| In this paper, the measurement of dynamic fracture toughness for titanium alloys was studied. The dimensions of the specimens as well as pre-nocth or pre-crack, which were suitable for either instrumented impact test or caustics method, were determined by a series of experiments. The dynamic fracture toughness under mode-I loading of TA15ELI, TC4 and TB10 alloys were evaluated by instrumented impact test, while the caustics method was adoped to test mode-Ⅱdynamic fracture toughness and averge velocity of crack propagating for TA15ELI alloy in plane-stress condition. It is also discussed that the effects of microstructure factors on dynamic fracture toughness.The results of study on instrumented impact test are shown as following:The dynamic fracture property for titanium alloys exhibits sensitivity to the notch-tip radius. Thus, the use of U-notched specimen for simplification is inappropriate for evaluating dynamic fracture toughness. The fracture of specimens usually happens after yield for titanium alloys. The Charpy specimen with short pre-crack (a/w=0.2) shows its feasibility for evaluating mode-I dynamic fracture toughness Jd by the method of instrumented impact test.The results of study on caustics method are shown as following:it is difficult to determine dynamic fracture toughness for titanium alloys in mode-I plane-stress condition in the loading rate extent that can be introduced by drop hammer system, while both dynamic fracture toughness and averge velocity of crack propagation in mode-Ⅱplane-stress condition can be tested by specimen with pre-notch in two sides. For TA15ELI alloy with lath-like microstructure, the dynamic fracture toughness K11d is 279MPa-m1/2, and the averge velocity of crack propagating is 32.6m/s.The respectively highest dynamic fracture toughness Jd values of TA15ELI, TC4 and TB10 alloys are in a similar level, in which the value of TA15ELI is higher. TA15ELI and TC4 alloy with Widmanstatten microstructure, TC4 alloy with (a+BβTrans) microstructure, TC4 alloy with a'martensite microstructure, and TB10 alloy with (distributed a+βmatrix) microstructure, the specimens with above types of microstructures can develop a better dynamic fracture property by microstructure parameter controlling, which show the Jd values about 350～400kJ/m2. TA15ELI alloy with Widmanstatten microstructure has a better dynamic fracture and lower notch sensitivity than that with lath-like microstructure. TC4 alloy with (a+a'martensite) microstructure and TB10 alloy with (a" martensite+βmatrix) exhibit poor dynamic fracture toughness. The dynamic fracture mode is influenced by alloy brand and microsturecture. TA15ELI alloy with lath-like microstructure, TC4 alloy with (a+βTans) microstructure, TC4 alloy with (a+a'martensite) microstructure, and TB10 alloy with (distributed a+βmatrix) microstructure, the specimens with above types of microstructures have a ductile and brittle mixed fracture (mainly ductile) under dynamic loading. The fracture surfaces are composed of dimples and tear ridges. TA15ELI alloy with Widmanstatten microstructure, TC4 alloy with a'martensite microstructure and TB10 alloy with (a" martensite+13 matrix) microstructure, the fracture of specimens with above microstructures shows a cleavage features, but for Widmanstatten microstructure the edges of cleavage facets are covered with very fine dimples. For mode-Ⅱfracture, The fracture behavior of TA15ELI alloy under mode-Ⅱdynamic loading in this experiment has a feature of mixed mode, mainly plastic and partly brittle. The fracture surface is composed of elongated dimple along loading direction and smoothly curved facet. Shear localization exists in the crack initiation process.It can be found a large amount of micro-voids as well as secondary micro-cracks on the fracture sureface. These voids play a role of energy dissipation in the dynamic fracture process when the voids nucleate. Micro-voids nucleate at phase boundaries in a phase. Fracture surface initiation results from voids coalescence.For TC4 alloy with (a+βTans) microstructure, the dynamic fracture toughness is influenced by microstructure details, such as volume of equiaxed a Phase and ratio of length/width for secondary a Phase; the value of volume of equiaxed a in 47～50% with short rod-like secondary a Phase leads to relative good dynamic fracture toughness in this experiment.
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