In the previous article, we introduced fatigue failure and explained most types of failure.
This time, unlike previous explanations, we will discuss a phenomenon where failure occurs even without any external load being applied.
Typical examples of this are creep fracture and brittle fracture.
I will explain the details later, but this creep and brittle fracture phenomenon occurs in relatively common machinery and structures.
While creep fracture and brittle fracture are generally not a concern under normal load conditions at room temperature, they do occur occasionally, so it's good to keep them in mind.
If you're unfamiliar with creep fracture and brittle fracture, you might get stuck in a maze of unanswered questions and waste a lot of time trying to figure out how the fracture occurred.
So you don't need to know all the details, but it's good to understand because it can actually happen.
What is "creep"?
Before understanding creep failure, it's obvious that you need to understand the creep phenomenon itself.
Creep is a phenomenon in which, when a constant stress is continuously applied to a material, the strain increases over time. This strain increase is particularly pronounced in high-temperature environments.
Furthermore, the decrease in stress generated within a material over time while maintaining a constant strain is called stress relaxation.
Up until now, we have explained that the relationship between stress and strain is constant according to Hooke's Law.
If we add a time axis to the graph above, we can see that the strain increases even if the stress remains constant.
A common example of creep is when a potted plant pot is left unused for a long time and cracks without any apparent cause (although this can be caused by various factors such as material corrosion and repeated thermal stress due to temperature differences between day and night, so it may not be entirely accurate to call it creep).
A common example of stress relaxation is when the bolts holding the handle of a pot lid become loose, which can be one of the causes of loose screws (however, repeated thermal stress due to temperature differences can also be a cause of loosening in pot lids, so it's not a simple matter).
To reiterate, creep is the phenomenon where deformation progresses when a constant stress is continuously applied, while stress relaxation is the opposite phenomenon where the stress inside a material decreases when deformation is continuously applied.
It's important to understand that creep does occur in common metal materials like aluminum, albeit at a small level .
To represent the creep phenomenon numerically, we use a graph like the one below.
This graph shows how a carbon steel (iron containing carbon, a common type of steel) deforms when subjected to constant stresses of 260 MPa, 240 MPa, 228 MPa, and 210 MPa under a constant temperature of 400°C. The horizontal axis represents time, and the vertical axis represents strain.
This type of graph is called a creep curve.
Next, I will explain the characteristics of creep.
First, as can be seen from the graph, the greater the stress on a material, the greater the deformation due to creep. In other words, structures that are continuously exposed to high loads will spontaneously increase in deformation due to creep.
Next, creep is not a constant increase in the amount of strain over time, but can be broadly divided into three categories.
In region I of the graph, the strain increases rapidly because stress has just been generated. This is called transition creep.
Next, in region II of the graph, the system is relatively stable, and the increase in strain and time maintain a proportional relationship. This is called steady-state creep.
Finally, in region III of the graph, the strain increases rapidly. This is called accelerated creep.
The reason why the amount of strain increase changes over time is that in many metallic materials, the internal crystals harden in response to the strain caused by stress.
The rate at which the crystal hardens remains constant up to a certain point, but as the hardness increases, the crystal rapidly increases in response to that increased hardness, causing this phenomenon to occur.
Further details will be provided in the section on metal materials.
While I won't show graphs for different temperatures, generally speaking, even with the same stress, continuous exposure to high temperatures increases the strain due to creep.
In other words , the amount of strain increases with higher stress levels and higher ambient temperatures during the creep phenomenon.
In mechanical design, steady-state creep in the second region is primarily dealt with, and the rate at which strain increases due to creep is called the creep rate (slope of the creep curve).
This concludes the explanation of the creep phenomenon.
Creep destruction
If the strain increases due to creep, it will eventually exceed the limit of strain within the material and lead to fracture.
Furthermore, the limit of creep that occurs without leading to destruction is called the creep limit (under certain environmental conditions and stresses, creep will eventually stop if left alone).
This creep limit is important to note because it significantly impacts the lifespan of buildings, machinery, and civil engineering structures that are constantly subjected to a large, constant load.
Power plant generators, in particular, operate under high loads and high temperatures for long periods of continuous operation, so they must require considerable care.
In the automotive industry, where I have experience, people don't pay much attention to creep (some people may not even know about it).
The creep failure that the author experienced occurred in a cylinder with a shaft pressed into it, as shown in the following diagram.
The materials used were a press-fit process with a large engagement amount (large overlap) using ultra-hard chromoly steel for both the shaft and cylinder.
The part was going to be tested later, so I left it in storage for about three days.
When we tried to use it, the cylinder was broken. Of course, it caused a huge commotion and a lot of trouble.
Of course, I was aware of the creep phenomenon at the time, but since the storage facility was kept at a temperature of around 20°C, nobody was concerned.
An examination of the cracked cylinder revealed that it was due to creep failure.
The cause of creep failure in this case was not temperature, but rather the large press-fit depth, which continuously subjected the cylindrical section to significant tensile stress. This led to large creep-induced deformation, resulting in stress exceeding the material's strength and ultimately causing it to break.
For details on cylindrical stress, please click here.
Until this incident occurred, the company was unaware that creep failure could occur during press-fitting at room temperature, so we were made to conduct extremely detailed experiments and compile the results. Based on those results, we were made to check all the press-fitted parts that had been mass-produced up to that point, which was so difficult I thought I was going to die.
This abundance of small, tangible know-how is what makes a company strong (it represents the true capabilities of a company that cannot be seen from its financial performance or business operations).
It's important to note that creep failure can occur even without high temperatures if high stress levels are present (few scholars are aware of this).
Up to this point, we've seen that stress due to creep increases with high load and high temperature, so one might think it would be fine at low temperatures, but unfortunately, that's not how things work in this world.
Now, let's explain what happens to metallic materials at low temperatures.
Low-temperature brittleness and brittle fracture
Let me explain what happens when you lower the temperature of a metal material to a low temperature (basically below 0°C).
Actually, among metallic materials, steel (an alloy of iron) hardens automatically at low temperatures because the binding force of its internal crystals increases.
When exposed to such low temperatures for extended periods, the body becomes increasingly hard, and its elastic modulus increases, but the plastic region shortens, making it brittle. In other words, it becomes hard and brittle.
This phenomenon is called low-temperature brittleness .
This phenomenon of metals hardening at low temperatures has been utilized since ancient times. Humankind has empirically known that cooling a metal through heat treatment causes its internal crystal structure to transform, making it harder and stronger (tempering).
However, it was not known that metals become progressively harder when exposed to low temperatures for extended periods (and the technology for continuous cooling is a very recent development).
This low-temperature embrittlement was actually discovered relatively recently, when a large number of transport ships (Liberty ships) built in the United States during World War II mysteriously sank in the Arctic Ocean.
The U.S. Navy took this seriously and, after analyzing it, determined that the cause was that metal materials harden and become brittle when exposed to low temperatures (below 0°C). This marked the beginning of the world's first research into the brittleness of metals at low temperatures.
There are several theories as to what actually caused the ship's destruction, including that it was due to a welding defect in the world's first welded hull structure, or that it suffered fatigue failure due to fluctuating stress caused by temperature differences as it traveled between warm California and the Arctic Ocean. However, since it is a US national secret, the fact that it remains a mystery is fascinating.
I'll go into more detail later in my article on the world's three biggest mechanical failures (two of which are in mechanics of materials).
Materials developed based on the lessons learned from this low-temperature brittle fracture include SP, SPCC, and JSC materials, which are commonly used in sheet metal and have very high resistance to low-temperature brittleness (toughness is increased by adding Mn, Si, etc.).
I'll introduce the materials commonly used in this type of machine later.
Incidentally, I have never experienced low-temperature brittle fracture.
However, I introduced this site in case, by any chance, someone might be designing containers for freezers, cryogenic gases, or liquids (such as liquid nitrogen or liquid oxygen).
Also, as of the time of writing this article, it's rumored that coronavirus vaccines need to be stored at -70 to -80°C, and the storage containers must be designed to account for this low-temperature brittle fracture, otherwise the containers will crack during storage and the vaccines will be ruined. I think if the design is poor, they'll probably crack in about 2-3 days (I wonder if that's okay?).
As you can see, materials mechanics is surprisingly relevant to our daily lives.
The value indicating the degree of low-temperature brittleness can be determined by the Charpy impact test, but since the Charpy test is not introduced here, it will be explained in detail in the section on metallic materials.
Summary
Let's summarize creep fracture and brittle fracture.
When a constant stress is applied to a material, the strain increases over time; this phenomenon is called creep.
When a material is subjected to a certain amount of strain for an extended period, the stress generated decreases over time; this is called stress relaxation.
Creep phenomenon increases with higher loads and higher temperatures.
Creep occurs in any material and under any stress.
The amount of creep is represented by a creep curve, which has three strain-increasing regions, called transitional creep, steady-state creep, and accelerated creep.
In mechanical design, steady-state creep is the primary focus, and the rate at which the strain increases is called the creep rate.
The maximum value of stress generated by creep is called the creep limit.
Creep failure is a type of failure in which increased stress due to creep causes the strain to grow, exceeding the strength of the material and causing it to break.
Creep occurs even at room temperature, and under high stress, creep failure occurs, which causes the material to break.
Metallic materials harden and become brittle when continuously exposed to low temperatures (below 0°C); this is called low-temperature brittleness.
Low-temperature brittle fracture is the term used to describe the failure of a component that has become brittle due to low-temperature brittleness.
This has turned out to be a bit long, but that concludes my summary.
The loosening of the boss due to stress relaxation will be explained in detail in the section on screws in mechanical element design.
Brittle fracture includes not only low-temperature brittle fracture but also hydrogen embrittlement due to corrosion, and there are many things to explain, such as the Charpy test used for evaluation, so I will explain them in detail using metallic materials.
Here, to help you understand brittle fracture, I explained low-temperature brittle fracture, which is the most representative type of brittle fracture.
This completes our discussion of the main types of fracture in materials mechanics.
This destruction series has become quite long, but it contains essential information for anyone involved with machinery or manufacturing, so I hope you will understand it.
Next time, I'll summarize the destruction process and explain how it's considered in actual work.
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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