Fatigue life prediction under multiaxis random loading is an extremely complex and intractable topic; only a few methods have been proposed in the literature. In addition, experimental results under multiaxis random loading are also scarce. In part one of this two-part paper, a multiaxial non-proportional cycle counting method and fatigue damage calculation procedure are proposed, which is compared with one published damage-searching method. Both theories are based on critical plane concepts, one being an extension of the local strain approach for uniaxial variable amplitude loading and the other employing a new counting algorithm for multiaxis random loading. In principle, these two methods can be considered as bounding solutions for fatigue damage accumulation under multiaxis random loading.

A path-independent multiaxial fatigue damage criterion is proposed based on critical plane concepts: fatigue crack growth is controlled by the maximum shear strain, and an important secondary effect is due to the normal strain excursion over one reversal of the maximum shear strain. The effect of loading path on fatigue endurance is quantified by the normal strain excursion. Only one multiaxial material constant is required in the model which can be determined from uniaxial test data plus one torsional result. The parameter can be easily integrated with a shear strain-life relationship to predict low cycle fatigue lifetime. Experimental data of four different materials: En1 5R steel, I % Cr-Mo-V steel, 304 stainless steel, and 316 stainless steel at two temperatures were used to verify the criterion. It is shown that the proposed parameter can satisfactorily correlate test results for various proportional and non-proportional straining paths. NOMENCLATURE b. c = fatigue strength and ductility exponents E = Young's modulus F = rotation factor g = cross hardening coefficient factor K ' , n' = cyclic strength coefficient and strain hardening exponent Nf = fatigue lifetime S = normal strain effect coefficient f = effective shear strain yma, = maximum shear strain t e q , ueq = equivalent strain and stress tn = normal strain across maximum shear plane t: = normal strain excursion between turning points of maximum shear strain v,, vpr vCR = elastic, plastic, and effective Poisson's ratios u;, t i = fatigue strength and ductility coefficients ofl, zfl = uniaxial and torsional fatigue limits

A new method of assessing urethral competence has been developed (Brown & Wickham 1969). It was originally devised to study the effect of electrical stimulation to the pelvic floor on the closure forces in the urethra. There seem to be applications of the technique in the study of incontinence and of strictures.The basic apparatus is an electric strain gauge manometer coupled to a pen recorder. This is used to record the pressure at the distal end of a single lumen urethral catheter through which a constant, but very slow, flow of water is forced by an infusion pump or a very high infusion bottle. The water forced through the catheter is expelled from the catheter eye against the urethral wall. The presure developed inside the catheter in order to o'rtrcome the wall pressure over the catheter eye, and the wall pressure itself can be shown to be very nearly equal.The flow of water is regulated to about 003 ml/sec by a standard infusion regulator situated on the water feed tube just distal to the point of pressure measurement. This flow rate is chosen for a compromise between the response time error, due to very slow flow rates, and the viscosity error due to higher flow rates. An advantage of this simple system is that fast response times (about 0O1 sec) will permit its use for continuous profile measurement. The catheter, calibrated in centimetres, is drawn slowly down the urethra and the recorder auto-40 matically plots the urethral wall pressure as a continuous graph. A complete profile takes about twenty seconds to make and a series of profiles can be plotted sequentially to trace the effect of electrical stimulation, chemotherapy, dilation or other surgery.

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