I have explained the destruction on several occasions so far.
That concludes the basics of how objects are destroyed.
If you want to learn about mechanics of materials in a step-by-step manner, even as a beginner, please start with this index.
This information regarding destruction is something that everyone involved in manufacturing, whether it's industrial products or handcrafted goods, should be aware of.
This is because it concerns the safety of everyone who uses industrial products, and everyone around them—it's an extremely important matter.
In some casesInsufficient consideration of the potential damage can lead to injuries or even deaths among those who use the object.In the worst-case scenario, it can involve other people around you.
In the author's area of expertise, transportation machinery such as automobiles, airplanes, and ships can cause multiple fatalities if they involve not only the users but also passengers or people nearby.
Buildings, stationary machinery, and equipment are in an even more precarious situation (if the fatigue calculations for a nuclear reactor are wrong, it could become uninhabitable for hundreds of years).
Depending on the item,Let's be aware that even small miscalculations can potentially lead to serious social problems that affect the lives and deaths of many people.
roadThe basic concept of a tool is that it is something that is convenient for people and aims to bring them happiness, so if it causes unhappiness to the person using it or those around them, it should not exist in this world.ノIt is.
It might sound exaggerated, but this is something that could really happen, so let's approach it with a great sense of responsibility.
I've included links for details on each type of destruction, so please refer to them if you have any questions or have forgotten anything.
I apologize if this sounds preachy, but please remember that ethical considerations will become very important from now on (the most recent major accident that raises questions about ethical considerations was the Fukushima nuclear disaster).
Summary of the fundamental theories of destruction
The fundamental theories of fracture consider how stress and strain within a material or component cause it to break, and there are basically about five types.
The following diagram shows the names and characteristics of each.
Each type of material and stress has its own appropriate fracture theory, so at least understand its characteristics.
When analyzing failure using manual calculations or spreadsheets like Excel, tensile strength and fatigue limit are generally used, so there's usually no need to worry too much about failure theory.
However, when using computer-aided engineering (CAE), which is expected to increase exponentially in the future, fracture theory will become extremely important.
In CAE, you must always choose the calculation method. The calculation methods available in CAE are basically based on destruction theory, so if you don't understand it, you won't be able to perform the calculations.
Choosing the wrong destruction theory at this stage can lead to serious problems, so be careful.
Next, we compare the computational complexity of each.
As you can see, the computational complexity varies greatly depending on the theory. In a business, time is money, so it's important to remember that it's crucial to obtain the correct result without wasting time (in large-scale calculations, this choice can take two to three days).
Next, let's look at the characteristics of the stresses obtained using fracture theory.
The important thing here isThe fracture theory commonly used in CAE is shear strain energy (because it's quick to calculate), but note that the resulting stresses are all scalars (they don't have a direction), so the type of stress (tension, compression, torsion, bending) is unknown.
Because the yield point, fracture stress, and fatigue limit can differ for the same material depending on the type of stress, it's easy to make mistakes, so be sure to read them accurately.
For example, let's say we solve a simple cantilevered beam using the shear strain energy theory and display the resulting stress (von Mises stress).
ThenThe display only shows the magnitude of the stress value, so at first glance, the compression at the base of the beam, highlighted in red, looks dangerous. However, the tensile stress is lower than the tensile stress, so the yellow area on the tensile side of the beam base is the point to focus on (red is conspicuous and easy to misinterpret).
This is a simple example, so it's fine, but it can get quite confusing when it becomes more complex.
Misunderstandings can easily occur like this, so be very careful when conducting CAE and when reviewing human results.
At the end of this section, let's review the fundamental theories of even more microscopic destruction.
that isAtomic dislocationThe result is as shown in the following diagram.
External forces generate shear stress.
The bond breaks due to shear stress.
Atoms that have moved due to deformation recombine with nearby atoms.
This is a fundamental concept of destruction, so remember it. Incidentally, machining is also an application of this principle.
One-shot destruction and fatigue destruction, etc.
Now, let's take a look at the diagrams illustrating the actual forms of destruction.
The major divisions of destruction areFailure can be categorized into single-shot failure, fatigue failure, creep failure, and brittle failure. Actual failure occurs as a combination of these processes.
So, remember this diagram and always keep it in mind.
Let's review the methods for analyzing each type of destruction.
Considering a one-shot destruction
Single-shot fracture can be broadly categorized into tensile, torsional, and bending fractures (plus yielding due to compression).
In either type of failure, the material will break when stress exceeding its respective strength is generated.
The method of considerationFor tension, use a stress-strain diagram; for torsion, use a shear force (torque)-torsion diagram; and for bending, use a stress-deflection diagram.
Material properties are always available from material suppliers or your own company's materials department, so be sure to obtain that data.
BasicallyAll materials must be handled below their yield strength (0.2% proof stress).However, in the case of disposable tools such as those used for plastic tightening of bolts (angle method), the yield point is not the only relevant factor, although opportunities to use it are infrequent.
Details for each can be found here.
If it's designed properly, it's highly unlikely that it would suddenly break down in one go.
Study of fatigue failure
Fatigue failure can be broadly divided into three types: tensile and compressive fatigue failure, torsional fatigue failure, and bending fatigue failure.
All types of failure, like single-shot failure, depend on the material and the form of stress.Fatigue limit (stress that can withstand 10^7 cycles)It has a limit, so exceeding it will destroy it.
The tools we will use for this examination are:We use an S/N diagram that shows the fatigue limit (bidirectional fatigue limit) of the material, a stress-time diagram that shows the fluctuating stress on the material, and a fatigue limit diagram created from the bidirectional fatigue limit, unidirectional fatigue source, yield point, and fracture point of the material.
Of these, only the S/N diagram is data held by the materials supplier or the company's own materials department; the remaining cyclic stress and fatigue limit diagrams are often created by the designer, so it's important to understand how to create these diagrams.
Each fatigue source has the following characteristics
Bending fatigue limit > Tensile and compressive fatigue limit > Torsional fatigue limit
Basically, I createdIf the stress falls within the fatigue limit curve, it can withstand $10^7$ cycles, so in machinery, it can be considered to last almost indefinitely (I don't know about architecture, civil engineering, etc.).
fundamentallySince fatigue failure is the trigger for object failure, any part subjected to repeated loads should be handled within its fatigue limit.However, this does not apply to disposable parts or situations where strict lifespan management of parts is in place, such as in car racing.
Details on fatigue failure can be found here.
Creep fracture, (low-temperature) brittle fracture
This creep-induced failure is a particularly troublesome type of failure because it occurs without any external force being applied.
Creep is the phenomenon where, if a constant stress is continuously applied to a material, the strain increases over time. Stress relaxation, on the other hand, is the phenomenon where, if a constant strain is continuously applied to a material, the generated stress decreases over time.
Creep failure is a type of fracture that occurs due to the stress generated by this creep phenomenon.
このTo examine creep failure, we use the following creep curve.
Basically, creep strain is large under high stress and high temperature, but even at room temperature, sufficient strain to break the material occurs under high stress, so caution is advised.It is necessary.
Next is brittle fracture. Basically, brittleness alone does not cause fracture; rather, brittle fracture refers to a situation where a material becomes brittle and fractures under a weaker load than expected. Of particular concern is low-temperature brittle fracture.
Metallic materials become hard and brittle when continuously exposed to low temperatures (below 0°C). This phenomenon is called low-temperature brittleness..
This analysis uses Charpy temperature characteristics, which will be explained in the section on metallic materials.
Details of this destruction can be found here.
Actual destruction
Let's review what the fracture surface looks like when actual destruction occurs.
The order in which the properly functioning machines are destroyed is,
Repeated load application → Repeated stress is generated within the member → Damage accumulates in the cross-section of the member → The effective area of the member's cross-section decreases → Stress exceeds the fracture point and causes a sudden fracture.
ThereforeThe fracture surface will always show signs of fatigue and the initial fracture. If there are no signs of fatigue at all, it is highly likely to be secondary damage.
As for what kind of traces of fatigue remain, they are beach marks (seashell patterns) like the one shown in the following diagram.
Furthermore, fatigue can be classified into low-cycle and high-cycle fatigue based on the number of beach marks. Low-cycle fatigue is caused by secondary damage, design flaws, or unexpected external forces.
BasicallyBeach marks (high-cycle fatigue failure) + traces of one-shot failureIt will definitely remain.
Details of the fatigue fracture cross-section can be found here.
Let's review what the cross-section looks like after a single-shot fracture. Basically, it depends heavily on the material properties.
Although the diagram only shows tension, torsion and bending are almost the same and basically depend heavily on the toughness and brittleness of the material.
またEven if no damage has occurred, check all parts that have undergone durability tests for cracks or runout lines.
If cracks or Lüders lines are found, it is a sign that the material has already exceeded its yield point, and in most cases, countermeasures will be necessary.
Details of the cross-section of tensile fracture and Lüders wire can be found here.
Choose the materials you use carefully, depending on the purpose.For example, if you want to destroy something quickly for safety reasons, you would use a hard and brittle material, while if the failure would directly affect people's safety, you would use a tough and strong material.
The materials are mostly determined by the manufacturing process and cost, but the designer's ideas and intentions should be kept in mind.
theseKeep the fracture surface in mind, and if you encounter a fracture site, try to identify the origin and form of the fracture to pinpoint the true cause.
Furthermore, let's combine all the knowledge we've gained from the beginning of this course to determine whether it's tension, compression, shear, torsion, bending due to deflection, buckling, or localized stress concentration.
That's the extent of the scope of materials mechanics.
yetAI and computers can perform calculations, but they cannot pinpoint the cause of destruction. In other words, when destruction occurs, knowledgeable humans are needed to understand the cause. Therefore, even if you can't perform calculations, you must understand the cause.
These kinds of experiences will be valuable assets for you as a designer and engineer, so take on these opportunities proactively.
Finally, I will explain the behavior of creep fracture and brittle fracture, but to be frank, they do not have any unique characteristics whatsoever.
It was quite a problem, or rather, it was a real problem.
The process of determining the cause of failure involves thoroughly investigating all possible factors, including unexpected external forces, manufacturing defects, and design errors. Only after finding no other issues is it determined to be creep fracture or brittle fracture.it can.
Moreover, the only solutions available are changing the materials or modifying the operating environment, which often requires a complete redesign, making the process extremely difficult.
It's a good experience, but it's the kind of person I'd rather not encounter.
If you don't anticipate encountering destruction at work or elsewhere, it's good practice to briefly consider the cause of any damage to your belongings or things you find in your daily life. I recommend it.
Summary and future prospects
With the knowledge you've gained so far, you should understand the meaning of most material specifications, so use them effectively.
However, I apologize if this sounds repetitive,Considering the possibility of destruction is an extremely important step that must be taken for every object.Furthermore, understanding fracture requires a thorough understanding of stress and stress concentration, as explained above.
This concludes the basics of material mechanics necessary for design.
I added an explanation of how to solve stress problems that require a slightly specialized approach.
However, triaxial stress (combined stress) and strain energy will be explained later. These two are also very important, but they require some knowledge of partial differentiation and matrices, so I will introduce them after I have finished explaining those.
Looking ahead, now that we finally have the necessary foundational knowledge to create drawings, we will explain the mechanical elements such as screws, springs, and gears, which are components of the machine.
Furthermore, having reaffirmed the importance of mathematical knowledge, I would like to start a course on industrial mathematics.
Furthermore, since the designer's way of thinking is useful not only for designers but for all business people, I would like to start a new course.
Ultimately, I'd like to explain everything up to the point where we can design an engine, but that seems like a long road ahead.
I hope you'll continue to support me in the future.
To those who found this article helpful in understanding design:
While there is basically no textbook covering this content, and it is my own original work, I will introduce the textbook that I have been using since I was a student.
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