Hole Clearance Research Articles

The development of fatigue damage around fastener holes in thick graphite/epoxy composite laminates

This paper discusses the mechanisms by which damage develops and grows around countersunk fastener holes in composite laminates under fatigue loading conditions. Experiments have shown that the erosion of material between ply layers nucleates delaminations which then grow through the laminate under the action of the fatigue loads. The damage at the hole bore surface produces sites for the nucleation of delaminations. The morphology of the delaminations and ply cracking was mapped extensively and from these maps it was found that the volume of material around the fastener hole, damaged by the fatigue loading, adopted a characteristic shape; the volume of damaged laminate increased towards the faying surface of the laminate and (metal) fixture. This characteristic damage volume was generated by the fastener rocking under the fatigue loads. Growth of the delaminations has been shown to be preceded by intraply cracking and, as fatigue loading proceeded, more delaminations were generated at the hole bore surface. The interfacial region between the composite laminate and the fixture also provided the nucleation site for fatigue cracks in the fasteners. The effects of initial fastener-hole clearances on damage nucleation and growth did not appear to follow any clear pattern. However, coupons with excessive initial hole clearances did appear to exhibit greater than expected damage growth after only one sequence block. Several techniques for the measurement of damage growth (development) were investigated. Stiffness measurements of the test system were found to show only small changes with hole wear and fastener rocking (using shadow moire techniques) also showed only small changes with hole wear. Ultrasonic C-scanning methods were used to map the extent of damage around the fastener holes with fatigue loading. The experimental work has shown that damage development around fastener holes is a complex process, usually producing several delaminations in the region of the fastener hole which grow and may ultimately lead to the failure of the coupon.

樹脂充填高力ボルト接合部に関する実験的研究 : その1 基本性能試験

The results of two types experiments on high strength bolted connections with filled up resin in bolt hole clearance are described herein. These two experiments are slip tests and statical two way loading tests. The resins are methacrylate resins and three strength types of resins are adopted. And in these experiments, size of bolt hole clearance, initial clamping force, thickness of plate, condition of friction surface and position of bolt shank in bolt hole are also adopted as the factors of experiments. The results would be used as useful data to design the high strength bolted connections with filled up resin.

Experimental Study on Impact/Fretting Wear in Heat Exchanger Tubes

The objective of this study is to provide qualitative impact/fretting wear information for heat exchanger tubes through the performance of a series of tests involving the pertinent parameters: force between the tube and its support; tube to support plate hole clearance; tube support plate thickness; preload; and tube vibration frequency. The characteristics of impact/fretting wear relative to material combinations and fluid environment were also investigated. The test apparatus consists of a cantilevered tube with a simulated tube support plate at the “free end.” Tube vibration is induced by an electromagnetic exciter to simulate the flow-induced tube motion occurring in a real heat exchanger at the tube/tube support plate interface. Tests are conducted in air, water, and oil, all at room temperature. Removable wear rings are attached to the tube free end and simulated support fixture. Wear ring materials include carbon steel, 304 stainless steel, Inconel 600 and brass. Wear is measured by a weight loss technique and wear rates are calculated and reported as functions of the various pertinent parameters. Based on the test results, general conclusions are drawn.

Stress analysis method for a clearance-fit bolt under bearing loads

In the present, simple analysis method for bolt hole clearance stresses in mechanically fastened joints, an inverse formulation is used with a finite element analysis. Conditions along the bolt hole contact arc are specified by displacement constraint equations. The method was applied to a single fastener clearance-fit joint with typical clearance values, for the cases of a rigid, frictionless bolt and a quasi-isotropic graphite/epoxy laminate. Results indicate that the contact arc, as well as the peak stresses around the hole (and their locations), were strongly influenced by the clearance. After a minor initial nonlinearity, peak stresses varied linearly with applied load. Clearance levels are found to have only a minor influence on overall joint stiffness.

Multirow joint fastener load investigation

A multirow joint subjected to the mechanical and thermal loadings was investigated to determine the loads taken by the fasteners in each row. The hole clearance was considered and its effect on the thermal expansion was defined. The friction in the interface between two plates in contact was analyzed and its effect on the distribution of fastener loads was determined. The primary joint design parameters, such as the material and size of the plates and of the fasteners and the patterns of fastener arrangement were considered. The principle of complementary virtual work was employed to derive the general equations involving these parameters, and the analytical solutions were obtained by solving the matrix equations. A computer program was developed to obtain the numerical solutions through the use of FORTRAN IV System 370/165 computer techniques. The application of the computer program to the practical joint design problems is discussed.

Vibro-Impact Responses of a Tube with Tube-Baffle Interaction

The relatively small, inherent tube-to-baffle hole clearances associated with manufacturing tolerances in heat exchangers affect the vibrational characteristics and the response of the tube. Numerical studies were made to predict the vibro-impact response of a tube with tube-baffle interaction. The finite element method has been employed with a non-linear elastic contact spring-dashpot to model the effect of the relative approach between the tube and the baffle plate. The numerical analysis results are in reasonably good agreement with those of the experiments.

Bolted Joints with Oversize or Slotted Holes

The effect of oversize and slotted holes is evaluated on the slip resistance and ultimate strength of bolted joints. Tests were performed with 21 samples with various types of joints. Oversize or slotted holes do not affect losses in bolt tension with time. Slip behavior of these joints is similar to slip behavior of joints with holes of nominal size. The average slip coefficient for joints with 1/4 in. hole clearance was about the same as the slip coefficient for hole joints. Changes in bolt tension were not greatly affected by oversize holes or slots. Slotted holes placed perpendicular to the line of load in the enclosed plates of a bolted joint did not reduce the tensile strength of the plates or the shear strength of the bolts.

Photoelastic Measurement of Surface Strain

The technique described in this paper consists of the examination of surface strains in a metal component with optically sensitive material bonded to the component and the analysis of the photoelastic pattern produced under load by means of polarized light reflected from the surface of the metal. The original investigations cover in considerable detail the development of the technique and the results obtained, using the photoelastic materials Catalin 800, C.R.39, and the Marco Resin, S.B.26C, and S.B.28C, in conjunction with light alloy, magnesium and mild steel. The fundamental problem of producing good adhesion between the photoelastic material and the metal surface coupled with satisfactory light reflexion from the latter comprised the major part of the initial investigations. Results of the measurement of both elastic and plastic stress concentrations at holes in plates subjected to uniform tension are presented, together with a qualitative analysis of the effect on stress distribution of the variation of the pin to hole clearance in lugs. Owing to the fairly extensive nature of the original investigations it has only been possible to outline the more salient features of the work undertaken. The term ‘Metaplastic’ is suggested to describe concisely the use of compound specimens for the photoelastic applications considered.