Product Design for the Environment: A Life Cycle Approach - Chapter 12 pot

Chapter 12 Environmental Characterization of Materials and Optimal Choice A product’s environmental impact is directly infl uenced by the environmental properties of the materials us

Chapter 12

Environmental Characterization of Materials

and Optimal Choice

A product’s environmental impact is directly infl uenced by the environmental

properties of the materials used, such as energy costs, emissions involved in

production and manufacturing phases, and recyclability The choice of

materi-als, therefore, assumes strategic importance and requires an extension of the

characterization of materials, integrating conventional characterization (aimed

at defi ning physical–mechanical properties) with a complete characterization

of environmental behavior To enable the designer to make an optimal choice

of materials that harmonizes performance characteristics and properties of

eco-compatibility, the selection process must take account of a wide range of

factors: constraints of shape and dimension, required performance,

techno-logical and economic constraints associated with the manufacturability of

materials, and environmental impacts of all the phases of the life cycle

In accordance with the Life Cycle Design approach, this chapter proposes

a defi nition of the environmental characterization of materials and processes,

and a systematic method that introduces environmental considerations in

the selection of the materials used in components This defi nition and method

are directed at meeting functional and performance requirements while

minimizing the environmental impact associated with the product’s entire

life cycle The proposed selection procedure elaborates data on the

conven-tional and environmental properties of materials and processes, relates this

data to the required performance of product components, and calculates the

values assumed by functions that quantify the environmental impact over

the whole life cycle and the cost resulting from the choice of materials As

shown in the case study presented, the results can then be evaluated using

multiobjective analysis techniques

12.1 Materials Selection and Environmental Properties

“New materials inspire designers; but even more, design drives material

development” (Ashby, 2001) This statement highlights the close connection

between materials and the design activity, confi rmed by the signifi cance of

the issues related to the effi cient integration of materials selection in the

product development process (Edwards, 2003; Lu and Deng, 2004)

The enormous variety of materials available for engineering applications and

the complexity of the requirements conditioning the choice of the most

appro-priate materials and processes lead to a taxing problem of multiple-criterion

optimization (Brechet et al., 2001) In recent years, several systematic methods

have been proposed to help the designer in the selection of materials and

processes (Charles et al., 1997; Farag, 1997; Asbhy et al., 2004) Of the more

com-monly used quantitative selection methods, that developed by Ashby is based

on the defi nition of material indices consisting of sets of physical–mechanical

properties which, when optimized, maximize certain performance aspects of

the component under examination (Ashby and Cebon, 1995) Defi ning these

indices makes it possible to compile selection charts summarizing the relations

between properties of materials and engineering requirements (Ashby, 1999)

Usually taking into consideration the physical-mechanical properties of

materials, these selection charts can be extended to introduce some

environ-mental properties (Navin-Chandra, 1991) From this standpoint, several

impor-tant studies have been based on the development of indices able to express the

environmental performance of materials by introducing the energy

consump-tion and emissions (into the atmosphere or water) associated with the

materi-als (Holloway, 1998), or eco-indicators developed on the basis of Life Cycle

Assessment methods (Wegst and Ashby, 1998) An alternative approach is that

of translating environmental impact in terms of economic cost of production,

introducing functions of environmental cost such as energy consumption and

toxicity that depend on the properties of the materials (Chen et al., 1994)

All the methods proposed are limited to quantifying the environmental

impact of the choice of materials on the basis of their environmental properties

associated with the production phase Only a few studies have considered the

infl uence of the choice of materials on the impact associated with the working

life of the component (Kampe, 2001) To date, the problem of choice of materials

from the viewpoint of Life Cycle Design (taking into account the environmental

impacts involved in all phases of the life cycle, from production to retirement)

has been considered only in general terms, with the aim of defi ning guidelines

for choices that integrate properties of materials, manufacturing demands, and

end-of-life impacts, and suggesting a distinction of selection criteria between

component design and assembled product design (Stuart, 1998)

12.2 Environmental Characterization of Materials and Processes

The infl uence that the materials used to manufacture a product have on its

envi-ronmental impact is manifested in the energy costs and emissions associated

with the production and end-of-life processes of the material, and in the intrinsic

properties of the material and production process that constrain its level of

recy-clability Complete environmental characterization of a material should,

there-fore, consist of defi ning the environmental impact linked to its production and

disposal, and of evaluating the margins of recyclability in terms of decline in

performance of the recycled material and recovery costs Therefore, the optimal

choice of materials, in relation to environmental demands, requires this complete

environmental characterization, with particular regard to the following aspects:

• Environmental impact associated with production processes (energy

costs and overall impact)

• Environmental impact associated with phases of end-of-life

(recy-cling or disposal)

• Suitability for recycling (expressed by the recyclable fraction)

Information on the energy costs and recyclable fractions of more common

materials can be obtained from commercially available databases, such as that

of the CES ® (Cambridge Engineering Selector, Granta Design Ltd., Cambridge,

UK) materials selection software Overall environmental impact can be

evalu-ated using the techniques of Life Cycle Assessment (LCA), the analysis method

used to quantify the environmental effects associated with a process or

prod-uct through the identifi cation and quantifi cation of the resources used and the

using these resources and of the emissions produced Quantifi cation of the

impacts is based on inventory data that is subsequently translated into

eco-indicators such as those used here These are evaluated according to the

Eco-SimaPro 5.0 ® software (Pré Consultants BV, Amersfoort, The Netherlands)

Environmental characterization is also extended to common primary

(forming) and secondary (machining) manufacturing processes, evaluating

the indicators that quantify the impacts of standard processes per unit of

process parameter or of the volume or weight of material processed

12.2.1 Data on Materials and Processes

For each material it is necessary to integrate the information used in

conven-tional design with that regarding environmental properties to obtain:

• General properties (density, cost)

• Mechanical properties (e.g., modulus of elasticity, hardness, fatigue

limit)

• Thermal and electrical properties (e.g., conductivity and thermal

expansion, operating temperature, electrical resistance) waste generated As was discussed in Chapter 4, LCA evaluates the impact of

indicator 99 method (Chapter 4, Section 4.2 and Table 4.3) and calculated using

• Environmental properties (energy cost, environmental impact,

recyclability)

As an example, the datasheet in Figure 12.1 relates to a widely used plastic

material (polypropylene) and shows the data on its environmental

proper-ties indicators were evaluated with SimaPro 5.0 software, using the

Eco-indicator 99 method and expressing impacts in mPt (milliPoint) With this

software it is possible to select the inventory data to be used for impact

eval-uation, in this specifi c case Buwal 250 data (Pré, 2003)

Likewise, the following information must be obtained for the primary and

secondary manufacturing processes:

• Physical attributes of the fi nal product

• Economic cost of standard process (fi xed and variable costs)

• Environmental properties (energy consumption, environmental

impact of standard process)

12.3 Summary of Selection Method

that quantify and interrelate the various performances required of the material

FIGURE 12.1 Material datasheet: Polypropylene.

The reference method depicted in Figure 12.2 is based on calculation models

in order to identify potential solutions, and a successive, multiobjective analysis

aimed at harmonizing the conventional performance, costs, and

environmen-tal performance of the product

The fi rst phase consists of defi ning the set of design requirements and

parameters:

• Primary performance (Pf1), in relation to the specifi c functionality of

the component

• Secondary performance (Pf2), which can impose further restrictions

to guide the selection

• Geometric parameters, distinguishing between fi xed (Gf) and

vari-able (Gv) geometric parameters

• Typology of shape and relative level of complexity (Sh), which

greatly affects the choice of forming processes

• Use of component (Us), which can infl uence an initial selection of

materials The set of design requirements constitutes the input for the procedure of

selecting potential solutions This procedure is based on two different types

of each hypothetical solution is evaluated by analyzing some of the

informa-tion given in the set of design requirements (in particular, the typology of

FIGURE 12.2 Summary of method.

of analysis, shown in Figure 12.3 In the fi rst stage, the production feasibility

shape required and the intended use) The solutions identifi ed in the analysis

of production feasibility must then be evaluated in terms of the required

performances (Pf1, Pf2) The potential solutions obtained are then analyzed

in subsequent phases of the selection method

Each potential solution S is defi ned by pairs of material–primary forming

process (M, FPr), and by the performance volume (PfV), representing the

minimum volume needed to meet the requirements of primary performance

If appropriate, the defi nition of the generic solution S can also include any

processes of secondary machining required after the initial forming

In the following phase, the calculation models are applied to each potential

solution in order to evaluate the indicators of environmental impact and cost

over the entire life cycle The fi nal phase of the method involves analyzing

the results and identifying the optimal choice

12.4 Analysis of Production Feasibility

The fi rst stage of the selection procedure must correlate material, process,

shape, and function The problem of the interaction between these factors is

considered central to the selection of materials and has been thoroughly

investigated (Ashby, 1999)

In the method proposed here, this problem is addressed by considering

shape (Sh) and use (Us) to be design requirements, expressed using binary

FIGURE 12.3 Procedure for selection of potential solutions.

vectors V Sh and V Us , and introducing binary matrices correlating shape–

process, material–use, and material–process:

als, processes, shape typologies, and uses Considering processes of

primary manufacture only, on the basis of the correlation matrices (12.1)

and vectors V Sh and V Us , and following the calculation scheme

Pr and V Mt , ing, respectively, the primary processes able to produce the required

indicat-typology of shape, and the materials suitable for the intended use The

subsequent application of the material–process correlation matrix gives a

matrix of producible solutions:

the set of producible solutions

The material–use correlation matrix constitutes a fi lter in the preselection

of possible solutions in that it limits the choice to those materials

convention-ally employed for the intended use For a broader preselection, it is possible

to bypass this fi lter In this case, the terms of matrix (12.2) would depend

solely on V Sh , ⌽ S-P , and ⌽ P-M

Using the above approach in the analysis of production feasibility, it is

possible to:

• Produce an analytical and exhaustive selection of all the possible

solutions that can satisfy the intended form and use

FIGURE 12.4 Summary of production feasibility analysis.

rized in Figure 12.4, it is possible to obtain the vectors V

• Separate the selection conditioned by production feasibility from

that conditioned by performance requirements, thereby evidencing the relationships between choice of material and effect on life cycle impacts; such relationships, as shown below, depend on the different performance capacities of the materials

This approach requires the prior compilation of the correlation matrices (12.1)

Given the ever-greater variety of engineering materials and related

manufac-turing processes, it is reasonable to consider compiling these matrices by

typol-ogy of material Alternatively, for a fi rst selection of material–process pairs, it

is possible to use existing software tools such as CES, which implements

Ashby’s methodology It must be remembered, however, that tools of this type

allow a selection that already takes account of the performances required

12.5 Analysis of Performance

The second stage of the selection procedure identifi es producible solutions

that respect the required performance characteristics In this way a set of

potential solutions is obtained, which are then analyzed by applying the

calculation models to evaluate their environmental and economic impacts

over the entire life cycle

In general, the analysis of performance can be simplifi ed by considering

three different typologies of mathematical relations:

• Function of performance volume (PfV)—Expresses the minimum

volume necessary to meet the primary performance requirements

Generally, it is a function of the primary performance (Pf1), the ric parameters (Gf, Gv), and the properties of the material (MtPp):

PfV⫽PfV(Pf1, Gf, Gv, MtPp) (12.3)

• Geometric conditions of performance—If the variable geometric

parameters Gv are directly correlated with primary performance Pf1, the geometric conditions of performance can be expressed by functions constrained by a range of values (defi ned by the design requirements):

Gv⫽Gv (Pf1, Gf, MtPp) Gv ∈(Gv , Gv )1 2 (12.4)

• Secondary conditions of performance—Conditions of this type can

be generally expressed using functions dependent on the properties

of the materials and the performance volume, to be compared with assumable limit values:

Pf2⫽Pf2 PfV , MtPp( ) Pf2ⱕⱖPf2LIM (12.5)

In conclusion, if a producible solution meets all the performance constraints

and requirements, it then becomes a performing solution and can be selected

consists of all the performing material–primary process pairs, integrated by

the corresponding performance volume The latter parameter acquires

particular relevance in the proposed method because it directly conditions

the values assumed by the life cycle indicators which, defi ned below, guide

the optimal choice Using this approach, it is possible to correlate the search

for environmentally and economically convenient solutions with the

perfor-mance characteristics of the materials

Only in the case of particularly simple design problems can the functions

of type (12.3) be defi ned in analytical form (Giudice et al., 2001) More

generally, the performance volume cannot be explicitly ascribed to the

factors affecting it; it is the result of design procedures employing modern

methods of engineering design, implemented in commonly used tools

based on parametric CAD and FEM software for structural performance

analyses

12.6 Life Cycle Indicators

The fi nal phases of the selection method consist of applying the calculation

models to the set of potential solutions, evaluating the indicators of

environ-mental impact and cost relative to the entire life cycle (Life Cycle Indicators),

and then analyzing the results and identifying the optimal choice The

indi-cators are functions of the quantities of material necessary to produce the

component, expressed by the performance volume

12.6.1 Environmental Impact Functions

The Environmental Impact of the Life Cycle (EI LC ) is expressed by:

EILC⫽EIMat⫹EIMfct⫹EIUse⫹EIEoL (12.6)

where EI Mat is the environmental impact of the material needed to produce

the component; EI Mfct is the impact associated with its manufacture; EI Use is

for fi nal evaluation As shown in Figure 12.3, the set of potential solutions

the impact related to the entire phase of use (which can depend on the choice

of material); and EI EoL is the impact of the end-of-life (recycling, disposal)

The fi rst two terms of Equation (12.6) constitute the Environmental Impact

of Production (EI Prod ), which can be expressed by:

EIProd⫽EIMat⫹EIMfct⫽eiMat⋅W⫹eiPrss⋅␮ ⫹( eiMchg⋅␩) (12.7)

where ei Mat is the eco-indicator per unit weight of material (expressed by W);

ei Pcss is the eco-indicator of the primary forming process per unit of µ, which

can represent the characteristic parameter of the process or the quantity of

material processed; and ei Mchg is the eco-indicator of the secondary machining

process per unit of characteristic parameter of process ␩ As mentioned above,

these eco-indicators can be evaluated using the Eco-indicator 99 method

The Environmental Impact of End-of-Life (EI EoL ) can be expressed by:

EIEoL⫽eiDsp·(1⫺ ␰)·W⫹eiRcl· ·␰ W (12.8)

where ei Dsp and ei Rcl are, respectively, the environmental impact of disposal

and of recycling processes per unit of weight of material (ei Rcl generally

includes a quota of environmental impact recovered), and ␰ is the recyclable

fraction So defi ned, Equation (12.8) refers to the optimal condition where, at

the end-of-life, all of the recyclable fraction of material is recovered

Considering a more realistic scenario, it is possible to introduce an appropriate coeffi

-cient of reduced recyclability to obtain the fraction actually recycled

Finally, the Environmental Impact of Use (EI Use ) cannot be expressed in

general terms and must be defi ned each time, according to the specifi c case

under examination In this chapter, it will be defi ned in relation to the

partic-ular case study discussed below

12.6.2 Cost Functions

Similar to the fi rst life cycle indicator, which quantifi es the environmental

impact, the second life cycle indicator quantifi es the economic cost related to

the entire life cycle Hypothesizing that both production and disposal costs

are paid by a single entity (the manufacturer), the Cost of the Life Cycle (C LC )

can be expressed as:

CLC⫽CProd⫹CEoL (12.9)

The Cost of Production (C Prod ) can be expressed in a form analogous to

Equation (12.7), as a function of the quantity of material to be employed and

of the more signifi cant process parameters Alternatively, it is possible to use

a conventional evaluation of the production costs of a component,

distin-guishing between variable and fi xed costs and dividing the latter by the size

of the production batch (Ulrich and Eppinger, 2000)

The Cost of End-of-Life (C EoL ) can be expressed as:

CEoL⫽cDsp·(1⫺ ␰)·W⫹(cRcl⫺rRcl)· ·␰ W (12.10)

where c Dsp , c Rcl , and r Rcl are, respectively, the cost of disposal, the cost of

recy-cling processes, and the proceeds from the sale of recycled material per unit

weight of the material; ␰ is the recyclable fraction

12.7 Analysis of Results and Optimal Choice

By applying these models, the life cycle indicators (EI LC , C LC ) are calculated

for each potential solution Various tools can be used to evaluate the fi tness

of each solution in order to identify the optimal choice Two tools that are

particularly simple but signifi cant in terms of the proposed method are

described below More sophisticated tools are discussed in references to

multiobjective optimization in general (Sawaragi et al., 1985), and in relation

to the specifi c case of materials selection (Ashby, 2000)

12.7.1 Graphic Tools

Graphs of C LC –EI LC can clearly visualize the different fi tness of the potential

solutions Graphic tools are particularly useful when a large number of

12.7.2 Multiobjective Analysis

In its simple form, multiobjective analysis is the analysis of a multiobjective

function ␥, which includes the more signifi cant product properties, suitably

normalized and weighted:

q 1

nqB

·

=

As already suggested for the comparison of alternative solutions in the

prob-lem of choice of materials (Farag, 2002), the following expression can be used

tions must be compared; an example is shown in Figure 12.5