Mechanical Failure Basics: Loads, Stress, and Weak Points
(Level 1)
Before you can prevent failure, you need to understand what makes a part vulnerable. Level 1 introduces the core concepts: how loads create stress, where stress concentrates, and why some locations in a part are more likely to fail than others.
At this stage, the focus is on building a mental model of how forces move through structures and where problems typically begin.
What You'll Learn
Loads: What They Are and Why Engineers Misjudge Them
A load is any force or couple applied to an object. What seems like a simple definition is actually the root of most failures. People can design with the stiffest material, the greatest factor of safety, and still have a part fail – all because the loads were not properly understood; failure occurs when the part is subjected to more than what was designed for – 5000 pounds versus 8000 pounds during a rapid deceleration, for example. Then comes the job of explaining this failure to management and/or customers.
Loads come in many different forms, some static, some dynamic, some concentric and some applied over an area. For example, a book on a shelf puts a static load on a shelf; vibration, impacts, and weight put a dynamic load on a structure. In many situations a point load is encountered (bolts, a hook under a crane, etc.); in other situations one finds an area load (atmospheric pressure on a liquid filled tank, a roof loaded with snow, etc.).
One major factor at play, however, is the unexpected load. Someone drops the entire assembly on the shaft, a jolt occurs when something in the machine jams and binds, or someone treats a flat panel as though it were a step. However, these kinds of events cannot be planned for, and yet they so often occur to cause failure. The designer must consider how a product will be used incorrectly, as that is often the only way to ensure it won't be misused excessively.
Load vs Stress: Same Force, Different Risks
I want to clarify something. I see a lot of people misuse the terms load vs stress. Load is typically thought of as something a part is subjected to (i.e. how much amperage will the cable carry, or in the video above, how much weight will the bridge hold). Stress on the other hand is something a material undergoes. So even though the load (weight) is the same, the stress experienced by the bridge is greatly different due to its varying width.
When tensioning two rods with the same length of stretch, one with a 10 mm dia. and the other with a 20 mm dia., and applying 10,000 N tension to both, the 10 mm rod develops 4 times the stress of the 20 mm rod - even though both are subjected to the same load. This is why bolts often snap when over-sized holes are used - in an attempt to make bolting and de-bolting easier. When a larger hole is used, the force required for de-bolting is reduced, but a greater stress is developed in the reduced cross-sectional area.
That part failed under load? Well, what was the load? A large load applied on a thick section may be okay whereas a modest load on a thin section with a neat little notch may cause it to crack immediately. Remember: failure occurs under stress and stress is what your geometry imposes upon a part.
Stress Concentrations: Where Failures Actually Start
Stress is not distributed evenly throughout a part or assembly. Stress tends to concentrate in particular regions such as within holes or sharp corners. In these areas stress can reach levels three times greater than normal operating stress - and this is typically where cracks develop.
Problem features include holes or cutouts, notches, sharp changes, threads, keyways in shafts, and welds. Stresses, when going through these features, must change direction over a small area, a pathological condition that stress detests. These are common trouble spots, and on many occasions, the problem originates in such locations. Therefore, it is wise to start investigating a failure at these types of locations.
Stress concentration factors are multipliers used to allow for the increased local stress above the nominal stress. To design 'blind' is to ignore them. A part which looks comfortably safe from stress calculations may fail under test for a trivial reason such as a fillet radius which is marginally too small.
Load Paths: How Forces Travel Through a Part
Forces do not vanish into thin air. They must enter the system, travel through the system (following specific paths – load paths), and then depart at the members' ends (supports or restraints). Water in pipes is a good analogy. Water follows the path of least resistance. When one pathway is blocked, water seeks an alternate route.
Understanding the load paths in a structural system can help the designer clarify which portions of the structure are actually performing work and which are merely present as unnecessary additional length. Viewing a simple fixed end beam, one can see that the load path extends from the applied load, downward through the length of the beam, and then through the fixed ends to the foundations. A simple free end cantilever is different; the load path extends from the free end tip down through the member length and continues back to the fixed end, bending the entire member.
Stress tends to follow the path of least resistance. This is particularly problematic when the load path is ambiguous or irregular. You spent a lot of time and careful planning to ensure that loads were transferred along a specific vector, only for a loose bolt (or missing clamp) or under-developed weld to suddenly change-vector for you – allowing stress to suddenly find an obscure, un-designed path of least resistance to follow. As a result, failure occurs at an obscure location, often opposite to where you would have expected.
Stress Types That Drive Different Failure Modes
Stress is not always "stress." There are many different forms of stress, and they can each have profoundly different effects on materials, sometimes allowing the very same part to survive high loads of one sort of stress while failing spectacularly under loads of another sort. Linear tension is one of the most commonly encountered forms of stress and also among the easiest on materials, with some types of parts even being designed to withstand purely linear tension with only minimal danger of catastrophic failure. But bending stress—where torque applies a varied force across a part—is an entirely different animal.
Brittle fracture occurs as a result of tensile stress, the force that pulls material apart. Compressive stress, or the squeezing force, can cause buckling in thin members and crushing in short members. Shear stress results in forces sliding layers of material, common in bolts, pins, and welds. Bending stress causes a combination of tension on one side and compression on the other. Torsional stress is responsible for twisting shapes, common in drive shafts and torqued fasteners.
Cast iron is brittle. That means it's very strong in compression but breaks easily in tension. Mild steel on the other hand is ductile and so is generally strong in both tension and compression but can yield at high stresses. Different materials have different failure modes. These failure modes are not just determined by the amount of stress that has been applied, but also by the type of stress that has been applied and how the stresses have combined together. A factor of safety that may seem perfectly adequate when considering strength in simple static loading becomes quite irrelevant when failure occurs in a completely different way.
Finding Weak Points: Geometry That Creates High Stress
As engineers, we recognize that everything has some inherent weaknesses. Just as a well designed building can have 'problem areas' that put unusual amounts of stress on a part, weaken a panel, or change from a simple to difficult geometry. By studying these fail points, we can design away the worst of the problems before something collapses under stress.
Check for any change in cross section where the part thickens or gets thinner - stress will concentrate on this boundary. Material discontinuities such as welds (particularly through thick sections where a heat affected zone with reduced mechanical properties will have developed during the weld process) , brazed joints and adhesive bonds all create ideal stress concentration sites. In addition check if there are locations of high stress due to design requirements. Typical areas of high stress include the root of cantilevers and bases of tall structures due to reverse bending load, and also around holes - especially in parts that have reduced area adjacent to these.
It's not just the supports that get worn out. Other, seemingly less critical areas can also fail, such as long spans that sag and crack over time or a cantilevered section that bends and fatigues at the point where it connects to the structure. Seasoned design engineers are notorious for scrutinizing the poorest areas of a design first. This is not due to a naturally pessimistic worldview; rather, it is because they know all too well where poorly designed structures tend to fail.
Why This Level Matters: Predicting Failure Before It Happens
For those pursuing the field of engineering, it is easy to compute stress in simple, text book style problems involving straightforward beams, uniform load, and ideal supports. However, real world parts consist of features such as holes, fillets, welds, and varying cross sections. For these sorts of components, there is not a simple answer to the question of where the part will fail. Instead, the answer revolves around the part's geometry, failure patterns, and how stress behaves in unexpected scenarios.
This level of understanding stress helps you to visualise stress as a distribution, not just as a simple calculation performed once and then forgot. It allows you to see stress as a distribution that you can use to estimate the probability of failures before you ever make a part. Understanding stress to failure also helps you to see how loads, internal stress and failure risk are all connected, and are simply different ways of looking at the same thing in a continuous chain of events.
Once we have established the initial foundation for our bridge by determining the loads, stress concentrations, critical points of weakness, and locations of failure we then begin to investigate the consequences of high stress upon materials. This includes an examination of yielding, fracture, material toughness and why some materials fail without warning whilst others fail catastrophically providing the engineer with little or no indication of impending disaster.