Dynamic Tensile Testing of Soft Materials

Determination of dynamic tensile response of soft materials has been a challenge because of experimental difficulties. Split Hopkinson tension bar (SHTB) is a commonly used device for the characterization of high-rate tensile behavior of engineering materials. However, when the specimen is soft, it is challenging to design the necessary grips, to measure the weak transmitted signals, and for the specimen to achieve dynamic stress equilibrium. In this work, we modified the SHTB on the loading pulse, the equilibrium-monitoring system, and the specimen geometry. The results obtained using this modified device to characterize a soft rubber indicate that the specimen deforms under dynamic stress equilibrium at a nearly constant strain rate. Axial and radial inertia effects commonly encountered in dynamic characterization of soft materials are also minimized.     

A Technique for Dynamic Tensile Testing of Human Cervical Spine Ligaments

An experimental study is undertaken to examine the dynamic stress–strain characteristics of ligaments from the human cervical spine (neck). Tests were conducted using a tensile split Hopkinson bar device and the engineering strain rates imposed were of the order of 10²∼10³/s. As ligaments are extremely soft and pliable, specialized test protocols applicable to Hopkinson bar testing were developed to facilitate acquisition of reliable and accurate data. Seven primary ligaments types from the cervical spines of three male cadavers were subjected to mechanical tests. These yielded dynamic stress–strain curves which could be approximated by empirical equations. The dynamic failure stress/load, failure stain/deformation, modulus/stiffness, as well as energy absorption capacity, were obtained for the various ligaments and classified according to their location, the strain rate imposed and the cadaveric source. Compared with static responses, the overall average dynamic stress–strain behavior foreach type of ligament exhibited an elevation in strength but reduced elongation.     

Dynamic Tensile Response of a Microwave Damaged Granitic Rock

Background

Understanding the dynamic tensile response of microwave damaged rock is of great significance to promote the development of microwave-assisted hard rock breakage technology. However, most of the current research on this issue is limited to static loading conditions, which is inconsistent with the dynamic stress circumstances encountered in real rock-breaking operations.

Objective

The objective of this work is to investigate the effects of microwave irradiation on the dynamic tensile strength, full-field displacement distribution and average fracture energy of a granitic rock.

Methods

The split Hopkinson pressure bar (SHPB) system combined with digital image correlation (DIC) technique is adopted to conduct the experiments. The overload phenomenon, which refers to the strength over-estimation phenomenon in the Brazilian test, is validated using the conventional strain gauge method. Based on the DIC analysis, a new approach for calculating the average fracture energy is proposed.

Results

Experimental results show that both the apparent and true tensile strengths increase with the loading rate while decreasing with the increase of the irradiation duration; and the true tensile strength after overload correction is lower than the apparent strength. Besides, the overload ratio and fracture energy also show the loading rate and irradiation duration dependency.

Conclusions

Our findings prove clearly that microwave irradiation significantly weakens the dynamic tensile properties of granitic rock.

     

基于区间因子法的不确定性桁架结构动力响应分析

研究了不确定性桁架结构的动力响应分析问题.在桁架结构的物理参数、几何尺寸和荷载幅值均为区间变量时,从结构响应的Dulaamel积分式出发,利用区间因子法、区间运算和振型叠加法导出了结构动力响应区间变量的表达式.通过算例考察了结构参数和作用荷载的区间分散性对结构动力响应分散性的影响,经与蒙特卡洛数值模拟结果的比较,表明文中所提出的计算模型和分析方法的合理性和可行性.     

Microstructure and Tensile Mechanical Properties of Anisotropic Rigid Polyurethane Foam

An understanding of the mechanical properties of solid foams facilitates effective use of such materials, which are often deployed in load-bearing applications such as impact absorbers, cushioning and sandwich structures. This study is an experimental investigation that focuses on the deformation response of rigid polyurethane foam to tension. Microstructural features such as the size and geometry of constituent cells and the struts that define the cell edges, as well as their stiffness and tensile strength, are examined. The nature of cell deformation and fracture are identified through microscopy and the associated micromechanics analyzed. Results show that the cells are elongated and thus the foam exhibits anisotropic properties. Flexure of the struts that define the cell edges is the primary mechanism governing deformation and failure. Information on the mechanical, microstructural and deformation characteristics elicited through this investigation will facilitate formulation of idealized cell-based models to account for the mechanical response of rigid polymeric foams.     

Re-Examination of Correlation between Hardness and Tensile Properties by Numerical Analysis

Contact, singular-field, and large-deformation numerical analyses were performed to re-examine the correlation between hardness and tensile properties. Materials have single hardness values, but continuous changes in stress-strain diagrams; it is impossible, in principle, to correlate the hardness to one stress-strain value. Therefore, there must exist an application limit, which is discussed in this study. Because discretization error is unavoidable in such analyses, a method for leveling the discretization error regardless of the analysis condition was introduced. Moreover, in order to generalize the analysis results, the stress-strain diagram used for this analysis was considered as a dimensionless expression in arranging the results. From the analytical results, the following conclusions were deduced. The range in which the empirical equation between hardness and tensile strength is applicable depends on only the value of the work hardening exponent. Moreover, for the 0.05–0.2 range of the work hardening exponent for general steel, the prediction of tensile strength from the hardness is possible with 10% error at indenter face angles between 110° and 136° in case of the wedge indentation.     

Flattened Brazilian Disc Method for Determining the Dynamic Tensile Stress-Strain Curve of Low Strength Brittle Solids

An indirect tensile testing method is proposed to measure the full dynamic tensile stress-strain curve of low strength brittle solids. In this method, the flattened-Brazilian disc (FBD) sample is loaded by modified split Hopkinson pressure bars (SHPB) system. Low amplitude dynamic forces were measured with a pair of piezoelectric force transducers embedded in the incident bar and the transmitted bar. The evolution of tensile stress at the center of the disc sample was determined through finite element analyses using the measured stress in SHPB as inputs. In a traditional Brazilian test, a strain gauge is mounted at the center of the specimen to measure the tensile strain, which is difficult to apply for low strength brittle materials. Thus, two types of non-contact methods, the Digital Image Correlation (DIC) technique and the Laser Gap Gauge (LGG), were used to measure the strain. The DIC method was used to monitor the displacement and the strain map of the specimen during the test, from which the strain at the center of the specimen can be obtained. The accuracy of the DIC results was assessed, and the displacement and strain uncertainties of our system were 0.003 mm and 0.003, respectively. LGG was used to monitor the expansion of the disc perpendicular to the loading axis, from which the average tensile strain is deduced. The numerical simulation revealed that the tensile strain at the center of the specimen is proportional to the average tensile strain and that the ratio is not sensitive to the material elastic parameters. The strain measured through LGG was compared with that measured by the DIC method using photos captured with a synchronized high-speed camera. The result of the LGG method was 20 % smaller than that of the DIC process. However, the latter was limited by the number of frames of the high-speed camera. The feasibility of this methodology was demonstrated using a polymer-bonded explosive (PBX).     

Dynamic Tensile Characterization of a 4330-V Steel with Kolsky Bar Techniques

Dynamic tensile experimental techniques of high-strength alloys using a Kolsky tension bar implemented with pulse shaping and advanced analytical and diagnostic techniques have been developed. The issues that include minimizing abnormal stress peak, determining strain in specimen gage section, evaluating uniform deformation, as well as developing pulse shaping for constant strain rate and stress equilibrium have been addressed in this study to ensure valid experimental conditions and obtainment of reliable high-rate tensile stress–strain response of alloys with a Kolsky tension bar. The techniques were applied to characterize the tensile stress–strain response of a 4330-V steel at two high strain rates. Comparing these high-rate results with quasi-static data, the strain rate effect on the tensile stress–strain response of the 4330-V steel was determined. The 4330-V steel exhibits slight work-hardening behavior in tension and the tensile flow stress is significantly sensitive to strain rate.     

铝合金材料AA-6061-T6和AA-6061-OA的动态拉伸力学性能

铝合金材料蜂窝夹层板结构具有在较低体重情况下的高硬度和高抗冲击性能力。近年来许多关于其在低应变率下冲击能量吸收性质的文献纷纷涌现，但是对于其在高应变率下的能量吸收力学性能的研究却非常贫乏。为了更好地研究铝合金材料蜂窝夹层板结构在高应变率下的能量吸收力学性能，其结构组成材料本身的动态力学性能必须首先得到充分研究。本文介绍和总结了铝合金材料AA-6061的两种热处理成品，T6与OA，在室温（24℃）与低温（-170℃）下的动态拉伸力学性能。在本研究中，霍普金生拉伸杆被应用，拉伸应变率为10^3每秒。     

铝合金材料AA-6061-T6和AA-6061-OA的动态拉伸力学性能(英文)

铝合金材料蜂窝夹层板结构具有在较低体重情况下的高硬度和高抗冲击性能力。近年来许多关于其在低应变率下冲击能量吸收性质的文献纷纷涌现,但是对于其在高应变率下的能量吸收力学性能的研究却非常贫乏。为了更好地研究铝合金材料蜂窝夹层板结构在高应变率下的能量吸收力学性能,其结构组成材料本身的动态力学性能必须首先得到充分研究。本文介绍和总结了铝合金材料AA-6061的两种热处理成品,T6与OA,在室温(24℃)与低温( -170℃)下的动态拉伸力学性能。在本研究中,霍普金生拉伸杆被应用,拉伸应变率为103每秒。     

Dynamic Fracture Properties of Titanium Alloys

Fatigue precracked specimens of three titanium alloys (6Al-4V, ELI, and Timetal 5111) were dynamically loaded in a drop weight tower system while the dynamic fracture toughness was inferred using Coherent Gradient Sensing, crack opening displacement, or strain gage methods. A comparison of the initiation toughness of the three materials as a function of loading rate and specimen thickness is made.     

Comparison of Test Specimens for Characterizing the Dynamic Properties of Rubber

Dynamic material properties inferred via experiment can be strongly influenced by the choice of test specimen geometry unless care is taken to ensure that mechanical fields (stress, strain, etc.) within the specimen adequately reflect the ideal homogeneous deformation state. In this work, finite element models of simple shear, cylindrical compression, simple tension, and bi-conical shear test specimens were analyzed in order to quantify the relative conformity of each specimen to its corresponding ideal. Three metrics of conformity were evaluated, based respectively on the distributions of stress, strain, and strain energy density. The results show that a simple shear specimen provides superior conformity. Other factors involved in the selection of test specimen geometry are also discussed. Such factors include relative linearity and symmetry of measured stress–strain data, grip slip, and heat build up.     

A New Shear-Compression-Specimen for Determining Quasistatic and Dynamic Polymer Properties

A new design of the shear compression specimen (SCS) for investigating the viscoelastic shear response of polymers is presented. The specimen consists of a polymer gage section with two metal ends that remain essentially rigid during deformation. Two closed-form analytic models are developed to predict the average stress and strain in the gage section from the deformation-load histories. This new SCS design and its analytic models are thoroughly evaluated via laboratory measurements and numerical simulations. These simulations show that the deformations in the gage section are more uniform than in the original design, and the distribution of the average shear stress and strain are highly homogenous. The simulation results yield good agreement with those of closed-form analytic results and the experiments demonstrate that the new SCS geometry and its analytic models are as reliable as other commonly employed specimens. It can also generate higher strain rates under usual loading conditions because of its smaller specimen gage length. The need for care in specimen preparation is also discussed in detail as illuminated by the experimental and simulation results.

Measurement of Dynamic Properties of Viscoelastic Materials

An improved method to measure the dynamic viscoelastic properties of elastomers is proposed. The method is based on the analysis of forced oscillation of a cylindrical sample loaded with an inertial mass. No special equipment or instrumentation other than the ordinary vibration measurement apparatus is required. Upper and lower surfaces of the viscoelastic material sample were bonded to a load disc and a rigid base plate, respectively. The rigid base plate was subject to forced oscillations driven by a vibration exciter. Two accelerometers were attached to monitor the displacement of the base plate and the load disc. The recorded magnitude ratio and the phase difference between the load disc and the base plate vibrations represent the axial, dynamic deformation of the sample. The data are sufficient to obtain the dynamic properties of the sample, oscillation properties of vibration exciter, whereas the sensitivity of gauges having no effect on the calculation results. For accurate calculation of the properties, a two-dimensional numerical model of cylindrical sample deformation was used. Therefore, a form factor, which takes into account the sample sizes in one-dimensional models, is not required in this method. Typical measurement of the viscoelastic properties of a silicone rubber Silastic® S2 were measured over the frequency range from 10 Hz to 3 kHz under deformations (ratio of vibration magnitude to sample thickness) from 10^?4% to 5%. It was shown that the modulus of elasticity and the loss tangent fall on a single curve when the ratio of load mass to sample mass changed from 1 to 20. When the sample diameter was varied from 8 to 40 mm, the modulus of elasticity fall on the same curve, but the loss tangent curves showed some degree of scatter. Studied temperature dependence and nonlinear behavior of viscoelastic properties is found not to be associated with this effect.     

A New Multiaxial Specimen for Determining the Dynamic Properties of Adhesive Joints

Adhesive joints are increasingly employed for bonding critical parts of industrial structures. Therefore, adhesive joints become a key element in design, and their mechanical characterization is of the utmost importance. Significant advancement has been realized for their characterization under quasi-static loadings; however characterization techniques are rather limited for dynamic loadings. Indeed, due to the complex paths of waves through structures, existing dynamic characterization techniques will not characterize only the adhesive joint, but instead will characterize the complete assembly containing the joint and the adherents. Moreover, multiaxiality control of the loading on the adhesive joint is difficult to achieve. This paper proposes an innovative experimental technique for the characterization of adhesive joints under dynamic multiaxial loadings. The experimental method relies on three main components: i) a conventional split Hopkinson pressure bar (SHPB) apparatus, ii) a novel specimen, denoted as DODECA, which enables testing of three distinct multiaxial loadings using the same method and iii) local strain and stress measurements performed by digital image correlation (DIC). The paper describes all steps of the experimental procedure, including the underlying preparation of the specimen and the measuring methods. The stress and strain in the adhesive joint are estimated directly from the experimental data both during loading and at the failure point. Finally, the dynamic material behavior of the adhesive joint is identified from the data.     

Extraction of Mechanical Properties with Second Harmonic Detection for Dynamic Nanoindentation Testing

In this article, a new method based on the detection of the second harmonic of the displacement signal to determine mechanical properties of materials from dynamic nanoindentation testing, is presented. With this technique, the Young’s modulus and hardness of homogeneous materials can be obtained at small penetration depths from the measurement of the second harmonic amplitude. With this innovative method, the measurement of the normal displacement is indirectly used, avoiding the need for very precise contact detection. Moreover, the influence of the tip defect and thermal drift on the measurements are reduced. This method was used for dynamic nanoindentation tests performed on fused silica and on an amorphous polymer (PMMA) because these materials are supposed not to exhibit an indentation size effect at small penetration depths. The amplitude of the second harmonic of the displacement signal was correctly measured at small depths, allowing to calculate the Young’s modulus and the hardness of the tested materials. The mechanical properties calculated with this method are in good agreement with values obtained from classical nanoindentation tests.