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Question: what factors determine the direction of propagation of a crack through a material?
Everyone knows that answer, Simon........the image of the the obese plumber bending over to fix the drain pipe.......or a red eyed dog that needs a fix CRACK KILLS!!!! :p
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Everyone knows that answer, Simon........the image of the the obese plumber bending over to fix the drain pipe.......or a red eyed dog that needs a fix CRACK KILLS!!!! :p
That picture is perhaps the most disgusting thing I've ever seen. I'm honestly interested in this topic as my best friends PhD was on fracture mechanics of leather. When we were writing up our respective works we spent many nights in the local pub splitting atoms with axes and discussing such things as crack propagation. As I recall, at one point he was thinking about lyapunov exponents and chaos mathematics. He is running his first half marathon this weekend, if he survives I'll give him a call to get his angle on it.
__________________ Science is the antidote to the poison of enthusiasm and superstition
A qualitative analysis of crack propagation in articular cartilage at varying rates of tensile loading.
Connect Tissue Res. 2003; 44(2):109-20 (ISSN: 0300-8207)
Stok K; Oloyede A
School of Mechanical, Manufacturing, and Medical Engineering, Queensland University of Technology, Brisbane, Queensland, Australia.
A custom-built miniature tensile testing apparatus was used to study the propagation of cracks through the articular cartilage matrix at various loading rates and initial crack lengths. The crack propagation mechanism was observed to be significantly dissimilar to that normally seen in traditional fracture mechanics opening mode, where fracture propagates through the thickness of samples or perpendicularly to the applied load. Instead, an artificially initiated microcrack in the surface layer of an articular cartilage sample grew laterally in the direction of the applied load, stretching about the crack tip, whose initial position remained unchanged throughout the fracture process. A progressive upward pull of the bottom layer toward the surface, which resulted in necking of the specimen, was observed. Our analysis revealed that the rate of necking was the same as that of the lateral stretch of the growing crack. We hypothesize that necking is due to the response of the collagen meshwork especially in the deep zones of the matrix to the tensile load. Our samples exhibited unstable fracture growth immediately after each microcrack grew to the base of the articular surface layer, with very fast crack propagation to failure, thereby indicating that the fracture toughness of the articular cartilage matrix is significantly determined by the toughness of its articular surface.
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Propagation of surface fatigue cracks in human cortical bone.
J Biomech. 2006; 39(5):968-72 (ISSN: 0021-9290)
Kruzic JJ; Scott JA; Nalla RK; Ritchie RO
Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331, USA.
An understanding of how fatigue cracks grow in bone is of importance as fatigue is thought to be the main cause of clinical stress fractures. This study presents new results on the fatigue-crack growth behavior of small surface cracks (approximately 75-1000 microm in size) in human cortical bone, and compares their growth rates with data from other published studies on the behavior of both surface cracks and many millimeter, through-thickness large cracks. Results are obtained with a cyclically loaded cantilever-beam geometry using optical microscopy to examine for crack growth after every 100-500 cycles. Based on the current and previous results, small fatigue cracks appear to become more resistant to fatigue-crack growth with crack extension, analogous to the way the fracture resistance of cortical bone increases with crack growth. Mechanistically, a theory attributing such behavior to the development of bridges in the wake of the crack with crack growth is presented. The existence of such bridges is directly confirmed using optical microscopy.
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Resistance to crack growth in human cortical bone is greater in shear than in tension.
J Biomech. 1996; 29(8):1023-31 (ISSN: 0021-9290)
Norman TL; Nivargikar SV; Burr DB
Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown 26506-9196, USA.
It has been proposed that longitudinal shear stresses create bone microdamage, which suggests that bone is weak in shear and may not be adapted to prevent crack growth under shear loading. However, based on the similarities between bone and other fiber-reinforced composites that are tough, i.e. resistant to crack growth, we hypothesized that resistance of human bone to crack growth under shear loading is greater than under tensile loading. Because bone from older individuals and women has demonstrated increased propensity to fracture, we also hypothesized that bone from these individuals has less resistance to crack growth under shear and tension loading. Using compact shear and compact tension specimens, the critical strain energy release rate (Gc) of human bone was measured for longitudinally oriented cracks under tension (mode I) and shear (mode II) loading for male and female cadavers ranging from 55 to 89 y. Average tensile fracture toughness (GIc) of male and female human bone was 339 Nm-1 (S.D. = +/- 132). Average shear fracture toughness (GIIc) of human bone over the same range was 4200 Nm-1 (S.D. = +/- 2516 Nm-1). Shear toughness was greater than tensile toughness (approximately 13 times), which is consistent with other fibrous composite materials. Fracture toughness decreased with age, but the fits were weak and significant for shear loading only. Tension and shear toughness did not depend on gender. We concluded that the resistance to crack propagation under shear loading is greater than under tensile loading, a finding which suggests that bone adapts to prevent crack growth in shear. We also found that bone toughness is equivalent in men and women and that bone toughness gradually decreases with age between 55 and 89 y.
Simon,
I couldn't resist the photo.....BTW, the chihuahua was previously a punk rocker in his youth. :p
__________________
Sincerely,
Kevin
**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Question: what factors determine the direction of propagation of a crack through a material?
Only just came across your thread on crack propagation.
This is a major consideration in structural engineering and especially in steel jackets in offshore installations. Crack detection, inspection and reair were a major part of the work when I was a sub sea inspection engineer in the oil industry. Methods used for detection include, visual / video, metalic partical inspection, ultrasonic detection and radiography. This comes under the heading of NDT non destructive testing (BTW Simon have you ever tried testing a crack to destruction, it's hard work and very messy but someone's got to do it eh! )
I'll have to see if I can dig out my old NDT books but of the top of my head-
Anyway as far as I remember cracks propagate at right angles to the lines of stress providing the material is of uniform stiffness and grain.
Cracks in steel often start where there are inclusions / contamination or gas bubbles in the steel left during production or included in a weld during construction. Also they start from pitting errosion due to oxidisation and galvanic corrosion or damage from mechanical errosion. These areas are weak and the crack will propagate from them. The crack itself acts as a stress raiser IE like a little lever for the internal forces to act on within the material.
These cracks once detected and providing they were small could be made safe by grinding or drilling a small hole to below the depth of the crack this has the effect of spreading, and therefore reducing the local stress concentration. If the cracks were large and particularly if they were full thickness then measures to increase the local stiffness of the member were undertaken EG fitting of a grout filled clamp that envelopes the node. Node are generally where crack start since they are areas of construction welding and the junction of 2 or more caissons or bracings and therefore also act as levers for external forces to act on and induce high internal forces.
Therefore one may see that cracks propagate wher the material is weakest or the internal forces are highest or a combination of the two. To stop propagation several things can be done.
One - reduce local internal stress eg negate the lever effect of the crack, Two - increase local node or bracing stiffness, Three- reduce external applied forces, Four - increase internal resisting forces, Five do not allow inclusions in the steel, Six - do not allow corrosion and errosion.
I'm sure one could apply similar principles to bones or orthotic materials.
(BTW Simon have you ever tried testing a crack to destruction, it's hard work and very messy but someone's got to do it eh! )
As a matter of fact I have. Ignoring the obvious double entendre, I spent many a Saturday morning "helping" my good friend Dr David O'Leary in his lab with his instrom and leather experiments. Really, I was watching Withnail and I, making up the health and safety numbers and tossing in a thought or two when asked.
Quote:
Originally Posted by David Smith
Anyway as far as I remember cracks propagate at right angles to the lines of stress providing the material is of uniform stiffness and grain.
Problem is I don't think the materials in question meet these criteria. Moreover, their fibrous nature means we also get things like fibre pull-out, which we don't see in metallic materials.
__________________ Science is the antidote to the poison of enthusiasm and superstition
Crack propagation
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Linear Mode
