Engineering Analysis in mechanical design

Analyzing attributes of a design are key activity in product lifecycle. What is engineering analysis and how is it different from design?

Difference between Design and analysis

The essence of engineering is analysis of the system or product. It is what distinguishes an engineer from a technician. Analysis is basically breaking down of systems into their mechanical elements and study of their behavior under various conditions. For a highly complex product, engineering analysis becomes crucial to establish the reliability, durability, safety and function of the product.

Design is the entity which represents the product it maybe in the form of a model or a prototype.

Analysis is the study which is done on design with respect to engineering principles and can be various domains from the strength of the part to study of motions and even costing.

Types of analysis – Based on Domain

Basic levels of engineering analysis

Strength – Analysis of designs based on concepts of mechanics of materials to evaluate the stress levels and deflections in the parts or assemblies during service
Fatigue – Behavior of designs under cyclic loading conditions which is an extension of the strength analysis but involves much deeper study.
Stability – Study of the static or dynamic stability of the parts in the product.
Dynamic – Study of internal and external forces on parts or assemblies and the motion caused by those forces
Vibration – Study of excitations and vibrations which occur in products that have dynamic moving parts.
Flow – Study of fluid flow along with change in other properties like temperature, pressure in a working fluid.
Thermal – Study of temperature and Heat flow in a part.

There are multiple specialized in-depth analyses which are done on designs. The type of analysis is chosen based on the nature problem.

Example: Static structural analysis can be either in Linear range or it can be in the non-linear range. Depending on the nature of problem the analysis type can be chosen.

Types of analysis – Based on Depth

Technical judgement – A judgement on a decision based on prior experience, without proper justification due to the simplicity of the problem or repetitiveness of the problem

Hand calculations – The problem and analysis is defined by “closed form expressions” These are the equations found in generic textbooks and for simple problems. The same procedure can be extrapolated to approximation of real life design problems. Knowledge of underlying assumptions and boundary conditions is a must while using hand calculations for meaningful results.

Computational analysis– An analysis requires the computational power of a computer to perform millions of calculations which cannot be done by hand reliably. Example – Finite element analysis of a complex Beam for stiffness and strength, CFD

Usage of each of these techniques for analysis depends on the stage of the project. When the project is at the concept stage and the amount of detail and certainty in product specifications is limited, its better to use technical judgement and hand calculations for concept validation.

Once the concept is detailed into a mature design then the usage of computational techniques can be justified. For critical applications that involve high costs and need very accurate and precise results, computer simulations become important to make decisions rather than invest in iterations in testing.

Computational methods

Called as Computer aided Engineering it is the use of High performance computing (HPC) for simulation of real world phenomena like the loading or unloading of a part, motion of links in a mechanism and flow of a fluid in an engine.

FEA – Finite element analysis is the method of breaking up a large geometry into smaller manageable chunks on which the engineering analysis is done using Linear algebra. This technique is mostly used for static stress analysis, dynamic analysis – vibrations and also non-linear stress analysis
CFD – The simulation of fluid flow through a volume along with the thermal effects using computational power of a power. The methodology of computation is mostly Finite volume method which is different from the Finite element method. Instead of elements of geometry here elements of volume are used to discretize the fluid flow region.
MBD – Multi body dynamics is the simulation of forces, moments and motion of a mechanism when it is acted upon certain forces. Multi body simulations show the effect of an input to the mechanism on how the motion, force and moments at various joints of the mechanism vary

Analyzability

Based on the stage of the design and amount of data available, there are decisions to be taken on whether a certain analysis is fruitful or not.

Sometimes there are so many interrelated variables that separating each and analyzing them separately does not give meaningful results.

Sometimes the amount of knowledge available about a design is not enough to analyze it without testing it physically. Hence it is first important to assess “Analyzability” before starting the analysis

Following questions are relevant to be asked while deciding:

Whether a design problem is sufficiently defined to go for an in-depth analysis?
Whether it is worth to spend time and resources on analyzing the design in a certain method in depth
Whether the amount of information regarding the design problem can define the problem completely or more data has to be collected?

Importance of Context, boundary conditions and assumptions

If the Data is wrong, then the analysis will not provide any meaningful outcome.

If the method is not relevant to design problem, then how much ever good the input data is the output will be wrong

If the assumptions made for the analysis are based on false contexts then the analysis will not yield any meaningful result

If the loading and boundary conditions considered for the analysis are not correct the analysis will not yield the right or accurate result

Design changes

The whole point of analysis is to arrive at a design configuration which is better than the earlier one.

The outcome of the analysis is used to decide the changes which should be done in the design. So, if there is an area which is weak according to the analysis then changes are done to counteract that weakness.

Design changes at the concept level are much easier to integrate than at a stage when the design has been sent for production. Also design changes for one analysis may affect the results of another. This situation calls for a trade off in design.

Design Trade-off

Conflicting design requirements for two separate analyses are not uncommon in mechanical design. In such situations the design engineer has to take a decision and prioritize the change which has a minimum impact on the overall performance of the product.

For example: For a moving assembly, A Strength (stress) analysis would direct an increase in material, but that increase in material is counterproductive to dynamics of the part generating more inertia. Hence a trade-off must be made prioritizing strength over inertia increase since durability is much more important.

Categories: : Engineering analysis and simulation