Numbers are exact, and engineering is full of numbers. The world, on the other hand is imprecise and full of all sorts of anomalies. Where does that leave engineering? Or, more to the point, how do we integrate practical engineering to fill the gap?
Stimulating The Thought
This article comes from a question about engineering simulation (FEA) and resolving the pretty pictures with a practical application. For this individual, the simulation of a trailer frame shows one material thickness, but advice from fabricators said much more is required. To their credit, they ask questions, which is a fantastic way to learn.
Theory Versus Practical Engineering
In most industries there is a difference between theoretical design and practical design. The void varies, but understanding differences and how to compensate is experience.
Example 1: In theory, something as thick as Aluminum foil works in some areas (in simulation) of a bicycle frame, but in practice, that's a disaster. For most analysis, in spite of how well you try, it won't (and can't) include everything. The dynamics of motion, customer interaction, environmental factors and impact are just a few areas of concern. Then there's abuse . . . .
Example 2: For a project a few years ago, we used a 12 ton hydraulic press and some special fixtures to "squish" a metal connection to just the right thickness. A 6 ton press (the theory) was insufficient, but in practice, it took a lot more. Interestingly, we also discovered that the impulse from small slide hammer dropping just a few inches, will accomplish the same thing.
With respect to a trailer frame, knowing that trailers frequently encounter minor impact, how does that fit in the simulation? Don't ever underestimate the effects of impact. A pothole, washboard, or debris in the road are impact examples. Bumping a post when backing is another form of impact. Loading objects onto the trailer can also have impact.
Example 3: In large structural design -- like a gantry crane or long trailer -- sometimes it's not strength that limits the design. Sometimes it's deflection. How much is acceptable? Stress analysis theory may say a beam is plenty strong, but practical engineering says that much deflection will cause problems.
Understanding Limits of Applied Theory
In simulation, assigning material properties is easy. However, applications for reality are different. Last time you analyzed a welded structure, did you include heat distress and annealing near each weld joint? Areas immediately around welds are the weakest areas for many materials. Of course, if the weld is good, in a test the parent material will break before the weld. Yet, often the break occurs in the heat distress areas immediately around the weld. And, for thinner material this is even more important.
The limits for most simulations include assumptions of consistent material -- consistent in properties as well as in cross section and surface condition. Practical engineering knows there are little dents, dings and scratches as well as inconsistencies in actual material properties. And, the practical engineering knows the fallacy of Unobtainium.
In every theoretical case, there are assumptions. Often we know the assumptions are not exactly true, but we use them anyway. That's where the practical engineering comes in. It allows us to use theory to learn and predict, while compensating with experience for reality.
The Most Important Practical Engineering
This discussion started with a void between the theory and suggested practice. How we get to the suggested practice and resolve that with the practical is often in the form of a "Safety Factor". And, the significance of the "Safety Factor" is (or at least should be) driven by the importance of the product use. Here are some examples.
Example 1: Think of a stand in a home or office that holds a plant or other object for display. Think about the needs, uses, and how a failure might affect the surroundings.
Example 2: Contrast the above display stand with the needs of a race obstacle like this one in the image. When there are 8 people clamoring to get up and over this obstacle, what are the implications of failure?
Example 3: Finally, another example of thinking through various practicalities to accomplish a task. It's not all about engineering, because it's the mix of practicality with constraints in engineering the elegant solution.
What we know and how well we can predict loading makes a big difference. Choosing the most important aspects of each design must drive the decisions in practical engineering.
Limits By Availability
Going back to the original question, that project uses standard available structural steel members. Standard materials are available only in designated increments. 16 gage might be readily available, but 15 gage not so much. 14 gage sometimes. 11 gage is quite commonly 1/8" and for him, is the next thickness that's easy to get. How much heavier is the trailer in 1/8" over 1/16"? Again, this is a practical matter. If the difference is only 100 pounds or something, then what does that really matter? And, if safety is a question, is 100 pounds worth the risk?
Materials and weights are a judgement call. However, thinking a little beyond the simulation, if slightly thicker material keeps a minor mishap from damaging (weakening) a beam, then it's worth considering. On the other hand, adding 100 pounds to your bicycle trailer can mean the difference between usable or not. There is always more to every project than the simple questions make us think.
Many designs are fluid where we can manipulate things like thickness, but others rely on specific requirements -- like available materials, or limits based manufacturing needs. (Things like making the design so it will also pull from a mold is a constraint.) Even making things that "look" right from a styling standpoint while functioning supreme. Practical engineering blends all these considerations together in the elegant solution.
Value in Experience
With many things in Engineering, knowing how to compensate the theory to meet the practical is key. That's experience.
Going back to the original question, the underlying request was for a safety factor to apply in the simulation. Unfortunately, safety factors rely on judgement and experience as well. I would love to know what safety number to apply in every situation. More importantly, I would love to know what safety factor is right for each area of a design.
In many cases only bad experience will teach you what's not right. And, sometimes that's just as valuable.
Another option is to seek, like the individual stimulating this post, for someone who has the experience. Interestingly enough, that is how Synthesis was started. Surround yourself with people with expertise in different areas, then use Design Reviews to bring it together.
Finally, experience teaches us to look beyond the obvious for considerations. For instance, finding customer use cases that are not the normal, but are certainly possible. In the trailer example, something like a fat guy sitting down on the end bumper. If this is an already loaded 1000# capacity trailer, a 350# dynamic addition to the back bumper could be disaster. Him stepping up onto the trailer or sitting down hard on the bumper will momentarily add far more than 350#.
Food for thought. Call Synthesis for experience in practical engineering. Good luck with your projects.