Background
When we think of a ‘structure’, we usually think of buildings or bridges. However, from an engineering perspective, structures are encountered in all walks of life. Apart from civil structures like buildings or bridges, other examples of structures include automobiles and aero planes.
Let us take an example of an automobile, and see how structural optimization plays an important role in its design. An automobile drives on the roads. And irrespective of how progressive the country is, there is a chance of the car encountering curb bumps, high gravitational curves for roads that turn sharply, the need to brake suddenly in case of an emergency, and so on. These are called instantaneous loads. Apart from this, an automobile encounters routine pot holes, twists and variable passenger weight. These loads are called as fatigue loads and are characterized by complex time histories with lower amplitudes but a greater number of occurrences. The automobile structure must be designed in such a way that it is capable of handling all such loads. Or consider a tall building. A building is subject to dead and imposed loads, wind loads and other loads. A building will also need to resist an earthquake. Likewise, aero planes are subjected to aerodynamic forces, thrust, turbulence, etc. In short, any structure – be it civil or other – is subject to various loads and stresses. The branch of engineering that deals with structures is called Structural Engineering. Structural Analysis is a subset of structural engineering, and ensures that a structure is capable of handling these loads and stresses without compromising on the safety and performance.
Structural Analysis
Structural analysis is a multidiscipline engineering endevour and involves the coordinated effort between designers and engineers from various disciplines in order to avoid failure in the intended use of any structure. Usually, durability and sustainability are two major priorities in the structural design and numerous models have been developed along these lines in different disciplines. A few engineering / mathematical theories that structural analysis is based upon include linear and non linear mechanics, multi-scale and multi-physics modelling, finite element analysis, meshless methods, and simulation of solids. On a broad level, there are two types of structural analysis – linear and non linear. In very simple terms, linear structural analysis deals with a direct ‘cause-and-effect’ kind of phenomenon. Let us take the example of a cast iron rod. If it is subject to tensile stress, it elongates in proportion to the force applied up to a certain thresh hold. Structural analysis under these conditions is called linear analysis. However, after the rod reaches its yield strength (the point beyond which it starts to deform permanently), the consequent elongation is no longer linear. Instead, it behaves in a manner which is not proportionate to the force applied; it is simply erratic. A simple additional force may make the rod elongate disproportionately, or in a non linear fashion. Structural analysis under such conditions is called non linear structural analysis.
The need for accurate predictions of resilience of structures subject to extreme events (typically non linear in nature) requires a new generation of engineering tools that permit a damage assessment below the limit loads for which structures are designed. Finite element analysis (FEA) is one such accepted way of analyzing stresses numerically. It involves dividing the model body into an equivalent system of many smaller bodies or units (finite elements) interconnected at points common to two or more elements (nodes or nodal points) and / or boundary lines and / or surfaces and solving their differential equations numerically. Structural analysis software is now very popular in engineering companies as well as companies that provide engineering services. While the erstwhile software was not very reliable for non linear structural analysis, the newer breeds of software like Altair OptiSctruct™ provide extremely reliable non linear analysis.
Topography Optimization
In order to make something structurally stable, it is also needed to design its surface optimally. Topology is a branch in mathematics which is concerned with the properties of space that are unaffected by elastic deformations. It is a mathematical technique that optimizes the material distribution for a structure within the given spatial constraints. Topology optimization has come to be known as a method to numerically solve problems wherein the optimal topology of a solid structure gets determined. In the context of a structure, the method gives us the layout along with the detailed geometric form. From an engineering viewpoint, topology optimization deals with problem formulation, design parameterization, sensitivity analysis, algorithms for solving the problem, and its implementation. Again, a software like OptiStruct is very efficient in topology optimization.
Noise, Vibration, and Harshness (NVH) Analysis
NVH analysis is a subset of structural analysis that deals with noise, vibration and harshness performance, mainly of automotive models. However, it should be noted that NVH analysis is needed for every system that moves or is put into motion (like manufacturing machines). All mass-elastic systems like the car body and chassis have natural frequencies that need to be optimized in order to create a pleasant, vibration free driving environment. These vibration sources are characterized by their time and frequency domain characteristics and can be periodic or random. For example, the vibration from a power unit in an automobile is periodic, while those from terrain inputs to the wheel are random. All these vibrations make an impact on the overall engineering and topology of the model. NVH performance is most often done with mode-based finite element procedures.
Structural Analysis / Topology Optimization / NVH Software
As you may have noted from the above topics, structures are quite complex systems. Since they are interlinked, the safety and performance of each component needs to be evaluated individually and as a cohesive unit. Automobiles and aero planes in particular are subjected to unpredictable loads and stresses. With powerful CPUs, abundant RAM and advances in structural analysis software, engineers and engineering services providers have abundant arsenal at their disposal today. The best in class software includes features like NASTRAN formulation for linear analysis, powerful non linear analysis algorithms, advanced optimization technology, specialty vibration and acoustics solvers, and many others. Engineers can explore various options that will solve structural issues using real-world multi-physics these software provide. Only a good software will provide an engineering solution that meets all the complex criteria that makes a structure sturdy and safe, so it should be chosen with care.