ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14544.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY

摘要:

The sources and calculation of thermal stresses are considered, together with the factors involved in thermal stress resistance factors. Properties affecting thermal stress resistance of ceramics are reviewed, and testing methods are considered.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14545.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY

摘要:

Practical experience has shown that thermal shock tests do not lead to generally useful test data. This is probably due to the fact that thermal shock failure is a complicated function of the external thermal shock conditions and of the temperature functions of five different material properties. These five material functions appear in a different combination in almost every thermal shock case and cannot be extracted from thermal shock test data. It is recommended that these five properties and their temperature dependence be determined by separate standard tests, not employing thermal shock. If the five property functions are known, thermal shock tests proper can be used to determine the maximum thermal shock stresses in any device.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14546.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY

摘要:

Present theories predicting the resistance of brittle materials to thermal shock are based on the premise that failure occurs on the attainment of a definite critical stress. However, the failure of many brittle materials has been shown to be dependent upon stress distribution within the body rather than upon the maximum stress criteria. Weibull's statistical theory of strength which accounts for this behavior is adapted to predict the strength of the circular disks of brittle materials subjected to peripheral thermal shock. This analysis shows, for those materials in which tensile strength differs appreciably from the bending strength, that considerable error can be introduced by the use of the conventional maximum stress theory of fracture when predicting thermal shock resistance over a wide range of quenching severities. Experimental thermal shock data for steatite are analyzed to show procedures for applying the theory.SummaryIn most analyses of the behavior of brittle materials under conditions of thermal shock, use is made of the maximum stress criterion of fracture. It has been demonstrated, however, that many brittle materials do not obey this criterion, and that stress distribution frequently affects the stress at which fracture occurs. For this reason the bending strength of many brittle materials is frequently twice as high as the tensile strength. Weibull has developed a statistical theory of strength to account for this behavior. In the present report use is made of the concepts of Weibull to predict the behavior of circular disks of brittle materials subjected to peripheral thermal shock.It is found that fracture most probably occurs not at the time when the surface stress is a maximum, but at a later time when the surface stress has fallen somewhat, but a greater volume of material in the interior of the disk has been brought up to moderate stress level. Utilizing the “risk of rupture” concept introduced by Weibull, a general relation is established for relating the conditions of fracture under varying degrees of quenching severity.The analysis indicates that for materials having low material homogeneity factors,m(or materials in which the tensile strength differs appreciably from the bending strength) considerable error can be introduced by the use of the conventional maximum stress theory of fracture to relate the fracture conditions under mild and severe quenchings. Errors as high as 30% can be expected in some practical cases. It is also suggested how thermal shock data can be used to evaluate the material constantm.Analysis of limited data on the thermal shock characteristics of steatite disks indicated that for this material the experimentally determined value ofmwas high enough to obscure possible small discrepancies arising from the use of the maximum stress theory of fracture. The value ofmas deduced from the thermal shock tests was in very good agreement with the value determined from a statistical study of the bending strengths of twelve small specimens. More data on a variety of materials would, however, be desirable for a full evaluation of the proposed theory, particularly for cases involving low values ofmfor which the largest discrepancies ar

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14547.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY

摘要:

The ring test consists in heating a pile of ceramic rings, each 2 in. in outside diameter, 1 in. in inside diameter, and 1½2 in. long, from the inside by a heating element and cooling it from the outside by a calorimetric chamber. The center rings of the pile have radial heat flow. The radial temperature gradient in these center rings can be measured and also the number of calories flowing through their unit surface area. Under equilibrium conditions these measurements yield the thermal conductivity of the ring material. The radial temperature gradient produces thermal stresses in the rings. If the gradient is increased slowly so as to maintain always approximate equilibrium conditions, there will be a maximum gradient which causes failure. The maximum gradient determined by thermal fracture and the thermal conductivity can be used to compute the two thermal stress resistivity constants,RandR1, of the material.RandR1have been determined for five ceramic materials: porous TiO2, dense TiO2, steatite, cordierite, and spodumene

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14548.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY

摘要:

Sintered alumina with from 4 to 50% porosity was prepared by incorporating crushed naphthalene in a casting slip. Thermal stress resistance was determined from the radial temperature difference required to fracture hollow cylinders under steady‐state conditions. Thermal stress resistance decreases with increasing porosity; resistance to thermal stresses at 50% porosity is about one‐third of that estimated for zero porosity.Summary and ConclusionsSamples with controlled porosity were prepared by incorporating naphthalene flakes in an alumina casting slip. Samples were prepared and fired under identical conditions so that the continuous solid phase was identical for all samples. Steady‐state thermal stress tests described lead to the following conclusions:(1) Resistance to thermal stress,R(maximum steady‐state Δtfor fracture), decreases with increasing porosity. The resistance for a sample with 50% porosity is about one‐third that for dense samples.(2) Resistance to thermal stress determined by the maximum heat flow permissible,R', decreases more rapidly thanRbecause increasing porosity lowers the thermal conductivity. This factor for a sample with 50% porosity is about one‐sixth the value for dense samples. This is the factor generally applicable to thermal shock conditions.(3) Resistance to thermal fracture with a constant rate of surface temperature rise is directly proportional toR, since thermal diffusivity is unaffected

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14549.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY

摘要:

For radial steady‐state heat flow through hollow circular cylinders, the usual application of thermoelastic theory indicates that the heat flow per unit length to cause fracture (Wmax) is simply the product of two factors:M, characterizing the material, andS, the shape. Studies sponsored by the Atomic Energy Commission are in progress at Battelle Memorial Institute to evaluate the practical worth of this relationship. In this paper, attention is given to its validity for cylinders with both circular and noncircular cross sections and to measuring the effect of dimensions onSfor circular cylinders. Thermal fracture experiments on circular cylinders of two ceramics indicated the manner in whichSvaries with the ratios of dimensions which define the shape. Changing all dimensions by factors up to 1.6, with these ratios constant, had no noticeable effect onS.Using theorems derived by Biot, it is shown that the product relationship is expected to hold for a considerably wider variety of shapes than the simple circular tube. Experimental results from tubes having cross sections with a circular inner boundary and a circular or square outer boundary support the product relation. Information was obtained on the shape factor for these cross sections and for similar tubes with a triangular outer boundar

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14550.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY

摘要:

A method is described for studying the thermal shock characteristics of a brittle material. An analysis of the thermal stresses developed in a homogeneous isotropic solid sphere has led to the formulation of an equation relating the physical properties of the body to the temperature difference causing failure and time to maximum stress in a single‐cycle unsteady‐state test. The thermal shock test consisted of plunging a sphere at one uniform temperature into a medium at another temperature. If fracture occurred, the time to fracture was recorded. A large number of tests were run to determine the temperature difference which caused 50% of the spheres to fracture. The thermal shock relationships were tested using a high‐alumina body. The physical properties relating to the thermal shock equations were measured, and calculated temperature differences causing failure and times to maximum stress were compared with measured values. Sufficient agreement was found to lend support to the theory.SummaryA method has been developed for studying the thermal shock characteristics of a brittle substance. The method consists of a single‐cycle test of unsteady‐state nature. Two testing conditions have been selected, one having a rather high surface heat transfer coefficient in a liquid bath and the other having a small finite surface heat transfer coefficient in an air bath. These two conditions are at either extremes regarding the thermal shock usually given a substance in practice.The only fair agreement found in the calculated and experimental data indicates that further investigation is necessary. The importance of certain factors, such as time to maximum stress, which was found to be in rather good agreement for the salt bath heat transfer, cannot be overlooked. There are many applications in the high temperature‐high stress field in which the concept of time to maximum stress might well be examined. For example, when repeated high‐temperature heatings are made on a refractory piece, the cycling might be arranged so that the time to maximum stress was never reached for the particular heating cycle, although the (ΔT) was higher than that necessary to cause failure.Much is to be learned from this type of study which may be applied to actual situations. It must be emphasized, however, that this work has been conducted on one single set of conditions and the factors found here do not necessarily apply to ot

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1955.tb14551.x

出版商:Blackwell Publishing Ltd    

年代:1955

数据来源: WILEY