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Sigma Key Crack
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In order to predict the crack growth life in microelectronics solder joints, an FE A (finite element analysis) program employing a new scheme for crack growth analysis is developed. Also some experimental data necessary for the practical application of this program are obtained. Above all, the data related to the crack growth rate play a key role and are obtained in terms of the maximum opening stress range Δσθ max as
where α = 2.0 and β = 2.5 x 10-9 mm 5/N 2 are independent of the test conditions, and γ is dependent on the solder material. The calculated values of the crack growth life by the FEA are in good agreement with the experimental ones. This indicates at the same time that the crack growth rate and path are certainly controlled, through the above equation, by Δσθ max measured at a certain radial distance from the crack tip.
Given the major requirements of clinical services and materials after fracture, an autonomous crack-healing ability has recently captured a lot of attention due to the recovery strength of biomaterials after being forced to break [24]. This approach employs a liquid healing agent encapsulated in a polymeric shell to form microcapsules, which are then incorporated into a matrix material [25]. When cracking occurs, the propagating crack will rupture the microcapsules, release the healing liquid that has its polymerization trigger when in contact with capsules, and lead to the healing of the composite [26,27]. More recently, novel self-healing poly (urea-formaldehyde) (PUF) microcapsules containing polymerizable TEGDMA and N,N-dihydroxy ethyl-p-toluidine (DHEPT) were synthesized and incorporated into dental resin good self-healing efficacy [28,29].
The self-healing approach has been investigated in previous reports, which indicate that healing efficiency is relative to the matrix used, and can vary tremendously due to different matrix-healed network interactions [24,30,31]. By concept, the self-healing polymeric materials have the built-in capability to substantially recover their load-transferring ability after damage. Such recovery can occur autonomously or be activated after an application of a particular stimulus (e.g., heat, radiation)[36,37]. Research into producing self-healing dental composites has been based on the release of a healing liquid after cracking produced via fatigue[38]. In the present study, microcapsules with a healing liquid of TEGDMA plus 1% DHEPT surrounded by a PUF shell were used via an in situ polymerization technique in an oil-in-water emulsion [28]. This method allowed the production of microcapsules with an average diameter of 70 24 µm [29], which respond to a massive rupture of the PUF shell when stress is required to propagate a pre-existing flaw. We proposed that this effect was efficient due to sufficient microcapsule stability inside the resin matrix promoted by the roughness of the external surface of microcapsule wall and a suitable thickness of the wall, which also protects the encapsulated healing agent from premature polymerization [29]. Approximately 65% of the original strength was recovered after repair with TEGDMA-DHEPT as a healing liquid. Efficient self-healing has been also demonstrated in previous studies using the applied method of fabrication of microcapsules [29,31]. The microencapsulated healing is quite stable and durable, which broadens the process window for fabricating self-healing composites and prevents the deterioration of the healing capability of the composites during storage [32]. The successful demonstration of this healing system will open pathways for healing in dental composites.
The incorporation of additives with different purposes into a resin matrix would inevitably affect its intrinsic properties. The fraction of each element is critical for the current system to achieve the highest healing efficiency. One major goal of this study was to combine the best performance of those two elements (healing system and protein repellent) without detrimental effects on their highlighted properties. High healing efficiency can be acquired at 10% capsules content so that the fundamental mechanical properties of the matrix are insignificantly affected. Previous studies showed a positive correlation between microcapsule content incorporation and the fracture toughness of the polymer matrix [26,27]. In a previous study, a nearly 40% increase in the original virgin KIC was achieved when the microcapsule mass fraction was increased from 0 to 10%, and slightly reduced when increased to 15% [29]. This finding was also consistent with a previous study showing that the incorporation of up to 6% of microcapsules into a host material did not affect the original flexural strength [30]. Mechanistically, we show that the addition of 10% microcapsules into the composite did not decrease its properties, and can be used as an optimal concentration that reaches up to 70% of healing efficiency. Also, because the healing agent possesses high flow ability and reactivity and belongs to the same family as the matrix polymer, crack healing is automatically conducted at or below room temperature, which offers satisfactory repair effectiveness. 153554b96e
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