This chapter will discusses about the literature review of the background study, theory and method of previous research with a lot of references gathering that are very important to prove the experiment method and result in the project.
Welding and joining processes are essential for the development of virtually every manufactured product. However, these processes often appear to consume greater fractions of the product cost and to create more of the production difficulties than might be expected. There are a number of reasons that explain this situation. First, welding and joining are multifaceted, both in terms of process variations such as fastening, adhesive bonding, soldering, brazing, arc welding, diffusion bonding, and resistance welding. Second, a very large percentage of product failures occur at joints because they are usually located at the highest stress points of an assembly and are therefore the weakest parts of that assembly (Thomas, 1993).
Shielded metal arc welding (SMAW), submerged arc welding (SAW), gas metal arc welding (GMAW) (except GMAW-S, short circuiting transfer and GMAW-P, pulse mode), and flux cored arc welding (FCAW) WPSs which conform to all of the provisions of section 3 shall be deemed as prequalified and are therefore approved for use without performing WPS qualification tests for the process (Richard, 2007).
Weld is done by melting the work pieces and adding a filler material to form a pool of molts en material the weld pool that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself to produce the weld. Welded part also face fracture problems that caused by fatigue. So in this study the life of welded jointed under random loading will be predict.
The automotive industry designs, develops, manufactures, markets, and sells the world's motor vehicles. Parts and structure in automotive are often welded together in some fashion, usually due to cost and weight effectiveness. Especially in transport parts like cars, almost of all join in their chassis are welded jointed.
There are always a macro or micro discontinuities in weldments that provide sites for cracks to nucleate. Some of these discontinuities may actually be planar, as in the case of crack, and hence crack nucleation fatigue life may be zero or small in this case. Fatigue life in weldment is then considered to involve only fatigue crack growth life (Jutla, 1996 and Maddox, 1991).
When crack like discontinuities do not exist, weldment fatigue life may be considered to consist of fatigue crack nucleation and fatigue crack growth of small crack leading to the growth of large crack (Rice, 1997 and Lawrence, 1996).
2.3 Automotive
2.3.1 Introduction
Automotive is become very familiar to the human life. Without automotive human's life will be so difficult. The automotive industry in Malaysia can be considered as one of the most important and strategic industries in the manufacturing sector. Compared with other industries in the manufacturing sector in Malaysia, the automotive industry has been earmarked to boost the industrialization process and by that Malaysia can be a developed nation by 2020 (Siti, 2007).
2.3.2 History About Automotive
In the year 1769, a French engineer by the name of Nicolas J. Cugnot invented the first automobile to run on roads. The automobile was a self-powered, three wheeled, which is the military tractor that made and used a steam engine. It could only run at a stretch for fifteen minutes. In addition, these automobiles were not fit for the roads and as the steam engines, it made them very heavy, large, and required impeller at starting time (Alain, 2009).
Oliver Evans was the first to design a steam engine to driven automobile in the U.S. A Scotsman, Robert Anderson, was the first to invent an electric carriage between 1832 and 1839. However, Thomas Davenport of the U.S.A. and Scotsman Robert Davidson were amongst the first to invent more applicable automobiles, making use of non-rechargeable electric batteries in 1842. Development of roads made travelling comfortable and as a result, the short ranged, electric battery driven automobiles were no more the best option for travelling over longer distances (Mohd, 2010).
The Automobile Industry finally came of age with Henry Ford in 1914 for the bulk production of cars. The several methods adopted by Ford, made the new invention, popular amongst the rich as well as the masses (Erjavec, 1999).
2.3.3 Chassis In Automotive
Chassis is an important part of any vehicle that serves as the support of heavy vehicles, machinery and passengers, so the power must be considered in making the chassis of a vehicle.
Chassis in this topic mean the rectangular, usually steel frame, supported on springs and attached to axle, that holds the body and motor of an automotive vehicle. Basic (Stripped) Chassis is an incomplete vehicle without occupant compartment, that requires the addition of an occupant compartment and cargo carrying, work performing, or load-bearing components to perform its intended function.
Chassis cab is an incomplete vehicle with completed occupant compartment, that requires only the addition of cargo carrying, work performing, or load-bearing components to perform its intended functions.
Cutaway chassis is an incomplete vehicle that has the back of the cab cut out for the intended installation of a structure that permits access from the driver's area to the back of the completed vehicle (Fisol, 2006).
2.3.4 Welded Join In Automotive
A space frame chassis is made from steel sections that are welded together to form the structure of the chassis. These chassis are typically MIG welded together and are either painted or powder coated for protection. By welding sheet metal panels together to the right design, a very stiff and strong chassis is constructed. It can be seen in Figure 2.1, how a car chassis being welded.
Figure 2.1 : Car chassis being weld. Erik (2008).
In the study, the fatigue strength of the welded join will be determine by analyze the fatigue life. The comparison of welded join to non welded join also are going to be determine to support the result.
2.4 Fatigue
2.4.1 Fatigue Introduction
The word fatigue comes from the Latin verb fatig are mean "to tire". There are many ways to define engineering fatigue (Radaj 2001 and Gurney 1998). According to General Principles for Fatigue Testing of Metals (1964), the definition evoking technical aspects of the word „fatigue‟ is applies to changes in properties which can occur in a metallic material due to the repeated application of stresses and strains, although usually this term applies specially to those changes which lead to cracking and failure. The definition related to the word engineering that fatigue shows the inability of an engineer to design structures that support relatively small cyclic loads
for a long time.
2.4.2 Fatigue History
There are many researches in the area of fatigue. Some of the earliest research was conducted during the first half of the nineteenth century. Some milestones during the development of fatigue science are well worth nothing and are listed herein (Suresh, 1991).
1829 Albert, the German mining engineer, performed the first probable study of metal fatigue. He rendered repeated load proof tests on mine-hoist chains made of iron.
1839 Poncelet introduced the term fatigue in connection with metal failure.
1843 W. J. M. Rankine, a British railway engineer, recognized the distinctive characteristics of fatigue fractures and noted the dangers of stress concentrations in machine components.
1860 Wöhler conducted systematic investigations of fatigue failure in railroad axles for the German Railway Industry. He observed that the strength of the steel axles subjected to cyclic loads was much lower than the static strength. His work also led to the characterization of fatigue behavior using stress amplitude-life (S-N) curves and the concept of the fatigue „endurance limit‟.
1910 Basquin proposed empirical laws to characterize the fatigue endurance limit of materials.
1913 Inglis, by using stress analyses, and 1921 Griffith, by using an energy concept, provided the mathematical tools for quantitative treatments of fracture in brittle solids.
1924 Palmgren developed a damage accumulation model for fatigue failure.
1939 Westergaard developed a method to determine the stress and displacement field ahead of the sharp crack tip.
1945 Miner developed a damage accumulation model for fatigue failure.
1954 Coffin and Manson discovered independently that plastic strains are responsible for cyclic damage. They proposed an empirical relationship between the number of load reversals to fatigue failure and the plastic strain amplitude.
1957 Irwin showed that the amplitude of the stress singularity ahead of the crack could be expressed in terms of a scalar quantity known as the stress intensity
factor (K).
1960 Dugdale and 1962 Barrenblatt introduced simple crack models.
1961 Paris, Gomez and Anderson were the first to suggest that fatigue crack propagation rate per stress cycle, da/dN, could be related to the range of the stress intensity factor, ΔK.
1970 Elber showed that fatigue cracks could remain closed even when subjected to cyclic tensile loads. His work created a base for development of the crack closure concept. According to Suresh (1991) Fatigue can be classified by the form in
mechanical, creep, thermo-mechanical, corrosion, rolling contact, and fretting fatigue. Fatigue can also be classified by the duration of the fatigue life. They are low-cycle and high-cycle fatigue. That study only examines mechanical and low cycle fatigue which is the most common fatigue types in civil engineering.
2.4.3 Fatigue life
Historically Low Fatigue Cycle most focused on situations that require more than 104 cycles to failure where stress is low and deformation primarily elastic. In high-cycle fatigue situations, materials performance is commonly characterized by an S-N curve, also known as a Wöhler curve. S-N curves are derived from tests on samples of the material to be characterized where a regular sinusoidal stress is applied by a testing machine which also counts the number of cycles to failure. This process is sometimes known as coupon testing. Each coupon test generates a point on the plot though in some cases there is a run-out where the time to failure exceeds that available for the test. Analysis of fatigue data requires techniques from statistics, especially survival analysis and linear regression (Miner, 1945). Low-cycle fatigue is where the stress is high enough for plastic deformation to occur, the account in terms of stress is less useful and the strain in the material offers a simpler description. Low-cycle fatigue is usually characterized by the Coffin-Manson relation (Coffin, 1954 and Manson, 1953).
2.4.4 Strain-Life Approach
Fatigue, a synonym word especially in automotive history is a failure caused by repeated cyclic load. Characterizing the capability of a material to survive the many cycles a component may experience during its lifetime is the aim of fatigue analysis (Rahman, 2009). The example of fatigue incident in the 1800s when several investigators in Europe observed that bridge and railroad components were cracking when subjected to repeated loading. In a general sense, there are several method of fatigue analysis that is Strain Life and Stress Life. Stress Life is based on empirical S-N curves and then modified by a variety of factors. Strain Life is based upon the Strain Life Relation Equation where the Strain Life Parameters are values for a particular material that best fit the equation to measured results (Raph, 2001).
For the Strain-life method, three strain-life models, Coffin-Manson, Morrow and Smith-Watson-Topper (SWT), are available in most of the study (Abdullah, 2008). The Coffin-Manson total strain-life is mathematically defined as in Equation 2.2 (Manson, 1953).
Figure 2.2 : Typical Strain-Life Curve. William (2003).
Equation 2.1
The theory or equation can be used for obtaining the fatigue life of a part when the strain and other cyclic characteristics are given, however it is of little use to the designer. More over it is necessary to compound several idealizations and so some uncertainties will exists in the results. This theory is more applicable to low cycle fatigue.
In designing for the durability, the presence of nonzero mean stress normal stress can influence fatigue behavior of materials due to a tensile or compressive normal mean stress. In conjunction with the local strain-life approach, many models have proposed to quantify the effect of mean stresses on fatigue behavior, Figure 2.2. The commonly used models in the ground vehicle industry are those by Morrow and by Smith Watson Topper. These two models are described in the following sections. Morrow has proposed the following relationship when a mean stress is expressed in Equation 2.2 (Morrow, 1968).
Figure 2.3 : Effect of mean stress on strain-life curve (Morrow Correction). William (2003).
Equation 2.2
Smith, Watson and Topper proposed another mean stress model which is expressed in Equation 2.3 called Smith- Watson-Topper (SWT) mean stress correction (Smith, 1979 and Ouk, 1997).
Equation 2.3
Where Equation 2.4
2.4.5 Fatigue of Weldment
Parts and structure are often welding together in some fashion, usually due to cost and weight effectiveness. Steels, followed by aluminum alloys, are the most frequently welded metals, while some metals cannot be effectively welded. Weldments present difficulties because of macro and micro discontinuities, residual stress and possible misalignment, all of which may vary between nominally equal parts (Raph, 2001).
Several typical weldments are shown schematically in Figure 2.4 with accompanying fatigue strengths for structural steel at 2 x cycles with R = 0 (Richard, 1969). These are rather simplified welded join, but they do represent many real parts and structure.
F:\academic\fyp\fatigue strength on weld.png
Figure 2.4 : Weld type and fatigue strengths for structural steel. Raph (2001).
2.5 Rain Flow Counting
2.5.1 Introduction To Rain Flow Counting
There are three major approaches to analyse fatigue damage or fatigue life. They are Stress-life approach (S-N), the strain-life approach (ε-N) and the linear elastic fracture mechanics (LEFM). However, the strain-life (ε-N) is used for the analysis because in the automotive field must be consider a small deflector to detect a parts of failure the case study was related to the low cycle fatigue.
Level crossing counting, peak counting, simple range counting and rainflow counting are the methods which are using stress or deformation ranges (Christian, 1999). One of the preferred methods is the rainflow counting method. Rainflow cycle counting method has initially been proposed by Matsuiski and Endo to count the cycles or the half cycles of strain-time signals (Matsuishi, 1968).
Counting is carried out on the basis of the stress-strain behavior of the material. This is illustrated in Figure 2.5. As the material deforms from point a to b, it follows a path described by the cyclic stress-strain curve. At point b, the load is reversed and the material elastically unloads to point c. When the load is reapplied from c to d, the material elastically deforms to point b, where the material remembers its prior history, i.e. from a to b, and deformation continues along path a to d as if event b-c never occurred (Secil, 2004).
Figure 2.5 : Stress-strain cycles. Secil (2004).
The signal measured, in general, a random stress S(t) is not only made up of a peak alone between two passages by zero, but also several peaks appear, which makes difficult the determination of the number of cycles absorbed by the structure. An example for the random stress data is shown in Figure 2.6.
Figure 2.6 : Random stress fluctuation. Secil (2004).
The counting of peaks makes it possible to constitute a histogram of the peaks of
the random stress which can then be transformed into a stress spectrum giving the number of events for lower than a given stress value. The stress spectrum is thus a representation of the statistical distribution of the characteristic amplitudes of the random stress as a function of time.
The origin of the name of rainflow counting method which is called 'Pagoda Roof Method' can be explained as that the time axis is vertical and the random stress S(t) represents a series of roofs on which water falls. The rules of the flow can be shown as in Figure 2.7.
Figure 2.7 : The drop released from the largest peak. Secil (2004).
The origin of the random stress is placed on the axis at the abscissa of the largest peak of the random stress. Water drops are sequentially released at each extreme. It can be agreed that the tops of the roofs are on the right of the axis, bottoms of the roofs are on the left.
If the fall starts from a peak:
The drop will stop if it meets an opposing peak larger than that of departure.
It will also stop if it meets the path traversed by another drop, previously determined as shown in Figure 2.8.
The drop can fall on another roof and to continue to slip according to rules a and b.
Figure 2.8 : Flow rule of the drop from a peak. Secil (2004).
If the fall begins from a valley:
The fall will stop if the drop meets a valley deeper than that of departure as shown in Figure 2.9.
The fall will stop if it crosses the path of a drop coming from a preceding valley as given in Figure 2.10.
The drop can fall on another roof and continue according to rules d and e.
The horizontal length of each rainflow defines a range which can be regarded as equivalent to a half-cycle of a constant amplitude load.
Figure 2.9 : Drop departure from a valley. Secil (2004).
Figure 2.10 : Flow rule of the drop from a valley. Secil (2004).
As the fundamentals of the original definition of the rainflow cycle counting given above, the cycles are identified in a random variable amplitude loading sequence in Figure 2.11 as an example. First, the stress S(t) is transformed to a process of peaks and valleys. Then the time axis is rotated so that it points downward. At both peaks and valleys, water sources are considered. Water flows downward according to the following rules (Paul, 1995):
(a)
(b)
Figure 2.11 : Rainflow cycle counting, (a) before rotate and (b) after rotate. Paul (1995)
A rainflow path starting at a valley will continue down the "pagoda roofs", until it encounters a valley that is more negative than the origin. From the figure, the path that starts at A will end at E.
A rainflow path is terminated when it encounters flow from a previous path. For example, the path that starts at C is terminated as shown.
A new path is not started until the path under consideration is stopped.
Valley-generated half-cycles are defined for the entire record. For each cycle, the stress range Si is the vertical excursion of a path. The mean Si is the midpoint.
The process is repeated in reverse with peak-generated rainflow paths. For a sufficiently long record, each valley-generated half-cycle will match a peak-generated half-cycle to form a whole cycle.
