Subcritical fatigue crack growth in High-Pressure Manifolds
High-Pressure Manifolds
are subjected for a long time to alternating loads between 40 MPa and 100 MPa. A component without a crack or with an initial crack will not fail when the stress intensity factor K produced by the combination of static external load and crack is lower than the fracture toughness Kc of the object. However, under repeated alternating loads, the crack slowly grows. When the crack grows from the initial length a0 to the critical size ac, unstable failure occurs. This is called fatigue failure. The crack-growth process from a0 to ac under alternating stress is called subcritical fatigue crack growth.
Research on crack growth behavior under alternating loads has very important significance for practical engineering structures. When initial damage exists in an engineering structure, such as internal defects, microcracks or inclusions, fracture may not occur under working load at the beginning of service. However, most engineering structures are subjected to cyclic alternating loads or even impact loads during service, and under such loads the initial damage in the structure grows slowly. Once the combination of external load and crack length makes the stress intensity factor reach the critical value, the structure fails. The number of load cycles experienced from initial crack to critical crack length is called the fatigue crack growth life of the structure. Before fracture mechanics appeared, traditional fatigue design was based on smooth small specimens. Although this traditional design method made major contributions to preventing fatigue failure of components, it did not consider possible defects in actual components and did not reflect crack-growth laws. Therefore, its results are sometimes unsafe and sometimes overly conservative. Applying fracture mechanics principles to study fatigue crack-growth laws and establishing fatigue design calculation methods on that basis is an important supplement and development of traditional fatigue analysis. To predict fatigue crack growth life more accurately, this article studies crack-growth laws under various types of alternating loads.
Crack initiation and growth under alternating load
The fatigue fracture process of metallic materials is very complex. For most practical engineering problems, including High-Pressure Manifolds and similar components, loads and stresses often vary alternately with time. Under this alternating load, components may experience fatigue failure. Generally, fatigue cracks start on the component surface and then gradually grow until the stress reaches a critical value, after which unstable growth develops and sudden fracture occurs.
Crack growth law under constant-amplitude alternating load
First, crack growth under constant-amplitude alternating load is studied. Although this fatigue form is rarely seen in actual structures, it is easy to test on ordinary fatigue testing machines. In addition, the fatigue crack-growth laws obtained from this condition can serve as the basis for studying crack growth under other types of fatigue loading. Therefore, most fatigue crack-growth data for materials are obtained under fatigue loading.
Factors affecting crack growth rate and interaction between loads
The above discussion shows that the stress intensity factor range at the crack tip is the main parameter determining crack growth rate. However, many other factors also affect fatigue crack growth rate. Although their influence is secondary compared with the stress intensity factor range, they are important for understanding crack-growth laws and improving material resistance to fatigue fracture. Several factors affecting crack growth rate are introduced below.
Effect of overload peaks
When an overload peak appears in alternating stress under a normal stress amplitude, it produces a retardation effect on subsequent crack growth under lower stress-amplitude cycles. Tests show that this retardation depends on overload magnitude. The higher the overload peak, the greater the retardation. Multiple overloads cause additional retardation. A very high overload may even completely stop crack growth during subsequent low-amplitude cycles.
Effect of loading frequency
Generally speaking, the lower the loading frequency, the higher the crack growth rate; the higher the loading frequency, the lower the crack growth rate. In the lower frequency range, frequency influence is small; in the higher range, frequency influence becomes more significant. Load interaction must also be considered.
Crack-growth behavior under constant-amplitude fatigue loading is only a starting point. In actual engineering structures, most cyclic loads are variable-amplitude loads. Because loads of different amplitudes appear in the load spectrum, there is interaction between these different-amplitude loads, and they influence fatigue crack growth rate. Many experimental studies show that after high-amplitude loads appear in the load spectrum, subsequent crack growth rate decreases and needs a certain number of cycles to recover to its normal growth rate. After a negative high-amplitude load appears, subsequent crack growth rate increases. Therefore, when studying crack-growth characteristics and predicting crack-growth life, the influence of load interaction must be considered.
The simplest case is adding one high peak load into constant-amplitude cyclic loading. This high peak load is called overload. The change characteristics of crack growth rate after overload have been studied extensively in the past twenty years. Research shows that after each overload, the crack growth rate decreases significantly.
The retardation effect caused by overload has the following conclusions:
1) Residual stress exists in the large plastic zone produced by overload. The combined effect of residual compressive stress ahead of the crack and applied tensile stress reduces the effective stress intensity factor, thereby reducing crack growth rate.
2) Residual compressive strain exists in the plastic zone produced by overload. When the crack tip enters the plastic wake, residual deformation closes the crack. Because the stress required for crack reopening is higher than the minimum stress of the cyclic load, crack growth rate decreases.
3) Overload causes crack-tip blunting, reducing crack growth rate.
Crack growth law under variable-amplitude alternating load
From the influence of overload peaks on fatigue crack growth, it can be inferred that under variable-amplitude loading, fatigue cycles of different amplitudes affect each other. Considering this effect makes fatigue crack-growth calculation very complex. In engineering, Miner's linear cumulative damage rule is widely used. It ignores the influence between fatigue cycles of different amplitudes, calculates crack-growth amount under each amplitude independently, and then obtains total growth according to linear superposition.
For fatigue crack growth under variable-amplitude cycles, no matter which analysis method is used, sufficient knowledge of the load is required. It should be noted that the order of cycles of different amplitudes cannot be reversed in calculation, and cycles of the same amplitude separated by another amplitude load cannot simply be added together.
In addition, because overload peaks retard fatigue crack growth, results calculated by Miner's linear cumulative damage rule are conservative.
Crack growth life analysis of High-Pressure Manifolds
When a crack does not produce single-event fracture, but under alternating loads the stress amplitude exceeds a certain value, a small initial crack in the structure, or even a crack generated under alternating load when there was no initial crack, gradually grows until failure. This process is crack growth, and the number of alternating-load cycles in this process is the crack growth life.
Conclusions and analysis
Based on material property tests and the above analysis and calculation, the following conclusions can be obtained for fracture safety of typical components:
1) Because the number of alternating load cycles during service is relatively low and the stress value is not high, analysis results from both fatigue theory and crack-growth theory show that safe service life is greater than 20 years.
2) Even if most pipe fittings fail, leakage occurs first and provides warning, which is favorable for safety.
3) It is recommended to perform NDT inspection on the manifold every 2 to 3 years. When cracks are found, compare them with the values given in the table. If the crack is in a safe state, the component can continue to be used, but crack-growth change should be observed so that problems can be diagnosed in time and the manifold can work safely and reliably.
