Thermal contact resistance with non-uniform interface pressures

This work considers the effect of roughness and waviness on interfacial pressure distributions and interfacial contact resistance. It is shown that for moderate roughness the contour area could be substantially different from the contour area calculated using the Hertzian theory. The model for pressure calculation assumes plastic deformation of surface irregularities and elastic deformation of a spherically wavy base. The calculations of pressure distributions cover the range of parameters of practical interest. Experimental contact resistance values have been determined and are compared with theoretical predictions. It was calculated that contact conductance for wavy surfaces can be increased for certain ranges of parameters by making surfaces rough.NASA DSR Projec

Areas of contact and pressure distribution in bolted joints

When two plates are bolted (or riveted) together these will be in contact in the immediate vicinity of the bolt heads and separated beyond it. The pressure distribution and size of the contact zone is of considerable interest in the study of heat transfer across bolted joints. The pressure distributions in the contact zones and the radii at which flat and smooth axisymmetric, linear elastic plates will separate were computed for several thicknesses as a function of the configuration of the bolt load by the finite element method. The radii of separation were also measured by two experimental methods. One method employed autoradiographic techniques. The other method measured the polished area around the bolt hole of the plate's caused by sliding under load in the contact zone. The sliding was produced by rotating one plate of a mated pair relative to the other plate with the bolt force acting. The computational and experimental results are in agreement and these-yield smaller zones of contact than indicated by the literature. It is shown that the discrepancy is due to an assumption made in the previous analyses. In addition to the above results this report contains the finite element and heat transfer computer programs used in this study. Instructions for the use of these programs are also included.Final technical report prepared for George C. Marshall Space Flight Center under DS

Thermal contact resistance

This work deals with phenomena of thermal resistance for metallic surfaces in contact. The main concern of the work is to develop reliable and practical methods for prediction of the thermal contact resistance for various types of surface characteristics under different conditions. In particular, consideration is restricted to the following cases: (i) rough nominally flat surfaces in a vacuum environment; (ii) rough nominally flat surfaces in a fluid environment; (iii) smooth wavy surfaces in a vacuum environment (with either of the following three types of waviness involved; spherical waviness, cylindrical waviness in one direction and cylindrical waviness in two perpendicular directions) and (iv) rough wavy surfaces in a vacuum environment. The problem is divided into three parts: thermal analysis, surface analysis and deformation analysis. The thermal analysis, based upon the proposed models, investigates the analytical solutions for the thermal contact conductance under steady state conditions. It was found convenient, due to the extensive analytical work connected with various models and different methods used here, to present all details of the thermal analysis separately in the appendices. The surface analysis, treating the surfaces as random processes with Gaussian distribution of height, relates the interface geometry to the actual contact area. The method suggested in this analysis has been checked against some autoradiographical experimental data. The deformation analysis, in its two parts, gives dependence between the load supported by the interface and (i) the actual contact area and (ii) the contact spots distribution for rough spherically wavy surfaces, respectively. The result of the first part of(cont.) the analysis is based on the plastic deformation of the surface asperities. The second part considers, through the model of the equivalent contour area, the combined effect of spherical waviness and roughness on the problem of contact spots spreading at the interface. Limitations and possible deviations of the proposed models are discussed. Prediction of the thermal contact conductance is compared with experimental data obtained in this work (in a vacuum environment) together with some data obtained by other investigators (for which necessary surface parameters were available). Agreement between the measured and predicted values was good in the whole tested range of system variables.Sponsored by the National Aeronautics and Space Administratio

Thermal contact resistance in a non-ideal joint

The contact conductance at an interface can be determined by knowing the material and surface properties and the interfacial pressure distribution. This pressure distribution can be influenced strongly by the roughness of the mating surfaces but until now this effect has been ignored in studies of joint conductance. This thesis considers this effect and shows the circumstances when it is an important factor. Furthermore, it is shown that one can either raise or lower the total resistance of a joint by changing the surface properties in the proper manner for the particular system being considered. Specifically, this thesis deals with three systems: the contact of two rough, wavy surfaces; the contact of two rough but nominally flat plates pressed together over a concentrated area; and the contact of two rough but nominally flat plates bolted together. In each case the pressure distribution is calculated as a function of the surface properties. In the case of wavy surfaces it is found that all necessary information for any combination of parameters can be reduced to one master graph. In the other two cases one such graph is needed for each geometry used. The resulting pressure distributions are used in a specific heat transfer example and the total joint resistance versus roughness is presented. It is shown how one can actually decrease the resistance by increasing the roughness - a seemingly contradictory phenomenon. Heat transfer experiments performed by Joseph Pigott qualitatively confirmed the theoretical findings.Sponsored by George C. Marshall Space Flight Center, NAS