| 1. |
- Hejman, Ulf, et al.
(author)
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Dissolution driven crack branching in polycarbonate
- 2011
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In: Fatigue & Fracture of Engineering Materials & Structures. - : Wiley. - 1460-2695 .- 8756-758X. ; 34:4, s. 227-239
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Journal article (peer-reviewed)abstract
- Stress corrosion, in the form of chemically assisted crack growth, in polycarbonate is examined with focus on crack branching characteristics. Cracks with finite width are observed; this is to be expected for dissolution driven cracking. The cracks branched repeatedly and crack widths before and after branching are measured. Both symmetric and asymmetric branching is found. The dissolution rate is assumed to be a linear function of the strain along the crack surface. In the literature, it is proposed that the crack width is proportional to the square of the mode I stress intensity factor. Energy considerations lead to that the sum of branch widths must equal the width of the unbranched crack. The results from this study correspond fairly well with this assumption. The branching angle is found to be 32 degrees +/- 12 degrees, which is in line with results for sharp cracks reported in the literature. The mean growth direction of the branches is found to deviate slightly from the expected straight. No significant correlation between angles and crack widths is found. The scatter in results is mainly addressed to the inherent perturbation sensitivity of stress corrosion cracking. Also numerically simulations of crack branching is performed. These results show promising agreement with the experiments.
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| 2. |
- Hejman, Ulf, et al.
(author)
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Environmentally assisted initiation and growth of multiple surface cracks
- 2010
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In: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 47:14-15, s. 1838-1846
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Journal article (peer-reviewed)abstract
- The initial stages of stress corrosion on an amorphous polymer is investigated. This is done by exposing stressed specimens of polycarbonate to an acetone and water solution. The surface develops two distinct features of degradation that appear on different length scales when subjected to tensile stress. Small pits form on the surface and make it rough. These pits are in the order of micrometers, and are found to be randomly distributed. They occur even without load and seem to slightly increase in number with increasing stress. In the millimeter domain, visible to the bare eye, surface cracks are formed transverse to the direction of loading. The occurrence of cracks is seen to have a positive stress threshold value, exceeding which, a linear increase of number of cracks with stress is found. The manners in which the cracks grow and coalesce on the surface are examined. It is seen that they do not meet crack tip to crack tip. Instead, they avoid each other initially and coalesce crack tip to crack side. The results are discussed in the light of mechanical considerations. A stress analysis for a few configurations of meeting cracks supports the experimental observations. With assumptions of stress corrosion crack growth and coalescence, a simulation of cracks growing from randomly distributed initiation sites is performed. Similar crack patterns as obtained in the experiments are found.
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| 3. |
- Hejman, Ulf, et al.
(author)
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Experiment and simulation of a crack growing by material dissolution
- 2007
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In: Abstracts 1st Bertram Broberg symposium on mechanics of materials;1.
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Conference paper (other academic/artistic)abstract
- Crack growth and crack path under a fracture mechanical experiment in polycarbonate is studied. The material, a polycarbonate, is photo elastic witch makes it possible to do an optical stress field analysis. The crack path and crack branching is also predicted by an adaptive finite element analysis. The crack growth is a result of stress induced, chemically activated material dissolution that completely determines the geometry of the specimen, including the shape and length of the crack. In this case, the mechanisms and processes leading to crack growth are assumed to be incorporated in a general material dissolution process. As a consequence of this assumption there is no need for a crack criterion or crack path hypothesis. The lack of crack path criterion imposes an arbitrary length scale where the crack width scales with the square of the applied load but the crack growth rate is independent of the load. Numerical calculations suggest that the crack is capable of branching and that the branches will continue to grow at the same rate as the original crack. The numerical results are verified by the photo elastic experiments and a reasonable agreement is established.
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| 4. |
- Hejman, Ulf, et al.
(author)
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Experiment och simulering av materialupplösningsspricka
- 2007
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In: Abstracts Svenska mekanikdagarna, Luleå, Sweden. - : Luleå tekniska universitet.
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Conference paper (other academic/artistic)abstract
- Sprickväg och sprickgrening studeras under brottmekaniska experiment i polykarbonat. Detta material är spänningsoptiskt och fotoelastiska analyser utförs. Sprickväg och sprickgrening beräknas också med hjälp av en adaptiv finit element-metod. Spricktillväxten är ett resultat av spänningsunderstödd, kemiskt eller biologiskt aktiverad, materialupplösning, som helt bestämmer provkroppens geometri inklusive sprickans form och längd. På så sätt är de processer som leder till sprickpropagering inkluderade i en allmän materialupplösningsprocess. Som en konsekvens av detta behövs varken brottkriterium eller sprickvägshypotes. Den fotoelastiska undersökningen jämförs med de numeriska resultaten. Avsaknaden av brottkriterium gör att sprickpropageringsprocessen saknar längdskala. Därmed följer att sprickans bredd skalar med kvadraten på den pålagda lasten medan spricktillväxthastigheten blir oberoende av pålagd last. Förhållandet gör vidare att sprickan förgrenas och att alla grenar fortsätter att växa utan att förlora fart.
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| 5. |
- Hejman, Ulf, et al.
(author)
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Investigation of Branching in Polycarbonate Due to Stress Corrosion
- 2008
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In: Proceedings 17th European Conference on Fracture, Brno 2008. - : VUTIUM Brno, Czech Republic.
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Conference paper (other academic/artistic)abstract
- In this study, experiments have been made to investigate how the width of stress corrosion cracks change when branching occurs. The amount of dissolved material is investigated using and optical microscope. Crack width before and after branching and characteristic crack angels are measured. It was found that the total width of the branches is preserved after branching and a connection to the stress intensity factor at the crack tips is suggested. It is assumed that quasistatic calculations for branched cracks, found in literature, can be applied to these cracks. Experiments have been performed on polycarbonate with acetone as a dissolving agent. Experimental results are compared with numerically simulated results as well as with data found in literature.
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| 6. |
- Hejman, Ulf
(author)
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On initiation of chemically assisted crack growth and crack propagation paths of branching cracks in polycarbonate
- 2010
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Licentiate thesis (other academic/artistic)abstract
- Stress corrosion, SC, in some cases gives rise to stress corrosion cracking, SCC, which differs from purely stress intensity driven cracks in many aspects. They initiate and grow under the influence of an aggressive environment in a stressed substrate. They grow at low load and may branch. The phenomenon of SCC is very complex, both the initiation phase and crack extension itself of SCC is seemingly associated with arbitrariness due to the many unknown factors controlling the process. Such factors could be concentration of species in the environment, stress, stress concentration, electrical conditions, mass transport, and so on.In the present thesis, chemically assisted crack initiation and growth is studied with special focus on the initiation and branching of cracks. Polycarbonate plates are used as substrates subjected to an acetone environment. Experimental procedures for examining initiation and branching in polycarbonate are presented. An optical microscope is employed to study the substrate.The attack at initiation is quantified from pits found on the surface, and pits that act as origin for cracks is identified and the distribution is analysed. A growth criterion for surface cracks is formulated from the observations, and it is used to numerically simulate crack growth. The cracks are seen to coalesce, and this phenomenon is studied in detail. Branching sites of cracks growing in the bulk of polycarbonate are inspected at the sample surface. It is found that the total width of the crack branches are approximately the same as the width of the original crack. Also, angles of the branches are studied. Further, for comparison the crack growth in the bulk is simulated using a moving boundary problem based algorithm and similar behaviour of crack branching is found.
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