Danian Chen

An investigation of the dynamic separation of spot welds under plane tensile pulses

We performed ultra-high-speed tests for purely opening spot welds using plane tensile pulses. A gun system generated a parallel impact of a projectile plate onto a welded plate. Induced by the interactions of the release waves, the welded plate opened purely under the plane tensile pulses. We used the laser velocity interferometer system for any reflector to measure the velocity histories of the free surfaces of the free part and the spot weld of the welded plate. We then used a scanning electron microscope to investigate the recovered welded plates. We found that the interfacial failure mode was mainly a brittle fracture and the cracks propagated through the spot nugget, while the partial interfacial failure mode was a mixed fracture comprised ductile fracture and brittle fracture. We used the measured velocity histories to evaluate the tension stresses in the free part and the spot weld of the welded plate by applying the characteristic theory. We also discussed the different constitutive behaviors of the metals under plane shock loading and under uniaxial split Hopkinson pressure bar tests. We then compared the numerically simulated velocity histories of the free surfaces of the free part and the spot weld of the welded plate with the measured results. The numerical simulations made use of the fracture stress criteria, and then the computed fracture modes of the tests were compared with the recovered results.

Spall Strength of Resistance Spot Weld for QP Steel

The spall tests under the plane tensile pulses for resistance spot weld (RSW) of QP980 steel are performed by using a gun system. The velocity histories of free surfaces of the RSW are measured with the laser velocity interferometer system for any reflector. The recovered specimens are investigated with an Olympus GX71 metallographic microscope and a scanning electron microscope (SEM). The measured velocity histories are explained and used to evaluate the tension stresses in the RSW applying the characteristic theory and the assumption of Gathers. The spall strength (1977–2784 MPa) of the RSW for QP980 steel is determined based on the measured and simulated velocity histories. The spall mechanism of the RSW is brittle fracture in view of the SEM investigation of the recovered specimen. The micrographs of the as-received QP980 steel, the initial and recovered RSW of this steel for the spall test are compared to reveal the microstructure evolution during the welding and spall process. It is indicated that during the welding thermal cycle, the local martensitic phase transformation is dependent on the location within the fusion zone and the heat affected zone. It is presented that the transformation at high strain rate may be cancelled by other phenomenon while the evolution of weld defects is obvious during the spall process. It may be the stress triaxiality and strain rate effect of the RSW strength or the dynamic load-carrying capacity of the RSW structure that the spall strength of the RSW for QP980 steel is much higher than the uniaxial compression yield strength (1200 MPa) of the martensite phase in QP980 steel. Due to the weld defects in the center of the RSW, the spall strength of the RSW should be less than the conventional spall strength or the dynamic load-carrying capacity of condensed structure.

Dynamic compressive behavior and constitutive relations of lanthanum metal

Abstract The dynamic compressive behavior and constitutive relations of Lanthanum (La) metal was determined by using the first compression in split Hopkinson pressure bar (SHPB) tests at different strain rates and temperatures. The constitutive relation of La metal determined in a certain range of strains was employed and adjusted in numerically simulating large deformations of La metal specimens generated by multi-compression in SHPB tests and recorded by a high-speed camera. The dynamic compressive behavior and constitutive relations of La metal under multiple SHPB tests loading was also revealed. The results of scanning electron microscope (SEM) investigation of the recovered La metal specimens for typical tests showed that there was a variety of deformation microstructures depending on strain rate, temperature and stress state.

Uniaxial compression properties of fusion zone martensite in resistance spot-weld for QP980 steel

The cross-sectional micrograph, microhardness profile, and microstructures of the resistance spot-weld (RSW) for QP980 steel are shown to reveal the effects of the content and tempering of the martensite in the fusion zone (FZ) and the heat-affected zone (HAZ) on the microhardness profile of the RSW. It is indicated that the transient peak temperature above the critical temperature during the welding thermal cycle induces the significant variations of material and mechanical properties in the FZ and HAZ. The quasi-static uniaxial compression stress-strain curve of the FZ martensite in the RSW for QP980 steel is obtained with digital image correlation (DIC) and compared with that of two micropillars of martensite phase in the base material (BM) of the RSW for QP980 steel which was given by Srivastava et al. It is attributed to the welding thermal cycle different from QP980 steel heat treatment that the flow stress of the FZ martensite in the RSW is higher than that of the martensite phase in the BM. The dynamic uniaxial compression experiments for the FZ martensite in the RSW for QP980 steel are performed on a modified split Hopkinson pressure bar (SHPB), in which the reflected and transmitted waves are improved. A dynamic compression constitutive equation is presented by analyzing the results of the quasi-static and dynamic uniaxial compression experiments. A Swift law for martensite phase is extended to high strain rates to describe the weak strain-rate dependence of the dynamic compression behaviors of the FZ martensite in the RSW for QP980 steel.

Failure mechanism of resistance-spot-welded specimens impacted on base material by bullets

The tests of bullet impact on the base material (BM) of a simple specimen with a single resistance-spot-welded (RSW) nugget of TRIP800 steel are performed to investigate the response of the RSW specimen to the ballistic debris impact on the RSW specimen. A one-stage gas gun is used to fire the bullets while a laser velocity interferometer system for any reflector (VISAR) is used to measure the velocity histories of the free surfaces of the RSW specimen. The recovered RSW specimens are examined with the three-dimensional super depth digital microscope (SDDM) and the scanning electro microscope (SEM). For the tests of small multiple-bullet impact, it is revealed that the wave train of the VISAR measured results and the detachment of the base material interfaces in the recovered RSW specimens are directly related to the reflection and refraction of the curved stress waves incoming to the interfaces and the free surfaces in the RSW specimens. The detachment of BM interfaces can lead to the impact failure of t...

Dynamic Constitutive Behavior and Fracture of Lanthanum Metal Subjected to Impact Compression at Different Temperatures and Impact Tension

Abstract Based on compressive tests, static on 810 material test system (MTS) and dynamic on the first compressive loading in split Hopkinson pressure bar (SHPB) tests at different strain rates and temperatures for Lanthanum (La) cylinder specimens, this study determined Johnson-Cook (J-C) type compressive constitutive equation of La metal. The determined compressive constitutive equation of La metal was calibrated in numerically simulating the recorded large deformations of La cylinder specimens generated by the multi-compressive loadings in SHPB tests. Based on tensile tests, static on MTS and dynamic on the first tensile loading in optimized tensile split Hopkinson bar (TSHB) tests at different strain rates for La sheet specimens, this study determined the J-C type tensile constitutive equation of La metal. It was found that the La sheet specimens were fractured during the first tensile loading in TSHB tests. The numerical simulations of transmitted and reflected pulses of TSHB tests for La sheet specimens using strain rate dependent failure strain criterion were consistent with the experimental data. The relation between dynamic failure strength and strain rates was discussed. From scanning electron microscope investigation of fractured La specimens, it was found that damage evolution patterns at high loading rates tend to become regular.

Dynamic compressive behavior of Pr-Nd alloy at high strain rates and temperatures

Based on compressive tests, static on 810 material test system and dynamic on the first compressive loading in split Hopkinson pressure bar (SHPB) tests for Pr-Nd alloy cylinder specimens at high strain rates and temperatures, this study determined a J-C type [G. R. Johnson and W. H. Cook, in Proceedings of Seventh International Symposium on Ballistics (The Hague, The Netherlands, 1983), pp. 541–547] compressive constitutive equation of Pr-Nd alloy. It was recorded by a high speed camera that the Pr-Nd alloy cylinder specimens fractured during the first compressive loading in SHPB tests at high strain rates and temperatures. From high speed camera images, the critical strains of the dynamic shearing instability for Pr-Nd alloy in SHPB tests were determined, which were consistent with that estimated by using Batra and Wei’s dynamic shearing instability criterion [R. C. Batra and Z. G. Wei, Int. J. Impact Eng. 34, 448 (2007)] and the determined compressive constitutive equation of Pr-Nd alloy. The transmitt...

Analysis of Thermoviscoplastic Effects on Dynamic Necking of Pure Copper Bars during Impact Tension

Abstract In order to make effective measure for the stress, strain and strain rate in the specimen, a tensile split Hopkinson bar (TSHB) was optimized in the first loading duration and the thermoviscoplastic models for a pure copper were determined. The deformation of the pure copper bar during the multi-tensile loading including the necking process in the optimized TSHB testing was recorded by a high speed camera and was used in providing an extensional check of the determined thermoviscoplastic models of the pure copper. The thermoviscoplastic models determined in a certain range of strain were employed in numerically simulating the large deformation of the pure copper bar to adjust the thermoviscoplastic model to describing large strain range. The instability strain of thermoviscoplastic materials in simple tension given by Batra et al was compared with the experimental strain at diffuse necking of the pure copper bar and the computed instability strain based on the load-average strain of the pure copper bar. The local necking of the pure copper bar in the optimized TSHB testing was numerically simulated using the micro-mechanical data given by Ragab and was compared with the recorded necking.

Dynamic Growth and Coalescence of Drilled Voids in Pure Copper Sheets

Abstract A series of tensile tests for pure copper sheets containing drilled holes of multi-configurations are carried out on a tensile split Hopkinson bar (TSHB) at a range of tensile velocities. The dynamic growth and coalescence of drilled voids were recorded by a high-speed camera. The results of scanning electron microscopical (SEM) investigation of microvoid evolution at edges of ruptured zones between drilled voids in recovered sheets show void coalescence mechanisms, which are similar to evolution of visualized drilled voids. The experimentally recovered and high-speed camera recorded evolution of drilled voids in pure copper sheets are compared with the numerical simulations involving different models for void coalescence in order to overcome the difficulty of assessing the validity of available models for void coalescence. The possible application of Thomason model and Brown–Embury model for void coalescence to thermoviscoplastic material was explored under impact tension, and the rapid local straining of intervoid ligament was revealed.