Crack initiation and growth

Due to the large thermal expansion mismatch between barium chromate and the sealing glass or FSS (e.g., AISI446) [213], the extensive formation of barium chromate has been shown to result in crack initiation and growth between the sealing glass and alloy coupons. 

Figure 16. SEM images of the fracture surface of an RH-cycled (80-120% RH) sample, showing crack initiation and growth from a craze site near the anode and membrane interface. A crazing site found on Hi-RH cycled samples. The half-penny shaped craze closely resembles crazes formed on free surfaces.

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

In corrosion fatigue experiments, a notch is cut in the specimen at which maximum stress will occur. Crack initiation and growth thus occur at the tip of this notch and may be measured with the aid of a microscope. 

Metal fatigue is the process of crack initiation and growth due to the action of a fluctuating mechanical stress. Corrosion fatigue is simply metal fatigue that... 

A Physically Based Methodology for Predicting Material Variability in Fatigue Crack Initiation and Growth... 

Polymer blends, in which the constituent polymers are incompatible, tend to improve fatigue resistance. For example, natural rubber and polybutadiene show good resistance to fatigue, crack initiation, and growth because of the formation of heterogeneous polymer phases a crack growth in one polymer phase is arrested at the boundary with the adjacent polymer phase. 

Fatigue failure in metals is known to consist of crack initiation and growth. Crack initiation starts with dislocation movement. Submicrocracks are then formed at slip bands. They subsequently grow and coalesce to form a crack of detectable size (short crack) to complete the crack initiation process [12]. This is then followed by the growth of a single crack until final rupture. The period of crack initiation and submicrocrack growth covers most of the fatigue life. 

Crack initiation and growth under creep and fatigue loading for Incoloy 800 alloy. 

The purpose of the work is to develop and validate methods to analyse crack initiation and growth at high temperature for LMR components. 

This programme was presented last year (ref. 2). The purpose of TERFIS 5 tests carried out in 1996 was to quantify the influence of residual stresses on crack initiation and growth in weldments. They were obtained on stainless steel 316 L tubes where a defect was machined in a welded joint. The tubes were submitted to a low uniform tension (29 Mpa) and to thermal stocks (ex in the case of a circunferential defect AT in the thickness 180°C and 10 cycles in the case of a semi-elliptic defect AT in the thickness 300°C and 1500 cycles). 

One of the shortcomings most adversely affecting the utilization of PP filled with flame retardant magnesium hydroxide (Mg(OH)2) is the steep reduction of subambient fracture toughness compared with that of neat polypropylene. Generally, an increase in the resistance to crack initiation and growth can be achieved... 

In a test under an imposed strain (relaxation test) crack initiation and crack growths, i.e. craze formation, is slowed down by simultaneous stress relaxation. Crack initiation and growth and stress relaxation are competitive processes both of which depend very strongly on temperature. Crack... 

Metallurgical factors can influence CF crack initiation and growth. Prominent in this regard are sensitization of grain boundaries in austenitic stainless steels [22,23], locally intense slip in aluminum alloys from dislocation interactions... 

Total load cycles to failure are measured, but crack initiation and growth are not monitored during an HCF experiment. Information on these stages of HCF is critical, but difficult to obtain. Methods for CF crack detection are discussed in the subsection on Small Crack CF Methods. 

The mechanistic differences between SSC and SCC (anodic type) have a significant influence on the test methods used. Stated simply, the critical vciriable in SSC of carbon and low-alloy steels is the stress (strain) required to initiate and sustain cracking in an arbitrarily selected severe environment. In SCC, the critical variables are the electrochemical polarization characteristics of the metal/environ-ment system that control the anodic processes involved in crack initiation and growth. Therefore, alloy SCC susceptibility comparisons are done by determining the critical environmental factors (e.g., temperature, H2S partial pressure, chloride concentration) required to cause failure of a severely stressed specimen. 

The Benefit of Hydrogen Addition to the Boiling Water Reactor Environment on Stress Crack Initiation and Growth in Type 304 Stainless Steel J. Eng. Mater. Technol. (Trans. ASME) 108 (1986) 1, S. 10-19 piO] Hettiarachchi, S. et al. 

Information is given on the frequency, level and formation of ground surface ozone concentrations in Central Europe, and a study is described of the weathering behaviour of various rubber materials (crack initiation and growth) under the effect of ozone. 20 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

The use of a fracture mechanics approach based on the strain energy release rate to assess failure in rubber-cord laminated structures is reviewed. The mechanics of crack propagation are considered for cracking either between the plies or around individual cords, and also for crack initiation and growth near cord ends. The ability of the approach to predict the effects of various design and construction parameters on laminate failure is also discussed. 9 refs. 

Figure 1 Schematic drawing of crack initiation and growth by shp-dissolntioa (Adapted from Ref. 25.)...

---