More Like this

1. Quasistatic strain fields in normally- and tangentially-loaded elastomeric rollers under impending slip

   https://doi.org/10.1016/j.ijsolstr.2024.112739  

   Mork, Nehemiah; Rajchel, Milosz K; Varenberg, Michael; Antoniou, Antonia; Leamy, Michael J (April 2024, International Journal of Solids and Structures)

   Elastomeric rollers are important components in applications such as printing and roll-to-roll manufacturing. To gain insight into roller mechanics and provide a basis for further investigations into dynamic rolling problems where rolling instabilities and rolling friction arise, we employ a specially designed apparatus to obtain displacement and strain fields via digital image correlation (DIC) under applied loads.We test loading scenarios leading to impending slip of an elastomeric roller, mounted on a steel hub, and in contact with a glass (rigid) substrate. We first examine strain fields under normal loading and compare them with the closest analytical predictions. We also analyze the strain fields under normal and tangential loading for which limited analytical predictions exist. For each loading scenario, we discuss the displacement and strain fields of the roller sidewall and contact interface. We implement a conceptual string model to demonstrate how stick and slip zones develop within the contact area as well as how memory effects arise during cyclic loading. This memory effect is then verified experimentally using the DIC strain fields. Additionally, we demonstrate a means for identifying the stick zone area between the roller and substrate using the experimentally-obtained displacement fields. We believe the apparatus, and the ability to obtain experimental displacement and strain fields, will prove valuable in understanding roller mechanics and associated instabilities. 

   more » « less
2. Phase field theory for fracture at large strains including surface stresses

   Jafarzadeh, Hossein; Farrahi, Gholam Hossein; Levitas, Valery I.; Javanbakht, Mahdi (November 2020, ArXivorg)

   null (Ed.)

   Phase field theory for fracture is developed at large strains with an emphasis on a correct introduction of surface stresses. This is achieved by multiplying the cohesion and gradient energies by the local ratio of the crack surface areas in the deformed and undeformed configurations and with the gradient energy in terms of the gradient of the order parameter in the reference configuration. This results in an expression for the surface stresses which is consistent with the sharp surface approach. Namely, the structural part of the Cauchy surface stress represents an isotropic biaxial tension, with the magnitude of a force per unit length equal to the surface energy. The surface stresses are a result of the geometric nonlinearities, even when strains are infinitesimal. They make multiple contributions to the Ginzburg-Landau equation for damage evolution, both in the deformed and undeformed configurations. Important connections between material parameters are obtained using an analytical solution for two separating surfaces, as well as an analysis of the stress-strain curves for homogeneous tension for different degradation and interpolation functions. A complete system of equations is presented in the undeformed and deformed configurations. All the phase field parameters are obtained utilizing the existing first principle simulations for the uniaxial tension of Si crystal in the [100] and [111] directions. 

   more » « less
3. Measuring vertical displacement using laser lines and cameras

   https://doi.org/10.1680/jphmg.21.00038  

   Sinha, Sumeet Kumar; Kutter, Bruce Lloyd; Ziotopoulou, Katerina (November 2021, International Journal of Physical Modelling in Geotechnics)

   Measuring displacements in model tests typically involves contact-based sensors such as linear potentiometers, where contact between two moving parts occurs at the sensing point. The sensor's finite mass, the limited stiffness of the beams and the clamping mechanism, and the slippage and hinging of the sensor body could affect the object's response and lead to measurement errors. Also, the physical mounting rack required to hold these sensors often obstructs the view and makes significant areas unavailable for conducting some other essential investigations. The advancement in high-speed, high-resolution and reasonably priced rugged cameras makes it feasible to obtain better displacement measurements by image analysis. This paper introduces a non-contact method that works by video recording the projection of laser lines on a test object to measure static and dynamic vertical displacements. The technique produces a continuous settlement distribution along the laser line passing through multiple objects of interest. This paper presents the theory for converting laser line images to displacements. The new method's validity is demonstrated by comparing the results from other measurement techniques: hand measurements, linear potentiometers and three-dimensional stereophotogrammetry. 

   more » « less
4. Application of Digital Image Correlation (DIC) to the Measurement of Strain Concentration of a PVA Dual-Crosslink Hydrogel Under Large Deformation

   https://doi.org/10.1007/s11340-019-00520-4  

   Liu, M.; Guo, J.; Hui, C.-Y.; Zehnder, A. T. (January 2019, Experimental Mechanics)

   Hydrogels are a class of soft, highly deformable materials formed by swelling a network of polymer chains in water. With mechanical properties that mimic biological materials, hydrogels are often proposed for load bearing biomedical or other applications in which their deformation and failure properties will be important. To study the failure of such materials a means for the measurement of deformation fields beyond simple uniaxial tension tests is required. As a non-contact, full-field deformation measurement method, Digital Image Correlation (DIC) is a good candidate for such studies. The application of DIC to hydrogels is studied here with the goal of establishing the accuracy of DIC when applied to hydrogels in the presence of large strains and large strain gradients. Experimental details such as how to form a durable speckle pattern on a material that is 90% water are discussed. DIC is used to measure the strain field in tension loaded samples containing a central hole, a circular edge notch and a sharp crack. Using a nonlinear, large deformation constitutive model, these experiments are modeled using the finite element method (FEM). Excellent agreement between FEM and DIC results for all three geometries shows that the DIC measurements are accurate up to strains of over 10, even in the presence of very high strain gradients near a crack tip. The method is then applied to verify a theoretical prediction that the deformation field in a cracked sample under relaxation loading, i.e. constant applied boundary displacement, is stationary in time even as the stress relaxes by a factor of three. 

   more » « less
5. Quantification of Errors in Applying DIC to Fiber Networks Imaged by Confocal Microscopy

   https://doi.org/10.1007/s11340-022-00870-6  

   Sarkar, M.; Notbohm, J. (January 2022, Experimental Mechanics)

   Background. An assumption of Digital Image Correlation (DIC) is that the displacement field within each subset is relatively smooth, captured with reasonable accuracy by, for example, linear or quadratic shape functions. Although this assumption works well for many materials, it becomes problematic for heterogeneous materials, such as fiber networks, wherein the length scale of heterogeneity matches the size of a subset. Objective. Here we applied DIC to fibrous networks made of collagen, for which displacements at the scale of a subset are highly heterogeneous, but errors caused by the heterogeneity are difficult to quantify. We developed a method to quantify such errors. Methods. We began by generating a synthetic three-dimensional fiber network with structure matching that of gels made of fibrous collagen. We then formulated an algorithm to mimic the way in which a confocal microscope images the fibers at its focal plane, thereby generating synthetic images similar to those obtained in experiments. Displacement boundary conditions were applied to the synthetic fiber networks, and the resulting displacement fields were computed using a finite element solver. DIC was applied to the synthetic images, and displacements were compared to the data from the finite element method, enabling rigorous quantification of error. Results. Point-wise errors in the DIC-measured displacements were substantial, often exceeding 40%, but over regions far larger than the length scales of heterogeneity or the DIC subset size, errors were modest, e.g., ≤15%. Conclusions. Although DIC can accurately measure displacements of fiber networks at length scales larger than the subset window, quantification of mechanical behavior at the scale of material heterogeneity will require new methods to complement or replace the use of DIC. 

   more » « less