Experimental Mechanics

There is a lack of reliable methods to obtain valid measurements of the tensile response of high performance materials such as fibre composites, ceramics and textile products at high rates of strain. We propose and assess two new test techniques aimed at measuring valid tensile stress versus strain curves at high and ultra-high strain rates. We conduct detailed, non-linear explicit Finite Element (FE) simulations of the transient response of the test apparatus and specimen during the tests and we develop simple analytical models to interpret the test measurements. We consider two test techniques: one based on the split Hopkinson bar apparatus, and suitable for strain rates of up to 1000 /s, and a second technique relying on projectile impact and aimed at measurements at strain rates higher than 1000 /s. The simulations are successfully validated using test data at strain rates of order 200 /s and then used to predict the test performance at strain rates up to approximately 5500 /s. We find that both techniques can give valid stress versus strain curves across a wide range of strain rates. We identify the limits of both techniques and recommend optimal measurement strategies for dynamic testing of materials with different ductility.

A novel method is presented for measuring the height profile of the surface of an object, even in the presence of relative motion between the surface and the height sensors. The method involves making height measurements of the surface using several sensors arranged along the scanned surface. The surface profile and the relative motions are mathematically separated by observing that the surface features appear sequentially among the sensor data, while the relative motions appear simultaneously. The proposed linear inverse technique overcomes several limitations of existing profiling methods based on curvature measurements. In contrast with other inverse calculations, the results are quite stable, with noise in the results only about twice the measurement noise. Regularization is introduced as a means of smoothing noise and for achieving solutions for ill-posed cases. This paper focuses on the profile calculation of one-sided objects, and initially uses the assumption that the rigid relative motion between object and sensors is purely translational.

Raman spectroscopy has become an effective experimental stress analysis method because of its high spatial resolution, nondestructive and noncontact, etc. However, the stress confidence of single point and single time detection is far from accurate. Especially for the component decoupling analysis of complex stress states, the characterization results of the stress components often have difficulty reflecting the real stress states. In this paper, based on stress characterization using polarized micro-Raman spectroscopy, we focus on the two main factors that affect the confidence of the Raman spectrum, namely, the frequency shift repeatability of the instrument and the polarization control accuracy. Through a combination of physical experiments and numerical ones, the influence of random error in the Raman measurement system, initial error and random error in the polarization angle on the decoupling characterization of the stress component is quantitatively analyzed. The results show that due to the existence of various error factors and the influence of polarization angle combinations, the error levels for the characterization results for each stress component are not only difficult to ignore but also the difference is obvious, and the true stress state of the sample surface cannot be accurately analyzed. In particular, the influence of the random error in the polarization angle on the characterization results obtained for the stress component is dominant. Although various experimental error sources are difficult to eliminate, the influence of the above factors on the characterization of stress components can be effectively weakened by improving the experimental design and analysis method.

Tỷ lệ gãy của các stent Nitinol trong động mạch đùi nông (SFA) cao hơn mức mong muốn. Việc phát triển các stent chịu lực gãy tốt hơn đòi hỏi một sự hiểu biết tốt hơn về tải trọng trong cơ thể, cách mà các stent biến dạng dưới các tải trọng này và ảnh hưởng của động mạch lên biến dạng của stent. Trong công trình báo cáo ở đây, các thiết bị thử nghiệm đã được thiết kế và chế tạo để đo tải trọng trên các stent chịu biến dạng kéo, uốn và xoắn. Các bài thử nghiệm để đo độ cứng cơ học đã được thực hiện trên các stent, động mạch giả và các stent được đặt trong động mạch giả. Sự tương tác đáng kể giữa stent và động mạch đã được quan sát thấy dưới tải trọng kéo và uốn; ít tương tác xảy ra dưới tải trọng xoắn. Các kết quả này được giải thích bởi sự thay đổi trong hình học của stent dưới tải trọng. Thông tin về biến dạng, tải trọng và sự tương tác giữa stent và động mạch sẽ hữu ích trong việc xác thực các mã phần hữu hạn để thiết kế và phân tích các stent.

Incremental and total-strain theories have recently been used by the authors to obtain numerical solutions for the hydrostatic bulging of circular diaphragms. These solutions have incorporated the effects of anisotropy in the direction of the thickness of the sheet. This investigation was conducted to obtain test data on aluminum-killed steel sheets which contain the above mentioned anisotropy and to compare these data with the predictions of the incremental and total-strain theories solutions. The results show that the incremental theory is in better agreement with test data than the total-strain theory. Strain-path data for several volume elements shows that each volume element follows an approximately proportional strain path. However, the strain paths obtained from the total-strain theory are in gross disagreement with the data.

In-plane surface displacements, when measured with 2D Digital Image Correlation (2D-DIC), are very sensitive to out-of-plane displacement components. Any out-of-plane motion of the surface can pollute the measured field by introducing artificial displacements. These displacements are difficult to separate from the underlying response of the surface and thereby limit the application of 2D-DIC in inverse problems where the test specimen has significant motion in the out-of-plane direction. In the context of inverse problems, we propose to partially relax this condition of no out-of-plane motion in 2D-DIC. With this approach, only the out-of-plane rigid-body motion of the specimen surface, which is initially in-plane, needs to be avoided while the requirement of surface deformations to be primarily in-plane is essentially waived. Compensation, based on the pinhole camera model, for out-of-plane displacements of the surface in response to applied load is included within the error function of the minimization problem. The improvements in material parameter estimation, obtained by using the proposed compensation strategy, are demonstrated by an example. The proposed technique makes it possible to utilize 2D-DIC with a simple conventional lens for an increased number of inverse problems; and in the process avoiding the computational and experimental difficulties associated with 3D measurement methods as well as the high cost and magnification limitations of a telecentric lens.

The evolution of anisotropy has an important influence on the forming of parts under large deformation. However, most of the current yield criteria do not consider the evolution. An anisotropic constitutive model based on non-associated flow rule (non-AFR) was established for orthotropic sheet metal. The classical quadratic Hill48 model was used to describe the yield anisotropy and plastic deformation anisotropy, respectively. According to the principle of equivalent plastic work, the existence and significance of anisotropy evolution with plastic deformation were revealed. In order to improve the prediction accuracy of the model, a continuous capture scheme considering anisotropic hardening was proposed. The evolution of directional yield stress, directional r-value and yield locus was well captured by the developed model. To further verify the model, square box deep drawing tests of different strokes of the punch were carried out. Compared with the experimental results, the developed model could predict the material flow behavior in flange area and thickness thinning behavior, which actually reflected the evolution behavior of directional flow stress and directional r-value of sheet metal respectively. The developed model improves the prediction accuracy of anisotropic sheet metal forming, and can provide an effective reference scheme for large deformation problems in industrial production.

The hole-drilling strain-gage method of measuring residual stresses in elastic materials can be termed semidestructive if holes of very small diameters are used. The method permits the magnitudes and principal directions of residual stresses at the hole location to be determined. This is accomplished by means of an emirpically determined relation between the magnitudes and directions of the principal stresses and the strain relaxation about the hole as the hole is drilled. This relation was obtained for a nondimensional model of the hole-gage assembly in order to make the results independent of hole size. A generalization was postulated to extend the use of this calibrated solution to the measurement of residual stresses in all elastic, isotropic materials.

It has been shown that theΣ δ p method can detect impending failure of structures under bending if loads are applied slowly and under constant temperature conditions. This method has been extended analytically to encompass dynamic test loadings with varying temperatures. Experimental verification for dynamic loads is needed.