Material Science - Creep and Fatigue

HNC Materials Steve Goddard

Diagnose causes of failure of materials

The project I’m going to concentrate my research on for this assignment will be helicopter rotor blades. I chose these because they are a critical and well developed part to every aircraft made in the AgustaWestland. Failure of a blade during flight would be considered extremely un-acceptable, this means that every possible aspect of failure would have been researched.

The rotor blades are aerofoils that provide lift and, unlike fixed wing aircraft, they can do this without needing to move forward. This means helicopters are ideally suited for flight operations in areas in which there is no usable runway. In addition to the single main rotor there is a tail rotor, or a device designed to fulfil a similar function, that prevents the helicopter simply spinning around the rotor blade. The tail rotor provides a force, which works against the direction of the torque produced by the main rotor blades. Dual rotor head helicopters overcome the torque force by having the two rotor heads spinning in opposite directions. Some rotor blade systems can move between the horizontal and vertical plane depending on the stage of flight. These are found in the tilt rotor aircraft.

The rotor blades fulfil two functions. Described in a simple way, they provide both lift and thrust. The configuration of the rotor blade will depend on the design of the helicopter and the purpose for which it is to be used, as well as the overall weight. Some helicopters have a single rotor head with 2 blades, some have four blades and some helicopters have two-rotor heads.

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AW139 – Main Rotor Blade

EH101 – ASW Italian Navy


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

Potential causes of failure that may occur in service could be:

Causes of failure in some engineering industry investigations

Origin % 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

38151511

8652

Summary of data from laboratory reports on failed aircraft components

Origin % Improper Maintenance Fabrication Defects Design Deficiencies Abnormal Service Damage Defective Material Undetermined Cause

44171610

76

Mechanisms of failure in some engineering industry investigations

Mechanism % Corrosion Fatigue Brittle Fracture Overload High Temperature Corrosion Stress Corrosion/ Corrosion Fatigue/ Hydrogen

Embrittlement Creep Wear, Abrasion and Errosion

29251611

7633

Mechanisms of failure of aircraft components

Origin % Fatigue Overload Stress-Corrosion Excessive Wear Corrosion High Temperature Oxidation Stress Rupture

6118

87321

All the data described above come from essential metallurgy for engineers by AC Vivian.

The data clearly shows that in general with aircraft components, fatigue is the main cause of failure followed by improper maintenance.

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

I have thought of various things that may affect the service life of a rotor blade. Although some of these reasons may seem quite general I have tried to categorise them into each part of the components life.

Design Material Selection Too much detail on the design for

the manufacturing process Size of weight saving cavities Aerodynamic shape Incorrect surface modelling Thickness Small Radii Sharp Corners Lack of finite element analysis*

Manufacturing Manufacturing Process (i.e. sand casting opposed to investment casting)

Machine Malfunction Inaccurate drawings Surface Imperfections Indentations Inclusions Poor surface finish (i.e. Corrosion)

Delivery Inadequate Packaging Inappropriate transportation

In Service Accidental Overloading Exposure to extreme heat Exposure to static mechanical

stresses Creep**

* Finite element analysis is the basically a computer programme that analyses a ‘solid model’ and highlights areas of stress concentrations usually using colours this is an example of Finite element analysis used on a gear box.

** Creep is defined as the time dependant and permanent deformation of components under a constant load or stress. Creep is an undesired property and will definitely have an impact on the service life of the component. Polymers are most sensitive to creep.

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Finite Element Analysis


Environmental factors will also affect the service life of the component. There are two main types of environmental fatigue; Thermal fatigue: occurs when the temperature is very varied, the changes in component temperature increase the thermal stress, which can cause expansion or contraction of the component; and Corrosion fatigue: this is caused by cyclic stresses and chemical attack. Even in a normal environment corrosion fatigue will play a part in the service life of the component. To start with there is a chemical reaction between the material and environment which will result in small pits being formed in the material, which then become stress concentration points. These pits will increase the crack propagation which will in turn increase the chance of failure.There are several ways to avoid corrosion fatigue they are:

Apply a protective coating e.g. Alocrom, Anodising, Powder coating. Select a more corrosive resistant material. Reduce the corrosiveness of the environment.

It is also worth noting that combining materials in design, for example having Al-Lith in contact with titanium can cause severe exfoliation corrosion.

3. Describe and illustrate the typical macroscopic 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 shown in Figure 2. Simple macroscopic observation of this fracture may be sufficient to identify the ductile fracture mode.

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Macroscopic Ductile Fracture


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

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 (below).

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Macroscopic Ductile Fracture surface

Macroscopic Brittle Fracture


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 5). Note that a crack may propagate at a speed approaching the speed of sound in the material!

Fatigue Fracture

A phenomenon which results in the sudden fracture of a component after a period of cyclic loading in the elastic regime. Failure is the end result of a process involving the initiation and growth of a crack, usually at the site of a stress concentration on the surface. Occasionally, a crack may initiate at a fault just below the surface. Eventually the cross sectional area is so reduced that the component ruptures under a normal service load, but one at a level which has been satisfactorily withstood on many previous occasions before the crack propagated. The final fracture may occur in a ductile or brittle mode depending on the characteristics of the material. Fatigue fractures have a characteristic appearance which reflects the initiation site and the progressive development of the crack front, culminating in an area of final overload fracture. Fig. la illustrates fatigue failure in a circular shaft. The initiation site is shown and the shell-like markings, often referred to as beach markings because of their resemblance to the ridges left in the sand by retreating waves, are caused by arrests in the crack front as it propagates through the section. The hatched region on the opposite side to the initiation site is the final region of ductile fracture. Sometimes there may be more than one initiation point and two or more cracks propagate. This produces features as in Fig. 1b with the the final area of ductile fracture being a band across the middle. This type of fracture is typical of double bending where a component is cyclically strained in one plane or where a second fatigue crack initiates at the opposite side to a developing crack in a component subject to reverse bending. Some stress-induced fatigue failures may show multiple initiation sites from which separate cracks spread towards a common meeting point within the section.

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Macroscopic view (x1000) of Intergranular brittle fracture


There are many types of corrosive attack, some are explained below:

Uniform attack is a form of electrochemical corrosion that will occur over the exposed surface, and will leave behind a deposit. In a macroscopic sense the oxidation occurs randomly over the surface. Good examples of these are the rusting of steel and iron and tarnishing of silver ware. This is probably the most common type of corrosion attack and can be easily predicted or rectified, it can be prevented by careful component design and material selection.

Galvanic corrosion is an electro chemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.

Crevice corrosion occurs in crevices and recesses or under dirt, where the solution becomes stagnant and there is localized depletion of dissolved oxygen. The crevice must be wide enough for the solution to penetrate, yet narrow enough to let the ions and dissolved gases to stagnate, usually within several thousands of an inch. Crevice corrosion can be prevented by using welded joints instead of riveting or bolted joints, using non absorbent gaskets if possible, removing

accumulated deposits and designing the product so there are no stagnant areas and good drains.

Pitting is another form of localized corrosion where small pits and holes can occur within the metal. Pitting usually penetrates along the top of the horizontal surface, downward in a vertical direction. This is hard to identify or spot and usually going undetected. The mechanism for pitting is the same as crevice corrosion in that the oxidation occurs within the pit itself. It is gravity that makes the pits grow downwards, a pit may occur by a surface defect such as a scratch or a variation in the composition of the material. Pitting can be prevented by ensuring that all materials used have a

good surface finish.

Erosion corrosion occurs from the combined action of chemical attack and mechanical abrasion

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or wear as a consequence of fluid motion. Virtually all metal alloys are susceptible to erosion corrosion. The best way to reduce erosion corrosion is to change the design features to eliminate fluid turbulence i.e. reducing tight corners, change in diameter

Stress corrosion cracking (SCC) is caused by the simultaneous effects o f tensile stress and a specific corrosive environment. Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both. To start with small cracks form and then propagate perpendicular to the stress with the end result occurring in failure.

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

The easiest 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 use.The 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 a pc.

The aim of the rig testing is to establish a set of points along an S-N Curve. By setting the maximum force to say 400MPa, and using a frequency that would be typical to the component you can determine how many cycles it takes before failure. This can sometimes be a very long process and often a high frequency is used to complete the test data in less time. This would then be repeated for

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Blade Test facility at Boeing


different forces, say 350, 300, 250, 200 MPa etc; until a good curve can be created, and extrapolated down until maximum cycles at low forces can be seen. This is done because it would take far too long to actually test with these conditions, it would take too long. Also after a certain point the curve or line becomes relatively constant.

The pictures below show how a 32 m rotor blade is fatigue tested by being bent cyclically in a flap wise direction for 5 million full cycles. A full flap wise test thus takes about three months. If you look closely to the left you can see another (shorter) rotor blade being bent cyclically in an edgewise (chord wise) direction. In both cases the blades are bent using a cycle close to the natural frequency of the blade. The natural frequency is the frequency with which the blade will oscillate back and forth, if you push it once in a certain direction and let go. The natural frequencies are different in the flap wise and edgewise direction: The blade tends to be much stiffer in the edgewise direction, thus it has a higher natural frequency for edgewise bending. Each blade is set in motion by an electric motor mounted on the blade which swings a weight up and down. The foundations which carry the blade socket have to be very solid: The foundation for the large blade socket consists of 2,000 tonnes of concrete. This video which these pictures were taken from was shot at the rotor blade test facility of the Risoe National Laboratory Sparkær Test Centre in Jutland, Denmark.

The pictures above show screen shots from a video illustrating fatigue tests on blades.

The measurement results from the strain gauges are continuously monitored on computers. Nonlinear variations in the pattern of bending may reveal damage in the rotor blade structure.

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Static Testing of Rotor Blades

Rotor blades are also tested for strength (and thus their ability to withstand extreme loads) by being bent once with a very large force. This test is made after the blades has been subject to fatigue testing, in order to verify the strength for a blade which has been in operation for a substantial amount of time.

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 blade.

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Bibliography

http://www.ami.ac.uk/courses/topics/0124_seom/index.html#2 – Fracture

http://www.windpower.org/en/tour/manu/bladtest.htm - Blade Test Info

Creep and Fatigue Notes from class

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http://www.windpower.org/en/tour/manu/bladtest.htm

http://www.ami.ac.uk/courses/topics/0124_seom/index.html#2

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Material Science - Creep and Fatigue