Thermal and Mechanical Response of Inner Cone Sample of ZrB₂-SiC Ceramic under Arc-Jet Conditions

Under arc-jet test conditions, ZrB2-SiC ceramic will undergo high temperature oxidation and develop an external glassy layer (SiO2), zirconia sub-layer (ZrO2) and SiC-depleted diboride layer (ZrB2). This study relates to finite element modeling of the effects of oxidation on heat transfer and mechanical behavior of ZrB2-SiC ceramic under arc-jet test conditions. A steady-state heat transfer FE method was employed to conduct the heat transfer analysis to obtain the temperature distribution in the inner body of the cone. The surface thermal conditions available in the literature were used in the heat transfer analysis. The resulting temperature distribution in the inner body of the cone is then applied to the thermomechanical finite element analysis to calculate the thermal stress distribution. The results show that the oxide layers affect both thermal and mechanical response of the ZrB2-SiC ceramic under arc-jet high temperature test conditions. Due to the mismatch of material properties between the bulk ZrB2-SiC and its new products after oxidation, the outer oxide layers constrain the thermal deformation of the inner bulk ZrB2-SiC thereby putting it in compression and outside oxide layers in tension

Effect of Polymer-Surface Mobility on Adhesion in Poly(Methyl Methacrylate)-Tape System

The interaction between two polymer layers, especially adhesion between them, plays an important role in polymer processing and other applications. Detailed knowledge of the molecular structure and dynamics of polymer interfaces, and how they relate to macroscopic mechanical properties, should help designers construct more functional systems. Unfortunately, there have been few studies where both molecular and macroscopic studies have been performed on similar systems. In previous studies from our group, we have probed the dynamics of poly(methyl acrylate) (PMA) and thermal behavior of poly(methyl methacrylate) (PMMA) on silica. These studies helped us paint a picture for strongly bound molecules on silica, where a motional gradient perpendicular to the surface was observed. More mobile, lower Tg, material was found at the air-polymer interface, while less mobile, higher Tg material was found at the polymer-substrate interface. A previous study from our group showed that the glass-transition temperature of PMMA changes with the thickness of the polymer layer on silica. by examining the thermal behavior of the polymer with modulated differential scanning calorimetry (MDSC), it was observed that the glasstransition temperature increases as the thickness of the polymer layer decreases. Blum and Lin have also used the deuterium NMR to probe the dynamics of bulk and silica-absorbed poly(methyl acrylate)-d3 (PMA-d3). It was found that, an increase in the absorbed amount of polymer increased the mobility of polymer at the air-polymer interface and a decrease in the absorbed amount of polymer, decreased the polymer mobility. It is not known how this change in mobility relates to the mechanical properties of polymers, especially adhesion between two polymer layers. Of the various methods available for measuring adhesion-related properties, peeling is considered to be the most convenient. Current theories consider peeling to be the principal mode of separation of an adhesive from the substrate. It is proposed that adhesion strength is proportional to the surface free energy. Research has been done to find the adhesion strength between films and substrates, but there are some questions which remain unanswered like: Does adhesion strength depend on the thickness of the polymer layer? Does adhesion strength relate to the mobility of polymer segments on the surface? in the present report, we summarize some of our macroscopic studies, namely peel tests, on systems similar to those previously studied. We find that the mobility of the polymer chains at the interface play an important role in determining the peel strength between the two polymer layers. We decided to use a 90° peel angle method. At this high angle, the lateral stress in the film is very negligible, thereby reducing the lateral elongation of the film. In addition, at this angle, there is no slip at the interface. Our studies show that as the polymer film thickness decreases, the force required to initiate fracture between two polymer surfaces increases and the fracture energy also increases proportionately. We have also observed how fracture energy changes with a change in film thickness

Failure Investigation of the Steel Strut of Paseo Suspension Bridge

A vertical strut of the 1232-foot long, self-anchored Paseo Suspension Bridge fractured when the temperature hit at a record low of 9°F below zero. During inspection the following day, it was found that its lower pin was frozen and did not allow for free movement of the superstructure. The objective of this study is to pinpoint one of the four reasons for this incidence or their combination: overstressing, thermal contraction, fatigue, and reduction in fracture toughness at low temperatures. To achieve this objective, material property and fatigue testing was performed on samples of strut material while the bridge and strut were analyzed under service loading conditions. This study indicated that the strut material practically has an infinite life under normal conditions. The root cause of the failure is overstressing of the vertical strut due to a frozen pin that became frozen because the design of the bridge did not allow the pin to be maintained. The mechanically frozen pin condition was attributable to salt and sand accumulation in the strut housing. To prevent this to the new struts and other similar structures, it is recommended that both upper and lower pins be greased during special inspections and the lower housings be partially sealed to prevent salt and sand accumulation near the pins. Alternatively, a rotation monitoring system can be installed to remotely monitor the rotation of all four vertical struts and alert officials should the pins become mechanically frozen

Analysis of Damage in Laminated Automotive Glazing Subjected to Simulated Head Impact

During vehicle accidents, occupant\u27s head impacting on windshield or side window is commonly observed. Any attempt to design glazing that minimizes injury to and death of occupants during a vehicle accident requires a thorough understanding of the mechanical behavior of automotive glazing subjected to head impact loads. A continuum damage mechanics (CDM) based constitutive model is developed and implemented into an axisymmetric finite element model to study the failure and impact resistance of laminated automotive glazing subjected to simulated head impact. An anisotropic elastic damage tensor with a linear damage evolution law is chosen to model the failure of the laminated glass by cracking. The damage patterns and zone size are predicted. Various geometric parameters are investigated to determine their effects on the impact resistance of laminated glass

Signal Mast Arm Fatigue Failure Investigation

The Missouri Department of Transportation (MoDOT) has discovered and documented failures in several cantilever mast arms in the recent years. The failures were primarily by fatigue at the weld of the arm to the base plate attached to the mast. With over 6000 mast arms in service in Missouri, the failures raised concerns with the existing mast arm inventory and future mast arm design. This report presents findings from an effort to determine the cause of unexpected cracking in Missouri mast arms. Three causes of premature failure were investigated: the stress ranges experienced at the weld detail were higher than anticipated, the number of cycles experienced at the weld detail were larger than anticipated and/or the weld quality was less than expected. The results show that the main culprit for the premature fatigue failure of mast arms in Missouri can be attributed to poor weld quality. The new fatigue-resistant weld detail, without quality welding techniques, does not improve the situation. The loads and cycles of loads experienced by the mast arms are not necessarily critical if the weld is of high quality. Recommendations for possible solutions for existing in-service mast arms and for new mast arms are presented. The recommendations range from insuring weld quality to dampers on the mast arms

Effect of Interfacial Mobility on Flexural Strength and Fracture Toughness of Glass/Epoxy Laminates

Mechanical testing and surface fractography were used to characterize the fracture of E-glass fiber reinforced epoxy composites as a function of the silane coupling agent used. gamma-Aminopropyltriethoxysilane (APS) and delta-aminobutyltriethoxysilane (ABS) were used because these have been shown to have different interfacial mobilities at multilayer coverage. The values of the properties studied generally increased from untreated c, as determined from a Mode I translaminar fracture toughness tests, for the untreated composites (10.5 ± 0.4 kJ/m2) was lower than that for the ABS-treated composites (14.3 ± 2.1 kJ/m2) which was lower than that for the APS-treated composites (17.1 ± 2.4 kJ/m2). Macroscopic observations showed that a larger fiber debonding area was formed in the crack tip region for the untreated glass composites, suggesting poorer bonding compared to those treated with coupling agents. Since these silanes have similar chemistry, the differences were attributed to differences in the interfacial mobility of the coupling agent layers

Analysis of Damage in Laminated Architectural Glazing Subjected to Wind Loading and Windborne Debris Impact

Wind loading and windborne debris (missile) impact are the two primary mechanisms that result in window glazing damage during hurricanes. Wind-borne debris is categorized into two types: small hard missiles such as roof gravel, and large soft missiles representing lumber from wood-framed buildings. Laminated architectural glazing (LAG) may be used in buildings where impact resistance is needed. The glass plies in LAG undergo internal damage before total failure. The bulk of the published work on this topic either deals with the stress and dynamic analyses of undamaged LAG or the total failure of LAG. The pre-failure damage response of LAG due to the combination of wind loading and windborne debris impact is studied. A continuum damage mechanics (CDM) based constitutive model is developed and implemented via an axisymmetric finite element code to study the failure and damage behavior of laminated architectural glazing subjected to combined loading of wind and windborne debris impact. The effect of geometric and material properties on the damage pattern is studied parametrically

Fatigue Assessment of Traffic Signal Mast Arms based on Field Test Data under Natural Wind Gusts

In recent years, several states including Missouri, Wyoming, California, and Texas experienced fracture failures of traffic signal mast arms. Almost all the failures are associated with the propagation of defects or cracks. It is therefore imperative to evaluate existing mast arms using a simple yet accurate procedure. A statistical methodology is proposed to predict the fatigue life of signal mast arm structures on the basis of field-measured strain data. The annual occurrence of various stress levels is determined using the historical wind speed data in the vicinity of a mast arm structure and the strain readings of the structure under specific wind gusts. For each stress level, the crack initiation and propagation lives are estimated with the strain-life approach and the Paris crack-growth-rate model. They are combined to account for variable stresses by means of Miner\u27s rule and the root-mean-square model, respectively. The stress concentration factor around the arm-post connection is determined using a finite element model. The parameters in the life prediction models are determined with ASTM flat tension and compact tension tests. The proposed methodology was applied to a 12.8-m (42-ft) long octagonal mast arm and a 16.5-m (54-ft) long circular mast arm in Missouri. It is concluded that signal structures in perfect condition will not crack under natural wind gusts during their service life. However, the 16.5-m-long arm is likely to be vulnerable to tiny defects around the weld connection, but the 12.8-m-long arm is safe unless a visible crack exists

Forensic Investigation of Failed Mast Arms of Traffic Signal Supported Structures

In Missouri, 11 traffic signal mast arms fractured at the arm-post weld connection in 7 years. To reduce this fatigue failure, the Missouri Department of Transportation developed a fatigue-resistant weld profile that increases the weld leg and reduces the slope of the weld at the toe. This study investigated causes of the failed arms, compared performance of new and old weld profiles, and suggested retrofitting measures for further investigation. The scope included a metallurgical investigation of one failed field mast arm, laboratory fatigue testing of five prototype mast arms (two new and three old profiles), and laboratory failure analysis of one arm tested to cracking. Metallographic and fractographic analyses indicated that the fatigue crack in the failed mast arm initiates near the weld toe of the arm due to undercutting, creating a sharp local toe angle. Location of undercutting at the heat-affected zone of the base material, where the material is softest, further contributed to early fatigue failure. Tests showed that the new weld profile does not consistently increase fatigue strength. Premature fracture surfaces of one tested arm indicated that the fatigue cracks initiate in an area at the weld toe as observed in the failed mast arm. Therefore, changing the weld profile alone is unlikely to increase mast arm fatigue life. Pinning the weldment surface at the weld toe of mast arms is suggested to increase the life of mast arms