Fatigue Crack Growth Under Variable Amplitude Loading Engineering Essay

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The report is the Feasibility study of Final Year Project on the title of '' Crack Growth Behaviour under Variable Amplitude Loading''

Crack behaviour is one of well known issue in materials science and there has been a lot of work done on this subject since early nineteenth century [1].It for the most part starts by different materials failure like: plastic deformation, fatigue, creep and multiple damage [1]. Mainly fatigue is one of the most important causes of crack in metals. In particular, fatigue describes the change in properties of metallic materials under repeated stresses and strains (cyclic loads) which leads to cracking and it can happen in many different forms.

Basically, during the time studying about the methods to stop the failures developed the '' science of materials'' and ''fracture mechanics'

The project Aims and objectives can be specified as following:


Review fundamental Fatigue failure and cyclic loads theory,

Analyse behaviour and concepts of materials in Aerospace structure,

The demonstration of initiative cracking growth in aero frames as well as complex loads existing in aerospace structure.


Research variable amplitude loads and fatigue behavior of materials,

Identify fatigue crack propagation in structures,

Examine and explore the different types of stress on chosen materials.

Identify some airframe structures and check current Design defects ,

Modify and amend a design if it is appropriate use fatigue stopping methods.

Investigate the complex dynamic loads with different numerical and experimental methods.

2. Background of the Fatigue Failure:

Fatigue is very slow process which occurs in long term period of time. For instance it may take up over 30 years for failure to completely happen. Since, fatigue failure is one of the hidden types of failures which take place without any obvious warning. Fatigue generally happen under appropriate conditions:

1. Presence of fluctuating/vibrating stresses

2. Presence of stress concentrations

3. Occurs at nominal stresses lower than the yield stress

4. Characteristic fracture surface

In view of the fact that fatigue occurs under cyclic loading, the general information about the behaviour of materials under variable loads is crucial. Variable Stress amplitude causes different types of fatigue over the time. The graph below is an understandable demonstration of the S-T plot for an aircraft wing.

Stress Ascent Cruise Descent


Taxing loads Gusts

Refuelling Touch down

Fig. 1.Typical stress-time plot for the lower wing skin of an aircraft

There are three common ways in which stresses may be applied: axial, torsion, and flexural. One of the main and known methods to visualize time to failure of the specific material is the S-N curve (also known as Wohler curve) which shows the amplitude stress verses number of cycle loads that indicates the Fatigue limit. The S-N curves are derived from tests on the material specimen subjected under regular sinusoidal stress by a testing machine.

3. Literature review:

It has being done lots of effort to cope with this important problem in material and mechanics; therefore there would be plenty amount of sources related to the crack and its growth behavior as it has being one of the most vital topic in mechanical and material science. Many famous failures happened primarily due to material failure in structures so many attempts has taken to eliminate the amount of lost made initially by material failures and perhaps stop it in wherever possible. For example de Havilland comet aircraft could be one well known crash for engineers occurred due to material fatigue. In this section difficulties that need to be considered throughout the project are highlighted.

Firstly, Understanding the behavior of materials in different environmental situation is a problem itself for scientists because there are many difficulties they have to face with. For instant identification of materials and failure characteristics are hard to achieve. Moreover, controlled types of experiment are costly and time consuming for example standard certification for aircraft fatigue test is around 5000 flight cycles [2].

It is also important to mention that, Fatigue crack growth in structure components, which is subjected to variable amplitude loading, is a very complex subject. Studying of fatigue crack growth rate and fatigue life calculation under spectrum loading is vital in life prediction of engineering structures. A number of load relations models have been developed to compare fatigue crack growth rates and predict crack growth under variable amplitude loading during past years. Mostly they are base on the calculations of the Yield zone size however it recently determined as a function of stress intensity factor. Possibly, there might be other techniques which will be needed to be covered in main report.

On the other hand, Understanding of materials used for aerospace structures and the reasons behind that is also crucial for this project. Aluminium alloys are the main alloys are used in aircraft and aerospace structures. The term of aircraft aluminium is usually referred to 7010 and 7075 Aluminium which are the composition of zinc, copper, magnesium, iron, titanium and chromium, etc. (there are a vast majority of aluminium alloy series such as 2,3,5 and 6 used widely in aviation).

4. Capital Resources:

As it has been suggested by supervisor the software require can be Afgrow which is fatigue crack growth analysis software tool. Within the project, any practical test can be desired as a case study which can be done either in material properties lab test or by the software.

The aim of experimental test in lab involves requirements that need to be considered. The process of fatigue test in lab therefore a specific drawing of material I am going to test should be design and be ordered from manufactured( if it is not provided within the school). Moreover, any require workshop activity like Drilling, milling, etc, can be done in the lab without any cost. In general, the budget of 50 pound will be essential for the project.

5. Project Plan:

Task no.




Week 1 (Induction week)



1 week

Introduction and general information,

Week 2&3 (Initial study)



2 weeks


Try to understand the title area.


Overview the subject area.


Figure out initial misunderstandings.

Week 4&5 (Feasibility study)



2 weeks


List of all topics and results related on the subject.


Find out about Materials in use in this manner include metallic and none-metallic.


Estimate amount of effort required for report.


Representation ideas and outline the work stages.


Revise subject and come across final decision about the title.

Week 6, 7 (Project Seminar and Initial progress check)





Review the process and amend it.


Achieve general knowledge


Understand the equipment, software or lab test need to be done for project


Find out an example part to be analyzed using software.


Develop sources.


Preparation for seminar.

Week 9, 10, 11&12 (Developments).



4 weeks


understand materials failures


Understand Complex loading act on structures by change of amplitude


Understand design and manufacturing defects


Understands new tech materials to eliminate the failure,


Prospect for farther improvement.

Week 13,14&



3 weeks


(Examination preparation)





Week 16,17,18&19 (Writing a report)



4 weeks


Complete a report for each part.


A report reviews.


A brief of Modifications & Developments.


Advantages & Disadvantages.




Comparisons, Summary & conclusion.

Week 23&24 (Final Finish).





Finishing all requirements and ensure that they are ready to be submitted.


Finalizing the report.


Poster Session



1 week

Fig. 2. Project time plan

Task Name




Initial understanding about the title

7 days

Fri 08/10/10

Mon 18/10/10

Start Planning and getting initial general information

7 days

Wed 13/10/10

Thu 21/10/10

Feasibility Study

7 days

Tue 19/10/10

Wed 27/10/10

Seminar preparation

14 days

Wed 27/10/10

Mon 15/11/10

Understanding the material and mechanical points of the title

28 days

Sat 20/11/10

Tue 28/12/10

writing up the main report

30 days

Tue 01/02/11

Mon 14/03/11

Finalizing the report

30 days

Tue 01/03/11

Mon 11/04/11

Fig. 3. General Task will be taken through project

Fig. 4. Gant chart of the project plan

6. Conclusion:

It concluded that, it is not feasible to cover all the topics and results on this subject in a single report due to different reasons. Studying and development of reliable life prediction models which are capable of handling complex service conditions is one of the taught challenges in fatigue research. As a result, it would be a recommendation to eliminate the title to more specific area by researching only about Fatigue failure, compare to other possibility in materials crack failures such as corrosion, creep, buckling, fracture, impact, mechanical overload, thermal shock, etc. Also, as this problem is a common failure in different area, a large number of experimental and theoretical studies on cumulative damage and fatigue crack growth under variable amplitude loading were carried out world-wide, especially in the fields of aircraft engineering and automobile engineering. However, in the report I am going to focus on aerospace structures as it is my course of studying s well as choosing used materials in aviation like AL, Ti and composites.

Secondly, for this project to be done successfully, that understanding of the Fatigue behaviour, as well as stress and loads, materials properties, fabrication, surface preparation of materials, Mean stress and its importance, crack initiation and propagation mechanics, fatigue growth rate, and also fatigue prevention methods are vital.

Additionally, the starting of this project is on 13/10/2010 and the final deadline is due on 20/04/2011 and as the project time plane is pretty much challenging it is only achievable if we reach the targets timed on the plan.

Finally, as the examination taking place on January, there will not be an efficient work on project during the time mentioned. Therefore, the resuming of the process is going to be by the beginning of the February as it is illuminated in project plan.