Category Materials and the Environment: Eco-Informed Material Choice

Stress and pressure

The SI unit of stress and pressure is the N/m2 or the Pascal (Pa), but from a materials point of view it is very small. The levels of stress large enough to distort or deform materials are measured in megaPascals (MPa). The table list the conversion factors relating MPa to measures of stress used in the older cgs and metric systems (dyne/cm2, kgf/mm2) by the Imperial system (lb/in2, ton/in2) and by atmospheric science (bar).

Conversion of units – stress and pressure

To ^

MPa

dyn/cm2

lb/in2

kgf/mm2

bar

long ton/in2

From j

Multiply by

MPa

1

107

1.45 X 102

0.102

10

6.48 X 10~2

dyn/cm2

10-7

1

1.45 X 10-5

1.02 X 10-8

10-6

6.48 X 10-9

lb/in2

6.89 X 10-3

6.89 X 104

1

7.03 X 1Q-4

6.89 X 10-2

4.46 X 10“4

kgf/mm2

9...

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Appendix – Useful numbers and conversions

CONTENTS

A1 Introduction A.1 Introduction

Quantitative analysis needs numbers. Many of those needed to understand and quantify eco-aspects of material production and use are presented in the Chapters of the text.

■ Table 2.1: approximate efficiency factors for energy conversion and the associated CO2 emission per useful MJ.

■ Table 6.5: the energy content of fossil fuels and the CO2 they emit when burnt.

■ Table 6.6: the energy efficiency of electricity generation and the related CO2 per useful kWhr.

■ Table 6.7: the energy and CO2 costs of alternative modes of transport.

■ Table 9.6: the energy and CO2 rating of cars as a function of mass.

■ Chapter 12: data sheets listing the attributes of 47 of the most widely used materials.

This appendix assembles further useful numbers and conversio...

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Plywood

The material. Plywood is laminated wood, the layers glued together such that the grain in successive layers are at right angles, giving stiffness and strength in both directions. The number of layers varies but is always odd (3, 5, 7 …) to give symmetry about the core ply; if it is asymmetric it warps when wet or hot. Those with few plies (3, 5) are significantly stronger and stiffer in the direction of the outermost layers; with increasing number of plies the properties become more uniform. High-quality plywood is bonded with synthetic resin. The following data describes the in-plane properties of a typical five-ply.

Composition

Cellulose/hemicellulose/lignin/12%H2O/adhesive.

Plywood dominates the market for both wood and steel stud construction...

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Paper and cardboard

The material. Papyrus, the forerunner of paper, was made from the flower stem of the reed, native to Egypt; it has been known and used for over 5000 years. Paper, by contrast, is a Chinese invention (105 AD). It is made from pulped cellulose fibers derived from wood, cotton, or flax. There are many types of paper and paperboard: tissue paper, newsprint, Kraft paper for packaging, office paper, fine glazed writing paper, cardboard—and a corre­spondingly wide range of properties. The following data spans the range of newsprint and Kraft paper.

Composition

Cellulose fibers, usually with filler and colorant. General properties

Density

480

– 860

kg/m3

Price

2.07

– 12.4

USD/kg

Mechanical properties

Young’s modulus

3

– 8.9

GPa

Yield strength (elastic lim...

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Flexible polymer foam

The material. Polymer foams are made by the controlled expansion and solidification of a liquid or melt through a blowing agent; physical, chemi­cal, or mechanical blowing agents are possible. The resulting cellular mate­rial has a lower density, stiffness, and strength than the parent material, by an amount that depends on its relative density—the volume fraction of solid in the foam. Flexible foams can be soft and compliant, the material of cush­ions, mattresses, and padded clothing. Most are made from polyurethane, although latex (natural rubber) and most other elastomers can be foamed.

Composition

Hydrocarbon.

General properties

Density

Price

Mechanical properties

Young’s modulus Yield strength (elastic limit) Tensile strength Compressive strength Elongation Hardness—Vickers Fatig...

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Rigid polymer foam

The material. Polymer foams are made by the controlled expansion and solidification of a liquid or melted through a blowing agent; physical, chem­ical, or mechanical blowing agents are possible. The resulting cellular mate­rial has a lower density, stiffness, and strength than the parent material, by an amount that depends on its relative density—the volume fraction of solid in the foam. Rigid foams are made from polystyrene, phenolic, poly­ethylene, polypropylene, or derivatives of polymethylmethacrylate. They are light and stiff and have mechanical properties that make them attrac­tive for energy management and packaging and for lightweight structural use. Open-cell foams can be used as filters, closed-cell foams as flotation...

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Sheet molding compound (SMC)

The material. Layup and filament winding methods of shaping composites are far too slow and labor intensive to compete with steel pressings for car body panels and other enclosures. Sheet molding compounds (SMCs) over­come this by allowing molding in a single operation between heated dies. To make SMC, polyester resin containing thickening agents and cheap par­ticulates such as calcium carbonate or silica dust is mixed with chopped fibers—usually glass—to form a sheet. The fibers lie more or less parallel to the plane of the sheet but are randomly oriented in-plane, with a volume fraction between 15% and 40%. This makes a "pre-preg" with leather – or doughlike consistency. When an SMC sheet is pressed between hot dies, it polymerizes, giving a strong, stiff sheet molding.

Composition

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GFRP (Isotropic)

The material. Composites are one of the great material developments of the 20th century. Those with the highest stiffness and strength are made of continuous fibers (glass, carbon, or Kevlar, an aramid) embedded in a ther­mosetting resin (polyester or epoxy). The fibers carry the mechanical loads, whereas the matrix material transmits loads to the fibers and provides duc­tility and toughness as well as protecting the fibers from damage caused by handling or the environment. It is the matrix material that limits the service temperature and processing conditions. Polyester-glass composites (GFRPs) are the cheapest and by far the most widely used...

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CFRP (Isotropic)

The material. Carbon fiber reinforced composites (CFRPs) offer greater stiffness and strength than any other type, but they are considerably more expensive than GFRP (see record). Continuous fibers in a polyester or epoxy matrix give the highest performance. The fibers carry the mechanical loads, whereas the matrix material transmits loads to the fibers and provides duc­tility and toughness as well as protecting the fibers from damage caused by handling or the environment. It is the matrix material that limits the ser­vice temperature and processing conditions.

Composition

Epoxy + continuous HS carbon fiber reinforcement (0, н— 45, 90), quasi­isotropic layup.

General properties

Density

1500 –

1600

kg/m3

Price

*40.0 –

44.0

USD/kg

Mechanical properties

Yo...

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Hybrids: composites, foams, and natural materials

Composites are one of the great material developments of the 20th century. Those with the highest stiffness and strength are made with continuous fibers of glass, carbon, or Kevlar (an aramid) embedded in a thermosetting resin (polyester or epoxy). The fibers carry the mechanical loads, whereas the matrix material transmits loads to the fibers, provides ductility and tough­ness, and protects the fibers from damage from handling or the environment. It is the matrix material that limits the service temperature and processing conditions. Polyester-glass composites (GFRPs) are the cheapest, epoxy-carbon (CFRPs) and Kevlar-epoxy (KFRPs) the most expensive...

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