Study On Spalling Response Of Materials Under Complicated-Stress States

It is still unsolved that how the damage parameters obtained under simple stress state are applied to complicated stress state. One-dimensional strain state is a relatively simple state and easy to be carried out experimentally, the spalling strength defined under this state is correlative to impact stress amplitude and strain rate. The pullback amplitudes of the free surface velocity have been well defined and are measured using various means. But ,how to determine and characterize the strength and critical damage of materials under multi-dimensional strain states, or how to apply the damage parameters obtained under simple stress state to complicated stress states , is still not solved theoretically or experimentally. For this purpose, this thesis studies the spherical spallation with theoretical analysis, numerical simulation and experiment. It shows that the difference of the spalling strength and critical damage is prominent from the former to the latter if we apply these parameters directly. Which give us a hint that it must be modified. In this thesis the fracture criterion under complicated stress state and the modificatory method from one-dimensional strain to spherical strain are discussed.The main work and innovative points of the thesis are summarized as follows:(1) Review briefly the former researches on spallation phenomena, point out the difficulty and shortage in this field, and put forward the significance of this study;(2) Raise a point that the geometric shape of loading evidently influence the spalling strength, and its essence consists in that different geometric shapes of loading correspond to different stress states. On this problem, few existing researches in this field discuss whether the geometric shape of loading can influence the spalling strength;(3) Educe the stress-strain expression for spherical samples under spallation other than that of plate samples. The result of stress analysis shows that an approximate research method can be used to study the stress state of spherical samples by using plate samples prestressed in the radial direction. It is a new method to transform the spherical loadings to planar loadings under different initial stress states;(4) Propose the method of excessively-conjugated heat assemblage and mechanical clamp, develop the device of load in the radial direction. A series of spalling test under different radial prestrains of (200～2000)μεare carried out with these devices. The test results show that the pullback amplitudes of free-surface velocity and spalling strength of samples obviously decrease under the same impact velocity, relative to those without initial prestresses. Even if the prestress is only 0.70 percent of shock pressure, the spalling strength is 18.1 percent lower than that of samples without initial prestresses. Numerical simulations are performed with an axial symmetry cell model, the simulated results show from the elementary principle of Void Growth that the expanded paths and consumed energy of prestressed voids differ from those of voids without initial prestresses, which can explain why the spalling strengths of samples decrease with the increase of the radial prestresses of samples; (5 ) Adopt the finite element method controlling macroscopic stress states and numerically investigate axial symmetry cells under the same mean stress and different stress triaxiality to explore effects of stress triaxiality on void-growth mechanism. Calculated results indicate that at the same strain rate, stress triaxialities affect remarkably the volume expansion and shape change of voids, and the lower the stress triaxiality, the more distinct the increase of the void volume. When the mean stress is 1.75 GPa, the stress triaxialities of 1.83 (λ=0.8) ,∞(λ=1.0) correspond to the void volume fractions of 0.897, 0.222 percent respectively. The former is larger than the latter. The traditional spherical void models are difficultly expanded and they overestimate the resistance to volume expansion of voids;(6) Analyse primarily the deficiency of the strain equivalence principle based on the simple curve assumption and advance that the strain equivalence principle must be reasonably modified before it is used in the constitutive models containing damage. The modificative method is tightly correlated with the determination of damage variables in the damage constitutive model. It is necessary to introduce the important parameter embodying the macroscopic stress fields into the damage constitutive models and combine the meso-damage theory based on mesomechanics with the phenomenological damage theory in order to describe damage evolution in materials preferably;(7) Present a simulated method to putting static radial prestress on samples and investigate numerically the effects of the radial prestress in samples on the spalling information. Computed results display that it is indispensable to change the fracture criterion so as to reproduce the experimental information of spalling signals of materials varied with the prestresses;(8) Mention two fracture theories describing complex stress states, including the void expansion ratio theory by Zhen Chang-qin and the fracture surface theory by Johnson, and discuss the relationship between these two theories. Anlyse the damage mechanism of samples under complicated stress states by combining the feasibility of experiments and introducing the parameter of stress triaxiality, and propose the method on how to establish the damage criterion of materials under no one-dimensional stress states by means of the prestressed spall experiments and utilize this damage criterion to explain other spall processes under multidimensional strain states.