Effects of Constituents and Lay-up Configuration on Drop-Weight Tests of Fiber-Metal Laminates

Impact responses and damage of various fiber-metal laminates were studied using a drop-weight instrument with the post-impact damage characteristics being evaluated through ultrasonic and mechanical sectioning techniques. The first severe failure induced by the low-velocity drop-weight impact occurred as delamination between the aluminum and fiber-epoxy layers at the non-impact side. It was followed by a visible shear crack in the outer aluminum layer on the non-impact face. Through-thickness shear cracks in the aluminum sheets and severe damage in the fiber laminated layers (including delamination between adjacent fiber-epoxy laminae with different fiber orientations) developed under higher energy impacts. The impact properties of fiber-metal laminates varied with different constituent materials and fiber orientations. Since it was punched through easily, the aramid-fiber reinforced fiber-metal laminates (ARALL) offered poorer impact resistance than the glass-fiber reinforced fiber-metal laminates (GLARE). Tougher and more ductile aluminum alloys improved the impact resistance. GLARE made of cross-ply prepregs provided better impact resistance than GLARE with unidirectional plies.     

Effect of repeated impacts on the response of plain-woven hybrid composites

The repeated low-velocity impact responses of hybrid plain-woven composite panels were studied by drop-weight experiments. Non-hybrid S2-glass-fiber/toughened epoxy and IM7 graphite fiber/toughened epoxy as well as hybrid S2-glass–IM7 graphite fiber/toughened epoxy composite panels were impacted repeatedly using a pressure-assisted Instron-Dynatup 8520 instrumented drop-weight impact tester. During the low-velocity impact tests, the time histories of impact forces, absorbed impact energies and panel central deflections were recorded. The relations between the impact force and central deflection, whose slope represented the dynamic contact stiffness, were then constructed. The damaged specimens were inspected visually and using the ultrasonic C-Scan method. The effects of hybridization and lay-up sequence on the repeated drop-weight impact responses of woven composites were investigated. It was observed that damage accumulations could be slowed down using hybridization. It was also witnessed that the lay-up configuration of a hybrid composite had a significant influence on damage accumulation rate. The hybrid specimens with glass–epoxy skins survived the double number of successive impacts compared to hybrid specimens with graphite–epoxy skins.     

Dilute solution properties of poly(3-dimethyl acryloyloxyethyl ammonium propiolactone)

The aqueous solution properties of an ampholytic poly(3-dimethylacryloyloxyethyl ammonium propiolactone), poly(DMAEAPL), are examined in this study by measuring reduced viscosity, intrinsic viscosity, degree of binding and dynamic light scattering. This polyampholyte's intrinsic viscosity is related to the type and concentration of the salt added. The intrinsic viscosity behaviour for the polyampholyte resulting from the associations of the polymer chains is in contrast with cationic and anionic polyelectrolyte. The polyampholyte in high concentration of NaCl has a low degree of binding, indicating that the proton ion (H⁺) has difficulty in binding to the carboxylate group (COO⁻) at the polymer end. The carboxybetaine, DMAEAPL, has a higher degree of binding than the corresponding sulfobetaine, DMAPS. Dynamic light scattering measurements indicate that the poly(carboxybetaine) diffusion coefficients decrease and the chain dimensions increase with an increasing salt concentration. The models proposed in this study can account for the poly(DMAEAPL) solution viscometrics and the degree of binding. © 1997 Elsevier Science Ltd.     

On the DSP-based switch-mode rectifier with robust varying-band hysteresis PWM scheme

This paper presents the development of digital signal processor (DSP) based switch-mode rectifier (SMR) with robust varying-band hysteresis current-controlled (HCC) pulse width modulated (PWM) schemes. First, a robust fixed-frequency HCC PWM control scheme is developed. Through robust harmonic spectrum shaping, the constant frequency control performance is insensitive to the changes of system parameters and operating condition, and the low frequency harmonics possess smaller magnitudes. Next, the robust HCC PWM schemes of SMR with linearly and randomly varying hysteresis bands are proposed. The harmonic spectra of the SMR are shaped to be uniformly distributed. As to the output voltage regulation control, the dynamic plant model is first estimated. Then accordingly, the controller is quantitatively designed to meet the prescribed control requirements. Significant power quality improvement in line drawn current for the SMR is achieved by using the developed HCC PWM schemes.     

Autologous tissue in initial type I and type III tympanoplasty operations in chronic suppurative otitis media

We reviewed 192 patients who had been treated for chronic otitis media. All operations were carried out at the Department of Otolaryngology, University of Tübingen. No patient had a previous ear operation and revision operations were excluded. The observation period varied from a minimum of 1 year to a maximum of 4 1/2 years. The ossicular chain was partially destroyed in 43 ears (22.4%). Ossiculoplasty was performed using autologous ossicles. An inens was used in 31 ears and a malleus in 12 cases. The overall failure rate was 20.8%. Most recurrent perforations were found in middle-aged patients. Children had only a 14% incidence of recurrent perforations. Fascia was the material associated with the highest failure rate (28.6%). After using perichondrium 6.8% of the cases had recurrent perforations, whereas the perichondrium-cartilage transplant was not successful in 4.8%. Overall, 62.7% of the patients were found to have an air-bone gap of 10 dB or less at 1.5 kHz 3-6 months after surgery. An air-bone gap of 20 dB or less was found in 91.2% of the patients. The main problem seen in the patients with chronic otitis media was not reconstruction of the ossicular chain but a lasting closure of the tympanic membrane. The perichondrium-cartilage transplant permitted the best results and is now recommended especially for patients with unfavorable middle ear conditions. A long-lasting closure of the tympanic membrane was also found in children (< or = 15 years of age). For this reason we also recommend an early operation in order to allow children to lead a normal life.     

Selective effects of methylphenidate in attention deficit hyperactivity disorder: a functional magnetic resonance study

Functional MRI revealed differences between children with Attention Deficit Hyperactivity Disorder (ADHD) and healthy controls in their frontal-striatal function and its modulation by methylphenidate during response inhibition. Children performed two go/no-go tasks with and without drug. ADHD children had impaired inhibitory control on both tasks. Off-drug frontal-striatal activation during response inhibition differed between ADHD and healthy children: ADHD children had greater frontal activation on one task and reduced striatal activation on the other task. Drug effects differed between ADHD and healthy children: The drug improved response inhibition in both groups on one task and only in ADHD children on the other task. The drug modulated brain activation during response inhibition on only one task: It increased frontal activation to an equal extent in both groups. In contrast, it increased striatal activation in ADHD children but reduced it in healthy children. These results suggest that ADHD is characterized by atypical frontal-striatal function and that methylphenidate affects striatal activation differently in ADHD than in healthy children.