Material Selection And Structural Analysis Engineering Essay

Material Selection And Structural Analysis Engineering Essay

2.1 Introduction

The basic map of an aircraft ‘s construction are to convey and defy the applied tonss ; to supply an aerodynamic form and to protect riders, warhead, systems, etc. From the environmental conditions encountered in flight. These demand, in most aircraft, consequence in thin shell constructions where the outer surface or tegument of the shell is normally supported by longitudinal stiffening members and cross frames to enable it to defy bending, compressive and torsional tonss without clasping. Such construction are known as semi-monocoque, while thin shells which rely wholly on their teguments for their capacity to resists tonss are referred to as monocoque.

Structures engineering encompasses a broad scope of constituent engineerings from stuffs development to analysis, design, proving, production and care. Materials and constructions have mostly been responsible for major public presentation betterments in many aerospace systems. For military aircraft, there will be a alteration in accent from best public presentation to low cost at acceptable public presentation. For infinite systems, new challenges are a consequence of a displacement in scheme from long term, complex, and expensive missions to those that are little, cheap and fast. Materials and constructions, in add-on to enabling engineerings for future aeronautical and infinite systems,

Continue to be the cardinal elements in finding the dependability, public presentation, testability, and cost effectivity of these systems. For some of the future air vehicles, the development and deployment of new constructions engineerings can hold more impact on cut downing the operating cost and the gross weight than any other engineering country. An overview of

The focal point of the present article is on developments in constituent engineerings that will better the vehicle public presentation, progress the engineering development procedure, and cut down system life-cycle costs. The constituent engineerings are grouped into six classs, viz. :

Smart stuffs and constructions

Multifunctional stuffs and constructions

Low-cost composite constructions

Extreme environment constructions

Flexible supporting constructions

Computational methods and simulation-based design

The development of each of the constituent engineerings is a multidisciplinary activity, which involves undertakings in other subjects.

2.2Material Selection –

The major focal point of structural design in the early development of aircraft was on strength, now structural design besides trade with fail-safety, weariness, corrosion, care and inspect ability, and producability. There are several factors which influence the choice of the structural stuff for an aircraft but amongst these strength allied to lightness is likely the most of import. Other belongingss holding varying, though sometimes critical significance are stiffness stamina, opposition to corrosion, weariness and the effects of environmental warming, easiness of fiction, handiness and consistence of supply and, non least of import, cost.

The basic structural layout of the aircraft is given in below figure. The structural model consisted of three distinguishable subdivisions:

the forward fuselage/hull and the aft fuselage/hull, and

the wing/engine mount/undercarriage support subdivision.

Each of these subdivisions required a different design doctrine but all were designed to reassign tonss swimmingly between the coupling constituents.

The aft fuselage subdivision is the simplest of the three structural constituents. It incorporates a fuselage shear web. This extends from the engine support bulkhead frame to the austere station beneath the tail. Positioned along this web are cross frames which support the aft fuselage tegument and reassign flexing tonss to the web. The non-load carrying chine deck and cabin floor make several watertight compartments in this portion of the fuselage.

The forward fuselage is built up from a planning hull on a cardinal box beam traveling from the forward-most cabin frame to the engine support bulkhead. Two upper longerons supplement the box beam in transporting frontward fuselage flexing tonss. Transverse frames encircle the construction to reassign tonss, enclose the cabin, and to back up cabin seating, instruments and controls. Two chief frontward frames besides back up the sponsons, flying pylon, and the flying frontward spar. All of the forward subdivision frames besides serve as watertight bulkheads in the forward fuselage hull country. These structural elements are constructed with chiefly of high strength aluminium/lithium metal.

The 3rd constituent is the most complex portion of the construction. It consists of the wing and engine support subdivision. The wing structural box is built from two consecutive spars, running continuously between each flying tip, and top and bottom profile teguments. The engine firewall and support construction is mounted to the aft flying spar. This, in bend, is supported by two chief fuselage frames. Like the other two structural subdivisions, all frames, longerons, spars, ribs, and the wing tegument are constructed of ferrous metals of aluminum. ( M.Subba Rao,2005 )

Mechanical belongingss of Aluminium-Lithium Alloys


The major issue for the aerospace industry is increasing warhead and fuel efficiency of aircrafts, which has increase the development of more advanced stuffs with high specific belongingss. The new coevals of low denseness Li-Al metal is the campaigner in the among material These low- denseness metals are attractive to the aerospace industry, as structural weight decrease is a really efficient agencies of bettering aircraft public presentation.

The combination of Li to Al offers the significantly cut down the weight of aerospace metals, since each 1 wt. % Li added to Al reduces denseness by 3 % and addition in elastic modulus. Al-Li alloys usage in aircraft applications. The weight nest eggs effected by utilizing these low-density metals really much cut down the vehicle fuel costs and increases public presentation. In comparison to new stuffs systems such as fibre-reinforced complexs, low denseness. Al alloys do non necessitate big capital investings by the aircraft manufacturer in new fabricating installations. This cost nest eggs can more than balance the greater public presentation increase. This composite may offer, ensuing in Al-Li metals being well more cost effectual than complexs in some applications. Fatigue cleft growing opposition in Al-Li metals by and large is really high ; this is of import in damage-tolerant constructions such as lower wing surfaces.

The major development began in the production of aluminium-lithium metals in the seventiess. The aim of presenting light weight, high stiffness aluminum alloys that could be fabricate on bing equipment and constituents could be handled and assembled utilizing established techniques. Some of the most of import commercial metal in this category include 2090, 2091, 8090, and Weldalite 049 that were introduced in the 1980s. The tabular array below shows the chemical composing of these metals. ( Amit Joshi 2005 )

Composition of aluminium-lithium metals ( wt. % )


















0.95 Mg






0.4 Ag

0.4 Mg


Li is the lightest metallic component and each 1 % of Li reduces alloy denseness by about 3 % and additions modulus by about 6 %

• 7-10 % Lower denseness.

• 10-15 % Higher Modulus.

• Excellent weariness and cryogenic stamina belongingss.

• Higher stiffness.

• Superior weariness cleft growing opposition.

• Reduced ductileness

• Low break stamina


1 ) Aircraft parts such as taking and draging borders, entree screens, place paths and flying teguments.

2 ) Military Applications: – Certain types of military aircrafts parts like chief wing box, centre fuselage, control surfaces are made by Al-Li metals. Al-Li metals are used as replacement for conventional Al alloys in choppers, projectiles and satellite systems.

3 ) Space Applications: – Of all the benefits offered, by the usage of Al-Li metals, weight nest eggs is the most critical in infinite applications. Al-Li metal is a candidate stuff for the cryogenic tankage of supporter systems. These metals are used in cryogenic applications for illustration, liquid O and H fuel armored combat vehicles for aerospace vehicles. ( John d.anerson, jr,1999 )

Fabrication of Al-Li metals

• Ingot Metallurgy.

• Rapid Solidification Metallurgy Technique.

Commercial Al-Li Alloys

1 ) Weldalite 049 – Composition ( wt % ) – 5.4Cu, 1.3Li, 0.4Ag, 0.4Mg, 0.14 Zr, bal. Al

2 ) Alloy 2090 – Composition ( wt % ) – 2.7 Cu, 2.2 Li, 0.12 Zr, bal. Al

3 ) Alloy 2091 – Composition ( wt % ) – 2.1 Cu, 2.0 Li, 0.10 Zr, bal Al

4 ) Alloy 8090 – Composition ( wt % ) – 2.45 Li, 0.12 Zr, 1.3 Cu, 0.95 Mg, bal Al

Physical Properties of Al-Li metals





Density, g/cm3




Melting scope, 0 C




Elastic modulus, GPa




Poisson ‘s ratio


Thermal conduction at 250C, W/m-k




Specific heat

at 1000C, J/kg-k




Physical Metallurgy

Li is the lightest metallic component and each 1 % Li reduces the metal denseness by about 3 % and additions modulus by about 5 % . Li in little sums allow the precipitation strengthening of Al when a homogenous distribution of coherent, spherical ? ‘ ( Al3Li ) precipitate is formed during heat intervention. Like other age-hardened Al metal, Al-Li alloys achieve precipitation strengthening by thermic aging after a solution heat intervention. The precipitate construction is sensitive to a figure of variables, including, but non limited to, the slaking rate following the solution heat intervention, the grade of cold distortion prior to aging, and the ageing temperature and clip.

The age hardening of Al-Li alloys involves the uninterrupted precipitation of ? ‘ ( Al3Li ) from a supersaturated solution. The Al and Li in the ? ‘ precipitates are positioned at specific locations. The eight shared corner sites are occupied by Li, and six shared faces are occupied by Al. Al-Li base metals are micro structurally alone. The major beef uping precipitate ( ? ‘ ) when homogeneously precipitated remains consistent even after extended ripening. The 8090 type Al-Li metal show a microstructure dwelling of metastable stages such as ? ‘ ( Al3Li ) , S ‘ ( Al2CuMg ) , T1 ( Al2CuLi ) and ? ‘ ( Al3Zr ) within the grains. In add-on several other constitutional stages such as T2 ( Al6CuLi3 ) , ? ( AlLi ) and dross stages occur at grain boundaries. Another of import micro structural characteristic is the presence of a ?’-PFZ near the high angle grain boundaries.


The advantages of Al-Li alloys over conventional aluminum metals

comparatively low densenesss,

high elastic modulus,

first-class weariness

cryogenic strength and stamina belongingss,

superior weariness cleft growing opposition.

The last belongings is a cardinal factor for damage-tolerant aircraft design. However, it has been discovered that the high opposition to tire cleft growing is due to a jaggy cleft way through the stuff that produces a big sum of roughness-induced cleft closing under tenseness dominated burden. Crack closing is a phenomenon foremost known in the seventiess that reduces the abrasiveness of the emphasis strength at the cleft tip under an externally applied burden. It is hence utile ; provided it can be counted on to be. Unfortunately, lading conditions that contain compaction or compressive overloads, that compress the cleft surfaces, cut down or extinguish cleft closing and do cleft growing rates to speed up significantly.


Another disadvantage of these metals is that in the strongest heat treated conditions, the mechanical belongingss are frequently extremely anisotropic. There exists significantly low ductileness and break stamina in the short transverse way. Another disadvantage is a really high cleft growing rate for micro structurally short clefts which potentially allows for fast cleft induction. This could intend comparatively early snap in high emphasis parts such as stud holes.