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MATERIALS SELECTION GUIDELINES

Today, engineers and designers are challenged with the development of light weight, low cost, high performance components and structures. There are more than 50,000 materials available for the design and manufacture of a product. Every material can not be a right choice for a given application, therefore there is a need for a suitable material selection. This section will illustrate how material properties and systematic selection methods are important for quick and effective selections of suitable raw materials, adhesives and manufacturing processes.

1. Raw material selection guidelines.

2. Manufacturing process selection guidelines.

           3. Prepreg selection guidelines.

4. Adhesive selection guidelines.

 

The need for material selection

There are mainly two reasons when an engineer is involved in a material selection process.

1. To redesign an existing product for better performance, lower cost, increased reliability, and       decreased weight.

2. To select a material for a new product or application.

 

1. Raw material selection guidelines

 

1.1 Steps in a Material Selection Process

There are four main steps involved in narrowing down a list of items for a suitable material of choice.

1.1.1 Understanding and determining the requirement

The first step to identify a material is to define the requirements such as cost, weight, service, performance, etc. of a product. There may be several benefits a material can offer but some requirements are critical to the application. For example, weight may not be critical for a consumer product, whereas weight is critical for an aerospace part. Similarly, for an application, high wear resistant requirement may be necessary, whereas for another application, wear resistance may not be a concern at all. So it is important to prioritize the requirements of a product in the beginning of the material selection process.

Here is a checklist for setting up the requirements:

1. Strength requirement

2. Type of loading

3. Cost Requirement

4. Weight Requirement

5. Impact resistance

6. Temperature resistance

7. Humidity, chemical and electrical resistance

8. Process requirements

9. Production rate requirements

 

1.1.2 Selection of possible materials

Based on the requirements of a particular application, list all possible materials, which can meet minimum or maximum requirements for the application. To narrow down the choice, set minimum or maximum requirements that the material must possess and should result in a positive yes or no. The purpose of this screening phase is to get definite answers as to whether a particular material should be considered for the application. For screening purposes, materials data from various sources are collected, reviewed and then a decision is made.

Materials data enters at various stages of the design process. But the level of accuracy required on material property differs at each stage. In the initial stage of the design process (conceptual stage), approximate data for a wide range of materials are gathered and design options are kept open. Tables 1 to 3 illustrate basic information about various types of thermoset and thermoplastic based composite materials. This information can be helpful for preliminary assessment of these materials. At preliminary stage, a designer identifies which matrix materials and fibers are more suitable. For example, for a certain application polyster resin could be a suitable choice and for another application epoxy may be the suitable choice. Similarly for any manufacturing process, a specific type of material system is more appropriate. Selection of potential materials is done from materials databases obtained from material suppliers and handbooks.

 

Table 1. General properties of thermoset and thermoplastic composites

Property

Thermoset composites

Thermoplastic composites

Fiber Volume

Fiber length

Solvent resistance

Heat resistance

Molding time

Molding pressure

Material cost

Safety/Handling

Storage life

 

Medium to High

Continuous and Discontinuous

High

Low to High

Slow: 1/2 hr to 4 hr

Low: 1 Bar to 7 Bars

Low to High

Good

Good (6 to 24 months with refrigeration)

Low to Medium

Continuous and Discontinuous

Low

Low to Medium

Fast: less than 5 mins

High: Greater than 14 Bars

Low to Medium

Excellent

Indefinite

 

  

Table 2. Strength and modulus comparison of common composite systems

 

Reinforcement/Matrix Resin

Tensile Strength (MPa)

Tensile Modulus (Gpa)

Fiber glass / Thermoplastic

Kevlar / Thermoplastic

Graphite / Thermoplastic

Fiber glass / Epoxy

Kevlar / Epoxy

Graphite / Epoxy

63

120

70

1400

875

1925

4.2

4.9

9.1

45

63

133

 

 

Table 3. Property comparison of common fabric reinforced composites (epoxy matrix resin)

 

Property

E-Glass

S-2 Glass

Kevlar 49

Graphite

Density, g/cm3

Fiber Volume

Tensile strength, Mpa

Tensile Modulus, Gpa

Compression Strength, Mpa

Compression Modulus, GPa

Short Beam Shear Strength, MPa

1.80

46%

385

23

560

26

84

1.83

45%

700

25

665

30

84

1.33

49%

455

29

245

25

35

1.52

54%

915

70

860

70

91

 

The performance of composite materials can be engineered by adjusting the volume fraction of the reinforcement and resin matrix. Table 4 shows the optimal fiber volume fraction achievable with various types of reinforcements. Thermoplastic composites do not offer higher fiber volume fraction.

Table 4. Optimal fiber volume fraction for various types of reinforcements.

Composite material

Optimal fiber volume fraction

Fiberglass / Epoxy

Aramid / Epoxy

Graphite / Epoxy

40 to 55%

40 to 50%

48 to 60%

 

 

1.1.3 Determine Candidate Materials

Once a list of materials is created, the next task is to determine candidate materials, which are best suited for the application. At this point, various manufacturing processes and design options are looked into to provide the best choice on performance, cost, weight and other requirements. For example, for an application, RTM, hand-lay-up and filament winding processes can be looked into to come up with the best design solutions. In each of the above cases, the starting materials are different. The cost, weight, and performance characteristics of each of these design concepts would vary as well and engineer must look at all of these options for right selection of material systems and manufacturing processes. Stress and other analysis must be performed to evaluate each design concept. Finite Element Analysis (FEA) software and other tool can greatly reduce the cost and time of product development phase. A good understanding of the product requirement, design and manufacturing processes would greatly help in deciding the right material and processing choices.

1.1.4 Testing and Evaluation

After the candidate materials for various types of feasible manufacturing processes are selected, prototype parts are made and then tested to validate the design. Depending upon the seriousness of an application, number of parts to be tested is decided. Aerospace and automobile parts generally require more number of tests to make sure that the part functions safely in various types of service conditions. For some consumer products where breaking of the part may not result any physical damage or injury require minimum number of tests.

 

 

2. Manufacturing Process Selection Guidelines

There are several composite manufacturing processes available, each providing distinct characteristics. Selection of a process is done depending upon the application need. The criteria for selecting a process are production rate, cost, strength, size and shape of the part. These criteria are described here briefly.

Production rate/Speed

Depending upon the application and market needs, the rate of production is different. For example, the automobile market requires a high rate of production say 10,000 units per year (40 per day) to 5,000,000 per year (20,000 per day). For the aerospace market, production requirements are less and are in the range of 10 to 100 per year.

Cost

Most of the consumer and automobile markets are cost sensitive and cannot afford higher cost of production. Factors influencing cost are tooling, labor, raw materials cost, process cycle time, and assembly time.

Strength

Each composite process utilizes different starting material and therefore final property of the part is different. Strength of the composite part strongly depends upon fiber type, fiber length, fiber orientation, and fiber content (60 to 70% is strongest as rule).

Size of the structure

Size of the structure is also a deciding factor in screening manufacturing processes. The automobile market typically requires smaller components compared to the aerospace market, which utilizes bigger structures.

Shape

Shape of the composite parts can also play a deciding role for the production of a part. For example, filament winding is most suitable for the manufacture of pressure vessels. Pultrusion can very economically produce long parts with uniform cross-section such as circular and rectangular.

 

Table 5. Manufacturing process selection guidelines

 

Process

Speed

Cost

Strength

Size

Shape

Common Materials

Filament Winding

 

Pultrusion

 

 

Hand-lay-up

 

Spray-up

 

RTM

 


SRIM

 

 

Copression Molding

 

 

Stamping

 


Injection Molding

 



RRIM

 

 

Roll Wrapping


Autoclave Processing

Medium to fast

 

Fast

 

 

Very slow

 

Fast

 

Medium

 


Medium

 

 

Fast

 

 


Fast

 


Med (Fast)

 

 


Fast

 

 

Fast

 


Slow

Low to medium

 

Medium

 

 

High

 

Low

 

Low

 


Low

 

 

Medium

 


Medium

 


Low to High depending upon volume


Low to High

 

 

Medium

 


Medium to High

High

 


High (along longitudinal direction)

 

High

 

Low

 

Medium

 


Medium

 

 

Low to medium

 

 

Medium

 



Low to medium

 


Med

 

 

Medium to High

 

High

Small to large

 

Medium to large

 


Small to big

 

Small to medium


Small to medium

 

Small to medium

 


Small to medium

 

 

Medium

 



Small

 



Med

 

 

Small to Medium

 

Small to Big

Cylindrical and axisymmetric

 

Constant cross-section

 


Simple to complex


Simple

 

Simple to complex

 

Simple to complex

 


Simple to complex

 

 

Simple to contoured


Complex

 

 


Complex

 

 

Tubular

 


Simple to complex

Continuous fibers with epoxy and polyster resins


Continuous fibers with polyster and vinyl ester as well as thermoplastic


Prepreg and fabric with epoxy resin


Short fiber catalyzed resin


Preform and fabric with vinyl ester and epoxy


Fabric or preform with isocynucralate resin

 

Molded compound (short fiber with polyster or vinyl ester resin)


Fabric impregnated with thermoplastic


Pallets (Short fiber with thermoplastic)



Preform with polyurethanes, epoxies

 

Prepreg (Continuous fiber with epoxy)

 

Prepreg

 

 

 

3. Prepreg selection guidelines

There are two important considerations in prepreg selection. First, the material variable selection and second, the processing considerations.

3.1 Material variable selection

Selection of materials depends upon the performance and other requirements as set in checklist above. Depending upon the laminate property requirements, the following parameters are selected.

Types of fibers (Glass, Carbon, Kevlar)

Fiber styles and orientations (woven, unidirectional, areal weight)

Thickness and width

Resin Content

 

3.2 Processing considerations

This strongly depends upon the resin characteristics.

Processing method (autoclave, roll wrapping, compression molding)

Tack (stickiness of the uncured prepreg)

Drape (the ability of prepreg to take a contoured shape)

Gel time

Cure temperature, pressure and cycle time

Flow characteristics

 

 

4. Adhesive Selection Guidelines

Adhesives are used to join two similar or dissimilar materials together. There are several considerations involved for effective use of adhesive bonding. A successful application requires good joint design, surface preparation, proper adhesive selection, and proper adhesive curing.

The goal of any bond is substrate failure. That means the bond is stronger than the joining materials themselves. In substrate failure, the parent materials fail either away from the joint or near the bond area by tearing away the parent materials. The next expectation on failure mode could be cohesive failure of the adhesive where adhesive splits in the bond area but remains firmly attached to both substrates. Adhesive failure, where adhesive releases from substrate materials is considered a weak bond and is generally unacceptable.

The first step in selecting an adhesive is to define the substrate materials and to set up durability requirements. See the above checklist for creating durability and performance requirements. Once above requirements are met then processing parameters for the adhesive bonding is defined. These include production rate, adhesive position, clamp time and position, surface preparation, fixture time, open time, cure parameters such as time, temeperature, and pressure, dispensing method, manual or automated assembly, and inspection method.

There are many types of adhesives avilable in the market. The most common adhesives are epoxies, acrylics, urethanes, silicones and polyolefins. Following Table 6 categorizes these adhesives by their relative properties.

 

Table 6: Adhesive selection guidelines

 

Characteristics

Standard Epoxies

Urethane

Acrylic

Silicones

Polyolefins (Vinylics)

Adhesive type*

Cure requirement

Curing speed

Substrate flexibility

Shear strength

Peel strength

Impact resistance

Humidity resistance

Chemical resistance

Temperature resistance(oC)

Gap filling

Storage (months)

L1, L2, F

Heat, Ambient

Poor

Very Good

Best

Poor - Fair

Fair

Poor

Very Good

Fair

Fair

6

L, W, HM

Heat, Ambient

Very Good

Very Good

Fair

Very Good

Very Good

Fair

Fair

Fair

Very Good

6

L1, L2, W

Heat, Ambient

Best

Good

Good

Good

Fair

Fair

Fair

Fair

Very Good

6

L1, L2

Heat, Ambient

Fair

Good

Poor

Very Good

Best

Best

Fair

Good

Best

6

F

Hot melt

Very Good

Fair

Poor

Fair

Fair

Fair

Good

Poor

Fair

12

* Adhesive Type: L1 = Liquid one part, L2 = Liquid two part, F = Film, W = Waterborne, HM = Hot melt

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This page last updated on Thursday, June 17, 1999 02:05:31 PM