Product failure

The process of analyzing designs includes the modes of failure analysis reviewed in this book. At an early stage the designer should try to anticipate how and where a design is most likely to fail. The most common conditions of possible failure are elastic deflection, inelastic deformation, and fracture. During elastic deflection a part fails because the loads applied produce too large a deflection. In deformation, if it is too great it may cause other parts of an assembly to become misaligned or overstressed. Dynamic deflection can produce unacceptable vibration and noise. When a stable structure is required, the amount of deflection can set the limit for buckling loads.

Because many plastics are relatively flexible, analysis should consider how much deflection might result from the loadings and elevated temperatures any given part might see in service. The equations for predicting such deflections should use the modulus of the material; its tensile strength is not pertinent. Usually, the most effective way to reduce deflection is to stiffen a part's wall by changing its cross-section.

Inelastic deformation causes part failure arising out of a massive realignment of the molecular structure. A part undergoing inelastic deformation does not return to its original state when its load is removed. It should be remembered that there are plastics that are sensitive to this situation and others that are not.

The existence of an elevated temperature, with or without long-term or continuous loading, would suggest the possibility that a material might exceed its elastic limits. Regarding momentary loading, the properties to consider are the proportional limit and the maximum shear stress.

The presence of fracture reflects a load that exceeds the strength of the design. The load may occur suddenly, such as upon impact, or at a low temperature, which will reduce the elongation of the material. A failure may develop slowly, from a steady, high load applied over a long time (creep rupture) or from the gradual growth of a crack from fatigue. If fracture is the expected mode of failure, analysis should examine the greatest principal stresses involved.

To ensure that a design functions as intended for the prescribed design lifetime and, at the same time, that it is competitive in the marketplace. Success in designing competitive products while averting premature mechanical failures can be reached consistendy only by recognizing and evaluating all potential modes of failure that might influence the design. To recognize potential failures a designer must be acquainted with the array of failure modes observed in practice, and with the conditions leading to these failures.

A failure may be defined as the physical process or processes that take place or that combine their effects to produce a failure. In the commonly observed failures it may be noted that some failures are a single phenomena, whereas others are combined phenomena. For example, fatigue is listed as a failure, moisture is listed as a failure, and moisture fatigue is listed as another failure. These combinations are included because they are commonly observed usually resulting in a synergistic response. Failures to be reviewed only occur when they generate a set of circumstances that interferes with the proper functioning of a product. With the proper use of the plastic material required to meet the designed product performance requirements the failures reviewed will be rare, actually should not occur.

Failures in products almost always initiate at sites of local stress concentration caused by geometrical or micro-structural discontinuities. These stress concentrations, or stress raisers, often lead to local stresses many times higher than the nominal net section stress that would be calculated without considering stress concentration effects. Thinking in terms of "force flow" through a member may develop an intuitive appreciation of the stress concentration associated with a geometrical discontinuity as it is subjected to external loads.

Force and/or temperature-induced elastic deformation failure occurs whenever the elastic (recoverable) deformation in a product, brought about by the imposed operational loads or temperatures, becomes large enough to interfere with its ability to perform its intended function satisfactorily.

Yielding failure occurs when the unrecoverable deformation in a ductile product, brought about by the imposed operational loads or motions, becomes large enough to interfere with the ability of the product to not meet its performance requirements.

Ductile rupture failure occurs when the plastic deformation, in a part that exhibit ductile behavior, is carried to the extreme so that the member separates into two pieces. Initiation and coalescence of internal voids slowly propagate to failure, leaving a dull, fibrous rupture surface.

Fatigue failure refers to the sudden and catastrophic separation of a part into two or more pieces as a result of the application of fluctuating loads or deformations over a period of time. Failure takes place by the initiation and propagation of a crack until it becomes unstable and propagates suddenly to failure. The loads and deformations that typically cause failure by fatigue are far below the static failure levels. When loads or deformations are of such magnitude that more or less than about 10,000 cycles are required to produce failure, it is usually termed high-cycle fatigue or low-cycle fatigue, respectively. When a fluctuating temperature field in the part produces load or strain cycling, the process is usually termed thermal fatigue. Surface fatigue failure, usually associated with rolling surfaces in contact, manifests itself as pitting, cracking, and spalling of the contacting surfaces as a result of the cyclic contact stresses that result in maximum values of cyclic shear stresses slighdy below the surface. The cyclic subsurface shear stresses generate cracks that propagate to the contacting surface.

Creep failure results whenever the deformation in a part accrues over a period of time under the influence of stress and temperature until the accumulated dimensional changes interfere with the ability of the part to perform satisfactorily its intended function. Three stages of creep are often observed: (1) transient or primary creep during which time the rate of strain decreases, (2) steady-state or secondary creep during which time the rate of strain is virtually constant, and (3) tertiary creep during which dme the creep strain rate increases, often rapidly, until rupture occurs. This terminal rupture is often called creep rupture and may or may not occur depending on the stress-time-temperature conditions.

Creep buckling failure occurs when, after a period of time, the creep process results in an unstable combinadon of the loading and geometry of a part so that the critical buckling limit is exceeded and failure ensues.

Buckling failure occurs when, because of a critical combination of magnitude and/or point of load application, together with the geometrical configuration of a part, the deflection of the member suddenly increases greatly with only a slight change in load. This nonlinear response results in a buckling failure if the buckled member is no longer capable of performing its design function.

Stress rupture failure is intimately related to the creep process except that the combination of stress, time, and temperature is such that rupture into two parts is ensured. In stress rupture failures the combination of stress and temperature is often such that the period of steady-state creep is short or nonexistent.

Thermal relaxation failure occurs when the dimensional changes due to the creep process result in the relaxation of a pre-strained or pre-stressed member until it no longer is able to perform its intended function.

Thermal shock failure occurs when the thermal gradients generated in a part are so pronounced that differential thermal strains exceed the ability of the material to sustain them without yielding or feature.

Impact failure results when a part is subjected to non-static loads that produce in the part stresses or deformations of such magnitude that the member no longer is capable of performing its function. The failure is brought about by the interaction of stress or strain waves generated by dynamic or suddenly applied loads, which may induce local stresses and strains many times greater than would be induced by the static application of the same loads. If the magnitudes of the stresses and strains are sufficiendy high to cause separation into two or more parts, the failure is called impact fracture. If the impact produces intolerable elastic or plastic deformation, the resulting failure is called impact deformation. If repeated impacts induce cyclic elastic strains that lead to initiation of a matrix of fatigue cracks, which grows to failure by the surface fatigue phenomenon, the process is called impact wear.

Brittle fracture failure occurs when the elastic deformation, in a part that exhibits britde behavior, is carried to the extreme so that the primary plastic structure bonds are broken and the member separates into two or more pieces. Pre-existing flaws or growing cracks form initiation sites for very rapid crack propagation to catastrophic failure, leaving a multifacctcd fracture surface.

Wear is the undcsired cumulative change in dimensions brought about by the gradual removal of discrete particles from contacting surfaces in motion, usually sliding, predominantly as a result of mechanical action. Wear is not a single process, but a number of different processes that can take place by themselves or in combination, resulting in material removal from contacting surfaces through a complex combination of local shearing, plowing, gouging, welding, tearing, and others. Adhesive wear takes place because of high local pressure and welding at contact sites, followed by motion induced plastic deformation and rupture of functions.

Abrasive wear takes place when the wear particlcs arc removed from the surface by the plowing, gouging, and cutting action of the harder mating surface or by hard particles entrapped between the mating surfaces. Deformation wear arises as results of repeated plastic deformation at the wearing surfaces, producing a matrix of cracks that grow and coalesce to form wear particles. Deformation wear is often caused by severe impact loading. Impact wear is impact-induced repeated clastic deformation at the wearing surface that produces a matrix of cracks diat grows.

Radiation damage failure occurs when the changes in material properties induced by exposure to a nuclear radiation field are of such a type and magnitude that the part is no longer able to perform its intended function, usually as a result of the triggering of some other failure mode, and often related to loss in ductility associated with radiation exposure.

The very broad term environment failure implies that a part is incapable of performing its intended function because of the undesired deterioration of the material as a result of chemical or electrochemical interaction with the environment. It often interacts with other failure modes such as wear or fatigue.

Spectrum Loading and Cumulative Damage

With engineering applications where fatigue is an important failure mode, the alternating stress amplitude may be expected to vary or change in some way during the service life. Such variations and changes in load amplitude, often referred to as spectrum loading, make the direct use of standard S-N curves inapplicable bccausc these curves are developed and presented for constant stress amplitude operation

(Chapter 3). Therefore, it becomes important to a designer to have available a theory or hypothesis, verified by experimental observations, that will permit good design esdmates to be made for operation under conditions of spectrum loading using the standard constant amplitude S-N curves.

It has been basically adopted by all fatigue investigators working with spectrum loading that operation at any given cyclic stress amplitude will produce fatigue damage, the seriousness of which will be related to the number of cycles of operation at that stress amplitude and also related to the total number of cycles that would be required to produce failure of an undamaged specimen at that stress amplitude. With this situation the damage incurred is permanent and operation at several different stress amplitudes in sequence will result in an accumulation of total damage equal to the sum of the damage increments accrued at each individual stress level. When the total accumulated damage reaches a critical value, fatigue failure occurs.

Although this concept is simple in principle, much difficulty is encountered in practice because the proper assessment of the amount of damage incurred by operation at any given stress level S, for a specified number of cycles N, is not straightforward. Many different cumulative damage theories have been proposed for the purposes of assessing fatigue damage caused by operation at any given stress level and the addition of damage increments to properly predict failure under conditions of spectrum loading. The fiat cumulative damage theory was proposed by Palmgren in 1924 and later developed by Miner in 1945. This linear theory, which is still widely used, is referred to as the Palmgren-Miner hypothesis or the linear damage rule. The theory may be described using the S-N plot.

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