Specimen Failures Research Articles

Torsional retrofit of spandrel beams with composite laminates

This paper describes the experimental and analytical findings of a project focused on the structural strengthening of reinforced concrete (RC) spandrel beams using carbon fibre reinforced polymer (CFRP) composite laminates, and subjected to pure torsion. Many current torsional strengthening and repair methods are time and resource intensive and quite often very intrusive. The proposed method, however, uses composite laminates to increase the torsional capacity of concrete beams. In traditional cast-in-place construction, the slab adjacent to the spandrel beam does not allow a complete CFRP wrap around the beam. To simulate this condition, the CFRP laminate was provided only on three sides of the beam, and special anchors were used to allow a continuous torsional shear flow around the specimens. The experiments showed that this method increased the torsional strength of the spandrel beams by up to 77%. Excessive concrete cracking followed by composite delamination caused the specimen failures. An equation was developed that, for most of the specimens, accurately predicted the composite contribution. This analytical procedure was based on the existing information on the torsional resistance provided by closed stirrups, and on the equations developed to design the composite shear retrofit of RC beams.

Effect of dentine depth on the fracture toughness of dentine-composite adhesive interfaces

Objectives: The fracture toughness test was recently introduced as a clinically relevant method for assessing the fracture resistance of the dentine-composite interface. The objective of this study was to evaluate the effect of dentine depth on the interfacial fracture toughness test of several dentine-composite interfaces using some new proprietary dentine bonding agents. Methods: Miniature short rod fracture toughness specimens containing a chevron-shaped dentine-composite-bonded interface were prepared for each group ( n = 12). Six different dentine bonding agents and two dentine depths were the variables assessed at the dentine-composite interfaces. After 24 h at 37 °C in water, the specimens were tested by loading at 0.5 mm/min in the Instron Universal Testing Machine. The interfacial K IC results were analysed by ANOVA, unpaired Student's t-tests and Fisher's LSD test ( P < 0.05). Results: The interfacial K IC results in MN·m −3 2 (S.D.) on superficial and deep dentine, respectively, were: All-Bond 2, 0.80 (0.21), 0.44 (0.13); Bond-It, 0.75 (0.20), 0.38 (0.19); Prime and Bond, 0.56 (0.11), 0.28 (0.10); Scotchbond Multi-Purpose, 0.45 (0.23), 0.26 (0.15); One-Step and OptiBond, insufficient results due to premature specimen failures. Conclusions: The results from this study should contribute to the development of the fracture toughness test as a method for assessing the integrity of the dentine-composite interface. The interfacial fracture toughness test determined significant differences among the different dentine bonding agents and between the superficial and deep dentine substrates. The dentine bonding agents showed significantly reduced interfacial fracture toughness results when bonding to deep versus superficial dentine.

Comparison of the biaxial strength properties of braided and laminated carbon fiber composites

Abstract This paper presents the results of a series of biaxial tests to measure the strength of 2-D braided carbon/ epoxy specimens, and compares the measured strength with that of laminations constructed with similar materials and fiber orientations. Braided preforms are currently being considered as replacements for laminated structures, because of the potential of cost saving and possible increases in interlaminar strength. However, lower in-plane strengths would be anticipated relative to laminations, particularly in the braid direction, because of the undulation in the braid fiber paths. It was found that in the braid specimen failures controlled by the axial fibers, reasonably similar values of ultimate strength were measured for braids and laminates, with the braid specimens being somewhat lower because of irregularities in the axial fibers that were related to the various braid designs. However, the strength of the braid specimens in failures controlled by the braid direction fibers was much lower than in laminates, corresponding to as much as 60–70% strength loss relative to laminates.

Thermal Shock Fracture Testing for Float Glass by Infrared Radiation Technique

An infrared radiation heating method was proposed previously as a new technique for estimating thermal shock resistance of ceramics, and a concept of thermal shock fracture toughness and stress intensity factors for a disk-shaped specimen with an edge crack was expressed. The method uses a thin circular disk which is heated by infrared ray at the central area with a constant heat flux. The technique makes it possible to evaluate the thermal shock strength, R1c, and thermal shock fracture toughness, R2c, directly from the electric power charged and the time to fracture, despite the fact that R1c and R2c consist of the thermal properties of the material tested. In this report, the thermal shock strength and the thermal shock fracture toughness for float glass at room and high temperature are evaluated by the infrared radiation heating technique, and shown as a function of temperature when the specimen failures. The thermal shock parameters obtained experimentally are also compared with the estimated values which are calculated by combining the temperature-dependent mechanical properties of float glass.

Effects of Layer Waviness on the Compression Strength of Thermoplastic Composite Laminates

Layer waviness was investigated in T300/P1700 carbon/polysulfone com posite laminates under static compression loading. A three-step procedure was used to fab ricate isolated layer waves into the central 0° layer of [902/02/902/02/902/ /02w]s laminates. Efforts were made to insure that this three-step procedure itself did not degrade the lami nate quality. Layer wave geometries up to 1.5 layer thicknesses in amplitude and as short as nine layer thicknesses in length were tested. The more severe wave geometries pro duced reductions in static strength as high as 36 % , although the wavy 0° layer accounted for only 20% of the load carrying capacity of the laminate. Specimen failures were sudden and catastrophic. Brooming failure, characterized by through-the-thickness splaying of the layers and numerous delaminations near the waviness, was the common failure mode.

The effect of stress amplitude on the cause of fatigue crack initiation in a spring steel

Hour-glass-shaped specimens of a hardened spring steel were fatigue tested with fully reversed loading at five different stress amplitudes. The corresponding Weibull diagrams for failure probability were determined and the failure causes were established by fractographic studies. The failure curves in the Weibull diagrams showed a double linear appearance at the lower stress amplitudes and a linear appearance for the higher stress amplitudes. The low-life part of the broken Weibull curve at low stress amplitude was associated with crack-initiating inclusions sitting close to the surface of the specimens. The high-life end of the curve was caused by inclusions sitting in the specimen interior. At high stress amplitudes the specimen failures were caused by inclusions situated at the specimen surface. The inclusion size distribution of the steel was determined with three different techniques based on optical microscopy, scanning electron microscopy and a chemical dissolution technique. The three techniques gave results in fair agreement with one another for large inclusion sizes. Long crack growth data were established with three-point bending. Residual stresses on the specimen surfaces were measured with X-ray techniques. A model for the failure probability of specimens is presented. It considers the growth of cracks from spherical pores. The pores are used to model inclusions. Crack growth is considered from a crack length of one Burgers vector to final failure of the specimen. Crack growth curves for short and long cracks are designed so that the growth can be calculated. The short cracks are assumed to grow at a constant rate until the threshold for long cracks is reached and growth proceeds according to the Paris law. The constant crack growth rate for short cracks is found to be a function of the applied stress amplitudes and different for inclusions sitting at the specimen surface and in the specimen interior. The failure probability is calculated for distributions of pores corresponding to the experimentally determined size distributions of inclusions. Reasonable agreement is obtained between experimental and theoretical failure probabilities.

Microstructure and fatigue crack growth behaviour of a solid solution strengthened nickel base superalloy: Bartosiewicz, L., Krause, A.R., Spis, A., Raghavan, J. and Putatunda, S.K. Conf. ISTFA/91, Los Angeles, California, USA, Nov 1991 (ASM International, Materials Park, Ohio 44073, USA, 1991) 531–540

Hour-glass-shaped specimens of a hardened spring steel were fatigue tested with fully reversed loading at five different stress amplitudes. The corresponding Weibull diagrams for failure probability were determined and the failure causes were established by fractographic studies. The failure curves in the Weibull diagrams showed a double linear appearance at the lower stress amplitudes and a linear appearance for the higher stress amplitudes. The low-life part of the broken Weibull curve at low stress amplitude was associated with crack-initiating inclusions sitting close to the surface of the specimens. The high-life end of the curve was caused by inclusions sitting in the specimen interior. At high stress amplitudes the specimen failures were caused by inclusions situated at the specimen surface. The inclusion size distribution of the steel was determined with three different techniques based on optical microscopy, scanning electron microscopy and a chemical dissolution technique. The three techniques gave results in fair agreement with one another for large inclusion sizes. Long crack growth data were established with three-point bending. Residual stresses on the specimen surfaces were measured with X-ray techniques. A model for the failure probability of specimens is presented. It considers the growth of cracks from spherical pores. The pores are used to model inclusions. Crack growth is considered from a crack length of one Burgers vector to final failure of the specimen. Crack growth curves for short and long cracks are designed so that the growth can be calculated. The short cracks are assumed to grow at a constant rate until the threshold for long cracks is reached and growth proceeds according to the Paris law. The constant crack growth rate for short cracks is found to be a function of the applied stress amplitudes and different for inclusions sitting at the specimen surface and in the specimen interior. The failure probability is calculated for distributions of pores corresponding to the experimentally determined size distributions of inclusions. Reasonable agreement is obtained between experimental and theoretical failure probabilities.

Criteria for optimal shot-peening of passenger automobile main springs: Hutter, W. and Kloos, K.H.Blech Rohre Profile June 1990 37, (6), 406–409 (in German)

Hour-glass-shaped specimens of a hardened spring steel were fatigue tested with fully reversed loading at five different stress amplitudes. The corresponding Weibull diagrams for failure probability were determined and the failure causes were established by fractographic studies. The failure curves in the Weibull diagrams showed a double linear appearance at the lower stress amplitudes and a linear appearance for the higher stress amplitudes. The low-life part of the broken Weibull curve at low stress amplitude was associated with crack-initiating inclusions sitting close to the surface of the specimens. The high-life end of the curve was caused by inclusions sitting in the specimen interior. At high stress amplitudes the specimen failures were caused by inclusions situated at the specimen surface. The inclusion size distribution of the steel was determined with three different techniques based on optical microscopy, scanning electron microscopy and a chemical dissolution technique. The three techniques gave results in fair agreement with one another for large inclusion sizes. Long crack growth data were established with three-point bending. Residual stresses on the specimen surfaces were measured with X-ray techniques. A model for the failure probability of specimens is presented. It considers the growth of cracks from spherical pores. The pores are used to model inclusions. Crack growth is considered from a crack length of one Burgers vector to final failure of the specimen. Crack growth curves for short and long cracks are designed so that the growth can be calculated. The short cracks are assumed to grow at a constant rate until the threshold for long cracks is reached and growth proceeds according to the Paris law. The constant crack growth rate for short cracks is found to be a function of the applied stress amplitudes and different for inclusions sitting at the specimen surface and in the specimen interior. The failure probability is calculated for distributions of pores corresponding to the experimentally determined size distributions of inclusions. Reasonable agreement is obtained between experimental and theoretical failure probabilities.