A modification to Brown and Miller's critical plane approach is proposed to predict multiaxial fatigue life under both in-phase and out-of-phase loading conditions. The components of this modified parameter consist of the maximum shear strain amplitude and the maximum normal stress on the maximum shear strain amplitude plane. Additional cyclic hardening developed during out-of-phase loading is included in the normal stress term. Also, the mathematical formulation of this new parameter is such that variable amplitude loading can be accommodated. Experimental results from tubular specimens made of 1045 HR steel under in-phase and 90" out-of-phase axial-torsional straining using both sinusoidal and trapezoidal wave forms were correlated within a factor of about two employing this approach. Available Inconel 718 axial-torsional data including mean strain histories were also satisfactorily correlated using the aforementioned parameter. NOMENCLATURE b = fatigue strength exponent c = fatigue ductility exponent E = modulus of elasticity G = modulus of rigidity K:, = cyclic shear strength coefficient n: = cyclic shear strain exponent n = constant Nf = cycles to failure R = strain ratio, minimum strain/maximum strain S = constant a, = crack angles c, , c2, c3 = principal strains t, = Ac /2 = axial strain amplitude tT = total strain ti = fatigue ductility coefficient AZ/2 = effective strain amplitude ya = Ay/2 = shear strain amplitude y,,, = maximum shear strain 1 = biaxial strain ratio, Ay/At v = Poisson's ratio v, = elastic Poisson's ratio vD = plastic Poisson's ratio u, = A u / 2 = axial stress amplitude (ry = yield strength ( u , ) , , ,~~ = maximum value of the largest principal stress u; = alternating normal stress on yma, plane u; = mean (static) normal stress on ymax plane u; = fatigue strength coefficient 4 = phase angle u y = maximum normal stress on yma, plane T~ = AT/^ = shear stress amplitude 149 F F E M S lI/%A

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Many factors are known to influence the mechanical fatigue life of rubber components. Four major categories of factors are reviewed here: the effects of mechanical loading history, environmental effects, effects of rubber formulation, and effects due to dissipative aspects of the constitutive response of rubber. For each category, primary factors are described, and existing literature is presented and reviewed. Rubber's fatigue behavior is extremely sensitive to both the maximum and minimum cyclic load limits. Other aspects of the mechanical load history are also discussed, including the effects of static loaded periods (“annealing”), load sequence, multiaxiality, frequency, and loading waveform. Environmental factors can affect both the short and long term fatigue behavior of rubber. The effects of temperature, oxygen, ozone, and static electrical charges are reviewed. A great range of behavior is available by proper manipulation of formulation and processing variables. Effects of elastomer type, filler type and volume fraction, antidegradants, curatives, and vulcanization are discussed. The role of dissipative constitutive behavior in the improvement of fatigue properties of rubber is also reviewed. Four distinct dissipative mechanisms are identified, and their effects on fatigue behavior are described.