A mixed-mode bending apparatus for delamination testing

A mixed-mode delamination test procedure was developed combining double cantilever beam mode I loading and end notch flexure mode II loading on a split unidirectional laminate. By loading the specimen with a lever, a single applied load simultaneously produces mode I and II bending loads on the specimen. This mixed mode bending (MMB) test was analyzed using both finite element procedures and beam theory to calculate the mode I and II components of strain energy release rate, G sub I and G sub II, respectively. The analyses showed that a wide range of G sub I/G sub II ratios could be produced by varying the applied load position on the loading lever. As the delamination extended, the G sub I/G sub II ratios varied by less than 5 percent. The simple beam theory equations were modified to account for the elastic interaction between the two arms of the specimen and to account for shear deformations. The resulting equations agreed closely with the finite element results and provide a basis for selection of G sub I/G sub II test ratios and a basis for computing the mode I and II components of measured delamination toughness. The MMB specimen analysis and test procedures were demonstrated using unidirectional laminates

Ganzflicker Reveals the Complex Relationship Between Visual Mental Imagery and Pseudo-Hallucinatory Experiences: A Replication and Expansion

Rhythmic visual flicker is known to elicit pseudo-hallucinations, making it an up-and-coming method to investigate anomalous perceptual experiences without pharmaceutical intervention. Ganzflicker is a full-screen visual flicker that can be experienced online. In the first exploratory Ganzflicker paper (N = 204), we investigated whether people with different self-reported visual mental imagery abilities report different visual experiences in the Ganzflicker. Results showed that people with no-to-low imagery (aphantasia distribution) were much less likely to experience complex and vivid pseudo-hallucinations compared to people with moderate-to-vivid imagery (imagery distribution). In this follow-up, I collected data from 6664 individuals from around the world, replicated the main results of the previous study, and additionally found that people from the imagery distribution report more frequent pseudo-hallucinations for a longer duration than people from the aphantasia distribution. I also conducted new analyses across individual imagery vividness ratings. This revealed a dramatic increase in susceptibility to pseudo-hallucinations from reports of “no imagery” to “low imagery vividness” within the aphantasia distribution. There is a positive linear relationship between imagery vividness and pseudo-hallucination vividness, whereas the relationship between imagery vividness and pseudo-hallucination complexity is categorical, as indicated by a jump in the likelihood to experience complex pseudo-hallucinations from the aphantasia distribution to the imagery distribution with no evidence for within-distribution variations. Finally, word cloud analyses of written descriptions of Ganzflicker experiences revealed unique language used by individuals from each distribution. In sum, Ganzflicker is an accessible, efficient, and effective method of investigating multiple aspects of anomalous perceptual experiences in people with different mental imagery abilities.</jats:p

Photos by J. C. Allen & Son

At the 1974 AAACE meeting in Indiana, 93-year-old John Allen showed up and took a bow

The Inclusion of Arbitrary Load Histories in the Strength Decay Model for Stress Rupture

Stress rupture is a failure mechanism where failures can occur after a period of time, even though the material has seen no increase in load. Carbon/epoxy composite materials have demonstrated the stress rupture failure mechanism. In a previous work, a model was proposed for stress rupture of composite overwrap pressure vessels (COPVs) and similar composite structures based on strength degradation. However, the original model was limited to constant load periods (holds) at constant load. The model was expanded in this paper to address arbitrary loading histories and specifically the inclusions of ramp loadings up to holds and back down. The broadening of the model allows for failures on loading to be treated as any other failure that may occur during testing instead of having to be treated as a special case. The inclusion of ramps can also influence the length of the "safe period" following proof loading that was previously predicted by the model. No stress rupture failures are predicted in a safe period because time is required for strength to decay from above the proof level to the lower level of loading. Although the model can predict failures during the ramp periods, no closed-form solution for the failure times could be derived. Therefore, two suggested solution techniques were proposed. Finally, the model was used to design an experiment that could detect the difference between the strength decay model and a commonly used model for stress rupture. Although these types of models are necessary to help guide experiments for stress rupture, only experimental evidence will determine how well the model may predict actual material response. If the model can be shown to be accurate, current proof loading requirements may result in predicted safe periods as long as 10(13) years. COPVs design requirements for stress rupture may then be relaxed, allowing more efficient designs, while still maintaining an acceptable level of safety

Composite Stress Rupture: A New Reliability Model Based on Strength Decay

A model is proposed to estimate reliability for stress rupture of composite overwrap pressure vessels (COPVs) and similar composite structures. This new reliability model is generated by assuming a strength degradation (or decay) over time. The model suggests that most of the strength decay occurs late in life. The strength decay model will be shown to predict a response similar to that predicted by a traditional reliability model for stress rupture based on tests at a single stress level. In addition, the model predicts that even though there is strength decay due to proof loading, a significant overall increase in reliability is gained by eliminating any weak vessels, which would fail early. The model predicts that there should be significant periods of safe life following proof loading, because time is required for the strength to decay from the proof stress level to the subsequent loading level. Suggestions for testing the strength decay reliability model have been made. If the strength decay reliability model predictions are shown through testing to be accurate, COPVs may be designed to carry a higher level of stress than is currently allowed, which will enable the production of lighter structure

A Critique of a Phenomenological Fiber Breakage Model for Stress Rupture of Composite Materials

Stress rupture is not a critical failure mode for most composite structures, but there are a few applications where it can be critical. One application where stress rupture can be a critical design issue is in Composite Overwrapped Pressure Vessels (COPV's), where the composite material is highly and uniformly loaded for long periods of time and where very high reliability is required. COPV's are normally required to be proof loaded before being put into service to insure strength, but it is feared that the proof load may cause damage that reduces the stress rupture reliability. Recently, a fiber breakage model was proposed specifically to estimate a reduced reliability due to proof loading. The fiber breakage model attempts to model physics believed to occur at the microscopic scale, but validation of the model has not occurred. In this paper, the fiber breakage model is re-derived while highlighting assumptions that were made during the derivation. Some of the assumptions are examined to assess their effect on the final predicted reliability

3-D Mixed Mode Delamination Fracture Criteria - An Experimentalist's Perspective

Many delamination failure criteria based on fracture toughness have been suggested over the past few decades, but most only covered the region containing mode I and mode II components of loading because that is where toughness data existed. With new analysis tools, more 3D analyses are being conducted that capture a mode III component of loading. This has increased the need for a fracture criterion that incorporates mode III loading. The introduction of a pure mode III fracture toughness test has also produced data on which to base a full 3D fracture criterion. In this paper, a new framework for visualizing 3D fracture criteria is introduced. The common 2D power law fracture criterion was evaluated to produce unexpected predictions with the introduction of mode III and did not perform well in the critical high mode I region. Another 2D criterion that has been shown to model a wide range of materials well was used as the basis for a new 3D criterion. The new criterion is based on assumptions that the relationship between mode I and mode III toughness is similar to the relation between mode I and mode II and that a linear interpolation can be used between mode II and mode III. Until mixed-mode data exists with a mode III component of loading, 3D fracture criteria cannot be properly evaluated, but these assumptions seem reasonable

the contents of the search template for category level search in natural scenes

Visual search involves the matching of visual input to a ''search template,'' an internal representation of taskrelevant information. The present study investigated the contents of the search template during visual search for object categories in natural scenes, for which low-level features do not reliably distinguish targets from nontargets. Subjects were cued to detect people or cars in diverse photographs of real-world scenes. On a subset of trials, the cue was followed by task-irrelevant stimuli instead of scenes, directly followed by a dot that subjects were instructed to detect. We hypothesized that stimuli that matched the active search template would capture attention, resulting in faster detection of the dot when presented at the location of a template-matching stimulus. Results revealed that silhouettes of cars and people captured attention irrespective of their orientation (08, 908, or 1808). Interestingly, strong capture was observed for silhouettes of categorydiagnostic object parts, such as the wheel of a car. Finally, attentional capture was also observed for silhouettes presented at locations that were irrelevant to the search task. Together, these results indicate that search for familiar object categories in real-world scenes is mediated by spatially global search templates that consist of view-invariant shape representations of category-diagnostic object parts

A bilinear failure criterion for mixed-mode delamination

Many different failure criteria have been suggested for mixed-mode delamination toughness, but few sets of mixed-mode data exist that are consistent over the full range of Mode 1 opening load to Mode 2 shear load range. The mixed-mode bending (MMB) test was used to measure the delamination toughness of a brittle epoxy composite, a state-of-the-art toughened epoxy composite, and a tough thermoplastic composite over the full mixed-mode range. To gain insight into the different failure responses of the different materials, the delamination fracture surfaces were also examined. An evaluation of several failure criteria that have been reported in the literature was performed, and the range of responses modeled by each criterion was analyzed. A bilinear failure criterion was introduced based on a change in the failure mechanism observed from the delamination surfaces. The different criteria were compared to the failure response of the three materials tested. The responses of the two epoxies were best modeled with the new bilinear failure criterion. The failure response of the tough thermoplastic composite could be modeled well with the bilinear criterion but could also be modeled with the more simple linear failure criterion. Since the materials differed in their mixed-mode failure response, mixed-mode delamination testing will be needed to characterize a composite material. This paper presents consistent sets of mixed-mode data, provides a critical evaluation of the mixed-mode failure criteria, and should provide general guidance for selecting an appropriate criterion for other materials

Comparison of the compressive strengths for stitched and toughened composite systems

The compression strength of a stitched and a toughened matrix graphite/epoxy composite was determined and compared to a baseline unstitched untoughened composite. Two different layups with a variety of test lengths were tested under both ambient and hot/wet conditions. No significant difference in strength was seen for the different materials when the gage lengths of the specimens were long enough to lead to a buckling failure. For shorter specimens, a 30 percent reduction in strength from the baseline was seen due to stitching for both a 48-ply quasi-isotropic and a (0/45/0/-45/90/-45/0/45/0)s laminate. Analysis of the results suggested that the decrease in strength was due to increased fiber misalignment due to the stitches. An observed increasing strength with decreasing gage length, which was seen for all materials, was explained with a size effect model. The model assumed a random distribution of flaws (misaligned fibers). The toughened materials showed a small increase in strength over the baseline material for both laminates presumably due to the compensating effects of a more compliant matrix and straighter fibers in the toughened material. The hot/wet strength of the stitched and baseline material fell 30 percent below their ambient strengths for shorter, nonbuckling specimen, while the strength of the toughened matrix material only fell 20 percent. Video images of the failing specimen were recorded and showed local failures prior to global collapse of the specimen. These images support the theory of a random distribution of flaws controlling composite failure. Failed specimen appearance, however, seems to be a misleading indication of the cause of failure