In this lecture we study about what is Fatigue failure With Example why it happens and how to avoid so let's start
Fatigue failure With Example
It is observed that matter fail under fluctuating stress at stress expanse is lower than ultimate tensile strength of the matter.
The expanse is even smaller than the yield strength.
What is fatigue failure?
It has been establish that the expanse of stress bring about fatigue failure decrease as the number of stress cycle in rise. This phenomenon of reduce resistance of the materials to fluctuating stress is called fatigue.
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| Fatigue failure |
In this basic variance between failure due to static load and that due to fatigue. The failure due to static load displayed by simple tension test. In this case load gently applied and there sufficient time for elongation of fibers.
In ductile material, there is considerable plastic flow prior to fracture. this outcome in smooth fibrous structure due to starching of crystal at fractured surface. The fatigue failure begins with a break at some point in material.
The crack is extra likely to occur in following regions
1) Regions of interruption, such as oil hole, keyways, screw thread etc.
2) Regions of deviation in machining working, such as scratches on plane, stamp mark, inspection marks etc.
3) Inside cracks due to deficiency in materials like blow holes.
These regions subjected to stress concentration because of break. The break spread due to fluctuating stresses, until the cross section of part is so reduced that the remaining portion is subjected to sudden fracture.
There are two areas of fatigue failure
a) Region showing extension of crack with fine fiber appearance
b) Region of unexpected fracture with a rough granular appearance
In case of failure of static load there is always sufficient plastic deformation prior to failure which gives warming well in advance.
Fatigue cracks are not evident till they reach the surface and by that time failure has so far occurred.
The fatigue failure is unexpected and complete.
It is relatively easy to design component for load.
This makes the design of components subjected to fluctuating stresses are more complex.
R.R. MOORE rotating beam fatigue testing machine
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| R.R. MOORE rotating beam fatigue testing machine |
The rotating beam machine developed by R.R. MOORE is shown.
Four Distinct Stages
Initiation Stage:
This stage begins with the formation of microcracks at stress concentrations within the material. These microcracks develop due to cyclic loading and can remain relatively small and localized.
Propagation Stage:
As the cyclic loading continues, the microcracks begin to grow and extend further into the material. This propagation stage involves the progressive growth of these cracks, often in the direction perpendicular to the applied stress.
Critical Crack Stage:
Eventually, the cracks reach a critical size where the remaining cross-sectional area of the material cannot support the applied stress. At this point, the crack grows rapidly until complete failure of the material occurs.
Failure Stage:
In the final stage, the material experiences sudden and catastrophic failure due to the coalescence of cracks or the propagation of a dominant crack, leading to fracture.
Understanding these stages is crucial in engineering to predict the lifespan and durability of materials subjected to cyclic loading.
Example:-
specific example of fatigue failure in a metal component, such as a steel axle shaft used in a vehicle:
Scenario: Imagine a steel axle shaft in a truck that regularly travels long distances over rough roads, experiencing frequent loading and unloading due to the vehicle's weight and road conditions.
Examples of fatigue failure
Aircraft Structures:
Aircraft components like wings, fuselage, and landing gear experience cyclic loading during flight cycles.
Fatigue cracks can develop at areas of stress concentration, such as fastener holes or structural joints.
Failure of critical aircraft parts due to fatigue can have severe consequences and is closely monitored and managed in aviation maintenance.
Automotive Components:
Vehicle parts like engine components (e.g., crankshafts, connecting rods), suspension systems, and chassis are exposed to cyclic loading from road conditions.
Fatigue failure can lead to fractures in parts like suspension springs, axle shafts, or wheel rims.
Continuous vibration and impact stresses contribute to the development and propagation of fatigue cracks over time.
Machine Parts:
Industrial machinery and equipment often operate under cyclic loading conditions.
Components such as gears, shafts, and bearings can experience fatigue failure, leading to unexpected breakdowns and production downtime.
Fatigue cracks can initiate from areas of stress concentration or surface defects and propagate under repeated loading cycles.
Bridge Structures:
Bridges and infrastructure subjected to vehicle traffic and environmental loads experience cyclic stresses.
Fatigue failure in bridge components like steel beams or cables can result from repetitive loading over many years.
Proper inspection and maintenance programs are critical to detect and prevent fatigue-related issues in bridge structures.
Pipelines:
Underground or underwater pipelines transporting fluids or gases are susceptible to fatigue failure.
Cyclic pressure fluctuations, along with environmental factors like soil movement or water currents, can lead to fatigue cracking in pipeline materials.
Failure of pipeline sections due to fatigue can cause leaks or ruptures, posing environmental and safety risks.
Consumer Products:
Everyday items like bicycles, tools, and appliances can experience fatigue failure.
For example, a bicycle frame may develop fatigue cracks over time due to repetitive stress from riding.
Similarly, hand tools subjected to frequent use and loading can exhibit fatigue-related issues such as cracking or deformation.
These examples highlight the broad range of applications and industries where fatigue failure can occur. Understanding the causes and mechanisms of fatigue is essential for designing durable and reliable structures and components, ensuring safety and longevity in engineering applications.
In this lecture we study the Fatigue failure With Example and why it happens. So hope you understand well.
Any queries comment below. Thanks for reading.
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