How to Design Knives that Do Not Fail

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Failure Modes

A common engineering technique is to evaluate designs based on how they failed, or may fail. Failure modes are identified and the designs are modified to prevent those failures. These ideas are simple in concept but it is surprising how easy it can be to miss the forest for the trees. In terms of predicting failure modes in some cases it is easy and in some it is difficult. You would expect a large chopping knife to require heavier edge geometry to prevent chipping and rolling, or a seawater diving knife to require high rust resistance. Other times failure modes are identified through testing of the knife or by returns from customers. Whatever the source of the failure, there are usually relatively simple methods for fixing each one, though the trade-offs for doing so may not be desirable.

Chipping of Edges

If knives are used only for fine slicing tasks a chipped edge is highly unlikely. Unfortunately, knives are used more roughly than that. I have an article on the microstructure mechanisms that occur during chipping of edges. The easiest way to make chipping more difficult is to make the edge thicker or more obtuse. However, there are many failure mechanisms that may make chipping easier within a given geometry. Steel that is higher in hardness has poorer toughness. Steel can be embrittled in heat treatment several ways which are covered in this article. A higher toughness steel can be selected which is controlled greatly by the carbide structure as described in this article. Using a high toughness steel given a good heat treatment will allow the thinnest edge geometry possible while preventing chipping, but in the end the edge geometry must be appropriate for the application.

Rolling of Edges

Rolling of edges is in some ways connected to chipping and in other ways the opposite. They are similar in that thicker or more obtuse geometry is the easiest way to help prevent rolling. Thicker geometry has a very strong effect on strength of edges, the simple calculations are described here. After geometry the prevention of rolling is controlled by the strength of the steel. The most common measure of knife steel strength is rockwell hardness, though that is not always a perfect measure of strength. Steel can have a low “yield strength” for a given hardness which means that rolling will occur more easily. Undertempering or excessive retained austenite are two reasons for low yield strength. Read more in this article.

Cutting Ability vs Strength and Toughness

Thinner and more acute edges cut better than thicker and more obtuse edges. This is independent of sharpness. A knife can be sharp but still have poor cutting ability. On the flip side, as described above, the thicker and more obtuse edges have better resistance to chipping and rolling. This is one of the primary design tensions in knives. Unfortunately, humans are not always predictable. One user may be fine with a very cutting ability knife while another will chip the edges or break the knife. And it is also dependent on the intended application, from choppers to kitchen knives to “general use” folding knives. It’s difficult to give broad recommendations, it is up to the knifemaker and customer to determine the best balance. The customer does have some control over the edge geometry through sharpening. The knife can be sharpened to a more acute edge, or through more work the edge can be thinned.

Cutting force (lower means better cutting ability) vs edge angle

Edge Stability

Roman Landes likes the term “edge stability” to describe an edge’s ability to resist rolling and chipping. A combination of tough steel at high hardness provides the best resistance to rolling and chipping, allowing the design of fine cutting knives with thin edge geometry. I recommend my linked article and Roman’s book if you want to learn more about edge stability.

Broken Knives and Tips

The mechanisms that control chipping of edges are somewhat similar to broken knives. Tougher steel in thicker stock is more difficult to break. However, sometimes broken blades are seen anyway. It is important to analyze where the knife broke and why to determine the source of the failure. I showed this process in an article on stress risers, which are inherently weaker parts of a knife because of its design. Read that article to learn more about how to avoid stress risers in knife design.

Knife tips have a similar design tension to edges, where thinner have better cutting and piercing ability but thicker are less prone to breaking. I have always liked the “concrete floor test” where a knife is dropped tip first to determine if it will break. This is a potential scenario for most knives. If prying is expected or a possibility the knife must be tested in that situation as well. The orientation of the knife relative to the rolling direction of the steel is also important for avoiding broken tips, as elongated impurities and carbides provide easy fracture paths.

Dull Edges

There is one way by which any knife will eventually fail – the edge is worn to the point where it no longer cuts effectively. Edge geometry is yet again an important property, thinner and more acute edges will cut longer than thicker edges. Thin edges both cut better and longer than thicker edges. Steels with higher wear resistance will have better resistance to edge “wear” of course. Slicing edge retention is controlled almost entirely by edge geometry and the wear resistance of the steel. Read about which steels have the best wear resistance for edge retention and how edges wear here: Part 1 and Part 2.

TCC is a measure of edge retention with the CATRA test

Worst Case Scenario vs Optimal Performance

Usually in product and engineering design the “worst case scenario” is determined so that failures can be avoided in that situation. However, that is somewhat more difficult with knife designs. For one thing, most knives can be broken or chipped if the customer wants to, or used it in a way the knife is not designed to withstand. Also, the heavier and less breakable the knife is designed to be, the worse it will be at actually cutting things and holding an edge. Knives are not tools that will see only one repetitive task, but used by people that will use it in a variety of ways. A steel is “tough enough” until it is used in a way where it wasn’t. Humans, unfortunately, are very unpredictable in how they will use knives. With custom knives it helps to have a discussion between the maker and customer. Or for the customer to determine their own use case before choosing a knife.

Edge Retention vs Ease in Sharpening

More wear resistant steels are also more time consuming to sharpen because sharpening is a process of wearing the steel back to sharpness. However, thinner edges also require less material to be removed in sharpening which makes it less time consuming. Time in polishing isn’t necessarily changed by thin geometry so highly finished edges with the highest sharpness are most affected by high wear resistance steels. Some customers are more sensitive to “hard to sharpen” steels than others. And some customers prefer a steel that is easy to sharpen and willing to give up some edge retention, such as those that like to regularly stop their knives to keep high sharpness in between light to medium cutting tasks. Other customers prefer high edge retention either because they want to sharpen less frequently, or the cutting tasks they perform are not conducive to regular sharpening, or the customers are good at sharpening and don’t mind a more difficult to sharpen steel. This is another case where the customer needs to understand her own preferences and behavior. A related but somewhat different issue is that sharpening can be more time consuming due to knives that are difficult to “deburr” which can happen due to low hardness or excessive retained austenite, both of which can be solved with heat treatment.

Image of a burr from [1]

Rust

In terms of design the biggest thing that affects corrosion is the finish of the knife. A rough finish makes corrosion more likely, polished steel is less prone to corrosion. Most consumers understand that there are differences between steels in terms of corrosion resistance since we have “stainless steels.” However, not all stainless steels are created equally, and heat treatment can also affect corrosion resistance. I wrote about the effects of heat treatment in this article, and I compared the corrosion resistance of different knife steels in this article. Better corrosion resistance is required for corrosive environments like around salt water or acidic foods. Even a humid environment can mean more rusting without more care of a knife, so a steel with some corrosion resistance can be beneficial for many customers. Even if tarnishing or rust is not a problem perceived by a customer, corrosion of edges can also reduce edge retention and sharpness. Rust can be avoided to some extent, depending on the environment, by the customer such as keeping the knife dry and using various rust preventatives like oil.

Broken Locks in Folding Knives

I am not an expert in lock mechanisms of folding knives, but these are failures that can occur. You will have to read “Folding Knife Lock Mechanism Nerds” to get more information on that. For light cutting tasks knives are unlikely to fail in that way. For “hard use” folders it can be a different story.

Uncomfortable Handles

Uncomfortable handles are perhaps not a failure mode. However, if the knife isn’t comfortable the customer will not use it, and perhaps not buy it. The handle design changes depends on the type of knife it is; choppers, kitchen knives, and whittling knives are likely to have somewhat different handles. I am not a knife designer so my contributions in this area are limited.

Steel Selection

Depending on the intended use and customer for a given knife, there are different steel types and categories to consider. While I do not have a ranking of every steel type, I do have a series of recommendations for different broad categories. One category is steel type, including stainless (corrosion is a factor), extra stainless (corrosive environments), tool steel (maximum performance ignoring corrosion), and low alloy steel (primarily forging bladesmiths). And for each of those types I have recommendations for high toughness, high edge retention, or in between:

Note that in terms of optimal cutting ability with thin edges, steels with both high toughness and high hardness do best. So using a “high edge retention” steel is not always the best choice for knives that are used primarily for low-impact cutting tasks.

Heat Treatment and Processing of Steel

There have been some general references to heat treatment in this article such as the requirement of sufficient hardness to avoid rolled edges. While some knifemakers spend a lot of time looking for the “ultimate” heat treatment, a major element is simply avoiding the many potential issues with heat treating. I have summarized many of these issues in an article called What a Good Heat Treatment Can and Cannot Do. That article introduces heat treatment and links to the many other articles I have written about the individual elements of heat treatment and the mechanisms by which heat treatment occurs. I also have articles about optimizing the heat treatment of a few different steels such as AEB-L, A2, 52100, Z-Wear/CPM CruWear, CruForgeV, and 5160.

Probability and Severity

As part of the decision-making for which failure modes have the highest risk, which is controlled by the probability of the failure and the severity of the failure. Failures that are highly likely and very severe have the highest risk. It can be useful to list out the potential failure modes of a knife and to assign a rating for probability and severity, then rank them by the risk. This can provide a clearer view of the knife design and which aspects of it need to be most closely analyzed and tested for.

Summary and Conclusions

It may seem obvious that if a knife fails then that failure needs to be corrected for. However, in practice these obvious problems can often be ignored. When a knife does not perform as intended, a simple process is used: identify what the failure was and modify the design to prevent the failure. In some cases the source of the failure may be somewhat subtle such as heat treatments that lead to brittle behavior, non-obvious stress risers, or knives that have poor edge retention due to edges that are too heavy. Prevention of chipping is helped by thicker or more obtuse edges and tougher steel. Rolling of edges is also helped by thicker edges but also through stronger, or higher hardness, steel. Edge stability is a combination of those two factors. While thicker and more obtuse edges are better at resisting rolling and chipping, they also have poor cutting ability and edge retention. Therefore, a major design element of a knife is the balance between edge stability and cutting ability. Steels that have a combination of high toughness and hardness are best able to handle the thinner edges required for high cutting ability and edge retention. Stress risers are design flaws that can lead to broken blades and should be eliminated. Rust and corrosion of edges can be avoided through the use stainless steels selected for the environment the knife will see and also by appropriately finishing the blade.

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[1] https://scienceofsharp.wordpress.com/2015/01/13/what-is-a-burr-part-2/