Investigation On Dynamic Fracture Toughness Experimental Technique Under Stress Wave Load

Fracture and damage of space vehicles,aircrafts,ships and bridges under dynamic load have been subjects of increased attention with the development of technology.Dynamic fracture toughness of material is the key point of dynamic fracture mechanics.Confined to test apparatus and technology,some defects are existing.At present,the dynamic fracture experiment under stress wave load is based on Hopkinson bar experimental devices.Engineering application and the test standard to study dynamic mechanical properties of materials cannot be established.Consequently,the study on material dynamic fracture experimental technology under stress wave load is very necessary.For dynamic fracture toughness experiment technology under stress wave load,some work were proceeded in this paper as follows.In order to obtain the dynamic fracture toughness of material,Hopkinson experimental devices were improved.For spilt Hopkinson tensile bar(SHTB),a method to modify Hopkinson tensile bar has been proposed.It realized the dynamic fracture toughness test of different size compact tensile specimen(CT specimen).And the method was numerical simulated using ANSYS/LS-DYNA which is a software used for finite element analysis.The modified method is proved to be feasible by comparing the results with tests.To avoid overlapping phenomenon of the incident wave and reflected wave under the long duration time stress pulse loading,the incident bar is extended with a threaded pipe.The influence of the threaded pipe geometry on the incident wave propagation was studied by experiments.For spilt Hopkinson pressure bar(SHPB),the problem that transmission wave obtained difficultly can be solved by hollow aluminum alloy transmission bar.To investigate the stress wave propagation behavior on a hollow transmission bar,the effect of wall thickness of the hollow transmission bar was studied numerically.The results demonstrated the amplitude of the transmitted pulse is larger in hollow transmission bar under the same loading conditions,compared with the solid transmission bar.The amplitude of the transmitted pulse decreases with the wall thickness increasing.Furthermore,the effect of the span on the stress wave propagation behavior is also addressed in the work.To restrain the geometry dispersion effect of the stress wave propagation,the pulse shape method was proposed for Hopkinson tensile and pressure bars,respectively.2A12-T4 aluminum alloy short rod and pulse-shaper were used in the SHTB test system.The experimental results indicated that high frequency oscillation of incident pulse was restrained.For SHPB,a modified striker bar was selected.Both the experiment and the simulation were performed to investigate the modified effect.It was found that the geometric dispersion effect of incident pulse could be restrained effectively by the modified striker bar.The dynamic fracture toughness of CT specimen was studied with theorectital and experimental method.Using loading point displacement combining with spring mass model,the dynamic stress intensity factor of CT specimen can be calculated.Three examples of simple dynamic loading were calculated,the results were in good agreement with their finite element model values.The dynamic fracture toughness of CT specimen under stress wave load was studied using digital image correlation and strain gage.The crack-tip strain field was obtained using digital image correlation.And by dynamic J-integral formula with crack-tip strain field data,the dynamic fracture toughness of 2A12-T4 aluminum alloy was obtained.Comparing the results of strain gage,it demonstrated that the digital image correlation could be used to obtain dynamic fracture toughness.The dynamic fracture toughness of two different crack forms and different initial crack lengths were compared and analyzed.It was found that the dynamic fracture toughness of wire-cutting gap specimen was larger than that of pre-fatigue crack specimen and the fracture toughness of 2A12-T4 aluminum alloy decreased with the initial crack length increasing under the same load condition.Pre-fatigue crack shape,loading hole position and size of CT specimen were analyzed using finite element method.The result indicated that stress-state equilibrium time was almost unchanged,the fracture initiation time and the dynamic fracture toughness were affected.To study the mixed I-II crack dynamic fracture toughness,mixed I-II crack dynamic fracture toughness experiments under tensile stress load were done by SHTB testing system combined with compact tensile shear specimen(CTS specimen)and its test equipments.The applicability of Richard theory was discussed compared with the experimental-numerical results.It could provide a valuable reference for mixed ?-? crack dynamic fracture toughness experiments.