Fatigue Test

Summary

In "Fatigue Test" by MaterialsScience2000, the video delves deep into the pervasive issue of fatigue loading on materials, a condition where materials are subjected to repeatedly changing, nonuniform loads that eventually lead to failure. Everyday examples like bicycle frames, railway bridges, and garage door springs highlight this phenomenon. The video outlines how these components are tested, detailing the process of determining fatigue strength using steel specimens and fatigue testing machines. Through controlled experiments, results are analyzed using SN diagrams to understand the fatigue limits of materials. These insights are crucial for ensuring the durability and safety of various engineering components, emphasizing the complex interplay of stress, material properties, and environmental factors.

Highlights

• Everyday materials face fatigue loading due to nonuniform, alternating loads. 🚴‍♂️
• Off-road bikes, garage doors, and railway bridges are typical examples. 🚪🌉
• Fatigue testing involves testing specimens under controlled cyclic loads. 🔄
• Understanding fatigue is essential for predicting when a material will fail. 📉
• Detailed SN diagram analysis is used to visualize fatigue strength and limits. 📈

Key Takeaways

• Fatigue loading leads to material failure under repeated stress even at lower stress levels. 🚴‍♂️
• Common items like bikes and bridges face fatigue loading without us noticing. 🌉
• Testing for fatigue strength involves specialized machines and precise measurements. 🛠️
• Accurate testing is crucial for safety and efficacy in engineering and manufacturing. 🏭
• Fatigue limits vary and are determined through graphical SN diagram analysis. 📊

Overview

Fatigue loading is an underappreciated yet critical aspect of material science that affects various daily use items. This unseen phenomenon occurs due to repeated application of stress. It’s the silent enemy of off-road biking gear, components of railway bridges, and even simple garage door springs. When such components fail, it is often due to what is known as fatigue loading where materials endure less when stressed repeatedly than with a single load.

The video explains this subtle yet significant issue by walking viewers through the fatigue testing process. Materials are put through rigorous testing in laboratories using standardized specimens made from steel rods. These are mounted on fatigue testing machines, subjected to alternating tensile and compressive forces, and monitored for the formation of fatigue cracks. With each load cycle, these cracks grow invisibly until failure ensues, demonstrating why fatigue is a major concern in engineering.

Results of such tests are captured in SN diagrams, which are crucial for engineers and manufacturers. These diagrams offer a visual representation of when and how materials might fail under specific stress amplitudes over extensive cycles. This methodical approach helps in developing materials and components that not merely meet functional requirements but also ensure long-term durability—an essential factor for safety in public infrastructure and transportation.

Chapters

• 00:00 - 00:30: Introduction to Fatigue Loading This chapter introduces the concept of fatigue loading, a condition where everyday items experience non-uniform, periodically changing loads. It provides examples such as off-road biking, where frames, handlebars, and wheels undergo high alternating mechanical loads, and a garage door spring, which alternates between being loaded and unloaded as the door opens and closes.
• 00:30 - 01:00: Examples of Fatigue Loading in Everyday Items The chapter discusses the concept of fatigue loading in everyday items, using a specific example of the connection rods in an old compressor. These rods endure numerous rapidly changing tensile and compressive forces. It highlights how even moderate cycling in a city involves components that undergo periodically varying loads. This includes the crank arms of bicycles, which experience load and load-free states in rhythm with pedaling.
• 01:00 - 01:30: Detailed Description of Fatigue Loading This chapter delves into the concept of fatigue loading, where individual elements like links in a chain, a railway bridge, or aircraft components undergo periodic and variable stress. The chapter gives examples, such as railway bridges experiencing varying loads from heavy freight trains or merely their own weight, and aircraft facing different stresses during take-offs and in turbulence. The core focus is on understanding how materials respond to such stress patterns, commonly referred to as 'fatigue.'
• 01:30 - 02:00: Material Testing for Fatigue Strength The chapter focuses on material testing for fatigue strength, emphasizing the difference in material endurance between continuous loads and repeated cyclic loads. It explains that while materials may endure a single load, they typically have a lower threshold under repeated loading, causing early failure. An example provided is a wire that doesn't break upon the first bend but eventually fractures when bent repeatedly. This concept is further illustrated through the failure of a bicycle crank arm and a garage door spring, both of which succumb to fracture after enduring cyclic loads for a certain period.
• 02:00 - 02:30: Preparation of Test Specimens This chapter delves into the critical process of preparing test specimens to assess their durability under cyclic loading conditions. It highlights various instances of material failure, such as a fractured garage door system after 60,000 uses, broken exercise equipment, and a fan blade with tears. The chapter underscores the significance of understanding how cyclic loading leads to product failures and stresses the importance of ensuring the structural integrity of components to withstand such repetitive stress.
• 02:30 - 03:00: Mounting and Initial Testing of Specimens This chapter discusses the mounting and initial testing of specimens to measure the fatigue strength of a commonly used structural steel, S235 JR. The text emphasizes the importance of preparing suitable specimens to eliminate surface influences and ensure well-defined material stress during testing. Hot rolled steel rods are identified as the base material for this testing process.
• 03:00 - 03:30: Specimen Fracture Analysis In the chapter titled 'Specimen Fracture Analysis', the focus is on the preparation and selection of test specimens for fracture analysis. The ideal specimen shape involves shoulder-shaped ends that facilitate safe gripping during tests. The central region tested is cylindrical with a diameter of 6 mm. The material tester ensures the production of 10 identical specimens from rods, emphasizing uniformity. With all specimens ready, the tester begins the analysis by selecting the first specimen for testing.
• 03:30 - 04:00: Fatigue Crack Propagation Investigation In the chapter titled "Fatigue Crack Propagation Investigation," the process of preparing a material specimen for fatigue testing is described. The specimen is secured into a fatigue testing machine using two half shells and union nuts. Once securely mounted into the right and left specimen grips, the preparation is complete. The material tester then prepares to select the test parameters on the control computer for the investigation.
• 04:00 - 04:30: Alternate Load Testing and Results The chapter titled 'Alternate Load Testing and Results' explores the process of alternately loading a specimen with a tensile force of 7 kons and a compressive force of 7 kons. This results in a mean force of zero. The procedure begins with a click on the start button, initiating the periodic stressing of the specimen by a fatigue testing machine. This machine is equipped with a mechanical resonance drive. The specimen is specifically connected at its left side for the test.
• 04:30 - 05:00: Stress Analysis and SN Diagram The chapter "Stress Analysis and SN Diagram" discusses a mechanical test setup where a grip is connected via a load cell to the machine's frame. A spring and mass are mounted on the setup, and an imbalance encourages the mass to vibrate. Longitudinal guides are used to restrict vibrations to one direction, preventing transverse movement. The test force, which acts on the specimen, is modulated by adjusting the excitation frequency of the imbalance, allowing precise control over the test conditions.
• 05:00 - 05:30: Understanding the Fatigue Limit This chapter explores the concept of the 'Fatigue Limit' in materials. It discusses a scenario where a specimen is subjected to about 25 load cycles per second, and mentions how the first specimen has endured over 300,000 load cycles. Although the specimen appears intact, fatigue cracks have begun forming starting from the surface. These cracks grow incrementally with each load cycle.
• 05:30 - 06:00: Operational Loading and Final Testing The chapter titled 'Operational Loading and Final Testing' describes a testing procedure where specimens are subjected to operational loads to induce fracture. The narrative follows a detailed process of observing the fracture as it occurs and progresses through multiple load cycles. After completing 381,000 load cycles, the test is concluded, and the fractured specimens are removed for inspection. Despite appearing unimpressive at first glance, a closer examination of the fracture surfaces reveals more detailed information which is critical to understanding the material's behavior under stress.
• 06:00 - 06:30: Influence of Additional Factors on Fatigue Strength The chapter discusses two main regions of a material undergoing fatigue: the flat fatigue crack propagation area and the rough residual fracture area. The residual fracture area is a result of the final catastrophic rupture, while the fatigue crack propagation area develops during cyclic stress starting at the surface of the specimen. The chapter also mentions examining the fatigue crack propagation area at higher magnification by placing a fragment carefully on a specimen stage.
• 06:30 - 07:00: Importance of Regular Inspections The chapter discusses the importance of regular inspections, particularly focusing on fatigue crack propagation. Using a scanning electron microscope, the investigation reveals that parallel grooves and ridges, known as fatigue striations, are formed by individual load cycles. This is a typical phenomenon observed in materials undergoing repeated stress. The chapter emphasizes understanding how many load cycles the material can endure under various loads to prevent failures.

Fatigue Test Transcription

• 00:00 - 00:30 fatigue test many everyday items are not loaded uniformly but rather nonuniformly with periodically changing loads this is quite obvious in off-road biking frames handlebars and wheels are subjected to high alternating mechanical loads it is just as obvious in the case of a garage door spring the spring is unloaded when opening the door and loaded when closing the
• 00:30 - 01:00 door the connection rods of this old compressor have endured a large number of rapidly changing tensile and compressive forces even when cycling quite moderately in the city many components are exposed to periodically varying loads without us being aware of it the crank arms are alternately underload then swing back almost load free are under load again with the rhythm of pedaling the spokes the pedals even the
• 01:00 - 01:30 individual links in the chain are stressed periodically also the Railway Bridge Over the River Ry seemingly totally addressed alternately has to Bear the additional burden of a heavy freight train and then only its own Mass aircraft experience quite different stresses during take off and in turbulence than on ground this type of loading is generally termed fatigue loading and the material response to it fatigue the term fatigue is used because
• 01:30 - 02:00 it takes some time for the failure to occur under repeated loads the problem with materials is that they normally endure less at repeated loading than with a once only load the wire does not break when first bent but bending it back and forth repeatedly leads to an early fracture this bicycle crank arm had only been able to endure the cyclic loading for a certain time before it broke the fate of of the garage door spring was
• 02:00 - 02:30 the same it had borne the opening and closing of the garage door about 60,000 times before it finally fractured the broken exercise equipment a torn blade of a fan a Banda Blade full of cracks the broken screw a defective universal joint and many other examples show how important this phenomenon is each day thousands of items break due to cyclic often repeated loading how can we make sure that all these components withstand
• 02:30 - 03:00 the cyclic loads to achieve this one has to test the materials appropriately we intend to measure the fatigue strength of a frequently used steel in our laboratory hot rolled steel rods made from the structural steel s235 Jr are used as base material in order to eliminate the influence of the surface and to stress the material in a well- defined manner suitable specimens have to be machine from the rods this
• 03:00 - 03:30 specimen type would be ideal the shoulder shaped ends have been designed for safe gripping the actually tested region is cylindrical with a diameter of 6 mm the material tester goes to the workshop to get 10 specimens manufactured from the rods not just one all of the specimens are as identical as possible back in the lab he has all specimens ready for use and picks out the first one
• 03:30 - 04:00 he mounts the specimen carefully and securely into the fatigue testing machine together with two half shells he inserts one end of the specimen into the right specimen grip with a union nut he locks the right end of the specimen likewise he mounts the Left End of the specimen into the left specimen grip and tightens the nuts the preparation is now complete the material tester then selects the appropriate test parameters on the control computer he intends to
• 04:00 - 04:30 load the first specimen alternately with a tensile force of 7 kons and a compressive force of 7 kons the mean force is zero a click on the start button and the testing machine begins to stress the specimen periodically our fatigue testing machine is equipped with a mechanical resonance Drive [Music] the specimen is connected at its left
• 04:30 - 05:00 grip via a load cell to the machine frame at the right grip a spring is mounted and there in turn a mass an imbalance stimulates the mass to vibrate longitudinal guides prevent the Mass from vibrating in the transverse Direction via the spring the test Force acts on the specimen by varying the excitation frequency of the imbalance the test force can be controlled in our
• 05:00 - 05:30 case the specimen is loaded with about 25 cycles per second meanwhile the first specimen has already endured over 300,000 load cycles and still seems to be intact in reality however several cracks have already developed starting from the surface the so-called fatigue cracks with each load cycle they grow a bit
• 05:30 - 06:00 further into the specimen the biggest crack gains the upper hand grows the fastest and finally leads to fracture after 381,000 load Cycles the first test is finished the material tester removes the fragments cautiously and inspects the fracture surfaces at first glance the fracture surface looks unimpressive a close look reveals two
• 06:00 - 06:30 regions the rather dull predominantly flat fatigue crack propagation area and the rough residual fracture area the residual fracture area results from the final catastrophic rupture the fatigue crack propagation area has developed during the cyclic growth of the fatigue cracks starting at the surface of the specimen what does the fatigue crack propagation area look like at higher magnification one of the fragments is carefully placed on the specimen stage
• 06:30 - 07:00 of our scanning electron microscope now we can investigate the fatigue crack propagation area in detail in some places but not everywhere parallel grooves and ridges can be seen the so-called fatigue striations each Groove and each Ridge has been formed by an individual load cycle a typical phenomenon of course now we want to know how many load Cycles this material can endure with other loads for this purpose the material
• 07:00 - 07:30 tester picks the next specimen from his Supply and installs it into the fatigue testing machine he chooses a higher alternating load for the second specimen a tensile force of 7.5 kons and a compressive force of 7.5 kons under the increased load the specimen is only able to withstand 43,600 Cycles to failure the third specimen is only tested with a cyclic load of 6.5 K under
• 07:30 - 08:00 this reduced load it does not break even after 3 million load Cycles additional specimens complete the series of measurements some of the specimens are tested with the same load we've already used in this way we get an impression of the range of scattering to compare materials in a Fair Way the different cyclic loads are
• 08:00 - 08:30 converted to the respective stresses acting in the specimen in this case the stress alternates sinusoidally around the mean value which is termed mean stress Sigma m in this series of tests the mean stress is zero the stress amplitude is labeled with a symbol Sigma a for graphical representation the SN diagram is used also termed vula diagram
• 08:30 - 09:00 in an unusual manner the predetermined variable is plotted upwards on the y- AIS it is the stress amplitude Sigma a and the result of the test the number of Cycles to failure n is plotted to the right on the x-axis to fit small and large numbers into the diagram the xaxis is not scaled linearly but logarithmically the first specimen was loaded with 7 kiltons
• 09:00 - 09:30 this corresponds to a stress amplitude of 248 Newt per square mm under this stress amplitude it broke after 381,000 load Cycles the second specimen was loaded with 7.5 kons this corresponds to a stress amplitude of 265 Newtons per square mm under this stress amplitude it broke after 43,600 load cycles and these are the results of the other specimens which have been tested
• 09:30 - 10:00 by us when additional specimens are tested and the results are displayed as a curve it becomes clear that this material will not break below a certain level of stress amplitude even after a very large number of Cycles this stress level is termed fatigue limit Sigma capital A for most Steels with body centered cubic structure the fatigue limit is reached after 10 million load Cycles at the latest
• 10:00 - 10:30 it must be noted that the experimental results scatter considerably therefore instead of a Sharp curve a statistically defined scatter band is commonly used of course most users don't load their components with a constant cyclic load but rather in an irregular manner therefore these components have to be tested under this type of loading termed operational loading in this test rig critical bicycle components are loaded similarly
• 10:30 - 11:00 to practical use stem and handlebar must Ure the typical bending and torsional loads for a sufficient number of Cycles only then they are approved for serial production moreover the fatigue strength does not only depend on the material the mean stress has an important influence the shape the surface quality environmental conditions and much more not an easy story before aircraft is allowed to take
• 11:00 - 11:30 off for the first time it has to go through extensive stress tests the wings and many other elements are cyclically loaded in large test rigs this later ensures the safe operation of the aircraft nevertheless particularly important components are repeatedly examined for fatigue cracks in the course of operation the tram for instance has to go into the test Hall at regular intervals experienced staff use ultrasound to examine special component onon especially the wheel set axles to
• 11:30 - 12:00 check whether they are still in perfect condition aircraft are also inspected for fatigue cracks at reasonable intervals this helps us to depart for air travel without worry and land again safely [Music]