Research On Determination Of Quasi-Static Mechanical Properties Of Ductile Metals From Spherical Indentation Tests

Quasi-static mechanical properties represented by uniaxial tensile properties and fracture toughness are the most commonly used and basic mechanical properties of materials.Accurate acquisition of them is of great significance for structural integrity assessment and service life extension of in-service devices.However,traditional uniaxial tensile tests and fracture tests all need destructive sampling,and thus cannot be used on in-service devices.The study of quasi-static mechanical properties testing technology without sampling is the experimental basis of structural integrity assessment and life extension of in-service devices.Thus,the author used extensively studied spherical indentation tests as the testing technology,and investigated the deformation behavior and damage mechanism of specimen materials.The Pharr-Oliver model that has been widely used in calculating the effective elastic modulus from spherical indentation tests was revised,the elastoplastic behavior and damage mechanism of sample material was investigated,and the effectiveness of characterizing material damage with effective Young's modulus was verified.A model that does not depend on any specific constitutive equations was established to determine the uniaxial tensile properties of specimen materials at room temperature,and a method to determine the softening index of materials at elevated temperatures from spherical indentation tests with monotonic loading was proposed.The intrinsic relationship between the indentation damage and the material damage in fracture tests was studied,and the energy release rate model that takes the damage mechanism into consideration was established to calculate the fracture toughness.The study provides theoretical support for quasi-static mechanical property determination from spherical indentation tests on ductile metals.The Pharr-Oliver model that has been widely used in effective Young's modulus calculation was revised by taking the plastic deformation into consideration to make it corresponds better with the actual indentation test on ductile metals.The phenomenon of pile-up and sink-in around the indentation was observed through 3D profile scanning,the effect of pile-up and sink-in on effective elastic modulus calculation was discussed,and then a method to determine the pile-up and sink-in coefficient was proposed to further propagates the calculation accuracy,which establish the foundation for uniaxial mechanical property and fracture toughness calculation.FE calculations of the spherical indentation test revealed that the concentrated distribution of shear stress caused by friction is the cause of damage concentration in the 'wing' region in spherical indentation tests.The unloading slope from different stage of unloading curve was used to investigate the relationship between the reduction on effective Young's modulus with the occurrence of damage,and verified the effectiveness of characterizing the material damage in spherical indentation tests through the reduction on effective Young's modulus.A simplified expanding cavity model,which considers the core region and elastoplastic region as a whole,was proposed,and thus the error caused by corresponding pi to pm was avoided.The digital image correlation(DIC)was introduced to measure the plastic zone radius rp for proportional limit ?o calculation.A function to map the stress-strain increment with the energy increment was established,and a model that does not depend on any specific constitutive equation was proposed to determine the uniaxial stress-strain curves from indentation tests.A comparison of the results from the microscope and telecentre lens revealed that for microscopic images,the strain threshold ?th setting needs to consider lens distortion and out-of-plane displacement,and thus different Eth must be used according to the phenomenon of pile-up and sink-in,while for images captured by a telecentre lens,only lens distortion needs to be considered in ?th setting,and thus ?th of different materials can be regarded as the same value.Compared with the uniaxial tensile test results at room temperature,the errors of yield strength Rp0.2 and tensile strength Rm from spherical indentation tests at room temperature were all less than 5%.Therefore,the model has reliable accuracy for testing uniaxial tensile properties at room temperature.The flow behavior of the sample material at elevated temperatures was investigated,based on the expanding cavity model and Johnson-Cook constitutive equation,and then a set of indentation governing equations to determine the material parameters from spherical indentation tests with monotonic loading was established.By analyzing the difference of the unloading slope when the displacement sensor was placed inside and outside the environment cabin,the frame flexibility at room temperature was obtained,and the frame flexibility at elevated temperatures was further determined by combining the reduction of elastic modulus of the frame material at high temperatures.The accuracy of the equivalent stress-equivalent strain calculation model described in Chapter 4 was compared with the indentation governing equation described in Chapter 5 in determining the stress-strain curves at room temperature,and the error sources of the indentation governing equation in Chapter 5 in determining the stress-strain relationship at room temperature were analyzed.Compared with the uniaxial tensile test results at elevated temperatures,the maximum error of the softening exponent obtained by the spherical indentation test at elevated temperatures was only about 10%,the maximum error of the RP0.2 and Rm calculation were also less than 20%,and the test results had good repeatability.Thus,it can meet the needs of engineering applications.The stress triaxiality in different specimen configurations was investigated through FE calculations and theoretical analyses,which reveals that the crack-tip in a mode I fracture specimen(compact tension specimen)is always in high stress triaxiality,the crack-tip in a mode ? fracture specimen(Arcan specimen),the uniaxial compression specimen,and the 'wing' region in a spherical indentation specimen are:always in low stress triaxiality,while there is a transition from intermediate stress triaxiality to high stress triaxiality in the central region of a tensile specimen.SEM observation revealed that the damage and fracture under high stress triaxiality and low stress triaxiality are obviously different.By contrast,the damage mechanisms for pre-cracked and un-cracked specimens under similar stress triaxiality are similar,and thus explain the reason why un-cracked specimens can be used to calculate the fracture toughness.The energy release rate was fitted from spherical indentation tests,and compared with that from uniaxial tensile tests and uniaxial compression tests.The comparisons proved that the spherical indentation test and uniaxial compression test with the same damage mechanism yielded almost the same energy release rate,while the uniaxial tensile test yielded a completely different energy release rate.The fracture toughness calculation model based on the energy release rate has obvious physical meaning and high repeatability,and the maximum error of the fracture toughness calculated with energy release rate model is less than 10%when compared with that from traditional compact tension tests.Thus,it can meet the repeatability and accuracy requirements of engineering applications.Above all,the Pharr-Oliver model that has been widely used in effective Young's modulus calculation was revised by taking the plastic deformation before unloading into consideration,the damage mechanism for specimen materials and its relationship with the reduction of effective elastic modulus was analyzed,and the effectiveness of characterizing material damage by effective Young's modulus reduction was verified.The deformation behavior of specimen material was studied.On the basis of considering the characteristics of spherical indentation tests at room and elevated temperatures,a model that does not depend on any specific constitutive equation was established to obtain the uniaxial mechanical properties at room temperature,and a monotonic loading spherical indentation test method was proposed to determine the softening index at elevated temperatures.The rationality for determining the fracture toughness of a material from un-cracked specimens was presented,the energy release rate from tests on un-cracked specimens was determined with continuum damage mechanics,and the fracture toughness calculation model in accordance with material damage mechanism was established.The study mentioned above provides theoretical support for testing the quasi-static mechanical properties of a material from spherical indentation tests,and has significant importance for the structural integrity assessment and service life extension of in-service equipment.