1、 SURFACE VEHICLE INFORMATION REPORT Thermal Transport Properties Germane to Friction Materials and Brakes SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and i
2、ts applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comme
3、nts and suggestions. Copyright 2003 SAE International All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of S
4、AE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-0790 Email: custsvcsae.org SAE WEB ADDRESS: http:/www.sae.org ISSUED SEP2003 Issued 2003-09 J2581 1. Scope This SAE Information report defines the thermal transport properties import
5、ant in the assessment of heat management capability of brake lining, shoe, disc and drum materials. The report discusses thermal diffusivity, specific heat capacity, thermal conductivity and thermal expansion. Measurement techniques for the appropriate ASTM standards are identified. The thermal tran
6、sport properties discussed are material sample properties, not the properties of entire components such as pad assemblies. 2. References 2.1 Applicable Publications The following publications form a part of this information report to the extent specified herein. Unless otherwise specified, the lates
7、t issue of SAE and ASTM publications shall apply. 2.1.1 SAE PUBLICATIONS None. 2.1.2 ASTM INTERNATIONAL PUBLICATIONS Available for purchase from: ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, Pennsylvania, USA 19428-2959. www.astm.org. ASTM E 1461Standard Test Method for
8、 Thermal Diffusivity of Solids by the Flash Method ASTM C 1114Steady-State Thermal Transmission Properties by Means of the Thin-Heater Apparatus ASTM E 1225Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique ASTM E 1269Determining Specific Heat Capacit
9、y by Differential Scanning Calorimetry ASTM E 228Linear Thermal Expansion of Solid Materials With a Vitreous Silica Dilatometer Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE J2581 Issued SEP2003
10、- 2 - 2.2 Related Publications The following publications are for information purposes only and are not a required part of this document. 2.2.1 SAE PUBLICATIONS SAE J160Swell, Growth and Dimensional Stability of Brake Linings 2.2.2 OTHER PUBLICATIONS JIS D4416Test procedure of thermal expansion for
11、brake linings and pads of automobiles ISO 6313Effects of heat on dimensions and form of disc brake pads 3. Definitions 3.1 Symbols Used Thermal diffusivity Thermal conductivity CpHeat capacity per unit mass at constant pressure CTE Coefficient of thermal expansion D Specimen thickness t Time Q Heat
12、energy M Specimen mass T Temperature rise H Rate of heat transfer across the area A A Sample area dxdTTemperature gradient Sample density LoSample original length 3.2 Summary of Thermal Transport Materials Properties Thermal properties of disc, drum and friction materials are typically required to a
13、ssess the heat management of brake components. These properties are often used to perform computer-aided design of brakes and computer simulations or modeling of brake performance. This document defines several standard thermal transport material properties: thermal diffusivity, specific heat capaci
14、ty, thermal conductivity and thermal expansion, and describes the techniques used to measure them. Table 1 is included below listing the equipment used to measure the material properties and the applicable ASTM International standards. Copyright SAE International Provided by IHS under license with S
15、AENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE J2581 Issued SEP2003 - 3 - TABLE 1SUMMARY TABLE OF THERMAL TRANSPORT PROPERTIES Property Symbol Typical Units Typical Measurement Equipment/Technique Measurement Standards Thermal diffusivity cm2/sec mm2/sec Laser
16、 Flash, Transient Plane Source ASTM E 1461 ASTM C 1114 Thermal conductivity W /mK cal /cm s K Calculation, Longitudinal Heat Flow ASTM E 1225 Specific heat Cpcal /g oC J /g oC Differential scanning calorimetry (DSC) ASTM E 1269 Coefficient of thermal expansion CTE mm/mm/oC Dilatometer ASTM E 228 It
17、should be noted that the transport properties discussed in this information report are material properties, and not the component properties measured by SAE J160, ISO 6313 and JIS 4416. 4. Thermal Diffusivity, Thermal diffusivity is a measure of how fast heat flows through a material. It is the rate
18、 of change of temperature during a transient heat flow event. The greater the thermal diffusivity value, the faster the rate of temperature propagation or heat flow through a material. Thermal diffusivity is a useful descriptor for brake materials. It is easy to measure and can be used to calculate
19、thermal conductivity. It should be noted that thermal diffusivity is temperature dependent, typically decreasing as temperature increases. The laser flash technique is one method used to measure thermal diffusivity. A small material sample of known thickness is subjected to an intense, short burst o
20、f energy from a laser or xenon flash lamp. The temperature rise at the rear surface of the sample is measured, and thermal diffusivity is computed from the temperature rise versus time data. The diffusivity, , is typically calculated at the time that it took the rear surface of the sample to achieve
21、 1/2 of its maximum temperature rise with the equation 25.01388.0tD= (1) where D is the specimen thickness and t0.5is the half rise time of the rear surface temperature. The laser flash technique requires that a small sample (e.g. a 10 mm diameter, 3 mm thick disc) be cut from the material to be mea
22、sured. Thermal diffusivity can be measured at room and elevated temperatures with this technique. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE J2581 Issued SEP2003 - 4 - The transient plane sour
23、ce (or hot disc) method allows nondestructive measurement of the thermal diffusivity, thermal conductivity and specific heat simultaneously. This new technique relies on a thin sensor that consists of an electrically conducting pattern in the shape of a double spiral; the sensor is used as both a he
24、at source and as a dynamic temperature sensor. It can be performed on brake linings without cutting small samples; the thin sensor can be placed between two whole, identical pads to do the measurements. Room temperature values are easy to measure with this technique, however for elevated temperature
25、 numbers, the linings must be held at temperature by an external heater, oven or hot plate. Another recent development in nondestructive thermal diffusivity measurement techniques is pulsed video thermography. There is no industry standard for this technique yet. 5. Specific Heat, CpHeat capacity is
26、 defined as the ratio of the amount of heat energy (Q) supplied to a body to its corresponding temperature rise (T). The heat capacity per unit mass of the body at constant pressure is known as the specific heat, or TmQCp= (2) Specific heat is the amount of energy per unit mass required to create a
27、unit temperature rise. Hence, the greater the value of the specific heat, the better the materials performance as a heat sink. Specific heat is temperature dependent; its value will be different depending upon the temperature interval over which it is measured. Specific heat is typically measured wi
28、th a Differential Scanning Calorimeter (DSC); the DSC measures the thermal response of a material specimen as compared with a standard (such as sapphire) when the two are heated uniformly at a constant rate. Data is typically recorded from 100 oC up to the user-specified maximum temperature. This me
29、asurement also reveals if any phase changes occur in the material over the temperature range applied. It should be noted that if phase transitions occur during the DSC test, subsequent testing on the same sample of material would yield different results. Specific heat values can be used in conjuncti
30、on with thermal diffusivity values to calculate thermal conductivity. (Discussed below.) One issue with measuring the specific heat of friction materials is that the typical sample volumes used for DSC are very small, e.g. 0.5 to 20 mm3. Individual material samples may not be representative of frict
31、ion materials with large size ingredients or coarsely mixed ingredients. Suggested approaches for addressing this problem are: 1) measure specific heat on numerous material samples, and make those samples as large as the equipment allows, or 2) grind a representative piece of the friction material i
32、nto a fine powder and lightly compact into pellets to be measured via DSC. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE J2581 Issued SEP2003 - 5 - 6. Thermal Conductivity, Thermal conductivity,
33、the steady state measure of heat flow or how fast heat moves through a material, is a commonly quoted assessment of a materials thermal transport ability. Thermal conductivity () is defined as the rate of steady state heat flow through a unit thickness of material induced by a unit temperature rise,
34、 or roughly dxdTAH= (3) where H is the rate of heat transfer across the area A, and dT/dx is the temperature gradient. A material with a high value of is a good thermal conductor. Thermal conductivity is a function of temperature, typically increasing as temperature increases. The simplest way to de
35、termine thermal conductivity is to measure , and Cpand calculate it from the following equation: = Cp(4) where is the thermal diffusivity, is the sample density and Cpis the specific heat capacity at constant pressure. Thermal conductivity can also be measured directly via the transient plane source
36、 technique mentioned above, or equipment such as a Longitudinal Bar Cryostat. A bar-shaped test specimen is mounted to a temperature-controlled heat sink, and a small heater is applied to the opposite end of the specimen to produce a temperature gradient. Thermal conductivity is calculated from the
37、temperature gradient (measured with thermocouples), the cross-sectional area of the specimen and the heater power dissipation. The disadvantage to this measurement technique is that relatively large bar samples of material are required; hence it is difficult to apply to friction materials. 7. Therma
38、l Expansion (Coefficient Thermal Expansion, CTE) Thermal expansion is defined as the change in length per unit length of a material resulting from a temperature change of one degree. The coefficient of thermal expansion is described in different ways: 1. Coefficient of linear expansion (L/Lo) is sim
39、ply the change in length, L, divided by the samples original length at some reference or starting temperature, Lo. 2. The mean coefficient of expansion is the average coefficient from the temperature range (T) evaluated, L/LoT. When using this value, the temperature range over which it applies must
40、be specified. 3. The instantaneous coefficient of linear thermal expansion is the expansion coefficient at any temperature T; it is the slope of the observed change in length versus the temperature curve at a particular instantaneous temperature. Copyright SAE International Provided by IHS under lic
41、ense with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE J2581 Issued SEP2003 - 6 - The value most commonly cited as the CTE is the mean coefficient of expansion (Number 2 above), TLLCTEo= (5) which is expressed in units such as in/inoC or mm/mmK, or occasion
42、ally 1/oC. A differential push rod dilatometer is used to measure linear thermal expansion. The thermal expansion of a test specimen is determined relative to that of a standard reference specimen, such as quartz. The two specimens are slowly heated side by side in a furnace while pressed against tw
43、o push rods that extend from the furnace to a thermally isolated linearly variable displacement transducer (LVDT) to measure their change in length. A difference in expansion between the two specimens results in a differential movement of the push rods, thus allowing the linear thermal expansion of
44、the unknown sample to be determined. The temperature of the specimen is measured with thermocouples. 8. Notes When performing finite element analysis (FEA) or modeling of brake system performance, material properties such as those described in this document should be used. Specifications for compone
45、nt testing of thermal expansion, dimensional stability, and swell and growth, such as SAE J160, JIS 4416 and ISO 6313 evaluate the behavior of lining components in response to heat. Component test values of thermal expansion, for example, should not be confused with the material property CTE. Also,
46、it should be noted that the results from thermal swell tests conducted on linings placed in a compressibility test frame or tested via ISO 6313 are not the thermal conductivity, . It should be noted that exposure to elevated temperature will change the properties of friction materials, including the
47、ir thermal transport properties. For example, material from a new pad and a used pad will have different ambient thermal transport properties. In addition, any changes in material properties resulting from elevated temperature may also be a function of time at that temperature. For these reasons, it
48、 is recommended that the condition of the pad, temperature exposure and time at temperature be recorded with all test results. PREPARED BY THE SAE BRAKE LININGS STANDARDS COMMITTEE DEVELOPED BY THE SAE BRAKE COMMITTEE Copyright SAE International Provided by IHS under license with SAENot for ResaleNo
49、 reproduction or networking permitted without license from IHS-,-SAE J2581 Issued SEP2003 - 7 - Rationale New document to provide information on the material properties typically required for the computer aided design of brake systems. Relationship of SAE Standard to ISO Standard Not applicable. Application This SAE document provides definitions of the thermal transport properties of materials that
