Influence Of Absorbed Moisture On Fatigue Crack WORK Propagation Behavior In Polyamides
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Thermoplastics and fiber-reinforced thermoplastics represent great deals in nowadays industries and applications where some of these applications are projected to wet environment. The present study investigates the effect of water moisture on the bearing strength (BS) of Polypropylene (PP) and glass fiber (GF) reinforced Polypropylene (GFRPP) composites. PP and GFRPP are produced by injection molding using different GF weight fractions (wt%), 10, 20, and 30 wt%, and two different initial fiber lengths 12 and 24 mm. A burnout test indicated that produced specimens with 12 mm long fibers have higher final fiber lengths than those made of 24 mm long fibers. More water was absorbed for higher GF weight fractions. The results of the dry bearing test showed higher bearing strengths for specimens with higher GF wt% and longer fibers. The same observation was obtained from wet tests, while, wet-tested specimens of all compositions have higher strengths than their dry counterparts. Strain-at-break seemed to be significantly reduced by water absorption for all specimens. Specimens tested in wet conditions have different fracture morphology than dry ones due to the change in the mechanical behavior of the materials after water immersion.
On the contrary, the strain-at-break between wet and dry conditions showed the opposite behavior. Where Fig. 8 represents the difference in the strain at break of the specimens tested in bearing between wet and dry conditions. It could be clearly observed from Fig. 8 that, the strain at break dramatically decreases when specimens are subjected to water immersion. The same behavior was observed for all specimens. The penetrated water fills the microcracks in the material which accelerates the crack propagation28,53. Abdelhaleem et al.28 noticed an accelerated crack propagation for wet specimens than dry ones when studying the fatigue behavior of PP and GFRPP composites with different GF content. Meng et al.53 found that water ingress occurs as an effect of capillary and the mass of water conserved during the loading cycle forbidding cracks from closing after load removal which in turn accelerates the crack propagation. The absorbed water chemically reacts with the GF in the composites. Chemical elements vanished as an effect of this reaction and were replaced with micro-flaws which are the seeds of fiber fractures28. Also, Ghasemzadeh-Barvarz et al.51 observed a decrease in both elongation-at-break and strain-at-yield of PP and GFRPP after being water aged for 1000 h.
This paper presents a literature review on fatigue in adhesively bonded joints and covers articles published in the Web of Science from 1975 until 2011. About 222 cited articles are presented and reviewed. The paper is divided into several related topics such as fatigue strength and lifetime analysis, fatigue crack initiation, fatigue crack propagation, fatigue durability, variable fatigue amplitude, impact fatigue, thermal fatigue, torsional fatigue, fatigue in hybrid adhesive joints, and nano-adhesives. The paper is concluded by highlighting the topics that drive future research.
Fatigue is undoubtedly a very important type of loading for many structural components that contain adhesive bonding systems. In a fatigue loading regime, a structure may fail at a small percentage of static strength. Therefore, fatigue analysis and fatigue strength prediction are highly required especially for the case of fail-safe or damage tolerance design. Accurate prediction of fatigue life is a challenge due to the complicated nature of fatigue crack initiation and propagation, geometry of bonded joints, and complex material behaviour under loading and unloading regimes.
This paper covers a literature review on fatigue in adhesively bonded joints during the last few decades and more precisely from 1975 until 2011 (or early 2012). All cited articles, 222 references, are published in the Web of Science (WoS). It is a difficult task indeed to classify these articles because of the overlap between the different topics and because there exist many ways to classify them. The classification chosen in this paper is mainly based on the type of analysis and fatigue loading regime. The topics that can be regarded as classified under type of analysis are (a) fatigue strength and lifetime prediction, (b) fatigue crack initiation, (c) fatigue crack propagation and (d) fatigue durability, while the topic that can be regarded as classified under fatigue loading regime are (a) variable fatigue amplitude, (b) impact fatigue, (c) thermal fatigue, and (d) torsional fatigue. Two additional topics that cannot be classified under those two broad headings are fatigue in hybrid adhesive joints and nanoadhesives.
In general, fatigue lifetime can be divided into two main phases, namely, crack initiation and crack propagation. Many authors have neglected the crack initiation phase and based their lifetime analysis only on the crack propagation phase. The main reason for doing this is that the crack initiation phase is more difficult to deal with due to the difficulties associated with modelling the nucleation of a crack and the ability to monitor and detect the initiation phase. Damage models can be either empirical based on plastic strain, or principal strain, or scientific based on continuum damage mechanics theory. Monitoring and detecting crack initiation has been performed in the literature using back-face strain and video microscopic. Although the research into crack initiation in adhesively bonded joints has been started since 1986, it has not yet been well developed and could be seen as in its early stage. This section is divided into two parts, namely, damage models and the monitoring of crack initiation. In some references, both crack initiation and propagation were studied, and therefore sometimes an overlap between them in Sections 3 and 4 is unavoidable.
Scientific-based damage models are normally derived from continuum damage mechanics theory using the principles of thermodynamics . The number of cycles to failure is expressed in terms of the stresses in the adhesive layer, calculated from FEA, and material constants. Abdel Wahab et al.  investigated the measurement of fatigue damage in adhesive bonding using bulk adhesive. They have carried out low cycle fatigue tests to determine the damage variable, , as a function of the number of cycles. Damage was evaluated using the decrease in stress range during fatigue lifecycles of a constant displacement amplitude test. The damage variable is given by [65, 66]where is the range of von Mises stress, is the triaxiality function, is the power constant in Ramberg-Osgood equation, and and are damage parameters to be determined experimentally. Wahab et al.  determined the damage parameters for crack initiation in an SLJ by combining continuous damage mechanics, FEA, and experimental fatigue data. They have studied the effect of stress singularity, due to the presence of corners at edges, on the complex state of stress and the variability of the triaxiality function along the adhesive layer. The damage parameters and determined in  for bulk adhesive were extended to take into account the multiaxial stress state in the adhesive layer, as calculated from FEA. Hilmy et al.  have shown that scarf joint test specimen could simulate constant triaxiality in the adhesive layer. Several types of adhesive joints have been modelled and analysed using FEA. From FEA, they showed that changed as a function of adhesive bond-line angle of the scarf joint and that its values were constant along adhesive line except at the free edges. Quaresimin and Ricotta  presented a model for the prediction of the fatigue life of composite bonded joints. The model was based on the actual mechanics of the fatigue damage evolution and divided the joint lifetime into a crack nucleation phase and a crack propagation phase. They modelled the nucleation phase using a generalised stress intensity factor approach. They have further studied the evolution of the fatigue damage in SLJs and observed that a significant fraction of the fatigue life of the joint was spent in the nucleation of one or more cracks . They found that the duration of nucleation process was from 20% up to 70% of the joint life. Imanaka et al.  proposed an estimation method of fatigue strength of adhesively bonded joints with various stress triaxialities using a damage evolution model for high cycle fatigue. Imanaka et al.  investigated the damage evolution of adhesively bonded butt joints under cyclic loading. They applied an isotropic continuum damage model coupled with a kinetic law of damage evolution, which was solved using analytical and numerical methods. Wahab et al. [65, 73] presented and programmed a procedure in order to predict the fatigue threshold in composite adhesively bonded joints. They considered two different joint configurations, namely, DLJs and LSJs. Software has been developed to automatically calculate the damage parameters and produce the required load number of cycle to failure curves. Lefebvre and Dillard [74, 75] have shown that the fatigue initiation criterion using stress singularity parameter, a generalized stress intensity factor and the singular eigenvalue lambda (see (1)), is only appropriate for adhesive contact angle smaller than 90 and modulus ratio between adhesive and substrate smaller than 0.1.
Back-face strain has extensively been used in the literature to monitor crack initiation in adhesively bonded joints. Khoramishad et al. [76, 77] monitored damage initiation and propagation phases in SLJs using the back-face strain (Figure 12) and in situ video-microscopy techniques. Graner Solana et al.  presented experimental fatigue data obtained from SLJ tests at a range of load levels. They used six strain gauges (SGs) placed along the overlap to monitor fatigue initiation and propagation within the adhesive layer. Shenoy et al.  used the back-face strain measurement technique to characterise fatigue damage in SLJs subjected to fatigue loading. They found that crack initiation dominated at lower fatigue loads, whereas crack propagation dominated at higher fatigue loads. They used the back-face strain and fatigue life measurement results to propose a simple predictive model, which divided the fatigue lifetime into different regions depending upon the fatigue loads. Solana et al.  presented experimental fatigue testes of a selection of adhesive joints. They used multiple strain gauges to record the change in back-face strain during the tests and measure damage in different locations. They found that damage first appeared in the fillet as a change in adhesive colour in a specific area. Deng and Lee  presented details of a fatigue test programme of a series of small-scale steel beams bonded with a CFRP plate. They used back-face strain technique to detect crack initiation and monitor crack growth. They found that crack initiated and propagated in mode I earlier than in mode II. Crocombe et al.  studied the fatigue damage evolution in adhesively bonded joints using the back-face strain technique. From the results of the fatigue tests, they found that there was an initiation phase of about half the total fatigue life of the joint. They further observed that removing the adhesive fillet eliminated the initiation phase and consequently reduced the fatigue life. Zhang et al.  developed a back-face strain technique to detect fatigue crack initiation in SLJs. From experimental measurements, they found that fatigue crack initiation lives at different stresses have greater proportion of the total fatigue life as the stress decreased. 153554b96e