39 research outputs found
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Rigid Square Inclusion Embedded within an Epoxy Disk: Asympototic Stress Analysis
The asymptotically singular stress state found at the tip of a rigid, square inclusion embedded within a thin, linear elastic disk has been determined for both uniform cooling and an externally applied pressure. Since these loadings we symmetric, the singular stress field is characterized by a single stress intensity factor, and the applicable calibration relationship has been determined for both fully bonded and unbended inclusions. A lack of interfacial bonding has a profound effect on inclusion-tip stress fields. A large radial compressive stress is generated in front of the inclusion tip when the inclusion is well bonded, whereas a large tensile hoop stress is generated when the inclusion is unbended, and frictionless sliding is allowed. Consequently, an epoxy disk containing an unbended inclusion appears more likely to crack when cooled than a disk containing a fully bonded inclusion. Elastic-plastic calculations show that when the inclusion is unbended, encapsulant yielding has a significant effect on the inclusion-tip stress state. Yielding relieves stress parallel to the interface and greatly reduces the radial compressive stress in front of the inclusion. As a result, the encapsulant is subjected to a nearly uniaxial tensile stress at the inclusion tip. For a typical high-strength epoxy, the calculated yield zone is embedded within the region dominated by the elastic hoop stress singularity. A limited number of tests have been carried out to determine if encapsulant cracking can be induced by cooling a specimen fabricated by molding a square, steel insert within a thin, epoxy disk. Test results are in qualitative agreement with analysis. Cracks developed only in disks with mold-released inserts, and the tendency for cracking increased with inclusion size
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How geometric details can affect the strength of adhesive lap joints
The durability of adhesively bonded joints--when utilized as blade attachments--has a significant impact on the performance of wind turbines. Accordingly, there is interest in determining how geometric details affect the strength of these joints. Finite element analyses were performed to aid in the selection of three composite-to-metal joint geometries for compressive axial testing. Both monotonic and low-cycle fatigue tests were conducted. Analysis and testing of these joints provide insight into the effects of adding extra adhesive to the end of the bond or tapering the metal adherend. The issue of whether the relative performance of different joints in monotonic tests can be used to predict the relative fatigue strength of these joints is also addressed
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A phenomenological finite element model of part building in the stereolithography process
The finite element method has been used to develop the framework for a tool that can be used to model the structural deformation arising from the stereolithography build process. Such a tool when fully developed can facilitate numerical studies aimed at evaluating build parameters and build styles. Although the current software makes no attempt to capture all the physics of the process, provisions for three important build features have been made: (1) laser path history including scanning rate and depth of cure, (2) structural linkage, and (3) time varying material behavior. For demonstration purposes, a three dimensional finite element code was modified to include a phenomenological material model of solidification. The model was based on cure shrinkage and stress relaxation data collected from in-situ tests on individual strands drawn using 3D Systems` stereolithography apparatus (SLA-250). To depict the directed path of solidification within layers, a finite element birthing scheme was conceived to activate elements along the predetermined coordinate path of the laser. Structural linkage was enforced by joining element strands of layers when laser paths connect or overlap, respectively. A limited number of analyses have been performed to contrast simple build styles
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Mechanical properties of Hysol EA-9394 structural adhesive
Dextor`s Hysol EA-9394 is a room temperature curable paste adhesive representative of the adhesives used in wind turbine blade joints. A mechanical testing program has been performed to characterize this adhesive. Tension, compression stress relaxation, flexural, butt tensile, and fracture toughness test results are reported
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Epoxy and acrylate stereolithography resins: In-situ measurements of cure shrinkage and stress relaxation
Cross-sections of resin strands. Techniques were developed to make in situ measurements of gelled resin to determine linear shrinkage, stress-strain response and stress relaxation of single strands of SL 5170 epoxy and SL 5149 photocurable resins. Epoxy strands shrank approximately 1.4% and the acrylate strands about 1.0% after a single exposure. No forces were measured during cure shrinkage of strands following the first laser exposure. In multiple laser exposures, the acrylate continues to shrink; whereas (University of Dayton data) no additional shrinkage is observed in epoxy strands on a second hit. In force relaxation tests, a strand is drawn and then a 0.5% step strain is applied after different elapsed times. The epoxy initial modulus evolves (increases) with elapsed time following draw of the strand, and this evolution in modulus occurs after linear shrinkage has stopped. On the other hand, acrylates show no evolution of modulus with elapsed time following a single laser draw; i.e., once shrinkage stops after one laser hit, the initial modulus remains stable with elapsed time. Finally, relaxation response times of epoxy strands get larger with increasing elapsed time after laser draw. In acrylate strands there was no evolution in initial modulus with elapsed time after a single draw so relaxation times are not a function of elapsed time after a single hit with the laser
The Effect of Peel Stress on the Strength of Adhesively Bonded Joints
Composite wind turbine blades are often attached to a metallic structure with an adhesive bond. The objective of this investigation is to determine which parameters affect the durability of these adhesively bonded joints. The composite-to-steel joint considered in this study typically fails when the adhesive debonds from the steel adherend. Previously, this joint was monotonically loaded in either compression or tension. Compressive and tensile axial loads of the same magnitude produce adhesive stresses with very similar magnitudes but opposite signs. (For the joint considered, tensile loads produce compressive peeh stresses in the adhesive at the location where debonding initiates.) The tensile specimens failed at much higher loads, establishing that the sign of the adhesive peel stresses strongly influences the single-cycle strength of these joints. Building on this earlier work, this study demonstrates that the adhesive peel stresses are also critical for fatigue loading. The results of low-cycle (axial) and high- cycle (bending) fatigue tests are presented. To complement the test results, finite element analyses demonstrate the localized nature of the peel stresses that develop in the adhesive. In addition, these analyses are used to investigate some of the causes of these peel stresses
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Isothermal aging of three polyurethane elastomers
Two polyurethane systems, EN-7 and L-100, have a long history as encapsulants and coatings in Sandia programs. These materials contain significant amounts of toluene diisocyanate (TDI), a suspect human carcinogen. As part of efforts to reduce the use of hazardous materials in the workplace, PET-90A, a polyurethane with less than 0.1% free TDI, was identified as a candidate for new applications and as a replacement for the more hazardous polyurethanes in selected programs. This report documents the results of a two-year accelerated aging study of PET-90A, EN-7, and L-100 polyurethane elastomers to characterize the effect of 135{degrees}F isothermal aging on selected physical, electrical, mechanical and thermal properties. In general, there was very little change in properties over the two year period for the three elastomers. The largest changes occurred in EN-7, which is the polyurethane with the longest service history in Sandia applications
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Analysis and testing of adhesively bonded lap joints
Detailed studies of adhesively bonded tubular lap joints subjected to axial loads can be efficiently performed with two-dimensional (2D) finite element analyses. However, three-dimensional (3D) analyses are required to model the bending of tubes and the axial loading of many other shapes such as airfoils. Unfortunately, these 3D analyses require significantly more time and computer resources than 2D analyses. Thus, it is of interest to determine whether some aspects of 3D behavior can be captured with 2D analyses. A series of finite element analyses will show that the shear stress in the adhesive of a tubular or an elliptic lap joint -- due to a bending load -- can be reasonably estimated with a 2D analysis even though the behavior is 3D. After the agreement between 2D and 3D analyses is detailed, preliminary efforts to assess the importance of adhesive geometry at the end of the bond will be discussed. Experimental measurements of the mechanical properties of a structural adhesive used in joint tests will also be presented. Tension, compression, and stress relaxation data for a filled, amine-cured epoxy adhesive will be discussed
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In-situ property measurements on laser-drawn strands of SL 5170 epoxy and SL 5149 acrylate
Material behavior plays a significant role in the mechanics leading to internal stresses and, potentially, to distortion (curling) of parts as they are built by stereolithography processes that utilize photocuring resins. A study is underway to generate material properties that can be used to develop phenomenological material models of epoxy and acrylate resins. Strand tests are performed in situ in a 3D System`s SLA-250 machine; strands are drawn by either single or multiple exposures of the resin to a laser beam. Linear shrinkage, cross-sectional areas, cure shrinkage forces and stress-strain data are presented. Also, the curl in cantilever beam specimens, built with different draw patterns, are compared