Practical Stress Analysis For Design Engineers Flabel Pdf
Mirtha Hinrichs
Jean-Claude Flabel is the author of the aerospace engineering textbook Practical Stress Analysis for Design Engineers; a handbook on practical stress analysis which is widely used within the aerospace industry.
Jean-Claude Flabel graduated from the California State University, Northridge in 1970 with a bachelor's degree in mechanical engineering. He has worked with a number of prominent aerospace companies including Rockwell International, Gulfstream American, Sikorsky Aircraft, American Jet Industries and Bell Aerospace and has specialized in stress analysis and safety-of-flight certification of primary airframe structures and components.
Since its publication, the textbook Practical Stress Analysis for Design Engineers has been adapted into a distance learning certificate course for practicing stress engineers. The emphasis of the course is on technical fundamentals and practical real-life examples of stress analysis, with less treatment given to the higher mathematical or derivative aspects of the subject.
In May 2013, after doing an exhaustive research and discussing with many industry professionals, I decided to enroll myself into the distance learning Practical Stress Analysis for Design Engineers Program in the area of Aircraft Stress Analysis by Jean-Claude Flabel. It took me 11 months to complete the full course along with the exams and I must say, it is one of those courses that you really enjoy going through.
This course is designed to place more emphasis on technical fundamentals and give less treatment to the theoretical and derivative aspects of the subject. It discusses topics such as structural methods and procedures, application of stress formulas, sizing techniques and detail design conceptual approach in more detail. In addition to that, the actual aircraft structural problems presented in the book allow to understand both, the technicality and cost analysis part of the problem.
Overall, this course allows me to make intelligent and sound engineering judgments to analyze the structural behavior of components. In addition, it also gives me the capability to optimize their performance in a cost-effective manner. Together with this Stress Analysis certification, I hope to gain practical and progressive stress analysis experience in actual engineering problems.
Hello Manoj,
For more information on this course, please visit
Many Aerospace companies like Boeing provide their engineers with this training. So learning this course will be a good addition to your profile. However, please keep in mind that Aerospace industry is very competitive. Your other skills and education in this field is much more important.
At a minimum you should be thinking about the following: What is the purpose of the fastener in this application? What loading conditions do I expect the joint to experience? What drives the design: Static strength? Stiffness? Fatigue? What is the worst case scenario? What is the target safety factor? What materials are present? Should corrosion be considered? Should thermal stresses be considered? How will it be assembled? Does it need to be taken apart or serviced? How long does it need to last? How much will it cost?
Bolts are ubiquitous in machine design and product engineering, and the vast majority of use cases will not require in depth analysis. However, for those cases where safety factors are lower due to strength, weight or other requirements, or where exact preload must be achieved, bolted joint design can be extremely difficult. Hand calculations can be challenging to reason through, and finite element models can be way off if not setup with proper inputs. Because of the complexity and unique nature of bolted joint design, many fastener related failures occur in the field. General Motors recalled approximately 500,000 vehicles across seven models (Impala, Camaro, Equinox, GMC Terrain, Cadillac SRX, Buick Regal and Lacrosse) in 2014 due to fastener related problems(2). Even the new eastern span of the San Francisco-Oakland Bay Bridge had a complex fastener related problem (hydrogen embrittlement) shortly after construction. \
Due to this complexity, we cannot possibly cover all design cases thoroughly in this post. However, in our experience the following design guidance can arm engineers with a basic joint design toolkit, an understanding of what to look out for when designing, as well as orient engineers to the areas of complexity that must be investigated further (empirically, or with nonlinear contact FE models).
In most cases it is best practice to design a slip-critical joint. This means that the bolts in the joint generate sufficient clamping load across the joint such that the shear through the joint is transferred through the joint member faces, not as direct shear through the fastener itself. This may seem obvious, but it is worth reiterating: bolts are designed to develop clamping loads between two or more components. Implicit in this statement is that bolts are not designed primarily to act as shear pins or in bending. For this reason, ensuring the proper preload across a bolted connection is especially important. As engineers, it is our job to understand and control the loading conditions across structures we design and assembly plays a large and often undervalued role in this.
To demonstrate the differences in loading condition as a function of bolt preload we can look at a double lap joint secured by a single bolt. The figure below illustrates the approximate stress flow in this joint with and without a preloaded bolt.
When fatigue life is a concern, be sure to design in a target preload across the joint, communicate that preload by documenting required bolt torques, and develop QC procedures to validate that the proper bolt torque was achieved.
Now apply this same reasoning to a bolted joint with a pattern of fasteners of varying grip lengths and diameters. The fasteners with shorter grip lengths and larger diameters will take more of the axial loads when clamped members are displaced. This means that varying stiffness across fasteners leaves the stiffest fasteners more susceptible to overload in certain conditions.
A useful guideline for bolted joints is to try to maximize the stiffness of the joint members (things being clamped) and both equalize and minimize the stiffness of the fasteners used for a given target preload. Minimizing fastener stiffness may sound like counter-intuitive advice, but it helps to think about the impact of member displacement and vibration across fasteners. Again, for a given preload, a joint with more compliant fasteners will see less variation in preload when clamping members are displaced. This joint will also see a more equal distribution of axial loads in fasteners when loaded dynamically, thereby sharing loads more uniformly and reducing local failures
As engineers we design bolted and riveted joints that include multiple fasteners. The fastener positions are often patterned uniformly because of the ease of developing simple linear patterns in CAD programs. In situations where installed stiffness is critical, or factors of safety are lower, more care must be put into faster layout. Always remember that the position of fasteners relative to the applied load dictates how much of that load is carried by each fastener. This is the fundamental reason we recommend maximizing member stiffness in the previous section. A stiff component being bolted down will transfer load more evenly among a given fastener group, independent of fastener position.
Unbuttoning is similar to unzipping, but is caused by shear loads (radial relative to the fastener). Again, the layout of fasteners and the stiffness of the clamped members is such that a fastener in close proximity to the applied load bears the majority of the load, in this case shear. The figure below illustrates the conditions leading to unbuttoning failure.
When using bolts and joint members made of different materials, galvanic corrosion can be an issue. For prototypes with short lifespans, this is rarely a problem, but solutions should be considered for production assemblies with long anticipated lifespans or harsh environmental exposure (ex: marine products). The best solution is to electrically isolate dissimilar materials from each other, either through paint, powder coat, insulating washers, plating and other surface coatings like anodizing. In instances where this is not possible, the surface area of the anode material should be minimized. In high moisture environments and especially salt water environments insulating methods become essential.
How much torque can a bolt take? Like all other mechanical components made from ductile materials, bolts have a yield strength. Assuming the nut or threads that a bolt is mating with are strong enough, it is possible to torque a bolt until it tears itself apart. Obviously this is suboptimal for the vast majority of cases, so we can rule out tightening until yield as a good design and assembly practice. This begs the question, how much less than the yield load should we target for our fasteners?
In order to prevent over-tightening, manufacturers often publish proof loads or proof torque charts for their bolts. Proof load is the maximum load a bolt can take without any elastic deformation. Proof load is typically in the range of 85% - 95% of the yield load. Proof load represents the absolute upper limit that a fastener should be subjected to. The illustration below shows a typical stress-strain curve for a ductile material (ex: low carbon steel) along with yield stress and proof stress indicated on the curve.
This leaves us with perhaps the most difficult problem of any bolted joint design; determining the optimal preload and associated torque for each bolt in the joint. We know the optimal preload for a bolt is somewhere between zero and proof load, but how do we determine it exactly? The answer lies in balancing the mean load due to preload, and the dynamic or cyclical load due to external forces across the joint. We can visualize the impact of mean load and cyclical load alongside the stress strain curve.
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