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Apparatus for Detecting Cracks in Connector Pins

IP.com Disclosure Number: IPCOM000049979D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 4 page(s) / 41K

Publishing Venue

IBM

Related People

Hammer, R: AUTHOR [+2]

Abstract

This article describes an apparatus and a method which use static beam deflection measurements to detect the presence of fatigue cracks in the bases of connector pins composed of a material such as phosphor-bronze.

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Apparatus for Detecting Cracks in Connector Pins

This article describes an apparatus and a method which use static beam deflection measurements to detect the presence of fatigue cracks in the bases of connector pins composed of a material such as phosphor-bronze.

Method: A typical connector pin may be thought of as a cantilever beam. For small lateral applied forces, the beam deflection is given by d = P1/3//3EI where d = deflection of the free end, P = force at the free end,

1 = length,

E = Young's modulus, and

I = sectional moment of inertia.

The quantity (EI) is referred to as the flexural rigidity. The sectional moment of inertia, and hence the flexural rigidity, depends on the transverse dimensions and shape of the pin. For example, a pin with a rectangular cross section of width W and thickness T will have a moment I for bending around the easy axis of I = WT /3//12.

If a lateral crack is present in the pin, the effect reduces the cross sectional area of the pin. If the crack is placed in tension by the bending process, it tends to open up, and reduces the rigidity. On the other hand, if the cracked area is compressed by the bending process, the crack will not affect the rigidity. This situation is illustrated in Figs. 1a-1c. Fig. 1a shows a cracked pin. Fig. 1b shows how the crack has opened up in response to pressure P. Fig. 1c shows how the crack remains closed when pressure P is in the opposite direction, to unload the crack. Thus an accurate measurement of the flexural rigidity for both positive and negative lateral deflections provides a sensitive test of the presence of cracking.

Structures such as connector pins always fail at their outer surface first, and then the cracks propagate inwardly. The presence of cracking always introduces an asymmetry, and once the symmetry is broken, it can never be regained. For a round pin, we would expect equal flexural rigidities in all directions. For a pin with rectangular cross section, the flexural rigidities should be equal for positive and negative deflections along each axis of symmetry. In a pin with rectangular cross section, cracking always occurs at the corners first. In such pins, cracks will most likely not propagate in the direction of the symmetry axes, and so may not cover large lateral areas. For this reason, an apparatus with great sensitivity and reproducibility must be used for the measurements. Fig. 2 illustrates cracking of a rectangular pin.

Apparatus: A general sketch of the apparatus is shown in Fig. 3. In the present embodiment, the connector array being tested is held in a fixture which moves a precise distance in the vertical direction. The distance traversed is defined by limit stops on a micrometer screw advancement mechanism. The total distance moved is restricted to keep the deflections well within the elastic

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regime of the connector pin under test. A carefully machined hook-like probe 10 engages the connector pin 11 on connector housing...