Investigation of Crack Problems in Steering Column Assemblies Part I
September 17th, 1981
Report to Lucas Electrical Systems Ltd.
S F Bush
Summary and Conclusions
- As commissioned, the report covers the following points: the experimental observation with strain-guages of the strains developed in a typical knob assembly; an estimate of the strains likely to be developed in the assembly by reference to the dimensions of the cap and case as found from moulded specimens; an estimate of the likely stresses resulting; and finally the implications for crack development.
- The strain-guages fitted performed well and reproducibly on the same specimen over a period of months. The estimates of strain are based on the considerable deviations from design recorded on six specimens and compare fabourably with the observed values. The application of the lubrication oil supplied did not make any apparent difference over a period of days and weeks on the specimen supplied. The experimental approach adopted appears satisfactory for the class of moulding supplied.
- The estimate of stress developed is very tentative, based as it is on point property data supplied by the manufacturer for the grade of ABS used. The tensile stresses in the worst fit case correspond to about 30% of quoted yield strength.
- This stress would probably be sufficient to propagate a microcrack of about 0.5 μ, which could correspond to a distinct, but invisible-to-the-naked-eye weld line feature. A macromolecular defect of say 0.06 μ corresponding to a good weld might require about 100-120% of quoted yield strength to propagate.
- A crack could thus result from a combination of circumstances: a more distinct weld line than usual, probably arising from a small unnoticed change of moulding conditions, together with a larger-than-average strain in the blister region of the case, of the magnitude estimated. The design should thus avoid weld lines in tension-stressed parts of the assembly, especially if as is sometimes the case, they correspond to the coolest part of a mould.
See also the section on Polymer Morphology & Fibre Reinforcement Mechanisms.