade. The turbine in an assembly of discs with blades that are attached to the turbine shafts, nozzle guide vanes casings and structures.

The turbine extracts energy from the hot gas stream received from the combustion chamber. In a turbofan this power is used to drive the fan and compressor.

Turbine blades convert the energy stored within the gas into kinetic energy. Like compressor, the turbine comprises of a rotating disc with blades and static vanes, called nozzle guide vanes. The gas pressure and temperature both fall as passes through the turbine.

HP turbine blades and nozzle guide vanes are designed with cooling passages and thermal barrier coatings, to ensure long life while operating at such high temperatures. Cooling air is taken from the compressor and is fed around the combustion chamber into the blades to cool the aerofoil.

HP turbine blade cooling flows

Blade cooling air

HP TurbineIP Turbine

LP Turbine

Then: 1. Identify potential causes of failure that may occur in service.

Engine blades may fail in service causing catastrophic damage to the engine resulting in further damage to the aircraft which could possibly result in loss of the aircraft and the loss of the personnel on board. Failure of the blades will occur when it is no longer able to maintain structural integrity through the stresses applied to it within the design limitations.

Several reasons related to stress concentrations failure are Improper material selection Fabrication Defects Faulty Heat Treatment Mechanical Design Fault Unforeseen Operating Conditions Inadequate Environmental Control Improper or Lack of Inspection And Quality Control Material Mix-Up

2. Discuss the factors that may influence the service life of the component.

There are many various things that may affect the service life of an engine compressor blade. While some of these reasons may seem quite general they are categorised into each part of the components life.

Design Material SelectionToo much detail on the design for the manufacturing processAerodynamic shapeIncorrect surface modellingThicknessSmall RadiiLack of finite element analysis

Manufacturing Manufacturing Process (i.e. sand casting opposed to investment casting)Machine MalfunctionInaccurate drawingsSurface ImperfectionsIndentationsInclusionsPoor surface finish (i.e. Corrosion)

Delivery Inadequate PackagingInappropriate transportation

In Service Accidental OverloadingExposure to extreme heatExposure to static mechanical stressesCreep**

3. Describe and illustrate the typical microscopic features that would be present in failures associated with:

Ductile fracture

Ductile fracture involves plastic deformation in the vicinity of an advancing crack, and is a slow process. It is stable, and will not continue unless there is an increase in the level of applied stress. It normally occurs in a trans-granular manner (across the grains) in metals that have good ductility and toughness. Often, a considerable amount of plastic deformation – including necking – is observed in the failed component. This deformation occurs before the final fracture.

Ductile fractures are normally caused by simple overloads or by applying too high a stress to the material, and exhibit characteristic surface features with a significant portion of the fracture surface having an irregular, fibrous face. They also have a small shear lip, where the fracture surface is at a 45° angle to the applied stress. The shear lip, indicating that slip occurred, gives the fracture the cup-and-cone appearance. Simple macroscopic observation of this fracture may be sufficient to identify the ductile fracture mode.

Examination of the fracture surface at a high magnification – using a scanning electron microscope (SEM) – reveals a dimpled surface. Figure 3 on the left hand side shows that under a normal tensile stress, these dimples are usually round or equiaxed (having the same dimensions in all directions) – while figure 3 on the right hand side shows if shear stress has been dominant, the dimples are oval-shaped or elongated, with the ovals pointing towards the origin of fracture.

Microscopic images of ductile fracture x 1000.

Brittle fracture. In brittle fractures, cracks spread very rapidly, with little or no plastic flow, and are so unstable that crack propagation occurs without further increase in applied stress. They occur in high strength metals, in metals with poor ductility and toughness, and in ceramics.

Even metals that are normally ductile may fail in a brittle manner at low temperatures, in thick sections, at high strain rates (such as impact), or when flaws play an important role. Brittle fractures are frequently observed when impact rather than overload causes failure.

Brittle fracture can be identified by observing the features on the failed surface. Normally, the fracture surface is flat and perpendicular to the applied stress in a tensile test. If a failure occurs by cleavage, each fractured grain is flat and differently oriented, giving a shiny, crystalline appearance to the fracture surface Initiation of a crack normally occurs at small flaws which cause a concentration of stress. Normally, the crack propagates most easily along specific crystallographic planes by cleavage. However, in some cases, the crack may take an inter-granular (along the grain boundaries) path, particularly when segregation or inclusions weaken the grain boundaries (Figure below). It has been identified that a crack may propagate at a speed approaching the speed of sound in the material.

Microscopic image of intergranular brittle fracture x 1000

Fatigue Fracture. Fatigue is a form of failure that occurs in materials subjected to fluctuating stresses – for example, solder joints under temperature cycling. Under these circumstances, it is possible for failure to occur at a

stress level considerably lower than the tensile or yield strength for a static load.

The term ‘fatigue’ is used because this type of failure normally occurs after a lengthy period of repeated stress cycling. It is the single largest cause of failure (approximately 90%) of metallic materials, and polymers and ceramics (other than glasses) are also susceptible to this type of failure. Although failure is slow in coming, catastrophic fatigue failures occur very suddenly, and without warning.Fatigue failure is brittle-like in nature – even in normally ductile metals – in that there is very little, if any, gross plastic deformation associated with failure. The process occurs by the initiation and propagation of cracks, and the fracture surface is usually perpendicular to the direction of an applied stress.

A major problem with fatigue is that it is dominated by design. Whilst it is possible to assess the inherent fatigue resistance of a material, the effects of stress-raisers such as surface irregularities and changes in cross-section, as well as the crucial area of jointing (solder joints!) can be a major problem.

Failure by fatigue is the result of processes of crack nucleation and growth, or, in the case of components which may contain a crack introduced during manufacture, the result of crack growth only brought about by the application of cyclical stresses. The appearance of a fatigue fracture surface is distinctive and consists of two portions, a smooth portion, often possessing conchoidal, or ‘mussel shell’, markings showing the progress of the fatigue crack up to the moment of final rupture, and the final fast fracture zone.

At higher magnifications, using a scanning electron microscope, fatigue striations can be observed below. Each striation is thought to represent the advance distance of the crack front during a single load cycle.

SEM image of fatigue striations x 1000

An important point regarding fatigue failure is that beach marks do not occur on the region over which the final rapid failure occurs. This region will exhibit either ductile or brittle failure – evidence of plastic deformation being present for ductile, and absent for brittle failure.

Corrosive attack.

Corrosion fatigue damage occurs more rapidly than would be expected from the individual effects of fatigue or corrosion. In general, different environments have different effects on the service life of a given material. Similarly, the corrosion fatigue behaviour of different materials is usually different in the same environment.

The behaviour established for a given material/environment system or for a given set of test conditions cannot be applied indiscriminately to other systems or conditions.

The corrosion fatigue behaviour of metallic materials has had considerable attention from researchers over the years, throughout the world. Most engineering materials are, to a greater or lesser extent, susceptible to corrosion, in the form of either general or localized corrosion. The reduction in fatigue lifetime of components as a result of presence of an aggressive environment is becoming very common. This emanates from the fact that corrosion fatigue is responsible for many service failures in a wide variety of industries, including aircraft design. This has led to an important consideration to be encountered by design engineers for safer design.

Several studies have examined pit initiation and growth behaviour during the corrosion fatigue process. It is well established that corrosion pit initiation and grow in the early stages of the corrosion fatigue process. Corrosion fatigue cracks start to grow from these corrosion pits and cause the final failure of the metallic structures.In the studies of the fatigue and corrosion fatigue, crack initiation mechanisms aimed to identify the preferential sites for crack initiation and microstructural particularities and/or peculiarities associated to these sites. These sites are possibly responsible for premature fatigue or corrosion fatigue crack initiation and contribute to reducing the fatigue life of the alloys. It has been identified that the main stages of damage leading to environment-assisted fatigue failure from defect-free surfaces include: breakdown of the surface passive film, pit development and growth, transition from pitting to cracking, crack growth and crack coalescence. The environment acts on the material through the surface, producing uniform or localized chemical attack by diffusive mass transfer. All alloys used in engineering develop surface passive films as a result of surface oxidation during processing. The degree of protection given by a surface film depends on the diffusion rates of various environmental constituents through the film and on the stability of the coating itself against environmental attack. In carbon steel with little or any alloying additions shows weak passive behaviour and is considered active when immersed in environments as benign as water. In these alloys, corrosion occurs very quickly following the immersion in aqueous environments. Additions of sufficient alloying elements such as Cr, Cu and Ni improve the corrosion resistance through the formation of a tightly adhered mixed

oxide film on the surface of the alloy. This increases the pitting resistance and more aggressive environments are required to break down the oxide film.

The pits formations are a major consideration for engineering components with high integrity surface finish. If a residual or applied mechanical stresses occur together with an aggressive environment, the early development of pits and subsequent cracks can play a major role in the total lifetime of a component. Various factors are important to corrosion fatigue behaviour. The effect of decreasing frequency on pit nucleation and growth can be observed in figure below.

Corrosion pits are typically smaller than a millimetre in depth and serve as micro notches with locally elevates the stress level. Furthermore, the pH level of the corrosive environmental inside the pit can be more acidic than that in the bulk, causing possible acceleration in the rate of fatigue crack growth. Once that the stage of development and growth of pits and the initiation of a crack from a pit happened, the subsequent stage in the accumulation of damage under environment-assisted fatigue following the stage of the transition from a pit to a crack. Figure below shows the transition from a pit to a crack.

4. Evaluate the methods that could be used to provide an estimate of the life of the component.

A simple method of estimating a components life it to carry out fatigue tests on it. By setting up a test rig that will replicate the typical forces the component will suffer under normal in service use. The turbine blade would be attached the test rig. Then you would have actuators creating forces up and down on the blade, using strain gauges to measure the forces applied. The actuators would be set to cycle through load cases determined by the operator.

The aim of the rig testing is to establish a set of points along an S-N Curve. Bysetting the maximum force to say 400MPa, and using a frequency that would betypical to the component you can determine how many cycles it takes beforefailure. This can sometimes be a very long process and often a high frequency isused to complete the test data in less time. This would then be repeated fordifferent forces, possibly 350, 300, 250, 200 MPa etc; until a good curve can beproduced, and extrapolated down until maximum cycles at low forces can be observed.

This is done because it would take far too long to actually test with theseconditions, it would take too long. Also after a certain point the curve or linebecomes relatively constant.

5. Suggest suitable methods for improving the service life of the component.

Suitable methods for improving the service life of the component would be to analysis the testing that is done on a previous product and look at areas that could be improved and modified to create a superior product. Other ways would be to ensure that the points mentioned in the list for question 1 are all considered in the life of the blade. Materials could also be changed and new alloys with specific properties could be used to increase the life of the component.To reduce the effect of corrosion addition of sufficient alloying elements such as Cr, Cu and Ni improve the corrosion resistance through the formation of a tightly adhered mixed oxide film on the surface of the alloy.

Bibliographyhttp://www.ami.ac.uk/courses/topics/0124_seom/index.htmlhttp://www.iasmirt.org/iasmirt-2/SMiRT19/S19_FinalPapers/G05_2.pdf