ASTM A723 Q & T steel with a yield strength and ultimate strength of 1170 and 1262 MPa respectively was evaluated for mean stress-strain effects under smooth specimen axial strain controlled low cycle fatigue conditions with strain ratios R of -2, -1,0,0.5 and 0.75. Cycles to failure ranged from 15 to lo5. Cyclic stress-strain response based upon half-life hysteresis loop peaks were similar for all R ratios. Mean stress relaxation occurred for R # -1 only when plastic strain amplitudes were present and this occurred above total strain amplitudes of 0.005. Thus, mean stress relaxation was completely dependent upon cyclic plasticity. Mean strains did not affect low cycle fatigue life unless accompanied by half-life mean stress. Tensile mean stress was detrimental and compressive mean stress was beneficial and these effects only occurred at strain ampltidues below 0.005. Three different mean stress models were used to evaluate the low cycle fatigue data and the SWT log-log linear model best represented the data. These results can be used with the local notch strain fatigue life prediction methodology. NOMENCLATURE b = fatigue strength coefficient c = fatigue ductility exponent A6 t, = -= strain amplitude 2 t,,, = mean strain tmln = minimum strain tmar = maximum strain At = strain range Ate = elastic strain range Atp = plastic strain range 6 ; = fatigue ductility coefficient E, = modulus of elasticity from the first quarter cycle E, = modulus of elasticity from tensile unloading E3 = modulus of elasticity from compressive loading HR, = Rockwell hardness C scale K = strain hardening coefficient K' = cyclic strain hardening coefficient n = strain hardening exponent n' = cyclic strain hardening exponent Nf = cycles to failure 2Nf = reversals to failure R = strain ratio (t,,JcmaX) Au = stress range ( T~ = stress amplitude urn = mean stress u; = fatigue strength coefficient (T, = ultimate tensile strength uys = monotonic yield strength uks = cyclic yield strength SWT = Smith-Watson-Topper parameter 413 414

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Mode I crack resistance behavior of fiber‐reinforced (graphite/epoxy) composites laminated unidirectionally and anti‐symmetrically was studied. Double cantilever beam (DCB) specimens of stacking sequences, [O12//O12] and [(O/90)3s//(90/O)3s] were used where // represents the initial crack location. Resistance curves (R‐curves) were constructed for three initial crack lengths in order to determine the effects of initial crack length on the resistance behavior. The resistance force, GR, for a crack increment was determined from the compliance calibration method. The results show that for the case of [(O/90)3s//(90/O)3s], the initial crack deviated from the midplane and propagated in a zigzag fashion within 13th(90‐deg), while the crack propagated along the midplane for a [O12//O12] case. The results also show that for both cases, GR was affected by the initial crack length before GR was stabilized. However, GR was not affected by initial crack length when GR was stabilized for each case.